Document [original]

i

Transition Metal Complexes with Imine-phenolate and Imino-

benzosemiquinone Ligands; Synthesis, Characterization

and Their Catalytic Reactivity

Dissertation for the degree of

Doktor der Naturwissenschaft

in the Fakultät für Naturwissenschaften

(Department Chemie)

at the Universität Paderborn

presented by

Soumen Mukherjee

Mülheim an der Ruhr 2003

ii

iii

To my Parents

iv

v

Knowledge, the object of knowledge

and the knower are the three factors

which motivate action; the senses,

the work and the doer comprise the

threefold basis of action.

" Bhagavad Gita "

vi

This work was independently carried out between December 1999 and July 2003 at the

Max-Planck-Institut für Bioanorganische Chemie, Mülheim an der Ruhr, Germany.

Papers published:

1. S. Mukherjee, T. Weyhermüller, E. Bothe, K. Wieghardt , P. Chaudhuri;

Single-atom O-bridged Urea in a Dinickel(II) Complex together with NiII4, CuII2 and CuII4

Complexes of a Pentadentate Phenol-containing Schiff Base with (O,N,O,N,O)-Donor Atoms.

Eur. J. Inorg. Chem. 2003, 863.

2. S. Mukherjee, T. Weyhermüller, E. Bothe, P. Chaudhuri;

Structural, Magnetochemical and Electrochemical Studies of Dinuclear Complexes

Containing the [VVO]2, [VIVO]2, CrIII2, MnIII2 and FeIII2 Cores of a Potentially Pentadentate

Phenol-Containing Ligand with (O,N,O,N,O)-Donor Atoms.

Eur. J. Inorg. Chem. 2003, 1956.

3. S. Mukherjee, E. Rentschler, T. Weyhermüller, K. Wieghardt , P. Chaudhuri;

A unique series of dinuclear transition metal-polyradical complexes with a m-

phenylenediamine spacer and their catalytic reactivity.

Chem. Comm., 2003, 1828.

4. S. Mukherjee, T. Weyhermüller, K. Wieghardt, P. Chaudhuri

The Molecular and Electronic Structure of [FeIII2(t-buLISQ)4(µ-O)] - a Dinuclear Ferric

Complex Containing Four, O,N-Coordinated o-Iminobenzosemiquinonate(1-) π Radical

Anions

Dalton Trans., 2003, 3483.

Examination Committee:

Prof. Dr. N. Risch

Prof. Dr. K. Huber

Prof. Dr. G. Henkel

Prof. Dr. P. Chaudhuri

Examination: 31st October, 2003.

vii

Acknowledgements

I would like to acknowledge everyone who extended their support and help during the course

of this work. My deep sense of gratitude goes to:

• Prof. Dr. P. Chaudhuri, for his encouragement, invaluable guidance, frequent

suggestions, continuous support and hour-long discussions.

• Prof. Dr. K. Wieghardt, for the opportunity of working in his research group and

providing with all needed laboratory facilities.

• Dr. T. Weyhermüller and Mrs. H. Schucht for their excellent work with the X-ray

crystallography.

• Dr. E. Bothe , Mrs. P. Höfer and Mr. H. Schmidt for their help during electrochemical

measurements.

• Dr. E. Rentschler and Dr. U. Schatzschneider for assistance with the fitting of some

magnetochemical data.

• Dr. E. Bill, Mr. A. Göbels, Mr. F. Reikowski and Mr. B. Mienert for discussions and

measurements of EPR, SQUID and Mössbauer spectroscopy.

• Dr. K. Hildenbrand for the measurement and simulation of room temperature EPR.

• Mr. U. Pieper and Mrs. R. Wagner for their help in the laboratory.

• Mrs. U. Westhoff, Mrs. G. Schmidt and Mrs. M. Trinoga for skilful GC and LC

analyses.

• Mrs. J. Teurich, Mrs. E. Hollander, Mrs. T. Montenbruck and Mrs. B. Deckers for

their helpfulness in general.

• Mr. K. Chlopek, Dr. N. Aliaga-Alcalde, Dr. U. Beckmann, Dr. C. N. Verani, Dr. R.

Schumacher, Dr. S. Kimura and Dr. T. Kruse for their interest and friendship during

this work.

• Dr. T.K. Paine, Mr. K. Ray, Dr. D. Ray, Mr. S. Khanra, Mrs. R. Kapre and Mr. S.

Kinge for a nice atmosphere in- and outside the laboratory.

• My parents for their constant inspiration and encouragement.

• Family Basak, Family Göhl, Family Gupta-Nehring, Family Ghosh and Dr. S.

Chatterjee for providing a nice atmosphere outside the laboratory.

• Deutsche Forschungsgemeinschaft (DFG) and Max-Planck-Gesellschaft (MPG) for

financial support.

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CONTENTS

Chapter 1

OBJECTIVES AND INTRODUCTION

1.1 METALS AND RADICALS IN BIOLOGICAL SYSTEMS 04

1.1.1 Phenol; the tyrosine mimic 05

1.1.2 Imidazole-bridged ligands 07

1.1.3 Dinucleating ligands 07

1.2 MOLECULAR MAGNETISM 10

1.2.1 Accidental ferromagnetism 10

1.2.2 Planned orthogonality by spin polarization 11

1.2.3 Spin-crossover of Iron(III) compounds 14

References 15

Chapter 2

TRANSITION METAL COMPLEXES WITH

IMINE-PHENOLATE LIGANDS

2.1 INTRODUCTION 21

2.2 SYNTHESIS AND CHARACTERIZATION OF LIGANDS 21

2.3 TRANSITION METAL COMPLEXES WITH H3L1 AND H3L2 23

2.3.1 Infrared and Mass spectroscopy of complexes 1-11 24

2.3.2 Crystal structure and characterization of the complexes 26

References 61

Chapter 3

TRANSITION METAL COMPLEXES WITH

IMINO -BENZOSEMIQUINONE LIGANDS

3.1 INTRODUCTION 67

3.2 SYNTHESIS AND CHARACTERIZATION OF LIGANDS 68

3.3 TRANSITION METAL COMPLEXES WITH H4L3 and H4L4 69

3.3.1 Infrared and Mass spectrometry of the complexes 12-17 70

3.3.2 Crystal structure and characterization of the complexes 72

References 96

x

Chapter 4

MnIV COMPLEXES WITH IMINO-BENZOSEMIQUINONE LIGANDS;

SYNTHESIS, CHARACTERIZATION AND REACTIVITY STUDY

4.1 INTRODUCTION 99

4.2 CATECHOL OXIDASE MODEL STUDIES 100

4.3 SYNTHESIS AND CHARACTERIZATION OF MnIV(LA6)(L6)2 (18) 104

4.4 OXIDATIVE STUDIES WITH 2,6-DI-TERT-BUTYL-PHENOL 112

References 113

Chapter 5

FeIII AND CoIII COMPLEXES WITH IMINO-BENZOSEMIQUINONE

LIGANDS; EFFECT OF SUBSTITUTION

5.1 INTRODUCTION 117

5.2 SYNTHESIS AND CHARACTERIZATION OF LIGANDS 117

5.3 SYNTHESIS AND CHARACTERIZATION OF COMPLEXES 118

References 131

Chapter 6

CONCLUSIONS AND PERSPECTIVES 135

Chapter 7

EQUIPMENT AND EXPERIMENTAL WORK

7.1 METHODS AND EQUIPMENTS 143

7.2 SYNTHESIS

7.2.1 LIGANDS 146

7.2.2 COMPLEXES 151

7.3 REACTIVITY STUDIES 172

Appendices

1) Crystallographic data 175

2) Magnetochemical data 181

3) Magnetic and EPR data 203

4) Curriculum Vitae 205

xi

Abbreviations:

technical terms:

aCU: antiferromagnetic coupler

AF : antiferromagnetic

Ag / AgNO3 : reference electrode

av. : average

B : magnetic field

CT : charge transfer

D : zero-field splitting

deg. : degree (°)

e- : electron

E : total energy

E.C. : enzyme classification

EHMO : extended Hückel molecular orbital

exp. : experimental

fCU: ferromagnetic coupler

F : ferromagnetic

fac. : facial

Fc+/Fc : internal electrochemical standard

g : Landé factor

H : Hamiltonian

HS /h.s: high spin

I : nuclear spin

IS : intermediate spin

J : coupling constant ( cm-1)

LS : low spin

m/z : mass per charge

[M]+: molecular ion peak

M : molar magnetization

m- : meta-

mer.: meridional

MP : melting point

PI : paramagnetic impurity

RT : room temperature (293K)

xii

S : electron spin

SCE : standard calomel electrode

sh. : shoulder

sim. : simulated

TIP : temperature independent paramagnetism

TON : turnover number

techniques:

CV : cyclic voltammetry

EA : elemental analysis

EI : electron ionisation

EPR : electron paramagnetic resonance

ESI : electrospray ionization

FTIR : Fourier transform infrared spectroscopy

GC : gas chromatography

HPLC : high performance liquid chromatography

IR : infrared spectroscopy

LC : liquid chromatography

MS: mass spectroscopy

NMR : nuclear magnetic resonance

OTTLE : optically transparent thin layer electrochemical

SQUID : superconducting quantum interface device

SQW : square wave voltammetry

UV-Vis : ultraviolet-visible spectroscopy

XRD : X-Ray Diffractometry

units:

Å : angstrom (10-10 m)

cm : centimeter

emu : electromagnetic unit

G : gauss

h : hour

K : Kelvin

m : meter

xiii

M : molar

min. : minute

mm : millimeter

nm : nanometer (10-9 m)

ppm : part per million

s : second

T : tesla

V : volts

µB : bohr magnetron

latin expressions:

ca. : around

et al : and coworkers

e.g. : for example

i.e. : that is

tert- : tertiary

vs. : versus, against

viz. namely

symbols:

λ : wavelength (nm)

ε : extinction coefficient (M-1cm-1)

θ : Theta- Weiss parameter

IS : isomer shift (mms-1)

µeff : magnetic moment (µB)

∆EQ : quadrupole splitting (mm/s)

δ : isomer shift (mm/s)

solvents and reagents:

Bu4NOMe : tetrabutylammoniummethoxide

cat.: catechol

CH2Cl2 : dichloromethane

CDCl3 : deuteriated chloroform

DTBP : 2,6-di-tert-butylphenol

xiv

3,5-DTBC : 3,5-di-tert-butylcatechol

3,5-DTBQ : 3,5-di-tert-butyl-o-benzoquinone

Et2O : diethylether

Et3N : triethylamine

EtOH : ethanol

H2O2 : hydrogen peroxide

KBr : potassium bromide

MeCN : acetonitrile

MeOH : methanol

TBAPF6 : tetrabutylammonium hexafluorophosphate

THF : tetrahydrofuran

TMS : tetramethylsilane

TTBD : 3,3´-5,5´-Tetra-tert-butyldiphenoquinone

ligands used in this work:

H3L1 :- 2,4-Di-tert-butyl-6-[(5-methyl-3H-imidazol-4-ylmethyl)-amino]-phenol

H3L2 :- 2,6-Bis-iminomethyl-(4,6-di-tertbutyl-2-iminophenol)-4-methyl-phenol

H4L3 :- 1,3-bis-(4,6-di-tert-butyl-2-iminophenol)benzene

H4L4 :- 4,4´-bis-(4,6-di-tert-butyl-2-iminophenol)diphenyl methane

H2L6-12 :- 2-(mono- to di- substituted)anilino-4,6-ditert-butylphenol

List of complexes synthesized with their numbers:

NiII4L14 (1)

CuII4L14 (2)

[NiII2 (L2)(NH2CONH2)(OAc)(MeOH) 2] (3)

[NiII4 (L2H) 2(OMe) 2(OAc) 2(MeOH) 2]

(4)

[CuII2 L2 (MeO) (THF)2] (5)

[CuII4L22 (µ4-O) ]

(6)

FeIII2 L22 (7)

MnIII2L22 (8a)

[MnIII2L22 (THF)2] (8b)

CrIII2L22

(9)

[(VIV=O)2 (µ-Oisoprop) L2)] (10)

xv

[(VV=O)2 L22] (11)

CuII2L32 (12)

NiII2L32 (13)

CoIII2L33 (14)

FeIII2L33 (15)

MnIV2(LA3)2L3 (16)

CoIII2L43 (17)

MnIV(LA6)(L6)2 (18)

MnIV(LA7)(L7)2 (19)

MnIV(LA8)(L8)2 (20)

MnIV(LA9)(L9)2 (21)

MnIV(LA10)(L10)2 (22)

CoIII(L6)3 (23)

FeIII(L6)3 (24)

FeIII(L11)3 (25)

[FeIII2(µ-O)(L12)4] (26)

xvi

1

Chapter 1

OBJECTIVES AND INTRODUCTION

Fe

2

3

OBJECTIVES AND OUTLINE OF THE THESIS

Two apparently dissimilar subjects viz. molecular magnetism and metal sites in

biology are at the center of this thesis. The principles of coordination chemistry e.g. ligand

field theory constitute a common ground for molecular magnetism, biomimicking and

bioinspired chemistry. Summarily, this thesis describes model complexes both of structural

and functional types, for various metalloprotiens containing paramagnetic metal ions, with

particular emphasis on the interactions of ligand radicals with transition metal ions using

different spectroscopic techniques which help one to learn how nature has employed common

transition metals in a number of intriguing catalytic transformations.

This work is divided in five chapters. The first chapter gives an introduction relevant

to this work and consists of two parts. The first part (Chapter1) considers the importance of

metals and radicals in biology. The importance of ligands as a backbone for metal centers and

also as an activation center for catalysis is discussed. A few examples of metalloenzymes,

whose structural and functional models are relevant to this thesis, is outlined. The second part

(Chapter1) discusses the building up of polynuclear “parallel spin coupled” system using

“accidental ferromagnetism” and “planned ferromagnetism” both governed by the common

principle of orthogonal orbital overlap.

The second and fourth chapters are relevant to “metallobiochemistry” and the third

and fifth chapters to “molecular magnetism”. The second chapter deals with the synthesis and

characterization of 3d- di- and tetranuclear homometallic complexes bridged by imidazolate,

phenolate and urea ligands. Some of the complexes act as structural models for enzymes like

the Cu-Zn superoxide dismutase and the dinickel containing enzyme urease. In the fourth

chapter, a functional model of the dicopper containing enzyme catechol dioxygenase, which

catalyses the oxidation of catechols to quinones , is discussed.

The third chapter concerns the synthesis and characterization of transition metal

containing imino-semiquinone radical complexes. All the complexes consists of

homodimetallic centers with four or six ligand-based radical centers. Here the concept of

spin polarization was used in an attempt to bring the metal or radical centers in spin aligned

arrangement i.e. coupling between the metal-metal or radical-radical centers are

ferromagnetic. Continuing with polyradical based metal complexes the fifth chapter

highlights the effect of substitution at different position in tuning the spin ground state of a

metal center. The complexes synthesized are mono- and dinuclear with three or four imino-

benzosemiquinone radicals.

4

1.1 METALS AND RADICALS IN BIOLOGICAL SYSTEMS

One of the major roles played in biochemistry is by the metalloenzymes.

Metalloenzymes are those enzymes, which require a metal center not only as an active site to

function but bind that metal ion (or ions) strongly even in the resting state. There are several

hundred metalloenzymes and they belong to the subclass of metalloprotiens i.e. proteins,

which incorporate one or more metal atoms as a normal part of the structure.1

As these metalloenzymes are large macromolecules with molecular weights ranging in

kilodaltons (1 dalton ≈ weight of 1 proton) the interest for the synthesis of smaller molecular

weight compounds, which can act as structural, and, perhaps as functional model, grew. These

smaller molecules can serve as good mimics for the metalloenzymes and can help in

understanding their nature and characteristics. The amino acid containing bases like histidine,

tyrosine and cystine sparked and fueled the synthesis of a large number of low molecular

weight ligands containing imidazole, phenol and thiol respectively. It is observed that the

active center of a metalloenzyme constitutes either a single metal center (e.g. Galactose

oxidase with a single copper center) or multi-metal centers (e.g. Urease with a dinickel

center). Thus, in order to mimic the structural model of the respective metalloenzyme, the

nature of ligand plays an important role.

Ligands should, therefore, be designed in such a way that the amino acid residue can

be replaced by commonly occurring donor groups and also fit multimetal centers. The most

common ligands used by bioinorganic chemists are based on phenolate (tyrosinase mimic)

and imidazolate (histidine mimic). In order to mimic metalloenzymes containing two metal

centers (the simplest!), dinucleating ligands have been used. The importance of these three

types of ligands, which are pertinent to this work, in bioinorganic chemistry is outlined below.

With the advancement about the knowledge of metals in biochemistry, the role of

radicals, both stable and transient, started to gather interest. Table 1.1 shows some of the

protein radicals along with some of their characteristics.2

5

O

.

S

.

NH

O

.

NH

ON

H

.+

Table 1.1:- Some protein radicals in enzymatic systems

1.1.1 Phenol; the tyrosine mimic

The better stability and easy detection of tyrosyl radicals in comparison to other

radical systems invoked the bioinorganic chemists to synthesize model complexes where

phenol based ligands were used. Phenoxyl radicals have been generated either chemically or

electrochemically and studied extensively. Electronic spectra of phenoxyl radicals show that

irrespective of the substitution pattern, the absorption maximum lies at ∼380 nm(ε ≈ 1.5 x 103

M-1 cm-1 ) and 400 nm (ε > 1.8 x 103 M

-1 cm-1 ) with a weak maximum at 600-700 nm

(ε ∼ 500 M-1 cm-1).7 The absorption range of phenoxyl radical at 380-400 nm can be

compared to that of tyrosyl radicals, which has absorbance at 407 nm [Table 1.1].

Electrochemical data (cyclic voltametry) for phenoxyl-phenolate and phenoxyl-phenol

couples in water have been obtained. Presence of electron donating substituents at 2 or 4

positions decreases the oxidative potentials of phenols making the phenoxyl radicals much

Radical

type

Structure Examples

(found in)

UV-vis, λmax (nm)

[εmax (M-1 cm –1) ]

Reference

Tyrosyl

Class I Ribonucleotide

Reductase (RNR)

Galactose oxidase

Plasma amine oxidase

407 [3200]

3

Thiyl

Class II RNR

300-330 [400-1200]

4

Glycyl

Class III RNR

Pyruvate formate lyase

365 [8000]

5

Trptophan

Cytochrom peroxidase

320 [2800]

6

GO , O2

Figure 1.1:- Ligands used by Chaudhuri et. al. as functional models of Galactose Oxidase

X

OH OH

more easily available.6,8 Tertiary butyl group has been a good choice as it provides excellent

electronic stability to the phenoxyl radicals by increasing the electron density at oxygen. The

most noteworthy observation was in electrochemistry where the oxidative peaks of the cyclic

voltammogram tend towards reversibility as one increases the electron donating properties of

the 2 and 4 substituent groups.

The main research goal of this group is to synthesize simple molecular models, which

can act both as structural and functional models for metalloenzymes. Most of the synthesized

ligands were phenol based and were redox active or “non-innocent” in character. By redox

active it is meant that the ligand itself is easily oxidized in mild conditions. Using “non-

innocent” ligands to bind to metal centers were first used for magnetism but once the role of

radicals in enzymatic catalysis skyrocketed 2, new complexes were synthesized which act as

possible functional model for metalloenzymes. One such metalloenzyme, which has been

studied in detail, is Galactose Oxidase (GO) [EC 1.1.3.9]. It is a copper-containing enzyme,

which catalyzes the two-electron oxidation of primary alcohols to their corresponding

aldehydes.10

RCH2OH RCHO + H2O2

The X-ray structure at 1.9 Å resolution shows that the copper is in a square pyramidal

geometry with two tyrosine residues, two histidine residues and a water or acetate at the 5th

position11. From a mechanistic view tyrosyl radical is generated in presence of substrate,

which actually catalyzes the oxidation12. Stack et. al.13 and Chaudhuri et. al.14,17 have

synthesized copper complexes with phenol containing ligands, which act as functional models

of Galactose Oxidase (Figure1.1). The ligand H3LN, originally reported by Girgis and Balch15,

was found to be redox active as it forms air stable free radical in presence of metal centers and

air. The copper complex of the ligand H2LS forms transient phenoxyl radicals in solution as

observed was found to by UV and EPR spectroscopy.

H3LN when X =NH

H2LS when X =S

H2LSe when X =Se

7

O2

Amine Oxidase

Another copper enzyme containing a tyrosinase residue is plasma amine oxidase. This

enzyme catalyzes the two electron oxidation of primary amines to the corresponding

aldehydes.

RCH2NH2 RCHO + H2O2 + NH3

However, the tyrosine is in a modified form acting as a co-factor and bound independently

from the copper center. The copper center lies in a distorted square pyramidal geometry

bounded to 3 histidine bases and two water molecules 16. The copper complex with both the

ligands H2LS and H2LSe were found to be good functional models for amine oxidases.17

Tyrosine residue and subsequently tyrosyl radicals were found to play important roles

in other metalloenzymes e.g. the iron containing enzyme class I Ribonucleotide Reductase 18

or in the Yz. component in manganese containing Photosystem II19 where a tyrosine residue is

oxidized to an intermediate tyrosyl radical which subsequently reduces back to tyrosine

before the next turnover.20

1.1.2 Imidazole-bridged ligands

The role of imidazole, a five membered nitrogen hetrocycle, is well known in chemical

and biological systems21. It occurs in proteins as part of the side chain in the amino acid

histidine, in nucleic acid structures as part of the purine ring of adenosine and guanine and in

the vitamin B12 coenzyme as benzimidazole. A good example where imidazole acts as

bridging ligand is the bovine erythrocyte superoxide dismutase (BESOD)22 in which it bridges

a Cu(II) and a Zn(II) ions. The main function of this enzyme is to protect animals from

microbial infection by destroying superoxides to yield molecular oxygen and hydrogen

peroxide. There are quite a number of imidazolate bridged metal complexes which provides

functional models23 for various enzymes but these complexes have been more actively studied

in order to understand the extent of exchange coupling between the two paramagnetic metal

centers through the imidazolate bridge.24,25 Unfortunately only a few of these structurally

characterized compounds have been subjected to a combined EPR and magnetic susceptibility

study.25

1.1.3 Dinucleating ligands

Since the first report by Robson26 in 1970 of a dinucleating Schiff-base ligand obtained

by condensation of 2,6-diformyl-4-methylphenol with 2-aminophenol, many examples of

8

similar compartmental ligands have been reported.27 The dinucleating ability of these ligands

stems from the readiness of the phenol to deprotonate and bridge two metal ions. Recently

trinuclear and tetranuclear complexes with the ligands derived from the condensation of 2,6-

diformyl-4-methylphenol and selected diamine or hydroxylamine were reported, which yield

heteronuclear complexes of the type MAMBMC and MAMBMBMA.28 Dinucleating ligands are

also helpful in providing the backbone for the synthesis of structural and functional models

for metalloenzymes having a homodinuclear center at the active site. Two such enzymes,

which are relevant to this work, are the dinickel containing hydrolase enzyme urease [E.C.

3.5.1.5]29 and the dicopper enzyme catechol oxidase30 [E.C. 1.10.3.1].

Urease, which is present in bacteria, fungi and higher plants, catalyzes the hydrolysis

of urea to ammonia and carbon dioxide. The structure of urease shows that each nickel ion is

coordinated to two histidine residues from the protein, and a carbamylated lysine residue

bridges the two metal ions. The second nickel ion is additionally ligated by an aspartate

H2N - C - NH2

O

2 NH3 + CO2

urease

H2O

residue. Two terminally coordinated water molecules and one bridging water molecule results

in a distorted square pyramidal environment for one nickel site and pseudooctahedral for the

other nickel site [Figure 1.2].29b,29c,29e,31

Figure 1.2:- Schematic view of the active site of urease from K. aerogenes.

There are quite a few dinickel complexes relevant to the active site of urease32. Several

dinuclear nickel(II) complexes with urea33 have been reported as models for possible binding

modes of urea in urease. But none of the complexes except one34 reported so far contains a

carbonyl single-atom bridging urea. In this work, a dinuclear nickel(II) complex with a

bridging urea through a single-atom O-bridging of the carbonyl oxygen is reported35a.

Unfortunately, presence of strong hydrogen bonding in the molecule inhibits the attack of

9

nucleophile (here ethanol) to allow any hydrolysis. This type of models indicates that there

might be an alternative mechanistic pathway for the hydrolysis of urea.

As mentioned earlier, the role of radicals in biological electron transfer processes

surged the interest for the synthesis of radical containing metal complexes. The ligand

reported by Girgis and Balch 15(H3LN) oxidizes in air with the formation of free radical. Later

on, Pierpont and coworkers synthesized a variety of complexes using 3,5-di-tert butyl

catechol37. Chaudhuri et. al.38 reported a new aminophenol containing ligand 2-anilino-4,6-di-

tert butylphenol, which oxidizes sequentially to imino-benzosemiquinato monocation (SR =

½) and finally the neutral o-imino-benzoquinone.

As a natural and obvious progression, a dinucleating redox active ligand is reported

here based on meta-phenylenediamine. Using the dimetallic complexes of this ligand, an

oxidative reactivity study was carried out in order to mimic the function of catechol

oxidase39,40, a type III copper protein that catalyzes the two-electron oxidation of catechols to

quinones in presence of air.

O

OH

Catechol Oxidase

O2

+H2O2

10

1.2 MOLECULAR MAGNETISM

It is now clear that magnetic property of a material can be determined by a way in

which the unpaired electrons interact with each other. In case of a ferromagnet, the unpaired

spins are parallely “high-spin” coupled to each other. From Pauli’s principle, electrons of like

spins are forbidden to occupy the same region of space i.e. nature always prefers

antiferromagnetic coupling of two weakly interacting electrons. So, on designing

ferromagnets, one cannot create a large number of spins, crystallize or condense them and

then hope for the best! The challenge for the chemist is therefore to go against nature and

create parallel spin coupled systems against common natural law.

The exchange pathway for magnetic interactions between metal centers are dependent

on the nature of pathway linking the centers.41,42 If the metal based orbitals are not close

enough to overlap directly, the nature of spin interactions are formalized by the Goodenough-

Kanamouri rules.43 A number of complexes with predictable magnetic properties have been

synthesized using this principle.42,44 However, if the metal centers are far apart to overlap

directly, the bridging ligand mediates in the interaction (superexchange process) i.e. even if

the pure metal orbitals cannot overlap directly, mixing of these orbitals with the orbitals of the

bridging ligands means that the magnetic orbitals may not be purely metal centered but also

have a significant ligand-based component, and in such cases direct overlap of the magnetic

orbitals can still occur. Thus the bridging ligand plays an important part by mediating in the

electronic interactions. This type of interactions was seen in organic radicals45 where it was

shown that special topology and structure was needed. For long range interactions, the

topological importance grew with the generation of the McConnell “spin-polarization”

mechanism.46

1.2.1 Accidental ferromagnetism

The Goodenough-Kanamouri rules is based on the nature of overlap between the

orbitals of the different magnetic centers via the intervening ligand orbitals. Two orbitals can

overlap either in an orthogonal fashion or a non-orthogonal fashion. If the orbitals interact

with each other in a non-orthogonal fashion, the interaction is anti-ferromagnetic. However, if

there is overlap between half filled orbital of one ion with either an empty or filled orbital of

another, ferromagnetic interaction occurs. This concept has been employed in controlling the

magnetic properties of polynuclear coordinated complexes. A good example is the

magnetostructural correlation associated with an octahedral coordinated dinickel center

11

bridged by an oxygen atom.47a There are several complexes reported with the [Ni4(OR)4] (R =

H, alkyl) cubane structure along with their magnetochemical property. It has been observed

that ferromagnetic exchange between the two nickel centers are generated when the Ni-O-Ni

angle tends towards orthogonality. Such type of “accidental ferromagnetism” of

superexchange transmitted through bridging atoms can also be observed through different

donor atoms e.g. nitrogen in azide 48b.

1.2.2 Planned orthogonality by spin polarization

The concept of spin polarization46,48was first applied to organic conjugated di-radicals

and was first observed in trimethylenemethane (1, Fig. 1.3) by Dowd.45b,46 Later on 1,2 and

1,4 connected diradicals were also synthesized. Unfortunately both of them turned out to have

antiferromagnetic interactions between them. Calculation for 1 showed that the single triplet

energy gap is +15 Kcal/mol.49 If m- benzoquinonedimethane diradical moiety is taken,

calculation predicts that the interaction is ferromagnetic and the energy between the singlet

and triplet state is +10 Kcal/mol [Figure 1.3]50.

A plausible explanation for such type of behavior can be given by spin polarization. It

....

..

fCU

aCU

fCU

aCU

1

2

3

4

Figure 1.3 :- Different organic π−conjugated di-radical systems with their coupling scheme

aCU :- antiferromagnetic coupler ; fCU :- ferromagnetic coupler

12

A plausible reason for such type of behavior can be given by spin polarization. It describes

how an unpaired electron polarizes the electron cloud on the adjacent atom in the opposite

sense. Spin densities at the adjacent atomic centers in a π-conjugated system always prefer

opposite signs α and β. This leads to an αβαβαβ spin pattern [Figure 1.4]. This concept of

spin polarization was adopted by the inorganic chemist although it lagged behind in the

αβαα

β

α

αβ

.

...αβ

α

β

.

αβ

α

β

..

β

antiparallel spin;

antiferromagnetic

parallel spin;

ferromagnetic

parallel spin;

ferromagnetic

antiparallel spin;

antiferromagnetic

Figure 1.4:- Spin propagation along substituted benzene moiety

development of organic ferromagnets. The advantages of inorganic over organic paramagnetic

centers include high spin density, improved chemical stability (organic radicals tends to

polymerize) and the possibility of reversible redox activity (for switching). Soon high spin

binuclear transition metal complexes with suitable bridging ligands as ferromagnetic coupler

were reported. Ferromagnetism was ascribed mainly to i) the meta-substitution pattern of the

ligand and ii) d(π) – p (π) overlap between metal and ligand which allowed propagation of the

exchange interaction by a simple spin polarization mechanism involving the d(π) unpaired

electrons in the metal centers and the p (π) electrons in the bridging ligand.51

There are a number of complexes, which have been synthesized using the spin

polarization mechanism. Different bridging ligands like 1,3 bipyridine, polypyridines, 1,3

substituted phenols and 1,3,5 substituted phenols have been used as ferromagnetic couplers

and are listed in Table 1.2.

Another ligand which acts as a ferromagnetic coupler is the meta-phenylenediamine

moiety. Here again, the 1,3 position helps in αβαβαβ spin propagation forcing the electron

centers in the metal to be spin parallel. Fernández et. al.52 synthesized a copper(II) dimer with

the ligand, a parent acid of N,N´-1,3-phenylenediaminebis(oxamate) where the electrons in

the copper atoms were aligned in a spin parallel orientation. From the susceptibility

measurement the value of magnetic interactions between the two copper centers were +8.4

cm-1. This was further supported by theoretical calculations which showed that the single-

triplet energy gap was +7.1 cm –1 [Figure 1.5].

13

NN

N

NN

N

O O

--

O O

O

--

-

Table 1.2:- Different substituted ligands used as ferromagnetic couplers

Figure 1.5 :- The dicopper(II) complex synthesized by Fernández et. al.52

Bridging ligands used as ferromagnetic coupler Reference

51b, 51c, 51d, 51f, 51g, 51h, 51i

51d, 51j

51d

51a, 51b, 51j

51j, 51e

Cu

N N

O

NaO

O

ONa

OO

N

NO

ONa

O

NaO O

O

Cu

14

Keeping a view on the above aspects, it is clear that in order to synthesize molecules

with ferromagnetic interactions between the two paramagnetic centers, the derivatization of

1,3 position is important. As mentioned earlier, it is also clear that imino-semiquinone

radicals are easily stabilized by a 3d-metal. These two aspects were conjoined and a new

ligand was synthesized based on a 1,3 diaminobenzene moiety where the ligand oxidizes

easily in presence of air and the radicals are stabilized by the 3d-metals. The 1,3 position

necessitates ferromagnetism between the paramagnetic centers.

1.2.3 Spin-crossover of Iron(III) compounds

The magnetic properties of mononuclear iron (III) complexes are now well

understood. Fe(III) has an electronic configuration of 3d5 and the complexes can be high spin

(S=5/2, HS), intermediate spin (S=3/2 , IS) or low spin (S= ½, LS). In an octahedral geometry,

presence of strong crystal field increases 10Dq (the energy difference between the eg and the

t2g set of orbitals) leading to the formation of low spin electronic ground states whereas in

presence of weak crystal field strength ligand, the eg states are also populated due to

weakening of 10Dq leading to the formation of high spin electronic ground states. Thus the

majority of the complexes are either high spin or low spin. Only a few complexes are known

where the iron is in the intermediate spin state.53

The spin state of a metal can also change within the same molecule. In Fe(II)55a,55d

and Co(II)54b systems this “spin crossover” can be observed. For Fe(III) the most widespread

example known is the iron(III) dithiocarbamate55c where a 6A1g (HS) ⇔ 2T2g(LS) crossover is

observed. In this work, an iron complex has been described which shows a HS ⇔ LS spin

crossover. The iron is in an six-coordinated or distorted octahedral geometry and the ground

state for iron is 1/2 .

15

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Pandeya, H. Singh, J. Ind. Chem. Soc., 2001, 78,525 (e) P.Gültich , J. Jung , Nuovo Cimento, 1996, 18, 107.

19

Chapter 2

TRANSITION METAL COMPLEXES WITH

IMINE-PHENOLATE LIGANDS

M

N

O

R

20

21

O

NH2

OH

N

O

H

N

OH H

H

NH4OH, Methanol,

1. NaBH4

2. Water

Argon

NaBH4,Argon

H3L1

2.1 INTRODUCTION

The homopolynuclear complexes are of interest to the inorganic chemists for their

relevance to biology and magnetism (Chapter 1). This chapter describes some 3d-transition

metal containing di- and tetranuclear complexes which are relevant to some metalloenzymes

and their importance in magnetostructural correlation. The ligands chosen are phenol based

with either imidazole- or imine-containing side chain.

2.2 SYNTHESIS AND CHARACTERIZATION OF LIGANDS

The interest for the synthesis of redox-active aminophenol ligands paved the way in

preparing H3L1 (2,4-Di-tert-butyl-6-[(5-methyl-3H-imidazol-4-ylmethyl)-amino]-phenol).

When 2,4-di-tert-butyl-6-aminophenol is condensed with 5-Methyl-3H-imidazole-4-

carbaldehyde and then further reduced by NaBH4 in methanol, the ligand H3L1 precipitates as

a white solid in an aqueous medium (Figure 2.1). It has a molecular peak at m/z 315 (EI-MS)

with the characteristic peaks at IR (Table 2.1). NMR spectroscopy unambiguously proves the

presence of 26 protons (3 protons are exchangeable).

Figure 2.1 :- Synthetic procedure for the preparation of H3L1

In a methanolic solution containing a few drops of triethylamine, this ligand is redox active

i.e. it forms radicals in presence of air. When this solution is subjected to EPR studies, a six

line hyperfine spectrum appears[Figure 2.2]. The radical center couples with the nitrogen

center (I = 1) and the proton centers (I= ½), giving rise to the six hyperfines. A simulation of

this spectrum shows that the coupling constants for nitrogen is 4.74 G and for the two protons

9.30 G and 3.59 G respectively. The g value is centered at 2.0049 which clearly shows that

the ligand is “non-innocent” and forms imino-semiquinone radical in air.

It has already been reported that tert-butyl substituents at the ortho and para positions

of the phenolates facilitate one-electron oxidation to the corresponding phenoxyl radicals,

22

333 334 335 336 337

g values

B (mT)

2.02 2.01 2 1.99

Experimental

Simulated

N N

OH OH

OH

CH3

OH

CH3

H

O O

2

NH2

OH

+

H3L2

Methanol

- 2H2O

Figure 2.2 :- EPR spectrum of H3L1 in methanol in presence of triethylamine.

because these substituents decrease the oxidation potential of the phenolates and provide

enough steric bulk to suppress bimolecular decay reactions of the generated phenoxyl

radicals. Accordingly, the Schiff-base ligand, H3L2, derived from the (1+2) condensation of

2,6-diformyl-4-methylphenol and 2,4-di-tert-butyl-6-aminophenol, was synthesized [Figure

2.3]. This ligand has been already reported by Robson 1 but other than a dimeric cobalt

complex2, no other crystal structure was reported. This ligand acts as a dinucleating ligand

with a pentadentate (O, N, O, N, O)-donor atoms and is as expected “innocent” i.e. non-redox

active. The ligand shows characteristic peaks in IR spectroscopy (Table 2.1). Mass

spectroscopy in the EI-mode shows the molecular peak at m/z 570 and NMR data reveals the

presence of 47 protons (3 are exchangeable).

Figure 2.3 :- Synthetic procedure for the preparation of H3L2

23

[L2(VIV=O)2(OCH(CH3)2)]

10

Urea

[L22(VV=O)2]

11

MeOH

[L2NiII2(µ2-Urea)(µ2-OOCMe)(MeOH)2]

3[(L2H)2Ni4II(µ3-OMe)2(OOCMe)2(MeOH)2]4

[L22CuII4(µ4-O)]6

[L2CuII2(OCH3)(THF)2]5

CH3CN /

CH2Cl2

[L22FeIII2]7

[L22MnIII2] 8a

MeOH / CH2Cl2

CH3CN / CH2Cl2

Cu(ClO4)2·6H2O

MeOH / CH2Cl2

CH3CN / CH2Cl2

VO(isopropoxide)3

VCl3(THF)3

CH3CN / CH2Cl2

Air

NiAc2, NaAc

CH3CN / CH2Cl2

Fe(II)(ClO4)2

Air

CH2Cl2 / CH3CN

"Mn(III)Acetate"

H3L2

Urea,

MeOH / CH2Cl2

8b

[L22MnIII2(THF) 2]

[L22Cr III2] 9 THF/ CH3CN

THF/ CH3CN

CrCl2

"Mn(III)Acetate"

2.3 TRANSITION METAL COMPLEXES WITH H 3L1 AND H 3L2

Using the ligand H3L1 , Ni(II) (1) and Cu(II) (2) complexes have been synthesized

using the corresponding metal salts and in the presence of triethylamine as base. The

complexes obtained were characterized by Infrared spectrometry, Mass spectrometry,

Elemental Analysis and Single-crystal X-ray diffractometric study. Magnetic susceptibility

and EPR studies were performed with the copper complex.

The dinucleating Schiff-base, H3L2 form complexes with all the 3d transition metals

ions. Figure 2.4 shows the complexes prepared and thus exemplifies the diversity of this

ligand. Reaction of metal salts and the ligand H3L2 in presence of the base triethyl amine or

tetrabutylammonium methoxide affords all the complexes in moderate yield.

When nickel acetate and H3L2 in presence of sodium acetate are allowed to react with

a slight excess of urea in methanol, complex 3 is obtained. In the absence of urea, the

tetranuclear complex 4 is obtained. Complex 4 can also be transformed to 3 by adding urea to

the red solution of 4 in methanol. Formation of 3 from 4 implies that the driving force for

formation of 3 lies in the strong tendency of urea to be incorporated in the dinickel center

embedded in the dinucleating ligand [L2]3-.

Figure 2.4 :- Complexes prepared with ligand H3L2.

24

The reaction of copper perchlorate and the Schiff-base ligand H3L2 afforded either 5 or

6, depending on the solvent system, in relatively high-yield. Lacking protic solvents, complex

5, a methoxo-bridged dicopper(II) species, is transformed easily to 6, a µ4-oxo-tetracopper(II)

complex, in presence of the solvent-mixture CH3CN/CH2Cl2 indicating the stability of 6.

Adventitious water in the solvent serves as a source for the µ4-oxo ligand.

The complexes 7-11 were obtained by using the salts shown in Figure 2.4. All these

dinuclear complexes were easily isolated and characterized by different methods. Remarkably

[L22MnII2] (8a), which is considered to possess identical coordination sphere and atom

connectivity as with the structurally characterized [L22FeIII2] (7), yields on recrystallization

from THF [L22MnIII2(THF)2] (8b), in which the phenolate-bridging between two manganese

centers prevailing in 8a does not persist any more, and thus resulting in a comparatively long

MnMn separation (6.45 Å).

2.3.1 INFRARED AND MASS SPECTROSCOPY OF COMPLEXES 1-11

The solid state FTIR spectra of the ligand H3L1 shows characteristic stretching peaks

for ν(OH) and ν(NH) in the region 3100 to 3400 cm-1 which is missing in 1 as well as for 2.

This indicates that that the phenol and amine character of the ligand is lost upon

complexation. The weak band at 1615 cm-1 due to (-C-N) stretch shifts to 1600 cm-1 in the

Compound ν (cm-1)

H3L1 3299 (-NH), 2959(tert-butyl), 1615(C-N) ,1591(C=C,aromatic), 1233(C-O)

H3L2 3523, 3492, 3348w, 2954, 2868s, 1625m, 1580m, 1482s, 1456s, 1361s, 1250s, 987s, 960s, 867m

1 2952, 1600, 1474, 1399, 1253,1130, 861

2 2951, 1595, 1470, 1398, 1257, 1126, 857, 647

3 3586w, 3382m, 3139m, 1670m, 1654m, 1568s, 1469s, 1444s, 1410s, 1250s, 1159m, 831m

4 3440br, 1624s, 1552m, 1477s, 1446s, 1410s, 1259s, 1039s, 805m

5 1597m, 1557m, 1474s, 1445s, 1256s, 1077m, 830m

6 1593m, 1557m, 1467s, 1254s, 1200m, 1077m, 829m, 564m, 539m, 510m

7 1610m, 1591m, 1565m, 1471s, 1408m, 1255s, 1163m, 834m, 537m, 492m

8 1610m, 1587m, 1442m, 1406m, 1360m, 1246s, 1161m, 836m, 561m, 537m

9 1598m, 1565m, 1445s, 1406m, 1256s, 1162m, 832m, 569m, 539m, 517m

10 1614m, 1569m, 1476s, 1446m, 1250s, 1161s, 990vs, 948m, 831s, 567m, 546s

11 1616m, 1591m, 1544s, 1476s, 1443s, 1361s, 1244s, 996s, 838m

Table 2.1 :- Characteristic FTIR peaks for the ligands H3L1 and H3L2 and complexes 1-11.

25

case of 1 and 1595 cm-1 for 2. Quite interestingly, these two peaks are stronger and sharper

than the weak peak observed at 1615 cm-1 for the ligand. Characteristic IR peaks are

summarized in Table 2.1.

The sharp peaks in the solid state FTIR spectrum due to ν(OH) of the ligand H3L2

occur at 3523, 3492 and 3348 cm-1. These bands are missing in 3 and 5-11, indicating that on

complexation the phenol-character of the ligand has been lost. For complex 4, the presence of

two phenolic protons, as evidenced from the crystal structure, could not be observed due to

the broadness of the bands at 3440 cm-1 . The solid state FTIR spectrum of complex 3 exhibits

a shift in the carbonyl stretching frequency of urea from 1690 to 1670 cm-1 upon coordination

to the dinickel center. For 6 the sharp band of medium intensity at 510 cm-1 is associated with

the νCu-O frequency in the Cu4O core. Support for this assignment is obtained by comparison

with the vibrational spectrum of 5, which exhibits, similar to that for 6, two sharp bands of

medium intensity at 564 and 539 cm-1 attributable to ν(Cu-O) vibrations for bonds to the

phenolate groups. These three bands are missing in the spectrum of the free ligand. Selected

IR data for complexes 3 - 11 are given in the Table 2.1. There are several peaks in the region

3000 - 2800 cm-1 due to the tert-butyl groups along with the other ν(C-H), ν(C=C), ν(C=N)

and ν(C-O) vibrations found in the normal range for these types of linkages. For 10 the sharp

strong-band at 990 cm-1 is associated with the ν(V=O) vibration. The corresponding band for

11 occurs at 996 cm-1.

When 1 and 2 were subjected to mass spectrometry in EI and ESI mode, it is clear that

the Ni (in 1) and Cu (in 2) are bounded to the ligand center. However, a large number of

peaks were observed ranging from mononuclear to tetranuclear complexes. Elemental

analysis of both 1 and 2 shows the metal to ligand ratio as 1:1. It is therefore probable that the

complexes are not monomers which is furthur proved by single crystal X-Ray diffractometric

studies.

Mass spectrometry in the EI mode indicates unambiguously the presence of urea and

acetate ion in 3. In the ESI positive mass spectrum of 3 the peak with an abundance of 100 %

is observed centered around m/z 743, corresponding to [L2Ni2(OOCCH3)]+, with expected

isotope pattern. EI-MS for 4 did not provide much useful information regarding its

tetranuclear nature, but exhibits signals around m/z 714-720 (100 %) with expected isotope

pattern for a dinickel compound, indicating clearly the presence of [L2Ni2(OCH3)]+ in the gas

phase. In the EI mass spectrum of 5 the molecule peak at m/z 724 with an abundance of 100%

corresponding to the LCu2(OCH3)-species is observed. Additionally, a clear indication of the

presence of THF in 5 is found at m/z 72. Mass spectrometry in the EI mode has been proved

26

to be very useful for 6, for which the molecule peak at m/z 1404 with expected isotope pattern

confirming the presence of [L22Cu4(O)] is observed. For complexes 7-11 mass spectrometric

analysis were also performed. In the EI mode for 7 exhibits the molecular ion peak centered

around m/z 1247 with expected isotope pattern, indicating unambiguously the composition to

be L22Fe2. For 8a the parent ion peak corresponds to a small peak centered around m/z 1244;

the peaks with dominating intensities are observed at m/z 206, 351 and 568. MS-EI indicates

unambiguously the presence of THF in 8b. Compound 9 shows the main peak at m/z 1239 in

the MS-ESI (positive) spectrum in CH2Cl2, indicating the composition to be L22Cr2 like that

of 7. In the EI mass spectrum of 10 the parent ion peak with an abundance of ∼21 % is

observed at m/z 760 with expected isotope pattern. The peak at m/z 718 with an abundance of

100 % corresponding to the L2V2O3 species is also observed. In addition there are other peaks

including at m/z 702 corresponding to the L2V2O2 species. MS-EI for 11 does not leave any

doubt about the composition L22V2O2 with the molecule ion peak at m/z 1268; the strongest

peak (100 %) with m/z 1251 corresponds to L22V2O. The other significant peaks at m/z 1236

and 618 are attributed to L22V2+ and L22V22+ ions.

2.3.2 CRYSTAL STRUCTURE AND CHARACTERIZATION OF COMPLEXES

[L1

4 NiII

4] (1)

Orange-red single crystals of 1, afforded from a dichloromethane-methanol solvent

mixture, was subjected to X-ray diffractometric studies. The structure (Figure 2.5) shows the

formation of a neutral tetranuclear nickel complex along with 9 dichloromethane molecules.

Each of the nickel center is in a distorted square planer geometry with N2O co-ordination

from the ligand and the 4th coordination sphere being occupied by the nitrogen atom of the

adjacent imidazolate ring. A closer look at the bond length of one of the ligand shows that the

C(8)-N(7) bond distance has shortened considerably to 1.297 (11) Å i.e. a double bond has

been formed. This indicates oxidation occurs at this bond, instead of the phenyl ring. The

average Ni-O(phenol), Ni-N (imine), Ni-N (imidazolate nitrogen) and Ni-N (imidazolate

nitrogen of adjacent ligand) bond lengths are 1.856Å, 1.850Å, 1.891Å and 1.850Å,

respectively which are comparable to other square planer Ni(II) complexes3a. The four nickel

centers forms a “butterfly structure”3b-d with Ni(1)-Ni(1A)-Ni(1B) making one plane and

Ni(1C)-Ni(1A)-Ni(1B) the other (shown in dashed line in Figure 2.5(b) ). The dihedral angle

between these two planes is 107.7 °. All the adjacent Ni-Ni were equidistant with a value of

5.851 Å. Selected bond distances and angles are given in Table 2.2.

27

N(7)

O(1)

N(12A)

N(10)

Ni(1)

Ni(1A)

N(10A)

N(12)

N(12C)

N(10B)

N(7A)

N(7B)

N(10C)

N(7C)

N(12B)

Ni(1B)

Ni(1C)

O(1C)

O(1A)

O(1B)

a) b)

Figure 2.5:- a) ORTEP diagram of 1 b) A view of 1 highlighting the “butterfly structure” of

the four nickel centers. The ligand L1 is denoted only by the donor atoms joined

by the curved lines.

Table 2.2:- Selected bond distances (Å) and angles (degree) for 1

Ni(1)-N(7) 1.850 (7) C(4)-C(5) 1.405 (12)

Ni(1)-N(12A) 1.891 (6) C(5)-C(6) 1.405 (11)

Ni(1)-N(10) 1.885 (7) C(1)-C(6) 1.407 (12)

Ni(1)-O(1) 1.856 (5) C(9)-N(10) 1.381 (10)

C(1)-O(1) 1.355 (9) N(12)-C(11) 1.332 (11)

C(6)-N(7) 1.391 (10) C(13)-N(12) 1.362 (10)

N(7)-C(8) 1.297 (11) C(11)-N(10) 1.297 (11)

C(1)-C(2) 1.396 (11) C(8)-C(9) 1.447 (11)

C(2)-C(3) 1.411 (12) C(9)-C(13) 1.370 (11)

C(3)-C(4) 1.368 (13) C(13)-C(14) 1.489 (11)

Ni(1)…Ni(1A) 5.851 Ni(1C)…Ni(1A) 5.851

Ni(1)…Ni(1B) 5.851 Ni(1C)…Ni(1B) 5.851

Ni(1A)…Ni(1B) 7.351 Ni(1)…Ni(1C) 7.351

Ni(1A)-Ni(1)-Ni(1B) 77.8 Ni(1B)-Ni(1C)-Ni(1A) 77.8

Ni(1)-Ni(1B)-Ni(1C) 77.8 Ni(1C)-Ni(1A)-Ni(1) 77.8

O(1)-Ni(1)-N(7) 86.5 (3) N(12A)-Ni(1)-N(10) 94.1 (3)

N(7)-Ni(1)-N(10) 84.0 (3) N(12A)-Ni(1)-O(1) 95.6 (3)

N(7)-Ni(1)-N(12A) 174.6 (3) N(10)-Ni(1)-O(1) 170.2 (3)

C(1)-O(1)-Ni(1) 111.8 (5) Ni(1)-N(10)-C(9) 112.4 (5)

C(8)-N(7)-C(6) 129.8 (7) C(11)-N(12A)-Ni(1) 123.8 (5)

Ni(1)-N(10)-C(11) 142.8 (6) C(13)-N(12A)-Ni(1) 130.5 (5)

C(8)-N(7)-Ni(1) 117.3 (6) C(6)-N(7)-Ni(1) 112.8 (5)

28

The tetramer Ni(II) complex is as expected diamagnetic and is proved by magnetic

susceptibility as well as 1H NMR studies. In 1H NMR spectra, the ratio of -CH proton (tert-

butyl, twice), -CH (Methyl), -CH (-N=CH-), -CH (aromatic proton) and –CH (imidazolate

ring) is 9:9:3:1:2:1 as expected from the deprotonated ligand.

[L1

4CuII

4 (THF)4] (2)

The crystal structure determination of 2 confirms that it is isostructural to 1 (Figure

2.6). However, even at low temperature, the crystal decomposes and as a result the R value is

high. Due to this high R value, it is not clear how many THF molecules are embedded in the

unit cell. Other solvent mixtures gave microcrystalline solid. Only a THF and Methanol or

THF and Ethanol mixture affords X-Ray quality crystals.

The relevant interatomic distances and angles are given in Table 2.3. As observed in 1,

oxidation of ligand occurs at the same position (C(8)-N(7)= 1.31 Å ). The structure consists

of a distorted square pyramidal tetranuclear neutral molecule with the Cu centers forming a

“butterfly structure” [Figure 2.6 (b)]. The fifth position of the coordination site in each copper

center is occupied by a THF molecule. The dihedral angle between the plane made by

connecting Cu(1)-Cu(2)-Cu(4) and Cu(3)-Cu(2)-Cu(4) is 150.1°. The average Cu-O (phenol),

Cu-N (imine), Cu-N (imidazolate nitrogen) and Cu-N (imidazolate nitrogen of adjacent

ligand) bond lengths are 1.942Å, 1.942Å, 1.992Å and 1.948Å, respectively which are

comparable to other square planer Cu (II) complexes 4. It is observed that the Cu(1)-N(10)

bond is elongated than the other coordinating atoms. This tendency is same for the other

copper centers and is due to Jahn-Teller effect which is present for d9 systems thus making 2

less symmetric than 1, a d8 system. The Cu(1) is displaced by 0.11 Å from the mean basal

plane of the three nitrogen [N(7), N(10), N(42)] and one oxygen atom [O(1)]. The dihedral

angles made between the plane N(42)O(1)N(7)N(10) containing the Cu(1) ion and the planes

N(42)C(43)C(39)N(40)C(41) and N(10)C(9)C(13)N(12)C(11) describing the two imidazolate

bridging groups are 47.2° and 82.2° respectively. It is interesting to note that angle between

the Cu(1)-Nim(42) and Cu(2)-Nim(40) vectors is 142.2° and that made between the Cu(1)-

Nim(10) and Cu(4)-Nim(12) vector is 132.6°. The plane made by Cu(1)O(1)N(7)N(10)N(42)

and Cu(2)O(31)N(37)N(40)N(72) makes an angle 139.1° between them.

29

Cu(1)

Cu

(

2

)

Cu

(

3

)

Cu

(

4

)

C(1)

C(6)

C

(

11

)

C

(

13

)

C

(

14

)

C(9)

C(8)

C(5)

C(4)

C(3)

C(2)

O(1)

O

(

91

)

O

(

61

)

O

(

31

)

N(7)

N(42)

N

(

40

)

N(37) N

(

72

)

N

(

70

)

N

(

67

)

N

(

97

)

N(102)

N(100)

N(12)

N(10)

C(41)

a) b)

Figure 2.6:- a) ORTEP diagram of 2 b) A view of 2 highlighting the “butterfly structure” of

the four copper centers. The ligand L1 is denoted only by the donor atoms

joined by the curved lines.

Table 2.3:- Selected bond distances (Å) and angles (degree) for 2

Cu(1)-N(7) 1.913 (17) Cu(2)-N(37) 1.964 (16)

Cu(1)-N(42) 1.925 (17) Cu(2)-N(72) 1.957 (16)

Cu(1)-N(10) 1.952 (16) Cu(2)-N(40) 2.020 (16)

Cu(1)-O(1) 1.955 (12) Cu(2)-O(31) 1.965 (13)

C(1)-O(1) 1.316 (25) C(9)-N(10) 1.462

C(6)-N(7) 1.420 N(12)-C(11) 1.347

N(7)-C(8) 1.310 C(13)-N(12) 1.408

C(1)-C(2) 1.422 C(11)-N(10) 1.381

C(2)-C(3) 1.399 C(8)-C(9) 1.461

C(3)-C(4) 1.365 C(9)-C(13) 1.341

C(4)-C(5) 1.399 C(13)-C(14) 1.525

C(5)-C(6) 1.409

C(1)-C(6) 1.377

Cu(1)…Cu(2) 6.054 Cu(3)…Cu(4) 6.034

Cu(1)….Cu(4) 6.044 Cu(1)…Cu(3) 8.524

Cu(2)…Cu(3) 6.019 Cu(2)…Cu(4) 8.245

Cu(1)-Cu(2)-Cu(3) 89.8 Cu(3)-Cu(4)-Cu(1) 89.8

Cu(2)-Cu(3)-Cu(4) 86.3 Cu(4)-Cu(1)-Cu(2) 85.9

O(1)-Cu(1)-N(7) 82.6 O(31)-Cu(2)-N(37) 81.6

N(7)-Cu(1)-N(10) 83.4 N(37)-Cu(2)-N(40) 81.7

N(7)-Cu(1)-N(42) 164.7 N(37)-Cu(2)-N(72) 163.2

N(42)-Cu(1)-N(10) 99.5 N(72)-Cu(2)-O(31) 93.8

N(42)-Cu(1)-O(1) 94.2 N(72)-Cu(2)-N(40) 100.9

N(10)-Cu(1)-O(1) 166.0 N(40)-Cu(2)-N(31) 162.7

Cu(1)-N(10)-C(11) 146.14 (16) Cu(2)-N(40)-C(41) 142.8 (17)

Cu(1)-N(42)-C(41) 123.59 (16) Cu(4)-N(12)-C(11) 123.89 (16)

O(1) Cu(1)

N(7)

N(10) N(12)

Cu(4)

N(97)

N(102)

N(72) N(70)

Cu(3)

N(42)

O(91)

N(40)

N(37)

O(31)

Cu(2)

N(67)

O(61)

30

The magnetic susceptibility data for polycrystalline sample of 2 was measured from

2-290 K in an applied magnetic field of 1 T to characterize the nature and magnitude of the

exchange interaction propagated by the bridging ligands. The Heisenberg spin Hamiltonian in

the form H = -2J A

S

r

B

S

r for an isotropic exchange coupling was used. The experimental

magnetic data were simulated using a least-square fitting computer program 5 with a full-

matrix diagonalization of exchange coupling, Zeeman splitting, and axial single-ion zero-field

interactions (D S

r

z2), if necessary. The susceptibility data were corrected for diamagnetism

(Pascal corrections), temperature-independent paramagnetism (TIP) and the presence of

paramagnetic monomer impurity (P) in the following way: χcalc = (1-P)χ + χTIP + Pχmono. It is

to be noted here that the conditions which are mentioned above are taken as the standard for

all the magnetic susceptibility measurements related to this work and their simulation. For all

the complexes, other than for magnetization measurements at different field strength, the

magnetic field applied was 1T.

The nature of the plot of 2 (Figure 2.7(a)) shows a behavior typical for

antiferromagnetic spin coupling. The µeff decreases monotonically with the decrease in

temperature; the values of µeff are 3.051 µB at 290 K and 0.180 µB at 2 K. Thus the magnetic

data reveal an energetically well-isolated ground state of total spin St = 0, discernible from the

decline of effective moments at temperatures below 100 K. The residual moment of 0.180 µB

at 2 K could be attributed to a monomeric (S =½) impurity (2.0%). Monomeric species

appeared also in weak abundance in the EPR spectra, as will be mentioned later.

The susceptibility data could be simulated between adjacent Cu(II) pairs with local

spins (S=½), as sketched in Figure 2.7(a)(inset). The multiplets |SASBSt> are labeled by the

“pair spins”, SA ( A

S

r= 1

S

r+ 3

S

r) and SB (B

S

r=2

S

r+4

S

r), and the total spin St (t

S

r= A

S

r + B

S

r). The

ground state is a singlet |110> and the first excited state at energy 2J is a triplet |111> [Figure

2.7(b)]. The states |011>, |000> and |101> are degenerate at energy 4J and the highest state at

energy 6J is a quintet |112>. In zero field the multiplets remain degenerate in magnetic

quantum numbers, according to their multiplicity. The simulation of the experimental

susceptibility data for the tetramer yielded an exchange coupling constant J = - 49 cm-1 , g =

2.022 , a paramagnetic impurity (P) of 2% and a temperature independent paramagnetism

(TIP) of 240 x10-3. In an imidazolate bridged system, there can be two type of pathways in

explaining the magnetic interaction viz. the σ and the π-exchange pathway. Generally, it is

agreed that the π-orbitals are not involved in the coupling6,4b,7. It has been stated that, since

31

NN

Cu

--

N

Cu

αα

β

N

Cu

δδ

N / O

θ

N Im

N / O

N / O Cu Im

γ

50 100 150 200 250 300

0.5

1.0

1.5

2.0

2.5

3.0

3.5

J

S3

S2

S4

S1

Simulated

Experimental

µ eff / µ B

T / K

112

101

011

000

111

110

>

|

>

|

>

|

>

|

>

|

>

|

0

2J

4J

6J

|SASBSt >

Energy

a) b)

Figure 2.7:- a) Magnetic measurement of 2 with the model of spin coupling.b) Corresponding

spin states in Zero Applied Field

the magnetic orbitals are σ -antibonding for square planar and square pyramidal complexes,

the relevant exchange pathway through the imidazolate bridge is of the σ -type. The

imidazolate orbitals that are responsible for this sort of interaction are of the type sketched in

Figure 2.8(a), and they are essentially parallel to the N-N' (or C-C) direction.

Several studies have already been reported4a,4b,7, in which structural data have been

used to find correlations between structure and exchange coupling in imidazolate-bridged

copper(II) complexes. In Table 2.4, the structural parameters which appear to be responsible

for the exchange coupling constant are listed for several structurally characterized

imidazolate-bridged complexes. Different angles listed in the Table 2.4 are defined in

Figure 2.8(b). Comparison of the numerical data shows that a simple magnetostrutural

correlation for imidazolate bridged Cu complexes does not exist. However, as the values of J

is independent of θ, a π−exchange pathway can be discarded. However it has been concluded

from EHMO calculations7 that the extent of anti-ferromagnetic coupling will increase when

a) b)

Figure 2.8 a) Schematic representation of the orbital orientations for the Cu-Im-Cu unit.

b) Different angles used in the magnetostructural correlation.

32

Table 2.4 Exchange Coupling Constants and Structural Parameters for Imidazolate-Bridged

Copper(II) Complexes

Im=imidazolate (1-), L=1,4,7-triazacyclononane, L' = 1.4,7-trimethyl-1,4,7- triazacyclononane, Bpim =4,5-

bis[2-[[(2-pyridyl)ethyl]imino]-methyl] imidazolate, TMDTN, N,N',N'-tetramethyldiethylene triamine, Pip= 2-

[2-[ [(2 pyridyl) ethyl] imino] methyl] pyridine, Macro = a 30-membered macrocyclic ligand derived from 2,6-

diacetylpyridine and 3,6-dioxaoctane-1,I-diamine,Gly-Glyo = glycylglycinate(2-), Schiff Base = macrocycle

prepared from 2 molecules of 2,6-diacetylpyridine and 2 molecules of m-xylenediamine, IH = Schiff Base

prepared from the condensation of 2.-imidazolecarboxaldehyde and histamine, IP = Schiff Base prepared from

the condensation of 2.-imidazolecarboxaldehyde and 2-amino ethyl pyridine, hfca = hexafluroacetylacetonate

anion.

the N-N and other Cu-N bonds are parallel to the imidazolate carbon-carbon bond, thus

favoring a σ-exchange pathway. Although the coupling constant for 2 is strong compared to

the other complexes, the values of J cannot be strictly attributed to the values of α and γ. This

indicates that only σ-superexchange cannot be the only mechanism for the exchange coupling

Complex J /cm-1 α β γ δ θ reference

catena- [Cu3(Im)2(ImH)8(Cl04)4]

-58.5 130.0

129.8

70.0

60.0

162.9

160.9

4e,7

catena- [ Cu (Im)(ImH)2Cl]

-42 135.3

128.5

90.0

169.4

168.8

4f,7

[L´3Cu3(Im)3](ClO4)3 -37.5 127.0

123.0

120.5

131.1

135.3

91.2

74.3

56.2

154.0

157.4

154.5

74.3

61.0

56.2

91.2

4a,4g

[L4Cu4(Im)4](ClO4)4 -35 130.4

126.6

126.7

134.6

142.1

76.9

50.2

95.7

26.9

157.7

164.4

156.9

157.6

63.8

4g

[Cu4(Bpim)2(Im)2](NO3)4 -35 120.0

128.0

97.3

100

153

4b

[Cu2(TMDT)2(Im)2](ClO4)3 -26 129.0

129.0

143.0 91.8

90.0

161.9

160.2

4b

[Cu2(Pip)2(Im)](NO3)3 -27 121.0

120.0

124.0

128.0

95.0

80.1

90.0

77.4

166.3

158.9

4b

[Cu2(Macro)(Im)](Cl04)3 -21 134.4

129.1

79.1

68.8

4j

[Cu2(Schiff Base)( Im)](CF3SO3)3 -19.3 126.5

128.9

133.0

128.0

140.0

91.4

93.4

45.2

4k

Na[Cu2(Gly-Glyo)2(Im)] -19 124.5

124.1

135

5.8

10.4

157.5

157.2

5.9 4l

[Cu3(IH)2(hfca)4] -30.3 145.6

166.3

11.7

41.6

4h

[Cu3(IP)2(hfca)4] -18.6 126.5

143.6

4i

[L14Cu4(THF)4] -49 146.14

142.80

142.2

132.6

44 139.1 this work

33

for this compound. The presence of ligand-ligand interaction may also play a part in this type

of exchange coupling.

X-band EPR spectra for 2 in solid state, from 11.7 K to 62.1 K , shows a isotropic

signal at g =2.08 which remains constant for the given temperature range. This is attributed to

the monomeric paramagnetic impurity (2%) present in the solid state. On increasing the

temperature additional changes in the spectra is observed between the 15-263 mT and

between 380-600 mT [Figure 2.9(a)]. Below 10 K no significant change in spectra was

observed between 380-600 mT; however, on increasing the temperature, the intensity of the

signal increases and reaches a maximum at 34.5 K.

In order to elucidate the correlation between the EPR spectra and the spin multiplets of

the coupled system, the change of intensity(I) at 480 mT was analyzed for all the temperature

(T) range. As the population of the excited state in first order is dependent on the Boltzmann

function of resonating levels , an IT vs. T was plotted [Figure 2.9 (b)]. The fading below 10

K and the strong rise at elevated temperatures prove that the signals arise from excited states

and that the ground state is diamagnetic and EPR silent, in accordance with the susceptibility

findings. For quantitative analysis, the experimental data was compared with the theoretical

Boltzmann functions, derived from the spin coupling model (Figure 2.7(b)). It is to be noted

that values till 40K were taken as saturation occurs above this temperature. The best

agreement was obtained for I111T, describing the thermal population of the first excited triplet

state |111>

I111 T ≈ (exp (-J/kT))/Z

where Z= 1+3 exp (-J/kT) + 7exp (-J/kT)

10 20 30 40

0

20

40

60

I * T

T (K)

a) b)

Figure 2.9:- a) EPR spectra of 2. b)Temperature dependence of the temperature- weighted

intensity of the EPR spectra. The solid line are calculated Boltzmann functions:

I111T with J = - 42 cm-1.

34

The solid line is a fit with J = - 42 cm-1, which is close to the exchange coupling constant of

J = - 49 cm-1 evaluated from the susceptibility measurements. Therefore, the complex EPR

spectra arising from 380-600 mT are assigned to the first excited triplet state |111> of the

tetramer.

[L2NiII

2(µ2-Urea)(µ2-OOCMe)(MeOH)2] (3)

The structure of 3 (Figure 2.10) shows the formation of a dinickel(II) complex with a

bridging urea through the carbonyl oxygen. Ni(1) and Ni(2) are additionally bridged by a

phenolate oxygen O(10) and an acetate ion. Each nickel is in a distorted octahedral NO5

environment, being equatorially bound to the NO3 donor set [Ni-N av. 1.975 Å; Ni-O av. 2.01

Å], and with axial interactions to an carboxylic O-donor of a bridging MeCO2- group [Ni-O

av. 2.09 Å] and a methanol O-donor [Ni-O 2.165, 2.159 Å] trans to the MeCO2- group.

Hydrogen atoms of the hydroxide group in MeOH and urea molecules were located from a

difference map and are shown as circles of arbitrary radii in Fig. 2.10. The hydrogen atoms on

the NH2 groups of urea enter into hydrogen bonding with the phenolate oxygens, O(1) and

O(17) with O(1)N(71) 2.784 and O(17)N(70) 2.813 Å, thus making urea presumably

nonsusceptible to alcoholysis (see later). The intramolecular hydrogen bond between the

methanol molecules cis-ligated to two different nickel(II) ions, O(40)O(50) 2.928 Å, is

shown as dotted lines. The metal-metal distance Ni(1)Ni(2) of 2.966(1) Å in 3 is

Figure 2.10 : - An ORTEP drawing of the neutral molecule 3.

35

Table 2.5 Selected bond distances (Å) and angles (degree) for 3

Ni(1)-O(1) 1.954(3) Ni(2)-O(17) 1.967(3)

Ni(1)-N(7) 1.971(4) Ni(2)-N(16) 1.978(4)

Ni(1)-O(10) 1.971(3) Ni(2)-O(10) 1.986(3)

Ni(1)-O(70) 2.080(3) Ni(2)-O(70) 2.065(3)

Ni(1)-O(60) 2.089(4) Ni(2)-O(61) 2.088(4)

Ni(1)-O(40) 2.165(4) Ni(2)-O(50) 2.159(4)

Ni(1)-Ni(2) 2.9656(11) O(70)-C(70) 1.285(6)

N(70)-C(70) 1.323(7) N(71)-C(70) 1.312(6)

Ni(1)Ni(2) 2.966(1)

O(17)-Ni(2)-N(16) 84.6(2) O(1)-Ni(1)-N(7) 84.7(2)

O(17)-Ni(2)-O(10) 174.54(14) O(1)-Ni(1)-O(10) 175.4(2)

N(16)-Ni(2)-O(10) 91.6(2) N(7)-Ni(1)-O(10) 93.2(2)

O(17)-Ni(2)-O(70) 99.33(13) O(1)-Ni(1)-O(70) 97.52(13)

N(16)-Ni(2)-O(70) 175.7(2) N(7)-Ni(1)-O(70) 176.4(2)

O(10)-Ni(2)-O(70) 84.39(13) O(10)-Ni(1)-O(70) 84.38(13)

O(17)-Ni(2)-O(61) 94.20(14) O(1)-Ni(1)-O(60) 95.39(14)

N(16)-Ni(2)-O(61) 94.2(2) N(7)-Ni(1)-O(60) 93.0(2)

O(10)-Ni(2)-O(61) 89.96(13) O(10)-Ni(1)-O(60) 88.84(14)

O(70)-Ni(2)-O(61) 87.25(14) O(70)-Ni(1)-O(60) 89.59(14)

O(17)-Ni(2)-O(50) 92.8(2) O(1)-Ni(1)-O(40) 94.6(2)

N(16)-Ni(2)-O(50) 95.1(2) N(7)-Ni(1)-O(40) 92.0(2)

O(10)-Ni(2)-O(50) 83.7(2) O(10)-Ni(1)-O(40) 81.33(14)

O(70)-Ni(2)-O(50) 83.08(14) O(70)-Ni(1)-O(40) 85.09(14)

O(61)-Ni(2)-O(50) 168.89(13) O(60)-Ni(1)-O(40) 169.22(13)

significantly shorter than those observed in comparable complexes.8,9 Bridging Ni-O (urea)

distances, 2.080(3) and 2.065(3) Å, are significantly shorter than those in the only other single

atom O-bridged urea compound, Ni-O (urea) 2.158(3) Å.9 The Ni-O (urea) bond distances for

compounds containing non-bridging urea lie within the range 2.05 - 2.13 Å.8 The angle at

bridging urea Ni(1)-O(70)-Ni(2) is 91.4(1)o whereas the phenoxide bridging angle Ni(1)-

O(10)-Ni(2) 97.1(2)o is significantly greater. Relevant bond distances (Å) and angles (in

degrees) are summarized in Table 2.5 .

The electronic spectra of 3 [Figure2.11(a)] show intense π-π* transitions below 480

nm, attributable to the ligand, as judged by their high absorption coefficients and comparison

with the spectrum of the ligand (Table 2.6). Additionally, there are three broad, weak

transitions in the visible-near IR region at 779, 858 and 1284 nm for 3, indicating that the

nickel ions are in an octahedral environment in solution as well as in the solid state (X-ray

structure).

Table 2.6:- Characteristic peaks in the electronic spectrum of [L2]3- and 3.

Ligand with TBAOMe 3

λ (nm)

ε(M-1cm-1)

300 350 496

6150 10,270 13,900

330 464 779 858 1284

14,770 16,930 19 27 12

36

600 800 1000 1200 1400 1600

0

10

20

30

40

50

1284;12.28

779;19.28

858;26.67

ε ( M -1 cm -1 )

λ (nm)

50 100 150 200 250 300

0

1

2

3

4

Simulated

Experimental

µeff / µB

T(K)

1000 800 600 400 200 0

10 µA

E( mV ) vs Fc+/Fc

800 600 400 200 0 -200 -400 -600

12 µA

E ( mV ) vs Fc+/Fc

a) b)

Figure 2.11 :- a) Electronic spectra of 3 showing the three lower energy bands. b)Magnetic

measurements of 3.

The magnetic susceptibility data for polycrystalline sample of 3 was collected to

characterize the nature and magnitude of the exchange interaction propagated by the bridging

ligands. The magnetic moment (µeff/molecule) for 3 of 4.280µB (χMT=2.291 cm3Kmol-1) at

290 K decreases monotonically with decreasing temperature until it reaches a value of 1.474

µB (χMT = 0.2717 cm3 K mol-1) at 2 K; this temperature dependence of µeff is a clear

indication of an antiferromagnetic exchange coupling between two paramagnetic centers

Ni(II) (SNi = 1) with a resulting diamagnetic ground state. Using only D (i.e. J = 0) a fit of a

poor quality with unusually large D was obtained and hence discarded. Attempts to fit the

data using both D and J yield physically meaningless D values. For example, a good fit was

obtained with the following parameters: J = -2.6 cm-1, D = +27.1 cm-1, g = 2.13, P = 0.5%. As

the zero-field splitting D is unusually large for a 6-coordinated Ni(II) ion, the solution was

discarded. Finally, the solid line in Figure 2.11(b) represents the best fit with the following

parameters: J = -3.5 cm-1, g1 = g2 = 2.137, P = 0.5%. The quality of the fit is insensitive to D

varying between 0 - 5 cm-1.

The nickel complex 3 shows waves at +0.970 ,+0.470 V and preceded by one broad

wave at +0.270 V in square wave voltammetry (SQW) mode (25 Hz) [Figure 2.12].

a) b)

Figure 2.12 :- a) Square wave and b) Cyclic Voltammogram of 3.

37

Additionally, with increasing scan rate the oxidation and re-reduction peaks in the cyclic

voltammogram (CV) was found to split into two components. This complicated behavior was

assigned to an involvement of the metal nickel(II)-ion in the redox processes leading to

radical through oxidation. Ligand centered oxidation to radical is inferred from the occurrence

of oxidation processes at potentials which are similar, irrespective of the nature of the central

metal ion, and involvement of central nickel ions in oxidation processes of radical complexes

was documented earlier10 in detail, including digital simulations of complex cyclic

voltammograms.

When an ethanolic solution of 3, in presence of 50 times of urea, was refluxed at 80°C

for 14 hours, a gas chromatography study showed that ethanol was converted to urethane

(ethyl carbamate). However, calculation with undecane (C11) as standard, showed that the

turnover number, defined as the ratio of the number of moles of product to that of catalyst

used, was one. This means that 3 does not catalyze the ethanolysis of urea to urethane

presumably due to hydrogen bonding network involving urea in 3, which is also maintained in

solution. Due to the presence of stable hydrogen bonded structure in solution, there is no

possibility of intramolecular EtO--delivery to the π* orbital of urea, as has been proposed for

the mechanism of action of urease (here OH-).11,9 It is noteworthy that the bond lengths within

the coordinated urea,8 bridging or non-bridging, are very similar, in contrast to the compound

which exhibits catalytic activity.8g

[(L2H)2NiII

4(µ3-OMe)2(OOCMe)2(MeOH)2] (4)

The crystal structure of 4 is shown in Figure 2.13 with selected bond distances and

angles provided in Table 2.7. The molecule is based on a roughly cubic Ni4O412 unit,

consisting of two interpenetrating tetrahedra, one of four nickel atoms and one of four µ3-

oxygen atoms originating from cresol-phenolates (O10, O50) part of the ancillary ligand L2

and methoxide groups (O81, O91). Each Ni(II) is in a distorted octahedral environment with

an NO5 donor set. For Ni(1) and Ni(4) a peripheral ligation is provided by a monodentate

acetate ion, whereas for Ni(2) and Ni(3) a methanol molecule (O82 and O92, respectively)

completes the sixth coordination position. The ancillary Schiff's base ligands are present in

monoprotonted form HL2 and protonated at O(1) and O(58) building hydrogen bonds with

O(84) and O(94) of the acetate ions, respectively, O(1)O(84) 2.426 Å and O(58)O(94)

2.449 Å. Although it was not possible to locate all hydrogen atoms in the structure, the intra-

molecular contacts between the oxygen atoms of the methanol molecules (O82 and O92) and

38

a) b)

Figure 2.13:-(a)Molecular structure of 4. The tert-butyl groups have been removed for clarity.

(b)A view of 4 highlighting the Ni4O4-cubane core. The pentadentate ligand

[HL2]-2 is denoted only by the donor atoms joined by the curved lines.

Table 2.7 Selected bond distances (Å) and angles (degree) for 4

Ni(1)-N(7) 1.988(7) Ni(3)-O(41) 1.970(6)

Ni(1)-O(91) 2.050(6) Ni(3)-N(47) 1.994(7)

Ni(1)-O(81) 2.052(6) Ni(3)-O(91) 2.052(6)

Ni(1)-O(10) 2.081(6) Ni(3)-O(50) 2.054(6)

Ni(1)-O(1) 2.089(6) Ni(3)-O(92) 2.109(6)

Ni(1)-O(83) 2.112(6) Ni(3)-O(10) 2.219(6)

Ni(2)-O(18) 1.970(6) Ni(4)-N(56) 1.979(7)

Ni(2)-N(16) 1.984(7) Ni(4)-O(91) 2.042(6)

Ni(2)-O(81) 2.024(6) Ni(4)-O(81) 2.055(6)

Ni(2)-O(10) 2.049(6) Ni(4)-O(50) 2.069(6)

Ni(2)-O(82) 2.123(6) Ni(4)-O(93) 2.092(7)

Ni(2)-O(50) 2.229(6) Ni(4)-O(58) 2.126(6)

Ni(1)Ni(2) 3.047

Ni(3)Ni(4) 3.077

Ni(1)Ni(3) 3.191

Ni(2)Ni(4) 3.155

Ni(1)Ni(4) 3.102

Ni(2)Ni(3) 3.227

O(91)-Ni(3)-O(50) 82.2(2) O(10)-Ni(1)-O(83) 96.4(2)

O(41)-Ni(3)-O(92) 94.5(3) O(1)-Ni(1)-O(83) 89.6(2)

N(47)-Ni(3)-O(92) 89.5(3) O(18)-Ni(2)-N(16) 83.8(3)

O(91)-Ni(3)-O(92) 93.3(2) O(18)-Ni(2)-O(81) 101.3(2)

O(50)-Ni(3)-O(92) 87.5(2) N(16)-Ni(2)-O(81) 174.3(3)

O(41)-Ni(3)-O(10) 96.8(2) O(18)-Ni(2)-O(10) 174.3(2)

N(47)-Ni(3)-O(10) 97.0(3) N(16)-Ni(2)-O(10) 90.9(3)

O(91)-Ni(3)-O(10) 78.9(2) O(81)-Ni(2)-O(10) 83.9(2)

O(50)-Ni(3)-O(10) 81.9(2) O(18)-Ni(2)-O(82) 96.4(3)

O(92)-Ni(3)-O(10) 167.6(2) N(16)-Ni(2)-O(82) 88.6(3)

N(56)-Ni(4)-O(91) 171.3(3) O(81)-Ni(2)-O(82) 93.2(2)

N(56)-Ni(4)-O(81) 99.8(3) O(10)-Ni(2)-O(82) 85.5(2)

O(91)-Ni(4)-O(81) 81.0(2) O(18)-Ni(2)-O(50) 96.9(2)

N(56)-Ni(4)-O(50) 89.4(3) N(16)-Ni(2)-O(50) 96.9(3)

O(91)-Ni(4)-O(50) 82.0(2) O(81)-Ni(2)-O(50) 80.2(2)

O(81)-Ni(4)-O(50) 83.4(2) O(10)-Ni(2)-O(50) 81.8(2)

N(56)-Ni(4)-O(93) 89.8(3) O(82)-Ni(2)-O(50) 166.1(2)

39

800 1000 1200 1400

0

20

40

60

917 nm

ε ( M -1 cm -1)

λ(nm)

400 600 800 1000 1200

0

2

4

405

474

ε * 10- 4 ( M-1 cm-1 )

λ (nm)

N(7)-Ni(1)-O(91) 100.1(3) O(81)-Ni(1)-O(83) 88.4(2)

N(7)-Ni(1)-O(81) 170.6(3) O(91)-Ni(4)-O(93) 89.0(2)

N(7)-Ni(1)-O(1) 80.9(3) O(81)-Ni(4)-O(93) 169.9(2)

O(91)-Ni(1)-O(1) 93.7(2) O(50)-Ni(4)-O(93) 93.7(2)

O(81)-Ni(1)-O(1) 108.4(2) N(56)-Ni(4)-O(58) 80.2(3)

O(10)-Ni(1)-O(1) 167.9(2) O(91)-Ni(4)-O(58) 108.5(2)

N(7)-Ni(1)-O(83) 90.5(3) O(81)-Ni(4)-O(58) 94.0(2)

O(91)-Ni(1)-O(81) 80.9(2) O(50)-Ni(4)-O(58) 168.7(2)

N(7)-Ni81)-O(10) 88.5(3) O(93)-Ni(4)-O(58) 90.7(2)

O(91)-Ni(1)-O(10) 82.3(2) O(81)-Ni(1)-O(10) 82.4(2)

O(91)-Ni(1)-O(83) 169.3(2)

the oxygen atoms of the acetate groups (O83 and O93, respectively) of 2.657 Å and 2.684 Å,

can be interpreted as hydrogen bonds. Ni-O-Ni angles vary between 94.4(2) and 102.2(3)o.

NiNi distances on different cubic faces are also different, the shortest being Ni(1)Ni(2)

3.047 Å and the longest Ni(2)Ni(3) 3.227 Å. Average Ni-N and Ni-O bond distances of

1.99 and 2.10 Å, respectively, lie well within the range of reported values for the

corresponding bond distances of the tetranuclear cubane-like Ni(II).12 No substantial

differences in bond lengths and angles are found between the two crystallographically

independent molecules .

The electronic spectra of 4 shows similar intense π-π* transitions, like 3, below 480

nm, which is again attributable to the ligand. Interestingly only one broad absorption in the

range 917 nm for 4 [Figure 2.14] is observed as compared to 3.

For 4, the effective magnetic moment µB of 6.08 µB (χMT = 4.629 cm3 K mol-1) at

290 K increases monotonically with decreasing temperature reaching a broad maximum at

20 - 30 K with µeff = 6.39 µB (χMT = 5.1073 cm3 K mol-1). Below 20 K µeff starts to decrease

reaching a value of 3.99 µB (χMT = 1.992 cm3 K mol-1) at 2 K (Figure 2.15a); it is clear that

the magnetic properties of 4 are dominated by a ferromagnetic exchange interaction between

four 3A2 nickel(II) ions as propagated by bridging phenoxides and methoxides. Structural

a) b)

Figure 2.14:- (a)Electronic spectra of 4 showing single band at 917 nm. (b) Electronic

spectra from 280 to 1200.

40

parameters of 4 strongly suggest a lower symmetry than Td (idealized D2d) for the molecule.

The magnetic analysis were carried out using either a two-J or three-J models based on the

diagram shown in inset [Figure 2.15 (a)].

The data were fit initially by using a higher symmetry model, i.e. J12 = J34 and J13 = J14

= J24 = J23, which yielded a good fit (not shown) with J12 = J34 = +1.5 cm-1, J13 = J14 = J24 = J23

= +0.25 cm-1, g = 2.133, D = 14.8 cm-1. This solution was discarded, as the zero-field splitting

D is unusually large for a 6-coordinated 3A2 Ni(II) ion.13 Therefore the data was fitted with

three different exchange parameters, which also corroborate with three different types of faces

present in the Ni4O4 cubane core of 4. The best fit parameters are: J12 = J34 = +0.47 cm-1, J13 =

J24 = +4.25 cm-1, J14 = J23 = -1.45 cm-1, g = 2.122 and D = 0 (fixed). The most important

parameter in the magnetostructural correlation of the Ni4O4 cubane cores has been described

in the literature12 to be the averaged Ni-O-Ni angle of a cubane-face. A ferromagnetic

exchange interaction is observed when this angle is close to orthogonality. On the other hand,

the Ni-O-Ni angles in the vicinity of and larger than 99o lead to antiferromagnetic interaction.

Thus the strongest ferromagnetic coupling of J13 = J24 = +4.25 cm-1 should be associated with

the average Ni(1)-O-Ni(3) angle of 99.0o, which would be not in agreement with the

magnetostructural correlation reported in the literature.12 This suggested that a better fitting

model should be used. So, the data was fitted again by using three different exchange

parameters, but with the following constraints: J12, J13 and J14 = J24 = J23 = J34, as three

different ranges of average Ni-O-Ni angles of 96o, 97-98o and 99o are present in 4. The best fit

obtained with the parameter set, J12 = +8.0 cm-1, J14 = J24 = J23 = J34 = +0.9 cm-1, J13 = -3.95

cm-1, g=2.120, D = 0 (fixed) is shown as a solid line in Figure 2.15(a). The differences and

nature of the signs of the exchange parameters are in full accord with the Ni-O-Ni angle

correlation and support the three-J model used, which takes into account the reduced

symmetry of the cubane core observed in the X-ray structure of 4. Thus these results for 4

predict a switch from ferromagnetic to antiferromagnetic coupling only for angles Ni-O-Ni

above 98o.

Recently it has been demonstrated12i that the major contributor to the superexchange

constants observed in [Ni4(OR)4]4+ cubanes is the Ni-O-Ni angle. A list of the nickel cubane

complexes is listed in Table 2.8. Accordingly, a linear correlation between J and Ni-O-Ni

angle has also been described12i,12m. A plot the observed J values for 4 and other structurally

characterized [Ni4(OR)4]4+ cubanes against average Ni-O-Ni angles [Figure2.15(b)] was done.

The solid line drawn as a guide to the eyes shows a fairly good correlation between J and the

41

91 98 105

-30

-20

-10

0

10

20

J (cm-1)

Ni - O - Ni (deg)

50 100 150 200 250 300

0

1

2

3

4

5

6

Simulated

Experimental

µ eff / µ B

T (K)

angles. The borderline between ferromagnetic and antiferromagnetic interaction, as seen here,

is 98 ° which agrees well when compared to other nickel cubanes. This guide also allows one

a) b)

Figure 2.15 :- a) Magnetic measurement at 1T for 4.(Inset:- The magnetic interaction (J)

model used in simulating the experimental curve.) b) Plot of J vs Ni-O-Ni angle in [Ni4(OR)4]

cubanes. The values depicted by filled squares and open triangles are from literature and that

of open circles are from this work.

Table 2.8 :- J as a function of the Ni-O-Ni angles for the [Ni4(µ3-OR)4] cubanes

Complex Ni-O-Niav (deg) J (cm-1) Reference

[Ni4(µ3-OMe)4 (sal)4(EtOH)4] 97.73 7.46 12a

[Ni4(µ3-OH)4 (chta)4(NO3)4] 99.0 -0.57 12b

[Ni4(µ3-OMe)4 (TMB)4(µ-O2CMe)4] 93.0

100.9

17.5

- 9.1

12d

[Ni4(µ3-OH)4 (tzdt)4(py)4] 95.89

103.2

17.5

-22.0

12f

[Ni4(µ3-OMe)4 (dbm)4(MeOH)4] 96.7

99.6

2.2

-3.4

12i

[Ni4(µ3-OH)2 (pypentO)(pym) (µ-Oac)2 (NCS)2(OH2)]

89.9

92.9

100.5

15.0

6.7

-3.09

12m

[Ni4(µ3-OMe)4 (LSe)2(MeOH)2(MeCN)2]

96.17

97.17

101.25

5.50

1.05

-4.1

12n

[Ni4(L2H)2(µ3-OMe)2 (OAc)2(MeOH)2]

95.94

97.85

99.0

8.0

1.0

-3.8

this work, 12o

salH = salicylaldehyde; chta = r-1-c-3-c-5 triaminocyclohexane; TMB=2,5-dimethyl-2,5-diisocyanohexane; dzdt

= 1,3-thiazolidine-2-thione; dbmH = dibenzoylmethane; py = pyridine; pypentO = 1,5-bis[(2-

pyridylmethyl)amino]-3-pentanolate; pym = 2-pyridylmethanolate; LSe = 2,2´-seleno-bis(4,6-di-tert-

butylphenol).

Ni1

Ni2

Ni3 Ni4

J13 J14

J24

J23

J34

J12

42

0.0 0.6 1.2 1.8 2.4

0.0

0.6

1.2

1.8

2.4

3.0 7 T

4 T

1 T

M / N g β

β H / k T

to notice that the antiferromagnetic interactions (-J) fall fairly well on the correlated line with

respect to Ni-O-Ni angles. In contrast, the positive J values scatter appreciably. This is not

surprising considering the high tolerance attached in general to the positive J values

(ferromagnetic interactions) evaluated through simulation of the susceptibility data.

The non-zero magnetic moment of 3.99 µB at 2 K and the broad maximum with µeff =

6.39 µB at 20 - 30 K indicate that 4 has a complicated low-lying magnetic structure with a

non-diamagnetic ground state smaller than the spin state of St = 4, expected for a

ferromagnetically coupled tetranuclear nickel(II) (SNi = 1). The ground state can be

determined by examining the magnetization of the compound at low temperatures as a

function of the applied magnetic fields. Magnetization measurements have performed at

applied magnetic fields of 1, 4 and 7 T. The field-dependent magnetizations as a function of

temperature and their simulations are depicted in Figure 2.16. It is clear from Figure 2.16 that

the saturated magnetization value reaches ∼3.035 in the temperature range 2.0 - 2.8 K at the

highest field of 7 T. The simulated parameters are: J12 = +8.0 cm-1, J13 = -3.95 cm-1, J14 = J24 =

J23 = J34 = +0.60 cm-1, g = 2.12, D = 3.0 cm-1. These values corroborate well with those

from the susceptibility measurements. As J12, J13 and D are of similar magnitude, it is not

possible to calculate the ground state in the form of an St value; S is not a good quantum

number to describe the ground state, but rather MS.

The compound 4 an irreversible oxidation wave at +0.163 V against Fc+/Fc with other

peaks at +0.44 V and 1.26 V [Figure 2.17]. At different scan rates, the peaks are scattered, the

same observation as seen for 3. Ligand center phenoxyl-radical formation is inferred.

Figure 2.16 :- Field dependent magnetization curve of 4.The open triangle, circle and square

represents the experimental curve at 1,4 and 7 T respectively. The solid line is

the simulated curve.

43

1500 1000 500 0 -500

8 µA

E ( mV ) vs Fc+/ Fc

-500 0 500

2 µA

E ( mV ) vs Fc+/ Fc

a) (b)

Figure 2.17 :- (a) Cyclic voltammogram of 4 at different scan rates b) Square wave

voltammogram of 4 (10 Hz).

[L2CuII

2(OCH3)(THF)2] (5)

Although the analytical and spectroscopic data showed the presence of a dinuclear Cu2

core as the smallest unit in 5, an X-ray analysis was undertaken to remove the doubts

regarding connectivity. Unfortunately, crystals of 5 diffract X-rays very weakly. In spite of

the high R factor and large standard deviations, the crystal structure analysis of 5 confirmed

its dinuclear structure. Selected bond lengths and angles are given in Table 2.9. The crystal

structure reveals that the copper atoms are doubly bridged by the phenoxo and by a methoxide

group (Figure 2.18). The penta-coordination of copper atoms is achieved by a tetrahydrofuran

oxygen with Cu(1)-O(60) and Cu(2)-O(50) distances of 2.444 Å and 2.566 Å, respectively,

which are oriented trans to each other in the dicopper complex. The coordination polyhedron

for the copper centers is distorted square pyramid with O(1)N(7)O(10)O(40) for Cu(1) and

O(10)N(16)O(17)O(40) for Cu(2) forming the equatorial planes, where Cu(1) and Cu(2) are

respectively located at 0.102 Å and 0.134 Å out of the equatorial planes. The ring

Cu(1)O(10)Cu(2)O(40) is not planar, the dihedral angle being 12.7o. The distances Cu-N and

Cu-O are in the ranges reported for comparable complexes. 14

Figure 2.18:-A perspective view of the neutral complex 5.

44

Table 2.9 Selected bond distances (Å) and angles (degree) for 5

Cu(1)-O(1) 1.899(6) Cu(2)-O(17) 1.880(7)

Cu(1)-O(40) 1.911(6) Cu(2)-O(40) 1.905(6)

Cu(1)-N(7) 1.915(8) Cu(2)-N(16) 1.931(8)

Cu(1)-O(10) 1.948(6) Cu(2)-O(10) 1.952(7)

Cu(1)-O(60) Cu(2)-O(50)

Cu(1)Cu(2) 2.974(2)

Cu(1)-O(10)-Cu(2) 99.38(31) O(10)-Cu(1)-N(7) 93.25(30)

Cu(1)-O(40)-Cu(2) 102.43(31) O(40)-Cu(2)-O(10) 78.55(27)

O(1)-Cu(1)-O(10) 173.32(27) O(40)-Cu(2)-N(16) 167.77(30)

O(40)-Cu(1)-N(7) 170.28(28) O(40)-Cu(2)-O(17) 101.62(29)

O(1)-Cu(1)-O(40) 100.83(26) O(10)-Cu(2)-N(16) 92.48(33)

O(1)-Cu(1)-N(7) 86.78(29) N(16)-Cu(2)-O(17) 86.24(34)

O(40)-Cu(1)-O(10) 78.49(27) O(10)-Cu(2)-O(17) 172.80(28)

The electronic spectra of 5 from 1000 to 200 nm shows strong π-π* band at 486nm

with a high absorbance value (17,000 M-1 cm-1) [Figure 2.19(a)]. These bands are all ligand

based and can be compared with the absorbance bands for the deprotonated ligand .

Variable-temperature magnetic data for 5 exhibit a steady decrease of µeff from 1.634

µB (χMT = 0.3338 cm3 K mol-1) at 290 K to 0.049 µB (χMT = 0.03 x 10-3 cm3 K mol-1) at 2

K, which is indicative of very strong intramolecular antiferromagnetic coupling. The

experimental data were simulated with the parameter set J = -192.1 cm-1, g = 2.055, TIP =30 x

10-6, P = 0 and is shown in Figure 2.19(b). The major factor controlling the exchange

interactions in hydroxo-, alkoxo- and phenoxo-bridged copper(II) is the Cu-O-Cu bridge

angle.15,16 The average Cu-O-Cu bridge angle of 100.9o for 5 would result in an approximate J

value of -250 cm-1 according to the empirical equation reported in the literature and it

deviates appreciably from the experimentally evaluated value of -192 cm-1 for 5. A possible

reason for the weak exchange coupling in 5 may be the deviation from co-planarity within the

central Cu2O2 ring. It seems plausible that the methoxide ion, with the shorter Cu(1)-O(40)

1.911 Å and Cu(2)-O(40) 1.905 Å distances and larger Cu(1)-O(40)-Cu(2) angle 102.43o than

those with the phenoxide ion, provides a better pathway for antiferromagnetic exchange

coupling.14

Electrochemical voltammetric measurements (cyclic voltammetry, CV, and square

wave voltammetry, SQW) were performed with the complexes in CH2Cl2 solutions containing

0.1 M TBAPF6 [Figure 2.20]. The copper(II) complex 5 exhibit an oxidation wave at +0.490

vs Fc+/Fc for 5. The waves have reversible appearance, however the peaks in CV and SQW

are broad; the peak separation of the oxidative and reductive peaks in the CVs is relatively

high(0.110 V even at low scan rates) and in SQW shoulders on the forward and reverse peaks

45

400 600 800 1000

0

3

6

9

12

15

18 486 nm

ε * 10- 3 (M-1cm-1)

λ (nm)

50 100 150 200 250 300

0.0

0.3

0.6

0.9

1.2

1.5

1.8

Simulated

Experimental

µ eff / µ B

T/K

600 400 200

6 µA

E (mV) vs Fc+/ Fc

800 600 400 200 0 -200

4 µA

E ( mV ) vs Fc+/ Fc

a) b)

Figure 2.19 :- a) Electronic spectra of 5. b) Magnetic data of 5.

are discernible. Since a copper(II)-oxidation is not feasible at such low potentials, these redox

processes were assigned to ligand centered oxidation yielding two phenoxyl radicals in the

complex.

[L2

2CuII

4(µ4-O)] (6)

The molecular structure of 6 is shown in Figure 2.21. Selected interatomic distances

and bond angles are listed in Table 2.10. Compound 6 consists of four copper(II) ions bridged

by a central µ4-oxygen atom in an approximately tetrahedral environment. The phenolate

oxygen atoms, O(10) and O(50), of the cresol-part of the ligand bridge two copper centers,

resulting in very similar angles Cu(1)-O(10)-Cu(2) and Cu(3)-O(50)-Cu(4) of 99.49(9)o and

99.12(9)o. Each copper center is coordinated by three oxygen atoms and one nitrogen atom,

with the bond lengths ranging from 1.891(2) to 1.953(2) Å. There are considerable deviations

(a) (b)

Figure 2.20:- (a) Cyclic and (b) square wave voltammogram (10 Hz) of 5.

46

of the geometry of the copper center from the ideal square plane, as indicated by the basal

plane angles (Table 2.10), although each CuO3N portion is essentially planar. The dihedral

angles between the Cu(1)/Cu(2) planes, and between the Cu(3)/Cu(4) planes are 8.5o and 3.2o,

respectively. The two Cu2O4N2 dinuclear units are nearly perpendicular as evidenced by the

dihedral angle of 75.2o between the Cu(1)O(10)Cu(2)O(99) and Cu(3)O(50)Cu(4)O(99)

planes. The copper tetrahedron around the µ4-oxygen O(99) is distorted, as the bond angles

Cu-O(99)-Cu range from 122.37(1)o for Cu(2)-O(99)-Cu(3) to 90.23(8) for Cu(4)-O(99)-

Cu(1), the smallest among the six angles at O(99). Perhaps as a consequence, the

Cu(2)Cu(3) distance of 3.356 Å is substantially longer than that of Cu(4)Cu(1), 2.759 Å.

The remaining CuCu distances are Cu(1)Cu(2) 2.963, Cu(1)Cu(3) 3.368,

a) b)

Figure 2.21:- a) Molecular structure of 6. b) The atom connectivity in the core of complex 6.

Table 2.10:- Selected interatomic distances (Å) and angles (deg) for 6

Cu(1)-O(1) 1.903(2) Cu(3)-O(41) 1.897(2)

Cu(1)-N(7) 1.919(3) Cu(3)-O(99) 1.915(2)

Cu(1)-O(10 1.925(2) Cu(3)-N(47) 1.932(2)

Cu(1)-O(99) 1.953(2) Cu(3)-O(50) 1.950(2)

Cu(2)-O(22) 1.897(2) Cu(4)-O(62) 1.891(2)

Cu(2)-O(99) 1.915(2) Cu(4)-N(56) 1.916(3)

Cu(2)-N(16) 1.927(3) Cu(4)-O(50) 1.920(2)

Cu(2)-O(10) 1.956(2) Cu(4)-O(99) 1.942(2)

Cu(1)Cu(4) 2.760(1) Cu(1)Cu(3) 3.368(1)

Cu(1)Cu(2) 2.963(1) Cu(2)Cu(4) 3.345(1)

Cu(3)Cu(4) 2.945(1) Cu(2)Cu(3) 3.356(1)

Cu(2)-O(99)-Cu(3) 122.37(11) N(7)-Cu(1)-O(99) 173.77(10)

Cu(2)-O(99)-Cu(4) 120.30(11) O(10)-Cu(1)-O(99) 80.02(9)

Cu(3)-O(99)-Cu(4) 99.58(9) O(1)-Cu(1)-Cu(4) 71.26(7)

Cu(2)-O(99)-Cu(1) 99.98(9) N(7)-Cu(1)-Cu(4) 137.58(7)

Cu(3)-O(99)-Cu(1) 121.07(11) O(10)-Cu(1)-Cu(4) 106.01(7)

Cu(4)-O(99)-Cu(1) 90.23(8) O(99)-Cu(1)-Cu(4) 44.71(6)

Cu(4)-O(50)-Cu(3) 99.12(9) O(1)-Cu(1)-Cu(2) 138.52(7)

Cu(1)-O(10)-Cu(2) 99.49(9) N(7)-Cu(1)-Cu(2) 134.27(8)

O(1)-Cu(1)-N(7) 86.44(10) O(10)-Cu(1)-Cu(2) 40.64(6)

O(1)-Cu(1)-O(10) 175.87(9) O(99)-Cu(1)-Cu(2) 39.53(6)

N(7)-Cu(1)-O(10) 93.86(10) Cu(4)-Cu(1)-Cu(2) 71.43(2)

47

O(1)-Cu(1)-O(99) 99.59(9) O(22)-Cu(2)-O(99) 102.40(9)

O(22)-Cu(2)-N(16) 86.48(11) N(47)-Cu(3)-Cu(4) 131.12(8)

O(99)-Cu(2)-N(16) 170.27(10) O(50)-Cu(3)-Cu(4) 40.07(6)

O(22)-Cu(2)-O(10) 174.81(9) O(62)-Cu(4)-N(56) 87.16(10)

O(99)-Cu(2)-O(10) 80.20(9) O(62)-Cu(4)-N(50) 178.00(9)

N(16)-Cu(2)-O(10) 91.26(10) N(56)-Cu(4)-O(50) 94.35(10)

O(22)-Cu(2)-Cu(1) 142.89(7) O(62)-Cu(4)-O(99) 97.66(9)

O(99)-Cu(2)-Cu(1) 40.49(6) N(56)-Cu(4)-O(99) 172.05(10)

N(16)-Cu(2)-Cu(1) 130.53(8) O(50)-Cu(4)-O(99) 80.69(9)

O(10)-Cu(2)-Cu(1) 39.86(6) O(62)-Cu(4)-Cu(1) 131.72(8)

O(41)-Cu(3)-O(99) 101.94(9) N(56)-Cu(4)-Cu(1) 104.47(6)

O(41)-Cu(3)-N(47) 86.63(10) O(50)-Cu(4)-Cu(1) 45.06(6)

O(99)-Cu(3)-N(47) 169.95(10) O(99)-Cu(4)-Cu(1) 137.53(7)

O(41)-Cu(3)-O(50) 174.51(10) O(62)-Cu(4)-Cu(3) 134.87(8)

O(99)-Cu(3)-O(50) 80.61(9) N(56)-Cu(4)-Cu(3) 40.81(6)

N(47)-Cu(3)-O(50) 91.31(10) O(50)-Cu(4)-Cu(3) 39.88(6)

O(41)-Cu(3)-Cu(4) 142.25(6) O(99)-Cu(4)-Cu(3) 72.29(2)

O(99)-Cu(3)-Cu(4) 40.54(6)

Cu(2)Cu(4) 3.345, Cu(3)Cu(4) 2.945 Å. Thus these differences can be explained by the

steric forces of the chelating ligand [L2]3- which contracts the Cu(1)-O(99)-Cu(4) angle with a

concomitant expansion of the opposite angle Cu(2)-O(99)-Cu(3). Examples of tetranuclear

copper(II) complexes with a µ4-O kernel are abundant,15,17,18 but all of them involve bridging

ligands like halides or carboxylates between the edges of the tetrahedron. Compound 6

without any such bridging ligand is an exception. Figure 2.21(b) highlights the core structure

for greater clarity and to emphasize the unique structure of the Cu4(µ4-O)-unit present in 6.

The effective magnetic moment µeff/molecule for 6 of 2.768 µB (χMT = 0.9582 cm3 K

mol-1) at 290 K decreases monotonically with decreasing temperature until it reaches a value

of 0.133 µB (χMT = 0.0022 cm3 K mol-1) at 2 K indicating dominant antiferromagnetic

interactions between four 2B2 copper(II) ions; the data clearly show an St = 0 ground state for

6.

Two different exchange pathways are envisagable for 6: Cu-phenoxo-Cu and Cu-µ4-

oxo-Cu. Figure 2.22(a), shows the connection of the copper centers through the phenoxo and

the oxo-group. As the pairs Cu(3)/Cu(4) and Cu(2)/Cu(1) are bridged by both phenoxo and

oxo groups and the pairs Cu(4)/Cu(1), Cu(2)/Cu(3), Cu(2)/Cu(4) and Cu(1)/Cu(3) only

through the oxo group, O(99), the experimental magnetic data were analyzed as a first

approximation with a three-J model: J12 = J34, J14 = J23 and J24 = J13. In this model the

differences in the Cu- O-Cu angles has been neglected. A very good fit (not shown) was

obtained with the following parameters: J12= J34= - 86.3 cm-1, J14 = J23 = +40.8 cm-1 and J24 =

J13 = - 86.1 cm-1, g = 1.976, D = 0 (fixed), P = 0.7% and TIP = 240 x 10-6 which imply that

the spin exchange is dominated by antiparallel coupling in 6. The low g value and the positive

coupling constants for both J14 and J23, which represent the couplings between the copper

48

J13

J24

O(99)

Cu (3)

J34

(50)O

Cu(4)

Cu (2)

J12 O(10)

Cu(1)

J23

J14

50 100 150 200 250 300

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Simulated

Experimental

µeff / µB

T/K

a) b)

Figure 2.22:- a) Connection of the copper centers through the phenoxo and the oxo-groups.

b) Magnetic data of 6.

centers with the angles Cu(4)-O(99)-Cu(1) 90.23o and Cu(2)-O(99)-Cu(3) 122.37o, however,

suggest that a better fitting model should be used. The data were consequently fitted to a more

accurate model. Therefore the data was fitted by using again a three-J model but with a

different constraint based on the three ranges of Cu-O-Cu angles, 90.2, 99.12 - 99.98o and

120.30 – 122.37o. Thus, the following correlations between the six coupling constants

depicted in Figure 2.22(a) were used to fit the experimental data: J12 = J34, J23 = J24 = J13 and

J14. The fit shown in Figure 2.22(b), was obtained with J12= J34 = -122.3 cm-1, J23 = J24 = J13=

-90.0 cm-1 and J14 = 0, g = 2.088, P = 0.3%, TIP = 240 x 10-6 and D = 0 (fixed). The

susceptibility data consistent with this data set were found to be relatively insensitive to J14,

but very sensitive to J12 and J23. J14-values lying in the range -20 to +20 cm-1 have no

influence on the quality of simulation.

For copper(II) centers bridged by a ligand oxygen atom (oxo, hydroxo, alkoxo,

phenoxo, etc.), a linear relationship of exchange coupling J with the average Cu-O-Cu angles

in the Cu2O2 ring has been reported.[17,20,23] It should be noted that although J12 of

-122 cm-1 assigned to an angle of 99.5o does not deviate significantly from the reported linear

relationship, the J23-value of -90.0 cm-1 deviates remarkably. This is not very surprising as the

electronic structures of the µ-OR and µ4-O groups are clearly different. J14 is non-determinant

for 6 within the measured temperature range 2 - 290 K.

The electronic spectra of 6 shows similar strong π-π* band at 485nm with a high

absorbance value (35,700 M-1 cm-1), similar to that as observed in 5 [Figure 2.23(b)]. These

bands are all ligand based and can be compared with the absorbance bands for the

deprotonated ligand . The tetranuclear copper complex (6) shows similar electrochemical

behavior as observed for the dinuclear copper complex (5). An oxidation peak is observed at

+0.523 mV against Fc+/Fc [Figure 2.23(a)]. Coulometry of 6 at –25 °C in presence of 0.2 M

49

1000 800 600 400 200 0 -200

5 µA

E ( mV ) vs Fc+/ Fc

200 400 600 800 1000

0.0

0.2

0.4

0.6

0.8

1.0

Absorbance

λ (nm)

a) b)

Figure 2.23 :- a) Cyclic voltammetry of 6 vs. Fc+/Fc b) Electronic spectra of 6 (bold line) and

its 1e- oxidized species.

TBAPF6 shows a single electron oxidation at +0.523 V with an increase in the intensity of the

band at 410 nm and also at 990 nm. These two bands are the fingerprints for the formation of

the phenoxyl radicals [Figure 2.23 (b)] (See 1.1.1).

[L2

2FeIII

2] (7)

Although H3L2 is estimably suited to form dinuclear complexes and the analytical and

spectroscopic data are in accord with the presence of a dinuclear Fe2L2 unit as the smallest

unit in 7, an X-ray analysis was undertaken to remove the doubts regarding connectivity.

Indeed, the structure analysis shows the presence of two 6-coordinated iron(III) centers with

the Fe2O6N4 coordination unit. An ORTEP drawing of the molecule is displayed in Figure

2.24 with selected bond distances and angles provided in Table 2.11.

The iron atoms, Fe(1) and Fe(2), are in distorted octahedral environments, having

FeN2O4 coordination spheres. The two octahedra share a common edge and are bridged by

two phenolate oxygen atoms O(50) and O(10). The six oxygen atoms of two ligands are

roughly coplanar with the two Fe atoms. Thus, for two iron centers, the

Fe(1)O(1)O(41)O(50)O(10) and Fe(2)O(62)O(22)O(50)O(10) atoms constitute the equatorial

planes. Each iron center is coordinated to two nitrogen atoms, e.g Fe(1)-N(7) and Fe(1)-

N(47), which are trans to each other with an angle N(47)-Fe(1)-N(7) of 162.7(2)o, and belong

to two different [L2] 3- ligands.

The non-bridging Fe(1)-O(1)/O(41) av. 1.913(4) and Fe(2)-O(62)/O(22) av. 1.923(7)

Å distances are distinctly different from the bridging Fe(1)-O(10)/O(50) av. 2.043(5) and

Fe(2)-O(10)/O(50) av. 2.043 Å distances. The symmetry in the bridging bond lengths in the

nearly planar Fe2O2 rhomb is noteworthy. Although the Fe-O-Fe angle of 108.9(2)o for 7 fall

50

into the range 105 - 110o observed for previously structurally characterized FeIII2(OPh)2

bridge, the FeFe separation of 3.32 Å is one of the longest yet found for complexes with

this bis(phenoxide) bridge.19 In the previously reported [FeIII2(salmp)2],19c where salmp3- is

the pentadentate ligand 2-bis(salicylideneamino)methylphenolate, the Fe-O-Fe angle of 97o is

the smallest yet found. As will be seen later, mostly because of the differences in the Fe-O-Fe

angle, the magnetic properties of 1 differ considerably from that of [FeIII2(salmp)2], although 7

and [FeIII2(salmp)2] are otherwise very similar. The Fe-N and Fe-O bond distances are

consistent with high-spin electron configuration of both Fe(III) centers in 7 with imine

nitrogen and phenolate oxygen donor ligands. The d5 h.s. electron configuration has also been

confirmed both by Mössbauer and magnetic susceptibility measurements.

Figure 2.24 :- An ORTEP drawing of the neutral molecule 7.

Table 2.11 Selected Interatomic Distances (Å) and Angles (degrees) in 7

Fe(1)-O(1) 1.913(4) Fe(2)-O(62) 1.920(4)

Fe(1)-O(41) 1.914(3) Fe(2)-O(22) 1.927(4)

Fe(1)-O(10) 2.042(4) Fe(2)-O(50) 2.041(4)

Fe(1)-O(50) 2.044(4) Fe(2)-O(10) 2.044(3)

Fe(1)-N(47) 2.161(4) Fe(2)-N(16) 2.167(5)

Fe(1)-N(7) 2.166(4) Fe(2)-N(56) 2.174(5)

Fe(1)Fe(2) 3.32(1)

O(1)-Fe(1)-O(41) 100.2(2) O(62)-Fe(2)-O(22) 122.5(2)

O(1)-Fe(1)-O(10) 150.9(2) O(62)-Fe(2)-O(50) 135.4(2)

O(41)-Fe(1)-O(10) 99.8(2) O(22)-Fe(2)-O(50) 94.3(2)

O(1)-Fe(1)-O(50) 99.9(2) O(62)-Fe(2)-O(10) 93.4(2)

O(41)-Fe(1)-O(50) 150.8(2) O(22)-Fe(2)-O(10) 135.9(2)

O(10)-Fe(1)-O(50) 71.1(2) O(50)-Fe(2)-O(10) 71.2(2)

O(1)-Fe(1)-N(47) 89.6(2) O(62)-Fe(2)-N(16) 88.8(2)

O(41)-Fe(1)-N(47) 79.1(2) O(22)-Fe(2)-N(16) 77.2(2)

O(10)-Fe(1)-N(47) 114.9(2) O(50)-Fe(2)-N(16) 126.3(2)

O(50)-Fe(1)-N(47) 80.0(2) O(10)-Fe(2)-N(16) 79.1(2)

O(1)-Fe(1)-N(7) 79.2(2) O(62)-Fe(2)-N(56) 77.6(2)

O(41)-Fe(1)-N(7) 89.9(2) O(22)-Fe(2)-N(56) 88.9(2)

O(10)-Fe(1)-N(7) 79.9(2) O(50)-Fe(2)-N(56) 79.1(2)

O(50)-Fe(1)-N(7) 114.6(2) O(10)-Fe(2)-N(56) 126.3(2)

N(47)-Fe(1)-N(7) 162.7(2) N(16)-Fe(2)-N(56) 151.3(2)

51

-4 -2 0 2 4

0.93

0.96

0.99

Simulated

Experimental

Relative Transmission

Velocity (mm sec -1)

0 50 100 150 200 250 300

0

1

2

3

4

5

6

7

Simulated

Experimental

µeff / µB

T/K

The Mössbauer spectrum of 7 , [L2FeIII2], at 80 K in zero applied magnetic field and

the nonlinear least-squares fit is shown in Figure 2.25(a). The spectrum was fitted with a

single quadrupole split doublet with an isomer shift of δ = 0.518 mm s-1 and a quadrupole

splitting of ∆EQ = 0.754 mm s-1. The isomer shift is consistent with those observed for high

spin iron(III) ions in an octahedral or distorted octahedral coordination.20 The magnitude of

the quadrupole splitting is a reflection of the unsymmetrical electric field gradient about each

high-spin iron(III) site, although the two metal sites are equivalent.

The magnetic behavior of 7, L22FeIII2, is characteristic of an antiferromagnetically

coupled dinuclear complex. At 290 K the µeff value of 6.784 µB (χMT = 5.755 cm3 K mol-1)

decreases monotonically with decreasing temperature until it reaches a value of 0.274 µB

(χMT = 9.380 x 10-3 cm3 K mol-1) at 2 K; this is a clear indication of exchange coupling

between two paramagnetic Fe(III) centers (SFe=5/2) with a resulting St=0 ground state. The

solid line in Figure2.25(b) represents the best fit with the following parameters:J = -12.7 cm-1,

g =2.00(fixed), P (S = 5/2) = 0.3%. The evaluated antiparallel exchange is in keeping with the

range observed for comparable diphenoxo-bridged ferric dimers. 19b-c,21

Two semi-empirical magnetostructural correlations relating the magnitude of the

exchange coupling to the iron-oxygen bond length in exchange coupled

phenoxo-, alkoxo- and hydroxo-bridged dinuclear iron(III) compounds have been

proposed.19a-c Using the empirical relationship J = -107 exp (-6.8 d),19c where d is the averaged

iron-oxygen distance, 2.043 Å for 7, the J value for 7 can be calculated to be -9.25 cm-1,

which differs from the experimentally observed value of -12.7 cm-1. The second equation19a

a) b)

Figure 2.25 :- a) Mössbauer spectra for 7. b) Magnetic data of 7

52

1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5

5 µA

E (mV) vs Fc+/ Fc

400 600 800 1000

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Absorbance

λ (nm)

a) b)

Figure 2.26 :- a) Cyclic voltammetry of 7 vs. Fc+/Fc ; b) Electronic spectra of 7 (bold line)

and its 1e- oxidized species.

leads to a calculated value of -5.1 cm-1. Regrettably none of these two correlations can

satisfactorily reproduce the exchange interaction in the present ferric dimer, 7.

The electrochemical properties of complex 7 was investigated in CH2Cl2 solutions

containing 0.1 M TBA(PF6). Voltammetric experiments (cyclic voltammetry, CV and square

wave voltammetry, SQW) reveal that 7 can be oxidized thrice in three consecutive steps and

reduced twice in two consecutive steps [Figure 2.26(a)]. The oxidation potential are found to

be +0.870 V, +0.586 V and +0.390 V and the reduction peaks are found to be –1.42V and

-1.95V against Fc+/Fc. In the spectroelectrochemistry experiment with 7 in an OTTLE cell,

increase in absorption at 350 - 450 nm and for >700 nm was observed during the first

oxidation, as expected for formation of phenoxyl radicals [Figure 2.26(b)]. However, the

maxima of the difference spectra were at 350 - 370 nm rather than at 400 - 420 nm. Most

possibly these spectra are distorted by small oxidation-induced shifts in the very intense

charge transfer bands of the complexes (the ε-values of the starting material are ten times

higher than those of phenoxyl radicals at the respective wavelengths).

[L2

2MnIII

2] (8a) and [L2

2MnIII

2 (THF)2] (8b)

Single crystals of deep orange 8b were obtained from a tetrahydrofuran solution of 8a

by slow evaporation. The structure of 8b is shown in Figure 2.27 with selected bond distances

and angles provided in Table 2.12. The molecule is based on two MnO4N2 octahedra, in

which two triply deprotonated ligands [O∩N∩O∩N∩O]3- span between two manganese centers.

The pentadentate ligand [O∩N∩O∩N∩O] acts as a tridentate (O,N,O)-donor ligand for one

manganese center, whereas the residual two donor atoms N,O coordinate to the second

manganese center, thus each of the manganese center attains only pentacoordination through

53

the ligand. The sixth coordination position of each manganese center is occupied by the

oxygen atom of a tetrahydrofuran molecule. That the manganese centers are in the +III

oxidation state is evident from the axial elongation of the octahedra long the

N(47)Mn(1)O(80) and the N(16)Mn(2)O(90) axes, as is expected for a Jahn-Teller distorted

high-spin d4 ion. The atoms Mn(1)N(7)O(1)O(41)O(14) and Mn(2)N(56)O(54)O(22)O(62)

comprise the equatorial planes for each octahedron around Mn(1) and Mn(2) centers,

respectively. The planarity of MnNO3 fragment is good, the maximum deviation from the

mean plane is 0.01 Å. Notable is the axial position of each coordinated tetrahydrofuran

molecule. The average equatorial Mn-O and Mn-N distances of 1.896(16) and 2.006(3) Å,

respectively, fall in the range reported for the structurally characterized d4 Mn(III)

complexes,22 which also corroborate with the susceptibility measurements. The smallest acute

bite angle between the phenolate O atom and the N atom of the L3- ligand has been found for

the Mn(1) center with the angle O(41)-Mn(1)-N(47) of 77.2(1)o.

Compound 8b is a rare example of a dinucleating ligand containing the central p-

cresol group,[10] in which the central phenolate oxygen, O(54) or O(14), of the cresol ring

does not coordinate as a bridging atom between two metal centers.

Figure 2.27:- Molecular structure of 8b.

Table 2.12 Selected Bond Distances (Å) and Angles (deg) for 8b.

Mn(1)-O(1) 1.879(2) Mn(2)-O(62) 1.888(2)

Mn(1)-O(41) 1.899(2) Mn(2)-O(22) 1.896(2)

Mn(1)-O(14) 1.907(2) Mn(2)-O(54) 1.906(2)

Mn(1)-N(7) 2.005(2) Mn(2)-N(56) 2.008(3)

Mn(1)-O(80) 2.291(3) Mn(2)-O(90) 2.271(3)

Mn(1)-N(47) 2.310(3) Mn(2)-N(16) 2.341(3)

Mn(1)Mn(2) 6.448(2)

O(1)-Mn(1)-O(41) 89.79(9) O(54)-Mn(2)-N(56) 91.52(10)

O(1)-Mn(1)-O(14) 174.17(9) O(62)-Mn(2)-O(90) 93.88(10)

O(41)-Mn(1)-O(14) 95.98(9) O(22)-Mn(2)-O(90) 88.62(10)

O(1)-Mn(1)-N(7) 82.77(10) O(54)-Mn(2)-O(90) 85.61(10)

54

50 100 150 200 250 300

0

2

4

6

8

Simulated

Experimental

8b

8a

µeff / µB

T / K

O(41)-Mn(1)-N(7) 172.21(10) N(56)-Mn(2)-O(90) 93.79(11)

O(14)-Mn(1)-N(7) 91.49(10) O(62)-Mn(2)-N(16) 91.94(10)

O(1)-Mn(1)-O(80) 94.12(10) O(22)-Mn(2)-N(16) 76.16(10)

O(41)-Mn(1)-O(80) 94.36(10) O(54)-Mn(2)-N(16) 90.17(10)

O(14)-Mn(1)-O(80) 84.60(10) N(56)-Mn(2)-N(16) 101.98(11)

N(7)-Mn(1)-O(80) 88.51(10) O(90)-Mn(2)-N(16) 163.78(10)

O(1)-Mn(1)-N(47) 92.67(10) O(80)-Mn(1)-N(47) 169.12(9)

O(41)-Mn(1)-N(47) 77.16(10) O(62)-Mn(2)-O(22) 90.85(10)

O(14)-Mn(1)-N(47) 89.46(10) O(62)-Mn(2)-O(54) 173.90(10)

N(7)-Mn(1)-N(47) 100.79(10) O(22)-Mn(2)-O(54) 95.21(9)

O(62)-Mn(2)-N(56) 82.45(10) O(22)-Mn(2)-N(56) 173.02(10)

The magnetic moment µeff /molecule for 8a [L22MnIII2] of 6.636 µB (χMT = 5.507 cm3

K mol-1) at 290 K decreases monotonically with decreasing temperature until it reaches a

value of 1.421 µB (χMT = 0.2523 cm3 K mol-1) at 2 K; this temperature dependence of µeff is

a clear indication of an antiferromagnetic exchange coupling between two paramagnetic

Mn(III) (SMn = 2) centers. A least-squares fit, shown as the solid line in Figure 2.28, with

J = - 2.95 cm-1, g = 1.98 was obtained. Thus a weak exchange coupling is operating between

the Mn(III) centers through the diphenoxo-bridge and as expected the exchange interaction is

weaker in 8a than that in 7. The exchange coupling operating in 8b [L2 2MnIII2(THF)2] is even

weaker than that in 8a, as is evident from the temperature-dependence of µeff for 8b. The

magnetic moment µeff varies only slightly (µeff = 6.94 to 6.54 µB) in the temperature range 290

- 40 K, but then starts to decrease monotonically reaching a value of 1.94 µB at 2 K. The

simulation of the experimental magnetic data yields J = -0.66 cm-1, g = 1.995 (the solid line in

Figure 2.28). The weaker antiferromagnetic coupling in 8b than that in 8a is in accord with

the dimeric solid state structure of 8b, in which the manganese(III) centers are 6.45 Å apart

from each other.

The electro- and spectroelectrochemistry of [L22MnIII2](8a) and [L22MnIII2(THF)2](8b)

Figure 2.28 :- Magnetic study of 8a and 8b.

55

1500 1000 500 0 -500

1 µA

E (mV) vs Fc+/ Fc

400 600 800 1000

0.0

0.2

0.4

0.6

0.8

Absorbance

λ (nm)

(a) (b)

Figure 2.29:- a) Cyclic voltammetry of 8a vs. Fc+/Fc ; b)Electronic spectra of 8a (bold line)

and its 1e- oxidized species.

was measured and it was found that both exhibits the same redox properties. The redox

potentials of the first oxidations are spread over a somewhat larger range. The oxidative peaks

in the square wave mode (Frequency 10Hz) are found to be +0.860V (irreversible), +0.542V

(reversible) and -0.060V (reversible) and one reductive peak is found in -1.30V vs. Fc+/Fc. In

particular the redox potential at -0.06 V of the Mn-complex 8a is low and could be due to

formation of the MnIV form [Figure 2.29 (a)]. Therefore spectroelectrochemical measurements

at -25 oC in an OTTLE cell was performed and it was found that the spectral changes upon the

first oxidation of 8a shows the fingerprints for phenoxyl radical formation.18 In the difference

spectrum a new peak with a maximum at 417 nm developed together with a broad band which

extends from 650 - 950 nm and has a maximum at ∼800 nm. Therefore, the first oxidation of

8a can be clearly assigned to phenoxyl radical formation [Figure 2.29 (b)]. The reductions at

potentials less than -1V are either electrochemically quasi-reversible (peak separation 0.15 -

0.25 V at 0.2 V/s scan rate) or chemically irreversible. They were not further investigated and

are most feasibly metal-centered reductions to the M2+ oxidation states.

[L2

2 CrIII

2] (9)

The experimental magnetic moment of 9, L22CrIII2 decreases from 4.95 µB (χMT =

3.064 cm3 K mol-1) at 290 K to an essentially diamagnetic value of µeff = 0.677 µB (χMT =

0.05736 cm3 K mol-1) at 2 K, resulting from the antiferromagnetic interaction between two

Cr(III) ions bridged by a diphenoxo-group in complex 9. The solid line in Figure 2.30

represents the best fit with the following parameters: J = -7.6 cm-1, g = 1.893, P(S=3/2 )=1%.

The evaluated antiparallel exchange falls in the range observed for comparable phenoxo-

alkoxo-bridged chromium(III) dimers.23

56

50 100 150 200 250 300

0

1

2

3

4

5

Simulated

Experimental

µ eff / µ B

T / K

1000 500 0 -500 -1000 -1500 -2000 -2500

E (mV) vs Fc+/ Fc

400 600 800 1000

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Absorbance

λ (nm)

Figure 2.30 :- Magnetic measurement of 9.

Electrochemical data of 9 in the square wave mode (25 Hz) shows the presence of

three oxidative peaks at +0.905V, +0.410V and +0.144V and a single reductive peak at -1.99

V vs. Fc+/Fc. The peaks at +0.905 and that of +1.99 V are irreversible in character with high

peak current and the other two peaks are reversible [Figure 2.31(a)]. The nature of these peaks

are similar to that obtained for 8 . The reductive peaks are attributed to the metal based

reduction to M2+ state.

The electronic spectra of 9 shows a ligand based peak at 458 nm with a shoulder at

526 nm. When 9 is subjected to spectroelectrochemistry in an OTTLE cell, it is found there is

an increase in the absorption at 350-450 nm and for >700 nm for the first oxidation as

observed for 7 [Figure 2.31(b)].

(a) (b)

Figure 2.31:-a) Square wave voltammogram of 9. b) Electronic spectra of 9 (bold line) and its

1e- oxidized species.

57

[L2 (VIV=O)2(OCHMe2)] (10)

The structure of 10 (Figure 2.32) shows the formation of a divanadyl(IV) complex

with a bridging isopropoxide originating from the starting material vanadium isopropoxide.

The atoms V(1) and V(2) are additionally bridged by a phenolate oxygen O(10). The penta-

coordination of each vanadium atom is achieved by oxygen atoms O(12) and O(13), with

V(1)-O(12) and V(2)-O(13) distances of 1.586(3) and 1.588(3) Å, respectively, indicating

their double-bond character. The V=O groups are oriented trans to each other in the

divanadium complex. The coordination polyhedron for the vanadium centers is distorted

square pyramid with O(1)N(7)O(11)O(10) for V(1) and O(22)N(16)O(11)O(10) for V(2)

forming the basal planes, in which both V(1) and V(2) are located at 0.60 Å out of the

equatorial planes. The ring V(1)O(11)V(2)O(10) is not planar, the dihedral angle being 10.3o.

The distances V-O and V-N are in the ranges reported for comparable complexes24 and are in

accord with the d1 electron configuration for the vanadium centers. This electronic structure

has also been confirmed by the magnetic susceptibility measurements. The separation

V(1)V(2) of 3.063 Å necessitates consideration of direct interaction between the metal

centers. Selected interatomic distances and bond angles are listed in Table 2.13.

The temperature dependence of the molar magnetic susceptibility χM for 10 shows a

clear maximum around 230 K, consistent with the presence of significant antiferromagnetic

coupling. That at low temperatures (< 30 K) χM increases again has been often seen in

Figure 2.32 :- ORTEP diagram of 10.

Table 2.13 Selected Bond Distances (Å) and Angles (deg) for 10.

V(1)-O(12) 1.586(3) V(2)-O(13) 1.588(3)

V(1)-O(1) 1.905(3) V(2)-O(22) 1.919(3)

V(1)-O(11) 1.970(3) V(2)-O(11) 1.953(3)

V(1)-O(10) 1.995(3) V(2)-O(10) 1.994(3)

V(1)-N(7) 2.066(4) V(2)-N(16) 2.065(4)

58

0 50 100 150 200 250 300

0.0

0.1

0.2

0.3

0.4

0.5

Simulated

Experimental

λ M* T (cm3 K mol-1)

T (K) 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0

Reductive Peak

Oxidative Peak

E (mV) vs Fc+/ Fc

V(1)V(2) 3.063(1)

O(12)-V(1)-O(1) 109.3(2) O(13)-V(2)-O(22) 107.3(2)

O(12)-V(1)-O(11) 110.6(2) O(13)-V(2)-O(11) 110.9(2)

O(1)-V(1)-O(11) 95.45(13) O(22)-V(2)-O(11) 94.61(13)

O(12)-V(1)-O(10) 107.86(14) O(13)-V(2)-O(10) 106.1(2)

O(1)-V(1)-O(10) 142.13(14) O(22)-V(2)-O(10) 146.22(13)

O(11)-V(1)-O(10) 77.92(12) O(11)-V(2)-O(10) 78.36(12)

O(12)-V(1)-N(7) 102.6(2) O(13)-V(2)-N(16) 105.7(2)

O(1)-V(1)-N(7) 80.16(14) O(22)-V(2)-N(16) 79.87(14)

O(11)-V(1)-N(7) 146.03(14) O(11)-V(2)-N(16) 142.89(14)

O(10)-V(1)-N(7) 85.12(13) O(10)-V(2)-N(16) 86.15(13)

V(2)-O(10)-V(1) 100.30(13)

V(2)-O(11)-V(1) 102.65(14)

strongly antiferromagnetically coupled systems and arises from a small amount of

paramagnetic impurity in the sample. The magnetic moment of 10 is 1.83 µB (χMT = 0.4430

cm3 K mol-1) at 290 K, which is significantly lower than the spin-only value of (2.45 µB) for

two uncoupled S = 1/2 spins, and µeff gradually decreases on decreasing the temperature,

reaching 0.15 µB at 2 K. The data were fitted by the following parameters: J = -128.5 cm-1,

g=1.90, PI (S = 1/2) = 1.8%, TIP = 80 x 10-6 cm3 mol-1, θ = -3.0 K [Figure 2.33(a)]. The

antiferromagnetic coupling constant of -128.5 cm-1 lies in the range of values found for other

dinuclear vanadyl(IV) complexes24,25 containing phenoxo, alkoxo and hydroxo-bridging

ligands.

The complex 10 was subjected to electrochemical study and it was observed that

broad, irreversible peaks at +0.436 V and +1.124V vs.Fc+/Fc appear for the oxidative range in

the square wave voltammogram. From the peak position, it can be inferred that the oxidations

are all ligand centered with the formation of phenoxyl radicals. For the reductive range in the

cyclic voltammogram multiple waves are observed at -0.830 V, -0.980 and –1.6 V vs. Fc+/Fc

[Figure 2.33(b)]. These peaks are probably due to the reduction of the metal centers to V+3.

a) b)

Figure 2.33 :- a)Magnetic susceptibility measurement of 10. b) Square wave voltammogram

of 10.

59

O

N

O

N

O

N

O

VV

N

[L2

2(VV=O)2] (11)

The crystals of [L2(VO)2]2CH3CN obtained by crystallizing 11 from an acetonitrile

solution were subjected to single-crystal X-ray crystallography at 100 K. Figure 2.34(a)

shows a perspective view and atom-labeling scheme of 11. Selected bond parameters are

listed in Table 2.14.

In the distorted square pyramidal VO4N coordination sphere the metal atom is

displaced toward the O(1) or O(5) atom from the equatorial planes O(2)O(3)N(4)O(4) for

V(1) and O(6)O(7)N(2)O(8) for V(2) by 0.33 and 0.31 Å, respectively. The V(1)-O(1) and

V(2)-O(5) distances of 1.603(2) and 1.595(2) Å correspond to a vanadium-oxygen multiple

bond and resemble closely vanadium(V) complexes containing a single V=O group.26 Each

ligand with its five donor atoms spans between two vanadium(V) centers and one N atom,

N(1) or N(4), does not coordinate to any of the metal centers, rendering each of the ligand to

be tetradentate. This type of behaviour for the ligands based on 2,6-diformyl-p-cresol is

observed for the first time. A schematic drawing of the coordination sphere is shown

[Figure 2.34(b)] to highlight the tetradentate coordination of the ligand. The V-O and V-N

bond distances (Table 2.14) are comparable to those of vanadium(V) complexes with a

monooxo- and cis-dioxo-vanadium moiety.27 The C-O (average 1.34 Å) and the aromatic C-C

b)

a)

Figure 2.34:-A perspective view of the neutral complex 11.b)Schematic view of the

coordination sphere of the deprotonated ligand [L2]-3 in 11.

60

0.5 0.0 -0.5 -1.0 -1.5 -2.0

E (mV)

Table 2.14. Selected Bond Distances (Å) and Angles (deg) for 11.

V(1)-O(1) 1.603(2) V(2)-O(5) 1.595(2)

V(1)-O(2) 1.8202(13) V(2)-O(6) 1.8340(14)

V(1)-O(4) 1.8787(14) V(2)-O(7) 1.874(2)

V(1)-O(3) 1.883(2) V(2)-O(8) 1.875(2)

V(1)-N(3) 2.108(2) V(2)-N(2) 2.124(2)

V(1)V(2) 6.882

O(1)-V(1)-O(2) 100.01(7) O(5)-V(2)-O(6) 98.01(7)

O(1)-V(1)-O(4) 101.12(7) O(5)-V(2)-O(7) 100.17(8)

O(2)-V(1)-O(4) 102.23(6) O(6)-V(2)-O(7) 102.69(6)

O(1)-V(1)-O(3) 105.51(7) O(5)-V(2)-O(8) 104.38(8)

O(2)-V(1)-O(3) 88.66(6) O(6)-V(2)-O(8) 89.26(7)

O(4)-V(1)-O(3) 148.96(7) O(7)-V(2)-O(8) 150.87(7)

O(1)-V(1)-N(3) 93.26(7) O(5)-V(2)-N(2) 95.41(7)

O(2)-V(1)-N(3) 163.17(7) O(6)-V(2)-N(2) 163.20(7)

O(4)-V(1)-N(3) 85.14(6) O(7)-V(2)-N(2) 84.75(6)

O(3)-V(1)-N(3) 77.78(6) O(8)-V(2)-N(2) 77.61(7)

(average 1.40 Å) bond lengths are normal. The ligand is thus chelated in the trianionic

phenolate form and the compound is correctly described with a physical oxidation state of +V

for the vanadium ion with a d0 electron configuration. This assignment also corroborates with

the diamagnetism and 51V NMR data for 11.

Compound 11 containing V(V) with d0 electron configuration is diamagnetic and was

subjected to 51V NMR measurements28 with VOCl3 in C6D6 as an internal standard. The

compound gives rise to a single signal at δ = -420 ppm, suggesting that there is only one

species in solution, as in the solid state.

A few electrochemical experiments were done with the Vanadium (V) compound

[L22(V=O)2] (11). The square wave voltammogram exhibits three oxidations in the accessible

potential range, two reversible ones at -0.136 V and +0.226 V. These oxidations, which

proceed in the same potential range as those of 7 - 9, and are assigned tentatively also to

phenoxyl radical formation. Reductions do also occur, in the potential range -0.7 to -0.9 V

which are most possibly metal-centered reductions to the VIV state. The square wave

voltammogram, however, exhibits complex multiple peak formations [Figure 2.35 ].

Figure 2.35:- Square wave voltammogram of 11.

61

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28) D. Rehder, Angew. Chem., Int. Ed. Engl., 1991, 30, 148.

64

65

Chapter 3

TRANSITION METAL COMPLEXES WITH

IMINO -BENZOSEMIQUINONE LIGANDS

M

N

O

R

.

66

67

N

O

M

Ph

1.36

1.397

1.396

1.409

1.396 1.409

1.347

1.418

N

O

M

Ph

H

N

O

M

Ph

.

N

O

M

Ph

1.46

1.382

1.386

1.398

1.397 1.412

1.352

1.400 1.34

1.423

1.367

1.430

1.379 1.431

1.304

1.443

1.30

1.44

1.36

1.46

1.36 1.46

1.24

1.52

3.1 INTRODUCTION

The reactions catalyzed by radical-dependent proteins are very diverse. The essential

tyrosyl radical in ribonucleotide reductase, modified tyrosyl radicals, tryptophan radicals,

glycyl radicals and thiyl radicals have been identified and shown to play important roles in

enzymes involved in primary metabolic pathways.

Since the report of the redox active ligand by Girgis and Balch (H2LN in Chapter1),

quite a number of complexes have been synthesized using this ligand1. Later on, Pierpont and

co-workers used 3,5-di-tert-butylcatechol as another redox active ligand and complexes

containing benzosemiquinone radicals were synthesized1a,2. This hybridization of organic-

inorganic molecules in which paramagnetic ions are coordinated to organic open shell radical

ligands helped in studying the development of molecular magnetic materials. 3

Lately, this field of chemistry interested this group with the synthesis of the ligand 2-

anilino-4,6-ditertbutylphenol. Quite a number of complexes have been prepared using this

ligand and all have been well characterized4. As a natural and obvious progress in this work, a

new ligand has been developed based on m-phenylenediamine. This ligand can best be

described as the dimeric form of 2-anilino-4,6-ditert-butylphenol. 5

The question, however, arises about ways to discern the presence or absence of

radicals in the benzene ring containing the tert-butyl substitution. A very powerful tool has

been proved to be high quality X-ray crystallography. Figure 3.1 shows the geometrical

features observed for i) [LAP]- containing an aromatic aminophenol (AP) ring with six

equivalent C-C bond, a long C-N bond at 1.47 Å and a relatively long C-O bond at 1.35 Å, ii)

[LAP-H]-2, where only the shorter C-N bond at 1.37 Å differs significantly from those of

[LAP]-, (iii)[LISQ]-, where the six-membered ring displays a quinoid-type distortion comprising

a short, a long, and another short C-C bond followed by three long ones and, in addition, both

the C-O and C-N distances are significantly shorter (1.30 and 1.34 Å, respectively) than those

in ]LAP]- and [LAP-H]-2 and iv) [LIBQ], where the ring is in a ortho-iminobenzoquinone form.

[LAP]- [LAP-H]-2 [LISQ]- [LIBQ]

Figure 3.1:- Geometrical features of the ligands.

68

NH N

H

OH OH

OH

2

NH2NH2

OH

NH

OH

NH

NH2NH2

NEt3 , Heptane,

Reflux

H4L3

H4L4

OH

O

OH

H

N

OH

H

R

NH

OH

R

NEt3RNH2

-H2O

3.2 SYNTHESIS AND CHARACTERIZATION OF THE LIGANDS

The ligands H4L3 and H4L4 were synthesized in the same procedure as described for

2-anilino-4,6-ditertbutyl phenol 4a. Refluxing 1,3 diaminobenzene or 4,4´-diaminodiphenyl

methane with 3,5-di-tert-butyl catechol (1:2) in heptane with triethyl amine as base afforded

both the ligands [Figure 3.2(a)]. A plausible mechanism for these type of condensation is

depicted in Figure 3.2(b).

Although both the ligands are tetradentate, however only dinuclear complexes could

be synthesized using this ligand. The –NH and –OH groups easily deprotonate to yield imino-

benzosemiquinone radicals in presence of metal and air. These iminobenzosemiquinone

radicals are stabilized by the tert-butyl groups placed at the 2 and 4 positions to the phenol.

For the ligand H4L3, the position of the amine group (1,3) necessitates spin polarization

a) b)

Figure 3.2 :-a) Synthetic procedure for ligands H4L3 and H4L4. b) Mechanism of the amine

condensation with 3,5-di-tert-butyl catechol.

(Chapter 1) and serves as a backbone in attempting to induce ferromagnetic coupling between

the dinucleating centers or between the radical centers. However , in H4L4 , the presence of –

CH2- group in between the π-conjugation inhibits spin polarization resulting in different

magnetic property.

Both the ligands were characterized by different spectroscopic techniques viz. IR,

NMR and Mass spectroscopy. The ligands show characteristic peaks in IR due to (-NH)

stretch and (-OH) stretch between 3330-3350 cm-1 and 3370-3392 cm-1 respectively. The

peaks at 2960-2860 cm-1 are due to the –C-H stretch of the tert-butyl groups. The -C-N stretch

is between 1610-1615 cm-1 and present in both the ligands. Selected IR peaks (in cm-1) are

listed in Table 3.1.

69

Cu [L ]

II 3

22

12

HL

4

3

Ni [L ]

II 3

22

1

3

Co [L ]

III 3

23

14

15

Fe [L ]

III 3

23

16

Mn

IV

2A2

[L ] [L ]

33

THF/CH OH

CuCl

3

CHCl/CHCN

[Co(HO)](ClO)

22 3

2642

CHCOCH

FeCl.4HO

33

22

CH Cl /CH CN

“Manganese acetate”

22 3

CH CN

[Ni(H O) ](ClO )

3

26 42

N N

O

H H

O

H

= H

4

L3

N N

O O

= [L3 ]

2-

N N

O O

= [L

A

3 ]

3-

ν(-OH) ν(-NH) ν(C-H) ν(C-N) ν(C-O)

H4L3 3392 3349 2959,2907,2868 1614 1420

H4L4 3372 3330 2958,2905,2867 1611 1418

Table 3.1 :- Characteristic IR bands for H4L3 and H4L4 ( in cm-1).

NMR spectra of both the ligands are given in the experimental section (Chapter 7).

Other than the four exchangeable protons , all the other protons (44 for H4L3 and 50 for H4L4)

are visible for both the ligands. Mass spectroscopy in EI mode clearly confirms the

composition of H4L3 as C34H48N2O2 (m/z-516) and for H4L4 as C41H54N2O2 (m/z-606).

3.3 TRANSITION METAL COMPLEXES WITH H 4L3 and H 4L4

The ligand H4L3 reacts with different metal ions in the presence of base and air to yield

dimers with either four or six iminobenzosemiquinone radicals [Figure 3.3]. With H4L4, only

the CoIII(LS) (17) complex has been synthesized in order to study the role played by the

radicals in a non-conjugated ligand system. Here L3 depicts the ligand in the diradical

dianionic form and LA3 ,the monoradical trianionic form [Figure 3.4].

Figure 3.3 :- Complexes prepared with the ligand H4L3.

Figure 3.4:- Different forms of the ligand H4L3, which have been observed in the complexes.

70

When the ligand H4L3 is reacted with CuCl in presence of triethylamine and air, a dark

green microcrystalline compound (12) separates out immediately which is recrystallized

further from a THF-Methanol solvent mixture. Using Nickel(II) perchlorate hexahydrate as

starting material, compound 13 precipitates out as dark green microcrystalline compound.

Magnetic data of 13 are given in the Appendix section. If Cobalt(II) perchlorate hexahydrate

is used, in an acetonitrile and dichloromethane solvent mixture, crystals of 14 separate out.

The cobalt compound with H4L4 (17) is synthesized using the same procedure. Using this

solvent mixture, 15 crystallizes out from solution when iron(II) chloride tetrahydrate is used.

In the synthesis of all these compounds, triethylamine was used as a base. However,

when triethylamine is used as base for the preparation of the manganese complex, no

precipitation occurs even after addition of tetrabutylammonium perchlorate. When

tetrabutylammonium methoxide, a strong base, is used, a dark brown microcrystalline

precipitate of 16 is obtained. With a weak base, like triethylamine, an equilibrium between the

MnII, MnIII and MnIV complexes presumably exists in solution. When a strong base is added,

this equilibrium is shifted towards the Mn+4 form with the lowering of the oxidation potential.

3.3.1 INFRARED AND MASS SPECTROMETRY OF THE COMPLEXES 12 - 17

Complexes 12-17 were all subjected to infrared and mass spectroscopic studies. The

most salient features observed for all the complexes were the absence of the (-NH) and (-OH)

peaks of both the ligands. This indicates coordination of the metal sites with the ligands. A

medium intense band between 1580-1560 cm-1, assigned to the ν(-C=N) and a sharp peak

between 1450-1430 cm-1, due to ν (C-O) appears for all the complexes. List of some of the

infrared peaks (in cm-1) are given in Table 3.2.

Complex ν(C=N) ν(C-O) Other characteristic peaks

12 1576m 1473m 1442m,1386m,1265m,1027m,697w

13 1576s 1476s 1437m,1360s,1244s,1024m,698m

14 1575s 1464s 1435s,1359s,1262m,1032w,706s

15 1579m 1478s 1437m,1357w,1262m,1028w,704m

16 1578s 1470s 1520m,1360s,1268s,1021m,703w

17 1578w 1500m 1429s,1358m,1255m,1030w,692w

Table 3.2 :- Characteristic IR peaks for 12-17 ( in cm-1).

71

1635 1640 1645 1650 1655 1660

Iso: C102H132FE2N6O6

100%

1647

1648

1649

1650

1651

1652

1635 1640 1645 1650 1655 1660

21% MW:1648/1649.90 C102H132Fe2N6O6 (C34H44N2O2)3Fe2

100%

1647

1648

1649

1650

1651

1652

1640 1645 1650 1655 1660

Iso: C102H132MN2N6O6

100%

1646

1647

1648

1649

1650

1640 1645 1650 1655 1660

12% MW:1646/1648.08 C102H132Mn2N6O6 Mn2(C34H44N2O2)3

100%

1644

1646

1647

1648

1649

1650

For all the complexes, mass spectroscopy in the EI and/or ESI mode were carried out.

The copper(II) complex (12) shows a molecular peak in the EI as well as in the ESI-positive

mode at m/z-1150 (in dichloromethane for ESI-positive mode) with other characteristic peaks.

For the nickel(II) complex (13), the peak occurs at m/z-1140 in the EI mode corresponding to

the molecular peak. Compounds 14, 15 and 16 show molecular peaks at 1654,1648 and 1646

respectively [Figure 3.5]. For the cobalt(III) compound (17), ESI-positive mode in

dichloromethane shows the molecular peak at 1924. The molecular peak along with the

characteristic peaks are shown in Table 3.3.

Table 3.3 :- Characteristic peaks in mass spectroscopy for 12-17.

Complex Molecular Weight Molecular Peak

m/z

Other characteristic Peaks

m/z

12 1150 1150[CuII2L32] 1087, 1047, 791, 616, 575 [CuL3]+

13 1140 1140[NiII2L32] 570 [NiL3]+

14 1654 1654 [Co2L33] 1142 [Co2L32]+

15 1648 1648[FeIII2L33] 1136 [Fe2L32]+, 824 [CoL3

1.5]+

16 1646 1646[MnIV2(L3)(LA3)2] 1134 [Mn2L32]+, 822 [MnL31.5]+

17 1924 1924[CoIII2L43] 1322 [Co2L42]+

a) b)

Figure 3.5:- Experimental (top) and simulated (bottom) mass spectrum of (a) Fe2L33 (15) and

(b) MnIV2(L3)(LA3)2 (16).

72

C(41)

C(46)

C(42)

C(43) C(44)

C(45)

C(47)

N(3)

O(3)

O(1)

Cu(1)

N(1)

N(2)

Cu(2)

N(4)

O(4)

O(2)

C(53)

C(58) C(57)

C(56)

C(55)

C(54)

3.3.2 CRYSTAL STRUCTURE AND CHARACTERIZATION OF COMPLEXES

[CuII

2L3

2] (12)

Dark green crystals of 12 were afforded from a solvent mixture of THF-Methanol in

the ratio 4:1. The crystal structure [Figure 3.6 (a)]shows the presence of 1.5 THF molecules

per unit cell of 12. The complex consists of two distorted square-planer copper ions co-

coordinated to two fully deprotonated ligands. The Cu(1) and Cu(2) centers is 0.457Å and

0.405Å above the plane made by joining the respective donor atoms for each center viz.

N(1)N(3)O(1)O(3) and N(2)N(4)O(2)O(4). The distance between the two copper centers is

6.697 Å. The average copper-oxygen bond distance is 1.919(2) Å and that for copper-

nitrogen distance is 1.945(2) Å. Therefore the oxidation state of the copper center is (+II) and

the copper-donor atom distances values are comparable for other copper complexes 4b. The

two nitrogen and the two oxygen atoms are cis co-coordinated to each copper center. In 12*

(the monomeric copper(II) complex with the ligand 2-anilino-4,6-ditertbutyl phenol), the two

nitrogen and the two oxygen atoms are in trans position4b. The geometry of [L3]2- forces the

homo-donor atoms in cis position. The two meta-phenylene rings, which act as the spacer, are

in a staggered position and make an angle of 8.5° between them [Figure 3.6(b)].

b)

a)

Figure 3.6 :- a) Molecular structure of 12.b) Perspective view of 12 showing the staggered

form of the two meta-phenylene spacers.

73

Table 3.4:- Selected bond lengths (Å) and angles (degree) for 12.

Cu(1)-O(3) 1.914(2) C(41)-C(42) 1.439(3)

Cu(1)-O(1) 1.9322(14) C(42)-C(43) 1.378(3)

Cu(1)-N(1) 1.945(2) C(42)-C(59) 1.530(3)

Cu(1)-N(3) 1.950(2) C(43)-C(44) 1.427(3)

Cu(2)-O(2) 1.901(2) C(44)-C(45) 1.369(3)

Cu(2)-O(4) 1.928(2) C(44)-C(63) 1.535(3)

Cu(2)-N(4) 1.934(2) C(45)-C(46) 1.423(3)

Cu(2)-N(2) 1.949(2) C(47)-C(48) 1.397(3)

O(1)-C(1) 1.292(2) C(47)-C(52) 1.397(3)

N(1)-C(6) 1.346(3) C(48)-C(49) 1.386(3)

N(1)-C(7) 1.415(3) C(49)-C(50) 1.384(3)

O(2)-C(14) 1.290(3) C(50)-C(51) 1.399(3)

N(2)-C(13) 1.349(2) C(51)-C(52) 1.392(3)

N(2)-C(11) 1.414(3) C(53)-C(58) 1.421(3)

O(3)-C(41) 1.294(3) C(53)-C(54) 1.451(3)

N(3)-C(46) 1.357(3) C(54)-C(55) 1.432(3)

N(3)-C(47) 1.413(3) C(55)-C(56) 1.379(3)

O(4)-C(54) 1.284(2) C(55)-C(67) 1.528(3)

N(4)-C(53) 1.349(3) C(56)-C(57) 1.435(3)

N(4)-C(51) 1.416(3) C(57)-C(58) 1.367(3)

Cu(1)….Cu(2) 6.697

O(3)-Cu(1)-O(1) 100.16(6) C(14)-O(2)-Cu(2) 112.50(13)

O(3)-Cu(1)-N(1) 143.83(7) C(13)-N(2)-C(11) 120.5(2)

O(1)-Cu(1)-N(1) 83.23(6) C(13)-N(2)-Cu(2) 110.95(14)

O(3)-Cu(1)-N(3) 84.41(7) C(11)-N(2)-Cu(2) 127.66(13)

O(1)-Cu(1)-N(3) 146.42(7) C(41)-O(3)-Cu(1) 112.58(13)

N(1)-Cu(1)-N(3) 112.51(7) C(46)-N(3)-C(47) 120.4(2)

O(2)-Cu(2)-O(4) 95.48(6) C(46)-N(3)-Cu(1) 111.25(14)

O(2)-Cu(2)-N(4) 149.98(7) C(47)-N(3)-Cu(1) 126.85(13)

O(4)-Cu(2)-N(4) 83.39(7) C(54)-O(4)-Cu(2) 112.58(13)

O(2)-Cu(2)-N(2) 84.43(7) C(53)-N(4)-C(51) 121.6(2)

O(4)-Cu(2)-N(2) 146.87(7) C(53)-N(4)-Cu(2) 112.63(13)

N(4)-Cu(2)-N(2) 112.57(7) C(51)-N(4)-Cu(2) 124.10(13)

A closer look at the bond distances and the angles at the tert-butyl substituted rings

shows that the average C-O bond distance (1.29 Å) is much shorter than a C-O bond distance

for a phenolate oxygen. This distance corresponds to the iminobenzosemiquinone form and is

supported by the planer coordination geometry of nitrogen and the C-N bond distance. The

nitrogen is deprotonated and three-coordinated (sp2 hybridization) with average C-N bond

distance (carbon of the phenyl ring containing tert-butyl as substituent) has shortened

considerably (1.35 Å) signifying the formation of a double bond. The phenyl ring with the

tert-butyl substituents has lost its aromaticity with the formation of long and short bond

distances between the six carbon centers describing the phenyl ring (average bond length of

C(42)-C(43), C(44)-C(45), C(57)-C(58), C(55)-C(56) is 1.373 Å and average bond length of

C(41)-C(42), C(41)-C(46), C(43)-C(44), C(45)-C(46), C(54)-C(55), C(56)-C(57), C(53)-

C(58), C(53)-C(54) is 1.434 Å ). It is therefore clear that the ligand is rendered in the

iminobenzosemiquinone form and the complex contains four iminobenzosemiquinone

radicals. Thus, 12 can be best described as the dimer containing m-phenylene bridges of the

74

analogous mononuclear Cu(II) complex with the ligand 2-anilino-4,6-di-tert-butylphenol 4a.

Selected bond lengths and angles (degree) for 12 are given in Table 3.4.

A magnetic susceptibility study of complex 12 was carried in from the temperature

range of 2-290 K. At higher temperatures, the value of µeff remains almost constant (2.38 µB

at 30K and 2.51µB at 290K). On further decreasing the temperature, a slow decrease in µeff is

observed till 10K (2.22 µB) that decreases further at 2K (1.53 µB)[Figure 3.7(a)].

The presence of four iminobezosemiquinone radicals along with the copper centers

and their interactions between them should be taken into consideration. In order to simplify

this complicated magnetic property, the scheme used in simulating the magnetic susceptibility

measurement of 12* (the mononuclear copper complex with the ligand 2-anilino-4,6-

ditertbutylphenol)4a was taken into consideration. Therefore, in 12, the environment around

each copper center is studied first and then the whole molecule.

Cu(1) or Cu(2) is surrounded by two iminosemiquinone radicals, each having a

spin of ½ ( SCu= SRad = ½ ). Therefore, it is a three-spin molecule in each metal center. The

states corresponding to each metal center are labeled by their total spins St = SCu +SRad1 +SRad2

and a pair subspin S*= SRad1 + SRad2, (St, S*) = ( 3/2 , 1), ( ½ ,1), ( ½ , 0) or in symbolic

fashion (↑↑↑ ), (↑↓↑ ), (↑↑↓ ), respectively. The energy of the corresponding states are given

by E( 3/2 ,1) = -J-2J´, E( ½ ,1) = 2(J-J´) and E( ½ ,0) = 0 where J correspond to the coupling

between the radical and copper ion and J´ between the two radical centers. It has been shown

that the value of J´ override the value of J i.e. the coupling between the radicals is much

stronger than the coupling between the radical and copper ion4a. The 1st excited state lies at an

energy difference of 595 cm-1. This important information is used in simulating the magnetic

susceptibility of data of 12. This indicates that the radical-radical coupling value remains

constant throughout the temperature range and is supported by the nickel (II) complex (13)

which is found to be diamagnetic for the whole temperature range of 2-290K (Appendix).

Incorporating this assumption in 12, a more simplified model can be drawn up; the

interaction which is observed arises from the two copper centers, Cu(1) and Cu(2) with

S = ½. From the experimental data it is clear that the value of µeff between 30K and 290K

corresponds to the two uncoupled S = ½ system (µeff = 2.45 ± 0.07µB). The values obtained

by simulating this data are J = 0 cm-1 (fixed), g1= g2 = 2.049 and a Theta-Weiss parameter(θ)

of –2.85K. The most plausible reason which can be given for no-coupling is the improper

mixing of the p(π) and the metal d(π) orbital. As mentioned earlier (Chapter 1), orthogonal

overlap between these types of orbitals is a necessity for the superexchange process. This type

of coupling in a meta-phenylene bridged system was also observed by Hendrikson, Stucky

75

Cu

O

Cu

ONN

NNO

O

.

50 100 150 200 250 300

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Simulated

Experimental

µ eff / µ B

T/K

S S

S

15

3

RR

Strong

antiferromagnetic

coupling

Strong

antiferroma

g

netic

coupling

No

coupling

[Cu2(L3 )+1(L3 )] [Cu2L32][Cu2(L3)-2(L3)]

[Cu2(L3)-1(L3)]

[Cu2(L3 )+2(L3 )] -1e--1e- +1e- +1e-

a) b)

Figure 3.7 :- a) Magnetic data for 12 b) Spin orientation diagram of 12.

and co-workers6. From the crystal structure, it is observed that the Cu(1) and Cu(2) lies about

0.457 A and 0.405 A respectively. This indicates that the geometry surrounding the copper

atoms are highly distorted and this leads to improper mixing between the dx2-y2 orbital of

copper and p(π)-orbital of the nitrogen atom.

In order to understand the electrochemical property of the complexes, the different

redox states of the ligand (H4L3) should be discussed. Figure 3.8 shows the different states.

The de-protonated ligand is oxidized by two electrons to the dianionic diradical form [L3]-2

and further by two electrons to the quinoid form [L3].

A number of redox waves were observed in the cyclic voltammogram (bold line) and

square wave voltammogram(dotted line) of 12 in dichloromethane and in presence of 0.1M

TBAPF6 [Figure 3.9(a), Table 3.5]. From coulometric measurements all these waves are

single-electron trasfer processes and are assigned to the oxidation and the reduction of the

radicals. The two reversible waves at –0.214 V and –0.126 V vs. Fc+/Fc are assigned to the

oxidation to the mono-cation and the di-cation respectively[Figure 3.8(b)].The redox

potentials for these oxidations are comparable to 12*(the mononuclear copper complex with

the ligand 2-anilino-4,6-ditertbutylphenol).The two reversible reduction peaks at –0.955 V

and -1.133V are also comparable to that of 12*. These two peaks are due to the formation of

the mono-anionic and di-anionic form of the complex 12. It is to be noted that the pairwise

redox process occurs at a single ligand. The different redox forms are given as

The cyclic voltammetry peaks assigned above can be corroborated with the electronic

spectrum of the oxidized and reduced form of 12 [Figure 3.9 (b)], generated by coulometry in

dichloromethane at –25 °C with 0.2 M TBAPF6. The electronic spectrum of 12 shows peaks

at 476nm (ε = 16,400 M-1cm-1) which is due to the partial quinone character at the radical site.

76

400 600 800 1000

0

1

2

3

4

[Cu2(L3)+2(L3)]

[Cu2(L3)+(L3)]

[Cu2L3

2]

ε * 10 - 4 ( M -1 cm -1 )

λ (nm)

Cu2+

O

N

t-Bu

But- t-Bu

t-Bu

Cu2+

+2

O

N

t-Bu

But- t-Bu

t-Bu

O

Cu2+ Cu2+

O

N

t-Bu

But- t-Bu

t-Bu

Cu2+ Cu2+

+2e-

-2e-

+2e-

-2e-

[Cu2(L3 )+2][Cu2L3][Cu2(L3-2H)-2]

-2

..

-

1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0

E (V)

Figure 3.8:- a) Redox states of ligand H4L3.

(b) Redox steps of 12 as observed from cyclic voltammetry and square wave voltammetry.

Table 3.5 :- Redox potentials (V) for 12 and 12* vs. Fc+/Fc.

Compound Oxidation Reduction

12 + 0.440

(irreversible)

- 0.120

(reversible)

- 0.210

(reversible)

- 0.960

(reversible)

- 1.13

(reversible)

- 1.56

(irreversible)

12* (the mononuclear

coppercomplex 2-anilino-

4,6 ditertbutylphenol)

+ 0.370 (reversible)

-0.260 (reversible)

- 1.02 (reversible)

- 1.32 (reversible)

The broad peak at 815nm (ε = 11,100 M-1cm-1) and another peak at 1120nm (ε=9645

M-1cm -1) are due to the intense π-π* intra- ligand charge transfer. When 12 is oxidized by one

electron, a bathochromic shift of the peak at 476 nm is observed to 509 nm and further to 520

nm upon the 2nd electron oxidation with an increase in the ε value (21,350 M-1 cm-1). This

agrees with the mechanism given in Figure 3.8b.

6µA

10µA

a) b)

Figure 3.9 :- a) Cyclic (bold line) and square wave (dotted line) voltammogram of 12.

b) Electronic spectra of 12 (bold line), its 1e- and 2e- oxidized species.

O

N

t-Bu

t-Bu t-Bu

t-Bu

O

N

t-Bu

t-Bu t-Bu

t-Bu

O

N

t-Bu

t-Bu t-Bu

t-Bu

..

+ 2 e -+ 2 e -

- 2 e -- 2 e -

--

- 4

--

- 2

77

.

C(1)

C(6)

C(5)

C(4) C(3)

C(2)

C(7)

O(1)

Co(1)

N(1)

N(5) N(3)

O(3)

O(5)

C(12)

C(11)

C(10)

C(9)

C(8)

N(2)

N(4)

N(6)

Co(2)

O(2)

O(4)

O(6)

C(14)

C(15)

C(16)

C(17)

C(18)

C(13)

CoIII

2L3

3 (14)

The neutral complex Co2L33 (14) [Figure 3.10] crystallizes from the reaction medium

as deep brown (nearly black) crystals. Each cobalt ion is in a distorted octahedral geometry

bounded by three oxygen and three nitrogen. A C3 axis passes through each of the cobalt

center, compared to the complex 14* (monomer cobalt complex using 2-anilino-4,6-ditert-

butylphenol) where a C2 axis passes through the cobalt center4a,b. The six short Co-O distance

at 1.899 ± 0.004 Å together with the six short Co-N distance 1.930 ± 0.009Å in 14 are

compatible with the low spin d6 configuration. Here again, it is observed that the ligands has

lost their amino-hydrogen atom together with the phenol proton. The coordination geometry

around the nitrogen donor atom is planer, showing that it is three coordinated. Each of the

phenyl rings containing the tert-butyl groups has lost its aromaticity. Thus the average

distance between C(3)-C(4), C(5)-C(6), C(15)-C(16), C(17)-C(18), C(43)-C(44), C(45)-

C(46), C(55)-C(56), C(57)-C(58), C(83)-C(84), C(85)-C(86), C(95)-C(96) and C(97)-C(98)

being 1.374Å which is shorter than the rest 24 C-C bond lengths of the tert-butyl containg

phenyl rings; the complex 14 consists of six iminobenzosemiquinone radicals.

Correspondingly the imino C=N bonds at 1.342 ± 0.012 Å which is shorter than the C-N

bonds to the spacer phenyl ring, (1.425 ± 0.012Å) along with the C-O bond length at 1.299 ±

0.012 Å, further supporting the formation of the iminosemiquinone character. Selected bond

distances are given in Table 3.6.

Figure 3.10 :- Molecular structure of 14.Tert-butyl groups have been removed for clarity.

78

Table 3.6 :- Selected bond distances (Å) and bond angles (degree) of 14.

Co(1)……Co(2) 6.791

Co(1)-O(5) 1.895(2)

Co(1)-O(3) 1.897(2)

Co(1)-O(1 1.905(2)

Co(1)-N(5) 1.916(3)

Co(1)-N(1) 1.926(3)

Co(1)-N(3) 1.934(3)

Co(2)-O(4) 1.898(2)

Co(2)-O(2) 1.898(2)

Co(2)-O(6) 1.900(2)

Co(2)-N(4) 1.927(3)

Co(2)-N(2) 1.937(3)

Co(2)-N(6) 1.939(3)

O(1)-C(1) 1.295(4)

O(2)-C(14) 1.301(4)

O(3)-C(41) 1.300(4)

O(4)-C(54) 1.298(4)

O(5)-C(81) 1.299(4)

O(6)-C(94) 1.301(4)

N(1)-C(2) 1.343(4)

N(1)-C(7) 1.425(4)

N(2)-C(13) 1.342(4)

N(2)-C(9) 1.429(4)

N(3)-C(42) 1.344(4)

N(3)-C(47) 1.431(4)

N(4)-C(53) 1.344(4)

N(4)-C(49) 1.425(4)

N(5)-C(82 1.338(4)

N(5)-C(87) 1.421(4)

N(6)-C(9 1.343(4)

N(6)-C(89) 1.421(4)

C(1)-C(6) 1.425(4)

C(1)-C(2) 1.438(5)

C(2)-C(3 1.432(4)

C(3)-C(4) 1.371(5)

C(4)-C(5) 1.425(5)

C(4)-C(23) 1.539(5)

C(5)-C(6) 1.379(5)

C(6)-C(19) 1.531(5)

C(7)-C(12) 1.396(5)

C(7)-C(8) 1.400(5)

C(8)-C(9) 1.389(5)

C(9)-C(10) 1.399(5)

C(10)-C(1 1.395(5)

C(11)-C(1 1.395(5)

C(13)-C(18) 1.421(5)

C(13)-C(14) 1.436(5)

C(14)-C(15) 1.430(5)

C(15)-C(16) 1.381(5)

C(15)-C(27) 1.539(5)

C(16)-C(17) 1.431(5)

C(17)-C(18) 1.373(5)

C(17)-C(31) 1.531(5)

C(19)-C(22) 1.536(5)

C(19)-C(20) 1.543(6)

O(5)-Co(1)-O(3) 87.64(10)

O(5)-Co(1)-O(1) 90.02(10)

O(3)-Co(1)-O(1) 90.61(10)

O(5)-Co(1)-N(5) 84.10(11)

O(3)-Co(1)-N(5) 171.70(11)

O(1)-Co(1)-N(5) 88.61(11)

O(5)-Co(1)-N(1) 172.92(11)

O(3)-Co(1)-N(1) 88.98(11)

O(1)-Co(1)-N(1) 83.80(11)

N(5)-Co(1)-N(1) 99.14(12)

O(5)-Co(1)-N(3) 87.03(11)

O(3)-Co(1)-N(3) 83.76(11)

O(1)-Co(1)-N(3) 173.74(11)

N(5)-Co(1)-N(3) 96.57(12)

N(1)-Co(1)-N(3) 98.78(12)

O(4)-Co(2)-O(2) 88.83(11)

O(4)-Co(2)-O(6) 89.35(11)

O(2)-Co(2)-O(6) 87.75(11)

O(4)-Co(2)-N(4) 83.96(11)

O(2)-Co(2)-N(4) 172.75(12)

O(6)-Co(2)-N(4) 91.49(11)

O(4)-Co(2)-N(2) 90.38(12)

O(2)-Co(2)-N(2) 84.03(11)

O(6)-Co(2)-N(2) 171.78(11)

N(4)-Co(2)-N(2) 96.65(12)

O(4)-Co(2)-N(6) 171.80(11)

O(2)-Co(2)-N(6) 86.92(12)

O(6)-Co(2)-N(6) 83.49(11)

N(4)-Co(2)-N(6) 100.15(12)

N(2)-Co(2)-N(6) 96.15(12)

C(1)-O(1)-Co(1) 113.1(2)

C(14)-O(2)-Co(2) 112.8(2)

C(41)-O(3)-Co(1) 111.8(2)

C(54)-O(4)-Co(2) 113.4(2)

C(81)-O(5)-Co(1) 113.2(2)

C(94)-O(6)-Co(2) 110.6(2)

C(2)-N(1)-C(7) 120.9(3)

C(2)-N(1)-Co(1) 112.0(2)

C(7)-N(1)-Co(1) 127.1(2)

C(13)-N(2)-C(9) 120.8(3)

C(13)-N(2)-Co(2) 111.9(2)

C(9)-N(2)-Co(2) 126.6(2)

C(42)-N(3)-C(47) 120.4(3)

C(42)-N(3)-Co(1) 111.0(2)

C(47)-N(3)-Co(1) 127.9(2)

C(53)-N(4)-C(49) 121.2(3)

C(53)-N(4)-Co(2) 112.6(2)

C(49)-N(4)-Co(2) 125.9(2)

C(82)-N(5)-C(87) 120.6(3)

C(82)-N(5)-Co(1) 112.4(2)

C(87)-N(5)-Co(1) 126.4(2)

C(93)-N(6)-C(89) 120.1(3)

C(93)-N(6)-Co(2) 110.6(2)

C(89)-N(6)-Co(2) 127.9(2)

79

Magnetic data for a polycrystalline sample of 14 is displayed in Figure 3.11(b). On

lowering the temperature, µeff (4.21 µB at 290 K) decreases rather slowly till 100 K (4.13 µB)

and then monotonically till it reaches a value of 1.99µB at 2K. This data shows that an overall

anti-ferromagnetic coupling dominates throughout the whole molecule and needs

rationalization for simulating this data.

The mononuclear CoIII complex (14*) prepared with the ligand 2-anilino-4,6-

ditertbutyl phenol4a with three iminobenzosemiquinone radicals has a quartet ground state

arising from the ferromagnetic coupling between the radicals. Out of the three planes possible,

two dihedral angles were found to be deviating from orthogonality and the 3rd angle was

found to be nearly orthogonal. Therefore, two different coupling constants (J) were needed in

order to simulate the data.

In case of 14, the presence of six radicals complicates the nature of interactions. As

CoIII (low spin) is a d6 ion (eg level is vacant), the interaction must occur through the metal π-

orbitals and, the coupling between the radicals in each ‘part’ can be ferro- or anti-

ferromagnetic in nature. When each cobalt center is considered, the dihedral angles made

between the two planes (each plane consisting the tert-butyl substituted phenyl rings)

containing the O(1) atom [C(1)-C(2)-C(3)-C(4)-C(5)-C(6)] and O(3) [C(41)-C(42)-C(43)-

C(44)-C(45)-C(46)] is 121.8°; that between O(1)[C(1)-C(2)-C(3)-C(4)-C(5)-C(6)]-O(5)

[C(81)-C(82)-C(83)-C(48)-C(85)-C(86)] and O(3)[C(41)-C(42)-C(43)-C(44)-C(45)- C(46)]-

O(5) [C(81)-C(82)-C(83)-C(84)-C(85)-C(86)] being 86.5° and 88.6°, respectively. From

these values it is clear that the angles deviate from orthogonality and the coupling between the

radicals at each cobalt center is probably anti-ferromagnetic. Examples of anti-ferromagnetic

coupling between radicals with cobalt as central ion is known. The dihedral angles made by

the phenyl rings between O(2) [C(13)-C(14)-C(15)-C(16)-C(17)-C(18)] - O(4)[C(43)-C(44)-

C(45)-C(46)-C(47)-C(48)] is 162°; that between O(2) [C(13)-C(14)-C(15)-C(16)-C(17)-

C(18)]-O(6)[C(93)-C(94)-C(95)-C(96)-C(97)-C(98)] and O(4)[C(43)-C(44)-C(45)-C(46)-

C(47)-C(48)]-O(6)[C(93)-C(94)-C(95)-C(96)-C(97)-C(98)] being 112° and 121.4°,

respectively. In order to simulate the experimental data, it was assumed that since each cobalt

center has a symmetric environment, a single coupling constant (J1) is needed for the radical-

radical interaction at each part. It is also to be noted that this coupling constant is the

summation of the ferromagnetic (if present) and antiferromagnetic coupling constants (J =

JFerro + JAntiferro). This simplifies the problem; the ground state at each ‘part’ is S = ½ with

anti-ferromagnetic coupling between the radicals. Naturally the question arises whether there

some interaction between these two residual spins with each other. If these two parts were

80

50 100 150 200 250 300

0

1

2

3

4

µ

eff

/µ

B

T / K

Experimental

Simulated

S (1)

R

Co

I I I

J

1

J

1

J

2

J

1

S (4)

R

Co

I I I

J

1

J

1

J

1

a)

b)

Figure 3.11 :- a) Spin coupling model used in simulating the magnetic data of 14.

b) Experimental and simulated data of 14.

assigned as two independent moiety having no interaction between them(as in 12), the value

of µeff should have reached 5.48 µB. This shows that there is some interactions and the nature

could be ferromagnetic (Stotal=1) or anti-ferromagnetic (Stotal=0). The coupling constant of

this interaction is denoted as J2 and is taken as the coupling between the radicals of the same

ligand for sake of simplification.

The spin coupling model is given in Figure 3.11(a). When J1 is anti-ferromagnetic

(negative J value ) and J2 is also taken to be anti-ferromagnetic, a good fit is not obtained.

This ensured that by spin polarization mechanism there is ferromagnetic coupling (positive J

value) between these two parts. Indeed a good fit is obtained when the value of J1 is taken to

be negative and J2 positive. The values obtained were J1 = -9.66 cm-1 and J2 = + 13.03 cm-1

with g = 2.0 (fixed) [Figure 3.11(b), bold line].

The complex 14 was subjected to square wave and cyclic voltammetric studies in

dichloromethane and in the presence of 0.1M TBAPF6. A number of redox peaks was

observed and all inferred to the radical oxidation or reduction process. Figure 3.12(a) depicts

the cyclic voltammogram (bold line) and square wave voltammogram (dotted line) of 14. All

the four peaks between –0.75V and 0.5V are due to the oxidation of radicals and each peak

correspond to a single electron transfer process, as observed from coulometric measurements.

The potentials of 14 and that of the parent mononuclear CoIII complex (14*)3 are given in

Table 3.7. All the anodic peaks are reversible in nature and therefore spectroelectrochemical

experiments were performed in order to characterize each of the oxidized species.

Coulometric experiments were performed at –25°C in the presence of 0.2M TBAPF6. During

the coulometry, the change in the electronic spectrum of 14 was monitored from190-1100 nm.

81

0.4 0.0 -0.4 -0.8 -1.2

12 µA

22 µA

E (V) vs. Fc+ /Fc

400 600 800 1000

0

2

4

6

8[CoIII

2(L3)3 ]

[CoIII

2(L3)2

+ 4(L3)]

[CoIII

2(L3)2

+ 3(L3)]

[CoIII

2(L3) + 2(L3)2]

[CoIII

2(L3)+(L3)2]

ε * 10 - 4 ( M - 1 cm - 1)

λ (nm)

Table 3.7 :- Redox potentials (V) for 14 and 14* vs. Fc+/Fc.

Compound E ½ (Oxidation) E ½ (Reduction)

14 + 0.247

(reversible)

0.174

(reversible)

- 0.277

(reversible)

-0.431

(reversible)

-1.214

(irreversible)

14*(the mononuclear

cobalt complex with 2-

anilino-4,6 ditert-

butylphenol)

+ 0.196 (reversible)

-0.363 (reversible)

- 0.933

(reversible)

- 1.32 (reversible)

The neutral complex 14, shows a spectrum with absorption maxima at 483 nm (ε = 8230 M-1

cm-1), 779 nm (ε = 5536 M-1 cm-1) and shoulders at 607 nm (ε =5980 M-1 cm-1), 700 nm (ε =

5095 M-1cm-1) and 874 nm (ε = 4229 M-1 cm-1) [Figure 3.12(b) (bold line)]. It is observed that

for each electron oxidation there is an increase in the intensity at 482 nm (ε = 21784 M-1 cm-1)

which shows the increase in quinoid character of the ligand. The bands lying above 600 nm

decreases in intensity and after the 4th electron oxidation, no absorption maxima above 600

nm was observed.

a) b)

Figure 3.12 :- a) Cyclic voltammogram (bold line) and square wave voltammogram (dotted

line) of 14. b)Electronic spectrum of 14 and the corresponding oxidized

products

82

C(20)

C(15)

C(16) C(19)

C(18)

C(17)

C(10)

C(9)

C(8)

C(13)

C(12) C(11)

C(3) C(4)

C(5)

C(6)

C(1)

C(2)

N(94) O(100)

O(60)

O(20)

N(54)

N(14)

N(7)

N(47)

O(41)

O(81)

O(1)

N(87)

Fe(1)

Fe(2)

Fe

O

N

NO

O

FeII

2L3

3 (15)

Although analytical and spectroscopic data are in agreement with the presence of a

dinuclear Fe2L33 unit as the smallest unit in 15, an X-ray analysis was undertaken to remove

the doubts. Indeed, the structure analysis shows the presence of two 6-coordinated iron(III)

centers. 15 crystallizes in the monoclinic space group P21/n. The iron center is in a distorted

octahedral geometry with three oxygen and three nitrogen donor atoms and a C3 axis passing

through each iron center [Figure 3.13]. The monomer iron complex 15*4c (with the ligand 2-

anilino-4,6-ditertbutylphenol) possess a C2 symmetry.

The average Fe-O and Fe-N bond lengths are 2.015 ± 0.009Å and 2.098 ± 0.009Å,

respectively. This shows that the complex 15 consists of two high spin ferric centers (d5) and

consequently the Fe-O and Fe-N bonds are larger than that of the CoIII complex (14). Selected

bond lengths are given in Table 3.8. That 15 consists of two high spin ferric centers has also

been confirmed by Mössbauer spectroscopy.

The geometrical features of the three ligands are identical within the small

experimental error and does not vary appreciably with the nature of the central metal ion (Cu,

a) b)

Figure 3.13 :- a) Crystal Structure of 15. The tert-butyl groups has been removed for clarity.

b) A view of 15 highlighting the coordination sphere around the two Fe(III) centers. The tetra-

dentate ligand L3 is denoted by the donor atoms and the meta-phenylene spacer.

83

Fe(1)-O(1) 2.006(3)

Fe(1)-O(41) 2.013(3)

Fe(1)-O(81) 2.019(3)

Fe(1)-N(87) 2.070(3)

Fe(1)-N(7) 2.083(3)

Fe(1)-N(47) 2.124(3)

Fe(2)-O(60) 2.004(3)

Fe(2)-O(20) 2.004(3)

Fe(2)-O(100) 2.046(3)

Fe(2)-N(94) 2.073(3)

Fe(2)-N(54) 2.097(3)

Fe(2)-N(14) 2.139(3)

O(1)-C(1) 1.285(5)

O(20)-C(20) 1.292(5)

O(41)-C(41) 1.287(5)

O(60)-C(60) 1.287(5)

O(81)-C(81) 1.283(5)

O(100)-C(100) 1.290(5)

C(6)-N(7) 1.335(5)

N(7)-C(8) 1.425(5)

N(14)-C(15) 1.326(5)

C(12)-N(14) 1.426(5)

C(46)-N(47) 1.332(5)

N(47)-C(48) 1.429(5)

N(54)-C(55) 1.344(5)

C(52)-N(54) 1.435(5)

C(86)-N(87) 1.338(5)

N(87)-C(88) 1.424(5)

N(94)-C(95) 1.339(5)

C(92)-N(94) 1.416(5)

C(1)-C(2) 1.428(6)

C(1)-C(6) 1.462(6)

C(2)-C(3) 1.377(6)

C(3)-C(4) 1.422(6)

C(4)-C(5) 1.368(6)

C(5)-C(6) 1.416(6)

C(8)-C(13) 1.390(5)

C(8)-C(9) 1.403(6)

C(9)-C(10) 1.375(6)

C(10)-C(11) 1.377(6)

C(11)-C(12) 1.401(6)

C(12)-C(13) 1.383(6)

C(12)-N(14) 1.426(5)

C(15)-C(16) 1.418(6)

C(15)-C(20) 1.459(6)

C(16)-C(17) 1.355(6)

C(17)-C(18) 1.433(6)

C(18)-C(19) 1.373(6)

C(19)-C(20) 1.420(6)

O(1)-Fe(1)-O(41) 88.65(12)

O(1)-Fe(1)-O(81) 92.98(11)

O(41)-Fe(1)-O(81) 86.08(11)

O(1)-Fe(1)-N(87) 170.17(12)

O(41)-Fe(1)-N(87) 94.49(12)

O(81)-Fe(1)-N(87) 77.97(12)

O(1)-Fe(1)-N(7) 77.68(13)

O(41)-Fe(1)-N(7) 163.61(13)

O(81)-Fe(1)-N(7) 103.44(12)

N(87)-Fe(1)-N(7) 100.49(13)

O(1)-Fe(1)-N(47) 90.05(12)

O(41)-Fe(1)-N(47) 77.09(12)

O(81)-Fe(1)-N(47) 162.82(13)

N(87)-Fe(1)-N(47) 99.73(13)

N(7)-Fe(1)-N(47) 93.72(13)

O(60)-Fe(2)-O(20) 90.51(11)

O(60)-Fe(2)-O(100) 98.66(11)

O(20)-Fe(2)-O(100) 83.14(11)

O(60)-Fe(2)-N(94) 101.26(12)

O(20)-Fe(2)-N(94) 157.97(12)

O(100)-Fe(2)-N(94) 76.76(12)

O(60)-Fe(2)-N(54) 77.44(12)

O(20)-Fe(2)-N(54) 97.80(12)

O(100)-Fe(2)-N(54) 175.97(13)

N(94)-Fe(2)-N(54) 102.87(13)

O(60)-Fe(2)-N(14) 164.92(12)

O(20)-Fe(2)-N(14) 77.19(12)

O

(

100

)

-Fe

(

2

)

-N

(

14

)

88.62

(

12

)

N(94)-Fe(2)-N(14) 93.24(13)

N(54)-Fe(2)-N(14) 95.41(13)

C(1)-O(1)-Fe(1) 115.7(3)

C(6)-N(7)-C(8) 119.6(4)

C(6)-N(7)-Fe(1) 113.5(3)

C(8)-N(7)-Fe(1) 125.4(3)

C(15)-N(14)-C(12) 118.4(3)

C(15)-N(14)-Fe(2) 111.4(3)

C(12)-N(14)-Fe(2) 127.9(3)

C(20)-O(20)-Fe(2) 115.3(2)

C(41)-O(41)-Fe(1) 116.9(3)

C(46)-N(47)-C(48) 119.6(3)

C(46)-N(47)-Fe(1) 112.7(3)

C(48)-N(47)-Fe(1) 126.3(3)

C(55)-N(54)-C(52) 117.5(3)

C(55)-N(54)-Fe(2) 112.6(3)

C(52)-N(54)-Fe(2) 128.7(2)

C(60)-O(60)-Fe(2) 116.9(3)

C(81)-O(81)-Fe(1) 115.2(3)

C(86)-N(87)-C(88) 118.9(3)

C(86)-N(87)-Fe(1) 113.9(3)

C(88)-N(87)-Fe(1) 125.7(2)

C(93)-C(88)-C(89) 120.2(4)

C(95)-N(94)-C(92) 119.9(3)

C(95)-N(94)-Fe(2) 114.5(3)

C(92)-N(94)-Fe(2) 123.7(3)

C(100)-O(100)-Fe(2) 115.5(3)

Table 3.8 :- Selected bond distances (Å) and bond angles (degree) of 15.

Fe(1)……Fe(2) 6.934

84

-4 -2 0 2 4

0.94

0.95

0.96

0.97

0.98

0.99

1.00

1.01

Simulated

Experimental

Relative Transmission

V (mms-1)

Co, Fe). Here again, the ligand is in the deprotonated form and the nitrogen is planer and three

coordinated. The average C-O and C-N bond lengths are 1.287 ± 0.009Å and 1.335 ± Å ,

respectively. The six ring C-C distances of the tert-butyl substituted phenyl rings are not

equidistant and a typical pattern of short (1.377 Å), a long (1.422 Å), and a short (1.368 Å)

together with three long C-C bonds are observed. This features show that the tert-butyl

substituted phenyl rings have lost its aromaticity. Thus, each iron (III) center has three O,N

coordinated iminobenzosemiquinone radicals; the complex consists of six

iminobenzosemiquinone radicals.

Zero-field Mössbauer spectrum of 15 was recorded at 80K and the nonlinear least-

squares fit is shown in Figure 3.14. The spectrum was fitted with a single quadrupole split

doublet with an isomer shift of δ = 0.56 mms-1 and a quadrupole splitting of ∆EQ = 1.011

mms-1. The isomer shift is consistent with those observed for high spin iron(III) ions in an

octahedral or distorted octahedral coordination.

In order to establish the spin ground state of 15, magnetization measurements at 1,

4 and 7 T was carried out. The field-dependent magnetizations as a function of temperature

and their simulations are depicted in Figure 3.15(a). The curve shows the value of

magnetization reaches ∼1.74 in the temperature range 2.0 - 2.8 K at the highest field of 7 T.

The simulated parameters are: S = 2.0, g = 2.0 (fixed), D = - 2.0 cm-1 and a Theta-Weiss

parameter (θ) of -1.73K. Complex 15 is X-band EPR silent at 10K, which is in conform with

an integer spin system. The magnetic behavior of 15 can be interpreted as the presence of

strong antiferromagnetic interactions between the three iminobenzosemiquinone radicals

[SR(total) = 3/2 ] and the high-spin ferric ion with SFe= 5/2 resulting in two S = 1 fragments on

each side of the m-phenylene spacer. The educated guess of strong antiferromagnetic

Figure 3.14:- Mössbauer spectrum of 15.

85

0.0 0.6 1.2 1.8 2.4

0.0

0.9

1.8

7T

4T

1T

M / N g β

βH / kT

N N

Fe Fe

SRSR

(3 x 1/2 ) (3 x 1/2 )

(S = 5/2) (S = 5/2)

S = 1 S = 1

S = 2

b)

a)

Figure 3.15:- a) Magnetization curve of 15 at 1,4 and 7T. b) Coupling scheme of 15.

interactions between the radicals and the iron(III) center is strongly supported by the similar

antiferromagnetic interactions present in the comparable mononuclear iron(III) compound

with the ligand 2-anilino-4,6-ditertbutylphenol(15*)4c. A ferromagnetic interaction arising

presumably from the spin-polarization effect due to the topology of the spacer between the S

= 1 fragments resulting in the ground state of St = 2 for 15. This quantitative picture is

schematically depicted in Figure 3.15(b).

Cyclic voltammograms (CV) and square-wave voltammograms (SQW) in

dichloromethane solution of 15 containing 0.1M TBAPF6 as supporting electrolyte was

recorded [ Figure 3.16(a)]. The nature of the anodic waves are similar to the Co(III) complex

(14) and consists of four one-electron-transfer reversible waves in the range 0.75 V to – 0.5 V.

However the cathodic waves which appear from –0.75 V to –1.75 V shows that two of these

waves are reversible in nature and the other is irreversible. Coulometry at –25°C in the

presence of 0.2M TBAPF6 supports the assignments of the oxidative and reductive peaks and

also shows that the all these peaks consists of a single electron transfer process. The nature of

the peak at –1.557V could not be characterized as it was irreversible even in the time scale of

CV. The peak potentials (E½ in V) vs. Fc

+/Fc for 15 and 15* (

mononuclear iron(III)

compound with the ligand 2-anilino-4,6-ditertbutylphenol) is given in Table 3.9.

Table 3.9 :- Redox potentials (V) for 15 and 15* vs. Fc+/Fc.

Compound E ½ (Oxidation) E ½ (Reduction)

15 + 0.481

(reversible)

+ 0.382

(reversible)

- 0.108

(reversible)

-0.288

(reversible)

-1.114

(reversible)

-1.278

(reversible)

-1.557

(irreversible)

15*

+ 0.27 (reversible)

- 0.35 (reversible)

-1.12

(reversible)

-1.31

(reversible)

-1.51

(irreversible

86

1.0 0.5 0.0 -0.5 -1.0 -1.5

E(V) vs. Fc+/Fc

1 µA

5 µA

400 600 800 1000

0

1

2

3

4

Absorbance

λ (nm)

a) b)

Figure 3.16 :- a) Cyclic (bold line) and square wave voltammogram (dotted line) of 15.

b) Electronic spectra of 15 (bold line) and its oxidized forms.

In order to characterize the species obtained after each electrochemical oxidation,

the UV spectrum was measured for each of the oxidized species. The reduced species were

found to be unstable during the time of coulometry. The electronic spectra of 15, shows a

broad but intense peak at 746nm (ε = 35,100 M-1 cm-1) with a shoulder at 441nm (ε = 16,100

M-1 cm-1). Upon four electron oxidation, the intensity of the peak at 746nm slowly decreases

and a new peak at 958nm develops. The intensity of the shoulder peak at 441nm also

increases indicating the formation of quinoid character in the ligand [Figure 3.16(b)].

Therefore the four oxidation peaks along with the two reduction peaks are ligand-centered

and the peak at –1.557V can be tentatively assigned to the metal centered reduction .

MnIV

2(LA

3)2L3 (16)

Dark brown crystals of 16 were afforded from a dichloromethane-acetonitrile

solution mixture. 16 crystallizes in a monoclinic crystal system with a P21/n space group,

same as that for 15. This leads to the assumption that manganese has an oxidation state of +3

(d4 system). However the metal-donor bond length reveals that the average Mn-O and the

Mn-N are 1.934 ± 0.009 Å and 1.961 ± 0.009Å, respectively. Each manganese center is in a

pseudo-octahedral O3N3Mn polyhedron and does not show any Jahn-Teller effect

[Figure 3.17] . Thus the oxidation state of each manganese center is +4 (d3 system).

An intriguing question regarding the neutrality of the complex naturally arises. If

all the donor atoms are deprotonated and the oxidation of the three ligands results in six

iminobenzosemiquinone radicals, then the complex should be a doubly charged cation! This is

87

Mn

O

N

NO

O

C(20)

C(15)

C(16) C(19)

C(18)

C(17)

C(10)

C(9)

C(8)

C(13)

C(12)

C(11)

C(3) C(4)

C(5)

C(6)

C(1)

C(2)

N(94) O(100)

O(60)

O(20)

N(54)

N(14)

N(7)

N(47)

O(41)

O(81)

O(1)

N(87)

Mn(1)

Mn(2)

C(100)

C(99)

C(98)

C(97)

C(96)

C(95)

C(86)

C(85)

C(84)

C(83)

C(82)

C(81)

not true and can be corroborated from the bond distances between the tert-butyl substituted

rings, and that between the donor atoms and the carbon with which it is attached.

The ligand containing the donor atoms O(41), N(47), O(60) and N(54) has the

typical characteristics of the iminobenzosemiquinone form; a short C-N and C-O bond

distance together with the loss of aromaticity at the phenyl ring containing the tert-butyl

substituent. So, this ligand consists of two iminobenzosemiquinone radicals. The second

ligand containing the donor atoms O(1),N(7),O(20) and N(14) consists of the deprotonated

ligand and two sp2 hybridized nitrogen donor atoms. However, the C(1)-O(1) and C(6)-N(7)

bond distance are 1.323Å and 1.381Å, respectively which corresponds to the amidophenolate

form of the phenyl ring. The six C-C bond distances of this amidophenolate ring are

equidistant at 1.404Å [Table 3.10]. Thus, this part of the ligand shows the characteristic of an

O,N-coordinated dianion [Figure 3.1]. The other part of the ligand has the typical

characteristic of iminobenzosemiquinone ring. So, this ligand consists of a single iminobenzo-

semiquinone radical. This features is also observed in the third ligand. One part consists of the

amidophenolate ring and the other the iminobenzosemiquinone ring; the complex consists of

four iminobenzosemiquinone radicals. Thus, each Mn(IV) center is coordinated by i) two

oxygen and two imine-nitrogen from two iminobenzosemiquinone rings and ii) one oxygen

and one amide-nitrogen from the amidophenolate ring. This renders the complex to be neutral.

Magnetic susceptibility measurement for 16 from 2-290 K for a powdered sample

was measured [Figure 3.18(a) as closed square]. The value of µeff at 290K was found to be

a) b)

Figure 3.17 :- a) Crystal Structure of 16. The tert-butyl groups has been removed for clarity.

b) A view of 16 highlighting the coordination sphere around the two Mn(IV) centers. The

tetradentate ligand L3 is denoted by the donor atoms and the meta-phenylene spacer.

88

Mn(1)-O(1) 1.884(3)

Mn(1)-O(41) 1.933(3)

Mn(1)-O(81) 1.967(3)

Mn(2)-O(100) 1.905(3)

Mn(2)-O(20) 1.921(3)

Mn(2)-O(60) 1.999(3)

Mn(1)-N(7) 1.936(3)

Mn(1)-N(47) 1.959(3)

Mn(1)-N(87) 1.972(3)

Mn(2)-N(94) 1.933(3)

Mn(2)-N(14) 1.975(3)

Mn(2)-N(54) 1.988(3)

O(1)-C(1) 1.323(5)

C(6)-N(7) 1.381(5)

N(7)-C(8) 1.427(5)

C(1)-C(2) 1.415(5)

C(1)-C(6) 1.423(6)

C(2)-C(3) 1.395(6)

C(3)-C(4) 1.410(6)

C(4)-C(5) 1.385(6)

C(5)-C(6) 1.397(6)

O(20)-C(20) 1.318(5)

C(12)-N(14) 1.436(5)

N(14)-C(15) 1.359(5)

C(15)-C(16) 1.406(6)

C(15)-C(20) 1.426(6)

C(16)-C(17) 1.368(6)

C(17)-C(18) 1.427(6)

C(18)-C(19) 1.382(6)

C(19)-C(20) 1.410(6)

O(41)-C(41) 1.310(5)

C(41)-C(42) 1.414(6)

C(41)-C(46) 1.427(6)

C(42)-C(43) 1.383(5)

C(42)-C(61) 1.535(6)

C(43)-C(44) 1.418(6)

C(44)-C(45) 1.369(6)

C(44)-C(65) 1.537(6)

C(45)-C(46) 1.414(6)

C(46)-N(47) 1.360(5)

N(47)-C(48) 1.438(5)

C(52)-N(54) 1.440(5)

N(54)-C(55) 1.343(5)

C(55)-C(56) 1.415(5)

C(55)-C(60) 1.445(5)

C(56)-C(57) 1.355(5)

C(57)-C(58) 1.429(5)

C(58)-C(59) 1.367(6)

C(59)-C(60) 1.421(6)

O(60)-C(60) 1.294(4)

O(1)-Mn(1)-O(41) 87.32(11)

O(1)-Mn(1)-N(7) 82.68(13)

O(41)-Mn(1)-N(7) 169.33(12)

O(1)-Mn(1)-N(47) 90.04(12)

O(41)-Mn(1)-N(47) 81.65(13)

N(7)-Mn(1)-N(47) 94.59(14)

O(1)-Mn(1)-O(81) 89.69(11)

O(41)-Mn(1)-O(81) 85.84(11)

N(7)-Mn(1)-O(81) 97.78(12)

N(47)-Mn(1)-O(81) 167.49(13)

O(1)-Mn(1)-N(87) 169.30(13)

O(41)-Mn(1)-N(87) 90.32(12)

N(7)-Mn(1)-N(87) 100.20(13)

N(47)-Mn(1)-N(87) 99.95(13)

O(81)-Mn(1)-N(87) 79.73(12)

O(100)-Mn(2)-O(20) 84.45(11)

O(100)-Mn(2)-N(94) 81.86(13)

O(20)-Mn(2)-N(94) 166.03(13)

O(100)-Mn(2)-N(14) 90.69(12)

O(20)-Mn(2)-N(14) 81.85(13)

N(94)-Mn(2)-N(14) 95.53(13)

O(100)-Mn(2)-N(54) 171.92(12)

O(20)-Mn(2)-N(54) 92.10(12)

N(94)-Mn(2)-N(54) 101.83(13)

N(14)-Mn(2)-N(54) 96.06(13)

O(100)-Mn(2)-O(60) 94.06(11)

O(20)-Mn(2)-O(60) 88.58(11)

N(94)-Mn(2)-O(60) 95.12(12)

N(14)-Mn(2)-O(60) 168.85(12)

N(54)-Mn(2)-O(60) 78.52(12)

C(1)-O(1)-Mn(1) 113.4(2)

C(6)-N(7)-C(8) 119.4(3)

C(6)-N(7)-Mn(1) 111.7(3)

C(8)-N(7)-Mn(1) 126.4(2)

C(15)-N(14)-C(12) 119.2(3)

C(15)-N(14)-Mn(2) 110.3(3)

C(12)-N(14)-Mn(2) 128.2(3)

C(20)-O(20)-Mn(2) 111.9(2)

C(41)-O(41)-Mn(1) 113.8(2)

C(46)-N(47)-C(48) 118.7(3)

C(46)-N(47)-Mn(1) 112.0(3)

C(48)-N(47)-Mn(1) 128.4(3)

C(55)-N(54)-C(52) 116.6(3)

C(55)-N(54)-Mn(2) 115.3(2)

C(52)-N(54)-Mn(2) 127.7(2)

C(60)-O(60)-Mn(2) 115.8(2)

C(81)-O(81)-Mn(1) 114.4(2)

C(86)-N(87)-C(88) 118.9(3)

C(86)-N(87)-Mn(1) 114.5(3)

C(88)-N(87)-Mn(1) 125.9(2)

C(95)-N(94)-C(92) 119.4(3)

C(95)-N(94)-Mn(2) 112.3(3)

C(92)-N(94)-Mn(2) 125.8(2)

C(100)-O(100)-Mn(2) 113.6(2)

Table 3.10 :- Selected bond distances (Å) and bond angles (degree) of 16.

Mn……Mn 6.762

89

0 20406080100

0

20

40

60

80

100

120

140

T / K

1 / χ (mol / cm3 )

µeff / µB

T / K

0 50 100 150 200 250 300

0.0

0.5

1.0

1.5

2.0

2.5

3.0

a) b)

Figure 3.18 :-(a) Magnetic data of 16. Filled squares shows the µeff value from 2-290 K and

filled triangles denotes 1/χ value from 2-100K. b) Coupling scheme for 16.

2.56µB which decreases rather slowly till 40 K (2.41 µB). A monotonous decrease was then

observed till 2K (0.93 µB). As seen in 15, coupling between the two radical centers (S=½) and

the metal center is antiferromagnetic. For 16 this also holds true. The two radicals (S = ½ ) are

strongly antiferromagnetically coupled with the electrons of the Mn(IV) ion (S = 3/2 ) which

leaves a residual spin of S = ½ in each part of the complex [Figure 3.18(b)]. The values of

µeff shows that the coupling between these two parts are weak. This data can be simulated

with coupling constants (J) between the two S = ½ centers ranging from +5 to –5 cm-1 but

with a high Theta-Weiss (θ) parameter. The presence of θ can be justified by plotting a 1/χ (χ

is the molar susceptibility) vs. Temperature [Figure 3.18(a) as triangle] till 100K. A linear

regression fit (bold line) shows that the line makes an intercept at T = - 7.9K i.e. – 5.53 cm-1.

This value indicated that the intermolecular interactions exceeds the intramolecular

interactions. From the nature of the curve it is probable that coupling between that coupling

between the two parts are weakly anti-ferromagnetic. The most plausible reason for

antiferromagnetism between the two centers is the non-orthogonal mixing between the d(π)

orbitals of the Mn(IV) center with that of the p(π) orbitals of the donor atoms [as seen for the

dicopper complex (12)].

X-band EPR spectrum of a dichloromethane solution of 16 at 10K in parallel mode is

shown in Figure 3.19. The spectrum consists of a broad signal centered at g = 2.0 and a weak

11-line hyperfine signal at g = 4.05. The spectrum needs thorough study and rigorous

calculation for simulation.

N N

Mn Mn

SRSR

(2 x 1/2 ) (2 x 1/2 )

(S = 3/2) (S = 3/2)

S = 1/2 S = 1/2

Very Weak Coupling

90

100 200 300 400

g values

B[mT]

65 4 3 2

0.5 0.0 -0.5 -1.0 -1.5

E (V) vs Fc+/Fc

18 µA

4 µA

400 600 800 1000

1

2

3

4

[ Mn2(LA

3)2(L3)+2 ]

[ Mn2(LA

3)2(L3)+1 ]

[ Mn2(LA

3)2(L3) ]

ε * 10 - 4 ( M -1 cm -1)

λ (nm)

Figure 3.19 :- X-band EPR spectra at 10K of 16.

Cyclic voltammetry and square wave voltammetry at 50mV/sec and 20Hz.,

respectively in dichloromethane in the presence of 0.1M TBAPF6 shows waves which are

similar to that of the dicopper complex (12). Four out of six peaks are reversible and two are

irreversible as observed from the variation of scan rate [Figure 3.20(a), Table 3.11]. A broad

irreversible peak [not shown] at –1.632V is observed and is probably due to the Mn centered

reduction. All the peaks are a single-electron transfer process and that occurring at – 0.12V

a) b)

Figure 3.20 :- a) Cyclic (bold line) and square wave voltammogram (dotted line) of 16.

b) Electronic spectra of 16 (bold line) and its oxidized forms.

140 160 180

-0.02

0.00

0.02

B[mT]

54

91

[Mn2(L3 )+1(LA3 )2][Mn2(L3)(LA3)2]

[Mn2(L3 )+2(LA3 )2]-1e--1e- +1e- +1e-

[Mn2(L3 )-1(LA3 )2] [Mn2(L3 )-2(LA3 )2]

and –0.341V corresponds to the oxidation of the ligand to the quinoid form; the peaks

observed at –1.019V and –1.234V are due to the reduction of the ligand to the monoanionic

and dianionic form of the ligand. These processes has been confirmed from coulometry. The

quasi-reversible peaks at +0.101V and +0.575V can be tentatively assigned to the oxidation of

ligand but due to its irreversibility in the time scale of coulometry, it was not studied further.

The different redox processes are

Table 3.11 :- Redox potentials (V) for 16 vs. Fc+/Fc.

Compound E ½ (Oxidation) E ½ (Reduction)

16 +0.575

(Irreversible)

+0.101

(Irreversible)

- 0.120

(Reversible)

- 0.341

(Reversible)

-1.019

(Reversible)

-1.234

(Reversible)

-1.632

(irreversible)

Spectroelectrochemistry of this complex was performed at –25°C in the presence

of 0.2M TBAPF6. The electronic spectrum shows a number of waves above 450nm with

maximums at 550 nm (ε=11,880 M-1 cm-1), 648 nm (ε=11,500 M-1cm-1), 807 nm (ε=10,700

M-1cm-1) and a shoulder peak at 953nm (ε=8430 M-1cm-1) ( bold line in Figure 3.20(b)] , all

arising due to the strong intra-ligand π-π* transitions. Upon two electron oxidation, these

waves disappear and a single maxima is generated at 720nm along with the development of a

peak at 420nm, characteristic for the formation of the quinoid form of the ligand.

CoIII

2L4

3 (17)

The neutral complex Co2L33 (17) crystallizes from a dichloromethane/acetonitrile

solution as deep brown crystals. The structure [Figure 3.21] consists of two cobalt atoms in a

distorted octahedral geometry with the oxygen and nitrogen atoms. The six short Co-O

distance at 1.893Å together with short Co-N distance at 1.923Å shows that the oxidation state

of cobalt is (+III) and low spin. Here again, the ligand is deprotonated as evidenced from the

planarity of the nitrogen atom. All the tert-butyl substituted phenyl rings has lost its

aromaticity with the formation of short (1.387Å), a long (1.444 Å), and a short (1.364 Å)

together with three long C-C bonds are observed. The average C-O and the average C-N

(N,O-attached to the tert-butyl substituted phenyl rings) bond distances are 1.303Å and

1.346Å which clearly shows that the tert-butyl substituted phenyl rings are in

iminobenzosemiquinone form. Thus, the molecule consists of six iminobenzosemiquinone

92

C(1)

C(6)

C(5)

C(4)

C(3)

C(2)

O(1)

N(3)

O(3)

O(5)

Co(1)

N(1)

N(5) C(9)

C(10)

C(11)

C(12)

C(7)

C(8) N(2)

N(4)

N(6)

Co(2)

O(6)

O(4)

O(2)

C(13)

C(24)

C(23) C(22)

C(21)

C(20)

Figure 3.21 :- Molecular structure of 17. Tert-butyl groups are omitted for clarity.

radicals. Selected bond distances (Å) and bond angles (degree) is given in Table 3.12.

Magnetic data of a polycrystalline sample of 17 is shown in Figure 3.22 (open circle).

The value of µeff at 290K (4.51 µB) increases with the decrease in temperature and reaches a

maximum at 5.07 µB at 30K whereupon it decreases till 4.80 µB at 2K. This indicates that an

overall ferromagnetic coupling occurs within the molecule. In order to simulate this data,

simplification is needed. The value of the magnetic moment at 290K is higher than that of six

non-coupled S=½ center (4.24 µB) and is due to the presence of temperature independent

paramagnetism (TIP) for CoIII ion. The value at 30K is however lower than that observed for

two uncoupled S = 3/2 spin states (5.24 µB) and is probably due to Theta-Weiss parameter (θ)

which accounts for the intermolecular interactions. Thus, the magnetic data of this molecule

can be simulated by calculating the µeff value for each CoIII center. This can be done by

dividing the molecular weight and the diamagnetic susceptibility by two. The resultant curve

(closed circle) is shown in Figure 3.22 (a). This data can be simulated by using a single

coupling constant value between the three iminosemiquinone radicals in each part of the

complex and no-coupling between the two parts of the ligand. The presence of a non-

conjugated sp3 carbon atom as a spacer between the two phenyl rings implies that interactions

through spin polarization is not possible. Furthermore, the two CoIII are at a distance of

11.319 Å which obliterates the possibility of any coupling between the two parts.

93

Co(1)-O(1) 1.882(6)

Co(1)-O(5) 1.891(5)

Co(1)-O(3) 1.903(6)

Co(1)-N(3) 1.910(7)

Co(1)-N(5) 1.912(7)

Co(1)-N(1) 1.928(7)

Co(2)-O(6) 1.891(6)

Co(2)-O(2) 1.895(6)

Co(2)-O(4) 1.900(6)

Co(2)-N(2) 1.917(7)

Co(2)-N(6) 1.932(7)

Co(2)-N(4) 1.943(7)

N(1)-C(2) 1.364(10)

N(1)-C(7) 1.417(10)

N(2)-C(20) 1.337(9)

N(2)-C(17) 1.430(10)

N(3)-C(52) 1.353(10)

N(3)-C(57) 1.429(10)

N(4)-C(70) 1.334(10)

N(4)-C(67) 1.408(10)

N(5)-C(102) 1.343(10)

N(5)-C(107) 1.463(10)

N(6)-C(120) 1.345(10)

N(6)-C(117) 1.403(10)

O(1)-C(1) 1.293(10)

O(2)-C(21) 1.295(9)

O(3)-C(51) 1.295(9)

O(4)-C(71) 1.309(9)

O(5)-C(101) 1.323(10)

O(6)-C(71) 1.295(10)

C(1)-C(6) 1.410(11)

C(1)-C(2) 1.451(11)

C(2)-C(3) 1.413(11)

C(3)-C(4) 1.382(11)

C(4)-C(5) 1.392(11)

C(4)-C(26) 1.513(12)

C(5)-C(6) 1.370(12)

C(20)-C(21) 1.409(11)

C(20)-C(25) 1.444(11)

C(21)-C(22) 1.411(11)

C(22)-C(23) 1.387(11)

C(22)-C(34) 1.535(11)

C(23)-C(24) 1.433(11)

C(24)-C(25) 1.364(11)

C(24)-C(38) 1.534(12)

O(1)-Co(1)-O(5) 87.5(3)

O(1)-Co(1)-O(3) 85.3(2)

O(5)-Co(1)-O(3) 88.9(2)

O(1)-Co(1)-N(3) 88.1(3)

O(5)-Co(1)-N(3) 171.9(3)

O(3)-Co(1)-N(3) 84.0(3)

O(1)-Co(1)-N(5) 170.6(3)

O(5)-Co(1)-N(5) 83.7(3)

O(3)-Co(1)-N(5) 91.4(3)

N(3)-Co(1)-N(5) 100.3(3)

O(1)-Co(1)-N(1) 84.0(3)

O(5)-Co(1)-N(1) 91.4(3)

O(3)-Co(1)-N(1) 169.3(3)

N(3)-Co(1)-N(1) 94.8(3)

N(5)-Co(1)-N(1) 99.3(3)

O(6)-Co(2)-O(2) 87.4(3)

O(6)-Co(2)-O(4) 87.6(3)

O(2)-Co(2)-O(4) 86.8(2)

O(6)-Co(2)-N(2) 171.3(3)

O(2)-Co(2)-N(2) 84.0(3)

O(4)-Co(2)-N(2) 90.7(3)

O(6)-Co(2)-N(6) 84.3(3)

O(2)-Co(2)-N(6) 91.4(2)

O(4)-Co(2)-N(6) 171.8(3)

N(2)-Co(2)-N(6) 97.1(3)

O(6)-Co(2)-N(4) 89.3(3)

O(2)-Co(2)-N(4) 169.8(3)

O(4)-Co(2)-N(4) 83.3(3)

N(2)-Co(2)-N(4) 99.0(3)

N(6)-Co(2)-N(4) 97.9(3)

C(2)-N(1)-C(7) 119.3(7)

C(2)-N(1)-Co(1) 113.4(6)

C(7)-N(1)-Co(1) 127.3(6)

C(20)-N(2)-C(17) 120.6(7)

C(20)-N(2)-Co(2) 111.7(5)

C(17)-N(2)-Co(2) 127.4(6)

C(52)-N(3)-C(57) 119.5(7)

C(52)-N(3)-Co(1) 108.8(6)

C(57)-N(3)-Co(1) 129.0(6)

C(70)-N(4)-C(67) 118.7(7)

C(70)-N(4)-Co(2) 111.0(6)

C(67)-N(4)-Co(2) 130.2(6)

C(102)-N(5)-C(107) 119.2(7)

C(102)-N(5)-Co(1) 112.2(6)

C(107)-N(5)-Co(1) 126.5(6)

C(120)-N(6)-C(117) 120.0(8)

C(120)-N(6)-Co(2) 111.8(6)

C(117)-N(6)-Co(2) 128.2(6)

C(1)-O(1)-Co(1) 113.4(5)

C(21)-O(2)-Co(2) 112.5(5)

C(51)-O(3)-Co(1) 109.6(5)

C(71)-O(4)-Co(2) 111.1(5)

C(101)-O(5)-Co(1) 113.0(5)

C(71)-O(6)-Co(2) 113.0(5)

Table 3.12 :- Selected bond distances (Å) and bond angles (degree) of 17.

Co(1) ….. Co(2) 11.319

94

100 200 300

2

3

4

5

µ eff / µ B

T / K

100 150 200 250 300 350 400

B[mT]

76 5 4 3 2

g values

a) b)

Figure 3.22:- a) Magnetic susceptibility data of 17. The open circle represents the value of

µeff for the total molecule. The closed circle and the bold line represents the calculated and

simulated µeff value for a single CoIII center. b) X-band EPR spectrum of 17 at 9.6K.

From the nature of the susceptibility data, it is clear that ferromagnetic interactions is

operating between the three radicals in each part of the ligand. When each cobalt center is

considered, the dihedral angles made between the two planes (each plane consisting the tert-

butyl substituted phenyl rings) O(1) [C(1)-C(2)-C(3)-C(4)-C(5)-C(6)] - O(3)[C(51)-C(52)-

C(53)-C(54)-C(55)-C(56)] is 106.6°; that between O(1)[C(1)-C(2)-C(3)-C(4)-C(5)-C(6)]-

O(5)[C(101)-C(102)-C(103)-C(104-C(105)-C(106)] and O(3)[C(51)-C(52)-C(53)-C(54)-

C(55)-C(56)] - O(5)[C(101)-C(102)-C(103)-C(104-C(105)-C(106)] being 103.4° and 111.4°,

respectively. For the other cobalt center, the dihedral angles made between the two planes

(each plane consisting the tert-butyl substituted phenyl rings) O(2) [C(21)-C(22)-C(23)-

C(24)-C(25)-C(26)] - O(4)[C(71)-C(72)-C(73)-C(74)-C(75)-C(76)] is 114.5°; that between

O(2)[C(21)-C(22)-C(23)-C(24)-C(25)-C(26)] - O(6)[C(120)-C(121)-C(122)-C(123-C(124)-

C(125)] and O(4)[C(71)-C(72)-C(73)-C(74)-C(75)-C(76)] - O(6)[C(120)-C(121)-C(122)-

C(123-C(124)-C(125)] being 100.8° and 93.9°, respectively. The angles do not deviate

widely from orthogonality and therefore the interaction between the radicals are taken as

ferromagnetic. Therefore, the coupling scheme for 14 [Figure 3.11(a)] can be used in

simulating this data. The values obtained are J1 = +22.0, J2=0, g =2.0 and a Theta-Weiss

parameter (θ) of –2.0 K. where J1 describes the coupling constant between the radicals at each

part and J2, the interactions between the radicals of each ligand. Therefore the ground state

95

400 600 800 1000

0

2

4

[CoIII

2(L4)2

+ 4(L4)]

[CoIII

2(L4)2

+ 3(L4)]

[CoIII

2(L4) + 2(L4)2]

[CoIII

2(L4)+(L4)2]

[CoIII

2(L4)3 ]

ε * 10 - 4 [ M -1 cm -1]

λ (nm)

0.5 0.0 -0.5 -1.0 -1.5 -2.0

4 µA

E(V) vs Fc+/Fc

4 µA

consists of two quartets ( S =3/2 ,3/2 ) arising from the ferromagnetic coupling of the radicals

from each part of the complex.

The X-band EPR spectra of a frozen dichloromethane solution of 17 was measured at

9.6K. Only a sharp signal is observed at g = 2 without any hyperfine structures [Figure

3.22(b)], indicating a very small zerofield splitting for the ground state.

Electrochemistry (cyclic and square wave voltammetry) was carried out with 17. The

peaks consists of three broad waves out of which two, between +0.5V and –0.75V, are

reversible and the other (-1.34 V) irreversible [Figure 3.23(a),Table 3.13]. All the reversible

redox processes consist of one-electron oxidation as observed from coulometry

measurements. The peak at –1.34 V is tentatively assigned to the reduction of the CoIII center.

The value of the E1/2 (oxidation) are similar to that of 14 and therefore can be inferred to the

oxidation of the radical centers. Spectroelectrochemistry of 17 also supports this assignment.

The electronic spectra of 17 [Figure 3.23(b) (bold line)] shows a similar pattern to that of 14

with maximas at 477nm(ε = 10,765 M-1cm-1), 701nm(ε = 5,773 M-1 cm-1), 782nm(ε = 6564

M-1cm-1) and 857nm(ε = 10,765 M-1cm-1). Upon four electron oxidation, the peak

corresponding to 477nm increases in intensity along with the decrease in the peak maximas

above 650nm, signifying the formation of iminobenzoquinone form of the ligand.

Table 3.13 :- Redox potentials (V) for 17 vs. Fc+/Fc.

Compound E ½ (Oxidation) E ½ (Reduction)

17

+ 0.085 (reversible)

- 0.37 (reversible)

-1.34 (irreversible)

a) b)

Figure 3.23 :- (a) Cyclic (bold line) and Square Wave voltammetry (dotted line) of 17.

(b)Electronic spectra of 17 and its oxidized forms.

96

A novel series of dinuclear (iminosemiquinone)metal complexes is described that

provides a suitable basis for further research in a systematic way, especially on the metal-

radical interactions. The concept of spin polarization was used in an attempt to induce

ferromagnetic coupling between the dinulceating centers or between the radical centers.

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(g) A. Racja, Chem. Rev., 1994, 94, 871.

4) (a) C. N. Verani, S. Gallert, E. Bill, T. Weyhermüller, K. Wieghardt, P. Chaudhuri, Chem. Commun.,1999,

1747; (b)P. Chaudhuri, C. N. Verani, E. Bill, E. Bothe, T. Weyhermüller ,K. Wieghardt, J. Am. Chem.

Soc., 2001, 123, 2213; (c) H. Chun, C. N. Verani, P. Chaudhuri, E. Bothe, E. Bill, T. Weyhermüller ,

K. Wieghardt, Inorg. Chem.,2001, 40, 4157. (d) H. Chun, T. Weyhermüller, E. Bill, K. Wieghardt,

Angew. Chem.,Int. Ed., 2001, 40, 2489. (e)H. Chun, P. Chaudhuri, T. Weyhermüller, K. Wieghardt,

Inorg. Chem., 2002, 41, 790. (f)X. Sun, H. Chun, K. Hildenbrand, E. Bothe, T. Weyhermüller,F. Neese,

K. Wieghardt, Inorg. Chem., 2002, 41, 4295.(g) H. Chun, E. Bill, E. Bothe, T. Weyhermüller ,K.

Wieghardt, Inorg. Chem., 2002, 41, 5091.h) K. S. Min, T. Weyhermüller ,K. Wieghardt, Dalton Trans.,

2003, 1126. (i) C. N. Verani, Dissertation, Bochum, 2001.

5) a)S. Mukherjee, E. Rentschler, T. Weyhermüller, K. Wieghardt , P. Chaudhuri, Chem. Commun.,2003, 1828.

b) A. Dei, D. Gatteschi, C. Sangregorio, L. Sorace, M. G. F. Vaz, Inorg. Chem. 2003, 42, 1701. c) A. Dei,

D. Gatteschi, C. Sangregorio, L. Sorace, Vaz, M. G. F. Chem. Phys. Lett. 2003, 368, 162.

6) (a)D. R. Corbin, L. C. Francesconi, D. N. Hendrikson, G. D. Stucky; Inorg. Chem. 1979, 18,3069. (b) L. C.

Francesconi, D. R. Corbin, D. N. Hendrickson, G. D. Stucky, Inorg. Chem. 1979, 18, 3074.

7) C. W. Lange, B. J. Conklin, C. G. Pierpont ,Inorg. Chem. 1994, 33, 1276.

97

Chapter 4

MnIV COMPLEXES WITH IMINO-BENZOSEMIQUINONE LIGANDS;

SYNTHESIS, CHARACTERIZATION AND REACTIVITY STUDY

Mn

N

O

R

.IV

98

99

I

His

Cu I

His

Cu

deoxy state

O2

II II

-

O

His

Cu

OHis

His

Cu

oxy state

O

H

+

OH2

OO

His

Cu

OH (?)

His

Cu

II II

OH

2H

+

2H

+

OH

-

O

His

Cu

OHis

His

Cu

O

His

Cu

OH (?)

His

Cu

II II

met state

3H

+

O

+

OH2

2 e - Transfer

4.1 INTRODUCTION

Out of the several types of metalloenzymes in biochemistry, the oxidase enzymes have

been widely studied. The role played by the metal center has always attracted interest and has

inspired bioinorganic chemists for synthesizing structural as well as functional models for

such type of metalloenzymes [Chapter 1].

One interesting oxidase enzyme is the dicopper containing catechol oxidase [E.C.

1.10.3.1] which catalyzes the two-electron oxidation of catechols to quinones 1. The different

states and the mechanism of catechol oxidase1 activity are given in Figure 4.1. The crystal

structure of catechol oxidase from I. batatas (sweet potato) reveals that the two copper

centers, in its active form, are at a distance of 3.8 Å.

In order to mimic the structural and functional model of this enzyme, a number of

dinuclear copper complexes have been reported. Intensive studies were undertaken so as to

elucidate the relationship between structure and reactivity of the natural active sites and to

develop new complexes with useful catalytic performance2.

In the previous chapter, the dinucleating ligand [H4L3] along with different transition

metal complexes has been described. In an attempt to mimic the function of catechol oxidase,

the dinuclear copper complex (12) was used as a possible catalyst with 3,5-di-tert-

butylcatechol as substrate. 3,5-di-tert-butyl-o-benzoquinone was the expected oxidation

Figure 4.1 :- Catalytic cycle of catechol oxidase 1b.

100

product. That changing the metal center improves not only the yield but also the turnover

number (TON) [expressed in mole product per mole catalyst] will be discussed in this chapter.

4.2 CATECHOL OXIDASE MODEL STUDIES

Among the different catechols used in catechol oxidase model studies, 3,5-di-tert-

butylcatechol (3,5-DTBC) is the most widely used substrate due to its low redox potential for

the quinone–catechol couple, which makes it easy to be oxidized to the corresponding

quinone, 3,5-di-tertbutyl-o-benzoquinone (3,5-DTBQ), and its bulky substitutents which

make further oxidation reactions such as ring opening slower. 3,5-DTBQ is considerably

stable and has a strong absorption at 408 nm.

To evaluate quantitatively a significant activity, 5 x 10-6 mole of catalyst in 25 ml of

dichloromethane was treated with 50 equivalents of 3,5-DTBC and stirred in air. It is to be

noted that 3,5-DTBC alone does not undergo aerial oxidation. The products were analyzed by

liquid chromatography. The retention time in liquid chromatography for 3,5-DTBC and 3,5-

DTBQ was found to be 8.0 and 10.5 minutes, respectively, (column:- Luna-5 phenylhexyl;

Eluant:- Methanol and Water in ratio 3:1, 0.8 ml/min) as observed from commercially

available compound. It was found that for all the oxidative reactions involving 3,5-DTBC as

the substrate, 3,5-DTBQ was the only oxidized product.

The dinuclear copper complex (12) was used as a catalyst for this oxidation. Stirring a

dichloromethane solution of 12 in presence of 3,5-DTBC, in the ratio 1:50, gave at the end of

24 hours 15.6 % of 3,5-DTBQ. The other peaks observed in the liquid chromatography are the

decomposed products of 12, as observed from LC-MS coupling. The turnover number is

therefore eight. This indicates that although the dicopper complex catalyzes the oxidation of

3,5-DTBC to 3,5-DTBQ, the poor TON obliterates it being a good catalyst for such type of

oxidation. A probable reason is the long distance between the two copper centers (~ 6.8 Å). It

is observed that several dinuclear copper complexes show catalytic activity if the distance

between the two copper centers is around 5Å 3 .

The involvement of manganese in biological process and its relevance to oxidation

state study, coordination behavior and its role as biological catalyst interested the inorganic

chemists in order to synthesize polynuclear high-valent manganese complexes. Several

manganese complexes have been studied to demonstrate the mechanism of catalase,

superoxide dismutases or photosytem II. The role of the phenoxyl radical during the catalysis

process in PS II has also being studied [Chapter 1]. Compound 16, a high-valent dimanganese

101

complex may be a suitable complex for catalyzing oxidation reactions. The substrate chosen

was 3,5-DTBC for reasons mentioned above.

When 16 was dissolved in dichloromethane and 3,5-DTBC was added in the ratio

1:50, the dark brown color slowly changes, in presence of air, to yellow and then finally red.

Analysis of this solution after 24 hours shows a 100 % conversion of 3,5-DTBC to 3,5-DTBQ

i.e. the turnover number (TON) of this reaction is 50. On increasing the amount of 3,5-DTBC

it was found that the TON increases and the maximum turnover reached after 24 hours is 500.

This shows that this complex can act as a good catalyst in catalyzing the oxidation of 3,5-

DTBC to 3,5-DTBQ in presence of oxygen. The dimanganese compound (16) acts as a much

better catalyst than the dicopper complex (12), thus mimicking the function of catechol

oxidase.5

Reaction kinetics was performed by observing the change in absorbance at 408 nm,

which is characteristic of 3,5-DTBQ. In a typical reaction the catalyst was dissolved in

dichloromethane and the substrate, 3,5-DTBC was added. 1ml from this solution was taken,

diluted to 10ml with CH2Cl2 and the resultant electronic spectral change was observed in a

1cm cuvette. For calculation of rate constants, initial rate method was used and the velocity of

the reaction was obtained by the slope of the tangent to the absorbance vs. time curve. This

procedure was taken as standard for all the kinetic measurements.

The kinetic of the oxidative reaction at a constant catalyst concentration was

investigated by using 2 x 10-7 mole of the catalyst (16) in dichloromethane (10ml after

dilution) with variation of the substrate concentration from 2 x 10-6 to 20 x 10-6 moles. The

electronic spectra for a catalyst to substrate ratio of 1:30 shows an increase in the intensity of

the peak at 408 nm with time [Figure 4.2(a)]. The least square fit plot of the difference in

absorbance (absorbance is directly proportional to concentration) vs. time (bold line) [Figure

4.2(b)] gives the velocity(r0) for that particular catalyst to substrate concentration ratio. The

different substrate concentration along with the rate constants are listed in Table 4.1. Plotting

the velocity vs. the concentrations shows that the rate of the reaction is first order with respect

to the substrate concentration. The best fit line (bold line, Figure 4.3) passes through the

origin and the slope is 9.13 x 103 mole-1 min-1. This value is the rate constant of the reaction at

a constant catalyst concentration (ksub).

102

400 600 800

0.2

0.4

0.6

0.8

1.0

Absorbance

λ (nm)

0 75 150 225 300

0.00

0.06

0.12

0.18

0.24

0.30

r0 = 8.61 x 10 - 4 sec-1

Time (sec)

∆ Absorbance

0 5 10 15 20

0

5

10

15

20 ksub= 9.13 x 103 mole-1 min-1

r0 ( x 10 2 ) ( min -1 )

Conc. of Substrate ( x 10 6 ) (moles)

a) b)

Figure 4.2 :- a) Change in the electronic spectra of 16 after the addition of 3,5-DTBC. b) Plot

of the difference in absorbance vs. time for the catalyst to substrate concentration ratio 1:30.

For evaluating the rate constant at a constant substrate concentration, similar reactions

were carried out with a constant substrate concentration (5 x 10-6 mole) and a varying catalyst

concentration in dichloromethane. The different velocities(r0´) obtained [Table 4.2] were

plotted against the catalyst concentration [Figure 4.4]. From the plot it is clear that the rate of

the reaction is also first order with respect to the substrate concentration. The best fit line

(bold line Figure 4.4) passes through the origin with a slope (kcat) of 2.26 x105 mole-1 min-1.

[Substrate] x 106

(mole)

Rate (r0) x 102

(min-1)

2 0.8

3.15 1.87

4 3.49

5 4.5

6 5.1

7.6 6

12 10.92

15 13.98

20.6 19.38

Table 4.1:-List of substrate concentration

and corresponding rate. Figure 4.3:-Plot of substrate concentration vs.

corresponding rate.

103

012345

0

1

2

3

4

5

kcat = 2.26 x 105 min -1 mole -1

r0´ ( x 10 2 ) ( min -1 )

[Catalyst] ( x 10 6) (moles)

[Catalyst] x 106

(mole)

Rate(r0´) x 102

(min-1)

2.6 2.46

4 3.59

4.5 3.98

5 4.5

5.5 5.02

Table 4.2:-List of catalyst concentration

and corresponding rate.

Figure 4.4:- Plot of catalyst concentration vs.

corresponding rate.

Thus the evaluated rate law takes the form

Rate = k [Catalyst] [Substrate]

From the rate law, it is probable that each MnIV center of the dimer (16) is catalytically active.

A reasonable way to prove this is to synthesize the monomeric MnIV form (16*) with two

iminobenzosemiquinone radicals and study the similar oxidation reaction. A series of MnIV

compounds were prepared with this type of bidentate ligands, but substituted at the 3,5–

position to the amine group, and reactivity with the similar substrate was studied.

The ligands H2L5-10 [Figure 4.5] were synthesized according to the same procedure

used in synthesizing H2L5 and all the Mn(IV) complexes were prepared. The complexes were

characterized by IR, Mass and EPR spectroscopy together with magnetic susceptibility

measurements. From EPR and magnetic susceptibility measurements, it is clear that the

oxidation state at the manganese center is +IV. Electrochemical data was also measured for all

these compounds. Since these compounds are isostructural, the Mn(IV) complex with ligand

H2L6 (18) has been structurally characterized and will be described in details in this

Figure 4.5 :- Ligands used in the synthesis of Mn(IV) complexes.

N

HX

X

OH

X = H H2L5

= -C(CH3)3 H2L6

= -CF3 H2L7

= -CH3 H2L8

= -Cl H2L9

= -OCH3 H2L10

H2L

104

chapter. The Mn(IV) complex, using the ligand H2L5 (16*) has already been reported.4

Magnetic susceptibility measurements and EPR data for the complexes 19-23 are given in the

Appendix section.

4.3 SYNTHESIS AND CHARACTERIZATION OF MnIV(L A

6)(L6) 2 (18)

When H2L6 is refluxed in air with “Manganese(III) Acetate” in methanol in the

presence of tetrabutylammonium methoxide, a dark brown solution is obtained. On slow

cooling, a microcystalline precipitate of 18 is obtained. This was recrystallized from a

diethylether and methanol solution mixture. The compound shows the characteristic IR-peaks

and EI-MS shows an m/z=1276, which corresponds to three fully deprotonated ligands with a

manganese center. Selected IR peaks of H2L6 and 18 are given in Table 4.3.

Characteristic IR peaks (cm-1)

H2L6 3339s,1598s,1361s,1233s,999m,774m,631b

18 1592w,1478m,1360m,1142b,756w,706w

Table 4.3 :- Characteristic IR bands for H2L6 and 18.

Dark brown crystals afforded by diethylether-methanol solution is subjected to single

crystal XRD studies. Selected metal-to-oxygen and metal-to-nitrogen bond lengths are

summarized in Table 4.4. The geometrical features of the ligand are within an experimental

error of ±0.015 Å identical to those shown in Figure 3.1 (Chapter 3). The neutral molecule in

crystals of 18 [Figure 4.6] contains two O,N-coordinated o-iminobenzosemiquinonate(-) π

radical ligands, (L6)-, as is clearly borne out by the observation that (i) both nitrogens are sp2

hybridized and not protonated, (ii) the six-membered ring of the iminobenzosemiquinonate

part displays the typical quinoid distortions, and (iii) the C-O and C-N bond lengths are short,

approaching double bonds. The third ligand is N-deprotonated. The six C-C bonds of the

amidophenolate ring are equidistant at 1.40 (0.009) Å, and the C-O and C-N distances at

1.330 (0.006) and 1.384 ( 0.006) Å are long. This ligand displays the characteristic features of

an O,N-coordinated aromatic dianion, (L6-H)2- [Figure 3.1 (Chapter3)]. If this assignment is

correct, the central Mn ion must be ascribed to +IV (d3) oxidation level. The observed Mn-O

and Mn-N bond distances in 18 support this view; they are short (average Mn-O bond

distance is 1.904Å, average Mn-N bond distance is 1.945Å) and the pseudooctahedral

O3N3Mn polyhedron does not show any Jahn-Teller distortion .The crystal structure of 18 is

in excellent agreement with the charge distribution of [MnIV(L6)2(L6-H)]. Selected bond

distances for 18 are given in Table 4.4.

105

C

(

66

)

C

(

61

)

C

(

62

)

C

(

65

)

C

(

64

)

C

(

63

)

O

(

61

)

O(1) O

(

31

)

N

(

67

)

N

(

37

)

N(7)

Mn

(

1

)

C

(

31

)

C

(

32

)

C

(

36

)

C

(

35

)

C

(

34

)

C

(

33

)

Mn(1)-O(31) 1.8676(13)

Mn(1)-O(1) 1.9050(13)

Mn(1)-N(37) 1.9189(16)

Mn(1)-O(61) 1.9394(13)

Mn(1)-N(7) 1.9522(16)

Mn(1)-N(67) 1.9664(17)

O(31)-C(31) 1.330(2)

C(31)-C(32) 1.412(3)

C(31)-C(36) 1.413(3)

C(32)-C(33) 1.390(3)

C(33)-C(34) 1.411(3)

C(34)-C(35) 1.385(3)

C(35)-C(36) 1.404(3)

C(36)-N(37) 1.384(2)

N(37)-C(38) 1.429(2)

C(61)-O(61) 1.297(2)

C(61)-C(62) 1.426(3)

C(61)-C(66) 1.441(3)

C(62)-C(63) 1.379(3)

C(63)-C(64) 1.424(3)

C(64)-C(65) 1.373(3)

C(65)-C(66) 1.414(3)

C(66)-N(67) 1.353(2)

N(67)-C(68) 1.422(2)

O(31)-Mn(1)-O(1) 178.12(6)

O(31)-Mn(1)-N(37) 83.45(6)

O(1)-Mn(1)-N(37) 96.27(6)

O(31)-Mn(1)-O(61) 89.43(6)

O(1)-Mn(1)-O(61) 90.87(6)

N(37)-Mn(1)-O(61) 172.84(6)

O(31)-Mn(1)-N(7) 96.61(6)

O(1)-Mn(1)-N(7) 81.56(6)

N(37)-Mn(1)-N(7) 96.08(7)

O(61)-Mn(1)-N(7) 85.44(6)

O(31)-Mn(1)-N(67) 96.44(6)

O(1)-Mn(1)-N(67) 85.44(6)

N(37)-Mn(1)-N(67) 98.95(7)

O(61)-Mn(1)-N(67) 81.07(6)

N(7)-Mn(1)-N(67) 161.09(7)

C(31)-O(31)-Mn(1) 115.33(12)

C(36)-N(37)-C(38) 118.18(16)

C(36)-N(37)-Mn(1) 113.00(12)

C(38)-N(37)-Mn(1) 126.78(12)

C(61)-O(61)-Mn(1) 114.57(12)

C(66)-N(67)-C(68) 121.10(17)

C(66)-N(67)-Mn(1) 113.12(12)

C(68)-N(67)-Mn(1) 124.12(13)

Figure 4.6 :- Crystal structure of 18.

Table 4.4 :- Selected bond distances (Å) and angles (degree) of 18.

106

50 100 150 200 250

1.2

1.4

1.6

1.8

2.0

Simulated

Experimental

µ eff / µ B

T /K

327 328 329 330 331 332

B [mT]

2.06 2.04

Simulated

Experimental

g values

320 340 360 380

2.2 2.1 2 1.9 1.8

g values

B [mT]

Figure 4.7:- Magnetic data of 18.

The electronic ground state of 18 have been established from variable-temperature(2-

290K) magnetic susceptibility measurements by using a SQUID magnetometer in an external

magnetic field of 1.0 T. Figure 4.7 shows the temperature dependence of the magnetic

moment, µeff, for 18. On lowering the temperature, µeff of 18 decreases monotonically from

1.8 µB at 290 K to 1.7 µB at 15 K. This behavior indicates an S = ½ for 18. As the overall

magnetic behavior is dominated by much stronger antiferromagnetic interactions between

ligand radicals and metal-based unpaired d-electrons, the decrease of µeff on lowering the

temperature could well be fitted by a single coupling constant J [coupling between the radical

center (S = ½) and MnIV center (S = 3/2 )] with J’ [coupling between the radical centers (S =

½)] set to zero. Using the following parameters: S1 = S3 = ½ , S2 = 3/2 , g1=2.0 (fixed), g2=1.97

(fixed), J13 = 0 (fixed), J = -292 cm-1 mol-1, a satisfactory fit was obtained (bold line, Figure

4.7). Hence, a strong antiferromagnetic coupling exists between the ligand radicals and the

MnIV ion, similar to that observed for the MnIV complex with the ligand 2-anilino-4,6-

ditertbutylphenol (16*).4

Figure 4.8 :- X-band EPR spectra of 18 at 298K.

107

01234

0

2

4

6

8

10

12

ksub = 2.8 x 104 mole -1 min -1

r0´´ (*102 ) (min -1)

Conc. of Substrate (*106) (mole)

The metal-based S = ½ ground state of 18 has also been established by X-band EPR

spectroscopy. Figure 4.8 shows the EPR spectrum of 18 in CH2Cl2 solution at 298K for the X-

band. The spectrum displays an isotropic signal at giso = 2.009 with hyperfine coupling to the

I = 5/2 55Mn nucleus of Aiso=106.6 G , and in addition, superhyperfine coupling to three 14N

donor atoms (I = 1) and two protons (I =½) was clearly detected, A(14N) =4.29 G, A(1H) =

5.15G, A(1H) =2.83 G .

Electrochemistry (Cyclic and square wave voltammetry) of 18 shows two reversible

waves at -0.508 and –0.953V, which are assigned to the oxidation and reduction at the radical

site of the ligand. These redox processes are one-electron transfer processes. The irreversible

peak at –1.31V is assigned to the metal centered reduction .

When 3,5-DTBC is added to a CH2Cl2 solution of 18, in the ratio 1:50, the deep red

brown color of the solution starts to fade to yellow and then slowly to red. The solution was

stirred for one hour and then subjected to Liquid Chromatography. 100% conversion of 3,5-

DTBC to 3,5-DTBQ was found and the maximum TON was 169. It is to be pointed out that

although the rate of conversion when 18 is used as a catalyst is faster than that of 16, however

the TON is less. The probable reason could be the presence of two metal centers and the

presence of four radicals in 16.

To evaluate the rate constants at a constant catalyst concentration (ksub) and at constant

substrate concentration (kcat), the same procedure, when 16 was used as a catalyst, was

applied. When the substrate concentration was changed, the amount of catalyst was 2 x 10-7

mole in dichloromethane (10ml after dilution) [Table 4.5]. For measuring kcat the amount of

substrate taken was 4 x 10-6 mole [Table 4.6]. The value of ksub and kcat are 2.8 x 104 mole-1

min-1 and 5.6 x 105 mole-1 min-1, respectively and are calculated from the slope of the

Table 4.5:- List of substrate concentration

and corresponding rate.

Figure 4.9:-Plot of substrate concentration vs.

corresponding rate.

[Substrate] x 106

(moles)

Rate (r0´´) x 102

(min-1)

1.1 2.78

2.4 6.78

3.2 9.3

4 11.04

108

0 5 10 15 20

0

2

4

6

8

10

12

kcat = 5.6 x 10 5 mole -1 min -1

r0´´´ ( * 10 2 ) (min -1)

Conc. of Catalyst (*108) (mole)

Table 4.6:- List of catalyst concentration

and corresponding rate.

Figure 4.10:-Plot of catalyst concentration vs.

corresponding rate.

best fit line. [Figure 4.9 and Figure 4.10]. It is clear that the rate of this reaction is first order

to the catalyst and substrate concentration. The rate constant values (ksub as the catalyst

concentration is same in both the experiments] obtained show that this reaction is faster than

that when 16 is used as a catalyst.

As a natural progress to this work, experiments were carried out with the complexes

16* and 19-22. The rate constant (ksub) was calculated by the same procedure as above and the

maximum TON was determined. The values are listed in Table 4.7. It is to be noted that only

ksub was measured in order to compare the rate of the reaction when the different catalysts

were used. From Table 4.7 it is clear that the reaction rate is the slowest when 16 is the

catalyst, the fastest being 22.

In order to understand the mechanism, the knowledge of stoichiometry for this

reaction is important. 10 equivalents of 3,5-DTBC was reacted with 1 equivalent of 16 (the

X Ligands Mn(IV) complexes

with the ligands

Ered ( Fc+/Fc) (V) ksub x 10-4

(mole-1 min-1)

Maximum TON

( 24 hours)

H4L3 16 -1.02, -1.23, -1.63 0.9 500

H H2L5 16* -1.03 ,-1.17 1.5 48

-C(CH3)3 H

2L6 18 -0.95 , -1.31 2.8 169

-CF3 H

2L7 19 -0.68, -1.12, -1.96 4 48

-CH3 H

2L8 20 -1.12, -1.32 4.6 84

-Cl H2L9 21 -0.94, -1.20, -1.93 5.3 45

-OCH3 H

2L10 22 -1.02, -1.23 9.1 40

Table 4.7 :- Values of Ered, ksub and maximum TON obtained for different Mn(IV) complexes

used as catalyst .

[Catalyst] x 108

(moles)

Rate (r0´´´) x102

(min-1)

8 4.38

12 6.96

16 8.94

20 11.04

109

200 240 280 320 360

Simulated

Experimental

dX'

dB

B [mT]

3.2 2.8 2.4 2

280 300 320 340 360 380

g values

Experimental

Simulated

dX'

dB

B [mT]

2.4 2.3 2.2 2.1 2 1.9 1.8

a) b)

Figure 4.11:- X-band EPR spectrum of 16* in dichloromethane at 10K (a)before addition of

3,5-DTBC and(b) after the reaction of one equivalent of 3,5-DTBC in glove box.

dimer) or 16* (the monomer) (as the reaction velocity are slow for both, Table 4.7) in a glove

box for 1hour, and divided into two parts. The first portion of the aliquot was subjected to LC

study and it was found out that 2 equivalents of 3,5-DTBQ have been formed when 16 was

used as catalyst; when 16* was the catalyst, 1 equivalent of 3,5-DTBQ was formed. Upon

exposure to air, it was found out that all the substrate has reacted.

The X-band EPR spectrum of a frozen dichloromethane solution of 16* was measured

at 10K [Figure 4.11(a)]. It shows the characteristic 6 line hyperfine structure with giso =2.01 4.

When a frozen solution of the second aliquot, containing 16* as catalyst, was subjected to X-

band EPR studies at 10K, an interesting spectral feature was observed [Figure 4.11(b)]. The

spectrum now shows a peak centered at g=3.15 and another low intensity peak centered at g =

2.0. This spectrum is typical for an S =3/2 system. The spectral feature arising at g=2.0 is

probably due to minor amount of MnII.

When 16 (the dimer) was used as the catalyst, a complicated EPR signal was observed

(Figure 4.12, dotted line). The spectrum showed broad peaks between g=20 to g=4 along with

another six hyperfine signals centered at 2.0 (probably due to minor amount of MnII). This

spectrum could not be simulated due to the unavailability of corresponding simulation

program. However a meaningful interpretation is the formation of S=3/2 at each Mn center of

the dimer.

Oxygen uptake measurements were performed using 18 (18 was fully characterized

and it was assumed that the oxidative reaction mechanism, with 18 as catalyst, was same

when 16 or 16* was used) as catalyst at -25°C and at -10°C (due to high vapour pressure of

CH2Cl2). It was observed that at -25°C, equivalent amount of oxygen was needed to convert

110

80 160 240 320 400 480

dX'

dB

B [mT]

20 10 642

Figure 4.12:- X-band EPR spectrum at 10K of 16 (bold line) before addition of 3,5-DTBC

and after the reaction of two equivalents of 3,5-DTBC in glove box (dotted line).

all (0.5 mmole) 3,5-DTBC to 3,5-DTBQ (the reaction was monitored till 320 minutes due to

saturation) [Table 4.8]. However, at -10°C, half equivalent of oxygen was required in order

to convert all (0.5mmole) of 3,5-DTBC to 3,5-DTBQ (the reaction was monitored till 152

minutes due to saturation) [Table 4.9]. At -25°C, the H2O2 formed did not undergo any

decomposition to H2O and O2. When the reaction was carried out at -10°C, catalase like

Table 4.9:- O2 uptake data when 18 is

used as catalyst at –10°C.

Table 4.8:- O2 uptake data when 18 is

used as catalyst at –25°C.

Time

(mins.)

Vol. of O2

consumed (ml)

0 0

4 1.1

6 2.5

8 2.7

10 2.9

14 3.5

20 4.5

23 4.8

26 5.1

28 5.3

30 5.5

35 5.9

40 6.5

100 9.6

140 11.5

200 11.7

320 12.9

Time

(mins.)

Vol. of O2

consumed (ml)

0 0

2 1

3 1.5

10 4

25 4.4

76 4.8

109 4.9

132 4.9

152 5

111

[MnIV(2 Radicals)] + [Catechol] {[MnIV(2 Radicals)]

Electron

transfer

[MnIV(2 Phenolates) (Quinone)]

O2

- Quinone

[Catechol]}#

activity of 18 decomposed H2O2, the oxygen formed being taken up by the reaction medium.

The presence of hydrogen peroxide in the catalysis solution was confirmed by addition of

water (5ml) to the reaction mixture [5 x 10-3 mmole of catalyst was added to 500 x 10-3

mmole of 3,5-DTBC in 10ml dichloromethane], extracting the aqueous layer 3 times by

dichloromethane (20ml), adding titanyl sulphate solution to the water layer (1ml) and

measuring the UV-spectrum of the resulting solution. The absorbance peak due to the titanyl-

peroxy complex appears at 400nm. Furthermore, if a freshly prepared, slightly acidic

potassium iodide solution is added to the aqueous layer and the resulting solution is extracted

with carbon tetrachloride, a violet coloration occurs in the carbon tetrachloride layer. Blank

reactions were carried out with 18 as well as with 3,5-DTBC. No H2O2 was observed.

Therefore, the stoichiometry of the reaction at lower temperature could be written as

At room temperature the stoichiometry changes to

due to incipient decomposition of H2O2.

It seems that an outer-sphere mechanism is involved in this reaction. This is supported

by the reduction potentials of the catalysts [Table 4.7]. The EPR signal shows that the

intermediate could be the two-electron reduced form of the complex. In presence of air, the

ligands are re-oxidized to the radical form of the complex which carries out the next turnover.

This probable mechanism is shown in Figure 4.13.

Figure 4.13 :- Tentative mechanism for the oxidation of 3,5-DTBC by the radical containing

Mn-complexes.

CH2Cl2

catalyst

3,5-DTBC+ O2 3

,

5-DTB

Q

+ H2O2

CH2Cl2

catalyst

3,5-DTBC+ ½ O2 3

,

5-DTB

Q

+ H2O

112

0123

0.0

0.5

1.0

1.5

2.0

Absorbance

Time (Hours)

320 340 360 380 400 420 440 460 480

0.0

0.4

0.8

1.2

1.6

2.0

2.4

Absorbance

λ(nm)

320 340 360 380 400 420 440 460 480

0.0

0.4

0.8

1.2

1.6

2.0

2.4

Absorbance

λ(nm)

0123

0.0

0.5

1.0

1.5

2.0

Absorbance

Time (Hours)

This type of catalysis, where radicals undergoes an “on-off ” mechanism, is observed for the

first time. More complexes with polyradical species can be synthesized which can act as a

better oxidative catalyst.

4.4 OXIDATIVE STUDIES WITH 2,6-DI-TERT-BUTYL-PHENOL

When 2,6-di-tert-butylphenol (DTBP) (412mg; 2mmole) is added to 16 (17mg;

0.01mmole) in dichloromethane/methanol (1:1) solvent mixture (50ml), the dark brown color

slowly turns to red. After 48 hours, it was found that 100% of the phenol has converted to

3,3´-5,5´-Tetra-tert-butyldiphenoquinone (TTBD). A maximum turnover number of 1284 was

observed.

The reaction was monitored by eluting 50µl from the aliquot at different time

intervals, passing it through a Amberlyst-resin column to remove the catalyst and then

washing it by 10ml of dichloromethane. Electronic spectrum of these solutions were

measured. TTBD shows a characteristic peak in the electronic spectrum at 425nm. The

resultant change in electronic spectrum and the plot of absorbance vs. time is shown in Figure

4.14(a) and 4.14(b) respectively. From the plot, it could be manifested that the reaction rate is

of higher orders and more complicated than that observed for the oxidative catalysis of 3,5-

DTBC.

a) b)

Figure 4.14:-a) Electronic spectrum of the eluted solution at different time.(b) Plot of

corresponding absorbance vs. time.

OH OO

16

Air, RT

DTBP TTBD

113

References

1 (a)C. Gerdemann, C. Eicken, B. Krebs, Acc. Chem. Res. 2002, 35, 183. (b) E. I. Solomon, U. M. Sundaram,

T. E. Machonkin, Chem. Rev.,1996, 96, 2563.

2.(a) R. Wegner, M. Gottschaldt, H. Görls, E.-G. Jäger, D. Klemm, Chem. Eur. J., 2001, 7, 2143.(b) S. C.

Cheng , H.H. Wei, Inorg. Chim. Acta 2002 , 340,105 and references therein. (c) R. Than, A. A.

Feldmann, B. Krebs, Coord. Chem. Rev. 1999, 182, 211. (d) J. Reedijk, Bioinorganic Catalysis, (Ed.:

J. Reedijk), 2nd ed., Dekker, New York 1998. (e) J. Reim, B. Krebs, J. Chem. Soc. Dalton Trans. 1997,

3793. f) E. Monzani, G. Battaini, A. Perotti, L. Casella, M. Gullotti, L. Santagostini, G. Nardin, L.

Randaccio, S. Geremia, P. Zanello, G. Opromella, Inorg. Chem. 1999, 38, 5359 . g) E. Monzani,

L. Quinti, A. Perotti, L. Casella, M. Gullotti, L. Randaccio, S. Geremia, G. Nardin, P. Faleschini, G. Tabbi,

Inorg. Chem. 1998, 37, 553. h) L. Casella, E. Monzani, M. Gullotti, D. Cavagnino, G. Cerina, L.

Santagostini, R. Ugo, Inorg. Chem. 1996, 35, 7516. i) F. Thaler, C. D. Hubbard, F. W. Heinemann, R. van

Eldik, S. Schindler, I. Fabian, A. M. Dittler-Klingemann, F. E. Hahn, C. Orvig, Inorg. Chem. 1998, 37,

4022. j) J. Mukherjee, R. Mukherjee , Inorg. Chim. Acta 2002, 337, 429. k) R. Wegner, M. Gottschaldt,

H.Görls, E.-G. Jäger, D. Klemm, Angew. Chem. Int. Ed. 2000, 39. l) M.R. Malachowski, H.B. Huynh, L.J.

Tomlinson, R.S. Kelly, J.W. Furbee, jun. J. Chem. Soc., Dalton Trans. 1995, 31. m) M.R. Malachowski, J.

Carden, M.G. Davidson, W.L. Driessen and J. Reedijk. Inorg. Chim Acta 1997, 257, 59. n) M.R.

Malachowski, B.T. Dorsey, M.J. Parker, M.E. Adams and R.S. Kelly. Polyhedron 1998, 17, 1289.

3. N. Oishi, Y. Nishida, K. Ida, S. Kida, Bull. Chem. Soc. Jpn. 1980, 53, 2847.

4. H. Chun, P. Chaudhuri, T. Weyhermüller, K. Wieghardt, Inorg. Chem., 2002, 41, 790

5. S. Mukherjee, E. Rentschler, T. Weyhermüller, K. Wieghardt , P. Chaudhuri, Chem. Comm., 2003, 1828.

114

115

Chapter 5

FeIII AND CoIII COMPLEXES WITH IMINO-BENZOSEMIQUINONE

LIGANDS; EFFECT OF SUBSTITUTION

N

H

OH X

116

117

5.1 INTRODUCTION

The existence of imino-benzosemiquinone radical complexes is now well established

and quite a few structurally as well as spectroscopically characterized metal complexes

containing the imino-benzosemiquinone radicals have been synthesized [Chapter 3]. The

parent ligand 2-anilino-4,6-ditert-butylphenol was used in synthesizing complexes with 3d-

metal ions. The octahedral iron complex [FeIII(L5)3] (24*) possesses an S=1 ground state

comprising a high-spin FeIII ion (SFe=5/2) coupled antiferromagnetically to three imino-

benzosemiquinone π-radical ligands.1a This ligand has also been used in synthesizing Fe(III)

complexes where the ground state of the iron centers are low spin (LS, SFe= ½)1b, intermediate

spin (IS, SFe=3/2)1c or high spin (HS, SFe= 5/2)1a. The complex with intermediate FeIII is five

coordinated with two ligands and an iodide at the fifth position.1b The O,N-donor atoms from

each of the ligand occupy the equatorial position with the iodide group in the axial position.

On changing the axial halogen group, the spin state of the iron center changes from

intermediate spin to high spin. This spin tuning was achieved by the position of the halides in

the spectrochemical series.

In this chapter, changing the substituent groups at the meta- or para- position of the

aniline ring in the ligand 2-anilino-4,6-ditert-butylphenol results in the formation of Fe(III)

complexes with different structural and magnetic property, is discussed.

5.2 SYNTHESIS AND CHARACTERIZATION OF LIGANDS

Refluxing 3,5-ditert-butylcatechol and the respective anilines(1:1) in heptane in the

presence of triethylamine as base resulted in the formation of the ligands H2L6, H2L11 and

H2L12. The IR spectroscopy of the ligands shows characteristic peaks for –OH and –NH

stretch from 3500-3200 cm-1 along with the typical peaks for –C-H and –C-N stretch. The

characteristic IR spectroscopy peaks are given in Table 5.1. The ligands are all characterized

by mass spectroscopy in EI-mode[Table 5.1]. NMR spectrum (experimental section, Chapter

7) clearly shows that the number of non-exchangeable hydrogen atoms corroborates to that of

the ligand.

118

N

H

OH

N

H

F

OH

N

H

Table 5.1 :- Ligands synthesized and their characterization.

Ligand Ligand

structure

IR peaks (cm-1) EI-MS

(Molecular peak) m/z

H2L6

3339s, 1598s, 1361, 1233s, 999m, 774m, 631b. 409

H2L11

3440s,3363s,1628s,1598s,1478s,1227m,1000s,

830s, 667m.

333

H2L12

3356m,1614s,1515s,1315m,1232m,825m, 765m. 353

5.3 SYNTHESIS AND CHARACTERIZATION OF COMPLEXES

The iron complex of H2L6 was synthesized by refluxing the ligand with

[FeII(H2O)6](ClO4)2 in methanol in the presence of triethylamine as base. Deep green

microcrystalline precipitate was obtained in moderate yield and repeated recrystallization

from a diethylether-acetonitrile solvent mixture resulted in X-Ray quality crystals of the

monomer 24. With this ligand the cobalt(III) complex, 23, was prepared in order to

understand the nature of interactions between the radical centers. The cobalt salt used was

[CoII(H2O)6](ClO4)2 and refluxing with H2L6 in the presence of NEt3 in acetonitrile yields

dark brown microcrystalline precipitate of 23. Recrystallization from a diethylether-

acetonitrile solvent mixture affords X-ray quality crystals of 23. The iron complexes, 25 and

26 using H2L11 and H2L12 as ligand, respectively, were synthesized using the same procedure

that for 24. Recrystallization from dichloromethane-acetonitrile solvent mixture gave

crystalline material of 25 and 26. X-ray quality crystals of 25 were obtained from a saturated

acetone solution. 26 was found to be a µ-oxo bridged dimeric FeIII complex with radical

containing ligand.

All these complexes have been characterized by various spectroscopic techniques viz.

IR, UV-Vis and MS in EI as well as ESI mode. The Infrared spectroscopy of all the

complexes shows the absence of the –OH and –NH peaks and appearance of ν(CN) bands

between 1615 to 1580 cm-1. Interestingly, for 25, this band is split into two sharp bands at

119

O(61)

O(31)

O(1)

N(67)

N(37)

N(7)

Co(1)

C(1)

C(6)

C(5)

C(4)

C(3)

C(2)

C(8)

C(13)

C(12)

C(11)

C(10)

C(9)

.

Table 5.2:- Characteristic IR and EI-MS peaks of the complexes 23-26.

Ligand Complex IR peaks (cm-1) EI-MS (m/z)

H2L6 Co(L6)3 (23) 1578,1433,1361,1247,707 1280

H2L6 Fe(L6)3 (24) 1581,1468,1362,1247,945 1277

H2L11 Fe(L11)3(25) 1612,1591,1468,1116,990 1049

H2L12 [{Fe(L12)2}2O] (26) 1466,1424,1256,842,557 1532

1612 cm-1 and 1591cm-1. The peak due to Fe-O-Fe asymmetric stretch, for 26, appears at 842

cm-1 and fits well in the plot of asymmetric stretch frequency vs. the Fe-O-Fe angle.3 Mass

spectroscopy for all the complexes shows the molecular peak in the EI–mode. Selected IR and

MS peaks for the complexes 23-26 are given in Table 5.2.

CoIIIL6

3 (23)

The single crystal X-ray structure of 23 (Figure 5.1) at 100K shows that the first

coordination sphere of cobalt has a C2 axis passing through O(61)–Co(1)–N(7); the ligands

have lost their aminohydrogen atoms. The structure determination unambiguously shows that

cobalt is hexa-coordinated to three deprotonated ligands. The distances C2–C3, C4–C5, C32–

C33, C34–C25, C62–C63 and C64–C65 are significantly shorter than the other C–C distances

in the original phenol ring[Table 5.3]. Correspondingly, the imino C-N bonds at 1.346 ±

0.006Å are shorter than the C–N bonds to the aniline rings, 1.423 ± 0.006 Å along with the

shortened C–O bond distances, 1.30 ± 0.006 Å (average) each of which contains an

iminosemiquinone radical anion. The average metal-to-donor atom distance indicates a formal

+3 oxidation state at the cobalt centre.

Figure 5.1:- Crystal structure of 23.

120

Co(1)-O(31) 1.8754(11)

Co(1)-O(1) 1.8818(11)

Co(1)-O(61) 1.8896(11)

Co(1)-N(37) 1.9118(14)

Co(1)-N(7) 1.9177(13)

Co(1)-N(67) 1.9389(14)

O(1)-C(1) 1.3007(19)

O(31)-C(31) 1.3020(19)

O(61)-C(61) 1.296(2)

C(6)-N(7) 1.346(2)

C(36)-N(37) 1.344(2)

C(66)-N(67) 1.347(2)

C(1)-C(2) 1.423(2)

C(1)-C(6) 1.438(2)

C(2)-C(3) 1.375(2)

C(3)-C(4) 1.430(2)

C(4)-C(5) 1.369(2)

C(5)-C(6) 1.415(2)

O(31)-Co(1)-O(1) 178.65(5)

O(31)-Co(1)-O(61) 90.19(5)

O(1)-Co(1)-O(61) 89.64(5)

O(31)-Co(1)-N(37) 84.68(5)

O(1)-Co(1)-N(37) 96.64(5)

O(61)-Co(1)-N(37) 85.28(5)

O(31)-Co(1)-N(7) 95.58(5)

O(1)-Co(1)-N(7) 84.69(5)

O(61)-Co(1)-N(7) 173.17(5)

N(37)-Co(1)-N(7) 91.59(6)

O(31)-Co(1)-N(67) 85.39(5)

O(1)-Co(1)-N(67) 93.26(5)

O(61)-Co(1)-N(67) 83.86(5)

N(37)-Co(1)-N(67) 165.24(6)

N(7)-Co(1)-N(67) 100.22(6)

Table 5.3 :- Selected bond distances (Å) and angles (degree) of 23.

Such type of electron density distribution in ortho-iminosemiquinone has been discussed

previously [Figure 3.1, Chapter 3]. Therefore, this complex consists of three imino-

benzosemiquinone radicals.

Magnetic data (SQUID) with H = 1 T for a polycrystalline sample of 23 are displayed

in Figure 5.2 as µeff vs. T. On lowering the temperature, µeff (3.20µB at 290 K) increases

monotonically approaching a maximum around 30K with a value of 3.76µB, which is close to

the spin-only value for St = 3/2, expected as the ground state for three ferromagnetically

coupled iminosemiquinone radicals. Below 15 K there is a decrease in µeff, which reaches a

value of 2.79µB at 2 K due to saturation effects and/or intermolecular antiferromagnetic

interactions. The experimental data could not be fitted with only one coupling constant (J) as

satisfactorily as with two ‘J’ values. Two exchange coupling constants have to be considered

for the simulation based on the Hamiltonian

)SS(J2)SSSS(J2H

ˆ31133221

rrrrrr −+−=

with S1 = S2 = S3 = ½ and the best fit shown as the solid line in Figure 5.2 yields J = J12 = J23

= +26.7 cm-1, J13 = +53.2 cm-1, g1 = g2 = g3 = 2.0 (fixed) and a Theta-Weiss parameter (Θ) of

–0.908K . Thus the quartet ground state is separated from the first doublet state by ~ 80 cm-1

owing to the exchange interactions. The coupling constants, J and J13, of 23* (mononuclear

CoIII complex with the ligand 2-anilino-4,6-ditert-butylphenol) were found to be +9.0 cm-1

and +59.5cm-1, respectively. Here the quartet ground state is separated from the first doublet

state by 27 cm-1.1d

121

S

R

2

S

R

3

S

R

1

J = +26.7 cm

-1

J = +26.7 cm

-1

J = +53.2 cm

13

-1

50 100 150 200 250

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Simulated

Experiment

µ eff / µ B

T/K

400 600 800 1000 1200

0

1

2

3

ε x 10 - 4 (M -1 cm -1)

λ (nm)

0.5 0.0 -0.5 -1.0 -1.5

410 nA

E (V) vs. Fc+ / Fc

b)

a)

Figure 5.2 :- (a) Magnetic data of 23. (b) Spin coupling model used in simulating the data.

Optical spectrum of 23 in CH2Cl2 consist of a series of intense bands and shoulders

[Figure 5.3(a)]; in particular two intense absorptions at low energies are noteworthy: λ/nm

(ε/M-1 cm-1) 883(7559) and 661(6916). A peak with weaker intensity at 477 nm (ε = 5716 M-1

cm-1) is assigned to the quinone to metal charge transfer band. The intensity suggests that

allowed electronic transitions are the ligand-to-ligand π-π* charge transfer bands, which has

also been observed with 23*( mononuclear CoIII complex with the ligand 2-anilino-4,6-ditert-

butylphenol)1d that dominate the spectra.

Cyclic voltammetric experiments at high scan rates exhibit three reversible one-

electron redox waves for 23 in CH2Cl2 [0.1M (NBu)4PF6]: E 1 ½ = 0.225, E 2 ½ = - 0.386, and

E 3 ½ = -1.464 V vs. Fc+/Fc [Figure 5.3(b)]. The first two redox potentials are associated with

two ligand centered oxidation processes and E 3 ½ is assigned to the Co(III)–Co(II) couple.

a) b)

Figure 5.3:- a) Electronic spectrum of 23. b) Cyclic voltammogram of 23.

122

C(1)

C(13) C(12)

C(11)

C(10)

C(9)

C(8)

C(6)

C(5)

C(4)

C(3)

C(2)

N(7)

N(67) N(37)

O(1)

O(61)

O(31)

Fe(1)

FeIIIL6

3 (24)

Dark green single crystals of 24, obtained from a diethylether-acetonitrile solvent

mixture, was subjected to X-Ray diffractometric studies at 100K (as well as at 293K for

reasons mentioned later) and it was found that the complex was iso-structural with the cobalt

complex (23) [Figure 5.4]. A C2 axis passes through O(1)-Fe(1)-N(37). Here again, it was

observed that the three phenyl rings containing the tert-butyl as substituents have lost their

aromaticity with the presence of two long and four short C-C bond distances for each of these

rings. The C-N as well as the C-O bond distances have shortened (Average C-O bond distance

is 1.30Å; average C-N bond distance is 1.351 Å); the three rings adopts a quinoid-type

structure. Thus the complex consists of three imino-benzosemiquinone radicals. Selected

bond distances are given in Table 5.4.

The most interesting structural feature of this complex is the metal-to-donor bond

distances. The average Fe-O and Fe-N bond distances are 1.904 and 1.922 Å, respectively

which are much shorter than the reported iron(III) complex with the ligand 2-anilino-4,6-

ditertbutylcatechol.1a Thus, the iron in 24 has a oxidation state of +3 but probably not in a

high spin state. In order to discern the spin state, magnetic susceptibility measurements as

well as Mössbauer spectroscopy were performed with this complex.

Figure 5.4:- Crystal structure of 24.

123

Table 5.4:- Selected bond distances(Å) and angles (degree) of 24 at 100K and 293K.

100K 293K

Fe(1)-O(31) 1.8869(11)

Fe(1)-O(61) 1.9013(11)

Fe(1)-N(37) 1.9083(13)

Fe(1)-N(67) 1.9144(13)

Fe(1)-O(1) 1.9228(11)

Fe(1)-N(7) 1.9429(13)

O(1)-C(1) 1.2950(19)

C(6)-N(7) 1.352(2)

C(1)-C(2) 1.429(2)

C(1)-C(6) 1.442(2)

C(2)-C(3) 1.381(2)

C(3)-C(4) 1.426(2)

C(4)-C(5) 1.373(2)

C(5)-C(6) 1.421(2)

O(31)-Fe(1)-O(61) 178.55(5)

O(31)-Fe(1)-N(37) 83.42(5)

O(61)-Fe(1)-N(37) 95.81(5)

O(31)-Fe(1)-N(67) 95.94(5)

O(61)-Fe(1)-N(67) 82.91(5)

N(37)-Fe(1)-N(67) 96.29(6)

O(31)-Fe(1)-O(1) 89.74(5)

O(61)-Fe(1)-O(1) 91.03(5)

N(37)-Fe(1)-O(1) 173.16(5)

N(67)-Fe(1)-O(1) 84.54(5)

O(31)-Fe(1)-N(7) 95.86(5)

O(61)-Fe(1)-N(7) 85.47(5)

N(37)-Fe(1)-N(7) 98.62(6)

N(67)-Fe(1)-N(7) 161.92(5)

O(1)-Fe(1)-N(7) 81.87(5)

Fe(1)-O(31) 1.987(2)

Fe(1)-O(61) 1.995(2)

Fe(1)-N(37) 2.067(3)

Fe(1)-N(67) 2.078(3)

Fe(1)-O(1) 2.006(2)

Fe(1)-N(7) 2.079(3)

O(1)-C(1) 1.287(4)

C(6)-N(7) 1.336(4)

C(1)-C(2) 1.423(5)

C(1)-C(6) 1.453(5)

C(2)-C(3) 1.368(5)

C(3)-C(4) 1.420(5)

C(4)-C(5) 1.355(5)

C(5)-C(6) 1.416(5)

O(31)-Fe(1)-O(61) 173.53(10)

O(31)-Fe(1)-N(37) 78.29(10)

O(61)-Fe(1)-N(37) 96.04(10)

O(31)-Fe(1)-N(67) 99.17(10)

O(61)-Fe(1)-N(67) 78.08(10)

N(37)-Fe(1)-N(67) 95.08(11)

O(31)-Fe(1)-O(1) 91.88(10)

O(61)-Fe(1)-O(1) 93.76(10)

N(37)-Fe(1)-O(1) 170.16(10)

N(67)-Fe(1)-O(1) 86.04(10)

O(31)-Fe(1)-N(7) 97.95(10)

O(61)-Fe(1)-N(7) 86.33(10)

N(37)-Fe(1)-N(7) 103.05(11)

N(67)-Fe(1)-N(7) 156.90(10)

O(1)-Fe(1)-N(7) 78.01(10)

Figure 5.5 shows the magnetic data of a powdered sample of 24 from 2 to 290K. The

value of µeff at 290K (3.017 µB) decreases monotonously till 260 K (2.89 µB). A sharp

decrease was then observed till 140 K (1.23µB) and then again a monotonous decrease till 2K

(0.64 µB). The nature of the curve clearly shows that spin transition occurs on raising or

lowering the temperature. This spin transition occurs rather rapidly between the temperature

range of 140K to 260K .The value of µeff at low temperatures corroborated to a St (total

spin)=0. The residual magnetic moment at 2K is probably due to the presence of a

temperature independent paramagnetism(TIP) (Mössbauer spectroscopy fortifies the purity of

24). The value at 290K shows that at room temperature St=1. The diamagnetic ground state of

this complex can be explained by the antiferromagnetic coupling of one radical center with

the low spin iron center (SFe=½) and very weak anti-ferromagnetic coupling between the two

residual radical centers. At higher temperature, the iron center changes its spin state to high

spin (SFe= 5/2) and its strong anti-ferrromagnetic interactions with the radicals renders a total

spin of St = 1. No hysteresis effect was observed [Figure 5.5].

124

50 100 150 200 250 300

0.5

1.0

1.5

2.0

2.5

3.0

µ eff / µ B

T/K

Figure 5.5 :- Magnetic data of 24. Filled and open squares indicates the values of µeff on

increasing and decreasing the temperature, respectively.

In order to establish that the iron(III) center in 24 has a high spin character, X-Ray

diffratometric studies were also carried out at 293K. It was observed that all the three ligands

retain their imino-benzosemiquinone character. However the average metal-to-donor bond

distance increases; average Fe-O bond distance is 1.996Å and average Fe-N bond distance is

2.075Å. The bond distances are comparable to high spin Fe(III) complex synthesized with the

ligand 2-anilino-4,6-ditertbutylcatechol. Thus, the complex at room temperature consists of a

high spin Fe(III) (SFe = 5/2 ) with three radicals (SR = ½). At temperature below ~140K, the

iron center is in an low spin ground (S = 1/2) with three imino-benzosemiquinone radicals.

Table 5.5 compares the iron-to-donor bond distances for this complex at 100K and 293K and

for the iron complex with the ligand 2-anilino-4,6-ditertbutylcatechol (24*).

Mössbauer spectroscopy was also carried out with 24 both at 80K and at 297K

(Figure 5.6(a) and(b)). At 80K, the doublet obtained could be simulated by using an isomer

shift (IS) value of 0.13 mms-1 and a quadrupole split (∆EQ) of 1.37 mms-1 indicating a low

spin ground state for iron. At 297K , the isomer shift value changes to 0.391mms-1 and ∆EQ

changes to 0.917mms-1. The value of isomer shift and ∆EQ at room temperature shows that the

Fe(III) center in 24 is in a high spin state.

Table 5.5 :- Average Fe-O and Fe-N bond lengths (Å) for 24 at 100K and 293K and

corresponding average bond length of 24*.

24 at 100K 24 at 293K 24*( iron complex with the ligand

2-anilino-4,6-ditert- butylcatechol) at 100K

Fe-O 1.904 1.996 2.014

Fe-N 1.922 2.075 2.099

125

400 600 800 1000 1200

0

1

2

3

ε x 10 - 4 ( M -1 cm -1)

λ (nm)

0.5 0.0 -0.5 -1.0 -1.5

2.0 V/s

1.0 V/s

0.5 V/s

50 nA

E (V) vs. Fc+ /Fc

-4 -2 0 2 4

0.96

0.98

1.00

Simulated

Experimental

Relative Transmission

Velocity (mm s-1)

-4 -2 0 2 4

0.996

0.998

1.000

Simulated

Experimental

Relative Transmission

Velocity (mm s-1)

a) b)

Figure 5.6:- Mössbauer spectrum of 24 at a) 80K and b) 297K.

Electronic spectrum of 24 in a dichloromethane solution at room temperature is shown

in Figure 5.7(a). Absorption maxima were obtained at 750nm (ε = 9217 M-1cm-1) and 441nm

(ε =6571 M-1cm-1) and the spectrum is similar to that obtained for 24*.

The CV of a dichloromethane solution of 24 in the presence of 0.1M TBAPF6, shown

in Figure 5.7 (b), has been recorded at fast scan rates because the reduced form is quite labile.

The CV is essentially identical to that of 24*- ( iron complex with the ligand 2-anilino-4,6-

ditertbutylcatechol) the oxidation potentials are +0.294V and -0.422V and the reduction

potential is at –1.293V vs. Fc+/Fc. All these redox-processes comprise a single electron

transfer as witnessed by coulometric studies.

It is probable that steric reasons are responsible for this behavior of 24. As a natural

progress, the iron complexes with the other two ligands, where the tert-butyl group has been

replaced by the fluro-substituent (H2L11) and where the tert-butyl group is now at the para-

position (H2L12), were synthesized.

a) b)

Figure 5.7 :- a) Electronic spectra of 24. b) Cyclic voltammogram of 24 at fast scan rates.

126

C(1)

C(2)

C(3)

C(4)

C(5)

C(6)

N(37)

N(7)

O(61)

C(8)

C(9)

C(10)

C(11)

C(12)

C(13)

F(22) F(23)

O(1) O(31)

N(67)

Fe(1)

Fe(1)-O(31) 1.9932(8)

Fe(1)-O(61) 2.0008(8)

Fe(1)-O(1) 2.0368(8)

Fe(1)-N(67) 2.0692(9)

Fe(1)-N(7) 2.0934(10)

Fe(1)-N(37) 2.0956(9)

O(1)-C(1) 1.2841(13)

C(6)-N(7) 1.3391(14)

O(31)-C(31) 1.2874(13)

C(36)-N(37) 1.3347(14)

O(61)-C(61) 1.2892(13)

C(66)-N(67) 1.3406(13)

C(1)-C(2) 1.4371(14)

C(1)-C(6) 1.4564(14)

C(2)-C(3) 1.3753(15)

C(3)-C(4) 1.4339(15)

C(4)-C(5) 1.3694(15)

C(5)-C(6) 1.4230(15)

C(8)-C(9) 1.3924(18)

C(8)-C(13) 1.3951(17)

C(9)-C(10) 1.3864(17)

C(10)-F(22) 1.3524(18)

C(10)-C(11) 1.371(2)

C(11)-C(12) 1.376(2)

C(12)-F(23) 1.3508(17)

C(12)-C(13) 1.3883(18)

O(31)-Fe(1)-O(61) 108.25(3)

O(31)-Fe(1)-O(1) 82.40(3)

O(61)-Fe(1)-O(1) 168.92(3)

O(31)-Fe(1)-N(67) 89.52(3)

O(61)-Fe(1)-N(67) 77.87(3)

O(1)-Fe(1)-N(67) 105.89(3)

O(31)-Fe(1)-N(7) 157.74(3)

O(61)-Fe(1)-N(7) 92.73(4)

O(1)-Fe(1)-N(7) 76.33(3)

N(67)-Fe(1)-N(7) 102.45(4)

O(31)-Fe(1)-N(37) 77.38(4)

O(61)-Fe(1)-N(37) 91.05(3)

O(1)-Fe(1)-N(37) 88.28(3)

N(67)-Fe(1)-N(37) 159.39(4)

N(7)-Fe(1)-N(37) 95.31(4)

FeIIIL11

3 (25)

The crystal structure of 25 at 100K is shown in Figure 5.8. A C2 axis passes through

O(31)-Fe(1)-N(7) and the donor atoms are in a meridional coordination sphere. Here again, it

was observed that the three phenyl rings containing the tert-butyl as substituents have lost

their aromaticity with the presence of two long and four short C-C bond distances for each of

these rings. The C-N as well as the C-O bond distances have shortened (Average C-O bond

distance is 1.30Å; average C-N bond distance is 1.351 Å); the three rings adopts a quinoid-

type structure. Thus the complex consists of three imino-benzosemiquinone radicals. The

Figure 5.8:- Crystal Structure of 25.

Table 5.6:- Selected bond distances(Å) and angles (degree) of 25 at 100K .

127

-4 -2 0 2 4

0.90

0.92

0.94

0.96

0.98

1.00

Simulated

Experiment

Relative Transmission

Velocity (mm s-1)

50 100 150 200 250

1.5

1.8

2.1

2.4

2.7

3.0

Simulated

Experiment

µ eff / µ B

T / K

average Fe-O and Fe-N bond distances are 2.010Å and 2.086Å which correspond to a high

spin Fe(III) center (Table 5.6). Magnetic susceptibility measurements and Mössbauer

spectroscopy supports this assignment. Selected bond distances are given in Table 5.6.

The iron containing species 25 contains a high-spin ferric ion (d5) as was clearly

established by its zero-field Mössbauer spectrum recorded at 80 K [Figure 5.9(a)]. An isomer

shift, δ, of 0.545 mms-1 and a quadrupole splitting parameter, Q

E∆, of 1.035 mms-1 are

diagnostic for octahedral high-spin ferric species.

Figure 5.9(b) displays the temperature dependence of the effective magnetic moments,

µeff , of 25. Complex 25 possesses an S =1 ground state. In the temperature range 30-200 K,

25 display nearly temperature-independent spin-only value of 2.80-2.84 µB for S =1. Above

200 K the magnetic moments increase monotonically: At 290 K the magnetic moment is 4%

larger than that at 200 K. Strong intramolecular antiferromagnetic coupling between a high-

spin ferric ion (S = 5/2) with three organic radical ligands (S=½) prevails yielding the

observed ground states of S = 1. It is possible to fit this temperature dependence by using

antiferromagnetic coupling constants for the coupling between the paramagnetic metal ion

and three semiquinonate ligand radicals. The fits in Figure 5.9(b) was obtained by using the

following parameters: J12= J13 = J14 = -184 cm-1, g = 2.0 (fixed) and a Theta-Weiss parameter

(θ) of –0.8K.

Electronic spectra of 25 , in a dichloromethane solution at room temperature is shown

in Figure 5.10(a). Absorption maximas were obtained at 745nm (ε = 9260 M-1cm-1) and

435nm (ε =7015 M-1cm-1) and the spectrum is very similar to that obtained for 24.

The CV of the ferric complex 25 in dichloromethane, in the presence of 0.1M

TBAPF6 is shown in Figure 5.10(b). The oxidation potentials are at +0.531V and -0.103V and

the reduction potential is at –0.874V vs. Fc+/Fc. The nature of the voltammogram is similar to

a) b)

Figure 5.9:- (a) Mössbauer spectroscopy of 25. b) Magnetic data of 25.

128

400 600 800 1000 1200

0

1

2

3

ε x 10 - 4 ( M -1 cm -1)

λ (nm)

0.6 0.3 0.0 -0.3 -0.6 -0.9

15 µA

E (V) vs. Fc+ /Fc

a) b)

Figure 5.10:-a)Electronic spectra of 25. b) Cyclic voltammogram at different scan rates of 25.

that of 24- however a shift of the waves to more positive values is observed for all three

processes. In 25, due to the strong -I effect of the fluro-group, the radical becomes less viable

for oxidation to the quinone form and more viable for reduction to amino-phenolate. A fourth

irreversible wave at -1.268 V (not shown) may involve a metal-centered reduction generating

the FeII species.

[(FeIIIL12

2)2(µ-O)] (26)

Figure 5.11 shows the structure at 100K of a neutral complex in crystals of

262.75CH2Cl2.4 Both ferric ions are coordinated to two o-imino benzosemiquinonate(1-) π

radical anions. This is conclusively deduced from the average C-N, C-C, and C-O distances

which are identical within experimental error in each of the four ligands. They are the same as

observed for the above iron complexes. The two halves FeIII(LISQ)2 in dinuclear 26 are bridged

by a nearly linear oxo group (Fe-O-Fe 175.2(2)o) which is substantiated by the presence of

asymmetric stretch peak in the IR at 842 cm-1. This value fits well in the asymmetric stretch

frequency vs. Fe-O-Fe angle plot from literature.3 The two Fe-Ooxo distances are short at

1.775(2) Å and indicative of a strongly covalent bond with double bond character. This

bonding situation has been observed in many dinuclear µ-oxo bridged complexes containing

two high spin ferric ions (SFe = 5/2).3c Table 5.7 summarizes Fe-O and Fe-N bond lengths in

26. The average Fe-Orad distance is 1.964 Å whereas the average Fe-N bond length is 2.061 Å.

These data are in excellent agreement with the notion that complex 26 contain high spin ferric

ions. Complex 26 provides the first example of a structurally characterized µ-oxo(diferric)

complex containing four organic ligand radicals.

129

Fe(1)-O(40) 1.776(2)

Fe(1)-O(31) 1.968(2)

Fe(1)-O(1) 1.970(2)

Fe(1)-N(37) 2.051(3)

Fe(1)-N(7) 2.057(3)

Fe(2)-O(40) 1.775(2)

Fe(2)-O(131) 1.958(2)

Fe(2)-O(101) 1.961(2)

Fe(2)-N(107) 2.066(3)

Fe(2)-N(137) 2.069(3)

O(1)-C(1) 1.296(4)

C(6)-N(7) 1.348(4)

O(31)-C(31) 1.300(4)

C(36)-N(37) 1.340(4)

O(101)-C(101) 1.299(4)

C(106)-N(107) 1.349(4)

O(131)-C(131) 1.292(4)

C(136)-N(137) 1.345(4)

C(1)-C(2) 1.434(4)

C(5)-C(6) 1.422(4)

C(1)-C(6) 1.447(5)

C(2)-C(3) 1.375(5)

C(3)-C(4) 1.426(5)

C(4)-C(5) 1.374(5)

Fe(2)-O(40)-Fe(1) 175.21(15)

O(40)-Fe(1)-O(31) 112.40(10)

O(40)-Fe(1)-O(1) 110.92(10)

O(31)-Fe(1)-O(1) 136.68(10)

O(40)-Fe(1)-N(37) 106.84(11)

O(31)-Fe(1)-N(37) 78.84(10)

O(1)-Fe(1)-N(37) 88.94(10)

O(40)-Fe(1)-N(7) 105.10(10)

O(31)-Fe(1)-N(7) 90.04(10)

O(1)-Fe(1)-N(7) 78.76(10)

N(37)-Fe(1)-N(7) 148.04(11)

C(1)

C(2)

C(6)

C(5)

C(4)

C(3)

O(1)

O(31)

N(37) N(7)

Fe(1)

C(13) C(12)

C(11)

C(10)

C(9)

C(8)

O(40)

Fe(2)

N(107) N(137)

O(101)

O(131)

a) b)

Figure 5.11:-a)Molecular structure of 26. b) Bonding environment at the Fe(III) centers in 26.

Table 5.7:- Selected bond distances(Å) and angles (degree) of 26.

The notion of the presence of high spin ferric ions in 26 is corroborated by their

Mössbauer spectra at 80 K . From simulation of this spectra, an isomer shift, δ, of 0.38 mms-1

and quadrupole splitting, ∆EQ, of 1.105 mms-1 [Figure 5.12(a)] was obtained. The fact that the

isomer shift decreases from 0.545 mms-1 for octahedral 25 to 0.38 mms-1 for five-coordinate

26 is rationalized by an increasingly covalent character of the iron-to-ligand bonds; in 26 it is

dominated by the covalently bound oxo bridge.

Figure 5.12(b) exhibits the temperature dependence of the magnetic moment of

dinuclear 26 in the range 2 - 290 K. It has been possible to fit the behavior successfully by

using a model which invokes very strong intramolecular antiferromagnetic coupling between

high spin ferric ions (SFe= 5/2) and - in each case - two o-iminosemiquinonate π radicals

N(37) O(31)

O(1)

N(7)

Fe(1)

O(40)

Fe(2)

N(107)

O(131)

N(137)

O(101)

130

300 600 900 1200

0

2

4

ε x 10 - 4 ( M -1 cm -1 )

λ (nm)

600 300 0 -300 -600

2 µA

E (V) vs. Fc+ /Fc

50 100 150 200 250 300

0.0

0.5

1.0

1.5

2.0

2.5

Simulated

Experimental

738

cm -1

738

cm -1

E

0

2J

6J

12J

20J

30J

S = 0

S = 1

S = 2

S = 3

S = 4

S = 5

µ eff / µ B

T / K

-4 -2 0 2 4

0.94

0.96

0.98

1.00

Simulated

Experimental

Relative Transmission

Velocity ( mm s -1)

a) b)

Figure 5.12:- a) Mössbauer spectrum of 26. b) Magnetic data of 26 (inset: Energy levels of

the spin states).

yielding a fictitious S* = 3/2 state at each iron ion. The two halves of 26 are then coupled

through the µ-oxo group. Thus, using the spin Hamiltonian H= -2J S1S2 (S1 = S2 = 3/2), yields

the following parameters: J = - 123(5) cm-1, g = 2.0 (fixed). A mononuclear paramagnetic

impurity of 0.6% (S = 5/2) was included. Thus, a moderately strong antiferromagnetic

coupling between the two halves in 26 prevails yielding the observed singlet ground state.

That the S = 3 state remains depopulated even at the highest temperature of the

susceptibility measurement and only upto the S =2 state is populated is established by

simulating the magnetic data considering another fictitious S* = 1 (St = 0, 1, 2) state at each

iron centre. A good fit can be obtained (not shown). However, if S* = ½ is considered at each

iron center (St = 0,1), no reasonable fit was obtained. Thus population occurs till S = 2, which

is separated at 738 cm-1 from the singlet state [Figure 5.12(b) (Inset)].

a) b)

Figure 5.13 :- a) Electronic spectra of 26. b) Square wave voltammogram of 26.

131

Figure 5.13(a) displays the electronic spectrum of 26 in CH2Cl2 solution. Interestingly,

the spectrum is very similar to 24 and 25 and indicates the presence of FeIII(LISQ)x units (ISQ-

imino-benzosemiquinone ; x = 2 or 3).

A few electrochemical experiments with a dichloromethane solution of 26 in presence

of 0.1M TBAPF6 were performed. The square wave voltammogram [Figure 5.13(b)] shows

two reversible oxidation peaks at –0.26V, +0.153V and broad peaks at 0.512V and 0.617V vs.

Fc+/Fc. All these processes can be attributed to ligand centered radical oxidations. An

irreversible peak (not shown) appears at –1.219V vs. Fc+/Fc and is ascribed to the metal

centered reduction.

In this chapter, the aniline ring of the ligand 2-anilino-4,6-ditert-butylphenol is

substituted at different position by different groups and the synthesis and characterization of

transition metal complexes is discussed. Although the difference in structural as well as

spectroscopic property could be attributed to the stereochemical effects, electronic effects

could also play a significant role for such a behavior. Theoretical study is needed in order to

explain the results.

References

1. a)H. Chun, C. N. Verani, P. Chaudhuri, E. Bothe, E. Bill, T. Weyhermüller, K. Wieghardt, Inorg. Chem.,

2001, 40, 4157. b) K. S. Min, T. Weyhermüller ,K. Wieghardt, Dalton Trans., 2003, 1126. c) H. Chun, T.

Weyhermüller, E. Bill, K. Wieghardt, Angew. Chem.,Int. Ed., 2001, 40, 2489.d) C. N. Verani, S. Gallert, E.

Bill, T. Weyhermüller, K. Wieghardt ,P. Chaudhuri, Chem. Commun.,1999,1747.

2. a) D. A. Summerville, I. A. Cohen, K. Hatano, W. R. Scheidt, Inorg. Chem. 1978, 17, 2906.b) W. R. Scheidt,

C. A. Reed, Chem. Rev. 1981, 81, 543; c) M. M. Maltempo, J. Chem. Phys. 1974, 61, 2540; d) M. E.

Kastner, W. R. Scheidt, T. Mashiko, C. A. Reed, J. Am. Chem. Soc. 1978, 100, 666; e) P. Gans, G.

Buisson, E. Duée, J.-R. Regnard, J.-C. Marchon, J. Chem. Soc. Chem. Commun. 1979, 393.

3. a)J. Sanders-Loehr, W. D. Wheeler, A. K. Shiemke, B. A. Averill, T. M. Loehr, J. Am. Chem. Soc. 1989, 111,

8084. b) J. E. Plowman, T. M. Loehr, C. K. Schauer, O. P. Anderson, Inorg. Chem. 1984, 23, 3553. c) D.

M. Kurtz, Jr., Chem. Rev. 1990, 90, 585.

4. S. Mukherjee, T. Weyhermüller, K. Wieghardt, P. Chaudhuri, Dalton Transactions , 2003, 3483.

132

133

Chapter 6

CONCLUSIONS AND PERSPECTIVES

134

135

6.1 Conclusions

The attempt to coalesce two different subjects viz. bio-inorganic chemistry and

molecular magnetism have been the main goal of this work. The complexes have interesting

magnetic properties and some act as structural as well as functional models for some

metalloenzymes. The main information and conclusions concerning this work are summarized

and some perspectives are suggested.

Chapter 2

The ligands H3L1 and H3L2 act as a backbone for the synthesis of polynuclear

complexes. The ligand H2L1 was found to be “non-innocent” and form radicals in air. With

H2L1

 Two new tetrameric, isostructural nickel(II)(1) and copper(II)(2) complexes have been

synthesized with the metals in a butterfly-formation. The imidazolate-group acts as a

bridge between the two metal centers.

 The nickel complex (1) is diamagnetic.

 The magnetic data of the copper complex (2) exhibits anti-ferromagnetic coupling

between the copper centers with the coupling constant of -49 cm-1. This value is

relatively high from other reported imidazolate-bridged copper complexes.

 The EPR spectrum of 2, at different temperatures, were recorded. A plot of IT vs. T

and its fit show that the value of the coupling constant is -42 cm-1 close to that

obtained from the magnetic susceptibility measurements (-49 cm-1).

With the dinucleating ligand H2L2, ten new dinuclear and tetranuclear complexes with 3d-

transition metal ions were synthesized.

 The dinuclear nickel (II) complex(3) has been synthesized where a single-atom

oxygen of urea bridges the two nickel atoms. The Ni…Ni bond distance at 2.966(1) Å

is significantly shorter than those observed in comparable complexes. This complex

acts as a structural model for the dinickel containing enzyme urease.

 Electronic spectra of 3 shows the three lower energy bands along with intense π-π*

transitions below 480 nm, attributable to the ligand.

 The magnetic susceptibility data for 3 exhibits an antiferromagnetic exchange

coupling between two paramagnetic Ni(II) (SNi = 1) centers (J = -3.5 cm-1).

 However, 3 is not able to catalyze the ethanolysis of urea presumably because of the

hydrogen bonding network involving urea in 3, which is also maintained in solution.

136

 4 consist of a low-symmetry cubane core with differing NiNi distances and Ni-O-

Ni angles.

 The couplings in 4 is dominated by a ferromagnetic exchange interaction between four

3A2 nickel(II) ions. The observation of three discrete exchange parameters for 4 is

consistent with the lower symmetry of the cubane. A plot of J vs Ni-O-Ni angles

indicates that the symmetry of the cubane core has a profound effect on the

magnetostructural correlation. Thus more such distorted cubanes of Ni(II) are

warranted to solve this open question.

 Complex 5 consists of a (µ-methoxo)(µ-phenoxo)dicopper(II) unit and belongs to an

ubiquitous class of coordination complexes for copper(II).

 The magnetic properties of 5 are antiferromagnetic with J value of -192 cm-1 and are

in agreement with the paramagnetic copper(II) centers with the (dx2-y2) magnetic

orbitals.

 The redox processes as observed from the cyclic voltammograms were assigned to

ligand-centered oxidation yielding phenoxyl radical in the complex.

 Complex 6, [L22Cu4(µ4-O)] is the first example of a (µ4-oxo)tetranuclear copper(II)

without any bridging ligand between the tetrahedral edges.

 The simulation of the magnetic data affords an overall anti-ferromagnetic interaction

in 6. In accordance with the three averaged Cu-O-Cu angles of 90o, 99.5o and 121.3o, a

three-J model was used to analyze the magnetic data. It is interesting to note that an

exchange coupling constant J14 corresponding to the Cu-O-Cu angle of 90o could not

be evaluated. The other J values are -122 cm-1 and –90 cm-1.

 Cyclic voltammetry of 6 shows a redox wave at +0.523 V which is assigned to the

oxidation of the ligand to phenoxyl radical.

 This ligand can adapt itself to various metal ion sizes to yield dinuclear complexes. It

makes a versatile building block for the construction of dimetal complexes, which

might be models for the study of biologically relevant dinuclear complexes as has

been shown from complexes 7-11. Although this dinucleating ligand has the intrinsic

property to complex metal ions of the first transition series mostly in a six-coordinate

fashion, however electronic preferences, e.g., Jahn-Teller effect, absence of LFSE, can

override this proclivity as is evidenced by the square-pyramidal 5-coordinated V(IV)

(10) and V(V) (11) complexes.

 The magnetic exchange values for complexes 7-11 show that the coupling is anti-

ferromagnetic between the paramagnetic centers.

137

 The electrochemical results for 7-11 suggest the generation of ligand-centered

oxidation processes attributable to the phenoxyl radicals, rather than the formation of

unusually high oxidation states at the central metal centers.

Chapter 3.

The ligand H4L3 is dinucleating and “non-innocent” with the amine groups at 1,3

positions. This serves as a backbone in attempting to induce ferromagnetic coupling between

the dinucleating centers or the radical centers by spin polarization. For H4L4, the methylene

bridge between the two phenyl rings inhibits spin polarization. The complexes synthesized

contain either four or six imino-benzosemiquinone radicals.

 The copper(II)(12) and Mn(IV)(16) complex consists of four imino-

benzosemiquinone radicals. Thus 12 and 16 are the dimers containing m-phenylene

bridges of the analogous mononuclear Cu(II) and Mn(IV) complexes with the ligand

2-anilino-4,6-di-tert-butylphenol.

 The magnetic data simulation for 12 shows that the antiferromagnetic coupling

between the radicals are strong throughout the whole temperature range. However, no

coupling was observed between the two copper centers. For 16, the coupling between

the radicals and the 3d-electrons at each ‘part’ is anti-ferromagnetic; very weak

coupling was observed between the residual fictitious two S= ½ centers. Improper

mixing between the respective d-orbitals with that of the p(π)-orbitals of the nitrogen

atom is a probable reason.

 Electrochemistry of both the dimers show interesting redox properties, all ascribed to

the oxidation or reduction of the radical centers.

 The dimeric cobalt(III)(14) and iron(III)(15) complexes consist of six imino-

benzosemiquinone radicals. Each metal center is six-coordinated with three radicals

residing at each ‘part’ of the dimer.

 Magnetic susceptibility studies show that for 14, the interaction between the three

radical centers at each ‘part’ is anti-ferromagnetic and a fictitious spin of ½ is

obtained. These two parts couple ferromagnetically giving a ground state of St=1. For

15, the coupling between the radical and the d-electrons of Fe(III) is strongly anti-

ferromagnetic. The residual SResidual=1 spin at each part interacts with each other

ferromagnetically and a ground state of S = 2 is obtained. Thus molecules with high

spins are possible using the ligand H4L3.

138

 Electrochemistry of 14 and 15 shows a number of oxidation and reduction peaks, all

attributed to radical centered oxidation or reduction.

Chapter 4.

A reactivity study using 12 and 16 as catalyst and 3,5-ditert-butylcatechol as substrate

is discussed in this chapter.

 12 and 16 can catalyze the aerial oxidation of catechol to quinone; 16, is a better

catalyst than, 12, for the aerial oxidation of catechol to quinone, thus mimicking the

function of catechol oxidase.

 The maximum turnover number obtained when 16 was used as a catalyst is 500. For

12, it is only 8.

 The rate law for this reaction was found to be first order for the catalyst concentration

and first order for the substrate concentration; rate = k [catalyst][substrate].

 The monomeric Mn(IV) complexes(16*,18-22), containing two imino-benzo-

semiquinone radicals, also catalyze the oxidation of 3,5-ditert-butylcatechol to 3,5-

ditert-butyl-ortho-benzoquinone. The rate constants (ksub) were measured and it was

found out that the velocity was the fastest, when 22 was used as catalyst.

 An outer sphere reaction mechanism is probable and is in conform with the

electrochemical data.

Chapter 5.

The substitution effect at the meta- and para- positions of the aniline ring of the parent

ligand 2-anilino-4,6-di-tert-butylphenol have been discussed in this chapter. A Co(III) along

with three Fe(III) complexes have been synthesized using these ligands.

 The cobalt(III) complex with the ligand H2L6(23), consists of three imino-

benzosemiquinone radicals. Magnetic susceptibility data and its simulation show that

the radicals are coupled ferromagnetically with a total spin of St=3/2 .

 The crystal structure for the iron(III) complex with H2L6(24) at 100K, again shows the

presence of three imino-benzosemiquinone radicals; however the metal-donor bond

lengths are much shorter than that for high spin Fe(III) complex with the ligand 2-

anilino-4,6-di-tert-butylphenol.

 Magnetic susceptibility data of 24 exhibits a thermally induced LSHS ⇔ spin

transition. The ground state is St = 0 and that at room temperature is St = 1. The crystal

structure of 24, at room temperature, shows that the bond distance between the metal-

donor atoms is consistent for a high spin Fe(III).

139

 Mössbauer measurements at 80K as well as 297K for 24 supports the spin transition

phenomenon.

 The Fe(III) complex with the ligand H2L11(25), shows the typical characteristics of the

iron(III) complex with the ligand 2-anilino-4,6-di-tert-butylphenol. The complex

consists of three imino-benzosemiquinone radicals and the ground state is S = 1

arising due to the antiferromagnetic coupling of the three radicals with the d5 electron

at the iron center.

 A µ-oxo diferric complex (26) was obtained when H2L12 was used as the ligand. Each

iron center is in the high spin state and is five coordinated. The complex consists of

four imino-benzosemiquinone radicals. Magnetic data and its simulation afford a

coupling constant value of -123 cm-1 which is comparable with those of other µ-oxo

diferric complexes. Complex 26 provides the first example of a structurally

characterized µ-oxo(diferric) complex containing four organic ligand radicals.

 Electrochemistry of all the above complexes show mainly ligand centered redox

processes.

6.2 Perspectives.

A few ideas and perspectives, in the continuation of this work, are outlined below.

 The synthesis of complexes with redox-active ligands play a very important role in

bio-inorganic chemistry as well as in molecular magnetism. The “Robson type” of

ligands, which have been used for synthesizing homo- and hetero- polynuclear

complexes, can be modified in order to synthesize complexes which can act as

structural model for various metalloenzymes.

 The knowledge of radical-metal interactions can be widened by synthesizing

complexes where the number of radicals, as well as metal centers, may be increased.

Conjoining spin polarization may help in synthesizing molecules with larger spins.

The ligand, obtained by the condensation of 1,3,5-triamino-benzene with 3,5-ditert-

butylcatechol may serve both a ferromagnetic coupler (1,3,5 position) and as a redox

active ligand.

 The radical complexes can be tested further as catalyst for oxidative reactions.

 Several different ligands could be synthesized by the condensation with 3,5-ditert-

butylcatechol with the ring substituted anilines. Transition metal complexes with these

ligands will probably show interesting structural as well as spectroscopic

characteristics.

140

141

Chapter 7

EQUIPMENT AND EXPERIMENTAL WORK

142

143

7.1 METHODS AND EQUIPMENTS

All the analyses were performed at the Max-Planck-Institut für Bioanorganische Chemie,

Mülheim an der Ruhr, unless otherwise mentioned. Commercial grade chemicals were used

for the synthetic purposes and solvents were distilled and dried before use.

Infrared Spectroscopy

Infrared spectra were measured from 4000 to 400 cm-1 as KBr pellets at room temperature on

a ‘Perkin-Elmer FT-IR-Spectrophotometer 2000’.

NMR Spectroscopy

1H- and 13C- NMR spectra were measured using a ‘Bruker ARX 250, DRX 400 or DRX 500’.

The spectra were referenced to TMS, using the 13C or residual proton signals of the deuterated

solvents as internal standards. VOCl3 was used as reference in 51V NMR spectra.

Mass Spectroscopy

Mass spectra in the Electron Impact mode (EI; 70 eV) were recorded on a Finnigan MAT

8200 mass spectrometer. Only characteristic fragments are given with intensities. The spectra

were normalised against the most intense peak having intensity 100. Electron Spray Ionization

(ESI) mass spectra were recorded either on a Finnigan Mat 95 instrument or a Hewlett-

Packard HP 5989 mass spectrometer. ESI- and EI- spectra were measured by the group of Dr.

W. Schrader at the Max-Planck-Institut für Kohlenforschung, Mülheim an der Ruhr.

Elemental Analysis

The determination of the C, H, N and metal content of the compounds was performed by the

‘ Mikroanalytischen Labor H. Kolbe’, Mülheim an der Ruhr, Germany.

UV-vis Spectroscopy

UV-Vis spectra were recorded on a ‘Perkin-Elmer UV-vis Spectrophotometer Lambda 19’or

on a Hewlett-Packard HP 8452A diode array spectrophotometer in the range 200-1200 nm.

For UV-vis spectro-electrochemical investigations the HP 8452A diode array

spectrophotometer was used, by employing a coulometry cuvette and Bu4NPF6 as supporting

electrolyte.

144

Electrochemistry

Cyclic voltammetry, square wave voltammetry and linear sweep voltammetry experiments

were performed using an ‘EG&G Potentiostat / Galvanostat 273A’. A standard three-

electrode-cell was employed with a glass-carbon working electrode, a platinum-wire auxiliary

electrode and Ag/AgCl (saturated LiCl in EtOH) reference electrode. Measurements were

made under an inert atmosphere at room temperature. The potential of the reference electrode

was determined using Fc+/Fc as the internal standard.

Magnetic Susceptibility Measurements

The measurements of the temperature or field dependent magnetization of the sample were

performed in the range 2 to 295 K at 1,4 or 7 T on a ‘Quantum Design SQUID-Magnetometer

MPMS’. The samples were encapsulated in gelatin capsules and the response functions were

measured four times for each given temperature, yielding a total of 32 measured points. The

resulting volume magnetization from the samples had its diamagnetic contribution

compensated and was recalculated as volume susceptibility. Diamagnetic contributions were

estimated for each compound by using Pascal’s constants. The experimental results were

fitted with the programme JULIUS calculating through full-matrix diagonalzation of the Spin-

Hamiltonian. The following Hamiltonian-operators were used:

HZE = µB∑ gi Ŝi .B

HHDVV = -2 ∑ Jij Ŝi . Ŝj

HZFS = ∑Di[Ŝiz2-{Si(Si+1)/3}+ Ei/Di(Ŝix2- Ŝiy2)]

Indexes i,j indicate individual spins. For the magnetic measurement the calculated g values

obtained during simulation is the isotropic.

EPR Spectroscopy

First derivative X-Band EPR spectra of powdered or frozen solution samples were measured

with a ‘Bruker ESP 300 Spectrometer’ coupled to an ‘Oxford Instruments ESR 910-Cryostat’.

Spin-Hamiltonian simulations of the EPR spectra were performed with a program which was

developed from the S = 5/2 routines of Gaffney and Silverstone and which specifically makes

use of the resonance search procedure based on a Newton-Raphson algorithm as described

therein.

145

57Fe-Mössbauer Spectroscopy

57Fe-Mössbauer spectra were measured with an Oxford Instruments Mössbauer spectrometer

in the constant acceleration mode. 57 Co/Rh was used as the radiation source. The minimum

experimental linewidths were 0.24 mm/s. The temperature of the sample was controlled by an

‘Oxford Instruments Variox Cryostat’. Isomer shifts were determined relative to α-iron at

300K. The measurements were carried out at 80K and 100K with solid samples containing the

isotope 57Fe.

Crystallography

X-ray diffraction data were collected on an ‘Enraf-Nonius CAD4 Diffractometer’ or on a

‘Siemens Smart System’. Graphite-monochromatized Mo-Kα with λ = 0.71073 Å was

employed. Data were collected by the 2θ-ω scan method (3≤2θ≤ 50°). The data were

corrected for absorption and Lorenz polarization effects.The structures were solved by direct

methods and subsequent Fourier-difference techniques, and refined anisotropically by full-

matrix least-squares on F2 with the program SHELXTL PLUS. Hydrogen atoms were

included at calculated positions with U < 0.08 Å2 in the last cycle of refinement.

GC / GC-MS Analysis

GC of the organic products were performed either on HP 6890 instruments using RTX-5

Amine 13.5 m S-63 columns respectively. GC-MS was performed using the above column

coupled with a HP 5973 mass spectrometer with mass selective detector.

LC Analysis

LC of the complexes were performed on HPLC instrumentation using a Gilson M305 pump,

and the Diode-Array-Detector (DAD) SPDM 10 AV (Shimadzu corporation). MeOH and

water in the ratio 3:1 with flow velocity 0.8 ml/ min was used as eluent through a Luna-5

phenylhexyl column.

146

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

7.2 SYNTHESIS

7.2.1 LIGANDS

Preparation of 2,4-Di-tert-butyl-6-[(5-methyl-3H-

imidazol-4-ylmethyl)-amino]-phenol [H3L1]

To de-aerated methanol (50 ml ),2.2 gms (10 mmole) of 2-Amino-4,6-di-tert-butyl-

phenol and 1.1 gms (10 mmole) of 5-Methyl-3H-imidazole-4-carbaldehyde was added and

the whole solution was refluxed for 2 hours under Argon. The red solution was cooled and

NaBH4 was added portion wise till the solution turns faint yellow. Drop wise addition of

water initiates the precipitation of a white solid . The white solid was collected and dried

under vacuum.

Yield : 2.8 gms (89 %)

MP: 186°C

Molecular Weight: 315.46 g/mol C19H29N3O

Elemental Analysis:

%C %H %N

Calculated 72.34 9.27 12.84

Found 71.0 8.2 12.8

Infrared Spectrum:

1H NMR (DMSO-d6): δ 1.21(s, 9H), 1.31(s,9H), 2.15(s,3H), 4.02(s,2H), 6.5(t,2H), 7.4 (s, 1H)

13C-NMR (DMSO-d6) : δ 30.0 (s), 31.6 (s), 34.1 (s), 34.5(s), 48.6(s), 107.5(s), 111.1(s),

133.2(s), 136.3(s), 140.3(s), 141.9(s)

147

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of 2,6-Bis-iminomethyl-(4,6-di-tertbutyl-2-iminophenol)-4-methyl-phenol

(H3L2)

In 70 ml of de-aerated methanol, 2,6-Diformyl p-cresol (1.6 gm, 10 mmole) and 2,4 Di-tert

Butyl-o-amino phenol (4.4 gm, 20 mmole) was added and the whole solution was refluxed for

1 hour. The solution colour changes to deep red from where yellow microcrystalline

compound precipitates. The solution was cooled , filtered and the residue was washed with

cold methanol.

Yield: 5.1 gm (90 %).

MP: 192 °C

Molecular Weight :- 570.82 C37H50N2O3

Elemental Analysis:

%C %H %N

Calculated 77.85 8.83 4.91

Found 77.7 8.7 4.8

Infrared Spectrum:

1H NMR Data (CDCl3):

δ 1.32 (s,18H), 1.43 (s,18H), 2.38 (s,3H,), 8.94(s,2H), 7.26 (m,4H), 7.12(d,2H).

13C NMR (CDCl3):

δ 29.4 (s), 51.6 (s), 34.6 (s,), 35.0(s,),111.4(s), 121.9(s), 123.4(d), 128.8(s), 133.8(s), 135.1(d),

142.1(s), 147.5(s), 158.8(s)

148

4000 3500 3000 2500 2000 1500 1000 500

ν (cm -1)

Preparation of 1,3-bis-(4,6-di-tert-butyl-2-iminophenol)benzene (H4L3)

To 150 ml of n-heptane, 3,5 Di-tert-Butyl Catechol (14.4 gm; 65 mmole) was added along

with 2 ml of NEt3.To this stirred solution, 1,3 phenylene diamine (3.24 gm; 30 mmole) was

added and the whole solution was refluxed for three hours. It was cooled and stirred in a

closed vessel for 2 days. It was filtered and the light grey solid residue was washed with cold

n-pentane.

Yield : 15 gm (96 %)

MP : above 200°C

Molecular Weight : 516.77 C34H48N2O2

Elemental Analysis:

%C %H %N

Calculated 79.02 9.36 5.42

Found 79.0 9.4 5.3

Infrared Spectrum :

1H NMR (CDCl3):

δ 1.22 (s, 18H), 1.37 (s, 18H), 6.12 (s, 1H), 6.25 (d, 2H), 7.02 (d, 3H), 7.18 (d, 2H).

13C-NMR (CDCl3):

δ 29.28, 31.33, 34.12, 34.74, 121.27, 122.20, 126.86, 141.1, 148.83

149

4000 3500 3000 2500 2000 1500 1000 500

ν (cm -1)

Preparation of 4,4´-bis-(4,6-di-tert-butyl-2-iminophenol)diphenyl methane (H4L4)

To 90 ml of n-heptane, 3,5 Di-tert-Butyl Catechol (6.6 gm; 30 mmole) was added along with

0.8 ml of NEt3.To this stirred solution, 4,4´-diaminodiphenyl methane(2 gm; 10 mmole) was

added and the whole solution was refluxed for three hours. It was cooled and stirred in a

closed vessel for 2 days. It was filtered and the light grey solid residue was washed with cold

n-pentane.

Yield :- 5 gms (89 %)

MP: above 200°C

Formula Weight :- 606.89 C41H54N2O2

Elemental analysis:

%C %H %N

Calculated 81.14 8.97 4.62

Found 80.7 9.0 4.7

Infrared Spectrum:

1H NMR (CDCl3):

δ 1.24 (s, 18H), 1.41 (s, 18H), 3.77 (d, 2H), 6.60 (p, 4H), 7.0 (p , 6H), 7.18 (d, 2H)

13C-NMR (CDCl3)

δ 29.5(s), 30.1(s), 31.6(s), 34.3(s), 34.9(s),115.2(d), 121.3(s), 121.8(s), 129.6(s), 142.1(s),

144.8(s), 149.3(s).

150

Preparation of H2L6-12

As the di-substituted or mono-substituted ligands were prepared by similar protocol, a

representative method is only described. A solution containing 3,5-di-tertbutylcatechol, the

substituted anilines(1:1) and triethylamine in n-heptane was refluxed for 3 hours, cooled,

filtered and then concentrated. Colorless crystalline solid precipitates which was filtered and

washed with cold n-pentane.

Yield (gms)/(%) Melting point (°C) Molecular Weight Molecular formula

H2L6 6.20 (75 %)a 180 409.66 C28H43NO

H2L7 3.42 (40%)a 158 433.44 C22H25F6NO

H2L8 4.25(65%)a 164 325.49 C22H31NO

H2L9 4.49(61%)a 175 366.33 C20H25Cl2NO

H2L10 10.99(77%)b 132 357.49 C22H31NO3

H2L11 9.09(68%)b 154 333.42 C20H25F2NO

H2L12 12 (68%)c 173 353.55 C24H35NO

a) 3,5-ditert-butylcatechol (4.4gm;20mmole), substituted aniline (20mmole),0.2ml NEt3,

40ml heptane.

b) 3,5-ditert-butylcatechol (8.8gm;40mmole), substituted aniline (40mmole),0.5ml NEt3,

40ml heptane.

c) 3,5-ditert-butylcatechol (11.1gm;50mmole), 4-tertbutylaniline (7.46gm;50mmole),0.5ml

triethylamine, 60ml heptane.

Synthesis of H2L5 is already described [Reference 4(a), Chapter 3].

NMR Data

[1H (CDCl3)]

δ

H2L6 1.26(s, 27H), 1.44(s ,9H), 5.02(s, 1H), 6.23(s, 1H), 6.54(s, 1H), 6.92(m, 1H), 7.06(m, 1H), 7.17(s, 1H)

H2L7 1.26(s ,9H), 1.43(s, 9H), 5.44(s, 1H), 5.94(b, 1H), 7.06(m, 3H), 7.28(s, 2H)

H2L8 1.38(s,9H), 1.57(s,9H), 2.32(s, 6H), 4.93(b,1H), 6.4(s,2H), 6.61(s,1H), 7.15(s,2H), 7.35(s,1H)

H2L9 1.26(s, 9H), 1.42(s, 9H), 5.08(s, 1H), 6.06(s, 1H), 6.52(m, 2H), 6.81(b, 1H), 6.96(b, 1H), 7.24(s, 1H)

H2L10 1.24(s, 9H), 1.41(s, 9H), 3.70(s, 6H), 5.01(s, 1H), 5.83(b, 2H), 5.98(b, 1H), 7.02(b, 1H), 7.18(b, 1H)

H2L11 1.26(s, 9H), 1.42(s, 9H), 5.15(s, 1H), 6.1(m, 3H), 6.26(m, 1H), 6.98(d, 1H), 7.24(s, 1H)

H2L12 1.33 (s, 9H), 1.38 (s, 9H), 1.56 (s,9H), 4.95 (s, 1H), 6.70 (d, 2H), 7.14 (d, 1H), 7.33 (s, 1H), 7.34 (m, 2H)

151

4000 3500 3000 2500 2000 1500 1000 500

ν (cm -1)

7.2.2 COMPLEXES

Synthesis of NiII4L14 (1)

The ligand (1 mmole; 315 mg) was dissolved in 25 ml of de-aerated methanol. Solid

[Ni(H2O)6](ClO4)2 (1mmole;365mg) was added along with NEt3 (0.15 ml).The light orange-

red solution was refluxed for 1 hour under Argon, cooled and then stirred in air for 30 minutes

from where orange-red microcrystalline solid precipitated .The solid was filtered, dried and

recrystallised from Dichloromethane-Methanol.

Yield: 210mg (58%)

Molecular Weight: 1480.47 C76H100N12O4Ni4

Elemental Analysis:

%C %H %N %Ni

Calculated 61.66 6.81 11.35 15.86

Found 58.74 5.83 10.71 14.47

Infrared spectrum:

152

4000 3500 3000 2500 2000 1500 1000 500

ν (cm -1)

Synthesis of CuII4L14 (2)

The ligand (155 mg;0.5 mmole) was dissolved in 15 ml of methanol. Solid

Cu(OAc)2.4H2O (0.5mmole;100mg) was added along with NEt3 (0.15 ml).The light orange-

red solution was refluxed for 1 hour under Argon, cooled and then stirred in air for 30 minutes

from where orange-red microcrystalline solid precipitated .The solid was filtered, dried and

recrystallised from THF-Methanol.

Yield: 120mg (64%)

Formula Weight: 1499.89 C76H100N12O4Cu4

Elemental Analysis:

%C %H %N %Cu

Calculated 60.86 6.72 11.21 16.95

Found 59.8 6.6 11.1 17.3

Infrared spectrum:

153

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of [NiII2 (L2)(NH2CONH2)(OAc)(MeOH) 2] (3)

Method 1.

Ni acetate (240mg, 1mmole), sodium acetate (160 mg, 2 mmole) and urea (180mg, 3

mmole) were dissolved in methanol (15ml). Dichloromethane (30ml) was added along with

the ligand H3L (280 mg, 0.5 mmole). The solution was refluxed for 15 minutes in air and

allowed to cool. It was filtered and the solution was allowed to evaporate slowly giving

orange red crystals.

Yield: 240 mg (60 %)

Molecular Weight : 1012.54 C46.5H80N4O12.5Ni2

Elemental Analysis:

%C %H %N %Ni

Calculated 55.16 7.91 5.53 11.59

Found 55.8 7.4 6.0 11.7

Infrared spectrum:

Method 2

The same compound (3) can be prepared from (4) by addition of urea and refluxing the

resulting solution.

154

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of [NiII4 (LH) 2(OMe) 2(OAc) 2(OHMe) 2] (4)

Nickel acetate (240 mg, 1mmole) and sodium acetate (240 mg, 3 mmole) was dissolved in

methanol (15ml). Dichloromethane was added along with H3L (280 mg, 0.5 mmole) and the

solution was refluxed for 15 minutes. It was then cooled and filtered. The volume of the

filtrate was reduced till orange red microcrystalline precipitation occurs. It was filtered and

washed with methanol. The compound was recrystallised from dichloromethane and

methanol.

Yield: 188 mg (47%)

Molecular Weight : 1616.60 C82H116N4O14Ni4

Elemental Analysis:

%C %H %N %Ni

Calculated 61.02 7.25 3.47 14.37

Found 58.9 7.0 3.2 13.1

Infrared Spectrum:

155

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of [CuII2 L2 (µ-MeO) (THF)2] (5)

To a Methanol/Dichloromethane solvent mixture(1:3), the ligand (0.5mmole; 290mg),

[Cu(H2O)6](ClO4)2 (1.0 mmole ; 370 mg) and NEt3 (0.3 ml) was added and refluxed for ½

hour in air. The deep red solution was cooled , filtered and the filtrate was concentrated by

slow evaporation of the solvent. Orange-red microcrystalline solid precipitates after a few

days. Single crystals was grown from a THF-MeOH solvent mixture.

Yield :130 mg (70%)

Formula Weight: 725.92 C38H50N2O4Cu2

Elemental Analysis:

%C %H %N %Cu

Calculated 62.96 6.96 3.87 17.38

Found 63.1 6.9 3.8 17.4

Infrared spectrum:

156

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of [CuII4 L22 (µ4-O) ] (6)

To a Acetonitrile / Dichloromethane solvent mixture (1:3) , the ligand (0.5mmole ;

290mg),[Cu(H2O)6](ClO4)2 (1.0 mmole ; 370 mg) and NEt3 (0.3 ml) was added and refluxed

for ½ hour in air. The deep red solution was cooled, filtered and the filtrate was concentrated

by slow evaporation of the solvent. Orange-red microcrystalline solid precipitates after a few

days. Single crystals was grown from a MeCN-DCM solvent mixture.

Yield : 210 mg (60%)

Molecular Weight: 1405.76 C74H94N4O7Cu4

Elemental Analysis :

%C %H %N %Cu

Calculated 63.23 6.74 3.99 18.08

Found 62.4 6.9 4.0 17.8

Infrared Spectrum:

157

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of FeIII2 L22 (7)

To a degassed solution of the ligand H3L2 (0.29 g; 0.5 mmol) and NEt3 (0.12 ml) in an

acetonitrile-dichloromethane solvent mixture (10 ml:15ml), Fe(ClO4)2.6H2O (0.18 g; 1 mmol)

was added. The resulting solution was refluxed for 0.5 h under argon, cooled and stirred in air

for an hour. The resulting deep reddish-brown solution was kept at ambient temperature for

crystallization. After two days red-brown microcrystals were collected by filtration and air-

dried.

Yield: 140 mg(45 %)

Molecular Weight : 1247.28 C74H94O6N4Fe2

Elemental Analysis:

%C %H %N %Fe

Calculated 71.27 7.54 4.49 8.98

Found 71.3 7.5 4.4 9.0

Infrared Spectrum:

158

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of MnIII2L22 (8a)

A methanolic solution (25 ml) of the ligand H3L (0.29 g; 0.5 mmol), manganese(III) acetate

(0.13 g; 0.2 mmol) and tetrabutylammonium methoxide (0.2 ml) was refluxed for 1 h, which

on cooling and furthur concentration at room temperature yielded red-brown microcrystals of

8a. The solid was collected by filtration and air-dried.

Yield : 70 mg (23 %)

Molecular Weight : 1245.46 C74H94O6N4Mn2

Elemental Analysis:

%C %H %N %Mn

Calculated 71.36 7.61 4.50 8.82

Found 71.5 7.5 4.5 8.7

Infrared Spectrum :

[MnIII2L22 (THF)2] (8b).

X-ray quality crystals of 8b4 CH3CN were obtained by crystallizing 8a from a

tetrahydrofuran-acetonitrile solution.

Elemental Analysis :

%C %H %N %Mn

Calculated 69.57 7.91 7.21 7.07

Found 69.7 7.9 6.7 7.3

159

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of CrIII2L22 (9)

A solution of the ligand (0.29 g; 0.5 mmol), NEt3 (0.1 ml) and CrCl2 (0.12 g; 0.5 mmol) in

tetrahydrofuran (25 ml) was refluxed under argon for 15 min and the refluxing was continued

further for 1 h in air. The resulting deep red solution was filtered and the filtrate was

evaporated to dryness in a rotary evaporator. The solid was dissolved in 5 ml of THF and the

solution was filtered to remove any solid particle. After addition of 2 ml of acetonitrile to the

filtrate, the solution on slow evaporation at room temperature yielded orange-red

microcrystalline solid of 9.

Yield : 130 mg ( 42 %)

Molecular Weight: 1239.58 C74H94O6N4Cr2

Elemental Analysis:

%C %H %N %Cr

Calculated 71.70 7.64 4.52 8.39

Found 71.4 7.6 4.5 8.3

Infrared Spectrum:

160

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of [(VIV=O)2 (µ-Oisoprop) L2)] (10)

A solution of the ligand H3L2 (0.29 g; 0.5 mmol), vanadyltris(isopropoxide) (0.25 ml; 1

mmol) and Et3N (0.3 ml) in a deaerated acetonitrile-dichloromethane (10ml:10ml) solvent

mixture was refluxed under argon for 1 h and then stirred in air for further 1 h. The solution

was filtered to remove any solid particle and the filtrate was allowed to evaporate slowly at

ambient temperature to provide deep red crystals.

Yield: 180 mg (47 %)

Molecular Weight : 760.76 C40H54O6N2V2

Elemental Analysis:

%C %H %N %V

Calculated 63.13 7.16 3.68 13.40

Found 63.1 7.1 3.6 13.4

Infrared Spectrum:

161

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of [(VV=O)2 L22] (11)

To a de-aerated solvent mixture of Acetonitrile/DCM (30 ml:10 ml),the ligand (290 mg;

0.5 mmole), V(THF)3Cl3 (190 mg;0.5 mmole) and NEt3 (0.25 ml) was added and refluxed

under Argon for 1 hour, cooled and then stirred in air for 1 hour. It was filtered and the filtrate

was allowed to evaporate slowly. Dark brown crystals appear after some days.

Yield: 200 mg (63 %)

Molecular Weight:1269.47 C74H94O8N4V2

Elemental Analysis:

%C %H %N %V

Calculated 70.00 7.47 4.42 8.03

Found 69.8 7.3 4.4 7.9

Infrared spectrum:

162

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of CuII2L32 .THF(12)

The ligand H4L3 (0.3 g, 0.6 mmol), CuCl (0.06 g, 0.6 mmol) and NEt3 (0.4 cm3) were

dissolved under argon in CH3CN/CH3OH (1:1) and the resulting solution was refluxed for 1 h

and filtered in the air. Slow evaporation of the filtrate afforded green microcrystals of 12.

Recrystallization from THF/CH3OH (4:1) afforded X-ray quality crystals.

Yield: 400 mg (53%)

Molecular Weight : 1258.62 C74H100Cu2N4O5.5

Elemental Analysis:

%C %H %N %Cu

Calculated 70.50 8.00 4.44 10.08

Found 69.1 8.1 4.4 9.8

Infrared Spectrum:

163

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of CoIII2L33 (14)

To a solvent mixture of dichloromethane and acetonitrile (4:5),the ligand (H4L3) (300 mg ; 0.6

mmole) and [Co(H2O)6](ClO4)2 (160 mg ;0.44 mmole) was added. Addition of NEt3 (0.4 ml)

turns the solution dark green. The solution was refluxed for 30 minutes, cooled and then

filtered. Slow evaporation of the filtrate afforded dark brown crystals of 14.

Yield: 265 mg( 80%)

Molecular Weight : 1654.89 C102H132Co2N6O6

Elemental Analysis:

%C %H %N %Co

Calculated 73.96 8.04 5.08 7.12

Found 73.8 8.0 5.1 7.2

Infrared Spectrum:

164

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of FeIII2L33 (15)

The ligand H4L3 (0.3 g, 0.6 mmol), FeCl2.4H2O (0.08 g, 0.44 mmol) and NEt3 (0.4 cm3) were

dissolved in a solvent mixture (40 cm3) of CH2Cl2/CH3CN (4:5) and the resulting solution

was refluxed for 0.5 h and filtered. Slow evaporation of the filtrate afforded green crystals of

15. Recrystallization from acetone afforded X-ray quality crystals.

Yield: 180 mg( 54%)

Molecular Weight : 1648.89 C102H132Fe2N6O6

Elemental Analysis:

%C %H %N %Fe

Calculated 74.23 8.07 5.10 6.78

Found 74.2 7.8 5.1 6.8

Infrared Spectrum:

165

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of MnIV2(LA3)2(L3) (16)

To a solution of the ligand (0.52 g, 1 mmol) in CH3OH (25 cm3) containing [Bu4N]OCH3 (0.9

cm3, 2.5 mmol) "manganese(III) acetate" (0.13 g, 0.2 mmol) was added to produce to a brown

solution, which was refluxed in air for 0.5 h and filtered to remove any solid particles. The

deep brown microcrystalline solid, separated after cooling was recrystallized from CH2Cl2/

CH3CN (1:1).

Yield: 320 mg (60%)

Molecular Weight : 1646.89 C102H132Mn2N6O6

Elemental Analysis:

%C %H %N %Mn

Calculated 74.34 8.07 5.10 6.67

Found 74.3 8.0 5.0 6.5

Infrared spectrum:

166

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of CoIII2L43 (17)

To a solvent mixture of Dichloromethane and Acetonitrile (4:5),the ligand (H4L4) (360 mg ;

0.6 mmole) and [Co(H2O)6](ClO4)2 (160 mg ;0.44 mmole)was added. Addition of NEt3 (0.4

ml) turns the solution dark green. The solution was refluxed for 1 hour, cooled and then

filtered. Slow evaporation of the filtrate afforded dark brown crystals.

Yield: 170 mg( 45%)

Molecular Weight : 1925.02 C123H150Co2N6O6

Elemental Analysis:

%C %H %N %Co

Calculated 76.67 7.85 4.36 6.12

Found 76.6 7.8 4.5 6.2

Infrared Spectrum:

167

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of MnIV(LA6)(L6)2 (18)

The ligand H2L6 (0.82 g; 2.0 mmol) and "manganese(III) acetate" (0.13 g, 0.2 mmol) were

dissolved in methanol (25 mL). Upon addition of [Bu4N]OCH3 (0.9 cm3, 2.5 mmol) the color

of the solution turned deep red. The solution was heated to reflux for 1 h in the presence of

air. From the cooled and filtered solution black microcrystals crystallized upon slow

evaporation of the solvent. The crude material was recrystallized from a Et2O-CH3OH solvent

mixture.

Yield: 430 mg (60%)

Molecular Weight : 1276.89 C84H123MnN3O3

Elemental Analysis:

%C %H %N %Mn

Calculated 78.95 9.70 3.29 4.30

Found 78.8 9.8 3.2 4.3

Infrared Spectrum:

168

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of CoIIIL63 (23)

The ligand H2L6 (0.41 g; 1.0 mmol) and [CoII(H2O)6](ClO4)2 (0.12 g, 0.33 mmol) were

dissolved in deaerated acetonitrile (25 mL) under argon. Upon addition of NEt3 (0.5 cm3) the

color of the solution turned light blue. The solution was heated to reflux for 1h under argon.

It was then cooled and stirred in air for ½ hour. From the solution deep blue microcrystals

crystallized upon slow evaporation of the solvent. The crude material was recrystallized from

a Et2O-CH3CN solvent mixture.

Yield: 280 mg (66%)

Molecular Weight : 1280.89 C84H123CoN3O3

Elemental Analysis:

%C %H %N %Co

Calculated 77.90 9.32 3.06 5.06

Found 78.7 9.7 3.3 4.6

Infrared Spectrum:

169

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of FeIIIL63 (24)

The ligand H2L6 (0.41 g ;1.0 mmol) and [FeII(H2O)6](ClO4)2 (0.06 g; 0.25 mmol) were

dissolved in methanol (25 mL). Upon addition of NEt3 (0.5 mL) the color of the solution

turned deep green. The solution was heated to reflux for 1 h in the presence of air. From the

cooled and filtered solution deep green microcrystals crystallized upon slow evaporation of

the solvent. The crude material was recrystallized several times from a Et2O-CH3CN solvent

mixture.

Yield: 220 mg (69%)

Molecular Weight : 1277.89 C84H123FeN3O3

Elemental Analysis:

%C %H %N %Fe

Calculated 78.88 9.7 3.29 4.38

Found 76.9 9.0 3.0 4.9

Infrared Spectrum:

170

4000 3500 3000 2500 2000 1500 1000 500

ν (cm -1)

Synthesis of FeIIIL113 (25)

The ligand H2L11 (0.34g ;1.0 mmol) and [FeII(H2O)6](ClO4)2 (0.06 g; 0.25 mmol) were

dissolved in methanol (20 mL). Upon addition of NEt3 (0.4 mL) the color of the solution

turned deep green. The solution was heated to reflux for 1 h in the presence of air. From the

cooled and filtered solution deep green microcrystals crystallized upon slow evaporation of

the solvent. The crude material was recrystallized several times from a CH2Cl2-CH3CN

solvent mixture.

Yield: 165mg (63%)

Molecular Weight : 1049.46 C60H69F6FeN3O3

Elemental Analysis:

%C %H %N %Fe

Calculated 68.61 6.63 4.0 5.33

Found 68.5 6.75 3.9 5.28

Infrared Spectrum:

171

4000 3500 3000 2500 2000 1500 1000 500

ν (cm-1)

Synthesis of [FeIII2(µ-O)(L12)4] (26)

The ligand H2L12 (0.36 g; 1.0 mmol) and [FeII(H2O)6](ClO4)2 (0.06 g; 0.25 mmol) were

dissolved in methanol (25 mL). Upon addition of NEt3 (0.5 mL) the color of the solution

turned deep red. The solution was heated to reflux for 1 h in the presence of air whereupon a

color change to deep green was observed. From the cooled and filtered solution black

microcrystals crystallized upon slow evaporation of the solvent within 2 days. The crude

material was recrystallized from a CH2Cl2-CH3CN (3:1) mixture.

Yield: 120 g (63 %).

Molecular Weight : 1532.89 C96H132N4O5Fe2

Elemental Analysis:

%C %H %N %Fe

Calculated 75.15 8.68 3.65 7.30

Found 75.0 8.6 3.5 7.4

Infrared spectrum:

172

7.3 REACTIVITY STUDIES

Catalytic oxidation of 3,5-di-tert-butylcatechol

In dichloromethane (25ml), the Mn(IV) complexes were dissolved and 3,5-

ditertbutylcatechol was added and the solution was stirred in room temperature for 24hours.

The solution was then subjected to liquid chromatography studies.

For kinetic measurements, 1ml from this solution was taken and diluted to 10ml with

dichloromethane. The change in the concentration of the product, 3,5-di-tert-butyl-o-

benzoquinone, was measured by UV-spectroscopy.

Catalytic oxidation of 2,6-di-tert-butylphenol

In a dichloromethane/methanol solvent mixture (1:1; 50ml), 16 was dissolved. To it

2,6-di-tert-butylphenol was added and the solution was stirred for 48 hours. The solution was

then filtered, and the filtrate was evaporated to dryness. 5ml of methanol was added to

dissolve the excess 2,6-di-tert-butylphenol (16 is insoluble in methanol) and the methanolic

solution was subjected to GC studies with the presence of hexadecane (C16) as standard.

For measuring the progress of the reaction, 50µl from the aliquot was passed through

an Amberlyst cationic ion-exchanger and washed with 10ml dichloromethane. The change in

the concentration of product, 3,3´-5,5´-Tetra-tert-butyldiphenoquinone, was measured by UV-

spectroscopy.

173

Appendices

1. Crystallographic data

2. Magnetochemical data

3. Magnetic and EPR data

4. Curriculum vitae

174

175

1) Crystallographic data

Crystal data and structure refinement for 1, 2.

1 2

Empirical formula C76H100N12Ni4O4.9CH2Cl2 C76H100N12Cu4O4.xTHF

Formula weight 2244.85 1499.xTHF

Temperature (K) 100(2) 100(2)

Wavelength (Å) 0.71073 0.71073

Crystal system Tetragonal Monoclinic

Space group P-421/c (No. 114) P21/c

Unit cell dimensions a = 20.74(3) Å

b = 20.74(3) Å

c = 12.1538 (10) Å

α = 90 deg.

β = 90 deg.

γ = 90 deg.

a = 24.916(5) Å

b = 17.296 (4) Å

c = 33.004(7) Å

α = 90 deg.

β = 91.72 deg.

γ = 90 deg.

Volume (Å3);Z 5227.9(12); 2 14216

Density (cal.) (Mg/m3) 1.426 1.401

Absorp. coeff. (mm-1) 1.220 1.24

F(000) 2324 6304

θ range for data collection 1.94 to 25.00 deg. 1.94 to 21.18 deg.

Index ranges -31<=h<=32,

-10<=k<=31,

-18<=l<=17

-21<=h<=21,

-17<=k<=12,

-33<=l<=33

Reflections collected 39793 13608

Independent reflections 4608 10569

Absorption correction SADABS

Data / restraints / parameters 4591 / 16 / 289 10212 / 0 / 1137

Goodness-of-fit on F2 1.048 2.895

Final R indices

[I>2σ(I)]

R1 = 0.0786, wR2 = 0.2180 R1 = 0.1513,

R indices (all data) R1 = 0.0982, wR2 = 0.2366 R1 = 0.2162, wR2 = 0.4469

176

Crystal data and structure refinement for 3, 4, 5, 6.

3 4 5 6

Empirical formula C42H62N4O8Ni2

4.5CH3OH

C82H116N4O14Ni4

3CH2Cl2

[C38H50N2O4Cu2.

(2THF)]  2THF

C74H94N4O7Cu4

3.3CH2Cl20.7CH3CN

Formula weight 1012.57 1871.41 1014.35 1715.56

Temperature 100(2) K 100(2) K 100(2) K 100(2) K

Wavelength (MoKα) 0.71073 Å 0.71073 Å 0.71073 0.71073 Å

Crystal System Triclinic Triclinic Monoclinic Triclinic

Space group P-1 P-1 P2(1)/n P-1

Unit cell dimensions a = 12.099(2) Å a = 19.9102(12) Å a = 11.7479 Å a = 16.4678(9) Å

b = 12.128(2) Å b = 21.545(2) Å b = 30.236 Å b = 17.2675(12) Å

c = 18.800(3) Å c = 22.076(2) Å c = 15.64 Å c = 17.3208(12) Å

α = 95.90(2)o α = 80.45(2)o α = 90.0(2)o α = 106.03(1)o

β = 104.16(2)o β = 80.88(2)o β = 110.19(2)o β = 112.14(1)o

γ = 91.48(2)o γ = 83.43(2)o γ = 90.0(2)o γ = 102.71(1)o

Volume (Å3); Z 2656.9(8); 2 9182.7(13); 4 5213.2; 4 4083.9(5); 2

Density (calc.) Mg/m3 1.266 1.354 1.292 1.395

Absorp. coeff. (mm-1) 0.769 1.043 0.87 mm-1 1.298

F(000) 1086 3944 2168 1785

Crystal size (mm) 0.20 x 0.19 x 0.10 0.18 x 0.10 x 0.09 0.13 x 0.09 x 0.06 0.33 x 0.24 x 0.15

θ range for data collect. 1.69 to 23.27o 3.13 to 22.50o 4 to 26o 2.87 to 30.0o

Reflections collected 18237 57320 27492 49175

Independent reflect. 7534

[R(int.) = 0.1014]

23751

[R(int.) = 0.0845]

17225

[R(int.) = 0.0918]

23491

[R(int.) = 0.0452]

Absorpt. correction Gaussian,

face indexed

Gaussian,

face indexed

not measured not measured

Data/restraints/param. 7524 / 25 / 672 23751 / 112 / 2033 5150 / 62 / 492 23356 / 166 / 964

Goodness-of-fit on F2 0.877 1.051 1.214 1.019

Final R indices[I>2σ(I)] R1 = 0.0508 R1 = 0.0837 R1 = 0.0919 R1 = 0.0569

wR2 = 0.0971 wR2 = 0.1851 wR2 = 0.2172 wR2 = 0.1434

R indices (all data) R1 = 0.1113 R1 = 0.1412 R1 = 0.2092 R1 = 0.0803

wR2 = 0.1134 wR2 = 0.2173 wR2 = 0.4007 wR2 = 0.1602

177

Crystal data and structure refinement for 7, 8(b), 10, 11.

7 8(b) 10 11

Empirical formula C74H94Fe2N4O6

2.5 CH3CN

C82H110Mn2N4O8

4 CH3CN

C42H59N3O6V2 C

74H94N4O8V2

2 CH3CN

Formula weight 1349.87 1553.84 803.80 1351.52

Temperature 100(2) K 100(2) K 100(2) K 100(2) K

Wavelength (MoKα) 0.71073 Å 0.71073 Å 0.71073 Å 0.71073 Å

Crystal system Triclinic Triclinic Triclinic Triclinic

Space group P-1 P-1 P-1 P-1

Unit cell dimensions a = 16.887(2) Å a = 11.6088(8) Å a = 8.456(1) Å a = 14.8895(8) Å

b = 17.026(2) Å b = 12.6390(8) Å b = 16.087(2) Å b = 16.047(1) Å

c = 17.040(2) Å c = 31.802(2) Å c = 16.685(2) Å c = 16.881(1) Å

α = 115.63(2)o α = 79.41(1)o α = 71.35(2)o α = 77.47(1)o

β = 113.09(2)o β = 83.17(1)o β = 89.26(2)o β = 79.50(1)o

γ = 90.69(2)o γ = 72.87(1)o γ = 79.26(2)o γ = 81.47(1)o

Volume (Å3); Z 3964.7(8); 2 4372.7(5); 2 2110.2(4); 2 3846.5(4); 2

Density (calc.) Mg/m3 1.131 1.180 1.265 1.167

Absorp. coeff. (mm-1) 0.417 0.346 0.490 0.298

F(000) 1442 1664 852 1440

Crystal size (mm) 0.39 x 0.37 x 0.34 0.44 x 0.36 x 0.24 0.33 x 0.14 x 0.09 0.35 x 0.11 x 0.10

θ range for data collect. 1.69 to 26.00o 2.31 to 27.11o 2.16 to 22.50o 4.21 to 32.50o

Reflections collected 26505 31923 13369 44652

Independent reflect. 13350

[R(int.) =0.0419]

18622

[R(int.) = 0.0482]

5434

[R(int.) = 0.0852]

27516

[R(int.) = 0.0452]

Absorpt. correction SADABS Gaussian,

face indexed

Gaussian,

face indexed

Gaussian,

face indexed

Data/restraints/param. 13308 / 36 / 805 18622 / 6 / 1000 5430 / 0 / 479 27259 / 34 / 839

Goodness-of-fit on F2 1.057 1.065 1.012 1.014

Final R indices

[I>2σ(I)]

R1 = 0.0862,

wR2 = 0.2532

R1 = 0.0747,

wR2 = 0.1899

R1 = 0.0527,

wR2 = 0.1194

R1 = 0.0681,

wR2 = 0.1539

R indices (all data) R1 = 0.1223,

wR2 = 0.2875

R1 = 0.0933,

wR2 = 0.2020

R1 = 0.0963,

wR2 = 0.1345

R1 = 0.1102,

wR2 = 0.1814

178

Crystal data and structure refinement for 12, 14, 15, 16.

12 14 15 16

Empirical formula C74H100Cu2N4O5.5 C102H132Co2N6O6

*3CH2Cl2

C102H132Fe2N6O6

* 0.5 C3H6O

C102H132Mn2N6O6

* 0.5 CH2Cl2

Formula weight 1260.66 1910.77 1678.88 1690.48

Temperature (K) 100(2) 100(2) 100(2) 100(2)

Wavelength (Å) 0.71073 0.71073 0.71073 0.71073

Crystal system Monoclinic Triclinic Monoclinic Monoclinic

Space group P21/c P-1 P21/n P21/n

Unit cell dimensions a = 15.2345(12) Å

b = 18.903 (2) Å

c = 24.840 (3) Å

α = 90 deg.

β = 90.51(2) deg.

γ = 90 deg.

a = 15.5627(8) Å

b = 16.2417(12) Å

c = 23.594(2) Å

α = 74.09(1) deg.

β = 76.48(1) deg.

γ = 66.45(1) deg.

a = 25.374(2) Å

b = 15.7754(12) Å

c = 26.287(2) Å

α = 90 deg.

β = 106.40(1) deg.

γ = 90 deg.

a = 25.0716(9) Å

b = 15.7152(6) Å

c = 26.2842(12) Å

α = 90 deg.

β = 105.43(1) deg.

γ = 90 deg.

Volume (Å3);Z 7153.1(13); 4 5205.1(6); 2 10094.2(13); 4 9982.8(7); 4

Density (cal.) (Mg/m3) 1.171 1.219 1.105 1.125

Absorp. coeff. (mm-1) 0.645 0.526 0.340 0.332

F(000) 2696 2028 3608 3620

Crystal size (mm) 0.33 x 0.31 x 0.31 0.28 x 0.27 x 0.24 0.11 x 0.06 x 0.04 0.25 x 0.20 x 0.16

θ range for data

collection

2.15 to 30.00 deg. 3.50 to 30.00 deg. 3.06 to 23.50 deg. 2.06 to 22.50 deg.

Index ranges -23<=h<=14,

-29<=k<=27,

-38<=l<=30

-23<=h<=22,

-19<=k<=24,

-33<=l<=36

-29<=h<=15,

-18<=k<=15,

-30<=l<=30

-27<=h<=27,

-16<=k<=17,

-29<=l<=23

Reflections collected 46103 47876 33764 56918

Independent reflections 19375

[R(int) = 0.0438]

29002

[R(int) = 0.0441]

14847

[R(int) = 0.0768]

13020

[R(int) = 0.0922]

Absorption correction Not measured Gaussian,

face indexed

Gaussian,

face-indexed

Not measured

Data / restraints /

parameters

19318 / 0 / 808 26325 / 0 / 1105 14670 / 0 / 1081 12926 / 0 / 1072

Goodness-of-fit on F2 1.033 1.034 1.045 1.027

Final R indices

[I>2σ(I)]

R1 = 0.0472,

wR2 = 0.1196

R1 = 0.0881,

wR2 = 0.2329

R1 = 0.0658,

wR2 = 0.1120

R1 = 0.0592,

wR2 = 0.1236

R indices (all data) R1 = 0.0755,

wR2 = 0.1307

R1 = 0.1275,

wR2 = 0.2708

R1 = 0.1303,

wR2 = 0.1372

R1 = 0.1005,

wR2 = 0.1444

179

Crystal data and structure refinement for 17, 18, 23.

17 18 23

Empirical formula C123 H150 Co2 N6 O6

* CH3CN

C84 H123 Mn N3 O3 C

84 H123 Co N3 O3

Formula weight 1967.40 1277.79 1281.78

Temperature (K) 100(2) 100(2) 100(2)

Wavelength (Å) 0.71073 0.71073 0.71073

Crystal system Monoclinic Monoclinic Monoclinic

Space group P21/c P21/c P21/c

Unit cell dimensions a = 15.077(1) Å

b = 35.459(4) Å

c = 21.803(3) Å

α = 90 deg.

β = 105.53(2) deg.

γ = 90 deg.

a = 16.6726(4) Å

b = 14.5993(4)Å

c = 33.4362(12)Å

α = 90 deg.

β = 97.20(1)deg.

γ = 90 deg.

a = 16.1355(4) Å

b = 14.5516(4) Å

c = 34.0664(8) Å

α = 90 deg.

β = 97.77(1) deg.

γ= 90 deg.

Volume (Å3);Z 11230(2); 4 8074.5(4); 4 7925.3(3); 4

Density (calculated) (Mg/m3) 1.164 1.051 1.074

Absorption coefficient (mm-1) 0.352 0.209 0.263

F(000) 4216 2788 2796

Crystal size (mm) 0.56 x 0.50 x 0.16 0.22 x 0.14 x 0.14 0.48 x 0.17 x 0.09

θ range for data collection 1.58 to 20.00 deg. 2.96 to 27.50 3.01 to 27.50 deg.

Index ranges -16<=h<=16,

-39<=k<=30,

-24<=l<=22

-21<=h<=21,

-18<=k<=18,

-43<=l<=43

-20<=h<=20,

-18<=k<=18,

-44<=l<=44

Reflections collected 31610 35212 86147

Independent reflections 10466 [R(int) = 0.1036] 18437 [R(int) = 0.0410] 18094 [R(int) = 0.0676]

Absorption correction SADABS Not measured Gaussian, face-indexed

Data / restraints / parameters 10429 / 0 / 1232 18437 / 0 / 856 18094 / 0 / 856

Goodness-of-fit on F2 1.037 1.062 1.046

Final R indices

[I>2σ(I)]

R1 = 0.0779,

wR2 = 0.1872

R1 = 0.0590,

wR2 = 0.1285

R1 = 0.0480,

wR2 = 0.0954

R indices (all data) R1 = 0.1470,

wR2 = 0.2231

R1 = 0.0830,

wR2 = 0.1394

R1 = 0.0674,

wR2 = 0.1027

180

Crystal data and structure refinement for 24, 24 (at room temperature), 25, 26.

24 24 (at RT) 25 26

Empirical formula C84 H123 Fe N3 O3 C84 H123 Fe N3 O3 C60 H69 F6 Fe N3 O3 C96 H132 Fe2 N4 O5

* 2.75 CH2Cl2

Formula weight 1278.70 1278.70 1050.03 1767.30

Temperature (K) 100(2) 293(2) 100(2) 100(2)

Wavelength (Å) 0.71073 0.71073 0.71073 0.71073

Crystal system Monoclinic Monoclinic Monoclinic Triclinic

Space group P21/c P21/c C2/c P-1

Unit cell dimensions a = 16.656(2) Å

b = 14.648(2) Å

c = 33.433(3) Å

α = 90 deg.

β = 97.32(1) deg.

γ = 90 deg.

a = 17.0012(2) Å

b = 14.8159(2) Å

c = 33.897(3) Å

α = 90 deg.

β = 96.76(1) deg.

γ = 90 deg.

a = 25.6710(4) Å

b = 24.8786(4) Å

c = 17.9867(3) Å

α = 90 deg.

β = 105.50(1) deg.

γ = 90 deg.

a = 11.9826(6) Å

b = 15.9612(9) Å

c = 27.5321(15) Å

α = 84.48(1) deg.

β = 82.47(1) deg.

γ = 77.64(1) deg.

Volume (Å3);Z 8090.4(16) ; 4 8478.9(16) ; 4 11069.6(3); 8 5086.7(5); 2

Density (calc.)

(Mg/m3)

1.050 1.002 1.260 1.154

Absorp. coeff. (mm-1) 0.232 0.221 0.339 0.479

F(000) 2792 2792 4432 1887

Crystal size (mm) 0.32 x 0.11 x 0.08 0.32 x 0.11 x 0.08 0.27 x 0.19 x 0.18 0.16 x 0.13 x 0.12

θ range for data

collection

2.95 to 28.32 deg. 2.91 to 23.50 deg. 2.97 to 31.07 3.35 to 27.50

Index ranges -22<=h<=22,

-19<=k<=19,

-44<=l<=44

-19<=h<=19,

-16<=k<=15,

-38<=l<=38

-37<=h<=37,

-36<=k<=36,

-26<=l<=26

-15<=h<=15,

-14<=k<=20,

-35<=l<=35

Reflections collected 92357 47308 138510 38197

Independent reflections 20013

[R(int) = 0.0493]

12467

[R(int) = 0.0586]

17667

[R(int) = 0.0479]

21829

[R(int) = 0.0482]

Absorption correction Gaussian,

face-indexed

Gaussian,

face-indexed

Gaussian,

face-indexed

Gaussian,

face-indexed

Data / restraints /

parameters

20013 / 0 / 820 12467 / 216 / 893 17667 / 30 / 700 21829 / 38 / 1065

Goodness-of-fit on F2 1.037 1.023 1.038 1.043

Final R indices

[I>2σ(I)]

R1 = 0.0485,

wR2 = 0.1093

R1 = 0.0688,

wR2 = 0.1563

R1 = 0.0384,

wR2 = 0.0946

R1 = 0.0682,

wR2 = 0.1562

R indices (all data) R1 = 0.0690,

wR2 = 0.1195

R1 = 0.1024,

wR2 = 0.1748

R1 = 0.0458,

wR2 = 0.0989

R1 = 0.1066,

wR2 = 0.1799

181

2) Magnetochemical data

Complex Cu4L14 (2)

MW= 1498 gm/mol; λdia= -1000 x 10-6 cm3mol-1 ;

m=32.12 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff (exp.) µeff(theo.)

1 1.998 0.00402 0.00753 0.17998 0.24639

2 5 0.00591 0.00753 0.21828 0.24639

3 10.003 0.00766 0.00753 0.24851 0.24643

4 14.999 0.00963 0.00778 0.27866 0.25043

5 20.001 0.01442 0.01014 0.34099 0.28591

6 30.001 0.04634 0.03463 0.61139 0.52853

7 39.998 0.1115 0.09353 0.9483 0.86854

8 50.005 0.19708 0.17669 1.26079 1.19378

9 060.01 0.29136 0.26936 1.53298 1.47397

10 70.044 0.38413 0.36179 1.76017 1.70823

11 80.073 0.46943 0.44847 1.94584 1.90189

12 90.09 0.54592 0.52734 2.09838 2.06237

13 100.11 0.61319 0.59818 2.2239 2.19651

14 110.13 0.67235 0.66137 2.32871 2.30961

15 120.15 0.72541 0.71764 2.41885 2.40586

16 130.16 0.77136 0.76775 2.49429 2.48844

17 140.18 0.81147 0.81256 2.55832 2.56004

18 150.19 0.84747 0.85269 2.61446 2.62248

19 160.19 0.88167 0.88874 2.66668 2.67736

20 170.21 0.91385 0.92136 2.71491 2.72605

21 180.22 0.94286 0.95089 2.75768 2.76938

22 190.23 0.96988 0.97774 2.7969 2.80822

23 200.24 0.99383 1.00225 2.83122 2.84319

24 210.23 1.01628 1.02464 2.86302 2.87478

25 220.2 1.03612 1.04518 2.89084 2.90345

26 230.24 1.05326 1.06423 2.91465 2.92979

27 240.26 1.07078 01.0818 2.93879 2.95387

28 250.25 1.08718 1.09803 2.96121 2.97595

29 260.27 1.10477 1.11315 2.98507 2.99637

30 270.25 1.12177 1.12717 3.00795 3.01518

31 280.29 1.13854 1.14033 3.03035 3.03273

32 290.25 1.15384 1.15254 3.05064 3.04892

182

Complex [Ni2 (L2)(NH2CONH2)(OAc)(MeOH) 2] (3)

MW= 866 gm/mol; λdia= -450 x 10-6 cm3mol-1 ;

m=40.4 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff (exp.) µeff(theo.)

1 1.998 0.27169 0.02841 1.48032 0.47869

2 5 0.61728 0.35823 2.2313 1.69981

3 10.003 0.9786 0.89348 2.80944 2.68449

4 15 1.21507 1.26015 3.13054 3.18808

5 20.002 1.38996 1.4917 3.34826 3.46864

6 30.001 1.62955 1.74783 3.62537 3.75464

7 40 1.78154 1.88131 3.79067 3.89537

8 50.002 1.88122 1.96201 3.89528 3.97804

9 60.032 1.94994 2.01587 3.96578 4.03227

10 70.046 2.0008 2.05415 4.01717 4.07037

11 80.067 2.04218 2.08277 4.0585 4.09863

12 90.091 2.07292 2.10495 4.08893 4.1204

13 100.11 2.09862 2.12263 4.1142 4.13767

14 110.09 2.11771 2.137 4.13287 4.15165

15 120.15 2.1372 2.14904 4.15184 4.16333

16 130.16 2.15098 2.15916 4.16521 4.17312

17 140.18 2.16485 2.16781 4.17861 4.18147

18 150.19 2.17675 2.1753 4.19008 4.18869

19 160.2 2.18789 2.18183 4.20079 4.19497

20 170.21 2.19893 2.18759 4.21138 4.2005

21 180.22 2.20801 2.19271 4.22006 4.20542

22 190.22 2.21813 2.19727 4.22972 4.20979

23 200.22 2.22626 2.20138 4.23747 4.21372

24 210.23 2.23375 2.20509 4.24459 4.21727

25 220.24 2.24198 2.20847 4.2524 4.2205

26 230.24 2.24904 2.21154 4.25909 4.22343

27 240.27 2.26024 2.21437 4.26968 4.22614

28 250.24 2.26586 2.21695 4.27499 4.2286

29 260.25 2.27094 2.21934 4.27978 4.23088

30 270.24 2.27934 2.22155 4.28769 4.23298

31 280.14 2.28173 2.22358 4.28993 4.23492

32 290.25 2.29061 2.22551 4.29827 4.23675

183

Complex [Ni4 (LH) 2(OMe) 2(OAc) 2(OHMe) 2] (4)

MW= 1612 gm/mol; λdia= -860 x 10-6 cm3mol-1 ;

m=12.27 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff (exp.) µeff(theo.)

1 2 1.99243 3.02359 4.00876 4.93833

2 4.999 3.46621 4.01102 5.28744 5.68782

3 9.996 4.56533 4.65646 6.06812 6.12839

4 15 4.96198 4.94213 6.32624 6.31358

5 20.003 5.09559 5.05135 6.41085 6.38296

6 30.001 5.10728 5.07334 6.4182 6.39684

7 40 5.0382 5.02374 6.37465 6.36549

8 50 4.96035 4.9668 6.3252 6.32931

9 60.031 4.89486 4.91575 6.28331 6.2967

10 70.052 4.84391 4.87261 6.25052 6.26901

11 80.071 4.80548 4.83651 6.22568 6.24575

12 90.081 4.77253 4.80622 6.2043 6.22616

13 100.07 4.7401 4.78062 6.18318 6.20955

14 110.12 4.71522 4.75861 6.16693 6.19524

15 120.14 4.69132 4.7397 6.15129 6.18292

16 130.16 4.67169 4.72325 6.1384 6.17218

17 140.17 4.6551 4.70885 6.12749 6.16277

18 150.19 4.64667 4.69611 6.12194 6.15442

19 160.19 4.6361 4.6848 6.11498 6.14701

20 170.2 4.63168 4.67468 6.11206 6.14037

21 180.21 4.62484 4.66557 6.10755 6.13438

22 190.15 4.61775 4.65738 6.10286 6.12899

23 200.23 4.61749 4.64984 6.10269 6.12403

24 210.23 4.61483 4.64302 6.10093 6.11954

25 220.24 4.61881 4.63677 6.10356 6.11542

26 230.24 4.62173 4.63103 6.10549 6.11163

27 240.26 4.62821 4.62572 6.10977 6.10813

28 250.25 4.63203 4.62083 6.11229 6.1049

29 260.25 4.63361 4.61628 6.11333 6.10189

30 270.24 4.63972 4.61206 6.11736 6.0991

31 280.24 4.63367 4.60812 6.11337 6.0965

32 290.26 4.62858 4.60442 6.11001 6.09405

184

Complex [CuII2(µ-MeO)L2 (THF)2] (5)

MW= 724 gm/mol; λdia= -375 x 10-6 cm3mol-1 ;

m=25.53 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff (exp.) µeff(theo.)

1 1.999 2.38659E-4 5.997E-5 0.04387 0.02199

2 5.002 4.48819E-4 1.5006E-4 0.06017 0.03479

3 10.003 6.13843E-4 3.0009E-4 0.07036 0.0492

4 15 7.08861E-4 4.5E-4 0.07561 0.06025

5 20.002 7.85329E-4 6.0006E-4 0.07959 0.06957

6 30 9.31086E-4 9.00031E-4 0.08666 0.0852

7 39.999 0.00111 0.0012 0.09447 0.09851

8 50.01 0.0026 0.00155 0.14478 0.11182

9 60.03 0.00516 0.00212 0.20402 0.13069

10 70.048 0.00493 0.00328 0.19944 0.16273

11 80.065 0.00636 0.00557 0.22647 0.21192

12 90.092 0.00996 0.00951 0.28345 0.27691

13 100.1 0.0158 0.0155 0.35694 0.35361

14 110.12 0.02385 0.02381 0.43861 0.4382

15 120.13 0.0341 0.03445 0.52447 0.5271

16 130.15 0.04631 0.04732 0.61119 0.61781

17 140.18 0.06049 0.0622 0.69847 0.7083

18 150.18 0.07555 0.0787 0.78063 0.7967

19 160.19 0.09095 0.09652 0.85649 0.88232

20 170.13 0.10514 0.11517 0.92088 0.96382

21 180.09 0.15125 0.1345 1.1045 1.04153

22 190.22 0.12857 0.1545 1.01833 1.1163

23 200.24 0.18132 0.17438 1.20932 1.18595

24 210.23 0.20026 0.19409 1.27091 1.25118

25 220.24 0.22112 0.21356 1.33545 1.31244

26 230.24 0.23972 0.23261 1.3905 1.36971

27 240.26 0.25684 0.25119 1.43931 1.42338

28 250.24 0.27393 0.26913 1.48642 1.47333

29 260.26 0.28833 0.28653 1.52498 1.52021

30 270.25 0.30181 0.30324 1.56021 1.5639

31 280.24 0.31502 0.31929 1.59399 1.60477

32 290.25 0.32557 0.33473 1.62047 1.6431

185

Complex [L22CuII4 (µ4-O) ] (6)

MW= 1402 gm/mol; λdia= -800 x 10-6 cm3mol-1 ;

m=30.85 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff (exp.) µeff(theo.)

1 1.999 0.0022 0.00311 0.13308 0.15849

2 4.999 0.00371 0.00383 0.17301 0.17586

3 10.002 0.00522 0.00503 0.20511 0.20152

4 15 0.00659 0.00623 0.2306 0.22424

5 20.002 0.00812 0.00744 0.25587 0.24495

6 30.001 0.01206 0.01023 0.31192 0.28719

7 40 0.01984 0.01593 0.40001 0.3584

8 50.007 0.03558 0.02881 0.53567 0.48208

9 60.031 0.06053 0.05171 0.69869 0.64584

10 70.05 0.09999 0.08475 0.89805 0.82678

11 80.07 0.13952 0.12628 1.06082 1.0092

12 90.081 0.18582 0.17387 1.22424 1.18422

13 100.06 0.23476 0.22505 1.37604 1.34729

14 110.13 0.28524 0.27849 1.51679 1.49873

15 120.14 0.33583 0.33188 1.64581 1.63609

16 130.16 0.38596 0.3845 1.76438 1.76103

17 140.17 0.43497 0.43554 1.87304 1.87427

18 150.18 0.48179 0.48461 1.97128 1.97704

19 160.2 0.52705 0.53152 2.06178 2.07051

20 170.2 0.57019 0.57602 2.14451 2.15544

21 180.22 0.61144 0.61827 2.22073 2.23309

22 190.22 0.65108 0.65816 2.29159 2.30401

23 200.25 0.6884 0.69598 2.35635 2.36928

24 210.24 0.72414 0.73156 2.41674 2.42909

25 220.24 0.75845 0.76522 2.47333 2.48434

26 230.24 0.79234 0.79704 2.52798 2.53546

27 240.25 0.82178 0.82717 2.57453 2.58294

28 250.24 0.85384 0.85564 2.62426 2.62702

29 260.26 0.88109 0.8827 2.66581 2.66824

30 270.26 0.90834 0.90832 2.70672 2.70669

31 280.25 0.93295 0.93264 2.74314 2.74268

32 290.25 0.95817 0.95579 2.77997 2.77651

186

Complex FeIII2 L22 (7)

MW= 1246 gm/mol; λdia= -800 x 10-6 cm3mol-1 ;

m=27.76 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff (exp.) µeff(theo.)

1 1.999 0.00938 0.01317 0.27506 0.32595

2 4.998 0.01856 0.01519 0.38687 0.35002

3 10 0.06601 0.08544 0.72965 0.83012

4 15 0.22492 0.22808 1.34688 1.35632

5 20.003 0.36954 0.37543 1.72642 1.74013

6 30.001 0.64667 0.65673 2.28381 2.3015

7 40.001 0.9198 0.93253 2.72373 2.74251

8 50.007 1.193 1.20706 3.10198 3.1202

9 60.032 1.46667 1.48139 3.43941 3.45663

10 70.053 1.74184 1.7546 3.7482 3.7619

11 80.069 2.01492 2.02567 4.03132 4.04206

12 90.088 2.2852 2.29317 4.29319 4.30067

13 100.11 2.55122 2.55521 4.5362 4.53975

14 110.12 2.80876 2.80954 4.75966 4.76032

15 120.15 3.05992 3.05538 4.96791 4.96422

16 130.17 3.29905 3.29081 5.15837 5.15193

17 140.18 3.52759 3.5151 5.33405 5.3246

18 150.19 3.74458 3.7281 5.49566 5.48355

19 160.19 3.94848 3.92953 5.6433 5.62974

20 170.21 4.14351 4.12013 5.78099 5.76466

21 180.21 4.3233 4.29948 5.90508 5.88879

22 190.22 4.4954 4.46858 6.02147 6.00348

23 200.24 4.65671 4.6279 6.12855 6.10957

24 210.23 4.80675 4.77739 6.2265 6.20746

25 220.24 4.95209 4.91838 6.31993 6.29839

26 230.24 5.08527 5.05099 6.40435 6.38273

27 240.27 5.21458 5.17628 6.48527 6.46141

28 250.25 5.3312 5.2938 6.55739 6.53435

29 260.26 5.44199 5.405 6.62517 6.60262

30 270.25 5.54933 5.50978 6.69019 6.66631

31 280.23 5.65361 5.60871 6.75276 6.72589

32 290.26 5.75484 5.70276 6.81295 6.78205

187

Complex MnIII2L22 (8a)

MW= 1244gm/mol; λdia= -600 x 10-6 cm3mol-1 ;

m=26.89 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff(exp.) µeff(theo.)

1 1.998 0.25211 0.04311 1.42597 0.58966

2 5.001 0.63175 0.40159 2.25731 1.79974

3 10 1.22521 0.98777 3.14357 2.82258

4 15.001 1.74069 1.5593 3.74696 3.54636

5 20.002 2.198 2.09149 4.21049 4.1072

6 30.003 2.94746 2.94793 4.87576 4.87615

7 39.998 3.49126 3.54096 5.30652 5.34415

8 50.01 3.88691 3.95348 5.59913 5.64687

9 60.031 4.18325 4.25001 5.80865 5.85482

10 70.052 4.4134 4.47086 5.9663 6.00501

11 80.073 4.59476 4.64077 6.08765 6.11805

12 90.084 4.74048 4.77496 6.18343 6.20588

13 100.11 4.85597 4.88366 6.2583 6.27612

14 110.12 4.94917 4.97313 6.31807 6.33335

15 120.11 5.02472 5.04796 6.36611 6.38082

16 130.16 5.08928 5.1119 6.40688 6.4211

17 140.18 5.14118 5.16669 6.43946 6.45542

18 150.19 5.1893 5.21422 6.46953 6.48505

19 160.2 5.2266 5.25589 6.49274 6.51091

20 170.22 5.26353 5.29273 6.51564 6.53369

21 180.21 5.2912 5.32542 6.53274 6.55383

22 190.24 5.31924 5.3548 6.55003 6.57189

23 200.24 5.34432 5.38119 6.56545 6.58806

24 210.23 5.36357 5.40505 6.57726 6.60265

25 220.25 5.3856 5.42681 6.59076 6.61593

26 230.24 5.40325 5.44663 6.60155 6.628

27 240.27 5.41758 5.46488 6.6103 6.63909

28 250.24 5.43033 5.48156 6.61807 6.64922

29 260.27 5.44188 5.49706 6.62511 6.65861

30 270.26 5.45774 5.51136 6.63475 6.66727

31 280.25 5.46841 5.52463 6.64124 6.67529

32 290.26 5.47791 5.53702 6.647 6.68277

188

Complex [MnIII2L22 (THF)2] (8b).

MW= 1388 gm/mol; λdia= -650 x 10-6 cm3mol-1 ;

m=3.15 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff (exp.) µeff(theo.)

1 1.998 0.47003 0.93522 1.94706 2.74647

2 4.999 1.67597 2.41695 3.67664 4.41522

3 10.001 3.44863 3.85251 5.27402 5.5743

4 15 4.26771 4.51283 5.867 6.03313

5 20.003 4.69551 4.871 6.15403 6.26798

6 30.001 5.13289 5.24137 6.43427 6.50191

7 39.994 5.35646 5.42935 6.5729 6.61747

8 50.005 5.49297 5.54274 6.65613 6.68622

9 60.024 5.5823 5.61841 6.71004 6.7317

10 70.046 5.64892 5.67243 6.74996 6.76399

11 80.071 5.70341 5.71291 6.78243 6.78808

12 90.084 5.75149 5.74434 6.81096 6.80673

13 100.11 5.78237 5.76949 6.82922 6.82161

14 110.13 5.81254 5.79003 6.84702 6.83374

15 120.14 5.83062 5.80713 6.85766 6.84383

16 130.16 5.84936 5.8216 6.86867 6.85235

17 140.12 5.86428 5.83392 6.87742 6.8596

18 150.18 5.8784 5.84471 6.8857 6.86594

19 160.2 5.88611 5.85411 6.89021 6.87146

20 170.21 5.89856 5.86238 6.89749 6.87631

21 180.21 5.90393 5.86973 6.90063 6.88062

22 190.22 5.90934 5.87631 6.90379 6.88447

23 200.23 5.9171 5.88223 6.90833 6.88794

24 210.23 5.91841 5.88758 6.90909 6.89107

25 220.25 5.93376 5.89246 6.91804 6.89393

26 230.23 5.93129 5.89689 6.9166 6.89652

27 240.26 5.95107 5.90097 6.92813 6.8989

28 250.26 5.94699 5.90471 6.92575 6.90109

29 260.25 5.93893 5.90816 6.92106 6.9031

30 270.25 5.957 5.91136 6.93158 6.90497

31 280.16 5.95511 5.91431 6.93048 6.9067

32 290.25 6.02226 5.9171 6.96944 6.90833

189

Complex CrIII2L22 (9)

MW= 1238 gm/mol; λdia= -700 x 10-6 cm3mol-1 ;

m=10.09 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff (exp.) µeff(theo.)

1 2 0.05736 0.01887 0.68017 0.39013

2 5.001 0.11509 0.05192 0.96345 0.64712

3 10.003 0.31612 0.25705 1.59677 1.43989

4 15 0.52389 0.47098 2.0556 1.94904

5 20.002 0.72819 0.67751 2.42348 2.33764

6 30.002 1.11131 1.07116 2.99389 2.93931

7 40.002 1.43313 1.4142 3.39986 3.37733

8 50.005 1.6906 1.6933 3.69266 3.6956

9 60.023 1.89877 1.91498 3.9134 3.93007

10 70.051 2.06498 2.09091 4.08109 4.10663

11 80.065 2.20015 2.23167 4.21254 4.24261

12 90.072 2.31339 2.34597 4.31959 4.3499

13 100.12 2.40744 2.44056 4.40652 4.43673

14 110.14 2.48994 2.51936 4.48139 4.50779

15 120.15 2.55765 2.58597 4.54191 4.56699

16 130.16 2.61839 2.64297 4.59553 4.61705

17 140.18 2.66825 2.69229 4.63908 4.65993

18 150.18 2.71428 2.73523 4.67892 4.69694

19 160.2 2.75576 2.77306 4.71454 4.72931

20 170.21 2.79317 2.80653 4.74643 4.75777

21 180.22 2.82974 2.83637 4.7774 4.78299

22 190.23 2.86029 2.86313 4.80312 4.8055

23 200.25 2.88934 2.88728 4.82745 4.82573

24 210.23 2.9176 2.90908 4.851 4.84391

25 220.25 2.93991 2.929 4.86951 4.86047

26 230.26 2.96553 2.94719 4.89068 4.87554

27 240.25 2.98076 2.96384 4.90323 4.88929

28 250.15 3.00401 2.97903 4.92231 4.9018

29 260.26 3.01805 2.99336 4.9338 4.91358

30 270.25 3.03828 3.00647 4.95031 4.92433

31 280.25 3.05423 3.01866 4.96328 4.9343

32 290.26 3.06364 3.03003 4.97092 4.94358

190

Complex [(VIV=O)2 (µ-Oisoprop) L2)] (10)

MW= 924 gm/mol; λdia= -450 x 10-6 cm3mol-1 ;

m=44.21 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff (exp.) µeff(theo.)

1 1.999 0.00312 0.00711 0.1586 0.23952

2 5 0.0038 0.00735 0.17503 0.24353

3 10.004 0.00457 0.00775 0.19192 0.25007

4 15 0.00515 0.00815 0.20386 0.25643

5 20.004 0.00566 0.00855 0.21366 0.26266

6 30.004 0.00668 0.00939 0.23212 0.27514

7 40.004 0.00864 0.01061 0.264 0.29251

8 50.008 0.01604 0.01329 0.35966 0.32736

9 60.03 0.02703 0.01886 0.4669 0.38999

10 70.042 0.03884 0.02837 0.55972 0.47839

11 80.029 0.05328 0.04214 0.65552 0.583

12 90.043 0.07076 0.05989 0.75544 0.69501

13 100.12 0.08783 0.08095 0.84168 0.80805

14 110.13 0.10569 0.10411 0.9233 0.91636

15 120.15 0.12487 0.12859 1.00358 1.01842

16 130.16 0.1464 0.15356 1.08664 1.11292

17 140.17 0.16992 0.17842 1.17067 1.19962

18 150.18 0.19442 0.20272 1.25224 1.27869

19 160.2 0.2188 0.22617 1.32845 1.35062

20 170.21 0.24233 0.24854 1.39804 1.41584

21 180.2 0.26531 0.26971 1.46282 1.4749

22 190.23 0.28632 0.28976 1.51965 1.52875

23 200.24 0.30672 0.30858 1.57284 1.57763

24 210.24 0.3257 0.32623 1.62079 1.62211

25 220.27 0.34264 0.34282 1.66241 1.66286

26 230.25 0.36043 0.3583 1.70502 1.69996

27 240.26 0.37345 0.37284 1.73555 1.73412

28 250.25 0.391 0.38645 1.77585 1.76548

29 260.26 0.40419 0.39924 1.80555 1.79447

30 270.25 0.41793 0.41123 1.83598 1.82122

31 280.27 0.43277 0.42254 1.86831 1.84608

32 290.24 0.44296 0.43313 1.89016 1.86908

191

Complex CuII2L32 (12)

MW= 1150 gm/mol; λdia= -688 x 10-6 cm3mol-1 ;

m=23.83 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff (exp.) µeff(theo.)

1 2 0.29294 0.32292 1.53711 1.61386

2 5.003 0.54075 0.50075 2.08841 2.00969

3 10 0.61789 0.6125 2.23241 2.22266

4 14.998 0.66212 0.66162 2.31093 2.31005

5 20.001 0.68506 0.68923 2.35062 2.35777

6 30.001 0.70903 0.71919 2.39139 2.40847

7 39.997 0.72393 0.73515 2.41639 2.43504

8 50.008 0.73466 0.74507 2.43423 2.45142

9 60.027 0.74195 0.75184 2.44628 2.46253

10 70.048 0.74804 0.75675 2.4563 2.47055

11 80.047 0.75442 0.76046 2.46675 2.4766

12 90.086 0.7588 0.76338 2.4739 2.48136

13 100.11 0.76307 0.76573 2.48086 2.48517

14 110.14 0.76605 0.76767 2.48568 2.48831

15 120.15 0.769 0.76928 2.49048 2.49092

16 130.11 0.77044 0.77065 2.4928 2.49314

17 140.17 0.7727 0.77183 2.49646 2.49505

18 150.18 0.77451 0.77286 2.49937 2.49671

19 160.19 0.7764 0.77376 2.50242 2.49816

20 170.2 0.77793 0.77455 2.50489 2.49945

21 180.21 0.77876 0.77526 2.50623 2.50059

22 190.22 0.77929 0.7759 2.50708 2.50161

23 200.23 0.77943 0.77647 2.5073 2.50253

24 210.24 0.77952 0.77699 2.50745 2.50337

25 220.25 0.77914 0.77746 2.50683 2.50413

26 230.24 0.7794 0.77789 2.50726 2.50482

27 240.25 0.77965 0.77829 2.50766 2.50546

28 250.14 0.77919 0.77865 2.50691 2.50604

29 260.26 0.78075 0.77899 2.50943 2.50659

30 270.24 0.78392 0.7793 2.51452 2.50709

31 280.25 0.79093 0.77959 2.52573 2.50756

32 290.24 0.78914 0.77986 2.52287 2.50799

192

Complex CoIII2L33 (14)

MW= 1654gm/mol; λdia= -1026 x 10-6 cm3mol-1 ;

m=55.39 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff (exp.) µeff(theo.)

1 2 0.12429 0.49797 1.99594 0.99717

2 5 0.61984 0.67461 2.32311 2.22681

3 10 0.85717 0.86468 2.6301 2.61865

4 15 1.04218 1.03844 2.88227 2.88746

5 20 1.22355 1.2039 3.10342 3.12864

6 30 1.51795 1.48728 3.44938 3.48477

7 40 1.71275 1.69612 3.68361 3.70162

8 50.01 1.83917 1.83994 3.83661 3.8358

9 60.03 1.92401 1.93759 3.93709 3.92327

10 70.05 1.98323 2.0062 4.0062 3.98319

11 80.05 2.02614 2.05755 4.05714 4.02605

12 90.08 2.05843 2.09089 4.08988 4.058

13 100.11 2.08332 2.11482 4.11322 4.08247

14 110.13 2.10298 2.12936 4.12734 4.10169

15 120.15 2.11883 2.14215 4.13971 4.11712

16 130.16 2.13183 2.14959 4.14689 4.12973

17 140.18 2.14267 2.15611 4.15318 4.14021

18 150.19 2.15181 2.15898 4.15594 4.14903

19 160.21 2.15961 2.16235 4.15919 4.15655

20 170.22 2.16634 2.1652 4.16192 4.16302

21 180.22 2.17219 2.16715 4.16379 4.16863

22 190.15 2.17728 2.16771 4.16433 4.17352

23 200.24 2.18186 2.1732 4.16961 4.1779

24 210.24 2.18589 2.17556 4.17187 4.18176

25 220.25 2.18949 2.17611 4.1724 4.18521

26 230.25 2.19273 2.17374 4.17013 4.1883

27 240.25 2.19566 2.17119 4.16767 4.1911

28 250.25 2.19832 2.16294 4.15975 4.19363

29 260.26 2.20075 2.15769 4.15469 4.19595

30 270.24 2.20296 2.15338 4.15055 4.19805

31 280.26 2.205 2.15181 4.14903 4.2

32 290.23 2.20686 2.15055 4.14781 4.20177

193

Complex FeIII2L33 (15)

MW= 1648 gm/mol; λdia= -1026 x 10-6 cm3mol-1 ;

m=39.23 mg ; H = 1T, 4T and 7T

Field(T) Temp. (K) λMT (exp.) λMT (calc.) Field(T) Temp. (K) λMT (exp.) λMT (calc.)

1 2 1.2814 1.32305 4 4.227 1.49312 1.47348

1 2.072 1.31592 1.35379 4 4.526 1.56467 1.54571

1 2.134 1.3449 1.37954 4 4.868 1.64043 1.62298

1 2.203 1.37737 1.40743 4 5.265 1.71871 1.70588

1 2.286 1.41386 1.43996 4 5.734 1.80019 1.79506

1 2.366 1.44711 1.47029 4 6.294 1.88313 1.89023

1 2.456 1.4847 1.50326 4 6.975 1.96641 1.99134

1 2.553 1.52373 1.53749 4 7.82 2.05028 2.0979

1 2.659 1.56482 1.57341 4 8.899 2.13159 2.20941

1 2.773 1.60681 1.61039 4 10.324 2.21394 2.3247

1 2.898 1.65117 1.64909 4 12.289 2.28835 2.44201

1 3.034 1.69721 1.68913 4 15.182 2.37049 2.55969

1 3.184 1.74504 1.73095 4 19.858 2.45989 2.67526

1 3.349 1.79436 1.77431 4 28.688 2.59474 2.78592

1 3.533 1.84599 1.81966 4 51.676 2.86958 2.88875

1 3.738 1.89948 1.86677 4 260.01 3.48723 2.98021

1 3.967 1.95316 1.91552 7 1.999 0.55623 0.56193

1 4.23 2.00777 1.96699 7 2.071 0.57576 0.58073

1 4.526 2.06555 2.01979 7 2.14 0.59429 0.59866

1 4.868 2.12686 2.07489 7 2.203 0.61123 0.61494

1 5.265 2.18697 2.132 7 2.285 0.63313 0.63602

1 5.734 2.24849 2.19146 7 2.365 0.65436 0.65646

1 6.293 2.30945 2.25295 7 2.456 0.67844 0.67956

1 6.973 2.37004 2.31668 7 2.553 0.70393 0.70401

1 7.819 2.43414 2.38271 7 2.658 0.73135 0.73026

1 8.9 2.49972 2.45101 7 2.772 0.76088 0.75851

1 10.324 2.57539 2.5213 7 2.897 0.79302 0.78919

1 12.289 2.65165 2.59361 7 3.033 0.82759 0.82222

1 15.182 2.75825 2.66796 7 3.183 0.86526 0.85821

1 19.859 2.90029 2.74414 7 3.349 0.90644 0.89749

1 28.689 3.11649 2.82184 7 3.533 0.95127 0.94037

1 51.679 3.43761 2.9009 7 3.738 1.00025 0.9873

1 260.01 3.74699 2.9809 7 3.968 1.05425 1.03887

4 2 0.78639 0.79131 7 4.229 1.11229 1.09601

4 2.072 0.81346 0.81717 7 4.527 1.17745 1.15942

4 2.133 0.83621 0.83891 7 4.869 1.24899 1.22974

4 2.203 0.86223 0.86365 7 5.267 1.32733 1.30827

4 2.286 0.89267 0.89269 7 5.734 1.41299 1.39587

4 2.369 0.92277 0.92142 7 6.295 1.50684 1.49465

4 2.456 0.954 0.95118 7 6.973 1.60728 1.60485

4 2.553 0.98846 0.98394 7 7.818 1.71529 1.72881

4 2.659 1.02553 1.01921 7 8.897 1.82814 1.86721

4 2.773 1.0647 1.05653 7 10.322 1.94655 2.02016

4 2.898 1.10687 1.09669 7 12.289 2.05885 2.18621

4 3.034 1.1517 1.13949 7 15.182 2.17588 2.36187

4 3.184 1.1999 1.18558 7 19.858 2.28822 2.54053

4 3.349 1.25121 1.23491 7 28.691 2.41846 2.712

4 3.533 1.30644 1.28825 7 51.67 2.65287 2.86321

4 3.738 1.36544 1.34558 7 260.01 3.38497 2.97932

4 3.967 1.4277 1.40701

194

Complex MnIV2(LA3)2L3 (16)

MW= 1646 gm/mol; λdia= -1026 x 10-6 cm3mol-1 ;

m=46.01 mg ; H = 1T

Temp. (K)

λM λMT µeff

1 2 0.02426 0.44046 1.45348

2 5.001 0.24774 1.40759 1.87883

3 10.003 0.48633 1.97216 2.13161

4 15 0.57824 2.15048 2.24132

5 20.002 0.624 2.23394 2.30295

6 30.001 0.66924 2.31351 2.36986

7 40.002 0.6921 2.35269 2.40557

8 50.004 0.70662 2.37723 2.42777

9 60.034 0.71705 2.39472 2.44295

10 70.053 0.72588 2.40941 2.45395

11 80.076 0.73448 2.42365 2.4623

12 90.09 0.74113 2.43459 2.46884

13 100.09 0.74665 2.44365 2.47411

14 110.13 0.7518 2.45205 2.47845

15 120.16 0.75601 2.45891 2.48209

16 130.17 0.75957 2.4647 2.48517

17 140.17 0.76271 2.46979 2.48782

18 150.19 0.76559 2.47444 2.49013

19 160.14 0.76791 2.47819 2.49214

20 170.22 0.77051 2.48239 2.49394

21 180.21 0.77307 2.48651 2.49553

22 190.23 0.77588 2.49101 2.49696

23 200.24 0.77931 2.49651 2.49824

24 210.24 0.78424 2.5044 2.49941

25 220.25 0.7896 2.51295 2.50047

26 230.24 0.77319 2.4867 2.50144

27 240.25 0.80057 2.53034 2.50233

28 250.26 0.80787 2.54185 2.50315

29 260.28 0.81605 2.55468 2.50391

30 270.26 0.82373 2.56668 2.50461

31 280.26 0.83497 2.58413 2.50526

32 290.25 0.84273 2.59611 2.50587

195

Complex CoIII2L43 (17)

MW/2= 962 gm/mol; λdia/2= -610 x 10-6 cm3mol-1 ;

m=35.04 mg ; H = 1T

Temp.(K)

λmT (exp.) λmT (theo.) µeff (exp.) µeff (theo.)

1 1.999 1.20088 0.90888 3.09905 2.69608

2 4.999 1.48573 1.32593 3.44706 3.25641

3 10 1.56517 1.55712 3.53802 3.52891

4 15.001 1.59172 1.64722 3.5679 3.62957

5 20.002 1.60308 1.6855 3.58061 3.6715

6 30.002 1.60473 1.68889 3.58245 3.67519

7 39.999 1.58877 1.65107 3.56459 3.63381

8 50.007 1.56656 1.60318 3.53959 3.58072

9 60.017 1.53916 1.55685 3.5085 3.5286

10 70.045 1.51082 1.51551 3.47605 3.48144

11 80.071 1.48645 1.47974 3.4479 3.44011

12 90.058 1.46145 1.44915 3.41878 3.40436

13 100.1 1.43819 1.42272 3.39147 3.37318

14 110.13 1.4174 1.39996 3.36686 3.34609

15 120.16 1.39838 1.3802 3.3442 3.32239

16 130.11 1.37794 1.36307 3.31967 3.30171

17 140.17 1.34986 1.34784 3.28567 3.28321

18 150.18 1.33625 1.33442 3.26906 3.26682

19 160.19 1.32542 1.32248 3.25579 3.25218

20 170.2 1.31445 1.31179 3.24229 3.239

21 180.22 1.30508 1.30216 3.23071 3.22709

22 190.23 1.29528 1.29346 3.21856 3.2163

23 200.23 1.28665 1.28557 3.20782 3.20647

24 210.24 1.27879 1.27837 3.198 3.19748

25 220.25 1.27137 1.27177 3.18871 3.18921

26 230.16 1.26307 1.26577 3.17829 3.18168

27 240.25 1.2554 1.26014 3.16862 3.1746

28 250.26 1.25063 1.25497 3.1626 3.16808

29 260.27 1.24408 1.25018 3.1543 3.16203

30 270.23 1.23859 1.24576 3.14734 3.15643

31 280.27 1.23284 1.2416 3.14002 3.15116

32 290.26 1.22827 1.23773 3.1342 3.14624

196

Complex MnIV(LA6)(L6)2 (18)

MW= 1276 gm/mol; λdia= -750 x 10-6 cm3mol-1 ;

m=43.71 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff (exp.) µeff(theo.)

1 1.999 0.18322 0.34439 1.21563 1.66664

2 5.004 0.34322 0.3546 1.66381 1.69117

3 10.004 0.45162 0.35612 1.90855 1.69478

4 15.001 0.35495 0.3564 1.69201 1.69546

5 20.002 0.35628 0.3565 1.69517 1.6957

6 30.003 0.35768 0.35657 1.69851 1.69587

7 39.994 0.35764 0.3566 1.6984 1.69594

8 50.012 0.35837 0.35662 1.70015 1.69597

9 60.036 0.35818 0.35663 1.69969 1.69599

10 70.036 0.35835 0.35663 1.70008 1.69601

11 80.062 0.35888 0.35664 1.70135 1.69602

12 90.099 0.35905 0.35664 1.70174 1.69604

13 100.11 0.35896 0.35666 1.70153 1.69607

14 110.13 0.35867 0.35668 1.70085 1.69613

15 120.16 0.35865 0.35674 1.7008 1.69626

16 130.16 0.35816 0.35684 1.69964 1.69651

17 140.18 0.35794 0.35703 1.69912 1.69696

18 150.18 0.35806 0.35734 1.6994 1.69769

19 160.2 0.35806 0.35781 1.69941 1.69881

20 170.2 0.35829 0.35849 1.69994 1.70041

21 180.22 0.35902 0.35942 1.70167 1.70262

22 190.23 0.36001 0.36065 1.70402 1.70553

23 200.23 0.36133 0.36222 1.70713 1.70924

24 210.15 0.35991 0.36415 1.70379 1.71379

25 220.25 0.36526 0.36653 1.71641 1.71939

26 230.24 0.36819 0.36934 1.72327 1.72596

27 240.26 0.37009 0.37262 1.72772 1.7336

28 250.26 0.37562 0.37637 1.74057 1.74231

29 260.03 0.37788 0.38052 1.74581 1.75188

30 270.27 0.38572 0.38537 1.76383 1.76303

31 280.24 0.38876 0.39061 1.77076 1.77497

32 290.22 0.40336 0.39635 1.80371 1.78796

197

Complex CoIII(L6)3 (23)

MW= 1282 gm/mol; λdia= -750 x 10-6 cm3mol-1 ;

m=33.01 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff (exp.) µeff(theo.)

1 2 0.97157 1.21805 2.78751 3.12113

2 5.003 1.67421 1.56491 3.65919 3.53773

3 10.002 2.16215 1.71231 4.15836 3.70059

4 15 1.75745 1.76464 3.74905 3.75671

5 20.003 1.76257 1.78928 3.7545 3.78285

6 30.001 1.76313 1.80106 3.7551 3.79528

7 40.008 1.75495 1.7848 3.74638 3.77811

8 50 1.73579 1.7541 3.72587 3.74547

9 60.026 1.7113 1.71715 3.6995 3.70581

10 70.052 1.6844 1.67895 3.6703 3.66436

11 80.094 1.65758 1.64202 3.64097 3.62384

12 90.097 1.62726 1.60772 3.60751 3.58579

13 100.1 1.59816 1.57631 3.57511 3.55059

14 110.14 1.56591 1.54772 3.53886 3.51824

15 120.18 1.53605 1.52189 3.50495 3.48876

16 130.16 1.50892 1.49872 3.47386 3.4621

17 140.18 1.48782 1.47771 3.44949 3.43775

18 150.19 1.46745 1.45872 3.42579 3.41559

19 160.2 1.4495 1.44149 3.40478 3.39536

20 170.21 1.43161 1.42582 3.3837 3.37685

21 180.14 1.4143 1.41164 3.36318 3.36002

22 190.23 1.39951 1.39846 3.34555 3.34429

23 200.23 1.38437 1.38648 3.3274 3.32994

24 210.24 1.37037 1.37545 3.31054 3.31667

25 220.25 1.35768 1.36527 3.29517 3.30437

26 230.26 1.34468 1.35586 3.27936 3.29296

27 240.25 1.33336 1.34715 3.26553 3.28237

28 250.26 1.31966 1.33903 3.24871 3.27246

29 260.27 1.31133 1.33147 3.23844 3.26321

30 270.26 1.30753 1.32443 3.23374 3.25457

31 280.27 1.2909 1.31783 3.21311 3.24645

32 290.16 1.28257 1.31171 3.20273 3.23891

198

Complex FeIII(L6)3 (24)

MW= 1277 gm/mol; λdia= -750 x 10-6 cm3mol-1 ;

m=36.24 mg ; H = 1T

Temp.(K)

λm(exp.) λmT(exp.) µeff (exp.)

1 2 0.02597 0.05193 0.6445

2 5.001 0.01737 0.08689 0.8336

3 10.003 0.01016 0.10161 0.9015

4 14.999 0.00708 0.10621 0.9216

5 20.003 0.00543 0.1087 0.9324

6 29.999 0.00372 0.11169 0.9451

7 40 0.00286 0.11428 0.956

8 50.009 0.00234 0.11718 0.9681

9 60.023 0.00202 0.12138 0.9853

10 70.041 0.00182 0.12719 1.0086

11 80.034 0.00166 0.13257 1.0297

12 90.083 0.00153 0.13821 1.0513

13 100.1 0.00144 0.14408 1.0735

14 110.12 0.00137 0.15099 1.0989

15 120.17 0.00133 0.15941 1.1291

16 130.16 0.00131 0.17066 1.1683

17 140.18 0.00135 0.18933 1.2305

18 150.19 0.00151 0.22696 1.3473

19 160.2 0.00185 0.29656 1.54

20 170.21 0.00238 0.40525 1.8003

21 180.09 0.00296 0.53316 2.0649

22 190.23 0.00339 0.64459 2.2705

23 200.23 0.00369 0.73845 2.4302

24 210.24 0.00387 0.81323 2.5503

25 220.27 0.00397 0.8751 2.6455

26 230.25 0.00402 0.92676 2.7225

27 240.26 0.00404 0.97116 2.7869

28 250.24 0.00404 1.0101 2.8423

29 260.25 0.00402 1.0469 2.8936

30 270.24 0.00399 1.0794 2.9381

31 280.26 0.00397 1.1121 2.9824

32 290.25 0.00392 1.1382 3.0171

199

Complex FeIII(L11)3 (25)

MW= 1049 gm/mol; λdia= -550 x 10-6 cm3mol-1 ;

m=40.29 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff (exp.) µeff(theo.)

1 1.999 0.28784 0.68833 1.51724 2.34627

2 5 0.69449 0.85478 2.35674 2.6146

3 10.005 1.0698 0.92394 2.92503 2.71832

4 15.001 0.94635 0.94859 2.7511 2.75435

5 20.001 0.96578 0.96124 2.77919 2.77265

6 30.003 0.9801 0.97409 2.79972 2.79113

7 40.002 0.98448 0.98059 2.80597 2.80042

8 50.006 0.98696 0.98452 2.80949 2.80602

9 60.028 0.98787 0.98715 2.8108 2.80977

10 70.039 0.9881 0.98903 2.81112 2.81244

11 80.087 0.98936 0.99045 2.81291 2.81446

12 90.099 0.99038 0.99157 2.81437 2.81605

13 100.08 0.98995 0.9925 2.81375 2.81738

14 110.15 0.99095 0.99336 2.81517 2.8186

15 120.16 0.99075 0.99422 2.81489 2.81982

16 130.16 0.99099 0.99519 2.81524 2.8212

17 140.18 0.99122 0.99637 2.81555 2.82287

18 150.19 0.99309 0.99786 2.81821 2.82497

19 160.19 0.9946 0.99975 2.82035 2.82764

20 170.21 0.99723 1.00214 2.82408 2.83102

21 180.22 1.00039 1.0051 2.82855 2.8352

22 190.14 1.00311 1.00868 2.83239 2.84025

23 200.23 1.0085 1.01304 2.83999 2.84638

24 210.23 1.01346 1.01813 2.84697 2.85352

25 220.25 1.02078 1.02404 2.85723 2.86179

26 230.23 1.02851 1.03075 2.86803 2.87115

27 240.25 1.03558 1.03835 2.87787 2.88172

28 250.26 1.04655 1.04681 2.89307 2.89343

29 260.26 1.05587 1.05614 2.90593 2.9063

30 270.26 1.06704 1.06634 2.92126 2.9203

31 280.25 1.07877 1.07739 2.93727 2.93539

32 290.26 1.08966 1.08931 2.95206 2.95158

200

Complex [FeIII2(µ-O)(L12)4] (26)

MW= 1532 gm/mol; λdia= -820 x 10-6 cm3mol-1 ;

m=25.28 mg ; H = 1T

Temp.(K)

λmT(exp.) λmT(theo.) µeff (exp.) µeff(theo.)

1 2 0.01033 0.02634 0.28863 0.46096

2 5.004 0.02255 0.02634 0.42645 0.46096

3 9.999 0.02871 0.02634 0.48121 0.46096

4 15.003 0.02738 0.02634 0.46992 0.46096

5 20.003 0.02573 0.02634 0.45556 0.46096

6 30.003 0.02689 0.02637 0.46569 0.46115

7 39.996 0.02776 0.02676 0.47322 0.46455

8 50.009 0.03036 0.02878 0.49485 0.48181

9 60.037 0.03566 0.03429 0.53627 0.52589

10 70.048 0.04639 0.04467 0.61166 0.60023

11 80.062 0.06261 0.06037 0.71065 0.6978

12 90.107 0.08344 0.08103 0.82034 0.80841

13 100.11 0.11627 0.10552 0.96839 0.92254

14 110.14 0.13358 0.13291 1.03797 1.03536

15 120.15 0.15994 0.16205 1.13578 1.14327

16 130.16 0.21356 0.19223 1.31242 1.24515

17 140.18 0.23133 0.22289 1.36596 1.34079

18 150.19 0.27284 0.25361 1.48344 1.43022

19 160.2 0.29906 0.28422 1.55309 1.51407

20 170.2 0.3259 0.31457 1.62129 1.59286

21 180.22 0.35259 0.34472 1.68637 1.66744

22 190.22 0.37975 0.37453 1.75012 1.73805

23 200.25 0.40613 0.40419 1.80989 1.80555

24 210.23 0.43451 0.43347 1.87205 1.86981

25 220.25 0.46061 0.46268 1.92746 1.93179

26 230.25 0.4855 0.49167 1.97885 1.99138

27 240.26 0.51565 0.52055 2.03937 2.04903

28 250.25 0.54393 0.54925 2.09454 2.10477

29 260.26 0.56956 0.57791 2.14332 2.15898

30 270.26 0.59213 0.60645 2.18538 2.21165

31 280.27 0.6237 0.63494 2.24288 2.263

32 290.26 0.6542 0.66329 2.29706 2.31298

201

Complex Ni2L32 (13) and MnIV(LA8)(L8)2 (20)

MW= 1140 (13) and 1024 (20) gm/mol; λdia= -680 x 10-6 (13) and -560 x 10-6 (20) cm3mol-1 ;

m=36.40(13) and 23.09(20) mg ; H = 1T

Temp.(K)

(13)

µeff(exp.)

(13)

Temp.(K)

(20)

µeff(exp.)

(20)

µeff(theo.)

(20)

1 1.998 0.3282 1 1.999 1.30222 1.68697

2 5.001 0.4292 2 5 1.68037 1.71264

3 10.003 0.5007 3 10.005 1.70354 1.71642

4 15 0.5269 4 14.999 1.70483 1.71712

5 20.002 0.5334 5 20.001 1.70684 1.71737

6 29.999 0.533 6 30.002 1.70944 1.71755

7 40 0.5295 7 39.999 1.71132 1.71761

8 50.007 0.5376 8 50.008 1.71349 1.71764

9 60.032 0.5502 9 60.029 1.71362 1.71766

10 70.047 0.5648 10 70.054 1.71521 1.71766

11 80.071 0.5799 11 80.071 1.71717 1.71767

12 90.084 0.5937 12 90.096 1.71848 1.71768

13 100.13 0.6055 13 100.1 1.71955 1.71768

14 110.13 0.6148 14 110.18 1.72112 1.71768

15 120.16 0.6247 15 120.13 1.72147 1.71769

16 130.17 0.6359 16 130.11 1.72069 1.7177

17 140.17 0.646 17 140.17 1.72094 1.71772

18 150.19 0.6547 18 150.19 1.71989 1.71776

19 160.2 0.6609 19 160.2 1.71847 1.71784

20 170.21 0.6627 20 170.2 1.71563 1.71797

21 180.18 0.6676 21 180.21 1.71226 1.71817

22 190.22 0.6727 22 190.21 1.71075 1.71849

23 200.24 0.6776 23 200.22 1.70885 1.71895

24 210.24 0.6858 24 210.15 1.70883 1.71958

25 220.26 0.6946 25 220.26 1.7137 1.72045

26 230.25 0.6983 26 230.26 1.7119 1.72157

27 240.28 0.7087 27 240.26 1.71445 1.72299

28 250.26 0.7193 28 250.25 1.72128 1.72475

29 260.27 0.7213 29 260.27 1.72563 1.72691

30 270.25 0.7293 30 270.26 1.73009 1.72947

31 280.25 0.7546 31 280.26 1.73354 1.73249

32 290.24 0.7425 32 290.24 1.74971 1.73599

202

Complex MnIV(LA9)(L9)2 (21) and MnIV(LA10)(L10)2 (22)

MW= 1147 (21) and 1120 (22) gm/mol; λdia= -600 x 10-6 (21) and -600 x 10-6 (22) cm3mol-1 ;

m=44.83(21) and 32.44(22) mg ; H = 1T

Temp.(K)

(21)

µeff(exp.)

(21)

µeff(theo.)

(21)

Temp.(K)

(22)

µeff(exp.)

(22)

µeff(theo.)

(22)

1 1.999 1.09263 1.77633 1 1.998 1.44496 1.6941

2 5.023 1.66037 1.80614 2 5.001 1.69474 1.71991

3 10.003 2.02483 1.81049 3 10.005 1.71419 1.7237

4 15.001 1.78425 1.81132 4 15.001 1.71721 1.72441

5 20.002 1.77662 1.81162 5 20.003 1.72004 1.72466

6 29.999 1.78627 1.81184 6 30 1.72276 1.72484

7 40 1.79329 1.81193 7 39.998 1.72462 1.7249

8 50.006 1.80006 1.81198 8 50.009 1.72716 1.72493

9 60.037 1.80532 1.81202 9 60.024 1.72783 1.72494

10 70.063 1.80889 1.81205 10 70.057 1.72878 1.72496

11 80.063 1.81462 1.81207 11 80.071 1.73104 1.72496

12 90.106 1.81937 1.8121 12 90.086 1.73266 1.72497

13 100.1 1.82295 1.81214 13 100.09 1.73375 1.72497

14 110.12 1.82677 1.8122 14 110.13 1.7348 1.72499

15 120.14 1.82987 1.81233 15 120.15 1.73499 1.72503

16 130.16 1.83263 1.81257 16 130.16 1.73429 1.7251

17 140.12 1.83465 1.81297 17 140.18 1.73264 1.72525

18 150.19 1.83761 1.81364 18 150.19 1.7302 1.72551

19 160.2 1.84008 1.81466 19 160.19 1.72753 1.72594

20 170.2 1.84221 1.81613 20 170.21 1.72569 1.72661

21 180.22 1.84389 1.81815 21 180.22 1.72672 1.72757

22 190.23 1.84542 1.82083 22 190.23 1.72604 1.72891

23 200.23 1.84685 1.82425 23 200.15 1.73008 1.73068

24 210.25 1.84882 1.82851 24 210.24 1.72997 1.73301

25 220.24 1.84798 1.83365 25 220.25 1.73436 1.73591

26 230.25 1.85139 1.83977 26 230.24 1.73374 1.73946

27 240.26 1.85403 1.8469 27 240.26 1.73623 1.74373

28 250.26 1.85907 1.85506 28 250.25 1.74067 1.74875

29 260.17 1.85601 1.86418 29 260 1.73667 1.75441

30 270.24 1.85992 1.8745 30 270.24 1.75211 1.76119

31 280.27 1.87352 1.88584 31 280.27 1.76335 1.76869

32 290.24 1.87872 1.89813 32 290.27 1.79002 1.77703

203

327 328 329 330 331 332

B[mT]

2.06 2.05 2.04 2.03

Experimental

Simulated

g values

327 328 329 330 331 332

B[mT]

Experimental

Simulated

2.06 2.05 2.04 2.03

g values

327 328 329 330 331 332

B[mT]

2.06 2.05 2.04 2.03

Experimental

Simulated

g values

327 328 329 330 331 332

Experimental

Simulated

B[mT]

2.06 2.05 2.04 2.03

g values

3) Magnetic and EPR data.

The complexes 19-22 exhibit similar EPR spectra within the range 310mT to 370mT

(Figure 4.8, Chapter 4). The simulation of the spectra between 326 mT and 332 mT are shown

below along with the parameters used.

19 20

AMn = 106.7 G AMn = 106.6 G

AN = 4.20 G AN = 4.36 G

AH = 4.62 G AH = 5.15 G

AH = 2.71 G AH = 2.83 G

21 22

AMn = 106.7 G AMn = 106.9 G

AN = 4.29 G AN = 4.23 G

AH = 5.15 G AH = 5.02 G

AH = 2.83 G AH = 2.78 G

204

50 100 150 200 250 300

0.3

0.4

0.5

0.6

0.7

0.8

µ eff / µ B

T / K

50 100 150 200 250 300

0.3

0.6

0.9

1.2

1.5

1.8

Simulated

Experimental

µeff / µB

T / K

50 100 150 200 250 300

0.0

0.5

1.0

1.5

2.0

Simulated

Experimental

µ eff / µ B

T / K

50 100 150 200 250 300

0.0

0.5

1.0

1.5

2.0

Simulated

Experimental

µeff / µB

T / K

The magnetic data for complexes 13, 20-23 is given below together with the

parameters used for simulation.

13 20

J12 = J23 = - 400 cm-1

g1 = g3 = 2.00

g2 = 1.99

21 22

J12 = J23 = - 295 cm-1 J12 = J23 = - 330 cm-1

g1 = g3 = 2.0 g1 = g3 = 2.00

g2 = 2.05 g2 = 1.99