Copper(II) complexes of sterically hindered Schiff base ligands: Synthesis, structure, spectra and electrochemistry

Article abstract of DOI:10.1016/j.poly.2005.07.047

The synthesis, structure and spectral and redox behaviour of copper(II) complexes of sterically constrained N₃, N₃S₂ and N₂S₂ ligands are described. The ligands 2,6-bis(2,4,6-trimethylphenyliminomethyl)pyridine (L1), 2,6-bis(2,6- diisopropylphenyliminomethyl)pyridine (L2), 2,6-bis(2- methylthiophenyliminomethyl)pyridine (L3), 1,3-bis(2- methylthiophenyliminomethyl)benzene (L4), 1,3-bis(2,4,6- trimethylphenyliminomethyl)benzene (L5) and 1,3-bis(2,6-diisopropyl- phenyliminomethyl)benzene (L6) have been synthesized. The 1:1 copper(II) complexes of L1-L5 have been isolated and characterized by spectral and electrochemical techniques. The X-ray crystal structure of the complex [Cu(L2)Cl₂] (2) has been successfully determined and is found to possess a distorted square pyramidal coordination geometry. The replacement of isopropyl groups in this complex by the less sterically demanding methyl groups as in [Cu(L1)Cl₂] leads to a decreased geometric distortion. The complexes show relatively high positive redox potentials with a low reorganizational energy barrier for electron transfer involving the Cu(II)/Cu(I) couple. The redox cycle of the complexes 1 and 2 has been explained by assuming a square scheme. The spectral and electrochemical properties of CuN ₃S₂ and CuN₂S₂ complexes generated by the incorporation of thioether sulfurs into L1/L2 and L4/L5, respectively, are also discussed. Both the complexes show positive redox potentials suggesting the coordination of at least one thioether sulfur to copper(II). The incorporation of a pyridine nitrogen donor into the CuN₂S₂ complexes leads to higher LF field strengths but with lower absorptivities. The coordination of Cl^- ions to the CuN₃S₂ and CuN₂S₂ complexes significantly alter the spectral and redox properties of the complexes.

Full text of DOI:10.1016/j.poly.2005.07.047

Polyhedron 25 (2006) 1077–1088
www.elsevier.com/locate/poly
Copper(II) complexes of sterically hindered Schiff base ligands:
Synthesis, structure, spectra and electrochemistry
a
a,
b
Ramalingam Balamurugan , Mallayan Palaniandavar *, Malcolm A. Halcrow
a
Department of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamilnadu, India
b
School of Chemistry, University of Leeds, Leeds LS2 9JT, UK
Received 17 January 2005; accepted 25 July 2005
Available online 17 October 2005
Abstract
The synthesis, structure and spectral and redox behaviour of copper(II) complexes of sterically constrained N3, N3S2 and N2S2 ligands
are described. The ligands 2,6-bis(2,4,6-trimethylphenyliminomethyl)pyridine (L1), 2,6-bis(2,6-diisopropylphenyliminomethyl)pyridine
(L2), 2,6-bis(2-methylthiophenyliminomethyl)pyridine (L3), 1,3-bis(2-methylthiophenyliminomethyl)benzene (L4), 1,3-bis(2,4,6-trim-
ethylphenyliminomethyl)benzene (L5) and 1,3-bis(2,6-diisopropyl-phenyliminomethyl)benzene (L6) have been synthesized. The 1:1 cop-
per(II) complexes of L1–L5 have been isolated and characterized by spectral and electrochemical techniques. The X-ray crystal structure
of the complex [Cu(L2)Cl2] (2) has been successfully determined and is found to possess a distorted square pyramidal coordination geom-
etry. The replacement of isopropyl groups in this complex by the less sterically demanding methyl groups as in [Cu(L1)Cl2] leads to a
decreased geometric distortion. The complexes show relatively high positive redox potentials with a low reorganizational energy barrier
for electron transfer involving the Cu(II)/Cu(I) couple. The redox cycle of the complexes 1 and 2 has been explained by assuming a
square scheme. The spectral and electrochemical properties of CuN3S2 and CuN2S2 complexes generated by the incorporation of thio-
ether sulfurs into L1/L2 and L4/L5, respectively, are also discussed. Both the complexes show positive redox potentials suggesting the
coordination of at least one thioether sulfur to copper(II). The incorporation of a pyridine nitrogen donor into the CuN2S2 complexes
leads to higher LF field strengths but with lower absorptivities. The coordination of Clꢀ ions to the CuN3S2 and CuN2S2 complexes
significantly alter the spectral and redox properties of the complexes.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Copper(II) complexes; X-ray structure; Tri- and pentadentate ligands; EPR spectra; Redox properties
1. Introduction
properties of the metalloproteins [2]. As the function of
a metalloprotein is closely related to the structure of its
active site, i.e., the geometrical arrangement of the amino
acid residues around copper, the objective of such studies
include synthesizing ligands that contain donor atoms
around copper in biomolecules so that the copper com-
plexes resemble the overall structure of the active site
[3]. Among the most extensively investigated ligands,
macrocyclic amines, imines and thioethers deserve atten-
tion. Their flexibility, ease of preparation and capability
of stabilizing unusual oxidation states of copper can ex-
plain their successful performance in mimicking the pecu-
liar geometries around the metal, leading to very
interesting spectroscopic properties [4]. Several Cu(II)
Copper compounds of chelating ligands incorporating
pyridine, thioether, imidazole and imine donors have
been of great interest in recent years. This is mainly be-
cause of their relevance to histidine coordinated copper
proteins such as blue copper proteins, hemocyanin, tyros-
inase, cytochrome c oxidase and laccase [1]. Very often
better structural and functional models for metallopro-
teins have been prepared by altering the substituents of
the ligands in order to match with the required spectral
*
Corresponding author. Tel.: +91 431 2407053; fax: +91 431 2407245.
E-mail address: palani51@sify.com (M. Palaniandavar).
0277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.07.047

1078
R. Balamurugan et al. / Polyhedron 25 (2006) 1077–1088
complexes of bis(benzimidazolyl)-thioether donors with
N2S [5–7], N2S2 [8–12] and N2S3 [13–17] donor sets have
been synthesized as models for the active sites in proteins
and their structures, spectra and redox behaviour have
been investigated with the goal of relating the electronic
structure properties to the active site reactivity.
2. Results and discussion
2.1. Synthesis
A series of systematically varied ligands have been syn-
thesized with the aim to understand the effect of ligand elec-
tronic and steric factors upon the structure and spectra of
the copper(II) complexes. The ligands L1, L2 and L3 have
been synthesized using the reported procedure with slight
modifications, by condensing 2,6-pyridinedicarboxalde-
hyde [22] with 2,4,6-trimethylaniline, 2,6-diisopropylaniline
and 2-(methylthio)aniline, respectively. The ligands L4, L5
and L6 have been obtained by the condensation of isoph-
thalaldehyde with the corresponding sterically hindered
anilines. The results of elemental analyses of L1–L6 were
In our laboratory we have undertaken a systematic
structural, spectroscopic and electrochemical study on a
series of mononuclear copper(II) complexes of linear
polydentate bis(benzimidazolyl)-thioether/amine/pyridine
ligands [7,18–20] offering N and S donor atoms. The
copper(II) complexes of nitrogen and sulfur containing
Schiff base ligands have been demonstrated as valid mod-
els for blue copper proteins. Also, there is a continuing
interest in investigating the relationship between the re-
dox potentials and other electrochemical parameters
and the spectral/geometrical parameters of Schiff base
metal complexes, which could be the result of steric
and electronic effects [21]. In this report, we describe
the synthesis of copper(II) complexes of di-, tri-, tetra-
and pentadentate ligands (Scheme 1) containing heterocy-
clic N, imine N and thioether S donors with different
and varying steric demands. The 3N ligands L1 and L2
have been designed as they have the ability to form cop-
per(II) complexes in a meridional rather than facial coor-
dination mode. Two thioether sulfur atoms have been
incorporated into the 3N framework of L1/L2 to offer
a N3S2 donor set (L3). The ligand L4 has been synthe-
sized to generate a novel CuN2S2 coordination geometry
with steric constraints, to help in understanding the role
of the central pyridine nitrogen in L3. The 2N ligands
L5 and L6 are sterically constrained to impose geometric
distortions in bis-complexes of copper(II). The X-ray
crystal structure of the copper(II) complex of L2 has
been successfully determined. The spectral and electro-
chemical properties of the complexes have been studied
and the results discussed.
1
satisfactory and the H and 13C NMR confirmed the iden-
tity of the ligands. All the Schiff base ligands, except L6,
readily form complexes on treating one or two equivalents
of them with copper(II) salts in methanol at room temper-
ature. The perchlorate complexes were synthesized in order
to avoid the competing coordination of chloride ions to
copper(II) in the presence of weakly coordinating thioether
sulfur donors. Our attempt to synthesize [Cu(L6)2]2+ was
unsuccessful and no pure solid product could be isolated
from the brown solutions obtained on mixing the ligand
L6 and copper(II) perchlorate. The stoichiometries of the
complexes were derived from elemental analysis and con-
ductivity studies and that of complex 2 was confirmed by
determining its X-ray crystal structure. The observed values
of molar conductance suggest [23] that the complexes 1, 2
and 4a (30–60 Xꢀ1 cm2 molꢀ1) behave as neutral species,
while the complexes 3–5 (160–205 Xꢀ1 cm2 molꢀ1) exist as
1:2 electrolytes in solution. The molar conductance
(90 Xꢀ1 cm2 molꢀ1) of 3a suggests that it behaves as 1:1
electrolyte in solution. The IR spectral data of the com-
plexes support the coordination of the ligands to copper(II).
The C@N (1700–1580 cmꢀ1) and C–S (L3, 690; L6,
N
N
N
N
N
N
S
N
S
N
N
L1
L2
L3
N
N
N
N
N
S
N
S
L4
L5
L6
Scheme 1.

R. Balamurugan et al. / Polyhedron 25 (2006) 1077–1088
1079
685 cmꢀ1) stretching vibrations of the ligands are shifted to
Table 1
lower frequencies (m(C@N), 1600–1470 cmꢀ1; m(C–S), 645–
635 cmꢀ1) upon complex formation. The intense bands
observed around 1095 and 625 cmꢀ1 for the perchlorate
complexes are due to m(Cl–O) of uncoordinated perchlorate
anions.
Crystal data and structure refinement for the complex [Cu(L2)Cl2] (2)
Empirical formula
Formula weight
Crystal system
Space group
C31H39Cl2CuN3
588.09
orthorhombic
P212121
11.0446(2)
14.9386(2)
18.7519(3)
90.000
90.000
90.000
3093.89(9)
4
1.263
˚
a (A)
˚
b (A)
˚
c (A)
2.2. Description of the structure of [Cu(L2)Cl2] (2)
a (ꢁ)
b (ꢁ)
c (ꢁ)
The ORTEP representation of the structure of the com-
plex including the atom numbering scheme is shown in
Fig. 1. The crystallographic data are given in Table 1 while
the bond lengths and bond angles are collected in Table 2.
The structure of the complex molecule involves a five-coor-
dinate CuN3Cl2 chromophore constituted by two imine
(N9, N23) and one pyridine (N2) nitrogen of the tridentate
ligand L2 and two chloride ions. The geometry around
copper(II) is best described as distorted square pyramidal
[24], as evident from the value of the trigonal index [11]
s of 0.18 [s = (b ꢀ a)/60, where a = N2–Cu1–Cl36 =
160.42(5)ꢁ and b = N23–Cu1–N9 = 149.63(7)ꢁ; for perfect
square pyramidal and trigonal bipyramidal geometries
the s values are zero and unity, respectively]. The N9,
N23 and N2 nitrogen atoms of the meridionally coordi-
nated L2 ligand and one of the chloride ions (Cl36) occupy
the corners of the square plane. The other chloride ion
(Cl37) is axially coordinated at an average angle of
3
˚
V (A )
Z
Dcalc (g cmꢀ3
)
Absorption coefficient (mmꢀ1
F(000)
)
0.902
1236
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
Ra
7002/16/361
1.035
0.0303
0.0755
wRb
P
P
a
R = iFo| ꢀ |Fci/ |Fo|.
P
P
2
2
1=2
b
wR2 ¼ f w½ðF 2o ꢀ F c2Þ ꢁ= w½ðF 2oÞ ꢁg
.
mized. Also, interestingly, the bulky isopropyl substituents
on the phenyl rings are pushed away from the chloride ions
and hence there is no steric clash between them.
˚
99.43ꢁ to the CuN3Cl plane. The Cu–Npy (1.9523 A) and
2.3. Spectral properties
˚
Cu–Nimine bond distances (2.1203, 2.1482 A) fall in the
range of values found for similar complexes [25,26]. The
The solid-state reflectance spectra of all the complexes
display either one broad or two poorly separated ligand-
field bands (11900–15800 cmꢀ1) in the visible region,
which is typical of Cu(II) located in a weakly tetragonal
field [27]. On dissolution in methanol or acetonitrile no
˚
axial chloride ion is located at a distance (2.391 A) longer
˚
than the equatorially located chloride ion (2.203 A). The
two phenyl rings are almost parallel to each other as in this
arrangement the steric interaction between them is mini-
Fig. 1. ORTEP drawing of [Cu(L2)Cl2] (2) showing the atom numbering scheme and thermal motion ellipsoids (50% probability level). The hydrogen
atoms are omitted for clarity.

1080
R. Balamurugan et al. / Polyhedron 25 (2006) 1077–1088
Table 2
coordination geometry. This is expected as copper(II) com-
plexes with a N4 donor set display a LF band in the range
14000–19000 cmꢀ1 [7,27].
˚
Selected Bond lengths (A) and angles (ꢁ) for [Cu(L2)Cl2] (2)
Cu(1)–N(2)
Cu(1)–N(23)
Cu(1)–N(9)
Cu(1)–Cl(36)
Cu(1)–Cl(37)
1.9523(16)
2.1203(16)
2.1482(17)
2.2033(5)
2.3905(6)
The polycrystalline EPR spectra of complexes 2, 4a and
5 are isotropic while those of complexes 3 and 3a are axial.
Interestingly, the complexes 1 and 4 show rhombic spectra
with R values greater than unity [R = (g3 ꢀ g2)/(g2 ꢀ g1) =
1.2 (1); 1.1 (4), Table 3]. The rhombicity is more likely to
arise from the interaction between differently oriented
Cu(II) complexes rather than a dz2 ground state for cop-
per(II) in the solid state [29,30]. The cryogenic solution
EPR spectra of all the complexes are axial [gi > g^ > 2.0,
G = (gi ꢀ 2)/(g^ ꢀ 2) = 3.0–4.5], suggesting the presence
of a dx2ꢀy2 ground state for copper(II) located in square
based geometries [31,32]. The complex [Cu(L2)Cl2] (2)
shows gi (2.254) and Ai (133 · 10ꢀ4 cmꢀ1, Fig. 2) values,
which are consistent with the ÔCuN3Cl + ClÕ chromophore
as observed in the X-ray crystal structure. The complex
[Cu(L)(H2O)2](ClO4)2.(H2O)2 (L = bis(benzimidazol-2-yl)
methylamine) [30] with the CuN3O2 chromophore shows
similar gi (2.268) and Ai (140 · 10ꢀ4 cmꢀ1) values. How-
ever, copper(II) complexes of tridentate ligands with a
CuN3Cl2 coordination sphere, synthesized in our labora-
tory very recently [33], exhibit gi and Ai values in the range
2.21–2.25 and 179–184 · 10ꢀ4 cmꢀ1, respectively. Though
the gi value of complex 2 falls within this range, the very
low Ai value observed indicates a large deviation from axial
symmetry, which is evident from its crystal structure. It is
interesting to compare the complexes 1 and 2, which are ex-
pected to have similar coordination geometries. On replac-
ing isopropyl groups in 2 by methyl groups as in 1 the gi
value decreases and the Ai value increases suggesting a de-
crease in geometric distortion or axial interaction with cop-
per(II). This is supported by the lower gi/Ai quotient
(131 cm) of complex 1, which falls in the range 105–
135 cm, as expected [34] for square-planar copper(II)
complexes. The gi and Ai values of 2, which are higher
and lower, respectively, than 1, suggest that the former pos-
sesses a coordination geometry distorted (gi/Ai, 169 cm)
more than 1 because of the sterically demanding isopropyl
groups, rather than an axial interaction [35]. This is in con-
trast to the similar electronic spectral parameters of 1 and 2
suggesting the usefulness of EPR spectroscopy in detecting
even small differences in coordination geometries.
N(2)–Cu(1)–N(23)
N(2)–Cu(1)–N(9)
N(23)–Cu(1)–N(9)
N(2)–Cu(1)–Cl(36)
N(23)–Cu(1)–Cl(36)
N(9)–Cu(1)–Cl(36)
N(2)–Cu(1)–Cl(37)
N(23)–Cu(1)–Cl(37)
N(9)–Cu(1)–Cl(37)
Cl(36)–Cu(1)–Cl(37)
77.39(6)
77.44(7)
149.63(7)
160.42(5)
96.82(5)
101.25(5)
93.66(5)
101.93(5)
96.36(5)
105.88(2)
appreciable change in the ligand field bands is observed for
the complexes 1, 2 and 4 (Table 3). However, noticeable
changes in the ligand field features are observed for the
complexes 3 and 5. For the complexes 1 and 2 two more
absorption bands are observed at higher energies (1, 23900
and 26200 cmꢀ1; 2, 26300 and 28000 cmꢀ1), which are pre-
sumably derived from nitrogen to copper(II) charge trans-
fer (CT) transitions. Replacing the methyl groups in 1 by
the more sterically demanding isopropyl groups, as in 2,
would be expected to effect distortions in the copper(II)
coordination geometry and hence lower the LF energy,
but no significant spectral differences are observed. Obvi-
ously, the free rotation of the phenyl rings in the complexes
(cf. above) minimize the steric constraints around cop-
per(II), which is independent of the phenyl ring substitu-
ents and hence similar LF features are observed for 1
and 2. For complex 3, two ligand to metal charge transfer
(LMCT) bands, characteristic of S(r) ! Cu(II) and
N(p) ! Cu(II) transitions, are observed (33200 and
32700 cmꢀ1). Complex 4 in acetonitrile solution shows an
absorption band (13500 cmꢀ1) with enhanced absorptivity
(2340 Mꢀ1 cmꢀ1) in the visible region, which may be attrib-
uted to intensity borrowing [28] from the S(r) ! Cu(II)
LMCT band, providing strong evidence for thioether coor-
dination to copper(II) in solution. The noteworthy obser-
vation is that the LF band energy of 4 is lower than that
of 3, with the absorptivity of 4 being 30-fold higher than
that of 3 (Table 3). This suggests the presence of an axial
interaction by solvent and/or distortion around copper(II)
effected by the N2S2 donor set of the L4 ligand, which is
unfavourable for square planar coordination. The differ-
ence in electronic spectra of 3 and 4 in methanol solution
indicates that complex 4 experiences partial or complete
replacement of one or two weakly coordinated thioether
donors by coordinating solvents. The chloride complex
4a exhibits a spectral pattern closely similar to 4, but with
very low absorptivity (Table 3). The bis-complex 5, which
does not have a pyridine donor as in L1–L3, exhibits a
broad LF band around 13900 cmꢀ1 with a shoulder
around 18900 cmꢀ1, implying a distorted square based
The copper(II) complexes containing bis(benzimidaz-
olyl)-thioether ligands [19] offering a N3S2 donor set
display gi and Ai values in the range 2.24–2.29 and 138–
187 · 10ꢀ4 cmꢀ1, respectively. Complex 3, also with a
N3S2 donor set, exhibits a gi value of 2.249 and an Ai value
of 140 · 10ꢀ4 cmꢀ1 (Fig. 2). The relatively low Ai value of
complex 3 supported by the relatively low value of LF
band energy indicates the presence of strong geometric dis-
tortion or strong axial interaction in the square-based coor-
dination geometry. The EPR spectral features of 3a are
very similar to that of 3; however, a lower gi/Ai value is ob-
served for 3a. On replacing the pyridine ring in the L3 com-
plex by a benzene ring as in complex 4, the gi (2.382) and Ai

R. Balamurugan et al. / Polyhedron 25 (2006) 1077–1088
1081
Table 3
Electronic and EPR spectral data for the copper(II) complexes
Complexes
Electronic spectraa
EPR spectrab
Solid
ꢀmmax ꢃ 103 cmꢀ1 ðe; Mꢀ1 cmꢀ1
Þ
Solid
Solution
Frozen solution
MeOH
12.2 (215)
23.9 (6070)
26.2 (6520)
34.1 (15770)
CH3CN
gi 2.192c
Ai 167
g^ 2.091
gi/Ai 131
[Cu(L1)Cl2] (1)
11.9
22.3
g3 2.243
g2 2.138
g1 2.052
MeOH
MeOH
[Cu(L2)Cl2] (2)
12.3
20.2
12.1 (205)
26.3 (2885)
28.0 (3605)
34.6 (10600)
gave 2.098
gi 2.254
Ai 133
g^ 2.084
gi/Ai 169
MeOH
CH3CN
14.5 (40)
29.4 (6165)
CH3CN
gi 2.249
Ai 140
g^ 2.082
gi/Ai 161
[Cu(L3)](ClO4)2 (3)
[Cu(L3)Cl]Cl (3a)
13.5
22.6
14.0 (135)
33.2 (10825)
32.7 (4000)
41.7 (20340)
gi 2.211
g^ 2.072
DMF
DMF
gi 2.242
Ai 153
g^ 2.082
gi/Ai 146
12.1
23.6
12.0 (215)
26.6 (10560)
32.6 sh
gi 2.232
g^ 2.084
37.2 (35890)
MeOH
13.5 (1105)
15.8 sh
28.0 (4095)
33.7 (6690)
CH3CN
13.5 (2340)
17.7 sh
CH3CNd
gi 2.382
Ai148
g^ 2.083
gi/Ai 161
[Cu(L4)](ClO4)2 (4)
13.8
23.0
g3 2.266
g2 2.138
g1 2.019
26.4 (6025)
MeOH
MeOHe
gi 2.485
Ai128
g^ 2.105
gi/Ai 193
[Cu(L4)Cl2] (4a)
13.6
22.8
13.4 (400)
17.8 (840)
28.0 (5065)
33.9 (15330)
gave 2.078
MeOH
13.9 (67)
18.9 sh
CH3CN
gi 2.201
Ai 178
[Cu(L5)2(H2O)](ClO4)2 (5)
15.8
19.0
gave 2.072
29.8 (6240)
g^ 2.069
gi/Ai 124
a
Concentration, 2 · 10ꢀ3 M for ligand field and 2 · 10ꢀ5 M for ligand based transitions.
b
c
Ai in 10ꢀ4 cmꢀ1
.
The species [Cu(solvent)4]2+ is also present (gi 2.418; Ai, 148 · 10ꢀ4 cmꢀ1).
Values in methanol, gi 2.408; Ai 134 · 10ꢀ4 cmꢀ1; g^ 2.099.
Values in DMF, gi 2.404; Ai 131 · 10ꢀ4 cmꢀ1; g^ 2.089.
d
e
(148 · 10ꢀ4 cmꢀ1) values are enhanced (Table 3). This re-
veals the presence of a severely distorted square planar ste-
reochemistry around copper(II) in 4 effected by the N2S2
donor atoms, the geometrical disposition of which are
unfavourable for square planar coordination around
Cu(II).
complexes the one electron reduction peak (Epc) corre-
sponding to Cu(II) to Cu(I) reduction occurs in the poten-
tial range 0.508–0.208 V, with a directly associated
reoxidation peak (Epa) in the reverse scan, whose potential
values fall within the range 0.702–0.296 V (Table 4). Fur-
ther, the original Cu(II) complex species undergoing reduc-
tion are completely regenerated following electrochemical
oxidation (the ratio of peak currents, ipa/ipc ꢂ 1.0, Table
4) suggesting a chemically reversible CuII/CuI couple.
However, the value of the limiting peak-to-peak separation
(DEp, 86–226 mV) is higher than that for the Fc/Fc+ couple
(DEp, 84 mV; E1/2, 0.414 V) under identical conditions.
2.4. Redox chemistry
The electrochemical data obtained in methanol or aceto-
nitrile solution of the present complexes using TBAP as a
supporting electrolyte are collected in Table 4. For all the

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R. Balamurugan et al. / Polyhedron 25 (2006) 1077–1088
This suggests that the heterogeneous electron-transfer
process in most of the present complexes is far from revers-
ible and that on electron transfer considerable stereochem-
ical reorganization of the coordination sphere occurs.
However, quantitative conclusions on the extent of such
reorganization are difficult to estimate [36]. Interestingly,
the complexes 1 (Fig. 3) and 2 (Fig. 4(a)) have a minimal
reorganizational energy barrier (DEp, 1, 88; 2, 86; Fc/
Fc+, 84 mV) indicating that the structural reorganization
between the two oxidation states is minimum and hence
electron transfer in these complexes is facile. It is relevant
to note that in naturally occuring electron transfer copper
proteins the redox is facile because of minimal structural
changes between the redox pairs.
Complex 2 exhibits a one electron reduction at 0.268 V
and the corresponding oxidation occurs at 0.348 V. Interest-
ingly, an additional anodic peak at 0.484 V is ob-
served (Fig. 4(a)), which can be explained by assuming a
square scheme (Scheme 2). As the conductance of 2 is
30 Xꢀ1 cm2 molꢀ1 it is probable that the complex is present
as a [CuLCl]+ species. The additional peak may arise from
the oxidation of a trigonal planar (tp) Cu(I) species. Thus,
when the scan rate is increased from 50 to 500 mV the addi-
tional peak grows in height suggesting that at higher scan
rates the tp Cu(I) species is preferentially formed immedi-
ately on reducing the square planar (sp) [CuLCl]+ species.
Further, on addition of Clꢀ ions (LiCl), the height of the
additional anodic peak decreases illustrating that the tp
Cu(I) species combine with the Clꢀ ion to form a tetrahedral
Cu(I) species, decreasing its concentration. It may be noted
that the tp Cu(II) species corresponding to the tp Cu(I) spe-
cies could not be detected, obviously because the isopropyl
substituents on the phenyl rings in the Cu(II) species would
be brought closer to exhibit strong steric clashes leading to
Fig. 2. X-band EPR spectra of the complexes [Cu(L2)Cl2] (2) (a) in
methanol/acetone and [Cu(L3)](ClO4)2 (3) (b) and [Cu(L5)2](ClO4)2 (5) (c)
in acetonitrile/acetone solution at 77 K.
Table 4
Electrochemical dataa for the copper complexes at 25 2 ꢁC in methanol solution
Compound
Epc (V)
Epa (V)
DEp (mV)b
E1/2 (V)
ipa/ipc
106D (cm2 sꢀ1
)
CV
DPVc
[Cu(L1)Cl2] (1)
+ LiCl
0.208
0.391
0.400
0.200
0.262
0.252
0.338
0.290e
0.164
0.508
0.230
0.420
0.380
0.296
0.492
0.502
0.310
0.348d
0.360
0.452
0.516
0.510
0.702
0.548
0.510
0.496
88
101
102
110
86
108
114
226
346
194
318
90
0.252
0.442
0.451
0.255
0.305
0.306
0.395
0.403
0.337
0.605
0.389
0.465
0.438
0.274
0.436
0.474
0.266
0.316
0.314
0.414
0.398
0.326
0.592
0.500f
0.490
0.432
0.9
0.9
1.0
0.8
0.8
1.0
1.3
1.1
1.3
1.1
0.8
1.1
0.9
12.3
12.3
8.9
1.1
8.0
0.8
3.0
2.7
2.4
13.5
5.4
3.8
[Cu(L2)Cl2] (2)
+ LiCl
[Cu(L3)](ClO4)2 (3)
[Cu(L3)Cl]Cl (3a)
+ LiCl
[Cu(L4)](ClO4)2 (4)
[Cu(L4)Cl2] (4a)
+ LiCl
[Cu(L5)2(H2O)](ClO4)2 (5)
a
116
Measured on a Pt working electrode vs. a non-aqueous Ag/Ag+ reference electrode; add 544 mV [300 mV, Ag/Ag+ to SCE; +244 mV, SCE to SHE] to
convert to standard hydrogen electrode (SHE); Fc/Fc+ couple in CH3CN, E1/2, 0.414 V (CV); 0.401 V (DPV); scan rate 50 mV sꢀ1; supporting electrolyte,
tetra-N-butylammonium perchlorate (0.1 mol dmꢀ3); complex concentration, 1 mmol dmꢀ3
.
b
DEp at 50 mV sꢀ1 scan rate.
c
Differential pulse voltammetry, scan rate 1 mV sꢀ1; pulse height 50 mV.
d
Additional anodic peak appears at 0.484 V. Cathodic peak does not apppear at high scan rate.
Values in DMF.
Additional peak at 0.316 V.
e
f

R. Balamurugan et al. / Polyhedron 25 (2006) 1077–1088
1083
Fig. 3. Cyclic (a) and differential pulse (b) voltammograms of 0.001 M complex [Cu(L1)Cl2] (1) before (——) and after (-----) the addition of excess LiCl
in methanol solution at 25 ꢁC at 0.05 V sꢀ1 scan rate.
its destability. Thus, in the X-ray structure of the tp
Cu(I) complex [Cu(bbtmp)](NO3) [bbtmp = 2,6-bis (ben-
zimidazol-20-ylthiomethyl)pyridine] the benzimidazole rings
are closer to each other than in the corresponding tetrahedral
[Cu(bbtmp)Cl] species [17]. From the difference in Epa values
(DEpa) for the oxidation of the two Cu(I) species, the DE1/2
values for the redox of [Cu(L2)Cl] and [Cu(L2)]+ species
was estimated assuming reversibility, from which the K1/
K2 value was calculated [37] as 7.9 · 10ꢀ3. The magnitude
of K1/K2 illustrates that the tp Cu(II) species rather than
the tp Cu(I) species readily combines with Clꢀ ions leading
to the non-detection of the tp Cu(II) species. Similarly, the
tp Cu(I) species shows a reluctance to bind to the Clꢀ ion
illustrating the above observation that the tp Cu(I) species
are directly formed from square planar [Cu(L2)Cl]+ species
on reduction. The decreased steric constraints in the tp
Cu(II) species of 1 leads to its detection in the CV (Epc,
0.391 V), in contrast to 2. Similarly, the tp Cu(I) species for
1 will be more stable than that for 2 and hence combine with
Clꢀ less strongly (K1/K2, 1.8 · 10ꢀ3, calculated from the two
E1/2 values (Table 4) obtained from differential pulse voltam-
metry (DPV)). As for 2, an increase in scan rate leads to the
formation of more tp Cu(I) species, as seen from the increase
in height of the additional anodic peak with increase in scan
rate. The tp Cu(II) species of 1 is more stable because the ste-
ric clash between the two phenyl rings carrying the methyl
groups is less compared to the corresponding tp Cu(II) spe-
cies of 2 and hence its detection in the CV.
Complex 3, bearing a CuN3S2 coordination sphere,
shows a positive redox potential of 0.395 V (Fig. 4(b)).
The incorporation of two sulfur atoms into 1 as in com-
plex 3 leads to a Cu(II)/Cu(I) redox potential higher than
1/2 suggesting the involvement of thioether coordination
in 3 [38]. The higher E1/2 value (0.403 V) observed for
the chloro complex 3a is almost the same as for 3 but,
interestingly, the DEp value of complex 3a (226 mV) is
higher than 3 (114 mV). This suggests that the chloride
ion coordinates to copper(II), as shown by the lower
conductance (90 Xꢀ1 cm2 molꢀ1) of 3a. Further, the addi-
tion of excess LiCl shifts the cathodic peak to a more

1084
R. Balamurugan et al. / Polyhedron 25 (2006) 1077–1088
Fig. 4. Cyclic (——) and differential pulse (-----) voltammograms of the complexes [Cu(L2)Cl2] (2) (a) and [Cu(L3)](ClO4)2 (3) (b) and methanol solution
at 25 ꢁC at 0.05 V sꢀ1 scan rate.
negative value confirming the coordination of the chlo-
ride ion to copper(II), which renders the reduction more
difficult.
The complexes 4 and 4a display redox potentials higher
than complexes 3 and 3a, which is expected with the incor-
poration of hard N donors into the CuN2S2 coordination
sphere of 4 and 4a. An interesting electrochemical behav-
iour was observed for the chloro complex 4a, which under-
goes reduction at a very low positive potential of 0.230 V
with a shoulder around 0.438 V. The high DEp value
(318 mV) and the observation of two waves in DPV at dif-
ferent potentials suggest the coordination of Clꢀ ion as
Scheme 2.
illustrated above for 2. A well-defined CV with a low
DEp value of 90 mV was obtained after the addition of ex-
cess of LiCl, supporting the dissociation of chloride ions as
proposed above. It should be noted that all these proposals
are only speculative in the absence of crystal structures of
the complexes. The bis-complex 5 also exhibits positive re-
dox potentials (0.438 V) because of the bulky N-phenyl
groups, which would favour the formation of tetrahedral
copper(I) species more than complex 1, which may be

R. Balamurugan et al. / Polyhedron 25 (2006) 1077–1088
1085
due to the presence of hard N donors in the latter complex
as discussed above. The high redox potential (0.592 V ver-
sus Ag/Ag+) for the Cu(II)/Cu(I) couple of complex 4 may
be attributed to tetrahedral distortion from a regular
square-based geometry.
isophthaladehyde (Lancaster) were used without further
purification.
4.2. Synthesis of ligands
4.2.1. 2,6-Bis(2,4,6-trimethylphenyliminomethyl)pyridine
(L1)
3. Conclusions
2,6-Pyridinedicarboxaldehyde (0.68 g, 5.0 mmol) and
2,4,6-trimethylaniline (1.35 g, 10.0 mmol) were refluxed
[26] in methanol (25 mL) for 2 h over molecular sieves
The distinct spectral and electrochemical behaviour of
copper(II) complexes of five sterically hindered Schiff base
ligands have been studied. The 1:1 copper(II) complex of
2,6-bis(2,6-diisopropylphenyliminomethyl)pyridine adopts
a distorted square pyramidal geometry. The replacement
of isopropyl groups in this complex by the less sterically
demanding methyl groups leads to a similar coordination
geometry as revealed by their similar spectral and electro-
chemical features. However, the redox potential of the
isopropyl complex is higher than the methyl complex,
which is consistent with the higher gi value [39] of the for-
mer complex, illustrating the importance of sterically
demanding isopropyl groups in distorting the coordina-
tion geometry around copper(II). Further, the interaction
of a thiolate moiety with the methyl/isopropyl complexes
would be expected to generate a Cu(I) trigonal pyramidal-
like or tetrahedral-like coordination geometry, which
would be a better copper(II) model for the active sites
of electron transfer copper proteins. Thus, Holland and
Tolman [40] have recently reported four-coordinate
copper(II) complexes with a thiolate and thioether con-
taining chelating ligand and a bidentate b-diketiminate
as an auxiliary ligand and these complexes are believed
to be close structural mimics for the type I copper
proteins. However, these complexes fail to reproduce the
positive redox potential of the blue copper proteins.
The copper(II) complexes of sterically constrained li-
gands providing N3S2 and N2S2 donor sets, which are unfa-
vourable for square planar coordination, have also been
synthesized and characterized. The spectral and electro-
chemical properties of these complexes suggest the presence
of thioether coordination in solution. The incorporation of
a pyridine nitrogen donor into the CuN2S2 complexes leads
to a higher LF band energy but with lower absorptivity,
suggesting lesser distortion in the coordination sphere.
Both the CuN3S2 and CuN2S2 complexes show positive re-
dox potentials suggesting the coordination of at least one
thioether sulfur. The bis-complex of a sterically hindering
bidentate ligand also shows a positive redox potential sug-
gesting that the distortion in the coordination sphere is sig-
nificantly large.
˚
(4 A). The reaction mixture was filtered while hot. The yel-
low crystals of L1 (1.64 g, 89%) obtained upon cooling the
reaction mixture were filtered off and dried. Anal. Calc. for
C25H27N3: C, 81.3; H, 7.4; N, 11.4. Found: C, 81.4; H, 7.5;
1
N, 11.4%. H NMR (CDCl3, 400 MHz): d, ppm 8.39 (s,
2H, HC@N), 8.38 (d, J = 7.8, 2H, HPy), 7.97 (t, J = 7.6,
1H, HPy), 6.91 (s, 4H, HPh), 2.30 (s, 6H, H3C–PhC4),
2.16 (s, 12H, H3C–PhC2). 13C NMR (CDCl3, 100 MHz):
d, ppm 163.6 (CH@N), 155.0, 148.2, 137.7, 134.0, 129.3,
127.2, 123.0 (CPy, CPh), 21.2 (CH3–C4), 18.7 (CH3–C2).
IR (KBr): m, cmꢀ1 2971s, 2914s, 2914s, 1640vs, 1481s,
1205vs, 1140s, 835s, 733s.
4.2.2. 2,6-Bis(2,6-diisopropylphenyliminomethyl)pyridine
(L2)
The ligand L2 was synthesized by the reported procedure
[41]. To a solution of 2,6-pyridinedicarboxaldehyde (0.68 g,
5.0 mmol) in ethanol (25 mL) were added 2,6-diisopropy-
laniline (1.77 g, 10.0 mmol) and one drop of glacial acetic
acid, and the resulting mixture was refluxed over molecular
˚
sieves (4 A). After 24 h the reaction mixture was filtered and
the ligand (1.78 g, 78%) was obtained upon cooling. Anal.
Calc. for C31H39N3: C, 82.1; H, 8.7; N, 9.3. Found: C,
1
81.5; H, 8.9; N, 9.5%. H NMR (CDCl3, 400 MHz): d,
ppm 8.41 (d, J = 7.8, 2H, HPy), 8.39 (s, 2H, HC@N), 8.01
(t, J = 7.8, 1H, HPy), 7.21–7.10 (m, 6H, HPh), 3.00 (sep,
J = 6.8, 4H, HC(H3C)2), 1.20 (d, J = 6.8, 24H, (H3C)2C).
13C NMR (CDCl3, 100 MHz): d, ppm 162.8 (CH@N),
154.6, 148.4, 137.5, 137.3, 124.7, 123.2, 122.9 (CPy, CPh),
28.1 (CH(H3C)2), 23.6 ((CH3)2C). IR (KBr): m, cmꢀ1
3065w, 2961vs, 1585vs, 1566w, 1456w, 1319s, 1184s, 748s.
4.2.3. 2,6-Bis(2-methylthiophenyliminomethyl)pyridine (L3)
A mixture of 2,6-pyridinedicarboxaldehyde (0.68 g, 5.0
mmol) and 2-(methylthio)aniline (1.47 g, 10.5 mmol) were
refluxed in benzene (20 mL) for 36 h. The reaction mixture
was filtered under hot conditions. Fine yellow crystals of
L3 (1.16 g, 62%) were obtained on recrystallization in ben-
zene, which was filtered off and dried. Anal. Calc. for
C21H19N3S2: C, 66.8; H, 5.1; N, 11.1. Found: C, 66.9; H,
1
4. Experimental
5.1; N, 11.2%. H NMR (CDCl3, 400 MHz): d, ppm 8.63
(s, 2H, HC@N), 8.41 (d, J = 7.8, H, HPy), 7.95 (t, J = 7.8,
1H, HPy), 7.36–7.11 (m, 8H, HPh), 2.49 (s, 6H, H3C). 13C
NMR (CDCl3, 100 MHz): d, ppm 159.7 (CH@N), 154.7,
147.8, 137.5, 135.1, 127.5, 125.4, 124.7, 123.6, 117.5 (CPy,
CPh), 14.9 (CH3). IR (KBr): m, cmꢀ1 3050w, 2978w,
2913s, 1619vs, 1570w, 1468s, 1331s, 800s, 724vs.
4.1. Materials
Copper(II) chloride dihydrate, copper(II) perchlorate
hexahydrate, 2,6-pyridinedimethanol, 2,4,6-trimethylani-
line, 2,6-diisopropylaniline, selenium dioxide (Aldrich),

1086
R. Balamurugan et al. / Polyhedron 25 (2006) 1077–1088
4.2.4. 1,3-bis(2-methylthiophenyliminomethyl)benzene (L4)
The ligand L4 was synthesized by the same method
adopted for L3, using isophthalaldehyde (0.67 g, 5.0 mmol).
The yellow solid precipitated was filtered off and dried
(1.43 g, 76%). Anal. Calc. for C22H20N2S2: C, 70.2; H, 5.4;
N, 7.4. Found: C, 70.2; H, 5.3; N, 7.5%. 1H NMR (CDCl3,
400 MHz): d, ppm 8.50 (s, 2H, HC@N), 8.41 (s, 1H, HPh),
8.14 (d, J = 7.8, 1H, HPh), 8.13 (d, J = 7.6, 1H, HPh), 7.59
(t, J = 7.6, 1H, HPh), 7.24–7.03 (m, 6H, HPh), 7.01 (d,
J = 7.6, 2H, HPh), 2.48 (s, 6H, H3C). 13C NMR (CDCl3,
100 MHz): d, ppm 159.3 (CH@N), 148.9, 136.9, 134.3,
131.5, 130.2, 129.4, 126.8, 125.4, 124.7, 117.7 (CPh), 14.9
(CH3). IR (KBr, m, cmꢀ1): 3050w, 2978w, 2905w, 1614vs,
1570s, 1466s, 1436s, 1351s, 1268s, 1071s, 804s, 754vs,
691s.
mole ratio. The volume of the solvent was reduced by ro-
tary evaporation and then cooled. The product formed
was filtered off, washed with small amounts of cold metha-
nol and then dried under vacuum. Single crystals of
[Cu(L2)Cl2] (2) suitable for X-ray diffraction were obtained
by diffusion of ether vapour into an acetonitrile solution of
2. The complex 5 was obtained by mixing the ligand L5 and
copper(II) perchlorate hexahydrate in a 2:1 (ligand:metal)
mole ratio.
[Cu(L1)Cl2](1). Yield: 86% (0.43 g). Anal. Calc. for
C25H27N3Cl2Cu: C, 59.6; H, 5.4; N, 8.3. Found: C, 59.6;
H, 5.5; N, 8.4%.
[Cu(L2)Cl2](2). Yield: 88% (0.52 g). Anal. Calc. for
C31H39N3Cl2Cu: C, 63.3; H, 6.7; N, 7.1. Found: C, 63.4;
H, 6.7; N, 7.2%.
[Cu(L3)](ClO4)2(3). Yield: 91% (0.58 g). Anal. Calc.
for C21H19N3O8S2Cl2Cu: C, 39.4; H, 2.9; N, 6.6. Found:
C, 39.5; H, 3.0; N, 6.7%.
4.2.5. 1,3-bis(2,4,6-trimethylphenyliminomethyl)benzene
(L5)
The ligand L5 was prepared by refluxing isophthalalde-
hyde (0.67 g, 5.0 mmol) and 2,4,6-trimethylaniline (1.42 g,
10.5 mmol) in benzene (20 mL) for 36 h over molecular
[Cu(L3)Cl]Cl(3a). Yield: 76% (0.39 g). Anal. Calc. for
C21H19N3S2Cl2Cu: C, 49.3; H, 3.7; N, 8.2. Found: C, 49.3;
H, 3.8; N, 8.3%.
[Cu(L4)](ClO4)2(4). Yield: 63% (0.40 g). Anal. Calc.
for C22H20N2O8S2Cl2Cu: C, 41.4; H, 3.2; N, 4.4. Found:
C, 42.4; H, 3.4; N, 4.4%.
[Cu(L4)Cl2](4a). Yield: 82% (0.42 g). Anal. Calc. for
C22H20N2S2Cl2Cu: C, 51.7; H, 3.9; N, 5.5. Found: C,
51.8; H, 4.0; N, 5.5%.
[Cu(L5)2(H2O)](ClO4)2(5). Yield: 62% (0.39 g).
Anal. Calc. for C26H28N2O8Cl2Cu: C, 48.1; H, 4.7; N,
4.3. Found: C, 48.1; H, 4.8; N, 4.3%.
˚
sieves (4 A). The reaction mixture was filtered and the sol-
vent removed under vacuum. The excess amine was evapo-
rated on warming gently under high vacuum. The 1H
NMR spectrum of the residue showed that the product
was sufficiently pure (1.62 g, 88%). Anal. Calc. for
C26H28N2: C, 84.7; H, 7.7; N, 7.6. Found: C, 84.8; H,
1
7.7; N, 7.7%. H NMR (CDCl3, 400 MHz): d, ppm 8.36
(s, 1H, HPh), 8.31 (s, 2H, HC@N), 8.10 (d, J = 7.6, 1H,
HPh), 8.09 (d, J = 7.8, 1H, HPh), 7.63 (t, J = 7.6, 1H,
HPh), 6.91 (s, 4H, HPh), 2.31 (s, 6H, H3C–C4), 2.15 (s,
12H, H3C–C2). 13C NMR (CDCl3, 100 MHz): d, ppm
162.1 (CH@N); 148.6, 136.9, 133.3, 130.9, 129.4, 128.9,
128.8, 127.1 (CPh), 20.9 (CH3–C4), 18.4 (CH3–C2).
4.4. Data collection and structure refinement
Single crystals of [Cu(L2)Cl2] were chosen for X-ray
data collection after careful examination under an optical
microscope. The X-ray diffraction intensities were mea-
sured by x and / scans using a Nonius Kappa CCD
four-circle diffractometer fitted with a CCD area detector
and a graphite monochromator for the Mo Ka radiation.
The unit cell parameters and the orientation matrix of the
crystal were determined using all data. Data reduction
was performed using the DENZO program [42], and a
semi-empirical multi-scan absorption correction was ap-
plied using the program SORTAV [43]. The structures were
solved by direct methods using the SHELXS-97 program
[44] and refined by the full-matrix least-squares method
on F2 using SHELXL-97 [45]. While three of four isopropyl
groups showed slightly higher thermal parameters, indica-
tive of librational motion, in only one of those were the
ellipsoids large enough to merit a disorder model. This
was refined over two equally occupied orientations,
C(33A)–C(35A) and C(33B)–C(35B). All disordered C–C
4.2.6. 1,3-bis(2,6-diisopropylphenyliminomethyl)benzene
(L6)
The ligand L6 was prepared using the method adopted
for L5. After the evaporation of the solvent the solid ob-
tained was washed with hexane to yield L6 (1.89 g, 83%).
Anal. Calc. for C32H40N2: C, 84.9; H, 8.9; N, 6.2. Found:
1
C, 85.0; H, 9.0; N, 6.2%. H NMR (CDCl3, 400 MHz): d,
ppm 8.36 (s, 1H, HPh), 8.32 (s, 2H, HC@N), 8.15 (d,
J = 7.8, 1H, HPh), 8.14 (d, J = 7.6, 1H, HPh), 7.68 (t,
J = 7.6, 1H, HPh), 7.21–7.15 (m, 6H, HPh), 3.02 (sep,
J = 6.8, 4H, HC(H3C)2), 1.21 (d, J = 6.8, 24H, (H3C)2C).
13C NMR (CDCl3, 100 MHz): d, ppm 161.8 (CH@N);
149.4, 137.9, 137.2, 131.5, 129.9, 129.5, 124.8, 123.5 (CPy,
CPh), 28.4 (CH(H3C)2), 23.9 ((CH3)2C). IR (KBr): m,
cmꢀ1 3057w, 2964vs, 2869s, 1636vs, 1581s, 1456w, 1361s,
1321s, 1147s, 840s, 762vs, 692s.
˚
4.3. Synthesis of copper(II) complexes
bond lengths were restrained to 1.53(2) A, and non-
bonded 1,3-Cꢄ ꢄ ꢄC distances between methyl groups on
˚
The copper(II) complexes 1–4 of the ligands L1–L4 were
prepared by adding the corresponding copper(II) salt in
methanol to a methanolic solution of the ligand in a 1:1
each disordered orientation to 2.50(2) A. All non-H atoms
except the disordered C atoms were refined anisotropi-
cally, while all sp2-H atoms were placed in calculated

R. Balamurugan et al. / Polyhedron 25 (2006) 1077–1088
1087
[13] P.J.M.W.L. Birker, E.F. Godefroi, J. Helder, J. Reedijk, J. Am.
Chem. Soc. 104 (1982) 7556.
[14] R.P.F. Kanters, R. Yu, A.W. Addison, Inorg. Chim. Acta 196 (1992)
97.
positions and refined using a riding model. Methyl H
atoms were treated as idealised groups with freely refined
torsions about their C–C bonds.
[15] C.R. Lucas, R.C. Hynes, J.P. Charland, Can. J. Chem. 70 (1992)
1773.
[16] A.W. Addison, P.J. Burke, J. Heterocyc. Chem. 18 (1981) 803.
[17] R. Balamurugan, M. Palaniandavar, R. Srinivasa Gopalan, Inorg.
Chem. 40 (2001) 2246.
5. Supplementary data
Full crystallographic data for 2 are available from the
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK on re-
quest, quoting the deposition number 255046.
[18] S. Usha, M. Palaniandavar, J. Chem. Soc., Dalton Trans. (1994)
2277.
[19] R. Balamurugan, M. Palaniandavar, R. Srinivasa Gopalan, G.U.
Kulkarni, Inorg. Chim. Acta 357 (2004) 919.
[20] M. Palaniandavar, Proc. Ind. Nat. Sci. Academy A 70 (2004) 293.
[21] (a) H. Toftlund, J. Becher, P.H. Olesen, J.Z. Pedersen, Isr. J. Chem.
25 (1985) 56;
Acknowledgements
The authors thank the Department of Science and Tech-
nology, India for financial support and the Council of Sci-
entific and Industrial Research, India for a Senior Research
Fellowship (to R.B.). One of them (M.P.) thanks the
Department of Science and Technology, India and The
Royal Society, UK for funding to visit Leeds. We thank
Professor P. Sambasiva Rao, Pondicherry University for
providing the EPR facility.
(b) M. Gullotti, L. Casella, A. Pintar, E. Suardi, P. Zanello, S.
Mangani, J. Chem. Soc., Dalton Trans. (1989) 1979;
(c) L. Casella, Inorg. Chem. 23 (1984) 2781;
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