Structural diversity in dinickel(II) complexes of thiosemicarbazones

Article abstract of DOI:10.1016/S0022-2860(02)00313-7

2,6-Diformyl-p-cresol serves as a starting point for the generation of multidentate N/O/S chelating agents. Condensation with 4-(X-phenyl) thiosemicarbazide yields the pentadentate ligand having SNONS donor sequences, capable of holding two metal ions in

Full text of DOI:10.1016/S0022-2860(02)00313-7

Journal of Molecular Structure 616 (2002) 119–127
www.elsevier.com/locate/molstruc
Structural diversity in dinickel(II) complexes of
thiosemicarbazones
*
Anil D. Naik, Satish M. Annigeri, Umesh B. Gangadharmath, Vidyanand K. Revankar ,
Vinayak B. Mahale
Department of Chemistry, Karnatak University, Dharwad 580 003, India
Received 10 December 2001; revised 14 May 2002; accepted 14 May 2002
Abstract
2,6-Diformyl-p-cresol serves as a starting point for the generation of multidentate N/O/S chelating agents. Condensation with
4-(X-phenyl) thiosemicarbazide yields the pentadentate ligand having SNONS donor sequences, capable of holding two metal
ions in close proximity. The ligands behave as mono/di/tri basic depending on the pH of the medium. Stereochemical diversity
in the reaction product of such ligands with nickel(II) chloride at different pH is observed. Sterically demanding substituted
ligands in association with various exogenous bridges dictate the geometry and coordination number of such complexes. The
compounds were investigated by elemental analysis, molar conductivities, electronic spectra, IR, NMR, FAB mass spectra, TG-
DTG, magnetic susceptibility measurements. Varieties of geometries such as square planar, square pyramidal, octahedral and
square planar–square pyramidal are observed. Cryomagnetic data for the complexes (79–296 K) can be reproduced by an
equation based on the Heisenberg model (H ¼ 22JS1S2, S1 ¼ S2 ¼ 1). The singlet–triplet splitting, J varies systematically
with the coordination geometry about the Ni2(SNONS) core, with the hydroxo bridged complex exhibiting the greatest degree
of antiferromagnetic coupling. The coupling is somewhat weaker for the chloro-bridged complexes. None of the complexes
have shown any appreciable antimicrobial activity. q 2002 Elsevier Science B.V. All rights reserved.
Keywords: Nickel complexes; Thiosemicarbazone; Antiferromagnetic coupling; Structural diversity
1. Introduction
sites of certain ureases, methyl-S-coenzyme-M-
methyl reductase, hydrogenases and dehydrogenases
and may play a role in the supposed mutagenicity of
nickel compounds. Several aspects of the chemistry of
these biological nickel ions are unusual in the context
of the known coordination chemistry of nickel: three
of the four enzyme are known to contain redox active
nickel centers that can cycle between the þ3, þ2,
and/or þ1 oxidation states, in thiolate rich or
tetrapyrrole ligand environments that were not
previously thought to favor metal centered redox
processes in the nickel complexes [2–5].
Thiosemicarbazones comprise an intriguing class
of chelating molecules, which possess a wide range of
beneficial medicinal properties [1]. In the last few
years, nickel(II) complexes containing sulfur donors
have received considerable attention due to the
identification of a sulfur rich coordination environ-
ment in biological nickel centers such as the active
*
Corresponding author. Tel.: þ91-836-747121; fax: þ91-836-
721275.
E-mail address: vkrevankar@rediffmail.com (V.K. Revankar).
Despite the plethora of spectroscopic, redox and
0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(02)00313-7

120
A.D. Naik et al. / Journal of Molecular Structure 616 (2002) 119–127
magnetic data that are now available many details of
the nature and role of the nickel centers in these
enzymes are still unclear. This continuing uncertainty
has inspired a concurrent growth of interest in the
coordination chemistry of nickel; significant efforts
towards the synthetic modeling of all the above nickel
biosites have been expended by many researchers [2].
In addition developing, understanding and exploit-
ing structural diversity, electronic communication
between metal centers, and stabilizing oxidation states
is a continuing area of interest to both synthetic and
theoretical chemist. The focuses of most synthetic
studies in this area are complexes in which metal ions
are linked via a phenoxo-bridged organic backbones
with sufficient flexibility to accommodate various
exogenous bridges [6–10].
solution (10 cm3) of nickel chloride (0.002) was
added drop wise. After addition of a small portion of
dil. NH3 (0.5 cm3) the resulting mixture was again
stirred for 1 h and then refluxed for 1 h. The
precipitated complexes were isolated by filtration,
washed with EtOH and dried in vacuo. Yield 95%.
3. Physical measurements
The complexes were analyzed for their metal
content by EDTA titration after decomposition with a
mixture of HCl and HClO4. Sulfur was estimated as
BaSO4. C, H, and N were estimated on a Thermoquest
CHN analyzer. Magnetic susceptibility measurements
were made at room temp on a Gouy balance using
Hg[Co(SCN)4] as calibrant. The magnetic suscepti-
bilities on powdered samples of the complexes were
measured in the temperature range 77–300 K using a
PAR model 155 vibrating sample magnetometer and
the instrument was calibrated with the use of metallic
nickel. Electronic spectra were recorded on a Hitachi
2001 in DMF. IR spectra were recorded in the 4000–
400 cm21 region (KBr disc) on a Nicolet 170 SX FT-
IR. Far IR spectra were recorded in the 500–
100 cm21 region on a Bruker IFS66V (Polythene
In continuation of our work on biologically
potential binucleating ligands thiosemicarbazones,
here we report the synthesis, spectral and magnetic
studies of 2,6-diformyl-p-cresol-(4-X phenyl) thiose-
micarbazones (Fig. 1) [11,12] to probe the influence
of different substitution on the phenyl ring of the
thiosemicarbazone limb on the structural, spectral and
magnetic properties of resulting complexes having the
M2N2OS2 chromophore.
1
discs). The H NMR spectra were obtained in d6-
2. Experimental
DMSO using TMS as an internal reference. Con-
ductance measurements were determined in DMF
(1023 M) using a ELICO-CM82 Conductivity Bridge.
TG-DTG studies were carried out in the 25–800 8C
range using a Rigaku TAS-100 Model thermal
analyzer with a heating rate of 10 8C per min. in a
N2 atmosphere. The FAB mass spectrum was
recorded on a JEOL EX 102/DA-6000 mass spec-
trometer using Ar as the FAB gas. m-Nitrobenzyl
alcohol (NBA) was used as the matrix.
The preparation of 2,6-diformyl-p-cresol was
carried out according to the reported method with
slight modification [13]. The synthesis of aryl
thiosemicarbazides and substituted thiosemicarba-
zides was carried out according to the procedure
reported elsewhere [14]. Preparation of aryl thiose-
micarbazones was based on our earlier report [11].
2.1. Preparation of the complexes
[Ni2(m-Cl)L0·nCl·m H2O] (1–10). A mixture of
nickel chloride (0.002 mol), appropriate thiosemicar-
bazone (0.001 mol) and few drops of dil. HCl in EtOH
(40 cm3) were refluxed for 2 h. The separated
complex were filtered, washed with EtOH and dried
in air. Yield 80%.
[Ni2(m-OH)L0·x H2O] (11–20). To a vigorously
stirred suspension of the appropriate thiosemicarba-
zone (0.001 mol) in EtOH (25 cm3), an ethanolic
4. Results and discussion
All the complexes are non-hygroscopic. The
complexes contain 2:1 metal-to-ligand ratio. They
are formed by the loss of one, two or three proton(s).
They are insoluble in water, EtOH, MeOH but soluble
in DMF, DMSO and MeCN. Analytical data are
presented in Tables 1 and 2. Molar conductivities in
DMF suggest that the complexes are non-electrolyte

A.D. Naik et al. / Journal of Molecular Structure 616 (2002) 119–127
121
Fig. 1. Structure of ligands (L1H3 to L10H3).
(10–16 mho cm2 mol21). Color of the complexes
ranges from pale yellow to dark brown. Melting
points of all the complexes are above 250 8C.
bands at around 850, 760, 620, and 370 nm suggesting
square pyramidal geometry. Complexes (2), (3), (5)
and (6) contain similar bands with an additional weak
shoulder around 570 nm. This is assigned to
1
4.1. Electronic spectral data
1A1g ! A2g transition of square planar nickel. In
square pyramidal complexes the lowest energy band
could not be located, as it might be shifted almost to
near-IR region. Squeezing of the SNONS compart-
ment by the appropriately substituted ligand might
generate cavities of the right size to accommodate two
nickel atoms under different stereochemical configur-
ation. A considerable stronger in-plane ligand field
strength of the square planar nickel(II) relative to that
of the square pyramidal nickel(II) is not unusual,
because the Ni–N and Ni–O distances at the square
nickel are shorter than those of the square pyramidal
nickel [17,18]. Complexes (15) and (17) are
According to their electronic spectral character-
istics (Tables 3 and 4), the complexes can be divided
into four groups, viz. I (1, 11–14, 16, 18–20), II (2, 3,
5, 6), III (4, 7–10) and IV (15–17). The complexes
(4) and (7–10) adopt hexa-coordination and are
evidenced by its electronic spectra. Three d–d bands
observed at around 960, 590, and 465 nm can be
attributed in an octahedral model to the transitions,
3
3
3
3
3
3A2g ! T2g, A2g ! T1g and A2g ! T1g(P),
respectively [15,16].
Complexes (1), (11–14), (16), (18–20) exhibits
Table 1
Results of elemental analysis (1–10)
Code
Complex
Found (%) (calcd (%))
C
H
N
S
M
Cl
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
[Ni2(m-Cl)L1H2·2Cl]
[Ni2(m-Cl)L2H·Cl]
40.31 (40.33)
46.29 (46.27)
46.22 (46.27)
40.13 (40.07)
42.40 (42.36)
42.38 (42.36)
38.49 (38.43)
36.77 (36.67)
36.69 (36.67)
36.78 (36.67)
3.09 (3.11)
3.77 (3.73)
3.71 (3.73)
3.78 (3.90)
3.45 (3.41)
3.48 (3.41)
3.80 (3.74)
3.38 (3.34)
3.37 (3.34)
3.38 (3.34)
12.30 (12.26)
8.67 (8.63)
9.38 (9.36)
9.78 (9.88)
9.75 (9.88)
8.62 (8.56)
9.11 (9.05)
9.15 (9.05)
8.25 (8.21)
8.43 (8.51)
8.42 (8.51)
8.66 (8.51)
17.2 (17.13)
18.11 (18.09)
18.18 (18.09)
15.55 (15.66)
16.61 (16.56)
16.59 (16.56)
15.03 (15.02)
15.55 (15.58)
15.43 (15.58)
15.61 (15.58)
15.41 (15.54)
10.88 (10.94)
10.91 (10.94)
14.22 (14.21)
10.11 (10.01)
10.12 (10.01)
13.69 (13.62)
4.75 (4.71)
[Ni2(m-Cl)L3H·Cl]
8.69 (8.63)
[Ni2(m-Cl)L4H2·2Cl·2H2O]
[Ni2(m-Cl)L5H·Cl]
[Ni2(m-Cl)L6H·Cl]
[Ni2(m-Cl)L7H2·2Cl·2H2O]
[Ni2(m-Cl)L8·4H2O]
[Ni2(m-Cl)L9·4H2O]
[Ni2(m-Cl)L10·4H2O]
11.22 (11.21)
11.90 (11.86)
11.79 (11.86)
10.77 (10.76)
11.21 (11.16)
11.23 (11.16)
11.18 (11.16)
4.77 (4.71)
4.73 (4.71)

122
A.D. Naik et al. / Journal of Molecular Structure 616 (2002) 119–127
Table 2
Results of elemental analysis (11–20)
Code
Complex
Found (%) (calcd (%))
C
H
N
S
M
Cl
(11)
(12)
(13)
(14)
(15)
(16)
(17)
(18)
(19)
(20)
[Ni2(m-OH)L1·2H2O]
[Ni2(m-OH)L2·2H2O]
[Ni2(m-OH)L3·2H2O]
[Ni2(m-OH)L4·2H2O]
[Ni2(m-OH)L5]
43.88 (43.85)
45.50 (45.49)
45.44 (45.49)
45.43 (45.49)
45.95 (45.91)
44.45 (44.42)
45.92 (45.91)
39.58 (39.53)
39.58 (39.53)
39.49 (39.53)
3.89 (3.84)
4.56 (4.58)
4.52 (4.58)
4.59 (4.58)
3.72 (3.7)
4.49 (4.47)
3.73 (3.7)
3.2 (3.17)
3.14 (3.17)
3.25 (3.17)
13.37 (13.34)
12.77 (12.73)
12.71 (12.73)
12.7 (12.73)
12.88 (12.85)
12.47 (12.43)
12.89 (12.85)
12.09 (12.03)
12.1 (12.03)
12.11 (12.03)
10.21 (10.18)
9.68 (9.72)
9.73 (9.72)
9.77 (9.72)
9.76 (9.81)
9.43 (9.49)
9.82 (9.81)
9.22 (9.18)
9.23 (9.18)
9.22 (9.18)
18.55 (18.63)
17.62 (17.78)
17.8 (17.78)
17.8 (17.78)
17.99 (17.95)
17.4 (17.36)
17.99 (17.95)
16.85 (16.8)
16.92 (16.8)
16.93 (16.8)
–
–
–
–
–
–
–
–
–
–
[Ni2(m-OH)L6·2H2O]
[Ni2(m-OH)L7]
[Ni2(m-OH)L8·2H2O]
[Ni2(m-OH)L9·2H2O]
[Ni2(m-OH)L10·2H2O]
diamagnetic. The electronic spectra exhibit a shoulder
band at around 570 nm due to a metal d–d transition,
which is characteristic of square planar nickel(II)
complexes. Bands below 350 nm, which were
assigned to intra-ligand transitions in ligands, remain
unaffected in all complexes.
For high spin d8 nickel dx22y2 and d2z are the two
magnetic orbitals. The dominant pathway involved in
exchange interaction between two nickel(II) centers is
d2x22y orbital of the metal ions and s and p orbitals of
the phenoxy oxygen. Magnetic analyses were carried
out using the magnetic susceptibility expression (1)
for an S1 ¼ S2 ¼ 1 system based on the Heisenberg
model [19]. In this expression N, b, k and g have their
usual meaning. Na is the temperature independent
paramagnetism. The second term accounts for the
non-coupled impurity. The xm vs. T plot shown in
Figs. 2 and 3 are typical of binuclear nickel(II)
complexes exhibiting significant antiferromagnetic
exchange. Fairly good magnetic simulations are
obtained with the equation, as seen in Figs. 2 and 3,
4.2. Magnetochemistry
The room temperature magnetic moments of the
hydroxy-bridged complexes ranges from 1.11 to
1.40 BM. These values are far below the spin only
value (2.87 BM) for Ni(II) indicating a high degree of
antiferromagnetic interaction between metal centers.
The chloro-bridged species show a lesser degree of
spin–spin coupling possessing room temperature
magnetic moment of 1.7–2.5 BM (Tables 3 and 4).
using magnetic parameter of J ¼ 260 cm21
,
g ¼ 2.11, Na ¼ 200 £ 1026 cm3 mol21 for (10) and
J ¼ 270 cm21
Table 4
,
g ¼ 2.12,
Na ¼ 200 £ 1026
Table 3
Electronic spectral data and magnetic moments of the complexes
(1–10)
Electronic spectral data and magnetic moments of the complexes
(11–20)
Complex lmax (nm)
m
eff per Niþþ
Complex
l
max (nm)
m
eff per Niþþ
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
860, 750, 620, 370, 348, 266
2.10
870, 755, 620, 570sh, 368, 343, 270 1.91
865, 751, 621, 568sh, 371, 344, 270 1.94
(11)
(12)
(13)
(14)
(15)
(16)
(17)
(18)
(19)
(20)
855, 745, 610, 371, 348, 266
842, 755, 615, 377, 350, 267
850, 748, 620, 372, 345, 260
848, 761, 618, 374, 348, 264
570, 350, 275
1.12
1.11
960, 592, 462, 350, 267
2.40
1.20
865, 755, 615, 567sh, 365, 349, 268 1.96
863, 753, 618, 570sh, 369, 348, 267 1.90
1.30
Diamagnetic
1.14
955, 589, 461, 343, 269
960, 590, 461, 349, 270
957, 595, 462, 344, 265
960, 597, 465, 350, 265
2.50
1.94
1.88
1.72
853, 744, 612, 372, 346, 268
581, 348, 261
Diamagnetic
1.17
858, 765, 628, 378, 349, 255
860, 770, 624, 372, 350, 245
863, 762, 621, 373, 340, 267
1.22
1.40
sh denotes shoulder.

A.D. Naik et al. / Journal of Molecular Structure 616 (2002) 119–127
123
The Ni–Ophenolate–Ni angle increases on going
from –OH bridge complex to halide bridge complex.
So the antiferromagnetic coupling should increase in
the same order [21]. The opposite trend observed in
our study can be rationalized on the basis that the Ni–
Cl–Ni angle decreases from Ni–OH–Ni to Ni–Cl–
Ni because both the Cl atom and the Ni–Cl distance
become large. The relevant structural trend here is that
the most strongly coupled complex possesses (a) the
smallest Ni–Ophenolate–Ni bridging angle (b) the
largest Ni–Cl–Ni bridging angle. So the Ni–Cl–Ni
angle is the most important determinant in the sign
and magnitude of the magnetic coupling between
metal centers [21–23]. Substantial decrease in the
value of meff occurs when a tetragonally elongated six
coordinated species transforms to a five coordinate
square pyramidal species. Report also suggests that
both distortion from the planar geometry and
reduction in electron density on the metal centers by
the electron-withdrawing effect of bridging exogen-
ous chloride are less favorable for effective spin
exchange interactions and, consequently, the super-
exchange interaction decreases [20].
Fig. 2. Molar paramagnetic susceptibility per binuclear Ni(II)
complex and effective magnetic moment per Ni(II) ion vs.
temperature for (Ni2(m-Cl)L10·4H2O]. The solid line results from
least-square fit to the theoretical Eq. (1) given in the text.
cm3 mol21 for (20).
ꢀ
ꢁ
Ng2b2
5 þ expð24J=kTÞ
Xm
¼
kT
5 þ 3expð24J=kTÞ þ expð26J=kTÞ
Ng2b2
Ginsberg [24,25] has shown that the evaluation of
accurate g and J parameters for a given nickel dimer
will not be influenced markedly by the value of either
the zero field splitting parameter D or the intercluster
interaction Z0J0. Hendrikson et al. [26] too showed that
the effect of a large variance in D and Z0J0 is only to
displace the xm vs. temperature curve along the x-axis.
The position of the maximum is essentially
unchanged and since this position determines the
value of J, the value of D or Z0J0 has little effect on the
evaluation of J. Owing to the large stabilization of
the singlet ground state, the effect of zero-field
splitting on the magnetic behavior is expected to be
negligible [27].
 ð1 2 rÞ þ
r þ Na
ð1Þ
3kT
P
The discrepancy factor defined as RðxÞ ¼ ½ ðXobs
2.27 £ 1023 for (20), respectively.
2
P
2
2 1=2
XcalcÞ = ðXobsÞ Š
was 2.3 £ 1023 for (10) and
In summary the two possible superexchange path-
ways are in opposite directions to one another. Over
all, the Ni–Ophenolate–Ni pathway dominates and give
rise to reasonably strong antiferromagnetic coupling.
The Ni–X–Ni (X ¼ –Cl, –OH) pathway modulates
the extent of coupling and is over whelmed by the Ni–
Ophenolate–Ni effect.
Fig. 3. Molar paramagnetic susceptibility per binuclear Ni(II)
complex and effective magnetic moment per Ni(II) ion vs.
temperature for (Ni2(m-OH)L10·2H2O]. The solid line results from
least-square fit to the theoretical Eq. (1) given in the text.
4.3. NMR spectral studies
The proton NMR, data for the ligand (L5H3, L7H3)

124
A.D. Naik et al. / Journal of Molecular Structure 616 (2002) 119–127
and nickel complexes (15, 17) are given in the Table
5. A singlet at 2.3 ppm (in L5H3) and 2.29 (in L7H3)
are accounted to –CH3 of p-cresol moiety of the
ligands. Multiplets in the region 6.91–7.76 ppm are
assigned to aromatic protons. The proton adjacent to
–CH3 group of p-cresol moiety have appeared as a
singlet at 7.76 ppm. Azomethine proton is observed
around 8.46 ppm. The phenyl –4NH proton signal is
observed around 10.18 ppm and hydrazine –2NH
signal at around 11.86 ppm. Phenolic proton signal is
not observed in the 0–15 ppm region. On D2O
exchange, both –2NH and –4NH signals disappears
[11].
ably in intensity suggesting involvement of sulfur in
coordination. The complexation is via thioketo sulfur
in (1), (4), and (7) complexes, where n(2N–H) is
retained but thioamide band IV has undergone slight
shift in position. In all hydroxy bridged complexes as
well as in few chloro bridged complexes (8–10), the
n(2N–H) has disappeared completely and thioamide
bands have suffered changes in both intensity and
positions. The hydroxy bridged complexes exhibit a
sharp band in the range 3480–3600 cm21 which is
assigned to the bridging O–H stretch on the basis of
previous observations [33]. The bridging O–H
stretching band is readily distinguished from the O–
H stretching bands for waters of hydration, which
occurs at 3200–3300 cm21. The spectrum is rather
complex in the region below 400 cm21 where the
various M–L atom-stretching vibrations are often
found in combination with other bands. The low
frequency bands in the region 430–410, 365–320,
and 295–205 cm21 are tentatively assigned to
n(M–O), n(M–N), and n(M–S), respectively. The
presence of chloro-bridge is evident from the
observation of IR bands at around 300 cm21 in
(1–10) [32,34].
The deprotonation and chelation of the ligand is
accompanied by the loss of the resonances at
11.86 ppm (NH). The azomethine proton undergoes
shift towards low field suggesting coordination of
azomethine nitrogen. These changes are attributable
to a variation of electronic density and steric
constraints brought about by the chelation [28,29].
Bridging inorganic –OH is identified around
3.45 ppm as a broad signal.
4.4. IR spectral studies
Pertinent IR absorptions are presented in Tables 6
and 7. The possibility of thione–thiol tautomerism
(H–N–CyS , NyC–SH) in all the ligands has been
ruled out, since there is no band around 2500–
2600 cm21 which is characteristic of thiol group. The
phenyl n(4NH) and hydrazine n(2NH) are observed
around 3300 and 3100 cm21, respectively [11,12].
The phenolic n(OH) is found around 2900 cm21 as a
weak broad band which disappear in all complexes
indicating deprotonation and coordination to metal.
Coupled vibration among thioamide bands (I, II, II
and IV) are distributed around 1535, 1422, 1374, and
1120 cm21. The sharp band at around 1618 cm21 was
assigned to n(CyN) of azomethine linkage [11,
30–32].
4.5. Thermal behavior and FAB mass spectra
Thermal behavior of few complexes (1, 8, and 11)
was studied in the temperature range 25–800 8C. All
the dehydrated complexes decompose in two major
steps. In complex (8) and (11) the weight loss between
150 and 210 8C is due to the loss of four and two
coordinated matter molecules, respectively. In the first
step of all the complexes under study, the weight loss
corresponds to combined loss of a thiosemicarbazide
limb plus chloride, if present. Partially decomposed,
rearranged, intermediate complex continues to loose
weight up to 750 8C. Residues were analyzed as NiO.
Weight losses from the TG, agrees well with
theoretical calculations.
In complexes (2), (3), (5), and (6) the band around
3130 cm21 which was assigned to n(2N–H) in the
ligand, decreases in intensity (nearly half) relative to
n(4N–H) on complexation. This suggests that only
one 2N–H proton has undergone deprotonation
through thioenolization on complexation. The band
around 950 cm21 in the ligand, which was assigned to
coupled vibration of n(CyS) has lowered consider-
FAB mass spectroscopic data (1 and 20) clearly
suggested that dinuclear complexes have been formed
in each case. The peaks at highest m/z values could not
always be assigned with certainty, but the isotopic
patterns in both cases are consistence with dinuclear
complexes. For (1), a molecular ion peak was
(m/z ¼ 683) observed corresponding to the mass of
the entire complex including bridging and axially

A.D. Naik et al. / Journal of Molecular Structure 616 (2002) 119–127
125
Table 5
NMR spectral data
Compound
CH3
OCH3
2NH (hydrazine)
4NH (phenyl)
H–CyN
Aromatic
OH bridging
L5H3
(15)
2.51
2.28
2.32
2.50
3.37
3.77
3.88
3.87
11.83
–
10.06
9.20
9.95
9.96
8.47
8.11
8.48
8.66
6.94–8.43
6.81–7.58
6.95–8.14
6.86–8.23
–
3.45
–
L7H3
11.96
–
(17)
3.42
coordinated anion. FAB mass spectra of complex (20)
reveal an intense peak for parent cation at m/z ¼ 697.
designed to enforce geometrical constraints on the
metal atoms. Also to verify the different behavior of
acidic protons (hydrazinic protons, which have
different pKa values) in the ligand towards bases, the
pH of the reaction medium is varied during complex
preparation. The nature of the second bridging
strongly depends on the experimental conditions
employed. The presence of excess of proton sponge
(base) gives rise to exclusively hydroxy-bridged
binuclear complexes. In (15) and (17), the in plane
ligand field strength, having bulkier substituted
ligands at o and p positions forced the high spin
Ni2þ ion to acquire low spin state. Surprisingly in m-
substituted complex (16), the metal enjoyed high spin
state probably due to steric relaxation around it due to
the out of plane orientation of –OCH3. Juxtaposition
of two nickel(II) centers in different spin states with
ligands having the same in-plane donor atoms as we
observed in most of the complexes is quite interesting
(Fig. 4).
4.6. Biological activity
The antimicrobial activities of few compounds
were assessed by Cup–plate method [35]. Two strains
of bacteria (Pseudomonas auregenosa, Bacillus
cirroflagellosus ) and fungi (Aspgergillus niger,
Candida albicanas ) were selected for the activity
study. None of the metal chelates have shown any
pronounced inhibitory effect on these microbes.
5. Conclusion
Controlling the geometry at a metal ion site is of
fundamental importance in many areas of chemistry.
The judicious choice of ligand, which forces metal
ions into unusual geometries, is of interest in
designing biomimics. In this communication, we
have probed the properties of nickel(II) complexes
prepared from a series of related ligands, which were
The electronic structure of the coupled polynuclear
systems is receiving ever-increasing attention for
designing magnetic materials and understanding the
Table 6
IR spectral results of the complexes (1–10)
Complex
n(4NH)
n(2NH)
n(CN)
Thioamide bands
n(C–S)
I
II
III
IV
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
3380s
3275s
3260s
3264s
3265s
3277s
3274s
3270s
3271s
3275s
3220m
3180w
3181w
3184m
3175w
3177w
3135m
–
1625m
1615m
1620m
1621m
1621m
1621m
1624m
1610s
1562s
1551s
1567s
1560s
1541s
1560s
1555s
1554s
1560s
1554s
1498m
1490m
1517m
1490m
1490m
1479m
1492m
1470m
1491m
1511m
1331m
1325m
1357m
1350m
1320m
1337m
1319m
1342m
1313m
1332m
905w
945w
940w
970w
942w
917w
945w
940w
948w
935w
–
725w
733w
–
650w
677w
–
751w
752w
700w
–
1603s
–
1622s
w denotes weak, s strong and m medium intensity band.

126
A.D. Naik et al. / Journal of Molecular Structure 616 (2002) 119–127
Table 7
IR spectral results of the complexes (11–20)
Complex
n(4NH)
n(OH)
n(CN)
Thioamide bands
n(C–S)
I
II
III
IV
(11)
(12)
(13)
(14)
(15)
(16)
(17)
(18)
(19)
(20)
3240s
3310s
3245s
3251s
3260s
3241s
3245s
3235s
3235s
3230s
3354m
3437m
3351m
3350m
3365m
3338m
3394m
3338m
3415m
3260m
1597m
1584m
1609m
1597m
1621s
1554s
1555s
1547s
1540s
1541s
1554s
1541s
1547s
1516s
1556s
1504m
1510m
1457m
1449m
1501m
1492m
1510m
1493m
1489m
1508m
1325m
1326m
1327m
1326m
1340m
1325m
1332m
1323m
1331m
1337m
940w
935w
930w
923w
922w
940w
935w
952w
961w
955w
689m
625m
691m
655m
622m
677m
680m
650m
644m
645m
1594s
1591s
1603m
1598m
1609m
w denotes weak, s strong and m medium intensity band.
Fig. 4. Proposed structure of the complexes (1–20).

A.D. Naik et al. / Journal of Molecular Structure 616 (2002) 119–127
127
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The authors thank the USIC, Karnatak University,
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