The Coordination Chemistry of Redox Noninnocent
o-Aminophenol and Dithiolene Ligands with
Transition Metal Ions
Dissertation for the degree of
Doktor der Naturwissenschaft
in the Fakultät für Naturwissenschaften
(Department Chemie)
at the Universität Paderborn
presented by
Swarnalatha Kokatam
Mülheim an der Ruhr 2006
i
ii
Asato ma sat gamaya
Tamaso ma jyotir gamaya
Mrityor ma amritam gamaya.
“ ……. Miles to go before I sleep.”
Robert Frost
iii
iv
This work was carried out between August 2003 and February 2006 at the Max-
Planck-Institut für Bioanorganische Chemie, Mülheim an der Ruhr, Germany.
Papers published:
1. Molecular and Electronic Structure of Four- and Five- Coordinate Cobalt
Complexes Containing Two o-Phenylenediamine- or two o-Aminophenol Type
Ligands at Various Oxidation Levels: An Experimental, Density Functional, and
Correlated ab initio Study.
E. Bill, E. Bothe, P. Chaudhuri, K. Chlopek, D. Herebian, S. Kokatam, K. Ray, T.
Weyhermüller, F. Neese, K. Wieghardt.
Chem. Eur. J. 2005, 11, 204
2. Structural Characterization of Four Members of the Electron-Transfer Series
[PdII(L)2)2]n (L= o-Iminophenolate Derivative; n = 2-, 1-, 0, 1+, 2+). Ligand Mixed
Valency in the Monocation and Monoanion with S = 1/2 Ground States.
Swarnalatha Kokatam, Thomas Weyhermüller, Eberhard Bothe, Phalguni Chaudhuri,
Karl Wieghardt.
Inorg. Chem. 2005, 44, 3709
3. Molecular and Electronic Structure of Square Planar Complexes:
[PdII(1LISQ)(tertbpy)](PF6), [PdII(1LIP)(tertbpy)], and [PdII(1LIBQ)(tertbpy)](PF6)(BF4) •
CH2Cl2 : An o-Iminophenol based Ligand Centered Three-membered Redox Series.
Swarnalatha Kokatam, Phalguni Chaudhuri, Thomas Weyhermüller, Karl Wieghardt.
In preparation
Examination Committee:
Prof. Dr. W. Bremser
Prof. Dr. K. Krohn
Prof. Dr. G. Henkel
Prof. Dr. P. Chaudhuri
Examination: 21.04.06
v
ACKNOWLEDGEMENT
The extensive discussions, effective supervision, and helpful suggestions
provided by the competent staff of the Max Planck Institute for Bioinorganic
Chemistry, Mülheim, have led my research work to bear fruits. I express my sincere
gratitude to the research group of Prof. Karl Wieghardt in general and to the following
in particular.
Prof. Dr. P. Chaudhuri, for not only giving me a chance to work under his able
guidance, but also for sparing his time and sharing his ideas with me from time to
time. His vast experience and deep knowledge in the field of magnetic chemistry has
left clear impression on this work.
Prof. Dr. K. Wieghardt, for his continuous guidance and constant encouragement
throughout this project. I reiterate my debt towards him for letting me use his well
equipped laboratories. His magnetic personality, fascinating ideas, and attitude "to
know more and to know all" will certainly have substantial impact on my subsequent
academic career.
Dr. T. Weyhermüller, for introducing me to the arena of crystal structure analysis.
His untiring help, both technical and fundamental, has resulted a tyro solving crystal
structures on her own! Mrs. H. Schucht, for patience to pick up the best crystal from
a bunch all the time!!
Dr. E. Bill, for teaching us basic magneto chemistry, EPR, SQUID, and Mössbauer
spectroscopies. His help in simulation of experimental data and his ever-standing
cheerfulness will remain unforgettable.
Dr. E. Bothe and Mrs. P. Höfer, for their help in electrochemical measurements.
Dr. J. F. Berry, for sparing his time to explain about the basics of X-ray
crystallography and for many valuable suggestions regarding the project.
Dr. L. P. Larsen, for careful revision of the manuscript.
vi
Dr. A. Patra and Dr. K. Ray, for their contributions to my work.
Dr. N. Aliaga-Alcade, Dr. S. Blanchard, Dr. K. Merz, Dr. L. Singerian, Dr. Y.
Song, Dr. I. Silvestre, Dr. K. Chlopek, Dr. S. Mukherjee, Dr. S. Khanra, Dr. T.
Petrenko, Dr. N. Muresan, Dr. R. Kapre, Mr. F. Benedito, Mr. S. Presow, Mr.
C. Mukherjee, Mr. B. Pluijmaekers, Mr. N. Roy, and Mr. V. Bisvas, for a sound
academic and friendly life inside the laboratory.
Mr. U. Pieper and Ms. R. Wagner, for their helpful hand in the laboratory.
Mr. F. Reikowski, Mr. A. Göbels, Mrs. U. Westhoff, and Mr. J. Bitter, for their
measurements of EPR, SQUID, GC and NMR.
Family Seayad, Family Göhl, Family Basak, Juli, Aruna, Hemanth, Michael,
Ingrid, Almuth, Christa, Murthy, Srinivas, Malleswari, Meghanath, and Raji for
enabling me to have a healthy social life half a globe away from my home.
My parents, my brother, and my sister, for their providential care.
Mishra, whose contribution gets underestimated, if acknowledged through words.
Finally to the Max-Planck- Gesellschaft (MPG) for financial support.
vii
CONTENTS
Chapter 1 Introduction
1.1. General introduction
1.2. Objective of the work
1.3. Legends used in this work
1.4. Complexes synthesized in this work
1.5. References
1
3
5
9
10
11
Chapter 2 Molecular and Electronic Structure of Square Planar
Complexes: [PdII(1LISQ)(tertbpy)](PF6), [PdII(1LIP)(tertbpy)], and
[PdII(1LIBQ)(tertbpy)](PF6)(BF4) • CH2Cl2 : An o-Iminophenol
based Ligand Centered Three-membered Redox Series
2.1 Introduction
2.2. Syntheses and X-ray crystal structures
2.3. Electro- and spectroelectrochemistry
2.4. Magnetic properties
2.5 Conclusions
2.6. References
15
17
19
24
29
31
32
Chapter 3 Structural Characterization of Square Planar Co, Ni, and Pd
Complexes with o-Aminophenol Type of Ligands in
Various Oxidation Levels
3.1. Introduction
3.2. Co complexes
3.2.1. Syntheses and X-ray crystal structures
3.2.2. Electro- and spectroelectrochemistry
3.2.3. Magnetic properties
3.3. Ni complexes
3.3.1. Syntheses and X-ray crystal structures
3.3.2. Electro- and spectroelectrochemistry
3.3.3. Magnetic properties
3.4. Pd complexes
3.4.1. Syntheses and X-ray crystal structures
3.4.2. Electro- and spectroelectrochemistry
3.4.3. Magnetic properties
35
37
43
43
48
51
53
53
57
60
63
63
70
74
viii
3.5. Discussion and conclusions
3.6. References
78
82
Chapter 4 Synthesis and Characterization of Octahedral Oxo-Mo and Oxo-
W Complexes with o-Aminophenol Type of Ligands
4.1 Introduction
4.2. Mo complexes
4.2.1. Results and Discussion
4.3. W complexes
4.3.1. Results and Discussion
4.4 Conclusions
4.5. References
85
87
89
89
94
94
100
101
Chapter 5 Square Planar Gold Dithiolene Complexes with cis-1,2–
disubstitutedethylene–1,2 dithiolato Ligands: A Combined
Experimental and Theoretical Study
5.1 Introduction
5.2. Syntheses and X-ray crystal structures
5.3. Electro- and spectroelectrochemistry
5.4. Magnetic properties
5.5 Infrared spectra
5.6. Calculations
5.7. Conclusions
5.8. References
103
105
106
109
112
115
116
126
127
Chapter 6 Summary 131
Chapter 7 Equipment and Experimental work
7.1. Methods and Equipment
7.2. Ligand syntheses
7.3. Complex syntheses
7.4. References
137
139
143
146
164
Chapter 8 Appendices
1. Crystallographic data
2. Magnetochemical data
3. Curriculum Vitae
165
167
177
185
ix
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 (293K)
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
nm : nanometer (10-9 m)
s : second
x
T : tesla
V : volts
μB : bohr magnetron
Symbols:
λ : wavelength (nm)
ε : extinction coefficient (M-1cm-1)
μeff : magnetic moment (μB)
Solvents and reagents:
AgBF4: sivertetraflouroborate
AgClO4: siverperchlorate
AgPF6: siver hexafluorophosphate
TBABr : tetrabutylammonium bromide
TBAPF6 : tetrabutylammonium hexafluorophosphate, supporting electrolyte
Cat.: catechol
CH3NO2 : nitromethane
CH2Cl2 : dichloromethane
CHCl3 : chloroform
CCl4 : carbontetrachloride
CoCp2 : cobaltocene
Et2O : diethylether
Et3N : triethylamine
EtOH : ethanol
FcPF6 : ferroceniumhexafluorophosphate
HCl : hydrogen chloride
KBr : potassium bromide
MeOH : methanol
MeCN : acetonitrile
NOBF4 : nitocenium tetrafluorophosphate
NaOMe : sodium methoxide
THF : tetrahydrofuran
xi
Techniques:
CV : cyclic voltammetry
EA : elemental analysis
EI : electron ionization
EPR : electron paramagnetic resonance
ESI : electrospray ionization
IR : infrared spectroscopy
MS: mass spectroscopy
SQUID : superconducting quantum interface device
SWV : square wave voltammetry
UV-Vis : ultraviolet-visible spectroscopy
Latin expressions:
ca. : around
et al. : and coworkers
e.g. : for example
i.e. : namely
tert- : tertiary
vs. : versus, against
xii
Introduction
1
2
Chapter 1
1.1. General Introduction:
The realization of the widespread occurrence of radicals in enzyme catalysis
has triggered considerable interest and research activity into metal-radical
interactions.1, 2 Additionally, investigations relating to organic radicals bound to metal
ions are relevant to the field of “molecular magnets”.3, 4 In this way, the coordination
chemistry of o-aminophenolates,5-9 o-diaminophenolates,10 o-catecholates11, and o-
benzenedithiolates12-14 has been well established and understood in recent years.
They constitute an archetypal class of redox-active, noninnocent ligands. The term
‘noninnocent’ is widely used to emphasize the idea that these ligands do not
necessarily possess a closed-shell electron configuration when coordinated to a metal
ion. They can coordinate to a central metal ion in distinctly different oxidation and
protonation levels, as single or doubly deprotonated forms and can be oxidized to the
monoanionic radical form and finally to a neutral quinone form. Consequently, a large
number of paramagnetic transition metal complexes have been synthesized,5-14 where
there are interactions of radicals with metal centres observed.
Formal and spectroscopic oxidation number (state) is a very important concept
here. The formal oxidation number (state) of a given metal ion in a mononuclear
coordination compound is a nonmeasurable integer, commonly defined as “the charge
left on the metal after all ligands have been removed in their normal, closed-shell
configuration that is with their electron pair”.15
In contrast, it is accepted practice that referring to e.g. an iron(III) complex
implies that this compound contains an iron ion with a d5 high-, intermediate-, or low-
spin electron configuration. Since n for a dn electronic configuration is, at least, in
principle, a measurable quantity (by various spectroscopies), Jörgensen has
suggested16 that an oxidation number which is derived from a known dn configuration
should be specified as spectroscopic (or physical) oxidation number (state).
In many cases formal and spectroscopic oxidation numbers are identical as
exemplified for [Co(NH3)6]3+, where the low-spin d6 cobalt ion possesses a +III
oxidation state, both formally and physically. This is not necessarily always the case.
Discrepancies arise when organic radicals with an open-shell electron configuration
are coordinated to a transition metal ion. For example, consider an O-coordinated
phenoxyl radical complex of an iron ion with a d5 configuration. According to the
above definition the formal oxidation number of the iron ion should be +IV when a
closed-shell phenolato anion is removed. On the other hand, Mössbauer and resonance
3
Chapter 1
Raman spectroscopies unequivocally prove the presence of a high-spin d5 electron
configuration at the metal ion and a phenoxyl ligand, respectively.26 Thus, the iron ion
has a physical oxidation number of +III, and clearly, an Fe(IV)-phenolato complex
has a distinctly different electronic structure than an Fe(III)-phenoxyl species.
Since the term physical (or spectroscopic) oxidation state is not accepted by the
community, both formal and physical oxidation numbers are often used as synonyms
which they are not. Unfortunately, in some areas of coordination chemistry this
practice leads to considerable confusion. In contrast, the terms innocent and
noninnocent ligands are widely used to emphasize the fact that some ligands do not
necessarily possess a closed-shell configuration. These terms can only be used
meaningfully in conjugation with the physical oxidation state of the metal ion. This
means these ligands can exist in different oxidation levels as shown in scheme 1.1.
Small but significant structural differences in the coordinated ligands (forms A, B and
C) are clearly detectable by high-quality, single crystal X-ray crystallography where
the experimental error of a given C-X, C-Y or C-C bond length should not exceed
+0.015 Å (3σ), and supplies evidence of these different oxidation levels.
Mn+
X
Y
X
Y
M(n-1)+
X
Y
M(n-2)+
ABC
where X, Y = N, N; N, S; S, S; N,O; O,O
Scheme 1.1.
A large number of transition metal complexes with one-, two- or three
coordinated noninnocent o-aminophenolates,5-9 o-diaminophenolates,10 o-
catecholates,11 and o-aminothiophenolates17-19 have been synthesized recently and
subjected to detailed characterization. In many cases, the complexes studied or closely
related complexes have been known since the 1960s. However, it is only recently
through a combination of theory and experiment, that the properties of the complexes
are well understood.19-22
On the other hand, ambiguities arise in many complexes with noninnocent
ligands regarding metal and ligand oxidation states. A few of the best examples
include: A number of low-quality crystal structures of [AuIII(LSS)2]- monoanion, have
4
Chapter 1
been reported,23 but these do not provide an unambiguous assignment of two closed-
shell o-dithiolate(2-) dianions coordinated to Au(III) ion. Interestingly, the crystal
structure of one-electron oxidized species is published as [AuIV(LSS)2]!24 This
assignment has been proven wrong recently, in an exactly analogues complex,
[AuIII(BuLSS)2],13 and reported as Au(III) containing complex with one open-shell
o-dithiobenzosemiquinonte(1-) radical ligand and one closed-shell o-dithiolate(2-)
dianionic ligand confirmed by AuIII Mössbauer, UV-vis, and infrared spectroscopies,
though an X-ray crystal structure is lacking. Additionally errors have been made by
assigning iron(IV)12c-d and iron(V)12e complexes synthesized by Sellmann et al. which
are in fact iron(II) or iron(III) complexes with coordinated radical ligands.14c, 19, 20b
Thus, proper determination of the electronic structure of transition metal
complexes with noninnocent ligands demands identification of structural and
spectroscopic features which allow for correct assignment of oxidation states to metal
ions and ligands.
1.2. Objective of the Work:
It is now well established5-9 that O,N-coordinated o-aminophenolate ligands
are noninnocent in the sense that they can be bound to transition metal ion either as an
o-aminophenolate monoanion (LAP)1-, or o-iminophenolate dianion (LIP)2-, or
o-iminobenzosemiquinonate π radical monoanion (LISQ)•1-, or as an
o-iminobenzoquinone (LIBQ)0. Though complexes with o-iminobenzoquinone ligands
have been investigated electrochemically20, 25a in solution, no square planar complex
containing an o-iminobenzoquinone ligand has been chemically isolated to date. Only
one octahedral complex with one o-iminobenzoquinone ligand has been reported7 to
the date. All these forms of the ligand are characteristic of their C-O, C-N, and C-C
bond distances. Thus, X-ray crystallography performed at cryogenic temperatures is
very helpful to distinguish these structural differences at different oxidation levels of
the ligand. Scheme 1.2. shows the significant bond distances for different oxidations
states of the o-aminophenolate ligand.
5
Chapter 1
O -
NH
[LAP ]1-
1.46
1.35
1.39
1.41
1.39
1.40
1.39
1.38
O -
N -
[LIP ]2-
1.35
1.37
1.39
1.41
1.40
1.39
1.40
1.42
-H+
+ H+
-e
-
+ e-
-e-
+ e
-
N
O -
[LISQ ]1-
1.30
1.35
1.46
1.43
1.43
1.43
1.38
1.37
•
•
O
N
[LIBQ]0
1.24
1.30
1.52
1.46
1.46
1.36
1.36
1.44
Scheme 1.2.
Important feature to mark on going from the N,O-coordinated (LIP)2- dianion to
the (LISQ)1-y monoanionic π radical, and then to the neutral quinone (LIBQ): a)
Decrease in the C-N bond lengths from 1.37 + 0.01 Å to 1.35 + 0.01 Å and, finally to
1.30 + 0.01 Å with increasing oxidation level. b) Similarly, decrease in the C-O bond
lengths decrease from 1.35 + 0.01 Å to 1.30 + 0.01 Å to 1.24 + 0.01 Å with increasing
oxidation level. c) Finally, the six C-C bonds of the aminophenolate six-membered
ring of (LIP)2- are nearly equidistant at 1.407 + 0.01 Å indicating the aromatic
character of the phenyl ring. One-electron oxidation to (LISQ)1-y results in two
alternating short C-C bonds at 1.375 + 0.01 Å of partially double bond character and
four longer bonds at 1.438 + 0.01 Å. This characteristic distortion is labeled "quinoid-
like". In the neutral quinone form (LIBQ), this distortion is more pronounced with two
alternating short C=C double bonds at 1.36 + 0.01 Å and four long C-C single bonds
one of which at 1.52 + 0.01 Å being a normal C-C single bond.
In this work, we aimed to structurally characterize o-aminophenolate ligands
in all the possible oxidation states. A series of square planar Pd complexes,
interrelated by one-electron transfer, are synthesized with singly N,O-coordinated
6
Chapter 1
o-aminophenolate ligand analogues and used to characterize the ligand in different
oxidation states. N,N- coordinated tertbpy was used as a second ligand in these
complexes. Syntheses and results from these complexes are discussed in Chapter 2.
As pointed out previously,12, 23, 24 ambiguities arise also in cobalt complexes
with noninnocent o-phenylenediamine ligands. The square planar, neutral,
mononuclear, paramagnetic complex, [Co{C6H4(NH)2}2] possessing an S = ½ ground
state has been reported.25 It has been suggested that both organic ligands are identical.
Their oxidation level was assigned as monoanionic π radicals (Srad = ½) of the
diiminobenzosemiquinonate(1-) type. The central cobalt ion possesses then a +II
oxidation state (low spin d7, SCo = ½). In this model the spins of the ligand π radicals
are assumed to be strongly intramolecularly antiferromagnetically coupled.25c The
question arises, is it really a CoII complex with two radicals? Because, a cobalt(II) ion
(d7, SCo = ½) coordinated to two radicals, where the two are coupled strongly
antiferromagnetically or a cobalt(III) ion (d6, SCo = 1) coordinated to one radical and
one dianionic ligand, where the radical is coupling strongly antiferromagnetically to
one of the electron of cobalt(III) ion; both options yield an St = ½ electronic state by
leaving the electron in the same d orbital of the central cobalt ion (Scheme 1.3.). In
order to clarify this ambiguity, we have decided to resynthesize and characterize the
analogous cobalt complexes with bulky noninnocent N,O-coordinate o-aminophenol
ligands and to assign a correct oxidation number to the metal ion. Chapter 3 discusses
these results.
CoIII
LIP LISQ
St= 1/2
CoII
LISQ LISQ
St= 1/2
Scheme 1.3.
Transition metal complexes of group 10 metal ions and noninnocent ligands
are known to form an electron-transfer series of five bis(chelate) metal complexes of
[M(L)2]n type exists where n = -2, -1, 0, +1, +2. These species are interrelated by one-
electron-transfer steps; L represents a redox-noninnocent derivative of o-
phenylenediamide,20, 25a [C6H4(N)2]2-,1-,0, or of o-iminophenolate, [C6H4O(N)]2-,1-,0, and
MII is a divalent metal ion with a d8 electron configuration as in NiII, PdII, and PtII.5,6
7
Chapter 1
We have attempted to isolate all the members of this electron transfer series. In
this regard, a series of Ni and Pd square planar complexes were synthesized with
noninnocent N,O-coordinating o-aminophenolate ligand analogue to understand the
electronic structures of the complexes in different oxidation states. Chapter 3 also
discusses the Ni and Pd square planar complexes.
Some octahedral Mo-oxo and W-oxo complexes with noninnocent o-
aminophenolate ligands have been synthesized and are discussed in Chapter 4.
Similarly, cis-1,2–disubstituted ethylene–1,2 dithiolato ligands also belong to
the class of noninnocent ligands. The coordination chemistry of these ligands is
similar to that of o-benzenedithiolates12-14 ligands and known since the 1960s. The
most characteristic feature of transition metal dithiolene complexes derived for d1-9
metal ions is the existence of an electron-transfer series whose members are
interrelated by reversible one electron steps27-29 Initial developments in the dithiolene
field, which have been summarized by McCleverty,27 led to the recognition and
experimental realization of the planar three member series shown below.
Many authors12c, 12f have attempted to discern between a dithiolato(2-) and its
monoanionic radical form in a given complex by their crystallographically determined
structural parameters. An increase in the ethylene C=C bond and a decrease in the C-S
bond was observed upon oxidation of a dianionic thiolate to a monoanionic radical.
This behaviour has been firmly established for the corresponding complexes
containing o-benzenedithiolates.12-14
Although bis(dithiolene) complexes of group 10 and 11 metals ions tend to
adopt a square planar geometry, favourable for extended π-π interactions and electron
delocalization in stacked structures, in some cases, e.g., for Fe and Co, other
structures and different metal coordination environments are possible. All known FeIII
bis(dithiolene) complexes are dimeric with a square-pyramidal coordination
geometry.27a, 30, 31 For CoIII, in addition to this dimeric structure, examples of
trimeric32 as well as polymeric structures are also known.33
During the last years single-crystal EPR studies were performed on the
dithiolene chelates [Ni(mnt)2]-,34 [Rh(mnt)2]2-,34a [Au(mnt)2]2-,35 (mnt =
maleonitriledithiolate) Single-crystal EPR spectra for [Pd(mnt)2]- and [Pt(mnt)2]- are
also reported.36, 37 Fascinating magnetic properties and interesting bonding properties
8
Chapter 1
of these square planar complexes have prompted us to do this study on Au complexes
with cis-1,2–disubstituted ethylene–1,2 dithiolate ligands. Chapter 5 discusses square
planar Au complexes with S,S-coordinating cis-1,2–disubstitutedethylene–1,2
dithiolate ligands. DFT calculations were done on these complexes to understand the
correct electronic structure of the complexes.
1.3. Ligands Used in this Work:
Three different ligands are used in this work. Synthetic procedures and
characterizations of the ligands are discussed in Chapter 7: Equipment and
Experimental Work.
1. 2-(2-trifluoromethyl)anilino-4,6-di-tert-butylphenol (1LH2)
2. 2-(4-fluoro)anilino-4,6-di-tert-butylphenol (2LH2)
3. cis-4,4´-di-tert-butylphenylethylene-1,2-dithiolene (3LH2)
OH
NH
CF3
[1LH2 ]
OH
NH
[2LH2 ]
F
SH
SH
[3LH2 ]
9
Chapter 1
1.4. Complexes Synthesized in this Work:
1. [Pd(1LISQ)(tertbpy)](PF6) (1a)
2. [Pd(1LIP)(tertbpy)] (1b)
3. [Pd(1LIBQ)(tertbpy)](PF6)(BF4) • CH2Cl2 (1c)
4. [Co(1L)2] (2a)
5. [Co(1L)2]- [Co(Cp)2]+ (2b)
6. [NiII(1LISQ)2] (3a)
7. [NiII(1LISQ)(1LIP)]-[CoCp2]+ (3b)
8. [NiII(1LIBQ)2(ClO4)2] • 2CH2Cl2 (3d)
9. [PdII(1LISQ)2] (4a)
10. [PdII(1LISQ)(1LIP)]- [CoCp2]+ (4b)
11. [PdII(1LISQ)(1LIBQ)]+(BF4)- (4c)
12. [PdII(1LIBQ)2]3(BF4)4{(BF4)2H}2 • 4 CH2Cl2 (4d)
13. [Mo(2LIP)(2LAP)](O)(OCH3)] • 2 MeOH (5)
14. [(2LIP)(2LAP)(O)Mo-(μ-O)-Mo(O)(2LIP)(2LAP)] (6)
15. [W(2LIP)(2LAP)](O)(Cl)] (7a)
16. {W(2LIP)(2LAP)](O)(OCH3)}2 • 0.5 MeOH (7b)
17. [Au(3L)2][N-(n-Bu)4] (8)
18. [Au (3L)2] (8i)
All the complexes are structurally characterized and studied by IR-, Mass-,
UV-vis spectroscopic techniques. Electrochemical measurements were performed to
understand the redox properties of the complexes. Spectroelectrochemical
measurements were performed to record the spectra of the electrochemically
generated reduced and oxidized species. All the paramagnetic complexes are well
studied by using SQUID and EPR techniques to understand the electronic structures
of the complexes.
10
Chapter 1
1.5. References:
(1) (a) Stubbe, J. Annu. Rev. BioChem. 1989, 58, 257. (b) Frey. P. A. Chem. Rev.
1990, 90, 1343. (c) Metalloenzymes involving amino acid-residue and related
radicals, ed Siegel. H and Siegel. A, Marcel Dekker, New Yoek, 1994, vol
30. (d) Pedersen J. Z.; Finazzi-Argo, A. FEBS Lett. 1993, 325, 53. (f) Prince,
R. C. Trends Biochem. Sci. 1988, 13, 152.
(2) (a) Babcock, G. T.; Espe, M.; Hoganson, C.; Lydakis-Simantiris, N.;
McCracken, J.; Shi, W.; Styring, S.; Thommos, C; Warncke, K. Acta Chem.
Scand. 1997, 51, 533. (b) Fontecave, M.; Pierre, J. L. Bull. Soc. Chim. Fr.
1996, 133, 653. (c) Goldberg, D.P.; Lippard, S. J.; in Mechanistic
Bioinorganic Chemistry, ed. Holden Thorp, H. and Pecoraro, V. L. Advances
in Chemistry, Series 246, American Chemical Society, Washington DC 1995,
p. 61. (d) Stubbe, J.; van der Donk, W. A. Chem. Rev. 1998, 98, 705. (f)
Holm, R. H.; Solomon, E. I.; Guest editors, Chem. Rev. 1996, 96, No. 7.
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Palacio, F.; Kluwer, Dordrecht, 1991.
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(6) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.; Weyhermüller, T.;
Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 2213.
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Chaudhuri, P. Chem. Commum. 1999, 1747.
(10) (a) Carugo, O.; Djinovic’, K.; Rizzi, M.; Castellani, C. B. J. Chem. Soc.
Dalton Trans. 1991, 1551. (b) Mederos, A.; Dominguez, S.; Hernandez-
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Herebian, D.; Bothe, E.; Neese, F.; Weyhermüller, T.; Wieghardt, K. J. Am.
Chem. Soc. 2003, 125, 9116. (c) Herebian, D.; Wieghardt, K.; Neese, F. J.
Am. Chem. Soc. 2003, 125, 10997.
11
Chapter 1
(12) (a) Sellmann, D.; Binder, H.; Häussinger, D.; Heinemann, F. W.; Sutter, J.
Inorg. Chem Acta. 2000, 300-302, 829. (b) Mrkvova, K.; Kameni, J.; Sindela,
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Inorg. Chem. 2003, 42, 4082.
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44, 5345. (b) Ray, K.; Begum, A.; Weyhermüller, T.; Piligkos, S.; Van
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(15) Hegedus, L. S. In Transition Metals in the Synthesis of Complex Organic
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(17) Herebian, D.; Ghosh, P.; Chun, H.; Bothe, E.; Weyhermüller, T.; Wieghardt,
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J. Am. Chem. Soc. 2003, 125, 10997.
(21) (a) Weber, J.; Daul, C.; von Zelewsky, A.; Goursot, A.; Penigault, E. Chem.
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Chapter 1
(22) Bill, E.; Bothe, E.; Chaudhuri, P.; Chlopek, K.; Herebian, D.; Kokatam, S.;
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Chem. Eur. J. 1999, 5, 2554.
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13
Chapter 1
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1157.
14
Molecular and Electronic Structure of Square Planar
Complexes: [PdII(1LISQ)(tertbpy)](PF6), [PdII(1LIP)(tertbpy)],
and [PdII(1LIBQ)(tertbpy)](PF6)(BF4) • CH2Cl2 : An
o-Iminophenol based Ligand Centered Three-membered
Redox Series
15
16
Chapter 2
2.1. Introduction:
The subject of transition metal ions coordinated to organic radicals, the
interactions between them, and the electronic structure elucidations with the help of
extensive ab initio and density functional calculations currently received much
attention.1-3 Recognition of the existence of such systems in the active sites of
metalloproteins2 drives much effort in this direction of research. A single Cu (II) ion
coordinated to a modified tyrosyl-radical3 in galactose oxidase is a well understood
example of such systems. However, it is very important to have a clear understanding
of the bonding and physical properties of such complexes if one desires to understand
their reactivities. Consequently, a large number of transition metal complexes with
one, two, or three coordinating radicals have been synthesized and well characterized
in recent years.4-18
It is well established10-19 that o-aminophenols are redox noninnocent, and bind
to transition metal ions in four forms: 1) paramagnetic o-iminobenzosemiquinonate
(1-) monoanionic radical ((LISQ)1-•; Srad = ½), 2) diamagnetic monoanionic
o-aminophenolate (LAP)1-, 3) dianionic o-iminophenolate (LIP)2-, and 4) neutral
o-iminobenzoquinone (LIBQ)0 forms. High-resolution, low-temperature X-ray
crystallography allows the assignment of protonation and oxidation level of the O,N-
coordinated ligand in a given complex. Scheme 2.1. shows the average bond distances
in the (LISQ)1-·, (LAP)1-, (LIP)2-, and (LIBQ)0 forms of the ligands in transition metal
complexes. Many complexes of Ni(II), Pd(II), Cu(II), Cr(III), and Co(III) containing
o-aminophenol ligands in different oxidation states, namely o-iminophenolates and
open-shell benzosemiquinonate π radical analogues, have been isolated.11, 14, 16, 18, 19
Complexes containing the closed-shell quinone type ligands like [MII(LIBQ)(bpy)]2+,
[MII(LIBQ)2]2+ have been proposed in many square planar complexes depending on
spectroelectrochemical results,5c, 6b, 7, 11, 16, 18, 21 however, only one crystal structure
[PdII(LIBQ)2]2+ containing two neutral closed-shell o-iminobenzoquinone (LIBQ) has
been reported recently.19 In fact, no crystal structure with a singly N,O-coordinated o-
iminobenzoquinone has been reported to date.
Our main aim is to characterize structurally as well as spectroscopically some
singly N,O-coordinated noninnocent o-aminophenol ligands, specifically, the 2-(2-
trifluoromethyl)anilino-4,6-di-tert-butylphenol ligand, which are bound to transition
metal ions in all possible oxidation states. We have synthesized square planar
complexes of divalent Pd having d8 electronic configuration containing one
17
Chapter 2
noninnocent N,O-coordinated 2-(2-trifluoromethyl)anilino-4,6-di-tert-butylphenol
ligand and one N,N-coordinated 4,4´-di-tert-butyl-2,2´-dipyridyl ligand. 4,4´-Di-tert-
butyl-2,2´-dipyridyl was used instead of simple bipyridyl (bpy) to prevent stacking of
molecules in the solid state.
O -
NH
CF 3
[1LAP ]1-
1.46
1.35
1.39
1.41
1.39
1.40
1.39
1.38
O -
N-
CF3
[1LIP ]2-
1.35
1.37
1.39
1.41
1.40
1.39
1.40
1.42
N
CF3
O -
[1LISQ ]1-
•
1.30
1.35
1.46
1.43
1.43
1.43
1.38
1.37
•
O
N
CF 3
[1LIBQ]0
1.24
1.30
1.52
1.46
1.46
1.36
1.36
1.44
-H+
+ H+
-e
-
+ e-
-e
-
+ e-
Scheme 2.1. Average bond distances in (1LISQ)1-·, (1LAP)1-, (1LIP)2- and (1LIBQ)0 forms of the 2-(2-
trifluoromethyl)anilino-4,6-di-tert-butylphenol ligand.
The synthesis, structural, and spectroscopic characterization of the
paramagnetic monocation [Pd(1LISQ)(tertbpy)](PF6) (1a), the diamagnetic neutral
species [Pd(1LIP)(tertbpy)] (1b), and the dicationic [Pd(1LIBQ)(tertbpy)](PF6)(BF4)
• CH2Cl2 (1c) complexes which are the members of the same electron transfer series,
are presented. Complexes 1a, 1b, and 1c are ideally suited to establish the structural
and spectroscopic features of the single O,N-coordinated monoanionic o-
iminobenzosemiquinonate(LISQ)1-· π radical, dianionic o-iminophenolate (LIP)2-, and
neutral o-iminobenzoquinone (LIBQ)0 in a coordination compound. Structures of 1a,
1b, and 1c give the geometrical features of an O,N-coordinated monoanionic π radical
(LISQ)1- versus its one-electron reduced dianion (LIP)2-, and its one-electron oxidized
neutral quinone (LIBQ)0. This allows one to discern structurally between the three
forms of A, B, and C (see below). Spectroscopic characterization of 1a, 1b, and 1c
18
Chapter 2
gives further the supporting information to distinguish spectroscopically the three
forms A, B, and C. Similar neutral and monocationic complexes of Pd with a single
N,O-coordinated o-aminophenolate and a N,N-coordinated bpy were reported
previously,16 but square planar, dicationic, singly N,O-coordinated 1c type complexes
with the quinone form of ligand were not structurally characterized before, though
they were proposed on spectroscopic data previously.16
O
N
Ph
N
O -
O -
N-
Ph Ph
[LISQ]1- [LIBQ]0
[LIP ]2-
or or
A
BC
Results and Discussion:
2.2. Syntheses and X-ray Crystal Structures:
Reflux of a solution of triethylamine, equivalent amounts of PdCl2, 1LH2, and
4,4´-Di-tert-butyl-2,2´-dipyridyl in MeOH for 1 h under Ar gives a green coloured
solution. Addition of excess of KPF6 followed by stirring at room temperature, in air,
for 3 h yields a reddish brown solution. On the slow evaporation of the solvent, red
coloured crystals of paramagnetic [Pd(1LISQ)(tertbpy)](PF6) (1a) are obtained in good
yield (53%). 1a can be reversibly oxidized as well as reduced by one-electron,
respectively, to give diamagnetic species. As these species were stable for many hours
on the coulometric time scale, attempts to oxidize and reduce 1a by using [Cp2Co]
and [NO]BF4 as one-electron reductant and oxidants, respectively, were made. One-
electron chemical reduction of 1a with one equivalent of cobaltocene, [Cp2Co], in dry
and degassed CH2Cl2 solution under an Ar blanketing atmosphere, gives a dark blue
coloured neutral diamagnetic [Pd(1LIP)(tertbpy)] (1b) species in excellent yield (92%).
One-electron chemical oxidation of 1a with one equivalent of [NO]BF4 in dry CH2Cl2
solution under Ar gives once again a diamagnetic brown coloured complex,
[Pd(1LIBQ)(tertbpy)](PF6)(BF4) • CH2Cl2 (1c) in good yield (56%).
19
Chapter 2
The crystal structures of 1a, 1b, and 1c have been determined at 100(2) K by
using Mo Kα radiation. Table 2.1. shows the selected bond lengths in complexes 1a,
1b, and 1c and Figure 2.1. (a-c) show the thermal ellipsoid diagrams of 1a, 1b, and
1c, respectively. These structures are shown without solvent molecules, but with
assigned bonding pattern of the iminophenolate type ligand. The asymmetric unit of
1a contains one crystallographically independent [Pd(1LISQ)(tertbpy)]+ cation and one
PF6- anion. The complex 1b crystallizes with one crystallographically independent
neutral molecule, [Pd(1LIP)(tertbpy)]. Compound 1c crystallizes with two CH2Cl2
molecules of crystallization. The asymmetric unit of 1c consists of one independent
[Pd(1LIBQ)(tertbpy)]+2 dication and one BF4- and one PF6– anion. However, the BF4-
molecule is disordered. BF4 molecule is located above the (1LIBQ) plane and involves
in two a weak F(74)•••C(1) and F(71X)•••Pd(1) interactions at 2.699 Å and 2.795 Å,
respectively (Figure 2.1.d.). In all three complexes, geometry around the Pd centre is
found to be square planar with a PdN3O donor set.
O1
C1
C2
C3
C4
C5 C6 N7 P
F
Pd
N31
N42
C8
•
Figure 2.1.a. Thermal ellipsoidal diagram of monoanionic 1a with the interpretation of the
bonding pattern in the ligand.
20
Chapter 2
O1
N7
C1
C2
C3
C4
C5
C6
Pd
N31
N42
C8
Figure 2.1.b. Thermal ellipsoidal diagram of neutral 1b with the interpretation of the
bonding pattern in the ligand.
O1
N42
C1
C2
C3
C4
C5
C6
Pd
P
F
F
B
N31
N7
C8
Figure 2.1.c. Thermal ellipsoidal diagram of dicationic 1c with the interpretation of the bonding
pattern in the ligand.
21
Chapter 2
2.699 Å
2.795 Å
Figure 2.1.d. Figure 3.15. Ion-pairing interactions in the solid state in 1c.
1a 1b 1c
Pd-N7 2.021(2) 1.9806(10) 2.050(3)
Pd-N31 1.994(2) 2.0109(10) 2.012(3)
Pd-N42 2.009(2) 2.0418(10) 1.989(3)
Pd-O1 1.983(2) 1.9692(9) 2.024(2)
N7-C8 1.424(4) 1.4039(15) 1.439(4)
N7-C6 1.360(4) 1.3870(16) 1.306(4)
C1-O1 1.307(4) 1.3531(14) 1.242(4)
C1-C2 1.422(4) 1.4038(17) 1.458(4)
C1-C6 1.439(4) 1.4154(16) 1.499(4)
C2-C3 1.381(4) 1.4127(17) 1.354(5)
C3-C4 1.439(4) 1.3951(17) 1.466(5)
C4-C5 1.355(4) 1.3989(17) 1.345(5)
C5-C6 1.422(4) 1.3961(17) 1.428(4)
Table 2.1. Selected bond distances (Å) in complexes 1a, 1b, and 1c.
22
Chapter 2
In the following we discuss the bond distances, specifically the C-C, C-N, and
C-O distances in the N,N- and N,O-coordinated tertbpy and aminophenolate derived
ligands in complexes 1a, 1b, and 1c. It is noteworthy that all corresponding C-C bond
distances in tertbpy and in the N-phenyl group of the aminophenolate ligands are
equidistant within the experimental error (+ 0.01 Å) in all three crystal structures.
Thus, the tertbpy and N-phenyl group of the aminophenolate ligand are internal
markers of structure determinations which do not vary either on oxidation from 1a to
1c or on reduction from 1a to 1b. On the other hand, the C-O, C-N, and C-C bond
distances in the o-aminophenol parts undergo characteristic changes upon oxidation
from 1a to 1c and on reduction from 1a to 1b. From this bond distance information it
is possible to assign the protonation and oxidation levels of the O,N-coordinated 2-(2-
trifluoromethyl)anilino-4,6-di-tert-butylphenol ligands by using the information
compiled in Scheme 2.1. and from previous results. 14, 16, 19
Short C1-O1 and C6-N7 distances in 1a at 1.307 + 0.01 Å and 1.36 + 0.01 Å,
respectively, indicate the considerable double bond character of these bonds. At the
same time, the average distances of C2-C3 and C4-C5 are short at 1.38 + 0.01 and
1.37 + 0.01 Å, respectively, whereas all other C-C distances in the ring are rather long
at 1.43 + 0.01 Å. Thus, this ring shows significant quinoid type distortion from the
aromatic behaviour. These bond distances are similar to the bond distances found
previously for Pt and Pd complexes with N,O-coordinated aminophenolate ligands
namely the π radical anion (LISQ)1-•.16 From the above structural markers complex 1a
is assigned also to contain a monoanionic iminobenzosemiquinonate(1-) π radical.
The presence of a π radical in this complex is further supported by EPR, SQUID and
UV-vis spectroscopic data.
In neutral 1b, the chemically one-electron reduced complex of 1a, all six C-C
distances in the ring: namely C1-C2, C2-C3, C3-C4, C4-C5, C5-C6, and C1-C6 are
equidistant at 1.40 + 0.01 Å (within the experimental error) reflecting the aromatic
nature of the ring. Significantly longer C1-O1 and C6-N7 distances of 1.35 + 0.01 Å
and 1.39 + 0.01 Å, respectively, as compared to those in 1a are observed. The N7-C8
distance is at 1.40 Å which indicates that N7 is non protonated sp2 hybridized amido
nitrogen. These results corroborate the presence of a dianionic o-iminophenolate(2-)
ligand in 1b. Thus, based on the above distances and referring to the previous
assignments,16 we can comfortably assign 1b as [PdII(1LIP)(tertbpy)].
23
Chapter 2
Dicationic 1c, exhibits significantly shorter C1-O1 and C6-N7 bond distances
at 1.24 + 0.01 Å and 1.30 + 0.01 Å, respectively, than those observed in 1a. This
indicates complete double bond character of the C1-O1 and C6-N7 bonds. The
quinoid type distortion becomes more pronounced than in 1a and yields alternate two
short C=C double bonds, namely C2=C3 and C4=C5 at 1.35 + 0.01 Å and four long
C-C single bonds at 1.44 + 0.02 Å to give the full closed-shell quinone structure to the
ligand. Using the above criteria it is easy to assign 1c as [PdII(1LIBQ)(tertbpy)]+2
containing a neutral iminobenzoquinone ligand and neutral tertbpy with a divalent Pd
ion (SPd = 0) with d8 electronic configuration.
2.3. Electro- and Spectroelectrochemistry:
Figure 2.2. shows the cyclic voltammogram of 1a recorded at different scan
rates varying from 50-800 mV s-1. All the potentials summarized in Table 2.2. and are
referenced versus the Ferrocenium/Ferrocene couple (Fc+/ Fc).
1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5
E [V] vs. Fc+/Fc
10 μA
Figure 2.2. Cyclic voltammogram of 1a in CH2Cl2 solution (0.1 M TBAPF6). Conditions: Scan
rate variation 50-800 mV s-1; Temperature 25 ۫ C. (glassy carbon as working electrode and
ferrocene (Fc) as an internal standard).
24
Chapter 2
Complex E1/2 (V) vs Fc+/Fc
Oxidation Reduction 1 Reduction 2
[Pd(1LISQ)(tertbpy)](PF6) +0.433 -0.605 -1.986
Table 2.2. Summary of redox potentials in volts vs Ferrocenium/Ferrocene couple for 1a
The cyclic voltammogram of 1a displays three completely reversible one-
electron waves in the range +1.0 V to -2.5 V vs. Fc+/Fc. These correspond to ligand-
centered oxidation and reductions (Eq 2.1.). This cyclic voltammogram is very similar
to that of [Pd(LIP)(bpy)] and [Pt(LIP)(bpy)] complexes.16 However, the redox
potentials are shifted by 60-90 mV because of the presence of the electron
withdrawing substituent (–CF3) in the ortho position of the N-phenyl ring. Controlled
potential coulometric measurements established that monocationic 1a undergoes one
reversible one-electron oxidation process corresponding to oxidation of monoanionic
o-iminobenzosemiquinonate, (1LISQ)1- to neutral o-iminobenzoquinone, (1LIBQ)0. the
first-electron reduction process at -0.605 V corresponds to the reduction of the
o-iminobenzosemiquinonate, (1LISQ)1- to the o-iminophenolate(2-) (1LIP)2- and the
second-electron reduction corresponds to the reduction of the N,N-coordinated tertbpy
to a monoanionic radical form (tertbpy•)1-. The existence of bpy radicals upon
reduction (Figure 2.3.) has been proposed previously in the literature.16, 17 Further
characterizations were not performed because of their instability in solution at
ambient temperature.
Eq 2.1
[PdII(1LISQ)(tertbpy)]1+ +e
-e
[PdII(1LIBQ)(tertbpy)]2+
[PdII(1LIP)(tertbpy)] +e
-e [PdII(1LIP)(tertbpy )]1-
+e
-e
25
Chapter 2
NN
_
NN
1-
0
-e
(tertbpy)1-
(tertbpy)0
Figure 2.3. Existence of tertbpy radicals on reduction
Figure 2.4. shows the spectroelectrochemistry of 1a together with its
electrochemically generated one-electron oxidized and one-electron reduced forms
recorded in CH2Cl2 solution containing 0.20 M [(n-Bu)4N] )PF6 at -25 ۫ C in the range
of 300-2000 nm. The potentials for the generation of neutral and dicationic species
were fixed at -0.7 V and +0.9 V, respectively. Table 2.3. summarizes the electronic
spectra of complexes. The electronic spectrum of 1a displays five bands in the visible
region above 400 nm at 435, 465 (sh), 761, 855, and 978 nm with molar extinction
coefficients (0.5-6.3) x 103 M-1 cm-1. This allows the characterization of a single O,N-
coordinated (1LISQ)1- radical. Alternatively, the dicationic species, which is the one-
electron oxidized form of 1a, shows no absorptions above 600 nm but shows a strong
band at 400 nm (ε = 0.51 x 104 M
-1 cm-1) with a shoulder at 517 nm which are
characteristic features of uncoordinated quinone CT bands in CH2Cl2 solution,11 thus
suggesting a coordinated neutral quinone form of the ligand, (1LIBQ)0 to be present in
1c. On the other hand, the neutral species, which is the one-electron reduced form of
1a, displays an interesting intense and broad absorption maximum in the visible
region at 685 nm (ε = 0.38 x 104 M-1 cm-1) with a shoulder at 355 nm. The former is
due to a spin- and dipole-allowed charge transfer transition between two bidentate
ligands in square planar geometry (Scheme 2.2.).20 These transitions are independent
of the nature of the central metal ion and have been observed for all square planar
complexes of type [MII(L•)2].11 Similar electronic spectra for [Pd(LIP)(bpy)],
[Pt(LIP)(bpy)], and their electrochemically one and two-electron oxidized forms are
reported in ref 16.
26
Chapter 2
Interestingly, [Cu(dmtacn)(LISQ)] PF6 also exhibits similar electronic spectrum
like 1a. However, unlike 1b, [Cu(dmtacn)(LAP-H)]0 exhibits no absorptions above 400
nm.11
N
CF3
O
Pd
N
NN
CF3
O
Pd
N
N
h.v
00*
ground state S = 0 excited state S = 0
Scheme 2.2. Inter ligand charge transfer transitions in 1b.
300 400 500 600 700 800 900 1000 1100
0.0
0.2
0.4
0.6
0.8
[PdII(1LIP)(tertbpy] (1b)
[PdII(1LIBQ)(tertbpy]2+ (1c)
ε [M-1cm-1X104]
λ [nm]
[PdII(1LISQ)(tertbpy]+ (1a)
Figure 2.4. The electronic spectra showing 1a (solid line) together with its electrochemically one-
electron reduced, 1b (dots line) and one-electron oxidized, 1c (dashed line) species in CH2Cl2 solution
containing 0.20 M [(n-Bu)4N] PF6 at -25 ۫ C.
27
Chapter 2
400 500 600 700 800 900 1000
0.0
0.2
0.4
0.6
0.8
ε [M-1cm-1X104]
λ [nm]
400 500 600 700 800 900 1000
0.0
0.2
0.4
0.6
0.8
1.0
ε [M-1cm-1X104]
λ [nm]
1b 1c
Figure 2.5. UV-vis spectra of (a) 1b and (b) 1c in CH2Cl2
UV-vis spectra of 1b and 1c are shown separately in Figure 2.5. Chemically
isolated 1b shows same absorptions as the electrochemically generated species with
little shift in the wave lengths. But, unlike electrochemically oxidized species, 1c
exhibits pronounced bands above 380 nm (ε = 0.08-0.52 x 103 M
-1 cm-1) than
electrochemically generated species. Results of these spectra are also summarized in
Table 2.3.
Complex λmax, nm (ε, 104 M-1 cm-1)
[Pd (1LISQ)(tert bpy)](PF6) (1a) 978 (0.04); 855 (0.07); 761 (0.05);
435 (0.64)
* [Pd (1LIP)(tertbpy)]0685 (0.38); 355 sh (0.31)
* [Pd (1LIBQ)(tertbpy)]2+ 508 (0.16); 400 (0.52)
[Pd (1LIP)(tertbpy)] (1b) 708 (0.39); 401 sh (0.31)
[Pd (1LIBQ)(tertbpy)](PF6)(BF4) (1c) 547 (0.08); 386 (0.52)
Table 2.3. Electronic spectra of the complexes 1a, 1b and 1c and electrochemically generated
one-electron oxidized as well as one-electron reduced species of 1a in CH2Cl2 solution.
* Electrochemically generated species
28
Chapter 2
2.4. Magnetic Properties:
The electronic ground states of complexes 1a, 1b, and 1c have been
established from the variable-temperature magnetic susceptibility measurements in
the range 3-300K by using a SQUID magnetometer. Complex 1b is diamagnetic (S =
0 ground state) due to the presence of the coordinated dianionic o-iminophenolate and
the neutral tertbpy to a diamagnetic, divalent, PdII metal centre (d8 electronic
configuration; SPd = 0). Complex 1c is diamagnetic due to N,O- and N,N-coordination
of neutral o-iminobenzoquinone and tertbpy ligands to the PdII centre. Complex 1a is
paramagnetic with an S = ½ ground state due to a coordinated
o-iminobenzosemiquinonate (1-) radical (Srad = ½) to a diamagnetic PdII metal centre.
50 100 150 200 250
1.4
1.5
1.6
1.7
1.8
μeff [μB]
T [K]
Sim
Exp
Figure 2.6. μeff vs T graph of 1a (4-300 K); External applied field is 1T. TIP = 0.1 X 10-5 cm3 Mol-1
θ = -1.1 K
Temperature dependent magnetic moments, μeff of 1a are shown in Figure 2.6.
In the range of 50-250 K, μeff is nearly independent of the temperature at ~ 1.72 μB
with g = 2.001, indicating an S = ½ ground state. This behaviour is typical for an
uncoupled single spin. Since the μeff is very close to the spin-only value for an S = ½
ground state and g value is very close to an isolated organic radical, 1a is assigned to
contain one ligand based radical (1LISQ)1-, coordinated to a diamagnetic PdII centre.
29
Chapter 2
The X-band EPR spectra of 1a in CH2Cl2 solution recorded at (a) 10K and (b)
298K are shown in Figure 2.7. At 10K, the spectrum shows a sharp signal at g =
2.0073 without hyperfine splitting, corresponding to an S = ½ spin state.
Alternatively, at 298K the EPR signal displays an hyperfine split S = ½ signal at giso =
2.0035 in agreement with the assignment that complex 1a contains an O,N-
coordinated o-iminobenzosemiquinonato(1-) radical. Satisfactory simulations were
obtained by using the following parameters: a(14N) = 19.6 MHz (6.99 G); a(1H) =
13.4 MHz (4.78 G); a(105Pd, I = 2.5, 22.2%) = 9.0 MHz (3.21 G) and line width of 2.0
G. Thus, the data further supports the assignment of the semiquinone character of the
ligand (1LISQ)1-. A similar spectrum with giso = 2.002 has been reported previously for
[Pd(LISQ)(bpy)] (PF6).16
280 300 320 340 360 380 400
1.8 2.0 2.2 2.4
Exp
Sim
B [mT]
dX´´
dB
3320 3340 3360 3380 3400
B [mT]
Sim
Exp
g values
ab
Figure 2.7. X-band EPR spectrum of the 1a in frozen solution of CH2Cl2 solution (a) at 10 K (b) at 298
K. Experimental conditions: microwave frequency 9.63 GHz; power 2 μW modulation 1 mT.
Simulation parameters are given in the text.
30
Chapter 2
2.5. Conclusions:
In summary, a series of square planar complexes 1a, 1b, and 1c containing
N,N- and N,O-coordinated a neutral tertbpy and a noninnocent 2-(2-
trifluoromethyl)anilino-4,6-di-tert-butylphenol ligands coordinated to a diamagnetic,
divalent Pd centre have been isolated. The complexes have been studied in detail,
structurally and spectroscopically. It has been shown that paramagnetic monocationic
1a contains an o-iminobenzosemiquinonate(1-) radical ligand. The most salient
structural features to identify this radical character are (i) C-O and C-N bond lengths
at 1.30 + 0.01 Å and 1.36 + 0.01 Å, respectively, and (ii) quinoid type distortion in the
aminophenolate ring. A sharp EPR signal at 10 K with a g = 2.0073 value indicating
the organic, i.e., ligand centred, radical. Neutral diamagnetic 1b contains the dianionic
o-iminophenolate ligand. Important structural features include (i) C-O and C-N bond
lengths at 1.35 + 0.01 Å and 1.39 + 0.01 Å, and (ii) the aromatic nature of the
aminophenolate ring. Intense broad band at 685 nm is the inter-ligand charge transfer
transition (LLCT). The dicationic diamagnetic complex 1c with neutral o-
iminobenzoquinone ligand has been isolated for first time. Identification of (1LIBQ)0
has been done by the observation of (i) very short C-O and C-N bond lengths at 1.24
+ 0.01 Å and 1.30 + 0.01 Å, (ii) a pronounced quinoid type distortion with two short
C=C and four long C-C in the aminophenolate ring, and (iii) quinone charge transfer
bands in UV-vis spectrum in between 350-550 nm. Thus, it is clear that the oxidation
levels of O,N-coordinated o-aminophenolate derivatives can be established primarily
from their high quality crystal structures and electronic spectra.
31
Chapter 2
2.6. References:
(1) Pierpont C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331.
(2) Sigel, H. S. A. Metalloenzymes Involving Amino Acid Residue and
Related Radicals; Marcel Dekker: Newyork, 1994; Stubbe, J.; Van der
Donk, W. A. Chem. Rev. 1998, 98, 705.
(3) (a) Jadzewski, B. A.; Tolman, W.B.; Coord. Chem. Rev. 2000, 200-202,
633. (b) Chaudhuri, P.; Wieghardt, K. Prog. Inorg. Chem. 2001, 50, 151.
(4) Beckmann, U.; Bill. E.; Weyhermüller, T.; Wieghardt, K. J. Inorg.
Biochem. 2001, 86, 141.
(5) (a) Chun, H. P.; Weyhermüller, T.; Bill. E.; Wieghardt, K. J. Inorg.
Biochem. 2001, 86, 182. (b) Chun, H. P.; Weyhermüller, T.; Bill. E.;
Wieghardt, K. Angew. Chem. Int. Ed. Engl. 2001, 40, 2489. (c) Herebian,
D.; Bothe, E.; Bill. E.; Weyhermüller, T.; Wieghardt, K. J. Am. Chem.
Soc. 2001, 123, 10012. (d) Herebian, D.; Ghosh, P.; Chun, H.; Bothe, E.;
Weyhermüller, T.; Wieghardt, K. Eur. J. Inorg. Chem. 2002, 1957.
(6) (a) Ghosh, P.; Bothe, E.; Neese, F.; Weyhermüller, T.; Wieghardt, K. J.
Am. Chem. Soc. 2003, 125, 1293. (b) Ghosh, P.; Begum, A.; Herebian, D.;
Bothe, E.; Weyhermüller, T.; Wieghardt, K. Angew. Chem. Int. Ed. Engl.
2003, 42, 563.
(7) Herebian, D.; Bothe, E.; Neese, F.; Weyhermüller, T.; Wieghardt, K. J.
Am. Chem. Soc. 2003, 125, 9116.
(8) (a) Bhattacharya, S.; Gupta, P.; Basuli, F.; Pierpont, C. G. Inorg. Chem.
2002, 41, 5810. (b) Kaim, W.; Wanner, M.; Knodler, A.; Zalis, S. Inorg.
Chem. Acta. 2002, 337, 163. (c) Frantz, S.; Hartmann, H.; Doslik, N.;
Wanner, M.; Kaim, W.; Kummerer, H. J.; Denninger, G.; Barra, A. L.;
Duboc-Toia, C.; Fiedler, J.; Ciofini, I.; Urban, C.; Kaupp, M. J. Am.
Chem. Soc. 2002, 124, 10563. (d) Kaim, W.; Dogan, A.; Wanner, M.;
Klein, A.; Tiritiris, I.; Schleid, T.; Stufkens, D. J.; Snoeck, T. L.;
McInnes, E. J. L.; Fiedler, J.; Zalis, S. Inorg. Chem. 2002, 41, 4139. (e)
Pierpont C. G. Inorg. Chem. 2001, 40, 5727. (f) Pierpont C. G. Coord.
Chem. Rev. 2001, 216, 99. (g) Glockle, M.; Hubler, K.; Kummerer, H. J.;
Denninger, G.; Kaim, W. Inorg. Chem. 2001, 40, 2263. (h) Abakumav, G.
A.; Cherkasov, V. K.; Nevodchikov, V. I.; Kurapatov, V. A.; Yee, G. T.;
32
Chapter 2
Pierpont C. G. Inorg. Chem. 2001, 40, 2434. (i) Pierpont C. G.; Attia, A.
S. Collect. Czech. Chem. Commum. 2001, 66, 33.
(9) (a) Lim, B. S.; Fomitchev, D. V.; Holm, R. H. Inorg. Chem. 2001, 40,
4257. (b) Fomitchev, D. V.; Lim, B. S.; R. H. Inorg. Chem. 2001, 40, 645.
(10) Verani, C. N.; Gallert, S.; Bill, E.; Weyhermüller, T.; Wieghardt, K.;
Chaudhuri, P. Chem. Commum. 1999, 1747.
(11) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.; Weyhermüller, T.;
Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 2213.
(12) Chun, H.; Verani, C. N.; Chaudhuri, P.; Bothe, E.; Bill, E.; Weyhermüller,
T.; Wieghardt, K. Inorg. Chem. 2001, 40, 4157.
(13) (a) Chun, H.; Chaudhuri, P.; Weyhermüller, T.; Wieghardt, K. Inorg.
Chem. 2002, 41, 790. (b) Chun, H.; Bill, E.; Bothe, E.; Weyhermüller, T.;
Wieghardt, K. Inorg. Chem. 2002, 41, 5091.
(14) (a) Min, K. S.; Weyhermüller, T.; Wieghardt, K. Dalton Trans. 2003,
1126. (b) Min, K. S.; Weyhermüller, T.; Wieghardt, K. Dalton Trans.
2004, 178. (c) Min, K. S.; Weyhermüller, T.; Bothe, E.; Wieghardt, K.
Inorg. Chem. 2004, 43, 2922.
(15) Mukherjee, S.; Weyhermüller, T.; Wieghardt, K.; Chaudhuri, P. Dalton
Trans. 2003, 3483.
(16) Sun, X.; Chun, H.; Hildenbrand, K.; Bothe, E.; Weyhermüller, T.; Neese,
F.; Wieghardt, K. Inorg. Chem. 2002, 41, 4295.
(17) (a) Tokel-Takvoryan, N.E.; Hemingway, R.E.; Bard, A. J. J. Am. Chem.
Soc. 1973, 95, 6582. (b) McInnes, E. J. L.; Welch, A. J.; Yellowlees, L. J.
Chem. Commun. 1996, 2393. (c) McInnes, E. J. L.; Farley, R. D.;
Macgregor, S. A.; Taylor, K. J.; Yellowlees, L. J.; Rowlands, C. C. J.
Chem. Soc., Faraday Trans. 1998, 94, 2985. (d) McInnes, E. J. L.; Farley,
R. D.; Rowlands, C. C.; Welch, A. J.; Rovatti, L.; Yellowlees, L. J. J.
Chem. Soc., Dalton Trans. 1999, 4203.
(18) Bill, E.; Bothe, E.; Chaudhuri, P.; Chlopek, K.; Herebian, D.; Kokatam,
S.; Ray, K.; Weyhermüller, T.; Neese, F.; Wieghardt, K. Chem. Eur. J.
2005, 11, 204.
(19) Kokatam, S.; Weyhermüller, T.; Bothe, E.; Chaudhuri, P.; Wieghardt, K.
Inorg. Chem. 2005, 44, 3709.
(20) Vogler, A.; Kunkely, H.; Comments Inorg. Chem. 1990, 9, 201.
33
Chapter 2
(21) Balch, A.; Holm, R. H. J. Am. Chem. Soc. 1966, 88, 5201.
34
Structural Characterization of Square Planar Co, Ni, and
Pd Complexes with o-Aminophenol Type of Ligands in
Various Oxidation Levels *
* This chapter is based on: (a) Bill, E.; Bothe, E.; Chaudhuri, P.; Chlopek, K.;
Herebian, D.; Kokatam, S.; Ray, K.; Weyhermüller, T.; Neese, F.; Wieghardt, K.
Chem. Eur. J. 2005, 11, 204. (b) Kokatam, S.; Weyhermüller, T.; Bothe, E.;
Chaudhuri, P.; Wieghardt, K. Inorg. Chem. 2005, 44, 3709.
35
36
Chapter 3
3.1. Introduction:
It is well known1-18 that o-diaminophenylenes,1 o-catacholates,2 as well as
o-aminophenolates12-18 belong to the class of redox-active, noninnocent ligands in the
transition metal chemistry. A large number of transition metal complexes are reported
with these type of noninnocent ligands. In many complexes, discrepancies arise in the
assignment of oxidation state to the central metal ion, because, formal and
spectroscopic (physical) oxidation states of the ligands are not differentiated carefully.
This chapter describes the synthesis and characterization of a series of square
planar Co, Ni, and Pd complexes with the noninnocent o-aminophenolate ligand
(1LH2) in different oxidation states. These complexes are the members in an electron
transfer series. 1LH2 represents the bulky 2-(2-trifluoromethyl)anilino-4,6-di-tert-
butylphenol. Dianionic o-iminophenolate (1LIP)2-, monoanionic
o-iminobenzosemiquinonate π radical (1LISQ)1-•, and neutral o-iminobenzoquinone
(1LIBQ)0 represent the different oxidation states of the coordinating ligand to a central
metal ion. These complexes are summarized in Table 3.0.
Co complexes Ni complexes Pd complexes
[Co(1LISQ)(1LIP)] (2a) [Ni(1LISQ)2] (3a) [Pd(1LISQ)2] (4a)
[Co(1LIP)2]- (2b) [Ni(1LISQ)(1LIP)]- (3b) [Pd(1LISQ)(1LIP)]- (4b)
-
-
-
[Ni(1LIBQ)2(ClO4)2](3d)*
[Pd(1LISQ)(1LIBQ)]+1 (4c)
[Pd(1LIBQ)2]+2 (4d)
Table 3.0. Summary of all complexes going to be discussed in this chapter. * Octahedral.
In 1966 Balch and Holm3 reported that the reaction of o-phenylenediamine
with CoCl2.6H2O (or [Co(CH3CO2)2].4 H2O)4 in the ratio 2:1 in an aqueous ammonia
affords, in the presence of air, a deep violet precipitate of [Co{C6H4(NH)2}2]. The
room-temperature crystal structure of this complex was reported by Peng et al.5 The
neutral, mononuclear complex is paramagnetic and possesses an S = ½ ground state as
was established by its X-band EPR spectrum and magnetochemistry.3 It has been
suggested3-5 that both organic ligands are identical and that they are monoanionic π
radicals (Srad = ½) of the diiminobenzosemiquinonate(1-) type and that, therefore, the
central cobalt ion possesses a +II oxidation state (low spin d7, SCo = ½). In this model
37
Chapter 3
the spins of the ligand π radicals are assumed to be strongly intramolecularly
antiferromagnetically coupled.5
Using a valence-bond picture, Balch and Holm suggested a different model, as
central low-spin cobalt (II) ion (S = ½) coordinated to two closed-shell [C6H4(NH)2]n-
ligands, which are formulated as two resonance hybrids between an aromatic dianion
(LIP)2- and its neutral diiminobenzoquinone form (LIBQ)0: [CoII(LIBQ)(LIP)] ↔
[CoII(LIP)(LIBQ)]. These conclusions are supported by a) the corresponding highly
coloured, isostructural but diamagnetic complexes of NiII, PdII, and PtII, namely
[M{C6H4(NH)2}2], had also been prepared,3 b) square planar complexes of CoII with
redox innocent ligands such as acetylacetonate(1-), (acac)3-6 or (salen)4-7 are known;
they also possess an S = ½ ground state.8
N
H
H
NN
N
H
Co
H0
1.37
1.36
1.40 1.42
1.41
1.41
1.36
1.37
1.82
1.82
S = 1/2
N
H
H
NN
N
H
CoIII
H
.
.0
Cl
1.42
1.34
1.35 1.42
1.42
1.42
1.32
1.32
1.86
1.86
S = 0
Figure 3.1. Schematic structures with bond lengths in Å of square planar [Co(L)2]0 from ref 5(top) and
square-base pyramidal [CoIII(L)2Cl] from ref 5. (bottom). The experimental
errors from the room temperature crystal structure are ~ + 0.03 Å (= 3σ) for the
former and ~ + 0.015 Å for the latter.
Chemical oxidation of [Co{C6H4(NH)2}2] with air or iodine, in the presence of
coordinating (solvent or anion) affords diamagnetic, neutral or monocationic five-
coordinated species: [Co{C6H4(NH)2}2X] (X = Cl,4,5 I,3,4 SCN-,4
[Co{C6H4(NH)2}2(PPh3)]PF6,4,9 and [Co{C6H4(NH)2}2(Py)]Cl10. All of these have
been characterized by X-ray crystallography (Figure 3.1.) at ambient temperature. The
proposed bonding picture requires then the presence of two π radical mono anions,
(LISQ)1- and an additional apical ligand, X. The neutral species are generated when the
38
Chapter 3
apical ligand is an anion, and the monocationic species are formed when the apical
ligand is neutral. In both the cases the central cobalt(III) ion possesses an SCo = 0
ground state. Thus, on going from four-coordinated to five-coordinated species, an
oxidation of the central Co(II) to Co(III) is believed to occur.
N
H
H
NN
N
H
CoII
H
.
.
N
H
H
NN
N
H
H
N
N
H
CoIII
H
.
N
H
N
H
.
CoIII
[CoII(LISQ)2]
A
B
[CoIII(LIP)(LISQ)] [CoIII(LISQ)(LIP)]
Scheme 3.1.
It has been observed that in these square-base pyramidal complexes of low-
spin cobalt(III), the monoanionic ligands [C6H4(NH2)]-• exhibit geometrical features
that are readily ascribed to o-diiminosemiquinonate(1-) π radicals:11 The average C-N
bond length is in the range 1.34 + 0.01 Å irrespective of the nature of the fifth apical
ligand. Furthermore, the six-membered rings display typical quinoid type distortions
with two alternating short C-C distances and four longer ones. It is surprising that the
geometrical features of the two ligands apparently differ slightly in the four- and five-
coordinate species (Figure 3.1.). The C-N bonds are longer in the four- than in the
five-coordinate species. However, the poor quality of the X-ray structure
determinations, (experimental error in C-C or C-N bond length is ~ + 0.03 Å (= 3σ)
does not allow to safely assign an oxidation level of the ligands in the four-coordinate
species. Thus, experimentally it was not possible to distinguish difference between the
electronic structures A and B (Scheme 3.1.) for four-coordinate species. The
difference between A and B is: A has two ligand π radicals and a central CoII ion (SCo
= ½), while B is described as a species containing a dianionic ligand, (LNIP)2-, a single
π radical monoanion, (LNISQ)1-, and a CoIII ion in a square planar ligand field (SCo = 1).
The ligand mixed valency in B may be delocalized (Class III)29, 30 ensuring the
observed structural equivalency of the two ligands on the time scale of the X-ray
diffraction experiment. Model B would explain the longer C-N distances as a
39
Chapter 3
consequence of the presence of an aromatic dianion and a monoanionic π radical.
Furthermore, one would expect the observed C-C and C-N distances to occur at the
arithmetic average of the corresponding distances in the mono- and dianion, (LISQ)1-
and (LIP)2- (Scheme 3.2.).
The same situation holds true for the o-aminophenolate ligand, H2[1LIP]. In
case of the neutral, paramagnetic (S = ½) complex, [Co(1L)2], where also the problem
arises of the correct assignment of oxidation state of the central Co ion.25 Bond
distances observed are at the arithmetic average of the corresponding distances in
(1LISQ)1-• and (1LIP)2-, as shown in Scheme 3.2. Bond distances seem to indicate a
charge distribution of this species as in [CoIII(LIP)(LISQ)] with charge delocalization
over both ligands (Model B). The Co(III) ion should possess an SCo = 1 ground state
as is readily deduced from ligand-field theoretical considerations (d6 in a square
planar field). Intramolecular antiferromagnetic coupling between the spins of the CoIII
ion and one ligand radical yields then the observed St = ½ ground state.
N
H
H
N
M
O
N
M
1.385
1.385
1.41
1.405
1.405
1.41 1.37
1.37 1.415
1.41
1.415
1.39
1.385
1.42
1.325
1.36
Scheme 3.2.
In order to clarify this ambiguity, we have synthesized a series of Co
complexes with the bulky ligand: 2-(2-trifluoromethyl)anilino-4,6-di-tert-butylphenol
(1LH2).12-18 The neutral, four coordinate, paramagnetic (S = ½), square planar complex
[Co(1L)2] (2a), is the exact analogue of [Co(L)2] from ref 25. It is possible to reduce
2a by one-electron by using [CoCp2] as a reductant affording the monoanionic, four
coordinate, paramagnetic (S = 1), square planar complex [Co(1L)2]- [Co(Cp)2]+ (2b),
in which Cp- is the cyclopentadienyl anion and [Co(Cp)2]+ is the cobaltocenium
cation.
40
Chapter 3
Similarly, Balch and Holm3 established that the reaction of
o-phenylenediamine, H2(LPDI), with MX2 salts in (ratio 2:1) (M = Ni, Pd, and Pt)
affords, in the presence of air, dark blue/black microcrystals of neutral, square planar,
and diamagnetic complexes of [M(LISQ)2] where (LISQ)1-• represents the o-
diiminobenzosemiquinonate oxidation level.
The electronic structure of these neutral, planar, diamagnetic molecules has
been studied theoretically21, 22 but only recently have DFT calculations23 shown that
[Ni(LISQ)2] is best described as diradical with a singlet ground state. Thus, the two o-
diiminobenzosemiquinonate(1-) π radicals couple intramolecularly, strongly, and
antiferromagnetically yielding the observed S = 0 ground state and a quinoid type
distorted ligand geometry. A singlet-triplet energy gap of ~3000 cm-1 has been
calculated.23
It has also been shown that an electron-transfer series of five bis(chelate) metal
complexes of the type [M(L)2]n exists where n = -2, -1, 0, +1, +2. These species are
interrelated by one-electron-transfer steps; L represents a redox noninnocent
derivative of o-phenylenediamide,3, 11 [C6H4(N)2]2-,1-,0, or of o-iminophenolate,
[C6H4O(N)]2-,1-,0, and MII is divalent metal ion with a d8 electron configuration as in
NiII, PdII, and PtII.14, 15
The nature of these redox steps as ligand-or metal-centered processes has been
a matter of considerable debate. Gray et al24 refuted the idea of the occurrence of
Ni(I), Ni(III), or Ni(IV) in this series and proposed that all the redox processes are
ligand-centered and that the spectroscopic oxidation of the central Ni ion is + II (d8)
throughout the above series of five complexes. The same holds true for the Pd and Pt
species of this series.
The diamagnetic dianions [M(LIP)2]2- have been described3, 11a as species
containing a divalent d8 metal ion (S = 0 in a square planar environment) and two
bidentate aromatic, closed-shell, dianions (LIP)2-. The crystal structure of diamagnetic
[PdII(bpy)(2LIP)]0 has recently been reported,13 which confirms the presence of a
single (2LIP)2- ligand and its aromatic character and a neutral 2,2´-bipyridine ligand.
Similarly, although structural evidence is lacking, the diamagnetic dications
have been proposed3, 11 to contain square planar [MII(LIBQ)]2+ ions with closed-shell
quinone-type ligands and a diamagnetic central metal ion.
41
Chapter 3
The most intriguing and electronically interesting species are the
paramagnetic, monoanions and monocations of the series, which both possess an St =
½ ground state. It has been proposed11, 14 that the former is best described as
[MII(LISQ)(LIP)]- and the latter is [MII(LISQ)(LIBQ)]+. Thus, in a formal sense we are
dealing with a case of mixed valency of the two ligands, which, in principle, can
comprise a localized or delocalized excess electron.
To the best of our knowledge no mononuclear square planar monocationic
form of this class of complexes has been structurally characterized to date. This is due
to their tendency to form diamagnetic dimers with a weak Ni···Ni or Pt···Pt bond at
~3.0 Å.11a Some of these have recently been structurally characterized, and it was
found that the four ligands in such a dimer, within experimental error are identical,
which is indicative of a delocalized electronic structure.11a The metrical details, i.e.,
the C-C and C-N bond distances, were found to be intermediate between an (LISQ)1-
and an (LIBQ)0 ligand.
As it is well known that divalent metal complexes of the type, [MII(L)2]n, with
noninnocent ligands, tend to form a complete five-membered electron-transfer series
with n = 2-, 1-, 0, +1, +2; attempts were made to chemically isolate these species
containing a noninnocent o-aminophenol derived ligand, 2-(2-trifluoromethyl)anilino-
4,6-di-tert-butylphenol (1LH2), where M = Ni or Pd. In this regard, neutral
diamagnetic, divalent, complexes of Ni and Pd, [NiII(1LISQ)2] (3a) and [PdII(1LISQ)2]
(4a), were synthesized according to the procedures reported in refs 14 and 15. Both
complexes can be oxidized and reduced by one- and two- electrons, respectively.
However, the one- and two-electron electrochemical waves are not well separated in
3a. One-electron reduction of 3a with [CoCp2] yields square planar, paramagnetic (S
= ½), monoanionic, [NiII(1LISQ)(1LIP)]-[CoCp2]+ (3b). Two-electron oxidation of 3a
with AgClO4 salt gives an octahedral, paramagnetic (S = 1), neutral complex,
[NiII(1LIBQ)2(ClO4)2] • 2CH2Cl2 (3d). Alternatively, four members of such an electron-
transfer series are isolated as solid materials in the case of Pd. One-electron chemical
reduction of 4a with [CoCp2] gives square planar, paramagnetic (S = ½),
monoanionic, [PdII(1LISQ)(1LIP)]- [CoCp2]+ (4b). One-electron chemical oxidation of
4a with AgBF4 gives square planar, paramagnetic (S = ½), monocationic,
[PdII(1LISQ)(1LIBQ)]+(BF4)- (4c). Interestingly, 4a does not dimerize on oxidation, but
gives monomeric 4c. Two-electron chemical oxidation of 4a with [NO]BF4 gives
42
Chapter 3
square planar, diamagnetic, dicationic, [PdII(1LIBQ)2]3(BF4)4{(BF4)2H}2 • 4 CH2Cl2
(4d).
3.2. Co complexes:
3.2.1. Syntheses and X-ray Crystal Structures:
Reflux of a solution of triethylamine, two equivalents of 1LH2, and one
equivalent of Co(ClO4)2 • 6H2O in MeOH for 1 h in air gives a deep blue precipitate
of neutral, paramagnetic (S = ½), [Co(1L)2] (2a) in good yield (55%). 2a can be
oxidized as well as reduced reversibly by one-electron, and irreversibly by two-
electrons, respectively. One-electron chemical reduction of 2a, using one equivalent
[CoCp2] as reductant, in degassed CH2Cl2 under an Ar atmosphere, gives purple
coloured, monoanionic, paramagnetic (S = 1), [Co(1L)2]- [CoCp2]+ (2b), in moderate
yield (30%). All the attempts were failed to isolate one-electron oxidized form of 2a.
The crystal structures of 2a and 2b have been determined 100(2) K by using
MoKα radiation. Figure 3.2. (a-b) show the thermal ellipsoid diagrams of 2a and 2b,
respectively, without solvent molecules. Table 3.1. shows the selected bond lengths in
complexes 2a and 2b. The asymmetric unit of 2a consists of one crystallographically
independent neutral molecule, [Co(1L)2], and two half molecules of [Co(1L)2] which
are located on the centre of inversion. Both CF3 groups are on the same side of CoL2
plane in the former (syn-rotamer), they are on the opposite sides of the CoL2 plane in
the latter (anti-rotamer). Complex 2b crystallizes with two, well separated CH3CN
molecules of crystallization. The asymmetric unit of 2b consists of well separated two
half anionic molecules, [Co(1L)2]-, each located on a centre of inversion and one
crystallographically independent full cation, [CoCp2]+. Complexes 2a and 2b are
found to be square planar with CoN2O2 donor set.
43
Chapter 3
Co1
O1
C1
C2
C3
C4
C5
C6
N7
N32
C31
C30
C33
C29
C28
C27
C26
O26
C8
F
Figure 3.2.a. Thermal ellipsoidal diagram of the neutral [Co(1L)2] in crystal structure 2a.
Co1
Co
O1
C1
C2
C3
C4
C5
C6
N7
C8
F
Figure 3.2.b. Thermal ellipsoidal diagram of the monoanionic [Co(1L)2] [CoCp2] in
crystal structure 2b.
44
Chapter 3
Figure 3.3. shows the C-N, C-O, and C-C bond distances in N,O-coordinated
ligands as a function of their respective oxidation level, namely (LIP)2- in
[PdII(bpy)(LIP)]14 and [PdII(tertbpy)(1LIP)]; (LISQ)1-• in [PdII(bpy)(LISQ)]PF614 and
[PdII(tertbpy)(1LISQ)]; and finally (LIBQ)0 in [NiII(tren)(LIBQ)](PF6)216 and
[PdII(tertbpy)(1LIBQ)](PF6)(BF4). Clearly, the C-N, C-O, and C-C bond distances vary
with the ligand oxidation level in a predictable manner and, conversely, the measured
distances in a given complex should allow the experimental determination of the
oxidation level of such a ligand in a given complex.11
It is noteworthy, that at this stage we do not assign oxidation state to the
central cobalt ion or the ligands in complex 2a, because the experimentally observed
C-O, C-N, and C-C distances do not closely resemble any pattern in Figure 3.3. But,
the observed C-O, C-N, and C-C distances are in better agreement with values of the
arithmetic mean between those of an aromatic dianion and the corresponding π radical
monoanion (Scheme 3.2.). Since the two ligands in the neutral molecule are
crystallographically identical (within the experimental error + 0.01 Å) and no
indication for static disorder has been detected, it appears that the unpaired electron is
delocalized over both ligands. We propose this charge distribution, [CoIII(1LISQ)(1LIP)]
for 2a. The observed C-O, C-N, and C-C bond distances in this compound closely
resemble those which were calculated from the arithmetic mean of one mono- and
dianion.
Complex 2b contains the cobaltocenium cation and the [Co(1L)2]- ion as
shown in Figure 3.2.b. The overall geometric features of the monoanion in the crystals
of 2b are similar to those of the neutral species in crystals of 2a; both are square
planar species. The C-O and C-N bond distances are slightly longer than those in the
neutral species in 2a (Table 3.1.). The C-C bond distances of the aminophenol ring in
2a and 2b also differ significantly from each other. In complex 2b, six C-C bonds are
equidistant within experimental error at 1.40 + 0.01 Å; this clearly indicates the
presence of two aromatic, closed-shell, dianionic (1LIP)2- ligands in 2b. In 2a these six
C-C distances show quinoid-type distortions with two alternating shorter C-C bonds.
Without any ambiguity the oxidation level of the ligands in 2b are therefore assigned
as aromatic dianions; this renders the oxidation state of the central cobalt ion +III. It is
interesting to note that the ligand dimensions in 2b are identical to those observed for
diamagnetic, square-planar [PdII(bpy)(LIP)],14 and [PdII(tertbpy)(1LIP)] in chapter 2. The
45
Chapter 3
charge distribution in the monoanion of 2b is best formulated as [CoIII(1LIP)2]-. It is
also worth noting that the average Co-O, and Co-N bond distances in 2a and 2b are
1.835, 1.822 Å and 1.837, 1.830 Å, respectively, and are identical within experimental
error of + 0.006 Å (3σ). The reduction of the neutral species, 2a, yielding 2b is
therefore unlikely to be a metal-centered process involving CoIII + e- → CoII.
O
N
PdII1.35
1.37
d (C-C) ~ 1.39 Å
[PdII(bpy)(3LIP)], ref. 14
O
N
NiII 1.24
1.30 1.52
1.46
1.44
1.36
1.36
1.46
[NiII(tren)(3LIBQ)](PF6)2, ref. 16
[PdII(bpy)(3LISQ)](PF6), ref. 14
O
N
PdII .
1.30
1.35 1.44
1.43
1.42
1.38
1.37
1.43
O
N
PdII .
CF3
1.30
1.36 1.44
1.42
1.42
1.38
1.36
1.44
[PdII(tertbpy)(3LISQ)](PF6), ref. Chapter 2
O
N
PdII
CF3
1.35
1.38
d (C-C) ~ 1.39 Å
[PdII(tertbpy)(3LIP)] ref. Chapter 2
O
N
PdII
CF3
1.24
1.30 1.50
1.46
1.43
1.35
1.35
1.47
[PdII(tertbpy)(3LIBQ)](PF6)(BF4), ref. Chapter 2
Figure 3.3. Average C-O, C-N, and C-C distances in Å in complexes containing (1L)n-
ligands (n = 2-, 1-, 0).
46
Chapter 3
2a 2b
Co1-N7 1.832(2) 1.854(3)
Co1-N32 1.840(2) -
Co1-O1 1.822(2) 1.845(2)
Co1-O26 1.823(2) -
N7-C8 1.434(4) 1.413(8)
N7-C6 1.373(3) 1.388(8)
C1-O1 1.329(3) 1.339(3)
C1-C2 1.415(4) 1.404(4)
C1-C6 1.424(4) 1.402(4)
C2-C3 1.384(4) 1.402(4)
C3-C4 1.420(4) 1.397(4)
C4-C5 1.382(4) 1.394(4)
C5-C6
N32-C31
N32-C33
C26-O26
C26-C27
C26-C31
C27-C28
C28-C29
C29-C30
C30-C31
1.402(4)
1.374(3)
1.432(3)
1.327(3)
1.419(4)
1.420(4)
1.387(4)
1.420(4)
1.378(4)
1.411(4)
1.393(4)
-
-
-
-
-
-
-
-
-
Table 3.1. Selected bond distances (Å) in complexes 2a and 2b.
47
Chapter 3
3.2.2. Electro- and Spectroelectrochemistry:
Figure 3.4. shows the cyclic voltammogram of 2a, recorded at 50 mV s-1. The
potentials are referenced versus the Ferrocenium/ Ferrocene couple (Fc+/ Fc); they are
summarized in Table 3.2.
Complex E1/2 (V) vs Fc+/Fc
Oxidation 2 Oxidation 1 Reduction 1 Reduction 2
[Co(1L)2] (2a) +0.536 -0.194 -0.849 -1.76
Table 3.2. Summary of redox potentials in volts vs Ferrocenium/Ferrocene couple for 2a
0.8 0.4 0.0 -0.4 -0.8 -1.2 -1.6 -2.0 -2.4
E [V]vs. Fc+/Fc
10 μA
Figure 3.4. Cyclic voltammogram of 2a in CH2Cl2 solution (0.1 M TBAPF6). Conditions: Scan rate 50
mV s-1 at 25º C. (glassy carbon as working electrode and ferrocene (Fc) as internal standard).
The cyclic voltammogram of 2a displays two completely reversible one-
electron waves in the range +0.5 V to -1.0 V vs. Fc+/Fc. The waves at the potentials
+0.536 V and -1.76 V are irreversible. Neutral species 2a, and its monoanion in 2b,
are structurally characterized, and shown to both contain a cobalt(III) ion, we assign
the charge distribution as shown in Eq 3.1.
48
Chapter 3
[CoIII(1LIP)2]- (2b)[CoIII(1LIP)(1LISQ)]0 (2a)[Co(1LISQ)2]+ (2c)
- e
+ e
- e
+ e Eq 3.1
S = 1 S=1
/
2S = 0
The oxidized and reduced species of 2a are stable in solution on the time scale
of a coulometric experiment. Therefore, it has been possible to record the spectra of
the monoanion and monocations of the neutral species 2a. Figure 3.5. shows the
spectroelectrochemistry of 2a together with its electrochemically generated one-
electron oxidized, and one-electron reduced form (2b) recorded in CH2Cl2 solution
containing 0.20 M [N(n-Bu)4] PF6 at -25 ۫º C in the range of 300-2000 nm. The
potentials for the generation of monoanion and monocationic species were fixed at
-0.95 V and +0.6 V, respectively. Table 3.3. summarizes the electronic spectra of
these complexes. From the crystal structure of 2b, which contains the square planar
and paramagnetic ion [CoIII(1LIP)2]-, we conclude that electrochemically generated ion
[CoIII(1LIP)2]- is also square planar and paramagnetic (S =1). All the attempts failed to
produce the salt of the four-coordinate monocationic form of 2a.
400 600 800 1000 1200 1400 1600 1800 2000
0.4
0.8
1.2
1.6
[CoIII(1LIP)2]1-
[Co(1L)2]1+
[CoIII(1LISQ)(1LIP)]0
ε (104 M-1cm-1)
λ (nm)
(2a)
(2b)
(2c)
Figure 3.5. The electronic spectra showing 2a (solid line) together with its electrochemically one-
electron reduced species, 2b (dashed line), and one-electron oxidized, 2c (dotted line) species in
CH2Cl2 solution containing 0.20 M [N(n-Bu)4] PF6 at -25 ۫ C.
49
Chapter 3
The electronic spectrum of 2a displays three absorption maxima in the visible
and near infrared region. Two very intense bands at 676 nm (ε = 1.05 x 104 M-1 cm-1)
and 916 nm (ε = 1.68 x 104 M-1 cm-1); as the molar extinction coefficients are very
high do not represent any d-d transitions. So we tentatively assign these two bands as
spin-allowed ligand-to-metal charge transfer (LMCT) bands. In addition to these two
bands, 2a also exhibits a very broad band around 1600 nm (ε = 0.16 x 104 M-1 cm-1)
and a shoulder at 285 nm (ε = 1.36 x 104 M-1 cm-1). The broad band around 1600 nm
is assigned as a ligand-to-ligand intervalence charge transfer band (LLIVCT), which
is a clear indication of complete delocalization of the unpaired electron over both
ligands. This kind of charge transfer bands are observed for all square planar
complexes with ligand mixed valency.14 Presence of this band supports the
assignment of ligand mixed valency in 2a. Alternatively, the electrochemically one-
electron reduced species, 2b exhibits two LMCT bands at 535 nm (ε = 0.35 x 104 M-1
cm-1) and at 827 (ε = 0.7 x 104 M-1 cm-1) with two shoulders at 400 nm (ε = 0.6 x 104
M-1 cm-1) and 720 nm (ε = 0.33 x 104 M-1 cm-1). 2b exhibits no absorption observed
above 1000 nm. This clearly indicates that there is no radical in 2b, and that both
ligands in 2b are equivalent. The electrochemically generated, one-electron oxidized
species, displays two intense absorption maxima at 620 nm (ε = 0.7 x 104 M-1 cm-1)
and at 960 nm (ε = 1.1 x 104 M-1 cm-1) with two shoulders at 300 nm (ε = 1.28 x 104
M-1 cm-1) and at 350 nm (ε = 0.5 x 104 M-1 cm-1).
Complex λmax, nm (ε, 104 M-1 cm-1)
2a 285 sh (1.36); 676 (1.05); 916 (1.68); 1600 (0.16)
2b 400 sh (0.6); 535 (0.35); 720 sh (0.33); 827 (0.7)
2c 200 sh (1.28); 350 sh (0.5); 620 (0.7); 960 (1.1)
Table 3.3. Electronic spectra of the electrochemically generated one-electron oxidized (2c)
as well as one-electron reduced (2b) species of 2a in CH2Cl2 solution.
50
Chapter 3
3.2.3. Magnetic Properties:
The electronic ground state of the complexes 2a and 2b has been established
from the variable-temperature magnetic susceptibility measurements in the range 3-
300 K by using a SQUID magnetometer. Complexes 2a and 2b are paramagnetic with
an S = ½ and S = 1 ground state, respectively.
Figure 3.6. shows the temperature dependence of the magnetic moment of 2a
and 2b. In the temperature range 80-298 K, complex 2a displays a temperature
independent magnetic moment of 2.35 μB ( g = 2.43) which is indicative of an S = ½
ground state.
Complex 2b shows a magnetic moment of ~2.9 μB (g = 2.26) which is
indicative of an S = 1 ground state. A satisfactory simulation of experimental data has
been obtained by using the following parameters: S = 1, D = 57 cm-1, and g = 2.26. A
similar large zero-field splitting parameter (D = 32 cm-1) has been reported for a
bis(benzodithiolato)cobalt(III) monoanion, which also has an S = 1 ground state.26
These results imply that the charge distribution in the monoanion is best described as
[CoIII(1LIP)2]1-, whereby two closed-shell dianions, (1LIP)2-, are bound in a square
planar fashion to a cobalt(III) ion affording an SCo = 1 ground state. A number of
square planar complexes with an S = 1 ground state, containing noninnocent N,O-
donor ligands have been described in the literature.27 These complexes also display
remarkably large zero-field splitting of more than 50 cm-1. This is in stark contrast to
the previously reported octahedral analogue [CoIII(LOISQ)3], which possesses an S =
3/2 ground state. This is typical for the presence of three orthogonally coordinated o-
iminobenzosemiquinonate(1-) radicals and a diamagnetic central cobalt(III) ion.18
The X-band EPR spectrum of 2a, in frozen CH2Cl2 solution recorded at 90K is
shown in Figure 3.7. 2a shows a rhombic signal with gx = 1.97, gy = 2.03, gz = 3.11
(giso = 2.43) corresponding to S = ½ ground state without resolvable 59Co hyperfine
splitting. This can be explained in two different ways; 1) a cobalt(III) ion in a square
planar ligand field possesses an S = 1 local spin state;26, 27 strong antiferromagnetic
coupling of a ligand radical with metal spin yields an St = ½ ground state with an
unpaired electron in the metal d orbital: 2) a cobalt(II) ion in a square planar ligand
field with two radicals possesses an S = ½ local spin state; strong antiferromagnetic
coupling of two radical spins yields an St = ½ ground state again with an unpaired
electron in the same metal d orbital. Thus, it is not straight forward to discern between
51
Chapter 3
A and B in Scheme 3.1. by EPR spectroscopy. Complex 2b is EPR silent in X-band
due to the large zero-field splitting of 57cm-1.
50 100 150 200 250
2.1
2.2
2.3
2.4
2.5
2.6
Exp
Sim
μeff / μB
T / K
0 50 100 150 200 250
0
1
2
3
Exp
sim
T / K
*
2a 2b
Figure 3.6. μeff vs T graph of 2a and 2b (4-300 K); External applied field is 1T.
TIP = 0.14 X 10-3 cm3 Mol-1and g = 2.43 for 2a; g = 2.25 and D = 57 cm-1 for 2b.
* Experimental artefact (?)
100 200 300 400
Exp
Sim
dX´´
dB
B (mT)
g values
1210 8 6 4 2
Figure 3.7. X-band EPR spectrum of the 2a in frozen CH2Cl2 solution at 90 K. Experimental
conditions: microwave frequency 9.52 GHZ; power 10 μW modulation 1 mT.
52
Chapter 3
3.3. Ni complexes:
3.3.1. Syntheses and X-ray Crystal Structures:
The diamagnetic complex 3a has been prepared according to the procedure
from reference14, 15 as follows: Reflux of a solution of triethylamine, two equivalents
of 1LH2, and one equivalent of [Ni(NO3)2]•6H2O in MeOH for 1 h, followed by
stirring at room temperature in air for 2 h gives a green precipitate of diamagnetic,
four-coordinated [Ni(1LISQ)2] (3a) in good yield (53%). 3a can be reduced reversibly
by one-, two-electrons, and can be oxidized quasireversibly by two-electrons. One-
electron chemical reduction of 3a, using one equivalent [CoCp2] as reductant, in
degassed CH2Cl2, under an Ar atmosphere, gives green, monoanionic, paramagnetic
(S = ½), [Ni(1L)2]- [CoCp2]+ (3b), in good yield (70%). Two-electron chemical
oxidation of 3a in degassed CH2Cl2 solution with two equivalents of AgClO4 under an
Ar atmosphere, gives a red solution. An orange-red microcrystalline precipitate of
neutral, paramagnetic (S = 1), six coordinated [Ni(1L)2(OClO3)2] • 2 CH2Cl2 (3d), was
obtained in moderate yield (40%) upon the addition of n-hexanes.
The crystal structures of 3a, 3b, and 3d have been determined at 100(2) K by
using Mo Kα radiation. Figure 3.8. (a-c) shows the thermal ellipsoid diagrams of 3a,
3b, and 3d, respectively, without solvent molecules. Table 3.4. summarizes the
selected bond lengths of the complexes. The asymmetric unit of 3a consists of neutral,
half molecule of [Ni(1LISQ)2] which is located on a centre of inversion. As the
molecule is located on the centre of inversion both CF3 groups are located on opposite
sides of the NiL2 plane (anti-rotamer). The asymmetric unit of 3b consists of
crystallographically independent anion, [Ni(1LISQ)2]-, and one cation, [CoCp2]+.
Complex 3c crystallizes with two CH2Cl2 molecules of crystallization. The
asymmetric unit of 3d consists of one crystallographically dependent neutral, half
molecule, [Ni(1L)2(OClO3)2] which is located on a centre of inversion. The
coordinated ClO4 anion of compound 3d was found to be disordered on two positions
with equal occupation factors. Equal anisotropic displacement parameters were
attributed to the corresponding atoms. Two N,O-coordinated 1L ligands are located on
an equatorial plane in trans positions relative to each other. The axial positions are
occupied by two oxygen atoms, each from an O-coordinated perchlorate anion.
Complexes 3a and 3b are found to be square planar with NiN2O2 donor set and 3d
found to be octahedral with NiN2O4 donor set.
53
Chapter 3
3a 3b 3d
Ni1-N8 1.8444(10) 1.843(3) 2.0487(13)
Ni1-N38 - 1.831(3) -
Ni1-O1 1.8365(9) 1.844(2) 2.0298(11)
Ni1-O31 - 1.846(3) 2.0934(18)
N8-C9 1.4271() 1.421(5) 1.428(2)
N8-C7 1.3550(15) 1.378(5) 1.296(2)
C2-O1 1.3148(15) 1.329(5) 1.2394(18)
C2-C3 1.4249(16) 1.428(5) 1.460(2)
C2-C7 1.4318(17) 1.407(6) 1.513(2)
C3-C4 1.3852(17) 1.384(6) 1.357(2)
C4-C5 1.4304(18) 1.407(6) 1.473(2)
C5-C6 1.3756(17) 1.386(6) 1.352(2)
C6-C7
N38-C37
N38-C39
C32-O31
C32-C33
C32-C37
C33-C34
C34-C35
C35-C36
C36-C37
1.4178(17)
-
-
-
-
-
-
-
-
-
1.410(6)
1.382(5)
1.429(5)
1.345(5)
1.403(6)
1.422(6)
1.411(6)
1.386(6)
1.401(6)
1.394(6)
1.438(2)
-
-
-
-
-
-
-
-
-
Table 3.4. Selected bond distances (Å) in complexes 3a, 3b, and 3d.
In the following, we discuss the bond distances, specifically the C-C, C-N, and
C-O distances in nickel complexes 3a, 3b, and 3d. It is remarkable that in the neutral
square planar complex, 3a, the C-O, C-N, and C-C distances (Table 3.4.), in both the
N,O-coordinate ligands, are in excellent agreement with the geometrical features of a
monoanionic π radical ligands (1LISQ)1-•, and therefore, the nickel ion possesses an
experimentally determined spectroscopic oxidation state of +II (d8). As shown in
Table 3.1. these bond distances differ slightly from those in the cobalt complex, 2a;
indicating that the respective ligand oxidation levels in both the species are
54
Chapter 3
Ni1
O1
C2
C3
C4
C5
C6
C7
C9
N8
Figure 3.8.a. Thermal ellipsoidal diagram of the neutral [NiII(1LISQ)2] in crystal structure 3a.
Ni1
O31
N38
C32
C33
C34
C35
C36
C37
C39
O1
N8
C2
C3
C4
C5
C6
C7
Co
C9
F
Figure 3.8.b. Thermal ellipsoidal diagram of the monoanionic [NiII(1LISQ)(1LIP)] [CoCp2] in
crystal structure 3b.
55
Chapter 3
Ni1
O1
C2
C3 C4
C5
C6
C7
N8
C9
O31
Figure 3.8.c. Thermal ellipsoidal diagram of the neutral [NiII(1LIBQ)2(OClO3)2] in
crystal structure 3d.
different as are the metal oxidation states. Very similar dimensions of the π radical
ligands have been reported14, 15 for [M(LoISQ)2] (M = NiII, PdII, PtII) complexes; thus,
the charge distribution in 3a is best described as [NiII(1LISQ)2].
In monoanionic 3b, C-O and C-N are slightly longer than the bond distances
observed in neutral 3a (shown in Table 3.4.). The C-C bond distances in the
aminophenolate ring are also slightly longer than the C-C bond distance observed in
neutral species, 3a. The observed C-O, C-N, and C-C distances closely resemble those
which were calculated from the arithmetic average of one monoanionic o-
iminobenzosemiquinonate(1-) π radical and one dianionic o-aminophenolate(2-)
(Scheme 3.2.) which are comparable to the bond distances found neutral cobalt
complex (2a) in this chapter. It is noteworthy, that Ni-O and Ni-N bond distances in
both 3a and 3b are identical within the experimental error + 0.005 Å, indicating no
change in the oxidation state of the central nickel ion. This clearly indicates the
presence of the one monoanionic π radical ligand and one dianionic o-
aminophenolate(2-) ligand in 3b. Thus, the charge distribution in the anionic part of
3b can be better described as [NiII(1LISQ)(1LIP)]- ↔ [NiII(1LIP)(1LISQ)]-; where the
geometry of the two ligands is identical and thus the excess electron is delocalized
(class III ligand mixed valency).
56
Chapter 3
The neutral complex 3d is octahedral, where the two apical positions are
occupied by two perchlorate anions, coordinated to a central nickel ion in trans
fashion. The observed C-O, C-O, and C-C bond distances in N,O-coordinated o-
aminophenol ligands, are identical to the observed bond distances for the (LIBQ)0 in
[NiII(tren)(LIBQ)](PF6)216, and (1LIBQ)0 in [PdII(tertbpy)(1LIBQ)](PF6)(BF4) from chapter
2. Alternatively, two short and four long C-C distances in the aminophenolate rings of
3d also clearly show the pronounced quinoid-type distortion; thereby indicating the
presence of two neutral iminobenzoquinone(0), (1LIBQ)0 ligands in 3d. Thus,
paramagnetic 3d is best described as [NiII(1LIBQ)2(OClO3)2], where central nickel
exhibits d8 electronic configuration with two unpaired electrons, SNi = 1.
3.3.2. Electro- and Spectroelectrochemistry:
Figure 3.9. shows the cyclic voltammogram of 3a, recorded at 400 mV s-1. The
potentials are summarized in Table 3.5.
1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0
E [V]vs. Fc+/Fc
10 μA
Figure 3.9. Cyclic voltammogram of 3a in CH2Cl2 solution (0.1 M TBAPF6). Conditions: Scan rate
400 mV s-1 at 25º C. (glassy carbon as working electrode and ferrocene (Fc) as internal standard).
57
Chapter 3
400 600 800 1000 1200 1400 1600
0.0
0.8
1.6
2.4
3.2
[3a]2-
[3a]2+
[3a]- (3b)
[3a]
ε [M-1cm-1 X 104]
λ
[
nm
]
Figure 3.10. The electronic spectra showing 3a (solid red line) together with its electrochemically one-
electron reduced, 3b (dashed line) species, two-electron reduced species (solid black line), and two-
electron oxidized (dotted line) species, in CH2Cl2 solution containing 0.20 M [(n-Bu)4N] PF6 at -25ºC.
(Electrochemical two-electron reduction was only 90%. So traces of one-electron reduction shows a
band around 1400 nm in the spectrum of two-electron reduced species)
Complex E1/2 (V) vs Fc+/Fc
Oxidation Reduction 1 Reduction 2
[NiII(1LISQ)2] (3a) +0.038 -1.012 -1.74
Table 3.5. Summary of redox potentials in volts vs Ferrocenium/Ferrocene couple for 3a
Complex λmax, nm (ε, 104 M-1 cm-1)
3a 325 sh (1.4); 588 (0.2); 894 (3.4)
3a- (3b) 382 sh (0.6); 611 (0.2); 900 (0.2); 1344 (2.1)
3a2-
3a2+
352 sh (1.4); 1356 (0.5)
483 (0.5)
Table 3.6. Electronic spectra of the electrochemically generated one-electron reduced (3b), two-
electron reduced, and two-electron oxidized (3c) species of 3a in CH2Cl2 solution.
58
Chapter 3
The CV of 3a is very similar to that reported for [NiII(LOISQ)2].14, 15 Two
successive one-electron reductions of 3a are observed at -1.01 V and -1.74 V.
Controlled potential coulometry at +0.6 V shows that 3a can be oxidized in a nearly
reversible one-step, two-electron oxidation. As we have isolated the neutral species,
3a and it’s one-electron reduced species 3b we assign all these electron transfer
processes as ligand centered processes as shown in Eq 3.2. It is noteworthy that upon
two-electron oxidation, geometry of the species changes from square planar to
octahedral in the presence of coordinating anions like ClO4-.
[NiII(1LIP)2]2- [NiII(1LIP)(1LISQ)]-[NiII(1LISQ)2]0
-e
+e
-e
+e Eq 3.2
Spectroelectrochemical measurements of 3a and of its oxidized and reduced
forms are shown in Figure 3.10. The spectra have been recorded in CH2Cl2 solution
containing 0.20 M [(n-Bu)4N] PF6 at -25º C in the range of 300-1700 nm. Table 3.6.
summarizes the electronic spectra of these complexes. It is interesting to first discuss
the absorption spectra of the neutral nickel complex 3a and it monoanion 3b. 3a and
3b exhibit an intense band at 894 nm (ε = 3.4 x 104 M-1 cm-1) and 1344 nm (ε = 2.1 x
104 M
-1 cm-1), respectively. For comparison, the corresponding complexes
[NiII(LNISQ)2] and [NiII(LNISQ)(LNIP)]- display these bands at 839 nm (ε = 4.0 x 104 M-1
cm-1) and 1119 nm (ε = 1.7 x 104 M-1 cm-1), respectively.
Figure 3.11. MO scheme for [NiII(LNISQ)(LNIP)]-
59
Chapter 3
For the neutral species [NiII(LNISQ)2], it has been established11 that the upper
valence region contains four doubly occupied MOs that are predominantly centered
on the central NiII ion (d8). The LUMO+1 orbital is dominated by the Ni dx2-y2 MO,
which is strongly σ antibonding with the ligands. The HOMO-LUMO transition
1b1u→2b2g represents the intense ligand-to-ligand charge transfer band (LLCT). No
other ligand-to-metal charge transfer bands (LMCT) are observed in the visible
region. In the corresponding monoanion, 3b, (Figure 3.11.) the former LUMO (2b2g)
becomes SOMO, which has ~15% Ni 3dxz character; the transition 1b1u→2b2g is again
electric dipole and spin-allowed and represents a ligand-to-ligand intervalence charge
transfer band (LLIVCT).
The electronic spectrum of the two-electron oxidized form of 3a, displays
single absorption maximum at ~480 nm, which is characteristic of an uncoordinated
quinone charge transfer band in CH2Cl2 solution.15 Thus, the presence of neutral o-
iminobenzoquinone(0) ligand in the two-electron oxidized complex is implicated.
3.3.3. Magnetic Properties:
The electronic ground state of the complexes 3a, 3b, and 3d have been
established from the variable-temperature magnetic susceptibility measurements in
the range 3-300K by using a SQUID magnetometer. Complex 3a is diamagnetic due
to presence of two o-iminobenzosemiquinonate(1-) π radical ligands. The two radicals
couple intramolecularly, antiferromagnetically, through a diamagnetic NiII (d8) centre
yielding a singlet ground state. Complexes 3b and 3d are both paramagnetic with an S
= ½ and S = 1 ground states, respectively.
Figure 3.12. shows the temperature dependence of the magnetic moment of 3b
and 3d. Complex 3a displays a temperature independent magnetic moment of ~1.8 μB
(g = 2.08), in the temperature range 60-298 K, which is indicative of an S = ½ ground
state. Whereas, 3d shows an effective magnetic moment of ~2.9 μB (g = 2.04), which
indicates a triplet ground state. Simulation parameters are: S = 1, D = 5.5 cm-1, g =
2.04 and TIP = 0.25 X 10-3 cm3 Mol-1. This indicates the presence of two neutral N,O-
coordinated o-iminobenzoquinone(0) ligands, and a paramagnetic nickel(II) (d8, S =1
in an octahedral geometry) centre in 3d, i.e., [NiII(1LIBQ)2(OClO3)2].
60
Chapter 3
50 100 150 200 250 300
1.0
1.2
1.4
1.6
1.8
2.0
2.2
Exp
Sim
μeff [μB]
T [K] 50 100 150 200 250
2.0
2.2
2.4
2.6
2.8
3.0
Exp
Sim
μeff [μB]
T [ K]
3b 3d
*
Figure 3.12. μeff vs T graph of 3b and 3c (4-300 K); External applied field is 1T.
g = 2.08 for 3b; TIP = 0.25 X 10-3 cm3 Mol-1, g = 2.04 and D = 5.5 cm-1 for 3d.
* Experimental artefact (?)
280 300 320 340 360 380
Exp
Sim
dX''
dB
B [mT]
2.4 2.3 2.2 2.1 2 1.9 1.8 1.7
g values
Figure 3.13. X-band EPR spectrum of the 3b in frozen CH2Cl2 solution at 90 K. Experimental
conditions: microwave frequency 9.52 GHZ; power 10 μW modulation 1 mT.
61
Chapter 3
Figure 3.13. shows the X-band EPR spectrum of 3b in frozen CH2Cl2 at 90K
exhibiting a rhombic signal (gx = 2.075, gy = 2.014, gz = 2.046 (giso = 2.045)), which
resembles closely those reported for many square planar bis(dioxolene)nickel(II)
monoanions with an S = ½ ground state.11 These spectra have recently been
interpreted11 in terms of a central, diamagnetic nickel(II) ion(d8), a (LOIP)2- ion, and a
π radical ion (LOISQ)1-, in which the unpaired electron is delocalized over both ligands.
The SOMO (b2g) of these compounds, under D2h symmetry, is basically the anti
symmetric combination of the SOMO of the free semiquinonate(1-) ligand; it
transforms “gerade” under inversion, and therefore, mixes with the out-of-plane dxz
orbital of the nickel(II) and, thereby, acquires some metal character (~15%), which in
turn gives rise to a sizable nickel hyperfine coupling. Since the ground state 2BB2g
readily mixes with relatively low-lying d-d excited states, it has a reasonably large
orbital angular momentum that manifests itself in a relatively large g anisotropy.
The charge distribution in the monoanion 3b is therefore correctly described by the
two resonance structures: [Ni ( L )( L)] ↔ [Ni ( L )( L)] .
11, 14
II 1ISQ 1IP -II 1IP 1ISQ -
62
Chapter 3
3.4. Pd complexes:
3.4.1. Syntheses and X-ray Crystal Structures:
The diamagnetic complex 4a has been prepared according to the reported
procedure14, 15 as follows: Reflux of a solution of triethylamine, two equivalents of
1LH2, and one equivalent of PdCl2 in MeOH for 1 h, followed by stirring at room
temperature in air, for 2 h gives a blue precipitate of diamagnetic, four-coordinated
[Pd(1LISQ)2] (3a) in a moderate yield (~30%). 4a can be reduced and oxidized
successively by one- and two-electrons, respectively. One-electron chemical reduction
of 4a with one equivalent of [CoCp2] in degassed CH2Cl2, under an Ar atmosphere,
gives green coloured, monoanionic, paramagnetic (S = ½), four-coordinated
[PdII(1LISQ)(1LIP)]-[CoCp2]+ (4b), in good yield (60%). One-electron chemical
oxidation of 4a in degassed CH2Cl2 solution using one equivalent of AgBF4 as an
oxidant, under an Ar atmosphere, gives a red coloured solution. Reddish-brown
microcrystalline precipitate of monocationic, paramagnetic (S = ½), four-coordinated
[PdII(1LISQ)(1LIBQ)]+ BF4- (4c), was obtained in good yield (70%), upon the addition of
n-hexanes. Two-electron chemical oxidation of 4a with two equivalent of [NO]BF4 in
degassed CH2Cl2, under an Ar atmosphere, gives green coloured solution. Green
coloured microcrystals of dicationic, diamagnetic four-coordinated [Pd(1LIBQ)2]3
(BF4)4 {(BF4)2H}2 * 4CH2Cl2 (4d) in good yield (66%) obtained on addition n-
hexanes at low temperature (-10° C). Complexes 4a, 4b, 4c, and 4d are four members
of a five membered electron-transfer series. All attempts to isolate the dianionic
species of this series failed due to the extreme oxygen sensitivity of this species.
The crystal structures of 4a, 4b, 4c, and 4d have been determined at 100(2) K
by using Mo Kα radiation. Table 3.7. shows selected bond lengths in complexes 4a,
4b, 4c, and 4d. Figure 3.14. (a-c) shows the thermal ellipsoid diagrams of 4a, 4c, and
4d, respectively, without solvent molecules, but with assigned bonding pattern of the
iminophenolate type ligands. In 4a, 4b, 4c, and 4d the palladium(II) ions are always
N,O-coordinated to two ligands in trans positions relative to each other at various
oxidation levels, namely, (1LIP)2-, (1LISQ)1-y, or (1LIBQ)0, giving rise to square planar
coordination polyhedra with PdN2O2 donor set.
63
Chapter 3
The asymmetric unit of 4a consists of one crystallographically dependent
neutral, half-molecule, [Pd(1LISQ)2] which is located on the centre of inversion. We
also note that the two crystallographically identical (trifluoromethyl)phenyl groups in
4a are in trans positions relative to each other, and are positioned in such a way that
one CF3 group is above the PdL2 plane and the other one is below the plane (anti-
rotamer). The observed C-O, C-N, and C-C bond distances (Table 3.7.) in the neutral
4a, are very close to the bond distances reported for neutral Ni and Pd
complexes11, 14, 15 for o-iminobenzosemiquinonate(1-) π radical ligands. Thus, these
results clearly indicate the presence of two N,O-coordinated, π radical ligands, which
are intramolecularly, antiferromagnetically coupled through a diamagnetic PdII (d8; S
= 0) centre to give a singlet ground state, i.e., [PdII(1LISQ)2].23
The structure of 4b consists of well separated monoanion [PdII(1LIP)(1LISQ)]-
and cation [CoCp2]+ in 1:1 ratio. The overall structure of the monoanion is very
similar to that of monocation, 4c. Therefore, the crystal structure is not shown
separately. The complex 4b was refined in centrosymmetric space group P⎯1 (No.2),
but this refinement suffered from disorder of (trifluoromethyl)phenyl groups (PhCF3)
of the anion and severe disorder of the cobaltocenium cation, each of them residing on
a centre of inversion. A closer inspection of the structure revealed that the disorder of
the cation disappears upon refinement in the chiral space group. P1 (No.1).
Refinement of the disordered PhCF3 groups of the anion clearly showed that the
occupation factors for both PhCF3 units were different on either side of the pseudo
inversion centre (52:48 vs 87:13). The structure is probably most accurately described
as a racemic twin consisting of two rotamers with respect to the orientation of the CF3
groups. It is quite remarkable that the C-C, C-N, and C-O distances of both ligands in
4b are (within experimental error + 0.01 Å) identical. It is also noteworthy that the
averaged C-C, C-N, and C-O bonds in 4b do not allow straight forwardly to assign the
ligand oxidation level using the data in Figure 3.3. Rather, it appears that these
distances are close to the arithmetic mean between those of (LIP)2- and (LISQ)1- as
shown in Scheme 3.2. This result implies that the excess electron in [PdII(1LIP)(1LISQ)]-
is delocalized over both ligands.
64
Chapter 3
Figure 3.14.a. Thermal ellipsoidal diagram of neutral 4a with the interpretation of the
bonding pattern in the ligands.
Figure 3.14.b. Thermal ellipsoidal diagram of monocation 4c with the interpretation of the
bonding pattern in the ligands.
65
Chapter 3
Figure 3.14.c. Thermal ellipsoidal diagrams of two conformers (syn- (top) and anti-(bottom) rotamers)
dication 4d with the interpretation of the bonding pattern in the ligands.
The structure of 4c reveals that monocation [PdII(1LISQ)(1LIBQ)]+ and BF4-
anion are present in a 1:1 ratio. Interestingly, the cation does not possess
crystallographically imposed symmetry; both N,O-coordinated ligands are in trans
position relative to each other and are crystallographically independent. It is
noteworthy that the CF3-groups of both ligands are located on the same side of PdL2-
plane (syn-rotamer). Unexpectedly, the C-C, C-N, and C-O bond lengths of both
ligands differ in a characteristic fashion, which is in stark contrast to the structure of
the monoanion in 4b. These bond distances allows us to assign (see Figure 3.3) ligand
oxidation levels as (1LISQ)1-y for one ligand (N7, O1, C1-C6) but (1LIBQ) for the other
66
Chapter 3
one (N37, O31; C31-C36). The ligand oxidation level distribution yields again a +II
oxidation state for the central palladium ion (d8, SPd = 0). The unpaired electron of the
St = 1/2 ground state is localized on the single (1LISQ)1-y ligand; it is not significantly
delocalized over both ligands in the solid state. This localization in the solid state is
due to an unsymmetrical ion pairing between the cation and the BF4- anion. The latter
is located above the (1LIBQ) plane and involves a weak F(63)•••C(31) interaction at
2.709 Å (Figure 3.15.).
N
O
Pd
Ph
1
2
3
4
56
4a 4b* 4c* 4d**
Pd-N 1.963(2) 1.960(2) 1.996(2) 2.011(2) 1.966(2) 1.998(5)
Pd-O 1.975(2) 2.001(2) 1.984(2) 1.997(2) 1.974(2) 2.005(2)
C2-N 1.354(4) 1.377(3) 1.378(3) 1.316(3) 1.345(3) 1.301(5)
C1-O 1.314(4) 1.329(3) 1.341(3) 1.255(3) 1.313(3) 1.252(5)
C1-C2 1.437(5) 1.422(4) 1.426(4) 1.491(3) 1.443(3) 1.506(6)
C2-C3 1.413(4) 1.406(4) 1.406(3) 1.424(3) 1.417(3) 1.430(6)
C3-C4 1.377(4) 1.388(4) 1.391(4) 1.353(3) 1.362(3) 1.350(6)
C4-C5 1.421(4) 1.401(4) 1.400(4) 1.478(3) 1.445(3) 1.472(6)
C5-C6 1.377(4) 1.403(4) 1.396(4) 1.350(3) 1.376(3) 1.353(6)
C6-C1 1.427(4) 1.426(4) 1.405(4) 1.450(3) 1.427(3) 1.447(6)
Table 3.7. Selected bond distances (Å) in complexes 4a, 4b, 4c, and 4d.
* Corresponding bond lengths are given for two crystallographically independent ligands in
[Pd(1LISQ)(1LIP)][CoCp2] (4b) and [Pd(1LISQ)(1LIBQ)](BF4) (4c).
** Averaged values.
67
Chapter 3
y
mmetric ion-pairing enforces localisation in the solid st
a
2.71Å
Figure 3.15. Unsymmetric ion-pairing enforcing electron localisation in the solid state in 4c.
It is noteworthy that the two Pd-N bond distances in 4c are at 1.966 + 0.006 Å
for the (1LISQ)1-y ligand and at 2.011 + 0.006 Å for the (1LIBQ) ligand; they differ
significantly; the same holds true for the corresponding two Pd-O distances at 1.974 +
0.006 Å and 1.997 + 0.006 Å. It is also noteworthy, that the dimensions of the
Pd(1LISQ) part in both 4a and 4c are identical within experimental error.
68
Chapter 3
Complex 4d crystallizes in the triclinic space group P1 with three
[PdII(1LIBQ)2]2+ dications and four well-separated BF4- anions as well as two
{(BF4)2H}- monoanions and four CH2Cl2 molecules of crystallization per unit cell.
One PdII ion lies on a centre of symmetry, whereas the other two occupy symmetry
related general positions. Interestingly, the conformations of the two
crystallographically different dications are not the same. While in the first, both CF3
groups are in anti position relative to each other, these are located on the same side of
the PdL2 plane in the second. The C-O, C-N, C-C, Pd-N, and Pd-O distances in both
rotamers are within experimental error identical. The bond lengths within the ligands
agree nicely with those given in Figure 3.3. for the quinone oxidation level, 1LIBQ.
Thus, an electronic structure as [PdII(1LIBQ)2]2+ is established unequivocally by X-ray
crystallography. The presence of two {(BF4)2H}- monoanions is clearly established by
the short F•••F distance of ∼2.6 Å between two BF4- groups. A few other examples for
the presence of this anion were identified by a search in the Cambridge X-Ray
Crystallographic Data Base.28
69
Chapter 3
3.4.2. Electro- and Spectroelectrochemistry:
Figure 3.16. shows the cyclic voltammogram of 4a, recorded at 100 mV s-1.
The potentials are summarized in Table 3.8. Four reversible one electron transfer
waves are observed at E1/2 values of +0.65, +0.17, -1.01, and -1.54 V vs. Fc+/Fc.
Controlled-potential coulometric measurements established that 4a can be twice one-
electron oxidized yielding a mono- and a dication and twice one-electron reduced
yielding a mono- and dianion. Thus, a complete electron transfer series consisting of
five species [PdII(1L)2]n (n = 2+,1+,0, 1-,2-) (Eq 3.3.) has been established as has been
previously done for [Pd(LN,O)2] with quite similar E1/2 values of +0.47, +0.08, -0.99, -
1.40.14, 15
-2.5-2.0-1.5-1.0-0.50.00.51.01.5
E [V]vs. Fc+/Fc
10 μA
Figure 3.16. Cyclic voltammogram of 4a in CH2Cl2 solution (0.1 M TBAPF6). Conditions: Scan rate
100 mV s-1 at 25º C. (glassy carbon as working electrode and ferrocene (Fc) as internal standard).
Complex E1/2 (V) vs Fc+/Fc
Oxidation 2 Oxidation 1 Reduction 1 Reduction 2
[Pd(1LISQ)2] (2a) +0.652 -0.17 -1.012 -1.539
Table 3.8. Summary of redox potentials in volts vs Ferrocenium/Ferrocene couple for 4a
70
Chapter 3
[PdII(1LIP)2]2- [PdII(1LIP)(1LISQ)]-[PdII(1LISQ)2]0
-e
+ e
-e
+ e
Eq 3.3
- e
+ e [PdII(1LIBQ)(1LISQ)]+- e
+e [PdII(1LIBQ)2]2+
The electrochemically oxidized and reduced forms of 4a are stable at ambient
temperature on the coulometric time scale which allowed the recording of the
electronic spectra of complexes. Figure 3.17. shows the spectrum of 4a as well as
those of its electrochemically generated one-electron reduced and oxidized forms
(CH2Cl2 solution containing 0.20 M [N(n-Bu)4] PF6 at -25 ۫º C in the range of 300-
2500 nm). Figure 3.18. shows the electronic spectra of the electrochemically
generated two-electron reduced as well as oxidized forms of 4a in the region 300–
1000 nm. Table 3.9. summarizes the electronic spectra of all the complexes.
500 1000 1500 2000
0.4
0.8
1.2
1.6
2.0
2.4
2.8
4a 0
4a -
4a +
ε [M-1cm-1 X 104]
λ [nm]
Figure 3.17. The electronic spectra showing 4a (solid line) together with its electrochemically one-
electron reduced, 4b (dotted line) and one-electron oxidized (dashed line) 4c,
species in CH2Cl2 solution containing 0.20 M [N(n-Bu)4] PF6 at -25ºC.
71
Chapter 3
300 400 500 600 700 800 900 1000
0.0
0.4
0.8
1.2
1.6
2.0
2.4
4a 2-
4a 2+
ε [M-1cm-1 X 104]
λ [nm]
Figure 3.18. The electronic spectra showing electrochemically generated two-electron reduced species
(solid line), and two-electron oxidized species (dashed line), 4d in CH2Cl2 solution
containing 0.20 M [N(n-Bu)4] PF6 at -25 ۫ C.
Complex λmax, nm (ε, 104 M-1 s-1)
4a 871 (2.7); 546 (0.2); 403 sh, (0.2)
4a- (4b) 1564 (2.4); 872 (0.13); 615 (0.21); 360 sh (0.9)
4a+ (4c) 1986 (2.5); 1432 (0.23); 1173 (0.2); 875 (0.3); 507 (1.3)
4a2+(4d) 625 (0.25); 394 (1.8); 331(1.8)
4a2- 592(0.2); 359(1.3)
Table 3.9. Electronic spectra of the electrochemically generated 4a, and its one-electron reduced,
(4b), two-electron reduced, one-electron oxidized (4c), and two-electron
oxidized (4d) species in CH2Cl2 solution.
The electronic spectrum of 4a exhibits a very intense, spin and dipole allowed,
ligand-to-ligand charge transfer band (LLCT) at 871 nm (ε = 2.7 x 104 M-1 cm-1). This
absorption band is characteristic for all square planar [MII(Ly)2] complexes.3, 11
72
Chapter 3
The electronic spectrum of 4b shows also a very intense band at 1547 nm (ε =
2.4 x 104 M
-1 cm-1), which is assigned as a ligand-to-ligand intervalence charge
transfer band expected for all complexes with ligand mixed valency.25
The electronic spectrum of 4c exhibits two important absorption maxima at
1999 nm (ε = 2.5 x 104 M
-1 cm-1) and 504 nm (ε = 1.28 x 104 M
-1 cm-1). The
dominating intense band at 1992 nm is an “intervalence band”, which supports the
assignment of ligand mixed valency in 4c. The medium intense absorption maximum
at 504 nm is characteristic for the presence of an N,O-coordinated
o-iminobenzoquinone, (1LIBQ)0, ligand. For the uncoordinated organic molecule
(LIBQ)0 in CH2Cl2 this maximum is observed15 at 488 nm (as well as a shoulder at 396
nm).
All attempts to generate a solid material containing the dianion [PdII(1LIP)2]2-
failed due to the extreme oxygen sensitivity of this species. The electronic spectrum
of the electrochemically generated dianion has been successfully recorded and is
shown in Figure 3.18. A single relatively intense ligand-to-metal charge transfer band
73
Chapter 3
at 592 nm (ε = 0.2 x 104 M-1 cm-1) and no LLCT band is observed > 600 nm (Figure
3.18.).
In contrast, the two-electron oxidation of 4a by two equivalents of [NO]BF4 in
CH2Cl2 successfully generated green, diamagnetic [PdII(1LIBQ)2]3(BF4)4{(BF4)2H}2*
4CH2Cl2 (4d). Its electronic spectrum exhibits two absorption maxima, characteristic
of (1LIBQ) ligands at 625 and 394 nm (Figure 3.18, Table 3.9.).
3.4.3. Magnetic Properties:
The electronic ground state of the complexes 4a, 4b, 4c, and 4d have been
established from the variable-temperature magnetic susceptibility measurements in
the range 3-300K using a SQUID magnetometer. Complex 4a is diamagnetic due to
presence of two o-iminobenzosemiquinonate(1-) π radical ligands. Two of the radical
couple intramolecularly, antiferromagnetically, though the diamagnetic PdII (d8)
centre yielding a singlet ground state as in the case of 4a. Complex 4d is also
diamagnetic due to presence of two neutral o-iminobenzoquinone(0) ligands
coordinated to a diamagnetic PdII ion (d8; SPd = 0). Complexes 4b and 4c both are
paramagnetic with an S = ½ ground state, respectively.
0 50 100 150 200 250 300
0.4
0.8
1.2
1.6
2.0
Exp
Sim
T [K]
0 50 100 150 200 250 300
0.4
0.8
1.2
1.6
2.0
Exp
Sim
μeff [μB]
T [K]
4b 4c
Figure 3.19. μeff vs T graph of 4b and 4c (4-300 K); External applied field is 1T.
g = 2.00(fixed) and TIP = 0.1 X 10-3 cm3 Mol-1 for 3b; g = 2.0006 for 3c.
74
Chapter 3
300 320 340 360 380
B [mT]
dX''
dB
2.3 2.2 2.1 2 1.9
Sim
Exp
320 340 360
B [mT]
2.2 2.1 2 1.9
Sim
Exp
4b 4c
g value
s
Figure 3.20. X-band EPR spectra of the 4b and 4c in frozen CH2Cl2 solution at 10 K. Experimental
conditions: microwave frequency 9.65 GHZ; power 10 μW modulation 1 mT.
Figure 3.21. Electronic structures of the five members of the electron-transfer series [PdII(1L)2]z (z =
2+, 1+, 0, 1-, 2-). The two highest energy molecular orbitals are only considered.
75
Chapter 3
Magnetic susceptibility measurements (4-300 K) (shown in Figure 3.19.)
revealed a temperature-independent magnetic moment of ~1.7 μB for both 4b and 4c,
indicating an S = ½ ground state for both complexes.
The X-band EPR spectrum of 4b (Figure 3.20.) in frozen CH2Cl2 at 10 K
reveals a rhombic signal with parameters g1 = 2.0715, g2 = 2.0167, g3 = 1.974 (giso =
2.021). Similar signals have been reported for many square planar monoanions
[M(L)2]- (ref. 11a). The unpaired electron resides predominantly on the ligands.11, 14
Whereas, The X-band EPR spectrum of 4c (Shown also in Figure 3.20.) in frozen
CH2Cl2 at 10 K displays a narrow signal at giso = 2.0007 (width 10 mT) which is
typical for the square planar monocationic species [M(L)2]+ (see Table 5 in ref. 11a).
This confirms the S = 1/2 ground state and indicates that the unpaired electron resides
predominantly on the ligand11, 14 as in (1LISQ)1-y. The electronic structure of 4b and 4c
is thus best described as [Pd(1LISQ)(1LIP)]- and [Pd(1LISQ)(1LIBQ)]+, respectively.
The electronic structures for this series of complexes and EPR spectra of the
monoanion, 4b and monocation 4c can be better understood in terms of a simple
model involving only two redox-active molecular orbitals (shown in Figure 3.21.),
which are basically the symmetric and antisymmetric combination of the SOMO of
the free (uncoordinated) semiquinonate(1-) ligand.
Thus, the diamagnetic dianion [Pd(1LIP)2]2- possesses a closed-shell
configuration (au)2(bg)2 with two iminophenolate(2-) ligands and a central PdII ion.
Similarly, the diamagnetic dication [PdII(1LIBQ)2]2+ consists of two closed-shell
quinone ligands and a central PdII ion. The ground state configuration possesses an
(au)0 LUMO which is a ligand π* orbital. Therefore, both the complexes are
diamagnetic. The structural parameters of 4d support this interpretation.
The monoanion [PdII(1LISQ)(1LIP)]- possesses an (au)2(bg)1 ground state (Figure
3.21.) which transforms "gerade" under inversion.11b, 14 It, therefore, mixes readily
with the out-of-plane dxz orbital of the Pd(II) ion and acquires thereby some metal
character. Since the above ground state mixes readily with energetically relatively
low-lying d-d excited states, it has a sizeable orbital angular momentum which
manifests itself in the relatively large g-shift observed experimentally for this
monoanion.
In contrast, the same bonding model predicts a ground state (au)1(bg)0 for the
monocation [PdII(1LISQ)(1LIBQ)]+ (Figure 3.21.) with S = 1/2 which represents a
76
Chapter 3
delocalized ligand mixed valence case. Since the au MO transforms "ungerade" under
inversion it cannot mix with any metal d orbital. As pointed out previously, the lack of
metal character in the SOMO of this delocalized monocation makes the spin-orbit
coupling for excited states with the ground state inefficient.11b, 14 Therefore, there is
little angular momentum in the presumed ground state wave function, and the
observed g-shift is very small and reflects organic radical character. From the X-ray
structure determination it is clear that 4c cannot have an (au)1 ground state in the solid
state since the unpaired electron is localized on one (LISQ)1-y ligand while the other is a
closed-shell quinone 1LIBQ.
77
Chapter 3
3.5. Discussion and Conclusions:
The molecular and electronic structures of the square planar Co, Ni, and Pd
complexes have been elucidated experimentally as well as with the help of density
functional calculations.11, 14, 23, 25 As we have pointed out previously, small structural
differences in the ligand C-O, C-N, and C-C distances between the corresponding
four-coordinate cobalt, nickel, and palladium complexes may exist and might point to
differing ligand oxidation levels (and concomitantly to the central metal ions) in the
complexes in different oxidation levels. This has been verified experimentally by low-
temperature (100 K) crystallography of all Co, Ni, and Pd complexes in different
oxidation levels.
The low temperature and high quality crystal structures of square planar cobalt
complexes (neutral 2a and monoanion 2b) clarify all the discrepancies that arised and
thereby, allowed assignment of the spectroscopic oxidation state of the central cobalt
ion as +III. But it is still catastrophic from the DFT calculations carried out on the
exact analogue of 2a and its monoanion.25 In the HOMO of the monoanionic Co
complex, the 2b2g orbital has almost equal contributions from the Co 3dxz and the
ligand b2g fragment orbitals (Table 6 from ref 25). The oxidation of monoanion to
neutral complex is accompanied by a 12% decrease in the 2b2g orbital. The unpaired
electron of the neutral species is located predominantly in a metal based 2b3g orbital
(Figure 3.22.), in agreement with the observed large anisotropy of the EPR g tensor of
[Co(L)2]0.25 In the nickel case these 2b2g and 2b3g orbitals are predominantly ligand-
based orbitals (Figure 3.22.).This is consistent with the considerably higher effective
nuclear charge of nickel(II) relative to cobalt(II). From DFT calculations the results
are not straightforward to assign the spectroscopic oxidation state to the central Co
ion. Taking the following resonance structures into account, one can describe the
electronic structure as follows.
[CoIII(1LISQ)(1LIP)] [CoIII(1LIP)2]-[CoII(1LIP)2]-2
[CoIII(1LIP)(1LISQ)] [CoII(1LISQ)(1LIP)]-
[CoII(1LISQ)2]
+e +e
- e - e
S = 1/2 S = 1/2
S = 1
78
Chapter 3
The resonance structures that formally contain CoIII ions have significant
weights which explain the observed ligand bond distances in four coordinate
complexes: 2a and 2b.
[Co(L)2]0[NiII(LISQ)2]0
Figure 3.22. MO schemes for both neutral Co complex (2a) and neutral Ni complex (2a). (M and L
represents metal and ligand based orbitals, respectively).
On the other hand, the electronic structures of [NiII(LNISQ)2] and of
[PtII(LOISQ)2] where (LNISQ)1-y represents the o-diiminobenzosemiquinonate(1-) and
(LOISQ)1-y represents the o-iminobenzosemiquinonate(1-) radical anion, have been
calculated by (relativistic) density functional theoretical and correlated ab initio
methods.11, 14, 23 It has been shown that their electronic structures and that of their
monocations and -anions can be understood in terms of a simple model which
involves only two redox-active molecular orbitals showed in Figure 3.21. which are
basically the symmetric and antisymmetric combination of the SOMO of the free
(uncoordinated) semiquinonate(1-) ligand. In 3a, 3b, 4a, 4b, 4c, and 4d the four filled
metal d-orbitals of lower energy indicate the presence of a diamagnetic d8
configuration of a central nickel(II) or palladium(II) ion. The large g shift (or g
anisotropy) observed for monoanions 3b and 4b is best explained with the molecular
orbitals showed in Figure 3.21. The structures of 3b and 4b are best described as
complexes with the ligand mixed valency; with one dianionic o-iminophenolate(2-)
79
Chapter 3
ligand and one monoanionic o-iminobenzosemiquinonate(1-) radical ligand. A
characteristic ligand-to-ligand intervalence charge transfer band observed in the near
infrared region for these complexes supports this formulation. An arithmetic average
bond distances between o-iminophenolate(2-) ligand and one
o-iminobenzosemiquinonate(1-) radical ligand are observed in complexes 3a and 4a,
indicating complete delocalization of the radical over both ligands.
Interestingly, in the crystal structure of the monocation in 4c, the excess
electron appears to be localized on one ligand, namely the (1LISQ)1-y monoanionic
radical. This is due to an unsymmetrical ion pairing in the solid state (Figure 3.15.).
The presence of the intense LLCT of 4c in CH2Cl2 solution at 1986 nm (ε = 2.5 x 104
M-1 cm-1) with its very narrow half-width at a half height of 366 cm-1 is more in
agreement with class III29 behaviour of 4c in solution.30 It is puzzling that the one-
electron oxidations of [M(3LISQ)2] (M = Ni, Pt) where H2(3LIP) represents 3,5-di-tert-
butyl-o-phenylenediamine and H2(4LIP) is N-phenyl-o-phenylenediamine, yields
diamagnetic dimers: {cis-[NiII(3LISQ)(3LIBQ)]2}2+ and {cis/trans-[Pt(4LISQ)(4LIBQ)]2}2+
containing weak Ni•••Ni and Pt•••Pt bonding interactions at 2.800(1) Å and 3.031(4)
Å, respectively.11a In these dimers, the dimensions of the four organic ligands were
found to be identical; they are the arithmetic mean of the two forms (LISQ)1-y and
(LIBQ). This indicates that now the one excess electron per half dimer is delocalized
over both ligands. The nature of the M•••M interaction is rather unclear at this point.
Interestingly, the density functional calculations on [PtII(LISQ)(LIBQ)]+ which explicitly
included relativistic effects by using the scalar ZORA method14 for the geometry
optimizations revealed that the above cationic form shows pronounced distortion
toward a quinoidal structure for both equivalent ligands. (LISQ)1-y and LIBQ represent
the one- and two-electron oxidized forms of the unsubstituted o-iminophenolate(2-)
dianion. Again, the C-N, C-O, and C-C bond distances reflect with arithmetic mean
between those observed for the (LISQ)1-y and (LIBQ) forms. Class III behaviour with an
(au)1 SOMO is calculated for the monocation.14, 29, 30
In summary, it has been clearly established that complexes containing N,O-
coordinated o-aminophenol ligands, the respective oxidation level may be defined by
high-quality X-ray crystallography. The C-O, C-N, and C-C bond lengths are found to
be characteristic for a given oxidation state. Thus, the following markers have been
identified on going from the N,O-coordinated (1LIP)2- dianion to the (1LISQ)1-y
80
Chapter 3
monoanionic π radical, and then to the neutral quinone (1LIBQ): a) The C-N bond
lengths decrease from 1.37 + 0.01 Å to 1.35 + 0.01 Å and, finally to 1.30 + 0.01 Å
with increasing oxidation level. b) Similarly, the C-O bond lengths decrease from 1.35
+ 0.01 Å to 1.30 + 0.01 Å to 1.24 + 0.01 Å. c) Finally, the six C-C bonds of the
aminophenolate six-membered ring of (1LIP)2- are nearly equidistant at 1.407 + 0.01 Å
indicating the aromatic character of the phenyl ring. One-electron oxidation to
(1LISQ)1-y results in two alternating short C-C bonds at 1.375 + 0.01 Å of partially
double bond character and four longer ones at 1.438 + 0.01 Å. This characteristic
distortion is labeled "quinoid-like". In the neutral genuine quinone form (1LIBQ), this
distortion is more pronounced with two alternating short C=C double bonds at 1.36 +
0.01 Å and four long C-C single bonds one of which at 1.52 + 0.01 Å being a normal
C-C single bond.
81
Chapter 3
3.6. References:
(1) (a) Carugo, O.; Djinovic, K.; Rizzi, M.; Castellani, C.B. Dalton Trans. 1991,
1551. (b) Mederos, A.; Domingguez, S.; Hernández-Molina, R.; Sanchiz, J.;
Britto, F. Coord. Chem. Rev. 1990, 193-195, 913.
(2) Pierpont C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331.
(3) Balch, A.; Holm, R. H. J. Am. Chem. Soc. 1966, 88, 5201.
(4) Warren, L. F. Inorg. Chem. 1977, 16, 2814
(5) Peng, S. -M.; Chen, C. -T.; Liaw, D. -S.; Chen, C. -I.; Wang, Y. Inorg. Chem.
Acta. 1985, 101, L31.
(6) Cariati, F.; Morazzoni, F.; Busetto, C.; Del Piero, G.; Zazzetta, A. J. Chem.
Soc. Dalton Trans. 1976, 342.
(7) Schaefer, W.P.; Marsh, R.E. Acta Crystallogr. Sect. B 1969, 25, 1675.
(8) Daul, C.; Schläpfer, C. W.; Zelewski, A. V.; Struc. Bonding 1979, 36, 12.
(9) Nemeth, S.; Simandi, L. I.; Argay, G.; Kálman, A. Inorg. Chem. Acta. 1989,
166, 31.
(10) Cheng, P. -H.; Cheng, H. -Y.; Lin, C. -C.; Peng, S. -M. Inorg. Chem. Acta.
1990, 169, 19.
(11) (a) Herebian, D.; Bothe, E.; Neese, F.; Weyhermüller, T.; Wieghardt, K. J.
Am. Chem. Soc. 2003, 125, 9116. (b) Herebian, D.; Wieghardt, K.; Neese, F.
J. Am. Chem. Soc. 2003, 125, 10997.
(12) Herebian, D.; Ghosh, P.; Chun, H.; Bothe, E.; Weyhermüller, T.; Wieghardt,
K. Eur. J. Inorg. Chem. 2002, 1957.
(13) Ghosh, P.; Begum, A.; Herebian, D.; Bothe, E.; Weyhermüller, T.;
Wieghardt, K. Angew. Chem. Int. Ed. Engl. 2003, 42, 563.
(14) Sun, X.; Chun, H.; Hildenbrand, K.; Bothe, E.; Weyhermüller, T.; Neese, F.;
Wieghardt, K. Inorg. Chem. 2002, 41, 4295.
(15) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.; Weyhermüller, T.;
Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 2213.
(16) Min, K. S.; Weyhermüller, T.; Wieghardt, K. Dalton Trans. 2003, 1126.
(17) Chun, H.; Verani, C. N.; Chaudhuri, P.; Bothe, E.; Bill, E.; Weyhermüller, T.;
Wieghardt, K. Inorg. Chem. 2001, 40, 4157.
(18) Verani, C. N.; Gallert, S.; Bill, E.; Weyhermüller, T.; Wieghardt, K.;
Chaudhuri, P. Chem. Commum. 1999, 1747.
82
Chapter 3
(19) Hegedus, L. S.; in Transition Metals in the Synthesis of Complex Organic
Molecules, University Science Books, Mill Valley, California 1994, p. 3.
(20) Jörgensen, C. K. in Oxidation Numbers and Oxidation States, Springer,
Berlin, Heidelberg, Germany. 1969.
(21) Weber, J.; Daul, C.; von Zelewsky, A.; Goursot, A.; Penigault, E. Chem.
Phys. Lett. 1982, 88, 78.
(22) Lelj, F.; Rosa, A.; Riccardi, G. P.; Casarin, M.; Cristinziano, P. L.; Morelli,
G. Chem. Phys. Lett. 1989, 160, 39.
(23) Bachler, V.; Olbrich, G.; Neese, F.; Wieghardt, K. Inorg. Chem. 2002, 41,
4179.
(24) Stiefel, E. I.; Waters, J. H.; Billig, E.; Gray, H. B. J. Am. Chem. Soc. 1965,
87, 3016.
(25) Bill, E.; Bothe, E.; Chaudhuri, P.; Chlopek, K.; Herebian, D.; Kokatam, S.;
Ray, K.; Weyhermüller, T.; Neese, F.; Wieghardt, K. Chem. Eur. J. 2005, 11,
204.
(26) Van der Put, P. J.; Schilperoord, A. A. Inorg. Chem. 1974, 13, 2476.
(27) (a) Gordon-Wylie, S. W.; Bominaar, E. L.; Collins, T. J.; Workman, J. M.;
Claus, B. L.; Patterson, R. E.; Williams, S. E.; Conklin, B. J.; Yee, G. T.;
Weintraub, S. T. Chem. Eur. J. 1995, 1, 528. (b) Gordon-Wylie, S. W.; Claus,
B. L.; Horwitz, C. P.; Leychkis, Y.; Workman, J. M.; Marzec, A. J.; Clark, C.
E.; Rickard, C. E. F.; Conklin, B. J.; Sellers, S.; Yee, G. T.; Collins, T. J.
Chem. Eur. J. 1998, 4, 2173. (c) Bour, J. J.; Beurskens, P. T.; Steggarda, J. J.
J. Chem. Soc. Chem. Commum. 1972, 221. (d) Collins, T. J.; Powell, R. D.;
Slebodnick, C.; Uffelman, E. S. J. Am. Chem. Soc. 1991, 113, 8419. (e)
Brewer, J. C.; Collins, T. J.; Smith, M. R.; Santarsiero, B. D. J. Am. Chem.
Soc. 1988, 110, 423. (f) Yagi, T.; Hanai, H.; Komorita, T.; Suzuki, T.;
Kaizaki, S. J. Chem. Soc. Dalton Trans. 2002, 1126.
(28) Goodfellow, R. G.; Hamon, E. M.; Howard, J. A. K.; Spencer, J. L.; Turner,
D. G.; Chem. Commum. 1984, 1604.
(29) Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem. 1967, 10, 247
(30) (a) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1. (b) Allen, G. C.; Hush, N. S.
Prog. Inorg. Chem. 1967, 8, 357.
83
84
Synthesis and Characterization of Octahedral Oxo-Mo and
Oxo-W Complexes with o-Aminophenol Type of Ligands
85
86
Chapter 4
4.1. Introduction:
The coordination chemistry of molybdenum and tungsten in their higher
oxidation states, IV to VI received a great deal of attention, due to its possible
relationship with redox active molybdenum1, 2 or tungsten3, 4 centres found in enzymes
and the general interest in polynuclear metal compounds.5, 6
A number of six coordinated (trigonal prismatic or antiprismatic) complexes of
Mo and W with noninnocent S,S-coordinated benzenedithiolenes,7-11 and N,S-
coordinated aminothiophenols12, 23 are reported in the literature. These molybdenum
and tungsten complexes are capable of deprotonating aromatic amino functionals12-19
to produce strong donor ligands from species such as H3A and H3B.
NH2
SH
N
H
OH HX
H3AH3B X = O, S
In complexes of molybdenum and tungsten the higher oxidation states V and
VI are normally stabilized by compact, strongly donating ligands such as oxo,20
sulfido20 or alkylidene.21 In the absence of such ligands, there is only moderately well-
developed chemistry17-19 of mononuclear molybdenum in such high formal oxidation
states, primarily involving the deprotonated aromatic ligands. The ligands hbpdH4,22
and haeH422 (Where hbpdH4 is N,N´-bis(2-hydroxybenzyl)-o-phenylenediamine and
haeH4 is 1,2-bis(2-hydroxyanilino)ethane) with an N2O2 donor sets, react with
MoO2(acac)2 and yields dioxo complexes with protonated aminoligands.22 Though
many octahedral Mo and W complexes with noninnocent o-benzenedithiolenes, and
o-aminothiophenols are reported previously, no compounds with o-aminophenolates
are reported in the literature to date. Thus, well characterized monomeric complexes
of molybdenum and tungsten with simple redox noninnocent O,N donor ligands are
still rather scarce.
Here, we aimed to synthesize octahedral M(2L)3 type of complexes where M =
Mo or W and 2L = 2-(4-fluoro)anilino-4,6-di-tert-butylphenol. Various synthetic
procedures and starting materials were utilized to synthesize this type of complexes
with noninnocent N,O donor o-aminophenolate ligands. Extreme oxygen sensitivity of
87
Chapter 4
the complexes, high stability of M=O bond, the presence of a donor atom set, N3O3,
and difficult reproducibility problems made the synthesis of this kind of Mo and W
complexes with noninnocent o-aminophenolate ligands unsuccessful. However,
finally we ended up with the following mononuclear oxo-molybdenum, binuclear μ-O
bridged oxo-Mo, and mononuclear or binuclear oxo-tungsten complexes.
1. [Mo(2LIP)(2LAP)(O)(OCH3)] • 2 MeOH (5)
2. [(2LIP)(2LAP)(O)Mo-(μ-O)-Mo(O)(2LIP)(2LAP)] (6)
3. [W(2LIP)(2LAP)(O)(Cl)] (7a)
4. {W(2LIP)(2LAP)(O)(OCH3)}2 • 0.5 MeOH (7b)
This chapter describes the synthesis and characterization of the above complexes;
5, 6, 7a, and 7b. It is known24-29 that o-aminophenol ligands (2LH2) can coordinate to
a metal ion in different ligand oxidation states namely, o-aminophenolate monoanion
(2LAP)1-, o-imidophenolate dianion (2LIP)2-, o-iminobenzosemiquinonate π radical
monoanion (2LISQ)•1-, and as an o-iminobenzoquinone (2LIBQ)0 (Scheme 4.1.). These
forms of the ligand are characteristic of their C-N, C-O, and C-C bond distances.
Scheme 4.1. shows the bond distances in the ligand 2LH2 in different oxidation states.
O -
N -
[LIP ]2-
F
1.35
1.37
1.39
1.41
1.40
1.39
1.40
1.42
-H+
+ H+
-e
-
+ e-
-e
-
+ e
-
•
N
O -
[LISQ ]1-
F
1.30
1.35
1.46
1.43
1.43
1.43
1.38
1.37
•
O
N
[LIBQ]0F
1.24
1.30
1.52
1.46
1.46
1.36
1.36
1.44
1.46
1.35
1.39
1.41
1.39
1.40
1.39
1.38
O-
NH
[L AP ]1-
F
Scheme 4.1
88
Chapter 4
4.2. Mo complexes:
4.2.1. Results and Discussion:
To a solution of three equivalents of the 2LH2 ligand in freshly distilled and
degassed MeOH, 5 mL of a MeOH solution of [Mo(O)2(acac)2] (one equivalent) was
added under Ar and stirred for 3 h. Further stirring in air for 1 h at room temperature
gave a purple-brown solution. Slow evaporation of the solvent at room temperature in
air gave a purple-brown precipitate in good yield (~50%). Recrystallization from a
mixture of CH2Cl2/CH3OH (1:1) afforded single crystals of neutral diamagnetic
[Mo(2LIP)(2LAP)(O)(OCH3)] • 2 MeOH (5) suitable for X-ray crystallography.
Interestingly, the same reaction carried out in CH2Cl2 solution instead of MeOH,
followed by recrystallization from a mixture of CH2Cl2/CH3NO2 (1:1) afforded single
crystals of the neutral, diamaganetic, μ-O bridged dinuclear [(2LIP)(2LAP)(O)Mo-(μ-
O)-Mo(O)(2LIP)(2LAP)] (6).
The crystal structures of 5 and 6 have been determined by X-ray
crystallography at 100(2) K by using Mo Kα radiation. Table 4.1. summarizes
selected bond distances in complexes 5 and 6. Figure 4.1. (a-b) shows the thermal
ellipsoid diagrams of 5 and 6, respectively, solvent molecules have been removed for
clarity.
Complex 5 is found to be octahedral with an N2O4 donor set consisting of one
deprotonated N,O-coordinated o-iminophenolate ligand, one N,O-coordinate N-
protonated monoanionic o-aminophenolate ligand (2LAP)1- ligand, one oxo group, and
one methoxy group. The oxo and methoxy ligands are coordinated in a cis fashion
relative to each other. Complex 6 is a dimer and found to be octahedral with an N2O4
donor set around each Mo ion; two Mo atoms are connected by a bridging μ-oxo
group; and each molebdenum ion is coordinated to one terminal oxo group, one
completely deprotonated o-iminophenolate ligand, and one N-protonated
monoanionic o-aminophenolate (2LAP)1- ligand. Each complex shows characteristic
M=O and N-H stretches at 935, 3413 cm-1 and 917, 3421 cm-1, respectively, in its
infrared spectrum.
The C-O and C-N bond distances (Table 4.1.) in the deprotonated 2-(4-
fluoro)anilino-4,6-di-tert-butylphenolate ligands in compounds 5 and 6 are close to
the bond distances observed for dianionic o-iminophenolate ligand, (2LIP)2-(Scheme
4.1.). All six C-C bonds are also equidistant within the experimental error. Thus, the
89
Chapter 4
oxidation states of the Mo in both monomeric 5 and dinuclear 6 is best described as
+VI.
Mo
O1 N7
C1
C2
C3 C4
C5
C6
C8
C31
O31
C32
C33
C34
C35
C36
N37
C38
H
F
O60
O70
Figure 4.1.a. Thermal ellipsoidal diagram of neutral mononuclear 5.
O1
O2
O3
Mo1
Mo2
O11
O71
O101
O41
N17
N47
N77
N107
C12
C13
C11
C14
C15
C16
C18
C48
C42
C43
C44
C45
C101
C102
C103
C104
C105
C106
C108
C71
C72
C73
C74
C75
C76
C78
H
H
Figure 4.1.b. Thermal ellipsoidal diagram of neutral dinuclear 6.
90
Chapter 4
5 6
Mo1-O60 1.6962(11) Mo1-O1 1.7037(14) Mo2-O2 1.7042(15)
Mo1-O70 1.9367(11) Mo1-O3 1.8734(13) Mo2-O3 1.8853(13)
Mo1-O1 2.0516(10) Mo1-O11 1.9736(13) Mo2-O71 1.9763(13)
Mo1-N7 2.0340(12) Mo1-N17 2.3236(17) Mo2-N77 2.2865(18)
Mo1-O31 1.9721(10) Mo1-O41 2.0280(14) Mo2-O101 2.0427(15)
Mo1-N37 2.3092(12) Mo1-N47 2.0434(15) Mo2-N107 2.0326(15)
O1-C1 1.3236(17) O11-C11 1.354(2) O71-C71 1.340(2)
C1-C6 1.4110(19) C11-C16 1.406(2) C71-C76 1.412(2)
C1-C2 1.412(2) C11-C12 1.397(3) C71-C72 1.404(3)
C2-C3 1.388(2) C12-C13 1.401(2) C72-C73 1.392(3)
C3-C4 1.418(2) C13-C14 1.402(2) C73-C74 1.410(3)
C4-C5 1.380(2) C14-C15 1.388(2) C74-C75 1.380(2)
C5-C6 1.4079(19) C15-C16 1.386(3) C75-C76 1.397(3)
C6-N7 1.3841(18) C16-N17 1.444(2) C76-N77 1.426(3)
N7-C8 1.4379(18) N17-C18 1.448(2) N77-C78 1.446(2)
O31-C31 1.3560(16) O41-C41 1.328(2) O101-C101 1.328(2)
C31-C36 1.3924(18) C41-C46 1.412(3) C101-C106 1.418(3)
C31-C32 1.4018(18) C41-C42 1.412(3) C101-C102 1.410(3)
C32-C33 1.3965(19) C42-C43 1.391(3) C102-C103 1.394(3)
C33-C34 1.4053(19) C43-C44 1.415(3) C103-C104 1.411(3)
C34-C35 1.3911(19) C44-C45 1.383(3) C104-C105 1.386(3)
C35-C36 1.3872(19) C45-C46 1.401(3) C105-C106 1.401(2)
C36-N37 1.4581(17) C46-N47 1.391(2) C106-N107 1.376(2)
N37-C38 1.4664(19) N47-C48 1.431(2) N107-C108 1.430(2)
Table 4.1. Selected bond distances (Å) in complexes 5 and 6.
Figure 4.2. shows the UV-vis spectrum of 6 recorded in CH2Cl2 at room
temperature in the range of 300-1000 nm while that of 5 is shown in Figure 4.3.
Interestingly, the UV-vis spectrum of 5 changes with time; in first the 10 min. the
spectrum changes very fast. After 25 min. the final spectrum is observed. This
91
Chapter 4
spectrum is similar to that of 6. Table 4.2. summarizes the electronic spectra of
complexes 5 and 6.
300 400 500 600 700 800 900 1000
0.0
0.4
0.8
1.2
1.6
2.0
ε [M-1cm-1X104]
λ [nm]
Figure 4.2. The electronic spectrum of 6 in CH2Cl2 solution at room temperature.
300 400 500 600 700 800 900 1000 1100
0.0
0.2
0.4
0.6
0.8
1.0
ε [M-1cm-1X104]
λ [nm]
Figure 4.3. The electronic spectral changes of 5 in CH2Cl2 solution at room temperature
recorded in 10 min. with 1 min. intervals. Blue colour spectrum
is the initial electronic spectrum of 5.
92
Chapter 4
Complex λmax, nm (ε, 104 M-1 cm-1)
5 820 (0.13); 606 sh (0.37); 450 (0.73); 360 (0.70)
6 815 (1.32); 490 (1.12); 374 (1.24)
Table 4.2. Electronic spectra of the complexes 5 and 6 in CH2Cl2 solution.
The observed electronic spectra of 5 clearly indicate that, monomeric 5
undergoes dimerization and forms 6 in solution in presence of a little water in CH2Cl2
by forming two equivalents of MeOH (Eq 4.1.); whereas in presence of excess of
MeOH it crystallizes as a monomer with –OCH3 as a sixth coordinating ligand.
[Mo(2LIP)(2LAP)](O)(OCH3)] + H2O[(2LIP)(2LAP)(O)Mo(-O-)Mo(O)(2LIP)(2LAP)]
+ 2 CH3OH Eq 4.1
Cyclic voltammograms of 5 and 6 look similar and Figure 4.4. shows the
cyclic voltammograms of 5 and 6.
1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5
E [V] vs. Fc+/Fc
10 μA
1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5
E [V] vs. Fc+/Fc
10 μA
56
Figure 4.4. Cyclic voltammograms of 5 and 6 in CH2Cl2 solution (0.1 M TBAPF6). Conditions: Scan
rate 100 mV s-1 at 25º C. (glassy carbon as working electrode and ferrocene (Fc) as internal standard).
Cyclic voltammograms of 5 and 6 show two quasi reversible waves at
potentials ~ +0.3 V and ~ -2.1V; and three completely irreversible waves at potentials
~ +0.9 V, 1.25 V, and -1.0 V. Further spectroelectrochemical measurements were not
93
Chapter 4
performed due to instability of the complexes upon electrochemical oxidation or
reduction.
The electronic ground states of complexes 5 and 6 have been established by
the variable-temperature magnetic susceptibility measurements in the range 3-300K
by using a SQUID magnetometer. Complexes 5 and 6 are diamagnetic (S = 0 ground
state) due to the coordination of the dianionic o-iminophenolate (2LIP)2- and
protonated monoanionic o-aminophenolate (2LAP)1- ligands to a diamagnetic Mo(VI)
centre (d0, SMo = 0).
A number of dinuclear complexes with Mo2O3 unit where two molybdenum
ions posses +VI oxidation state are reported in the literature.32 In any case an intense
broad band is observed at ~800 nm is observed. But complex 6 exhibits an intense
band at 815 nm. Very high molar extinction coefficient value of this band does not
indicate any d-d transition. This can be explained in terms of: an internal electron
transfer from one 2LIP to MoVI ion will generate a ligand radical 2LISQ and MoV (d1)
ion. A strong intramolecular antiferromagnetic coupling of this radical with the metal
electron will yield to an S = 0 ground state. Thus this band at 815 nm can be an
intervalence charge transfer band. But there is no other spectroscopic evidence for this
phenomenon as the complex is diamagnetic.
4.3. W complexes:
4.3.1. Results and Discussion:
A solution of three equivalents of 2LH2 ligand in degassed CCl4 and one
equivalent WCl6 was stirred under Ar for 1 h. Subsequent stirring in air for 2 h at
room temperature yielded a purple-brown solution. Slow evaporation of the solvent at
room temperature in air gave a purple-brown precipitate in excellent yield (92%).
Recrystallization from a CH2Cl2/CH3CN mixture afforded single crystals of neutral
diamagnetic [W(2LIP)(2LAP)(O)(Cl)] (7a) suitable for X-ray crystallography. All
attempts to deprotonate the o-aminophenolate ligand in 7a were unsuccessful. Stirring
of 7a in MeOH with NaOCH3 under an argon blanketing atmosphere for 16 h gave a
neutral, diamagnetic dimer {W(2LIP)(2LAP)(O)(OCH3)}2 • 0.5 MeOH (7b) where the –
OCH3 ligand coordinated in the position of Cl.
The crystal structures of 7a and 7b have been determined by X-ray
crystallography at 100(2) K by using Mo Kα radiation. Table 4.3. summarizes
selected bond distances in complexes 7a and 7b and Figure 4.5. (a-b) shows the
94
Chapter 4
thermal ellipsoid diagrams of 7a and 7b, solvent molecules have been removed for
clarity. Two neutral molecules in the asymmetric unit of 7b are identical, so only one
molecule is shown.
Complex 7a is found to be octahedral with an N2O3Cl donor set; it consists of
one deprotonated N,O-coordinated o-iminophenolate ligand, one N,O-coordinated, N-
protonated monoanionic o-aminophenolate ligand (2LAP)1- ligand, one oxo group, and
one coordinated chloride anion. The oxo and chloride ligands are coordinated to the
central W ion in a cis fashion with respect to each other. Complex 7b is a dinuclear
where two neutral monomer molecules are connected by an intermolecular hydrogen
bonding through N-H protons of one molecule to the oxo group of the second
molecule. Figure 4.6. shows the formation of this dinuclear complex by
intermolecular by hydrogen bonds between the two neutral molecules in the crystal
structure of 7b. Heavy atom distances N37-O160 and N137-O60 in hydrogen bonds
are found to be 2.85 Å and 2.71 Å, respectively.
The geometry around each W ion was found to be octahedral with an N2O4
donor set. Each of the tungsten ions in the asymmetric unit is coordinated to one oxo
group, one methoxy group, one deprotonated o-iminophenolate ligand, and one N-
protonated monoanionic o-aminophenolate (2LAP)1- ligand. Complexes 7a and 7b
show characteristic W=O, and N-H stretches at 930, 3415 cm-1 and 917, 3416 cm-1,
respectively, in their infrared spectrum.
W
O1
O60
O31
N7
N37
H
Cl
C1
C2
C3
C4
C5
C6
C8
C31
C32
C33
C34
C35
C36
C38
Figure 4.5.a. Thermal ellipsoidal diagram of neutral monomer 7a.
95
Chapter 4
C31
C32
C33
C34
C35
C36 C1
C3
C4
C5
C6
C8
O1
O31
O60
O70
F
W1
HN7
N37
Figure 4.5.b. Thermal ellipsoidal diagram of neutral molecule in crystal structure 7b.
299 01
Figure 4.6. Diagram showing formation of a dimer by intermolecular by hydrogen bonds between two
neutral molecules in the crystal structure of 7b.
96
Chapter 4
7a 7b
W-O60 1.7299(15) W1-O60 1.735(4) W2-O160 1.745(4)
W-Cl 2.3325(5) W1-O70 1.895(4) W2-O170 1.876(3)
W-O1 2.0061(14) W1-O1 2.004(4) W2-O101 2.010(4)
W-N7 2.0192(18) W1-N7 2.015(4) W2-N107 2.016(4)
W-O31 1.9502(15) W1-O31 1.953(3) W2-O131 1.956(3)
W-N37 2.2822(18) W1-N37 2.320(4) W2-N137 2.388(4)
O1-C1 1.336(2) O1-C1 1.349(7) O101-C101 1.354(6)
C1-C6 1.403(3) C1-C6 1.409(7) C101-C106 1.396(7)
C1-C2 1.410(3) C1-C2 1.392(3) C101-C102 1.393(8)
C2-C3 1.389(3) C2-C3 1.391(7) C102-C103 1.406(7)
C3-C4 1.415(3) C3-C4 1.416(7) C103-C104 1.401(7)
C4-C5 1.383(3) C4-C5 1.389(8) C104-C105 1.389(8)
C5-C6 1.399(3) C5-C6 1.392(7) C105-C106 1.389(7)
C6-N7 1.403(3) C6-N7 1.394(7) C106-N107 1.408(7)
N7-C8 1.444(3) N7-C8 1.441(6) N107-C108 1.442(6)
O31-C31 1.361(3) O31-C31 1.380(6) O131-C131 1.354(6)
C31-C36 1.388(3) C31-C36 1.380(7) C131-C136 1.386(7)
C31-C32 1.407(3) C31-C32 1.394(7) C131-C132 1.408(7)
C32-C33 1.398(3) C32-C33 1.391(7) C132-C133 1.394(7)
C33-C34 1.401(3) C33-C34 1.390(7) C133-C134 1.398(7)
C34-C35 1.392(3) C34-C35 1.405(7) C134-C135 1.396(7)
C35-C36 1.389(3) C35-C36 1.375(7) C135-C136 1.386(7)
C36-N37 1.467(3) C36-N37 1.465(6) C136-N137 1.465(6)
N37-C38 1.463(3) N37-C38 1.454(6) N137-C138 1.461(6)
Table 4.3. Selected bond distances (Å) in complexes 7a and 7b.
The C-O and C-N bond distances (Table 4.3.) in the deprotonated 2-(4-
fluoro)anilino-4,6-di-tert-butylphenolate ligand in 7a and 7b are very similar to the
bond distances observed for dianionic o-iminophenolate (2LIP)2- ligands (Scheme
4.1.); all six C-C bonds are also equidistant 1.39 Å within the experimental error +
97
Chapter 4
0.01 Å. Thus, the oxidation states of the W ion in both mononuclear 7a and dinuclear
7b is best described as +VI.
Figure 4.7. shows the UV-vis spectra of 7a and 7b recorded in CH2Cl2 at room
temperature in the range of 300-1000 nm. Table 4.4. summarizes the electronic
spectra of these complexes. The electronic spectrum of 7a displays three bands in the
visible region above 300 nm at 548, 390, and 338 (sh) nm with molar extinction
coefficients in the range of 0.36-0.7 x 103 M-1 cm-1. The electronic spectrum of 7b
displays four bands in the visible region above 300 nm at 360, 490 (sh), 600 (sh), and
700 (sh) nm with molar extinction coefficients in the range of 0.3-3.2 x 103 M-1 cm-1.
300 400 500 600 700 800 900 1000
0.0
0.2
0.4
0.6
0.8
ε [M-1cm-1X104]
λ [nm]
300 400 500 600 700 800 900 1000
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
ε [M-1cm-1X104]
λ [nm]
7a 7b
Figure 4.7. The electronic spectra 7a and 7b in CH2Cl2 solution at room temperature.
Complex λmax, nm (ε, 104 M-1 cm-1)
7a 548 (0.36); 390 (0.7); 338 sh (0.57)
7b 700 sh (0.33); 600 sh (0.96); 490 sh (1.76); 360 (3.28)
Table 4.4. Electronic spectra of the complexes 7a and 7b in CH2Cl2 solution.
98
Chapter 4
1.0 0.5 0.0 -0.5 -1.0 -1.5
E [V] vs. Fc+/Fc
10 μA
Au
GC
Pt
Figure 4.8. Cyclic voltammogram of 7a in CH2Cl2 solution (0.1 M TBAPF6). Conditions: Scan
rate 100 mV s-1 at 25 ۫ C. (glassy carbon (GC), Pt, and Au as working electrodes and
ferrocene (Fc) as internal standard).
1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0
E [V] vs. Fc+/Fc
10 μA
Figure 4.9. Cyclic voltammogram of 7b in CH2Cl2 solution (0.1 M TBAPF6). Conditions: Scan rate
400 mV s-1 at 25 ۫ C. (glassy carbon working electrode and ferrocene (Fc) as an internal standard).
Figure 4.8. shows the cyclic voltammograms of 7a recorded at a scan rate of
100 mV s-1 at 25 ۫ C in CH2Cl2 solution containing 0.1M TBAPF6 as the supporting
99
Chapter 4
electrolyte, different working electrodes i.e., a glassy carbon (GC), Pt, and Au, and a
Ag/AgNO3 reference electrode. Ferrocene was used as an internal standard. The
potentials are referenced versus the Ferrocenium/Ferrocene couple (Fc+/ Fc). All three
cyclic voltammograms of 7a show a quasi reversible one-electron transfer wave at a ~
+0.49 V potential.
The cyclic voltammogram of 7b is shown in Figure 4.9. and recorded at a scan
rate of 400 mV s-1 at 25º C in CH2Cl2 solution containing 0.1M TBAPF6 as the
supporting electrolyte, a glassy carbon working electrode, and a Ag/AgNO3 reference
electrode. Ferrocene was used as an internal standard. The potentials are referenced
versus the Ferrocenium/Ferrocene couple (Fc+/ Fc). Cyclic voltammogram of 7b
shows a reversible one-electron transfer wave at a potential of + 0.655 V.
The electronic ground states of complexes 7a and 7b have been established by
variable-temperature magnetic susceptibility measurements in the range of 3-300K
using a SQUID magnetometer. Complexes 7a and 7b are diamagnetic (S = 0 ground
state) due to coordination of the dianionic o-iminophenolate ligand, (2LIP)2-
N-protonated monoanionic o-aminophenolate ligand (2LAP)1- to a diamagnetic WVI
(d0, Sw = 0) centre .
4.4. Conclusions:
All attempts to prepare the desired octahedral M(2L)3 type of complexes where
M = Mo or W and 2L = 2-(4-fluoro)anilino-4,6-di-tert-butylphenol failed. However,
the monomeric complexes 5 and 7a and dinuclear complexes 6 and 7b were
synthesized. Complexes 5 and 6 contain a Mo=O bond and complexes 7a and 7b a
W=O bond. All complexes are octahedral with two N,O-coordinated noninnocent o-
aminophenolate ligands coordinated to a central Mo or W ion in cis fashion. Among
these ligands one is an N-protonated monoanionic o-aminophenolate(1-), (2LAP)1- and
one ligand is the dianionic o-iminophenolate(2-), (2LIP)2-. Thus oxidation states of the
central molybdenum ions in 5 and 6 as well as the tungsten ions in 7a and 7b are best
described as Mo(VI) and W(VI), respectively.
100
Chapter 4
4.5. References:
(1) Spence, J. T. Coord. Chem. Rev. 1969, 4, 475.
(2) Stiefel, E. I. Prog. Inorg. Chem. 1977, 22, 1.
(3) Kletzin, A.; Adams, M. W. W. FEMS Microbiol. Rev. 1996, 18, 5.
(4) Johnson, M. K.; Rees, D. C.; Adams, M. W. W. Chem. Rev. 1996, 96, 2817.
(5) (a) Stevenson, D. L.; Dahl, L. F. J. Am. Chem. Soc. 1967, 89, 3721. (b) Dahl,
L. F.; Frisch, D.; Gust, G. Proceedings of the Climax First International
Conference on the Chemistry and Uses of Molebdenum, University of
Readings, Mitchell, P. C. H. Ed. Climax Molebdenum Co. London, 1973,
134.
(6) Bishop, M. W.; Chatt, J.; Dilworth, J. R.; Hursthouse, M. B.; Motevalli, M. J.
Chem. Soc. Chem. Commun. 1976, 780.
(7) Cervilla, A.; Llopis, E.; Marco, D.; Perez, F. Inorg. Chem. 2001, 40, 6525.
(8) Cowie, M.; Bennett, M. J. Inorg. Chem. 1976, 15, 1584.
(9) (a) Knoch, F.; Sellmann, D.; Kern, W. Inorg. Chem. 1992, 202, 326. (b)
Knoch, F.; Sellmann, D.; Kern, W. Inorg. Chem. 1993, 205, 300.
(10) HanVinhHuynh; Lugger, T.; Hahn, F. E. Eur. J. Inorg. Chem. 2002, 3007.
(11) Lorber, C.; Donahue, J. P.; Goddard, C. A.; Nordlander, E.; Holm, R. H. J.
Am. Chem. Soc. 1998, 120, 8102.
(12) Yamanochi, K.; Enemark, J. H. Inorg. Chem. 1978, 17, 2911.
(13) Balch, A. L.; Rohrsheid, F.; Holm, R. H. J. Am. Chem. Soc. 1965, 87, 2301.
(14) Stiefel, E. I.; Waters, J. H.; Billig, E.; Gray, H. B. J. Am. Chem. Soc. 1965,
87, 3016.
(15) Balch, A.; Holm, R. H. J. Am. Chem. Soc. 1966, 88, 5201.
(16) Spence, J. T.; Minelli, M.; Kroneck, P. J. Am. Chem. Soc. 1980, 102, 4538.
(17) (a) Pariyadath, N.; Newton, W.; Stiefel, E. I. J. Am. Chem. Soc. 1966, 98,
5388. (b) Gardner, J. K.; Pariyadath, N.; Stiefel, E. I. Inorg. Chem. 1978, 17,
897.
(18) Yamanochi, K.; Enemark, J. H. Inorg. Chem. 1978, 17, 1981.
(19) Rajan, O. A.; Spence, J. T.; Minelli, M.; Sato, M.; Enemark, J. H.; Kroneck,
P. ‘Proceedings of the Climax Fourth International Conference on the
Chemistry and Uses of Molebdenum’ (Eds Barry. H. F. and Mitchell, P. C.
H.) Climax Molebdenum Co. Ann Arbor, Michingan, 1982, 139.
(20) Stiefel, E. I. Prog. Inorg. Chem. 1977, 22, 1.
101
Chapter 4
(21) Schrock, R. R. Science. 1983, 219, 13.
(22) Rajan, O. A.; Spence, J. T.; Leman, C.; Minelli, M.; Sato, M.; Enemark, J. H.;
Kroneck, P. M. H.; Sulger, K. Inorg. Chem. 1983, 22, 3065.
(23) Kapre, R. Ph.D. Thesis. 2005.
(24) Herebian, D.; Ghosh, P.; Chun, H.; Bothe, E.; Weyhermüller, T.; Wieghardt,
K. Eur. J. Inorg. Chem. 2002, 1957.
(25) Sun, X.; Chun, H.; Hildenbrand, K.; Bothe, E.; Weyhermüller, T.; Neese, F.;
Wieghardt, K. Inorg. Chem. 2002, 41, 4295.
(26) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.; Weyhermüller, T.;
Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 2213.
(27) Min, K. S.; Weyhermüller, T.; Wieghardt, K. Dalton Trans. 2003, 1126.
(28) Chun, H.; Verani, C. N.; Chaudhuri, P.; Bothe, E.; Bill, E.; Weyhermüller, T.;
Wieghardt, K. Inorg. Chem. 2001, 40, 4157.
(29) Verani, C. N.; Gallert, S.; Bill, E.; Weyhermüller, T.; Wieghardt, K.;
Chaudhuri, P. Chem. Commum. 1999, 1747.
(30) (a) Tatsumisago, M.; Matsubayashi, G. E.; Tanaka, T.; Nishigaki, S.;
Nakatsu, K. Dalton Trans. 1982, 121. (b) Dahlstrom, P.L.; Hyde, J. R.; Vella,
P. A; Zubieta. J. Inorg. Chem. 1982, 21, 927. (c) Cindric, M.; Matkovic-
Calogovic, D.; Vrdoljak, V.; Kamenar, B. Inorg. Chem. Commun. 1998, 1,
237. (d) Kamenar, B.; Korpar-Colig, B.; Penavic, M. Cryst. Struct. Commun.
1982, 11, 1583. (e) Craig, J. A.; Harlan, E. W.; Synder, B. S.; Whitener, M.
A.; Holm, R. H. Inorg. Chem. 1989, 28, 2082.
102
Square Planar Gold Dithiolene Complexes with cis-1,2–
disubstitutedethylene–1,2 dithiolato Ligands: A Combined
Experimental and Theoretical Study
103
104
Chapter 5
5.1. Introduction:
The last few years have seen a renewed interest in the study of transition metal
bis(dithiolene) complexes which, in their square planar coordination geometry, can be
used as building blocks for conducting and magnetic molecular materials.1
Bis(dithiolene) complexes based on extended and delocalised π-ligands are expected
to be more promising as conducting materials due to the accessibility of a wide range
of oxidation states and the possibility of large solid-state interactions.1-4
Although bis(dithiolene) complexes of metals from groups 10 and 11 tend to
adopt a square-planar geometry, favourable for extended π-π interactions and electron
delocalization in stacked structures, in some cases, e.g., for Fe and Co, other
structures and different metal coordination environments are possible. All known FeIII
bis(dithiolene) complexes are dimeric with a square-pyramidal coordination
geometry.5, 6, 34 For CoIII, in addition to this dimeric structure, examples of trimeric7 as
well as polymeric structures are also known.8
An extensive range of bis(dithiolene) complexes of Au and their properties
have been reported in the literature.9-33 The majority of them are diamagnetic [Au
(dithiolene)2]- complexes with a square planar {AuS4} core. In general they are
considered to involve a diamagnetic AuIII (d8) centre. The redox properties of
[Au(dithiolene)2]- complexes have been investigated and related [Au(dithiolene)2]z
complexes with (z = 0 or z = -2) have been identified.10, 12, 15, 18, 21, 22, 26, 28, 29, 39b The
electronic structure of these systems, notably the role of the dithiolene ligands in the
redox processes, has been investigated by extensive ab initio and density functional
calculations.18, 29
A number of strategies are available to synthesize dithiolene complexes.30 We
have adapted the procedure developed by W. Heinrich et. al.34 to synthesize new
[Au(dithiolene)2]- complexes, which involve the metal ion coordinated by two
symmetrically substituted dithiolene ligands, cis-1,2–disubstitutedethylene–1,2
dithiolates, where 4-tert-butyl phenyl group acts as a bulky substituent on the ethylene
backbone. Here, the synthesis and characterisation of the [Au(3L)2][N(n-Bu)4] (8), its
chemically and electrochemically one-electron oxidized neutral complex [Au(3L)2]0
(8a), and the properties of its electrochemically generated two-electron oxidized
cationic [Au(3L)2]+ (8b) form are presented. The complex 8b shows very interesting
features consistent with a diradical character which is not yet known in gold-
dithiolene chemistry.22, 26, 28
105
Chapter 5
Results and Discussion:
5.2. Syntheses and X-ray Crystal Structures:
Reflux of 4,4´-di-tert-butyl benzoin with P4S10 in 1,4-dioxane for 3h gives the
ligand (cis-4,4´-di-tert-butylphenylethylene-1,2-dithiolene) in situ.35 Addition of
Na[AuCl4].2H2O with 1mL of H2O followed by reflux for 1 h gives a green solution
of [Au(3L)2]- on filtration. A golden-yellow, micro-crystalline precipitate of
[Au(3L)2][N(n-Bu)4] (8) was obtained in poor yields upon addition of [N(n-Bu)4]Br to
this solution. Electrochemical one-electron and two-electron oxidation of 8 gives a
neutral species [Au(3L)2] (8a) and a cationic species [Au(3L)2]+ (8b), respectively.
Chemical oxidation of 8 with one-equivalent of ferrocenium hexafluorophosphate in
CH2Cl2 gives neutral complex [Au(3L)2] (8a) in excellent yield. Although, similar
monoanionic and neutral AuIII complexes have been reported (with o-benzene
dithiolate ligand28, 36), a cationic species like 8b was not reported previously in gold-
dithiolene chemistry.
The crystal structures of 8 and 8a were determined at 100(2) K by using Mo
Kα radiation. The asymmetric unit of 8 consists of one crystallographically
independent anions of [Au(3L)2]- and two half molecules [Au(3L)2]- located on the
centre of inversion along with two fully occupied cations, [N(n-Bu)4]+. Also consists
of four independent CH2Cl2 molecules and one CH2Cl2 being disordered next to an
inversion centre. Figure 5.1. shows the important structural features of 8 (only one
molecule of 8 is shown in the figure). All three Au centres in 8 are found to be square
planar and have equivalent bond lengths within the experimental error (+ 0.01 Å).
Complex 8a crystallizes with one CH2Cl2 molecule of crystallization. However, the
quality of the crystal structure determination is poor and only heavy atoms like Au, S
and CH2Cl2 molecules were refined with anisotropic thermal parameters. Only one
neutral Au complex crystal structure of this type with benzenedithiolene ligands is
known in the literature.36 Crystal structure of 8a is shown in Figure 5.2. Selected bond
lengths in complexes 8 and 8a are given in Table 5.1.
106
Chapter 5
1-
Figure 5.1. Structure of the mono anionic [Au (3L)2]1- in crystal structure 8.
Au-S bond lengths in all the three molecules in the crystal structure of 8 are
found equidistant with an average length of 2.310 Å. The corresponding C-S and
ethylinic bonds are average 1.766 Å and 1.35 Å in length (within the experimental
error of + 0.01 Å). This indicates the presence of S,S-coordinated closed-shell
dianionic 1,2-dithiolato ligands. Similar bond distances of Au-S and C-S were found
for the complexes where AuIII ion is coordinated to other bis(dithiolene) ligands. 28, 36
These bond lengths are different from the bond lengths found in case of NiII and PdII
bis(dithiolene) square planar complexes with the same cis-4,4´-di-tert-
butylphenylethylene-1,2-dithiolene ligand41 and also from the bond distances reported
in Ref. 36 for neutral square planar [AuL2]. Significantly shorter C-S bond lengths of
average 1.712 Å were found in case of NiII and PdII neutral complexes41, whereas an
average C-S bond length of 1.735 Å (which is the arithmetic average of 1.766 Å and
1.712 Å) is reported for the neutral Au complex in Ref 36. However, no observable
change was found in the Au-S bond lengths in both 8 and [Au(L)2]0 reported.36 Thus,
the quality of the X-ray crystal structure corroborates the presence of a π radical
(open-shell semiquinonate) on each of the ligands in the first case,41 where as the
107
Chapter 5
S(1)
S(2)
S(32)
S(31)
C(1)
C(2)
C(31)
C(32)
Au
Figure 5.2. Structure of the neutral [Au 3L2] in crystal structure 8a.
8 8a
Au1- S1 2.3067(5) Au2- S61 2.3188(5) Au- S1 2.284(5)
Au1- S2 2.3227(5) Au2- S62 2.3111(5) Au- S2 2.288(5)
Au1- S31 2.3162(5) Au3- S91 2.3066(5) Au- S31 2.303(5)
Au1- S32 2.3136(5) Au3- S92 2.3020(5) Au- S32 2.290(5)
S1-C1 1.7671(19) S61-C61 1.7664(19) S1-C1 1.74(2)
S2-C2 1.7722(19) S62-C62 1.7617(19) S2-C2 1.745(18)
S31-C31 1.7671(19) S91-C91 1.7626(19) S31-C31 1.751(19)
S32-C32 1.7671(19) S92-C92 1.7737(19) S32-C32 1.76(2)
C1-C2 1.350(3) C61-C62 1.357(3) C1-C2 1.38(2)
C31-C32 1.355(3) C91-C92 1.356(3) C31-C32 1.33(2)
Table 5.1. Selected Bond Distances (Å) in the crystal structures of 8
(in all the three molecules in the asymmetric unit) and 8a.
presence of one open-shell π radical ligand and one closed-shell dianionic ligand in
the latter case. That’s why, an intermediate average bond length of 1.735 Å is
observed which suggests the delocalized mixed valent electronic structure (class III).
Therefore, 8 can be best explained in terms of AuIII ion coordinated to two closed
shell dianionic cis-4,4´-di-tert-butylphenylethylene-1,2-dithiolene ligands and one-
108
Chapter 5
electron chemically oxidized 8a can be explained in terms of AuIII ion coordinated to
one open-shell radical (3L)1-• and one closed-shell dianionic (3L)2- ligand. These
crystallographic differences clearly indicate the noninnocent nature of this ligand
which will be further supported by spectroscopic data as well as density-functional
calculations. Scheme 5.1. shows the proposed localization of the noninnocent S,S-
coordinated cis-4,4´-di-tert-butylphenylethylene-1,2-dithiolene ligands towards AuIII,
NiII and PdII metal centres.
MII/III
S
S
S
S
MII/III
S
S
S
S
MII/III
S
S
S
S
Z
Z
Z
Z= -1or -2 Z=0 or-1
Z=+1 or 0
Scheme 5.1. Showing noninnocence of the cis-4,4´-di-tert-butylphenylethylene-1,2-dithiolene ligands.
Where M= AuIII or NiII or PdII. Radicals are completely delocalised over two sulphur atoms as well as
over two ligands, results in all equivalent C-S bond lengths.
5.3. Electro- and Spectroelectrochemistry:
Figure 5.3. shows the cyclic voltammogram of 8, recorded at 200 mV s-1.
Potentials are summarized in Table 5.2.
The cyclic voltammogram of 8 displays three completely reversible one-
electron waves at potentials +0.451 V, -0.108 V and -2.093V vs Fc+/Fc. Controlled
potential coulometric measurements established that monoanionic 8 undergoes two
reversible one-electron oxidation processes and one irreversible one-electron
109
Chapter 5
reduction process unlike other dithiolene complexes where the second oxidation
process was never observed.42 We assign these two reversible one-electron oxidation
processes of 8 as mostly ligand-centred (Eq 5.1), since the AuIII centre is known to
remain unchanged upon oxidation.28 The irreversible one-electron reduction is
assigned as metal centred.
[AuIII(3L)2]-[AuIII(3L)(3L)]0[AuIII(3L)2]+
[AuII(3L)2]2- - e
+ e
- e
+e
-e
+ e Eq 5.1
1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.
5
E [V] vs. Fc+/Fc
10 μA
Figure 5.3. Cyclic voltammogram of 8 in CH2Cl2 solution (0.1 M TBAPF6). Conditions: Scan rate 200
mV s-1 at 25º C. (glassy carbon as working electrode and ferrocene (Fc) as internal standard).
Complex E1/2 (V) vs Fc+/Fc
Reduction Oxidation 1 Oxidation 2
[AuIII(3L)2]--2.093 -0.108 +0.451
Table 5.2. Summary of redox potentials in volts vs Ferrocenium/Ferrocene couple for 8
The spectroelectrochemistry of 8 have been recorded in CH2Cl2 solution at
-25º C in the range of 300-2000 nm; the results are summarized in Table 5.3. Figure
5.4. shows the electronic spectra of 8, together with its electrochemically generated
one-electron oxidized species 8a and electrochemically generated two-electron
110
Chapter 5
oxidized species 8b. Species 8a and 8b were stable in CH2Cl2 solution at -25º C for at
least 30 min. The electrochemically generated one-electron reduced form of 8 was,
however, not stable on the time scale of coulometry, hence its electronic spectrum
could not be recorded. The assignment of the major absorption bands follows in
analogy to the assignments made in Refs. 28 and 37.
The electronic spectrum of 8 displays two d-d transitions in the visible range at
696 nm (ε = 0.04 x 104 M-1 cm-1) and 441 nm (ε = 0.41 x 104 M-1 cm-1) with low
intensities and a shoulder at 331 nm. No charge transfer transitions were observed in
the NIR for this complex. Similar electronic spectra have been observed for other
diamagnetic square planar complexes of AuIII with d8 electronic configuration.16, 28
Interestingly, the electronic spectrum of 8a displays a very intense absorption
maximum in the near-infrared region at 1495 nm (ε = 2.12 x 104 M-1 cm-1) along with
rather-weak maxima at 426 nm (ε = 0.84 x 104 M-1 cm-1) and 1184 nm (ε = 0.2 x 104
M-1 cm-1). We tentatively assign the intense band at 1495 nm to an intervalence
transition of the type [AuIII(3L•)(3L)] [AuIII(3L)(3L•)] which corresponds to a spin-
allowed transition from 1b1u to 2b2g as suggested for [Au(L)2]0 previously.29 It is
noteworthy that, the [N(n-Bu)4][NiII(Lt-Bu•)(Lt-Bu)] complex displays this intervalence
charge transition (IVCT) at 860 nm (ε = 1.2 X 104 M-1 cm-1)29 and it is absent (>700
nm) in case of [N(n-Bu)4][CoIII(Lt-Bu)2]40 due to the presence of two closed-shell
dithiolene ligands. The electronic spectrum of 8b, the two-electron oxidized form of
8 also displays a very intense absorption maximum in the near-infrared region at 1066
nm (ε = 4.25 x 104 M-1 cm-1) along with a moderately strong absorption maximum at
640 nm (ε = 0.6 X 104 M-1 cm-1) and a shoulder at 383 nm. We assign the intense band
at 1066 nm as ligand-to-ligand charge transition (LLCT). To the best of our
knowledge, this kind of LLCT had never been observed before in the gold-dithiolene
chemistry.
111
Chapter 5
300 600 900 1200 1500 1800
0.0
0.8
1.6
2.4
3.2
4.0
4.8
ε [M-1cm-1X104]
λ [nm]
[Au]1-
[Au]0
[Au]1+
8
8a
8b
Figure 5.4. The electronic spectra showing [AuIII(3L)2] [N(n-Bu)4], 8 (solid line) one-electron oxidised,
8a (dashed line) form and two-electron oxidized, 8b (dots line) forms in CH2Cl2 solution containing
0.20 M [(n-Bu)4N] PF6 at -25º C.
Complex λmax, nm (ε, 104 M-1 cm-1)
8 693 (0.04); 441 (0.41); 331 sh
8a 1495 (2.12); 1184 (0.2); 426 (0.84)
8b 1066 (4.25); 640 (0.6); 383 sh
Table 5.3. Electronic spectra of the complexes 8 and electrochemically generated
8a and 8b in CH2Cl2 solution.
5.4. Magnetic Properties:
Complex 8 is diamagnetic due to the presence of two closed-shell dianionic
dithiolato ligands (3L)2- and AuIII centre with d8 electronic configuration (SAu = 0).
The complex 8b is also diamagnetic, due to the presence of two S,S-coordinated to
1,2-dithioethylenesemiquinonato(1-) radical ligands. The two spins are
intramolecularly and strongly anti-ferromagnetically coupled via a super-exchange
mechanism mediated by the diamagnetic AuIII (d8; SAu = 0) ion. [NiII(3L•)2] and
[PdII(3L•)2] complexes with the same electronic configuration (d8; SNi/Pd = 0) are also
112
Chapter 5
found to be diamagnetic due to strong anti-ferromagnetic coupling of two radicals
through the diamagnetic metal centre.41 8a is paramagnetic and possesses S = ½
ground state. Figure 5.5. shows the X-band EPR spectrum of 8a, the
electrochemically generated one-electron oxidized form of 8, recorded in a frozen
solution of CH2Cl2:Toluene (1:3) at 90 K. The large g anisotropy of the signal
(2.0653, 2.0299, 1.9436) arises from spin-orbit interactions and indicates significant
spin density at the Au centre. Surprisingly, the spectrum exhibits a well-resolved
hyperfine splitting at gmax with very unusual appearance. The unusual splitting and
intensity pattern of these hyperfine lines owe their origin to large electric quadrupole
interactions of the AuIII ion, which has a nuclear spin of 3/2 (100% natural
abundance). Every nucleus possessing I > ½ has an nuclear quadrupole moment Q
due to non-spherical electric charge distribution in the nucleus. Nuclear electric
quadrupole interactions arise when the quadrupole moment of a given nucleus
interacts with a nonzero electric field gradient (EFG) generated by ligand (radical)
field. The major components of the EFG coupling tensor P is found in the direction of
gmin. All three A tensors are fairly isotropic. The numerical values of the g, A, and P
tensors obtained from the simulation are summarized in the caption of Figure 5.5. The
spectrum also shows fairly resolved hyperfine splitting (1:2:2:1 patten) at gmin.
Whereas in the case of [AuIII(Lt-Bu•)(Lt-Bu)] complex in Ref. 28 with g1 = 2.069, g2 =
2.032 , g3 = 1.911 (giso = 2.003) without any detectable 197Au (I = 3/2, 100% natural
abundance) hyperfine splitting is observed.
In Ref. 29 it has been explained that the magnetic orbitals of the Au complex
contain only 8% of metal contribution which describes a very small 197Au hyperfine
coupling in its EPR spectrum.28 In case of 8a, only 5% of metal contribution in the
singly occupied molecular orbital is observed from the relativistic DFT calculations,
which agrees with the EPR spectrum of 8a. In analogy to Ref. 28 (where the evidence
comes from 197Au Mössbauer spectroscopic data with isomeric shift δ = 3.36 mm s-1
and quadruple splitting Δ E
Q = 2.92 mm s-1 for [AuIII(Lt-Bu)2] [N(n-Bu)4] isomeric
shift δ = 3.20 mm s-1and quadruple splitting ΔEQ = 3.06 mm s-1 for
[AuIII(Lt-Bu•)(Lt-Bu)]), the EPR spectrum of 8a can be used to describe the presence of
AuIII, coordinated to one closed-shell dianionic dithiolato ligand and one open-shell
semiquinonate monoanionic π radical ligand instead of AuIV ion with a low spin d7
electronic configuration with two closed-shell dianionic dithiolene ligands.36
113
Chapter 5
3200 3250 3300 3350 3400 3450 3500
dX´´
dB
B [mT]
Sim
Exp
1.942.03
2.06
g
values
Figure 5.5. X-band EPR spectrum of the electrochemically generated 8a in frozen solution of
CH2Cl2:Toulene (1:3) solution at 90 K. Experimental conditions: microwave frequency 9.43 GHZ;
power 40 μW modulation 1 mT. Simulation parameters: g = (2.0653, 2.0229, 1.9436);
A(197Au) = (6.8, 6.7, 6.3) x 10-4 cm-1; P = (-150, 50, 100) x 10-4 cm-1.
0 50 100 150 200 250
0.0
0.4
0.8
1.2
1.6
2.0
μeff [μB]
T [K]
Sim
Exp
Figure 5.6. μeff vs T graph of 8a (4-300 K); External applied field is 1T.
114
Chapter 5
Figure 5.6. shows the temperature dependent magnetic susceptibility
measurements of 8a done in between 4-300 K by SQUID magnetometer in external
field of 1.0 T, showing a spin ½ ground state of the system with a temperature
independent magnetic moment of 1.72 μB. This indicates the presence of a single
paramagnetic neutral species which does not pack with formation of Au•••Au
interactions in the solid state. This further supports the results of the EPR spectrum.
B
5.5. Infrared Spectra:
1800 1600 1400 1200 1000 800 600
%T
wave number,cm-1
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
8
8a
8a
8
ab1149
1375
1106
1362
Figure 5.7. (a) Infrared spectra of 8 and 8a (KBr pellets) in region 4000-400 cm-1
(b) closer look in the region 1800-500 cm-1
The two isolated Au complexes 8, 8a where, 8a is one-electron oxidation
product of 8, are further investigated using infrared spectroscopy. Figure 5.7.a. shows
the infrared spectra (KBr pellets) of 8 and 8a in the range of 400-4000 cm-1 and
Figure 5.7.b. shows the magnified view of the spectra in the region 500-1800 cm-1.
The spectra for both complexes have many features in common, but, they carry
notable differences that include very strong bands at 1375 cm-1, 1149 cm-1 and 857
cm-1 in the IR spectrum of 8a. The comparison between the IR spectra of 8 and 8a
reveals the strengthening of bands at 1375 cm-1, 1106 cm-1 and the origin of an
altogether new band at 1149 cm-1 in the latter. We assign this strong band at 1149
cm-1 to the υ(C=S•) stretching mode of S,S-coordinated 1,2-
dithioethylenesemiquinonato(1-) radicals related with the ligand based oxidation for
115
Chapter 5
the [Au(3L)2]-/ [Au (3L)2] couple. This assignment is further in agreement with the
prior IR study of S,S-coordinated o-dithiobenzosemiquinonate(1-) complex.28 The
presence of this strong band corresponding to (C=S•) stretching mode would certainly
have substantial effect on the oxidation state of the ligand.
5.6. Calculations:
All the calculations on these complexes were done by Dr. Kallol Ray from our
group.
Structure Optimizations:
Density functional theoretical calculations have been carried out by employing
BP86 and B3LYP functionals for the dianionic [Au(L′)2]2- complex and its one-, two-
and three-electron oxidized counterparts [Au(L′)2]1-, [Au(L′)2]0, and [Au(L′)2]1+,
respectively, L′ represents the 1,2-diphenyl-ethylene-1,2-dithiolate ligand. Table 5.4.
summarizes the calculated bond lengths.
The agreement between the experimental and calculated structural parameters
for the monoanionic [Au(L′)2]1- complex is reasonably good. The metrical parameters
of the ligands, in particular, are very accurately predicted with the typical error in
bond length not exceeding 0.02 Å. The over estimation of the metal-sulfur bond
distances, varying between 0.05-0.06 Å, is typical of DFT functionals. The optimised
structures of [Au(L′)2]1-, [Au(L′)2]0, and [Au(L′)2]1+ complexes feature similar Au-S
av distances at ~2.36 Å for all the three complexes. The corresponding dianionic
[Au(L′)2]2- complex, in contrast, features a longer Au-S av distance at 2.41 Å.
Moreover, the ethylenic bond distances increase and the C-S distances decrease on
going from the monoanionic [Au(L′)2]1- complex, to the neutral [Au(L′)2]0, and to the
cationic [Au(L′)2]1+ complexes. The metrical parameters of the ligands in both
[Au(L′)2]1- and [Au(L′)2]2- are however, calculated to be very similar. These are
consistent with predominantly ligand based oxidations for the [Au(L′)2]1-/0/+1 series
and metal based reduction for the [Au(L′)2]2-/1- couple. It is important to note that in
[Au(L′)2]0 the calculated average C-S bond length of 1.756 Å corresponds to the
arithmetic average of calculated C-S bond distance at 1.788 Å for the [Au(L′)2]1-
complex containing only closed-shell (L′)2- dianions and at 1.736 Å for the [Au(L′)2]1+
complex containing only (L′●)1- radicals. Thus, the [Au(L′)2]0 complex behaves like
class III delocalised ligand mixed-valent systems [AuIII(L′)(L′●)]0↔[AuIII(L′●)(L′)]0 in
116
Chapter 5
the calculation. Similar results have been reported previously29a for the corresponding
[Au(LBu)2]0 complex.
Bonding Scheme and Ground State Properties:
For the MO description of the complexes within the Cs point group we choose
the coordinate system as shown in Figure 5.8.
The qualitative bonding scheme derived from the spin unrestricted B3LYP
DFT calculation on the [Au(L′)2]0 complex is shown in Figure 5.8. The compositions
of the important MOs are given in Table 5.5. The ground state electronic
configuration of the complex as evident from Figure 5.8, is thus predicted to be
(1a´)2(1a´´)2(2a´)2(3a´)2(4a´)2(2a´´)2(5a´)2(3a´´)1(4a´´)0
1a´(1ag)dx2-y2
2a´(1b3g)dyz
3a´ (2ag) dz2
1a´´(1b2g)dxz
4a´(2b3g)
2a´´(1au)
3a´´(2b2g)
5a´(1b1u)
4a´´(1b1g)dxy
X
Y
Figure 5.8. Energy scheme for the [Au(L´)2] complex as obtained from a ZORA B3LYP DFT
calculation. A Cs point group is considered for the complex. In parenthesis the symmetries of the
molecular orbitals in D2h point group are given.
117
Chapter 5
S
S
Au S
S
Ph
Ph
Ph
Ph
1
2
3
4
Complex Au-S C-S C1-C2
[Au(L´)2]2- 2.410 1.780 1.362
[Au(L´)2]1- 2.362(2.3162(5)) 1.788(1.7732(18)) 1.359(1.350(3))
[Au(L´)2]02.360 1.756 1.383
[Au(L´)2]1+ 2.357 1.736 1.408
Table 5.4. Calculated and experimental (in brackets) metrical parameters for the complexes in Å
obtained from scalar relativistic DKH2-BP86 DFT methods using large uncontracted gaussian bases at
the metal centre and uncontracted all electron polarized triple ξ (TZVP) Gaussian
bases for the remaining atoms.
The bonding scheme of the [Au(L′)2]0 complex is found to be very similar to
what has been obtained previously for the corresponding [Au(LBu)2]0 29a complex (the
point group in the two cases is, however, different providing different symmetries of
the molecular orbitals in the two complexes). The Au 5d manifold is lying very deep
in energy in [Au(L′)2]0 owing to high effective nuclear charge of gold. The metal d
and the ligand p orbitals in [Au(L′)2]0 are well separated from each other and the
superexchange interaction, observed in the corresponding [M(LBu)2]1- (M= Ni, Pd and
Pt) complexes29a is reduced to a minimum. The singly occupied molecular orbital
(SOMO) in [Au(L′)2]0 is thus, predominantly ligand based with only 5% of Au 5dxz
character. In the calculation four doubly occupied orbitals, namely 1a´ , 2a´
, 1a´´ , and 3a´´ are found to be mostly of metal d-origin and
hence, the valence state of the metal is best represented as d
)3( 22 yx
d−
)3( yz
d)3( xz
d)3( 2
z
d
8 AuIII ion. The LUMO of
the complex is the antibonding combination of the metal dxy and the ligand orbitals.
Due to the ligand geometry the overlap between these two orbitals is favourable,
providing an efficient pathway for ligand-to-metal σ donation.
Upon one electron reduction of the [Au(L′)2]0 complex the additional electron
enters the 3a´´orbital which becomes the HOMO of the reduced species. The
composition of the SOMO and the LUMO remains unaltered as a result of reduction
(Table 5.5.). Since the electron enters an orbital which is almost Au-S nonbonding the
reduction process is accompanied by no change in the Au-S distances. The electronic
118
Chapter 5
structure of the reduced species can thus be appropriately described as a AuIII ion
attached to two closed shell 1,2-diphenylethylene-1,2-dithiolate ligands.
Complex MO Au(5dyz) Au(5dxz) Au(5dxy) S(3pz) S(3px,y) C*(2pz) C*(2px,y)
[Au(L´)2]0
4a´´
3a´´
5a´
4a´
5.9
5.1
25.1
57.6
58.2
64.2
58.4
24.1
23.8
17.1
4
[Au(L´)2]1-
4a´´
3a´´
5a´
4a´
5.9
5.1
25.3
57.2
58.7
64.9
58.4
23.1
23.9
16.2
3
[Au(L´)2]2-
4a´´
3a´´
5a´
4a´
5.9
5.1
25.7
57.4
58.1
64.1
58.1
24.1
24.8
17.1
6
[Au(L´)2]1
* *
*) ethylenic carbon atoms * *)from Broken symmetry DFT calculation
4a´´
3a´´
5a´
4a´
5.7
5.0
24.9
57.8
58.5
65.1
60.2
24.7
22.8
17.3
2
Table 5.5. Composition of selected molecular orbitals of [Au(L´)2]z Complexes(%) as obtained from
the scalar relativistic ZORA-B3LYP DFT calculations using large uncontracted gaussian bases at the
gold and uncontracted all electron polarized triple-ξ (TZVP) gaussian bases for the remaining atoms.
Further one-electron reduction of the monoanionic [Au(L′)2]1- complex results
in the single occupancy of the 4a´´orbital. This orbital is strongly Au-S antibonding
and thus explains the calculated elongation in the Au-S bond lengths on moving from
the monoanionic [Au(L′)2]1- to the dianionic [Au(L′)2]2-. The ground state of the
[Au(L′)2]2- complex is thus calculated to be 2A´´ with the following electronic
configuration.
(1a´)2(1a´´)2(2a´)2 (3a´)2(4a´)2(2a´´)2(5a´)2(3a´´)2(4a´´)1
This is in contrast to the previously15 suggested 2A2g (D2h symmetry) ground
state for the corresponding [N(n-Bu)4]2[Au(mnt)2] complex (mnt = 1,2-
dicyanoethylene-1,2-dithiolate). Rather, it strongly supports van Ren’s assignment of
a b1g (D2h symmetry) orbital symmetry39a of the ground state of the unpaired electron
119
Chapter 5
in [N(n-Bu)4]2[Au(mnt)2]. The spin population at the gold in [Au(L′)2]2- is calculated
to be larger (25%) than what has been obtained experimentally (10%)43 from the
single crystal EPR studies on the [N(n-Bu)4]2[Au(mnt)2] complex. The electronic
structure of the [Au(L′)2]2- dianion can thus be best explained in terms of a Au(II) ion
attached to two closed shell 1,2-diphenylethylene-1,2-dithiolate ligands.
It has recently been shown44 that the neutral, diamagnetic, ortho-
semiquinonato type square planar Ni complexes with O-, N-, and S-ligands is best
represented as a low spin Ni(II) ion (d8, SNi= 0) coordinated by two ligand radicals
(singlet diradicals). Thus a single determinant closed-shell approximation of the
density functional methods is not an appropriate starting point for a quantitative
description of these complexes. Therefore the broken symmetry formalism as
introduced by Noodlemann45 is applied to determine the diradical index37a of the
complexes. The broken symmetry B3LYP wave functions of the neutral complexes44
show increasing diradical character upon going from S to N and to O. At these levels
of theory, the diradical character44 of the neutral Ni dithiolene complex is practically
zero and it does not break symmetry spontaneously. Large antiferromagnetic coupling
between the unpaired electrons is considered to be responsible for a negligible
diradical character in the nickel dithiolene complex. In a subsequent paper37a it has
been shown that the super-exchange interaction through the central metal ion
contributes strongly to the observed antiferromagnetism.
The extent of the superexchange interaction is dependent on the effective
nuclear charge of the central metal ion involved. Due to the small effective nuclear
charge of the Ni(II) ion the Ni d orbitals in [Ni(L)2] are placed very close to the sulfur
orbitals resulting in considerable mixing between the metal and ligand orbitals, and
hence significant superexchange interaction leading to a very small diradical
character. Therefore, the [Ni(L)2] complex does not have any broken symmetry
solution.
[Au(L′)2]+1 complex is isoelectronic with [Ni(L)2]. However, due to the high
effective nuclear charge of Au(III) ion the Au d orbitals are situated very deep in
energy and are well separated from the ligand orbitals. The antiferromagnetic
coupling between the ligand radicals mediated by the super-exchange interaction
through the central Au(III) ion is thus, expected to be very small. Accordingly, a
broken symmetry solution is found to exist for the [Au(L′)2]+ complex. A qualitative
bonding scheme derived from the B3LYP broken symmetry DFT calculation on the
120
Chapter 5
[Au(L′)2]+ complex is given in Figure 5.9. One finds six doubly occupied canonical
molecular orbitals, four of which are of predominant Au character and are situated
very low in energy. The remaining two orbitals are situated higher in energy and
predominantly ligand based. The analysis of the corresponding orbitals yields a spin-
coupled (magnetic) pair, formed between two ligand based orbitals (see Figure 5.9.).
The mutual overlap of these two orbitals is 0.35. A diradical character of 88% is
calculated (by procedures discussed in the literature37a for the complex with a singlet-
triplet (-2JGS) gap of 1024 cm-1. The electronic structure of the [Au(L′)2]+1 complex
can thus be best described in terms of a Au(III) d8 ion attached to two
antiferromagnetically coupled ligand radicals. It is important to note that calculations
have also been performed for the corresponding [Pd(L′)2] complex which is
isoelectronic to [Au(L′)2]+1. In contrast to the latter species no broken symmetry
solution is, however, obtained for the former. Due to a lower effective nuclear charge
of Pd(II) as compared to Au(III) the Pd d orbitals in [Pd(L′)2] are situated closer to the
ligand based orbitals and a Pd contribution of 20% has been calculated for the SOMO.
Thus the superexchange interaction in [Pd(L′)2] is still too high to obtain any broken
symmetry solution for the complex.
dx2-y2
dyz
dxz
dz2
Au 5d orbital manifold
dxy
Ligand
orbitals
1a´
2a´
1a´´
3a´
4a´
2a´´
π*π*
4a´´
Figure 5.9. Qualitative bonding scheme for [Au(L´)2]1+ as derived from BS-B3LYP ZORA DFT
calculations. The doubly occupied MOs are canonical orbitals and the singly occupied
MO’s result from a corresponding orbital transformation.
121
Chapter 5
The d populations and the spin densities at the central metal ion obtained from
the natural population analysis46 of the B3LYP densities for the complexes have been
summarised in Table 5.6. The d population remains constant on moving from the
cationic [Au(L′)2]+1 to the neutral [Au(L′)2] and to the monoanionic [Au(L′)2]1-
complex. However, it changes significantly on moving from the monoanionic
[Au(L′)2]1- to the dianionic [Au(L′)2]2- complex. This is consistent with a ligand based
oxidation for the [Au(L′)2]1-/0/+1 series and a metal based oxidation for the [Au(L′)2]2-/1-
couple.
Electrons-5d
Electrons-6s Spin-5d
[Au(L´)2]2- 9.34 0.62 0.25
[Au(L´)2]1- 8.91 0.58 0.00
[Au(L´)2]08.89 0.60 0.05
[Au(L´)2]1+ 8.84 0.64 0.00
Table 5.6. Comparison of the charge and spin populations at the metal ion resulting from a natural
population analysis of the one-electron density of the ground state obtained from scalar relativistic
ZORA-B3LYP DFT calculations.
Calculation of EPR parameters:
The calculated EPR parameters for the neutral [Au(L′)2] complex is in
reasonable agreement with the experiment. The SOMO of the complex is
predominantly ligand based (only 5% of Au dxz character) and correspondingly
together with the rather small nuclear moment of the 197Au nucleus, hyperfine
coupling constants of only –10, -15 and –11 MHz are calculated (see Table 5.7.)
which agree with the experimental values. There is also little angular momentum in
the ground state wave function, and thus, the calculated and observed g-shifts are
small and reflect the organic radical character of the ground state.
On the other hand, a large 197Au (I = 3/2, 100% natural abundance) hyperfine
splittings of –193, -203, and –204 MHz are however, predicted for the corresponding
dianionic [Au(L′)2]2- complex. The calculated g anisotropies (2.00, 2.08, and 2.05) in
[Au(L′)2]2- are also quite large as compared to what has been calculated for the
corresponding [Au(L′)2] complex. The experimental EPR spectrum of the [Au(L′)2]2-
could not be recorded due to stability reasons and hence the calculated EPR
parameters can not be directly compared with experiments. However, the calculated
EPR parameters for the [Au(L′)2]2- complex is very similar to what has been observed
122
Chapter 5
experimentally for the corresponding [Au(mnt)2]2- complex; where mnt2- represents
1,2-dicyanoethene-1,2-dithiolate(2-) (g1 = 1.98, g2 = 2.01, g3 = 2.02 and A1(197Au) = -
118.72, A2 = -121.11, A3 = -123.22 MHz).15
gxgygzAx
MHz
Ay
MHz
Az
MHz
[Au(L´)2] 2.028
(2.065)
2.023
(2.030)
1.962
(1.944)
-10 (20) -15 (20) -11 (19)
[Au(L´)2]2-
[Au(mnt2]**
[Au(mnt2]-2 **
2.00 *
2.0758
2.02
2.08 *
2.0381
2.01
2.05 *
1.9279
1.98
-193 *
16
-123
-203 *
9
-121
-204 *
24
-119
*) Experiments not done * *) Values from the Ref 13 and 39.
Table 5.7. Calculated and experimental (in parenthesis) EPR parameters for the complexes as obtained
from the scalar relativistic ZORA-B3LYP DFT calculation using large uncontracted Gaussian bases at
the gold and uncontracted all electron polarized triple ξ (TZVP) Gaussian
bases for the remaining atoms.
Excited State Calculations:
The optical electronic spectra of the [M(L)2]0/1-/2- (M = Ni, Pd, Pt) and
[Au(L)2]1-/0 complexes have been considered in detail before. Assuming D2h
symmetry for the complexes, all the spin and electric dipole allowed transitions
together with the corresponding excited states are summarized in Table 5.8. The
1b1u→2b2g transition in the [M(L)(L●)]z complexes is predominantly ligand-to-ligand
intervalence charge transfer (IVCT) in origin and is assigned to the intense band in the
near-IR region.28, 29 For the neutral diradical [M(L●)2] complexes the 1b1u→2b2g
transition is ligand-to-ligand charge transfer (LLCT) in origin and also occurs in the
near IR region.29a Interestingly, the intensity of the LLCT band is always twice as that
of the IVCT band.29a Here we will consider in detail the absorption spectra for the
corresponding [Au(L′)2]2-/1-/0/+1 complexes.
123
Chapter 5
Complex type Ground State Excited States Polarization
[MII(L)(L●)]z 2BB2g 2BB1u(1b1u→2b2g)
2Au(1au→2b2g)
2BB3u(1au→1b1g)
X
Y
Z
[MII(L)2]z 1Ag
1BB1u(1au→1b1g) Z
[M(L●)2]0 1A1g 1BB3u(1b1u→2b2g)
1BB2u(1au→2b2g)
1BB1u(1au→1b1g)
X
Y
Z
Table 5.8. Spin and electric dipole allowed transitions possible in the [M(L)(L●)]z (M = Ni, Pd, Pt, z=-
1; M= Au, z=0), and [M(L)2]z (M = Ni, Pd, Pt, z=-2; M = Au, z=-1), and [M(L●)2]0
(M= Ni, Pd, and Pt) complexes studied in this paper.
The calculated symmetry of the [Au(L′)2]2-/1-/0/+1 complexes is Cs. The nature
of the transitions in the complexes are, however, found to be very similar to their
[M(L)2]0/1-/2- counterparts in D2h symmetry. Thus, it is more appropriate to describe
the [Au(L′)2]2-/1-/0/+1 complexes as having an effective D2h symmetry with a small
perturbation.
The calculated spectra of the complexes as obtained from the time-dependent
DFT calculations are in reasonable agreement with the experiments (Table 5.9.). For
the neutral [Au(L′)2]0 complex the most intense transition in the near IR region is
IVCT in origin and corresponds to the 5a´(1b1u in D2h point group)→ 3a´´(2b2g in D2h
point group) transition as has been suggested previously for the corresponding
[Au(L)2]0 complex. This transition is predicted at 7020 cm-1 in the calculation and is
observed at 6666 cm-1. The next transition is calculated at 7300 cm-1 with an oscillator
strength of 0.03. This correspond to 2a´(1au in D2h point group) →3a´´(2b2g in D2h
point group) transition and is experimentally observed at 7100 cm-1 with an oscillator
strength of 0.025.
For the [Au(L′)2]1-/2- complexes the 3a´´(2b2g in D2h point group) orbital is
doubly occupied and correspondingly no transitions are calculated in the near infrared
region in agreement with the experiments. It is important to note that the 2a´(1au in
D2h point group) → 4a´´(1b1g in D2h point group)transition, expected to occur for all
the above three [Au(L′)2]0/1-/2-complexes appears at a high energy and is not observed
in the first 25 calculated (energy range 5000-25000 cm-1) states for these complexes.
As explained previously37a the broken symmetry formalism crudely models the
multireference character of the diradical systems like [Au(L′)2]1+ and is not an entirely
124
Chapter 5
satisfactory substitute for a genuine multiconfigurational treatment. A broken
symmetry time-dependent density functional calculation on the [Au(L′)2]1+ complex
will not therefore yield reasonable results as the wave function associated with the
broken symmetry state is not entirely satisfactory. Hence no time-dependent density
functional calculations have been performed on the [Au(L′)2]1+ complex. To calculate
the electronic spectrum of the complex a genuine multiconfigurational treatment is
necessary which is beyond the scope of the present work.
Energy(cm-1) Oscillator strength Complex
Transition
Exp Calc Exp Calc
[AuIII(L´)(L´●)] 5a´→3a´´
2a´→3a´´
6666
7800
7020
7300
0.129
0.025
0.20
0.03
Table 5.9. Results of the ZORA-B3LYP TDDFT calculation on the [AuIII(L´)(L´●)] complex and its
comparison with the experiments.
125
Chapter 5
5.7. Conclusions:
The electronic structure of the square planar diamagnetic monoanionic
complex 8 and its electrochemically and chemically generated paramagnetic one-
electron oxidized species 8a and diamagnetic two-electron oxidized species 8b, have
been elucidated experimentally as well as by density functional theory and correlated
ab initio methods. Complex 8 is formulated as a AuIII ion with d8 electronic
configuration (SAu = 0) coordinated to two closed shell dianionic cis-4,4´-di-tert-
butylphenylethylene-1,2-dithiolene ligands. Electrochemically and chemically
generated neutral paramagnetic (S = 1/2) one-electron oxidized species 8a consists of
a trivalent metal ion, one S,S-coordinated 1,2-dithioethylene semiquinonato(1-)
radical and one closed-shell dianionic cis-4,4´-di-tert-butylphenylethylene-1,2-
dithiolene ligand. This ligand mixed valency is of class III (delocalized) which gives
an intense intervalence charge transfer band at 1495 nm in the near-infrared region.
5% Au contributions in the SOMO orbital from the relativistic DFT calculations
supports the EPR spectrum of 8a with no Au hyperfine coupling. Electrochemically
generated diamagnetic monocationic two-electron oxidized species 8b consists of a
trivalent Au ion and two S,S-coordinated to 1,2-dithioethylene semiquinonato (1-)
radical ligands. The two spins are intramolecularly, strongly antiferromagnetically
coupled via a super exchange mechanism mediated by diamagnetic AuIII (d8; SAu = 0)
ion. It should be noted that the diradical character of this species found significantly
larger (~90%) with a singlet-triplet gap of 1024 cm-1 as we have not seen any time
before in Au(dithiolene) chemistry. The electronic spectrum of 8b displays a very
intense ligand to ligand charge transfer band at 1066 nm in the NIR region. Electronic
structure of the paramagnetic one-electron reduced species, elucidated only
theoretically, as AuII ion coordinated to two closed shell dianionic (3L)2- ligands.
126
Chapter 5
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129
130
Summary
131
132
Chapter 6
6.1. Summary:
This study has shown that N,O-coordinated o-aminophenol ligands are
noninnocent in the sense that they exist in four different protonation and oxidation
levels in the coordination compounds: (i) N-protonated monoanionic o-
aminophenolates (LAP)1-, (ii) dianionic o-iminophenolates (LIP)2-, (iii) monoanionic o-
iminobenzosemiquinonate (LISQ)1- π radicals, and (iv) neutral o-iminobenzoquinones
(LIBQ)0. These oxidation states of the ligands in the coordination compounds are
characterized by their differing C-O, C-N, and C-C bond distances. Low temperature
X-ray crystal studies are capable of providing identification for these different
oxidation levels of the respective ligands in a given transition metal complex. A
number of 1st, 2nd, and 3rd row transition metal complexes with mono- and di- N,O-
coordinate o-aminophenol ligands in different oxidation states has been synthesized
and spectroscopically characterized. The combination of electron spin resonance,
UV-vis absorption, and relativistic DFT calculations has given insight into the
electronic structures of these complexes.
Chapter 2:
A series of square planar complexes [Pd(1LISQ)(tertbpy)](PF6) (1a),
[Pd(1LIP)(tertbpy)] (1b) , and [Pd(1LIBQ)(tertbpy)](PF6)(BF4) (1c) containing N,N- and
N,O-coordinated a neutral tertbpy and a noninnocent 2-(2-trifluoromethyl)anilino-4,6-
di-tert-butylphenol ligand coordinated to a diamagnetic, divalent, Pd centre has been
synthesized and structurally characterized. The crystal structures have allowed the
determination of the redox level at the individual ligand. We have presented the
structural and spectroscopic evidence for the existence of o-
iminobenzosemiquinonate(1-) π radicals, o-iminophenolates(2-), and
o-iminobenzoquinones(0) in singly N,O-coordinated Pd complexes. Square planar,
dicationic, singly N,O-coordinated 1c type complexes with the quinone form of ligand
are now structurally characterized for the first time.
133
Chapter 6
Chapter 3:
The synthesis and characterization of a series of square planar Co, Ni, and Pd
complexes with the bulky noninnocent o-aminophenolate ligand, 2-(2-
trifluoromethyl)anilino-4,6-di-tert-butylphenol (1LH2) have been achieved. 1LH2
assumes different oxidation levels. The complexes prepared are summarized in Table
6.1.
Co complexes Ni complexes Pd complexes
[Co(1LISQ)(1LIP)] (2a) [Ni(1LISQ)2] (3a) [Pd(1LISQ)2] (4a)
[Co(1LIP)2]- (2b) [Ni(1LISQ)(1LIP)]- (3b) [Pd(1LISQ)(1LIP)]- (4b)
-
-
-
[Ni(1LIBQ)2(ClO4)2](3d)*
[Pd(1LISQ)(1LIBQ)]+1 (4c)
[Pd(1LIBQ)2]+2 (4d)
Table 6.1. Summary of all complexes that are discussed in chapter 3. * Octahedral.
The C-O, C-N, and C-C bond lengths are found to be characteristic for each
oxidation level. Thus, the following markers have been identified on going from the
N,O-coordinated (1LIP)2- dianion to the (1LISQ)1-y monoanionic π radical, and then to
the neutral quinone (1LIBQ): a) The C-N bond lengths decrease from 1.37 + 0.01 Å to
1.35 + 0.01 Å and, finally to 1.30 + 0.01 Å with increasing oxidation level. b)
Similarly, the C-O bond lengths decrease from 1.35 + 0.01 Å to 1.30 + 0.01 Å to 1.24
+ 0.01 Å. c) Finally, the six C-C bonds of the aminophenolate six-membered ring of
(1LIP)2- are nearly equidistant at 1.407 + 0.01 Å indicating the aromatic character of
the phenyl ring. One-electron oxidation to (1LISQ)1-y results in two alternating short C-
C bonds at 1.375 + 0.01 Å of partially double bond character and four longer bonds at
1.438 + 0.01 Å. This characteristic distortion is labeled "quinoid-like". In the neutral
genuine quinone form (1LIBQ), this distortion is more pronounced with two alternating
short C=C double bonds at 1.36 + 0.01 Å and four long C-C single bonds one of
which at 1.52 + 0.01 Å being a normal C-C single bond.
The neutral, four-coordinate, paramagnetic (S = ½), square planar complex
[Co(1L)2] (2a) and its one-electron reduced, square planar, paramagnetic (S = 1)
[Co(1L)2]- [Co(Cp)2]+ (2b) complexes were synthesized. The low temperature crystal
structures of square planar cobalt complexes (neutral 2a and monoanion 2b) clarify all
the discrepancies that have arisen. Therefore assignment of the spectroscopic
134
Chapter 6
oxidation state of the central cobalt ion as +III has been achieved. Unfortunately, the
results of DFT calculations could not conclusively help us assign the oxidation state
of +II or +III to the central cobalt ion.
Square planar complexes of [MII(L)2]n type with group 10 metal ions and
noninnocent ligands are known to form a complete five-membered electron-transfer
series where n = 2-, 1-, 0, +1, +2. The series of Ni and Pd complexes 3a, 3b, 3d, 4a,
4b, 4c, and 4d were synthesized and structurally characterized. The Pd complexes 4a,
4b, 4c, and 4d constitute four members of such an electron transfer series. The
electronic structures of the neutral, square planar complexes 3a and 4a are best
described as singlet diradicals. Both ligands in these complexes couple
intramolecularly, antiferromagnetically to give a singlet ground state. Electronic
structures of the monoanionic and monocationic complexes (3b, 4b, and 4c) have
been elucidated using simple MO diagrams. A monocationic species like 4c and
dicationic species like 4d have been structurally characterized for the first time. A
localized radical was observed for the first time in complex 4c due to unsymmetrical
ion pairing.
Chapter 4:
Diamagnetic, octahedral Mo-oxo complexes [Mo(2LIP)(2LAP)(O)(OCH3)] • 2
MeOH (5) and [(2LIP)(2LAP)(O)Mo-(μ-O)-Mo(O)(2LIP)(2LAP)] (6) along with W-oxo
complexes [W(2LIP)(2LAP)(O)(Cl)] (7a) and {W(2LIP)(2LAP)(O)(OCH3)}2 • 0.5 MeOH
(7b) have been synthesized using a noninnocent N,O-coordinating 2-(4-fluoro)anilino-
4,6-di-tert-butylphenol ligand, (2LH2). The X-ray crystal structures confirms the
presence of one N-protonated o-aminophenolate(1-), (2LAP)1-, ligand and one
o-iminophenolate(2-), (2LIP)2-, ligand in all four complexes.
Chapter 5:
A square planar Au complex [Au(3L)2][N(n-Bu)4] (8) and its chemically and
electrochemically one-electron oxidized neutral complex [Au(3L)2]0 (8a), with S,S-
coordinate cis-4,4´-di-tert-butylphenylethylene-1,2-dithiolene ligand, have been
structurally characterized. The properties of 8 and the electrochemically generated,
two-electron oxidized cationic [Au(3L)2]+ (8b) species are discussed. The electronic
structure of 8, 8a, and 8b have been elucidated experimentally as well as by density
functional theory and correlated ab initio methods. The complex 8b shows very
135
Chapter 6
interesting features consistent with a diradical character, not previously known in
gold-dithiolene chemistry. Complex 8 is formulated as a AuIII ion with d8 electronic
configuration (SAu = 0) and two coordinated closed-shell dianionic cis-4,4´-di-tert-
butylphenylethylene-1,2-dithiolene ligands. 8a consists of a trivalent metal ion, one
1,2-dithioethylene semiquinonato(1-) radical, and one closed-shell dianionic cis-4,4´-
di-tert-butylphenylethylene-1,2-dithiolene ligand. 8b consists of a trivalent Au ion
and two S,S-coordinated to 1,2-dithioethylene semiquinonato(1-) radical ligands. The
two spins are intramolecularly, strongly antiferromagnetically coupled via a super
exchange mechanism mediated by the diamagnetic AuIII (d8; SAu = 0) ion.
Spectroscopic and structural markers for the presence of such radicals in
coordination complexes have been established and are as follows: (i) Short C-S bonds
at ~1.72 Å and longer C-C ethylene bond distances, (ii) ν(C=S•) stretching frequency
at ~1149 cm-1 in the infrared spectrum, and (iii) a very intense intervalence charge
transfer band in the near infrared region.
136
Equipment and Experimental Work
137
138
Chapter 7
7.1. Methods and Equipment:
All the analyses were performed at the Max-Planck-Institut für
Bioanorganische Chemie, Mülheim an der Ruhr, Germany, unless otherwise
specified. Commercial grade chemicals were used for the synthetic procedures and
solvents were distilled before use. Degassed and dry solvents were used for the
synthesis of oxygen and moisture sensitive complexes. Air and moisture sensitive
syntheses were performed under argon using standard Schlenk techniques or in a
‘MBRAUN Labmaster 130’ glovebox utilizing Ar 4.6 as inert gas.
Infrared Spectroscopy:
Infrared spectra were measured from 4000 to 400 cm-1 as KBr pellets at room
temperature using a Perkin-Elmer FT-IR-Spectrophotometer 2000.
Mass Spectroscopy:
Mass spectra using Electron Impact ionization (EI; 70 eV) were recorded on a
Finnigan MAT 8200 mass spectrometer. Only characteristic fragments are provided
here with their intensities. The spectra were normalized against the most intense peak,
assigned to an intensity of 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, Germany.
Elemental Analysis:
The determination of the C, H, N, and metal content of the compounds were
performed by the “Mikroanalytischen Labor, H. Kolbe”, Mülheim an der Ruhr,
Germany.
Electrochemistry:
Cyclic voltammograms and square wave voltammograms were recorded by
using a EG&G potentiostat/galvanostat 273 A. A three electrode cell was employed
with a glassy carbon working electrode, a platinum-wire auxiliary electrode and a
Ag/AgNO3 reference electrode (0.01 M AgNO3 in MeCN). TBAPF6 was used as a
supporting electrolyte. Ferrocene was used as an internal standard. All potentials are
referenced versus the Ferrocenium/Ferrocene couple (Fc+/Fc). UV-vis-assisted,
controlled-potential coulometries were performed with the same potentiostat, in a
thermostatic 5 mm quartz cell equipped with a Pt grid as work electrode, a Pt brush
139
Chapter 7
separated from the work electrode compartment by a Vycor frit as counter-electrode,
and a Ag/AgNO3 electrode (0.01 M, MeCN) as reference.
UV-Vis Spectroscopy:
UV-vis spectra and near infrared coulometric measurements were performed
on a Perkin-Elmer UV-vis Spectrophotometer Lambda 19 or a Hewlett-Packard HP
8452A diode array spectrophotometer in the range 200-2500 nm. UV-Vis
spectroelectrochemical investigations were performed by employing a coulometry
cuvet and Bu4NPF6 as a supporting electrolyte.
Magnetic Susceptibility Measurements:
Measurements of temperature or field dependent magnetization of samples
were performed in the range 2 to 295 K at 1 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
were compensated for diamagnetic contribution compensated and recalculated as
volume susceptibilities. Diamagnetic contributions were estimated for each compound
using Pascal’s constants. The experimental results were fitted with the JULIUS1
program, calculating through full-matrix diagonalzation of the Spin-Hamiltonian.
EPR Spectroscopy:
First derivative X-band EPR spectra of frozen solution samples were recorded
on a Bruker ESP 300 equipped with a Bruker ER 041 XK-D microwave bridge and a
Oxford Instruments 910 EPR-cryostat. The simulation of the spectra was performed
with help of the programs “ESIM/GFIT” from Dr. E. Bill and “EPR” from Dr. F.
Neese.
Crystallography:
All X-ray crystal structures are solved by Dr. T. Weyhermüller. X-ray single
crystal diffraction data were collected by Mrs. H. Schucht on a ‘Nonius-Kappa CCD
Diffractometer’ (with a graphite monochromator, Mo-Kα with λ = 0.71073 Å) or on a
‘Siemens Smart System’ (with a Cu fine focus tube, Cu-Kα: 1.54178 Å). Data were
collected by the 2θ-ω scan method (3 ≤ 2θ ≤ 108°). The data were corrected for
absorption and Lorenz polarization effects2, 3 The Siemens SHELXTL software
package2 was used for solutions and artwork of the structure, SHELXL974 was used
for the refinement. The structures were solved by direct and Patterson methods,
140
Chapter 7
subsequent Fourier-difference techniques, and refined anisotropically by full-matrix
least-squares on F2. Hydrogen atoms were included at calculated positions with U <
0.08 Å2 in the last cycle of refinement. The ellipsoid plots of crystal structures were
done by ORTEP-32 and POV-Ray programs.
GC Analysis:
GC of the organic products were performed either on HP 5890 II or HP 6890
instruments using RTX-1701 15 m S-41 or RTX-5 Amine 13.5 m S-63 columns
respectively. GC-MS was performed using the above columns coupled with a HP
5973 mass spectrometer with mass selective detector.
NMR Spectroscopy:
1H- NMR spectra were measured using a Bruker ARX 250, DRX 400 or DRX
500 NMR spectrometer. The spectra were referenced against TMS, using the residual
proton signals of the deuterated solvents as internal standards.
Calculations:
All calculations in this work were performed by Dr. Kallol Ray, with the
electronic structure program ORCA.5 As will be further discussed in the text, the
geometry optimizations were carried out at the BP86 level6 of DFT. These functionals
have proved in many applications their ability to reliably predict structures of
transition metal complexes. Since we are dealing with heavy transition metal
complexes we have carried out the present calculations with inclusion of scalar
relativistic effects at the second order Douglas-Kroll-Hess level (DKH2).7 In the
geometry optimizations the one-centre approximation was used which eliminates
DKH2 contributions to the analytic gradients. In the ZORA8a context it has been
shown that the one-centre approximation introduces only minor errors in the final
geometries.8b Large uncontracted Gaussian basis sets were used for the gold atom
which were derived from the Well-tempered basis sets of Huzinaga.9 For the
remaining atoms we used the all-electron polarized triple-ξ (TZVP)10 Gaussian basis
sets of the Ahlrichs group but uncontracted them in order to allow for a distortion of
the inner shell orbitals in the presence of the relativistic potential.
The property calculations at the optimized geometries were done with the
B3LYP functional.11 In this case the same basis sets were used but the quasi-
relativistic ZORA method8 was used since in this formalism magnetic properties are
more readily formulated.12 For the calculation of the EPR parameters the Fermi
141
Chapter 7
contact, dipolar and metal spin-orbit contributions are included. TD-DFT calculations
were carried out according to ref 13.
Experimental Section:
7.2. Ligand Syntheses:
Three different ligands used in the work mentioned in this theses. The ligands
syntheses and characterizations are given below.
7.2.1. 2-(2-trifluoromethyl)anilino-4,6-di-tert-butylphenol (1LH2):
To a solution of 3,5-di-tert-butylcatacol 11.1 g (50 mmol) in n-heptane
(60 mL), triethylamine (0.5 mL), and 2-triflouromethylaniline 6.2 mL (50 mmol) were
added and stirred for 5 days at room temperature. The excess solvent was evaporated
using a rotary evaporator. The residue was layered with n-pentane (5 mL) and kept
under refrigeration at 4º C. A crystalline solid was obtained, filtered off, washed with
cool n-pentane, and air dried.
Yield: 43% (7.6 g)
Molecular weight: 365
EI-MS: m/z = 365 {M}+ 100%
Elemental Analysis: C21H26NOF3
%C %H %N
Calculated 69.10 7.18 3.83
Found 69.32 7.5 3.73
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
OH
NH
CF3
142
Chapter 7
Melting Point: 78-80º C
Gas Chromatography: 98% pure
H1 NMR (400 MHz, CD2Cl2, at room temp.,): δ =1.27 (s, 9H); 1.436 (s, 9H); 5.68 (s,
1NH); 6.19 (s, 1H); 6.50 (t, 1H); 6.89 (m, 1H); 7.03 (d, 1H); 7.29 (t, 1H); 7.55 (d,
1H).
7.2.2. 2-(4-fluoro)anilino-4,6-di-tert-butylphenol (2LH2):
To a solution of 3,5-di-tert-butylcatacol 11.1 g (50 mmol) in n-heptane
(60 mL), triethylamine (0.5 mL), and 4-flouroaniline 4.8 mL (50 mmol) were added
and stirred for 24 h at room temperature. White precipitate formed was filtered off,
washed with cool n-hexanes, and air dried.
Yield: 81% (12.74 g)
Molecular weight: 315
EI-MS: m/z = 315 {M}+ 100%
Elemental Analysis: C20H26NOF
%C %H %N
Calculated 76.16 8.31 4.44
Found 74.51 8.49 4.36
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
NH
OH
F
143
Chapter 7
Melting Point: 131-133º C
Gas Chromatography: 98% pure
H1 NMR (400 MHz, CD2Cl2, at room temp.,): δ =1.284 (s, 9H); 1.432 (s, 9H); 5.05 (s,
1NH); 6.63(m, 2H); 6.928(m, 2H); 7.02 (d, 1H); 7.225 (d, 1H).
7.2.3. cis-1,2-(4,4´-di-tert-butylphenyl)ethylene-1,2-dithiolene (3LH2):
This ligand synthesis was performed in two steps:
1. Preparation of 4,4´-di-tert-butyl benzoin
2. Obtaining ligand in situ.
Step 1: 4,4´-di-tert-butyl benzoin has been prepared by using simple benzoin
condensation. To a solution of 4-tert-butylbenzaldehyde (50 g; 0.302 mol) in EtOH
(45 mL) an aqueous solution (30 mL) of NaCN (3.2 g) was added and gently heated to
reflux for 0.5 hr. After cooling in an ice bath, crude benzoin formed, was filtered off,
and washed with water. The crude product was recrystallized by redissolving in EtOH
(200 mL) and heated until it completely dissolve. Upon cooling slowly to room
temperature, pure crystalline 4,4´-di-tert-butyl benzoin formed. The product was
filtered off, washed with water and air dried.
Step 2: This ligand is very difficult to isolate in solid state. The procedure was
adapted from the literature14 to obtain ligand in situ. To a solution of 1,4-dioxane (5
mL) and 4,4´-di-tert-butyl benzoin (0.413 g, 1.25 mmol) P4S10 (0.4 g, 0.91 mmol) was
added. The solution was heated to reflux for 2.5 h in air. A yellow solution obtained,
cooled down to the room temperature, and filtered to remove excess P4S10 from the
solution. The yellow solution contains an intermediate compound (see below) which
will give ligand in solution on providing protons. Protons are provided by either
aqueous or alcoholic solution of metal salt, and drive the reaction to completion.
Further characterizations on this ligand were not done as the ligand was not isolated in
its metal free form.
S
S
P
S (O)
S/2
SH
SH
H+
(O/2)
2
Mz+
S
S
M
2
144
Chapter 7
7.3. Complex Syntheses:
[Pd(1LISQ)(tertbpy)](PF6) (1a):
To a solution of PdCl2 (0.177g, 1 mmol) in MeOH (20 mL) triethylamine (0.8 mL),
ligand 1LH2 (0.37 g, 1 mmol), 4,4´-di-tert-butyl-2,2´-dipyridyl (0.268 g, 1 mmol)
were combined and stirring for 20 min under Ar, followed by reflux for 1 h. Then
solution was allowed to cool to room temperature and exposed to air. Further, KPF6
(0.8 g in 10 mL MeOH) was added to the mixture. The mixture was stirred for 3 h;
resultant reddish brown solution, was filtered, and allowed to stand in an open vessel,
yielding X-ray quality crystals.
Yield: 53% (0.460 g)
Molecular weight: 882
ESI mass spectrum: Positive mode m/z (100%) = 737 [Pd (1LISQ)(tertbpy)]+; Negative
mode m/z (100%) = 145 [PF6]-
Elemental Analysis: C39H48F9N3OPPd
%C %H %N %Pd
Calculated 53.05 5.48 4.76 12.05
Found 53.65 5.28 4.60 11.70
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave numbe
r
,
cm-1
145
Chapter 7
[Pd(1LIP)(tertbpy)] (1b):
To a degassed and dry solution of 1a (0.255 g, 0.29 mmol) in CH2Cl2 (8 mL),
cobaltocene, [Cp2Co] (0.055 g, 0.29 mmol) was added under Ar atmosphere. After
stirring for 1 h, the solution was allowed to stand for 1 h, and evaporated the solvent
using vacuum. Blue powder was obtained and redissolved in MeOH (6 mL). Residual
yellow material was removed by filtration. X-ray quality crystals were obtained by
slow evaporation of MeOH. The resultant blue compound is sensitive to air.
Yield: 92% (0.198 g)
Molecular weight: 737
EI mass spectrum: m/z {M}+ (30%) = 737
Elemental Analysis: C39H48F3N3OPd
%C %H %N %Pd
Calculated 63.45 6.55 5.69 14.42
Found 63.46 6.55 5.62 14.37
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
146
Chapter 7
[Pd(1LIBQ)(tertbpy)](PF6)(BF4) * 2 CH2Cl2 (1c):
To a degassed and dry solution of 1a (0.1 g, 0.11 mmol) in CH2Cl2 (7 mL), [NO]BF4
(0.013 g, 0.11 mmol) was added under Ar atmosphere. After stirring for 2 h, the
brown colour solution was filtered, and layered with n-heptane. Slow evaporation
under Ar offered X-ray quality crystals.
Yield: 56% (0.460 g)
ESI mass spectrum: Positive mode m/z (20%) = [Pd(1LISQ)(tertBPy)]2+; Negative mode
m/z (100%) = 145 (PF6)- and m/z (20%) = 87 (BF4)-
Molecular weight: 969
Elemental Analysis: C41H52F13N3Cl4OBPPd
%C %H %N %Pd
Calculated 43.23 4.60 3.68 9.25
Found 44.93 4.67 3.67 8.68
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
147
Chapter 7
[Co(1LISQ)(1LIP)] (2a):
The ligand, 1LH2 (2.19 g; 6 mmol), and Co(ClO4)2 • 6H2O (0.73 g; 2.0 mmol) were
added to a degassed solution of methanol (50 mL) and NEt3 (0.8 mL). The solution
was heated to reflux for 1 h and then stirred at room temperature in the presence of air
for 2 h. A deep blue precipitate formed and was collected by filtration.
Recrystallization from a CH2Cl2/CH3OH (1:1) mixture afforded single crystals
suitable for X-ray crystallography.
Yield: 54% (810 mg)
Molecular weight: 785
EI mass spectrum: m/z {M}+ (100%) = 785
Elemental Analysis: C42H48N2O2F6Co
%C %H %N %Co
Calculated 64.2 6.1 3.56 7.5
Found 64.0 5.84 3.43 7.93
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
148
Chapter 7
[Co(1LIP)2] [Co(Cp)2]* 2 CH3CN (2b):
To 15 mL of degassed CH2Cl2 2a (0.785 g, 1 mmol) and Cobaltocene (0.189 g, 1
mmol) were added and stirred under Ar for 3 h. The purple colour precipitate obtained
on filtration. Recrystallization from CH3CN/Ether (1:3) mixture solution afforded X-
ray quality crystals.
Yield: 30% (316 mg)
Molecular weight: 1056
ESI mass spectrum: Positive mode m/z (100%) = 737 [CoCp2)]+; Negative mode m/z
(100%) = 785 [Co(1LIP) 2]-
Elemental Analysis: [CoC42H48N2O2F6] [Co(Cp)2]
%C %H %N %Co
Calculated 63.69 6.10 5.30 11.17
Found 62.43 6.01 4.63 10.84
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
149
Chapter 7
[Ni(1LISQ)2] (3a):
To the solution of methanol (35 mL) and triethylamine (0.8 mL), 1LH2 (0.730 g; 2
mmol) and [Ni(NO3)2]•6H2O (0.291 g; 1 mmol) were added and refluxed for 1h,
followed by stirring in air for 2 h. The green precipitate obtained on filtration.
Recrystallization from Ether/MeOH (1:1) mixture solution afforded X-ray quality
crystals.
Yield: 53% (414 mg)
Molecular weight: 784
EI mass spectrum: m/z {M}+ (100%) = 784
Elemental Analysis: NiC42H48N2O2F6
%C %H %N %Ni
Calculated 64.33 6.17 3.57 7.39
Found 64.35 6.21 3.54 7.32
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
150
Chapter 7
[Ni(1LISQ)(1LIP)] [Co(Cp)2] (3b):
To 15 mL of degassed CH2Cl2 3a (0.784 g; 1 mmol) and Cobaltocene (0.189 g; 1
mmol) were added and stirred under Ar for 3 h. A green coloured precipitate was
obtained on filtration. Recrystallization from CH3CN:Ether mixture solution afforded
X-ray quality crystals.
Yield: 69% (671 mg)
Molecular weight: 973
ESI mass spectrum: Positive mode m/z (100%) = 189 [CoCp2]+; Negative mode m/z
(100%) = 784 [Ni(1LISQ)(1LIP)]-
Elemental Analysis: [NiC42H48N2O2F6][Co(Cp)2]
%C %H %N %Ni %Co
Calculated 64.20 6.00 2.88 6.0 6.0
Found 63.98 5.89 2.95 6.02 6.05
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
151
Chapter 7
[Ni(1LIBQ)2(ClO4)2] * 2 CH2Cl2 (3c):
To a degassed solution of 3a (0.250 g; 0.32 mmol) in CH2Cl2 (30 mL), AgClO4
(0.132 g; 0.64 mmol) was added an under Ar blanketing atmosphere. The mixture was
stirred at room temperature for 2 h and filtered. The solvent was stripped off by rotary
evaporation and a reddish-orange solid obtained was recrystallized from
CH2Cl2/Hexanes mixture (1:2).
Yield: ~40% (120 mg)
Molecular weight: 1152
ESI mass spectrum: Positive mode m/z (100%) = 883 [Ni(1LIBQ)2(ClO4)2] – [ClO4]+;
Negative mode m/z (100%) = 99 [ClO4]-
Elemental Analysis: NiC42H48N2O10F6Cl2 * 2 CH2Cl2
%C %H %N %Ni
Calculated 45.87 4.55 2.43 5.09
Found 46.55 4.78 2.34 4.94
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
152
Chapter 7
[Pd(1LISQ)2] (4a):
To a solution of acetonitrile (25 mL) and triethylamine 0.8 mL (excess) the ligand,
1LH2 (1.465 g; 4 mmol), and anhydrous PdCl2 (0.354 g; 2 mmol) were added. The
solution was stirred under Ar for 1 hr, heated to reflux for 1h, and stirred in air at
room temperature for 2 h. Blue green precipitate formed on standing at room
temperature for over night. The solid was collected by filtration. Recrystallization
from Ether/MeOH (1:1) mixture solution afforded blue coloured, X-ray quality
crystals.
Yield: ~30% (453 mg)
Molecular weight: 832
EI mass spectrum: m/z {M}+ (100%) = 832
Elemental Analysis: PdC42H48N2O2F6
%C %H %N %Pd
Calculated 60.57 5.81 3.37 12.8
Found 59.9 5.77 3.42 12.9
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
153
Chapter 7
[Pd(1LISQ)(1LIP)] [Co(Cp)2] (4b):
To a degassed solution of 4a (160 mg; 0.19 mmol) in CH2Cl2 (15 mL), Cobaltocene
(36 mg; 0.19 mmol) was added under an Ar blanketing atmosphere. After stirring for
1 h a green precipitate obtained, and collected by filtration. Recrystallization from a
CH3CN/Ether mixture (1:3) afforded X-ray quality crystals.
Yield: 61% (120 mg)
Molecular weight: 1021
ESI mass spectrum: Positive mode m/z (100%) = 188.8 [CoCp2]+; Negative mode m/z
(100%) = 832 [Pd(1LISQ)(1LIP)]-
Elemental Analysis: [PdC42H48N2O2F6] [Co(Cp)2]
%C %H %N %Pd
Calculated 61.16 5.72 2.74 10.38
Found 61.06 5.64 2.76 10.33
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
154
Chapter 7
[Pd(1LISQ)(1LIBQ)](BF4) (4c):
To a degassed solution of 4a (90 mg; 0.11 mmol) in CH2Cl2 (15 mL), AgBF4 (21 mg;
0.11 mmol) was added under an Ar blanketing atmosphere. The mixture was stirred at
room temperature for 2 h and filtered. The solvent was stripped off by rotary
evaporation and a reddish-brown solid obtained was recrystallized from
CH2Cl2/Hexanes mixture (1:3).
Yield: ~70% (0.070 g)
Molecular weight: 919
ESI mass spectrum: Positive mode m/z (100%) = 832 [Pd(1LISQ)(1LIBQ)]+; Negative
mode m/z (100%) = 87 [BF4]-
Elemental Analysis: [PdC42H48N2O2F6] [BF4]
%C %H %N %Pd
Calculated 54.84 5.22 3.04 11.53
Found 54.69 4.99 2.84 11.10
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
155
Chapter 7
[Pd(1LIBQ)2]3 (BF4)4 {(BF4)2H}2 * 4CH2Cl2 (4d):
To a distilled and degassed solution of 4a (75 mg; 0.091 mmol) in CH2Cl2 (15 mL),
[NO]BF4 (23 mg; 0.197 mmol) was added under Ar an blanketing atmosphere. The
mixture was stirred at room temperature for 3 h followed by bubbling through the
solution. The concentrated solution was layered with hexanes and allowed to stand at
-10o C. The green solid obtained was recrystallized from CH2Cl2/Hexanes mixture
(1:3). The complex must be stored at low temperatures.
Yield: ~66% (0.060 g)
Molecular weight: 3359
ESI mass spectrum: Positive mode m/z (100%) = 832.3 [Pd(1LISQ)(1LIBQ)]+; Negative
mode m/z (100%) = 87 [BF4]-
Elemental Analysis [Pd(LIBQ)2]3 [BF4]4 [BF4HF4B]2 * 4CH2Cl2
%C %H %N %Pd
Calculated 46.48 4.56 2.50 9.47
Found 47.10 4.76 2.50 9.56
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
156
Chapter 7
[Mo(2LIP)(2LAP)(O)(OCH3)] * 2 MeOH (5):
To a solution of 2LH2 (0.950 g; 3 mmol) in freshly distilled MeOH (20 mL),
[Mo(O)2(acac)2] (0.328 g; 1 mmol in 5 mL MeOH) was added under Ar and stirred
for 3 h. The solution was allowed to stir in air for 1 h at room temperature. The
resultant purple-brown solution was then allowed to stand in an open vessel, to give a
purple-brown precipitate. Recrystallization from a CH2Cl2/CH3OH (1:1) mixture
afforded single crystals suitable for X-ray crystallography.
Yield: ~50% (0.400 g)
Molecular weight: 834.9
Elemental Analysis: C43H60F2N2O6Mo
%C %H %N %Mo
Calculated 61.8 7.24 3.35 11.48
Found 61.30 7.26 3.10 10.72
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
157
Chapter 7
[(2LIP)(2LAP)(O)Mo-(μ-O)-Mo(O)(2LIP)(2LAP)] (6):
To a solution of 2LH2 (0.475 g; 1.5 mmol) in CH2Cl2 (10 mL), [Mo(O)2(acac)2] (0.164
g; 0.5 mmol in 5 mL CH2Cl2) was added under Ar and stirred for 3 h. The solution
was then allowed to stir in air for 1 h at room temperature. The resultant brown
solution was layered with CH3NO2 (1:1) allowed to evaporate slowly. Dark brown
coloured X-ray quality crystals obtained from the flask after 24h.
Yield: ~20% (0.290 g)
Molecular weight: 1495
Elemental Analysis: C80H98F4N4O7Mo2
%C %H %N %Mo
Calculated 64.25 6.60 3.75 7.49
Found 64.18 6.68 3.70 12.69
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
158
Chapter 7
[W(2LIP)(2LAP)(O)(Cl)] (7a):
To a solution of 2LH2 (0.950 g; 3 mmol) in degassed CCl4 (15 mL), WCl6 (0.4 g; 1
mmol) was added under Ar and stirred for 1 h yielding reddish-brown solution. The
solution was allowed to stir in air for 2 h at room temperature. Unreacted WCl6 was
removed by filtration. Solvent was evaporated from solution under vacuum. The
resultant brown precipitate was recrystallized from a CH2Cl2/CH3CN (1:1) mixture, to
afforded single crystals suitable for X-ray crystallography.
Yield: 92% (0.800 g)
Molecular weight: 862
Elemental Analysis: C40H49F2N2O3ClW
%C %H %N %Mo
Calculated 55.66 5.72 3.25 21.48
Found 54.85 5.71 3.03 18.83
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
159
Chapter 7
{W(2LIP)(2LAP)(O)(OCH3)}2 * 0.5 MeOH (7b):
To a solution of 7a (0.250 g; 0.3 mmol) in MeOH (8 mL), NaOCH3 (0.017 g in 0.6
mL MeOH) and TBAPF6 (0.116 g in 2.5 mL MeOH) were added under an inert
atmosphere and stirred at room temperature for 16 h. Orange precipitate formed was
filtered off, and recrystallized from a Ether/CH3OH (1:1) mixture.
Yield: ~17% (0.090 g)
Molecular weight: 1733
Elemental Analysis: C82H104F4N4O8W2 * 0.5 CH3OH
%C %H %N %W
Calculated 57.17 6.17 3.23 21.13
Found 56.82 6.44 3.03 20.28
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
160
Chapter 7
[Au(3L)2] [N(n-Bu)4] * 2.25 CH2Cl2 (8):
To a solution of 1,4-dioxane (5 mL) and 4,4´-di-tert-butyl benzoin (0.413 g; 1.25
mmol) P4S10 (0.4 g; 0.91 mmol) was added. The solution was heated to reflux for 2.5
h in air. The solution cooled to the room temperature. The excess of P4S10 was
removed from the solution by filtration and washed with 2 mL of 1,4-dioxane. To this
yellow solution, 1 mL of aqueous solution of Na[AuCl4].2H2O (0.223 g; 0.56 mmol)
was added. This mixture was refluxed for 1 h. A greenish-brown precipitate formed
was removed from the green solution. This green solution was layered with 3 mL
EtOH solution of TBABr (0.186 g; 0.56 mmol. Golden-yellow coloured
microcrystalline precipitate obtained in 24 h, was separated with filtration, and
washed with hexanes. Recrystallization from a CH2Cl2/Hexanes (1:1) mixture
afforded yellow coloured X-ray quality crystals.
Yield: ~13% (~80 mg)
Molecular weight: 1147
ESI mass spectrum: Positive mode m/z (100%) = 242 [N(n-Bu)4]+; Negative mode
m/z (100%) = 905 [Au(3L)2]-
Elemental Analysis: [AuC44H52S4] [TBA]
%C %H %N %S %Au
Calculated 62.74 7.72 1.22 11.17 17.15
Found 63.68 8.02 0.90 9.16 15.9
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
161
Chapter 7
[Au(3L)2] * CH2Cl2 (8a):
To a degassed solution of 8 (35 mg; 0.031 mmol) in CH2Cl2 (5 mL), FcPF6 (11 mg;
0.031 mmol) was added under an Ar blanketing atmosphere. The mixture was stirred
at room temperature for 2 h to yield a green solution. Microcrystalline green
precipitate obtained on layering with MeOH.
Yield: ~80% (0.022 g)
Molecular weight: 905
ESI mass spectrum: Positive mode m/z (100%) = 242 [N(n-Bu)4]+; Negative mode
m/z (100%) = 905 [Au(3L)2]-
Elemental Analysis: [AuC44H52S4]* CH2Cl2
%C %H %S %Au
Calculated 58.32 5.78 14.16 21.74
Found 58.10 5.67 14.01 21.55
Infrared Spectrum:
4000 3500 3000 2500 2000 1500 1000 500
%T
wave number,cm-1
162
Chapter 7
7.4. References:
(1) Krebs, C. Dissertation; Rhur-Universität Bochum: Bochum, Germany, 1997;
Birkelbach, F. Dissertation; Rhur-Universität Bochum: Bochum, Germany,
1995.
(2) SHELXTL V.5, Siemens Analytical X-Ray Instruments, Ins. Madison,
Wisconsion, USA, 1994.
(3) Sheldrick, G. M. SADABS, University of Göttinggen, Germany, 1994
(4) Sheldrick, G. M. SHELXL97, University of Göttinggen, Germany, 1994
(5) Neese, F. Orca- an ab initio, DFT and semiempherical Electronic Structure
Package. Version 2.4, Revision 16, Max-Planck Institute für Bioanorganische
Chemie, Mülheim, Germany, November 2004.
(6) (a) Becke, A. D. J. Chem. Phys. 1988, 84, 4524. (b) Perdew, J. P. Phys. Rev. B
1986, 33, 8522.
(7) Hess, B. A.; Marian, C. M. In Computational Molecular Spectroscopy; Jensen,
P.; Bunker, P. R.; Eds.; John Wiley & sons: New York, 2000, pl69ff.
(8) (a) van Lenthe, E.; Snijders, J. G.; Baerends, E. J. J. Chem. Phys. 1996, 105,
6505. (b) van Lenthe, E.; Faas, S.; Snijders, J. G. Chem. Phys. Lett. 2000,
328, 107.
(9) (a) Huzinaga, S.; Miguel, B. Chem. Phys. Lett. 1990, 175, 289. (b) Huzinaga,
S.; Klobukowski, M. Chem. Phys. Lett. 1993, 212, 260.
(10) (a) Schäfer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571. (b)
Schäfer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5289.
(11) (a) Lee. C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (b) Becke, A.
D. Chem. Phys. 1993, 98, 5648.
(12) van Lenthe, E.; van der Avoird, A. J. Chem. Phys. 1998, 108, 4783.
(13) Neese, F.; Olbrich, G. Chem. Phys. Lett. 2202, 362, 170.
(14) (a) Schrauzer, G. N.; Mayweg, V. P.; Heinrich, W. Inorg. Chem. 1965, 4,
1615. (b) Schrauzer, G. N.; Mayweg, V. P.; Finck, H. W.; Heinrich, W. J.
Am. Chem. Soc. 1966, 88, 4604.
163
164
165
1. Crystallographic data
2. Magnetochemical data
3. Curriculum Vitae
166
Appendices
167
1. Crystallographic data:
1a 1b
Empirical formula C39 H48 F9 N3 O P Pd C39 H48 F3 N3 O Pd
Formula weight 883.13 738.20
Temperature 100 (2) K 100 (2) K
Wavelength (MoKα) 0.71073 Å 0.71073 Å
Crystal system Orthorhombic Monoclinic
Space group Pbca P21/c
Unit cell dimensions a = 17.7791 (5) Å a = 14.8500 (4) Å
b = 20.9990 (7) Å b = 19.2194 (6) Å
c = 21.6489 (7) Å c = 13.7915 (4) Å
α = 90 deg α = 90 deg
β = 90 deg β = 110.538 (5) deg
γ = 90 deg γ = 90 deg
Volume (Å3), Z 8082.5 (4), 8 3686.02 (19), 4
Density (calc.) Mg/m3 1.452 1.330
Absorption coeff mm-1 0.574 0.552
F(000) 3624 1536
Crystal size (mm) 0.03 x 0.03 x 0.03 0.42 x 0.40 x 0.28
θ Range for data collection 2.94 to 26.00 deg 2.93 to 31.02 deg
Limiting indices -21<=h<=21 -21<=h<=21
-25<=k<=25 -27<=k<=27
-26<=l<=26 -19<=l<=19
Reflections collected 88473 62403
Independent reflections 7921 [R(int) = 0.0811] 11678 [R(int) = 0.0303]
Absorption correction None None
Data/restraints/parameters 7921 / 0 / 499 11678 / 31 / 452
Goodness-of-fit on F2 1.037 1.075
Final R indices R1 = 0.0417 R1 = 0.0261
[I>2σ(I)] wR2 = 0.0803 wR2 = 0.0603
R indices (all data) R1 = 0.0746 R1 = 0.0303
wR2 = 0.0928 wR2 = 0.0623
Appendices
168
1c 2a
Empirical formula C41 H52 B Cl4 F13 N3 O P Pd C42 H48 F6 N2 O2 Co
Formula weight 1139.84 785.75
Temperature 100 (2) K 100 (2) K
Wavelength (MoKα) 0.71073 Å 0.71073 Å
Crystal system Monoclinic Triclinic
Space group C2/c P⎯1
Unit cell dimensions a = 37.901 (2) Å a = 17.0501 (8) Å
b = 11.5338 (6) Å b = 17.3146 (6) Å
c = 26.740 (2) Å c = 18.0229 (8) Å
α = 90 deg α = 64.51 (1) deg
β = 124.352(6) deg β = 85.16 (1) deg
γ = 90 deg γ = 60.78 (1) deg
Volume (Å3), Z 9650.4 (10), 8 4137.9 (3), 4
Density (calc.) Mg/m3 1.569 1.261
Absorption coeff mm-1 0.726 0.477
F(000) 4624 1644
Crystal size (mm) 0.15 x 0.06 x 0.06 0.20 x 0.16 x 0.12
θ Range for data collection 3.03 to 29.00 deg 3.20 to 27.50 deg
Limiting indices -51<=h<=51 -22<=h<=22
-15<=k<=15 -22<=k<=22
-36<=l<=36 -23<=l<=23
Reflections collected 76981 56392
Independent reflections 12804 [R(int) = 0.0628] 18976 [R(int) = 0.0402]
Absorption correction None Gaussian, face indexed
Data/restraints/parameters 12804 / 94 / 614 18976 / 0 / 982
Goodness-of-fit on F2 1.043 1.030
Final R indices R1 = 0.0536 R1 = 0.0561
[I>2σ(I)] wR2 = 0.1201 wR2 = 0.1388
R indices (all data) R1 = 0.0791 R1 = 0.0813
wR2 = 0.1329 wR2 = 0.1548
Appendices
169
2b
Empirical formula C56 H64 F6 N4 O2 Co2
Formula weight 1056.97
Temperature 100 (2) K
Wavelength (MoKα) 0.71073 Å
Crystal system Triclinic
Space group P⎯1
Unit cell dimensions a = 12.3198 (6) Å
b = 13.5850 (6) Å
c = 17.6577 (9) Å
α = 93.31 (1) deg
β = 105.23 (1) deg
γ = 110.58 (1) deg
Volume (Å3), Z 2632.4 (2), 2
Density (calc.) Mg/m3 1.333
Absorption coeff mm-1 0.696
F(000) 1104
Crystal size (mm) 0.30 x 0.25 x 0.14
θ Range for data collection 3.20 to 27.50 deg
Limiting indices -16<=h<=16
-17<=k<=17
-22<=l<=22
Reflections collected 35495
Independent reflections 11968 [R(int) = 0.0338]
Absorption correction Gaussian, face indexed
Data/restraints/parameters 11968 / 60 / 676
Goodness-of-fit on F2 1.083
Final R indices R1 = 0.0658
[I>2σ(I)] wR2 = 0.1796
R indices (all data) R1 = 0.0790
wR2 = 0.1893
Appendices
170
3a 3b
Empirical formula C42 H48 F6 N2 O2 Ni C54 H61 F6 N3 O2 Co Ni
Formula weight 785.53 1015.70
Temperature 100 (2) K 100 (2) K
Wavelength (MoKα) 0.71073 Å 0.71073 Å
Crystal system Monoclinic Orthorhombic
Space group P21/c P212121
Unit cell dimensions a = 9.7701 (6) Å a = 10.2271 (4) Å
b = 13.8343 (8) Å b = 20.4853 (10) Å
c = 15.2068 (10) Å c = 23.6043 (14) Å
α = 90 deg α = 90 deg
β = 94.75 (1) deg β = 90 deg
γ = 90 deg γ = 90 deg
Volume (Å3), Z 2048.3 (2), 2 4945.2 (3), 4
Density (calc.) Mg/m3 1.274 1.364
Absorption coeff mm-1 0.537 0.782
F(000) 824 2124
Crystal size (mm) 0.18 x 0.16 x 0.06 0.04 x 0.02 x 0.02
θ Range for data collection 4.44 to 30.97 deg 3.11 to 25.00 deg
Limiting indices -14<=h<=13 -12<=h<=12
-19<=k<=20 -24<=k<=24
-22<=l<=22 -28<=l<=28
Reflections collected 23258 62008
Independent reflections 6485 [R(int) = 0.0344] 8669 [R(int) = 0.0698]
Absorption correction Gaussian, face indexed None
Data/restraints/parameters 6485 / 0 / 247 8669 / 0 / 617
Goodness-of-fit on F2 1.067 1.035
Final R indices R1 = 0.0383 R1 = 0.0491
[I>2σ(I)] wR2 = 0.0892 wR2 = 0.0718
R indices (all data) R1 = 0.0484 R1 = 0.0856
wR2 = 0.0938 wR2 = 0.0817
Appendices
171
3d 4a
Empirical formula C44 H52 Cl6 F6 N2 O10 Ni C42 H48 F6 N2 O2 Pd
Formula weight 1154.29 833.22
Temperature 100 (2) K 100 (2) K
Wavelength (MoKα) 0.71073 Å 0.71073 Å
Crystal system Monoclinic Monoclinic
Space group P21/n P21/n
Unit cell dimensions a = 12.8217 (3) Å a = 11.1042 (3) Å
b = 15.1119 (3) Å b = 8.1767 (3) Å
c = 13.2926 (3) Å c = 21.8740 (3) Å
α = 90 deg α = 90 deg
β = 98.210 (5) deg β = 91.81 (5) deg
γ = 90 deg γ = 90 deg
Volume (Å3), Z 2549.18 (10), 2 1985.07 (9), 2
Density (calc.) Mg/m3 1.504 1.394
Absorption coeff mm-1 0.773 4.327
F(000) 1188 860
Crystal size (mm) 0.08 x 0.06 x 0.04 0.12 x 0.12 x 0.06
θ Range for data collection 3.10 to 30.97 deg 4.327 to 68.96 deg
Limiting indices -17<=h<=18 -11<=h<=13
-21<=k<=21 -9<=k<=7
-19<=l<=19 -26<=l<=22
Reflections collected 48195 11753
Independent reflections 8089 [R(int) = 0.0474] 3439 [R(int) = 0.0451]
Absorption correction Gaussian, face indexed SADABS (G. Sheldrick 2004)
Data/restraints/parameters 8089 / 0 / 335 3436 / 0 / 242
Goodness-of-fit on F2 1.029 1.151
Final R indices R1 = 0.0423 R1 = 0.0464
[I>2σ(I)] wR2 = 0.0984 wR2 = 0.1352
R indices (all data) R1 = 0.0592 R1 = 0.0479
wR2 = 0.1071 wR2 = 0.1373
Appendices
172
4b 4c
Empirical formula C52 H58 F6 N2 O2 Co Ni C42 H48 B F10 N2 O2 Pd
Formula weight 1022.33 920.03
Temperature 100 (2) K 100 (2) K
Wavelength (MoKα) 0.71073 Å 0.71073 Å
Crystal system Triclinic Monoclinic
Space group P1 C2/c
Unit cell dimensions a = 10.1173 (6) Å a = 21.5769 (6) Å
b = 10.4874 (6) Å b = 13.2251 (3) Å
c = 12.6206 (6) Å c = 30.9126 (8) Å
α = 67.012 (5) deg α = 90 deg
β = 73.738 (5) deg β = 95.586 (5) deg
γ = 88.769 (5) deg γ = 90 deg
Volume (Å3), Z 1177.57 (11), 1 8779.2 (4), 8
Density (calc.) Mg/m3 1.442 1.392
Absorption coeff mm-1 0.800 0.501
F(000) 527 3768
Crystal size (mm) 0.20 x 0.17 x 0.07 0.10 x 0.05 x 0.05
θ Range for data collection 3.14 to 31.02 deg 3.08 to 28.28 deg
Limiting indices -14<=h<=14 -28<=h<=28
-15<=k<=15 -17<=k<=17
-18<=l<=18 -41<=l<=41
Reflections collected 31169 76289
Independent reflections 14161 [R(int) = 0.0474] 10873 [R(int) = 0.0760]
Absorption correction Gaussian, face indexed None
Data/restraints/parameters 14161 / 147 / 604 10873 / 0 / 535
Goodness-of-fit on F2 1.024 1.140
Final R indices R1 = 0.0300 R1 = 0.0482
[I>2σ(I)] wR2 = 0.0736 wR2 = 0.0819
R indices (all data) R1 = 0.0308 R1 = 0.0668
wR2 = 0.0742 wR2 = 0.0870
Appendices
173
4d
Empirical formula C43.33 H51.33 B2.67 Cl2.67
F16.17 N2 O2 Pd
Formula weight 1022.33
Temperature 100 (2) K
Wavelength (MoKα) 0.71073 Å
Crystal system Triclinic
Space group P⎯1
Unit cell dimensions a = 13.5833 (5) Å
b = 16.8101 (7) Å
c = 18.3743 (7) Å
α = 78.548 (5) deg
β = 72.946 (5) deg
γ = 78.209 (5) deg
Volume (Å3), Z 3883.1 (3), 3
Density (calc.) Mg/m3 1.512
Absorption coeff mm-1 0.593
F(000) 1788
Crystal size (mm) 0.64 x 0.24 x 0.18
θ Range for data collection 3.02 to 27.50 deg
Limiting indices -17<=h<=17
-21<=k<=21
-23<=l<=23
Reflections collected 54535
Independent reflections 17684 [R(int) = 0.0351]
Absorption correction Gaussian, face indexed
Data/restraints/parameters 17684 / 415 / 1012
Goodness-of-fit on F2 1.128
Final R indices R1 = 0.0568
[I>2σ(I)] wR2 = 0.1331
R indices (all data) R1 = 0.0675
wR2 = 0.1401
Appendices
174
5 6
Empirical formula C43 H60 F2 N2 O6 Mo C80 H98 F4 N4 O7 Mo2
Formula weight 834.87 1495.50
Temperature 100 (2) K 100 (2) K
Wavelength (MoKα) 0.71073 Å 0.71073 Å
Crystal system Triclinic Triclinic
Space group P⎯1 P⎯1
Unit cell dimensions a = 10.6787 (4) Å a = 13.3213 (4) Å
b = 11.1528 (4) Å b = 16.4986 (6) Å
c = 19.5265 (6) Å c = 18.1139 (8) Å
α = 104.127 (5) deg α = 97.114 (4) deg
β = 91.646 (5) deg β = 100.857 (4) deg
γ = 110.366 (5) deg γ = 96.549 (4) deg
Volume (Å3), Z 2097.66 (13), 2 3841 (2), 2
Density (calc.) Mg/m3 1.322 1.293
Absorption coeff mm-1 0.369 0.390
F(000) 880 1564
Crystal size (mm) 0.07 x 0.05 x 0.04 0.20 x 0.20 x 0.08
θ Range for data collection 2.95 to 30.54 deg 3.15 to 30.55 deg
Limiting indices -15<=h<=15 -11<=h<=19
-15<=k<=15 -23<=k<=23
-27<=l<=27 -25<=l<=25
Reflections collected 53818 50778
Independent reflections 12769 [R(int) = 0.0424] 23250 [R(int) = 0.0300]
Absorption correction None None
Data/restraints/parameters 12769 / 31 / 520 23250 / 139 / 924
Goodness-of-fit on F2 1.133 1.062
Final R indices R1 = 0.0322 R1 = 0.0417
[I>2σ(I)] wR2 = 0.0788 wR2 = 0.0936
R indices (all data) R1 = 0.0369 R1 = 0.0537
wR2 = 0.0810 wR2 = 0.1001
Appendices
175
7a 7b
Empirical formula C40 H49 Cl F2 N2 O3 W C41 H52 F2 N2 O4 W * 0.25
CH3OH
Formula weight 863.11 866.71
Temperature 100 (2) K 100 (2) K
Wavelength (MoKα) 0.71073 Å 0.71073 Å
Crystal system Triclinic Triclinic
Space group P⎯1 P⎯1
Unit cell dimensions a = 10.4242(2) Å a = 11.6428 (6) Å
b = 3.9710 (3) Å b = 16.2578 (10) Å
c = 15.1260 (4) Å c = 22.060 (2) Å
α = 110.518 (4) deg α = 73.493 (4) deg
β = 101.034 (4) deg β = 84.906 (4) deg
γ = 101.157(4) deg γ = 84.695 (4) deg
Volume (Å3), Z 1941.94 (8), 2 3977.9 (5), 4
Density (calc.) Mg/m3 1.476 1.447
Absorption coeff mm-1 3.091 2.955
F(000) 872 1762
Crystal size (mm) 0.08 x 0.06 x 0.05 0.12 x 0.05 x 0.04
θ Range for data collection 3.23 to 31.04 deg 2.94 to 27.50 deg
Limiting indices -15<=h<=15 -14<=h<=15
-20<=k<=20 -20<=k<=21
-21<=l<=21 -28<=l<=28
Reflections collected 56902 42486
Independent reflections 12375 [R(int) = 0.0516] 18064 [R(int) = 0.0638]
Absorption correction Gaussian Gaussian
Data/restraints/parameters 12375 / 0 / 457 18064 / 0 / 947
Goodness-of-fit on F2 1.042 1.084
Final R indices R1 = 0.0270 R1 = 0.0507
[I>2σ(I)] wR2 = 0.0504 wR2 = 0.0771
R indices (all data) R1 = 0.0328 R1 = 0.0836
wR2 = 0.0522 wR2 = 0.0862
Appendices
176
8 8a
Empirical formula C62.25 H92.5 N S4 Cl4.5 Au C45 H54 Cl2 S4 Au
Formula weight 1339.60 990.99
Temperature 100 (2) K 100 (2) K
Wavelength (MoKα) 0.71073 Å 0.71073 Å
Crystal system Triclinic Tetragonal
Space group P⎯1 P41212
Unit cell dimensions a = 17.7270 (4) Å a = 11.705 (1) Å
b = 17.8283 (4) Å b = 11.705 (1) Å
c = 24.6899 (6) Å c = 64.778 (5) Å
α = 69.394 (5) deg α = 90 deg
β = 89.774 (5) deg β = 90 deg
γ = 65.875 (5) deg γ = 90 deg
Volume (Å3), Z 6575.9 (3), 4 8875.0 (13), 8
Density (calc.) Mg/m3 1.353 1.483
Absorption coeff mm-1 2.584 3.654
F(000) 2770 4008
Crystal size (mm) 0.24 x 0.20 x 0.16 0.03 x 0.03 x 0.01
θ Range for data collection 2.95 to 35.00 deg 2.92 to 22.50deg
Limiting indices -28<=h<=28 -12<=h<=10
-28<=k<=28 -11<=k<=9
-29<=l<=39 -69<=l<=57
Reflections collected 181310 17723
Independent reflections 57536 [R(int) = 0.0323] 4515 [R(int) = 0.1475]
Absorption correction Gaussian Gaussian
Data/restraints/parameters 57536 / 77 / 1405 4515 / 7 / 254
Goodness-of-fit on F2 1.033 1.029
Final R indices R1 = 0.0338 R1 = 0.0746
[I>2σ(I)] wR2 = 0.0784 wR2 = 0.0971
R indices (all data) R1 = 0.0426 R1 = 0.1641
wR2 = 0.0830 wR2 = 0.1196
Appendices
177
2. Magnetochemical data:
[Pd(1LISQ)(tertbpy)](PF6) (1a)
MW = 882; χdia = -377 X 10-6 cm3 mol-1;
m = 28.11 mg; H = 1T
Temp. (K) χmT (exp.) χmT (calc.) μeff (exp.) μeff (calc.)
1 2 0.22434 0.35678 1.34043 1.69039
2 4.958 0.30519 0.36769 1.56341 1.71605
3 10.042 0.34326 0.36937 1.65806 1.71995
4 15.009 0.3516 0.36967 1.67808 1.72065
5 20.005 0.35658 0.36977 1.68991 1.72089
6 30.002 0.3611 0.36985 1.70058 1.72107
7 40.002 0.36362 0.36988 1.70652 1.72113
8 50.01 0.36521 0.36989 1.71024 1.72116
9 60.016 0.36649 0.3699 1.71323 1.72118
10 70.053 0.36735 0.3699 1.71525 1.72119
11 80.064 0.3682 0.3699 1.71723 1.72119
12 90.096 0.36861 0.3699 1.71818 1.7212
13 100.13 0.36899 0.36991 1.71907 1.7212
14 110.09 0.36915 0.36991 1.71944 1.7212
15 120.17 0.36959 0.36991 1.72046 1.7212
16 130.16 0.37005 0.36991 1.72155 1.72121
17 140.18 0.37044 0.36991 1.72244 1.72121
18 150.19 0.37087 0.36991 1.72345 1.72121
19 160.2 0.3715 0.36991 1.7249 1.72121
20 170.21 0.37192 0.36991 1.72589 1.72121
21 180.21 0.3726 0.36991 1.72746 1.72121
22 190.23 0.373 0.36991 1.72838 1.72121
23 200.24 0.37356 0.36991 1.72969 1.72121
24 210.15 0.37409 0.36991 1.7309 1.72121
25 220.24 0.37482 0.36991 1.73259 1.72121
26 230.25 0.37585 0.36991 1.73498 1.72121
27 240.24 0.37635 0.36991 1.73612 1.72121
28 250.26 0.37707 0.36991 1.73778 1.72121
29 260.25 0.37832 0.36991 1.74068 1.72121
30 270.26 0.37936 0.36991 1.74306 1.72121
31 280.23 0.381 0.36991 1.74681 1.72121
32 290.25 0.38385 0.36991 1.75334 1.72121
Appendices
178
[Co(1LISQ)(1LIP)] (2a)
MW = 785; χdia = -303 X 10-6 cm3 mol-1;
m = 48.22 mg; H= 1
Temp. (K) χmT (exp.) χmT (calc.) μeff (exp.) μeff (calc.)
1 1.967 0.56612 0.69513 2.0917 2.31782
2 5.037 0.71628 0.71693 2.35281 2.35388
3 9.978 0.89535 0.72004 2.63052 2.35898
4 15.015 0.90744 0.72064 2.64822 2.35996
5 20.003 0.84824 0.72085 2.56037 2.3603
6 30.001 0.77322 0.721 2.44454 2.36055
7 39.999 0.74189 0.72105 2.39449 2.36063
8 49.998 0.73008 0.72108 2.37536 2.36067
9 60.041 0.72721 0.72109 2.37069 2.3607
10 70.052 0.72445 0.7211 2.36619 2.36071
11 80.079 0.72197 0.7211 2.36214 2.36072
12 90.09 0.7211 0.72111 2.3607 2.36072
13 100.08 0.72059 0.72111 2.35987 2.36073
14 110.11 0.72058 0.72111 2.35986 2.36073
15 120.15 0.7209 0.72111 2.36038 2.36073
16 130.16 0.72078 0.72112 2.36019 2.36073
17 140.17 0.72076 0.72112 2.36015 2.36074
18 150.18 0.7212 0.72112 2.36087 2.36074
19 160.19 0.72113 0.72112 2.36076 2.36074
20 170.21 0.72161 0.72112 2.36154 2.36074
21 180.14 0.72131 0.72112 2.36106 2.36074
22 190.22 0.72137 0.72112 2.36115 2.36074
23 200.23 0.7215 0.72112 2.36137 2.36074
24 210.23 0.72154 0.72112 2.36142 2.36074
25 220.25 0.72177 0.72112 2.36181 2.36074
26 230.24 0.72183 0.72112 2.36191 2.36074
27 240.25 0.72242 0.72112 2.36286 2.36074
28 250.25 0.72312 0.72112 2.36402 2.36074
29 260.26 0.72334 0.72112 2.36437 2.36074
30 270.23 0.72394 0.72112 2.36536 2.36074
31 280.25 0.72467 0.72112 2.36654 2.36074
32 290.26 0.72555 0.72112 2.36798 2.36074
Appendices
179
[Co(1LIP)2] [Co(Cp)2]* 2 CH3CN (2b)
MW = 974; χdia = -530 X 10-6 cm3 mol-1;
m = 44.15 mg; H = 1T
Temp. (K) χmT (exp.) χmT (calc.) μeff (exp.) μeff (calc.)
1 1.954 0.06349 0.06029 0.70048 0.68261
2 5.081 0.16383 0.15678 1.12522 1.10075
3 10.136 0.31648 0.31286 1.56393 1.55495
4 15.045 0.46084 0.46371 1.88721 1.89308
5 20.004 0.59699 0.60793 2.14797 2.16755
6 29.999 0.81762 0.83901 2.51375 2.54641
7 39.999 0.96458 0.9852 2.73032 2.75934
8 50.011 1.05633 1.07348 2.85722 2.88033
9 60.031 1.11281 1.12803 2.93261 2.9526
10 70.066 1.14959 1.16321 2.98069 2.99829
11 80.084 1.17444 1.18681 3.01273 3.02856
12 90.099 1.19201 1.20332 3.03518 3.04954
13 100.13 1.20408 1.21527 3.05051 3.06465
14 110.12 1.21287 1.22413 3.06163 3.0758
15 120.15 1.22018 1.23091 3.07084 3.08431
16 130.12 1.22492 1.23616 3.0768 3.09089
17 140.18 1.22933 1.24037 3.08233 3.09614
18 150.19 1.2334 1.24374 3.08743 3.10034
19 160.21 1.23601 1.24649 3.0907 3.10377
20 170.21 1.23917 1.24877 3.09464 3.1066
21 180.21 1.24092 1.25067 3.09683 3.10897
22 190.24 1.24303 1.25229 3.09946 3.11097
23 200.24 1.24498 1.25366 3.10189 3.11268
24 210.24 1.24608 1.25483 3.10325 3.11414
25 220.26 1.24822 1.25585 3.10592 3.1154
26 230.27 1.24939 1.25674 3.10738 3.1165
27 240.25 1.2504 1.25752 3.10863 3.11747
28 250.16 1.25102 1.25819 3.1094 3.11831
29 260.26 1.25224 1.25881 3.11092 3.11907
30 270.25 1.25294 1.25935 3.11179 3.11973
31 280.27 1.2547 1.25983 3.11397 3.12033
32 290.28 1.25578 1.26026 3.11532 3.12086
Appendices
180
[Ni(1LISQ)(1LIP)] [Co(Cp)2] (3b)
MW = 973; χdia = -571 X 10-6 cm3 mol-1;
m = 28.42 mg; H = 1T
Temp. (K) χmT (exp.) χmT (calc.) μeff (exp.) μeff (calc.)
1 1.965 0.23794 0.39128 1.38045 1.77024
2 5.01 0.39129 0.40507 1.77025 1.80117
3 9.997 0.44816 0.40704 1.89454 1.80554
4 14.954 0.48586 0.40741 1.97261 1.80635
5 20.003 0.48929 0.40754 1.97957 1.80665
6 30.002 0.44895 0.40763 1.89621 1.80685
7 40 0.42813 0.40767 1.85172 1.80692
8 50.009 0.41809 0.40768 1.82987 1.80696
9 60.035 0.41321 0.40769 1.81917 1.80697
10 70.048 0.4103 0.4077 1.81276 1.80698
11 80.069 0.40941 0.4077 1.81077 1.80699
12 90.083 0.40856 0.4077 1.8089 1.807
13 100.07 0.40787 0.4077 1.80737 1.807
14 110.12 0.40767 0.4077 1.80693 1.807
15 120.16 0.39633 0.4077 1.78161 1.807
16 130.17 0.40724 0.40771 1.80598 1.80701
17 140.18 0.4072 0.40771 1.80589 1.80701
18 150.19 0.40727 0.40771 1.80605 1.80701
19 160.19 0.40753 0.40771 1.80661 1.80701
20 170.21 0.40745 0.40771 1.80644 1.80701
21 180.21 0.40761 0.40771 1.8068 1.80701
22 190.23 0.40821 0.40771 1.80813 1.80701
23 200.16 0.40639 0.40771 1.8041 1.80701
24 210.21 0.40636 0.40771 1.80402 1.80701
25 220.16 0.40573 0.40771 1.80262 1.80701
26 230.24 0.40452 0.40771 1.79994 1.80701
27 240.28 0.40321 0.40771 1.79702 1.80701
28 250.24 0.40178 0.40771 1.79382 1.80701
29 260.27 0.40033 0.40771 1.79059 1.80701
30 270.25 0.39898 0.40771 1.78756 1.80701
31 280.16 0.39669 0.40771 1.78243 1.80701
32 290.26 0.39606 0.40771 1.78101 1.80701
Appendices
181
[Ni(1LIBQ)2(ClO4)2] * 2 CH2Cl2 (3d)
MW = 1152; χdia = -536 X 10-6 cm3 mol-1;
m = 21.54 mg; H = 1T
Temp. (K) χmT (exp.) χmT (calc.) μeff (exp.) μeff (calc.)
1 1.965 0.5206 0.48784 2.04192 1.97663
2 5.098 0.84185 0.88782 2.5966 2.66654
3 10.154 0.97991 1.00011 2.80143 2.83015
4 15.038 1.01346 1.02235 2.84898 2.86145
5 20.004 1.02691 1.03043 2.86783 2.87273
6 30 1.034 1.0362 2.87771 2.88076
7 39.997 1.03682 1.03819 2.88163 2.88354
8 50.008 1.03851 1.03911 2.88398 2.88481
9 60.039 1.0404 1.03961 2.88661 2.8855
10 70.042 1.03898 1.0399 2.88462 2.88591
11 80.082 1.03965 1.0401 2.88556 2.88618
12 90.109 1.03928 1.04023 2.88505 2.88636
13 100.11 1.03892 1.04032 2.88455 2.88649
14 110.14 1.039 1.04039 2.88466 2.88658
15 120.09 1.03865 1.04044 2.88418 2.88666
16 130.16 1.03882 1.04048 2.88441 2.88671
17 140.18 1.03909 1.04051 2.88479 2.88676
18 150.19 1.03906 1.04054 2.88474 2.88679
19 160.19 1.03994 1.04056 2.88596 2.88683
20 170.22 1.03968 1.04058 2.8856 2.88685
21 180.24 1.04045 1.04059 2.88666 2.88687
22 190.23 1.04057 1.04061 2.88684 2.88689
23 200.25 1.04088 1.04062 2.88727 2.8869
24 210.24 1.04186 1.04063 2.88862 2.88691
25 220.26 1.04184 1.04063 2.8886 2.88692
26 230.17 1.04223 1.04064 2.88913 2.88694
27 240.25 1.04589 1.04065 2.8942 2.88694
28 250.25 1.04815 1.04065 2.89733 2.88695
29 260.31 1.04867 1.04066 2.89806 2.88696
30 270.26 1.05098 1.04066 2.90123 2.88697
31 280.23 1.05365 1.04066 2.90492 2.88697
32 290.26 1.05853 1.04067 2.91164 2.88697
Appendices
182
[Pd(1LISQ)(1LIP)] [Co(Cp)2] (4b)
MW = 1021; χdia = -482 X 10-6 cm3 mol-1;
m = 19.89 mg; H = 1T
Temp. (K) χmT (exp.) χmT (calc.) μeff (exp.) μeff (calc.)
1 1.927 0.33693 0.36065 1.6427 1.69954
2 5.164 0.35984 0.37304 1.69763 1.72849
3 9.993 0.36859 0.37458 1.71813 1.73204
4 14.999 0.37198 0.37489 1.72602 1.73277
5 20.006 0.37357 0.375 1.7297 1.73302
6 30.002 0.37478 0.37508 1.7325 1.7332
7 40 0.37721 0.37511 1.73812 1.73327
8 50.01 0.38914 0.37512 1.76539 1.73329
9 60.036 0.40187 0.37513 1.79403 1.73331
10 70.064 0.40034 0.37513 1.7906 1.73332
11 80.064 0.39535 0.37514 1.77941 1.73333
12 90.095 0.39277 0.37514 1.7736 1.73333
13 100.13 0.39048 0.37514 1.76842 1.73333
14 110.14 0.48788 0.37514 -- 1.73334
15 120.14 0.38127 0.37514 1.74745 1.73334
16 130.17 0.37715 0.37514 1.73796 1.73334
17 140.12 0.37507 0.37514 1.73317 1.73334
18 150.19 0.37458 0.37514 1.73204 1.73334
19 160.2 0.37445 0.37514 1.73175 1.73334
20 170.21 0.37428 0.37514 1.73135 1.73334
21 180.23 0.37457 0.37514 1.73201 1.73334
22 190.24 0.37468 0.37514 1.73226 1.73334
23 200.24 0.375 0.37514 1.73301 1.73334
24 210.24 0.37548 0.37514 1.73411 1.73334
25 220.25 0.37584 0.37514 1.73495 1.73334
26 230.24 0.37734 0.37514 1.73842 1.73335
27 240.27 0.37824 0.37514 1.74049 1.73334
28 250.15 0.37981 0.37514 1.74408 1.73334
29 260.26 0.38182 0.37514 1.7487 1.73335
30 270.27 0.384 0.37514 1.75369 1.73334
31 280.14 0.38699 0.37514 1.7605 1.73335
32 290.26 0.39198 0.37514 1.77181 1.73335
Appendices
183
[Pd(1LISQ)(1LIBQ)] (BF4) (4c)
MW = 919; χdia = -375 X 10-6 cm3 mol-1;
m = 18.35 mg; H = 1T
Temp. (K) χmT (exp.) χmT (calc.) μeff (exp.) μeff (calc.)
1 1.926 0.35676 0.36064 1.69035 1.69951
2 5.162 0.36036 0.37304 1.69885 1.72848
3 10.016 0.36485 0.37458 1.70939 1.73205
4 15.009 0.36358 0.37489 1.70642 1.73277
5 20.005 0.36349 0.375 1.70622 1.73302
6 30 0.36272 0.37508 1.7044 1.7332
7 40.002 0.36199 0.37511 1.70268 1.73327
8 50.008 0.36178 0.37512 1.70219 1.73329
9 60.035 0.36164 0.37513 1.70185 1.73331
10 70.035 0.3618 0.37513 1.70224 1.73332
11 80.063 0.36216 0.37514 1.70308 1.73333
12 90.112 0.36214 0.37514 1.70304 1.73333
13 100.11 0.361 0.37514 1.70035 1.73333
14 110.12 0.35978 0.37514 1.69749 1.73334
15 120.13 0.35847 0.37514 1.6944 1.73334
16 130.17 0.35751 0.37514 1.69212 1.73334
17 140.13 0.3567 0.37514 1.6902 1.73334
18 150.19 0.35673 0.37514 1.69027 1.73334
19 160.2 0.35644 0.37514 1.68958 1.73334
20 170.22 0.35641 0.37514 1.6895 1.73334
21 180.21 0.35645 0.37514 1.68962 1.73334
22 190.22 0.35735 0.37514 1.69174 1.73334
23 200.24 0.35563 0.37514 1.68766 1.73334
24 210.25 0.35732 0.37514 1.69167 1.73334
25 220.16 0.35763 0.37514 1.6924 1.73335
26 230.27 0.35909 0.37514 1.69585 1.73334
27 240.26 0.36019 0.37514 1.69845 1.73334
28 250.27 0.36139 0.37514 1.70127 1.73334
29 260.06 0.36388 0.37514 1.70713 1.73335
30 270.25 0.36641 0.37514 1.71305 1.73334
31 280.25 0.36996 0.37514 1.72132 1.73334
32 290.23 0.37478 0.37514 1.73251 1.73334
Appendices
184
[Au(3L)2] * CH2Cl2 (8a)
MW = 989; χdia = -375 X 10-6 cm3 mol-1;
m = 15.33 mg; H = 1T
Temp. (K) χmT (exp.) χmT (calc.) μeff (exp.) μeff (calc.)
1 2 0.17713 0.17137 1.18684 1.16738
2 5.014 0.25204 0.2539 1.41574 1.42096
3 10.043 0.29519 0.3019 1.53214 1.54946
4 15.035 0.31265 0.32197 1.5768 1.60012
5 20.006 0.32408 0.33301 1.60537 1.62733
6 30.002 0.33681 0.34491 1.6366 1.65616
7 40.001 0.34424 0.35118 1.65455 1.67114
8 50.013 0.35025 0.35505 1.66894 1.68034
9 60.013 0.35475 0.35768 1.67962 1.68654
10 70.073 0.35799 0.35959 1.68728 1.69103
11 80.107 0.36083 0.36103 1.69395 1.69441
12 90.066 0.36297 0.36215 1.69897 1.69704
13 100.09 0.3644 0.36305 1.70231 1.69916
14 110.17 0.36604 0.3638 1.70613 1.70091
15 120.11 0.36664 0.36442 1.70753 1.70235
16 130.16 0.36757 0.36495 1.7097 1.70359
17 140.19 0.36838 0.3654 1.71158 1.70465
18 150.19 0.36932 0.3658 1.71376 1.70557
19 160.19 0.37034 0.36614 1.71614 1.70638
20 170.22 0.35777 0.36645 1.68675 1.70709
21 180.23 0.3722 0.36672 1.72042 1.70772
22 190.24 0.37356 0.36696 1.72357 1.70829
23 200.24 0.37532 0.36718 1.72764 1.70879
24 210.15 0.37674 0.36738 1.73088 1.70926
25 220.26 0.37931 0.36756 1.73678 1.70968
26 230.24 0.38261 0.36773 1.74433 1.71006
27 240.24 0.38637 0.36788 1.75288 1.71042
28 250.23 0.3904 0.36802 1.762 1.71074
29 260.26 0.39579 0.36815 1.77411 1.71104
30 270.24 0.41839 0.36827 1.82406 1.71132
31 280.25 0.24049 0.36838 1.38292 1.71157
32 290.15 0.34675 0.36848 1.66056 1.71182
Appendices
185
Curriculum Vitae
Personal:
Name: Swarnalatha Kokatam
Date of Birth: 23.06.1980
Place of Birth: Pulivendla, Andhra Pradesh, India
Nationality: Indian
Marital Status: Unmarried
Education:
1986-1990 Elementary School, Kadapa, India.
1991-1995 Z. P. P. G High School, Pulivendla, India.
1995-1997 Inter Mediate, Vidhyadharshini Jr. College Pulivendla, India.
1997-2000 Kranthi Degree College, Hyderabad, India.
(Bachelor of Science in Chemistry, Osmania University)
2001-2003 University of Hyderabad, Hyderabad, India
(Master of Science, Project: In inorganic chemistry with Dr. Samar. K.
Das, University of Hyderabad, Dec 2002 to Apr 2003)
August
2003 - Ph. D student at the Max Planck Institute for Bioinorganic Chemistry,
Muelheim an der Ruhr, Germany (with Prof. Dr. P. Chaudhuri and
Prof. Dr. K. Wieghardt).
Scholarship:
2003 Qualified in GATE (Graduate Aptitude Test in Engineering) in
Chemistry.
2003 Qualified in NET (National Eligibility Test for Lectureship)
August 2003 Max Planck fellowship for Ph. D work with Prof. Dr. P. Chaudhuri and
Prof. Dr. K. Wieghardt.
