Crystallographic, spectroscopic and theoretical investigations on Ni(II) complexes of a tridentate NNS donor thiosemicarbazone

Article abstract of DOI:10.1016/j.poly.2018.11.023

Five new nickel(II) complexes [Ni(L)NCS] (1b), [Ni(L)N₃] (2), [Ni(L)N(CN)₂] (3), [Ni(L)OAc] (4) and [Ni(HL)SO₄] (5) of a thiosemicarbazone HL (where HL = 2-benzoylpyridine-N(4)-phenylthiosemicarbazone) including pseudohali

Full text of DOI:10.1016/j.poly.2018.11.023

Accepted Manuscript
Crystallographic, spectroscopic and theoretical investigations on Ni(II) com-
plexes of a tridentate NNS donor thiosemicarbazone
Ranjana Venugopal, S.S. Sreejith, M.R. Prathapachandra Kurup
PII:
S0277-5387(18)30748-4
https://doi.org/10.1016/j.poly.2018.11.023
POLY 13570
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
12 September 2018
7 November 2018
8 November 2018
Please cite this article as: R. Venugopal, S.S. Sreejith, M.R. Prathapachandra Kurup, Crystallographic, spectroscopic
and theoretical investigations on Ni(II) complexes of a tridentate NNS donor thiosemicarbazone, Polyhedron (2018),
doi: https://doi.org/10.1016/j.poly.2018.11.023
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1
Crystallographic, spectroscopic and theoretical investigations on
Ni(II) complexes of a tridentate NNS donor thiosemicarbazone
a
a
a,b,
Ranjana Venugopal , S.S. Sreejith , M.R. Prathapachandra Kurup
aDepartment of Applied Chemistry, Cochin University of Science and Technology, Kochi 682 022, Kerala, India
bDepartment of Chemistry, School of Physical Sciences, Central University of Kerala, Tejaswini Hills, Periye,
Kasaragod, 671316, India
Abstract
Five new nickel(II) complexes [Ni(L)NCS] (1), [Ni(L)N ] (2), [Ni(L)N(CN) ] (3), [Ni(L)OAc]
3
2
(
4) and [Ni(HL)SO ] (5) of a thiosemicarbazone HL (where HL = 2-benzoylpyridine-N(4)-
4
phenylthiosemicarbazone) including pseudohalides like thiocyanate, azide and dicyanamide have been
synthesized and characterized by elemental analyses, magnetic susceptibility, FT-IR, electronic and NMR
spectroscopic techniques. The interesting amido-iminoltautomerism enables the pro-ligand to exist in
amido form as well as deprotonated imino form thereby functioning as tridentate NNS donor,
coordinating through pyridyl nitrogen atom, azomethine nitrogen atom and amido/iminolate sulfur atom
in the complexes. The crystal structures of 1 and 2 were established by single crystal XRD analysis. Two
different single crystals of the compound 1 were obtained with and without DMF solvent molecule having
triclinic and monoclinic lattices with P1and P2 /n space groups respectively. Also, the compound 2 is
1
having a monoclinic lattice with P2 /n space group. Nickel(II) exists in distorted square planar geometry
1
in all the complexes. Further DFT calculations were carried out at B3LYP/TZVP level of theory to obtain
an insight on the stability and the nature of frontier orbitals. Finally, electrostatic potential plots mapped
on optimized geometries substantiate the noncovalent interactions found in their respective crystal
structures.
Keywords: Crystal structures; Nickel(II) complexes; 2-Benzoylpyridine; Thiosemicarbazone; DFT
1
. Introduction
Thiosemicarbazones (TSCs) and their metal complexes show a lot of applications in
pharmacology and therefore they have been extensively studied due to which there are now a
considerable number of coordination compounds in which TSCs behave as neutral or anionic ligands with
transition or non-transition metals [1-6]. The chelation property exhibited by these thiosemicarbazones
with metal ions is responsible for the biological activities [7]. Also the ability of the thiosemicarbazone
*Corresponding author at: Department of Chemistry, School of Physical Sciences, Central University of Kerala,
Tejaswini Hills, Periye, Kasaragod, 671 316, India E-mail address: mrpcusat@gmail.com; mrp@cukerala.ac.in
(M.R.P. Kurup) Phone: +91-9895226115

2
(-)
moiety to coordinate in the anionic form as C=N–N=C–S –N increases the biological activity of these
compounds [8]. These thiourea derivatives find substantial applications in different facets of
contemporary scientific research such as in enzyme modeling mechanism, corrosion inhibitory studies [9-
1
1], etc. These TSCs being NNS donors show carcinostatic efficiency and in vivo activity against tumor
cells [12-14]. It is also evidenced that some structural variations on the TSCs due to their chelation may
damage or decrease its therapeutic value [15]. Nickel is an essential nutrient and act as the active site in
some enzymes seen in plants. Various nickel(II) complexes find a potential application in the production
of anticancer agents.
In the present study,
phenylthiosemicarbazone has been used to synthesize five different Ni(II) complexes viz., [Ni(L)NCS]
1), [Ni(L)N ] (2), [Ni(L)N(CN) ] (3), [Ni(L)OAc] (4), [Ni(HL)SO ] (5). The chosen ligand shows both
a
tridentate NNS donor, HL
=
2-benzoylpyridine-N(4)-
(
3
2
4
anionic coordination (imino form) in complexes 1 to 4, as well as neutral coordination (amido form) in
the complex 5.
2
2
. Experimental
.1. General remarks
Commercially available AR grade reagents and chemicals were used as received without further
purification. The ligand was prepared in situ through an equimolar (1:1) condenzation reaction of 2-
benzoylpyridine with N(4)-phenyl-3-thiosemicarbazide and one pot method was adopted for the syntheses
of all the complexes.
2
.2. Synthesis of 2-benzoylpyridine-N(4)-phenylthiosemicarbazone, HL
An equimolar (1:1) methanolic solutions of 2-benzoylpyridine and N(4)-phenyl-3-
thiosemicarbazide with 3 drops of glacial acetic acid were mixed and refluxed for 5 hours. On slow
evaporation, the yellow crystalline compound formed was filtered, washed with ether and recrystallized
from methanol and dried over P O in vacuo.
4
10
Yield: 87%. M.Pt.: 140 ºC. Elemental analysis (%): (C H N S) (332.42) Calc.: C, 68.65; H, 4.85;
1
9
16
4
N, 16.85; S, 9.65; Found: C, 68.25; H, 4.77; N, 16.48; S, 10.50.
2
.3. Synthesis of nickel(II) complexes of HL
Complexes 1-3 were synthesized by the following method. A solution of the thiosemicarbazone, HL
0.332 g, 1 mmol) in 10 mL hot methanol and nickel(II) acetate hexahydrate (0.2488 g, 1 mmol). in methanol
(
was mixed and heated under reflux for 1 h. and allowed to cool. After cooling, a solution of the corresponding
sodium salt (1 mmol) was added and refluxed for 3 h. The complexes formed were filtered, washed with few
drops of methanol and ether and dried over P O in vacuo. The acetato complex (4) was synthesized by
4
10
refluxing methanolic solutions of equimolar amounts of nickel acetate hexahydrate and HL for 1 h and the
sulfato complex by an identical method by using nickel sulfate hexahydrate.
The complexes were kept for crystallization by slow evaporation in solvents like DMF, acetonitrile,
etc. Brownish crystals suitable for single crystal X-ray diffraction were obtained for compounds 1 and 2. The
complexes 3, 4 and 5 were formed as brown colored precipitates. The elemental analysis data are given in
Table S1.

3
Caution! Azide complexes of metals with organic ligands are potentially explosive and should be handled
with care.
2
.4. Methods and instrumentation
Elemental analyses were performed on a Vario ELIII elemental analyzer. Conductivity
measurements were done at 298 K with a Systronic model 303 direct reading conductivity bridge.
Magnetic susceptibility measurements were done in the polycrystalline state at room temperature on a
Sherwood magnetic susceptibility balance using Hg[Co(SCN) ] as a calibrant. The IR spectra were
4
-1
recorded on a JASCO FT/IR-4100 Spectrometer in the 4000-400 cm range using KBr pellets. Electronic
spectra were recorded on a Thermo scientific Evolution 201 UV-Vis double beam spectrophotometer in
the 200-900 nm range using solutions in DMF. Emission studies of were done using Shimadzu RF-5301
PC spectrofluorimeter. Single crystal X-ray diffraction studies were carried out using Bruker SMART
APEX II diffractometer. NMR spectra were obtained with a Bruker DRX-400 spectrometer using
DMSO-d as the solvent and TMS as internal reference.
6
2
.4.1 DFT simulations
Initial structures were taken from either crystal structures (1a, 1b and 2) or drawn using
Avogadro software package [16]. DFT simulations were done using ORCA 3.0.3 software [17] and all
the geometries were optimized in the gaseous state. The Becke’s three-parameter hybrid exchange
functional including the Lee–Yang–Parr nonlocal correlation functional (B3LYP) [18] and the triple zeta
valence basis set with one set of polarization function (TZVP) [19] was chosen based on the known fact
that it reduces the basis set superposition error (BSSE) to negligible in the calculation of systems with
noncovalent interactions. To accelerate the calculations, we utilized the resolution of identity (RI)
approximation with the decontracted auxiliary def2–TZV/J Coulomb fitting basis sets and the chain–of–
spheres (RIJCOSX) approximation to exact exchange as implemented in ORCA [20]. Orbital
compositional analysis using Mulliken method and AIM charge analysis of the optimized structures were
carried out using Multiwfn program [21]. For this molden files generated from ORCA (using orca_2mkl
functionality) were used as input and a proper conversion from cartesian into pure spherical harmonics of
basis functions with proper normalization of basis functions was achieved.
2
.5. X-ray crystallography
The single crystal X-ray diffraction data were collected using Bruker SMART APEXII CCD
diffractometer, equipped with graphite-crystal, incident beam monochromator, and a fine focus sealed
tube with Mo Kα (λ = 0.71073 Å) and the X-ray source at the SAIF, Kochi, India. Single crystal X-ray
diffraction data were collected at 293(2) K. The Bruker SMART software for data acquisition and Bruker
SAINT software was used for data reduction. The structure was solved by direct methods and refined by
full-matrix least-square calculations with the SHELXL-2014/7 software package [22]. All non-hydrogen
atoms were refined anisotropically, and all hydrogen atoms on carbon were placed in calculated positions,
guided by difference maps and refined isotropically. The molecular and crystal structures were plotted
using DIAMOND version 3.2f [23]. The crystallographic data along with details of structure solution
refinements of the three compounds are given in Table 1 and the selected bond angles and bond lengths
are given in Table 2.

4
Table 1. Crystal data and structure refinement parameters for complexes 1a, 1b, 2
Parameters
[Ni(L)NCS]∙DMF, 1a
[Ni(L)NCS], 1b
[Ni(L)N
3
], 2
Empirical formula
C
23
H
22
N
6
NiOS
2
C
20
H
15
N
5
NiS
2
19 15 7
C H N
NiS
Formula weight
Temperature/K
521.29
296(2)
448.18
296(2)
432.13
296(2)
Crystal system, space group
Triclinic, P1
10.4205(12)
10.6405(12)
11.6468(10)
71.212(3)
89.389(4)
77.756(4)
1192.4(2)
2
Monoclinic, P2
11.1117(6)
15.6019(8)
11.1635(6)
90
1
/n
Monoclinic, P2
13.7013(12)
8.9327(7)
16.2396(15)
90
1
/n
a/Å
b/Å
c/Å
α/º
β/º
93.333(2)
90
105.207(4)
90
γ/º
Volume/ Å3
Z
1932.07(18)
4
1918.0(3)
4
ρ
calc/Mgm-3
1.452
1.541
1.497 Mg m-3
1.140
μ/ mm-1
1.017
1.236
F(000)
540
920
888
Crystal size/mm3
θ range for data collection/º
Limiting indices
0.350 x 0.300 x 0.300
2.642 to 28.360
-13≤h≤13,
0.300 x 0.200 x 0.200
2.833 to 28.418
-14≤h≤14,
0.300 x 0.200 x 0.200
2.625 to 28.366
-18≤h≤18,
-
14≤k≤14,
14≤l≤15
-18≤k≤20,
-11≤k≤11,
-
-14≤l≤14
-21≤l≤21
Reflections collected
Unique Reflections
14657
22864
22732
5384 [R(int) = 0.0225]
5384/1/305
1.005
4856 [R(int) = 0.0519]
4738/1/257
4799 [R(int) = 0.0416]
4799/1/257
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I>2σ (I)]
R indices (all data)
1.009
1.005
R
R
1
=0.0365,wR
=0.0538,wR
2
=0.0930
=0.1069
R
1
1
=0.0347,wR
=0.0485,wR
2
2
=0.0911
=0.1044
R
1
=0.0321,wR
=0.0460,wR
2
=0.0822
=0.0928
1
2
R
R
1
2
Largest diff. peak and hole/e.Å-3 0.359 and -0.336
0.367 and -0.431
0.318 and -0.352
1 0 c 0 2 0 c 0
R = Σ||F | - |F || / Σ|F |; wR = [Σw(F ) ]
2-F 2)2/Σw(F
2 2 1/2

5
Table 2. Selected bond lengths and bond angles of compounds 1a, 1b and 2 obtained through X-ray and
DFT studies
Bond lengths (Å)
[Ni(L)NCS]∙DMF, 1a
[Ni(L)NCS],1b
3
[Ni(L)N ], 2
Experimental
DFT
1.323
Experimental
1.304(2)
DFT
1.337
Experimental
1.307(2)
DFT
1.336
C(6)–N(2)
1.306(3)
C(6)–N(2)
C(6)–N(2)
C(13)–N(3) 1.310(3)
C(13)–N(4) 1.345(3)
C(13)–S(1) 1.754(2)
N(1)–Ni(1) 1.919(2)
1.315
C(13)–N(3) 1.302(3)
C(13)–N(4) 1.362(3)
C(13)–S(1) 1.752(2)
N(1)–Ni(1) 1.9184(16)
1.364
1.359
1.759
1.934
1.354
1.872
1.838
2.178
C(13)–N(3) 1.313(2)
C(13)–N(4) 1.348(2)
C(13)–S(1) 1.7485(19)
N(1)–Ni(1) 1.9110(16)
1.316
1.362
1.759
1.944
1.291
1.868
1.885
2.182
1.360
1.624
1.845
1.357
1.874
1.907
2.181
N(2)–N(3)
1.369(2)
N(2)–N(3)
1.375(2)
N(2)–N(3)
1.375(2)
N(2)–Ni(1) 1.8473(18)
N(5)–Ni(1) 1.846(2)
S(1)–Ni(1) 2.1481(7)
N(2)–Ni(1) 1.8502(16)
N(5)–Ni(1) 1.8541(18)
S(1)–Ni(1) 2.1602(6)
N(2)–Ni(1) 1.8549(15)
N(5)–Ni(1) 1.8723(17)
S(1)–Ni(1) 2.1436(6)
Bond angles (º)
N(5)–Ni(1)–N(2) 177.46(9) 179.77 N(5)–Ni(1)–N(2) 176.73(8)
179.03
95.45
83.58
93.78
87.18
170.77
N(5)–Ni(1)–N(2)
176.19(7)
92.43(7)
84.06(7)
96.55(6)
86.99(5)
170.97(5)
174.87
91.75
83.12
98.15
86.97
170.09
N(5)–Ni(1)–N(1) 95.54(10) 96.18
N(5)–Ni(1)–N(1) 95.10(8)
N(2)–Ni(1)–N(1) 83.81(7)
N(5)–Ni(1)–S(1) 94.60(6)
N(2)–Ni(1)–S(1) 86.54(5)
N(5)–Ni(1)–N(1)
N(2)–Ni(1)–N(1)
N(5)–Ni(1)–S(1)
N(2)–Ni(1)–S(1)
N(1)–Ni(1)–S(1)
N(2)–Ni(1)–N(1) 83.91(8)
N(5)–Ni(1)–S(1) 93.35(8)
N(2)–Ni(1)–S(1) 87.30(6)
83.61
96.18
87.04
N(1)–Ni(1)–S(1) 170.84(6) 170.65 N(1)–Ni(1)–S(1) 170.29(5)
3
. Results and discussion
The ligand coordinates with the metal in the iminolate form in complexes 1, 2, 3 and 4 whereas it
coordinates in the amido form in complex 5. The magnetic moments of the five complexes are in the
range 0.3-0.7 B.M., i.e. almost close to zero; hence these are concluded to be diamagnetic and square
-1
2
-1
planar. The data obtained from conductivity measurements are in the range 5-16 Ω cm mol for the

6
complexes. This indicates that the complexes are non-conductors in nature since the anions are
coordinated to the Ni(II) ion(Table S2) [24].
3
.1. IR spectral studies
The presence of a ν(C=S) band and the absence of a ν(S–H) absorption in the region 2600–
500ꢀcm−1 in the spectrum of HL is considered as an evidence that the thiosemicarbazone exists in its
amido form in the solid state [25]. The band at 3046 cm observed in the spectrum is due to the ν(N–H)
2
−
1
band which again confirms the amido existence of the thiosemicarbazone in the solid state [26]. The
4
−1
ν( N–H) band is at 3299 ꢀcm (Table S2).
-1
For all the complexes ν(C=N) bands were observed in the range 1510-1600 cm . The strong IR
-1
band appearing at 1588 cm in the free ligand spectrum due to ν(C=N) has been observed to shift towards
lower wave numbers in the spectra of complexes due to the involvement of azomethine nitrogen
coordination to a Ni(II) center [27,28]. A positive shift of the ν(N–N) bands in the free ligand to higher
frequencies also supports coordination through azomethine nitrogen in all the complexes. The increase in
the frequency of this band in the spectra of the complexes is due to the increase in the bond strength [29].
The coordination of the pyridine nitrogen is indicated by shift in the in-plane modes of pyridine ring to
-1
higher energies [25,30]. The in-plane bending vibration of the pyridine ring in the ligand at 638 cm is
shifted to higher frequencies on complexation, confirming the coordination of ligands to the metal via the
pyridine nitrogen [31].The coordination of the sulfur atom to the metal is indicated by the shifts of the
-1
two bands considered to have contributions from ν(C–S). The bands observed at 1364 and 784 cm of
the spectrum of HL due to ν(C=S) undergo downward shift in complexes implying sulfur in coordination
with the metal [32,33].
-1
Complex 1 showed a sharp band at 2101 cm , which is assigned to ν(C–N) of the thiocyanate
group, indicating coordination through thiocyanate nitrogen [34]. The azido complex 2 exhibited a strong
-
1
band at 2059 cm , corresponding to the ν (NNN) stretching vibration of the coordinating azido group
a
[
35]. The band at 2168 cm-1 for the complex 3 is due to the coordination of the nitrogen of the
dicyanamide with the metal centre [36].The strong bands at 1499 and 1307 cm of the complex 4 can be
assigned to ν (COO) and ν (COO) of the acetate ion and it supports the unidentate nature of acetate group
-1
a
s
-1
in complex [35,37]. The presence of a band at 1396 cm which corresponds to ν(C=S) in complex 5 may
be due to the coordination of the ligand in the amide form to nickel(II) [29].
3
.2. Electronic spectral studies
The electronic spectra of the thiosemicarbazone and its complexes were recorded in DMF (Table
S3). The electronic spectrum of HL has an absorption band at 343 nm due to the n→π* transitions of the
thiosemicarbazone moiety. This band suffers a hypsochromic shift upon complexation (Fig. 1a). The
observed shifts the result of the weakening of C=S bond and the extension of conjugation upon
complexation. In the spectra of the Ni(II) complexes, a high intense broad band is present at ca. 420 nm
which may be due to the LMCT band. The broadness of these bands can be explained as due to the
1
combination of N→Ni and S→Ni LMCT transitions. All the three spin allowed transitions A
2
g
1
1
1
1
1
A , B
A and E
A could not be observed, probably due to masking by the high-
1g
1
g
2g
1g
g
intensity charge transfer bands [38]. Fig. 1b shows the visible spectra of the complexes. For a
diamagnetic Ni(II) complex, as a consequence of eight electrons being paired in the four low-lying d

7
orbitals with the upper orbital being dx2–y2, the four lower orbitals are often so close in energy that
individual transition from them to the upper d level cannot be distinguished resulting in a single
absorption band. The d–d bands appearing as weak shoulders in the range 500 to 700 nm are typical of
square planar Ni(II) complexes [39].
(b)
(a)
Fig. 1. (a)Electronic spectra of HL and its complexes, (b) visible spectra of Ni(II) complexes of HL.
3
.3Solvatochromism of HL and its complexes
-3
Electronic spectra of all the complexes were taken in DMF (1 x 10 M) at room temperature.
Solvents of different polarity were taken and they belong to different classes; non-polar (CHCl , toluene),
3
polar protic (methanol), polar aprotic (acetonitrile, acetone, ethyl acetate, THF).
All compounds show a hypsochromic shift with increasing polarity which is called negative
solvatochromism (Table S4, Fig. S1). A solvatochromic shift is always associated with a change in
dipole moment between electronic ground and excited states of the chromophore. In this case, increment
in solvent polarity results in a decrease in dipole moment of the excited state leading to the stabilization of
the electronic ground state when compared to the first excited state [40]. This phenomenon can be briefed
as the transfer of electron from the donor to the acceptor with simultaneous change in polarizability and
dipole moment giving rise to a destabilized excited state with a lower dipole moment [41].
3
.4. X-ray crystallographic studies
3
.4.1 Crystal structure of [Ni(L)NCS]∙DMF, 1a and [Ni(L)NCS], 1b
The compound [Ni(L)NCS]∙DMF crystallized into a triclinic P1space group with Z = 2 and the
complex exhibits a distorted square planar geometry (Fig. 2) while the compound [Ni(L)NCS] has the
space group P2 /n and its crystal system is monoclinic with Z = 4 and the coordination gives rise to two
1
five membered chelate rings with central metal having a square planar geometry (Fig. 3). The anionic
thiosemicarbazone, HL acts as a tridentate ligand and coordinates to the central nickel ion via the pyridyl
nitrogen, N(1) the azomethine nitrogen, N(2) and the thioiminolato sulfur, S(1) with the fourth

8
coordination site occupied by the nitrogen, N(5) of the thiocyanate group. The stronger coordination of
the azomethine nitrogen to metal compared to the pyridine nitrogen is attributed to the higher basicity of
N(2). The Ni(II) complex adopts a distorted square planar shape which is evident from the two bond
angles N(5)–Ni(1)–N(2): (177.46(9)º for 1a and 176.73(8)º for 1b); N(1)–Ni(1)–S(1): (170.84(6)º for
1
aand 170.29(5)º for 1b). Also the τ(4) value of the crystal (0.00 for square planar and 1.00 for
tetrahedral), [42] which is calculated as
3
60 ‒ (훽 + 훼)
휏(4) =
(α and β are the largest basal trans angles)
1
41
is 0.04 for 1a and 0.05 for 1b which confirms the distorted square planar shape of the complex. The
summation of bond angles; N(2)–Ni(1)–S(1), N(2)–Ni(1)–N(1), S(1)–Ni(1)–N(5),N(1)–Ni(1)–N(5) gives
a value of 360.10° (1a) and 360.05° (1b) and the low dihedral angle between the chelate rings illustrates
that the complexes are planar.
The tridentate ligand shows a Z, E, Z configuration about the bonds C(5)–C(6), C(6)–N(2) and
N(3)–C(13) for the donor centres nitrogen, nitrogen and sulfur respectively. The ligand has undergone
keto-enol tautomerism and the torsional angle C(5)–C(6)–N(2)–N(3), 176.70(18)º for 1a and -178.54(16)º
for 1b (the C(7)–C(6)–N(2)–N(3) torsional angle is -5.2º and 2.6º) shows that the complex has E
conformation with respect to the pyridyl group. Also the torsion angle S(1)–C(13)–N(3)–N(2) of 0.9º
(1a) and 2.1º (1b) shows that N(3)–C(13) double bond has Z conformation with respect to the sulfur
atom. The C–N and N–N bond lengths are intermediate between formal single and double bond and
consistent with the fact that charge delocalization extends throughout the entire molecular skeleton,
affecting the thiosemicarbazone chain as well as the ring.
Fig. 2. Structure and labeling diagram of [Ni(L)NCS]∙DMF, 1a.

9
Fig.3. Structure and labeling diagram of [Ni(L)NCS], 1b.
The molecule 1a shows one intermolecular hydrogen bonding interaction in which the polar
hydrogen atom of N(4) is involved in strong intermolecular H-bonding interaction with the O(1) atom of
the solvent molecule (Fig. S2 and Table S5). The crystal packing is also stabilized by weak π∙∙∙π as well
as M∙∙∙π interactions (Table S6). The five membered ring Cg(1) is involved in π∙∙∙π interactions with
another Cg(1) and a five membered Cg(2) rings of an adjacent unit with a distance of 3.8912(11) Å and
3
.7095(12) Å respectively. In addition to these, the six membered Cg(4) is also in π∙∙∙π interaction with
another Cg(4) ring of the neighboring unit with a distance 3.6935(16) Å (Fig.4a). The Ni(II) ion also
shows interaction with the Cg(1) ring with the ring-metal distance 3.928 Å (Fig.4b).
(b)
(a)
Fig. 4. (a) π∙∙∙π interactions in 1a, (b) ring∙∙∙metal interactions in 1a.

1
0
The complex 1b is stabilized by intermolecular hydrogen bonds (Fig. S3) involving the
uncoordinated nitrogen N(4) and sulfur atom S(1) of another molecule. The uncoordinated nitrogen N(4)
is a hydrogen donor, while sulfur S(1) is an acceptor (Table S7). Cg···Cg interactions found between five
membered chelate rings, pyridine rings and phenyl rings are shown in Fig. 5a (Table S8). Packing of the
molecule is also stabilized by strong ring···metal interactions of chelate ring Cg(5) with Ni(1) and is
observed at a distance of 3.929 (Fig. 5b). In addition to the π∙∙∙π interactions, the compound possesses a
C–H∙∙∙π interaction between hydrogen connected to C(2) and Cg(4) ring as well as a C–X∙∙∙π interaction
between the sulfur bonded to C(20) and Cg(2) ring (Figs. S4 and S5).
(b)
(a)
Fig. 5 .(a) π∙∙∙π interactions in1b, (b) ring∙∙∙metal interactions in 1b.
3
.4.2 Crystal structure of [Ni(L)N ], 2
3
The compound crystallizes in monoclinic space group P2 /n. The nickel ion in the mononuclear
1
complex is four coordinated (Fig. 6a). The coordination geometry around Ni can be best described as
square planar in which the thiosemicarbazone coordinates to the metal in a tridentate manner (NNS)
forming two five membered chelate rings. The fourth coordination position is occupied by the
pseudohalide azide group. Due to the conjugated π-electron system in the thiosemicarbazone moiety they
are coplanar with the coordination sphere around nickel. The packing diagram of the molecule viewed
along ‘a’ axis is given in Fig. 6b. In the unit cell the nickel atoms are facing the opposite sides. A torsion
angle value of -0.8(2)° corresponding to N(2)–N(3)–C(13)–S(1) is a clear evidence for the existence of
the thiosemicarbazone moiety in the Z conformation. The dihedral angle between the two metal chelate
rings is equal to 1.337(29)°, which further supports the distorted square planar structure of the complex.
The τ(4) value for this complex is 0.05.

1
1
(
a)
(b)
],2,(b) the packing diagram viewed along crystallographic ‘a’ axis.
Fig.6. (a) Structure and labeling diagram of [Ni(L)N
3
The complex shows an intermolecular H bonding between the hydrogen attached to the N(4) with
N(7) atom of azido group and forms a one-dimensional chain along the ‘b’ axis which is depicted in Fig.
7
a(Table S9). The metal chelate ring Cg(1) is involved in π∙∙∙π interactions with the Cg(2) chelate ring of
the neighboring unit (Fig. 7b). In addition to the π∙∙∙π stacking, ring∙∙∙metal interactions between the
nickel atom and the Cg(1) and Cg(3) chelate rings as well as C–H∙∙∙π interactions between the C(18)–
H(18) and the Cg(4) (phenyl ring) contribute to the stability of the unit cell packing (Figs. 7c, 7d and
Table S10).
(a)
(b)

1
2
(c)
(d)
Fig. 7.(a) Intermolecular hydrogen bondingextending along ‘b’ axis, (b) π∙∙∙π interactions, (c) ring∙metal
interactions and (d) C–H∙∙∙π interactions in 2.
3
.5. NMR studies
The NMR spectrum of HL was done in CDCl solvent whereas those of the complexes 3-5 were
3
1
recorded in DMSO-d solvent. The H peaks were assigned on the basis of chemical shifts and
6
1
13
multiplicities. Due to the existence of both Z and E conformations in solution, the H and C signals of
HL are duplicated. The peaks at δ = 14.01 and δ = 9.53 corresponds to the N3-H of the Z and E forms
[25,43].This N3-H signal is absent for the complexes evidently due to the deprotonation and formation of
the anionic ligand. However only one signal is seen for all the protons and carbon atoms for the
complexes. The peaks corresponding to the N4-H and the three protons of the pyridine ring undergo shift
during complex formation indicating the NNS coordination of the ligand with the nickel ion [25].
1
3
Comparing the C NMR spectra of the TSC and the complexes, the shifts occurring for the C=S,
C=N and the carbon atoms in the pyridine ring is clearly due to the complexation with nickel atom in
which the TSC adopts E conformation, as is shown by their crystal structures.
3
.6. DFT studies
3
.6.1 Frontier orbital analyses
All the complexes were optimized in gaseous phase at B3LYP/TZVP level of theory (Fig.8).
Experimental and theoretical bond angles and lengths agree with each other with minor discrepancies
Table 2) which may be due to the fact that the experimental structure is in solid phase while the
(
theoretical ones are in isolated gaseous form. Energy of HOMO usually dictates the ability of the
molecule to donate electron while that of LUMO decides its electron accepting nature. Frontier orbital
analyses of the complexes and reactivity descriptors derived from them (Table 3) reveal the respective
chemical reactivity and stability [44]. Chemical hardness is an important reactivity descriptor which
predicts the potential of the system to resist the electron cloud deformation during a chemical reaction

1
3
[45]. According to the energy gap trend, one may assume that 3 will be the stable complex after 4 but
highest electronegativity of the complex renders it more reactive. This is also reflected in the
electrophilicity index which is measure of energy stabilization due to the electron flow between HOMO
and LUMO. From the comparison, the order of kinetic stability is 4>2>3>1a>5>1b.
Fig. 8. Optimized structures of 1-5 at B3LYP/TZVP level of theory.
Table 3.Global reactivity descriptors
Ionization Electron
Hardness
(eV)
Chemical
Potential
(eV)
Electronegativity
(eV)
Global
electrophilicity index
(eV)
Potential
eV)
Affinity
(eV)
(
1
a
5.589
5.493
5.618
6.345
5.847
5.444
2.991
3.198
2.968
3.407
2.905
3.135
1.299
1.148
1.325
1.469
1.471
1.155
-4.29
4.29
14.168
1
b
-4.346
-4.293
-4.876
-4.376
-4.289
4.346
4.293
4.876
4.376
4.289
16.456
2
13.909
3
4
5
16.184
13.018
18.399
Orbital compositional analysis was performed on these compounds using Mulliken method. On
comparing 1a and 1b, it was found that the even though both the complexes are of the same structure the
presence of DMF moiety has brought difference in the energy of the frontier orbitals. It is to be noted that
in 1a, the distance between DMF and the Ni complex significantly decreased when compared to that of
the crystal structure. The anomaly can be attributed to both gas phase calculation and also to the fact that
only a single complex and DMF molecule is considered for calculation which is not the case in the
experimentally obtained structure. Even though the HOMO-LUMO contribution in both 1a and 1b are
pretty similar, out of the two, 1a is more stable with a relatively large HOMO-LUMO gap (Fig. 9). Here
HOMO is mainly concentrated on the NCS coligand (π type) with a cumulative contribution of 82.6% in
1
a, 87.14% in 1b and the metal centre is contributing to about 7.44% in 1a and 5.86% in 1b (d type). In
LUMO, the contribution from the coligand is very minimal (1.32% in 1a and 1.61% in 1a) and apart from

1
4
a minor contribution from the metal centre (3.58% in 1a and 3.68% in 1b), it is centered on the
thiosemicarbazone ligand. In the case of 2, the HOMO is mainly composed of azide coligand (π type)
with a cumulative contribution of ca. 32.84% followed by the Ni centre (d type, 18.65%). The next
significant contribution is from the thioiminolato sulfur atom (p type) with a contribution of 11.13%. In
the case of LUMO, the contribution from the azide group is minimum and it is located mainly on the
thiosemicarboazone ligand (91.75%) followed by Ni metal centre (3.93%). Among the ligand atoms, the
main contribution is from the nitrogen atom of the thiocyanato group (16.19%) followed by the C atom of
the azomethine group (13.15%) and finally from the N atom of pyridine group (10.39%). HOMO of 3 is
mainly composed of N(CN) coligand (p type, 88.23%) with metal and the thioiminolato sulfur atom
2
having minor contributions (d type, 4.68% and p type, 1.58% respectively). However, LUMO of 3 is
completely devoid of any contribution from coligand and is completely centered on the ligand (95.38%).
In 4, there is a change in the trend as seen in 2 and 3 and HOMO is mainly on ligand group (65.34%) with
thioiminolato sulfur atom (p type) having a hefty share of 50.79% followed by the metal centre (d type,
2
(
3.09%). When it comes to LUMO, it is more evenly distributed among different groups in the ligand
87.88%) with azomethine nitrogen contributing the most (16.32%). For 5,HOMO is concentrated mainly
on the Ni metal centre (d type) with contribution of 40.68% closely followed by the SO coligand (p type,
4
3
6
2.82%) but LUMO is mainly comprised of ligand (90.3%) with the metal contribution is minimized to
.68% and coligand to 3.02%. So in general as evident from Fig. 9 it can be said that in all the complexes
except 5, HOMO is mainly on coligands closely followed by the metal while LUMO is on the
thiosemicarbazone ligand.
Fig. 9. Frontier orbital plots of 1-5 with their energies.
3
.6.2 AIM charge analysis and Electrostatic potential maps (ESP)
Charge distribution on atoms in the complexes was calculated using AIM (Atoms In Molecules)
charge analysis [46]. In all the cases, the nitrogen atoms of the thiosemicarbazone ligand have charges
ranging from -1.247 to -0.755 while sulfur atoms exhibit low negative charges spanning from -0.103 to -
0
.138. In the case of 1a and 1b, Ni is having a charge of +0.861 while the coligand donor N atom in NCS
has a charge -1.432 and the nonbonded S on the other end has a positive charge of 0.203 showing

1
5
effective polarization. In the case of 2, the metal centre is at +0.817 while the coordinated nitrogen atom
of the azide coligand has a charge of -0.412 and the nonbonded one has a charge of -0.022. In 3, Ni metal
centre has a charge of +0.824 and the coligand donor N atom has a charge of -1.293 and in 4, Ni has a
charge of about +0.8863 and the donor O atom of the OAc coligand has -1.163 charge on it. It was found
that the Ni is exhibiting the highest positive charge on 5 of about +0.963 and the two bonded O atoms of
coligand SO has a charge of -1.247 and -1.108 each.
4
The molecular electrostatic potentials were mapped on electron density isosurface of optimized
structure of the complexes using B3LYP/TZVP level of theory (Fig. 10). Blue regions mark the areas
with positive electrostatic potential while the red ones indicate negative electrostatic potentials. In the Ni
complex with NCS coligand (1), the highest negative potential is seen near S atom of the NCS coligand (-
4
9.1 kcal/mol) while the next highest negative potential is near the thioiminolato sulfur atom of the ligand
(-35.8 kcal/mol). The highest positive potential seen near the hydrogen atom of the azomethine group
(+48.6 kcal/mol). This results in the intermolecular hydrogen bonding in 1b (Table S7). But in 1a, due to
the presence of crystallized DMF solvent, intermolecular interaction between the O atom of the solvent
DMF which is in a negative potential region (-47.3 kcal/mol) and azomethine hydrogen occurs (Table
S5). The metal centre is at a potential of +21.2 kcal/mol. In the case of 2, the metal centre is having a
potential of 16.5 kcal/mol and the highest positive potential is again at the proximity of azomethine
hydrogen atom with a value of +49.3 kcal/mol and the highest negative potential is near nonbonded N
atom of the azide coligand (-48.9 kcal/mol) which results in intermolecular hydrogen bonding interaction
as discussed above (Table S9). For the complexes 3-5, similar trend can be seen where the maximum
negative potential is seen near the nonbonded end of the respective coligand and the highest positive
potential seen near the hydrogen atom of the azomethine group (values given in Fig.10).
Fig. 10. Molecular electrostatic potential plots of the complexes.

1
6
4
. Conclusion
Five four coordinate, diamagnetic nickel(II) complexes with an NNS donor thiosemicarbazone
and different anions were synthesized and characterized by analytical and spectroscopic techniques. The
ligand coordinates in anionic form in the first four complexes but exists as a neutral ligand in the complex
5
. The single crystal X-ray diffraction studies reveal that the Ni(II) adopts a distorted square planar
geometry. The solvatochromic studies reveal that the complexes show negative solvatochromism. As a
proof of the experimental results, DFT calculations were done atB3LYP/TZVP level of theory and the
optimized geometries were compared with the experimental ones. The energies of HOMO and LUMO
and their composition analysis reveal that HOMO lies mainly on coligands and hence dictates the
reactivity in these complexes. Further electrostatic potential plots mapped on the electron density
isosurface corroborate and quantifies noncovalent interactions found in the respective crystal structures.
Acknowledgements
Ranjana Venugopal is grateful to the Cochin University of Science and Technology for providing
financial assistance in the form of a fellowship. The authors are thankful to the Sophisticated Analytical
Instruments Facility, Kochi, India for single crystal XRD measurements and elemental analyses.
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1
9
Crystallographic,
spectroscopic
and
theoretical
investigations on Ni(II) complexes of a tridentate NNS donor
thiosemicarbazone
Ranjana Venugopal, S.S. Sreejith, M.R. Prathapachandra Kurup
Index Abstract
Five new nickel(II) complexes of a thiosemicarbazone functioning as tridentate NNS donor are
synthesized and their structures established by single crystal XRD analysis and other physic-
chemical characterizations. Molecular structures showed that all nickel(II) complexes displayed
distorted square planar geometry.

2
0
Graphical abstract