Synthesis, spectroscopic and single crystal X-ray studies on three new mononuclear Ni(II) pincer type complexes: DFT calculations and their antimicrobial activities

Article abstract of DOI:10.1016/j.molstruc.2017.03.114

Three new mononuclear square planar Ni(II) complexes, containing pincer type tridentate Schiff base ligands, having general formula [(NiL₁(4-MePy)] (1), [(NiL₁(2-AzNp)] (2), and [(NiL₂(4-MePy)] (3) [where L₁?=?a

Full text of DOI:10.1016/j.molstruc.2017.03.114

Journal of Molecular Structure 1141 (2017) 428e435
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis, spectroscopic and single crystal X-ray studies on three new
mononuclear Ni(II) pincer type complexes: DFT calculations and their
antimicrobial activities
Samaresh Layek a, Bhumika Agrahari a, Abhrajyoti Tarafdar b, Chanda Kumari a,
,
*
Anuradha a, Rakesh Ganguly c, Devendra D. Pathak a
a Department of Applied Chemistry, Indian Institute of Technology (Indian School of Mines), Dhanbad 826004, India
b Department of Environmental Science, Indian Institute of Technology (Indian School of Mines), Dhanbad 826004, India
c Division of Chemistry & Biological Chemistry, Nanyang Technological University, Singapore 639798, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new mononuclear square planar Ni(II) complexes, containing pincer type tridentate Schiff base
ligands, having general formula [(NiL1(4-MePy)] (1), [(NiL1(2-AzNp)] (2), and [(NiL2(4-MePy)] (3) [where
L1 ¼ anion of N-(2-hydroxy-3-methoxybenzylidene) benzoylhydrazide (HL1), L2 ¼ anion of N-(2-
Received 27 February 2017
Received in revised form
30 March 2017
Accepted 30 March 2017
Available online 1 April 2017
hydroxy-3-methoxybenzylidene) thiosemicarbazide (HL2), 4-MePy
¼
4-Methylpyridine and 2-
AzNp ¼ 2-Azanapthalene] have been synthesized and fully characterized by FT-IR, UVevisible, NMR,
single crystal X-ray diffraction studies and elemental analysis. All the three complexes show square
planar geometry around the nickel atom. The pincer type ligand occupies three coordination sites, while
the fourth site is occupied by the monodentate nitrogen containing ligand. The Quantum chemical DFT
calculations have also been carried out using DFT/B3LYP method and 6-311þþG(d,p) basis set. The
synthesized nickel complexes were screened for antimicrobial activities by agar well diffusion method
against E. coli bacteria. Out of three complexes, [(NiL2(4-MePy)] (3) only showed the antimicrobial ac-
tivity against E. coli bacteria.
Keywords:
Schiff base
Nickel(II) pincer
Crystal structure
DFT calculations
Antibacterial activities
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
azomethine group is accountable for antimicrobial activities and
plays a significant chemical and biological role [18]. Our persistent
In recent decade, multi-dentate Schiff bases and their metal
complexes have allured a lot of attention of chemists not only
because of their an assortment of applications in coordination
chemistry [1], but also for their miscellaneous biological activities
such as antifungal, antibacterial, analgesic, sedative, antipyretic and
anti-inflammatory properties [2e8]. These are also in huge stipu-
late in catalysis [9e12], agriculture, analytical chemistry, dye
polymer industry [13e15], luminescent probes [16] etc. Owing to
multi-dentate nature of these ligands, they have been used in the
synthesis of model complexes for the active sites of various en-
zymes. The contribution of metal ions in biological reactions is well
known [17]. A perusal of the literature survey reveals that, presence
of azomethine (-C]N-) linkage in Schiff bases and lone pair of
electrons on a sp2-hybridized orbital of nitrogen atom of the
interest in transition metal complexes [19e21] and their diverse
applications, has lead us to synthesis hydrazide and carbazide
containing Schiff base ligands and their complexes. Recently, nickel
has emerged as a striking transition metal for the development of
catalysts in biological and industrial applications [22e24]. During
the present studies, we have chosen two potential tridentate Schiff
base ligands, N-(2-hydroxy-3-methoxybenzylidene) benzoylhy-
drazide (HL1) and N-(2-hydroxy-3-methoxybenzylidene) thio-
semicarbazide (HL2). Both ligands contain two potentially
dissociable acidic protons, viz. the phenolic OeH or SeH and the
hydrazinic NeH protons. Herein, we describe the synthesis and
characterization of three new complexes of general formula
[(NiL1(4-MePy)] (1), [(NiL1(2-AzNp)] (2) and [(NiL2(4-MePy)] (3)
obtained by the reaction of nickel(II) acetate and the corresponding
ligand and a monodentate co-ligand in methanol at room tem-
perature. Keeping in mind the significance and development of
new therapeutic reagents in biological field [25e27], all three
* Corresponding author.
E-mail address: ddpathak@yahoo.com (D.D. Pathak).
http://dx.doi.org/10.1016/j.molstruc.2017.03.114
0022-2860/© 2017 Elsevier B.V. All rights reserved.

S. Layek et al. / Journal of Molecular Structure 1141 (2017) 428e435
429
complexes have been tested for their antibacterial activities against
common bacteria Escherichia coli using agar well diffusion method.
In addition to these the structural geometry and binding mode of
ligands with Ni(II) in all complexes are presented by DFT
calculations.
Yields: 0.230 gm, 73%. Anal. Calc. for C21H19N3NiO3: C, 60.04; H,
4.56; N,10.00. Found: C, 60.24; H, 4.42; N,10.24. Selected FT-IR (KBr),
cmꢀ1: 2915 (CHar), 1579 (C]N), 1230 (CeO), 582 (NieO),
n
(NieN)
max(nm), ε (L molꢀ1 cmꢀ1)]: 303 (18400), 363
(18800), 413 (9800).1H NMR (CDCl3, 25 ꢁC, 400 MHz):
473. UVeVis [
l
d
¼ 8.56 (s,1H,
HeCN), 7.84e8.00 (d, 3H, Ar H), 7.01e7.46 (m, 5H, Ar H), 6.96 (s, 1H,
Ar H), 6.61e6.73 (d, 3H, Ar H), 3.81 (s, 3H, OCH3), 2.44 (s, 3H, CH3).
2. Experiment
2.1. Materials and physical measurements
2.5. Synthesis of [(NiL1(2-AzNp)](2)
All reagents and solvents for the synthesis and analysis were
commercially available and used as received without further puri-
fication. The FT-IR spectra were recorded on a Perkin Elmer Spec-
trometer (Model: Cary 660) in the range of 400e4000 cmꢀ1 using
KBr pellets in which MCT used as a detector with scan number 20,
and resolution 4 cmꢀ1. The UV visible spectra were obtained on an
Agilent 8453 diode array spectrophotometer using DMF as solvent.
Elemental analyses were carried out using a Heraeus CHN-Rapid
elemental analyzer. The NMR spectra of complexes were recorded
in CDCl3 on a Bruker 75 AvIII HD-400 MHz spectrometer using TMS
as the internal Standard.
A solution of Schiff base ligand (HL1) (0.144 mg, 0.50 mmol) in
methanol (4 mL) was added drop-wise to a methanolic solution
(4 ml) of Ni(OAc)2$4H2O (0.124 mg, 0.50 mmol) with constant
stirring at room temperature. After stirring the solution for 10 min
at room temperature, co-ligand 2-azanaphthalene (isoquinoline)
(0.059 ml, 0.50 mmol, dissolved in 2-mL methanol) was added via a
syringe to the reaction. On addition of co-ligand, the color of so-
lution had changed from greenish to dark-brown. After 6 h of
stirring at rt, the resultant precipitate was filtered, washed with
cold ethanol and dried in vacuum over anhydrous CaCl2. The pre-
cipitate was re-crystallized from DMF. Suitable single crystals for X-
ray crystallography were grown over a period of two weeks from a
concentrated solution of the complex in DMF.
2.2. Synthesis of N-(2-hydroxy-3-methoxybenzylidene)
benzohydrazide (HL1)
Yields: 0.216 gm, 65%. Anal. Calc. for C24H19N3NiO3: C, 63.73; H,
4.71; N, 8.92; Found: C, 63.55; H, 4.84; N, 8.99. Selected FT-IR (KBr),
The Schiff base ligand HL1 was synthesized by slight modifica-
tion of the reported method [28]. A solution of benzoylhydrazine
(0.1362 gm, 1 mmol) in ethanol (5 mL) was added to an ethanolic
solution (5-mL) of ortho-vaniline (0.1522 gm, 1 mmol) at room
temp. The resultant solution was heated to reflux for a period of
3e4 h to produce off-white solid. The reaction mixture was cooled
to room temperature and the precipitate was filtered, washed with
cold ethanol and dried in vacuum over anhydrous CaCl2. Yield:
0.233 g, 81%. Selected FT-IR (KBr), cmꢀ1: 3564 (NH), 3380 (OH),
2839-3062 (CHar), 1646 (C]O), 1596 (C]N), 1470 (C¼Cring).
cmꢀ1: 1580 (C]N), 1217 (CeO), 560 (NieO), 468 (NieN). UVeVis [
l
max(nm), ε (L molꢀ1 cmꢀ1)]: 309 (20900), 361 (19100), 414 (9600).
1H NMR (CDCl3, 25 ꢁC, 400 MHz):
d
¼ 9.44 (s, 1H, isoquinoline
H), 8.61 (s, 1H, HeCN), 7.72e8.10 (m, 8H, Ar H), 7.34e7.37 (t, 2H, Ar
H), 6.98 (d, 1H, Ar H), 6.75e6.77 (t, 2H, Ar H), 6.64 (t, 1H, Ar H), 3.89
(s, 3H, OCH3).
2.6. Synthesis of [(NiL2(4-MePy)] (3)
A solution of Schiff base ligand (HL2) (0.113 mg, 0.50 mmol) in
methanol (4 mL) was added drop-wise to a methanolic solution
(4 ml) of Ni(OAc)2$4H2O (0.124 mg, 0.50 mmol) with constant
stirring at room temperature. After stirring the solution for 10 min
at room temperature, co-ligand 4-methylpyridine (0.048 ml,
0.50 mmol, dissolved in 2-mL methanol) was added via a syringe to
the reaction. On addition of co-ligand, the color of solution had
changed from greenish to dark-brown. After 6 h of stirring at rt, the
resultant precipitate was filtered, washed with cold ethanol and
dried in vacuum over anhydrous CaCl2. The precipitate was re-
crystallized from DMF. Suitable single crystals for X-ray crystal-
lography were grown over a period of two weeks from a concen-
trated solution of the complex in DMF.
2.3. Synthesis of N-(2-hydroxy-3-methoxybenzylidene)
thiosemicarbazide (HL2)
The Schiff base ligand HL2 was synthesized by slight modifica-
tion of the reported method [29]. A solution of thiosemicarbazide
(0.091 g, 1 mmol) in ethanol (5 mL) was added to an ethanolic
solution (5-mL) of ortho-vaniline (0.1522 gm, 1 mmol) at room
temp. The resultant solution was heated to reflux for a period of
2e3 h to produce off-white solid. The reaction mixture was cooled
to room temperature and the precipitate was filtered, washed with
cold ethanol and dried in vacuum over anhydrous CaCl2. Yield:
0.185 g, 76%. m.p. 79-82 ꢁC. Selected FT-IR (KBr), cmꢀ1: 3443 (NH),
3326, 3157 (NH2), 3021 (OH), 1596 (C]N), 1045 (C]S).
Yields: 0.198 gm, 70%. m.p: 195 ꢁC. Anal. Calc. for
C
15H16N4NiO2S: C, 48.03; H, 4.30; N, 14.94; Found: C, 48.15; H, 4.41;
N, 14.80. Selected FT-IR (KBr), cmꢀ1: 3362, 3265 (NH2), 1530 (C]N),
1312 (CeO), 568 (NieO), 481 (NieN). UVeVis [l max(nm), ε (L molꢀ1
cmꢀ1)]: 297 (19700), 367 (16900), 415 (7300). 1H NMR (CDCl3,
2.4. Synthesis of complex [(NiL1(4-MePy)] (1)
A solution of Schiff base ligand (HL1) (0.144 mg, 0.50 mmol) in
methanol (4 mL) was added drop-wise to a methanolic solution
(4 ml) of Ni(OAc)2$4H2O (0.124 mg, 0.50 mmol) with constant
stirring at room temperature. After stirring the solution for 10 min
at room temperature, co-ligand 4-methylpyridine (0.048 ml,
0.50 mmol, dissolved in 2-mL methanol) was added via a syringe to
the reaction. On addition of co-ligand, the color of solution had
changed from greenish to dark-brown. After 6 h of stirring at rt, the
resultant precipitate was filtered, washed with cold ethanol and
dried in vacuum over anhydrous CaCl2. The precipitate was re-
crystallized from DMF. Suitable single crystals for X-ray crystal-
lography were grown over a period of two weeks from a concen-
trated solution of the complex in DMF.
25 ꢁC, 400 MHz):
(m, 2H, Ar H), 6.83 (d, 1H, Ar H), 6.66 (d, 1H, Ar H), 6.48e6.57 (t, 2H,
Ar H), 4.57 (s, 2H, eNH2), 3.67 (s, 3H, OCH3), 2.36 (s, 3H, CH3).
d
¼ 8.61 (s, 1H, HeCN), 7.79 (s, 1H, Ar H), 7.10e7.11
2.7. Crystallographic studies
Diffraction quality crystals of the complexes (1-3) were grown
over a period of two weeks from a concentrated solution of the
complex in DMF at room temperature and the structure of the
complex have been elucidated by single-crystal X-ray diffraction.
The X-ray diffraction intensity data were measured at 103 K with a
Bruker Kappa diffractometer equipped with a CCD detector,
employing Mo K
a
radiation (
l
¼ 0.71073 Å), with the SMART suite

430
S. Layek et al. / Journal of Molecular Structure 1141 (2017) 428e435
of programs [30]. All data were processed and corrected for Lorentz
and polarization effects with SAINT and for absorption effects with
SADABS [31]. Structural solution and refinement were carried out
with the SHELXTL suite of programs [32]. The structures were
refined (weighted least squares refinement on F2) to convergence.
All the non-hydrogen atoms in all the compounds were refined
anisotropically till convergence is reached. Precise unit-cell pa-
rameters were determined by a least-squares fit of 2500 (1), 3662
(2) and 1053 (3) reflections of the highest intensity, chosen from
the whole experiment. A summary of the crystallographic and
refinement data of these three nickel complexes are given in
Table 1.
Nutritional Agar was made with the following components [40]:
After the setting of plates with the agar media, they were well
dried at 37 ꢁC in a laminar airflow under UV light. For preparing the
stock, Escherichia coli ATCC 25922 (American Type Culture Collec-
tion, Rockville, MD) cultures were kept for 24 h at 36 ꢁC 1 ꢁC and
the purity of cultures was checked after 8 h of incubation of E. coli.
And then plates were washed out with stock. After 24 h of incu-
bation, bacterial suspension (inoculums) was diluted with sterile
physiological solution, for the diffusion and indirect bioautographic
tests, to 108 CFU/mL (turbidity ¼ McFarland barium sulfate stan-
dard 0.5) [41]. Plates inoculated with the E. coli had three 6 mm
wells cut into the surface of the agar using a cork borer dipped in
alcohol and flamed. The wells were filled with 0.5 ml of test com-
pounds with different concentration solutions. All plates were
incubated (35 ꢁC) overnight [42]. After incubation, the diameters of
any clear zones around the antimicrobial compound-containing
wells were measured using calipers.
2.8. Computational details
For supporting the experimental work, geometrical optimiza-
tion calculation of three nickel complexes were carried out in
Gaussian 09 program with DFT/B3LYP [33,34] method. The geom-
etries of all the three complexes were optimized using 6-
311þþG(d,p) basis set [35,36] and the stationary point structures
were visualized with the help of Gauss-View 5.0 program [37].
Using DFTcalculations, the energy level HOMO and LUMO diagrams
and molecular chemical stability of the Ni(II) complexes were
determined.
3. Results and discussion
Two Schiff base ligands HL1 and HL2 were synthesized by
modification of the reported methods [28,29]. Both ligands were
obtained as off-white crystalline solid in good yields (76 and 81%)
on heating an ethanolic solution of corresponding reactants just for
2e4 h. The synthesis of nickel complexes is outlined in Scheme 1.
All three complexes were obtained as dark-brown crystalline solids
in high yields (65e75%). The complexes were found to be air stable,
soluble in common organic solvents such as MeOH, CH3COCH3,
Et2O, CHCl3, CH2Cl2, CH3CN, DMF and DMSO and insoluble in water
and benzene. All complexes were fully characterized by various
spectroscopic techniques like FT-IR, UVevisible, 1H NMR, elemental
analysis. The solid state structures of complexes [(NiL1(4-MePy)]
(1), [(NiL1(2-AzNp)] (2) and [(NiL2(4-MePy)] (3) were determined
by single crystal X-ray diffraction analysis.
2.9. Antimicrobial assays
The in-vitro antibacterial activity and MIC value of the synthe-
sized nickel complexes were evaluated against Escherichia coli by
agar well diffusion method [38].
2.9.1. Determination of minimum inhibitory concentrations (MIC)
Agar dilutions were mostly prepared in sterilized Petri dishes as
it has advantage to test antimicrobial activity of several concen-
trations of the same compound on each plate. One can also get an
overview idea about the MIC of the test compound. MIC is defined
as the lowest concentration of an antimicrobial that will inhibit the
visible growth of a microorganism after overnight incubation [39].
3.1. Single crystal X-Ray studies
The proposed structures of three nickel(II) complexes 1, 2 and 3
Table 1
Crystallographic and refinement data for [(NiL1(4-MePy)] (1), [(NiL1(2-AzNp)] (2) and [(NiL2(4-MePy)] (3) complexes.
Complex
[(NiL1 (4-MePy)]
[(NiL1 (2-AzNp)]
[(NiL2 (4-MePy)]
CCDC deposition number
Chemical formula
Formula weight
Temperature
Wavelength
Crystal size
Crystal habit
Crystal system
Space group
Unit cell dimensions
a (Å)
1470164
1477281
1477282
C21H19N3NiO3
420.10 g/mol
103 (2) K
C24H19N3NiO3
456.13 g/mol
103 (2) K
C15H16N4NiO2S
375.09 g/mol
103 (2) K
0.71073 Å
0.71073 Å
0.71073 Å
0.060 ꢂ 0.140 ꢂ 0.160 mm
0.040 ꢂ 0.220 ꢂ 0.400 mm
0.140 ꢂ 0.180 ꢂ 0.320 mm
red plate
red plate
red block
orthorhombic
P b c a
orthorhombic
P b c a
monoclinic
P 1 21/c 1
21.1983 (14)
14.0989 (8)
25.1003 (15)
90
90
90
8.8501 (7)
17.0214 (15)
26.151 (2)
90
10.792 (2)
9.655 (2)
15.556 (4)
90ꢁ
101.932 (12)ꢁ
90ꢁ
b (Å)
c (Å)
a
b
g
(ꢁ)
(ꢁ)
(ꢁ)
90
90
V (Å3)
7501.8 (8)
16
3939.4 (6) Å3
8
1585.9 (7)
4
Z
Density (calculated)
Absorption coefficient
F (000)
1.488 g/cm3
1.062 mmꢀ1
3488
1.538 g/cm3
1.018 mmꢀ1
1888
1.571 g/cm3
1.369 mmꢀ1
776
R
0.1184
0.0866
0.0879
int
R1[I > 2
d
(I)]
0.0652
0.0501
0.0811
wR2 (all data)
Largest diff. peak and hole
R.M.S. deviation from mean
0.2435
0.1106
0.2236
1.519 and ꢀ0.715 eÅꢀ3
0.552 and ꢀ0.685 eÅꢀ3
1.332 and ꢀ0.579 eÅꢀ3
0.158 eÅꢀ3
0.089 eÅꢀ3
0.143 eÅꢀ3

S. Layek et al. / Journal of Molecular Structure 1141 (2017) 428e435
431
Ingredients
gLꢀ1
Peptic digest of animal tissue
5
Sodium chloride
5
Beef extract
1.5
Yeast extract
1.5
Agar
15
Final pH at 25 ꢁC
7.4 0.2
based on the above techniques were unambiguously confirmed by
X-ray diffraction studies. The ORTEP representations of three nickel
(II) complexes including the atom numbering scheme are shown in
Fig. 1. Molecular structures of these complexes revealed that the
nickel atoms in all complexes have distorted square planar coor-
dination geometry. Distortion is mainly caused by the presence of
the double deprotonated tridentate Schiff base ligand, which forms
one five-membered and one six-membered chelating ring with
nickel atom. The neutral complexes contain one Ni(II) ion with four
coordination number legated to one tridentate Schiff base ligand
and the appropriate amine i.e. coligand, and have similar structures.
This coordination is fulfilled by one nitrogen and two phenolic
oxygen atoms from the tridentate Schiff base ligands (L1), with one
nitrogen atoms from 4-methylpyridine (1) and 2-azanapthalene
molecule (2). In case of complex 3, the nickel is coordinated by
one nitrogen, one phenolic oxygen atom and one thionate sulphur
atom from the tridentate Schiff base ligand (L2) along with one
nitrogen atom of 4-methylpyridine. The selected bond lengths and
bond angles are listed in Table 2. The N(1)-Ni(1)-O(1), N(1)-Ni(1)-
O(2) and N(1)-Ni(1)-N(3) chelate bite angles of the complexes 1 and
2 are in the range of 83.7(2)ꢁ - 95.2(2)ꢁ, 83.61(10)ꢁ - 94.62(10)ꢁ and
174.6(2) ꢁ - 174.88(11) ꢁ respectively. While the N(1)-Ni(1)-O(1) and
O(1)-Ni(1)-S(1) chelate bite angles are 95.8(2)ꢁ and 169.69(17)ꢁ
respectively for complex 3. The NieO and NieN bond lengths in all
three complexes (1-3) are in the range of 1.807(5)-1.868(5) Å and
1.819(5)-1.952(3) Å respectively which are in close agreement with
the values reported for other nickel(II) complexes [43,44]. The NieS
bond length of complex 3 is 2.145(2) Å, is nearly similar to
Scheme 1. Preparation of nickel(II) complexes of Schiff base ligands.
Fig. 1. ORTEP diagram of (a) [(NiL1(4-MePy)] (1), (b) [(NiL1(2-AzNp)] (2) and (c) [(NiL2(4-MePy)] (3) complexes with the atom labeling scheme (Thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms have been omitted for clarity).

432
S. Layek et al. / Journal of Molecular Structure 1141 (2017) 428e435
Table 2
and from this result it may be concluded that the obtained struc-
tures to be local minima on the potential energy surfaces. Frontier
molecular orbital (FMO) play an important role in determining the
reactivity of compounds. The HOMO and LUMO contours of com-
plexes [(NiL1(4-MePy)] (1), [(NiL1(2-AzNp)] (2) and [(NiL2(4-
MePy)] (3) are shown in Fig. 2. HOMO generally behaved as an
electron donor and LUMO as an electron acceptor. In all the three
complexes, Schiff base ligand contained HOMO orbital around it
while heterocyclic aromatic co-ligand contained LUMO orbital.
Aromatic rings, especially heterocyclic bases, are suitable electro-
philic sites while the oxygen of methoxy moiety and nitrogen and
sulphur of hydrazine are potential negative sites. The interaction of
ligand with the metal is confirmed by availability of these sites.
Molecular chemical stability of the complexes was determined by
the difference of energy between HOMO and LUMO. Complex
which contained larger energy difference have a lower chemical
reactivity. A suitable cavity was formed for encapsulating Ni(II) by
both the ligands in presence of two co-ligand such as 4-
methylpyridine or 2-azanapthalene. The computed optimization
energy values of the three Ni(II) complexes [(NiL1(4-MePy)] (1),
Selected bond distances (Å) and bond angles (ꢁ) for [(NiL1(4-MePy)] (1), [(NiL1(2-
AzNp)] (2) and [(NiL2(4-MePy)] (3) complexes.
[(NiL1(4-MePy)]
[(NiL1(2-AzNp)]
[(NiL2(4-MePy)]
Bond distances
Ni(1)eN(1)
Ni(1)eN(3)
Ni(1)eN(4)
Ni(1)eO(1)
Ni(1)eO(2)
Ni(2)eO(4)
Ni(2)eO(5)
Ni(1)eS(1)
1.826 (6)
1.934 (6)
e
1.823 (3)
1.952 (3)
1.860 (5)
e
e
1.922 (6)
1.833 (5)
1.828 (5)
1.828 (5)
1.807 (5)
e
1.824 (2)
1.851 (2)
1.868 (5)
e
e
e
-
-
-
2.145 (2)
Bond angles
N(1)eNi(1)eO(1)
N(1)eNi(1)eO(2)
O(2)eNi(1)eO(1)
N(1)eNi(1)eN(3)
N1eNi1eN4
O(2)eNi(1)eN(3)
O(1)eNi(1)eN(3)
N(1)eNi(1)eS(1)
O(1)eNi(1)eS(1)
83.7 (2)
95.2 (2)
175.4 (2)
174.6 (2)
e
94.62 (10)
83.61 (10)
177.80 (10)
174.88 (11)
e
95.8 (2)
-
-
-
169.2 (2)
89.9 (2)
91.1 (2)
e
91.35 (10)
90.44 (10)
-
-
-
86.97 (18)
169.69 (17)
e
-
[(NiL2(4-MePy)]
(2)
and
[(NiL1(2-AzNp)]
(3)
were ꢀ73740.62 eV, ꢀ76851.78 eV and ꢀ70401.27 eV respectively
previously reported Ni(II) complex published by I. Kilic-cikla et al.
[45]. Packing of molecules of 1e3 is shown in ESI Fig. S1, Fig. S2 and
Fig. S3 respectively.
(Fig. 2). The metal binding energy of all three metal complexes 1, 2
and
3
were found to be ꢀ41006.66 eV, ꢀ41006.20 eV
and ꢀ33661.55 eV, respectively. It may be concluded that the
higher the binding energy of the complex, the greater will be the
stability. Since, complex 1 has highest metal binding energy among
the three complexes, it is likely to have the highest stability.
3.2. Computational study
Quantum chemical DFT calculations of all the nickel(II) com-
plexes were performed in Gaussian 09 program using DFT/B3LYP
method with 6e311þþG(d,p) basis set. The energy of the mole-
cules was calculated by simply optimization of complex structures
and the optimized structures of all three complexes were shown in
ESI Fig. S4. The optimized structures are at global minima was
confirmed by lack of imaginary vibration in frequency calculation
3.3. FT-IR spectra of the complexes
The FT-IR spectrum of the ligand HL1 exhibited a broad band at
3380 cmꢀ1 and a weak band at 3564 cmꢀ1 due to
n
OH and
n NH
groups, respectively. Interestingly, HL1 exhibited a sharp and high
Fig. 2. Energy level HOMO and LUMO diagrams of Ni(II) complexes.

S. Layek et al. / Journal of Molecular Structure 1141 (2017) 428e435
433
intensity band at 1646 cmꢀ1 due to
3564 cmꢀ1 and C]O at 1646 cmꢀ1 clearly indicated that the
n
C]O. The presence of
n
NH at
complexes displayed three strong absorption bands in the region
297e309 nm, 360e367 nm and 412e417 nm [49] (Supporting in-
formation, Figs. S11eS13). The strong and high energy peaks in the
regions 297e309 nm were attributed to the intra-ligand transition.
n
ligand HL1 existed predominantly in keto form in the solid state
[28]. The FT-IR spectrum of the HL1 ligand also showed an intense
band at around 1596 cmꢀ1 due to the azomethine group
n
(C]N).
The n-p* transition corresponding to the azomethine groups
The FT-IR spectra of the nickel(II) complexes (1) and (2) are given in
Fig. S5 and Fig. S6 (Supporting information). The FT-IR spectra of
complexes (1) and (2) exhibited bands at 1579 cmꢀ1 and 1580 cmꢀ1
observed in the range of 360e367 nm have been attributed to
ligand to metal charge transfer (LMCT) transition (3A2g/ 3T2g) and
/
the shoulder at 412e417 nm to forbidden (3A2g 3T1g) transition
assignable to
n
(C]N) stretching frequency, respectively. A com-
[50,51].
parison of the FT-IR of the free ligand HL1 and complexes clearly
indicated that the C]N stretching frequency were shifted to lower
wave number by 17 cmꢀ1 and 16 cmꢀ1, in complex (1) and (2),
respectively. The shifting of azomethine stretching frequency to
lower wave number may be taken as evidence for the coordination
3.5. 1H NMR spectra
The 1H NMR spectra of the complex [(NiL1(4-MePy)] in CDCl3 at
room temperature showed a sharp singlet at
d 8.56 due to CeH of
azomethine. The absence of OH protons in the 1H NMR of the
complex indicated the de-protonation of the hydroxy group and
coordination of the oxygen atom to the metal. Multiplet resonances
of the nitrogen atom to the metal [46]. The disappearance of
n OH
peak and appearance of a peak at 582 cmꢀ1 and 560 cmꢀ1 (NieO) in
the FT-IR spectra of complex (1) and (2), respectively, supports the
formation of complexes (1) and (2) [47].
observed in the range
d 6.6e8.0 were assigned to the aromatic
protons. The 1H NMR of Complexes [(NiL1(2-AzNp)] (2) and
[(NiL2(4-MePy)] (3), exhibited the azomethine proton (HC]N-)
The FT-IR spectrum of the free ligand (HL2), showed an intense
band at 3443 cmꢀ1 and 1045 cmꢀ1 assignable to
n (NeH) and n(C]
S) groups, respectively. It confirms the presence of the thione form
signal at
eOCH3 of ortho-vaniline were observed in the range of
3.90. The eCH3 protons of the co-ligand 4-methylpyridine were
observed as singlet at 2.30 and 2.50, in complex 1 and complex
(3), respectively. However, the complex (3) exhibited a signal at
4.57 for two protons of the eNH2 group. It is obvious from the 1H
NMR of complex (3), that no de-protonation of the NH2 group of
thiosemicarbazide moiety had occurred, confirming the non-
participation of the NH2 group in coordination. The fact that the
eNH2 group remained intact is unambiguously supported by single
crystal structure of the complex (3) [Fig. 1(c)]. The 1H NMR spectra
of all three complexes are given in Fig. S14-Fig. S16 (Supporting
information), respectively.
d
8.61 and 9.44, respectively. In all three complexes, the
of ligand in the solid state. The FT-IR spectrum of the HL2 ligand also
d
3.67 to
showed an intense band at around 1590 cmꢀ1 due to the
n
(C]N)
stretching frequency. The bands at 3331 cmꢀ1 and 3157-3326 cmꢀ1
were assigned to the OH and NH2 vibrations of the ligand. The
FT-IR spectra of complex (3) showed a band at 1584 cmꢀ1 assign-
able to (C]N) group. A comparison of the FT-IR of the free ligand
d
n
n
d
n
HL2 and complex (3) indicated that the C]N stretching frequency
were shifted to lower wave number by 6 cmꢀ1 (Fig. S7) which
confirmed the coordination of the nitrogen atom to the metal. The
disappearance of n OH stretching frequency and the appearance of a
peak at 588 cmꢀ1 (NieO) in FT-IR of the complex support the for-
mation of the complex (3) [48].
Theoretical FT-IR spectra of all complexes, calculated with DFT/
B3LYP method using a 6-311þþG(d,p) basis set, exhibited more
similarities with the experimental FT-IR spectra as shown in
Figs. S8eS10. The calculated vibrational frequencies and experi-
mental FT-IR frequencies are presented in Table 1S. These values
were found in good agreement with observed group frequencies.
3.6. Antimicrobial activity
In order to study the potential biological properties of the syn-
thesized nickel complexes, the antibacterial properties were stud-
ied. As the test complexes were insoluble in water, we used 5%
DMSO solution to make a homogeneous aqueous solution. The ef-
fect of 5% DMSO on the growth of E. coli is negligible and it is re-
ported that at 5% DMSO, only a slight reduction in growth rate was
observed over a 6 h [52]. Since aqueous solutions of DMSO were
known to be alkaline, the pH was carefully monitored each and
3.4. UVevisible spectra of the complexes
The electronic spectra of nickel(II) complexes were recorded in
DMF solutions (10ꢀ4 M) in the range of 270e800 nm. Each of the
Fig. 3. (a) Control 5% DMSO, 10 ppm and 20 ppm concentration of the [(NiL2(4-MePy)] (3) complex shows no significant antimicrobial effect on E. coli. (b) 30 ppm, 50 ppm and
100 ppm shows increasing inhibition zone for the antimicrobial effect of the test compound.

434
S. Layek et al. / Journal of Molecular Structure 1141 (2017) 428e435
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every time [53]. A control study was done with 5% DMSO without
the addition of any compound to eliminate the effect of DMSO on
bacterial growth inhibition. The study shows that MIC value of the
test compound must be between 20 and 30 ppm and more pre-
cisely around 30 ppm. Control 5% DMSO has no significant effect on
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30 ppm of concentration. These observations are more or less
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complexes. The result for the antimicrobial activity of complex (3)
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Three new heteroleptic nickel(II) complexes containing pincer
type of Schiff base ligand have been synthesized and characterized
by various spectroscopic techniques. The structures of all three
complexes (1), (2) and (3) were unambiguously confirmed by single
crystal X-ray diffraction studies. All complexes possess a square
planar geometry in which three coordination site are occupied by a
pincer type Schiff base ligand and the forth site by a nitrogen atom
of a co-ligand. The quantum chemical DFT calculations supple-
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ligands. Antimicrobial studies indicated that complex [(NiL2(4-
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Acknowledgments
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We are thankful to the SAIF Panjab University, Chandigarh and
IISER, Bhopal for providing help in the analysis of the samples and
we also grateful to NTU, Singapore for the single crystal X-ray
analysis. Samaresh acknowledge the receipt of IIT (ISM), Dhanbad
research fellowship.
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
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