Studies on N-picolinoyl-N′-benzothioylhydrazide and its Zn(II) complex: Synthesis, structure, antibacterial activity, thermal analysis and DFT calculation

Article abstract of DOI:10.1016/j.molstruc.2014.01.043

A new Zn(II) complex [Zn(pbth)₂] (where Hpbth = N-picolinoyl-N′-benzothioylhydrazide) has been synthesized and characterized by elemental analyses, IR, UV-Visible and single crystal X-ray data. The distorted octahedral complex [Zn(pbth)₂] crystallizes in monoclinic system with space group C2/c and is stabilized by various types of inter and intramolecular extended hydrogen bonding providing supramolecular framework. The optimized molecular geometry of N-picolinoyl-N′- benzothioylhydrazide (Hpbth) and the zinc complex in the ground state have been calculated by using the DFT method using B3LYP functional with 6-311 G(d,p){C,H,N,O,S}/Lanl2DZ basis set. The results of the optimized molecular geometry are presented and compared with the experimental X-ray diffraction data. In addition, quantum chemical calculations of Hpbth and the complex, molecular electrostatic potential (MEP), contour map and frontier molecular orbital analysis were performed. The solid state electrical conductivity and thermal behaviour (TGA) of the complex were investigated. The bioefficacy of the complex has been examined against the growth of bacteria in vitro to evaluate its anti-microbial potential.

Full text of DOI:10.1016/j.molstruc.2014.01.043

Accepted Manuscript
Studies on N-picolinoyl-N’-benzothioylhydrazide and its Zn(II) complex: Syn-
thesis, structure, antibacterial activity, thermal analysis and DFT calculation
S.K. Kushawaha, R.K. Dani, M.K. Bharty, U.K. Chaudhari, V.K. Sharma, R.N.
Kharwar, N.K. Singh
PII:
S0022-2860(14)00077-5
DOI:
http://dx.doi.org/10.1016/j.molstruc.2014.01.043
MOLSTR 20320
Reference:
Journal of Molecular Structure
To appear in:
Received Date:
Revised Date:
Accepted Date:
16 November 2013
15 January 2014
15 January 2014
Please cite this article as: S.K. Kushawaha, R.K. Dani, M.K. Bharty, U.K. Chaudhari, V.K. Sharma, R.N. Kharwar,
N.K. Singh, Studies on N-picolinoyl-N’-benzothioylhydrazide and its Zn(II) complex: Synthesis, structure,
antibacterial activity, thermal analysis and DFT calculation, Journal of Molecular Structure (2014), doi: http://
dx.doi.org/10.1016/j.molstruc.2014.01.043
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

REVISED MANUSCRIPT
Studies on N-picolinoyl-N'-benzothioylhydrazide and its Zn(II) complex:
Synthesis, structure, antibacterial activity, thermal analysis and DFT
calculation
S.K. Kushawahaa, R.K. Dania, M.K. Bhartya*, U.K. Chaudharia, V.K. Sharmab, R.N. Kharwarb, N.K. Singha*
a Department of Chemistry,bDepartment of Botany, Faculty of Science, Banaras Hindu University, Varanasi-221005,
India
Abstract
A new Zn(II) complex [Zn(pbth)2] (where Hpbth = N-picolinoyl-N'-benzothioylhydrazide)
has been synthesized and characterized by elemental analyses, IR, UV-Visible and single crystal
X-ray data. The distorted octahedral complex [Zn(pbth)2] crystallizes in monoclinic system with
space group C 2/c and is stabilized by various types of inter and intramolecular extended
hydrogen bonding providing supramolecular framework. The optimized molecular geometry of
N-picolinoyl-N'-benzothioylhydrazide (Hpbth) and the zinc complex in the ground state have
been calculated by using the DFT method using B3LYP functional with 6-311
G(d,p){C,H,N,O,S}/Lanl2DZ basis set. The results of the optimized molecular geometry are
presented and compared with the experimental X-ray diffraction data. In addition, quantum
chemical calculations of Hpbth and the complex, molecular electrostatic potential (MEP),
contour map and frontier molecular orbital analysis were performed. The solid state electrical
conductivity and thermal behavior (TGA) of the complex were investigated. The bioefficacy of
the complex has been examined against the growth of bacteria in vitro to evaluate their anti-
microbial potential.
Keywords: NNS donor ligand, Zn(II) complex, Supramolecular architecture, Bactericidal
property, DFT calculation.
1

*Corresponding author. Tel.: +91 542 6702447, E-mail: mkbharty@bhu.ac.in (M.K. Bharty),
singhnk_bhu@yahoo.com (N.K. Singh)
1. Introduction
N'-Thioaroyl derivatives of acyl hydrazine are the next higher homolog of potential biologically
important compounds after semicarbazides and thiosemicarbazides which is due to their
flexibility in assuming different conformations [1-3]. They display beneficial biological
activities, such as antitumor, antibacterial, antiviral and antimalarial activities [4-7]. The
biological activities of these compounds are considered to be due to the presence of HNCS
moiety and their ability to form chelates with metal ions. Biological activities of metal
complexes generally differ from those of either ligands or the metal ions and the changes in
biological activities are reported for several transition metal complexes [8, 9]. Furthermore, the
above type of ligands can coordinate either in the neutral or in the anionic form because of the
possibility of tautomerism which makes their study extremely interesting [10]. Due to the
presence of potential donor sites, N-aroyl derivatives of thiocarbohydrazides are interesting
ligands and generate mononuclear, dinuclear and even tetranuclear complexes which provide a
new dimension to supramolecular chemistry [11, 12]. Investigations into the structural stability
of these compounds using both experimental techniques and theoretical methods have been of
interest for many years. In view of this, we have synthesized and characterized the Zn(II)
complex of a new NNS tridentate ligand N-picolinoylN'-benzothioylhydrazide. DFT calculation,
thermal behavior, electrical conductivity has been studied. The antimicrobial properties of the
ligand and the Zn(II) complex have also been examined against the growth of several bacterial
pathogens.
2

2. Experimental
2.1. Chemicals and starting materials
Commercial reagents were used without further purification. Picolinic acid hydrazide
(Sigma Aldrich), Zn(OAc)2·2H2O (BDH Chemicals) were used as received. Carboxymethyl
dithiobenzoate was prepared by the reported method [13]. All the synthetic manipulations were
carried out in open atmosphere and at room temperature. The solvents were distilled before use
following the standard procedure.
2.2. Physical measurements
Carbon, hydrogen, nitrogen and sulphur contents were estimated on a CHN Model CE-
440 Analyzer and on an Elementar Vario EL III Carlo Erba 1108. IR spectra were recorded in
the 4000–400 cm-1 region as KBr pellets on a Varian Excalibur 3100 FT-IR spectrophotometer.
13
1H and C NMR spectra were recorded in DMSO-d6 on a JEOL AL300 FT-NMR spectrometer
using TMS as internal reference. Electrical conductance of pressed pellets was measured by a
conventional two-probe method in the 312-443 K range with contacts made on the pellet surfaces
by means of silver paint. A Keithley 236 SMU was used to measure the sample resistance.
Thermogravimetric analysis of the complex was done using a Perkin Elmer-STA 6000 thermal
analyser in a nitrogen atmosphere at a heating rate of 15 ºC min-1.
2.3 Antibacterial tests
Five human bacterial pathogens Salmonella typhi (MTCC 3216), Shigella flexneri (ATCC
12022), Staphylococcus aureus (ATCC 25323), Aeromonas hydrophila (ATCC 7966) and
Enterococcus faecalis were used to test the antibacterial activity of the ligand, Zn(OAc)2·2H2O
and the zinc complex. The antibacterial assay was done according to the reported method [14]
3

with some slight modifications [15]. The test compounds were dissolved in DMSO to a final
concentration of 5 mg/ml. Sterilized Whatman no. 1 filter paper discs (5 mm) were impregnated
with different volume (1, 2, 4, 6, 8 and 10 µL) of compounds to get a final concentration of 5,
10, 20, 30, 40 and 50 µg per disc. Sterilized paper disc loaded with the 10 µL of DMSO was
taken as a control. The bacterial test pathogens were spread on fresh Mueller Hinton Agar
(MHA) plates with the help of cotton swabs to form an even lawn of the test bacteria. The filter
paper disc impregnated with the test compound were placed on the surface of the MHA plates
seeded with test bacteria and the plates were incubated in a B. O. D. incubator (Caltan-152,
Narang Scientific Works, New Delhi, India) for 24 h at 37 2 ºC. The inhibition zones around
each disc were measured after 24 h of incubation. Commercial antibacterial drugs streptomycin
sulphate and neomycin sulphate (Himedia) were used in same concentration of 5-50 µg/disc to
compare effectiveness of the test compounds.
2.4. Synthesis
2.4.1. Synthesis of N-picolinoyl-N'-benzothioylhydrazide (Hpbth)
N-picolinoyl-N'-benzothioylhydrazide (Hpbth) was prepared by adding a methanol
solution of carboxymethyl dithiobenzoate (2.12 g, 10 mmol) dropwise to 1N NaOH solution of
picolinic acid hydrazide (1.37 g, 10 mmol). The reaction mixture was stirred continuously for 1 h
and acidified with dil. acetic acid. The precipitate obtained was filtered off, washed with water,
dried under reduced pressure and recrystallized from a mixture of MeOH-CHCl3 (50:50 v/v)
[16]. Yield: 75 %; m.p. 184 ºC. Found: C, 60.73; H, 4.31; N, 16.30; S, 12.47 %. Calc. for
C13H11N3SO (257.31): C, 60.70; H, 4.28; N, 16.33; S, 12.44 %. IR ( cm-1, KBr): ν(N-H) 3236;
ν(C=O) 1661; ν(N-N) 1075; ν(C=S) 921. 1H NMR (DMSO-d6; δ ppm): 10.63, 10.05 (s,2H, NH),
8.68 (C1H), 7.90 (C2H), 7.92 (C3H), 8.044 (C4H), 7.65 (C9H), 7.36 (C10H), 7.34 (C11H),
4

7.22 (C12H), 7.50 (C13H), 13C NMR (DMSO-d6; δ ppm): 190.71 (C=S), 163.07 (C=O), 147.53
(C1), 131.17 (C2), 137.53 (C3), 131.72 (C4), 149.08 (C5), 138.05 (C8), 129.16 (C9, C13),
127.97 (C10, C12), 131.06 (C11).
2.4.2. Synthesis of [Zn(pbth)2]
A methanol-acetonitrile solution (20 ml, 1:1 v/v) of the ligand N-picolinoyl-N'-
benzothioylhydrazide (2 mmol) was added slowly to the methanol (10 mL) solution of
Zn(OAc)2.2H2O (1 mmol), stirred for 2 h, and then refluxed for 1h at 60 ºC. A clear solution thus
obtained was filtered and kept for crystallization. Crystals suitable for X-ray analyses were
obtained by slow evaporation of the above solution over a period of 25 days. Yield: 56 %; m.p.
236 ºC. Anal. Found: C, 53.96; H, 3.47; N, 14.58; S, 11.05 %. Calc. for C26H20ZnN6S2O2
(578.01): C, 53.98; H, 3.46; N, 14.53; S, 11.07 %. IR (KBr, cm-1): ν(NH) 3284, ν(C=O) 1621,
ν(N-N) 1090, ν(C=S) 918, (Zn-N) 508, (Zn-S) 421. 1H NMR (DMSO-d6; δ ppm): 10.00 (s, 1H,
NH), 8.24 (C1H), 8.13 (C2H), 7.91 (C3H), 8.14 (C4H), 7.51 (C9H), 7.34 (C10H), 7.32 (C11H),
7.25 (C12H), 7.49 (C13H). 13C NMR (DMSO-d6; δ ppm): 185.88 (C=S), 162.14 (C=O), 146.14
(C1), 131.75 (C2, C11), 136.63 (C3), 128.77 (C4), 148.50 (C5), 138.84 (C8), 127.16 (C9, C13),
126.43 (C10, C12).
5

C10
C9
C8
C11
C12
O
O
N
H
C4
H
N
C5
S
C6
C3
C2
NaOH, stand for 2 h
dil. AcOH
N
N
C7
OH
C13
+
H2N
H
N
S
S
O
C1
Hpbth
Carboxymethyl dithiobenzoate
MeOH, CH3CN
Zn(OAc)2.2H2O
C10
C9
C8
C11
C12
O
H
N
C4
C5
N
S
C3
C2
C6
N
C7
C13
S
Zn
C1
N
N
N
H
O
Scheme 1 Synthesis of the ligand and its Zn(II) complex
3. Crystal structure determination
The X-ray data collection for the complex were performed on an Oxford Diffraction
Gemini diffractometer equipped with CrysAlis Pro., using a graphite mono-chromated Mo Kα (λ
= 0.71073 Å) radiation source. The structures were solved by direct method (SHELXL-2008)
and refined against all data by full matrix least-squares on F2 using anisotropic displacement
parameters for all non-hydrogen atoms. All the hydrogen atoms were included in the refinement
at geometrically ideal positions and refined with a riding model [17]. The MERCURY package
and ORTEP-3 programs were used for generating molecular structures [18, 19].
4. Quantum chemical calculation
The calculations based on DFT have been applied in many areas and the results also are in great
agreement with the experimental ones in calculating spectroscopic properties [20]. The purpose
of the present study is to compare the experimental data with theoretical and to calculate optimal
molecular geometry of N-picolinoyl-N'-benzothioylhydrazide and its Zn(II) complex. All
calculations were performed using the Gaussian 03 and Gauss View 4.1 [21] program. The
6

structure optimization of N-picolinoyl-N'-benzothioylhydrazide has been done using DFT
method with functional B3LYP and basis set 6-311G(d,p) [22] whereas the geometry
optimization for the complex has been done using DFT method with functional B3LYP and basis
sets 6-311G(d,p){C,H,N,O,S}/ Lanl2DZ. The basis set Lanl2DZ [23] serves the purpose of
including the pseudo potential of the core electrons in the metal atoms. The input geometries of
the zinc complex of N-picolinoyl-N'-benzothioylhydrazide for the DFT calculations were
generated from single crystal X-ray data. The results obtained from theoretical calculations and
experiments have been compared. The first task for the computational work was to determine the
optimized geometry of N-picolinoyl-N'-benzothioylhydrazide and the Zn(II) complex. Analytic
frequency calculations at the optimized geometry were done to confirm the optimized structures
to be an energy minimum. At the optimized geometry for the N-picolinoyl-N'-
benzothioylhydrazide and the Zn(II) complex, no imaginary frequency modes were obtained, so
there is a true minimum on the potential energy surface is found. It is well known in the quantum
chemical literature that the hybrid B3LYP method based on Becke’s three parameter functional
of DFT [24] yields a good description of harmonic vibrational wave numbers. This method is
based on fairly large and flexible basis set 6-311G(d,p){C,H,N,O,S} /Lanl2DZ level to perform
accurate calculations on the molecule were chosen. The HOMO–LUMO energies were
calculated using B3LYP method of the DFT. In order to show NLO activity of the Zn(II)
complex, the dipole moment, linear polarizability and first hyperpolarizability were obtained
using the same level of theory.
5. Results and discussion
The ligand N-picolinoyl-N'-benzothioylhydrazide (Hpbth) reacts with Zn(OAc)2·2H2O to give
[Zn(pbth)2] and is stable towards air and moisture. The ligand and the complex melt at 184 and
7

236 ºC, respectively. The Scheme 1 depicts the formation of the ligand and Zn(II) complex. The
colorless Zn(II) complex is insoluble in water, partially soluble in ethanol but highly soluble in
chloroform, acetone and DMSO. In complex, the Zn(II) ion is coordinated through pyridyl
nitrogen, hydrazinic nitrogen and thione sulphur of the ligand. Composition and identity of the
assembled system were deduced from elemental analysis and IR, NMR, UV-vis and TGA.
5.1. IR spectra
The IR spectrum of the ligand N-picolinoyl-N'-benzothioylhydrazide (Hpbth) shows absorptions
due to the stretching modes of NH (3236); C=O (1702); C=S (921) and N-N 1042 cm-1. The IR
spectrum of the [Zn(pbth)2] shows the absence of ν(NH) band together with the appearance of
two new bands for ν(Zn-N) and ν(Zn-S) at 508 and 421 cm-1 respectively which suggests
bonding of Zn(II) with hydrazine nitrogen [25]. The ν(C=S) band suffers a negative shift of 53
cm-1, indicating that pbth is acting as uninegative tridentate ligand, bonding through thione
sulfur. The bonding through thione sulphur and hydrazinic nitrogen in the Zn(II) complex is
further supported by X-ray crystallography.
5.2 1H and 13C NMR spectra
1
The H NMR spectrum of Hpbth exhibits two signals at δ 10.63 and 10.05 ppm for the amide
and thioamide protons, respectively. Aromatic protons appear as multiplet between δ 7.2-8.8
13
ppm. The C NMR spectrum of Hpbth shows two signals for two carbon atoms of which the
signals at δ 190.71 and 163.07 ppm are due to the >C=S and >C=O, carbons, respectively. The
aromatic carbons appear at δ 122.62-149.08 ppm. The 1H NMR spectrum of [Zn(pbth)2] exhibits
one signal at δ 10.00 ppm for the thioamide proton and the loss of amide proton indicates
bonding of amide nitrogen to the zinc ion. Aromatic protons appear as multiplet between δ 7.25-
8.24 ppm. The 13C NMR spectrum of [Zn(pbth)2] shows several signals for various carbon
8

atoms out of which the signals at δ 203.12 and 163.90 ppm are due to the >C=S and >C=O,
carbons, respectively. The signals appearing between δ 119.65-150.88 ppm are due to aromatic
carbons.
5.3. Solid state electrical conductivity
Zn(II) complex shows solid state electrical conductivity at room temperature. Fig.1 shows that
log σ of the complex decreases with increase in 1000/T, indicating semiconducting behaviour.
This may be due to the inclusion of zinc(II) ion in to the π electron delocalization of N-
picolinoyl-N'-benzothioylhydrazide on complexation by the production of charge carriers which
may take place via C-H···π interactions (Fig.4) [26, 27]. The band gap evaluated from the plot
was found to be 0.64 eV for the Zn(II) complex, respectively. Furthermore, lower conductivity of
the complex shows weaker inter-molecular contact and less extended delocalization and rules out
any possibility of partial oxidation of the central metal and also the unpaired electrons are not
participating in the electron conduction mechanism [28].
5.4. Antibacterial activity
Antibacterial activity of N-picolinoyl-N'-benzothioylhydrazide, Zn(OAc)2·2H2O and zinc(II)
complex and commercial antibacterial drugs streptomycin sulphate and neomycin sulphate were
tested against five human bacterial pathogens Salmonella typhi (MTCC 3216), Shigella flexneri
(ATCC 12022), Staphylococcus aureus (ATCC 25323), Aeromonas hydrophila (ATCC 7966)
and Enterococcus faecalis. The inhibition zones are given in the Table 1. The ligand and the
complex were found active against all pathogens at higher concentration. The highest zone of
inhibition (1.4 cm) was recorded in case of the complex against Shigella flexneri at 50 µg/disc.
The ligand shows little activity against S. flexneri and A. hydrophila at 30 µg/disc but is more
active at 50 µg/disc in other cases. The result of antibacterial activity is also expressed as Mean
9

standard deviation (SD) at the concentration range 30-50 μg/disc, [Table 2]. On comparing the
inhibitory activity it is found that the Zn(II) complex is more effective than that of free ligand
and Zn(OAc)2·2H2O. The order of activity is Zn(II) complex >Hpbth (ligand) >Zn(OAc)2·2H2O.
Activity of the ligand and the Zn(II) complex are lower as compared to streptomycin sulphate
and neomycin sulphate. It is clear that the zone of inhibition area is somewhat larger for the
complex than those of the ligand and zinc acetate salt. Such increased activity of the metal
chelate can be explained on the basis of chelation theory. On chelation, the polarity of the metal
ion is reduced to a great extent due to the overlap of the ligand orbital. Furthermore, it increases
the delocalization of π-electrons over the whole chelate ring and enhances the lipophilicity of the
complex [29, 30]. This increased lipophilicity leads to breakdown of the permeability barrier of
the cell and thus retards the normal cell processes [31] and show enhance antibacterial activity.
Thus, the zinc complex may serve as moderate therapeutic agent antibacterial activity.
5.5. Thermal analysis
The thermal stability of the Zn(II) complex was studied by heating the sample at a control rate of
15 ºC per minute under nitrogen atmosphere. The complex is stable up to 200 ºC, indicating the
absence of lattice water as well as coordinated water [32]. Generally in TGA, lattice water loss
occurs at low temperature in the range 60-120 ºC, whereas coordinated water requires a
temperature in the range 120-230 ºC. Absence of water molecule in Zn(II) complex is supported
by the DTA curve also, which represents weight loss by endothermic process [33].
Thermogravimetric analysis of zinc(II) complex shows that the complex starts decomposing
above 200 ºC and thermogram exhibits two distinct decompositions at 200 and 480 ºC. The first
weight loss (Cald; 68.16%, Obs; 67.55%) could be due to loss of two picolinamide molecule
along with two phenyl molecule and a corresponding endothermic peak at 223 ºC was obtained.
10

The second decomposition between 480 to 700 ºC shows the weight loss (Cald; 16.31%, Obs;
16.36 %) and endothermic peak observed at higher temperature in DTA is due to loss of organic
moieties in the form of gaseous product and finally metal is converted into its oxide [34]. In
addition to endothermic peaks, the DTA curve of the complex also shows exothermic peaks
which appear at high temperature and represent oxidation and/or decomposition of the complex.
The thermogravimetric curve for the complex is presented in Fig 2.
5.6. Crystal structure description
Fig. 3 shows the ORTEP diagram of the complex [Zn(pbth)2] together with the atom numbering
scheme. The crystallographic data and structural refinement details for 1 are given in Table 3 and
selected bond distances and bond angles of the complex are compiled in Table 4 and hydrogen
bonding parameters in Table 5. The complex [Zn(pbth)2] contains a pair of N-picolinoyl-N'-
benzothioylhydrazide ligand coordinated to Zn(II) ion via thione sulphur, pyridyl nitrogen and
hydrazinic nitrogen forming two five membered chelate rings. The C=O bond length [1.238(5)
Å] is purely a double bond which remains uncoordinated but takes part in the formation of
supramolecular architecture through hydrogen bonding [35, 36]. The C7-S1 bond distance
presence in the both units of the ligand is 1.678 (4) Å which suggest that it is intermediate
between single and double bond [37]. The complex may be described to have a distorted
octahedral geometry with the ligand-metal-ligand angle varying between 81.10(10) [N(2)-Zn(1)-
S(1)] to 77.65(14) [N(2)-Zn-N(1)] in the chelate rings. The average bond lengths of N2-N3 =
1.387, N3-C7 = 1.315, C7-S1 = 1.678 Å and N1-C5 = 1.344, C5-C6 = 1.491, C6-N2 = 1.322 Å,
suggest considerable delocalization of charge in the both five membered chelate rings [38]. The
dihedral angles formed between the pyridine and the phenyl ring is found to be 77.35° which
suggest that the two rings are nearly perpendicular to each other. The structure of the complex is
11

stabilized by N-H···O intermolecular hydrogen bonding occurring between the carbonyl oxygen
and hydrogen of hydrazinic nitrogen and C-H···O intermolecular hydrogen bonding between
carbonyl oxygen and phenyl hydrogen. In addition, the C-H···π interactions at a distances of
2.855 and 3.638 Ǻ act as a linker between two units of the ligand molecule in the formation of
supramolecular architecture (Fig. 4). Furthermore, the crystal packing shows that both sulphur
atoms of each molecule in every row are arranged in such a way that they face each other (Fig.5).
5.7. DFT calculations of N-picolinoyl-N'-benzothioylhydrazide (Hpbth) and its Zn(II) complex
The optimized geometrical parameters are listed in Table 4. The optimized energy of
Hpbth and Zn(II) complex are -1134.3745 and -1566.9097 a.u respectively indicating that the
complex is more stable than the ligand. The optimized geometry of complex accounts for
distorted octahedral geometry with Zn-N, Zn-S and Zn-Npy bond distances of 1.981, 2.454 and
2.127 Å, respectively. The C=O and C-S bond lengths 1.236 and 1.671 Å accounts for double
o
and single bond character and Nhy-Zn-Npy and Nhy-Zn-S chelate angles are 77.627 and 81.117 ,
respectively. The slight disagreement in the bond lengths is due to the fact that the DFT
calculations are done for an isolated molecule in gaseous phase while the X-ray crystallographic
data were obtained from crystal lattice of complex molecules [39, 40]. The optimized geometries
of Hpbth and the Zn(II) complex along with the atomic labelling scheme are shown in Fig 6a and
7a. As it can be seen from the experimental results that the most probable coordination site are
N, S and O atom which are also verified by theoretical calculations of optimized complexes.
There is good agreement between the geometrical parameters obtained by X-ray crystallography
to those generated by DFT method as shown in Table 4. The charge distribution on atoms of
Hpbth and its Zn(II) complex are also calculated and the charges on the ligand decreases, as a
result of redistribution on complexation [41]. The negative charges on binding atoms shift to
12

lower values: Npyridyl (-0.482 to -0.185), Nhydrazine (-0.375 to -0.318) and Sthione (-0.148 to 0.021)
which indcates that the charge redistribution takes place after complex formation.
5.8. Molecular electrostatic potential and Contour maps:
To predict reactive sites [42] for electrophilic and nucleophilic process for the ligand (Hpbth),
electrostatic surface potentials were obtained at the B3LYP/6-311G(d,p) optimized geometry.
The MEP mapped surface of the molecule are calculated by DFT/B3LYP/6-
311G(d,p){C,H,N,O,S}/Lanl2DZ method at the 0.02 isovalues and 0.004 density values. The red
colour scheme of MEP is the negative electrostatic potentials. The red region show atoms with
lone pairs; the intensity of the colour is proportional to the absolute value of the potential energy.
The positive electrostatic potentials are shown in blue colour region which characterize the C-H
bonds. The yellow/green areas cover parts of the molecule where electrostatic potentials are
close to zero (C-C, C-N and C-S bonds). The MEP mapped surface for Hpbth and the Zn(II)
complex are shown in Figs 6b and 7b. The negative red regions of MEP are related to
electrophilic reactivity and the positive (blue) ones to nucleophilic reactivity. As can be seen
from the figures, this molecule has several possible sites for electrophilic attack [43]. Negative
regions in the Hpbth were found around the thione S, carbonyl oxygen, hydrazinic and pyridyl
nitrogens. Thus, it would be predicted that an electrophile would preferentially attack to the
ligand Hpbth at the O, S and N positions. According to the calculated results, the MEP map
shows that the negative potential sites are on electronegative oxygen, sulphur and nitrogen atoms
as well as the positive potential sites are around the hydrogen atoms. These sites give
information about the region from where the compound can have noncovalent interactions. A
contour plot or map is a two-dimensional plot of a three-dimensional surface showing lines
where the surface intersects planes of constant elevation (Z) and is used to show lines of constant
13

density or brightness, such as electrostatic potentials. The contour maps are also calculated by
DFT/B3LYP/6-311G(d,p) {C,H,N,O,S}/Lanl2DZ method at the 0.02 isovalue and 0.004 density
values at same level of calculations of the MEP mapped surface of the molecules. The contour
maps of the molecules are shown in Fig 6c and 7c. The red lines are more around thione sulphur,
hydrazinic and pyridyl nitrogens than on carbonyl side. Hence, the positive zinc(II) ions
preferably bind through the thione sulphur, hydrazine and pyridyl nitrogen. The red lines
decreases after complexation.
5.9. Frontier molecular orbitals and NLO analysis
The Fig. 6d and 7d show distribution and energy levels of the HOMO and LUMO orbitals
computed at the B3LYP/6-311G(d) level for the ligand and Zn(II) complex. As seen from
(Fig.1d), the HOMO of the ligand, electrons are localized on the thione sulphur with EHOMO = -
8.0344 eV. For the LUMO, electrons are mainly localized on the pyridyl nitrogen with ELUMO = -
2.9989 eV. The value of the energy separation between the HOMO and LUMO is 5.0355 eV.
Whereas for HOMO of Zn(II) complex, electrons are mainly delocalized on the carbonyl
oxygen, hydrazinic nitrogen and thione sulphur with EHOMO = -5.4657 eV and for LUMO,
electrons are delocalized on the carbonyl oxygen, hydrazinic nitrogen, thione sulphur and pyridyl
ring nitrogen with ELUMO = -1.8833 eV and the energy gap is 3.5824 eV. This small HOMO–
LUMO energy gap means low excitation energy for the complex, a good stability and a low
chemical hardness for the complex than the ligand [44]. The molecules having a large energy gap
are known as hard, and molecules having a small energy gap are known as soft molecules. The
soft molecules are more polarizable than the hard ones, because they need small energy for
excitation. The hardness value of a molecule can be determined by the formula [45]
η = {-EHOMO + ELUMO}/2
14

where EHOMO and ELUMO are the energies of the HOMO and LUMO molecular orbitals. The
value of η (Table 6) of Hpbth and the Zn(II) complex, are 2.51775 and 1.79120 eV, respectively
indicates that the Zn(II) complex is a soft material than the ligand Hpbth.
The non-linear optical (NLO) properties of materials associated with the delocalized π-
electrons of a molecule play an important role in the design of materials used in communication
technology and optical devices [46]. An increase in delocalization of electron on the molecule is
resulted in the change in the NLO properties which in turn are related to the energy gap between
HOMO and LUMO. An addition of substituent to the conjugated systems or intermolecular
hydrogen bonding, could affect the nonlinear optical properties by changing the energy gap
between HOMO and LUMO where the small HOMO-LUMO gap requires small excitation
energy and so the absorption bands of a molecule are shifted towards the visible region [47]. Due
to the presence of various types of hydrogen bonding and CH···π interaction in Zn(II) complex,
electron delocalization becomes easier, which decreases the value of the energy gap, so the
absorption bands in the electronic spectrum of complex is shifted towards the visible region and
consequently, increases the nonlinear optical properties as compared to Hpbth. This suggests that
the metal modify the charge distribution of the π-electronic structure versus that of the free
ligand Hpbth. Further the calculated hardness value of complex indicates for soft material and
first hyperpolarizability (βtot) value for the complex (10.827x10-30 esu), indicates for the complex
to be a potential applicant in the development of NLO materials [48].
6. Conclusion
The ligand N-picolinoyl-N'-benzothioylhydrazide (Hpbth) and its complex [Zn(pbth)2] have been
synthesized and characterized by various physicochemical methods. Crystal structure of the
complex is stabilized through weak intermolecular N-H…O and C-H…O interactions between
15

hydrogens of hydrazine nitrogen and phenyl ring with carbonyl oxygen. Also the crystal
structure of the complex is stabilized by CH···π interactions occurring between phenyl hydrogens
with π electrons of adjacent phenyl ring. The presence of C-H···π interaction at a short distance
of 2.855 Ǻ may be responsible for semiconducting behaviour of the Zn(II) complex. The Hpbth
and Zn(II) complex are found to be active against all the test pathogens at higher concentration.
TGA shows that the complex is stable up to 200 ºC, indicating the absence of lattice water as
well as coordinated water and finally zinc oxide is left as the residue. The optimized energy of
Hpbth and Zn(II) complex is -1134.3745 and -1566.9097 a.u, respectively, indicating that the
complex is more stable than the ligand. MEP map and contour plot shows the negative potential
sites are on oxygen, sulphur and nitrogen atoms and the positive potential sites are around the
hydrogen atoms. Hence, the positive zinc(II) ions preferably bind through the thione sulphur,
hydrazine and pyridyl nitrogen sides thereby forming stable five membered chelate ring. The
nonlinear optical property is addressed theoretically and the complex may be a potential
applicant in the development of NLO materials.
7. Acknowledgements
Dr. S.K. Kushawaha is thankful to the Department of Science and Technology, New Delhi, India
for the award of a Young Scientist Project (No. SR/FT/CS-110/2011). R.K. Dani thanks to UGC,
New Delhi for the award of RGNF (SRF).
8. Supplementary material
CCDC 948488 contains the supplementary crystallographic data for [Zn(pbth)2] These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 IEZ, UK; fax: (+44)1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk.:
16

References
[1.] A. Bacchi, A. Bonini, M. Carcelli, F. Ferraro, E. Leporati, C. Pelizzi, G. Pelizzi, J.Chem.
Soc., Dalton Trans. (1996) 2699-2704.
[2.] M. Carcelli, D. Delledonne, A. Fochi, G. Pelizzi, M.C. Rodriguez-Arguelles, U. Russo, J.
Organomet. Chem. 544 (1997) 29-35.
[3.] B. Moubaraki, K.S. Murray, J.D. Ranford, X. Wang, Y. Xu, Chem. Commun. (1998) 353-
354.
[4.] A. Shrivastava, N.K. Singh, S.B. Singh, S.M. Singh, N. Singh, A. Shanker,
Bioinorg.Chem. Appl. 1 (2003) 255-270.
[5.] N.K. Singh, S.B. Singh, Transition Met. Chem. 26 (2001) 487-495.
[6.] M. Patil, R. Hunoor, K. Gudasi, Eur. J. Med. Chem., 45(2010), 2981-2986.
[7.] Z.H. Chohan, S.H. Sumrra, M.H. Youssoufi, T.B. Hadda, Eur. J. Med. Chem., 45(2010),
2739-2747.
[8.] A .E. Liberta, D.X. West, BioMetals 5 (1992) 121-126.
[9.] D.X. West, A.E. Liberta, S.B. Padhye, R.C. Chikate, P.B. Sonawane, A.S. Kumbhar, R.G.
Yerande, Coord. Chem. Rev. 123 (1993) 49-53.
[10.] R.L. Dutta, Md.M. Hossain, J. Sci. Ind. Res. 44 (1985) 635-674.
[11.] B. Moubaraki, K. S. Murray, J. D. Ranford, J. J Vittal, X. Wang, Y. Xu, Chem. Soc.
Dalton Trans. (1999) 3573-3578.
[12.] H. Cheng, D. Chun-ying, F. Chen-Jie, L. Yong-jiang, M. Qing-jin, J. Chem. Soc. Dalton
Trans. (2000) 1207-1212.
[13.] N.K. Singh, A.K. Srivastava, P. Tripathi, A. Shrivastava, R.K. Sharma, Eur.J.Med.
Chem.43 (2008) 577-583.
[14.] A.W. Bauer, W.M. Kirby, J.C. Sherris, M. Turck Am J. Clin. Pathol, 45 (1966), 493-496.
17

[15.] S. K. Gond, Mishra A. V.K. Sharma, S.K. Verma, J. Kumar, R.N. Kharwar, A. Kumar,
Mycoscience 53 (2012), 113-121.
[16.] N.K. Singh, D.K. Singh, S. B. Singh Synth. React. Inorg.Met.-Org. Chem 32 (2002) 703-
720.
[17.] G.M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112.
[18.] I.J. Bruno, J.C. Cole, P.R. Edgington, M. Kessler, C.F. Macrae, P. McCabe, J. Pearson, R.
Taylor, Acta Crystallogr., Sect. B 58 (2002) 389.
[19.] L.J.J. Farrugia, Appl. Crystallogr. 30 (1997) 565.
[20.] Y. Zhang, Z.J. Guo, X.Z. You, J. Am. Chem. Soc. 123 (2001) 9378–9387.
[21.] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
V.G. Zakrzewski, J.A. Montgomery, R.E. Stratmann, J.C. Burant, S. Dapprich, J.M.
Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J.B. V. Tomasi, M. Cossi, R.
Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson,
P.Y. Ayala, Q. Cui, D.K. Morokuma, A.D. Malick, K. Rabuck, J.B. Raghavachari, J.
Foresman, J.Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G.L.A. Liu, P. Piskorz, I.
Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A.
Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L.
Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian 03, Revision
A.1, Gaussian, Inc., Pittsburgh, 2003.
[22.] A.D. Becke, J. Chem. Phys. 98 (1993) 5648-5652.
[23.] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B. 37 (1988) 785-789.
[24.] M.A. McAllister, Theochem 427 (1998) 39-53.
[25.] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th
ed., Wiley Interscience, New York, 1986.
18

[26.] M. G. A. E. Wahed, A. M. Hassan and S. A. Hassan, J. Mater. Sci. Lett., 12 (1993) 453-
454.
[27.] R.K. Dani, M.K. Bharty, S.K. Kushawaha, Om Prakash, R.K. Singh, N.K. Singh.,
Polyhedron 65 (2013) 31-41.
[28.] N.K. Singh, S.K. Kushawaha, Trans. Met. Chem. 26 (2001) 140-146.
[29.] R. S. Srivastava, Inorg. Chim. Acta. 56 (1981), 65-67.
[30.] N. Gupta, R. Swaroop, R. V. Singh, Main Group Met. Chem.14 (1997), 387-390.
[31.] W.E Levinson, E. Jawetz, Medical Microbiology and Immunology, 4th edit., Stamford,
1996.
[32.] A.H. Kianfar, L. Keramat, M. Dostani, M. Shamsipur, M. Roushani, F. Nikpour,
Spectrochem. Acta A 77 (2010) 424-429.
[33.] A. Anthonysamy, S. Balasubramanian, Inorg. Chem. Commun. 8 (2005) 908-911
[34.] A.V. Nikolaev, V.A. Logvienko, L.T. Myachina, Thermal analysis Vol. 2, (Academic
Press, New York 1969) 779.
[35.] T.S. Lobana, G. Bawa, A. Castineiras, R.J. Butcher, Inorg. Chem. Comm., 10 (2007) 506-
509.
[36.] T.S. Lobana, P. Kumari, R.J. Butcher, T. Akitsu, Y. Aritake, J. Perles, F.J. Fernandez,
M.C. Vega, J. Organomet. Chem., 701 (2012) 17-26.
[37.] L.E. Sutton, Tables of Interatomic Distances and Configuration in Molecules and Ions,
Special Publication No. 18. The Chemical Society, London, 1965.
[38.] J. R. Dilworth, J. Hyde, P. Lyford, P. Vella, K. Venkatasubramaman, J.A. Zubieta, Inorg.
Chem., 18 (1979) 268.
[39.] A.D. Becke, J. Chem. Phys. 98 (1993) 5648-5652.
19

[40.] R. Gupta, R.P. Chaudhary, J. Mol. Struct. 1049 (2013) 189-197.
[41.] Om Prakash, S.K. Singh, B. Singh, R.K. Singh, Spectrochimica Acta Part A 112 (2013)
410-416.
[42.] J.S. Murray, K. Sen, Molecular Electrostatic Potentials, Concepts and Applications,
Elsevier, Amsterdam, 1996.
[43.] M. Govindarajan, M. Karabacak, A. Suvitha, S. Periandy, Spectrochimica Acta Part A 89
(2012) 137-148.
[44.] M. Arivazhagan, R. Meenakshi, Spectrochim. Acta 91A (2012) 419-430.
[45.] K. Fukui, Science 218 (1982) 747–754.
[46.] D. Sajan, J. Hubert, V.S. Jayakumar, J. Zaleski, J. Mol. Struct. 785 (2006) 43-53.
[47.] R.G. Pearson, Proc. Natl. Acad. Sci. 83 (1986) 8440-8441.
[48.] H. Tanak, M. Toy, Spectrochimica Acta Part A 115 (2013) 145-153.
20

Table 1 Antibacterial assay of the Hpbth and [Zn(pbth)2] against human bacterial pathogens
Test pathogen
Concentration of
compound
Compound
Strandard drug
Control
Zn(OAc)2·2H2O
(µg/disc)
Hpbth
[Zn(pbth)2]
Streptomycin Neomycin DMSO
sulphate
sulphate
1.8
(10µl/
disc)
Salmonella
typhi
(MTCC 3216)
5
-
-
-
-
-
2.4
-
-
-
-
10
20
30
40
50
5
-
2.5
2.7
2.8
2.9
3.0
0.8
1.0
1.6
2.0
2.3
2.5
1.8
2.0
2.3
2.4
2.5
2.8
2.5
2.6
2.7
2.8
1.9
-
0.6
0.7
0.8
1.0
-
0.1
0.2
0.4
0.6
-
2.0
0.5
0.6
0.7
-
2.1
2.2
2.4
Shigella
flexneri
(ATCC 2022)
1.5
10
20
30
40
50
5
-
-
-
1.8
-
0.8
0.9
1.0
1.4
-
0.3
0.4
0.6
0.7
-
2.0
0.3
0.5
0.6
-
2.1
2.2
2.5
Staphylococcus
aureus
(ATCC 5323)
1.75
1.8
10
20
30
40
50
5
-
-
-
-
-
0.2
0.3
0.4
0.6
-
1.9
-
0.7
0.8
0.9
-
2.1
0.5
0.6
-
2.2
2.3
Aeromonas
hydrophila
(ATCC 7966)
1.75
2.0
10
20
30
-
-
-
-
-
-
2.15
2.3
0.4
0.6
0.3
21

40
50
5
0.5
-
0.7
0.9
-
0.4
0.6
-
2.9
3.0
1.9
2.2
2.5
2.6
3.0
3.2
2.4
2.6
1.8
1.9
2.0
2.1
2.3
2.5
Enterococcus
faecalis
-
-
10
20
30
40
50
-
-
-
-
-
0.1
0.3
0.4
0.7
0.5
0.7
0.8
0.6
0.8
1.1
- No zone was observed, *inhibition zone is the average of the diameter of zone from two sides in cm.
22

Table 2 Inhibition zone of antibacterial activity for compounds
Conc. (μg/disc)
Zone of inhibition (mean SD)
Ligand
Zn(OAc)2·2H2O
0.3 0.07
[Zn(pbth)2]
0.7 0.12
0.8 0.11
1.0 0.22
30
40
50
0.4 ± 0.22
0.5 ± 0.11
0.7 ± 0.36
0.4 0.10
0.6 0.07
23

Table 3 Crystallographic data for [Zn(pbth)2]
Empirical formula
Formula weight
T (K)
C26H20N6O2S2Zn
578.01
293(2)
0.71073
Λ (Mo Kα) (Å)
Crystal system
Space group
a (Å)
Monoclinic
C 2/c
24.178(4)
8.9617(6)
15.381(3)
129.49(3)
2572.0(13)
4
b (Å)
c (Å)
β (º)
V (Å3)
Z
Dcalc(mg m-3)
μ (mm-1)
F(000)
1.493
1.154
1184
Crystal size(mm3)
θ range (º)
Index ranges
0.27x0.24x0.22
3.42-29.08
-32≤ h ≤32
-12≤ k ≤12
-19≤ l ≤21
3563
Reflections collected
Independent reflections
Data/restraints/ parameters
Goodness-of-fit on F2
1898
2503/0/168
0.854
Final R indices wR2 [I>2σ(I)](Rint)
0.0572, 0.1120
Final R indices (all data)
Largest diff. peak/hole (e Å-3)
0.0892, 0.1340
0.659, -0.951
aR1 = Σ||Fo| – |Fc||Σ|Fo|, bR2 = [Σw (|F2 | – |F2 |)2/Σw|F2 |2]1/2
.
o
c
o
24

Table 4 Selected bond lengths (Å) and bond angles (o) for [Zn(pbth)2]
Bond lengths (Å)
Exp.
Bond angles (°)
Cal.
Exp.
Cal.
Zn-N2
Zn-N2
Zn-N1
Zn-N1
Zn-S1
Zn-S1
S1-C7
O1-C6
N2-C6
1.982(4)
1.982(4)
2.128(4)
2.128(4)
2.454(13)
2.454(13)
1.678(4)
1.238(5)
1.322(5)
1.981
1.981
2.127
2.127
2.454
2.454
1.671
1.236
1.322
N2-Zn-N2
N2-Zn-N1
N2-Zn-N1
N1-Zn-N1
N2-Zn-S1
N2-Zn-S1
N1-Zn-S1
N1-Zn-S1
S1-Zn-S1
178.5(2)
178.519
77.627
103.493
84.984
81.117
97.843
95.561
158.241
91.867
77.65(14)
103.48(15)
84.98(19)
81.10(10)
97.85(12)
95.56(10)
158.24(11)
91.87(7)
25

Table 5 Hydrogen bond parameters [Å and °] in [Zn(pbth)2]
D-H…A
d(D-H)
d(H…A)
d(D…A)
<(DHA)
Intermolecular hydrogen bond
N3-H3A…O1
C12-H12…O1
0.860
0.931
1.911
2.664
2.766
3.530
172.78
155.19
Intramolecular hydrogen bond
N3-H3A…O1
C4-H4…O1
0.860
0.931
2.490
2.572
2.746
2.836
98.04
96.72
26

Table 6 Calculated Frontier Molecular Orbital energies (eV) for Hpbth and complex [Zn(pbth)2]
Molecule
Energies (ev)
LUMO
Degree of Hardness
η
HOMO
-8.0344
-5.4657
Energy Gap
5.0355
Hpbth
-2.9989
-1.8833
2.51775
1.79120
[Zn(pbth)2]
3.5824
27

Fig. 1 Solid state electrical conductivity
28

Fig. 2 Thermogram of Zn(II) complex
29