Syntheses, structural and thermal studies on Zn(II) complexes of 5-aryl-1,3,4-oxadiazole-2-thione and dithiocarbamates: Antibacterial activity and DFT calculations

Article abstract of DOI:10.1016/j.poly.2015.05.045

The new complexes [Zn(pyot)₂(en)₂] (1), [Zn(mphot)₂(en)₂] (2), [Zn(bhpzdtc)₂(py)] (3), [Zn(bhpzdtc)₂(4-pic)] (4) and [Zn(ecpzdtc)(bpy)(Cl)] (5) {pyot = 5-(4-pyridyl)-1,3,4-oxadiazole-2-thio

Full text of DOI:10.1016/j.poly.2015.05.045

Polyhedron 98 (2015) 84–95
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Syntheses, structural and thermal studies on Zn(II) complexes
of 5-aryl-1,3,4-oxadiazole-2-thione and dithiocarbamates:
Antibacterial activity and DFT calculations
a,
⇑
a
a
a
a
b
b
M.K. Bhartyc , R.K. Dani , P. Nath , A. Bharti , N.K. Singh , Om Prakash , Ranjan K. Singh ,
R.J. Butcher
a
Department of Chemistry, Banaras Hindu University, Varanasi 221005, India
Department of Physics, Banaras Hindu University, Varanasi 221005, India
Department of Chemistry, Howard University, 525 College Street NW, Washington, DC 20059, USA
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The new complexes [Zn(pyot) (en) ] (1), [Zn(mphot) (en) ] (2), [Zn(bhpzdtc) (py)] (3),
2
2
2
2
2
Received 28 February 2015
Accepted 28 May 2015
Available online 6 June 2015
2
[Zn(bhpzdtc) (4-pic)] (4) and [Zn(ecpzdtc)(bpy)(Cl)] (5) {pyot = 5-(4-pyridyl)-1,3,4-oxadiazole-2-thione,
mphot = 5-(4-methoxy phenyl)-1,3,4-oxadiazole-2-thione, bhpzdtc = 1-benzhydrylpiperazine dithiocar-
bamate, ecpzdtc = 1-ethoxycarbonyl piperazine dithiocarbamate} have been synthesized and character-
ized by various physicochemical techniques. Complexes 1 and 2 have been prepared from the
potassium salt of N-aroylhydrazine carbodithioate (RCONHNHCSSK) which was cyclized into the
corresponding 5-substituted-1,3,4-oxadiazole-2-thione during complexation in the presence of ethylene-
diamine (en), in which Zn(II) is bonded to the oxadiazole nitrogen atom. The potassium salts of
Keywords:
Oxadiazole and dithiocarbamato complexes
Mixed ligand zinc(II) complexes
Thermogravimetry
DFT calculations
Antibacterial activity
substituted piperazine dithiocarbamates formed complexes 3–5, which contain pyridine/4-picol-
0
ine/2,2 -bipyridyl as the co-ligand, and the zinc(II) center forms a four-membered CS
2
Zn ring with the
dithiocarbamate ligand. The crystal structures of the complexes are stabilized by various types of inter
and intramolecular hydrogen bonding, leading to supramolecular frameworks. Geometry optimization,
geometrical parameters, molecular electrostatic potential maps (MEP) and FMO analyses of the
complexes have been carried out by DFT methods, and these are presented and compared with the exper-
imental X-ray diffraction data. The thermal decompositions of the complexes have been studied using
thermogravimetric analysis (TGA) and the results show that complexes 1–2 and 3–5 form ZnO and
ZnS, respectively, as the final residue. The bioefficacy of the complexes has been examined using the
growth of bacteria to evaluate their anti-microbial potential. Complex 5 shows the highest antibacterial
activity as compared to the other complexes.
Ó 2015 Elsevier Ltd. All rights reserved.
1
. Introduction
The zinc(II) ion plays an important role in the structure and
bioinorganic chemistry [5–7]. New researches on the interaction
of amino acid and proteins with Zn(II) ions have been carried out
in the areas of zinc finger proteins, nucleic acid binding and gene
regulatory proteins [8–10]. The coordination chemistry of different
types of nitrogen–sulfur ligands is an important and developing
area of research during recent years since their complexes possess
semiconducting behavior, molecular magnetism, optoelectronic
and electrochemical properties [11–13]. Complexes of this ligand
system are involved in the catalysis of biological and non-
biological processes [14,15]. A variety of nitrogen sulfur ligands
are known, which include thiohydrazide, thiosemicarbazide,
function of many biomolecules. It is found as the active site of
enzymes, plays an important role in photosynthesis, acts as a sec-
ondary messenger, promotes and stabilizes native conformations
of nucleic acids and facilitates protein–DNA binding [1,2]. Zinc-
based compounds have also been used as potent antibacterial,
antifungal, anticancer drugs and as imaging agents [3,4]. Zinc(II)
stabilizes many zinc containing enzymes and zinc finger proteins,
due to which it has generated much interest in the field of
0
N-acyl-N -thioaroylhydrazides, simple as well as N and S substi-
tuted dithiocarbazates, dithiocarbamates and 1,3,4-oxadiazoles.
In the deprotonated complexes these ligands bind to the metal ions
⇑
Corresponding author. Tel.: +91 542 6702447.
E-mail address: mkbharty@bhu.ac.in (M.K. Bharty).
http://dx.doi.org/10.1016/j.poly.2015.05.045
277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
0

M.K. Bharty et al. / Polyhedron 98 (2015) 84–95
85
through either the sulfur or nitrogen atoms, or through both. 1,3,4-
Oxadiazole derivatives show diverse biological activities, such as
anti-tuberculostatic, anti-inflammatory, anticonvulsant, antibacte-
rial and antifungal activities [16,17]. Potassium salts of N-(aroyl)-
hydrazine carbodithioate are important precursors for the syntheses
of 1,3,4-oxadiazole and its derivatives [18,19]. Since 1,3,4-oxadia-
zole-2-thiones can exist in both the thione and thiol forms in solu-
tion [20,21], it is of interest to investigate the bonding modes in
their complexes. Dithiocarbamates have a wide range of applica-
tions in medicine, industry and rubber vulcanization, and can serve
as an antioxidant for increasing the longevity and photo-stability
of a variety of polymers, oils and other materials [22]. Several
dithiocarbamates and N-substituted dithiocarbamato complexes
and salts have been used as agrochemicals, mainly due to their
high efficiency in controlling plant fungal diseases and showing
relatively low toxicity [23,24]. Recent work has established that
Zn(II) complexes of dithiocarbamate are useful precursors for the
synthesis of ZnS nanoparticles and have application as non-linear
carried out by a dedicated computer using Gram Wire software.
The Origin 6.1 software was used for further analysis of band shape
and peak positions.
2.3. Computational details
All calculations were performed using the Gaussian 03 and
Gauss View 4.1 [29] programs. The structure optimization of the
ligand has been done using the DFT method with the functional
B3LYP and basis set 6–311G(d,p) [30], whereas the geometry opti-
mization for the complexes have been done using the DFT method
with the functional B3LYP and basis sets 6–311G(d,p) {C,H,N,O,S}/
Lanl2DZ {Zn(II)}. The basis set Lanl2DZ [31] serves the purpose of
including the pseudo potential of the core electrons in the metal
atoms. The input geometries of the complexes for the DFT calcula-
tions were generated from single crystal X-ray data. The first task
for the computational work was to determine the optimized geom-
etry of the ligand and the Zn(II) complexes. Analyses of frequency
calculations at the optimized geometry were done to confirm the
optimized structures to be an energy minimum. At the optimized
geometry no imaginary frequency modes were obtained, so a true
minimum on the potential energy surface was found. The HOMO–
LUMO energies were calculated using the B3LYP method of the DFT.
2
optical materials [25]. The treatment of Zn(dithiocarbamato) com-
plexes with nitrogen donor ligands generally results in the isola-
tion of adducts which are usually dimeric in the solid-state [26].
Some complexes of 1,3,4-oxadiazole-2-thione and dithiocarba-
mates have been reported, but no work seems to have been carried
out on the mixed ligand complexes of Zn(II) involving these ligands
[
27,28]. Therefore, it will be of great interest to study the theoret-
2.4. Synthesis of the potassium salt of N-aroylhydrazine carbodithioate
ical and experimental vibrational data based on the structures of
these molecules, to assign the fundamental bands in the experi-
mental FT-IR and FT-Raman spectra and to compare the mode of
bonding of the ligand and structures of the complexes. In view
of this, we have synthesized and characterized Zn(II) complexes
of 5-(4-pyridyl)-1,3,4-oxadiazole-2-thione, 5-(4-methoxy phe-
nyl)-1,3,4-oxadiazole-2-thione, potassium 1-benzhydrylpiperazine
The potassium salts of the N-aroylhydrazine carbodithioates
were prepared by the literature method [32].
2.5. Syntheses of complexes 1 and 2
A solution of the freshly prepared potassium salt of N-(4-pyri-
dyl/4-methoxyphenyl)hydrazine carbodithioate (1 mmol) in
methanol–acetonitrile (1:1, 20 mL) was added to a methanol solu-
+
ꢀ
+
ꢀ
[
(K (bhpzdtc) ] and 1-ethoxycarbonyl piperazine [(K (ecpzdtc) ]
0
dithiocarbamate using ethylenediamine/2,2 -bipyridyl/pyridine/4-
picoline as the co-ligand.
tion (10 mL) of Zn(OAc)
2
ꢃ2H
2
O (0.110 g, 0.5 mmol) and the mixture
was stirred for 1 h at room temperature. The resulting white pre-
cipitate was filtered off and washed thoroughly with methanol. A
methanol solution (15 mL) of ethylenediamine (0.2 mL, 2 mmol)
was added to the methanol suspension of the above compound
and refluxed for 2 h at 75 °C. The resulting clear solution was
cooled to room temperature, filtered off and kept for crystalliza-
tion. Single crystals of complexes 1 and 2 suitable for X-ray analy-
sis were obtained by slow evaporation of the above methanolic
solution over a period of 3–4 weeks.
2
. Experimental section
2.1. Chemicals and starting materials
Commercial reagents were used without further purification
and all experiments were carried out in the open atmosphere.
Isonicotinic acid hydrazide, methyl 4-methoxybenzoate, 1-ben-
zhydrylpiperazine and 1-ethoxycarbonylpiperazine (Sigma
Complex 1: Yield: 48%; M.p.: 266 °C. Anal. Found: C, 39.82; H,
0
Aldrich), ethylenediamine, pyridine, 4-picoline, 2,2 -bipyridyl,
4
(
m
.48; N, 26.03; S, 11.77. Anal. Calcd for
542.00): C, 39.85; H, 4.43; N, 25.98; S, 11.80%. IR ( cm , KBr):
(NH) 3232; (C–N) 1598; (N–N) 1070s; (C–S) 740; m(Zn–N) 544.
18 24 10 2 2
C H N S O Zn
(
CDH), CS
2
(SD Fine Chemicals) and KOH (Qualigens) were used
ꢀ1
m
as received.
m
m
m
Complex 2: Yield: 54%; M.p.: 276 °C. Anal. Found: C, 43.92; H,
5.04; N, 18.73; S, 10.71. Anal. Calcd for C22 Zn (600.07):
C, 43.96; H, 5.01; N, 18.67; S, 10.77%. IR (
3266; (C–N) 1604; (N-N) 1099s; (C–S) 769;
2
.2. Physical measurements
30 8 2 4
H N S O
ꢀ1
m
cm , KBr):
m(NH)
Carbon, hydrogen, nitrogen and sulfur contents were estimated
m
m
m
m
(Zn–N) 547.
on a CHN Model CE-440 Analyser and on an Elementar Vario EL III
Carlo Erba 1108. Thermogravimetric curves of the complexes were
recorded using a Perkin Elmer-STA 6000 thermal analyzer, TA
2.6. Syntheses of potassium 1-benzhydrylpiperazine-4-carbodithioate
+
ꢀ
[(K (bhpzdtc) ] and potassium 1-ethoxycarbonyl piperazine-4-
ꢀ1
+
ꢀ
Instruments. FT-IR spectra were recorded in the 4000–400 cm
carbodithioate [K (ecpzdtc) ]
region as KBr pellets on a Varian Excalibur 3100 FT-IR spectropho-
+
tometer. The laser Raman set-up consists of a 514.5 nm Ar laser as
The preparation and characterization of the potassium salts of
+
ꢀ
the excitation source delivering ꢁ5 mW intensity on the sample, a
microscope with 50ꢂ objectives from Olympus for proper focusing
of the laser beam on the desired portion of the sample and collect-
ing the Raman scattered signal. The Raman spectrometer was the
Renishaw Model: RM 1000 with a grating of 2400 grooves/mm,
1-benzhydrylpiperazine [(K (bhpzdtc) ] and 1-ethoxycarbonyl
+
ꢀ
piperazine [(K (ecpzdtc) ] are described elsewhere [33,34].
2.7. Syntheses of complexes 3 and 4
ꢀ1
+
ꢀ
giving a spectral resolution of ꢁ1 cm
Raman spectra were recorded in the range 200–3500 cm
Spectrometer scanning, data collection and processing were
at 50
l
m slit opening.
A solution of freshly prepared [(K (bhpzdtc) ] (1 mmol) in
methanol–acetonitrile (1:1, 20 mL) was added to a methanol solu-
ꢀ1
.
tion (10 mL) of Zn(OAc)
2
2
ꢃ2H O (0.5 mmol) and the mixture was

8
6
M.K. Bharty et al. / Polyhedron 98 (2015) 84–95
2
stirred for 3 h at room temperature. The resulting precipitate was
filtered off, washed thoroughly with a H O-MeOH mixture
20 mL, 50:50, v/v) and air dried. The resulting solid was dissolved
in a minimum amount of warm pyridine or 4-picoline. The solution
was filtered off and kept for crystallization. Colorless single crystals
suitable for X-ray analysis were obtained by slow evaporation of
the above solutions over a period of 25 days.
refined against all data by full matrix least-square on F using ani-
2
sotropic displacement parameters for all non-hydrogen atoms. All
hydrogen atoms were included in the refinement at geometrically
ideal positions and refined with a riding model [35]. The MERCURY
package and ORTEP-3 for Windows program were used for analysis
of the structures and drawing of the figures [36,37].
(
Complex 3: Yield: 64%. M.p: 266 °C. Anal. Calc. for C40
42 5 4
H N S Zn
5
. Results and discussion
(
5
9
786.39): C, 61.04; H, 5.34; N, 8.90; S, 16.28. Found: C, 61.18; H,
ꢀ
1
.38; N, 8.96; S, 16.34%. IR (
63; (pyridine) 755; (Zn–N) 590;
Complex 4: Yield: 68%. M.p.: 254 °C. Anal. Calc. for C42
813.44): C, 61.96; H, 5.53; N, 8.61; S, 15.74. Found: C, 62.04; H,
m
cm , KBr):
m(C–N) 1496; m(C@S)
The potassium salts of N-aroylhydrazine carbodithioates
m
m
m
(Zn–S) 498.
react with Zn(OAc)
dissolve in a methanolic solution of en yielding the octahedral
complexes [Zn(pyot) (en) (1) and [Zn(mphot) (en) (2).
Complexes [Zn(bhpzdtc) (py)] (3), [Zn(bhpzdtc) (4-pic)] (4) and
Zn(ecpzdtc)(bpy)(Cl)] (5) were obtained by reacting the potas-
2
2
ꢃ2H O to form white precipitates which
H
45
N
5
4
S Zn
(
5
1
2
2
]
2
2
]
ꢀ1
.67; N, 8.70; S, 15.82%. IR (
m cm , KBr): m(C–N) 1573; m
(C@S)
2
2
015; (Zn–N) 557; (Zn–S) 488.
m
m
[
sium salt of the respective piperazine dithiocarbamate with the
zinc(II) salt, which were further reacted with pyridine/4-picol-
ine/2,2-bipyridyl, forming five coordinate ternary complexes.
Schemes 1 and 2 depict the formation of the complexes, which
are stable toward air and moisture. Complexes 3–5 adopt a dis-
torted square pyramidal geometry at the metal center. All the com-
plexes are insoluble in common organic solvents, but are soluble in
DMF and DMSO and can be kept in desiccators over a prolonged
period without any sign of decomposition.
2
.8. Synthesis of complex 5
A solution of freshly prepared potassium 1-ethoxycarbonyl
piperazine-4-carbodithioate (1 mmol) in methanol–acetonitrile
1:1, 20 mL) was added to a methanol solution (10 mL) of
ZnCl O (0.5 mmol). This mixture was stirred for 3 h at room
(
2
ꢃ2H
2
temperature. The resulting precipitate was treated with a methano-
lic solution of 2,2 -bipyidyl. The clear solution obtained was filtered
off and kept for crystallization. Colorless single crystals suitable for
X-ray analysis were obtained by slow evaporation of the above
solution over a period of 30 days. Yield: 75%. M.p.: 305 °C. Anal.
0
5.1. IR spectra of the complexes
Calc. for C18
3.05. Found: C, 44.12; H, 4.37; N, 11.50; S, 13.09%. IR (
KBr): (C–N) 1548; (C@S) 978; (Zn–N) 534; (Zn–S) 416.
4 2 2
H21ClN S O Zn (490.37): C, 44.04; H, 4.28; N, 11.42; S,
The IR spectra of complexes 1 and 2 show bands in the region
266–3232 cm due to the NH stretching vibrations of en, which
ꢀ
1
1
m cm ,
ꢀ1
3
m
m
m
m
are shifted to lower frequencies than those encountered in free
ethylenediamine [38]. The appearance of a new band near
ꢀ
1
3
. Antibacterial assay
Five human bacterial pathogens, Salmonella typhi (MTCC 3216),
545 cm due to
en in complexes 1 and 2. A slight negative shift in
plexes 1 and 2 may be due to the involvement of the thione sulfur
atom in hydrogen bonding. Disappearance of (C@O) in complexes
m
(Zn–N) suggests the formation of a chelate with
m(C@S) in com-
Shigella flexneri (ATCC 12022), Staphylococcus aureus (ATCC 25323),
Aeromonas hydrophila (ATCC 7966) and Enterococcus faecalis (ATCC
m
1 and 2 indicates conversion of the C@O bond to C–O during
cyclization, which is further confirmed by the X-ray diffraction
data. In complexes 1 and 2, peaks appearing at 1592–1520 due
2
5923), were used to test the antibacterial activities of complexes
1–5. The minimum inhibitory concentration (MIC) of the com-
ꢀ
1
plexes was determined using the broth dilution method with an
MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bro-
mide) assay. The test compounds were dissolved in DMSO to a final
concentration of 2 mg/ml. The test compounds were diluted to var-
to m(C@N) (endocyclic), 1286–1250 (C–O–C) and 889–844 cm
(C@S) (exocyclic) suggest cyclization of the dithiolate moiety to
1,3,4-oxadiazole-2-thione via desulfurization [39].
The IR spectra of complexes 3–5 show a v(C@N) (thioureide)
ꢀ1
ious concentrations (2, 4, 8, 16, 32, 64, 100
l
g/mL) in a 5 mL cul-
band at higher frequencies viz. 1496, 1498 and 1441 cm , respec-
ture tube containing 2 mL Mueller–Hinton broth. One drop of an
exponentially growing culture of the test bacterium was inocu-
lated in each concentration of the compounds and incubated at
tively, indicating an increase in the partial-double-bond character
of the
parent ligand. The
2 2 2
v(–CH ) N–CS ) bond in the complexes as compared to the
v
(C@N) band has been used as a measure of
3
7 ± 2 °C for 24 h. A control was prepared using the test bacterium
and an equal volume of DMSO (100 L) in Mueller–Hinton broth
without any test compound. After the incubation period, 200
MTT solution (5 mg/ml) prepared in phosphate saline buffer
PBS) was added to each vial and incubated at 37 ± 2 °C for one
the contribution of the thioureide form of the ligand in the com-
plexes. The IR spectra of complexes 3 and 4 are nearly superimpos-
able, suggesting that the metal complexes behave in the same way.
l
lL
ꢀ
1
The v(C@S) band, observed around 1000 cm in the ligand, suffers
ꢀ1
(
a negative shift of 15–26 cm in the respective complexes, indi-
cating bonding through both dithio sulfur atoms with the zinc(II)
hour. The culture was centrifuged at 10,000 rpm for 10 min and
the absorbance was observed at 570 nm. The minimum concentra-
tion of the compounds which was able to inhibit 50% activity of
bacteria is considered as the IC50 value.
ion. The appearance of a new band in complex 5 for
v
(Zn–N) at
ꢀ
1
0
455 cm suggests bonding of the Zn(II) ion with the 2,2 -bipyridyl
nitrogen atom. The characteristic bands due to the pyridine and 4-
ꢀ1
methyl pyridine ring appear at 771 and 763 cm in complexes 3
and 4, respectively.
4
. Crystal structure determination
Crystals suitable for X-ray analyses of the complexes were
5.2. Raman Spectra of complexes
grown at room temperature. The crystal data were collected on
an Oxford Gemini diffractometer equipped with CrysAlis CCD soft-
The Raman spectra of complexes 1 and 2 were recorded to
obtain information on the binding sites of the ligand and coligand
with the Zn(II) ion (Supple. Figs. 1 and 2). Complex 1 shows a band
corresponding to the stretching of Nen–Zn, which is observed at
ware using a graphite mono-chromated Mo K
a (k = 0.71073 Å)
radiation source at 293 K. Multiscan absorption corrections were
applied to the X-ray data collections for all the compounds. The
structures were solved by direct methods (SHELXL-2013) and
ꢀ1
250 cm . Two bands are observed for complex 2 at 208 and

M.K. Bharty et al. / Polyhedron 98 (2015) 84–95
87
Ar
S
HN
HN
NH
NH
N
O
O
H
S-K+ Zn(OAc)
.2H
O
Zn
O
CS2
KOH
N
2
2
O
N
N
NH2
Ar
N
Ar
N
H
H
MeOH-CH
en
CN
N
3
S
S
Ar
Ar = 4-pyridyl, 4-methoxy phenyl
1
2
Scheme 1. Syntheses of the oxadiazole ligands and their Zn(II) complexes.
Y
CS2
S
N
S
S
Zn(OAc) .2H O
S
S
2
2
X
NH
X
N
N
X
KOH
X
N
Zn
SK
X= (Ph) CHN, Y = H
3
4
2
Y= CH3
Cl
S
O
O
O
S
O
O
N
CS2
KOH
Zn(Cl) .2H O
2
2
N
N
Zn
N
NH
N
N
SK
bpy
O
S
5
N
Scheme 2. Syntheses of the dithiocarbamates ligands and their Zn(II) complexes.
ꢀ
1
2
72 cm , which are attributed to the bending of Noxa–Zn, whereas
m(C–N) + m(C–S), and bending of (S–Zn–Cl) and (N–Zn–N) in the
ꢀ
1
this band is observed at 270 cm in complex 1. Complexes 1 and 2
Raman spectrum of 5. In the Raman spectra of complexes 3 and
ꢀ1
ꢀ1
show a band at 370 and 371 cm respectively, assigned to the
bending of Noxa–Zn, with an additional contribution of a deforma-
tional motion of the corresponding oxadiazole ligands perpendicu-
lar to the plane of the oxadiazole ligands. The bending of Nen–Zn is
4, the C–H stretching mode (3100–2700 cm ), mainly due to the
presence of the methyl group, shows a considerable difference at
the para position of the pyridine ring in 4-picoline because of the
change in the charge cloud distribution on complexation. The
ꢀ1
ꢀ1
observed at 429 and 400 cm in the Raman spectra of complexes 1
sharp bands at 2913 and 2806 cm
corresponding to the CH
ꢀ1
ꢀ1
and 2, respectively whereas the bands at 434 and 455 cm appear
due to additional contribution of bending of Noxa–Zn with the
bending of Nen–Zn. Similarly, in the Raman spectrum of complex
, a band appears at 505 cm due to
at 534 cm due to b(Zn–N) + m(C–S). Bands at 1117 and 1123 cm
stretching mode in 4 appear at 2886 and 2827 cm in complex
3, showing a red and blue shift respectively. These differences in
the Raman bands of complexes 3 and 4 give the evidence that they
have different molecular structures, which is confirmed by their
crystal structures.
ꢀ
1
1
m(Zn–N) +
m
(C–O) and another
ꢀ1
ꢀ1
in the Raman spectra of complexes 1 and 2, respectively are attrib-
uted to b(N–N–C) and (C–O) of the oxadiazole ring, and the bands
at 1349 and 1362 cm are assigned as the vibrational modes of
m
5.3. Antibacterial activity
ꢀ
1
the oxadiazole ring, viz.
and trigonal b(N–N–Zn).
m
(C@N) +
m
(C–S) in addition to b(S–C–O)
In vitro antibacterial activities were measured using the mini-
mum inhibitory concentration method. The minimum inhibitory
concentrations (IC50) are given in Table 1. Complex 5 was found
to be the most effective against all pathogens with low IC values
in comparison to those of the other complexes. The higher
antibacterial activity of 5 than the other complexes, suggests that
The Raman spectra of complexes 3–5 (Supple. Figs. 3–5) were
recorded to obtain the binding sites of the Zn(II) ion to the dithio-
carbamate sulfur and nitrogen bases py/4-pic/bpy, present as a
secondary ligand. Again we have focused on those bands which
are expected to undergo a change upon binding with Zn and the
corresponding vibrational modes involving the metal ion. In the
0
2,2 -bipyridine and Cl assist in the action of the complex against
all bacteria, possibly via enhanced membrane transport into the
Raman spectra of complexes 3 and 4, the band due to
observed near 276 cm , whereas it is observed at 265 cm in
m
(Zn–S) is
ꢀ1
ꢀ1
Table 1
ꢀ1
the case of complex 5. The bands at 518, 520 and 541 cm in com-
plexes 3–5, respectively are mainly due to (Zn–N). The band due
to (Zn–N) is also observed with an additional contribution of the
bending of py/4-pic at ꢁ 645 cm in 3 and 4 and bending of bpy at
IC50 values of complexes 1–5 against different human bacterial pathogens.
m
Test bacterium
IC50
(l
g/mL)
m
1
2
3
4
5
ꢀ
1
ꢀ1
ꢀ1
Salmonella typhi (MTCC 3216)
Shigella flexneri (ATCC 12022)
Staphylococcus aureus (ATCC
49.95 47.68
32.79 35.08
54.98 45.26
51.25
48.70
67.98
44.53
21.72
43.22
29.26
16.39
27.83
7
54 cm in complex 5. The bands at ꢁ 1000, 1011 and 1024 cm
are attributed to the mixed contribution of trigonal bending of the
four membered (CS Zn) chelate ring and breathing modes of ben-
zhydryl and pyridine in both 3 and 4, but these bands are shifted
2
25323)
Aeromonas hydrophila (ATCC
7966)
107.47 98.35 111.41 106.76
65.10
ꢀ1
to higher wavenumbers at 1119, 1126 and 1152 cm in 5. The
ꢀ1
Enterococcus faecalis (ATCC
64.27 53.85 37.60 49.802 33.23
bands appearing in the region 1315–1440 cm
correspond to
25923)
the vibrational modes of the chelate ring, which is mainly due to

8
8
M.K. Bharty et al. / Polyhedron 98 (2015) 84–95
Fig. 1. Thermograms of complexes 1–5.
2 2
Fig. 2. ORTEP diagram of [Zn(pyot) (en) ] (1).
2 2
Fig. 3. Molecular structure of complex [Zn(mphot) (en) ] (2).
cell. 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 orbitals. Furthermore, it increases the delocalization of
the preparation of the test solutions, does not affect the results.
Thus, complex 5 may serve as a moderate antibacterial agent.
5.4. Thermal analysis
p-electrons over the whole chelate ring and enhances the
lipophilicity of the complex [40,41]. This increased lipophilicity
leads to a breakdown of the permeability barrier of the cell and
thus retards the normal cell processes [42] and shows enhanced
antibacterial activity. Blank tests have shown that DMSO, used in
The thermal stability of the complexes was studied by heating
the samples at a control rate of 15 °C per minute under a nitrogen
atmosphere. The complexes are stable up to 200 °C, indicating the
absence of lattice water as well as coordinated water [43].

M.K. Bharty et al. / Polyhedron 98 (2015) 84–95
89
2
Fig. 4. ORTEP diagram of [Zn(bhpzdtc) (py)] (3).
2
Fig. 5. Space fill model of [Zn(bhpzdtc) (py)] (3).
2
Fig. 6. ORTEP diagram of [Zn(bhpzdtc) (4-pic)] (4).
Generally in TGA, lattice water is lost at a low temperature, 60–
20 °C, whereas coordinated water requires a temperature in the
than complex 1, however the mode of bonding of the ligands are
the same in both complexes.
1
range 120–230 °C. Thermograms of complexes 1 and 2 (Fig. 1)
show similar behavior and start decomposing above 250 °C with
two distinct decompositions in the regions 250–350 and 350–
Thermograms of complexes 3 and 4 (Fig. 1) show similar behavior
and reveal that the complexes decompose in three steps in the
temperature range 200–900 °C. Initially the coordinated pyri-
dine/4-picoline molecules are lost just after 200 °C in the first step
[39]. The ligand loses 2 mol of benzhydryl/piperazine moieties
(calc.: 63.85% and exp 64.12% for 3 and calc.: 61.85% and exp
62.41% for 4) in the second step in the range 250–360 °C. The
remaining organic moiety is lost by the breakdown of C–N and
C–C bonds with an accompanying endothermic effect and the
6
50 °C. The first weight loss (major) could be due to loss of both
oxadiazole ligand molecules. The second decomposition in the
region 350–650 °C shows a weight loss (minor) due to loss of both
en molecules. The presence of more intermolecular N–Hꢃ ꢃ ꢃN, C–
Hꢃ ꢃ ꢃO, N–Hꢃ ꢃ ꢃO and C–Hꢃ ꢃ ꢃS hydrogen bonding interactions makes
complex 2 more stable and it possesses a higher thermal stability

9
0
M.K. Bharty et al. / Polyhedron 98 (2015) 84–95
Fig. 7. ORTEP diagram of [Zn(ecpzdtc)(bpy)(Cl)] (5).
Table 2
Crystallographic data for complexes 1–5.
Parameters
1
2
3
4
5
Formula weight
T (K)
542.00
293(2)
600.07
293(2)
0.71073
Monoclinic
P 21/n
9.6819(10)
12.1901(10)
11.5671(12)
90.00
107.742(11)
90.00
1300.3(2)
2
1.533
786.39
173(2)
0.71073
Monoclinic
P 21/c
10.3560(3)
18.0608(4)
24.1583(7)
90.00
94.080(2)
90.00
4507.1(2)
4
1.159
813.44
293(2)
0.71073
Triclinic
P1ꢀ
9.5527(5)
15.1467(9)
17.3502(9)
110.331(5)
100.183(4)
105.307(5)
2168.2(2)
2
490.37
173(2)
0.71073
Triclinic
P1ꢀ
8.1043(7)
8.7475(7)
15.5323(11)
83.855(7)
82.404(7)
73.899(7)
1045.78(15)
2
K
(Mo K
a
) (Å)
0.71073
Monoclinic
P 21/n
10.913(4)
10.153(4)
11.964(4)
90.00
114.09(4)
90.00
1210.2(9)
2
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
3
V (Å )
Z
D
l
calc (Mg mꢀ3)
1.487
1.224
1.246
0.793
1.557
1.523
ꢀ
1
(mm
)
1.151
0.761
F(000)
560
624
1644
852
504
Crystal size (mm)
h (°)
Index ranges
0.23 ꢂ 0.21 ꢂ 0.19
3.28–29.10
ꢀ14 6 h 6 14
0.25 ꢂ 0.22 ꢂ 0.19
3.28–29.08
ꢀ7 6 h 6 13
ꢀ14 6 k 6 16
ꢀ15 6 l 6 15
3485
0.41 ꢂ 0.27 ꢂ 0.15
3.14–29.21
ꢀ11 6 h 6 12
ꢀ24 6 k 6 12
ꢀ32 6 l 6 21
10324
0.35 ꢂ 0.25 ꢂ 0.13
3.065–29.17
ꢀ7 6 h 6 12
ꢀ20 6 k 6 19
ꢀ23 6 l 6 22
9924
0.31 ꢂ 0.27 ꢂ 0.23
3.07–32.50
ꢀ11 6 h 6 11
ꢀ13 6 k 6 11
ꢀ21 6 l 6 22
11997
ꢀ
ꢀ
7 6 k 6 13
12 6 l 6 16
Reflections collected
3237
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F
2148
2757/0/151
1.209
0.0466, 0.1243
0.0616, 0.1505
0.404, ꢀ0.645
2270
2966/0/169
1.979
0.0413, 0.0902
0.0814, 0.1107
0.317, ꢀ0.441
6364
10324/148/451
1.034
0.0629, 0.1632
0.1057, 0.1912
0.681, ꢀ0.362
6079
9924/0/470
1.031
0.0598, 0.1461
0.1061, 0.1730
0.712, ꢀ0.431
6783
6783/166/288
1.032
0.0740, 0.1548
0.1582, 0.1949
0.580, ꢀ0.502
2
a,b
R
R
1
, wR
, wR
2
[(I > 2
r(I)]
a,b
1
2
(all data)
Largest difference in peak and hole (e Åꢀ3)
a
R
R
1
=
= [
R
R
||F
w (|F
o
| ꢀ |Fc||
R
|F
|) /
o
2
|.
R
b
2
2
2|2]1/2.
2
o
| ꢀ |F
c
w|F
o
intermediate is then decomposed exothermally in the temperature
range 400–800 °C (third step) liberating gaseous products. The
third step is slower and corresponds to oxidation, indicating that
the thermolysis is exothermic in nature. Finally the complex
decomposes at higher temperatures to give zinc sulfide (calc.:
2
of the CS moiety, giving a final residue whose mass corresponds
to zinc sulfide (calc.: 19.86% and exp 19.98%).
5.5. Crystal structure description
1
1.86% and exp 12.04% for 3 and calc.: 11.98% and exp 12.41% for
The molecular structures of the complexes (Figs. 2–4, 6 and 7)
were determined from single crystal X-ray diffraction data.
Details of the data collection, structure solution and refinement
are listed in Table 2. Selected bond lengths and angles are given
in Tables 3 and 4. The hydrogen bonding parameters are listed in
Tables 5 and 6.
4) as a residue [44]. The thermal decomposition patterns of these
complexes suggest that they could be employed for the syntheses
of zinc sulfide/zinc oxide nanoparticles.
The thermogram of complex 5 shows (Fig. 1) a first major
weight loss of 65.74% (calc.: 64.29%) from 235 to 265 °C, which is
consistent with the loss of the 1-ethoxycarbonyl piperazine moiety
0
of the ligand along with the 2,2 -bipyridyl molecule. The interme-
5
.5.1. Crystal structure description of [Zn(pyot)
Zn(mphot) (en) ] (2)
The ORTEP diagram of complex 1 and the molecular structure of
complex 2, with atom numbering schemes, are shown in Figs. 2
2 2
(en) ] (1) and
diate products are not stable over a long range of temperature and
decompose at a slower rate soon after their formation. The
observed mass loss corresponds to loss of chlorine and breakage
[
2
2

M.K. Bharty et al. / Polyhedron 98 (2015) 84–95
91
Table 3
Selected bond lengths (Å) and bond angles (°) for complexes 1 and 2.
Table 5
Intermolecular hydrogen bonding parameters [Å and °] for 1 and 2.
.
. .
. . .
d(H A)
. . .
d(D A)
Bond lengths (Å)
Exp.
Bond angles (°)
D–H
A
d(D–H)
<(DHA)
Calc.
Exp.
Calc.
Complex 1
C8H8Aꢃ ꢃ ꢃO1
0.970
0.900
2.667
2.506
3.368
3.247
129.51
139.99
Complex 1
N4H4Bꢃ ꢃ ꢃN1
Zn1–N5
Zn1–N5
Zn1–N4
Zn1–N4
Zn1–N3
Zn1–N3
2.084(3)
2.09
2.09
2.11
2.11
2.12
2.12
N5–Zn1–N5
N4–Zn1–N4
N3–Zn1–N3
N5–Zn1–N3
N5–Zn1–N4
N4–Zn1–N3
180.0
180.0
180.00(8)
90.26(11)
83.13(12)
90.45(11)
180.10
180.10
180.05
90.31
83.20
90.51
2.084(3)
2.103(3)
2.103(3)
2.115(3)
2.115(3)
Complex 2
C9H9Cꢃ ꢃ ꢃS1
N8H8Bꢃ ꢃ ꢃS1
C9H9Aꢃ ꢃ ꢃO2
N8H8Aꢃ ꢃ ꢃO2
0.960
0.900
0.960
0.900
2.892
2.756
2.621
2.178
3.840
3.620
3.483
3.050
169.35
161.28
149.72
163.17
Symmetry operations: ꢀx, 1ꢀy, ꢀz
Complex 2
Zn1–N8
Zn1–N8
Zn1–N5
Zn1–N5
Zn1–N1
Zn1–N1
2.031(3)
2.031(3)
2.011(2)
2.011(2)
2.491(2)
2.491(2)
2.04
2.04
2.02
2.02
2.51
2.51
N5–Zn1–N5
N8–Zn1–N8
N1–Zn1–N1
N5–Zn1–N8
N8–Zn1–N1
N5–Zn1–N1
180.00(11
180.00(1)
180.0
84.56(11)
86.64(10)
91.90(10)
180.06
180.06
180.06
84.61
86.72
91.97
Table 6
Intermolecular hydrogen bond parameters [Å and °] for complexes 4 and 5.
. . .
. . .
d(H A)
. . .
d(D A)
D–H
A
d(D–H)
<(DHA)
Complex 4
C20H20A S1
.
. .
0.970
2.763
3.590
143.59
Symmetry operations: 1ꢀx, ꢀy, 2ꢀz
Complex 5
.
. .
C5H5A Cl1
0.990
0.950
2.799
2.790
3.583
3.575
136.58
140.55
.
. .
C12H12A Cl1
and 3, respectively. Single crystal X-ray diffraction studies indicate
that the ligands N-aroylhydrazine carbodithioate (RCONHNHCSSK)
have cyclized to the corresponding 5-substituted-1,3,4-oxadiazole-
atom and the CH hydrogen atom of a nearby en molecule
Supple. Fig. 6). The molecules of complex 2 are associated in the
2
-thione. The molecular structures of complexes 1 and 2 show that
Zn(II) is six coordinated, in the centrosymmetric unit of
Zn(pyot) (en) ] (1)/[Zn(mphot) (en) ] (2), and is bonded through
(
solid state by C–Hꢃ ꢃ ꢃS, N–Hꢃ ꢃ ꢃS, C–Hꢃ ꢃ ꢃO and N–Hꢃ ꢃ ꢃO hydrogen
[
2
2
2
2
bonds formed between the thione sulfur, oxadiazole ring oxygen,
four equatorial nitrogen atoms of two en ligands and two oxadia-
zole nitrogen atoms of the 5-(aryl)-1,3,4-oxadiazole-2-thione
anions in the axial positions. The bond distances Zn(1)–N(oxa)
methoxy oxygen and hydrogen atoms of OCH
Supple. Fig. 7). The geometrical parameters relevant for these
interactions are given in Table 5.
3 2
and NH groups
(
{2.115(3) Å}, Zn(1)–N(en) {2.103(4) and 2.084(4) Å} for complex 1
and Zn(1)–N(oxa) {2.491(4) Å}, Zn(1)–N(en) {2.031(4) and 2.011 Å}
for complex 2, with bite angles of 83.14(15)° and 84.58(17)°, rep-
resent the deviation from an octahedral geometry. These axial
and equatorial Zn–N distances (Table 3) suggest a distorted octahe-
dral geometry for both the complexes. The soft donor site of the
ligand (pyot and mphot ) shows no tendency for coordination
with the Zn(II) ion as it is a hard metal ion and prefers to bind com-
paratively with the hard nitrogen site of the ligand. In the solid
state complex 1 is stabilized via an intermolecular N–Hꢃ ꢃ ꢃN inter-
action between pyridyl nitrogen atom and NH hydrogen atom of en
and a C–Hꢃ ꢃ ꢃO interaction between the oxadiazole ring oxygen
5
[
.5.2. Crystal structure descriptions of [Zn(bhpzdtc)
Zn(bhpzdtc) (4-pic)] (4) and [Zn(ecpzdtc)(bpy)(Cl)] (5)
ORTEP diagrams of complexes [Zn(bhpzdtc)
Zn(bhpzdtc) (4-pic)] (4) and [Zn(ecpzdtc)(bpy)(Cl)] (5), with their
2
(py)] (3),
2
2
(py)] (3),
[
2
ꢀ
ꢀ
atom numbering schemes, are given in Figs. 4, 6 and 7,
respectively. The five coordination number of complexes 3 and 4
is fulfilled by four sulfur atoms from two carbodithioate ligands
and one nitrogen atom from a pyridine/4-picoline ring. In complex
5
, the coordination number of five is fulfilled by two sulfur atoms
from carbodithioate ligands and two nitrogen atoms from a bipyr-
idyl ring and one chlorine atom. Values of the geometrical param-
eter tau {s = 0.513 (3), 0.356 (4) and 0.271(5)} indicate that
complex 3 is almost half way between a trigonal bipyramidal
and square pyramidal geometry, whereas complexes 4 and 5 adopt
a distorted square pyramidal geometry [45]. Complex 3 forms a
sheet-like structure which is shown as a space fill model (Fig. 5).
The S,S-anisobidentate coordination mode of the carbodithioate
ligands leads to the formation of four-membered chelate rings
Table 4
Selected bond lengths (Å) and bond angles (°) for complexes 3–5.
Bond lengths (Å)
Exp.
Bond angles (°)
Calc.
Exp.
Calc.
Complex 3
Zn–N5
Zn–S4
Zn–S3
Zn–S2
Zn–S1
2.078(3)
2.08
2.36
2.61
2.33
2.54
S1–Zn–S3
S4–Zn–S2
S4–Zn–S3
S2–Zn–S1
N5–Zn–S3
N5–Zn–S4
169.05(4)
138.30(4)
72.90(3)
73.38(3)
93.69(9)
111.37(9)
168.99
138.24
72.88
73.35
93.64
(
2
ZnS C) in all the complexes. In complexes 3–5, there are consider-
2.3455(11)
2.5975(10)
2.3595(10)
2.5547(10)
able differences between the pairs of Zn–S and C–S bonds and it is
interesting to note that the longer Zn–S bonds are associated with
the shorter C–S bonds. This is due to the combined effects of the
steric constraints induced by the carbodithioate ligands, electron
pair repulsion and the presence of the coligand. The stronger –N–
C(@S)S– bonds reflect the contribution of double bonding to the
111.32
Complex 4
Zn1–N5
Zn1–S4
Zn1–S3
Zn1–S2
Zn1–S1
2.077(3)
2.08
2.35
2.56
2.35
2.56
S1–Zn1–S3
S2–Zn1–S4
S1–Zn1–S2
S3–Zn1–S4
N5–Zn1–S2
N5–Zn1–S3
160.98(4)
138.74(4)
72.79(3)
73.09(3)
113.42(9)
102.75(9)
160.97
138.54
72.75
73.02
113.40
102.71
2.3538(11)
2.5829(11)
2.3606(10)
2.5635(10)
2
formally ordinary single bond (due to the mixing of the sp and
3
sp hybrid states of the nitrogen atom). All six membered piper-
azine heterocyclic moieties adopt a ‘chair conformation’ in the
complexes. The Zn–N(py/4-pic) distance of 2.078(3) Å in 3 and
Complex 5
Zn1–N4
Zn1–N3
Zn1–S2
Zn1–S1
Zn1–Cl1
2
.077(3) Å in 4 (Table 4) is shorter than that found in the six coor-
dinate [Zn(pipdtc) (1,10-phen)] and [Zn(pipdtc) (bipy)] complexes
(2.196(3) Å) due to a change in coordination number, but it is sim-
ilar to the five coordinate [Zn(thqdtc) (py)], [Zn(thqdtc) (py)] and
Zn(3,5-di-t-butyl-4-hydroxybenzene-dithio carboxylate) (py)]
complexes [46–48]. Also in complex 5, the Zn–N(bpy) distances of
2.116(3)
2.139(4)
2.5069(15)
2.4207(13)
2.2549(13)
2.12
2.14
2.51
2.43
2.24
N3–Zn1–S2
N4–Zn1–S1
N4–Zn1–N3
S1–Zn1–S2
N4–Zn1–Cl1
Cl1–Zn1–S2
151.25(11)
135.00(12)
76.40(14)
72.97(5)
109.23(12)
106.61(6)
150.04
136.89
76.36
72.93
109.20
106.57
2
2
2
2
[
2

9
2
M.K. Bharty et al. / Polyhedron 98 (2015) 84–95
Fig. 8. Optimized structures of complexes 1–5.
2
.116(3) and 2.139(4) Å are found to be shorter as compared to the
delocalization within a chelate ring is present, it would exhibit
some degree of metalloaromaticity [49]. The delocalized charge
in the chelate ring of complexes 3 and 4 forms an unusual
above complexes. In the complexes, the chelate rings and the
piperazine rings lie nearly in the same plane. In the solid state,
complexes
3
and
4
are stabilized by C–Hꢃ ꢃ ꢃ
p
interactions
intramolecular C–Hꢃ ꢃ ꢃ
p interaction with the hydrogen atom of
the pyridine/picoline ring, having distances of 2.763–2.799 Å in 3
(Supple. Figs. 8 and 9) between
p
electrons of the chelate and phe-
nyl rings with the hydrogen atoms of the pyridine ring. Also, com-
plex 4 contains an intermolecular C–Hꢃ ꢃ ꢃS interaction (Supple.
Fig. 10) between the dithio sulfur and piperazine ring hydrogen
atoms, leading to a linear structure. Complex 5 is stabilized via
and 2.868–2.907 Å in complex 4 [50–52]. The above observed
intermolecular hydrogen bonding, C–Hꢃ ꢃ ꢃ
p
and
pꢃ ꢃ ꢃp interactions
have stabilized the molecular crystal packing, giving a supramolec-
ular network. The geometry and bonding parameters agree with
those of other zinc complexes containing dithio as a ligand and
pyridine as a coligand.
C–Hꢃ ꢃ ꢃCl, CHꢃ ꢃ ꢃO and weak
pꢃ ꢃ ꢃ
p interactions (Supple. Fig. 11)
forming a linear structure. It is observed that if active electron
Fig. 9. Frontier molecular orbital diagram of complexes 1, 2 and 3.

M.K. Bharty et al. / Polyhedron 98 (2015) 84–95
93
Table 7
is distorted octahedral. A comparison of the X-ray structures of
the complexes with their optimized counterparts show slight con-
formational discrepancies between them. In complexes 3–5, the
calculated bite angles (S–Zn–S) are found to be between 72.53°
and 73.04° which shows that the complexes have a distorted
square pyramidal geometry around the metal center. The calcu-
Calculated optimized and molecular orbital energies (eV) for complexes 1–5.
Molecule Optimized
energy (a.u.)
Energy (eV)
HOMO LUMO
Degree of
hardness
g
Energy
gap
1
2
3
4
5
ꢀ1483.994
ꢀ1680.968
ꢀ1966.922
ꢀ2006.137
ꢀ1168.09
ꢀ5.877 ꢀ2.282 3.595
ꢀ5.289 ꢀ1.444 3.845
ꢀ5.110 ꢀ1.496 3.614
ꢀ5.043 ꢀ1.279 3.764
ꢀ5.284 ꢀ2.973 2.311
1.798
1.923
1.807
1.882
1.155
lated values of the geometrical parameter tau {s = 0.514 (3),
0.374 (4) and 0.219 (5)} indicate that complex 3 is almost half
way between a trigonal bipyramidal and square pyramidal geom-
etry, whereas complexes 4 and 5 adopt a distorted square pyrami-
dal geometry. This discrepancy between the calculated and
experimental bond parameters may arise from the basis sets which
were approximated during the calculations or may be due to the
influence of the crystal packing [53]. The theoretical calculations
do not consider the effects of the chemical environment. The devi-
ations in the geometrical parameters may be due to the fact that
the DFT calculations are for an isolated molecule in the gaseous
phase while the X-ray crystallographic data were obtained from
the crystal lattice of complex molecules. These small discrepancies
in bond lengths and bond angles are attributable to H-bonding and
packing interactions within the lattice, which are not modeled dur-
ing the computational studies [54].
5.6. DFT calculations of the complexes
The optimized geometrical parameters are listed in Tables 3 and
, and the optimized geometries of complexes 1–5 are shown in Fig
. The optimized geometries of the ligands are shown in Supple.
4
8
Figs. 12 and 13. The geometry optimization for the ligands and
the complexes has been done to understand the chemical reactiv-
ity and to predict the outcome of the molecular interactions. The
optimized energies of complexes 1 and 2 are ꢀ1483.997 and
ꢀ1680.965 a.u., respectively indicating that complex 2 is more
stable than complex 1. The optimized energies of complexes 3–5
are ꢀ1966.928, ꢀ2006.137 and ꢀ1168.012 a.u., respectively indi-
cating that the order of stability of the complexes is 4 > 3 > 5.
Complexes 1 and 2 possess zero dipole moment, whereas those
of complexes 3–5 are 3.4, 5.2 and 10.7 Debye, respectively. The
Zn–N(oxadiazole) bond lengths in the coordination sphere for com-
plexes 1 and 2 are 2.115(8) and 2.491(8) Å in the X-ray structures,
whereas the corresponding values in the optimized geometry are
5.7. Frontier molecular orbital analysis
3D plots of the HOMO and LUMO for complexes 1 and 2 are
shown in Fig. 9. Analysis of the frontier molecular orbitals
(FMOs) of complexes 1 and 2 shows that they exhibit similar local-
ized electron density since their HOMOs are mainly localized on
the oxadiazole thione moiety with very little contribution from
phenyl ring and in LUMO the entire phenyl and oxadiazole ring
have more electron density than the thione group, without any
contribution from the en molecule to either molecular orbital.
2
.213 and 2.238 Å, respectively. The calculated C@S(thione) bond
lengths in complexes 1 and 2 are 1.728 and 1.729 Å, respectively
indicating partial double bond character. The calculated angles in
complexes 1 and 2 show that the geometry around the Zn(II) ion
Fig. 10. Frontier molecular orbital diagram of complexes 4 and 5.

9
4
M.K. Bharty et al. / Polyhedron 98 (2015) 84–95
Both the HOMO and LUMO are located primarily on the ligand
pyot and mphot) with little or no orbital coverage on the metal
the residue. The X-ray crystallographic results of complexes com-
pare well to those obtained by the DFT method. The non-linear
optical property is addressed theoretically, which indicates that
complex 5 may be a potential applicant in the development of
NLO materials.
(
center. Also, there is no direct contribution from the substituted
phenyl rings to the HOMO coverage; however, the para-methoxy
group does appear to impart an influence on the overall energy
level. Complex 2 has greater EHOMO energy (ꢀ5.289 eV) than com-
plex 1 (ꢀ5.877 eV) (Table 7). Similarly, the LUMO energies also
reflect this variation (ELUMO = ꢀ1.444 eV for 2 and ELUMO = ꢀ2.282
for 1). Complex 2 contains an electron donating para-methoxy
group which can lead to a degree of tunable optical properties.
Supple. Figs. 12 and 13 show that the HOMO orbital is heavily
Acknowledgements
Dr. M.K. Bharty is thankful to the Department of Science and
Technology, New Delhi, India for financial support through a
Young Scientist Project (No. SR/FT/CS-63/2011). The authors thank
Prof. R.N. Kharwar, Department of Botany, Faculty of Science, BHU
for the antibacterial activity study.
2
localized mainly at the CS moiety of both the piperazine ligands
and the electronic density is localized on potassium in the LUMO.
The energy gap between the HOMO and LUMO is less for
+
ꢀ
[
(K (ecpzdtc) ], which suggests for its lower chemical hardness
Appendix A. Supplementary data
+
ꢀ
than [K (bhpzdtc) ]. The frontier molecular orbital (FMOs) of com-
plexes 3–5 (Figs. 9 and 10) show that they exhibit an electron dis-
tribution similar to their respective dithio ligands since their
CCDC 899305, 899306, 994814, 994815 and 994817 contain the
supplementary crystallographic data for 1–5, respectively. These
data can be obtained free of charge from the Cambridge
Crystallographic Data Center via www.ccdc.cam.ac.uk/data_
request/cif.
HOMOs are mainly localized on the CS
2
group and on chlorine in
5, however, this contribution is absent in case of the LUMOs, in
which the secondary ligand py/4-pic/bpy is heavily localized.
Complex 5 has a larger energy for the HOMO and LUMO than 3
and 4, due to the presence of bpy and chlorine. The small
HOMO–LUMO energy gap for excitation energy indicates a better
stability and a lower chemical hardness than complexes 3 and 4.
All these results can lead to an electronic transition from the
ground state to the excited state due to a transfer of electrons from
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2015.05.045.
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