Synthesis, characterization and X-ray crystal structures of thiolate sulfur-bridged dimeric copper(II) complexes of the 2-aminoacetophenone Schiff base of S-methyldithiocarbazate

Article abstract of DOI:10.1016/j.poly.2012.08.024

Copper(II) complexes of general empirical formula, Cu(apsme)X (apsme = monodeprotonated form of the 2-aminoacetophenone Schiff base of S-methyldithiocarbazate (Hapsme); X = Cl^-, Br^-, NCS ^-) have been prepared and characterised by a variety of physico-chemical techniques. The crystal and molecular structures of the Schiff base, Hapsme (1), [Cu(apsme)Br] (2), [Cu(apsme)(NCS)] (3) and [Cu(apsme)Cl] (4) have been determined by X-ray diffraction. In the solid state the copper(II) complexes 2, 3 and 4 are dimers in which the anionic Schiff base, (apsme ^-) coordinates with a copper(II) ion as a tridentate NNS chelating agent via the amino nitrogen atom, the azomethine nitrogen atom and the thiolate sulfur atom. The fourth and fifth coordination positions of a copper atom are occupied by a co-ligand and bridging thiolate sulfur atoms, respectively.

Full text of DOI:10.1016/j.poly.2012.08.024

Polyhedron 47 (2012) 79–86
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Polyhedron
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Synthesis, characterization and X-ray crystal structures of thiolate
sulfur-bridged dimeric copper(II) complexes of the 2-aminoacetophenone
Schiff base of S-methyldithiocarbazate
a
a
a
Mohammad Akbar Ali a, , Aminul Huq Mirza , Malai Haniti S.A. Hamid , Nuzuhath Aminath ,
⇑
Paul V. Bernhardt b
a Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, BE 1410, Brunei Darussalam
b School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, QLD 4072, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Copper(II) complexes of general empirical formula, Cu(apsme)X (apsme = monodeprotonated form of the
2-aminoacetophenone Schiff base of S-methyldithiocarbazate (Hapsme); X = Clꢀ, Brꢀ, NCSꢀ) have been
prepared and characterised by a variety of physico-chemical techniques. The crystal and molecular struc-
tures of the Schiff base, Hapsme (1), [Cu(apsme)Br] (2), [Cu(apsme)(NCS)] (3) and [Cu(apsme)Cl] (4) have
been determined by X-ray diffraction. In the solid state the copper(II) complexes 2, 3 and 4 are dimers in
which the anionic Schiff base, (apsmeꢀ) coordinates with a copper(II) ion as a tridentate NNS chelating
agent via the amino nitrogen atom, the azomethine nitrogen atom and the thiolate sulfur atom. The
fourth and fifth coordination positions of a copper atom are occupied by a co-ligand and bridging thiolate
sulfur atoms, respectively.
Received 22 June 2012
Accepted 8 August 2012
Available online 17 August 2012
Keywords:
2-Aminoacetophenone Schiff base of
S-methyldithiocarbazate
Thiolate sulfur-bridged dimeric copper(II)
complexes
Copper(II) complexes of
2-aminoacetophenone Schiff base of
S-methyldithiocarbazate
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
to exhibit very high inhibitory activities against the pathogenic
fungi, Alternaria alternata and Curvularia geniculata [3].
Metal complexes of dithiocarbazic acid and its derivatives have
received considerable attention because of their interesting phys-
ico-chemical properties [1], varied structural features [2] and
potentially useful chemotherapeutic properties [3,4]. However,
most work on this class of compounds involved tridentate NNS
Schiff bases formed by condensation of heterocyclic aldehydes
and ketones with S-methyl- and S-benzyldithiocarbazates [5–9].
Schiff bases formed by condensation of 2-aminobenzaldehyde or
2-aminoacetophenone with S-alkyl/aryl dithiocarbazates did not
receive much attention. The presence of an amino nitrogen atom
in the NNS set of donor atoms in a tridentate dithiocarbazate li-
gand offers additional possibilities of intermolecular hydrogen
bond formation which may lead to metal complexes with three
dimensional network structures exhibiting interesting physical
and chemical properties. Also, transition metal complexes of this
type of ligands may exhibit useful chemotherapeutic properties.
In fact, several copper(II) complexes of the 2-aminobenzaldehyde
Schiff base of S-methyldithiocarbazate have already been found
In view of the paucity of X-ray structural data on copper(II)
complexes of this type of ligands and as part of our continued
interest on metal complexes of dithiocarbazates, we report here
the preparation, spectroscopic characterization and X-ray crystal
structures of three dimeric copper(II) complexes of Hapsme, to-
gether with the structure of the free ligand, 1.
2. Experimental
2.1. Reagents
Chemicals and solvents used were of analytical reagent grades
and used without any further purification. 2-Aminoacetophenone
and the copper salts were purchased from the Aldrich Chemical
Company and S-methyldithiocarbazate was prepared according
to the published procedure [10].
2.2. Physical measurements
Microanalyses for C, H and N were performed by the Elemental
Analysis Laboratory, National University of Singapore, Singapore.
Molar conductances of ca. 10ꢀ3 M solutions of the complexes in
⇑
Corresponding author. Present address: Apt. #101A, Plot #11/A, Road #71,
Gulshan-2, Dhaka 1212, Bangladesh. Tel.: +880 2 8822665.
E-mail address: palonkuri@yahoo.com (M. Akbar Ali).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.08.024

80
M. Akbar Ali et al. / Polyhedron 47 (2012) 79–86
Table 1
Crystal and refinement data.
Hapsme (1)
10H13N3S2
239.35
293
[Cu(apsme)Br] (2)
[Cu(apsme)(NCS)] (3)
[Cu(apsme)Cl] (4)
Formula
Formula weight
T (K)
C
C10H12BrCuN3S2
381.80
293
C11H12CuN4S3
359.97
293
C10H12ClCuN3S2
337.34
293
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
P1
monoclinic
P21/n
7.5409(6)
11.426(2)
15.235(2)
monoclinic
P21/a
7.926(7)
16.201(5)
12.183(6)
monoclinic
P21/n
7.5124(9)
11.326(2)
15.066(2)
ꢀ
5.7972(5)
11.688(2)
17.040(3)
102.171(1)
93.736(9)
99.151(9)
1108.4(2)
4
a
(°)
b (°)
95.110(10)
108.33(6)
94.99(2)
c
(°)
V (Å3)
Z
1307.5(3)
4
1485(2)
4
1277.0(3)
4
k (Å)
0.71073
1.434
0.450
0.71073
1.940
5.025
0.71073
1.610
1.882
0.71073
1.755
2.225
q
l
(g cmꢀ3
)
(mmꢀ1
)
hmin,max (°)
F(000)
3.37–25.00
504
2.23–24.97
756
1.76–24.99
732
2.25–24.99
684
Measured reflections
Independent reflections
Rint
Observed reflections
R1 (observed data)
wR2 (all data)
GOF
7235
3874
2482
2482
2819
2615
2413
2231
0.029
1379
0.0410
0.1032
0.993
0.0369
2574
0.0565
0.1321
1.029
0.0605
1326
0.0659
0.1723
0.979
0.0473
1332
0.0772
0.2352
0.997
CCDC No.
885661
885662
885663
885664
DMSO were measured by means of a CMD400 conductivity meter.
Magnetic susceptibilities were measured at 300 K using a Sher-
wood Scientific balance. Hg[Co(NCS)4] was used as a standard.
The IR spectra in KBr discs and the electronic spectra either as Nu-
jol mulls or in solution were recorded on a Perkin-Elmer 1600 FT IR
spectrometer and a Shimadzu UV-3100 spectrophotometer respec-
tively. The 1H NMR spectra were recorded on a Bruker Avance
300 MHz FT-NMR DPX 300 K spectrometer using either DMSO-d6
or CDCl3 as solvents and TMS as an internal standard.
2.4. Preparation of the complexes
2.4.1. General method of preparation of 2 and 4
The appropriate hydrated copper(II) salt (2 mmol) dissolved in
hot acetonitrile (20 mL) was mixed with a boiling solution of the
Schiff base (2.2 mmol) in the same solvent (50 mL) and the mixture
heated on a water bath for about 5 min and filtered. The filtrate on
being left to stand overnight, yielded shining dark green crystals of
the complexes which were filtered off, washed with ethanol and
dried in a desiccator over anhydrous silica gel. Recrystallization
of the crude product from a 1:1 mixture of DMF and methanol
afforded diffraction quality crystals.
2.3. Preparation of the ligand
2: Yield, 0.382 g (50%).
Anal. Calc. for C10H12CuBrN3S2: C, 29.39; H, 2.74; N, 11.43. Found:
C, 29.50; H, 2.80; N, 11.20%. UV–Vis: kmax/nm, (log
(dm3 cmꢀ1
molꢀ1): 628 (1.60), 445 (4.71), 326 (5.03), 306 (5.05); IR/cmꢀ1
3234 m ( NH), 1598s ( CN), 1072s ( NN), 730s ( CS).
4: Yield, 0.40 g (60%). eff (BM): 1.73.
ꢀ1 cm2 molꢀ1): 18.3;
Anal. Calc. for C10H12CuClN3S2: C, 35.61; H, 3.68; N, 12.46. Found: C,
35.59; H, 3.58; N, 12.35%. UV–Vis: kmax/nm, (log
(dm3 cmꢀ1
molꢀ1): ca. 580 sh, 450 (4.40), 330 (4.87), ca. 320 sh; IR/cmꢀ1
3300 br ( NH), 1572 s ( CN), 1096 w ( NN), 720 s ( CS).
K (X leff (BM): 1.73.
ꢀ1 cm2 molꢀ1): 27.0;
2.3.1. Preparation of 1
A solution of 20-aminoacetophenone (2.0 g; 0.0148 mol) in abs.
ethanol (15 mL) was mixed with a boiling solution of S-meth-
yldithiocarbazate (1.81 g; 0.0148 mol) in the same solvent
(50 mL). Two drops of conc. HCl were added and then the mixture
was refluxed for ca. 3.5 h and cooled. The resulting crystals of the
Schiff base were filtered off, washed. Yield, 1.80 g (50%). M.p.
183 °C. Anal. Calc. for C10H13N3S2: C, 48.36; H, 5.68; N, 16.92.
Found: C, 47.92; H, 5.91; N, 16.48%, 1H NMR [DMSO-d6] dH: 2.54
(s, 3H, SCH3), 2.39 (s,3H, C–CH3), 6.90 (s, 2H, NH2), 6.55 (t, 1H,
ArH), 6.54 (d, 1H, ArH), 7.47 (d, 1H, ArH), 7.90 (t, 1H, ArH),
12.43H (s, NH); 13C NMR [DMSO-d6]: 16.66 (–CH3), 17.58 (SCH3),
115.50, 116.73, 117.10, 130.44, 130.97, 148.21 [Ar C], 197.56
(CS), 157.21 (C@N).
e
:
m
m
m
m
K
(X
l
e
:
m
m
m
m
2.4.2. Preparation of 3
A solution of copper(II) nitrate tetrahydrate (0.41 g; 2.2 mmol)
in ethanol (25 mL) was added to a solution of the Schiff base
NH2
NH2
SH
S
N
N
N
SMe
N
H
SMe
R
R
thiol
thione
(1) Hapsme R = CH3
(2) Habsem R = H
Fig. 1. The thione and thiol tautomers of Hapsme (R = CH3).

M. Akbar Ali et al. / Polyhedron 47 (2012) 79–86
81
(0.30 g; 2.2 mmol) in the same solvent (50 mL) and the mixture
was heated on a water bath for ca. 5 min and filtered while hot.
A solution of anhydrous lithium thiocyanate (0.285 g; 4.4 mmol)
in boiling abs. ethanol (20 mL) was added to the filtrate and the
resulting dark green solution was heated for about 5 min and then
left to stand for 7 days whereupon the dark green crystals that had
formed were filtered off and recrystallized from acetonitrile. Yield,
0.50 (63%).
11H12CuN4S3: C, 34.45; H, 3.36; N, 15.61. Found: C, 36.70; H, 3.06;
N, 15.57%. UV–Vis: kmax/nm, (log
(dm3 cmꢀ1 molꢀ1): 639 (2.59),
445 (4.87), 326 (5.19), ca. 340 sh, 306 (5.36); IR/cmꢀ1: 3300 br
NH), 1584 s ( CN), 1065 s ( NN), 720 s ( CS).
K (X leff (BM): 1.73. Anal. Calc. for
ꢀ1 cm2 molꢀ1): 35.2;
C
e
(m
m
m
m
Fig. 2. Crystal structure of 1 (30% probability ellipsoids) with the general atom
numbering scheme used in this paper.
2.5. Crystallography
For compound 1 data were collected on an Oxford Diffraction
Gemini S Ultra CCD diffractometer using graphite monochromated
and deprotonated thiolate [19] forms. There are even examples of
metal–thiosemicarbazone complexes in which both the protonated
thione and deprotonated thiolate forms of a ligand are present in
the same complex [20]. However, Schiff bases derived from S-al-
kyl/aryl dithiocarbazates have invariably been found to coordinate
with nickel(II) and copper(II) ions in their deprotonated forms [5–9]
which reflects significant differences in the electronic effects of the
terminal (–SR as opposed to –NHR) substituents.
Reaction of hydrated copper(II) salts with the Schiff base, Haps-
me in a boiling mixture of methanol and acetonitrile yield
copper(II) complexes of empirical formula, [Cu(apsme)X] (X = Cl,
Br, NCS). The copper complexes are stable at room temperature
and show no sign of decomposition on standing in desiccators for
a long period of time. Their room-temperature magnetic moments
(1.73–1.75 BM) are consistent with the 3d9 electronic configura-
tion of copper(II) in a magnetically dilute coordination environ-
ment. Their molar conductivities in DMF show that, although
some dissociation of the complexes occur in this solvent, the con-
ductance values are still much lower than that observed for 1:1
electrolytes in this solvent indicating that the anions are coordi-
nated to the copper(II) ion. Coordination of the anions to the
copper(II) ion is also supported by the IR spectrum of the [Cu(aps-
me)(NCS)] complex which exhibits a strong band at 2100 cmꢀ1
characteristic of an N-bonded thiocyanate group [21].
Mo Ka radiation (0.71073 Å) and data reduction and absorption
correction (multi-scan) was performed with the CRYSALISPRO software
(vers. 171.35.31). Cell constants for compounds 2, 3 and 4 were
determined by a least-squares fit to the setting parameters of 25
independent reflections, measured on an Enraf–Nonius CAD4
four-circle diffractometer using graphite monochromated Mo K
radiation (0.71073 Å) and operating in the –2h mode within the
range 2° < 2h < 50°. An empirical absorption correction ( scans)
a
x
w
and data reduction [11] were performed within the WINGX [12] suite
programmes. The structures were solved by direct methods with
SHELX86 and refined by full-matrix least-squares analysis with SHEL-
XL97 [13]. Non-hydrogen atoms were refined with anisotropic ther-
mal parameters whereas H-atoms were included at estimated
positions. Drawings of all molecules were produced with ORTEP
3
[14] and PLATON [15]. A summary of the crystal data, structure solu-
tion and refinement parameters are given in Table 1.
3. Results and discussion
Compounds containing the thioamide (–NH–C@S) moiety can
display thione–thiol tautomerism. However, an X-ray crystallo-
graphic structure determination of Hapsme (vide infra) shows that,
in the solid state, it exists solely as the thione form. However, in
solution, 1 may exist either as the thione (–NH–C@S) or the thiol
(–N@C–SH) tautomeric forms or as a mixture of these two forms
(Fig. 1).
The mode of coordination of the ligand to the copper(II) ion in
the present copper complexes can be ascertained by examining
their IR spectra. The absence of the N–H stretching band of the
sec N–H group of the uncomplexed ligands in the IR spectra of
the complexes supports their deprotonation upon coordination.
In addition to the thione and thiol forms, the Schiff base may
also exist as E or Z conformational isomers or a mixture of both iso-
mers in solution. Thiosemicarbazones having structural backbones
similar to the present Schiff bases have been found to exist in solu-
tion as a mixture of E and Z forms [16] and NMR spectroscopy has
been successfully used to distinguish between these two isomers
in solution [17]. The 1H NMR spectrum of 1 in DMSO-d6 exhibits
the sec-NH resonance at 12.34 ppm, indicating that in solution, it
exists only as the Z isomer [17].
Table 2
Selected bond lengths (Å) and bond angles (°).
1a
2
3
4
Cu–Seq
Cu–Sax
Cu–X
Cu–Nazomethine
Cu–Namine
C1–S1
C1–N1
N1–N2
N2–C3
2.245(3)
2.871(3)
2.4325(17)
1.995(8)
2.007(9)
1.75(1)
1.30(1)
1.38(1)
1.31(1)
1.44(1)
2.260(3)
2.747(4)
1.942(9)
1.977(8)
2.030(8)
1.751(10)
1.295(12)
1.394(9)
1.324(11)
1.45(1)
2.254(2)
2.895(2)
2.282(2)
2.010(4)
1.994(5)
1.745(3)
1.281(6)
1.391(5)
1.280(6)
1.427(6)
87.12(17)
166.05(14)
89.89(12)
86.48(13)
170.19(13)
94.35(6)
The IR spectrum of 1 lacks a
m
(S–H) band at ca. 2600 cmꢀ1
1.631(3)
1.321(4)
1.355(4)
1.276(4)
1.337(4)
(Table 3) indicating that, in the solid state, it exists only as the thi-
one tautomer. A single crystal X-ray crystal structure determination
(vide infra; Fig. 2) of 1 unequivocally proves that, in the solid it re-
mains in the E configuration about the azomethine C7a–N2a bond.
The 1H NMR spectrum (in DMSO) of 1 lacks any signal at ca. 4 ppm
attributable to the –SH proton indicating that it remains solely in its
thione tautomeric form even in a polar solvent like DMSO. However,
in solution and in the presence of copper(II) salts, it readily converts
to the thiol form, deprotonates and coordinates to the copper(II) ion
in its thiolate form. Thiosemicarbazones having structural back-
bones similar to the present Schiff bases, however, have been found
to coordinate with metal ions in both their protonated thione [18]
C6–N3
N
N
N
N
N
amine–Cu–Nazo
amine–Cu–Seq
amine–Cu–Xeq
azo–Cu–Seq
87.9(4)
88.5(3)
166.0(3)
89.7(3)
86.3(3)
169.6(2)
93.74(9)
96.4(1)
164.9(2)
91.4(4)
85.9(2)
168.3(3)
91.3(3)
97.1(1)
azo–Cu–Xeq
Seq–Cu–Xeq
Saxial–Cu–Seq
96.47(5)
a
Data for molecule A only shown (bond lengths for molecule B not significantly
different).

82
M. Akbar Ali et al. / Polyhedron 47 (2012) 79–86
Table 3
Comparison of Cu-donor atom distances in some copper(II) complexes of tridentate NNS dithiocarbazate and thiosemicarbazone ligands.
Compound
Cu–
Cu–Saxial
Cu–
Cu–Npy/pyrazine/
Cu–Xequatorial
Cu–Xaxial
Ref.
Sequatorial
Nazomethine
quinoline
[Cu(apsme)Br]2
[Cuapsme)NCS]2
[Cu(apsme)Cl]2
2.245(3)
2.260(3)
2.2544(6)
–
1.995(8)
1.977(8)
1.994(4)
2.007(9)
2.030(7)
2.010(4)
2.4325(17)
X = Brꢀ
1.942(9)
X = NCSꢀ
2.2826(5)
X = Clꢀ
1.901(4)
1.912(4)
X = NCSꢀ
2.212(1)
X = Clꢀ
–
–
–
–
this
work
this
work
this
work
[8]
2.746(3)
–
[Cu(acpyrsme)NCS]2
2.265(1)
2.263(2)
3.003(2)
3.126(2)
1.950(4)
1.964(4)
2.030(4)
2.032(4)
[Cu(acpyrsme)Cl]2
2.270(1)
2.260(4)
2.314(2)
2.278(1)
2.275(2)
2.248(6)
2.2318(8)
2.2600(6)
2.924(1)
1.965(3)
1.96(1)
2.029(3)
2.03(1)
–
[8]
ꢀ
[Cu(acpyrsbz)NO3]1
[Cu(quinolsme)NCS]2
[Cu(pytsc)Cl]2
–
1.92(2) X = NO3
2.39(1) N-
pyrazine
–
[8]
2.783(2)
2.760(2)
2.743(2)
–
1.945(6)
1.975(3)
1.987(3)
1.9784(14)
1.963(2)
1.9524(15)
1.940(4)
1.945(4)
1.978(5)
2.139(5)
2.034(4)
2.023(4)
2.0325(14)
2.014(2)
2.0063(15)
2.102(4)
2.108(4)
2.088(6)
1.897(6)
X = NCSꢀ
2.240(2)
X = Clꢀ
[6]
–
[24]
[24]
[37]
[38]
[38]
[7]
[Cu(pytsc)Br]2=
2.405(1)
–
X = Brꢀ
[Cu(4Nphpytsc)Cl]2
[Cu(4Nphbenzpytsc)Cl]2ꢂH2O
[Cu(4Nphbenzpytsc)NCS]
2.2587(7)
X = Clꢀ
2.7245(12)
–
2.2459(7)
X = Clꢀ
1.9240(19)
X = NCSꢀ
1.972(3)
[Cu(6mepy2aldsme)NO3]2ꢂ0.5CH3COCH3 2.2974(14)
2.7987 (15)
ꢀ
X = NO3
[Cu(6mepy2aldsme)(NCS)]1
[Cu(6mepy2aldtsc)Cl]
2.2969(15)
2.282(2)
2.7445(14) S of
NCS
–
1.926(4)
X = NCSꢀ
2.234(2)
X = Clꢀ
[7]
[34]
[Cu(Lhexim)Br]
2.236(3)
1.963(5)
2.016(6)
2.359(2)
X = Brꢀ
[36]
Cu(Lpip)Br]
[Cu(6mepy2aldsme)(CH3COO)]
2.2544(16)
2.295(4)
–
–
1.989(1)
1.95(1)
2.001
2.08(1)
2.344(3) X = Brꢀ
1.98(1)
[32]
[35]
X = CH3COOꢀ
1.943(2)
[Cu(Bpsme)NO3]
[Cu(Apsbz)(NO3)]
2.244(9)
–
–
1.957(2)
1.938(3)
2.010(2)
2.010(3)
[33]
[39]
ꢀ
X = NO3
2.2420(3)
1.982(3)
ꢀ
X = NO3
acpyrsme = anionic form of the 2-acetylpyrazine Schiff base of S-methyldithiocarbazate; acpyrsbz = anionic form of the 2-aqcetylpyrazine Schiff base of S-benzyldithioc-
arbazate; quinolsme = anionic form of the 2-quinoline carboxaldehyde Schiff base of S-methyldithiocarbazate; py2aldsme = anionic form of the 2-pyridinecarboxaldehyde
Schiff base of S-methyldithiocarbazate; 6mepy2aldtsc = anionic form of the 6-methyl-2-pyridinecarboxaldehyde thiosemicarbazone; 6mepy2aldsme = anionic form of the 6-
methylpyrine-2-aldehyde Schiff base of S-methyldithiocarbazate; Lpip = anionic form of 2-acetylpyridine piperidinyl thiosemicarbazone; Lhexim = anionic form of 2-ace-
tylpyridine hexamethyleneiminyl thiosemicarbazone; Bpsme = anionic form of the 2-benzoylpyridine Schiff base of S-methyldithiocarbazate; Apsbz = anionic form of the 2-
acetylpyridine Schiff base of S-benzyldithiocarbazate; 4Nphbenzpytsc = anionic form of 4-N-phenyl-2-benzoyldpyridine thiosemicarbazone; 4Nphpytsc = anionic form of 4-
N-phenyl pyridine-2-aldehyde thiosemicarbazone.
The azomethine
m
(C@N) bands in the IR spectra of the free ligand
3.1. Description of the crystal structures
appears at 1602 cmꢀ1 which shifts to lower frequencies supporting
coordination of the azomethine nitrogen atom to the copper(II) ion
[6–8]. This is further corroborated by the shift of the hydrazinic
3.1.1. The structure of 1
Fig. 2 depicts the molecular geometry, thermal ellipsoids and
numbering system of 1 and selected bond lengths and bond angles
are given in Table 2.
m
(N–N) band of the free ligand at 1060 cmꢀ1 to higher wave num-
bers 1065–1096 cmꢀ1 in the spectra of the complexes as noted pre-
viously [22]. The splitting of the
m
(SCS) band of the free ligands at
The structure comprises two molecules in the asymmetric unit
which exhibit identical geometries and conformations. Both mole-
cules are essentially planar with all non-H atoms lying within 0.1 Å
of each least squares plane of the ligand. The bond distances in the
dithiocarbazate chain [NNCSSCCH3] are close to those observed in
the thiosemicarbazone analogue [28] and in other Schiff bases de-
rived from S-alkyl/aryldithiocarbazates [6–9]. The C9a–S1a dis-
tance [1.631(3) Å] is by far the shortest value observed in this
class of compounds, but the value is still intermediate between a
C–S single bond [1.82 Å] and a C@S double bond [1.56 Å] [29].
The C9a–N3a and C37a–N2a bond distances are 1.321(4) Å and
1.276(4) Å, respectively indicating that the bond order of the
former is greater than one while the latter is a true double bond.
A comparison of the N2a–N3a distance [1.355(4) Å] with that in
ꢁ961–965 cmꢀ1 in the spectra of the complexes also reflects coor-
dination of one of the sulfur atoms [23].
The electronic spectrum of 2, 3 and 4 exhibit d–d maxima in the
range 580–640 nm. A strong S ? CuII charge transfer band is also
observed at about 450 nm in each spectrum supporting coordina-
tion of the sulfur to the copper(II) ion. Similar ligand-to-metal
charge transfer bands have also been observed in complexes of
other NNS thiosemicarbazones [24–27] and dithiocarbazates
[6–8]. The related five-coordinate dimeric copper(II) complex,
[Cu(pytsc)Br]2 (pytsc = anionic form of the 2-pyridinecarboxalde-
hyde thiosemicarbazone), whose structure has been determined
by X-ray diffraction, also displays a single d–d band at 640 nm in
its electronic spectrum [24].

M. Akbar Ali et al. / Polyhedron 47 (2012) 79–86
83
mer (Fig. 4B). Selected bond lengths and bond angles are given in
Table 2. The coordination environment around each copper(II)
ion is square pyramidal comprising a N2S2Br set of donor atoms.
Each ligand coordinates with a copper(II) ion as a tridentate
NNS chelating agent via the amino nitrogen atom, the azomethine
nitrogen atom and the thiolate sulfur atom. The fourth and fifth
coordination positions of a copper atom are occupied by a bromide
and bridging thiolate sulfur atoms, respectively.
There are several structurally characterised dimeric copper(II)
complexes of tridentate NNS thiosemicarbazones [24–27] and
dithiocarbazates [6–8] bearing a similar four-membered metallo-
cyclic thiolate bridging motif. The Cu–S1, Cu–N2, Cu–N3 and Cu–
Br1 bond distances 2.245(3) Å, 1.995(8) Å, 2.007(9) Å and
2.4325(17) Å, respectively, compare well with that of the related
dimeric copper(II) complexes of related NNS ligands [6–8,24–26].
The copper atom sits 0.22 Å above the basal plane displaced to-
ward the axially coordinated thiolate S-donor. The Cu–S–Cu bridg-
ing angle 83.6° and the Cu–Cu separation 3.44 Å, respectively, are
close to that in the related dimeric copper(II) complex,
[Cy(pytsc)Br]2 [(87.1(1)°) and [3.474(1) Å] [24].
The bridging arrangement observed in this complex is very sim-
ilar to those of other dimeric copper(II) complexes of analogous
NNS dithiocarbazates [5–7]. The Cu–Sequatorial [2.245(3) Å], Cu(1)–
N(2a) [1.995(8) ÅÅ], Cu(1)–N(3a) [2.007(9) ÅÅ], Cu–Saxial
[2.746(3) ÅÅ] and Cu–Br [2.4325(17) ÅÅ] bond distances compare
well with those observed in the dimeric copper(II) complexes of
[6–8,24–27]. The coordination geometry around each copper atom
can be considered as approximately square-pyramidal on the basis
Fig. 3. Intra- and intermolecular hydrogen bonding in 1.
S-methyldithiocarbazate [30] shows that the bond is shorter than a
single bond indicating significant p-charge delocalization along the
C(S)NNC moiety. Conjugation with the phenyl ring is probably
responsible for shortening of the N1–N2 distance. The aromatic
primary amino group is trigonal planar as a result of the N3 lone
pair being conjugated with the phenyl ring. Consequently the
C6–N3 bonds (ꢁ1.33 Å) have partial double bond character.
The structure of 1 also shows that the azomethine nitrogen
atom, N2a and the thione sulfur atom, S1a are in anti conformation
with respect to the azomethine C7a–N2a bond. The amino nitrogen
atom N1a and the azomethine nitrogen atom, N2a are, however in
the syn conformation. In this anti–syn conformation, the Schiff base
is unable to act as an NNS tridentate chelating agent. However, in
solution rotation about the C9–N2 bond can place all the three do-
nor atoms on one side and thereby enable it to act as a tridentate
NNS chelating agent.
There are both intra- and intermolecular hydrogen bonds
(Fig. 3). One of the NH2hydrogen atoms in the Schiff base is in-
volved in bifurcated hydrogen bonding between the azomethine
nitrogen atom N2a and the thioether sulfur atom, S2a. In addition
to the intramolecular hydrogen bond, the hydrogen atom on N3a of
one molecule forms a H-bond with the thione sulfur atom, S1a of
another molecule leading to dimers (Fig. 3).
of the parameter,
being the two largest coordinate angles N(2)–Cu(1)–Br(1)
s
= 0.051 [31] where
s
= (b ꢀ
a)/60°; a and b
[169.6(2)°] and N(3)–Cu(1)–S(1) [166.0(3)°]. For ideally square
pyramidal geometry
s = 0 while for trigonal bipyramidal s = 1.
One striking feature of this structure is the distinctly buckled
conformation of the tridentate coordinated apsme ligand. The coor-
dinated primary amine N-atom (N3) clearly adopts a tetrahedral
geometry (on the basis of the H-atoms that were identified during
the structure refinement) compared with the trigonal geometry
seen in the free ligand. This leads to a contraction of the preferred
Cu–N3–C6 angle (i.e. from 120° to ꢁ109.5°) for the now sp3 hybri-
dised atom. The observed Cu–N3–C6 angle [117.5(7)°] indicates
considerable strain in the chelate ring which is also revealed in a
twisting of the phenyl ring well out of the basal coordination plane.
Moreover, the lone pair on N3, formerly conjugated with the phenyl
ring, now contributes to the Cu–N3 coordinate bond and so C6–N3
lengthens to be consistent with a typical C–N single bond.
3.1.2. Crystal structures of the dimeric copper(II) complexes
3.1.2.1. The structure of [Cu(apsme)Br]. The asymmetric (mono-
meric) unit comprises the Cu ions coordinated to the tridentate
deprotonated Schiff base plus a monodentate bromido ligand
(Fig. 4A). The S-donor forms an additional coordinate bond to the
axial site of an adjacent Cu ion resulting in a centrosymmetric di-
An examination of bond angle data in Table 2 indicates that the
complex does not have a perfect square-pyramidal geometry. Be-
Fig. 4. (A) ORTEP view (30% probability ellipsoids) of the asymmetric monomeric unit of 2 and (B) PLATON view of the centrosymmetric dimer.

84
M. Akbar Ali et al. / Polyhedron 47 (2012) 79–86
Fig. 5. (A) ORTEP view of the asymmetric unit of 3 and (B) PLATON view of the centrosymmetric dimer.
cause of the restricted bite angles imposed by five-membered che-
late rings, the coordinate angles deviate significantly from 90° or
180°: N(3)–Cu–S1 [166.0(3)° versus 180°], N2–Cu–Br1: [169.6(2)°
versus 180°], N2–Cu–N3 [87.9(4)° versus 90°] and N2–Cu–S1
[86.3(3)° versus 90°].
thione nitrogen atom N2, the nitrogen atom N4 of the thiocyanate
ligand and the thiolate sulfur atom S1. The apical site of the five-
coordinate copper atom is occupied by the bridging thiolate sulfur
atom of the other monomeric unit in the dimer. The Cu–S–Cu bridg-
ing angle [82.88(9)°] and the Cu–Cu distance [3.33 Å] is close to that
observed in 2. The bond angle data in Table 2 show that the complex
does not have a perfect square-pyramidal geometry since both the
trans angles, S1–Cu1–N3 and N2–Cu1–N4 deviate substantially
from that required for an ideal square-pyramidal geometry.
3.1.2.2. The crystal structure of 3. The structure of the asymmetric
monomeric unit of 3 is depicted in Fig. 5A and that of the centro-
symmetric dimer is shown in Fig. 5B.
In this complex, the Schiff base is also coordinated to the cop-
per(II) ion in its iminothiolate form via the amino nitrogen atom,
the azomethine nitrogen atom and the thiolate sulfur atom. While
coordinating in this form the negative charge generated on the
thiolate sulfur atom of the Schiff base is delocalized over the N1–
C1–S1 chain as indicated by the intermediate C1–S1 [1.73(1) Å]
and C1–N1 [1.28(1) Å] bond distances.
The thiocyanate ion in this complex is coordinated with the
copper(II) ion as a monodentate ligand via the nitrogen atom N4.
The Cu1–N4 distance [1.942(9) Å] is close to that observed in the
related thiolate sulfur-bridged dimeric [Cu(quinolsme)(NCS)]2
complex (quinolsme = anionic form of the 2-quinolinecarboxalde-
hyde Schiff base of S-methyldithiocarbazate) [6], which also has
a structure similar to that of 3.
3.1.2.3. The crystal structure of 4. The structure of 4, together with
the atom numbering scheme adopted, is shown in Fig. 6 and se-
lected bond lengths and bond angles are given in Table 2.
The structure shows that, like 2 and 3, 4 is also dimeric in the
solid state. In this case a chlorido ligand occupies the coordination
site trans to N2. The Schiff base, Hapsme is coordinated to the cop-
per(II) ion in its iminothiolate form as an NNS tridentate chelating
agent via the amino nitrogen atom, the azomethine nitrogen atom
and the thiolate sulfur atom. The complex has a long N1–N2 bond,
a short C1–N1 bond and a long C1–S1 bond. This is indicative of the
dithiocarbazate’s greater conjugation and therefore, more delocal-
ized electron density. The Cu1–S1, Cu1–N2 and Cu1–Cl1 bond
distances, 2.254(2) Å, 2.010(4) Å and 2.283(2) Å, respectively. A
comparison of these bond distances with those observed in other
copper(II) complexes of NNS ligands whose structures have been
determined by X-ray diffraction (Table 3) shows that the Cu-
donor atom distances in the present dimeric copper(II) complexes
The overall geometry adopted by each copper atom in the dimer
may be considered as approximately square-pyramidal since the
geometric factor,
s has a value of 0.027. The basal plane of the
square-pyramid contains the amino nitrogen atom N3, the azome-
Fig. 6. (A) The crystal structure of the asymmetric unit of 4 (30% probability ellipsoids) with the atom numbering scheme and (B) PLATON view of the centrosymmetric dimer.

M. Akbar Ali et al. / Polyhedron 47 (2012) 79–86
85
[4] (a) M.T. Basha, J.D. Chartres, N. Pantara, M. Akbar Ali, A.H. Mirza, D.S.
Kalinowski, D.R. Richardson, P.V. Bernhardt, Dalton Trans. 41 (2012) 6536;
(b) T.B.S.A. Ravoof, K.A. Crouse, M.I.M. Thahir, A.R. Cowley, M. Akbar Ali,
Polyhedron 23 (2004) 2491.;
are close to those observed in other related copper(II) complexes
of NNS thiosemicarbazones and dithiocarbazates. The bond
distance data in Table 3 also shows that despite the different
stereochemistries adopted by the copper(II) complexes of NNS
thiosemicarbazone and dithiocarbazate ligands, there is not much
variation in the Cu-donor atom distances in these complexes. The
strength of the Cu–Namine bond is also very similar to that of a
Cu–Npyridine/pyrazine/quinoline bond.
(c) M. Akbar Ali, A.H. Mirza, R.J. Butcher, M.T.H. Tarafder, T.B. Keat, A.M. Ali, J.
Inorg. Biochem. 92 (2002) 141;
(d) M. Das, S.E. Livingstone, Br. J. Cancer 37 (1978) 101;
(e) A. Saxena, J.K. Koacher, J.P. Tandon, Antibact. Antifung. Agents 9 (1981)
435;
(f) A. Saxena, J.K. Koacher, J.P. Tandon, J. Toxicol. Environ. Health 15 (1985)
503.
The bond angles in 4 (Table 2) indicates that it has a distorted
square-pyramidal geometry. Thus, the two trans angles, viz. N1–
Cu1–S1 [166.05(14)°] and N2–Cu1–Cl1 [170.19(13)°] deviate by
about 10–14° from the ideal value of 180°, but the cis angles
(Table 2) are close to that required for an ideal square-pyramidal
geometry.
[5] (a) R. Takjooa, R. Centoreb, M. Hakimic, S.A. Beyramabadid, A. Morsalid, Inorg.
Chim. Acta 371 (2011) 36;
(b) M. Akbar Ali, D.J. Phillips, S.E. Livingstone, Inorg. Chim. Acta 6 (1972) 39;
(c) M. Akbar Ali, S.E. Livingstone, Coord. Chem. Rev. 13 (1974) 101;
(d) M. Akbar Ali, M.T.H. Tarafder, J. Inorg. Nucl. Chem. 39 (1977) 1785;
(e) M. Akbar Ali, A.H. Mirza, R.J. Butcher, K.A. Crouse, Transition Met. Chem. 31
(2006) 79;
(f) M.E. Hossain, M.N. Alam, J. Begum, M. Akbar Ali, M. Nazimuddin, Transition
Met. Chem. 18 (1993) 497.
As in the two other Cu complexes reported here, the Cu–S–Cu
angle (83.53(5)°) and Cu. . .Cu separation 3.46 Å are similar.
[6] M. Akbar Ali, A.H. Mirza, F. Hj Bujang, M.H.S.A. Hamid, P.V. Bernhardt,
Polyhedron 25 (2006) 3245.
[7] M. Akbar Ali, A.H. Mirza, R.J. Fereday, R.J. Butcher, J.M. Fuller, S. Drew, L.R.
Gahan, G.R. Hanson, B. Moubaraki, K.S. Murray, Inorg. Chim. Acta 358 (2005)
3937.
4. Conclusion
[8] M.H.S.A. Hamid, M. Akbar Ali, A.H. Mirza, P.V. Bernhardt, B. Moubaraki, K.S.
Murray, Inorg. Chim. Acta 363 (2009) 3648.
[9] M. Akbar Ali, A.H. Mirza, M.H.S.A. Hamid, P.V. Bernhardt, Polyhedron 24 (2005)
383.
[10] (a) G. Bahr, G. Schleitzer, Z. Anorg. Allg. Chem. 280 (1955) 176;
(b) M. Das, S.E. Livingstone, Inorg. Chim. Acta 19 (1974) 5.
[11] K. Harms, S. Wocadlo, XCAD-4, Program for Processing CAD-4 Diffractometer
Data, University of Marburg, Germany, 1995.
[12] L. Farrugia, J. Appl. Crystallogr. 32 (1999) 837.
[13] G.M. Sheldrick, SHELX, Programs for Crystal Structure Analysis, University of
Göttingen, Germany, 1997.
[14] L. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.
[15] A.L. Spek, PLATON, A Multipurpose Crystallographic Tool, Utrecht University, The
Netherlands, 1998.
[16] (a) D.X. West, J.P. Scovill, J. Silverton, A. Bavoso, Transition Met. Chem. 11
(1986) 123;
The 2-aminoacetophenone Schiff base of S-methyldithiocarbaz-
ate (Hapsme), upon reacting with copper(II) salts, does not lead to
the formation form any monomeric four-coordinate copper(II)
complexes but self-assemble to yield thiolato sulfur-bridged di-
meric copper(II) complexes in which each copper atom adopts an
approximately square-pyramidal geometry with the N2S2X (X = Cl,
NCS, Br) donor environment. While the related NNS thiosemicarba-
zones have been shown to yield both thiolato sulfur-bridged as well
as anion-bridged dimeric copper(II) complexes, the NNS dithiocar-
bazates have been found to yield only thiolato sulfur-bridged com-
plexes, which reflects significant differences in the electronic
effects of the terminal (–SR as opposed to –NHR) substituents.
(b) G.F. de Sousa, D.X. West, C.A. Brown, J.K. Swearingen, J. Valdés-Martínez,
R.A. Toscano, S. Hernández-Ortega, M. Hörner, A.J. Bortuluzzi, Polyhedron 19
(2000) 841;
Acknowledgements
(c) E. Bermejo, A. Carballo, A. Castíñeiras, R.R. Domíniguez, C. Maichle-
Mössmer, J. Strähle, D.X. West, Polyhedron 18 (1999) 3695;
(d) D.X. West, M.A. Lockwood, A.E. Liberta, X. Chen, R.D. Willet, Transition
Met. Chem. 18 (1993) 221;
(e) D.X. West, C.S. Carlson, C.P. Galloway, A.E. Liberta, C.R. Daniel, Transition
Met. Chem. 15 (1990) 91.
M. Akbar Ali, A.H. Mirza and Malai Haniti S.A. Hamid thank
the University of Brunei Darussalam for supporting this work.
Support from the Australian Research Council is also gratefully
acknowledged.
[17] (a) K.R. Koch, J. Coord. Chem. 22 (1991) 2;
(b) D.X. West, C.S. Carlson, K.J. Bouck, A.E. Liberta, Transition Met. Chem. 16
(1991) 271;
(c) D.X. West, B.L. Mokijewski, H. Gebremedhin, T.J. Romack, Transition Met.
Chem. 17 (1992) 384;
Appendix A. Supplementary data
(d) C.R. Kowol, R. Trondl, V.B. Arion, M.A. Jakupec, I. Lichtscheidl, B.K. Keppler,
J. Chem. Soc., Dalton Trans. 39 (2010) 704.
CCDC 885661, 885662, 885663 and 885664 contain the supple-
mentary crystallographic data for 1, 2, 3 and 4, respectively. These
data can be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
[18] (a) R. Gronback-Hazel, Acta Chem. Scand. 26 (1992) 1653;
(b) M. Mohan, A. Agarawal, N.K. Jha, J. Inorg. Biochem. 34 (1988) 41;
(c) A. Bino, N. Cohen, Inorg. Chim. Acta 210 (1993) 11;
(d) P. Sengupta, R. Dinda, S. Ghosh, W.S. Sheldrick, Polyhedron 22 (2003)
447;
(e) S.K. Chottopadhya, D. Chattopadhyay, T. Banerjee, R. Kuroda, S. Ghosh,
Polyhedron 16 (1997) 1925.
[19] (a) P. Pelagatti, A. Venturini, A. Leporati, M. Carcelli, M. Costa, A. Bacchi, G.
Pelizzi, C. Pelizzi, J. Chem. Soc., Dalton Trans. (1998) 2715;
(b) M.D. Timken, S.R. Wilson, D. Hendrickson, Inorg. Chem. 24 (1985) 3450;
(c) D.K. Sau, R.J. Butcher, N. Chaudhuri, N. Saha, Polyhedron 23 (2004) 5;
(d) E. Labisbal, K.D. Haslow, A. Sousa-Pedrares, J. Valdes-Martinez, S.
Hernandez-Ortega, D.X. West, Polyhedron 22 (2003) 2831;
(e) D. Kovala-Demertzis, J.R. Miller, C.S. Frampton, J.P. Jasinski, D.X. West, J.
Inorg. Biochem. 92 (2002) 137;
References
[1] (a) Y. Tian, C. Duan, C. Zhao, X. You, T.C.W. Mak, Z.Y. Zhang, Inorg. Chem. 36
(1997) 1247;
(b) Z.A. Abu-Raqabah, G. Davies, M.A. El-Sayed, A. El-Toukhy, S.N. Shaikh, J.
Zubieta, Inorg. Chim. Acta 193 (1992) 43;
(c) Z.H. Liu, C.Y. Duan, J. Hu, Inorg. Chem. 38 (1999) 1719;
(d) C.Y. Duan, X.Z. You, Chem. Lett. Jpn. 10 (1999) 1063;
(e) Y. Tian, C.Y. Duan, X. You, T.C.W. Mak, Q. Luo, J. Zhou, Transition Met. Chem.
23 (1998).
(f) D.X. West, J.S. Ives, J. Krejci, M.M. Salberg, T.L. Zumbahlen, G.A. Bain, A.E.
Liberta, J. Valdes-Martinez, S. Hernandez-Ortega, R.A. Toscano, Polyhedron 14
(1995) 2189.
[20] (a) P. Souza, A.I. Matesanz, V. Fernandez, J. Chem. Soc., Dalton Trans. (1996)
3011;
[2] (a) M. Akbar Ali, A.H. Mirza, Hjh Junaidah Hj Abu Bakar, P.V. Bernhardt,
Polyhedron 30 (2011) 556;
(b) D.S. Timken, S.R. Wilson, D.N. Hendrickson, Inorg. Chim. Acta 24 (1985)
3450.
(b) M. Akbar Ali, A.H. Mirza, R.J. Butcher, P.V. Bernhardt, M.R. Karim,
Polyhedron 30 (2011) 1478;
[21] R.A. Bailey, S.L. Kojak, T.W. Michelson, W.N. Mills, Coord. Chem. Rev. 6 (1971)
407.
(c) M. Akbar Ali, A.H. Mirza, M.H.S.A. Hamid, I.C. Siew, L.P. Sia, P.V. Bernhardt,
Inorg. Chem. Commun. 13 (2010) 1445;
[22] A. Monaci, F. Trali, Dalton Trans. (1977) 417.
[23] R. Ugo, J Organomet. Chem. 4 (1967) 395.
(d) M. Akbar Ali, A.H. Mirza, R.J. Butcher, M.T.H. Tarafder, M.A. Ali, Inorg. Chim.
Acta 320 (2001) 1;
[24] J. Garcia-Tojal, M.K. Urtiaga, R. Cortes, L. Lezama, M.I. Arriortua, T. Rojo, J.
Chem. Soc., Dalton Trans. (1994) 2233.
(e) M. Yazdanbakhsh, R. Takjooa, Struct. Chem. 19 (2008) 895.
[3] M. Rahman, M. Akbar Ali, Transition Met. Chem. 19 (1994) 237.

86
M. Akbar Ali et al. / Polyhedron 47 (2012) 79–86
[25] J. Garcia-Tojal, L. Lezama, J.L. Pizarro, M. Insausti, M.I. Arriortua, Polyhedron 18
(1999) 3703.
[33] M.E. Hossain, M.N. Alam, J. Begum, M. Akbar Ali, M. Nazimuddin, F.E. Smith,
R.C. Hynes, Inorg. Chim. Acta 249 (1996) 207.
[26] E.W. Ainscough, A.M. Brodie, J.D. Ranford, J.M. Waters, K.S. Murray, Inorg.
Chim. Acta 197 (1992) 107.
[34] M. Akbar Ali, K.K. Dey, M. Nazimuddin, F.E. Smith, R.J. Butcher, J.P. Jasinski, J.M.
Jasinski, Polyhedron 15 (1996) 3331.
[27] V.P. Philip, V. Sani, M.R.P. Kurup, M. Nethaji, Polyhedron 25 (2008) 1381.
[28] A.A. Nassar, S. Hernandes-Ortega, J. Valdez-Martinez, D.X. West, J. Mexican
Chem. Soc. 46 (2002) 251.
[35] M. Akbar Ali, R.J. Butcher, J.C. Bryan, Inorg. Chim. Acta 287 (1999) 8.
[36] D.X. West, H. Gebremedhin, R.J. Butcher, J.P. Jasinski, A.E. Liberta, Polyhedron
12 (1993) 2489.
[29] F.H. Allen, O. Kennard, D.G. Watson, L. Bremmer, A.G. Orpen, R. Taylor, J. Chem.
Soc., Perkin Trans. II (1987) S1.
[30] R. Mattes, H. Weber, J. Chem. Soc., Dalton Trans. (1980) 423.
[31] A.W. Addison, T.N. Rao, J. Reedijk, J. Rijn, G.C. Verschoor, J. Chem. Soc., Dalton
Trans. (1984) 1349.
[37] P.F. Rapheal, E. Manoj, M.R.P. Kurup, Polyhedron 26 (2007) 818.
[38] M. Joseph, M. Kuriakose, M.R.P. Kurup, E. Suresh, A. Kishore, S.G. Bhat,
Polyhedron 25 (2006) 61.
[39] M.E. Hossain, M.N. Alam, M. Akbar Ali, M. Nazimuddin, F.E. Smith, R.C. Hynes,
Polyhedron 15 (1996) 973.
[32] R.J. Bucther, D.X. West, Transition Met. Chem. 18 (1993) 449.