Document [original]

Transition Metal-Radical Complexes 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

Chandan Mukherjee

Mülheim an der Ruhr, 2006

iii

To my parents and Debanjali

"If we knew what it was we were doing, it

would not be called research, would it?"

Albert Einstein

vii

Acknowledgements:

I would like to mention that submission of this Thesis would not have been possible at all had

I not been bestowed with the benign association of the scientific elite of the Max-Planck-

Institut für Bioanorganische Chemie, Mülheim an der Ruhr, Germany.

xFirst of all, I humbly offer my deepest respect to Prof. Dr. P.Chaudhuri, who

offered me the scope to be a Research scholar under him. His constant guidance,

perpetual inspiration with occasional reformatory thrashings always acted as

motivating factors to my research work.

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

providing me with all needed laboratory facilities.

xDr. T. Weyhermüller and Mrs. H. Schucht for their excellent work with the X-

ray crystallography.

xDr. E. Bothe,Mrs. P. Höfer and Mr. H. Schmidt for their help during

electrochemical measurements.

xProf. Dr. E. Rentschler for assistance with the fitting of some magnetochemical

data.

xDr. E. Bill, Mr. A. Göbels, Mr. F. Reikowski Mr. J. Bitter, Mrs. Sand and Mr.

B. Mienert for discussions and measurements of EPR, SQUID, NMR and

Mössbauer spectroscopy.

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

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

analyses.

xMrs. J. Theurich,Mrs. T. Montenbruck and Mrs. B. Deckers for their

helpfulness in general.

xDr. J. F. Berry and Dr. P. L. Larsen, for careful revision of the manuscript.

xDr. K. Ray, Mr. N. Roy, Mr. B. Biswas, Dr. A. Patra, Dr. K. Chlopek, Dr. S.

Khanra, Mr. S. Presow, Dr. K. Merz, Dr. T. Petrenko, Dr. R. Kapre, Dr. S.

Kinge, Mr. B. Pluijmaekers, Dr. S. Kokatam, Dr. S. Blanchard, Dr. L.

Benisvy, Dr. Y. Song, Dr. N. Aliaga-Alcade, Dr. K. S. Min, Mr. F. Benidito,

Dr. N. Muresan, Dr. J. Sandor, and all the other members of the institute for

their help to my work and friendly life inside and outside the laboratory.

viii

xFamily Mukherjee and Family Ghosh for constant inspiration and

encouragement.

xFamily Basak for their help in general.

xMy parents for their constant inspiration and encouragement.

x I am highly indebted to my girlfriend Debanjali for her understanding,

inspiration, invaluable support and having faith in me.

xMy elder Brother and elder Sister for their encouragement.

xDeutsche Forschungsgemeinschaft (DFG) and Max-Planck-Gesellschaft (MPG)

for financial support.

This work was carried out between August 2003 and June 2005 at the Max-Planck-Institut

für Bioanorganische Chemie, Mülheim an der Ruhr, Germany.

Papers published:

1. A review article: Biomimetic metal-radical reactivity: aerial oxidation of alcohols,

amines, aminophenols and catechols catalyzed by transition metal complexes

Phalguni Chaudhuri, Karl Wieghardt, Thomas Weyhermüller, Tapan K. Paine, Soumen

Mukherjee and Chandan Mukherjee. Biol. Chem. 2005, 386, 1023-1033.

2. A trinuclear complex containing MnIIMnIIIMnIV, radicals, quinone and chloride

ligands potentially relevant to PS II

Chandan Mukherjee, Thomas Weyhermüller, Karl Wieghardt and Phalguni Chaudhuri,

Dalton Trans., 2006, 2169-2171.

Examination Committee:

Prof. Dr. W. Bremser

Prof. Dr. K. Huber

Prof. Dr. G. Henkel

Prof. Dr. P. Chaudhuri

Examination: 7th August, 2006.

Contents:

Chapter 1:

Introduction, Background and Objectives

1.1Introduction 3-4

1.2 Background 5-16

1.3 Objectives 17-20

1.4 References 21-23

Chapter 2:

Synthesis, Characterization and Catalytic Reactivity of Transition Metal

Complexes formed with N, N

-bis (2-hydroxy-3,5-di-tert-butylphenyl)-

2,2

-diaminobiphenyl amine, (H4L)

2.1 Synthesis and characterization of the ligand (H4L) 27-28

2.2 Methoxide bridged dinuclear non-oxovanadium (V)

complex (1) 29-33

2.3 Five coordinate square pyramidal high-spin Fe(III)

complex(2) 34-40

2.4 Distorted square planar Ni(II) and Pd(II) complexes(3and 4) 41-50

2.5 Distorted square planar Cu(II) complex(5) and

its catalytic reactivity

2.5.1 Synthesis and characterization of the complex (5) 51-56

2.5.2 Catalytic reactivity, Aerial oxidation of benzylalcohol;

mimicking the function of Galactose Oxidase 57-63

2.6 References 64-67

xii

Chapter 3:

Synthesis, Characterization and Catalytic Reactivities of the

Tetracoordinate Cu(II)-Complexes formed with the Ligands, N(2-

hydroxy-3,5-di-tert-butylphenyl)-3,5-di- substituted-aniline, H2LX(X = -

CF3, -F, -Cl, -OMe, -tBu)

3.1 Introduction 72-73

3.2 Synthesis and characterization of ligands and complexes 73-85

3.3 Catalytic reactivities, aerial oxidation of benzyl alcohol, ethanol

and methanol; mimicking the function of Galactose Oxidase 86-90

3.4 References 91-91

Chapter 4:

Synthesis, Characterization and Catalytic Reactivity of Polynuclear

Transition Metal Complexes formed with N (2-hydroxy-3,5-di-tert-

butylphenyl)-2-aminobenzylalcohol, H3LCH

4.1 Synthesis and characterization of the ligand (H3LCH

OH) 94-96

4.2 Alkoxide bridged dinuclear oxovanadium (V) complex (1) 97-102

4.3 A trinuclear Mn-cluster(2) 103-109

4.4 A Cu4O4 cubane complex (3)

4.4.1 Synthesis and characterization of the complex (3) 110-121

4.4.2 Catalytic reactivity, Aerial oxidation of 2-aminophenol;

mimicking the function of Phenoxazinone Synthase 122-125

4.5 References 126-127

xiii

Chapter 5:

Synthesis, Characterization and Catalytic Reactivities of a Monoradical-

Containing Mononuclear Mn(IV) complex

5.1 Synthesis and characterization of the ligand (H3LCOOH) 130-131

5.2 Synthesis and characterization of the complex (1) 132-138

5.3 Catalytic reactivities,

5.3.1 Aerial oxidation of primary amines; mimicking the function of

Amine Oxidases 139-142

5.3.2 Aerial oxidation of 2-aminophenol; mimicking the function of

Phenoxazinone Synthase 143-146

5.4 References 147-148

Chapter 6:

Conclusions and Perspectives

6.1 Conclusions 154-156

6.2 Perspectives 157-158

Chapter 7:

Equipment and Experimental work

7.1 Methods and equipments 162-164

7.2 Experimental works

7.2.1 Synthesis of ligands 165-168

(i) Synthesis of N, N

-bis(2-hydroxy-3, 5-di-tert-butylphenyl)-

2,2

-diaminobiphenyl compound (H4L)

(ii) Synthesis of N(2-hydroxy-3,5-di-tert-butylphenyl)-

benzylalcohol compound (H3LCH

OH)

(iii) Synthesis of N(2-hydroxy-3,5-di-tert-butylphenyl)

anthanilic acid compound (H3LCOOH)

xiv

7.2.2 Synthesis of complexes 169-179

(i) Synthesis of [V(L)(

2OMe)2(L)V]0

(ii) Synthesis of [Fe(HL

)Cl]0

(iii) Synthesis of [Ni(L

)]0

(iv) Synthesis of [Pd(L

)]0

(v) Synthesis of [Cu(L

)]0

(vi) Synthesis of [Cu(L

)][PF6]

(vii) General method for the Synthesis of Cu(II) complexes

, [Cu(LX

)2] formed with H2Lx ligands

(viii) Synthesis of [V2O2(LCat

OH )2]0

(ix) Synthesis of [Mn3(LSQ

)2(LCat

OH ) (LBQ

OH )Cl]0

(x) Synthesis of [Cu4(LSQ

)4]0

(xi) Synthesis of [Mn(LSQ

COOH

)(LCat

COOH )][HNEt3]

7.2.3 Reactivity studies 170-182

Appendices: 186-202

(1) Crystallographic data

(2) Magnetochemical data

(3) Curriculum Vitae

Abbreviations:

Technical terms:

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

exp. : experimental

Fc+/Fc : internal electrochemical standard

H : Hamiltonian

J : coupling constant ( cm-1)

m/z : mass per charge

RT : room temperature (298 K)

S : electron spin

sim. : simulated

TIP : temperature independent paramagnetism

Units:

Å : angstrom (10-10 m)

cm : centimeter

emu : electromagnetic unit

G : gauss

h : hour

K : Kelvin

m : meter

M : molar

min. : minute

mm : millimeter

xvi

nm : nanometer (10-9 m)

s : second

T : tesla

V : volts

ȝB : bohr magnetron

Symbols:

Ȝ : wavelength (nm)

İ : extinction coefficient (M-1cm-1)

ȝeff : magnetic moment (ȝB)

:Isomer shift (mm/s)

'EQ : quadrupole splitting (mm/s)

Solvents and reagents:

TBAPF6 : tetrabutylammonium hexafluorophosphate, supporting electrolyte

Cat.: catechol

SQ : iminosemiquinone

BQ : iminobenzoquinone

CH2Cl2 : dichloromethane

CHCl3 : chloroform

Et2O : diethylether

Et3N : triethylamine

EtOH : ethanol

HCl : hydrogen chloride

KBr : potassium bromide

MeOH : methanol

MeCN : acetonitrile

Techniques:

CV : cyclic voltammetry

EA : elemental analysis

EI : electron ionisation

EPR : electron paramagnetic resonance

ESI : electrospray ionisation

IR : infrared spectroscopy

xvii

MS: mass spectroscopy

SQUID : superconducting quantum interface device

SWV : square wave voltammetry

UV-VIS/NIR : ultraviolet-visible/ near infrared spectroscopy

p.i : Paramagnetic impurity

Latin expressions:

ca. : around

et al. : and co-workers

e.g. : for example

i.e. : namely

tert- : tertiary

vs. : versus, against

via : through

xviii

Chapter 1

Introduction, Background and Objectives

Introduction, Background and objectives

1.1 Introduction

Bioinorganic chemistry constitutes the discipline at the interface of the more classical

areas of inorganic chemistry and biology.1 Although biology is generally associated with

organic chemistry, inorganic elements and especially metal ions are essential to life processes.

Bioinorganic chemists study these inorganic species, with special emphasis on how they

function in vivo. Inorganic elements have also been artificially introduced into biological

systems as probes of structure and function. Bioinorganic chemistry thus has two major

components namely the study of the naturally occurring inorganic elements in biology and the

introduction of metals into biological systems as probes and drugs. Metalloenzymes are a

subclass of metalloproteins that perform specific catalytic functions. Many metalloenzymes

catalyze reactions that involve either oxidation or reduction of substrates. In most of these

enzymatic catalytic processes aerial oxygen plays a crucial role to bring the enzyme to its

initial active form.

Widespread occurrence of amino-acid radicals in enzyme catalysis is now well

documented.2-6 Enzymes containing amino-acid radicals are generally associated with

transition metal ions typically iron, manganese, cobalt or copper. In some instances the metal

is absent; it is apparently replaced by redox-active organic cofactors such as S-

adenosylmethionine or flavins. Functionally, their role is analogous to that of the metal ion in

metalloproteins. The metal ion containing active sites help to generate and stabilize the

amino-acid radical and the radical, in turn, initiates catalysis by abstracting a hydrogen atom

from the substrate. There are embellishments to this principle. Nevertheless, leaving aside

Photosystem II, the general scheme of ‘metal generates radical that initiates catalysis by

hydrogen-atom abstraction’ remains the underlying principle. Tyrosine-based radical enzymes

are among the best characterized.6-7 The tyrosine residue functions as a redox-active cofactor

by interconverting between the oxidized phenoxyl radical and the normal phenol or phenolate

states. The fungal enzymes galactose oxidase, amine oxidases, cytochrome c oxidase, etc. are

examples of the enzymes that use both tyrosine residues and metals as partners in affecting

redox chemistry.

The field of biomimetic chemistry covers a large area, quite as large as

biochemistry itself. Non-exhaustively, we can emphasize the following topics: (i) abiotic

models for the active sites of enzymes, (ii) models for biological processes.

Abiotic models may serve: (i) to elucidate structures of biomolecules (enzymes) not

known or partially known to date, (ii) to elucidate biological mechanisms which remain black

boxes (still unknown), (iii) to prepare authentic reagents (catalysts) usable for chemical

Introduction, Background and objectives

syntheses. The applied fields that ensue may be: (i) new catalysts (syntheses), (ii) new drugs

(medicinal chemistry), (iii) new nutrients (agrochemistry), (iv) tools for biological studies. It

should also be emphasized that biomimetic chemistry not only serves biology, but may also

be a conceptual approach for chemical problems. As an example, organic syntheses may be

inspired by known biosynthetic pathways.

Two types of models for the active sites in

metalloenzymes can be envisaged: structural models and functional models (of course, ideal

models are relevant to both types!). Structural models for the active sites in metalloenzymes

can be used to help the determination of the active site molecular structure. The design of

these models starts from partial spectroscopic data concerning the enzyme itself and from the

ensuing hypothesis for the molecular formula of the active site. The comparison of the

spectroscopic data obtained from the model and from the enzyme respectively, confirms or

invalidates the hypothesis. So, step by step, the hypothesis for the active site structure is

improved. A lot of structures have been established by this approach, which has been further

confirmed by crystallographic structural determinations. Structural models may also be

functional models. Mimicking the known (crystallographic data) structure of the active site of

the enzyme can lead to models that exhibit the catalytic function of the enzyme. These models

may be valuable catalysts for syntheses. They are also tools for studies concerning the

enzymatic mechanism: some transient species are often involved for a given enzymatic

catalytic cycle, which are more or less characterized. Good models for these reaction

intermediates may be key steps for the understanding of the enzymatic processes.

The design of functional

models for redox metalloenzymes can start from the known, partially known or unknown

structure of the active site of the enzyme. The stoichiometry of the catalyzed reaction and the

products of the reaction are the only properties which have to be known.

Thus, the study and modeling of the active site of metalloproteins is a field of great

interest within the scientific community. Indeed, proteins containing iron, manganese, cobalt

or copper ions at the active sites are mainly involved as redox catalysts in the range of

biological processes, such as electron transfer, dioxygen transport, Photosystem II and

oxidation of various bio-substrates. New and very effective bio-inspired homogeneous

catalysts for common reactions under mild conditions may be discovered.8

Introduction, Background and objectives

1.2 Background

Galactose Oxidase (GOase) from filamentous heat-rot fungus Fusarium ssp. was first

isolated in 1959, 9

that selectively catalyzes the aerial oxidation of primary alcohols to the

corresponding aldehydes with concomitant reduction of molecular oxygen to hydrogen

peroxide(eq 1). 10

GOase is a single polypeptide with a molecular mass of 68.5 kDa, and is unusual

because, in contrast to most copper proteins that affect multi-electron redox reactions by using

multinuclear active sites, GOase uses an isolated single copper center to carry out the required

two-electron redox chemistry.

Figure 1.1. Structure of GOase. (A) The active site showing the copper ligands, the thioether

bond between Tyr272 and Cys228, and the stacking interaction with Trp290. (B) An overview

of the GOase monomer showing domains 1 and 2 and showing the locations of the residues

Cys383, Tyr436 and Val 494 (shown in mauve and highlighted by yellow labels), which form

the basis of the present study. The active site residues are shown in atom colouring and are

labelled by arrows. Non-covalent bond interactions are shown as dotted lines.11-12 (C)

Schematic diagram of GOase.

GOase

RCH2OH + O2 RCHO + H2O2............eq 1

Introduction, Background and objectives

The copper ion is square pyramidal, with the tyrosinate ligand (or protonated form)

weakly bound in the axial position and two histidine imidazole ligands, a second tyrosinate,

and either water (pH 7) or acetate (from buffer, pH 4.5)12-13 in the equatorial sites, the latter

being the binding site of the substrate to the copper center. Interestingly, the equatorial

tyrosinate is linked to a nearby cysteine via an ortho CS bond to afford a thioether-modified

phenolate ligand (Y272-C228), which is within S-stacking distance of a tryptophan residue

(Figure 1.1). The active site of GOase contains one copper (II) ion and a tyrosine 272 radical.

Hence, two redox sites are present in the active form of this mononuclear metalloenyzme. The

mechanism of GOase is now well established (Figure 1.2).14-16 As a first step of the catalytic

process, an alcoholate is formed through the deprotonation of the substrate by the axial

tyrosinate ligand. In the next reaction step, a hydrogen atom

Figure 1.2. The proposed mechanism for Galactose Oxidase.14

is abstracted from the substrate to the equatorial tyrosine 272 radical and subsequently or

simultaneously the copper center is reduced to Cu(I) by the single electron transfer from the

ketyl radical. The catalytic cycle is closed by recovering the initial copper oxidation state

under concomitant reduction of molecular oxygen to hydrogen peroxide. Measurement of the

Introduction, Background and objectives

kinetic isotope effect (KIE) reveals that the H-atom abstraction from the D-C atom of the

coordinated substrate is rate limiting for this catalytic process.

Another copper-containing mononuclear metalloenzyme is Amine Oxidase (AOs),

which catalyzes the oxidative deamination of amines to the corresponding aldehydes, with

subsequent reduction of molecular oxygen to hydrogen peroxide (eq 2).15

Amine Oxidases can be divided into two groups based on the cofactors they utilize,

quinone and copper containing Amine Oxidases (CuAOs) (Figure 1.3, 1.4) and flavin-

dependent mono Amine Oxidases (MAOs). MAOs can oxidize primary, secondary, and

tertiary amines either by concerted covalent catalysis or by a single electron transfer

mechanism, both requiring flavin adenine dinucleotide (FAD) as a cofactor.17-18 Quinone

copper containing Amine Oxidases generally oxidize primary amines and can be subdivided

based on the cofactor present in the active site. The first group contains 2,4,5-trihydroxy-

phenylalanine quinone (TPQ) or topa-quinone, which is produced by post-translational

modification of an invariant tyrosine residue (Figure 1.5.i).21 The second class of quinone

copper-containing amine oxidases uses lysyl tyrosyl quinone (LTQ) as their cofactor and are

referred to as lysyl oxisases (Figure 1.5.ii).

Figure 1. 3. Crystal structures of WT-ECAO adduct I (A) and Y369F adduct II (B) (magenta

sphere, copper; blue spheres, water).20

E + RCH2NH2E. RCH2NH2Ered + RCHO

+ O2

- H2O2& NH3

.......eq 2

Introduction, Background and objectives

Figure 1. 4. Active site of E. coli copper Amine Oxidase.19

Figure 1.5. (i) 2,4,5-trihydroxy-phenylalanine quinone (TPQ) (ii) lysyl tyrosyl quinone(LTQ).

Figure 1.6. Mechanism for the biogenesis of the TPQ cofactor in Phenylethylamine Oxidase

(PEAO).

Introduction, Background and objectives

For the biogenesis of the TPQ cofactor from tyrosine residue (Figure 1.6), copper (II)

initially binds to the apoprotein to give a copper (II)-enzyme complex (A). The complex

exists in equilibrium with a copper (I)-tyrosine radical (B). In the next step copper (I) ion

reacts with dioxygen to produce an activated oxygen complex, which is copper (II)-

superoxide (C). This activated complex attacks the tyrosine radical to yield dopa quinone (D).

This copper oxide/ hydroxide species may be in rapid exchange with solvent water, allowing

solvent oxygen incorporation in TPQ. Rotation about the E-carbon would move the C2 ring

near the copper bound oxidase (E). Nucleophilic attacks by the copper oxide on the dopa

quinone would yield topa (F). In presence of dioxygen, topa would then rapidly be oxidized to

TOQ (G).

RCH2NH2

NHCH2R

H+

NH=CHR

RCHO

NH2

TPQ Substrate

Schiff base

Product

Schiff base

H2O

Reduced TPQ

(aminoquinol)

Figure 1.7. The proposed mechanism for Amine Oxidases.14

Introduction, Background and objectives

The role of the TPQ cofactor in amine oxidation is well understood. To start the

reductive half reaction, TPQ reacts with the substrate amine to form the substrate schiff base,

activating the C-1 proton of the substrate for abstraction. Proton abstraction results in the

product schiff base (PSB), with reduction of the cofactor. Hydrolysis of this intermediate

releases the aldehyde product, leaving the enzyme in aminoquinol form (TPQred). In the

oxidative half-reaction, oxygen interacts with TPQred to produce hydrogen peroxide and

iminoquinone.

NH2

O2O2

-Cu2+

His His

His

NH2

Cu2+

His His

His

NH2

Cu2+

His His

His

OH2

NH4O2

Cu2+

His His

His

H2OOH2

Reduced TPQ

(aminoquinol)

+++

H2O

,H2

Scheme A. The proposed mechanism for the oxidative half reaction viz. direct involvement of

oxygen.

Introduction, Background and objectives

Finally, hydrolysis of the iminoquinone releases ammonia and regenerates TPQox

(Figure 1.7). Detailed spectroscopic and kinetic studies show that only TPQ cofactor takes

part in the reductive half-reaction.22-30 The mechanism of the oxidative half-reaction is less

well understood. During this half reaction it has been proposed that the TPQred cofactor is

either oxidized directly by oxygen (Scheme A), or oxidised indirectly via the active site

copper (Scheme B).

NH2

Cu+

His His

His

NH2

Cu2+

His His

His

NH2

Cu2+

His His

His

OH2

NH4O2

Cu2+

His His

His

H2OOH2

Reduced TPQ

(aminoquinol)

H2O

,H2

Cu+-Topaquinone

-semiquinone

Scheme B. The proposed mechanism for the oxidative half reaction viz. the active site

copper(II).

Introduction, Background and objectives

In the first scenario, oxygen is bound in a non-metal site prior to reaction with reduced

cofactor, whereas in the second proposal the copper (II) takes one electron from aminoquinol

to produce copper (I) and TPQ-semiquinone form. The evidence for the later proposal

consists of direct observation of the copper (I)-semiquinone species by EPR spectroscopy in

anaerobic conditions, in the substrate-reduced enzyme from various sources. Support for the

former hypothesis includes extensive kinetic experiments with bovine serum amino oxidase

(BSAO). It is quite clear the copper (II) center in the active site of amine oxidases not only

converts the tyrosine residue to topa quinone (TPQ) but also plays a crucial role for the

oxidative half-reaction.

Phenoxazinone synthase (PHS), an oligomeric multicopper oxidase produced by

streptomyces antibioticus, is responsible for the six-electron oxidative coupling of two

molecules of 4-methyl 3-hydroxy anthraniloyl petapeptide to form antineoplastic agent

actinomycin D ( eq 3).31a

NH2

CONHR CONHR

CONHR

NH2

PHS

2+3/2 O2

+3 H2O

…..eq 3

PHS exists in two different forms. One is a dimeric form and the other is a hexameric

form. The dimers and hexamers are distinct stable molecular forms and are not related by a

simple equilibrium-aggregation phenomenon. The regulation and structural differences

between these two oligomeric forms is currently unknown. The hexametic form is much more

catalytically active than that of the dimeric form, probably due to the accessibility of the

active site, geometry of the copper centers, and the availability of the proper solvent channels.

Recently, J. P. Allen, W. A. Francisco and co-workers 31b have published an incomplete but

conclusive crystal structure of the hexameric form of PHS.

The hexameric form consists of six equivalent subunits which are not

crystallographycally related but follows an approximate 6-fold rotation about the center.

Figure 1.8 shows the structure of the hexameric form.

Introduction, Background and objectives

Figure 1.8. Hexameric structure of PHS. The cross-sectional diameter is 185 Å with a 50 Å

inner cavity that is unoccupied. The exterior of the hexamer is formed by domain 3 (red), with

domain 1 (blue) largely forming the interior as well as the protein-protein contact region with

domain 2 (green). The copper cofactors (purple) as well as the coordinating ion or molecule

X and water molecule (red) are shown as spheres. Note that the loop extending from domain

1 to 2 (black) contributes to the binding site between the subunits.

Each subunit of the hexamer contains five copper atoms as speculated earlier by

spectroscopic studies.32-33 These copper atoms form one mononuclear type 1 center, two

mononuclear type 2 centers, and one binuclear type 3 center.

The type 1 center, identified as Cu1, has distorted square pyramidal geometry with

His524, His608, Cys603, Met613 and a missing axial ligand common to type 1 copper

centers. Among these residues, only axial methionine ligand is not conserved among the

multicopper oxidases. The type 1 copper center is ~12.5 Å away from Cu2 of the type 3

center and depicted in Figure 1.9. The type 1 copper center is connected to type 3 center

through the HCH motif that facilities the transfer of electrons between these two centers.

Introduction, Background and objectives

Figure 1.9. Copper centers including the coordinating ligands. (A) Diagram of the

arrangement of the four conserved copper atoms, the distances to the ligating atoms, and the

distances between the copper atoms. (B) Four conserved copper atoms and their surrounding

ligands (shaded by atom type). (C) New type 2 copper atom and the three histidine ligands

(shaded by atom type). X represents an unidentified bridging ligand such as OH.

The distance between two copper centers in the binuclear type 3 copper center is 3.88

Å. Two copper atoms of the unit are identified as Cu2 and Cu3 and are linked with each other

via an unidentified molecule or group denoted by X. The Cu2-X-Cu3 bond angle is 153.13q.

His163, His201 and His604 act as ligands to Cu2 center and His929, His203 and His602 are

the ligands attached to the Cu3 center.

The copper atom of one of the type 2 centers, identified as Cu4, is 3.63 and 3.86 Å

away from Cu2 and Cu3 centers, respectively. The Cu4 center is surrounded by His161,

His527 and a water molecule.

Introduction, Background and objectives

The fifth copper atom, identified as Cu5, is part of type 2 center and bound at a loop

that establishes interactions between the subunits of the hexamer and stabilises the form. The

Cu5 center has T-shape coordination with three histidines and depicted in Figure 1.10.

The distance between the Cu5 and both Cu1 and Cu4 centers is ~ 25 Å and is unlikely to play

a role in the transfer of electrons to the substrates.

Figure 1.10. Stereoview; the three histidines, 434, 438, and 440, and the Cu5 center.

The type 2 centers seem to be involved in catalytic activity, and can be reduced by

substrate under anaerobic conditions. The mechanism for the formation of the phenoxazinone

chromophore is studied using isolated enzyme and the proposed mechanism is illustrated in

Scheme C.33-34

Introduction, Background and objectives

NH2

OOH

NH2

OOH

NH2

+H2O2

H2O + 1/2 O2

-2e-

r.d.s

Scheme C . The proposed mechanism for the formation of phenoxazinone chromophore by 6-

electron oxidation of 2 molecules of 2-aminophenol.

Introduction, Background and objectives

1.3 Objectives

In 1991 Ito and co-workers characterized Galactose Oxidase crystallographycally.

Since then, structural and functional model compounds of Galactose Oxidase have drawn

attention from bioinorganic chemists. The search for low molecular weight phenoxyl Cu(II)

complexes as functional models for GO which would mimic this reactivity had a promising

start in 1996 when Tolman and co-workers35 reported a stoichiometric oxidation of benzyl

alcohol to benzaldehyde by a phenoxyl radical complex. In the same year Wang and Stack36

reported the first truly catalytic system, which in presence of a base and an oxidizing agent

oxidizes benzyl alcohol to benzaldehyde with up to 10 turnovers. A different system was

described by Pierre and co-workers in 1998.37 They showed that the electrochemically one-

electron oxidized phenoxyl radical complex electrocatalyzes, in the presence of KOH, the

oxidation of primary alcohols to the corresponding aldehydes (where > 30 turnovers were

observed).

In 1998 Stack38 et al. and Chaudhuri, Wieghardt and co-workers 39 described the first

biomimetic catalytic systems for the aerial oxidation of primary alcohols. Among the

structural models of the active site of GOase described 35-49 only a few contain two phenolic

arms and involve the [N2O2] copper coordination sphere of the enzyme. Some of the

functional models of GOase involve salen-type ligands and exhibit interesting catalytic activity

only with activated alcohols as substrates. These model complexes do true GOase chemistry

but are not, strictly speaking, structural models. The best results have been obtained by

Chaudhuri, Wieghardt et al.39, 45,46 with a set of complexes in which the redox chemistry

during the catalytic cycle is ligand based and the proposed mechanism is similar to the

mechanism proposed for GOase itself.

Very recently, a few structural and functional models of GOase have appeared in

literature 50-52 which uses a phenol containing tripodal ligand. The models mimics the native

enzyme structurally and also the functional aspect by oxidizing primary alcohols to the

corresponding aldehydes with moderate turnover numbers.

Functional as well as structural models for Amine Oxidases are very rare in

literature. Largeron et al.53 have used controlled-potential electrolysis with an organic

cofactor, 3, 4-azaquinone, to oxidize various primary amines. A copper containing functional

model complex was reported by Chaudhuri et al.51 Investigations of the mechanism for the

catalytic oxidative deamination of amines by amine oxidases is very important as functional

model compound(s) and pure proteins do not provide clear ideas yet. The ambiguity

regarding the mechanism arises in the oxidative half-reaction (Figure 1.11). W. Kaim et al.

Introduction, Background and objectives

have shown the existence of temperature-dependent valence tautomerism between copper(I)-

semiquinone and Cu(II)-quinone forms.54 It may be a clue that amine oxidases follow

different mechanistic pathways at different temperatures. Hence, it is essential to investigate

the above postulation with a good quality functional model complex of amine oxidases.

NH2

-Cu2+

His His

His

NH2

Cu+

His His

His

+++

Figure 1.11. Two different mechanistic path ways for the oxidative half reaction for AOs.

Structural model compound(s) of Phenoxazinone Synthase has yet to be synthesized.

A few functional models exist in the literature.55-59 In most of the cases oximatecobalt(II)57

has been used. Oximateiron(II)57 is also found as a good functional model for Phenoxazinone

Synthase. The mechanism that has been proposed using model compounds for the formation

Introduction, Background and objectives

of the phenoxazinone chromophore from 2-aminophenol is a radical-mechanism. 55-57 Though

the formation of Phenoxazinone chromophore by catalytic activity of Phenoxazinone

Synthase is an aerial oxidation process, molecular oxygen has been used together with

functional model compounds to oxidize 2-aminophenol to 2-amino-phenoxazine-3-one.

It is important to note that the author L. I. Simándi 55-56 has shown use of

oximatecobalt(II), 2-aminophenol and molecular oxygen for not only catalytic activation of

dioxygen but also for kinetics and mechanistic study of the catalytic oxidation of 2-

aminophenol. It is quite clear that to have a functional model compound for Phenoxazinone

Synthase aerial oxidation of 2-aminophenol is important. Hence, a functional model that can

have catalytic activity for aerial oxidation of 2-aminophenol to 2-amino-phenoxazine-3-one is

necessary.

The aim of this work is two fold: (i) to gain deeper insight into the electronic structure

of the metal core and the ligand surrounding of these radical-containing complexes and,

thereby, to better understand such metal-radical interactions in natural systems; and (ii) to

explore the aerial-oxidation chemistry of well-characterized metal complexes, which is

expected to provide the basis for new catalytic oxidation systems for synthetic and industrial

processes.

To produce radical containing metal complexes, which will act as functional as well as

structural model compounds of some radical containing metalloenzymes, e.g. Galactose

Oxidase, Amine Oxidases etc. following non-innocent (which will change their redox

property in the presence of metal ion and oxygen) ligands have been synthesised. These

ligands are H4L, H2Lx, H3LCOOH, H3LCH2OH (Figure 1.12).

The ligand H4L was synthesized in order to prepare distorted square planar

transition metal +II complexes. Owing to its steric constraint, radical-containing Cu(II)

complex with distorted square planar geometry will be formed. The complex would be an

ideal example to study the active site of GOase and also its function as a catalyst for aerial

oxidation of alcohols.

Radical-containing Ni(II), Pd(II), Pt(II) complexes with distorted square planar

geometry are important to find out the effect of distortion on the electronic structure to those

of perfect or nearly perfect corresponding square planar complexes.

Furthermore the ligand, H4L can be employed to prepare pentacoordinate square

pyramidal or trigonal bipyramidal, or hexacoordinate-octahedral complexes with transition

metals like iron, cobalt, manganese, chromium etc. These complexes are important in

understanding metal-radical interactions and also as bio-inspired catalysis.

Introduction, Background and objectives

H4L

3LCH2OH

H4L H3LCH2OH

H2Lx H3LCOOH

Figure 1.12. Sketch of ligands.

The ligands, H2Lx, are bidentate ligands. These ligands may form square planar copper

complexes with 2:1 ligand: metal ratio. To find out the effect of substituent on the geometry

and the catalytic processes (if they catalyze aerial oxidations of alcohols or amines), a few

ligands with electron donating or withdrawing groups at the 3,5-positions of the N-phenyl

ring have been synthesized.

Besides synthesizing non-innocent tetradentate and bidentate ligands, tridentate

ligands were synthesized, H3LCOOH and H3LCH2OH. The ligand, H3LCH2OH may have a tendency

to form polynuclear transition metal complexes in its deprotonated form containing an

alcoholate group with a strong S-donor ability.

Contrary to the ligand, H3LCH2OH, a weakly S-donating group (corboxylate) will be a

part of the ligand, H3LCOOH. Hence, formation of polynuclear transition metal complexes is

less likely with the ligand, H3LCOOH.

H N

N H

O H H O

X = -tBu

-OMe

-H

-F

-CF3

N H

O H

N H

O H

Introduction, Background and objectives

1.4 References

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Introduction, Background and objectives

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Introduction, Background and objectives

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Chapter 2

Synthesis, Characterization and Catalytic Reactivity of

Transition Metal Complexes formed with N, N

-bis (2-

hydroxy-3,5-di-tert-butylphenyl)-2,2

-diaminobiphenyl

amine, (H4L)

Chapter 2

2.1 Synthesis and characterization of the ligand, N, N

-bis(2-

hydroxy-3,5-di-tert-butylphenyl)-2,2

-diaminobiphenylamine,

H4L

The ligand, H4L (shown in Figure 2.1) was synthesized by stirring 2,2c

diaminobiphenyl with 3,5-di-tert-butyl-catechol (1:2) in n-hexane, under air, with

triethylamine as a base. The ligand was characterized by different spectroscopic methods by

IR, NMR, GC, GC-MS and mass spectroscopy. The ligand shows characteristic peaks in IR

spectrum due to –O-H and –N-H stretches at 3479 cm-1and 3371 cm-1 respectively. The peaks

at 2957 to 2868 cm-1 are due to the –C-H stretching of the tert-butyl group, a sharp band at

1595 cm-1 is observed due to the -C-C stretching of the aromatic rings, and bands at 1509-

1426 cm-1 from skeletal vibrations. The –C-N stretching appears at 1579 cm-1. The sharp

peak at 753 cm-1 is due to the –C-H stretching of the tetra substituted aromatic rings. The 1H

NMR spectrum in CD2Cl2 for H4L exhibits signals at 1.23 ppm (s, 18 t-Bu H), 1.40 ppm (s,

18 t-Bu H), 5.16 ppm (s, 2H), 7.30-6.56 ppm (12 H from aromatic ring). GC and GC-MS

measurements have been performed to check the purity and the composition of the ligand.

Mass-spectroscopy in EI mode confirms the composition of C40H52N2O2 (592 gm / mole) for

H4L. Showing its non-innocent character, the ligand can exist in the five different oxidation

states depicted in Figure 2.2. The tetradentate ligand (H4L) is able to lose both phenolic

protons and amine protons thereby producing two iminosemiquinone radicals in presence of

transition metal(s) ions and oxygen.1 The tert-butyl groups at the 2 and 4 positions of the

phenol stabilize these iminosemiquinone radicals.2

H4L

Figure 2. 1. The non-innocent ligand H4L.

Chapter 2

O- -O

O-O

N- N

O- -O

HNN

O-

+4H+

-4H

-1e +1e

-1e

+1e

-1e +1e

-1e

+1e

S = 0 S = 0

S = 1/2

S = 2 or 0

S = 0

S = 1/2

Figure 2.2. Different oxidation states of H4L.

This chapter concerns synthesis, characterization and catalytic reactivities of the

complexes formed with the first row and Pd from second row transition metals. These

complexes are, [V V2(P2OMe)2 (L)2] (1), [Fe III(HLx)Cl] (2), [Pd II (Lxx)] (3), [Ni II (Lxx)] (4)

and [Cu II (Lxx)] (5)

Chapter 2

2.2 Methoxide bridged dinuclear non-oxovanadium (V)

complex (1)

It is well established that vanadium is present at the active site of certain enzymes,

including haloperoxidases in sea algae and lichens 3-7 and some nitrogenases in nitrogen-

fixing azoto-bactor.8-9 Furthermore, vanadium ions are bound to tyrosinate residues in

vanadium-modified transferrin 10 and amavadim, 11-13 and certain sea squirts or tunicatess

(polyphenol ascidians) are able to store vanadium in high concentrations. In view of the

importance of the interaction of V(III), V(IV) and V(V) with tyrosinate residues,10,14

vanadium-phenolate chemistry is receiving considerable attention.14-17 There is well

developed chemistry for V(IV)/V(V) “oxo” species but very few “bare” i.e. “non-oxo”,

vanadium complexes with phenol or phenoxyl as chelating ligand have been prepared and

fully characterized. 18-32, 36 Hence, synthesis and full characterization of non-oxo vanadium(V)

complexes with phenol bites are worth investigation.

Complex 1was prepared by sequentially adding H4L (2 mmol), VOSO4.5H2O (2

mmol) and 0.2 ml triethylamine in 30 ml methanol under air. Deep blue colored,

microcrystalline solid precipitated after stirring this solution for 2h.

IR spectrum of complex 1 is different than that of the ligand IR spectrum. The peaks,

in the 2952-2866 cm-1 region arise due to the –C-H stretching of the tert-butyl group, and are

seen to be present in the complex IR spectrum, and thereby clearly indicate the coordination

of the ligand with the central metal. The absence of 3478 cm-1 and 3370 cm-1 bands,

characteristic for -O-H and –N-H stretching, clearly indicates the deprotonation of the

phenolic and amino protons after coordination of the ligand to the metal. However, the

presence of a broad band at approximately 3400 cm-1 is possibly due to the coordinated water

molecule in complex 1. Complex 1 does not exhibit any sharp band(s) in the 900-1000 cm-1

region. This feature is indicative for the absence of the V=O structural unit in the complex.

The elemental analysis for 1 suggests a composition of C82H102N4O6V2 (1340 g/mol), in good

agreement with the ESI (pos, in CH2Cl2) spectrum where an m/z signal at 1340.5 is attributed

to M+.

X-ray quality single crystals of 1 were isolated from a 1:1:1 MeOH, CH3CN and

CH2Cl2 solution mixture. ORTEP diagram of the molecule with atom labeling scheme is

shown in Figure 2.3 and Table 2.1 contains selected bond lengths and bond angles. The

Chapter 2

binuclear vanadium complex is neutral with pseudo octahedral geometry around the both

vanadium (V(1), V(2)) ions. The two vanadium ions are connected through two P2methoxide

bridges. Complex 1 contains a C2 axis that passes vertically through the V(1), O(45), V(2) and

O(95) atoms containing plane. The oxidation state of the vanadium ions could be assigned to

+III, +IV or +V with two iminosemiquinone radicals, one iminosemiquinone radical or the

amidophenolate form of the ligand respectively. The respective O(1)-C(2), C(7)-N(8), O(27)-

C(28), and C(22)-N(21) bond lengths are 1.326(5), 1.369(7), 1.331(7) and 1.406(5) Å. From

these bond lengths V(III) with two iminosemiquinone composition can be ruled out easily for

the neutral complex 1. The average V-phenolic oxygen bond length is 1.885(3) Å. From the

V-phenolic oxygen bond length it is not possible to assign the oxidation state of the vanadium

ion.33

Figure 2.3. ORTEP diagram of 1.

Chapter 2

The average C-C bond length of the tert-butyl containing phenyl rings supports assignment of

the amidophenolate form of the coordinating ligands.34 Therefore, from a structural

perspective, oxidation state of the both vanadium ions in the neutral complex 1 is assigned as

V(V) and coordinated ligands are categorized as the fully reduced amidophenolate form.

Table 2.1. Selected bond distances (Å) and angles (degree) for 1

V(1)-O(28) 1.884(3) V(1)-N(21) 1.950(3)

V(1)-O(45) 1.908(3) V(1)-95(1) 2.057(3)

V(1)-O(1) 1.919(3) V(1)-V(2) 3.1195(10)

O(28)-V(1)-O(45) 106.68(13) N(21)-V(1)-N(8) 91.84(15)

O(28)-V(1)-O(1) 88.22(13) O(28)-V(1)-O(95) 82.24(12)

O(45)-V(1)-O(1) 159.10(13) O(45)-V(1)-O(95) 75.82(12)

O(28)-V(1)-N(21) 81.4(2) N(21)-V(1)-O(95) 159.32(14)

O(45)-V(1)-N(21) 96.96(13) N(8)-V(1)-O(95) 107.29(13)

O(28)-V(1)-N(8) 162.75(15) O(1)-V(1)-O(95) 92.23(12)

O(45)-V(1)-N(8) 89.82(14) V(1)-O(95)-V(2) 103.85(13)

O(1)-V(1)-N(8) 77.26(14) V(1)-O(45)-V(2) 103.99(13)

Variable temperature (2-290 K) magnetic susceptibility measurements using a SQUID

magnetometer at 1 T indicate that complex 1 is diamagnetic. The diamagnetic nature of the

complex was further confirmed by measuring 1H NMR spectrum of 1 in CD2Cl2 solution at

300 K(see experimental section). The 51V NMR spectrum is depicted in Figure 2.4.

The spectrum of complex 1 exhibits a broad peak at ~ 120 ppm (relative to VOCl3).

Figure 2. 4.51V NMR in CD2Cl2 at room temperature.

Chapter 2

The cyclic voltammogram (CV) of complex 1 has been recorded in CH2Cl2 containing

0.1M [nBu4N] PF6 as a supporting electrolyte at a glassy carbon working electrode and a

Ag/AgNO3 reference electrode. Ferrocene was used as an internal standard, and potentials are

referred versus the ferrocenium/ferrocene couple (Fc+/Fc). Figure 2.5 shows the CV of 1

recorded in the potential range +0.7 V to –1.2 V. Within that potential range complex 1 shows

one reversible one-electron-transfer wave and one irreversible one-electron-transfer wave.

Figure 2.5. Cyclic voltammogram of 1 at room temperature under an anaerobic condition.

From coulometric measurement at an appropriately fixed potential it has been

established that the wave at 0.23 V corresponds to one electron oxidation and the wave at –

0.822 V corresponds to one-electron reduction.

The electronic spectra of the neutral species and electrochemically oxidized species

are shown in Figure 2.6. In the neutral complex, the intense peak at 630 nm is assigned to

phenolate-to-metal (V) charge transfer (LMCT) 34, 55 which shifts to smaller wavelength, 600

nm, upon one-electron oxidation. The absorption coefficient also increases from 10700 M-

1cm-1 to 12400 M-1cm-1. As both vanadium ions are in the +V oxidation state, the oxidation

must be ligand centered. The new band appearing at 400 nm (H = 7000 M-1cm-1) is

presumably due to the formation of phenoxyl radical. Erasmus-Buhr et. al. 35 and Chaudhuri

et. al .36 have reported the band at 400 nm is due to the ligand to metal charge transfer.

0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2

E(V) vs Fc+/Fc

Chapter 2

300 400 500 600 700 800 900 1000

A/0.5 cm

Wavelength / nm

Figure 2.6. Change in UV-VIS spectrum of complex 1 during coulometric 1 e- oxidation.

Thus the electrochemical behaviour of the complex can be summarized as follows.

[LxL VV2(P2OMe)2]+ [L2VV2(P2OMe)2] [L2VVVIV(P2OMe)2]-

Complex 1+ Complex 1 Complex 1-

-1e-

+1e-

Chapter 2

2.3 Five coordinate square pyramidal high-spin Fe(III)

complex(2)

The ligand H4L (2 mmol), anhydrous FeCl3(4 mmol) and 0.2 ml triethylamine were

added in 30 ml CH3CN. After stirring under air for 1 hour, deep greenish-blue

microcrystalline solid was precipitated out with an yield of 75%.

The IR spectrum of complex 2 shows a sharp band at 3280 cm-1 and indicates the

presence of –NH group. The bands in the 2960-2870 cm-1 region are due to the –C-H

stretching of tert-butyl groups and confirm the coordination of the ligand to the metal. The IR

spectrum can also serve as an important tool in understanding the oxidation state of the

coordinated ligand qualitatively. The iminosemiquinone form of the ligand, i.e. one electron

oxidized ligand, while chelated to the Fe(III) ion usually shows strong band for C-Ox at

around 1485 cm-1 and a less intense band at approximately 1247 cm-1 arises due to phenolic

C-O stretching. 37-45 In the IR spectrum of complex 2 both bands are observed and hence,

provides qualitative evidence for the presence of mixed valence iminosemiquinone-

aminophenolate form of the coordinated ligand. The mass spectrum in ESI (Pos, CH2Cl2)

mode shows two intense peaks at 679.45 corresponding to M+( C40H49N2O2FeCl) and at 644.2

corresponding to [M-Cl]+. Elemental analysis confirms further the composition

C40H49N2O2FeCl (679.5 g/ mol) and supports the result obtained by mass spectrum analysis.

Figure 2.7. ORTEP diagram of 2.

Chapter 2

Dark greenish-blue crystals of complex 2 were obtained by slow evaporation of a 1:1

CH2Cl2 / CH3CN solution mixture. The X-ray crystal structure has been determined at 100(2)

K to minimize standard deviations as compared to the room temperature structure. ORTEP

diagram of the complex molecule is shown in Figure 2.7, and important bond distances and

angles appear in Table 2.2. The geometry of the central Fe ion is square-pyramidal where two

N-atoms and two O-atoms occupy basal positions. The distance between the plane comprising

of N(8)-N(21)-O(1)-O(28) and Fe ion is 0.55 Å. A chloride ion, which is attached to Fe ion, is

at the apex position of the pyramid. The respective Fe(1)-O(1), Fe(1)-O(28), Fe(1)-N(21),

Fe(1)-N(8), and Fe(1)-Cl(1) bond distances are 1.943(2), 1.905(2), 2.044(2), 2.206(3) and

2.2277(3) Å and are in accord with the pentacoordinate high-spin ferric species.46 The C-O

and C-N bond distances for the chelated ligand containing O(1), O(28), N(21), N(8) atoms are

1.305(3), 1.347(3), 1.452(4) and 1.345(4) Å respectively. These above bond distances indicate

that one of the tert-butyl containing phenyl rings is in iminosemiquinone form and the other is

in aminophenolate form. C(9)-N(8)-C(7), C(7)-N(8)-Fe(1), C(9)-N(8)-Fe(1) bond angles are

119.1(2), 113.4(2) and 126.34(18)q respectively. 114.9(2), 102.9(2) and 110.7(2)q are the

respective C(22)-N(21)-C(20), C(22)-N(21)-Fe(1) and C(22)-N(21)-Fe(1) bond angles.

Table 2.2. Selected bond distances (Å) and angles (degree) for 2.

Fe(1)-N(8) 2.044 (2) C(4)-C(5) 1.444 (4)

Fe(1)-N(21) 2.206 (3) C(5)-C(6) 1.364 (4)

Fe(1)-O(1) 1.943 (2) C(6)-C(7) 1.422 (4)

Fe(1)-O(28) 1.905 (2) C(22)-N(21) 1.424 (4)

C(2)-O(1) 1.305 (3) C(22)-C(23) 1.381 (4)

C(7)-N(8) 1.345 (4) C(23)-C(24) 1.393 (4)

C(2)-C(7) 1.453 (4) C(24)-C(25) 1.398 (4)

C(2)-C(3) 1.426 (4) C(25)-C(26) 1.402 (4)

C(3)-C(4) 1.371 (4) C(26)-C(27) 1.411 (4)

C(9)-N(8) 1.424 (4) C(27)-O(28) 1.347 (4)

C(9)-C(10) 1.389 (4) C(20)-C(15) 1.398 (4)

C(10)-C(11)

C(11)-C(12)

C(12)-C(13)

C(13)-C (14)

C (14)-C (9)

1.383 (4)

1.384 (4)

1.382 (4)

1.396 (4)

1.409 (4)

C(15)-C(16)

C(16)-C(17)

C(17)-C(18)

C(18)-C(19)

C(19)-C(20)

1.392 (4)

1.393 (4)

1.383 (4)

1.387 (4)

1.386 (4)

O(1)-Fe(1)-N(8) 79.79 (9) O(1)-Fe(1)-O(28) 99.20 (9)

N(8)-Fe(1)-N(21) 79.33 (9) O(1)-Fe(1)-N(21) 146.61 (9)

N(21)-Fe(1)-O(28) 85.3 (11) O(28)-Fe(1)-N(8) 148.62 (10)

C(27)-O(28)-Fe(1) 114.78 (19) C(2)-O(1)-Fe(1) 112.21 (18)

C(22)-N(21)-C(20) 114.9 (2) C(7)-N(8)-C(9) 119.1 (2)

Fe(1)-N(21)-C(20) 110.72 (18) C(9)-N(8)-Fe(1) 126.34 (18)

C(22)-N(21)-Fe(1) 102.9 (17) C(7)-N(8)-Fe(1) 113.4 (2)

Chapter 2

From the above bond angles it is quite clear that N(8)-atom is sp2 hybridized and

supports the amido form while the N(21)-atom is sp3 and in amino form. Moreover, from the

crystal structure, the H-atom attached to N(21) atom has been observed directly. Therefore,

from the crystal structure it can be concluded that the neutral pentacoordinate mononuclear

iron complex is a monoradical-containing Fe(III) ion.46

Zero-field Mössbauer spectrum of the solid sample of complex 2 has been recorded at

80 K to obtain independent spectroscopic information on the local spin and the oxidation state

of the iron ion. Figure 2.8 shows this spectrum. Complex 2 exhibits a single quadrupole

doublet and the simulation of the experimental results gives an isomer shift,G, of 0.436 mms-1,

and a quadruple splitting, «'EQ«, of 1.542 mms-1. These parameters unequivocally indicate the

presence of a high-spin ferric ion in the neutral mono-radical-containing complex.46-47

Figure 2.8. Mössbauer spectrum of 2at 80 K and at zero-field.

Figure 2.9. Magnetic measurements of 2 at different field and different temperature.

0 50 100 150 200 250 300

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4 Sim

Exp

eff / P%

T / K

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.0

0.4

0.8

1.2

1.6

2.0

1 T

4 T

7 T

M/Ng

H/kT

-4 -2 0 2 4

0.90

0.92

0.94

0.96

0.98

1.00

Sim

Exp

Relative Transmission

Velocity / mm sec-1

Chapter 2

Variable temperature magnetic susceptibility measurements on a solid sample were

performed in the range 2-290 K for complex 2 using SQUID magnetometer at external field 1

T. Figure 2.9 shows Peff vs T and field dependant magnetization measurements. At room

temperature as well as 9 K the Peff value is 4.88 PB. This value indicates a total spin state St =

2 throughout 9-290 K temperature range. No change in Peff value with respect to temperature

indicates two interesting features. First, a strong antiferromagnetic coupling between high

spin FeIII(d5) ion and the radical anion, and second, excited state with spin multiplicity more

that 5 is not populated in the temperature range, i.e. the energy difference between the ground

state (St= 2) and excited higher energy state (Stc = 3) is more than 200 cm-1(1 cm-1 = 0.69 K).

Fit to the experimental results (magnetic moment at 1 T and VTVH measurements) were

obtained using the Heisenberg spin exchange Hamiltonian ƨ = 26i<j Jij ǅixǅj in which Jij

represents the exchange constants and the subscripts iand j number the pairwise interacting

paramagnetic centres. Simulation to the experimental results provides the following fitting

parameters; gFe = 2.03, zero field splitting, «D« of 2.15 cm-1.

200 400 600 800 1000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

H

/ 104/ M-1cm-1

Wavelength / nm

Figure 2.10. Change in electronic absorption spectra of 2 after 1e- reduction.

Figure 2.10 displays the electronic spectrum of complex 2 in CH2Cl2 solution at

ambient temperature. The spectrum displays two very intense absorption maxima (H = 11100

Chapter 2

M-1cm-1 (710 nm) and H = 11000 M-1cm-1(443 nm)) in the visible range, 400-800 nm. The

intense absorption at 443 nm is presumably a ligand-to-metal charge transfer where as that at

710 nm may be assigned to an intervalence ligand-to-ligand charge transfer.

The electrochemical activity of complex 2 has been studied by cyclic voltametry (CV)

in CH2Cl2 containing 0.01 M [TBA]PF6 as a supporting electrolyte, a Pt working electrode

and a Ag/AgNO3 reference electrode. Figure 2.11 shows the cyclic voltammogram of

complex 2 recorded in the potential range 0.8 to –1.6 V.

0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0

5PA

100 mv/s

200mv/s

400mv/s

800mv/s

1600mv/s

E (v) vs Fc+/Fc

Figure 2.11. Cyclic voltammograms of 2 at different scan rates.

Complex 3 shows one one-electron quasi-reversible wave at 0.215 V and two

reversible one-electron-transfer waves at –0.615 V and -1.065 V. The redox potentials are

referred versus the ferrocenium/ferrocene couple. Controlled potential coulometic analysis

shows that the electron-transfer at 0.215 V corresponds to one one-electron oxidation while

that at –0.615 V and –0.1.065 V correspond to one-electron reductions. The change in UV-

VIS/NIR spectrum after the coulometric one electron reduction appears in Figure 2.10. Upon

one electron reduction the sharp and strong bands at 710 and 443 nm diminish and a broad

band at around 530 nm (H = 6700 M-1cm-1) appears. The new band is assigned to phenolate-to-

metal charge transfer band.48

Complex 2 is X-band EPR silent, as expected for an integer spin system (St = 2) with

presumably high D value. The one-electron reduced species should be a non-integer spin

system and EPR active in nature. Therefore, X-band EPR spectrum of electrochemically one-

0.5 0.0 -0.5 -1.0 -1.5

5PA

3200mv/s

E (mv) vs Fc+/Fc

Chapter 2

electron-reduced species of 3 in CH2Cl2 solution at 10 K has been measured. Figure 2.12

shows the X-band EPR spectrum of the one-electron reduced species. Clearly, an axial S =

1/2 signal is observed. This type of X-band EPR signal appears for low spin FeIII(d5).49-50 Two

types of electronic configurations, (dxz, dyz)4(dxy)1 and (dxy)2 (dxz, dyz)3, exist in low-spin

Fe(III) species (Figure 2.12). Major factor determining the ground state electronic

configurations are; (i) type of complex i.e. octahedral, square pyramidal, and trigonal

bipyramidal, (ii) the ligand field strength of the axial ligand, (iii) the deformation of the basal

plane. From simulation of the experimental spectrum the following parameters were obtained,

gx = 1.9157, gy = 1.9157 and gz = 2.041, with line width, Wx, Wy, and Wzvalues of 0.17 mT,

0.09 mT, and 0.01 mT respectively. Thus, the reduced species of 2 possesses an S = 1/2

ground state and the unpaired electron predominantly resides on a metal d orbital and most

probably in dxy orbital as the complex is square pyramidal with an axial chloride ligand. If the

reduction were metal centred, and there were no change in the spin state of metal ion, then the

signal should appear as an S = 3/2 system and would be an S = 5/2 system if the reduction

were ligand centred. The observed S = 1/2 signal may be described as follows; after one-

electron reduction the iminosemiquinone part of the ligand becomes an amidophenolate and

hence the ligand field strength has increased. Therefore, high- spin FeIII(d5) shifts to low-spin

FeIII(d5).

Figure 2.12. X-band EPR spectrum of one electron oxidized species of 2 in frozen CH2Cl2 10

K and electron configurations of low-Spin Iron(III) complexes.

300 320 340 360 380

Exp

Sim

dX´´/dB

B , mT

Chapter 2

From the above experimental result it is clear that the first reduction is ligand centered

and therefore, the second reduction has to be metal centered as the center is the only one

which can accept electron. In the case of oxidation process, three options are valid. First,

oxidation of the aminophenolate part of the ligand to iminosemiquinone form, second,

oxidation of iminosemiquinone form of the ligand to iminoquinone form, third, oxidation of

Fe(III) to Fe(IV). Among these options the first option is presumably valid. The

electrochemical transformation can be resumed as follows in Figure 2.13.

Figure 2.13. Different possible redox states of complex 2.

(III)

(II) NN

(III)

-1e

+1e

-1e

+1e

-1e

-1H

+1H

Chapter 2

2.4 Distorted square planar Ni(II) and Pd(II) complexes

It is well known that Ni(II), Pd(II) and Pt(II) ions (d8) form square planar complexes

with ligands having O,O, N,N, N,O, N,S, and S,S donor sets. The square planar complexes of

the above stated bidentate ligands with Ni(II), Pd(II) and Pt(II) have been studied exclusively.

47, 51-62 Instead of the above bidentate ligands chemists have also employed tetradentate

ligands 53 and have examined the properties of the square planar complexes. To understand

the electro and spectroelectrochemical behaviour of the distorted square planar complexes of

Ni(II) and Pd(II) the ligand H4L have been introduced. In this section the two distorted square

planar complexes, [Ni II (Lxx)] (3) and [Pd II (Lxx)] (4) will be discussed.

Complex 3 and 4 were synthesized in the following way; In 30 ml CH3CN, H4L (2

mmol), NiCl2. 6H2O(2 mmol) or PdCl2(2 mmol) and 0.2 ml triethylamine were added and the

solution was stirred under air for two hours. Microcrystalline solid of 3 or 4 was precipitated,

was filtered and washed with CH3CN. Yield ~75%.

The IR spectra of these complexes do not provide conclusive information about the

oxidation state of the ligand except the coordination of the ligand. The absence of the sharp

peaks in the 3434 cm-1 to 3320 cm-1 indicates the deprotonated form of the phenolic and

amino groups and bands in the 2952-2866 cm-1 region are strong evidence for the

coordination of the ligand to the metal ions. The complexes were examined by mass

spectrometry in ESI (pos) mode in CH2Cl2, and in EI-mode. The peaks obtained for the

complexes in the mass spectrum have been calculated theoretically and shown in Figure 2.14.

Chapter 2

Figure 2.14.Experimental and theoretical mass spectra for complex 3 and 4.

The mass spectra for both complexes (3 and 4) confirm that the composition of the

complexes is same, C40H48N2O2M1(M= Ni, Pd). The elemental analysis for both complexes

has been performed and the results support the composition obtained from the mass spectra.

The neutral complex [NiII(Lxx)] (3) was crystallized from 1:1 dichloromethane/

acetonitrile solution as a deep brown crystal. The structure consists of one nickel atoms in a

distorted square planar geometry with the dihedral angle between the C(7)-C(2)-O(1)-Ni(1)-

N(8) and C(22)-C(27)-O(28)-Ni(1)-N(21) planes at 30.8 º. ORTEP diagram with the labeling

scheme and selected bond distances and angles are shown in Figure 2.15 and in Table 2.3.

Chapter 2

Figure 2.15. ORTEP diagram of 3.

The ligand coordinates to the nickel centre through two deprotonated oxygen atoms

and two deprotonated nitrogen atoms. The Ni(1)-O(1), Ni(1)-O (28), Ni(1)-N(8), Ni(1)-N(21)

distances are 1.8435 (15), 1.8516(15), 1.8247(17) and 1.8276 (18) Å respectively. The radical

nature of the ligand is supported by the average C=N imine bond lengths 1.360(2), and by the

average angle of 121.33(17)q at C(7)-N(8)-C(9), indicative of a sp2 –hybridisation for the N

atom. The shortening of the C(3)-C(4), C(5)-C(6), C(23)-C(24) and C(25)-C(26) bonds in

contrast to the average C-C bond distances of the N-phenyl rings (1.39 r 0.01 Å) clearly

implies the collapse of the aromaticity of tert-butyl containing phenyl rings in the complex.

Moreover, the shortening of the C(2)-O(1) and C(27)-O(28) bond lengths from the normal C-

O single bond length is another indication of the radical nature of the ligand. Hence, the

oxidation state for the nickel centre in the neutral complex is +II.

Chapter 2

Table 2. 3. Selected bond distances (Å) and angles (degree) for 3

Ni(1)-N(8) 1.8276 (18) C(4)-C(5) 1.416 (3)

Ni(1)-N(21) 1.8247 (17) C(5)-C(6) 1.376 (3)

Ni(1)-O(1) 1.8435 (15) C(6)-C(7) 1.407 (3)

Ni(1)-O(28) 1.8516 (15) C(22)-N(21) 1.360 (3)

C(2)-O(1) 1.318 (2) C(22)-C(23) 1.410 (3)

C(7)-N(8) 1.369 (3) C(23)-C(24) 1.376 (3)

C(2)-C(7) 1.418 (3) C(24)-C(25) 1.417 (3)

C(2)-C(3) 1.421 (3) C(25)-C(26) 1.376 (3)

C(3)-C(4) 1.388 (3) C(26)-C(27) 1.423 (3)

C(9)-N(8) 1.411 (3) C(27)-O(28) 1.311 (2)

C(9)-C(10) 1.405 (3) C(20)-C(15) 1.410 (3)

C(10)-C(11)

C(11)-C(12)

C(12)-C(13)

C(13)-C (14)

C (14)-C (9)

1.380 (3)

1.382 (3)

1.380 (3)

1.400 (3)

1.405 (3)

C(15)-C(16)

C(16)-C(17)

C(17)-C(18)

C(18)-C(19)

C(19)-C(20)

1.396 (3)

1.383 (3)

1.391 (3)

1.375 (3)

1.403 (3)

O(1)-Ni(1)-N(8) 85.95 (7) O(1)-Ni(1)-O(28) 94.49 (7)

N(8)-Ni(1)-N(21) 101.54 (8) O(1)-Ni(1)-N(21) 160.06 (7)

N(21)-Ni(1)-O(28) 85.52 (7) O(28)-Ni(1)-N(8) 157.70 (7)

C(27)-O(28)-Ni(1) 112.47 (13) C(2)-O(1)-Ni(1) 112.65 (14)

C(22)-N(21)-C(20) 121.07 (17) C(7)-N(8)-C(9) 121.33 (17)

Ni(1)-N(21)-C(20) 124.19 (14) C(9)-N(8)-Ni(1) 122.34 (14)

C(22)-N(21)-Ni(1) 113.75 (14) C(7)-N(8)-Ni(1) 113.2 (14)

X-ray crystal analysis reveals the complex, [Pd (Lxx)], a mononuclear Pd (II) complex

with a tetracoordinate distorted square-planar geometry. The dihedral angle between the C(7)-

C(2)-O(1)-Pd(1)-N(8) and C(22)-C(27)-O(28)-Pd(1)-N(21) planes is 22.2º. ORTEP diagram

with the labeling scheme is shown in Figure 2.16 and Table 2.4 contains selected bond

distances and bond angles. In the discrete molecule, the central Pd ion is coordinated by the

N, O, O, N donor set of the ligand. The Ni(1)–O(1), Ni(1)–O(28), Ni(1)–N(8), Ni(1)–N(21)

are 1.993(3), 2.001(3), 1.951(4), 1.956(4) Å respectively. The C(7)-N(8), C(22)-N(21) bond

distances are shorter then that of C(9)-N(8), C(20)-N(21) bond distances. This indicates that

the tert-butyl group containing phenyl rings are different than the N-phenyl rings. From the

given bond distances and bond angles (Table 2.4) it is quite clear that the complex contains

two iminosemiquinone radicals.

Chapter 2

Figure 2.16. ORTEP diagram of 4.

Table 2.4. Selected bond distances (Å) and angles (degree) for 4.

Pd(1)-N(8) 1.951 (4) C(4)-C(5) 1.434 (5)

Pd(1)-N(21) 1.956 (4) C(5)-C(6) 1.368 (5)

Pd(1)-O(1) 1.993 (3) C(6)-C(7) 1.417 (5)

Cu(1)-O(28) 2.001 (3) C(22)-N(21) 1.367 (6)

C(2)-O(1) 1.311 (5) C(22)-C(23) 1.398 (6)

C(7)-N(8) 1.378 (6) C(23)-C(24) 1.363 (6)

C(2)-C(7) 1.422 (6) C(24)-C(25) 1.420 (5)

C(2)-C(3) 1.431 (6) C(25)-C(26) 1.369 (6)

C(3)-C(4) 1.382 (6) C(26)-C(27) 1.432 (6)

C(9)-N(8) 1.409 (5) C(27)-O(28) 1.306 (5)

C(9)-C(10) 1.401 (6) C(20)-C(15) 1.402 (6)

C(10)-C(11)

C(11)-C(12)

C(12)-C(13)

C(13)-C (14)

C (14)-C (9)

1.373 (7)

1.379 (7)

1.361 (5)

1.407 (6)

1.408 (6)

C(15)-C(16)

C(16)-C(17)

C(17)-C(18)

C(18)-C(19)

C(19)-C(20)

1.397 (6)

1.366 (7)

1.371 (7)

1.381 (6)

1.399 (6)

O(1)-Pd(1)-N(8) 81.96 (14) O(1)-Pd(1)-O(28) 97.66 (12)

N(8)-Pd(1)-N(21) 101.95 (15) O(1)-Pd(1)-N(21) 167.68 (14)

N(21)-Pd(1)-O(28) 81.38 (13) O(28)-Pd(1)-N(8) 165.95 (14)

C(27)-O(28)-Pd(1) 111.8 (3) C(2)-O(1)-Pd(1) 112.0 (3)

C(22)-N(21)-C(20) 122.5 (4) C(7)-N(8)-C(9) 122.5 (4)

Pd(1)-N(21)-C(20) 121.0 (3) C(9)-N(8)-Pd(1) 119.1 (3)

C(22)-N(21)-Pd(1) 114.3 (3) C(7)-N(8)-Pd(1) 113.3 (3)

Chapter 2

Figure 2.17. Electronic spectrum of complex 3 (left) and complex 4 (right) in CH2Cl2 at room

temperature.

The most interesting feature in the UV–VIS/NIR spectrum for complex 3 and 4

(Figure 2.17) is the ligand-to-ligand charge transfer band that appears in the lower energy than

that of the previously reported complexes.51-53 The Table 2.5 summarizes these results. This is

presumably because of the highly distorted square planar geometry around the Ni(II) and

Pd(II) ions relative to that of the previously reported complexes. The other bands appear due

to the ligand-to-metal and ligand S-S* transitions.51-53

Figure 2.18. Cyclic voltammogram of complex 3 (left) and 4(right) in CH2Cl2 at room

temperature under an argon atmosphere.

400 600 800 1000 1200

0.0

0.5

1.0

1.5

/ 104/ M-1 cm-1

Wavelength /nm

400 600 800 1000 1200 1400 1600 1800

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

/104/ M-1cm-1

Wavelength / nm

1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0

100mv/s

E (V) vs Fc+/Fc

1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5

50 mV/s

100 mV/s

200 mV/s

400 mV/s

E (V) vs Fc+/Fc

Chapter 2

Figure 2.18 displays the cyclic voltammograms (CVs) of 3 and 4 in CH2Cl2 solutions

containing 0.1 M [N (n-Bu) 4

] PF6 as supporting electrolyte at a glassy carbon working

electrode and a Ag/AgNO3 reference electrode. Ferrocene was used as an internal standard,

and potentials are referenced versus the ferrocenium/ ferrocene (Fc+/ Fc) couple. Table 2.6

summarizes these results. In the case of complex 3, one reversible two-electron-transfer wave

and two reversible one-electron-transfer waves are observed at E1/2 values 0.04, -0.82 and –

1.45V vs (Fc+/ Fc). Controlled potential coulometric measurements established that E1/2,

0.04V corresponds to one two-electron oxidation and the potentials at E1/2 –0.82 V and –1.45

V correspond to two successive one-electron reductions of complex 3. It has been well

established previously that the reduction processes are ligand centered.51-53 In the case of the

oxidation process, there are two options, first, direct oxidation of the ligand from

iminosemiquinone to iminoquinone at the same potential, or; second, the Ni(II) ion oxidizes

and becomes Ni(III) and then intramolecular electron transfer takes place between the Ni(III)

center and iminosemiquinone part of the ligand (Figure 2.19). It is clear from the reduction

potentials shown in Table 2.6 that the E1/2 values for the reduction processes are affected by

the geometry around the Ni(II) center. As the oxidation potential does not change for all the

biradical containing Ni(II) complexes (references in Table 2.6) the mechanism for the

oxidation process of 3 should be similar as that suggested by Chaudhuri-Wieghardt et. al.52

Hence, the electrochemical behaviour of complex 3 can be summarized as shown in Figure

2.19.

Table 2.5. Spectroelectrochemical data of the complexes.

, nm (

, M-1 cm-1) X 104Metal ion (II) Ref.

860 (1.8), 450 (0.28), 300(1.35) Ni 52

900 (1.6), 600 (0.2), 350 (0.3) Ni 51

981 (1.4), 746 (0.2), 505 (0.17), 336 (0.64) Ni This work

854 (3.0), 581 (0.17), 416 (0.16), 293(1.4) Ni 53

700 (2.5) Pd 52

871 (2.7), 546(0.2), 403 (0.2) Pd 51

961 (2.7), 756 (0.62), 554 (0.31), 475 (0.44), 345 (1.45) Pd This work

843 (3.1), 559 (0.29), 552 (0.24), 434 (3.1), 302 (1.9) Pd 53

Chapter 2

Table 2.6. Redox potentials of the complexes vs Fc+/ Fc.

Ox(V) E1

Red(V) E2

Red(V) Dihedral angle (

) Kd (103) Metal ion Ref.

0.040* -1.1 -1.64 0 Ni(II) 52

0.040* -1.0 -1.74 0 Ni(II) 51

0.040* -0.82 -1.45 30.2 Ni(II) This work

0.042* -1.07 -1.67 - Ni(II) 53

0.47 0.08 -1.0 -1.45 0 0.74 Pd(II) 52

0.65 0.17 -1.0 -1.54 0 3.4 Pd(II) 51

0.39 0.13 -0.8 -1.27 22.2 0.08 Pd(II) This work

0.40 -0.03 -1.13 -1.68 - 1.4 Pd(II) 53

*For 2e process

Complex 4 shows four reversible one-electron-transfer waves which are observed at

E1/2 value of 0.39, 0.13, -0.80 and –1.27 V vs. (Fc+/ Fc). Clearly, two reversible one-electron

oxidations and two reversible one-electron reductions are observed, as established by

coulometry at fixed potential. The electrochemical transformations of complex 4 can be

summarized as shown in Figure 2.20. The difference in E1/2 values of complex 4 with the

previously reported E1/2 values (Table 2.6) may be assigned by considering the geometry

around the Pd(II) center.

The disproportionation constant (corresponding to a conproportionation constant Kc =

Kd –1) for the equilibrium between monocataonic forms of complexes 3,4 and those in ref.51-

53 and their neutral and dicationic forms. These values can be calculated by use of the

following equation,

Kd = exp(nF(E2Ox - E1Ox))/RT = exp((E2Ox - E1Ox))/0.059

Chapter 2

.Ni

(II)

(III)

(II) (III)

(II)

1+ 2+

(II)

-1e-

-2e-

+2e-

+1e-

+1e--1e-

Figure 2.19. Different redox states of complex 3.

Chapter 2

The results are summarized in Table 2.6. From these results it is clear that there is a

correlation between the Kd values and dihedral angles. The Kd value for the complex (ref 51)

differs a little bit from complexes (ref 52, 53) presumably due to electron-withdrawing -CF3

substituent attached to the ligands. The appreciable difference in Kd values of complex 4 than

the complexes in references (ref 51-53) emphasize the fact that due to the distortion from

square planar geometry the electronic stabilization from delocalization of the S radical

throughout the whole molecule is low.

+1e-

-1e-

+1e-

-1e-

-1e-+1e-+1e-

-1e-

(II)

(II) (II)

Figure 2.20. Different redox states of complex 4.

Variable temperature (2 K-298 K) magnetic susceptibility measurement using SQUID

magnetometer at 1T indicates that complexes 3 and 4are diamagnetic. The 1H NMR for both

complexes 3 and 4 were measured in CH2Cl2 at RT and shown in the experimental section.

The 1HNMR spectra are quite similar with the ligands. No broadening of the lines is observed.

Hence, the diamagnetic character of 3 and 4 was further proved from the NMR spectroscopy.

Chapter 2

2.5 Distorted square planar Cu (II) complex (5) and its

catalytic reactivity

To mimic the distorted square-planar coordination geometry and the catalytic activity

of the copper-containing fungal enzyme Galactose Oxidase 63-73 (GOase) a tetradentate ligand,

N, Nc (2-hydroxy-3, 5-di-tert-butyl-phenyl) 2,2c diaminobiphenyl (H4L) (Figure 2.1), has been

synthesized and used to form a complex with a copper(II) ion. The expectation is that the

designed ligand with an N2O2 donor set will provide a distorted square planar copper(II)

complex owing to its steric constraint. The ligand is non-innocent and will form mononuclear

copper (II) complex, [CuII(Lxx)], with two iminosemiquinone radical centres. Hence, the

radical containing-copper(II) complex can be used as a true functional and structural model

for Galactose Oxidase (GOase).

2.5.1 Synthesis and characterization of the complex (5)

Complex 5, [Cu II (Lxx)] was synthesized in CH3CN by 1:1 reaction of the ligand, H4L

and [Cu I (CH3CN)4] ClO4 salt in presence of triethylamine as a base under air or under argon

atmosphere and followed by air exposure. X-ray quality single crystals were obtained by

recrystallization of microcrystalline 5 from a 1:1 CH2Cl2 and CH3CN solvent mixture.The

elementalanalysis data indicate a composition of C40H48N2O2Cu1 for 5. The ESI (m+/z, in

CH2Cl2) shows a signal at 651.5(100%) and also supports a composition of C40H48N2O2Cu1.

The structure of complex [CuII (L

)] (5) was determined by X-ray crystallography.

The analysis confirms the formation of a copper complex with M : L ratio of 1:1. Complex 5

consists of a discrete neutral unit in which the copper ion adopts quite distorted square planar

geometry with the dihedral angle between the C(7)-C(2)-O(1)-Cu(1)-N(8) and C(22)-C(27)-

O(28)-Cu(1)-N(21) planes is 35.5º. ORTEP diagram with the atom labeling scheme is shown

in Figure 2.21 and selected bond distances and angles are listed in Table 2.7. The tetradentate

ligand coordinates through two oxygen and two nitrogen atoms, which are fully deprotonated.

From the C-C bond distances of the tert-butyl containing phenyl rings, it is evident that the

tert-butyl containing phenyl rings have lost their aromaticity after formation of the complex.

The average C-O and the average C-N (N, O attached to the tert-butyl containing phenyl

rings) bond distances are 1.295(3) and 1.356(4) Å, respectively.

Chapter 2

Figure 2.21. ORTEP diagram of 5.

These clearly show that the tert-butyl substituted phenyl rings are in iminosemiquinone form.

It is also quite clear from the structure that the ligand is non-innocent. The complex is neutral

therefore the copper centre is in a +II oxidation state. The Cu(1)-O(1), Cu(1)-(28), Cu(1)-N(8)

and Cu(1)-N(21) distances are 1.925 (2), 1.925(2), 1.925(3) and 1.919 (3) Å respectively.

Table 2. 7. Selected bond distances (Å) and angles (degree) for 5.

Cu(1)-N(8) 1.925 (3) C(4)-C(5) 1.434 (5)

Cu(1)-N(21) 1.919 (3) C(5)-C(6) 1.368 (5)

Cu(1)-O(1) 1.925 (2) C(6)-C(7) 1.417 (5)

Cu(1)-O(28) 1.925 (2) C(22)-N(21) 1.356 (5)

C(2)-O(1) 1.295 (4) C(22)-C(23) 1.418 (5)

C(7)-N(8) 1.342 (4) C(23)-C(24) 1.373 (5)

C(2)-C(7) 1.452 (5) C(24)-C(25) 1.420 (5)

C(2)-C(3) 1.436 (5) C(25)-C(26) 1.386 (5)

C(3)-C(4) 1.377 (5) C(26)-C(27) 1.429 (5)

C(9)-N(8) 1.405 (5) C(27)-O(28) 1.304 (5)

C(9)-C(10) 1.403 (5) C(20)-C(15) 1.410 (5)

C(10)-C(11)

C(11)-C(12)

C(12)-C(13)

C(13)-C (14)

C (14)-C (9)

1.413 (5)

1.392 (5)

1.376 (5)

1.403 (5)

1.413 (5)

C(15)-C(16)

C(16)-C(17)

C(17)-C(18)

C(18)-C(19)

C(19)-C(20)

1.406 (5)

1.380 (5)

1.382 (5)

1.392 (5)

1.406 (5)

O(1)-Cu(1)-N(8) 84.49 (11) O(1)-Cu(1)-O(28) 99.72 (10)

N(8)-Cu(1)-N(21) 100.23 (12) O(1)-Cu(1)-N(21) 155.12 (10)

N(21)-Cu(1)-O(28) 85.3 (11) O(28)-Cu(1)-N(8) 157.41 (11)

C(27)-O(28)-Cu(1) 111.2 (2) C(2)-O(1)-Cu(1) 111.5 (2)

C(22)-N(21)-C(20) 123.2 (3) C(7)-N(8)-C(9) 122.8 (3)

Cu(1)-N(21)-C(20) 121.8 (3) C(9)-N(8)-Cu(1) 124.5 (3)

C(22)-N(21)-Cu(1) 111.6 (3) C(7)-N(8)-Cu(1) 112.3 (2)

Chapter 2

The cyclic voltammogram (CV) of complex (5) has been recorded in CH2Cl2 solutions

containing 0.1 M [N (n-Bu) 4

] PF6 as supporting electrolyte at a glassy carbon working

electrode and a Ag/AgNO3 reference electrode. Ferrocene was used as an internal standard,

and potentials are referenced versus the ferrocenium/ ferrocene (Fc+/ Fc) couple. Table 2.8

summarizes these results. Figure 2.22 shows the CV of 5 recorded in the potential range +1.5

V to –1.5 V. Four reversible one-electron-transfer waves are observed. Two reversible one-

electron oxidations and two reversible one-electron reductions are observed, as established by

coulometry at fixed potential. The oxidation and reduction processes are ligand centered in

nature.1, 52 The electrochemical transformations of complex (5) can be summarized as follow

(Figure 2.23).

1e-

(II)

Figure 2.23.Different redox states of [CuII(L

)], 5.

[CuII(L.)]1-

[CuII(L)]2-

[CuII(L)]2+

[CuII(L.)]1+

[CuII(L..)]0

Chapter 2

The results of spectroelectrochemistry investigations in the UV-VIS/NIR regions are

summarized in Figure 2.24. Upon reduction of [CuII (Lxx)] or more clearly 50 to 5- the long-

wavelength band attributed to intraligand (IL) and ligand-to-metal charge transfer (LMCT)

are diminished with shifting of maxima from 882 nm to 825 nm and from 592 nm to 542 nm

respectively. Spectroelectrochemically, one electron oxidation gives rise to a strong, broad,

Figure 2.24. Spectroelectrochemistry studies of complex 5 in CH2Cl2/ 0.1 M [N (nBu) 4] PF6.

intervalence charge transfer band at around 1280 nm in addition to a strong, sharp absorption

at 570 nm, possibly associated with the formation of a quinone ligand. Upon the second

oxidation to [CuII(L)]2+ only the band, at 507 nm, arising from quinone S-S* transition, is

observed.

The electronic ground state of complex, [Cu (Lxx)] (5), has been established from

variable-temperature (2-290 K) magnetic susceptibility measurements by using a SQUID

magnetometer. The distorted square-planar complex (5) displays a temperature dependence of

E2Ox =

E1Ox

E1Red

E2Red

+ 0.347V

-0.16V

- 0.614V

-0.984V

vs Fc+/Fc

1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5

E(V) vs Fc+/Fc

E2Ox

E1Ox

E1Red

E2Red

AE2Ox =

E1Ox

E1Red

E2Red

+ 0.347V

-0.16V

- 0.614V

-0.984V

vs Fc+/Fc

1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5

E(V) vs Fc+/Fc

E2Ox =

E1Ox

E1Red

E2Red

+ 0.347V

-0.16V

- 0.614V

-0.984V

vs Fc+/Fc

E2Ox =

E1Ox

E1Red

E2Red

+ 0.347V

-0.16V

- 0.614V

-0.984V

vs Fc+/Fc

1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5

E(V) vs Fc+/Fc

E2Ox

E1Ox

E1Red

E2Red

Figure 2.22. Cyclic Voltammogram of the complex [Cu (L

)] in CH2Cl2 at 298K under anargon

atmosphere.

[CuL..]0882 nm (7000 M-1cm-1), 492 nm (5970 M-1cm-1)

[CuL.]-825 nm (3520 M-1cm-1), 542 nm (1800 M-1cm-1)

[CuL.]+ 1279 nm (8900 M-1cm-1), 571 nm (17200 M-1cm-1)

[CuL]2+ 507 nm (8465 M-1cm-1)

[CuL..]0882 nm (7000 M-1cm-1), 492 nm (5970 M-1cm-1)

[CuL.]-825 nm (3520 M-1cm-1), 542 nm (1800 M-1cm-1)

[CuL.]+ 1279 nm (8900 M-1cm-1), 571 nm (17200 M-1cm-1)

[CuL]2+ 507 nm (8465 M-1cm-1)

400 600 800 1000 1200 1400 1600

0.0

0.5

1.0

1.5

2.0

/ 104 / M-1cm-1

Wavelength /nm

Table 2.8.

Chapter 2

Figure 2.25. Temperature dependence of the magnetic moment of solid [CuII (L

)].

the effective magnetic moment as shown in Figure 2.25. At temperature < 50K, a nearly

constant value of 1.83 PB (gcu(II) = 2.13) is indicating of an St = 1/2 ground state. With

increasing temperature (> 50K) Peff increases. This is an indication of thermal population of

excited states with higher spin multiplicity than 2. This behaviour indicates the presence of

more than one paramagnetic centre in complex 5. Obviously, the complex (5) is a “multispin”

system which has been proved previously from the high quality X-ray crystal structure.

Indeed, it is a three spin molecule with St = 1/2 ground state. Such a system possesses three

coupled states provided that exchange is the dominating spin interaction. The states are

labelled by their total spin St= SCu + Srad1 + Srad2 and a pair subspin S* = Srad1 + Srad2, (St, S*) =

(3/2, 1), (1/2, 1), (1/2,0), or, in a more symbolic fashion, by (nnn), (npn) and (nnp)

respectively. Considering symmetric coupling scheme for 5 with two coupling constants the

energy of the coupled states are given by E (3/2, 1) = -J-2Jc, E (1/2, 1) = 2(J-Jc), and E (1/2,0)

= 0. J describes the coupling between a radical and an adjacent copper (II) ion whereas Jc

describes that between two remote radical anions. Hence, the relative order of these states

depends critically on the values of J and Jc. To determine the ground state electronic

configuration, EPR spectroscopy is a useful tool. The nature of the spin ground state of the

complex (5) can be sensitively probed by X-band EPR spectroscopy. This is possible because

Cu (II) (d9) ions exhibit significant g anisotropy and large hyperfine splitting contrasting in

this respect to the coordinated organic radical anions. The EPR spectrum of 5 in a 1:1 CH2Cl2

and toluene solution mixture at 10 K (Figure 2.26) exhibits g values with rhombic distortions

(gx = 2.064, gy = 2.048, gz = 2.195). Interestingly, the spectrum shows a distinct ligand

50 100 150 200 250 300

1.6

1.7

1.8

1.9

2.0

2.1

2.2

2.3

2.4

2.5

eff /

T / K

Spin-ladders

E2 (npn) =800 cm-1

E1 (nnn) = 740 cm-1

Energy

0 (nnp) = 0

Chapter 2

hyperfine splitting which is barely resolved for the g|| lines but clearly so at gx and gy. A

satisfactory simulation was obtained with two equivalent 14N nuclei (I =1) with AN = (45, 40,

20 X 10-4 cm-1) for complex (5). Still one question remains unclear. What is the nature of

coupling between Cu(II) ion and radical anion? To get the answer the complex was oxidised

by one electron using ferrocenium hexafluorophosphate (See experimental section).

Figure 2.26. X-band EPR Spectrum of the [CuIIL

] in frozen CH2Cl2at 10 K.

The one electron oxidized species could have St = 1 or 0 ground state depending on the nature

of coupling between two S = 1/2 spins [Cu(II) ion and radical anion]. The magnetic

susceptibility measurement shows diamagnetic character of the one electron oxidized species.

Moreover, the one electron oxidized species is EPR silent. These two experimental evidences

help to conclude the antiferromagnetic coupling between Cu (II) ion and radical anion. This is

presumably due to the distortion from square planar geometry. The X-band EPR result

indicates that the lowest spin state has predominantly copper character, from which it can be

concluded that the ground state electronic configuration of the complex is (nnp). Thus the

antiferromagnetic radical anion-radical anion interaction (Jc) must be higher than that of the

copper (II) and adjacent radical anion. A satisfactory simulation of the magnetic susceptibility

was obtained with gCu = 2.13, grad = 2.0, Jc = -327 cm-1 (or -273 cm-1), J = -62 cm-1 (or-123 cm-

1), TIP = 6.5 X 10-4 cm3 mole-1. It is noteworthy that a variety of Jc and J values can be

employed to simulate the experimental results. But from the metal centered X-band EPR

result it can be argued that Jc> J.

2800 3200 3600

SIM

Exp

dX''/dB

B / Gauss

Chapter 2

2.5.2 Catalytic reactivity,

Aerial oxidation of benzyl alcohol; mimicking the function

of Galactose Oxidase(GOase)

When complex 5 was used as a catalyst for the aerial oxidation of the primary alcohol,

benzyl alcohol (PhCH2OH), it has been found that the complex is capable of oxidising benzyl

alcohol in the presence of air at room temperature (22 ± 1 º C) to benzaldehyde (PhCHO) with

an yield % 65. So, it is clear that complex 5 can act as a good functional model of Galactose

Oxidase (GOase), which, a fungal copper containing mononuclear metalloenzyme, catalyses

the two electron oxidation of primary alcohols to their corresponding aldehydes in the

presence of air (eq 1). The active site of the GOase contains one copper (II) ion and one Tyr

272 radical and here the copper complex is a copper (II)-biradical complex.

Figure 2.27. Effect of concentration of base on the % of conversion to PhCHO.

This catalytic process is base-dependant. Using a weak base like triethylamine (Et3N)

only 7 TON (Turnover Number, ratio between the total concentration of product and catalyst)

has been obtained. Alternatively, using a strong base like tetrabutylammonium methoxide

(nBu4NOMe) or tetrabutylammonium hydroxide (nBu4NOH) a good TON can be achieved.

The concentration of PhCHO was measured by LC. By varying the concentration of base the

percentage of yield can be improved and a maximum yield is obtained when the concentration

of base (nBu4NOMe) is 90% of the total concentration of substrate (PhCH2OH). Further

0246810

[S] = 5 X 10-2 M (Fixed)

[C] = 5 X 10-4 M (Fixed)

% of Conversion to PhCHO

Concentration of Bu4NOMe/ M , 10-2

[S] = Concentration of PhCH2OH

[C] = Concentration of [Cu II (L¨)]

Concentration of product was

measured by LC using Column,

Nucl.-5-C18 Sel-214, UV = 250 nm,

gas = MeOH /H2O 1/1 (V/V).

GOase

RCH2OH + O2 RCHO + H2O2............eq 1

Chapter 2

increase in the concentration of base inhibits the catalytic process (Figure 2.27). The most

probable reason is the coordination of excess base (MeO-) to the copper (II) centre, which

protects deprotonated benzyl alcohol to coordinate to the copper (II) centre and hence, the

percent yield decreases. It is noteworthy that 5 is incapable of oxidising methoxide to

formaldehyde.

Using initial rate method at constant concentration of substrate (PhCH2OH), the kinetic

of the catalytic oxidation of benzyl alcohol has been measured by varying the concentration of

[Cu II (L

)] under air at room temperature (22 ± 1 º C) in dichloromethane (Figure 2.28) or

vice versa. The product concentration was monitored by UV-VIS spectroscopy at 290 nm (H =

1220 M-1cm-1) taking same concentration of catalyst ([Cu II (L

)]) that has been used for the

reaction as a blank. During kinetic measurements the concentration of base has been kept as a

90% of the total concentration of substrate. From these data the following rate law was

deduced,

Rate = k [Cu II (L

)] [PhCH2OH]

Figure 2.28. Kinetic data.

The TON (Turnover Number, Figure 2.29) of this catalytic process has been measured

by varying the total concentration of substrate at constant concentration base 90 % of the total

concentration of substrate. The catalyst concentration was kept constant.

With the selectively deuterated substrate PhCD2OH kinetic isotope effect (KIE = kH/kD)

of about 14 was evaluated (Figure 2.29). This indicates that the H-atom abstraction from the

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Kobs /10-2 M-1min.-1

[Cu(L



)]/ 10-4M

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

kobs / 10-2 M-1 Min-1

[PhCH2OH]/ 10-2 M

Chapter 2

D-C atom of the coordinatively bound alcoholate is the rate-determining step for this catalytic

oxidation process. When the reaction was done under an anaerobic condition by allowing

excess substrate to react with copper (II)-biradical complex, it has been found that the copper

(II)-biradical complex has been converted to copper (II)-monoradical mono anionic species

(Figure 2.30).

Figure 2.29. KIE plot and TON vs concentration plot for aerial oxidation of benzyl alcohol.

Interestingly, in the above reaction condition the yield of product (PhCHO) is just half of

the total concentration of copper (II)-biradical complex. When that solution was kept under

air, the copper (II)-monoradical mono anionic species goes back to copper (II)-biradical

neutral complex. Here, air acts as an oxidant by reducing itself to hydrogen peroxide.

(I) (II)

Figure 2.30. (I) Change in UV-VIS spectrum of the complex, [Cu II (L

)], during

stoichiometric type reaction with excess benzyl alcohol under an argon atmosphere.

(a)Before addition of substrate (b) after addition of substrate (II) H2O2 obtained with the

reaction of the air and reduced catalyst.

400 600 800 1000 1200

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

/M-1 cm-1

Wavelength /nm

350 375 400 425 450 475

0.00

0.05

0.10

0.15

0.20

at 408 nm = 675 M-1 cm-1

A / cm

Wavelen

th /nm

024681012

TON

[PHCH2OH] (M / 10-2)

Concentration of product was

measured by LC using Column,

Nucl.-5-C18 Sel-214, UV = 250

nm, gas = MeOH /H2O 1/1 (V/V).

0 1020304050

0.00

0.05

0.10

0.15

0.20

0.25

PhCH2OH

PhCD2OH

A / 290 nm

Time / min.

Chapter 2

Extracting in water and using titanyl sulphate in 9 M sulphuric acid medium the

concentration of hydrogen peroxide was then determined by colorimetric method (Figure

2.30). The glove-box reaction i.e. the reaction under an anerobic condition is an indication

that the mechanism of this catalytic reaction is not similar to GOase.72, 74-75 The UV-VIS/NIR

spectral measurements during the reaction show that in the reaction time the complex exists as

copper (II)-monoradical mono anionic species and after the reaction time, 16h, the species

goes back to copper (II)-biradical neutral complex (Figure 2.31). No further conversion is

observed; even if the catalytic solution is kept for more than 48 hours under air. This leads to

the fact that the copper (II)-biradical neutral complex is not the active species but some other

species is catalytically active.

Figure 2.31. Change in UV-VIS spectrum of [Cu II (L

)] during the catalysis. (a) Before

addition of substrate, PHCH2OH, and base, nBu4NOMe. (b) During the catalysis under an

aerial condition. (c) After 16h of the catalysis.

If additional base (nBu4NOMe) is added to the solution after 16h of the reaction time,

the percent of yield increases from 65 to 98 within 48 hours. No change has been observed

when the same reaction was repeated with triethylamine.

400 500 600 700 800 900 1000

1000

2000

3000

4000

5000

6000

7000

8000

9000

/M-1 cm-1

Wavelength/ nm

Chapter 2

Effect of base

It has been stated earlier that the catalytic process is base-dependant, an attempt has

been made to investigate the effect of the base. The effect of the base was observed when

Et3N and nBu4NOMe were added to the two different solutions containing the same

concentration of 5 under an argon atmosphere and exposing those solutions to air. There is no

change in the UV-VIS/NIR spectrum of the copper (II)-biradical complex in the presence of

Et3N but changes were observed in case of nBu4NOMe (Figure 2.32). To determine the effect

of the base, three solutions with different concentration of [Cu II (L

)] were made. To each

solution different concentration of base (nBu4NOMe) was added under an argon atmosphere

keeping the nBu4NOMe: [Cu II (L

)] concentration ratio (6:1) fixed. The change in UV-

VIS/NIR spectrum is given in Figure 2.32. If [Cu II (L¨)] disproportionates to [Cu II (Lx)]- and

[Cu II (Lx)]+ in presence of base (nBu4NOMe) then the absorption value observed at 880 nm

would be equal to the sum of the absorption for 50% of [Cu II (Lx)]- and 50% of [Cu II (Lx)]+

species relative to the total concentration of [CuII(L

)]0. Though the mechanism of this

disproportionation reaction is not known, the results shown below support the assignment of a

disproportionation reaction.

Effect of base and an indication of disproportionation reaction

x Bold and italic are the calculated values.

Concentration

of base

(Bu4NOMe,

Concentration

of 5

(M, C0)

Base: 5

ratio

Initial

absorption

of 5 at 880

Final

absorption

of 5 at 880

Difference

absorption

('A) at

880 nm

Concentration

monoposative

species.( M,

X2)

Concentration

mononegative

species( M,

X3)

7.5 X 10-3 1.267 X 10-3 5.92 : 1 0.8873

[l = 0.1cm]

0.413

[l = 0.1cm]

0.474 5.6 X 10-4

6.33 X 10-4

5.6 X 10-4

6.33 X 10-4

4.2 X 10-3 0.694 X 10-3 6.04 : 1 0.486

[l = 0.1cm]

0.189

[l = 0.1cm] 0.3 3.49 X 10-4

3.47 X 10-4

3.49 X 10-4

3.47 X 10-4

2.86 X 10-3 0.475 X 10-3 6.02 : 1 0.333

[l = 0.1cm]

0.13

[l = 0.1cm] 0.2 2.35 X 10-4

2.35 X 10-4

Chapter 2

' A = [7000 C0 -{7000 (C0- X)+ 3300 X2 + 2200 X3}]

' A = {7000 (X2 + X3)-(3300 X2 + 2200 X3)}

' A = 3700 X2 + 4800 X3 [When, X = X2 + X3]

' A = 8500 X2 = 8500 X3[When, X2 = X3]

(I) (II)

Figure 2.32. (I)Change in UV-VIS/NIR spectrum of complex, [Cu II (L

)], on addition of

Et3N under an argon atmosphere and then allow the solution to be in the air. (II) Change in

VU-VIS/NIR spectrum of complex, [Cu II (L

)], on addition of nBu4NOMe under an argon

atmosphere (a to b) and then exposure of the solution to air (b to c).

This above result helps to support a proposal that in the presence of strong base the

copper (II)-biradical complex is converted to some other form that is catalytically active. The

most probable species that can be catalytically active is the copper (II)-monoradical mono

positive species as only copper (II)-monoradical mono negative species was obtained after

reaction under anaerobic condition and that goes back to copper (II)-biradical complex in

presence of air. From all these results a mechanism can be proposed as given below (Figure

2.33).

400 500 600 700 800 900 1000

2000

4000

6000

8000

/M-1 cm-1

Wavelength /nm

400 500 600 700 800 900 1000

1000

2000

3000

4000

5000

6000

7000

8000

Air (O2)

/M-1 cm-1

Wavelength / nm

Chapter 2

Cu .

N- N

Cu .

PhCH2O-

Cu .

HPh

-N

PhCHO

H2O2

r.d.s

Disproportionation in base

H-atom abstraction

H+

(II)

(II) (II)

(II)

Figure 2.33. The Proposed mechanism for the aerial oxidation of benzyl alcohol.

Chapter 2

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Chapter 2

Chapter 3

Synthesis, Characterization and Catalytic Reactivities of the

Tetracoordinate Cu (II)-Complexes formed with the

Ligands,N(2-hydroxy-3,5-di-tert-butylphenyl)-3,5-di-

substituted-aniline, H2LX(X = -CF3, -F, -Cl, -OMe, -tBu)

Is there any effect of substituents at the 3,5 positions of the N-phenyl ring to

the physical and chemical properties of the complexes?

CF3CF3

CF3

CF3CF3

CF3

RCH2O

CF3CF3

CF3

CF3CF3

CF3

HO2

RCHO

-R = H, CH3, Ph

H-atom abstraction

r.d.s

Chapter 3

3.1 Introduction

Radical containing Cu(II) complexes of the type shown in Figure 3.3 are known to

catalyze 6 oxidation of primary alcohols to the corresponding aldehydes(eq 1)

RCH2OH + O2RCHO + H2O2 (eq 1)

A natural question to ask in the optimization of this catalytic process is how the steric

and/or electronic effect of the ligand influences the geometries of the catalysts and also the

catalytic activity.

To show these effects a few ligands (1-5) have been synthesized by varying the nature

of the substituents at 3,5- positions of the N-phenyl group. Figure 3.1 shows the schematic

diagram of the ligands (1-5).

Figure 3.1. Schematic diagram of the ligands (1- 5).

To understand the nature of the effects, i.e. inductive effect or mesomeric effect,

isomeric ligands (6,7) have been synthesized by changing the position of the substituent from

the 3,5 positions to the 2, and the 4 positions. Figure 3.2 shows the schematic diagram of the

ligands (6, 7).

X = - CF3 (1)

- F (2)

- Cl (3)

- OMe (4)

-tBu (5)

Chapter 3

Figure 3.2. Schematic diagram of the ligands (6-7).

3.2 Synthesis and characterization

All the ligands were synthesized and characterized previously.1-2

The reaction of [CuI(CH3CN)4]ClO4 with N(2-hydroxy-3,5-di-tert-butylphenyl)-3,5-

di-substituted-aniline and triethylamine in dry CH3CN under argon produces a yellow solution

that becomes deep green/blue-green upon exposure to air. From these solutions crystalline

complexes listed in Table 3.1 precipitated out.

Table 3.1. The complexes.

Ligand Complex Complex No.

CF3

F3C

CF3

F3C

F3CCF3

II 1

X = - CF3 (6)

- OMe(7)

Chapter 3

CF3

F3C

CF3

II 2

Chapter 3

Cl Cl

OMe

MeO

OMe

MeO

MeO OMe

II 4

OMe

MeO

OMe

II 4a

Chapter 3

OMe

II 4b

Chapter 3

Figure 3.3. ORTEP diagram of 1.

All complexes 1-5 have very similar structures with square-planar or distorted square

planar geometry around the copper ion. Here, X-ray crystal structure of complex 1 is

described.

X-ray quality single crystals were grown from a 1:1 dicholoromethane, acetonitrile

mixture. Figure 3.3 displays ORTEP diagram of the mononuclear neutral complex 1and

Table 3.2 contains selected bond lengths and bond angles. The X-ray structure of 1 shows that

the copper ion is in a slightly distorted square-planar N2O2 environment. The dihedral angle

between Cu1-O1-C2-C1-N1 and Cu1-O2-C24-C23-N2 planes is 24.2 º. The C(4)–C(5), C(1)–C

(6), C(2)–C(3), C(5)–C(6), C(7)–C(8), C(8)–C(9), C(9)–C(10), C(10)–C(11) and C(11)–C(12)

bond lengths are 1.434 (4), 1.414 (4), 1.432 (4), 1.364(4), 1.390 (4), 1.385 (4), 1.388 (4),

1.383 (4) and 1.391 (4) Å respectively. From the above C-C bond lengths it is quite clear that

the aromaticity of the tert-butyl group containing phenyl ring has been destroyed in the

Chapter 3

complex. From the bond angles around the N(1) and N(2) (shown in Table 3.2) it is clear that

the hybridization of the both N atoms are sp2 and they are deprotonated. The respective C(1)-

N(1) and C(2)-O(1) bond lengths are 1.342(3) and 1.300(3) Å. These bond lengths indicate

that C(1)-N(1) bond has double bond character and the C(2)-O(1) bond length is in between

C-O and C=O, and hence suggests radical nature of the ligands. The Cu(1)-O(1), Cu(1)-O(2),

Cu(1)-N(1) and Cu(1)-N(2) bond lengths are 1.932(2), 1.932(2), 1.925(2) and 1.929(2) Å

respectively. From the Cu-O and Cu-N bond lengths it is clear that the oxidation state of the

copper ion in neutral complex 1is +II.

Table 3.2. Selected bond distances (Å) and angles (degree) for 1.

Cu(1)-N(1) 1.925 (2) C(4)-C(5) 1.434 (4)

Cu(1)-N(2) 1.929 (2) C(5)-C(6) 1.364 (4)

Cu(1)-O(1) 1.932 (2) C(6)-C(1) 1.414 (4)

Cu(1)-O(2) 1.932 (2) C(23)-N(2) 1.345 (3)

C(2)-O(1) 1.300 (3) C(26)-C(27) 1.443 (4)

C(1)-N(1) 1.342 (3) C(23)-C(24) 1.456 (4)

C(2)-C(1) 1.457 (4) C(24)-C(25) 1.434 (4)

C(1)-C(6) 1.414 (4) C(25)-C(26) 1.368 (4)

C(3)-C(4) 1.369 (4) C(28)-C(27) 1.368 (4)

C(7)-N(1) 1.415 (4) C(24)-O(2) 1.297 (4)

C(8)-C(9) 1.385 (4) C(29)-C(30) 1.390 (4)

C(10)-C(9)

C(11)-C(10)

C(12)-C(11)

C(7)-C(12)

C(7)-C(8)

1.388 (4)

1.383 (4)

1.391 (4)

1.388 (4)

1.390 (4)

C(30)-C(31)

C(31)-C(32)

C(32)-C(33)

C(33)-C(34)

C(29)-C(34)

1.387 (4)

1.394 (4)

1.382 (4)

1.402 (4)

1.381 (5)

O(1)-Cu(1)-N(1) 83.84 (8) O(1)-Cu(1)-O(2) 162.27 (10)

N(1)-Cu(1)-N(2) 174.98 (10) O(2)-Cu(1)-N(2) 83.62 (9)

N(2)-Cu(1)-O(1) 99.62 (11) O(2)-Cu(1)-N(1) 94.19 (9)

C(24)-O(2)-Cu(1) 111.2 (2) C(2)-O(1)-Cu(1) 111.8 (2)

C(23)-N(2)-C(29) 119.9 (2) C(1)-N(1)-C(7) 123.0 (2)

Cu(1)-N(2)-C(23) 112.9 (2) C(7)-N(1)-Cu(1) 122.5 (2)

C(29)-N(2)-Cu(1) 127.1 (2) C(1)-N(1)-Cu(1) 113.0 (2)

The dihedral angle between the two coordinating planes of the Cu(II) ion differs

according to the subsituent attached at the 3 and 5 or 2 or 4 positions of the N-phenyl ring.

Table 3.3 summarizes the dihedral angles for different complexes.

Chapter 3

C1O1

Cu1

Table 3.3. Dihedral angles between Cu1-O1-C1-C2-N1 and Cu1-O2-C3-C4-N2 planes.

Complex The dihedral angle between Cu1-O1-C1-C2-N1 and Cu1-O2-C3-C4-N2 planes

1(3,5-CF3)24.2 q

1a(2- CF3)25.4 q

2(3,5-F) 0q

3(3,5-Cl) 0q

4(3,5-OMe) 8.4 q

5(3,5-tBu) 1.1 q

From Table 3.3 it is clear that steric effects do not play any role in the distortion

around the Cu(II) center as described in the literature3 but electronic effects are responsible

for the distortion.

X-band EPR spectra were measured for complexes 1, 2, 3, 4 and 5. The nature of

entire spectrum is the same in the sense that all the signals are anisotropic with 14N (I = 1) and

1H (I=1/2) hyperfine coupling observed. As it has been shown by X-ray crystallography, all

the complexes are neutral with a Cu(II) ion and two coordinated radical anions, hence, the

system is in spin frustrating condition with three S = 1/2 spins. X-band EPR spectra provide

information that the residual unpaired electron which is responsible for the paramagnetic

nature of complexes, 1, 2, 3, 4 and 5 is on the Cu(II) center. Moreover, from the spectrum it is

clear that antiferromagnetic interactions between the Cu(II) center and the radical anions are

weaker than that of the antiferromagnetic interactions between the two radical anions and the

ground state electronic configuration for all the complexes is (npp). The interaction between

two radical anions in this type of systems must be antiferromagnetic while the interaction

Chapter 3

between the Cu(II) and radical anion(s) may be ferromagnetic obeying the Goodenough-

Kanamori rule or antiferromagnetic as shown in various cases4-6.

Figure 3.4. A

eff vs T plot for complex 1-5.

From Table 3.3 it is clear that there are distortions between the coordinating planes which

depend on the nature of the substituent at the 3,5-positions of N-phenyl group. Hence,

ferromagnetic interactions between the Cu(II) and radical anion(s) can not be accepted for all

cases. Though ferromagnetic or antiferromagnetic coupling constants between Cu(II) and

radical anion can be introduced to simulate the experimental magnetic behaviour of the

complexes, an assumption was taken that there is neither ferromagnetic nor antiferromagnetic

interaction between Cu(II) and radical anion(s), i.e., JCu-R is zero, to investigate if the

antiferromagnetic interaction constant between two radical anions depends on the nature of

the substituent. Considering the above assumption the experimental magnetic susceptibility

behaviour that has been measured at 1 T in the temperature range 2-290 K for complexes 1- 5

has been simulated. Figure 3.4 shows the plots and Table 3.4 summarizes the fitting

parameters. From these parameters it is clear that the JR-R exchange coupling constant is

antiferromagnetic and the value is similar for all the complexes except complexes 5 and 4a

which have that coupling constants –435 and -440 cm-1 respectively. It is also noticeable that

for complex 5 the gCu(II) value is 2.03 and smaller than that of others. Hence, from the JR-R and

gCu(II) values it is quite clear that the magnetic behaviour is not affected appreciably by the

nature of the substituent at the 3,5 positions.

(a) (b)

0 50 100 150 200 250 300

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2-CF3 (g = 2.0)

2-CF3 (g = 2.11)

4-CF3 (g = 2.11)

eff /

T/K

0 50 100 150 200 250 300

1.5

1.6

1.7

1.8

1.9

2.0

2.1

tBu

OMe

CF3

eff /

T/K

Chapter 3

2600 2800 3000 3200 3400 3600 3800

Sim

Exp

dX''/dB

Field / G

Figure 3.5. X-band EPR spectrum of complex 1 in frozen CH2Cl2 solution at 10 K.

Figure 3. 4b shows the magnetic behaviour of the complexes 1a and 1b. The nature of

magnetic behaviour for 1b is similar to 1with St = 1/2 ground state but that for 1a is totally

different. In the case of complex 1 and 1b the Peff value, 1.83 PB, remains constant in the

temperature range 2-150 K and then increases to 2.1 PB at 290 K. Bur for 1a the Peff value is

0.8 PB at 2 K and increases to 1.71 PE at 87 K. After that, the Peff value increases gradually

with increase of temperature and reaches to 1.94 PB at 290 K. The magnetic behaviour in the

temperature range 2-90 K for 1b may be due to intermolecular interactions. The X-band EPR

spectrum together with the simulation appears in Figure 3.5 for complex 1. The Fitting

parameters are, gx= 2.058, gy = 2.043, gz = 2.197, Wx = 12.5 G, Wy = 4.5 G, Wz =18.5 G, Ax

= 21 X 10-4 cm-1, Ay = 21 X10-4 cm-1, Az = 165 X 10-4 cm-1, AN(13, 10, 3 X10-4 cm-1).

The dihedral angle between the Cu1-O1-C1-C2-N1 and Cu1-O2-C3-C4-N2 planes for

complex 1 is 24.2q and that is 35.5q for the similar type of complex described in the Chapter

1. Both show a Cu(II) centered X-band EPR spectra. It has been shown in the literature that a

biradical-containing Cu(II) complex with dihedral angle 32.8q exhibits ligand (radical)

centered X-band EPR spectrum and the most probable reason behind of having that spectrum

is the distortion around the Cu(II) center.3It is clear from the Cu(II) centered X-band EPR

Chapter 3

spectra of the described complexes that the dihedral angle is not solely responsible for the

ligand centered EPR spectrum.

Table 3.4. Fitting parameters for the experimental temperature-dependent magnetic

behaviours of the complexes.

Complex No. J Cu-R (cm-1) JR-R (cm-1) gCu(II) g

1(3,5-CF3)0 (Fixed) -353 2.11 2.00

1a(2-CF3)0 (Fixed) -422(-326) 2.11(2.00) 2.00

1b(4-CF3)0 (Fixed) -352 2.11 2.00

2(3,5-F) 0 (Fixed) -320 2.11 2.00

3(3,5-Cl) 0 (Fixed) -323 2.08 2.00

4(3,5-OMe) 0 (Fixed) -340 2.11 2.00

4a(2-OMe) 0 (Fixed) -342 2.1 2.00

4b(4-OMe) 0 (Fixed) -440 2.09 2.00

5(3,5-tBu) 0 (Fixed) -435 2.03 2.00

1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5

100mV/s

CF3

tBu

E (V) vs Fc+/Fc

Figure 3.6. Cyclic voltammograms of complex 1and 5in CH2Cl2 under argon atmosphere.

Cyclic voltammograms (CVs) of complexes (1-5) have been recorded in CH2Cl2

solutions containing 0.1 M [N (n-Bu) 4

] PF6 as supporting electrolyte at a glassy carbon

working electrode and a Ag/AgNO3 reference electrode. Ferrocene was used as an internal

Chapter 3

standard, and potentials are referenced versus the ferrocenium/ ferrocene (Fc+/ Fc) couple.

Figure 3.6 shows the CVs of 1 and 5 recorded in the potential range +1.75 V to –1.75 V. Four

reversible one-electron-transfer waves are found. Two reversible one-electron oxidations and

two reversible one-electron reductions are observed, as established by coulometry at fixed

potential. Table 3.5 summarizes the results where Ox1 and Ox2 correspond to first and second

oxidation potentials respectively and successive reduction potentials are represented by Red1

and Red2. The oxidation and reduction processes are ligand centered and the redox behaviours

of the complexes can be given as shown in Scheme 1.

.Cu

(II)

(II) (II)

(II)

-1e-+1e-

-1e-

+1e-

Scheme 1. Different redox states for the complexes.

Chapter 3

To explain the cyclic voltammogram results for complexes 1-5, electrochemical

behaviour of complex 1 and complex 5 will be discussed. It is well known that any electron

donating or withdrawing group attached to the 3,5-or 3-or 5- positions of a phenyl ring system

exhibits an inductive effect (V-effect). Tert-butyl group shows +I effect (donation of electron)

whereas, trifluoromethyl group exhibits –I effect (withdrawing of electron.). From Table 3.5

it is clear that the reduction potentials of complex 5 are higher than those of complex 1. On

the other hand, opposite is true for oxidation processes. The presence of tert-butyl group at the

3,5-positions of the N-phenyl rings increases the electron density of the N-phenyl rings. The

opposite situation is valid for N-phenyl rings when 3, 5-positions of the rings are occupied by

–CF3 group. Hence, the Lewis acidity of complex 5is lower than that of complex 1. It is

known 7 that stronger Lewis acidity favours the acceptance capacity of electron and disfavour

removal capacity of electron by the complex. Therefore, complex 5 should have higher

reduction potentials and lower oxidation potentials than those in complex 1.

Table 3.5. Redox potentials of the complexes vs Fc+/ Fc.

Complex Ox1(V) Ox2(V) Red1(V) Red2(V)

1(3,5-CF3) -0.06 0.41 -0.804 -1.07

1a(2-CF3)-0.13 0.54 -0.965 -1.14

1b(4-CF3)-0.12 0.36 -0.84 -1.05

2(3,5-F) -0.12 0.365 -0.99 -1.445

3(3,5-Cl) -0.11 0.36 -0.87 -1.1

4(3,5-OMe) -0.30 0.318 -1.043 -1.289

4a(2-OMe) -0.327 0.360 -1.035 -1.420

4b(4-OMe) -0.116 0.57 -0.83 -1.18

5(3,5-tBu) -0.393 0.47 -1.16 -1.51

Chapter 3

Figure 3.7. UV



VIS/NIR spectra of complex 1, 1-and 12-.

The UV-VIS/NIR spectra of all the complexes are quite similar. Figure 3.7 displays

the spectra of complex [CuII(LCF3xx)]0, [CuII(LCF3x)]1-, [CuII(LCF3)]2- in CH2Cl2 solution and

Table 3.6 contains the results for electronic spectra of all complexes. After Controlled

potential one-electron reduction of complex 1, the band assigned to ligand-to-ligand charge

transfer band at around 800 nm diminishes. The ligand S-S* charge transfer band at around

440 nm also diminishes and a new sharp band at 355 nm (H = 23000 M-1cm-1) appears. Further

reduction of complex 1to 12- the band at 800 nm disappears and a sharp, strong band which

can be assigned to ligand-to-metal charge transfer at 378 nm (H = 37200 M-1cm-1) arises. The

band at 656 nm (H = 900 M-1cm-1) is probably due to the d-d transition of Cu(II)(d9) ion.

Table 3.6. Spectroelectrochemical data of the complexes.

Solvent = Dichloromethane, Temperature = 22 r 1q C, Sh =shoulder

Complex

, nm (

, M-1cm-1)

1, [CuII(L CF3

)]0 440sh (5170), 800 (10200), 1040sh (3300)

1-, [CuII(LCF3

)]1- 355 (23000), 790 (6500)

12-, [CuII(LCF3)]2- 378 (37200), 656 (900)

2, [CuII(LF

)]0 435 (4720), 797 (7700), 1100sh (2180)

4, [CuII(LOMe

)]0 456 (5400), 791 (6650), 1085sh (2570)

5, [CuII(LtBu

)]0 350 (21500), 450 (10500), 780 (8200), 1050 (4000)

400 600 800 1000 1200 1400 1600

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

12-

H

104/ M-1cm-1

Wavelength/



Chapter 3

3.3 Catalytic reactivities,

Aerial oxidation of primary alcohols; mimicking the

function of Galactose Oxidase(GOase)

To get an idea if biradical-containing Cu(II) complexes with N2O2 environments and

different substituents at the different positions of the N-phenyl ring can act as a catalyst

complexes 1, 1a, 1b, 2, 3, 4 and 5 were used for catalytic oxidation of primary as well as

secondary alcohols to the corresponding two-electron oxidised products. The catalytic

reaction was performed as describe below. In 20 ml CH2Cl2, 5.0 X10-4 M complex and 5.0

X10-2 M substrate, benzyl alcohol, ethanol, methanol or isopropanol were added. After

stirring the resulting solution under an aerial atmosphere for 16 hour and then passing an

aliquot of the solution through a neutral Al2O3 column the concentration of aldehyde was

analysed by LC or GC or by spectrophotometric methods.8-9 Table 3.7 summarizes the

product yields for different substrates.

Table 3.7. Percent of Yileds.

Complex PhCHO(%yield ) HCHO(%yield) MeCHO(%yield ) Me2CO(%yield )

1(3,5-CF3)

1a(2-CF3)

1b(4-CF3)

2(3,5-F)

-H(3,5positions)

4(3,5-OMe)

5(3,5-tBu)

<10

From Table 3.7, it is clear that for the oxidation of benzyl alcohol to benzaldehydes all

the complexes are equally efficient by yielding 63-72% benzaldehyde. It is to be noted that

during oxidation of benzyl alcohol to benzaldehyde tetrabutylammonium methoxide (4.50 X

10-4) was used as a base. That is why the formation of benzaldehyde with little amount of

formaldehyde (3-16%) was found using the complexes. Though the concentration of

methoxide was approximately same as the concentration of benzyl alcohol the yield of

benzaldehyde is higher than that of formaldehyde. This fact can be explained easily from the

C-HD bond dissociation energy for benzyl alcohol and methanol/methoxide (105.5 and 124

kcal mole-1 respectively).

Chapter 3

In case of oxidation of methoxide to formaldehyde or ethoxide to acetaldehyde using

tetrabutylammonium methoxide or tetrabutylammonium ethoxide as substrates maintaining

above stated reaction conditions the maximum yield of 25 % was obtained for complex 1. The

yield can be improved further and 75-80% formaldehyde or acetaldehyde can be obtained

using 1:25 complex: substrate ratio. The lesser yields for other complexes are due to the low

stability of those complexes in the catalytic solution.

Very low yield, 1% or < 1%, for the oxidation of isopropanol to acetone of C-C

coupled product, pinacol, (which is not formed at all after the catalysis) emphasized the fact

that the complexes are not capable of oxidizing secondary alcohols to their corresponding

aldehyde or C-C coupled product.4

Figure 3.8. Kinetics measurement for the formation of formaldehyde at room temperature.

From Table 3.7, it is clear that complex 1 is the best catalyst. Hence complex 1 was

used as a catalyst for oxidation of methoxide, ethoxide and benzyl alcohol. Kinetic studies

were done using methoxide or ethoxide as a substrate. In both cases spectrophotometric

methods were used to find out the concentration of formaldehyde or acetaldehyde with time.

The formation of formaldehyde or acetaldehyde was verified by GC, GC-MS, Mass spectra

(EI, ESI) using 2,4-dinitro-phenylhydrazine or 3-methyl-2-benzothiazolone hydrazine as

reagent to form Schiff base products.

350 375 400 425 450 475 500

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

2,2

A / cm

Wavelength / nm

0 5 10 15 20 25

[Formaldehyde] / m M

Time / h

Chapter 3

Figure 3.9. Kinetics measurement for the formation of acetaldehyde at room temperature.

Figures 3.8 and 3.9 show the changes in UV/VIS spectrum with time and increase of

concentration of formaldehyde and acetaldehyde with time. The exact procedures for kinetics

measurements using methoxide and ethoxide as substrates are written in the experimental

section. From the experimental results at 22 r1q C in CH2Cl2 keeping the concentration of

complex 1 (5.0 X10-4 M) fixed and varying the concentration of substrate or vice versa, the

rate law for the catalytic oxidation of methoxide or ethoxide was deduced as;

Rate = k [Substrate][Complex]

The kinetic of the oxidation of methoxide to formaldehyde using selectively

deuterated substrate CD3O- has been measured under the same reaction condition.

Figure 3.10. Plot of KIE (~ 5) at room temperature.

012345678910

CD3O-

CH3O-

[Formaldehyde] / mM

Time / h

0 2000 4000 6000 8000 10000 12000 14000

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

[CH3CHO]/M

Time /sec

550 600 650 700 750

0.0

0.4

0.8

1.2

1.6

2.0

A /cm

Wavelength/ nm

Chapter 3

The kH/kD ratio of ~ 5 (Figure 3.10) indicates a small but significant kinetic isotope

effect (KIE). This is a clear evidence that H-atom abstraction from the D-carbon atom of the

coordinated alcoholate is the rate-determining step. The resulting ketyl radical anion is known

to be a strong one-electron reductant 10-15 which is converted to aldehyde via. an

intramolecular electron-transfer step. In this case the intramolecular electron-transfer step

could reduce the ligand or metal center. Here ligand center reduction takes place.

Figure 3.11. Change in UV



VIS/NIR spectrum after passing air through the solution

obtained after glove-box reaction.

When a solution of complex 1 in dry CH2Cl2 was added to a 12.5 fold excess of

methoxide under anaerobic conditions at 22 r1q, the original blue-green color changed to

yellow within a minute. The final UV/VIS spectrum of this solution corresponds to that of

[CuII(LCF3)]2- generated electrochemically. Figure 3.11, the UV/VIS spectrum of the solution

obtained after the reaction under anaerobic condition. Subsequent exposure of the yellow

solution to air, the band at 800 nm(ligand-to-ligand charge transfer band), which arises due to

having radical in the complex, regenerated and thereby, proves the regeneration of radical in

complex 1. This fact also indicates that the radical takes part in the catalytic oxidation

reaction. Using the spectrophotometric method 9 the concentration of formaldehyde of the

resulting solution was measured and indicates the formation of about same concentration of

formaldehyde as the concentration of complex 1, [CuII(LCF3xx)]0 in the solution. The overall

reaction can be summarized as shown in the following equation.

CH3O- + [CuII(LCF3xx)]0 HCHO + [CuII(LCF3)]2- + H+

400 600 800 1000 1200 1400 1600

0.00

0.25

0.50

0.75

1.00

1.25

Air to the sol. of glove box

after glove box reaction

H

4/ M-1cm-1

Wavelength



Air

Chapter 3

It is important to note that during aerial catalysis of the reaction stated above no H2O2 was

detected. The catalytic reaction was also performed at 0 qC, –10 qC and -25 qC, even in these

conditions no H2O2 was detected. The complex does not show any catalase activity but due to

use of strong base, CH3O-, as a substrate H2O2 was decomposed to H2O and 1/2O2 as shown

below,

H2O2 H2O + 1/2O2

Thus, complex 1 is capable of oxidizing primary alcohols to the corresponding

aldehydes and forming the two electron reduced form [CuII(LCF3)]2-.

From the above results the mechanism for the catalytic oxidation of methoxide

ethoxide and benzylalcoholate can be proposed as follows (Figure 3.12),

Figure 3.12. The proposed mechanism for the catalytic oxidation of benzyl alcohol, methanol

and ethanol.

CF3CF3

CF3

CF3CF3

CF3

RCH2O

CF3CF3

CF3

CF3CF3

CF3

HO2

RCHO

-R = H, CH3, Ph

H-atom abstraction

r.d.s

Chapter 3

3.4 References

1. (a) Transition Metal Complexes with Imino-Phenolate and Iminobenzosemiquinone

Ligands; Synthesis, Characterization and Their Catalytic Reactivity. Soumen Mukherjee;

Thesis; 2003, University of Paderborn.

(b) Pieper, U.; Chaudhuri, P; unpublished results.

2. Kokatam, S.; Weyhermüller, T.; Bothe, E.; Chaudhuri, P.; Wieghardt K. Inorg. Chem.

2005,44, 3709.

3. Ye, S.; Sarkar, B.; Lissner, F.; Schleid, T.; Slageren, J. Van.; Fiedler, J. Kaim, W.Angew.

Chem. Int. Ed.2005,44, 2103.

4. Chaudhuri, P.; Hess, M.; Flörke, U.; Wieghardt, K. Angew. Chem. Int. Ed. 1998,37, No.16,

2217.

5. Chaudhuri, P.; Hess, M.; Weyhermüller, T.; Wieghardt, K. Angew. Chem. Int. Ed. 1999,

38, No. 8, 1095.

6. Chaudhuri, P.; Hess, M.; Müller, J.; Hildenbrand, K.; Bill, E.; Weyhermüller, T.;

Wieghardt, K. J. Am. Chem. Soc.1999,121, 9599.

7. Itoh, S.; Kumei, H.; Nagatomo, S.; Kitagawa, T. Fukuzumi, S. J. Am. Chem. Soc. 2001,

123, 2165.

8. Sawicki, E.; Hauser, T. R.; Stanley, T. W.; Elbert, W. Anal. Chem. 1961, 33. No 1. 93.

9. Nash, T. Biochem. J.1953,55, 416.

12. Whittaker, J. W. In Metal Ions in Biological Systems; Sigel, H. and Sigel, A., Eds. Marcel

Dekker: New York, 1994, Vol. 30, pp 315.

13. Knowles, P. F.; Ito, N. In Perspectives in Bio-inorganic Chemistry, Jai Press Ltd.

London, 1994, Vol. 2, 207.

14.Sokolowski, A.; Leutbecher, H.; Weyhermüller, T.; Schnepf, R.; Bothe, E.; Bill, E.;

Hildebrandt, P.; Wieghardt, K. J. Biol. Inorg. Chem. 1997,4, 444.

15. Saint-Aman, E. Biol. Inorg. Chem. 1997,2, 46.

Chapter 4

Synthesis, Characterization and Catalytic Reactivity of

Polynuclear Transition Metal Complexes formed with N (2-

hydroxy-3,5-di-tert-butylphenyl)-2-aminobenzylalcohol,

H3LCH

Chapter 4

4.1 Synthesis and characterization of the ligand, N (2-

hydroxy-3,5-di-tert-butylphenyl)-2-aminobenzylalcohol,

H3LCH

Condensation of 2-amino benzyl alcohol with 3,5-di-tert-butyl-catechol in the

presence of triethylamine in air in n-hexane produces the tridentate ligand, H3LCH2OH(Figure

4.1). The IR spectrum of the ligand shows -O-H and -N-H stretching frequencies at 3514 and

3383 cm-1 respectively, strong bands for tert-butyl groups appear at 2962-2865 cm-1, a sharp

band at 1603 cm-1 which is the -C-N stretch and bands at 1504-1426 cm-1 of the skeletal

vibration of the aromatic phenyl rings. The observed peak at 750 cm-1 is attributed to the

tetrasubstituted phenyl rings. Mass spectroscopy in the EI-mode shows the molecular peak at

m/z 327. GC and GC-MS analysis were performed to confirm purity and composition of the

ligand. Elemental analysis shows the composition of the ligand as C21H29NO2 and supports

mass and GC-MS analysis.

N H

O H

Figure 4.1. N (2-hydroxy-3,5-di-tert-butylphenyl)-2-aminobenzylalcohol.

The mechanism for the formation of this type of ligand is well established18 and is

shown in scheme 1.

Chapter 4

H2O

Et3N

N H R

RNH2

OH2

Scheme 1. The proposed mechanism for the formation of N-substituted aminophenol type

ligands.

In the first step, 3,5-di-tert-butylcatechol is oxidized to the corresponding quinone in

the presence of air (O2) and base. Condensation of the amine with the less sterically hindered

carbonyl moiety of 3,5-di-tert-butylquinone results in the 3,5-di-tert-butyl-iminoquinone

product. This iminoquinone is reduced by two electrons by 3, 5-di-tert-butylcatechol and N(2-

hydroxy-3,5-di-tert-butylphenyl)amine is generated together with regeneration of 3,5-di-tert-

butyl-quinone. According to the above mechanism, catalytic amounts of oxygen and base are

necessary for the formation of the ligand, which can be considered as non-innocent because of

its ability to change its redox state in the presence of metal ions and oxygen. Figure 4.2 shows

the different redox states of the deprotonated ligand, [(LCH2OH)] 3-.

Chapter 4

N-

O-

-1e-

+1e-

[LCatCH2OH ]3- [LBQCH2OH ]1-

[LSQCH2OH

]2-

Figure 4.2. Different redox states of the fully deprotonated ligand [LCH2OH] 3-.

Polynuclear complexes are of interest in bioinorganic chemistry due to their structural

and functional relevance to some metalloenzymes, e.g. Phenoxazinone Synthases (PHS),

Photosystem II (PS II), Bromoperoxidase, etc.1-16 and they are also important in understanding

metal-metal and metal-ligand interactions.17 As the deprotonated ligand contains a strong S-

donating alcoholate group, the ligand has very high tendency to form polynuclear metal

complexes.

Using the above ligand a series of polynuclear complexes were synthesized. These

complexes are, [VV2O2(P2-O)2(LCatCH2OH)2] (1), [MnIIMnIIIMnIV(LSQCH2OHx)2(LBQCH2OH)

(LCatCH2OH) Cl] (2) and [CuII4(LSQCH2OHx)4] (3).

This chapter involves synthesis, characterization by X-ray crystallography, magnetic

moments, spectro and spectroelectrochemical methods and catalytic reactivity of the above

polynuclear complexes.

Chapter 4

4.2 Alkoxide bridged dinuclear oxovanadium(V) complex (1)

Stirring a mixture of VOSO4.5 H

2O (2 mmol), H3LCH2OH (2 mmol) and triethylamine

(0.2 ml) in acetonitrile (30 ml), under aerobic conditions for two hours yields the deep blue

complex 1.

The sharp bands between 2961 cm-1 and 2869 cm-1 in the IR spectrum of complex 1

confirm coordination of the ligand to the metal. Complex 1 exhibits a sharp and strong band at

1001 cm-1 which is a clear indication of the V=O unit in the complex. The absence of -O-H

and -N-H stretching bands that are present in the ligand IR spectrum confirms deprotonated N

and O atoms as chelating sites. The sharp, strong band at 1114 cm-1 can be assigned to C-O

stretching of the alkoxide group attached to the metal ion. A sharp band observed at

approximately 760 cm-1 is characteristic of [V-(P-O)-V] stretching. ESI-positive mass

spectroscopy in dichloromethane yields a molecular ion peak at 782.3 (100%) and confirms

C42H52N2O6V2 composition for complex 1.

Deep blue colored single crystals were obtained by recrystallization of the

microcrystalline solid from a 1:1 CH2Cl2 and CH3CN solvent mixture. X-ray analysis shows

that complex 1 is a binuclear vanadyl complex. ORTEP diagram of the molecule with the

atom labeling scheme is shown in Figure 4.3. Selected bond lengths and bond angles are listed

in Table 4.1. In the discrete neutral binuclear complex 1, two five coordinate vanadium atoms

are each ligated to one [LCatCH2OH] 3- , one P2 alkoxide group and one oxo group. Both

vanadium ions are linked through two bridging alkoxide groups to form a V2O2 ring, which is

asymmetric in nature as the V(1)-O(16) and V(1)-O(16A) bond distances are 1.994(3) and

1.958(3) Å respectively. The dihedral angle between the O(1)-N(8)-O(16)-O(16A) and

O1(A)-N(8A)-O(16)-O(16A) planes is 0q and each vanadium ion is 0.636 Å away from the

plane. Hence, each pentacoodinate vanadium center can be described as square-pyramidal

one with oxygen atom at the apex. The V

}

V distance is 3.118(1) Å.

Chapter 4

Figure 4.3. ORTEP diagram of complex 1.

The vanadium ions in neutral complex 1 may have +IV or +V oxidation state with a

one-electron oxidized form or a fully reduced amidophenolate form of the ligand respectively.

The respective V(1)-O(1), V(1)-N(8), V(1)-O(16), V(1)-O(30) bond lengths are 1.9160(15),

1.9609(16), 1.9160(15) and 1.5886(15) Å. An amidophenolate form of the ligands is

evidenced by C-C bond distances within the tert-butyl containing phenyl rings.19 The C(7)-

N(8)-V(1), C(7)-N(8)-C(9) and C(9)-N(8)-V(1) bond angles are 112.57(13), 123.68(16), and

122.63(16)q respectively and support amidophenolate form of the N(8) atom. The O(1)-C(2),

C(7)-N(8) and N(8)-C(9) bond lengths are 1.331(2), 1.362(3) and 1.419(3) Å respectively.

The O(1)-C(2) and C(7)-N(8) bond distances do not correspond to either amidophenolate or

iminosemiquinone form of the tert-butyl containing phenyl rings. From the crystal structure it

is not possible to assign the oxidation states of the ligands and metal ions unambiguously but

from the V-O, V-N and V=O 20-21 bond lengths it can be concluded that the neutral complex is

composed of two vanadium (V) ions and two fully reduced ligands.

Chapter 4

Table 4.1. Selected bond distances (Å) and angles (degree) for 1.

C(2)-C(3) 1.414(3) C(4)-C(5) 1.421(3)

C(2)-C(7) 1.436(3) C(5)-C(6) 1.383(3)

C(3)-C(4) 1.386(3) C(6)-C(7) 1.408(3)

O(30)-V(1)-O(1) 112.39(8) O(30)-V(1)-O(16) 107.92(8)

O(1)-V(1)-O(16) 139.62(6) O(30)-V(1)-N(8) 81.1(6)

O(16)-V(1)-N(8) 87.35(7) O(30)-V(1)-O(16A) 104.75(7)

V(1)-O(16)-V(1A) 104.14(7) V(1)-O(16A)-V(1A) 107.88(7)

Variable temperature (2-290 K) magnetic susceptibility measurement using a SQUID

magnetometer at 1T indicates that complex 1 is diamagnetic. The diamagnetic nature was

further confirmed by recording 1H NMR spectrum in a CD2Cl2 solution at 300 K.

To further characterize the complex, a 51V NMR spectrum in CD2Cl2 was recorded

against VOCl3 in C6D6. Figure 4.4 shows the NMR spectrum. Owing to the quadrupole

moment (I = 7/2) of the 51V nucleus, the resonances are somewhat broadened. The

oxovanadium complex 1 shows resonances at G = -477.5 ppm and G = -507 ppm. These two G

values are expected for a dioxovanadium(V) complex with O,N,O donor set.22-23

-510 -500 -490 -480 -470 -460

ppm

Figure 4. 4. 51V NMR spectrum of 1 in CD2Cl2 at room temperature.

Chapter 4

1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0

800 mV/S

400 mV/S

200 mV/S

100 mV/S

50 mV/S

E(V) vs Fc+/Fc

Figure 4.5. Cyclic voltammogram of complex 1 in CH2Cl2 at room temperature under an

argon atmosphere.

The cyclic voltammogram (CV) of complex 1 has been recorded in CH2Cl2 solution

containing 0.1 M [N (n-Bu) 4

] PF6 as supporting electrolyte at a glassy carbon working

electrode and a Ag/AgNO3 reference electrode. Ferrocene was used as an internal standard,

and potentials are referenced versus the ferrocenium/ ferrocene (Fc+/ Fc) couple. Figure 4.5

shows the CV of 1 recorded in the potential range +1.75 V to –2.0 V. A reversible two-

electron oxidation and two reversible one-electron reductions are observed, as established by

coulometry at fixed potential. Hence, E1/2 value at 0.423 V corresponding to one two-electron

oxidation and E1/2 values at -0.65 and -0.86 V are due to two successive one-electron

reductions. As complex 1 contains two vanadium (V) ions with two fully reduced

coordinating ligands ([LCatCH2OH]3-) the oxidations should be ligand centered and reduction

processes will be metal centered.

100

Chapter 4

Figure 4.6. Electronic spectrum of complex 1, 1+, 12+,1-and 12- in CH2Cl2 at -25

The UV-VIS/NIR spectra of complex 1 together with its one (electron counted during

coulometry) and two-electron oxidized as well as reduced species are depicted in Figure 4.6.

Table 4.2 summarizes the spectroelectrochemical behaviours. Neutral complex 1shows three

absorption bands at 315, 467 and 760 nm. All the bands can be assigned to ligand-to-metal

charge transfer bands or more clearly the bands at 467 and 760 nm are likely phenolate-to-

vanadium (V) charge transfer bands 24 whereas the band at 315 nm is possibly an alkoxide to

vanadium (V) charge transfer band.25 Upon reduction by one electron the bands at 760 nm

and 467 nm diminish while that at 315 nm increases slightly. The decrease in absorption on

reduction could be due to reduction of vanadium (V) to vanadium (IV). Further reduction

leads to a sharp and strong band at 345 nm and the assigned metal-to-phenolate charge

transfer bands (760 and 476 nm) almost vanish indicating nonexistence of the vanadium (V)

moiety after reduction in complex 12-. Upon oxidation by one electron the band at 760 nm

diminishes slightly and the final absorption at that wavelength could be due to both ligand-to-

metal and intervalence ligand-to-ligand (amidophenolate-to-imonosemiquinone) charge

transfers. Similarly, ligand S-S* and phenolate-to-vanadium (V) charge transfers explanation

could be given for slight increase in absorption at 473 nm. After two electron oxidation of 1

the band at 760 nm becomes very broad and a new band at 495 nm with a shoulder at 563 nm

appears. The band at 495 nm can be assigned to a ligand S-S* transition, while, that at 563 nm

is presumably due to ligand (iminosemiquinone)-to-vanadium (V) charge transfer.

300 400 500 600 700 800 900 1000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

12-

H

/ 104/ M-1cm-1

Wavelength / nm

400 500 600 700 800 900 1000

0.0

0.5

1.0

1.5

2.0

300

12+

/ 104/ M-1cm-1

Wavelength / nm

101

Chapter 4

Table 4. 2. Spectroelectrochemical data of the complexes.

Complex

, nm (

, M-1cm-1, 104)

1315 (1.95), 467(0.7), 760(0.56)

1-314(2.0), 467(0.443), 760 (0.31)

12- 345 (2.5)

1+311(2.0), 473(0.73), 765(0.467)

12+ 495(0.9), 563(0.77)

From all these experimental results the electrochemical behaviour of complex 1 can be

summarized as shown in Figure 4.7.

[(LcatCH2OH)VIV(P2 O)VIV(LcatCH2OH)]2- Complex 12-

+1e--1e-

[(LcatCH2OH)VIV(P2 O)VV(LcatCH2OH)]-Complex 1-

+1e--1e-

[(LcatCH2OH)VV(P2 O)VV(LcatCH2OH)]0 Complex 1

+1e--1e-

[(LSQCH2OHx)VV(P2 O)VV(LcatCH2OH)]+ Complex 1+

+1e--1e-

[(LSQCH2OHx)VV(P2 O)VV(LSQCH2OHx)]2+ Complex 12+

Figure 4.7. Different redox states of 1.

102

Chapter 4

4.3 A trinuclear Mn-cluster(2)

The reaction of anhydrous MnCl2 (2 mmol) with the ligand (2 mmol), H3LCH2OH, in

methanol in the presence of triethylamine produces a brown colored solution. Addition of

acetonitrile to this solution followed by slow evaporation gives deep brown colored solid.

The IR spectrum of 2 is complicated because both the quinone and iminosemiquinone

forms of the ligand are attached to the metal. The bands at 1643 cm-1 and 1621 cm-1 are due to

the C=O and C=N stretching vibrations respectively. The sharp peak at 1475 cm-1 is attributed

to the presence of C-Ox units. The moderate intensity peak at 1022-1074 cm-1 can be assigned

to C-O stretching of the alkoxide group attached to the metal ions.

Complex 2·2MeCN crystallizes in the triclinic spacegroup P¯1. The neutral complex 2

(Figure 4.8) has nine anions (two monoradicaldianions [LSQCH2OHx]2í, one trinegative ligand

[LCatCH2OH]3í , one mononegative ligand [LBQCH2OH]1íand one chloride) requiring that the sum

of the charges on the three manganese ions to be +9 and resulting in the manganese oxidation

state formulation of [MnIIMnIIIMnIV]9+. The alternative formulation of MnIII3 can be

unambiguously discarded from the different M–X bond lengths observed in the X-ray crystal

structure.26-27 The structure contains a central six-coordinatd Mn(II) ion, Mn(2), which is

flanked by one six-coordinated Mn(IV), Mn(3), and one five-coordinated Mn(III), Mn(1)

ions; manganese ions are linked to each other through two bridging P2-alkoxo oxygens. The

coordination sphere around Mn(3) is N2O4, in which the equatorial plane is comprised of

O(61)O(91)O(76)O(106) and the nitrogen atoms N(68) and N(98) are disposed in trans

positions with an angle N(68)–Mn(3)–N(98) of 168.2(2)ƕ. The short Mn(3)–O and Mn(3)–N

bond lengths (Table 4.3) and highly octahedral ligand arrangement are consistent with a

Mn(IV)26-27 state for Mn(3) that shows no Jahn–Teller distortion, which would be a

characteristic for a d4 h.s. Mn(III) ion. The O(91)–C(92), C(97)–N(98), O(61)–C(62) and

C(67)–N(68) bond lengths at 1.343(6),1.393(7), 1.308(7) and 1.361(7) Å, respectively,

indicate clearly that the two ligands attached to Mn(3) are in two different oxidation states,

i.e. one in fully reduced amidophenolate form and the other in iminosemiquinone radical

form. Thus, the coordination environment of Mn(3) is best described as

Mn(3)(LCatCH2OH)(LBQCH2OHx). O(76) and O(106) make P2-alkoxo bridges between the

Mn(3)and Mn(2) centers. The Mn(2)–O and Mn(2)–N bond lengths lying between 2.103(4)–

2.231(5) Å strongly indicate the high-spin Mn(II)-character for the Mn(2) center. The

103

Chapter 4

Figure 4.8. ORTEP diagram of complex 2.

spective O(31)–C(32) and C(37)–N(38) bond lengths at 1.227(6) and 1.288(7) Å are in

accord with the assignment that the ligand attached to the Mn(2) centre is in fully oxidized

iminoquinone form(LBQCH2OH)1í.27 The Mn(1) centre is in a square-pyramidal geometry with

an axial Clí ion, Cl(1). The Mn(1) atom is about 0.437 Å above the NO3 equatorial plane.

The Mn(1)–O and Mn(1)–N bond lengths are in full conformity with the reported parameters

for h.s. d4 Mn(III). O(1)–C(2) and N(8)–C(9) bond lengths at 1.310(6) and 1.376(7) Å give a

clear indication that the ligand chelating with the Mn(1) centre in the iminosemiquinone

radical form, thus making Mn(1) a five-coordinated Mn(III) centre bound to a p-radical anion.

Summarily, the neutral trinuclear complex 2with the composition [Mn3L4CH2OHCl] consists of

valence-trapped Mn(IV), Mn(II) and Mn(III) ions with two iminosemi uinone radical

dianion, one iminoquinone monoanion and one amidophenolate trianion ligand system.

104

Chapter 4

Table 4.3. Selected bond lengths (Å) and angles (degree) for 2.

Mn(1)–O(46) 1.884(4) Mn(3)–O(76) 1.917(3)

Mn(1)–O(16) 1.898(3) Mn(3)–N(98) 1.928(4)

Mn(1)–N(8) 1.927(4) Mn(3)–N(68) 1.950(4)

Mn(1)–O(1) 1.929(4) Mn(1) ···Mn(2) 3.121(1)

Mn(1)–C1(1) 2.366(2) Mn(2) ···Mn(3) 3.117(1)

Mn(2)–O(16) 2.103(4) Mn(1) ···Mn(3) 5.856(1)

Mn(2)–O(76) 2.136(4) O(1)–C(2) 1.310(6)

Mn(2)–O(106) 2.139(3) N(8)–C(7) 1.376(7)

Mn(2)–O(46) 2.190(4) O(31)–C(32) 1.227(6)

Mn(2)–N(38) 2.231(5) N(38)–C(37) 1.288(7)

Mn(2)–O(31) 2.293(4) O(61)–C(62) 1.308(6)

Mn(3)–O(91) 1.883(4) N(68)–C(67) 1.361(7)

Mn(3)–O(61) 1.905(3) O(91)–C(92) 1.343(6)

Mn(3)–O(106) 1.915(4) N(98)–C(97) 1.393(7)

Mn(1)–O(16)–Mn(2) 102.4(2) Mn(2)–O(76)–Mn(3) 103.1(2)

Mn(1)–O(46)–Mn(2) 99.7(2) Mn(2)–O(106)–Mn(3) 103.1(2)

0 50 100 150 200 250 300

J12

J23

J13

Sim

Exp

eff/

T/K

Figure 4.9. A Plot of

eff vs. T for complex 2.

Variable-temperature solid-state magnetic susceptibility was measured on a

powdered sample of 2in the temperature range 2–290 K in an applied magnetic field of 1 T,

shown as Figure 4.10, to discern the type of exchange interactions for this valencetrapped

trinuclear complex. The effective magnetic moment, Peff of 2increases gradually from 7.6 PB

at 290 K to 8.48 PB at 10 K, below which Peff decreases, reaching 5.8 PB at 2 K. To describe

the magnetic properties of 2, the Heisenberg spin exchange Hamiltonian in the form ƨ = í2JS

105

Chapter 4

ǅixSǅjwas used. It has been shown previously that the interactions, Mn(III)–radical (R) and

Mn(IV)–radical (R) are very strongly antiferromagnetic in nature.26-27 Hence, the

experimental magnetic moment curve was fitted using only three (instead of five) spin-

carriers with S1= 1.0 [Mn(IV)–R], S2= 2.5 [Mn(II)] and S3 = 1.5 [Mn(III)–R]. As the

distances Mn(1)···Mn(2) of 3.121(1) and Mn(2)···Mn(3) 3.177(1) Å result in a too long

distance 5.856 Å between Mn(1) and Mn(3) to be significant for the exchange interaction, we

have considered a “2-J” model for simulation: J12 represents the exchange interaction between

the spin S1 and S2, whereas J23 the exchange parameter between S2 and S3. To avoid

overparametrization and to include the single-ion zero-field splitting parameter D1 of the

manganese(III) centre, whose inclusion in simulation is evidenced by nesting of the variable-

temperature variable field (1, 4 and 7 T) plots, we have used the constraint g1 = g2 = g3 during

simulation.

0.00.51.01.52.02.5

M/Ng

H/kT

0,0 0,5 1,0 1,5 2,0 2,5 3,0

ST= 4.0

Exp

Sim

M/Ng

H/kT

ST= 5.0

Figure 4.10. M/ Ng

H/kT plot for complex 2(dotted lines are for experimental and

solid lines are for simulation).

A satisfactory simulation, shown as the solid line in Figure 4.10, is obtained with the

fitting parameters, J12 = 2.7 cmí1,J23 = í0.30 cmí1,g1 = g2 = g3 = 2.05, D1 = 8.0 cmí1. Thus,

the magnetic measurements also corroborate the three different oxidation states of the

manganese centers in 2. Variable-temperature (2–290 K), variable-field (1, 4 and 7 T)

magnetization measurements were also simulated with very similar parameters. Figure 4.10

shows the simulation of the experimental data at 7 T with the parameters, J12 = +2.6 cmí1,J23

=í0.1 cmí1,g1 = g2 = g3 = 2.05, D= 8.0 cmí1, together with the theoretical Brillouin curves

with g= 2.05. Figure 4.10 shows clearly that the ground state of the molecule is higher than St

106

Chapter 4

= 4.0 and the low-lying states are very near to each other in energy. It has been noticed that

keeping J23 fixed at zero, the quality of simulation reduces and becomes unacceptable. As J’s

and Dare of similar magnitude it is not possible to calculate the ground state in the form of an

Stvalue; Sis not a good quantum number to describe the ground state, but rather MS.

0.4 0.2 0.0 -0.2 -0.4 -0.6

E(V) vs Fc+/Fc

Figure 4.11. Cyclic voltammogram of complex 2 in CH2Cl2 at room temperature under an

argon atmosphere.

The cyclic voltammogram (CV) of complex 2 was recorded at room temperature in

CH2Cl2 solutions containing 0.1 M [N (n-Bu) 4

] PF6 as supporting electrolyte at a glassy

carbon working electrode and an Ag/AgNO3 reference electrode. Ferrocene was used as an

internal standard, and potentials are referenced versus the ferrocenium/ ferrocene (Fc+/ Fc)

couple. Figure 4.11 shows the CV of 2 recorded in the potential range +0.70 V to –0.5 V. In

that potential range two one-electron-transfer waves were found. Controlled potential

coulometric analysis at –25 qC in CH2Cl2 indicates that the E1/2 value at -0.335 V corresponds

to reversible one-electron reduction of 2 whereas, the E1/2 value at +0.263 V corresponds to a

two-electron oxidation of complex 2. It is interesting to note that the wave at +0.263 V

becomes reversible and double in height with decreasing the temperature of the medium

(CH2Cl2). This behaviour is generally associated with a metal centered redox process(es).28

107

Chapter 4

400 600 800 1000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

H

/ 104/ M-1cm-1

Wavelength/ nm

Figure 4.12. Electronic spectrum of complex 2and 2- in CH2Cl2 at -25

Figure 4.12 shows the UV-VIS/NIR spectra of 2 and its electrochemically generated

one-electron reduced species. In complex 2, a broad band at 800 nm (H = 13500 M-1cm-1) with

a shoulder at around 940 nm (H = 7700 M-1cm-1) and another broad band at 490 nm (H =

22700 M-1cm-1) are observed. The first two bands may be assigned as intervalence ligand-to-

ligand charge transfer bands while the latter is likely ligand-to-metal charge transfer. Upon

reduction of complex 2 by one electron the band at 800 nm decreases slightly (H = 11870 M-

1cm-1) and a new broad band at around 910 nm (H = 11500 M-1cm-1) appears. The appearance

of the new, broad low energy band could be due to the reduction of the fully oxidized ligand,

iminoquinone, to iminosemiquinone and hence provides intervalence ligand-to-ligand charge

transfer. The band at 490 nm shifts to 466 nm (H = 22300 M-1cm-1) after one-electron

reduction of 2. The exact assignment of the electrochemical behaviours of the complex is not

possible because it is polynuclear with multiredox centers. But from the coulometric and

spectroelectrochemical results (Figure 4.12) two proposals shown in Scheme 2 can be given.

108

Chapter 4

(A)

[ClMnIII(LSQCH2OHx)MnIV(LBQCH2OH)MnIV(LSQCH2OHx) (LcatCH2OH)]2+

- 2e + 2e

[ClMnIII(LSQCH2OHx)MnII(LBQCH2OH)MnIV(LSQCH2OHx) (LcatCH2OH)]0

+1e -1e

[ClMnIII(LSQCH2OHx)MnII(LSQCH2OHx)MnIV(LSQCH2OHx) (LcatCH2OH)]-

(B)

[ClMnIV(LSQCH2OHx)MnIII(LBQCH2OH)MnIV(LSQCH2OHx) (LcatCH2OH)]2+

- 2e + 2e

[ClMnIII(LSQCH2OHx)MnII(LBQCH2OH)MnIV(LSQCH2OHx) (LcatCH2OH)]0

+1e -1e

[ClMnIII(LSQCH2OHx)MnII(LSQCH2OHx)MnIV(LSQCH2OHx) (LcatCH2OH)]-

Scheme 2. Different redox states of complex 2.

109

Chapter 4

4.4 A Cu4O4 cubane complex (3)

4.4.1 Synthesis and characterization of the complex(3)

To synthesize complex 3, H3LCH2OH (2 mmol) was added to a solution of Copper(II)

acetate monohydrate (2 mmol) in acetonitrile (30 ml). The solution was stirred under air for

24 h; a dark red / pink color powder was filtered and recrystallized from a 1:1 methanol,

dichloromethane solution mixture.

In the IR spectrum of complex 3, the bands at 1025 cm-1 arises due to the Cu-OR

stretching vibration.3 The sharp bands at 2961-2868 cm-1 are due to the –C-H stretching of the

tert-butyl group and the band at 1627 cm-1 is an indicative of the presence of -C=N unit in

complex 3.

X-ray quality single crystals were obtained after recrystallization of the

microcrystalline solid from a 1:1 MeOH / CH2Cl2 solution mixture. ORTEP diagram of the

tetrameric copper complex is shown in Figure 4.13. Selected bond lengths as well as bond

angles are listed in Table 4.4. The diagram shows cubane geometry of the complex with a

Cu4O4 core. The complex contains C2 axes that pass vertically through the planes of the

cubane (Figure 4.14). Complex 3 contains copper ions in the alternating corners of the cube

and each copper ion is linked with each other through P3oxygen atoms of the alcoholate

group of the ligands. The respective Cphenyl-O and Cphenyl-N bond lengths are 1.305(3),

1.367(3) Å. The six C-C distances in the tert-butyl groups containing phenyl rings are not

equidistant, whereas those distances in the N-phenyl part are same within experimental error

(1.390(3) Å). The C(1)-C(2), C(2)-C(3), C(3)-C(4), C(4)-C(5), C(5)-C(6) and C(6)-C(2) bond

lengths are 1.434(3), 1.379(3), 1.434(3), 1.368(3), 1.423(3) and 1.452(3) Å respectively. As

similar kind of bond lengths are observed for all the tert-butyl containing phenyl rings, hence,

it can be concluded that all the tert-butyl containing phenyl rings are in the iminosemiquinone

form with quinoid type distortion, i.e. alternative long and short C-C bonds. Each copper ion

is pentacoordinate with one imine N atom, one phenoxyl atom and three P3alkoxo O atoms

from the three ligands. The geometry around each copper ion is distorted square pyramidal [W

=D-E/ 60, D and E are two largest angles around the central atom, W = 0 and 1 for perfect

square pyramidal and trigonal bipyramidal geometries, respectively. W = 0.2 (Cu1), W = 0.23

(Cu2), W = 0.18 (Cu3), W = 0.26 (Cu4)]. The molecule is a neutral one and so, the oxidation

state of all copper ions, which have more or less same Cu-O and Cu-N bond lengths, should

110

Chapter 4

be +II and all the ligands are in iminosemiquinone form.29 The Cu-Oal bond lengths and Cu-

Oal-Cu (Oal stands for alkoxo oxygen atom) bond angles are shown in Figure 4.15.

Figure 4.13. ORTEP diagram of 3.

Table 4.4. Selected bond lengths (Å) and angles (degree) for 3.

Cu(1)-O(1) 1.9076(15) Cu(4)-N(97) 1.9448(18)

Cu(1)-N(7) 1.9579(14) Cu(1)…Cu(2) 2.956(3)

Cu(2)-O(31) 1.9295(14) Cu(1)…Cu(3) 3.089(3)

Cu(2)-N(37) 1.9441(17) Cu(1)…Cu(4) 3.332(3)

Cu(3)-O(61) 1.9137(15) Cu(2)…Cu(3) 3.324(3)

Cu(3)-N(67) 1.9562(16) Cu(2)…Cu(4) 3.119(3)

Cu(4)-0(91) 1.9211(15) Cu(3)…Cu(4) 2.954(3)

111

Chapter 4

Figure 4.14. Symmetry diagram for complex 3.

Cu2

O45

O15

Cu1

O105

O75

Cu4

Cu3

102

93.06

97.77

94.5

101.08

97.40

97.6

102.08

92.73

(A) (B)

Figure 4.15. (A) The Cu-Oal-Cu bond angles and (B) Cu-Oal bond lengths.

Cu2

O45

O15

1.990

Cu1

O105

O75

Cu4

Cu3

2.315

2.284 1.987

1.946 1.939

1.952

1.988 1.928

2.314

2.289

1.993

112

Chapter 4

Variable-temperature magnetic susceptibility measurements were carried out in the

temperature range of 2-290 K at a magnetic field of 1 T on a solid sample of 3 with a SQUID

magnetometer. Figure 4.16 illustrates the temperature dependence of the magnetic

susceptibility of the tetraradical-containing tetranuclear copper (II) distorted cubane cluster in

the form of the FMT vs T. The curve for complex 3 shows antiferromagnetically coupled

tetraradical-containing tetranuclear copper(II) cluster with a decrease of FMT from a high

temperature value of 3.13 cm3K mol-1 to a value close to 0 at low temperature indicating that

all the eight paramagnetic centers are coupled together to form an St = 0 ground state.

0 50 100 150 200 250 300

0.0

0.5

1.0

1.5

2.0

2.5

3.0

g = 2.18

J1 = - 41.7 cm-1

J2 = - 0.03 cm-1

J3 = - 0.03 cm-1

J4 = - 38.4 cm-1

p.i. = 0.08 in FT

g = 2.22

J1 = - 32.2 cm-1

J2 = - 39.7 cm-1

p.i. = 0.08 in FT

FMT / cm3 mol-1 K

T / K

Figure 4.16. A plot of

MT vs T for complex 3.

This result can be rationalized by considering (Figure 4.17) the orientation of the dx2-y2

orbital of the Cu(II) ions deduced from the structural analysis and presented in Figure

4.17(A). It appears that both of the oxygen bridges of coppers of the same pairs (1, 4 and 2, 3)

are axial to one metal atom and equatorial to the other; this gives rise to the overall situation

shown in Figure 4.17(C). It is clear that this arrangement does not lead to a strong overlap

between the dx2-y2 orbitals on any bridging atom, and, therefore, the interaction cannot be

important. The interpair interaction, on the other hand, implies two different types of bridges:

one of the axial-equatorial type as above, but also, one of the equatorial-equatorial type. As

illustrated in Figure 4.17(B), the latter combination produces a significant overlap on one

bridging oxygen, thus providing a pathway to the magnetic interaction.

113

Chapter 4

(A)

(B)

Cu Cu

(C)

Figure 4.17. A. Orientation of the dx2-y2 orbitals of Cu(II) ions within the tetranuclear

cluster.

B. Equatorial- equatorial and axial- equatorial interpair interactions.

C. Axial- equatorial intrapair interactions.

114

Chapter 4

Coupling schemes (3):

Scheme A R

CuCu

J 1 = Cu-Cu J 2 = Cu-Rad

Scheme B

RJ2

105

115

Chapter 4

Cu(1)-O(15)-Cu(2) = 97.60 J3 Cu(1)-O(15)-Cu(3) = 92.73 J2

Cu(1)-O(45)-Cu(4) = 102.08 J1 Cu(1)-O(45)-Cu(2) = 97.40 J3

Cu(1)-O(75)-Cu(4) = 101.08 J1 Cu(1)-O(75)-Cu(3) = 93.06 J2

Cu(2)-O(15)-Cu(3) = 100.82 J1 Cu(2)-O(45)-Cu(4) = 94.05 J2

Cu(2)-O(105)-Cu(3) = 102.00 J1 Cu(2)-O(105)-Cu(4) = 94.50 J2

Cu(3)-O(75)-Cu(4) = 97.77 J3 Cu(3)-O(105)-Cu(4) = 97.18 J3

Cu(1)-Rad J4 Cu(2)-Rad J4 Cu(3)-Rad J4 Cu(4)-Rad J4

To simulate the experimental magnetic properties of complex 3, the Heisenberg spin-

exchange Hamiltonian ƨ = 26i<j Jij ǅixǅj was used, in which Jij represents the exchange

constants and the subscripts iand j number the pairwise interacting paramagnetic centers.

Two different assumptions are taken into account. Firstly, it has been assumed that the

interactions between the Cu-Cu centers are the identical and the Cu-radical interaction for the

individual Cu-radical unit is same to the other Cu-radical units (Scheme 3A). The following

parameters are obtained from the best fit to the experimental result taking above assumption

into account. g = 2.22, J1 = - 32.2 cm-1, J2 = - 39.7 cm-1, p.i. = 0.08. Considering all the Cu-O

bond lengths and Cu-Oal-Cu bond angles (Scheme 3B) three different types of exchange-

interaction have been taken for the interactions among the copper ions. As the Cu-radical

bond lengths and the bond angle Cu-O-CR (R= radical containing-carbon center) are same,

radical-copper interaction is taken as same for all the four copper-radical units. Hence, four

different interactions were the second assumption (Scheme 3B). According to this assumption

the best fit result is, g = 2.18, J1 = - 41.7 cm-1, J2 = - 0.03 cm-1, J3 = - 0.03 cm-1, J4 = - 38.4

cm -1, p.i. = 0.08. Both simulations provide a good fit to the experimental results.

93 94 95 96 97 98 99 100 101 102 103 104 105 106

-400

-300

-200

-100

100

This work

References

J/ cm-1

Cu-Oal-Cu / degree

Figure 4.18. A plot of J vs Cu-Oal-Cu bond angles.

116

Chapter 4

Hence, it is conclusive that the exchange interaction constant between two copper(II)

ions, which are linked through

3-alkoxo bridges, remains approximately zero in the range of

Cu-Oal-Cu bond angle 92.73 to 97.77

. These results fairly close to the previously reported

results. 30-34 No correlation between the exchange coupling constant (J) and Cu-Oal-Cu angle

is possible as shown in Figure 4.18. Table 4.5 summarizes the exchange coupling results.

Table 5 . Cu-Oal-Cu bond angle and J data.

Cu-O-Cu ,

J, cm-1 Reference

99.4 -21.4 33

104 12.3 33

94.2 12.3 33

97.1 -176 34

99.8 -5 34

104.2 -335 30

105 -400 30

99.1 -122.5 30

95.7 -17.4 30

97.6 -79.6 30

99.7 -75.5 31

98.9 -42.35 31

101.9 18.5 31

97.2 11.7 31

101.5 -41.7 This work

97.5 0 This work

93.6 0 This work

117

Chapter 4

Cylic voltammogram (CV) and square wave voltammogram(SQW) of complex 3 have

been measured in CH2Cl2 solution containing 0.1 M [N (n-Bu) 4] PF6 as supporting electrolyte

at a glassy carbon working electrode and an Ag/AgNO3 reference electrode. Ferrocene was

used as an internal standard, and potentials are referenced versus the ferrocenium/ ferrocene

(Fc+/ Fc) couple. Figure 4.19 shows the SQWs of complex 3 recorded in the potential range

+0.70 V to –0.4 V and -0.6 V to -1.4 V.

-0.6 -0.8 -1.0 -1.2

E(V) vs Fc+/Fc

0.6 0.4 0.2 0.0 -0.2 -0.4

E(V) vs Fc+/Fc

Figure 4.19. Square wave voltammograms of complex 3 in CH2Cl2 at room temperature

under an argon atmosphere.

Coulometric experiments at fixed potential show that the waves at E1/2 , 0.34, 0.094

and -0.110 V are corresponding to oxidations while that at -1.166, -0.94 and -0.84 V are

corresponding to reductions of complex 3. E1/2, -0.94, -0.84 and 0.094 V are two-electron

oxidation processes and others are one electron processes. All the oxidations and reductions

processes are reversible.

Figure 4.20 shows UV-VIS/NIR spectral changes during stepwise four-electron

coulometric reduction of complex 3 and Table 4.6 summarizes the results.

118

Chapter 4

400 500 600 700 800 900 1000

5000

10000

15000

20000

25000

30000

35000

34-

33-

32-

/ M-1 cm-1

Wavelength



Figure 4.20. Electronic spectrum of complex 3, 3-, 32-, 33- and 34- in CH2Cl2 at -25

The UV-VIS/NIR spectrum of complex 3 shows strong and sharp absorption bands at

380, 550 and 507 nm with absorption coefficient (H, M-1cm-1) 33600, 7065, and 7500

respectively. All the bands may be assigned to ligand-to-metal charge transfer band. The

broad band cantered at around 840 nm is probably arising due to ligand-to-ligand charge

transfer. During four stepwise one-electron reductions the broad band at around 840 nm shifts

to longer wavelength and decreases in intensity. The final absorption coefficient after four-

electron reductions is 1950 M-1cm-1 and most probably arising due to the d-d transition of the

four copper(II) ions. Hence, ligand centered reduction is the speculation.

Table 4.6. Spectroelectrochemical data of the complexes.

Complex

, nm (

, M-1cm-1)

3845(8600), 550(7065), 507(7500), 447(9830), 380(33600)

3-952(5430), 840(6140), 546(7475), 507(7600), 380(33300)

32- 860(4500), 544(7800), 507(7800), 362(31600)

33- 875(2764), 541(8190), 511(8294), 355(33000)

34- 875(1950), 523(8900), 355(33000)

The X-band EPR spectra, recorded in frozen solution of CH2Cl2, of one-electron and

three-electron reduced species of complex 3, symbolically 3-and 33- are depicted in Figure

4.21.

119

Chapter 4

It is noteworthy that complex 3contains little bit impurity of mononuclear Cu(II) species

which is EPR active. Hence, the X-band EPR spectrum of the starting material (3) contains a

weak Cu(II)-centered signal.

250 275 300 325 350 375

Exp

Sim

dX''/ dB

Field / mT

200 250 300 350 400

Exp

Sim

dX''/dB

Field / mT

Figure 4.21. X-Band EPR spectra of 33-(left) and 3- (right), measured at 30 K.

Indeed, the intensity of the X-band EPR spectra obtained for 3-and 3

3- species are

much stronger than that of the starting material (3). The simulations of the experimental

results for complexes 3-and 33- provide the following parameters;

Parameters Complex 3-Complex 33-

gx2.06 2.0275

gy2.033 2.0575

gz2.245 2.245

Wx (G) 25 30

Wy(G) 30 25

Wz(G) 60 80

Ax(10-4 cm-1)20 10

Ay(10-4 cm-1)020

Az(10-4 cm-1)180 180

120

Chapter 4

Hence, from the X-band EPR spectra it is clear that the first four reduction processes

are ligand centered in nature and the spectroelectrochemical behaviors of complex 3can be

summarized as shown in Figure 4.22.

Figure 4.22. Different redox states of complex 3.

Cu Cu

CuCu

Cu Cu

CuCu

Cat

Cu Cu

CuCu

Cat

Cu Cu

CuCu

Cat

Cu Cu

CuCu

Cu Cu

CuCu

Cu Cu

CuCu

Cu Cu

CuCu

Cat

(II)(II)(II)

(II)(II)

(II)

+2e-+2e-

(II)(II)(II)

(II)(II)

-2e-(II)

-2e-

-1e-

+1e-

-1e-

(II)

(I)

(II)(I)

(II)

(II)-1e-

(II)(I)

(II)(II)

+1e-(II)

+2e--2e-

(II)

(II)(II)

(II)

SQ = iminosemiquinone

Cat = amidophenolate

Q = iminoquinone

+1e-

(II)

(II)(II)

-1e-(II)

121

Chapter 4

4.4.2 Catalytic reactivity,

Aerial oxidation of 2-aminophenol; Mimicking the

function of Phenoxazinone Synthase(PHS)

Actinomycin D, one of the most potent antineoplastic agents known, is synthesized by

the actinomycete Streptomyces antibioticus. This compound inhibits DNA-dependent RNA

synthesis by intercalation of the phenoxazinone chromophore to DNA. The last step in the

biosynthesis of actinomycin D, namely, the oxidative condensation of two molecules of 3-

hydroxy-4-methylanthranilic acid pentapeptide lactone to form actinocin (Scheme 4), is

catalyzed by the enzyme Phenoxazinone Synthase (PHS). PHS is a multicopper oxidase

produced in two distinct oligomeric forms: low activity dimers and high activity hexamers.

The relative amount of the two forms is regulated; young cultures that do not produce

actinomycin predominantly contain the dimeric form, whereas older actinomycin-producing

cultures mostly contain the hexameric form.35 The dimers and hexamers are distinct stable

molecular forms and are not related by a simple equilibrium-aggregation phenomenon.35 The

regulation and structural differences between these two oligomeric forms is currently

unknown.

Scheme 4. Reaction catalyzed by PHS.

When Begley et.al.36 have proposed the mechanism of the six-electron oxidative

coupling as radical-base process, Sim%ndi and co-workers 37 have proposed similar type of

mechanism using Co(II), Fe(II)-salen or oximate complexes as functional models. Using

various Cu(II) and Cu(I)-salts and molecular oxygen at 60 qC, phenoxazinone chromophore

can be synthesized catalytically using 2-aminophenol as substrate.38 Indeed, PHS is a

multinuclear copper containing metalloenzyme. The crystal structure of PHS has been

described in Chapter 1. It is noteworthy that still to date there is not a single structural as well

122

Chapter 4

123

as functional model complex in literature that can oxidise 2-aminophenol to phenoxazinone

chromophore catalytically in the presence of air.

Considering active site structural feature and proposed mechanism for the catalytic

processes, complex 3 has been employed as a catalyst for the aerial oxidation of 2-

aminophenol.

When 2-aminophenol (2.35 X 10-2 M) was added to complex 3 (2.38 X 10-4 M) in

methanol (20 ml), the color of the solution slowly turns to deep yellow-red color. After 40 h,

it was found that ~100% of the 2-aminophenol has been converted to 2-amino-phenoxazine-3-

one.

NH2

0 5 10 15 20 25 30

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

kobs/ 10- 4/ M-1h-1

[2-aminophenol]/ 10 -3 M

0 5 10 15 20 25 30

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

kobs / 10- 4 / M-1 h-1

[Complex 3] / 10-5 M

Using initial rate method the rate equation of the above catalytic reaction has been

determined by varying the concentration of substrate (2-aminophenol) keeping the

concentration of catalyst (complex 3) as constant and vice versa. From the experimental

results shown in Figure 4.23 the rate equation was deduced as,

Rate = k [2-aminophenol]2[Complex 3]

Figure 4.23. Kinetic data.

OH OO

Air, RT, 40h

Chapter 4

When 1 equivalent of complex 3was allowed to react with approximately 10

equivalent of 2-aminophenol under an argon atmosphere (Glove Box) for 40 h, one equivalent

of product to the concentration of complex 3 has been found to form by spectrophotometric

method at 435 nm and depicted in Figure 4.24. The above experiment implies that two

molecules of 2-aminophenol reacts with one molecule of complex 3to form one molecule of

phenoxazinone chromophore. Exposure of the solution to air increase the concentration of the

phenoxazinone chromophore, thus the oxidation process is an aerial oxidation process. This is

the first example of aerial oxidation of 2-aminophenol to phenoxazinone chromophore takes

place. No hydrogen peroxide was detected. This could be due to the catalase activity of the

complex 3. Moreover due to sluggish nature of the reaction determination of H2O2at lower

temperature than that of room temperature was not possible.

It has been discussed earlier that the first four oxidation processes are ligand centered,

hence, reduction of four radical centers together with reduction of two Cu(II) centers should

happen as product : complex 3 concentration ratio obtained after Glove Box reaction is

approximately 1. Moreover, the participation of Cu(II) centers during the oxidative reaction

can be speculated by comparing the UV-VIS/NIR spectrum (Figure 4.24) of the solution

obtained after Glove Box reaction with the UV-VIS/NIR spectrum obtained after four-

electron reductions of complex 3. The band at around 775 nm with an absorption coefficient,

3400 M-1cm-1 can be taken as an intervalence metal-to-metal charge transfer band as shown

by Thomson and co-workers. 39

400 600 800 1000 1200 1400

0.0

0.5

1.0

1.5

2.0

2.5

3.0

H

104



M-1cm-1

wavelength / nm

Figure 4.24. UV-VIS/NIR spectrum obtained after Glove-Box reaction.

124

Chapter 4

Hence, from the above experimental results the mechanism for the oxidative coupling of two

molecules of 2-aminophenol by catalytic activity of complex 3 in the presence of air can be

proposed as shown in Figure 4.25.

Cu Cu

CuCu

Cu Cu

CuCu

Cat

Cu Cu

CuCu

Cat

Cu Cu

CuCu

Cat

NH2

HNH2

OH OH

HNH2

(II)

NNH2

(II)

(II)(II)

(II)

3H2O + 3/2 O2

(II)

(I)(II)

(II)

3O2

(II)

(I)(II)

(II)

Figure 4.25. The proposed mechanism for the formation of phenoxazinone chromophore.

125

Chapter 4

4.5 References

1. Holm, R.H.; Solomon, E.I. Chem. Rev. 2004,104, 347.

2. Vanadium Compounds, Chemistry, Biochemistry and Therapeutic Applications (Eds.:

A. S. Tracey, D. C. Crans), ACS Symp. Ser. 1998, 711.

3. J. Inorg. Biochem. 2000,80 (Special Issue on Biological Aspects of Vanadium; Guest

Eds.: D. Rehder, V. Conte).

4. Vanadium and Its Role in Life (vol. 31 of Metal Ions in Biological Systems; Eds.: H.

Sigel, A. Sigel), Marcel Dekker, New York, 1995.

5. Rehder, D. Coord. Chem. Rev. 1999,182, 297.

6. Vanadium in the Environment (Ed.: J. O. Nriagu), Wiley, New York, 1998.

7. Hirao, T. Chem. Rev. 1997, 97, 2707.

8. Arends, J. W. C. E.; Pellizon, M.; Sheldon, R. A. Stud. Surf. Sci. Catal. 1997,110,

1031.

9. Thompson, K. H.; McNeill, J. H.; Orvig , C. Chem. Rev. 1999 , 99 , 2561 .

10. Shechter , Y.; Li , J.; Meyerovitch , J.; Gefel , D.; Bruck , R.; Elberg , G.; Miller , D.

S.; Shisheva , A. Mol. Cell Biochem. 1995,153, 39.

11. Brink , H. B. ten; Tuynman , A.; Dekker , H. L.; Hemrika , W.; Izumi , Y.; Oshiro , T.;

Shoemaker , H. E.; Wever , R. Inorg. Chem. 1998,37, 6780.

12. Andersson , M. A.;. Allenmark , S. G. Tetrahedron. 1998 , 54 , 15293 .

13. Schmidt, H.; Bashirpoor, M.; Rehder, D. J. Chem. Soc., Dalton Trans. 1996, 3865.

14. Bolm, V.; Bienewald, V. Angew. Chem. Int. Ed. Engl. 1995,34, 2883.

15. Nakajima , K.; Kojima , K.; Fujita ,V. Bull. Chem. Soc. Jpn. 1990,63, 2620.

16. Xie, Y.; Jiang, H.; Chan , A. S.-C.; Liu, Q.; Xu , X.; Du, C.; Zhu;Y. Inorg. Chim. Acta

2002,333, 138.

17. Fallon, G.D.; Moubaroki, B.; Murra, K.S.; Van Den Bergen, A. M.; West, B. O.

Polyhedron.1993, 12, 1989.

18. (a) Corei, E.J.; Achiva; K. J. Am. Chem. Soc. 1969,91, 1429.(b) Girgis, A.Y.; Balch;

A.L. Inorg. Chem. 1975,14, 2724.

19. Chun, H.; Verani, C. N.; Chaudhuri, P.; Bothe, E.; Bill, E.; Weyhermüller, T.;

Wieghardt, K. Inorg. Chem. 2001,40, 4157.

20. Asgedom, G.; Sreedhara, A.; Kivikoski, J.; Valkonen, J.; Kolehmainen, E.; Rao, C.P.

Inorg. Chem. 1996, 35, 5674.

126

Chapter 4

21. Cornman, C. R.; Colpas, G.J.; Hoeschele, J. D.; Kampf,J.; Pecoraro. V.L. J. Am.

Chem. Soc. 1992,114, 9925.

22. Rehder , D.; Weidemann , C.; Duch , A.; Priebsch , W. Inorg. Chem. 1988 , 27 , 584 .

23. Howarth , O. W. Progr. Magn. Reson. Spectrosc. 1990,22, 453.

24. Asgedom, G.; Sreedhara, A.; Rao , C. P.; Kolehmainen, E. Polyhedron.1996,15,

3731.

25. Asgedom, G.; Sreedhara, A.; Kivikoski, J.; Valkonen, J.; Kolehmainen, E.; Rao, C. P.

Inorg. Chem. 1996, 35, 5674.

26. Mukherjee, S.; Weyhermüller, T.; Bothe, E.; Wieghardt, K.; Chaudhuri, P. J. Chem.

Soc., Dalton Trans. 2004 , 3842 .

27. Chun, H.P.; Chaudhuri, P.; Weyhermüller, T.; Wieghardt, K. Inorg. Chem. 2002, 140,

790.

28. Slep, L.D.; Mijovilovich, A.; Meyer-Klaucke, W.; Weyhermüller, T.; Bill, E.; Bothe,

E.; Neese, F.; Wieghardt, K. J. Am. Chem. Soc. 2003,125, 15554.

29. Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.; Weyhermüller, T.; Wieghardt, K. J.

Am. Chem. Soc. 2001,123, 2213.

30. Merz, L.; Haase, W. J. Chem.Soc.,Dalton Trans.,1980, 875.

31. Schwabe, L.; Haase,W. J. Chem. Soc., Dalton Trans.,1985, 1909.

32. Wang, S.; Zheng, J.C.; Hall, J.R. Polyhedron.1994,13, 1039.

33. Merz.L.; Haase, W. J. Chem. Soc., Dalton Trans.,1978, 1594.

34. Trzebiatowska, B.J; Olejnik, S.; Lis, T. J. Chem.Soc., DaltonTrans.,1981, 251.

35. Choy, H. A.; Jones, G. H. Arch. Biochem. Biophys. 1981,211, 55.

36. Barry, C. E.; Nayar, P. G.; Begley, T. P. Biochemistry, 1989,28, 6323.

37. Sim%ndi, T.M.; Sim%ndi, L.I.; Gyor, M.; Rockenbauer, A.; Gomory, A. J. Chem. Soc.,

Dalton Trans., 2004, 1056 and there in.

38. Horv%th, T.; Kaiser, J.; Speier, G. Journal of molecular catalysis A: Chemical. 2004,

215, 9.

39. Farrar,J.A.; Lappalainen, P.; Zumft, W.G.; Saraste, M.; Thomson, A.J. Eur J Biochem.

1995,232,294.

127

Chapter 5

Synthesis, Characterization and Catalytic Reactivities of a

Monoradical-Containing Mononuclear Mn(IV) complex

•

Chapter 5

129

Chapter 5

5.1 Synthesis and characterization of the ligand, N (2-

hydroxy-3,5-di-tert-butylphenyl) anthranilic acid, H3LCOOH

Radical-containing metal complexes are of interest in understanding the metal-radical

interactions of natural systems. These complexes can be useful as functional and/or structural

models of metalloenzymes and can be studied not only to gain deeper insight into the

electronic structure but also as catalysts for new oxidation reactions for synthetic and

industrial purposes.

In this chapter synthesis, characterization and catalytic activities of a monoradical-

containing Mn(IV) complex (1), having a new non-innocent ligand, N (2-hydroxy-3,5-di-tert-

butylphenyl) anthranilic acid, H3LCOOH (Figure 5.1) , will be discussed.

Figure 5.1. Schematic representation of the ligand N (2-hydroxy-3,5-di-tert-butylphenyl)

anthranilic acid.

130

Chapter 5

Condensation of 3,5-di-tert-butyl-catechol with anthranilic acid in the presence of

triethylamine for 24 h under air in n-hexane produces the ligand, H3LCOOH. It is important to

note that an excess amount of triethylamine causes decomposition and polymerization of the

ligand, therefore only two drops of triethylamine (0.01 ml) should be added for 10 mmol scale

reaction (see experimental section). The ligand was characterized by IR, NMR, GC, GC-MS

and mass spectroscopy. The ligand shows characteristic peaks in the IR spectrum due to –O-H

and –N-H stretches at 3456 cm-1and 3336 cm-1 respectively. The peaks in the range from 2959

to 2868 cm-1 are due to the –C-H stretching of the tert-butyl group, the sharp bands at 1667

cm-1 and 1595 cm-1 are C=O and -C-C stretching bands of the carboxylic acid group of the

aromatic rings, respectively. The bands in the range from 1500 to 1420 cm-1 appear from the

skeletal vibrations of phenyl rings. The –C-N stretch appears at 1577 cm-1. The band at 1255

cm-1 is due to C-O stretch. The sharp peak at 753 cm-1 is due to the –C-H stretching of the

tetrasubstituted aromatic rings. GC and GC-MS coupling measurements have been done to

check the purity and composition of the ligand. EI mass-spectroscopy confirms the

composition C21H27NO3 (341 gm / mole) for H3LCOOH. The tridentate ligand (H3LCOOH) is able

to lose its acidic proton, phenolic proton and the proton on the nitrogen atoms. The different

oxidation states ranging from 1- to 3- exhibited by the ligand owing to its non-innocent

character are illustrated in Figure 5.2.

N-

O-

-1e-

+1e-

[LBQCOOH]1-

[LCatCOOH]3-

[LSQCOOH

]2-

Figure 5.2. Different redox States of the fully deprotonated ligand, [LCOOH] 3-.

131

Chapter 5

5.2 Synthesis and characterization of the complex (1)

2 mmol of MnII(acetate)2 .4H2O was dissolved in 30 ml of MeOH, and 2 mmol of

H3LCOOH was added. To this solution 0.3 ml Et3N was added and the mixture was stirred

under air for 2 h. A Deep brown colored microcrystalline solid precipitated. Yield: 1.05 gm

(63%).

The IR spectrum of complex 1 was taken as a KBr pellet. In the IR spectrum of 1

sharp and strong bands in 2950 to 2870 cm-1 region arise due to –C-H stretches of the tert-

butyl group. This could be taken as a preliminary indication for coordination of ligand(s) to

metal. The sharp band at 1594 cm-1 arises due to the –C=N group. The strong, sharp bands at

1574, 1255, and 1104 cm-1 show the existence of the ligands in the complex in both

iminosemiquinone and amidophenolate forms.1-9 ESI-positive and negative mode mass

spectra in dichloromethane show molecular peaks at 102 (100%) and 735 (100%) respectively

and confirm the composition C42H48N2O6Mn as the negatively charged ion and (C2H5)3NH+

as the positively charged ion. C, H, N and Mn analysis show the composition of complex 1 is

C48H64N3O6Mn.

Single crystals suitable for X-ray analysis were obtained from a 1:1 CH2Cl2, CH3CN

solution mixture. The result of the crystallographic study shows that a manganese complex is

formed in the M:L ratio 1:2. ORTEP drawing of the complex with the atom labeling scheme

is shown in Figure 5.3. Selected bond lengths and bond angles are listed in Table 5.1. The

geometry around the metal is six coordinate with an N2O4. Each ligand acts as a meridional

O,N,O donor. The MnN2O4 coordination sphere has a nearly perfect N2O2 equatorial with

respect to which the two O atoms are located in trans positions. The equatorial plane is

formed by O(1), N(8), O(17), N(38) all of which have bond lengths (Mn(1)-O(1), 1.881(3) Å,

Mn(1)-N(8), 1.913(3) Å, Mn(1)-O(17), 1.925 Å, Mn(1)-N(38), 1.933 Å. In terms of angles

with coordination sphere MnN2O4 is very close to an ideal octahedral geometry. The dihedral

angle between the planes, N(8)-O(17)-N(38)-O(1) and O(47)-O(17)-O(31)-O(1), is 88.7q. The

short metal-ligand bond lengths and highly octahedral ligand arrangement are consistent with

a Mn (IV) centre with no evidence of an axial Jahn-Teller distortion, characteristic of Mn (III)

ions. Thus, structurally this complex is best described as a Mn (IV) species.

132

Chapter 5

Figure 5.3. ORTEP diagram of complex 1.

Table 5.1. Selected bond lengths (Å) and angles (degree) for 1

Mn(1)-N(8) C(4)-C(5) 1.401 (5)

Mn(1)-N(38) C(5)-C(6) 1.381 (6)

Mn(1)-O(1) C(6)-C(7) 1.396 (5)

Mn(1)-O(47) C(37)-N(38) 1.378 (5)

Mn(1)-O(17)

C(2)-O(1)

C(39)-N(38) 1.398 (5)

C(7)-N(8) C(32)-C(33) 1.418(5)

C(2)-C(7) C(32)-C(37) 1.430 (6)

C(2)-C(3) C(33)-C(34) 1.387 (5)

C(3)-C(4) C(34)-C(35) 1.419 (6)

C(9)-N(8) C(35)-O(36) 1.373 (5)

C(9)-C(10) C(36)-C(37) 1.411 (6)

C(10)-C(11)

C(11)-C(12)

C(12)-C(13)

C(13)-C(14)

C(14)-C(9)

C(15)-O(16)

O(31)-C(32)

C(39)-C(40)

C(39)-C(44)

C(40)-C(41)

C(41)-C(42)

C(42)-C(43)

C(43)-C(44)

C(45)-O(46)

1.402 (6)

1.417 (6)

1.370 (6)

1.394 (7)

1.375 (7)

1.394 (6)

1.233 (5)

O(1)-Mn(1)-N(17)

O(31)-Mn(1)-O(47)

O(1)-Mn(1)-O(31)

O(1)-Mn(1)-O(47)

87.95 (12)

89.99 (12)

N(8)-Mn(1)-N(38) O(1)-Mn(1)-N(8) 83.96 (13)

N(8)-Mn(1)-O(17)

O(1)-Mn(1)-N(38)

1.913 (4)

1.933 (3)

1.881 (3)

1.911 (3)

1.925 (3)

1.336 (5)

1.393 (5)

1.423 (6)

1.405 (5)

1.391 (6)

1.391 (5)

1.408 (6)

1.379 (5)

1.391 (8)

1.377 (8)

1.400 (6)

1.419 (6)

1.239 (5)

1.314 (5)

175.15 (13)

174.83 (13)

170.92 (14)

91.19 (14)

90.23 (13)

O(31)-Mn(1)-N(8)

O(47)-Mn(1)-N(8)

89.90 (13)

94.60 (14)

•

133

Chapter 5

The MnN2O4unit is mono-negative. The cationic part is triethylammonium. There is H-

bonding between the carbonyl oxygen atom of the ligand and the H-atom of the

triethylammonium cation (not shown in the ORTEP diagram). The bond lengths, C(37)-N(38)

(1.378(5) Å) and C(31)-C(32) (1.315(5) Å), are shorter than C(7)-N(8) (1.393(5) Å), O(1)-

C(2) (1.336(5) Å). The C(37)-N(38) (1.378(5) Å) and O(31)-C(32) (1.314(5) Å) bond lengths

are typical for an iminosemiquinone system. The alternating short and long C-C bond lengths

(Table 1) also support the assignment of the iminosemiquinone form. On the other hand,

C(7)-N(8) (1.393(5) Å) and O(1)-C(2) (1.336(5) Å) bond lengths support amidophenolate

system. So, it is quite clear that among the two of the tridentate ligands (O,N,O) one is in

iminosemiquinone form with 2- charge and the other is in amidophenolate form with 3-

charge.

(A) (B)

0 50 100 150 200 250

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Exp

Sim

eff /

T/K

ST = 1

D = 6.3 cm-1

0,0 0,4 0,8 1,2 1,6 2,0 2,4

0,0

0,2

0,4

0,6

0,8

1,0

M / Ng

H / kT

ST = 1

D = 6.3 cm-1

(C)

Figure 5.4. Plots of

eff /

B vs. T/K and magnetization measurements at 1T, 4T and 7T. B

0.0 0.5 1.0 1.5 2.0 2.5

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

7 T

M/Ng

H/kT

ST = 1

D = 5.8 cm-1

134

Chapter 5

The temperature dependence of the magnetic susceptibility of 1 in the solid state was

measured in the temperature range 2-290 K (Figure 5.4). The magnetic moment in

temperature range 20-290 K is constant with a value of 2.82 r 0.03 PB. This value is

indicative of an St = 1 ground state for complex 1. As there is no change in the Peff value with

increasing temperature, it can be concluded that there is no thermal population of the higher

excited states within the temperature range. Complex 1 contains of two paramagnetic centres.

One is Mn (IV), S = 1.5 and the other is a radical-anion, S = 0.5. The strong antiferromagnetic

interaction between the two paramagnetic centres is the origin of ST= 1 ground state.10-12

Temperature dependent magnetic susceptibility measured at 1 T external field as well

as VTVH results were also simulated by taking St = 1 as there are no changes in Peff values in

the temperature range. The parameters obtained for the simulation of the temperature

dependent magnetic susceptibility measured at 1 T external field are; gMn = 1.985 and D =

µ6.3µcm-1. The parameters obtained for the best fit of the experimental results obtained at 7

T and 1T are; gMn = 1.985 and D = µ6.3µ cm-1(Figure 5.4). It is noteworthy, that using gMn =

1.987, and D = µ5.8µ cm-1 the experimental result obtained at 4 T can be simulated. Hence,

from the fitting parameters is can be said that the zero field splitting, D, for the system is

µ6.05 r 0.25µ cm-1.

At room temperature the electronic spectrum of 1in CH2Cl2 and ClCH2CH2Cl solution

exhibits a distinct band in the NIR range with a maximum at 945 nm. The intensity of this

band depends on the concentration of 1, being higher at higher concentrations (Figure 5.5 A)

where the molar absorption coefficients H (945) are plotted vs. the concentration(c) of 1.

Apparently complex 1 can form dimers in solution. It is seen, that the limiting H(945) value

for infinite dilution is zero and the value at high concentrations approaches a value of H

(945)lim = 4.8 x103 M-1 cm-1. The fraction H(945)/ H(945)lim should then correspond to dimeric

fraction of 1 and (1 - H(945)/ H(945)lim ) to the degree of dissociation, D. If the D values thus

obtained are plotted vs. the concentration of 1 according to Ostwald´s law of dilution, the

same association constant Ka = 4x104 M-1 is obtained for all concentrations and degrees of

dissociations (0.12<D<0.85), within the experimental error. This clearly shows that

dimerisation occurs indeed.

135

Chapter 5

(A) (B)

Figure 5.5. A plot of

vs. concentration (left) and (1-

2C vs. log c (right).

The height of the band at 945 nm can also be influenced by the temperature: when the

temperature is decreased, the intensity of the band increases, i.e. low temperatures favour

dimerisation. The spectral changes were found to be reversible and isosbestic behaviour at

415 nm and 475 nm showed that only two species are present in the solution, which are

reversibly interconverted by varying the temperature. At a concentration of 1 of 1.8 x 10-4 M

the OD (945) values as a function of temperature are shown in Figure 5.6.

Figure 5.6. Temperature dependence of the electronic absorption spectrum of a

dichloromethane solution of complex 1 (left) and absorption versus 1/T plot at 945 nm (right).

40 600 800 1000

0.0

0.5

1.0

1.5

2.0

2.5

-60 0 C

30 0 C

A / cm

Wavelength / nm

30 0 C

-60 0 C

0.0035 0.0040 0.0045

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1 / T/ K-1

= 945 nm

0.00.20.40.60.81.01.21.4

1000

2000

3000

4000

5000

0.000 0.005 0.010 0.015 0.020

500

1000

1500

2000

-5.6 -5.2 -4.8 -4.4 -4.0 -3.6

log c

(1-

DD

c, 103

H

M-1 cm

-1

Conc. 10-3, M

H

M-1 cm-1

c, 10-3, M

136

Chapter 5

It is seen that in the accessible temperature range (+30 °C to -60 °C) the

limiting values of the high and low temperature forms are not reached. However, the diagram

suggests that the limiting OD values of the pure forms are close to zero at high temperatures

and around 1.6 for low temperatures. Using these values, equilibrium constants K for the

different temperatures could be calculated. If they are plotted in the form lnK vs. T-1

according to van´t Hoff´s law (d lnK/d T-1 = ¨H/R), a reasonable straight line is indeed

obtained (Figure 5.7). The ¨H value obtained from the plot amounts to -22 kJ mol-1. Using

the equilibrium constant at 300 K, it is calculated that ¨G300 = +1.2 kJ mol-1 and ¨S300 = -77

J mol-1 T-1 (from the relationships ¨G300 = RT lnK300 and ¨G300 = ¨H - T¨S300). The signs of

the thermodynamic data were taken for the direction of cooling i.e. for formation of

([MnIV(LcatCOOH)(LSQCOOHx)]-)2.

Scheme 1

Species A Complex 1 Species B

III

Figure 5.7. A plot of lnK vs. T-1.

3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

ln K

1/T, 10-3/ K-1

If the process was a valence tautomerism then the complex 1 would be in equilibrium

with species A or species Bas shown in Scheme 1.The stabilization of species A is quite

137

Chapter 5

unlikely as it contains ligands in the strong oxidizing iminoquinone form and the strong

reducing amidophenolate form. Hence, disproportionation will be the result. Moreover,

entropy of the system decreases on cooling. On the other hand, if complex 1shifts to species

A or species B then entropy should increase as oxidation state of the metal decreases from

+IV to either +III or +II(for 2e transfer process). Therefore, the thermodynamic data support

again dimerization not valence tautomerism.13-28

2[MnIV(LcatCOOH)(LSQCOOHx)]- [Mn

IV(LcatCOOH)(LSQCOOHx)]2-2

(St = 1) (St = 2, 1 or 0)

240 260 280 300 320 340 360

3.7

3.8

3.9

4.0

4.1

4.2

4.3

4.4

4.5

eff /

T/ K

Concentration = 9 mM, M wt = 833 gm/ mol

Solvent = Dichloroethane d4

Figure 5.8. Change in

eff with temperature.

Further support for the formation of dimer on cooling the solution was obtained by 1H

NMR (ClCD2CD2Cl) spectra measured in the temperature range +70 °C to -40 °C. The

monomeric form has an St = 1 ground state whereas the dimeric form could have St = 2 , 0 or

two degenerate St = 1 ground states. Figure 5.8 shows Peff vs. T plot. The Peff values were

calculated using Evan´s method.29 At -40 °C Peff of the 9 mM ClCD2CD2Cl solution of 1is

4.5 PB. On increasing the temperature of the solution Peff value decreases gradually and

reaches to 3.82 PBat +70 °C. The decrease in Peff value with increase in temperature of the

solution indicates dissociation of the dimeric species to monomeric species and at +70 °C

both dimeric and monomeric species exist in the solution. Measurements of 1H NMR spectra

above +70 °C were not possible as the complex decomposed above the temperature.

138

Chapter 5

5.3 Catalytic reactivities,

5.3.1 Aerial oxidation of primary amines; mimicking the

function of Amine Oxidases (AOs)

Copper-containing amine oxidases (AOs) play a crucial role in metabolic oxidative

deamination of primary amines to the corresponding aldehydes, with the concomitant

production of hydrogen peroxide and ammonia (eq 1).30

RCH2NH2 + O2 + H2O RCHO + NH3 + H2O2…………………eq 1

The reaction mechanism for the oxidative deamination reaction is not yet understood. It is

evident that the reaction proceeds through a transamination mechanism mediated by the active

site, 2,4,5-trihydroxy-phenylalanine quinone (TPQ), a organic cofactor.

To mimic the function of amine oxidases complex 1 has been used. Employment of

primary amines with two D-H atoms, like benzyl amine, ethylenediamine, 2-aminoethanol,

etc, provides the corresponding Schiff base or aldehyde product. No oxidation of secondary

amines or amines with one D-H atom was found. Three primary amines, ethylenediamine, 2-

aminoethanol, benzyl amine were introduced as substrates for the catalytic oxidative

deamination reaction. Employment of ethylenediamine provides glyoxal as product, which

was identified by GC, GC-MS after formation of the corresponding Schiff base products with

2,4-dinitrophenylhydrazine (DNPH) and 3-methyl-2-benzothiazolene hydrazine

hydrochloride. The percent of yield 35 was determined by spectrophotometic methods using

the standard reagent, 3-methyl-2-benzothiazolene hydrazine hydrochloride. 31 Glyoxal and 2-

hydroxy-acetaldehyde in a 2:1 ratio were identified by GC, GC-MS as Schiff base products

with DNPH and 3-methyl-2-benzothiazolene hydrazine hydrochloride, when 2-aminoethanol

was used as a substrate. After 15 hours there was no more conversion and the final percent of

yields, 54 and 27 for glyoxal and 2-hydroxy-acetaldehyde were determined by

spectrophotometic methods taking the total absorption as the sum of absorptions of the

species (At = Aglyoxal + A2-hydroxy-acetaldehyde,Hglyoxal = 28000 M-1cm-1 and H2-hydroxy-acetaldehyde =

51000 M-1cm-1). When catalytic amount of complex 1 (10-5 mole) was introduced for the

aerial oxidation of a large excess of benzyl amine (10-3 mole), 20 % of

benzylidinebenzylamine was detected by GC using nhexadecane as an internal standard in 6

hours. No more conversion was noted after 6 hours. This is because of the decomposition of

139

Chapter 5

complex 1 after that period. Though decomposition of hydrogen peroxide occurred due to the

catalyse activity of the manganese complex at room temperature, hydrogen peroxide is

detected at -25 qC. Ammonia was detected qualitatively. The kinetics for this catalytic

reaction were studied by varying the total concentration of benzyl amine (10-2 to 2.5 X 10-6

mole) keeping the concentration of complex 1 as constant and vice versa. The rate law as

deduced from the experimental data is,

Rate = k [Amine][Complex]

Figure 5.10. Kinetic data for the aerial oxidation of benzyl amine.

The maximum TON (the ratio between the concentration of product and catalyst) that has

been achieved for the above catalytic process is 72 and shown in Figure 5.10. With the

selected deuterated substrate at the D-C atom, PhCD2NH2, 32 a kinetic isotope effect (KIE =

kH/kD) of about 1 was evaluated and depicted in Figure 5.11. This indicates clearly that the H-

atom abstraction from the D-C atom of the substrate is not the rate-determining step for the

catalysis. The rate determining step could be;

(i) One of the two steps for the formation of Schiff base, benzylidinebenzylamine,

PhCH2NH2 + PhCHO PhCH2N=CHPh.

(ii) The formation of H-bond between the substrate and catalyst.

0246810121416

0.0

0.4

0.8

1.2

1.6

2.0

2.4

[C] = 0.0005 M

[S] = 0.05 & 0.01 M

mg pdt / ml

Time/h

0.00 0.05 0.10 0.15 0.20 0.25 0.30

TON

[PhCH2NH2]/ M

140

Chapter 5

Figure 5.11. Kinetic isotope effect for the aerial oxidation of benzyl amine.

0 2 4 6 8 10 12 14 16 18

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

PhCH2N=CHPh (mg / ml)

Time/ h

When 12.5 equivalent of benzyl amine was allowed to react with 1 equivalent of

complex 1 under anerobic condition, about 1.5 equivalent benzylidinebenzylamine was found

to form by GC analysis. The X-band EPR spectrum at 10 K and UV-VIS/NIR spectrum at RT

of the solution were measured and depicted in Figure 5.12. The X-band EPR spectrum shows

a signal indicative of a Mn(II) species with six major hyperfine lines centered around g = 2

which originate from the hyperfine splitting of the central transition Ms = 1/2 to Ms = -1/2 of

S = 5/2 paramagnet with I = 5/2 of the 55Mn nucleus. From the simulation of the EPR

spectrum the following parameters are obtained; gx= gy= gz = 2.0, Ax = Az= 95 X 10-4 cm-1,

Ay = 85 X 10-4 cm-1,~D~ = 200 X 10-4 cm-1 and E/D = 0.06.

Figure 5.12. Change in UV-VIS/NIR spectrum during stoichiometric-type reaction under Ar-

400 600 800 1000

atmosphere (left) and X-band EPR spectrum at 10K of the solution obtained after Glove-box

reaction (right) .

C = 2.2 X 10-3M

Glove box reaction with

PhCH2NH2 and complex 1

Air (O2)

Complex 1

A/ 0.1 cm

Wavelength / nm

2000 2500 3000 3500 4000 4500

Exp

Sim

dX"/dB

Field /G

141

Chapter 5

Hence, from the X-band EPR spectrum it is quite clear that the Mn(IV) center has

been converted to Mn(II) in presence of benzyl amine under anerobic condition. The UV-

VIS/NIR spectrum shows clearly that there is no absorption band at around 930 nm that was

present in complex 1. This band may be assigned to the intervalence ligand-to-ligand charge

transfer band (between iminosemiquinone and amidophenolate forms of the ligand).12 The

absence of that charge transfer band in the presence of substrate under anaerobic conditions is

an indication that the radical involved in the catalysis and iminosemiquinone form of the

ligand has been reduced to its amidophenolate form. When the solution was exposed to air the

absorption spectrum feature changes and shows the regeneration of radical containing species

by reproducing the band at around 930 nm.

From the above experiments a mechanism for the formation of

benzilidinebenzylamine can be proposed as shown in Figure 5.13.

2[Mn

IV(LCatCOOH)(LSQCOOH

)]-

2 [Mn II(LCatCOOH)(LCatCOOH)]4-

3H2O2

3 PhCH2NH2

3 O2

3 PhCH2N=NCHPh

Figure 5.13. The proposed mechanism for the catalytic oxidation of benzyl amine.

142

Chapter 5

5.3.2 Aerial oxidation of 2-aminophenol; mimicking the

function of Phenoxazinone Synthase (PHS)

Actinomycin D is a member of an interesting class of natural products in which the

yellow-red 2-aminophenoxazinone chromophore is linked to two cyclic pentapeptides.33

These compounds are among the most potent antineoplastic agents known.33

The biosynthesis of actinomycin D involves the conversion of tryptophane to 3-

hydroxy-anthanilic acid in a multistep sequence.33 The pentapeptide lactone is then attached

and resulting 2-aminophenol undergoes a 6-electron oxidative coupling to form actinomycin

D. The latter reaction is catalyzed by Phenoxazinone Synthase (PHS), a multicopper

containing metalloenzyme.

NH2

CONHR CONHR

CONHR

NH2

PHS

2+3/2 O2

+3 H2O

This enzyme has been isolated from Streptomyces antibioticus. It has a subunit

molecular weight of 88 Kda and in its native form is a mixture of oligomers, with the dimer

and the hexamer predominating.35-36 The crystal structure of the enzyme has been described

in Chapter 1.

When complex 1was used as a catalyst for the oxidation of 2-aminophenol to 2-

aminophenoxazine-3-one by aerial oxygen at room temperature (RT) in methanol or a 1:1

dichloromethane, methanol solvent mixture 84% conversion of 2-aminophenol to 2-

aminophenoxazine-3-one was found within 2 hours. The reaction was monitored by

spectrophotometric methods (H = 24000 M-1 cm-1 at 435 nm) at 435 nm (Figure 5.14).

143

Chapter 5

Figure 5. 14. Formation of phenoxazinone chromophore with time.

Moreover, after 2 hours of the catalytic reaction, the solvent was removed and the solid was

examined by mass spectrometry. The mass spectrum shows a peak that has a composition

similar to the 2-aminophenoxazine-3-one compound.35-36 GC and GC-MS analysis was

performed after passing the solution through a neutral Al2O3 column (0.5 gm neutral Al2O3

was taken) in order to find out the number of products, percent of yield and the composition.

GC and GC-MS results showed the formation of 2-aminophenoxazine-3-one by aerial

oxidation of 2-aminophenol as a sole product with a composition of C12H8N2O2.

To find out the rate equation of the above stated catalytic reaction the concentration of

the substrate, i.e. 2-aminophenol, was varied keeping the concentration of complex 1 constant

and vice versa. The rate law that has been deduced from the experimental results (Figure 5.15)

is shown below;

Rate = k [2-Aminophenol][Complex 1]

Figure 5.15. Kinetic data for aerial oxidation of 2-aminophenol.

250 300 350 400 450 500 550 600

0.0

0.2

0.4

0.6

A / 0.1 cm

Wavelength/ nm

0 50 100 150 200 250

0,0

0,1

0,2

0,3

0,4

0,5

0,6

A / 0.1 cm

Time / min

0 5 10 15 20 25 30

kobs / 10-4/ M-1 min-1

[2-aminophenol]/ 10-4 M

012345

kobs / 10-5 / M-1 min.-1

[complex 1] / 10-5 M

144

Chapter 5

Catalase activity by manganese complexes is quite common. Complex 1is not

exceptional as confirmed by allowing methanolic solution of the complex to react with 30%

H2O2. Moreover, in the aerial oxidation of 2-aminophenol by catalytic activity of the complex

1 no H2O2 was detected using TiO2+ in 9 M H2SO4.

Oxygen uptake measurements using complex 1 as catalyst at 25qC and -25qC were

carried out using a gas-burette method until saturation of O2 uptake occurred. At 25 qC, there

is very small amount of oxygen up taken by the system. This is probably due to the

decomposition of hydrogen peroxide due to the catalase activity of complex 1. Moreover, no

hydrogen peroxide is detected using TiO2+. Hence, from the above experiments, it can be

proposed that the oxidation process at room temperature follows the following equation;

NH2

MeOH, 2h, RT

, Air (O2)

When the reaction was monitored at –25 qC in a 39:1 dichloromethane, methanol

solvent mixture the volume of oxygen up taken by the system increased with time linearly and

then saturation appeared. The volume of O2 is up taken by the system at –25 qC vs. time plot

is illustrated in Figure 5.14. Extraction of the solution into water and addition of TiO2+ in 9 M

H2SO4 produced yellow color and the UV–VIS spectrum (Figure 5.16) confirmed the

formation of H2O2 during the catalysis. The concentration of hydrogen peroxide as

determined by spectrophotometric method (O = 415 nm H= 675 M-1 cm-1) is approximately 3/2

times to concentration of the substrate and is same to the amount of oxygen up taken by the

system. Hence, the stoichiometry at -25 qC was found to follow the equation;

NH2

2+3 O2

+3 H2O

CH2Cl2, 2h, -250C

145

Chapter 5

Figure 5.16. Vol of O2/ml vs time plot (left) and measurement of conc. of H2O2 using TiO2+ in

9M H2SO4.

When 1 equivalent of 1 was allowed to react with approximately 10 equivalent of 2-

aminophenol under an argon atmosphere at room temperature in MeOH for 2 h, 0.5

equivalent of phenoxazinone chromophore to the concentration of complex 1 was found to

form spectrophotometrically at 435 nm. The X-band EPR spectrum feature of the solution at

10 K is same as shown in Figure 5.12,i.e. the formation of Mn(II) species. Exposure of the

solution to air increases the concentration of phenoxazinone chromophore. Hence, the process

is an aerial oxidation process. A few functional model complexes are know to catalyze the

oxidation reaction in the literature.37-38 This is the first example where aerial oxidation of 2-

aminophenol to phenoxazinone chromophore takes place catalytically by complex 1. A

mechanism can be proposed from the above experimental results as shown in Figure 5.17.

When 1 equivalent of 1 was allowed to react with approximately 10 equivalent of 2-

aminophenol under an argon atmosphere at room temperature in MeOH for 2 h, 0.5

equivalent of phenoxazinone chromophore to the concentration of complex 1 was found to

form spectrophotometrically at 435 nm. The X-band EPR spectrum feature of the solution at

10 K is same as shown in Figure 5.12,i.e. the formation of Mn(II) species. Exposure of the

solution to air increases the concentration of phenoxazinone chromophore. Hence, the process

is an aerial oxidation process. A few functional model complexes are know to catalyze the

oxidation reaction in the literature.

Figure 5. 17. The proposed mechanism for the formation of phenoxazinone chromophore. Figure 5. 17. The proposed mechanism for the formation of phenoxazinone chromophore.

300 350 400 450 500 550 600 650

0.0

0.3

0.6

0.9

1.2

1.5

1.8

300 350 400 450 500 550 600 650

0.0

0.3

0.6

0.9

1.2

1.5

1.8

2SO4.

37-38 This is the first example where aerial oxidation of 2-

aminophenol to phenoxazinone chromophore takes place catalytically by complex 1. A

mechanism can be proposed from the above experimental results as shown in Figure 5.17.

A / cm

Wevelength / nm

0 20406080100

0.0

0.5

1.0

1.5

2.0

2.5

Vol of O2 / ml

Time / min

NH2

2[Mn

IV(LCatCOOH)(LS

COOH

)]-

2 [Mn II(LCatCOOH)(LCatCOOH)]4-

3 O2

3H2O2

146

Chapter 5

5.4 References

1. Larsen, S. K.; Pierpont, C. G.; DeMunno, G.; Dolcetti, G. Inorg. Chem. 1986,25, 4828.

2. Lynch, M. W.; Valentine, M.; Hendrickson, D. N. J. Am. Chem. Soc. 1982,102, 6982.

3. Chang, H.-C.; Ishii, T.; Kondo, M.; Kitagawa, S. J. Chem. Soc.,Dalton Trans.1999,

2467.

4. Chang, H.-C.; Kitagawa, S. Angew. Chem., Int. Ed. 2002,41, 130.

5. Chang, H.-C.; Kitagawa, S. Angew. Chem., Int. Ed. 2002,41, 4444.

6. Cador, O.; Chabre, F.; Dei, A.; Sangregorio, C.; Van Slageren, J.; Vaz, M. G. F. Inorg.

Chem.2003,42, 6432.

7. Lynch, M. W.; Hendrickson, D. N.; Fitzgerald, B. J.; Pierpont, C. G. J. Am. Chem. Soc.

1984,106, 2041.

8. Larsen, S. K.; Pierpont, C. G. J. Am. Chem. Soc. 1988,110, 1827.

9. Lynch, M. W.; Valentine, M.; Hendrickson, D. N. J. Am. Chem. Soc. 1982,104, 6982.

10. Chun, H.; Verani, C. N.; Chaudhuri, P.; Bothe, E.; Bill, E.; Weyhermüller, T.;

Wieghardt, K. Inorg. Chem. 2001,40, 4157.

11. Mukherjee, S.; Weyhermüller, T.; Bothe, E.; Wieghardt, K.; Chaudhuri, P.; J. Chem.

Soc., Dalton Trans. 2004 , 3842 .

12. Chun, P.; Chaudhuri, P.; Weyhermüller, T.; Wieghardt, K. Inorg. Chem.2002, 140, 790.

13. Kahn, O.; Launay, J. P. Chemtronics, 1988,3, 140.

14. Day, P. Int. Rev. Phys. Chem. 1981,1, 149.

15. Creutz, C. Prog. Inorg. Chem. 1983,30, 1.

16. Richardson, D. E.; Taube, H. Coord. Chem. Rev. 1984,60, 107.

17. Prassidis, K. (Ed.) Mixed Valency Systems: Applications in Chemistry, Physics, and

Biology; NATO ASI Series C343; Kluwer Academic Publishers: Dordrecht, The

Netherlands, 1991.

18. Gütlich, P. Struct. Bonding, 1981,44, 83.

19. Konig, E. Progr. Inorg. Chem. 1987, 35, 527.

20. Beattie, J. K. Adv. Inorg. Chem. 1988, 32, 1.

21. Toftlund, H. Coord. Chem. 1989, 94, 67.

22. Gütlich, P.; Hauser, A.; Spiering, H. Angew. Chem., Int. Ed. Eng. 1994,33, 2024.

23. Kahn, O.; Martinez, C. J. Science, 1998,279, 44.

24. Pierpont, C. G.; Lange, C. W. Prog. Coord. Chem. 1993,41, 381.

147

Chapter 5

25. Adams, D. M.; Dei, A.; Rheingold, A. L.; Hendrickson, D. N. J. Am. Chem. Soc. 1993,

115, 8221.

26. Gütlich, P.; Dei, A. Angew. Chem., Int. Ed. Engl. 1997,36, 2734.

27. Hendrickson, D. N.; Adams, D. M.; Wu, C.-C.; Aubin, S. M. J. A Supramolecular

Function; Kahn, O., Ed.; NATO ASI Series C484; Kluwer Academic Publishers:

Dordrecht, The Netherlands, 1996, p 357.

28. Dei, A.; Gatteschi, D.; Sangregorio, C.; Sorace, L. Acc. Chem. Res., 37 (11), 2004. 827.

29. Evans, D. F. J. Chem. Soc. 1959, 2003.

30. Janes, S. M.; Mu, D.; Wemmer, D.; Smith, A. J.; Kaur, S.; Maltby, D.; Burlingame, A.

L.; Klinman, J. P. Science, 1990,248, 981.

31. E. Sawicki, T. R.; Hauser, T.; Stanley, W.; Elbert, W. Anal. Chem. 33. No 1. 1961, 93.

32. Edward, A.; Wintner, A.; Belinda, T.; Rebek, J. Jr. J. Org. Chem. 1995,60, 7997.

33. Hollstein. U. Chem. Rev, 1974,74, 625.

34. Barry, C.E., Nayar, P.G., and Begley, T.P. Biochemistry,1989,28, 6323.

35. Smith, A.W., Camara-Artigas, A., Olea, C. Jr., Francisco, W.A., and Allen, J.P. Acta

Cryst. 2004. D 60, 1453.

36. Barry, C.E., Nayar, P.G., and Begley, T.P. J. Am. Chem. Soc.1988.110, 3333.

37. Simandi, T.M.; Simandi, L.I.; Gyor, M.; Rockenbauer, A.; Gomory, A. J. Chem. Soc.,

Dalton Trans., 2004, 1056 and there in.

38. Horv%th,T.; Kaiser,J,; Speier,G. Journal of molecular catalysis A: Chemical. 2004,

215,9.

148

Chapter 5

149

Chapter 6

Conclusions and Perspectives

X = - tBu, -H

Conclusion and Perspectives

153

Conclusion and Perspectives

154

6.1 Conclusions

The main aim of this work was to synthesize of radical-containing transition metal

complexes which are relevant as functional and structural model of some metalloenzymes.

Use of these structural and functional model complexes can bring new information regarding

the active site(s) of the metalloenzymes. Moreover, radical-containing transition metal

complexes can be used as catalysts for aerial oxidation of some small organic molecules,

hence, invention of new systems as oxidants and utilization of bioinspired catalytic processes.

A few non-innocent ligands symbolically, H4L, H2Lx, H3LCH2OH, and, H3LCOOH, were

synthesized and used to prepare radical-containing transition metal complexes. The

conclusions that have been drawn from the structural, spectroscopic studies and catalytic

reactivities of the complexes are summarized below.

Chapter 1.

1. A monoradical-containing pentacoordinate high-spin Fe(III) complex has been

synthesized and characterized by X-ray crystallography, magnetic, spectro and

spectroelectrochemical methods. The complex has ST= 2 ground state with Peff =

4.92 PB, that remains constant in 4-290 K temperature range. This feature leads to

the conclusion that there is strong antiferromagnetic coupling between the Fe(III)

center and radical anion.

2. Oxovanadium complexes are common and well studied but non-

oxovanadium or “bare” vanadium complexes are very rare. Due to biological

relevance, non-oxovanadium vanadium complexes are important. A dinuclear

methoxide bridged non-oxovanadium(V) complex has been synthesized and

characterized. VO2+ and VO2+ ions do not have intense metal-to-ligand charge

transfer bands. Moreover, vanadium-containing metalloenzymes,

Bromoperoxidases, and tyrosinate proteins Transferrin do not show any strong

absorption band in 600 nm region, whereas the complex shows an intense metal-

to-ligand charge transfer band in the UV-VIS region. This information can be used

to determine that vanadium(IV) and vanadium(V) must both be bound to the

tyrosinate proteins Transferrin and Bromoperoxidases, as the VO2+ and VO2+

forms and tyrosine may not be a ligand to the vanadium in the Bromoperoxidases,

or the active site vanadium may be in the form of the pervanadyl moiety rather

Conclusion and Perspectives

155

than bare vanadium(V) or VO3+. Hence, complex 1 can be used as marker to

assign the active site of tyrosinate proteins Transferrin and bromoperoxidases.

3. Two distorted square planar complexes, Ni(II) and Pd(II), have been

synthesized and characterized via. IR, mass, X-ray crystallography, magnetism,

NMR, spectro and spectroelectrochemically. It shown that the lower energy

ligand-to-ligand charge transfer band around 970 nm for both complex 3 and 4 has

similar absorption coefficient to the reported complexes but appear at lower energy

compared to that complexes (Table 2.5). This feature leads to the fact that the

symmetry of the orbitals involved for the charge transfer is same as previously

reported (ref 51-53) but the energy difference between two orbitals are presumably

lower in energy in this case. It is also shown that the geometry around the metal

ion affects the delocalization of the S radical through the whole molecule and

therefore, shows more destabilized monoanionic or monocationic species than that

of the reported square planar complexes.50-52

4. A distorted square planar copper (II) complex having N2O2 environment has

been synthesized. A high quality X-ray crystal structure, measured at 100(2) K and

various spectroscopic methods have proved unambiguously that complex 5

contains two S radicals coordinated to the central copper (II) ion. When complex 5

was used as a functional model for Galactose Oxidase, to oxidize primary alcohol,

benzyl alcohol, it has been found that 5can oxidize benzyl alcohol to a two

electron oxidised product, benzaldehyde, with a good TON (65). Hence, biradical-

containing copper (II) complex with N2O2 not only behaves as structural model

but also as functional model for Galactose Oxidase.

Chapter 2.

1. Biradical-containing Cu(II) complexes with N2O2 coordination sites and

different substituents at the different positions of the N-phenyl ring have been

synthesized and characterized viz. X-ray crystallography and various spectroscopic

methods. Distortion from the square planar geometry has been found for –CF3

substituted complexes. Zero degree dihedral angle between two coordination

Conclusion and Perspectives

156

planes to Cu(II) ion for complex with -tBu substituent at 3,5 positions of the N-

phenyl ring emphasizes that the distortion around the Cu(II) ion is an electronic

effect instead of steric effect. Primary alcohols like benzyl alcohol, ethanol, and,

methanol, can be oxidized to their corresponding aldehydes using the biradical-

containing Cu(II) complexes. All the complexes are equally efficient for oxidising

benzyl alcohol to benzaldehyde but for ethanol and methanol respective 75% and

78% yield have been achieved using complex 1 in 25:1 substrate: complex ratio.

Kinetics and methanistic studies shows that the complexes can be considered as

functional as well as structural models for Galactose Oxidase.

Chapter 3.

1. A valence-trapped trinuclear manganese complex has been synthesized and

characterized. Complex 2 is the first example of discrete trinuclear manganese ions

in three different oxidation states MnIII, MnII, MnIV and iminosemiquinone,

iminoquinone, amidophenolate and chloride ligands. Moreover, it has been shown

from the magnetic susceptibility and magnetization measurements that the

exchange coupling between the MnIII and MnII is ferromagnetic whereas that for

MnII and MnIV is antiferromagnetic.

2. A tetraradical-containing Cu4O4 cubane core crystal structure of a new

tridented non-innocent ligand containing amino, alkoxo and phenoxo

functionalities has been shown. This is the first example of a tetraradical-

containing Cu4O4 cubane core. The unique magnetic properties have been

discussed. These results give further investigation of metal-metal and metal-radical

(ligand) interactions and insights into the design and synthesis of polynuclear

complex with new structure and magnetic properties. Moreover, this cubane core

can be used as a functional model for Phenoxazinone synthase.

Chapter 4.

1. Monoradical-containing Mn(IV) complex can be used as a functional model for

Amine Oxidases and Phenoxazinone Synthase. Variable temperature UV-VIS/NIR

spectra in CH2Cl2 solution of 1 show that the complex can exhibit valence

tautomerism in solution.

Conclusion and Perspectives

157

6.2 Perspectives

1. Addition of H4L ligand to VOSO4.5H2O in methanol produces “non-oxo”

dinuclearvanadium(V) complex. This above reaction can be carried out in the presence

of PPh3 to find out if during reaction the complex acts as an oxo-transfering agent of

not.

2. Titanium and Chromium complexes with the H4L ligand can be synthesized

and their coordination chemistry can be studied.

3. Pentacoordinated square pyramidal iron, cobalt and manganese complexes

can be utilized as starting materials to synthesized similar types of complexes by

varying the nature of the ligand situated at the axial position. These complexes can be

useful to synthesize high-spin, intermediate-spin or low spin complexes. Iron and

manganese complexes can be used as catalysts for the catalytic epoxide formation

from the corresponding ene compounds.

4. Theoretical calculation can be performed to find out the molecular energy

levels in the distorted square planar Cu(II), Ni(II) and Pd(II) complexes formed with

the H4L ligand and the bands appeared in the UV-VIS/NIR spectrum of those

complexes can be assigned unambiguously.

5. Theoretical calculations can be performed to find out the effect of substituents

to the 3, 5-positions of the N-phenyl group to the geometry of the Cu(II) complexes

formed with the H2LX ligand.

6. Having strong S-donation capability of the ligand, H3LCH2OH, forms

polynuclear complexes. Cobalt, chromium, titanium and other transition metals can be

used to synthesize polynuclear complexes and their coordination chemistry can be

studied as polynuclear complexes are useful in the study of metal-ligand interactions

and functional model for metalloenzymes.

7. [V2O2(LCH2OH )2]0 Complex can be oxidized and reduced chemically and all

these species can be isolated and their coordination chemistry can be studied. Racal-

containing vanadium complexes are rare hence, the radical containing vanadium

complex is of significant interest. Moreover, the complex can be examined as a drug

for antiameobic effect and its activity towards insulin can be examined.

8. nBu4N[MnO4] compound can be prepared from KMnO4 and can be used as an

oxidant to oxidize [Mn3(LIQCH2OH x)2(LAPCH2OH ) (LBQCH2OH )Cl]0 and forming a

Conclusion and Perspectives

158

tetranuclear Mn-complex. That complex can be examined as a functional model for

PS II.

9. Without radical-containing star-shaped tetranuclear iron complex can be

synthesized using the following ligand. That complex may have single molecule

magnetic behavior.

X = - tBu, -H

10. Ligand H3LCOOH can be introduced to synthesize corresponding vanadium,

iron, chromium, palladium, nickel and copper complexes and their coordination

chemistry as well as catalytic activities can be studied.

Chapter 7

Equipment and Experimental work

Equipment and Experimental Work

161

Equipment and Experimental Work

162

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- ,13C- and 51V 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

MAT8200 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.

Equipment and Experimental Work

163

Electrochemistry

Cyclic voltammetry, square wave voltammetry and linear sweep voltammetry

experiments were performed using an ‘EG&G Potentiostat / Galvanostat 273A’. A standard

threeelectrode-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 = ȝBȈ gi ǅi .B

HHDVV = -2 Ȉ Jij ǅi . ǅj

HZFS = ȈDi[ǅiz2-{Si(Si+1)/3}+ Ei/Di(ǅix2-ǅiy2)]

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.

Equipment and Experimental Work

164

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 Į-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-KĮ with Ȝ = 0.71073 Å was

employed. Data were collected by the 2ș-Ȧ scan method (32ș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 fullmatrix 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.

Equipment and Experimental Work

165

7.2 Experimental Work

7.2.1 Synthesis of Ligands

(i) Synthesis of N, N

-bis(2-hydroxy-3,5-di-tert-butylphenyl)-

2,2

-diamino Biphenyl compound (H4L)

Step 1: Synthesis of 2,2

dianitrobiphenyl compound

2-Iodonitribenzene (35.5 gm, 0.14 mol) and pure dry copper powder (25.4 gm, 0.4

mol) were ground together and then transfer to a flask which was immesered in an oil-bath

pre-heated to 150-160qC, when the internal temperature was raised during 15 minutes to 190-

200qC and maintained there for 24 hours. The solid cake was obtained on cooling to room

temperature dissolved in chloroform. The solution was passed through a alumina column (30

u 4 cm) and orange eluate was evaporated .Yield: 55 % (10.9 mg, 0.055 mol)

Step 2: Synthesis of 2,2

diamino biphenyl compound

A slurry of 1.8 gm of 10 % Pd-C catalyst was stirred in 250 ml of methanol while

bubbling argon through the mixture. A solution of 3.6 gm (96 mmol) of sodium borohydride

in 100 ml of water was added, and the purging continued. Over the next five minutes, 3.8 gm

(16 mmol) of 2,2c-dinitrobiphenyl in 300 ml of methanol was added in three portions. The

mixture was stirred at room temperature for one hour, filtered under suction through a pad of

celite filter aid and taken to dryness by rotary evaporation at 80q C. The residue was extracted

with dichloromethane (200 ml) and ether (200 ml). The combined extracts were taken to

dryness by rotary evaporation yielding 2.7 gm (94 %) of 2,2c-diaminobiphenyl.

Recrystrallized from n-hexane. H NMR (CD2Cl2): 7.31 (dd, 2h, J= 2.3, 7.1 HZ), 7.59 (m,

2H), 7.69 (m, 2H), 8.3 (dd, 2H, J= 1.5, 8.5). 13C NMR (CD2Cl2): 143.2 (C-1), 147.2 (C-2),

124.8 (C-3), 129.2(C-4), 130.9(C-6), 133.4(C-5).

Equipment and Experimental Work

166

Step 3: Synthesis of H4L

2,2c-Diaminobiphenyl (2 gm; 10 mmol), 3,5-di-tert-butylcatechol (4.5gm; 20 mmol)

and 1 ml Et3N were taken in 100 ml n-hexane. The solution was stirred under air for 15 hours.

Filtered and washed with n-hexane to get white color solid, N, Nc (2-hydroxy-3, 5-di-tert-

butyl-phenyl)-2,2c-diaminobiphenyl.

Yield: 4.75 gm (80%)

Mass (EI) : 592 gm/mol

m.p: 196-198q C.



H NMR (CD2Cl2, ppm): 1.23 (s, 18 tBu H), 1.40 (s, 18 tBu H), 5.16 (s, 2H), 6.56-7.3 (12 H

from aromatic ring).

Elemental analysis. C40H52N2O2

%C %H %N

Calculated 81.08 8.78 4.73

Found 80.57 9.07 4.85

4000 3500 3000 2500 2000 1500 1000 500

cm-1

Equipment and Experimental Work

167

(ii) Synthesis of N(2-hydroxy-3,5-di-tert-butylphenyl)-2-

aminobenzylalcohol compound (H3LCH

OH)

2-Aminobenzylalcohol (1.25 gm; 10 mmol), 3, 5-di-tert-butyl-catechol (2.25gm; 10

mmol) and 0.2 ml Et3N was taken in 50 ml n-hexane. The solution was stirred under air for 2

days. Filtered and washed with n-hexane to get white color solid, N (2-hydroxy-3, 5-di-tert-

butylphenyl)-2aminobenzylalcohol.

Yield: 58% (1.9 gm).

m.p: 126-128qC.

Mass (EI): 327.



H NMR (CD2Cl2, ppm): 1.29 (s, 9 tBu-H), 1.45 (s, 9 tBu-H), 4.81 (s, CH2 -H).6.31-7.25(6-H

from aromatic rings).

Elemental analysis. C21H29NO2

%C %H %N

Calculated 77.00 8.85 4.30

Found 76.9 8.80 4.30

4000 3500 3000 2500 2000 1500 1000 500

cm-1

Equipment and Experimental Work

168

(iii) Synthesis of N(2-hydroxy-3,5-di-tert-butylphenyl)-

anthranilic acid compound (H3LCOOH)

Anthranilic acid (1.45 gm; 10 mmol), 3,5-di-tert-butyl-catechol (2.25gm; 10 mmol) and

0.01 ml Et3N was taken in 50 ml n-hexane. The solution was heated to reflux for 5 h and then

stirred under air for 2 days. Filtered and washed with water and then n-hexane to get white

color solid, N (2-hydroxy-3, 5-di-tert-butylphenyl) anthranilic acid.

Yield: 56% (1.9 gm),

m.p: >220q C.

Mass (EI): 341.



H NMR (CD2Cl2, ppm): 1.27 (s, 9 tBu H), 1.44 (s, 9 tBu H), 6.12 (s, 1 NH H) 6.5-8.4 (6 H

from aromatic ring), 8.75 (s, 1 COOH H).

Elemental analysis. C21H27NO3

%C %H %N

Calculated 73.90 8.0 4.10

Found 74.5 7.67 3.93

4000 3500 3000 2500 2000 1500 1000 500

cm-1

Equipment and Experimental Work

169

7.2.2 Synthesis of complexes

(i) Synthesis of [V(L)(

2OMe)2(L)V]0

To a solution of the ligand, H4L (1.05 gm, 2 mmole) in MeOH (30 ml) under an argon

atmosphere VO(SO4).5H2O ( 0.51gm, 2 mmole) was added. Upon addition of Et3N the color

of the solution changed to dark blue. After being stirred under argon atmosphere air for 1h

and then under air for 2h, the dark blue solution was allowed to stand at room temperature.

Microcrystalline complex was precipitated which was collected by filtration.

Yield: 0.47 g (35%).

Molecular Weight: 1340.0

m/z : 1340.0 [M]+.



H NMR (CD2Cl2, ppm): 1.23 (s, 18 tBu H), 1.40 (s, 18 tBu H), 5.16 (s, 2H), 6.56-7.3 (12 H

from aromatic ring).

Elemental analysis. C82H102N4O6V2

%C %H %N %V

Calculated 73.43 7.67 4.18 7.61

Found 73.1 7.54 4.25 7.82

4000 3500 3000 2500 2000 1500 1000 500

cm-1

Equipment and Experimental Work

170

(ii) Synthesis of [Fe(HL

)Cl]0

A CH3CN solution (30 ml) of anhydrous FeCl3 (0.33gm, 1 mmol), H4L (0.592g, 1

mmol), and triethylamine (0.1 ml) was stirred under an aerial atmosphere at room temperature

for 1h. A dark greenish-blue microcrystalline solid precipitated out. Filtered and washed with

CH3CN. Single crystals suitable for X-ray structure analysis were obtained by slow

evaporation of a 1:1 CH2Cl2, CH3CN solution mixture.

Yield: 0.520 g (76%).

Molecular Weight: 680.5

m/z : 679.5 {M}+.

Elemental analysis. C40H49N2O2ClFe

%C %H %N %Fe

Calculated 70.69 7.21 4.12 7.22

Found 70.81 7.22 4.45 7.24

4000 3500 3000 2500 2000 1500 1000 500

cm-1

Equipment and Experimental Work

171

(iii) Synthesis of [Ni(L

)]0

To a solution of the ligand, H4L (1.185 gm; 2 mmol) in CH3CN (30 ml) was added

NiCl2.6H2O (0.475 gm; 2 mmol) at room temperature in the presence of air and 0.2 ml

triethylamine. From this solution, a microcrystalline brown precipitate formed with in 1h

which was collected by filtration and washed with CH3CN thoroughly. Dried under air. Single

crystals suitable for X-ray crystallography were obtained by slow evaporation of 1:1 CH3CN,

CH2Cl2 solution.

Yield: 0.960 g (74%).

Molecular Weight: 646.0

m/z : 646 [M]+.



H NMR (CD2Cl2, ppm): 1.14 (s, 9 tBu H), 1.26 (s, 9 tBu H) 1.53 (s, 9 tBu H), 1.56 (s, 9 tBu

H), 6.75-7.55 (12 H from aromatic ring).

Elemental analysis. C40H48N2O2Ni

%C %H %N %Ni

Calculated 74.2 7.47 4.33 9.06

Found 73.9 7.54 4.25 9.00

4000 3500 3000 2500 2000 1500 1000 500

cm-1

Equipment and Experimental Work

172

(iv) Synthesis of [Pd(L

)]0

To a solution of PdCl2 (0.345 gm; 2 mmol) in CH3CN (30 ml) was added with stirring

solid H4L (1.185 gm; 2 mmol) and triethylamine (0.2 ml) at room temperature in the presence

of air. From this solution a microcrystalline dark brown precipitate formed with in 1h which

was collected by filtration, washed with CH3CN, and air dried. Single crystals suitable for X-

ray crystallography were obtained by slow evaporation of 1:1 CH3CN, CH2Cl2 solution.

Yield: 1.05 g (75%).

Molecular Weight: 696.0

m/z : 696 [M]+.



H NMR (CD2Cl2, ppm): 1.19 (s, 9 tBu H), 1.26 (s, 9 tBu H) 1.54 (s, 9 tBu H), 1.6 (s, 9 tBu H),

6.8-7.61 (12 H from aromatic ring).

Elemental analysis. C40H48N2O2Pd

%C %H %N %Pd

Calculated 69.1 6.96 4.03 15.31

Found 68.75 7.00 4.15 15.45

4000 3500 3000 2500 2000 1500 1000 500

cm-1

Equipment and Experimental Work

173

(v) Synthesis of [Cu(L

)]0

The ligand, H4L (1.185 gm; 2 mmol), [Cu I(CH3CN) 4] ClO4 (0.655 gm; 2 mmol) and

0.1 ml Et3N were taken in 30 ml dry CH3CN. The solution was stirred under argon

atmosphere for 0.5 h and then under air for 1h. On slow evaporation of a 1:1 CH3CN, CH2Cl2

solution mixture crystalline complex precipitated.

Yield: 0.975 g (75%).

Molecular Weight: 651.5

m/z : 651.4 {M}+.

Elemental analysis. C40H48N2O2Cu

%C %H %N %Cu

Calculated 73.62 7.36 4.29 9.73

Found 73.09 7.54 4.25 9.82

4000 3500 3000 2500 2000 1500 1000 500

cm-1

Equipment and Experimental Work

174

(vi) Synthesis of [Cu(L

)][PF6]

To the stirring complex [CuII(Lxx)]0 (0.65 gm; 1mmol), ferrocenium

hexafluorophosphate (0.33 gm; 1 mmol) was added under argon blanketing atmosphere in 20

ml degassed CH2Cl2 solution. The solution was stirred for 1h and then to that solution 10 ml

degassed n-hexane was added. Slow evaporation of the solution provides microcrystalline

solid.

Yield: 0.270 g (34%).

Molecular Weight: 796.5

m/z : 651.4 [M]+, 145 [PF6]-

Elemental analysis. C40H48N2O2CuPF6

%C %H %N %Cu % P % F

Calculated 60.26 6.02 3.51 7.97 3.89 14.31

Found 60.19 6.14 3.35 7.82 3.90 14.1

4000 3500 3000 2500 2000 1500 1000 500

cm-1

Equipment and Experimental Work

175

(vii) General method for the Synthesis of Cu(II) complexes

with H2Lx ligands, [Cu(LX

)2]0

To a solution of H2LX (2 mmol) in CH3CN (30 ml) were added [CuI(CH3CN)4]ClO4

(0.655 gm; 2 mmol) and 0.2 ml triethylamine. The solution was stirred for 2h in the presence

of air. From the filtered solution a dark green, microcrystalline precipitate was formed. Single

crystals suitable for X-ray crystallography were obtained by slow evaporation of 1:1 CH3CN,

CH2Cl2 solution.

Yields and elemental analysis (C, H, N, Cu, X) results are in Table below.

24.61

14.42

10.46

17.9

Cu%

6.86

8.04

8.75

8.02

8.2

8.9

7.3

3.02

3.54

3.86

3.54

3.62

3.92

3.2

5.01

6.12

6.38

5.85

7.55

7.62

9.4

57.05

63.82

66.14

60.65

68.23

70.61

76.54

Yield %

Mol. Wt

925

789

725

791

773

713

879

Composition

C44H46F12N2O2Cu

C42H48F6N2O2Cu

C40H46F4N2O2Cu

C40H46Cl4N2O2Cu

C44H58N2O6Cu

C42H54N2O4Cu

C56H82N2O2Cu

Complex

Complex No. is same as used in Chapter 3

Equipment and Experimental Work

176

(viii) Synthesis of [V2O2(LCat

OH )2]0

To a stirred solution of H3LCH2OH (0.655gm; 2 mmol) in MeOH (30 ml) was treated

with VOSO4, 5H2O (0.51 gm; 2 mmol). The mixture was stirred under an aerial atmosphere

for 2h in the presence of triethylamine. Deep blue solid was separated out and filtered off,

washed with MeOH. X-ray quality deep blue colored single crystals were obtained by

recrystallization of the microcrystalline solid from a 1:1 CH2Cl2 and CH3CN solvent mixture.

Yield: 1.095 g (70%).

Molecular Weight: 783.0

m/z : 782.7 [M]+.



H NMR (CD2Cl2, ppm): 1.14 (s, 9 tBu H), 1.26 (s, 9 tBu H) 1.53 (s, 9 tBu H), 1.56 (s, 9 tBu

H), 6.75-7.55 (12 H from aromatic ring).

Elemental analysis. C42H52N2O6V2

%C %H %N %V

Calculated 64.45 6.70 3.58 13.02

Found 64.1 6.54 3.5 13.10

4000 3500 3000 2500 2000 1500 1000 500

cm-1

Equipment and Experimental Work

177

(ix)Synthesis of [Mn3(LSQ

)2(LCat

OH )(LBQ

OH )Cl]0

The stirring ligand, H3LCH2OH (0.655 gm; 2 mmol), anhydrous MnCl2 (0.25 gm; 2

mmol), and 0.2 ml triethylamine were added in MeOH (30 ml). The solution was stirred for

2h under air. Slow evaporation of the methanolic solution followed by addition of CH3CN to

the concentrated methanolic solution produces crystalline complex.

Yield: 0.48 gm (32%).

Molecular Weight: 1496.5

m/z : 1496.2 [M]+, 1461 [M-Cl]+.

Elemental analysis. C84H104N4O8Mn3

%C %H %N %Mn Cl%

Calculated 67.35 7.00 3.74 11.00 2.37

Found 67.3 6.82 3.62 11.2 2.4

4000 3500 3000 2500 2000 1500 1000 500

cm-1

Equipment and Experimental Work

178

(x) Synthesis of [Cu4(LSQ

)4]0

To CH3CN (30 ml) solution of H3LCH2OH (0.655 gm; 2 mmol), CuII(OAc)2.H2O (0.4

gm; 2 mmol), and 0.2 ml triethylamine were added. After stirring for 24h the solution was

filtered and residue was washed thoroughly with CH3CN.

Yield: 0.55 gm (71%).

Molecular Weight: 1550.0

m/z : 1550.2 [M]+.

Elemental analysis. C84H104N4O8Cu4

%C %H %N %Cu

Calculated 65.00 6.75 3.61 16.38

Found 65.80 6.10 3.12 15.62

4000 3500 3000 2500 2000 1500 1000 500

cm-1

Equipment and Experimental Work

179

(xi) Synthesis of [Mn(LSQ

COOH

)(LCat

COOH )][HNEt3]

MnII(acetate)2 . 4H2O (0.49 gm; 2 mmol) was dissolved in 30 ml of MeOH, and

H3LCOOH (0.68 gm; 2 mmol) was added. To the solution 0.3 ml Et3N was added and stirred

under air for 2 h. Microcrystalline solid precipitated.

Yield: 1.05 gm (63%).

Molecular Weight: 833.0

m/z : 102 [HNEt3]+, 731 [M]-.

Elemental analysis. C48H64N3O6Mn

%C %H %N %Mn

Calculated 69.2 7.73 5.05 6.60

Found 69.00 7.75 4.97 6.56

4000 3500 3000 2500 2000 1500 1000 500

cm-1

Equipment and Experimental Work

180

7.2.3 Reactivity studies,

General procedure for the oxidation of benzyl alcohol and

determination of the concentration of benzaldehyde

In 20 ml dicholoromethane, 5 X 10-4 M catalyst, 5 X10-2 M benzyl alcohol and 4.5 X

10-2 M tetrabutylammonium methoxide was added. The resulting solution was stirred under

air for 15 h and then LC was measured. Concentration of product (benzaldehyde) was

calculated against the area obtained for a standard benzaldehyde solution.

General procedure for the oxidation of ethanol (ethoxide)

and determination of the concentration of acetaldehyde

In 20 ml dicholoromethane, 5 X 10-4 M catalyst and 5 X 10-2 M tetrabutylammonium

ethoxide was added. The resulting solution was stirred under air for 3 to 5 h and then 0.5 ml

of the catalytic solution was extracted by 10 ml water. To the 0.5 ml water part, 0.5 ml 1 %

aqueous 3-methyl-2-benzothiazolone hydrazine hydrochloride was added and the mixture was

allowed to stand for 30 min. 2.5 ml 2 % freshly prepared aqueous FeCl3 solution was added.

Following a five minutes waiting period, the mixture is diluted to 10 ml with acetone. The

absorbance is then determined at 635 nm and 670 nm against a blank.

General procedure for the oxidation of methanol

(methoxide) and determination of the concentration of

formaldehyde

(a) Preparation of Hentzsch solution: 25 gm (0.325 mol) ammonium acetate, 3 ml 100%

acetic acid, 0.2 ml (19.4 mmol) acetyl acetone was taken in 100 ml volumetric flux. Volume

was made up 100 ml by water.

(b) Determination of formaldehyde: In 20 ml dicholoromethane, 5 X 10-4 M catalyst and 5

X 10-2 M tetrabutylammonium methoxide was added. The resulting solution was stirred under

air for 15 h. To 0.5 ml reaction solution 10 ml water is added. Water part was separated out.

Equipment and Experimental Work

181

To the 5 ml water part 5 ml Hentzsch reagent was added. The mixture was heated at 60 qC for

5 min., cooled at room temperature and UV-VIS spectrum was measured (H = 8000 M-1 cm-1

at O = 414 nm) against blank.

toluene. To that 4.5 ml (114 mmol) CD3OD was added and stirred for 8h (until total

potassium dissolved). 4 ml KOCD3 / CD3OD is separated out. To that solution 9.35 gm (27.4

mmol) tetrabutylammonium perchlorate is added. On addition of dry ether potassium

perchlorate and excess tetrabutylammonium perchlorate were separated out as precipitate.

Filtered under argon atmosphere and excess CD3OD was evaporated.

General procedure for the oxidation of benzyl amine and

determination of the concentration of

benzilidinebenzylamine

In 20 ml methanol, 5 X 10-4 M catalyst and 5 X 10-2 M benzylamine was added. The

resulting solution was stirred under air for 15 h and then 1 ml of the catalytic solution was

passed through a neutral alumina (0.5 gm) and the column was washed with 9 ml methanol.

To the resulting 10 ml solution 5 Pln-hexadecane was added as standard. GC and GC-MS

was measured. Concentration of benzilidinebenzylamine was calculated against the standard.

General procedure for the oxidation of ethylenediamine and

2-aminoethanol and determination of the concentration of

glyoxal and 2-hydroxy-acetaldehyde

In 20 ml methanol, 5 X 10-4 M catalyst and 5 X 10-2 M ethylenediamine or 2-

aminothanol was added. The resulting solution was stirred under air for 15 h and then 0.5 ml

of the catalytic solution was extracted by 10 ml water. To the 0.5 ml water part, 0.5 ml 1 %

aqueous 3-methyl-2-benzothiazolone hydrazine hydrochloride was added and the mixture was

allowed to stand for 30 min. 2.5 ml 2 % freshly prepared aqueous FeCl3 solution was added.

Equipment and Experimental Work

182

Following a five minutes waiting period, the mixture is diluted to 10 ml with acetone. The

absorbance is then determined at 634 nm and 664 nm against a blank.

General procedure for the oxidation of 2-aminophenol and

determination of the concentration of phenoxazinone

chromophore

In 20 ml methanol or dichloromethane, 2.5 X 10-4 M catalyst and 2.5 X 10-2 M 2-

aminophenol was added. The resulting solution was stirred under air for 2 h or 40 h. The

solution was diluted as required and UV-VIS spectrum at 435 nm (H = 24000 M-1 cm-1) was

measured against blank.

Equipment and Experimental Work

183

Appendices

(1) Crystallographic data

(2) Magnetochemical data

(3) Curriculum Vitae

Appendices

185

Appendices

186

(1)Crystallographic data

[Cu(L

)]0 [Pd(L

)]0

Identification code

Empirical formula

4633

C40H48CuN2O2

4850

C40H48N2O2Pd

Formula weight 652.34 695.2

Temperature 100(2) K 100(2) K

Wavelength (MoKD)0.71073 Å 0.71073 Å

Crystal system Monoclinic Monoclinic

Space group P21/n P21/n, No. 14

Unit cell dimensions a = 9.4200(6) Å, a = 9.5172(6) Å,

b = 18.5916(14) Å, b = 18.5949(12) Å,

c = 19.784(2) Å, c = 20.29(2) Å,

D = 90oD = 90o

E = 101.55(1)oE = 100.88(1)o

J = 90oJ = 90o

Volume (Å3), Z 2394.7(5), 4 3526.2(5), 4

Density (calc.) Mg/m3 1.276 1.310

Absorption coeff 0.68 mm-1 0.562 mm-1

F(000) 1388 1456

Crystal size (mm) 0.18 x 0.06 x 0.06 0.14 x 0.02 x 0.02

T range for data collect. 3.42 to 26.00 deg 3.00 to 23.35

Index range -10<=h<=11, -10<=h<=10,

-22<=k<=22, -20<=k<=20,

-24<=l<=24 -22<=l<=21

Reflections collected 45253 20043

Independent reflect. 6630 [R(int) = 0.1060] 5081 [R(int) = 0.0889]

Absorption correction Not corrected Gaussian, Face-indexed

Data/restraints/param. 6630 / 0 / 406 5081 / 31 / 419

Goodness-of-fit on F2 1.039 1.018

Final R indices [I>2V(I)] R1=0.0580,

wR2=0.1244

R1=0.0461,

wR2=0.0829

R indices (all data) R1=0.0936,

wR2=0.1409

R1=0.0813,

wR2=0.0943

Appendices

187

[Ni(L

)]0 [Fe(HL

)Cl]0

Identification code

Empirical formula

4682

C40H48N2Ni O2

5142

C40H49 Cl Fe N2O2

Formula weight 647.51 681.11

Temperature 100(2) K 100(2) K

Wavelength (MoKD)0.71073 Å 0.71073 Å

Crystal system Monoclinic Triclinic

Space group P21/n P-1, No. 2

Unit cell dimensions a = 9.4934(6) Å, a = 9.1566(5) Å,

b = 18.4206(14) Å, b = 10.1701(12) Å,

c = 19.7027(2) Å, c = 19.7866(2) Å,

D = 90oD = 94.374(5)o

E = 101.49(1)oE = 96.805(5)o

J = 90oJ = 91.558(5)o

Volume (Å3), Z 2376.4(2), 4 1823.07(15), 2

Density (calc.) Mg/m3 1.274 1.241

Absorption coeff 0.612 mm-1 0.523 mm-1

F(000) 1384 724

Crystal size (mm) 0.12x 0.06 x 0.06 0.14 x 0.06 x 0.03

T range for data collect. 3.42 to 27.50 deg 2.96 to 27.50

Index range -12<=h<=12, -11<=h<=11,

-23<=k<=23, -13<=k<=13,

-25<=l<=25 -25<=l<=25

Reflections collected 53654 29940

Independent reflect. 7708 [R(int) = 0.1047] 8345 [R(int) = 0.0786]

Absorption correction Not corrected None

Data/restraints/param. 7708 / 0 / 406 8345 / 0 / 430

Goodness-of-fit on F2 1.029 1.070

Final R indices [I>2V(I)] R1=0.0462,

wR2=0.0910

R1=0.0617,

wR2=0.1196

R indices (all data) R1=0.0659,

wR2=0.0986

R1=0.0930,

wR2=0.1324

Appendices

188

[V(L)(

2OMe)2(L)V]0 [Cu(L3,5-CF

)2]

Identification code

Empirical formula

5059

C83.25H104.5 Cl2.5N4O6V2

3269

C44H46 Cu F12 N2O2

Formula weight 1447.71 926.37

Temperature 100(2) K 100(2) K

Wavelength (MoKD)0.71073 Å 0.71073 Å

Crystal system Monoclinic Triclinic

Space group P21/n, No. 14 P-1

Unit cell dimensions a = 23.6897(6) Å, a = 11.426(1) Å,

b = 24.5147(7) Å, b = 14.197(2) Å,

c = 27.9324(9) Å, c = 14.726(2) Å,

D = 90oD = 67.95(2)o

E = 104.534(6)oE = 81.28(2)o

J = 90oJ = 83.86(2)o

Volume (Å3), Z 15704.2(8), 8 2185.2(5), 2

Density (calc.) Mg/m3 1.225 1.408

Absorption coeff 0.377 mm-1 0.589 mm-1

F(000) 6148 954

Crystal size (mm) 0.14 x 0.04 x 0.02 0.36 x 0.16 x 0.15

T range for data collect. 2.91 to 22.50 deg 2.21 to 26.00

Index range -25<=h<=25, -13<=h<=17,

-26<=k<=26, -19<=k<=22,

-30<=l<=30 -18<=l<=22

Reflections collected 114758 17366

Independent reflect. 20469 [R(int) = 0.0777] 8154 [R(int) = 0.0368]

Absorption correction Not corrected SADABS

Data/restraints/param. 20469 / 64 / 1798 8146 / 0 / 577

Goodness-of-fit on F2 1.048 0.994

Final R indices [I>2V(I)] R1=0.0603,

wR2=0.1359

R1=0.0464,

wR2=0.1025

R indices (all data) R1=0.0886,

wR2=0.1512

R1=0.0742,

wR2=0.1142

Appendices

189

[Cu(L2-CF

)2]0 [Cu(L3,5-F

)2]0

Identification code

Empirical formula

4816

C44H51 Cu F6N3O2

3297

C40H46 Cu F4 N2O2

Formula weight 831.42 726.33

Temperature 100(2) K 100(2) K

Wavelength (MoKD)0.71073 Å 0.71073 Å

Crystal system Monoclinic Monoclinic

Space group P21/n P2(1)/n

Unit cell dimensions a = 17.1824(3) Å, a = 5.8867(5) Å,

b = 11.8651(2) Å, b = 17.533(2) Å,

c = 21.0332(5) Å, c = 17.854(2) Å,

D = 90oD = 90.00 o

E = 101.501(4)oE = 95.32(2)o

J = 90oJ = 90.00 o

Volume (Å3), Z 4201.96(14), 4 1834.8(3), 2

Density (calc.) Mg/m3 1.314 1.315

Absorption coeff 0.586 mm-1 0.652 mm-1

F(000) 1740 762

Crystal size (mm) 0.40 x 0.28 x 0.20 0.32 x 0.21 x 0.14

T range for data collect. 2.97 to 30.99 deg 2.29 to 32.50

Index range -24<=h<=24, -9<=h<=9,

-17<=k<=17, -26<=k<=26,

-30<=l<=30 -27<=l<=27

Reflections collected 82338 36228

Independent reflect. 13340 [R(int) = 0.0448] 6617 [R(int) = 0.0889]

Absorption correction Gaussian, Face-indexed Not measured

Data/restraints/param. 13340/ 0 / 518 6610 / 0 / 229

Goodness-of-fit on F2 1.058 0.984

Final R indices [I>2V(I)] R1=0.0385,

wR2=0.0947

R1=0.0409,

wR2=0.0870

R indices (all data) R1=0.0461,

wR2=0.0987

R1=0.0861,

wR2=0.0991

Appendices

190

[Cu(L3,5Cl

)2]0 [Cu(L3,5-tBu

)2]0

Identification code

Empirical formula

3285

C40H46 Cl4 Cu N2O2

3280

C60H92 Cu N2O3

Formula weight 992.13 952.90

Temperature 100(2) K 100(2) K

Wavelength (MoKD)0.71073 Å 0.71073 Å

Crystal system Monoclinic Triclinic

Space group P21/c P-1

Unit cell dimensions a = 12.1419(8) Å, a = 9.5092(7) Å,

b = 10.2848(6) Å, b = 16.365(1) Å,

c = 15.3145(10) Å, c = 19.024(1) Å,

D = 90oD = 83.078(3)o

E = 92.543(4)oE = 79.902(3)o

J = 90oJ = 79.567(3)o

Volume (Å3), Z 1910.5(2), 2 2854.2(4), 2

Density (calc.) Mg/m3 1.377 1.109

Absorption coeff 0.888 mm-1 0.425 mm-1

F(000) 826 1038

Crystal size (mm) 0.33 x 0.22 x 0.16 0.30 x 0.19 x 0.14

T range for data collect. 2.39 to 27.50 deg 1.75 to 23.27

Index range -15<=h<=18, -10<=h<=10,

-14<=k<=14, -18<=k<=15,

-18<=l<=23 -19<=l<=21

Reflections collected 15776 19197

Independent reflect. 4265 [R(int) = 0.0498] 8051 [R(int) = 0.0378]

Absorption correction Gaussian, Face-indexed SADABS

Data/restraints/param. 4249 / 0 / 229 8044 / 0 / 624

Goodness-of-fit on F2 1.012 0.957

Final R indices [I>2V(I)] R1=0.0380,

wR2=0.0809

R1=0.0391,

wR2=0.0910

R indices (all data) R1=0.0719,

wR2=0.0919

R1=0.0644,

wR2=0.1097

Appendices

191

[V2O2(Lcat

OH )2]0[Mn3(LSQCH2OH

)2(LCatCH2OH )

(LBQCH2OH )Cl]0

Identification code

Empirical formula

5236

C42H52 N2O6V2

5167

C88H110 Cl Mn3 N6O8

Formula weight 782.74 1580.9

Temperature 100(2) K 100(2) K

Wavelength (MoKD)0.71073 Å 0.71073 Å

Crystal system Monoclinic Triclinic

Space group P21/c, No. 14 P-1, No. 2

Unit cell dimensions a = 11.3171(5) Å, a = 16.1719(8) Å,

b = 9.7889(4) Å, b = 16.1921(9) Å,

c = 17.9384(8) Å, c = 16.7326(5) Å,

D = 90oD = 103.201(6)o

E = 100.349(4)oE = 92.060(6)o

J = 90oJ = 97.692(6)o

Volume (Å3), Z 1954.92(15), 2 4217.03(15), 2

Density (calc.) Mg/m3 1.330 1.244

Absorption coeff 0.527 mm-1 0.531 mm-1

F(000) 824 1672

Crystal size (mm) 0.07 x 0.02 x 0.02 0.10 x 0.06 x 0.02

T range for data collect. 3.11 to 29.00 deg 3.33 to 24.00

Index range -15<=h<=15, -18<=h<=18,

-13<=k<=12, -18<=k<=18,

-24<=l<=24 -19<=l<=18

Reflections collected 27955 32092

Independent reflect. 5185 [R(int) = 0.0570] 12983 [R(int) = 0.0789]

Absorption correction None None

Data/restraints/param. 5185/ 31 / 251 12983 / 39 / 1015

Goodness-of-fit on F2 1.033 1.073

Final R indices [I>2V(I)]

R indices (all data)

R1=0.0476,

wR2=0.1035

R1=0.0742,

wR2=0.1161

R1=0.0687,

wR2=0.1448

R1=0.0717,

wR2=0.1327

Appendices

192

[Cu4(LSQ

)4]0 [Mn(LSQ

COOH

)(LCat

COOH)][HNEt3]

Identification code

Empirical formula

4936

C87.5H109.5 Cl1N4.5O8Cu4

4903

C48H64 Mn N3O6

Formula weight 1655.92 683.96

Temperature 100(2) K 100(2) K

Wavelength (MoKD)0.71073 Å 1.54178 Å

Crystal system Monoclinic Monoclinic

Space group P21/n P21/c, No. 14

Unit cell dimensions a = 16.3153(5) Å, a = 9.6003(6) Å,

b = 29.2224(8) Å, b = 10.2524(6) Å,

c = 17.7048(5) Å, c = 44.910(3) Å,

D = 90oD = 90o

E = 100.281(5)oE = 92.29(1)o

J = 90oJ = 90o

Volume (Å3), Z 8305.6(4), 4 4416.8(5), 4

Density (calc.) Mg/m3 1.324 1.254

Absorption coeff 1.100 mm-1 2.835 mm-1

F(000) 3480 1784

Crystal size (mm) 0.37 x 0.33 x 0.22 0.14 x 0.06 x 0.03

T range for data collect. 2.94 to 30.98 deg 3.94 to 69.16

Index range -23<=h<=23, -11<=h<=8,

-32<=k<=42, -12<=k<=12,

-25<=l<=25 -49<=l<=53

Reflections collected 96664 27457

Independent reflect. 24851 [R(int) = 0.0325] 8029 [R(int) = 0.0460]

Absorption correction Gaussian, face-indexed SADABS, 2004/1

Data/restraints/param. 24851 / 10 / 1025 8029 / 144 / 561

Goodness-of-fit on F2 1.038 1.266

Final R indices [I>2V(I)] R1=0.0505,

wR2=0.1210

R1=0.0872,

wR2=0.1975

R indices (all data) R1=0.0717,

wR2=0.1327

R1=0.0901,

wR2=0.1991

Appendices

193

[Mn(LSQ

COOH

)(LCat

COOH)][HNEt3]

Identification code

Empirical formula

5079

C48H64 Mn N3O6

Formula weight 683.96

Temperature 293(2) K

Wavelength (MoKD)0.71073Å

Crystal system Monoclinic

Space group P21/c, No. 14

Unit cell dimensions a = 9.706(2) Å,

b = 10.323(2) Å,

c = 45.132(6) Å,

D = 90o

E = 92.03(2)o

J = 90o

Volume (Å3), Z 4419.2(14), 4

Density (calc.) Mg/m3 1.226

Absorption coeff 0.342 mm-1

F(000) 1784

Crystal size (mm) 0.09 x 0.05 x 0.03

T range for data collect. 2.91 to 22.50

Index range -10<=h<=10,

-10<=k<=11,

-48<=l<=48

Reflections collected 20414

Independent reflect. 5834 [R(int) = 0.0562]

Absorption correction None

Data/restraints/param. 5834 / 145 / 563

Goodness-of-fit on F2 1.047

Final R indices [I>2V(I)] R1=0.0537,

wR2=0.1254

R indices (all data) R1=0.0773,

wR2=0.1389

Appendices

194

(2) Magnetochemical data

[Cu(L

)]0

MW = 651.5 gm/mol, Ȥdia = -416.60 x 10-6 cm3 mol-1

m = 32.67 mg , H = 1.000 T

No T(K) Ȥ.Texp.Ȥ.Tcalc. µ

exp µ

calc.

1 1.961 0.32519 0.40885 1.61267 1.80826

2 5.008 0.41422 0.42588 1.8201 1.84554

3 9.996 0.42852 0.43127 1.85125 1.85718

4 14.994 0.43541 0.43492 1.86607 1.86502

5 20.004 0.44022 0.43832 1.87636 1.8723

6 30 0.44779 0.44492 1.89241 1.88634

7 40.003 0.45515 0.45146 1.90792 1.90014

8 50.012 0.46139 0.45805 1.92094 1.91382

9 60.021 0.46744 0.46493 1.93349 1.92738

10 70.054 0.47319 0.47258 1.94536 1.94088

11 80.07 0.4796 0.48157 1.95848 1.95426

12 90.077 0.48546 0.49244 1.9704 1.96757

13 100.13 0.49115 0.50569 1.98193 1.98091

14 110.1 0.49664 0.52134 1.99297 1.99419

15 120.12 0.50234 0.5396 2.00438 2.00768

16 130.16 0.50844 0.56028 2.0165 2.02147

17 140.18 0.5152 0.58304 2.02987 2.03564

18 150.19 0.52264 0.60756 2.04446 2.05033

19 160.2 0.53126 0.63352 2.06127 2.06569

20 170.21 0.54035 0.66055 2.07881 2.08183

21 180.22 0.55054 0.68832 2.09834 2.09884

22 190.22 0.56108 0.71652 2.11832 2.11675

23 200.24 0.57237 0.74496 2.13952 2.13565

24 210.23 0.58421 0.77328 2.16154 2.15546

25 220.24 0.59606 0.80146 2.18335 2.17625

26 230.26 0.6087 0.8293 2.20638 2.19795

27 240.25 0.62093 0.85659 2.22843 2.22043

28 250.27 0.63344 0.88341 2.25078 2.24373

29 260.27 0.64661 0.90956 2.27405 2.26766

30 270.24 0.65926 0.93497 2.29618 2.29211

31 280.23 0.67146 0.95975 2.31734 2.31709

32 290.15 0.68506 0.98365 2.34069 2.34228

Appendices

195

[Fe(HL

)Cl]0

MW = 680.0 gm/mol, Ȥdia = -430.0 x 10-6 cm3 mol-1

m = 24.58 mg , H = 1.000 T

No T(K) Ȥ.Texp.Ȥ.Tcalc. µ

exp µ

calc.

1 2,001 1,50058 2,50601 3,46425 4,47683

2 5,106 2,928 2,90361 4,8391 4,81891

3 9,999 3,25025 2,97159 4,87499 4,87499

4 15,001 3,26307 2,98545 4,88635 4,88635

5 20 3,23313 2,99034 4,89035 4,89035

6 29,991 3,17456 2,99385 4,89322 4,89322

7 40,002 3,13172 2,99509 4,89423 4,89423

8 50,005 3,10279 2,99566 4,8947 4,8947

9 60,033 3,08006 2,99598 4,89496 4,89496

10 70,049 3,06439 2,99617 4,89511 4,89511

11 80,076 3,05473 2,9963 4,89522 4,89522

12 90,093 3,04735 2,99638 4,89528 4,89528

13 100,11 3,03948 2,99645 4,89534 4,89534

14 110,12 3,03307 2,9965 4,89538 4,89538

15 120,1 3,02678 2,99654 4,89541 4,89541

16 130,16 3,02069 2,99658 4,89545 4,89545

17 140,18 3,01512 2,99662 4,89548 4,89548

18 150,19 3,01149 2,99667 4,89552 4,89552

19 160,2 3,00535 2,99675 4,90261 4,89559

20 170,21 3,00354 2,99687 4,90113 4,89568

21 180,23 2,99914 2,99704 4,89754 4,89582

22 190,23 2,99606 2,99729 4,89502 4,89603

23 200,23 2,99416 2,99765 4,89347 4,89632

24 210,24 2,99096 2,99812 4,89085 4,8967

25 220,22 2,99022 2,99875 4,89025 4,89722

26 230,25 2,98853 2,99955 4,88887 4,89787

27 240,24 2,98702 3,00055 4,88763 4,89869

28 250,27 2,98684 3,00178 4,88748 4,89969

29 260,24 2,98475 3,00325 4,88577 4,90089

30 270,26 2,98634 3,00499 4,88708 4,90231

31 280,15 2,98637 3,007 4,8871 4,90395

32 290,24 2,98619 3,00935 4,88695 4,90587

Appendices

196

[Cu(L3,5-CF

)2]0

MW = 925.5 gm/mol, Ȥdia = -535.0 x 10-6 cm3 mol-1

m = 50.24 mg , H = 1.000 T

No T(K) Ȥ.Texp.Ȥ.Tcalc. µ

exp µ

calc.

1 2,003 0,30958 0,40083 1,57373 1,79071

2 5,004 0,41022 0,41465 1,81156 1,82132

3 9,999 0,41522 0,41672 1,82257 1,82586

4 15 0,41592 0,41711 1,82412 1,82671

5 20,002 0,41642 0,41724 1,82519 1,82701

6 30 0,41679 0,41734 1,82601 1,82722

7 40,001 0,41751 0,41738 1,82759 1,8273

8 50,007 0,41799 0,41739 1,82864 1,82733

9 60,022 0,41783 0,4174 1,82828 1,82735

10 70,046 0,41845 0,41741 1,82964 1,82736

11 80,068 0,41911 0,41742 1,83109 1,82739

12 90,081 0,4193 0,41745 1,83149 1,82745

13 100,11 0,41983 0,41753 1,83265 1,82763

14 110,09 0,41973 0,41771 1,83243 1,82802

15 120,15 0,42027 0,41805 1,83363 1,82877

16 130,18 0,42031 0,41864 1,83371 1,83006

17 140,18 0,42083 0,41955 1,83485 1,83205

18 150,19 0,42168 0,42087 1,8367 1,83493

19 160,2 0,42333 0,42268 1,84028 1,83886

20 170,21 0,42645 0,42504 1,84705 1,84399

21 180,15 0,42967 0,42798 1,85401 1,85036

22 190,23 0,43431 0,4316 1,86399 1,85818

23 200,24 0,43951 0,43586 1,87513 1,86733

24 210,24 0,44466 0,44078 1,88607 1,87783

25 220,26 0,45118 0,44636 1,89985 1,88968

26 230,25 0,45393 0,45255 1,90563 1,90273

27 240,26 0,46409 0,45934 1,92684 1,91695

28 250,26 0,46176 0,46667 1,92199 1,9322

29 260,26 0,47708 0,47451 1,95362 1,94836

30 270,26 0,48133 0,4828 1,96231 1,96531

31 280,24 0,48817 0,49147 1,9762 1,98287

32 290,26 0,49727 0,50052 1,99453 2,00105

Appendices

197

[Cu(L2-CF

)2]0

MW = 789.5 gm/mol, Ȥdia = -413.0 x 10-6 cm3 mol-1

m = 19.57 mg , H = 1.000 T

No T(K) Ȥ.Texp.Ȥ.Tcalc. µ

exp µ

calc.

1 2 0,08409 0,36165 0,82005 1,70068

2 4,978 0,12467 0,37288 0,99854 1,7269

3 10,053 0,19102 0,37459 1,23601 1,73084

4 15,033 0,23736 0,37489 1,37779 1,73155

5 20,007 0,26834 0,375 1,46495 1,7318

6 30,002 0,30473 0,37508 1,56113 1,73198

7 40 0,32521 0,37511 1,61273 1,73204

8 50,003 0,33907 0,37512 1,64672 1,73207

9 60,036 0,34968 0,37513 1,67229 1,73209

10 70,05 0,35801 0,37514 1,69209 1,73211

11 80,069 0,36464 0,37516 1,70771 1,73216

12 90,054 0,3695 0,37523 1,71905 1,73231

13 100,11 0,37366 0,37539 1,72869 1,73269

14 110,13 0,37633 0,37573 1,73486 1,73348

15 120,15 0,37849 0,37635 1,73982 1,7349

16 130,17 0,38031 0,37734 1,744 1,73719

17 140,18 0,38277 0,37882 1,74963 1,74059

18 150,2 0,38556 0,38088 1,75601 1,74532

19 160,2 0,38909 0,3836 1,76403 1,75154

20 170,21 0,3932 0,38705 1,77331 1,75938

21 180,16 0,39733 0,39122 1,78262 1,76884

22 190,22 0,4025 0,39621 1,79416 1,7801

23 200,23 0,4079 0,40196 1,80615 1,79295

24 210,25 0,41378 0,40845 1,81913 1,80738

25 220,2 0,41994 0,4156 1,83263 1,82313

26 230,24 0,42684 0,42347 1,84762 1,84031

27 240,25 0,43355 0,43191 1,86208 1,85857

28 250,26 0,4405 0,44089 1,87695 1,87778

29 260,25 0,44801 0,45032 1,89289 1,89774

30 270,27 0,45611 0,46016 1,90992 1,91838

31 280,24 0,46455 0,47029 1,9275 1,93937

32 290,24 0,47447 0,4807 1,94798 1,96073

Appendices

198

[Cu(L4-CF

)2]0

MW = 789.5 gm/mol, Ȥdia = -413.0 x 10-6 cm3 mol-1

m = 30.59 mg , H = 1.000 T

No T(K) Ȥ.Texp.Ȥ.Tcalc. µ

exp µ

calc.

1 2,002 0,26705 0,40094 1,46143 1,79068

2 5,013 0,44438 0,41479 1,82521 1,82134

3 9,999 0,51852 0,41685 1,82364 1,82586

4 15 0,48625 0,41723 1,8272 1,82671

5 19,999 0,41631 0,41737 1,82468 1,82701

6 29,995 0,41686 0,41747 1,82588 1,82722

7 39,998 0,41734 0,4175 1,82694 1,82729

8 50,007 0,41724 0,41752 1,82672 1,82733

9 60,035 0,41707 0,41752 1,82635 1,82735

10 70,059 0,41691 0,41753 1,82599 1,82736

11 80,075 0,41711 0,41754 1,82643 1,82738

12 90,099 0,41725 0,41757 1,82675 1,82745

13 100,13 0,41729 0,41766 1,82684 1,82763

14 110,16 0,41734 0,41784 1,82694 1,82803

15 120,17 0,4177 0,41818 1,82774 1,82878

16 130,17 0,41779 0,41877 1,82791 1,83007

17 140,12 0,41836 0,41968 1,82917 1,83205

18 150,2 0,4195 0,42101 1,83167 1,83496

19 160,21 0,42095 0,42282 1,83482 1,8389

20 170,21 0,4233 0,42519 1,83993 1,84404

21 180,22 0,42606 0,42816 1,84593 1,85047

22 190,23 0,42969 0,43177 1,85377 1,85826

23 200,22 0,43407 0,43604 1,8632 1,86741

24 210,25 0,43935 0,44098 1,87449 1,87797

25 220,24 0,44543 0,44655 1,88742 1,88979

26 230,23 0,45221 0,45275 1,90173 1,90286

27 240,25 0,4593 0,45956 1,91659 1,91712

28 250,26 0,46694 0,46691 1,93246 1,9324

29 260,26 0,47509 0,47476 1,94924 1,94858

30 270,25 0,48412 0,48306 1,96768 1,96553

31 280,07 0,49302 0,4916 1,98569 1,98283

32 290,25 0,50252 0,5008 2,00473 2,0013

Appendices

199

[Cu4(LSQ

)4]0

MW = 1550 gm/mol, Ȥdia = -913.0 x 10-6 cm3 mol-1

m = 39.54 mg , H = 1.000 T

No T(K) Ȥ.Texp.Ȥ.Tcalc. µ

exp µ

calc.

1 2 0,05913 0,08 0,68768 0,79988

2 5,04 0,07441 0,08 0,77144 0,79988

3 10,03 0,12091 0,0869 0,98337 0,83366

4 15,04 0,22632 0,148 1,34536 1,08795

5 20 0,35677 0,284 1,68917 1,50709

6 30 0,64184 0,649 2,26565 2,27825

7 40 0,93607 0,9879 2,73611 2,81084

8 50,01 1,21688 1,2722 3,11963 3,18975

9 60,03 1,46795 1,5132 3,42638 3,47878

10 70,04 1,6844 1,7196 3,6703 3,70846

11 80,07 1,87221 1,8975 3,86952 3,89556

12 90,1 2,03224 2,0508 4,0315 4,04987

13 100,11 2,17024 2,1827 4,16613 4,17808

14 110,15 2,29007 2,2973 4,27961 4,28636

15 120,16 2,39428 2,3968 4,37589 4,3782

16 130,16 2,48441 2,4837 4,4575 4,45686

17 140,18 2,56498 2,5602 4,5292 4,52498

18 150,2 2,63544 2,6279 4,59099 4,58441

19 160,2 2,69967 2,688 4,64659 4,63654

20 170,21 2,75503 2,7417 4,69399 4,68262

21 180,21 2,80482 2,79 4,73622 4,72369

22 190,23 2,84947 2,8335 4,77377 4,76037

23 200,22 2,88919 2,8729 4,80693 4,79336

24 210,23 2,92653 2,9088 4,83789 4,82321

25 220,23 2,95708 2,9416 4,86307 4,85033

26 230,26 2,98671 2,9717 4,88738 4,87508

27 240,26 3,01069 2,9993 4,90696 4,89767

28 250,26 3,03252 3,0248 4,92472 4,91844

29 260,25 3,05209 3,0483 4,94058 4,93751

30 270,25 3,06801 3,0702 4,95345 4,95522

31 280,26 3,08345 3,0906 4,9659 4,97165

32 290,24 3,10146 3,1095 4,98038 4,98683

Appendices

200

[Mn3(LSQ

)2(LCat

OH ) (LBQ

OH )Cl]0

MW = 1536.0 gm/mol, Ȥdia = -200.0 x 10-6 cm3 mol-1

m = 36.35 mg , H = 1.000 T

No T(K) Ȥ.Texp.Ȥ.Tcalc. µ

exp µ

calc.

1 2,001 4,757 5,22842 6,16802 6,46643

2 4,996 8,71285 8,70844 8,34756 8,34545

3 10,012 10,1567 10,0795 9,01272 8,9784

4 15,024 10,0934 10,263 8,98459 9,05976

5 20,004 9,96596 10,1385 8,92769 9,00464

6 30,002 9,66814 9,72195 8,79328 8,81772

7 40,001 9,39417 9,35972 8,66779 8,65189

8 50,012 9,16127 9,08581 8,55967 8,52435

9 60,025 8,9609 8,87973 8,46555 8,42712

10 70,051 8,79655 8,72134 8,38756 8,35163

11 80,086 8,67369 8,59676 8,32878 8,29176

12 90,093 8,56999 8,49694 8,27884 8,24348

13 100,08 8,47064 8,4153 8,23071 8,20378

14 110,12 8,39395 8,34694 8,19337 8,17039

15 120,13 8,32024 8,28935 8,15732 8,14216

16 130,16 8,26226 8,23999 8,12884 8,11788

17 140,18 8,20387 8,19735 8,10007 8,09685

18 150,19 8,16035 8,16015 8,07856 8,07846

19 160,2 8,11648 8,1274 8,05681 8,06223

20 170,21 8,07841 8,09835 8,0379 8,04781

21 180,23 8,04513 8,07238 8,02132 8,03489

22 190,15 8,00447 8,04928 8,00103 8,02339

23 200,24 7,98283 8,02806 7,9902 8,01281

24 210,24 7,9556 8,00898 7,97656 8,00328

25 220,24 7,93338 7,99158 7,96542 7,99458

26 230,25 7,91564 7,97565 7,95651 7,98661

27 240,24 7,89489 7,96103 7,94607 7,97929

28 250,28 7,88504 7,9475 7,94111 7,9725

29 260,24 7,86756 7,93508 7,93231 7,96627

30 270,26 7,8495 7,92349 7,9232 7,96045

31 280,26 7,83878 7,91274 7,91778 7,95505

32 290,25 7,8286 7,90272 7,91264 7,95001

Appendices

201

[Mn(LSQ

COOH

)(LCat

COOH )][HNEt3]

MW = 833 gm/mol, Ȥdia = -231.0 x 10-6 cm3 mol-1

m = 24.15 mg , H = 1.000 T

No T(K) Ȥ.Texp.Ȥ.Tcalc. µ

exp µ

calc.

1 1.96 0.31579 0.40892 1.58921 1.80843

2 5.092 0.70218 0.80402 2.36975 2.53579

3 10.164 0.91161 0.93583 2.70012 2.73575

4 15.05 0.94587 0.96288 2.7504 2.77501

5 20.005 0.96049 0.97275 2.77157 2.78921

6 30 0.97332 0.97984 2.79003 2.79934

7 40 0.98113 0.98228 2.8012 2.80284

8 50.011 0.98687 0.98341 2.80937 2.80444

9 60.027 0.98953 0.98401 2.81315 2.8053

10 70.051 0.99128 0.98438 2.81565 2.80582

11 80.046 0.99473 0.98461 2.82054 2.80616

12 90.089 0.99618 0.98477 2.82259 2.80639

13 100.13 0.99748 0.98489 2.82443 2.80655

14 110.14 0.9976 0.98497 2.82461 2.80667

15 120.16 0.99831 0.98504 2.82561 2.80676

16 130.16 0.9974 0.98509 2.82431 2.80684

17 140.18 0.99729 0.98513 2.82417 2.80689

18 150.13 0.99573 0.98516 2.82195 2.80694

19 160.19 0.99591 0.98519 2.82221 2.80697

20 170.22 0.99477 0.98521 2.8206 2.807

21 180.22 0.99376 0.98523 2.81916 2.80703

22 190.22 0.99238 0.98524 2.8172 2.80705

23 200.24 0.99129 0.98525 2.81566 2.80707

24 210.24 0.99064 0.98526 2.81474 2.80709

25 220.25 0.98967 0.98528 2.81336 2.8071

26 230.25 0.98825 0.98528 2.81134 2.80711

27 240.26 0.98674 0.98529 2.80919 2.80712

28 250.23 0.98668 0.9853 2.80911 2.80713

29 260.27 0.98437 0.9853 2.80581 2.80714

30 270.26 0.98286 0.98531 2.80365 2.80715

31 280.15 0.98176 0.98531 2.80209 2.80716

32 290.25 0.98357 0.98532 2.80468 2.80716