Cobalt(II) complexes of the antibiotic sulfadiazine, the X-ray single crystal structure of [Co(C10H9N4O2S)2(CH3OH)2]

Article abstract of DOI:10.1016/j.ica.2006.03.030

Cobalt(II) complexes of sulfadiazine formulated as [Co(C₁₀H₉N₄O₂S)₂(CH₃OH)₂] and [Co(C₁₀H₉N₄S)₂(H₂O)₂] have been synthesized and characterized by elemental analysis, infrared and UV-Vis spectroscopy and magnetic susceptibility measurements. The crystal structures of the complex [Co(C₁₀H₉N₄O₂S)₂(CH₃OH)₂] and of free sulfadiazine are also reported. The cobalt complex and the sulfadiazine ligand both crystallize in the monoclinic space group, P21/c, with sulfadiazine acting as a bidentate ligand. Cobalt is coordinated to two-sulfonamide nitrogen and the pyrimidine nitrogen of the sulfadiazine. Two molecules of methanol complete the octahedral geometry around the cobalt, with interligand hydrogen bonding between methanol and sulfadiazine. Infrared spectroscopy confirmed the presence of water molecule in the coordination sphere of [Co(C₁₀H₉N₄S)₂(H₂O)₂]. The electronic spectra and magnetic moments of both complexes were similar, indicating that both complexes have similar structure.

Full text of DOI:10.1016/j.ica.2006.03.030

Inorganica Chimica Acta 359 (2006) 3111–3116
www.elsevier.com/locate/ica
Cobalt(II) complexes of the antibiotic sulfadiazine, the X-ray
single crystal structure of [Co(C10H9N4O2S)2(CH3OH)2]
a
a,
b
Peter A. Ajibade , Gabriel A. Kolawole *, Paul O’Brien ,
b
b
Madeleine Helliwell , James Raftery
a
University of Zululand, Department of Chemistry, Faculty of Science and Agriculture, Kwadlangezwa, Kwazu-Natal 3886, South Africa
b
University of Manchester, Department of Chemistry, UK
Received 14 June 2005; received in revised form 14 March 2006; accepted 24 March 2006
Available online 4 April 2006
Abstract
Cobalt(II) complexes of sulfadiazine formulated as [Co(C10H9N4O2S)2(CH3OH)2] and [Co(C10H9N4S)2(H2O)2] have been synthesized
and characterized by elemental analysis, infrared and UV–Vis spectroscopy and magnetic susceptibility measurements. The crystal struc-
tures of the complex [Co(C10H9N4O2S)2(CH3OH)2] and of free sulfadiazine are also reported. The cobalt complex and the sulfadiazine
ligand both crystallize in the monoclinic space group, P21/c, with sulfadiazine acting as a bidentate ligand. Cobalt is coordinated to two-
sulfonamide nitrogen and the pyrimidine nitrogen of the sulfadiazine. Two molecules of methanol complete the octahedral geometry
around the cobalt, with interligand hydrogen bonding between methanol and sulfadiazine. Infrared spectroscopy confirmed the presence
of water molecule in the coordination sphere of [Co(C10H9N4S)2(H2O)2]. The electronic spectra and magnetic moments of both com-
plexes were similar, indicating that both complexes have similar structure.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Cobalt complexes; Sulfadiazine; Crystal structure; Malaria; Sulfanilamide; Antibiotics
1. Introduction
mostly used now in combination therapy with pyrimeth-
amine to treat chloroquine-resistant malaria parasite [3].
Malaria is one of the three major infectious diseases rav-
aging the world; the others are tuberculosis and AIDS [4].
Despite more than a century of efforts to eradicate or con-
trol malaria, the disease remains a major and growing
threat to the public health and economic development of
countries in the tropical and subtropical regions of the
world with 40% of the world at risk of infection [5]. There
are an estimated 300–500 million cases, up to 2.7 million
deaths and 42 million Disability Adjusted Life Years
(Daly’s) each year. The mortality levels are greatest in
sub-Saharan Africa, where children under 5 years of age
account for 90% of all deaths due to malaria [6]. In coun-
tries with endemic malaria, the annual economic growth
rates over a 25-year period were 1.5% lower than those in
other countries. This implies that the cumulative effect of
the lower annual economic output in a malaria endemic
country was a 50% reduction in per capital GNP compared
Sulfonamides are among the most widely used antibac-
terial agents in the world, chiefly because of their low cost,
low toxicity, and excellent activity against bacterial dis-
eases. The clinically useful sulfonamides are derived from
sulfanilamide, which is similar to para-aminobenzoic acid,
a factor required by bacteria for folic acid synthesis [1]. Sul-
fanilamide was introduced in therapy about half a century
ago for prevention and cure of bacterial infections in
humans [2]. Its importance was greatly reduced due to
the advent of modern antibiotics. Sulfadiazine is a sulfanil-
amide that is used as an antibacterial as well as an antima-
larial drug. Its use as antimalarial depends on its long
half-life in the blood of between 60 and 200 h, which obvi-
ates the need for frequent administration. However, it is
*
Corresponding author. Tel.: +27 35 902 6150; fax: +27 35 902 6568.
E-mail address: gayokola@pan.uzulu.ac.za (G.A. Kolawole).
0020-1693/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.03.030

3112
P.A. Ajibade et al. / Inorganica Chimica Acta 359 (2006) 3111–3116
to a non-malarious country [7]. The emergence and spread-
ing of parasites resistant to antimalarial drugs currently in
use indicates that novel compounds need development by
identification of novel chemotherapeutics targets. The
frightening spread of the parasitic resistance has led the
World Heath Organization to predict that without new
antimalarial drug intervention, the number of cases of
malaria will have doubled by the year 2010 [8]. Hence,
the search for new antimalarial therapies is a high-priority
for the control of the disease.
temperature magnetic susceptibility measurements were
carried out using Sherwood Scientific Magnetic Suscepti-
bility balance using Hg[Co(SCN)4] as the calibrant.
Diamagnetic corrections were estimated from Pascal’s con-
stants [21].
2.2. Synthesis
Sodium salt of sulfadiazine was obtained from Aldrich.
The complex was obtained by dissolving (2.723 g,
10 mmol) sodium salt of sulfadiazine in 50 mL of water.
This is followed by drop-wise addition of CoCl2 Æ 6H2O
(1.186 g, 5 mmol) or Co(CH3COO)2 Æ 4H2O (1.245 g,
5 mmol) in water with constant stirring at 40 ꢁC for 1 h.
A pink precipitate was formed, filtered and washed with
water, methanol and diethyl ether successively and dried
under a vacuum. Yield: 2.61 g (96%), m.p. 334 ꢁC.
Elemental Anal. Calc. for [Co(C10H9N4O2S)2(H2O)2]
(CoC20H24N8O6S2): C, 40.33; H, 4.06; N, 18.82; S, 10.76;
Co, 9.90. Found: C, 40.07; H, 3.92; N, 18.76; S, 10.54;
Co, 9.62%. The above procedure was repeated using meth-
anol as the solvent. The product was pale pink in color and
The use of metal complexes as pharmaceutical has
shown promise in recent years particularly as anticancer
agents [9]. Research is ongoing in fields such as cancer
[10], arthritis [11] and cardiovascular medicine. In the
search for novel drugs against chloroquine-resistant
malaria parasites, the modification of existing drug by
coordination to a metal centre has attracted considerable
attention recently [12–14]. Thus, the search for metal-based
drug with low level of toxicity and high biological activity
against parasites responsible for malaria and chagas disease
has gained prominence [15]. Interest in the metal complexes
of sulfadiazine is due to its use as pharmaceuticals. Zinc
sulfadiazine is used to prevent bacterial infection in burned
animals [16] and silvadene (2-sulfanilamidopyrimidine-sil-
ver (I)) is used commercially for the treatment of topical
burn [17]. The crystal structures of silver sulfadiazine [18],
zinc sulfadiazine [19] and cadmium sulfadiazine [20] have
been reported. In the silver complex, the active binding sites
of sulfadiazine are the pyrimidine N atoms, the sulfon-
amido N, and one sulfonic O, giving rise to a polymeric
arrangement. In zinc(II) compound, one molecule of sulfa-
diazine coordinates through the sulfonamido N and the
other through one N atom of the pyrimidine ring. In the
case of the cadmium complex, the cadmium atom lies on
the twofold axis and is coordinated to two sulfonamido
and amino N atoms from four symmetry-related sulfadia-
zine anions. The coordination geometry around these metal
ions is basically distorted tetrahedral. In our effort to con-
tribute to metal-based drug as alternative therapies in the
management of chloroquine-resistant malaria, we present
the synthesis and characterization of cobalt(II) complexes
of sulfadiazine. We also presented the single-crystal X-ray
structures of one of the complexes and sulfadiazine.
Table 1
Crystal data and structure refinement parameters
[Co(C10H9N4O2S)2(CH3OH)2] Sulfadiazine
Empirical formula
Formula weight
Temperature (K)
C
621.56
100(2)
0.71073
monoclinic
P21/c
22H26CoN8O6S2
C10H10N4O2S
250.28
100(2)
0.71073
monoclinic
P21/c
˚
Wavelength (A)
Crystal system
Space group
Unit cell dimensions
˚
a (A)
7.1287(9)
9.0628(11)
20.287(3)
90
94.085
90
13.6830(16)
5.8190(7)
14.7950(17)
90
115.037(2)
90
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
Volume (A )
1307.3(3)
2
1067.3(2)
4
Z
Dcalc (Mg/m3)
Absorption
coefficient (mmꢀ1
F(000)
1.579
0.871
1.558
0.299
)
642
520
Crystal size (mm)
h Range for data (ꢁ)
Index ranges
0.35 · 0.35 · 0.20
2.01–26.36
ꢀ8 6 h 6 5,
ꢀ11 6 k 6 11,
ꢀ25 6 l 6 25
7253
0.30 · 0.20 · 0.10
1.64–28.32
ꢀ17 6 h 6 18,
ꢀ7 6 k 6 6,
ꢀ19 6 l 6 16
6348
2. Experimental
2.1. General procedure
Reflection collected
Independent
7256 (0.0000)
2492 (0.0588)
Solvents of analytical grade were used as obtained. Ele-
mental analyses were performed at the micro analytical
laboratory, School of Chemistry, the University of Man-
chester, UK. FT-IR spectra were obtained on a Perkin–
Elmer Paragon 1000 FT-IR spectrophotometer in the
range 4000–250 cmꢀ1. UV–Vis spectra were obtained on
a Perkin–Elmer Lambda 20 spectrophotometer equipped
with an integrating sphere for diffuse reflectance spectra.
The solution spectra were on DMF solutions. Room
reflections (Rint
Completeness to
h = 26.36 (%)
Data/restraints/
parameters
)
99.4
94.0
7256/0/192
2492/0/166
Goodness-of-fit on F2 1.093
0.947
R indices [I > 2d(I)]
wR1 = 0.0433,
R1 = 0.403,
wR2 = 0.0917
R1 = 0.0549,
wR2 = 0.0966
wR2 = 0.1053
R1 = 0.0487,
wR2 = 0.1081
R indices (all data)

P.A. Ajibade et al. / Inorganica Chimica Acta 359 (2006) 3111–3116
3113
is formulated as [Co(C10H9N4O2S)2(CH3OH)2]: CoC22H26-
3.2. Crystal structures
N8O8S2. Yield: 2.58 g (95%); m.p. = 345 ꢁC. Elemental
Anal. Calc. for CoC22H26N8O8S2.: C, 42.51; H, 4.22; N,
18.02; S, 10.32; Co, 9.48. Found: C, 42.44; H, 4.01; N,
17.92; S, 10.21; Co, 9.35%.
The molecular structures of sulfadiazine and the com-
plex, [Co(C10H9N4O2S)2(CH3OH)2], are shown in Figs. 1
and 2 with the atom-numbering scheme used in Table 2,
which also contains the most relevant bond distances and
angles.
2.3. X-ray crystallography
In the centrosymmetric complex, the sulfadiazine
behaves as a bidentate anionic ligand. The central ion coor-
dinated via the sulfonamido N and pyrimidine N on each
of the sulfadiazine anion, with the fifth and sixth coordina-
tion sites occupied by two molecules of methanol. This
indicates that in the medium of the reaction, the N(1)–H
proton is acidic. The crystal structure of the complex con-
sists of discrete neutral [Co(C10H9N4O2S)2(CH3OH)2] mol-
ecules (Fig. 2), which gives rise to network of chains by
means of intermolecular hydrogen bonding (Fig. 3). The
Co(II) atom has a distorted octahedral coordination in
which the sulfadiazine ligands occupy asymmetric positions
in the equatorial plane and the methanol molecules, sulfon-
amide N and the pyrimidine N of each sulfadiazine are
trans to one another. Selected distances and angles are
given in Table 2. The bond lengths between Co(II) and
each of the four coordinated N atoms are the same and
within experimental error and also the bond lengths of
the two coordinated O atoms of the methanol molecules
are the same, since they are related by inversion symmetry.
The bond distances and angles in this complex are slightly
larger, in most cases, than those published for Ag(I) [18],
The crystals for X-ray analyses were obtained by dis-
solving CoCl2 Æ 6H2O (0.238 g, 1 mmol) in 30 mL of meth-
anol and layered with sodium salt (0.545 g, 2 mmol) of
sulfadiazine in 50 mL of methanol in a 150 mL conical
flask. The mixture was allowed to stand at room tempera-
ture for two weeks after which tiny needle-like pink crystals
formed. The crystals of sulfadiazine were obtained from a
solution of methanol. The data sets for the single-crystal
X-ray studies were collected with Mo Ka radiation at
100 K on a Bruker SMART APEX CCD diffractometer
equipped with an Oxford Cryosystems low temperature
device. For the complex, a semi-empirical absorption cor-
rection was applied using SADABS [22] with maximum and
minimum transmissions of 0.8450 and 0.7502, respectively;
no absorption correction was used for the ligand. The
structures were solved by direct methods using SHELXS-97
and completed by iterative cycles DF syntheses, using the
SHELXTL package [22]. Hydrogen atoms were included in
calculated positions and the structures were refined using
full-matrix least-squares refinement against F2 [22]. Crys-
tallographic data are presented in Table 1.
3. Results and discussion
3.1. Synthesis
The sample for the single crystal structure of the ligand
was obtained by recrystallization from methanol. Attempts
were made to use different metal salts with the intention of
establishing the effect of varying the counter-anion on the
composition of the complexes, but irrespective of the metal
salt used (including halides, acetate, nitrate and sulfate),
the product was essentially the same. In each case, two sol-
vent molecules were always accommodated within the
coordination sphere of the complex. The formation of the
complexes may be represented by the general equation
below
CoX2 ꢁ nH2O þ 2NaðC10H9N4O2SÞ
! ½CoðC10H9N4O2SÞ2ðAÞ2ꢂ þ 2NaX þ nH2O
ðX ¼ Clꢀ; NOꢀ; SOꢀ; CH3COOꢀand A ¼ H2O; CH3OHÞ
3
4
The complexes were all pink in color and are completely
insoluble in water and non-coordinating solvents but are
easily soluble in polar solvents such as DMF, with strong
donor strength. The complexes were non-electrolytes in
DMF with Km values of 2.0 for [Co(C10H9N4O2S)2(H2O)2]
and 1.0 Xꢀ1 cm2 molꢀ1 for [Co(C10H9N4O2S)2(CH3OH)2],
respectively.
Fig. 1. Molecular structure of sulfadiazine showing the numbering
scheme.

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P.A. Ajibade et al. / Inorganica Chimica Acta 359 (2006) 3111–3116
Table 2
Selected bond lengths and angles of [Co(C10H9N4O2S)2(CH3OH)2] and
sulfadiazine
˚
Bond lengths (A)
Bond angles (ꢁ)
Co(1)–N(3)
Co(1)–N(1)
Co(1)–O(3)
S(2)–O(2)
S(2)–O(1)
S(2)–N(3)
S(2)–C(5)
O(3)–C(11)
O(3)–H(5)
N(1)–C(4)
N(1)–C(1)
N(3)–C(1)
C(11)–H(11A)
2.1163(15)
2.1166(15)
2.1377(13)
1.4411(14)
1.4420(13)
1.6025(15)
1.7593(17)
1.442(2)
0.75(2)
1.329(2)
1.364(2)
1.367(2)
N(3A)–Co(1)–N(3)
N(3A)–Co(1)–N(1A)
N(3)–Co(1)–N(1A)
N(3A)–Co(1)–N(1)
N(1)–Co(1)–N(1)
N(3A)–Co(1)–O(3A)
N(3)–Co(1)–O(3A)
N(1A)–Co(1)–O(3A)
N(1)–Co(1)–O(3A)
N(3A)–Co(1)–O(3)
C(3)–Co(1)–O(3)
180
62.99(6)
117.01(6)
117.02(6)
180.0
91.16(6)
88.84(6)
89.26(5)
90.74(5)
88.84(6)
91.16(6)
180
N(1A)–Co(1)–O(3)
O(3A)–Co(1)–O(3)
0.9800
107.12(8)
Sulfadiazine
C(8)–N(4)
C(1)–N(2)
C(1)–N(1)
N(3)–S(2)
O(1)–S(2)
O(2)–S(2)
C(5)–S(2)
1.383(3)
1.345(2)
1.381(2)
1.6496(16)
1.4374(13)
1.4282(14)
1.7412(19)
C(10)–C(5)–C(6)
N(1)–C(1)–N(2)
N(1)–C(1)–N(3)
N(2)–C(1)–N(3)
O(1)–S(2)–N(3)
O(2)–S(2)–C(5)
O(2)–S(2)–O(1)
120.80(17)
127.15(17)
118.70(17)
114.14(16)
101.73(8)
108.65(9)
119.57(8)
Fig. 2. Molecular structure of [Co(C10H9N4O2S)2(CH3OH)2] with the
numbering scheme.
Zn(II) [19] and Cd(II) [20] sulfadiazine complexes, presum-
ably because of the differences in modes of coordination.
The packing of the [Co(C10H9N4O2S)2(CH3OH)2] mole-
cules in the crystal structure consists of parallel chains,
which are formed by mutual interactions through hydrogen
bonding between molecules. Each molecule interacts
through 16 hydrogen bonds with five neighbouring mole-
cules (Table 3).
and the metal complexes were studied (Table 4) and
assigned on the basis of careful comparison of their spectra
with that of free ligand. The bands at about 3400 cmꢀ1 in
the complexes are due to symmetrical and asymmetrical
vibrations of the NH2 group [23,24]. The shift to higher
wave numbers in the complexes as compared to the free
ligand is probably due to the presence of H-bonding as
confirmed by the X-ray crystal structure in one of the com-
plexes. In addition to the three peaks observed in the sulfa-
diazine, [Co(C10H9N4O2S)2(H2O)2] showed a generally
broader band with an additional band at 3558 cmꢀ1 attrib-
utable to the coordinated water with possible overlaps with
3.3. Infrared spectra
In order to clarify the mode of bonding and the effect of
the metal ion on the ligand, the IR spectra of the free ligand
Fig. 3. Packing diagram for [Co(C10H9N4O2S)2(CH3OH)2].

P.A. Ajibade et al. / Inorganica Chimica Acta 359 (2006) 3111–3116
3115
Table 3
Bond lengths (A), and angles (ꢁ) and position of atoms of hydrogen bonds
˚
D–Hꢁ ꢁ ꢁA
d(D–H)
d(Hꢁ ꢁ ꢁA)
d(Dꢁ ꢁ ꢁA)
\(DHA)
N(4)–H(5N)ꢁ ꢁ ꢁO(1)#2
N(4)–H(4N)ꢁ ꢁ ꢁO(2)#3
O(3)–H(5)ꢁ ꢁ ꢁO(2)#4
O(3)–H(5)ꢁ ꢁ ꢁN(2)#4
0.92(2)
0.85(2)
0.75(2)
0.75(2)
2.08(2)
2.18(2)
2.69(2)
2.16(2)
2.993(2)
3.021(2)
3.134(2)
2.876(2)
172.5(19)
171.5(19)
119.6(19)
159.0(2)
Symmetry transformations used to generate equivalent atoms: #1 ꢀx, ꢀy + 2, ꢀz + 1; #2 ꢀx, y ꢀ 1/2, ꢀz + 1/2; #3 ꢀx + 1, y ꢀ 1/2, ꢀz + 1/2.
Table 4
Selected IR data (cmꢀ1) for sulfadiazine and the complexes
Sulfadiazine
[Co(C10H9N4O2S)2(H2O)2]
[Co(C10H9N4O2S)2(CH3OH)2]
m(NH2)
m(C@N)
m(SO2)asym.
m(SO2)sym.
m(SN)
3422vs, 3258ms, 3355vs,
1652vs, 1580vs
1325vs
1157vs
942s
3558vs, 3480m, 3378s, 3248m
3431m, 3347s, 3236m
1625w, 1595vs
1267s, 1251m
1129vs
987vs
588vs
1621s, 1598s
1263vs, 1240vs
1124s
1018s, 989vs
590vs
M–N
M–O
426s
Abbreviations: v = very; s = strong; m = medium; w = weak.
the stretching frequencies of the NH2 group on the pyrim-
idine ring. The effect of the coordinated metal is also
noticeable on the SO2 symmetrical and asymmetrical
stretching modes that are shifted to lower wave numbers
(about 33–85 cmꢀ1), while the m(S–N) shifted by about
47 cmꢀ1 to higher wave numbers in both complexes. These
observations confirm the coordination of Co(II) through
the sulfonamido N atom. The m(C@N) stretching vibration
bands that occur at 1652 and 1580 cmꢀ1 in the free ligand
shift to 1621–1595 cmꢀ1 in the complexes, this supports the
coordination of the metal ions through the quinoline N(1)
atom. The complexes show medium to weak mCo–N bands
between 588 and 500 cmꢀ1 and Co–O bands between 450
and 330 cmꢀ1, respectively [23]. [Co(C10H9N4O2S)2(H2O)2]
gave an additional intense band at 426 cmꢀ1 which is com-
pletely absent in [Co(C10H9N4O2S)2(CH3OH)2] probably
due to strong coordination of the water molecule as com-
pared to methanol. The bands at 3347–3347 cmꢀ1 and
those at about 732 cmꢀ1 are due to stretching vibration
and out of plane bending modes of coordinated water
and methanol molecules. The involvement of N and O in
coordination is further strengthened by the appearance of
bands at 590–297 cmꢀ1 assigned to m(Co–N + Co–O).
18,656 cmꢀ1 and 22,988 cmꢀ1, respectively, which may ten-
4
4
4
tatively be assigned to T1g(F) ! T2g and T1g(F) !
4T1(P) transitions of pseudo-octahedral cobalt(II).
4. Conclusion
The complexes [Co(C10H9N4O2S)2(CH3OH)2] and
[Co(C10H9N4O2S)2(H2O)2] have been synthesised and
characterised. The structure of [Co(C10H9N4O2S)2-
(CH3OH)2] was established by single-crystal X-ray, while
that of [Co(C10H9N4O2S)2(H2O)2] was inferred from its ele-
mental analysis, conductivity in solution, electronic and IR
spectra, and magnetic susceptibility measurements. The IR
confirmed the presence of water molecule in the coordina-
tion sphere of [Co(C10H9N4O2S)2(H2O)2]. The electronic
spectra and magnetic moments of both complexes were sim-
ilar, indicating that both complexes have similar structure.
5. Supplementary data
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic Data
Centre, CCDC No. 271431 for compound [Co(SD)2-
(CH3OH)2] and CCDC No. 271432 for sulfadiazine (SD).
Copies of this information may be obtained free of charge
from The Director, CCDC, 12 Union Road, Cambridge,
CB2 1EZ, UK (fax: +44 1223 336 033; e-mail: depos-
it@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk.).
3.4. Electronic spectra and magnetic susceptibility
measurement
Magnetic susceptibility measurements were carried out
on powdered samples at 297 K. The effective magnetic
moments, 4.86 B.M. for [Co(C10H9N4O2S)2(H2O)2] and
4.93 B.M. for [Co(C10H9N4O2S)2(CH3OH)2], are consis-
tent with high-spin octahedral cobalt(II) complexes. The
electronic spectra of the complexes in DMF confirmed
their octahedral stereochemistry. The electronic spectra of
both show two bands of weak to medium intensity at
Acknowledgements
The authors gratefully acknowledge the financial sup-
port from National Research Foundation (South Africa)
and the Royal Society (UK). PAA thanks the University
of Ado Ekiti, Nigeria, for a study leave.

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P.A. Ajibade et al. / Inorganica Chimica Acta 359 (2006) 3111–3116
[14] N.H. Gokhale, S.B. Padhye, S.L. Croft, H.D. Kendrick, W. Davies,
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