Synthesis, structures, spectral and electrochemical properties of copper(II) complexes of sterically hindered Schiff base ligands

Article abstract of DOI:10.1016/j.ica.2008.03.083

The Schiff base ligands 2-(2,6-diisopropylphenyliminomethyl)phenol H(L1), 5-diethylamino-2-(2,6-diisopropylphenyliminomethyl)phenol H(L2), 2,4-di-tert-butyl-6-(2,6-diisopropylphenyliminomethyl)phenol H(L3), 3-(2,6-diisopropylphenyliminomethyl)naphthalen-2

Full text of DOI:10.1016/j.ica.2008.03.083

Inorganica Chimica Acta 362 (2009) 199–207
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, structures, spectral and electrochemical properties of copper(II)
complexes of sterically hindered Schiff base ligands
Karuppasamy Sundaravel a, Eringathodi Suresh b, Mallayan Palaniandavar a,
*
a School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamilnadu, India
b Analytical Science Discipline, Central Salt and Marine Chemicals Research Institute, Bhavnagar 364 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 July 2007
Received in revised form 6 March 2008
Accepted 17 March 2008
Available online 25 March 2008
The Schiff base ligands 2-(2,6-diisopropylphenyliminomethyl)phenol H(L1), 5-diethylamino-2-(2,
6-diisopropylphenyliminomethyl)phenol H(L2), 2,4-di-tert-butyl-6-(2,6-diisopropylphenyliminometh-
yl)phenol H(L3), 3-(2,6-diisopropylphenyliminomethyl)naphthalen-2-ol H(L4) and 4-(2,6-diisopropylph-
enyliminomethyl)-5-hydroxymethyl-2-methylpyridin-3-ol H(L5) have been synthesized by the
condensation, respectively, of salicylaldehyde, 4-(diethylamino)salicylaldehyde, 3,5-di-tert-butylsalicyl-
aldehyde, 2-hydroxy-1-napthaldehyde and pyridoxal with 2,6-diisopropylaniline. The copper(II) bis-
ligand complexes [Cu(L1)2] 1, [Cu(L2)2] 2, [Cu(L3)2] 3, [Cu(L4)2] 4 and [Cu(L5)2] ꢀ CH3OH 5 of these ligands
have been isolated and characterized. The X-ray crystal structures of two of the complexes [Cu(L1)2] 1
and [Cu(L5)2] ꢀ CH3OH 5 have been successfully determined, and the centrosymmetric complexes possess
a CuN2O2 chromophore with square planar coordination geometry. The frozen solution EPR spectra of the
complexes reveal a square-based CuN2O2 chromophore, and the values of g and g /A index reveal
enhanced electron delocalization by incorporating the strongly electron-releasing –NEt2 group (2) and
fusing a benzene ring on sal-ring (4). The Cu(II)/Cu(I) redox potentials of the Cu(II) complexes reveal that
the incorporation of electron-releasing –NEt2 group and fusion of a benzene ring lead to enhanced stabil-
ization of Cu(II) oxidation state supporting the EPR spectral results. The hydrogen bonding interactions
between the two molecules present in the unit cell of 5a generate an interesting two-dimensional hydro-
gen-bonded network topology.
Keywords:
Schiff base ligands
Copper(II) bis-ligand complexes
X-ray structures
Steric hindrance
Square planar geometry
EPR spectra
k
k
k
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
chemical parameters and the spectral/geometrical parameters of
Schiff base metal complexes, which could be the result of steric
In recent years there has been considerable interest in the
chemistry of transition metal complexes of Schiff bases [1–4], as
these ligands offer opportunities for inducing substrate chirality,
tuning the metal-centered electronic factor, and enhancing the sol-
ubility and stability of either homogeneous or heterogeneous cat-
alysts [5–8]. Also, since the past few years the Schiff base
complexes have become increasingly important as biochemical,
analytical and antimicrobial reagents [9]. The cobalt(II), nickel(II)
and copper(II) complexes of Schiff bases derived from 4-hydroxy-
salicylaldehyde and amino acids have been shown to have stronger
anticancer activity against Ehrlich ascites carcinoma (EAC) [10]. The
promising class of bis(salicylaldiminato) metal Schiff base com-
plexes [11] has been shown to exhibit quadratic non-linear optical
(NLO) properties, which are currently attracting considerable
interest. In addition, there is a continuing interest in investigating
the relationship between the redox potentials and other electro-
and electronic effects. The transition metal complexes having oxy-
gen and nitrogen donor Schiff bases possess unusual configurations
and structural lability and are sensitive to molecular environment.
Four-coordinated copper(II) complexes usually form a square pla-
nar coordination geometry that may be distorted to pseudo-tetra-
hedral geometry depending on the ligand environment [12]. The
X-ray crystal structures of copper(II) bis-ligand complexes of
bidentate Schiff bases derived by condensing simple and substi-
tuted salicylaldehydes with various alkyl and aromatic amines
have been determined [7,11–13] to understand the effect of ligand
steric hindrance on the copper(II) coordination geometry. In all
these complexes copper(II) is coordinated by the phenolate oxygen
and imine nitrogen donor atoms of the two ligand molecules in the
usual trans arrangement. For bis(salicylaldiminato)copper(II) com-
plexes the steric and electronic characteristics of the substituents
on both the phenol and imine moieties lead to various geometries
intermediate between square planar and tetrahedral [14,15].
Thus, the bis(N-methylsalicylaldiminato)copper(II) complex [12b]
has a planar geometry whereas bis(N-tert-butylsalicylaldiminato)
copper(II) has a flattened tetrahedral geometry [12c]. Also, the
* Corresponding author. Tel.: +91 431 2407053.
E-mail
addresses:
palaniandavarm@gmail.com,
palanim51@yahoo.com
(M. Palaniandavar).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.03.083

200
K. Sundaravel et al. / Inorganica Chimica Acta 362 (2009) 199–207
HO
N
R
R
N
N
N
OH
OH
OH
X
CH3
R = alkyl
R = 2,6-di(i-Pr)phenyl X = H
R = 2,6-di(i-Pr)phenyl X = 5-NEt2
X = H
R = cycloakyl
R = 2,6-di(i-Pr)phenyl H(L4)
H(L5)
H(L1)
H(L2)
R = 2,6-di(i-Pr)phenyl X = 2,4-di-t-Bu H(L3)
R
N
R = 4-nitrophenyl
R = 2,6-di(i-Pr)phenyl X = 6-nitro
R = 2-methylphenyl
X = 5-NEt2
H(L6)
H(L7)
OH
X = 2,4-dibromo H(L8)
R = cycloakyl
Scheme 1. Certain bidentate Schiff base ligands employed in the present study and in the literature.
complexes [Cu(L1)2] [16] and [Cu(L6)2] [11] (Scheme 1) possess a
square planar geometry while [Cu(L7)2] [7] possesses a distorted
square planar geometry; it is obvious that the –NO2 group present
ortho to the phenolic hydroxyl group in the latter distorts the
square planar geometry. Further, the bulky o-tolyl substituents at
the imine nitrogen atom in [Cu(L8)2] [13] distort the coordination
geometry from square planar to tetrahedral. A series of copper(II)
bis-ligand complexes of Schiff base ligands derived from 2-hydro-
xy-1-napthaldehyde, 3-hydroxy-1-napthaldehyde and salicylalde-
hyde with linear- and cycloalkylamines (Scheme 1) have been
also isolated and the structural distortions observed in the X-ray
crystal structures have been correlated with the Cu(II)/Cu(I) redox
Ligand H(L1) and complex [Cu(L1)2] 1 were prepared according
to the literature procedure [16].
2.2. Synthesis of ligands
2.2.1. 5-Diethylamino-2-(2,6-diisopropylphenyliminomethyl)phenol
H(L2)
To 4-(diethylamino)salicylaldehyde (0.97 g, 5.0 mmol) in meth-
anol (25 mL) was added dropwise 2,6-diisopropylaniline (0.89 g,
5.0 mmol) in methanol (25 mL). The mixture was stirred for 24 h
and then refluxed for 30 min. The red-orange oil of the ligand
H(L2) obtained upon rotary evaporation of the reaction mixture
was purified by column chromatography (Rf = 0.60 (methanol/hex-
ane 6:4), SiO2 column, eluent: methanol/hexane (4:6–6:4)) and
then used for complex preparation. Yield, 1.32 g, (75%). Anal. Calc.
for C23H32N2O: C, 78.36; H, 9.15; N, 7.95. Found: C, 78.52; H, 9.26;
N, 8.06%. 1H NMR (CDCl3, 200 MHz): d, ppm 8.25 (s, 1H), 5.12 (br s,
1H), 3.15 (sep, 2H), 1.10 (t, 6H), 3.35(q, 4H), 6.9–7.5 (m, 6H), 1.25
(d, 12H).
potentials and the A values from EPR spectral data [17].
k
Previously, we had been interested in understanding the effect
of varying steric requirements of the ligands on the extent of
distortion of copper(II) centers from square planar to tetrahedral
in order to throw light on the electron transfer in blue copper
proteins with distorted tetrahedral active site geometries. We
had previously shown that in mixed ligand complexes trichloro-
acetic acid forms chelate rings with high electron density and
tends to promote a distortion away from square planar towards
tetrahedral [18]. In the present work we have isolated and studied
the properties of Cu(II) bis-ligand complexes of the Schiff base
ligands (H(L1)–H(L5)) (Scheme 1) with an aim to understand the
effect of bulky substituents in the ligand on the Cu(II) coordination
geometry. The sterically hindering alkyl substituents at 2,4-
(H(L3)) and 2-positions (H(L5)), respectively, in the sal-ring in
the ligands are expected to impose a geometric distortion on the
complexes with a CuN2O2 chromophore. However, interestingly,
the copper(II) coordination geometry in [Cu(L5)2] is square planar,
as revealed in the X-ray crystal structure. The substituents on the
sal-ring in the present complexes are shown to confer electronic
and steric effects upon the coordination geometry and spectra of
the complexes.
2.2.2. 2,4-Di-tert-butyl-6-(2,6-diisopropylphenyliminomethyl)phenol
H(L3)
The ligand H(L3) was prepared by the method adopted for the
preparation of H(L2) using 3,5-di-tert-butylsalicylaldehyde
(1.17 g, 5.0 mmol) instead of 4-(diethylamino)salicylaldehyde.
The pale yellow oil obtained was purified by column chromatogra-
phy (Rf = 0.66 (methanol/hexane 6:4), SiO2 column, eluent: metha-
nol/hexane (4:6–6:4)). Yield, 1.38 g (70%). Anal. Calc. for C27H39NO:
C, 82.39; H, 9.99; N, 3.52. Found: C, 82.59; H, 9.87; N, 3.43%. 1H
NMR (CDCl3, 200 MHz): d, ppm 8.21 (s, 1H), 5.12 (br s, 1H), 3.15
(sep, 2H), 6.9–7.5 (m, 5H), 1.32 (s, 18H), 1.25 (d, 12H).
2.2.3. 3-(2,6-Diisopropylphenyliminomethyl)naphthalen-2-ol H(L4)
The ligand H(L4) was prepared by the method adopted for the
preparation of H(L2) except that 2-hydroxy-1-napthaldehyde
(0.86 g, 5.0 mmol) was used instead of 4-(diethylamino)salicylal-
dehyde. Upon rotary evaporation of the reaction mixture an yellow
crystalline solid was obtained. The crude product was recrystal-
lized from dichloromethane/hexane mixture and then used for iso-
lating the copper(II) complex. Yield, 1.32 g (80%). Anal. Calc. for
C23H25NO: C, 83.34; H, 7.60; N, 4.23. Found: C, 83.39; H, 7.73; N,
4.30%. 1H NMR (CDCl3, 200 MHz): d, ppm 8.39 (s, 1H), 5.2 (br s,
1H), 3.17–3.23 (sep, 2H), 6.5–7.9 (m, 9H), 1.29 (d, 12H).
2. Experimental
2.1. Materials
Copper(II) chloride dihydrate (BDH, India), triethylamine
(Merck, India), salicylaldehyde (LOBA Chemie, India), 2,6-diisopro-
pylaniline,
3,5-di-tert-butylsalicylaldehyde,
4-(diethylamino)
salicylaldehyde, 2-hydroxy-1-napthaldehyde and pyridoxal hydro-
chloride were purchased from Aldrich and used as received. Dry
methanol was prepared by distillation from Mg turnings and
iodine. N,N-Dimethylformamide (DMF), dichloromethane (DCM)
and acetone were purchased from Merck, India.
2.2.4. 4-(2,6-Diisopropylphenyliminomethyl)-5-hydroxymethyl-2-
methylpyridin-3-ol H(L5)
The free pyridoxal was obtained by the dropwise addition of
aqueous potassium hydroxide solution (2 N) to pyridoxal hydro-

K. Sundaravel et al. / Inorganica Chimica Acta 362 (2009) 199–207
201
chloride (1.01 g, 5.0 mmol) in water (15 mL) with constant stirring
[19]. To this solution was added dropwise 2,6-diisopropylaniline
(0.89 g, 5.0 mmol) in methanol (10 mL). The mixture was stirred
for 24 h, refluxed for 30 min and then rotaryevaporated. The yel-
low oil of the ligand H(L5) was purified by passing through silica
gel column (Rf = 0.58 (methanol/hexane 6:4), SiO2 column, eluent:
methanol/hexane (4:6–6:4)). Yield, 1.06 g (65%). Anal. Calc. for
C20H26N2O2: C, 73.59; H, 8.03; N, 8.58. Found: C, 73.65; H, 8.07;
N, 8.61%. 1H NMR (CDCl3, 200 MHz): d, ppm 7.52 (s, 1H), 5.20 (br
s, 1H), 2.10 (s, 1H), 3.14–3.27 (sep, 2H), 6.9–7.25 (m, 3H), 1.23
(d, 12H), 2.58 (s, 3H), 8.75 (s, 1H).
then in double distilled water to remove the impurities. The tem-
perature of the electrochemical cell was maintained at 25 0.2 °C
by a cryocirculator (HAAKE D8 G). The solutions were deoxygen-
ated by bubbling research grade nitrogen, and an atmosphere of
nitrogen was maintained over the solution during measurements.
The instruments utilized included an EG&G PAR 273 Potentiostat/
Galvanostat and Pentium IV computer along with EG&G M270 soft-
ware to carry out the experiments and to acquire the data.
2.5. Data collection and structure refinement
X-ray quality crystals of complex 1 were mounted on a glass fi-
ber and all measurements were made on a Rigaku RAXIS RAPID dif-
fractometer attached with a plate area detector and graphite
2.3. Synthesis of complexes
All the complexes were prepared using the following general
procedure. To a methanolic (5 mL) solution of the ligand (1 equiv.)
was added half-an-equivalent of copper(II) chloride dihydrate
(0.17 g, 1 mmol) in methanol (5 mL) in the presence of an equiva-
lent amount of triethylamine (0.20 g, 2 mmol), and the solution
was then stirred for 2 h. The resulting precipitate of all the com-
plexes, except 3 and 5, was filtered off, washed with small amounts
of cold methanol and dried over P4O10 under vacuum. Complexes 3
and 5 were obtained as dark green crystalline blocks from the reac-
tion mixture by slow evaporation. Crystals of complex 1 suitable
for X-ray diffraction analysis were obtained by dissolving the com-
plex in DMF and leaving it to stand for a week. The X-ray quality
crystals of complex 5 were obtained by the slow evaporation of
the reaction mixture (methanol + ethanol: 10 mL + 5 mL) layered
with diethyl ether.
monochromated Mo K radiation. The details regarding the data
collection and processing are collected in Table 1. The intensity
a
data were collected at a temperature of ꢁ165 1 °C using the
x–
2h scan technique to a maximum of 2h value of 55.0°. A total of
44 oscillation images were collected. A sweep of data were col-
lected using
= 45.0° and / = 0.0° and the exposure rate was 60.0 (s/°). A sec-
ond sweep was performed using scans from 0.0 to 160.0° in
5.0° step, at = 45.0° and / = 180.0°, and the exposure rate was
x scans from 130.0 to 190.0° in 5.0° step, at
v
x
v
60.0 (s/°). The data were collected and processed using the Crystal
Structure Analysis Package (Rigaku). The linear absorption coeffi-
cient, l
, for Mo K radiation is 7.245 cmꢁ1. The data were corrected
a
for Lorentz and Polarization effects. The structure was solved by
heavy-atom Patterson methods [20] and expanded using Fourier
techniques [21]. The final cycle of full-matrix least-square refine-
ment on F2 was based on observed reflections and variable param-
eters. The non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were refined using rigid model. Neutral atom
scattering factors were taken from Cromer and Waber [22]. All
other calculations were performed using the Crystal Structure
[23,24] Crystallographic Software Package.
[Cu(L1)2] 1. Yield 88% (0.55 g). Colour: Olive-green powder.
Anal. Calc. for C38H44N2O2Cu: C, 73.11; H, 7.10; N, 4.49. Found: C,
73.26; H, 7.09; N, 4.44%.
[Cu(L2)2] 2. Yield 74% (0.57 g). Colour: Reddish brown powder.
Anal. Calc. for C46H62N4O2Cu: C, 72.08; H, 8.15; N, 7.31. Found: C,
71.88; H, 7.97; N, 7.15%.
[Cu(L3)2] 3. Yield 63% (0.53 g). Colour: Dark green crystalline
solid. Anal. Calc. for C54H76N2O2Cu: C, 76.42; H, 9.03; N, 3.38.
Found: C, 76.22; H, 8.95; N, 3.25%.
[Cu(L4)2] 4. Yield 81% (0.59 g). Colour: Greenish brown powder.
Anal. Calc. for C46H48N2O2Cu: C, 76.27; H, 6.68; N, 3.87. Found: C,
76.31; H, 6.72; N, 3.90%.
[Cu(L5)2] ꢀ CH3OH 5. Yield 60% (0.45 g). Colour: Brown crystal-
line solid. Anal. Calc. for C41H54N4O5Cu: C, 65.97; H, 7.29; N, 7.51.
Found: C, 66.01; H, 7.33; N, 7.54%.
The single crystal diffraction experiments for 5 were carried
out on a Bruker SMART APEX CCD diffractometer at 100 K, and
the scan range was 1.08–28.37°. The crystallographic data and
details of data collection are given in Table 1. The data integra-
tion and reduction were processed with SAINT [25] software. An
empirical absorption correction was applied to the collected
reflections with SADABS [26]. The structure was solved by direct
methods using SHELXTL [27] and was refined on F2 by the full-ma-
trix least-squares technique using the SHELXL-97 [28] program
package. The molecular structure was drawn using ORTEP. All
the non-hydrogen atoms were refined anisotropically till conver-
gence is reached, and the hydrogen atoms attached to the ligand
moieties are stereochemically fixed and refined isotropically. The
selected bond lengths and bond angles are collected in Table 2
for 1 and 5.
2.4. Physical measurements
Elemental analyses (CHN) were performed in an elemental ana-
lyzer at Bharathiar University, Coimbatore. The diffuse reflectance
spectra were measured on a Hitachi U-3410 double-beam UV–Vis–
NIR spectrophotometer. The solution spectra were measured on
Varian 300 double beam UV–Vis spectrophotometer. FT-IR spectra
(4000–400 cmꢁ1 range) were recorded on a Perkin–Elmer FT-IR
spectrometer. EPR spectra were obtained on a JEOL JES-TE 100 X-
band spectrometer, calibrated with diphenylpicrylhydrazyl (dpph).
The spectra of the polycrystalline samples were measured at ambi-
ent temperature, while those of the solutions were recorded at li-
quid nitrogen temperature. Conductivity measurements on DMF
solution of complexes 1, 2, 4 and 5 and dichloromethane solution
of complex 3 were made using Elico conductivity bridge. Cyclic
voltammetry (CV) and differential pulse voltammetry (DPV) were
performed using a three-electrode cell configuration. A platinum
sphere, a platinum plate and SCE were used as working, auxiliary
and reference electrodes, respectively. The supporting electrolyte
used was NBu4ClO4. Platinum sphere electrode was sonicated for
two minutes in dilute nitric acid, dilute hydrazine hydrate and
3. Results and discussion
3.1. Synthesis
The Schiff base ligands H(L1)–H(L5) were synthesized by the
condensation of simple [16] and differently substituted salicylalde-
hydes, 2-hydroxy-1-napthaldehyde and pyridoxal [19] with the
bulky 2,6-diisopropylaniline in 1:1 molar ratio in methanol. The
1H NMR spectra of the ligands confirmed the identity of the
ligands. The Schiff base ligands H(L1), H(L2) and H(L4) readily
formed complexes [Cu(L1)2] 1, [Cu(L2)2] 2 and [Cu(L4)2] 4, respec-
tively, on treating 2 equiv. of them with 1 equiv. of copper(II)
chloride dihydrate in methanol at room temperature. Complexes
[Cu(L3)2] 3 and [Cu(L5)2] 5 were obtained as crystalline solids on
slow evaporation of the solution obtained by treating copper(II)

202
K. Sundaravel et al. / Inorganica Chimica Acta 362 (2009) 199–207
Table 1
eral formula [CuL2], which are confirmed by determining the X-ray
crystal structures of [Cu(L1)2] 1 and [Cu(L5)2] ꢀ CH3OH 5. Conduc-
tivity studies show that complexes 1, 2, 4 and 5 in DMF and com-
Crystal data and structure refinement details for [Cu(L1)2] 1 and [Cu(L5)2] ꢀ CH3OH 5
1
5
plex 3 in dichloromethane (4–12
X
ꢁ1 cm2 Mꢁ1) behave as non-
Empirical formula
Formula weight
Crystal colour, habit
Crystal size (mm)
Crystal system
Space group
a (Å)
C38H44N2O2Cu
624.32
green, block
C40H50N4O4Cu ꢀ CH3OH
746.45
brown, block
electrolytes. The IR spectral data of the complexes support the
coordination of the ligands to copper(II). The disappearance of a
broad band in the 2500–2800 cmꢁ1 region due to intramolecular
H-bonds involving phenolic OH group in the ligands on complexa-
tion indicates that coordination takes place via the phenolic OH
group. The C@N stretching vibration of the ligands (1625–
0.45 ꢂ 0.40 ꢂ 0.20 0.12 ꢂ 0.34 ꢂ 0.47
triclinic
P1 (#2)
triclinic
P1 (#2)
ꢀ
ꢀ
7.881(2)
10.151(4)
11.137(3)
113.58(1)
97.22(1)
97.63(2)
793.5(5)
1
8.4219(10)
12.4577(15)
18.905(2)
95.653(2)
92.035(2)
104.323(2)
1908.8(4)
2
b (Å)
c (Å)
1635 cmꢁ1) are shifted to lower frequencies (1596–1608 cmꢁ1
)
a
(°)
upon complex formation revealing coordination of the imine nitro-
gen atom.
b (°)
c
(°)
V (Å3)
Z
3.2. Description of the structures of [Cu(L1)2] 1 and [Cu(L5)2] ꢀ CH3OH
Mo K
a
, k (Å)
0.71073
1.306
1.015
0.71073
1.299
1.050
5
qcalc (g cmꢁ3
)
Goodness-of-fit on F2
The ORTEP representations of structures of complexes [Cu(L1)2]
1 and [Cu(L5)2] 5a including atom numbering scheme are shown in
Figs. 1 and 2, respectively. The crystallographic data are given in
Table 1, while the selected bond lengths and bond angles are col-
lected in Table 2. Complex 1 contains a square planar CuN2O2 chro-
mophore constituted by two imine nitrogen (N1, N1_2) and two
phenolate oxygen (O1, O1_2) atoms of the symmetrically disposed
bidentate H(L1) ligands. The molecule has a center of symmetry
with copper(II) located at the center of inversion of the perfect
CuN2O2 coordination plane with the angle between the two
N–Cu–O planes being 0.02°. The Cu–Ophenolate (1.8733(10) Å) and
Cu–Nimine (2.0015(13) Å) bond distances are within the range
Total number of reflections
7635 (0.018)
16633 (0.059)
measured (Rint
)
Number of unique reflections
Number of refined parameters
3571
218
8698
478
Residuals [I > 2
r
(I)]
Ra
0.028
0.068
0.0724
0.1619
b
wR2
P
P
a
R ¼
R ¼
jj Foj ꢁ jFcjj= jFoj.
ꢂ
ꢃ
ꢄ
1=2
hꢀ
ꢁi
ꢀ
ꢁ
2
2
P
P
w
F2o ꢁ F2c
w
Fo2
.
b
chloride dihydrate with 2 equiv., respectively, of H(L3) and H(L5)
pretreated with 2 equiv. of triethylamine in methanol to abstract
the ligand phenolic hydrogen atom. The stoichiometries of the
complexes derived from elemental analysis correspond to the gen-
reported earlier [11]. The sum
R of the six inter-bond angles [29]
for complex 1 is 720° suggesting that the geometry around
copper(II) ion is square planar. The cis angles in the basal plane
involving the phenolate oxygen and imine nitrogen (O1–Cu1–
N1 = 91.80(5)°, O1–Cu(1)–N1_2 = 88.20(5)°) show deviation from
the ideal value of 90° suggesting that the square planar coordina-
tion geometry in 1 is slightly distorted due to chelation effect. As
expected, the two N-phenyl rings are arranged in trans rather than
cis positions in order to avoid the steric hindrance exerted by the
isopropyl groups at 2,6-positions of the aniline moieties in cis
arrangement. Also, interestingly, the N-phenyl ring of aniline moi-
ety is rotated with the mean plane angle between CuN2O2 (N1–O1–
Cu1–N1_1–O1_1) and 2,6-diisopropylphenyl ring being 76.92°, and
the isopropyl groups are pushed away from the sal-moieties to
avoid any steric clash between sal-moiety and the bulky isopropyl
Table 2
Selected bond lengths (Å) and bond angles (°) for [Cu(L1)2] 1 and [Cu(L5)2] ꢀ CH3OH 5
1
5
Cu(1)–N(1)
Cu(1)–O(1)
O(1)–C(1)
N(1)–C(7)
N(1)–C(8)
C(1)–C(6)
C(6)–C(7)
2.0015(13)
1.8733(10)
1.296(2)
1.289(2)
1.4470(16)
1.422(2)
Cu(1)–N(2)
Cu(1)–O(1)
O(1)–C(1)
N(2)–C(6)
N(2)–C(7)
C(1)–C(5)
1.987(3)
1.904(2)
1.299(4)
1.292(5)
1.454(4)
1.404(5)
1.444(5)
1.997(3)
1.884(3)
1.304(4)
1.308(5)
1.451(4)
1.392(5)
1.441(5)
1.4334(18)
C(5)–C(6)
Cu(2)–N(4)
Cu(2)–O(3)
O(3)–C(21)
N(4)–C(26)
N(4)–C(27)
C(21)–C(25)
C(25)–C(26)
C4
C3
C5
C15
C14
C16
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(1_2)
O(1_2)–Cu(1)–N(1_2)
O(1_2)–Cu(1)–N(1)
N(1)–C(7)–C(6)
91.80(5)
88.20(5)
91.80(5)
88.20(5)
126.47(14)
124.10(12)
124.48(9)
119.38(10)
129.96(10)
116.14(13)
O(1)–Cu(1)–N(2)
O(1)–Cu(1)–N(2_2)
O(1_2)–Cu(1)–N(2_2)
O(1_2)–Cu(1)–N(2)
N(2)–C(6)–C(5)
91.30(11)
88.69(11)
91.31(11)
88.69(11)
126.6(3)
124.8(3)
125.0(2)
121.9(2)
129.2(2)
113.1(3)
91.49(11)
88.51(11)
91.49(11)
88.51(11)
126.3(3)
126.0(3)
125.0(2)
121.9(2)
129. 5(2)
C2
C6
C9
C7
C1
O1
C10
N1
O(1)–C(1)–C(6)
O(1)–C(1)–C(5)
Cu(1)–N(1)–C(7)
Cu(1)–N(1)–C(8)
Cu(1)–O(1)–C(1)
C(7)–N(1)–C(8)
Cu(1)–N(2)–C(6)
Cu(1)–N(2)–C(7)
Cu(1)–O(1)–C(1)
C(6)–N(2)–C(7)
C8
Cu1
C11
O1_2
C13
C17
N1_2
O(3)–Cu(2)–N(4)
O(3)–Cu(2)–N(4_2)
O(3_2)–Cu(2)–N(4_2)
O(3_2)–Cu(2)–N(4)
N(4)–C(26)–C(25)
O(3)–C(21)–C(25)
Cu(2)–N(4)–C(26)
Cu(2)–N(4)–C(27)
Cu(2)–O(3)–C(21)
C(26)–N(4)–C(27)
C12
C18
C19
Fig. 1. ORTEP drawing of [Cu(L1)2] 1 showing the atom numbering scheme and the
thermal motion ellipsoids (50% probability level) for the non-hydrogen atoms.
Hydrogen atoms are omitted for clarity.

K. Sundaravel et al. / Inorganica Chimica Acta 362 (2009) 199–207
203
along with one molecule of methanol as solvent of crystallization.
It is interesting to note that both the molecules possess a center of
inversion located at copper(II) atom with the symmetrically dis-
posed ligand molecules wrapped around the metal. The copper
atom in the complex molecule is coordinated to two phenolate
oxygen and two imine nitrogen atoms. The coordination geometry
around copper(II) is similar to that in 1 and is best visualized as
O2
C3
C13
C4
N1
C2
C5
C16
C1
C6
C14
C15
C17
square planar (Cu1,
R = 720°; Cu2, R = 720°). The molecule has a
center of symmetry and the angle between the two N–Cu1–O che-
late ring planes is 0.02°. The Cu1–Ophenolate (1.904(2) Å) and Cu1–
Nimine (1.987(3) Å) bond distances and O1–Cu1–N2 (91.30(11)°),
O1–Cu1–N2_2 (88.69(11)°) bond angles fall in the range of values
found for [Cu(L6)2] [11]. The Cu–Ophenolate bond distance
(1.904(2) Å) in 5 is longer than that (1.8733(10) Å) in 1. Concomi-
tantly, the Cu(1)–Nimine bond is shorter than that in 1. This may be
due to the electronic effects of heterocyclic ring of the pyridoxal
moiety and/or the steric hindrance to phenolate coordination from
the methyl substituent in the pyridoxal ring. Also, the mean plane
angle between the CuN2O2 coordination plane (O1–N2–Cu1–
O1_2–N2_2) and the 2,6-diisopropylphenyl ring (Cu1, 80.60°) in
5 is higher than that in 1 (76.92°), supporting the presence of steric
hindrance to phenolate coordination from the methyl substituent
in the pyridoxal ring leading to the higher deviation of 2,6-diiso-
propylphenyl ring from the CuN2O2 plane. Further, for the second
complex molecule the Cu2–Nimine (1.997(3) Å) and Cu2–Ophenolate
(1.884(3) Å) bond distances are longer and shorter than those in
the first molecule with Cu1 center in the same unit cell. Also, the
O1–Cu2–N2 (91.49(11)°), O1–Cu2–N2_2 (88.51(11)°) bond angles
O1
C8
N2
C9
C7
C20
C18
Cu1
C10
C12
C19
C11
N2_2
O1_2
Fig. 2. ORTEP diagram of one of the molecule present in the unit cell of [Cu(L5)2] 5a
showing the atom numbering scheme (50% probability factor for the thermal elli-
psoids for the non-hydrogen atoms). Lattice methanol solvent and hydrogen atoms
and are omitted for clarity.
groups. The bond lengths and bond angles are comparable with
those (Cu–O, 1.859(5) Å; Cu–N, 2.008(6) Å; O–Cu–N, 90.9(2)° and
O–Cu–N, 89.1(2)°) in the analogous complex [Cu(L6)2], where
H(L6) is 5-dimethylamino-2-(4-nitrophenyliminomethyl)phenol
(Scheme 1), and the geometry around the copper(II) center is
planar [11]. On the other hand, the coordination geometries of
[Cu(L7)2] [7], where H(L7) is 2-(2,6-diisopropylphenyliminomethyl)-
6-nitrophenol, and [Cu(L8)2] [13], where H(L8) is 2,4-dibromo-6-
(o-tolyliminomethyl)phenol, both with CuN2O2 chromo-phore are
distorted as evident from the sum of the six inter-bond angles
681.7° and 677.9°, respectively, which deviate from the ideal angle
of 720°.
and the
R value (720°) are similar to those in the first molecule.
The mean plane angle between the CuN2O2 coordination plane
(N4–Cu2–O3_2–N4_2) and the 2,6-diisopropylphenyl ring
(83.73°) is higher than that in the first molecule.
In an attempt to understand the molecular association the vari-
ous hydrogen bonding interactions between the two molecules
present in the asymmetric unit cell in 5 have been analyzed in
detail. As mentioned earlier, each of the two molecules present in
the unit cell possesses a center of symmetry located at the metal
center. The packing diagram of the compound viewed down a-axis
shows various hydrogen bonding interactions as depicted in Fig. 2a.
The two molecules of the complex moiety present in the unit cell
are oriented and aligned almost diagonal to the bc-plane and are
Complex [Cu(L5)2] ꢀ CH3OH 5 crystallizes in the triclinic space
ꢀ
group P1 with two independent molecules in the same unit cell
Fig. 2a. Packing diagram of the [Cu(L5)2] ꢀ CH3OH 5 viewed down a-axis showing the various H-bonding interactions in the compound (only the hydrogen atoms involved in
hydrogen bonding interactions are included for clarity).

204
K. Sundaravel et al. / Inorganica Chimica Acta 362 (2009) 199–207
Fig. 2b. Close-up view of the hydrogen bonding interactions (dotted black line) between the two independent molecules of [Cu(L5)2] present in the unit cell via O–Hꢀ ꢀ ꢀN
interaction generating a two-dimensional hydrogen bonded network along bc-plane. (only hydrogen atom involved in hydrogen bonding interactions are shown for clarity).
involved in intermolecular O–Hꢀ ꢀ ꢀN hydrogen bonding interactions
from the ligand moiety symmetrically wrapped around the metal
center from either side generating a layered hydrogen-bonded net-
work. The pertinent hydrogen bonding interactions are O(2)–
H(2)ꢀ ꢀ ꢀN(3): H(2)ꢀ ꢀ ꢀN(3) = 1.97 Å, O(2)ꢀ ꢀ ꢀ N(3) = 2.773(4) Å,
\O(2)–H(2)ꢀ ꢀ ꢀN(3) = 164° (symmetry code: x,1 + y, z). These layers
Table 3
Electronic absorption and EPR spectral properties of Copper(II) complexes
Complex
Electronic spectraa
EPR spectrab
kmax, nm (
Solid
e )
max, Mꢁ1 cmꢁ1
Solution
LF
Solid
Frozen solution
DMF
LF
CT
[Cu(L1)2] 1
[Cu(L2)2] 2
[Cu(L3)2] 3
[Cu(L4)2] 4
DMF
680
505
710 (75)
560 sh
380 (6150)
g
2.211
g
A
2.260, g\ 2.059
153
k
k
k
g\ 2.046
g =A 148 cm
G = 4.406
k
k
DMF
680 sh
475 sh
DMF
g
A
g =A 118 cm
G = 4.434
675
540
350 (15840)
395 (10780)
405 (17890)
g
2.198
2.204, g\ 2.046
187
k
k
k
g\ 2.051
k
k
DCM
650 sh
490 (1200)
DCM
g
A
g =A 142 cm
G = 3.904
670
510
g
2.238
2.246, g\ 2.063
158
k
k
k
g\ 2.061
k
k
DMF
DMF
gk 2.257, g\ 2.060
A
665
505
660 shc
gk 2.215
g\ 2.076
175
k
A
195
g =A 129 cm
k
k
G = 4.283
k
g =A 114 cm
k
k
G = 2.820
[Cu(L5)2] ꢀ CH3OH 5
DMF
700 sh
625
DMF
gk 2.254, g\ 2.057
A 169
k
690
395 (10650)
g 2.298
g\ 2.073
j
A
187
g =A 134 cm
k
k
G = 4.456
k
g =A 123 cm
k
k
G = 4.082
a
Concentration, 4 ꢂ 10ꢁ3 M for ligand field and 2.4 ꢂ 10ꢁ5 M for ligand based transition.
b
c
A
values in 10ꢁ4 cmꢁ1 units.
k
broad.

K. Sundaravel et al. / Inorganica Chimica Acta 362 (2009) 199–207
205
are further cross-linked via another intramolecular O–Hꢀ ꢀ ꢀN inter-
action from the second terminal alcoholic moiety of the ligand with
the pyridyl nitrogen N4 generating a two-dimensional hydrogen-
bonded network (Fig. 2b). The details of this hydrogen bonding
interactions are O(4)–H(4)ꢀꢀꢀN(1): H(4)ꢀꢀꢀN(1) = 1.99 Å, O(4)ꢀꢀ ꢀN(1)
= 2.774(4) Å, \O(4)–H(4)ꢀ ꢀ ꢀN(1) = 159°. The lattice methanol
hydrogen H5 is involved in a strong intermolecular O–Hꢀ ꢀ ꢀO hydro-
gen bonding with O2, which acts as an acceptor to stabilize the mol-
ecule in the crystal lattice (Fig. 2a). The details of this interaction
with symmetry code are O(5)–H(5)ꢀ ꢀ ꢀO(2): H(5)ꢀ ꢀ ꢀO(2) = 2.04 Å,
O(5)ꢀ ꢀ ꢀO(2) = 2.857(5) Å, \O(5)–H(5)ꢀ ꢀ ꢀO(2) = 176° (symmetry
code: ꢁ1 + x, y, z).
3.3. Spectral properties
3.3.1. Electronic absorption spectra
The solid state reflectance spectra of all the complexes exhibit
two well or poorly separated ligand field bands (690–505 nm) in
the visible region (Table 3), which are typical of Cu(II) present in
a square planar coordination geometry [30]. On dissolution of 2
and 4 in DMF only one visible band is observed and the high-en-
ergy ligand field bands are possibly merged with the intense
ligand-to-metal charge transfer band (lmct). There is no significant
change in the low-energy ligand field band of 2, 4 and 5 on disso-
lution suggesting that the square planar coordination geometry is
maintained in solution. On the other hand, the ligand field bands
undergo very small changes on dissolution of 1 in DMF suggesting
negligible changes in coordination geometry. Also, previous
electronic absorption spectral study on [Cu(L1)2] has shown that
the complex is square planar in both the solid state and in chloro-
form solution [16]. On dissolution of 3 in dichloromethane the
ligand field bands are observed as two shoulders with small
changes. The intense band observed for all the complexes in the
region 350–405 nm is assigned to phenolate-to-Cu(II) lmct
transition.
3.3.2. EPR spectra
The ambient temperature polycrystalline EPR spectra of all the
complexes are axial, which is consistent with the square planar
geometries of 1 and 5 found in their X-ray structures. The cryo-
genic solution EPR spectra (Table 3) of all the complexes (Fig. 3)
are axial ½gk > g? > 2:0, G ¼ ðgk ꢁ 2Þ=ðg? ꢁ 2Þ ¼ 3:9—4:5ꢃ suggest-
ing the presence of a dx2
ground state for copper(II) located in
ꢁy2
square-based geometries [31–34]. The gk and A values for square
k
planar CuN4 chromophore [35] are around 2.200 and
200 ꢂ 10ꢁ4 cmꢁ1, respectively. On replacing the nitrogen donors
Fig. 3. X-band EPR spectra of the complexes [Cu(L1)2] 1 (a), [Cu(L2)2] 2 (b) in DMF/
acetone glass at 77 K.
by oxygen donors the g value increases and the A value decreases
k
k
and thus the gk and A values for a CuO4 chromophore [18] are
k
around 2.420 and 145 ꢂ 10ꢁ4 cmꢁ1, respectively. Several CuN2O2
chromophores have been shown to possess g and A values around
oxidation state in the planar geometry rather than to distort the
planarity of the CuN2O2 chromophore. On incorporating a fused
aromatic ring in 1 to obtain 4 the geometrical distortion is lowered
as revealed by the lower values of the g =A quotient (129 cm) for
4. Thus the electronic effects in 2 overwhelm the steric effects in 1,
3 and 4 leading to impose a regular square planar coordination
geometry on the former. The bulky isopropyl groups on 2,6-posi-
tions in N-aryl groups as in the present complexes tend to confer
only a slight geometrical distortion on the CuN2O2 coordination
plane.
k
k
2.253 and 160 ꢂ 10ꢁ4 cmꢁ1 [36,15]. Thus all the present com-
plexes, except 2, exhibit gk (2.246–2.260) and A values (153–
k
175 ꢂ 10ꢁ4 cmꢁ1) consistent with a CuN2O2 chromophore. On the
k
k
other hand, the gk (2.204) and A (187 ꢂ 10ꢁ4 cmꢁ1) values of 2
k
are, respectively, lower and higher than those observed for all
other complexes. Such lower g and higher A values suggest delo-
k
k
calization of the unpaired electron on copper(II) making the coor-
dination plane more planar [15]. This is supported by the lower
values of the g =A index (118 cm) for the complex, which falls
k
k
within the range (105–135 cm) expected for perfectly square pla-
nar copper(II) coordination geometry [15]. In contrast to 2, the
However, only 1, 3, 4 and 5 show slight distortions in solution.
On introducing the –NEt2 group in para position of the phenyl ring
as in 2 the delocalization of the unpaired electron density (Scheme
2) is enhanced leading to a more planar coordination geometry.
This is in contrast to the similarity in electronic spectral parame-
ters of 1 and 2 suggesting the usefulness of EPR spectroscopy in
detecting even small differences in the coordination geometries.
g =A values for 1, 3, 4 and 5 are higher (129–148 cm) revealing
k
k
that the square planar coordination geometries of the complexes
are distorted. Thus, on incorporating the tert-butyl groups in 1 to
obtain 3, the g and g =A values decrease suggesting that the elec-
k
k
k
tron-releasing effect of the tert-butyl group is to stabilize Cu(II)

206
K. Sundaravel et al. / Inorganica Chimica Acta 362 (2009) 199–207
N
N
N
Cu
Cu
Cu
1/2
O
1/2
1/2
N
O
N
O
[Cu(L2)2] 2
[Cu(L4)2] 4
Scheme 2. Electron delocalization in the Sal-ring of L2 in [Cu(L2)2] 2.
Scheme 3. Electron delocalization in the aromatic rings of L2 in [Cu(L2)2] 2 and L4
in [Cu(L4)2] 4.
Table 4
Electrochemical dataa for copper(II) complexes at 25 0.2 °C in DMF solution
4. Conclusions
Complex
Epc (V)
E1/2 (V)
D (10ꢁ6 cm2 sꢁ1
)
c
A few copper(II) bis-ligand complexes of sterically hindered
bidentate Schiff base ligands obtained by the condensation of sim-
ple and substituted salicylaldehydes, 2-hydroxy-1-napthaldehyde
and pyridoxal with 2,6-diisopropylaniline have been isolated and
studied. The results from electronic and EPR spectral studies are
consistent with the X-ray crystal structures of two of the com-
plexes with a square planar CuN2O2 chromophore. Electrochemical
studies reveal that the introduction of electron-releasing groups
and fusion of a benzene ring in the sal-ring of the complexes render
the reduction of Cu(II) difficult. The bulky 2,6-diisopropyl groups
on the N-aryl ring in the Schiff base complexes tend to impose only
a slight geometric distortion on the square planar coordination
geometry.
[Cu(L1)2] 1
[Cu(L2)2] 2
[Cu(L3)2] 3b
[Cu(L4)2] 4
[Cu(L5)2] 5
ꢁ0.898
ꢁ0.996
ꢁ1.222
ꢁ1.088
ꢁ1.124
ꢁ0.825
ꢁ0.851
ꢁ1.019
ꢁ0.931
ꢁ0.937
0.90
0.60
0.12
0.37
0.15
a
Potential measured vs. SCE reference electrode; scan rate 50 mV sꢁ1; supporting
electrolyte: tetra-N-butylammonium perchlorate (0.1 M); complex concentration:
0.001 M.
b
solvent used: dichloromethane/DMF mixture (1:4 v/v).
c
Differential pulse voltammetry (DPV); scan rate 5 mV sꢁ1; pulse height: 50 mV.
50
a
Acknowledgements
40
30
b
We sincerely thank the Council of Scientific and Industrial Re-
search, New Delhi for a Junior Research Fellowship to K.S. Professor
M. Palaniandavar is a recipient of Ramanna Fellowship of Depart-
ment of Science and Technology, New Delhi. We thank Professor
Y. Ishii, Department of Applied Chemistry, Chuo University, Japan
for providing X-ray facility. One of us (MP) thanks Professor M.
Chikira, Department of Applied Chemistry, Chuo University, Japan
for hospitality and DST-JSPS for an Exchange Fellowship. We thank
Dr. P. Sambasiva Rao, Pondicherry University, Pondicherry for pro-
viding EPR facility and Professor K. Natarajan, Bharathiar Univer-
sity, Coimbatore for CHN analysis.
20
10
0
-10
-0.3
-0.5
-0.7
-0.9
-1.1
-1.3
E (V)
Fig. 4. Differential pulse voltammograms of the complexes [Cu(L1)2] 1 (a), and
[Cu(L2)2] 2 (b) in DMF solution at 25 °C at a scan rate of 0.05 V sꢁ1. Complex con-
centration 0.001 M.
Appendix A. Supplementary material
3.3.3. Electrochemical behaviour
CCDC 619122 and 643359 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2008.03.083.
The electrochemical behaviour of all the complexes, except 3,
was investigated in DMF:dichloromethane (4:1 v/v) solution by
employing cyclic (CV) and differential pulse voltammetry (DPV)
on a stationary platinum sphere electrode. All the complexes exhi-
bit a reduction wave at negative potentials, which is characteristic
[17] of irreversible CuII ? CuI couple. The E1/2 values (DPV, Table 4,
Fig. 4) of the CuII/CuI couple fall in the range ꢁ 0.825 to ꢁ 1.019 V,
which is consistent with those of copper(II) bis-ligand complexes
with CuN2O2 chromophore, and the E1/2 value of the complexes
in DMF solvent increases in the order 1 > 2 > 4. Thus, upon intro-
ducing the electron-releasing –NEt2 group in 1 to obtain 2 the E1/2
value decreases suggesting that the Cu(II) oxidation state in 2 is
stabilized more than that in 1 due to the electron-releasing –
NEt2 group (Scheme 3). Similarly, upon fusing a planar benzene
ring in 1 to obtain 4, the enhanced electron delocalization in the
aromatic ring (Scheme 3) leads to stabilize the Cu(II) oxidation
state in 4 more than that in 1, as observed from the more negative
E1/2 value (ꢁ0.931 V) of 4. These observations are consistent with
the enhanced planarity of CuN2O2 chromophore in 2 and 4 as
References
[1] (a) Y. Sunatsuki, Y. Motoda, N. Matsumoto, Coord. Chem. Rev. 226 (2002) 199;
(b) Chi-Ming Che, Jie-Sheng Huang, Coord. Chem. Rev. 242 (2003) 27.
[2] (a) H. Khanmohammadi, S. Amani, H. Lang, T. Rüeffer, Inorg. Chim. Acta 360
(2007) 579;
(b) P. DiBernardo, P.L. Zanonato, S. Tamburini, P.A. Vigato, Inorg. Chim. Acta
360 (2007) 1083.
[3] (a) R.T. Ruck, E.N. Jacobsen, J. Am. Chem. Soc. 124 (2002) 2882;
(b) S. Adsule, V. Barve, D. Chen, F. Ahmed, Q.P. Dou, S. Padhye, F.H. Sarkar, J.
Med. Chem. 49 (2006) 7242.
[4] (a) D.J. de Geest Andy Noble, B. Moubaraki, K.S. Murray, D.S. Larsen, S. Brooker,
Dalton Trans. (2007) 467;
(b) U. Beckmann, S. Brooker, Coord. Chem. Rev. 222 (2003) 17;
(c) S. Brooker, Coord. Chem. Rev. 222 (2001) 33.
[5] B. De Clercq, F. Verpoort, Macromolecules 35 (2002) 894.
[6] T. Opstal, F. Verpoort, Angew. Chem., Int. Ed. 42 (2003) 2876.
revealed by their lower values of g =A index (cf. above).
k
k

K. Sundaravel et al. / Inorganica Chimica Acta 362 (2009) 199–207
207
[7] C. Carlini, S. Giaiacopi, F. Marchetti, C. Pinzino, A.M.R. Galleti, G. Sbrana,
Organometallics 25 (2006).
[8] S.N. Pal, S. Pal, Inorg. Chem. 40 (2001) 4807.
[9] (a) M. Tümer, B. Erdog˘an, H. Köksal, S. Serin, M.Y. Nutku, Synth. React. Inorg.
Met. Org. Chem. 28 (1998) 529;
[21] P.T. Beurskens, G. Admiraal, G. Beurskens,W.P. Bosman, R. De Gelder, R. Israel,
J.M.M. Smits, DIRDIF99: The DIRDIF-99 program system, Technical Report of the
Crystallography Laboratory, University of Nijmegen, The Netherlands, 1999.
[22] D.T. Cromer, J.T. Waber, International Tables for X-ray Crystallography, vol. IV,
The Kynoch Press, Birmingham, England, 1974. Table 2.2 A.
(b) J.W. Pyrz, A.I. Roe, L.J. Stern, J.R. Que, J. Am. Chem. Soc. 107 (1985) 614.
[10] (a) W. Zishen, W. Huixia, Y. Zhenhuan, H. Changhai, XXV, International
Conference on Coordination Chemistry, Book of Abstracts, 1987, p. 663.;
(b) W. Zishen, L. Zhiping, Y. Zhenhuan, Trans. Met. Chem. 18 (1993) 291.
[11] P.G. Lacroix, F. Averseng, I. Malfant, K. Nakatani, Inorg. Chim. Acta 357 (2004)
3825, and references therein.
[23] Crystal Structure 3.00: Crystal Structure Analysis Package, Rigaku and Rigaku/
MSC, 2000–2002.
[24] D.J. Watkin, C.K. Prout, J.R. Carruthers, P.W. Betteridge, CRYSTALS Issue 10:
Chemical Crystallography Laboratory, Oxford, UK, 1996.
[25] G.M. Sheldrick, SAINT and XPREP, 5.1 ed., Siemens Industrial Automation Inc.,
Madison, Wisconsin, 1995.
[12] (a) R. Knock, A. Wilk, K.J. Wannowius, D. Reinen, H. Elias, Inorg. Chem. 29
(1990) 3799;
[26] SADABS, Empirical Absorption Correction Program, University of Göttingen,
Göttingen, Germany, 1997.
(b) E.C. Lingafelter, G.L. Simmons, B. Morosin, C. Scheringer, C. freiburg, Acta
Crystallogr., Sect. C 14 (1961) 1222;
[27] G.M. Sheldrick, SHELXTL Reference Manual: Version 5.1, Bruker AXS, Madison,
WI, 1997.
(c) T.C. Cheeseman, D. Hall, T.N. Waters, J. Chem. Soc. A (1966) 685.
[13] V. Manriquez, J. Costamagna, J. Vargas, H.G. Von Schnerir, K. Peters, Acta
Crystallogr., Sect. C 46 (1990) 1823.
[14] (a) H.S. Maslen, T.N. Waters, Coord. Chem. Rev. 17 (1975) 137;
(b) A.D. Garnovskii, A.L. Nivorozhkin, V.I. Minkin, Coord. Chem. Rev. 126
(1993) 1.
[15] (a) U. Sakaguchi, A.W. Addison, J. Chem. Soc., Dalton Trans. (1979) 600;
(b) A.W. Addison, in: K.D. Karlin, J. Zubeita (Eds.), Copper Coordination
Chemistry: Biochemical and Inorganic Perspectives, Adenine Press,
Guilderland, New York, 1983, p. 109.
[16] S. Yamada, K. Yamanouchi, Bull. Chem. Soc. Jpn. 55 (1982) 1083.
[17] M. Aguilar-Martinez, R. Saloma-Aguilar, N. Marcias Ruvalcaba, R. Cetina-
Rosado, A. Navarrete-Vasquez, V. Gomez-Vidales, A. Zentella-Dehesa, R.A.
Toscano, S. Hernandez-Ortega, J.M. Fernandez-G, J. Chem. Soc., Dalton Trans.
(2001) 2346.
[18] (a) D.X. West, M. Palaniandavar, Inorg. Chim. Acta 71 (1983) 61;
(b) D.X. West, M. Palaniandavar, Inorg. Chim. Acta 76 (1983) L149;
(c) D.X. West, M. Palaniandavar, Inorg. Chim. Acta 77 (1983) L97.
[19] J.T. Wroleski, G.J. Long, Inorg. Chem. 16 (1977) 2752.
[20] P.T. Beurskens, G. Admiraal, G. Beurskens, W.P. Bosman, S. Garcia-Granda, R.O.
Gould, J.M.M. Smits, C. Smykalla, PATTY: The DIRDIF program system, Technical
Report of the Crystallography Laboratory, University of Nijmegen, The
Netherlands, 1992.
[28] G.M. Sheldrick, SHELXL-97: Program for Crystal Structure Refinement, University
of Göttingen, Göttingen, Germany, 1997.
[29] E.C. Constable, C.E. Housecroft, B.M. Kariuki, N. Kelly, C.B. Smith, C.R. Chim. 5
(2002) 425.
[30] F.J. Richardson, C.N. Payne, Inorg. Chem. 17 (1978) 2111.
[31] (a) W. Fitzgerald, B.J. Hathaway, J. Chem. Soc., Dalton Trans. (1981) 567;
(b) B.J. Hathaway, D.E. Billing, Coord. Chem. Rev. 5 (1970) 143.
[32] M. Palaniandavar, T. Pandiyan, M. Lakshminarayanan, H. Manohar, J. Chem,
Soc., Dalton Trans. (1995) 455.
[33] A.W. Addison, in: K.D. Karlin, J. Zubieta (Eds.), Copper Coordination Chemistry:
Biochemical and Inorganic Prespectives, Adenine Press, New York, 1983.
[34] M. Vaidyanathan, R. Viswanathan, M. Palaniandavar, T. Balasubramanian, P.
Prabhaharan, P.T. Muthiah, Inorg. Chem. 37 (1998) 6418.
[35] (a) J. Schugar, in: K.D. Karlin, J.
A Zubieta (Eds.), Copper Coordination
Chemistry: Biochemical and Inorganic Perspectives, Adenine Press,
Guilderland, New York, 1983, p. 49;
(b) H.J. Scholl, J. Huttermann, J. Phys. Chem. 96 (1992) 9684;
(c) A.W. Addison, M. Carpenter, L.M.K. Lau, M. Wicholas, Inorg. Chem. 17
(1978) 1545;
(d) P. Tamil Selvi, M. Murali, M. Palaniandavar, M. Kockerling, G. Henkel, Inorg.
Chim. Acta 340 (2002) 139.
[36] M. Vaidyanathan, M. Palaniandavar, R. Srinivasa Gopalan, Inorg. Chim. Acta
324 (2001) 241.