Synthesis, characterization, crystal structure and bioactivities of a new potential tridentate (ONS) Schiff base ligand N-[2-(benzylthio) phenyl] salicylaldimine and its Ni(II), Cu(II) and Co(II) complexes

Article abstract of DOI:10.1016/j.poly.2013.04.062

A series of new bichelated complexes of the formula [M(L) ₂]·H₂O were synthesized from the reaction of N-[(2-benzylthio)-phenyl] salicylaldimine (HL), 1, a new tridentate (O, N, S donor) potential Schiff base with metal salts in methanol medium. These compounds were characterized by elemental analyses, FT-IR, UV-Visible and magnetic susceptibility studies. The crystal structures of the ligand 1 as well as two nickel(II) complexes [Ni(L)₂]·H₂O, 2 and 3, were determined by X-ray diffraction studies. The nickel atom is coordinated to two oxygen atoms, two nitrogen atoms and two sulfur atoms in bichelated fashion of the ligand, 1 indicating that the metal atom is in a slightly distorted octahedral environment. The bioactivity studies showed that the ligand and the complexes exhibit considerable antibacterial and antifungal activities against Bacillus subtilis (Gram +ve), Klebsiella pneumonia (Gram -ve) and Candida albicans.

Full text of DOI:10.1016/j.poly.2013.04.062

Polyhedron 60 (2013) 47–53
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization, crystal structure and bioactivities of a new
potential tridentate (ONS) Schiff base ligand N-[2-(benzylthio) phenyl]
salicylaldimine and its Ni(II), Cu(II) and Co(II) complexes
b
Mukul Kalita a, Tirtha Bhattacharjee a, Prasanta Gogoi a, Pranjit Barman a, , Ranjan D. Kalita ,
⇑
Bipul Sarma c, Sanjib Karmakar d
a Department of Chemistry, National Institute of Technology, Silchar 788010, Assam, India
b Department of Molecular Biology and Biotechnology, Tezpur University, Tezpur 784028, Assam, India
c Department of Chemical Sciences, Tezpur University, Tezpur 784028, Assam, India
d Instrumentation & USIC, Gauhati University, Guwahati 781014, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new bichelated complexes of the formula [M(L)2]ÁH2O were synthesized from the reaction of
N-[(2-benzylthio)-phenyl] salicylaldimine (HL), 1, a new tridentate (O, N, S donor) potential Schiff base
with metal salts in methanol medium. These compounds were characterized by elemental analyses,
FT-IR, UV–Visible and magnetic susceptibility studies. The crystal structures of the ligand 1 as well as
two nickel(II) complexes [Ni(L)2]ÁH2O, 2 and 3, were determined by X-ray diffraction studies. The nickel
atom is coordinated to two oxygen atoms, two nitrogen atoms and two sulfur atoms in bichelated fashion
of the ligand, 1 indicating that the metal atom is in a slightly distorted octahedral environment. The bio-
activity studies showed that the ligand and the complexes exhibit considerable antibacterial and anti-
fungal activities against Bacillus subtilis (Gram +ve), Klebsiella pneumonia (Gram Àve) and Candida
albicans.
Received 3 January 2013
Accepted 27 April 2013
Available online 22 May 2013
Keywords:
N-[(2-benzylthio) phenyl] salicylaldimine
ONS donor ligand
Schiff base
Crystal structure
Antibacterial
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
biological systems as well as for assisting in the development of
new homogeneous catalysts for selective oxidation [6]. Nickel
Schiff bases are indispensible ligands in coordination chemistry
due to their ease of synthesis and ability to be readily modified
both electronically and sterically. The Schiff bases derived from sal-
icylaldehydes are well known as polydentate ligands coordinating
in the neutral form [1,2]. The multidenticity of these ligands plays
an important role in the formation of transition metal complexes,
the stability of these complexes and allowing for the manipulation
of steric and electronic parameters to influence the reactivity at the
metal ions. Metal complexes with an ONS donor set are of much
interest because of their potential applications in fundamental
and applied sciences, especially in coordination chemistry, due to
their diverse roles in metallo-enzymes [3], varied catalytic [3,4]
and pharmacological properties [5]. The potential role played by
nickel ions, being present in the active sites of a large number of
metallo-proteins, has stimulated efforts to design new ligands
and complexes as models to provide a better understanding of
complexes of some bivalent ONS thiosemicarbazone ligands have
catalyzed important reactions related to CO-dehydrogenase and si-
lane alcoholysis. Copper complexes with ONS donor atoms show
interesting magnetic properties as well as biocidal activities (anti-
bacterial, antifungal) [7,8]. These complexes may be used as func-
tional models or may statically mimic the spectroscopic or other
physical properties of the enzyme [9]. Conversely, the various
coordination structures preferred and induced by different metal
configurations significantly alter the conformations and coordina-
tion modes of the ligands and eventually lead to rather diverse
kinds of complexes. Neutral tridentates are shown to form octahe-
dral complexes with transition metals, where two ligands encom-
pass the metal ion in an octahedral array.
Herein, we report the synthesis and characterization of a new
ONS donor Schiff base ligand, N-[(2-benzylthio)-phenyl] salicyl-
aldimine and its nickel(II), copper(II) and cobalt(II) complexes.
Crystal structures of the ligand and two nickel(II) complexes
have been solved by X-ray diffraction. The antibacterial and anti-
fungal activities of the ligand and complexes have also been
tested against Bacillus subtilis, Klebsiella pneumonia and Candida
albicans.
⇑
Corresponding author. Tel.: +91 9435374128; fax: +91 3842 224797.
E-mail addresses: mukulkalita88@gmail.com (M. Kalita), Tirtha_07@yahoo.com
(T. Bhattacharjee), prasantagogoi799@gmail.com (P. Gogoi), barmanpranjit@
yahoo.co.in (P. Barman), dkranjan@gmail.com (R.D. Kalita), bcsarma@tezu.ernet.in
(Bipul Sarma), sanjibkab@rediffmail.com (S. Karmakar).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.04.062

48
M. Kalita et al. / Polyhedron 60 (2013) 47–53
crystals, suitable for X-ray diffraction, that formed were filtered off,
2. Experimental
washed with 25% methanol in water solution several times to
remove any impurities and dried under a vacuum (10À2 torr) (the
purity was checked by TLC). Yield 67.2%; M.p: 296 °C. IR (KBr,
cmÀ1): 3425 (s), 1604 (s), 1384 (s), 1143 (s), 754 (s), 538 (m),
2.1. Material and methods
Nickel chloride hexahydrate, nickel nitrate trihydrate, copper
nitrate trihydrate, cobaltous sulfate heptahydrate and salicylalde-
hyde were purchased from Merck India. 2-(Benzylthio) aniline
was prepared according to a literature method [10]. Elemental
analyses were recorded on a Perkin-Elmer Model 240C elemental
analyzer. Electrical conductivities were measured on a CM-180
Conductivity meter (Elico India). Electronic spectra were measured
on a Cary 100 Bio UV–Visible spectrophotometer. Magnetic suscep-
tibilities were measured on a conventional Gouy balance using
freshly prepared Hg[Co(NCS)4] as the calibrant with a Magway
MSB MK1 Magnetic susceptibility balance, Sherwood Scientific,
Cambridge, UK. Infrared spectra of the ligand and the complexes
were recorded on an IR-affinity-I FTIR spectrometer SHIMADZU
as KBr pellets. Melting points were recorded on a Veego melting
point apparatus and were uncorrected. The 1H NMR spectrum
was recorded on a Bruker 400 DRX spectrometer in CDCl3 solution,
using TMS as the internal standard. All other reagents and solvents
used were of commercially grade and employed as received or
purified by standard methods prior to use. All reactions were car-
ried out at room temperature in the open atmosphere.
478 (m). UV–Vis [DCM, kmax
,
nm
(emax
,
L molÀ1 cmÀ1)]:
251(25970), 305(13030), 454(9900). Anal. Calc. for C40H32O3N2S2-
Ni: C, 67.53; H, 4.50; N, 3.93; S, 9.00. Found: C, 66.52; H, 4.48; N,
3.90; S, 8.98%.
2.4. Synthesis of [NiL2]ÁH2O, complex (3)
The ligand 1 (0.638 g, 2 mmol) was dissolved in boiling metha-
nol (30 mL), followed by the addition of 2 mmol of methanolic so-
dium hydroxide. The solution was then added to a hot solution of
nickel nitrate trihydrate (0.182 g, 1 mmol) dissolved in methanol
(30 mL). The solution was stirred for 40 minutes, whereupon the
color of the solution changed to brown-black. The solution was
kept undisturbed for 3 days. The black-brown needle-like crystals,
suitable for X-ray diffraction, that formed were filtered off, washed
with 25% methanol in water solution several times to remove any
impurities and dried under a vacuum (10À2 torr) (the purity was
checked by TLC). Yield 62.1%; M.p: 286 °C. IR (KBr, cmÀ1): 3425
(s), 1604 (s), 1384 (s), 1143 (s), 754 (s), 549 (m), 478 (m). UV–Vis
[DCM, kmax, nm (e
max, L molÀ1 cmÀ1)]: 256(25680), 303(12980),
453(9880). Anal. Calc. for C40H32O3N2S2Ni: C, 67.53; H, 4.50; N,
3.93; S, 9.00. Found: C, 67.51; H, 4.49; N, 3.89; S, 8.96%.
2.2. Synthesis of N-[2-(benzylthio)-phenyl] salicylaldimine (1)
2-(Benzylthio) aniline (0.215 g, 1 mmol) was dissolved in etha-
nol and salicylaldehyde (0.1 mL, 1 mmol) was added dropwise
with continuous stirring. The solution was then stirred for another
10 minutes and the color of the solution changed to yellow. The
solution was then kept undisturbed for 6 hours at room tempera-
ture. The yellow crystalline product that formed was filtered off,
washed several times with ethanol and dried in a vacuum
(10À2 torr) (the purity was checked by TLC). Needle-like crystals
suitable for XRD analysis were grown by recrystallization from eth-
anol. Yield 82.9%; M.p: 98 °C. IR (KBr pellet, cmÀ1): 3797 (m), 3421
(s), 1607 (s), 1451 (s), 1277 (s), 1166 (s), 747 (s), 701 (s). 1H NMR
(CDCl3, 400 MHz) d: 11.39 (1H, s, OH), 8.69 (1H, s, CH@N), 6.91–
7.42 (13 H, m, Ar–H), 4.11 (2H, s, CH2). UV–Vis [DCM, kmax, nm
2.5. Synthesis of [CuL2]ÁH2O, complex (4)
The preparation of complex 4 follows the same procedure as
that of 3, except that copper nitrate trihydrate (0.241 g 1 mmol)
was used. When the color of the solution changed to dark green,
it was kept undisturbed for 24 hours. Green micro crystals, suitable
for X-ray diffraction, were obtained (the purity was checked by
TLC). Yield 65.9%; M.p: 238 °C. IR (KBr, cmÀ1): 3420 (s), 1600 (s),
1523 (s), 1440 (s), 1330 (m), 1144 (m), 858 (m), 750 (s), 538 (m),
499 (m). UV–Vis [DCM, kmax
,
nm
(emax
,
L molÀ1 cmÀ1)]:
242(39620), 308(37840), 423(19340). Anal. Calc. for C40H34O3N2S2-
Cu: C, 66.91; H, 4.73; N, 3.90; S, 8.92. Found: C, 66.98; H, 4.66; N,
3.81; S, 8.87%.
(e
max, L molÀ1 cmÀ1)]: 232(15320), 271(10150). Anal. Calc. for
C
20H17ONS: C, 75.23; H, 5.32; N, 4.38; S, 10.03. Found: C, 75.16;
2.6. Synthesis of [CoL2]ÁH2O, complex (5)
H, 5.34; N, 4.40; S, 10.07%. The reaction scheme for the synthesis
of ligand is given in Scheme 1.
The preparation of complex 5 follows the same procedure as
that of 3, except that cobalt sulfate heptahydrate (0.281 g 1 mmol)
was used. When color of the solution changed to black, it was kept
undisturbed for 2 days. Black micro crystals, suitable for X-ray dif-
fraction, were obtained (the purity was checked by TLC). Yield
58.7%; M.p: 265 °C. IR (KBr, cmÀ1): 3440 (s), 3050 (m), 1600 (s),
1523.2 (s), 1440 (s), 1330 (m), 1140 (m), 989 (m), 750 (s), 544
2.3. Synthesis of [NiL2]ÁH2O, complex (2)
The ligand 1 (0.638 g, 2 mmol) was dissolved in boiling metha-
nol (30 mL) followed by addition of 2 mmol methanolic ammonia
solution. This solution was then added to a hot solution of nickel
chloride hexahydrate (0.237 g, 1 mmol) dissolved in methanol
(30 mL). The resulting solution was stirred for 30 minutes and
the color of the solution changed to brown-black. The solution
was then kept undisturbed for 3 days. The black-brown needle-like
(s), 491 (m). UV–Vis [DCM, kmax, nm (emax,
L molÀ1 cmÀ1)]:
231(14630), 370(17560), 422(3430). Anal. Calc. for C40H34O3N2S2-
Co: C, 67.32; H, 4.76; N, 3.92; S, 8.97. Found: C, 67.41; H, 4.69; N,
3.84; S, 8.81%. The reaction scheme for the synthesis of all the me-
tal complexes is given in Scheme 2.
2.7. Antibacterial and antifungal activity analysis
In the antimicrobial study, the MIC values of the ligand and the
complexes were determined according to the literature method of
Rangasamy et al. [11]. Klebsiella pneumoniae (ATCC 13883) as Gram
negative and Bacillus subtilis (ATCC 6633) as Gram positive strains
of bacteria and a fungal strain – Candida albicans (ATCC 2091) were
used for the study. These bacteria and fungi were cultured in Luria
broth (LB) media and cultures were grown until they obtained a
Scheme 1. Synthesis of the Schiff base, N-[2-(thiobenzyl) phenyl] salicylaldimine.

M. Kalita et al. / Polyhedron 60 (2013) 47–53
49
benzaldimine proton (CH@N). The aromatic (Ar–H) and benzylic
protons (CH2) appear as a multiplet and singlet at d 6.91–7.42
and 4.11 ppm, respectively.
3.1.2. Infrared Spectra
The spectrum of ligand 1 exhibits a broad band at 3797 cmÀ1
,
attributed to
to (C@N), and other strong bands belonging to
(C–S) are found at 1451 and 747 cmÀ1 respectively. The infrared
m
(O–H), a strong band at 1607 cmÀ1, which belongs
(C@C) and
m
m
m
spectra of the prepared complexes exhibit a band for m(C@N) in
the range 1600–1604 cmÀ1, which shows a shift to lower frequen-
cies by 7–5 cmÀ1 compared with the ligand HL, indicating the coor-
dination of the ligand to the metal atom through the nitrogen
atom. The disappearance of the band for the
the spectra of the complexes indicates the coordination of the phe-
nolic oxygen to the metal atom. Moreover, the phenolic (C–O)
m(O–H) vibration in
m
Scheme 2. Synthesis of the metal complexes.
stretching vibration in the free Schiff base is observed at
1166 cmÀ1, and this is shifted by 20–32 cmÀ1 towards lower wave-
numbers in the complexes, thus indicating the coordination of the
phenolic oxygen to the metal ion [7]. The broad band in the range
3420–3440 cmÀ1 shows the presence of water molecules in the
growth equivalent to 0.5 McFarland’s Standard. A stock concentra-
tion of 10 mg/mL of each of the compounds in DMF was prepared.
The MIC was determined using 96 well plates. 200 lL of the com-
metal complexes. The m(C–S) band of the ligand is shifted to higher
pound from the stock solution was added to the first well. From the
frequencies, from 748 to 749–754 cmÀ1, in the complexes which is
evidence for a M–S bond [4]. The appearance of a new band at
549–538 cmÀ1 in the complexes supports the coordination to an
oxygen atom [7]. Again, the appearance of a new band in the range
499–478 cmÀ1 confirms the coordination via a nitrogen atom to
the metal atom [5]. Metal sulfur stretching frequencies occur in
the (480 210 cmÀ1) range, but are often difficult to detect as they
tend to be weak and quite broad [14].
second well to the eighth well, 100 mL of LB media was added.
Now from the first well 100
to the second well and subsequently serially diluted until the
eighth well had a concentration of 195
g mLÀ1. To all the wells,
lL of the solution was taken and added
l
100 mL of bacterial inoculum were added. The plates were then
incubated at 310 K overnight. After the incubation period, 40 mL
of MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium
bromide] solution [at a concentration of 0.2 mg mLÀ1 of phosphate
buffer saline (pH 7.4)] was added to each well. The plates were
again incubated for 40 min at 310 K. After the incubation period,
the plates were checked for any change in color. A change of colour
to blue indicated bacterial growth, signifying that the compounds
do not have any inhibitory activity, while no change of colour
means a positive result, indicating inhibitory activity of the
compounds.
3.1.3. Electronic spectra
The ligand shows sharp bands at 232 and 271 nm which are
assigned to an n–p⁄ transition of the imine moiety and a p⁄ tran-
p–
sition of the phenyl rings respectively [15]. The electronic spectra
of the Ni(II) complexes 2 and 3 exhibit three bands in the ranges
251–256, 303–305 and 453–454 nm, which may be assigned to
n–p⁄ p⁄, d–d transitions respectively. The electronic spectrum
, p–
of 4 shows a band at 242 nm, which is assigned to the n–p⁄ band
of the imine moiety. Complex 4 has another two bands centered
2.8. Crystal structure determination and refinement
at 308 and 423 nm, which are assigned to a
p–
p⁄ transition of
Single-crystal X-ray diffraction data for compound 1 at 296 K
the phenyl rings and a d–d transition respectively. Complex 5
and compounds 2 and 3 at 100 K were collected with Mo K
ation (k = 0.71073 Å) using a Bruker Smart Apex II CCD diffractom-
eter equipped with graphite monochromator. SMART [12]
a radi-
shows bands at 231, 370 and 422 nm, which may be assigned to
n–p⁄ p⁄ and d–d transitions, respectively. The broad d–d transi-
, p–
a
tion band at 422 nm supports the presence of an octahedral cobal-
t(II) ion in the complex. The band of the azomethine chromophore
n–p⁄ transition is shifted to lower frequencies, thus indicating that
the imine nitrogen atom is coordinated to a metal ion [16].
software was used for data collection and also for indexing the
reflections and determining the unit cell parameters. Collected
data were integrated using SAINT [12] software. The structures
were solved by direct methods and refined by full-matrix least-
square calculations using SHELXTL [13] software. Absorption cor-
rections were done by the multi-scan method (SADABS) [12]. All
the non-H atoms were refined in the anisotropic approximation
against F2 of all reflections. The H atoms were placed at their calcu-
lated positions and refined in isotropic approximations. Crystal
data and structure refinements are given in Table 1.
3.1.4. Molar conductivity
The molar conductivities of
2 and 3 are in the range
20.5–27.5 s cm2 molÀ1, for 4 it is 21.5 s cm2 molÀ1 and for 5 it is
32.1 s cm2 molÀ1. These values for the molar conductivities in ace-
tonitrile reveal that all the complexes behave as non-electrolytes
[5].
3. Results and discussion
3.1.5. Magnetic measurement
3.1. Characterization
The magnetic moments (leff) for the nickel complexes 2 and 3
are 2.82 and 2.76 BM. These values are close to the spin only value
of 2.83 BM expected for a d8 system in Oh symmetry. The value ob-
tained for 4 is in the expected range (1.64 BM) for a d9 configura-
tion and thus the octahedral nature of the complexes can be
assumed [17,18]. The magnetic moment of 5 is 5.15 BM. This value
fairly well agrees with the value suggested for a high spin octahe-
dral Co(II) complex.
3.1.1. 1H NMR spectrum of the ligand (1)
The structure of N-[(2-benzylthio)-phenyl] salicylaldimine was
established by the 1H NMR (400 MHz) spectrum of the ligand
(HL) 1, recorded in CDCl3 in the range 0–15 ppm. The phenolic
proton appears downfield compared to the other peaks as a singlet
at d (Ar–OH) 11.39 ppm. The singlet at d 8.69 ppm is due to the

50
M. Kalita et al. / Polyhedron 60 (2013) 47–53
Table 1
Crystal data and structure refinement details for 1–3.
Compound
1
2
3
CCDC entry No.
Empirical formula
Formula weight
T (K)
903765
935095
C40H32N2NiO3S2
711.51
100(2)
0.71073
monoclinic
P21/c
901325
C40H32N2NiO2S2O
711.51
100(2)
0.71073
C
20H17NOS
319.4
296(2)
0.71073
monoclinic
P21/c
k (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
monoclinic
P21/c
15.0328(15)
5.3255(6)
20.348(2)
90
12.6290(3)
30.4235(8)
17.3032(4)
90
12.6369(4)
30.4430(9)
17.3151(5)
90
a
(°)
b (°)
92.915(7)
90
1626.9(3)
4
1.304
0.203
102.985(2)
90
6478.2(3)
8
1.459
0.772
102.991(2)
90
6490.7(3)
8
1.456
0.770
c
(°)
V (Å3)
Z
Dcalc (Mg/m3)
l
(mmÀ1
)
F(000)
672
2960.0
2960
Crystal size (mm3)
h (°)
0.52 Â 0.25 Â 0.14
4.20–35.09
À23 6 h 6 24,
À8 6 k 6 8,
À32 6 l 6 30
26444
0.38 Â 0.14 Â 0.12
1.34–27.95
À16 6 h 6 15,
À40 6 k 6 34,
À22 6 l 6 22
15567
0.42 Â 0.16 Â 0.10
1.34–27.15
À14 6 h 6 15
À30 6 k 6 36
À15 6 l 6 22
80439
Index ranges
Reflections collected
Independent reflections (Rint
Completeness (%)
)
7232 (0.0428)
100
15464 (0.094)
99.3
21549 (0.0510)
81.7
Absorption correction
Max and min transmission
Refinement method
multiscan (SADABS
)
multiscan (SADABS
)
multiscan (SADABS
0.873 and 0.749
)
0.972 and 0.941
0.9131 and 0.7580
full-matrix least-squares on F2
15464/0/865
full-matrix least-squares on F2
7031/0/213
full-matrix least-squares on F2
11761/0/865
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
1.081
1.043
1.072
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0736, wR2 = 0.1640
R1 = 0.1392, wR2 = 0.2311
1.06 and À0.74
R1 = 0.0631, wR2 = 0.1767
R1 = 0.1329, wR2 = 0.2470
1.00 and À1.41
R1 = 0.0631, wR2 = 0.1767
R1 = 0.1179, wR2 = 0.2277
0.78and À2.14
Largest difference in peak and hole (e ÅÀ3
)
3.2. X-ray crystallography
2-hydroxyphenyl rings are planar (Fig. 1). Due to repulsion
between the sulfanyl sulfur and azomethine or imine moiety of
the ligand 1, the benzylsulfanyl unit moves away and the molecule
attain a trans-geometry about the imine double bond (N1@C14),
creating the possibility of coordination of the ligand with a metal
ion through its O, N and S donor sites. As there are no significant
intermolecular interactions, van der Waals interactions are
believed to be the stabilizing force in the crystal packing.
We have synthesized two nickel complexes from two different
nickel salts, i.e. nickel chloride hexahydrate and nickel nitrate tri-
hydrate, with the ligand 1, but X-ray diffraction analysis reveals
that both the complexes are structurally the same, except some
parameters such as bond lengths, bond angles, cell volume, density
(Fig. 2 and Fig. 3).
Prospective views of the molecular structure of ligand 1 and the
two Ni complexes 2 and 3 are shown in Figs. 1–3 respectively.
Compound 1 crystallizes in the monoclinic space group P21/c, with
one molecule in the asymmetric unit. The distance between the
atoms C14 and N1 is 1.275(2) Å, suggesting the formation of an
azomethine bond. The C@N bond length is in the expected range
for typical C@N bond length of imines [Table 2] [3]. Again, the bite
angle [C14–N1–C13] is 123.03(14)°, suggesting that the atoms N1
and C14 are sp2 hybridized. The N-methylbenzenamine and
In both complexes, the coordination spheres consist of N2S2O2
donor sets. The metal center is octahedrally coordinated to the
two deprotonated phenolic oxygen atoms, two thioether sulfur
atoms and two imine nitrogen atoms in bichelated fashion with
the tridentate ligand. In the octahedral arrangement, the two
deprotonated phenolic oxygen atoms of the ligand bonded to the
Ni(II) center are cis to each other, as are the two thioether sulfur
atoms occupy cis positions, and these atoms constitute the basal
plane, whilst two imine nitrogen atoms are in axial positions to
this plane [19]. The observed distortion around Ni in 2 is low, as
judged from the spread of the cis angles, 82.18(8)–93.73(8)°, and
the trans angles of 174.23(8) and 172.08(8)°, whilst the bite angles
S1–Ni1–O2, O1–Ni1–O2, S1–Ni1–S2 and S2–Ni1–O1 of 2 are
90.18(8)°, 93.9(1)°, 92.74(4)° and 83.75(8)° respectively, which
deviate significantly from 90° indicating the distortion from a
regular octahedron. The Ni–S (thioether) distances in
2 are
Fig. 1. Molecular structure of N-[2-(thiobenzyl)phenyl] salicylaldimine, shown
with 50% probability ellipsoids.
2.466(1) and 2.485 Å, which are quite typical of Ni(II)–S (thioether)

M. Kalita et al. / Polyhedron 60 (2013) 47–53
51
Fig. 2. Molecular structure of complex 2, shown with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity.
Fig. 3. Molecular structure of complex 3, shown with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity.
complex 2, 1.777(4) Å, is slightly longer than the C8–S1 bond of
the free ligand, 1.7656(16) Å. The C14–S1 bond also shows the
same trend. This shows a weak coordination of S with the Ni atom.
The Ni–N(imine) distances of 2 [2.031(3) and 2.035(3) Å] are well
within the range of typical Ni–N(imine) distances. The imine bond
length of 1.302(5) Å in the complex increases compared to the free
ligand, 1.275(2) Å, due to delocalization of the imine bonded elec-
tron. The Ni–O bond lengths of 1.981(3) and 2.016(3) Å are in the
range of those found for similar complexes with an octahedral
arrangement [6]. Unusually, the C–O bond length, 1.316(5) Å, in
the complex is decreased, compared to the free ligand O1–C16
bond length of 1.339(2) Å. This may be due to delocalization of
the negative charge to the ligand backbone, anticipating partial
double bond character. Moreover, the Ni–O bond length is the
shortest as compared to the Ni–S and Ni–N bond lengths. This is
indicative of very strong coordination of O with the Ni atom. The
Table 2
Selected bond lengths (Å) and bond angles (°) for 1.
Bond lengths
Bond angles
S1–C7
S1–C8
1.8115(16)
1.7656(16)
1.339(2)
0.91(3)
1.411(2)
1.275(2)
0.97(2)
1.402(2)
0.93
1.445(2)
C7–S1–C8
C16–O1–H1A
S1–C7–C1
N1–C13–C8
N1–C14–H14
N1–C14–C15
O1–C16–C15
C13–N1–C14
103.10(8)
97.5(18)
108.6(11)
115.99(13)
118.8
122.49(15)
121.69(16)
123.03(14)
O1–C16
O1–H1A
N1–C13
N1–C14
C7–H7A
C8–C13
C14–H14
C14–C15
distances observed in complexes of multidentate ligands with one
or more thioether S donor center [20]. The C13–S1 bond in

52
M. Kalita et al. / Polyhedron 60 (2013) 47–53
Table 3
active only to 2.5 mg/mL. The compounds were also found to pos-
Selected bond lengths (Å) and bond angles (°) for 2.
sess antifungal properties. Compounds 1–5 inhibited Candida albi-
cans (ATCC 2091) with an inhibitory concentration of 0.039 mg/mL.
The results demonstrate the antimicrobial properties of the com-
pounds. Biological activity studies show that the ligand itself has
an obvious efficiency in restraining bacteria. In general the anti-
fungal activities of the complexes are better than the antibacterial
activities. The results are shown in [Table 5]. The Schiff base ligand
is biologically active and its activity may arise from the hydroxyl
group which may play an important role in the antibacterial activ-
ity [21,22].
Bond lengths
Bond angles
Ni–O1
Ni–O2
Ni–N1
Ni–N2
Ni–S1
Ni–S2
2.016(3)
1.981(3)
2.031(3)
2.035(3)
2.466(1)
2.485(1)
S1–Ni–S2
S1–Ni–O1
S1–Ni–O2
S1–Ni–N1
S1–Ni–N2
S2–Ni–O1
S2–Ni–O2
S2–Ni–N1
S2–Ni–N2
O1–Ni–O2
O1–Ni–N1
O1–Ni–N2
O2–Ni–N1
O2–Ni–N2
N1–Ni–N2
92.74(4)
172.08(8)
90.18(8)
82.18(8)
93.73(8)
83.75(8)
174.23(8)
95.75(8)
82.35(8)
93.9(1)
91.1(1)
92.8(1)
89.6(1)
92.5(1)
4. Conclusion
A new sterically and electronically tunable mixed hydroxyl–
imine–thioether tridentate ligand and its four mononuclear Ni(II),
Cu(II) and Co(II) complexes have been prepared and characterized
by physio-chemical analysis. Single crystal X-ray diffraction data,
magnetic susceptibility measurements and electronic spectra re-
veal a slightly distorted octahedral arrangement for complexes 2
and 3. Antimicrobial studies showed that the ligand and the four
metal complexes are active against Bacillus subtilis, Klebsiella pneu-
monia and Candida albicans.
175.4(1)
Table 4
Selected bond lengths (Å) and bond angles (°) for 3.
Bond lengths
Bond angles
Ni–O1
Ni–O2
Ni–N1
Ni–N2
Ni–S1
Ni–S2
1.981(3)
2.010(3)
2.033(4)
2.026(4)
2.486(1)
2.464(1)
S1–Ni–S2
S1–Ni–O1
S1–Ni–O2
S1–Ni–N1
S1–Ni–N2
S2–Ni–O1
S2–Ni–O2
S2–Ni–N1
S2–Ni–N2
O1–Ni–O2
O1–Ni–N1
O1–Ni–N2
O2–Ni–N1
O2–Ni–N2
N1–Ni–N2
92.72(5)
174.3(1)
84.0(1)
82.5(1)
95.7(1)
90.4(1)
172.1(1)
93.8(1)
82.2(1)
93.5(1)
92.5(1)
89.4(1)
92.8(1)
91.0(1)
175.6(2)
Acknowledgments
Authors thank IIT Guwahati, Tezpur University, Tezpur and B.
Borooah College, Guwahati for spectral and analytical data.
Authors also thank Director of NIT, Silchar for financial assistance
(M. K & P. G) and the Council of Scientific and Industrial research
(CSIR), New Delhi for the award of senior research Fellowship (T.B).
Appendix A. Supplementary data
CCDC 903765, 935095 and 901235 contain the supplementary
crystallographic data for compounds 1–3, respectively. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/con-
ts/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. Supplementary data
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.poly.2013.04.062.
Table 5
Antibacterial and antifungal activities of the ligand and its complexes as MIC values
(mg/ml).
Compound Gram positive bacteria Gram negative bacteria Fungus
(mg/ml)
Bacillus subtilis
(mg/ml)
Klebsiella pneumoniae
(mg/ml)
Candida
albicans
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