Hg(II) complexes of 4-phenyl-5-(3-pyridyl)-1,2,4-triazole-3-thione and 5-(4-pyridyl)-1,3,4-oxadiazole-2-thione and a Ni(II) complex of 5-(thiophen-2-yl)-1,3,4-oxadiazole-2-thione: Synthesis and X-ray structural studies

Article abstract of DOI:10.1016/j.poly.2012.11.043

Three new mixed ligand complexes, [Hg(en)(4-pptt)₂] (2) {4-pptt = 4-phenyl-5(3-pyridyl)-1,2,4-triazole-3-thione}, [Hg(en)(4-pot)₂] (3) {4-pot = 5-(4-pyridyl)-1,3,4-oxadiazole-2-thione} and [Ni(en) ₂(5-thot)₂] (4

Full text of DOI:10.1016/j.poly.2012.11.043

Polyhedron 50 (2013) 582–591
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Hg(II) complexes of 4-phenyl-5-(3-pyridyl)-1,2,4-triazole-3-thione and
5-(4-pyridyl)-1,3,4-oxadiazole-2-thione and a Ni(II) complex of
5-(thiophen-2-yl)-1,3,4-oxadiazole-2-thione: Synthesis and X-ray structural studies
b
b
c
a,
A. Bharti a, M.K. Bharty a, , S. Kashyap , U.P. Singh , R.J. Butcher , N.K. Singh
⇑
⇑
a Department of Chemistry, Banaras Hindu University, Varanasi 221005, India
b Department of Chemistry, Indian Institute of Technology, Roorkee 247667, Uttarakhand, India
c Department of Chemistry, Howard University, 525 College Street NW, Washington, DC 20059, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new mixed ligand complexes, [Hg(en)(4-pptt)2] (2) {4-pptt = 4-phenyl-5(3-pyridyl)-1,2,4-triazole-
3-thione}, [Hg(en)(4-pot)2] (3) {4-pot = 5-(4-pyridyl)-1,3,4-oxadiazole-2-thione} and [Ni(en)2(5-thot)2]
(4) {5-thot = 5-(thiophen-2-yl)-1,3,4-oxadiazole-2-thione}, have been prepared containing en as a co-
ligand. It is observed that 5-(pyridine-4-carbonyl) hydrazine carbodithioic acid methyl ester undergoes
cyclization during complexation with Hg(II) in the presence of ethylenediamine and formed complex 3
containing 5-(4-pyridyl)-1,3,4-oxadiazole-2-thione. The metal complexes have been characterized with
the aid of elemental analyses, IR, magnetic susceptibility and single crystal X-ray studies. The ligand 4-
pptt (1) and complexes 2, 3 and 4 crystallize in the monoclinic system, space group P21/n, P21/c,
P121/c1 and P21/n respectively. The ligand is present in the deprotonated thiol form in complexes 2
and 3, and is bonded covalently through sulfur, while in complex 4 the ligand 5-(thiophen-2-yl)-1,3,4-
oxadiazole-2-thione is present as the thione form and is bonded to Ni(II) through the oxadiazole nitrogen.
Complex 4 shows quasi-reversible redox behavior assignable to a Ni2+/Ni3+ one electron transfer. The
photoluminescence properties indicate that the complexes are fluorescent materials with maximum
emissions at 433, 376 and 465 nm for complexes 2, 3 and 4 at excitation wavelengths of 293, 273 and
327 nm, respectively. All the complexes contain extended hydrogen bonding that provides a supramolec-
ular framework.
Received 12 September 2012
Accepted 16 November 2012
Available online 8 December 2012
Keywords:
Oxadiazole complexes
Triazole complex
Mixed ligand complexes
Hg(II) and Ni(II) complexes
Intermolecular and intramolecular
hydrogen bonding
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
centers and own the capability to exhibit luminescence or other
properties, (3) they show a variety of bridging modes with extra
The coordination chemistry of nitrogen–sulfur containing het-
erocyclic ligands, such as 1,2,4-triazoles, 1,3,4-oxadiazoles and
1,3,4-thiadiazoles, is an emerging and rapidly developing area of
research [1–3]. These compounds have received much attention
on account of their important pharmacological activities, such as
antiviral, analgesic, antimicrobial, antidepressant and antifungal
effects [4]. Triazole derivatives have been used as starting materi-
als to synthesize a variety of compounds and also as ligands to ob-
tain metal complexes with specific properties [5]. It has been
reported that the thione form of 1,2,4-triazole-3-thiones and
1,3,4-oxadiazole-2-thiones are important for their antifungal activ-
ity [6,7]. 1,2,4-Triazoles and their derivatives have received much
interest because of the fact that (1) a number of functionalized
triazoles are readily accessible, (2) they have heterocyclic
potential donor sites provided by the functional groups substituted
on the triazole rings and (4) they contain nitrogen atoms as hydro-
gen bond acceptors and aromatic systems which contribute to p–p
stacking, leading to secondary non-covalent interactions in metal–
triazole complexes [8]. 1,2,4-Triazole-3-thiones are known to act
as bridging ligands in complexes of Cu(II), Fe(II), Pt(II) and Ru(II)
through their nitrogen and sulfur sites, forming coordination com-
pounds that are interesting from both magnetic and chemical as-
pects [9]. 1,3,4-Oxadiazole derivatives, which belong to an
important group of heterocyclic compounds, have been the subject
of extensive study in recent years [10]. The oxadiazole molecules
act as spacers via coordination and hydrogen bonding, and show
intermolecular cooperative interactions [11]. Although some work
has been reported on the binary complexes of 1,3,4-oxadiazole-2-
thiones and 1,2,4-triazole-3-thiones [9(b,g),12], little is known
about the mixed ligand complexes of these ligands. In view of this,
we have synthesized and characterized Hg(II) complexes of the
versatile ligands 4-phenyl-5-(3-pyridyl)-1,2,4-triazole-3-thione
(Hpptt) and 5-(4-pyridyl)-1,3,4-oxadiazole-2-thiol (Hpot) and a
p-conjugated systems with good coordination abilities to metal
⇑
Corresponding authors.
E-mail addresses: mkbharty@bhu.ac.in (M.K. Bharty), singhnk_bhu@yahoo.com
(N.K. Singh).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.11.043

A. Bharti et al. / Polyhedron 50 (2013) 582–591
583
Ni(II) complex of 5-(thiophen-2-yl)-1,3,4-oxadiazole-thione
(Hthot), containing ethylenediamine as a co-ligand.
was dissolved in ethanol and suitable crystals of 1 were obtained
from this solution after 12 days [14]. Yield (1.12 g, 60%). M.p.
190 °C; Anal. Calc. for C13H10N4S (254.31): C, 61.40; H, 3.96; N,
22.03; S, 12.60. Found: C, 61.43; H, 4.01; N, 21.98; S, 12.58%. IR
2. Experimental
(KBr, m m(NH), 1613 m(C@N), 950 m(C@S), 740 (pyri-
, cmꢀ1): 3219
dine ring), 686 (phenyl ring). 1H NMR (DMSO-d6, d, ppm): 7.25
(2H), 7.15 (1H), 7.13 (2H) (phenyl ring); 8.35 (1H), 8.34 (1H),
7.47 (1H) (pyridine ring); 8.37 (1H, NH). 13C NMR (DMSO-d6, d,
ppm): 150.56, 150.45 (C5, C7), 121.04, 118.63 (C4, C6), 140.27
(C3) (pyridine ring); 138.40 (C8), 117.28 (C11), 121.55 (C9, C13),
122.18 (C10, C12) (phenyl ring); 201.05 (C1), 160.46 (C2) (triazole
ring). UV–Vis. [DMSO, kmax, nm]: 287.
2.1. Materials and methods
Commercial reagents were used without further purification
and all experiments were carried out in the open atmosphere.
Ethyl nicotinate, ethyl 2-thiophene carboxylate, isonicotinic acid
hydrazide, phenyl isothiocyanate (Sigma–Aldrich), CS2 (SD Fine
Chemicals, India) and KOH (Qualigens) were used as received.
[Ni(en)2(SCN)2] was prepared by the reported method [13]. All
the solvents were purchased from Merck and used after purifica-
tion. The yield calculation is based on the weight of the ligand or
the initial starting material where intermediates were not isolated.
Carbon, hydrogen and nitrogen contents were estimated on a Carlo
Erba 1108 model microanalyser. Magnetic susceptibility measure-
ment was performed at room temperature on a Cahn Faraday bal-
ance using Hg[Co(NCS)4] as the calibrant. Electronic spectra were
recorded on a Shimadzu 1700 UV–Vis spectrophotometer in DMSO.
IR spectra were recorded in the 4000–400 cmꢀ1 region as KBr pel-
lets on a Varian Excalibur 3100-FT IR spectrophotometer. 1H and
13C NMR spectra were recorded in DMSO-d6 and CDCl3 on a JEOL
AL 300 FT NMR spectrometer using TMS as an internal reference.
The fluorescence spectra were measured at room temperature with
a Varian CARYECLIPSE spectrophotometer in DMSO. The electro-
chemical experiments were performed with a BAS CV-50W vol-
tammetric analyzer (Bioanalytical System, West Lafayette, USA),
using a three-electrode system with a Pyrolytic Glass electrode
as the working electrode, a platinum wire as the counter electrode
and Ag/AgCl was used as the reference electrode.
2.2.2. Synthesis of N0-(pyridine-4-carbonyl) hydrazine carbodithioic
acid methyl ester (4-pchcdme)
The preparation and characterization of N0-(pyridine-4-car-
bonyl) hydrazine carbodithioic acid methyl ester (4-pchcdme) is
described elsewhere [15].
2.2.3. Synthesis of potassium N0-(thiophene-2-carbonyl) hydrazine
carbodithioate [K(H2tchcd)]
Potassium N0-(thiophene-2-carbonyl) hydrazine carbodithioate
[K(H2tchcd)] was prepared by the dropwise addition of CS2
(1.2 mL, 20 mmol) to a solution of thiophene 2-carboxylic acid
hydrazide (2.84 g, 20 mmol) in methanol (30 mL) in the presence
of KOH (1.2 g, 20 mmol), stirring the reaction mixture for 30 min.
The solid which separated was filtered off, washed with 10% (v/
v) mixture of ethanol–ether and dried. Yield (1.54 g, 85%). M.p.
220 °C; Anal. Calc. for C7H6N3OS2K (257.00): C, 28.01; H, 1.94; N,
10.89; S, 37.35. Found: C, 28.02; H, 1.95; N, 10.90; S, 37.34%. IR
(KBr,
985s
m m(NH), 1666s m(C@O), 1062s m(N–N),
, cmꢀ1): 3213s, 3140s
m
(C@S). 1H NMR (DMSO-d6, d, ppm): 11.74, 10.42 (s, 2H,
NH), 7.92-7.14 (m, 3H, aromatic ring). 13C NMR (DMSO-d6, d,
ppm): 178.50 (>C@S), 157.19 (>C@O), 128.40 (C3), 127.68 (C4),
125.97 (C5), 129.18 (C6).
2.2. Synthesis
2.2.1. Synthesis of 4-phenyl-5-(pyridin-3-yl)-2H-1,2,4-triazole-3-
thione (Hpptt) (1)
2.2.4. Synthesis of [Hg(en)(pptt)2] (2)
To a suspension of finely powdered nicotinic acid hydrazide
(2.74 g, 20 mmol) in benzene was added phenyl isothiocyanate
dropwise (2.4 mL, 20 mmol) and the mixture was then refluxed
for 8 h at 80 °C. The solid 1-nicotinoyl-4-phenyl thiosemicarbazide
obtained upon cooling was filtered off, washed with water and
diethyl ether, and dried. 1-Nicotinoyl-4-phenyl thiosemicarbazide
(2.72 g, 10 mmol) was added to the ethanol solution of NaOH
(0.5 g, 12 mmol) and refluxed for 6 h. After cooling, the resulting
clear solution was acidified with dil. HCl and the obtained precip-
itate was filtered off, washed with water and air dried. The solid
A methanol solution (20 mL) of HgCl2 (0.235 g, 1 mmol) was
stirred with a methanol solution (20 mL) of Hpptt (0.5 g, 2 mmol).
The resulting white precipitate was filtered off, washed with meth-
anol and then suspended in methanol, to which an excess (0.30 mL,
5 mmol) of ethylenediamine was added and stirred until total dis-
solution of the precipitate was observed. The resulting solution
was filtered off and kept for crystallization. White crystals of 2
suitable for X-ray analysis were obtained by slow evaporation of
the solution over a period of 10 days. Yield (0.78 g, 52%). M.p.
280 °C; Anal. Calc. for C28H30HgN10S2 (771.35): C, 43.55; H, 3.88;
O
O
H
N
NH2
NH
PhNCS
N
N
H
H
Benzene, reflux
S
N
N
NaOH, Reflux
Acidify
H2N
NH2
N
NH
N
N
Hg
4
N
N
3
S
S
5
N
S
2
1
8
N
HgCl2
en
N
13
12
7
9
6
N
N
N
10
11
Hpptt (1)
Scheme 1. Preparation of the ligand Hpptt (1) and its complex [Hg(en)(4-pptt)2] (2).

584
A. Bharti et al. / Polyhedron 50 (2013) 582–591
O
O
H
N
SK
NH2
MeOH
N
H
N
H
CS
+
+
KOH
2
N
S
N
CH3I
MeOH
O
NH2
S
H2N
S
H
N
SCH3
N
7
N
1
6
N
HgCl2
en
3
2
Hg
H
O
N
N
N
S
O
4
N
5
N
Scheme 2. Preparation of [Hg(en)(4-pot)2] (3).
5
S
1
4
H
N
H
N
MeOH
3
CS
+
2
+
KOH
6
NH2
2
S
N
SK
S
H
O
O
Ni (en)2 (NCS)2
MeOH
S
H2N
NH2
O
S
N
N
Ni
N
N
H2N
NH2
S
O
S
Scheme 3. Preparation of [Ni(en)2(5-thot)2] (4).
N, 18.15; S, 8.29. Found: C, 43.53; H, 3.86; N, 18.14; S, 8.28%. IR
(KBr, (NH) (en), 1600 (C@N); 835 (C–S); 772
, cmꢀ1): 3229
2.2.6. Synthesis of [Ni(5-thot)2(en)2] (4)
m
m
m
m
A methanolic solution (10 mL) of [Ni(en)2(NCS)2] (0.237 g,
1 mmol) was mixed with an aqueous–methanol solution (50:50
v/v) (10 mL) of potassium N0-(thiophene-2-carbonyl) hydrazine
carbodithioate [K+(H2tchcd)ꢀ] (0.514 g, 2 mmol) and the reaction
mixture was filtered. The filtrate was kept for crystallization,
whereupon red crystals of complex 4 were obtained after 10 days.
Yield (0.63 g, 60%). M.p. 250 °C; Anal. Calc. for C16H22N8O2S4Ni
(545.37): C, 35.20; H, 4.04; N, 20.54; S, 23.47. Found: C, 35.21; H,
(py. ring), 698 (phenyl ring). 1H NMR (DMSO-d6, d, ppm): 6.59–
7.51 (m, 5H) (phenyl ring); 8.45 (1H), 7.61 (1H), 7.58 (1H) (pyri-
dine ring); 2.72 (4H, NH2) 2.40 (4H, CH2). 13C NMR (DMSO-d6, d,
ppm): 150.56, 150.45 (C5, C7), 121.04, 118.63 (C4, C6), 140.27
(C3) (pyridine); 135.35 (C8), 117.28 (C11), 128.12 (C9, C13),
129.57 (C10, C12) (phenyl); 56.41 (CH2); 154.76 (C@N); 183.77
(C@S) (triazole ring). UV–Vis. [DMSO, kmax, nm; e
max, Mꢀ1 cmꢀ1]:
293 (9000) (Scheme 1).
4.03; N, 20.53; S, 23.46%. IR (KBr,
(C@N), 1088s (N–N), 962 (C@S), 518s
kmax, nm (
max, Mꢀ1 cmꢀ1)]: 327 (40000), 361 (36000), 526 (102),
m
, cmꢀ1): 3231s
(Ni–N). UV–Vis. [DMSO,
m(NH), 1593s
m
m
m
m
e
793 (55) (Scheme 3).
2.2.5. Synthesis of [Hg(en)(4-pot)2] (3)
HgCl2 (0.270 g, 1 mmol) and freshly prepared N0-(pyridine-4-
carbonyl) hydrazine carbodithioic acid methyl ester (0.454 g,
2 mmol) were dissolved separately in 15–20 mL methanol, mixed
together and stirred for 20 min. The yellow precipitate was sepa-
rated by filtration, washed successively with methanol–water mix-
ture (50:50) and finally with methanol. A methanol solution
(10 mL) of ethylenediamine (0.15 mL, 2 mmol) was added to the
methanol suspension of the above compound and stirred for 1 h.
A clear solution was obtained which was filtered and kept for crys-
tallization. White crystals of 3 suitable for an X-ray analyses were
obtained by slow evaporation of the above solution over a period of
10 days. Yield: (0.74 g, 55%). M.p. 260 °C; Anal. Calc. for C16H16N8-
HgO2S2 (617.08): C, 31.10; H, 2.59; N, 18.14; S, 10.37. Found: C,
3. X-ray crystallography
Data for compounds 1 and 2 were recorded at 296(2) K and
those of 3 and 4 were obtained at 295(2) K on a Bruker three-circle
diffractometer/Oxford Diffraction Gemini diffractometer equipped
with SMART 6000/CCD CrysAlis Pro software using a graphite
monochromated Mo Ka (k = 0.71073 Å) radiation source. The
structures were solved by direct methods and refined (SHELX-08)
against all data by full matrix least-square on F2 using anisotropic
displacement parameters for all non-hydrogen atoms. All hydrogen
atoms were included in the refinement at geometrically ideal posi-
tions and refined with a riding model [16]. The MERCURY package
was used for the molecular graphics [17]. Molecular structure dia-
grams were generated using the ORTEP-3 for Windows program [18].
31.11; H, 2.58; N, 18.15; S, 10.36%. IR (KBr,
(NH), 1606s (C@N), 1214s (C–O–C), 1080s (N–N), 824
485
m
, cmꢀ1): 3232s
(C–S),
m
m
m
m
m
m
(Hg–S). 1H NMR (DMSO-d6, d, ppm): 8.74 (d, 2H), 7.79 (d,
4. Results and discussion
2H) pyridine ring, 2.72 (s, 4H, NH2), 2.33 (s, 4H, CH2). 13C NMR
(DMSO-d6, d, ppm): d 178.50 (C@S), 156.05 (C@N), 128.40 (C3),
127.68 (C5, C6), 119.31 (C4, C7), 47.64 (C1, C2). UV–Vis. [DMSO,
It has been observed that 5-(3-pyridyl)-1,2,4-triazole-3-thione
and 5-(pyridine-4-carbonyl) hydrazine carbodithioic acid methyl
ester react with HgCl2 and then with ethylenediamine to form four
kmax, nm (e
max, Mꢀ1 cmꢀ1)]: 273 (34000) (Scheme 2).

A. Bharti et al. / Polyhedron 50 (2013) 582–591
585
coordinate tetrahedral Hg(II) complexes (2 and 3) containing en as
a co-ligand. The potassium N-(thiophene-2-carbonyl) hydrazine
carbodithioate reacts with [Ni(en)2(NCS)2] to form the six coordi-
nate octahedral Ni(II) complex 4. The elemental analyses and phys-
ical measurements of complexes 3 and 4 indicate that both
hydrazinic hydrogens from the acylhydrazine carbodithioate
[–C(O)NHNHC(S)Sꢀ–] moiety and one sulfur atom were missing
from the resulting complexes. Complexes 2 and 3 were obtained
by shaking [Hg(L)2] (L = 5-(3-pyridyl)-1,2,4-triazole-3-thione/5-
(4-pyridyl)-1,3,4-oxadiazole-2-thione) with a methanol solution
of ethylenediamine taken in a 1:2 molar ratio, whereas complex
4 was obtained by the reaction of potassium N0-(thiophene-2-car-
bonyl)] hydrazine carbodithioate [K+(H2tchcd)ꢀ] with [Ni(en)2
(NCS)2]. Schemes 1–3 depict the synthesis of the ligands and their
Hg(II) and Ni(II) complexes. The complexes are stable towards air
and moisture for several days. Complexes 2, 3 and 4 are insoluble
in ethanol, methanol and chloroform, but are soluble in DMSO, and
melt at 280, 260 and 250 °C, respectively. The complexes were fully
characterized by magnetic susceptibility measurements, IR, UV–
Vis and X-ray spectroscopies. The analytical data of the complexes
corroborate well with their respective formulations.
of which the signals at d 204.11 and 164.04 ppm are due to the
>C@S and >C@O, carbons, respectively. The signal for the CH3 car-
bon is observed at d 16.92 ppm in the ester, which is absent in
complex 3 showing the absence of the methyl group in complex
3. The absence of both the hydrazinic protons and the methyl
group for complex 3 suggests cyclization of the S-methyl hydrazine
carbodithioate moiety to 5-(4-pyridyl)-1,3,4-oxadiazole-2-thiol
during formation of the complex. Complexes 2 and 3 show NCS
carbon signals at 183.77 and 178.50 ppm respectively, indicating
1
metal sulfur bonding. The H NMR spectrum of N0-(thiophene-2-
carbonyl) hydrazine carbodithioate K(H2tchcd) exhibits two sig-
nals at d 11.74 and 11.42 ppm for the amide and thioamide pro-
tons. The three protons of the thiophene ring appear as
a
multiplet between d 7.92 and 7.14 ppm. The 13C NMR spectra of
[K(H2tchcd) shows six signals for six carbon atoms, of which the
signals at d 178.50 and 157.50 ppm are due to the >C@S and
>C@O carbons, respectively. The thiophene ring carbons appear
at C3:128.40, C4:127.68, C5:125.97 and C6:129.18 ppm.
5. Magnetic moment and electronic absorption
The [Hg(en)(4-pptt)2] and [Hg(en)(4-pot)2] complexes are dia-
magnetic and show absorptions in the high energy region due to
intraligand/charge transfer transitions. [Ni(en)2(5-thot)2] shows a
4.1. IR spectra
The IR spectrum of the free ligand 5-(3-pyridyl)-1,2,4-triazole-
3-thione (Hpptt) in the thione form is expected to give rise to char-
acteristic bands due to m(NH), m(C@N) and m(C@S), which occur at
3219, 1613 and 950 cmꢀ1, respectively. The bands at 740 and
686 cmꢀ1 are due to the pyridine and phenyl rings, respectively.
The IR spectrum of the ligand potassium N0-(thiophene-2-car-
bonyl) hydrazine carbodithioate [K(H2tchcd)] shows absorptions
due to the stretching modes of NH (3213 (m) and 3140 (m)
cmꢀ1), C@O (1666 cmꢀ1), N–N (1062 cmꢀ1) and C@S (985 cmꢀ1).
The IR spectra of complexes 2 and 3 show no bands due to
m
(NH), indicating conversion of the thione form to the thiol form
of triazole/oxadiazole upon complexation. A negative shift of about
110 cmꢀ1 in
(C–S) shows that the thiol sulfur is participating in
m
bonding in complexes 2 and 3. The IR spectra of complexes 2, 3
and 4 show bands around 3230 cmꢀ1 due to the N–H stretching
vibrations of ethylenediamine. In [Ni(en)2(5-thot)2], m(C@S) shows
a small negative shift indicating that the thione sulfur does not
participate in bonding, but this shift can be attributed to the
involvement of sulfur in hydrogen bonding with the NH2 hydro-
gens of ethylenediamine [19].
Fig. 1. Cyclic voltammogram of [Ni(en)2(5-thot)2] in DMSO at PGE using 0.2 M KCl
as the supporting electrolyte.
4.2. 1H and 13C NMR spectra
The 1H NMR spectrum of Hpptt shows a signal at 8.37 ppm due
to the triazole NH proton. The protons of the phenyl ring appear as
multiplets between 7.86 and 7.36 ppm, and of the pyridine ring be-
tween 8.35 and 7.47 ppm. The signals at 201.05 and 160.46 ppm in
the 13C NMR spectrum are attributed to the >C@S and >C@N car-
bons, respectively. The 1H NMR spectrum of [Hg(en)(4-pptt)2] (2)
exhibits two signals at d 2.72 and 2.40 ppm for the CH2 and NH2
protons of ethylenediamine along with a slight downfield shift
for the aromatic protons. The disappearance of the triazole NH pro-
ton at 8.37 ppm shows conversion of the thione form of the ligand
to the thiol form, and subsequent bonding of the thiol sulfur with
Hg(II) in complex 2. The 13C NMR spectrum of 2 shows various sig-
nals for the carbon atoms, of which the signals at d 183.77 and
154.8 ppm are due to the >C–S and >C–N carbons, respectively.
The 1H NMR spectrum of 5-(pyridine-4-carbonyl) hydrazine carbo-
dithioic acid methyl ester exhibits two signals at d 11.75 and
11.55 ppm for the amide and thioamide protons, respectively,
and one signal at 2.0 ppm due to the methyl protons. The 13C
NMR spectrum of the ester shows six signals for six carbon atoms,
Fig. 2. UV–Vis excitation spectra of 2, 3 and 4.

586
A. Bharti et al. / Polyhedron 50 (2013) 582–591
and the E1/2 value [(Epa + Epc)/2] of 1.041 V indicate a quasi-revers-
ible redox process assignable to Ni(II)/Ni(III). However, it is to be
noted that the peak separation (
D
Ep = Epa ꢀ Epc) is quite large as
compared to that expected for a one electron process and hence
it can be concluded that this quasi-reversible redox process is a
coupled chemical reaction to the electrochemical change. The vol-
tammogram also exhibits an additional ill defined oxidation peak
at 1.375 V and a well defined reduction peak at 0.414 V.
6.1. Photoluminescent properties
The photoluminescent properties of complexes 2, 3, 4 and the
Hpptt ligand were examined in the solution state at room temper-
ature (30 °C). The ligand (Hpptt) displays a photoluminescent
emission at 433 nm upon excitation at 287 nm. The main chromo-
sphere of the ligand is the aromatic five-membered triazole ring
and its conjugation degree is further enhanced by the electron-
donating thiol group and the phenyl ring. The photoluminescence
of the Hpptt ligand has been assigned as originating from intrali-
gand (IL) p–
p⁄ transitions. The complexes 2, 3 and 4 exhibit weak
Fig. 3. Photoluminescence emission spectra of 2, 3 and 4.
luminescence properties. In complexes 2 and 4 excitation at 293
and 327 nm at room temperature result in emissions at 435 and
465 nm, respectively (Figs. 2 and 3). Complex 3 exhibits a fluores-
cent property, the emission maximum being observed at 376 nm
upon excitation at 273 nm due to a ligand transition. The photolu-
minescent quantum yields for complexes 2, 3 and 4 measured in
DMSO versus anthracene were found to be 0.60%, 0.34% and
0.33%, respectively. The origin of the emission bands for complexes
2 and 4 may be mainly ascribed to an intraligand emission state
[21]. The violet-blue emission of complex 2 and blue emission of
complex 4 in the solution state imply that these complexes may
be potentially applicable as materials for light emitting diode
devices.
magnetic moment of 2.83 BM and exhibits two d–d bands at 793
and 526 nm assigned to the 3A2g ? 3T2g(F) ( 1) and 3T1g(F) (m2
transitions, respectively, in the distorted octahedral geometry of
complex 4. The other two high energy bands may be assigned to
intraligand/charge transfer transition [20].
m
)
6. Electrochemical studies
The cyclic voltammogram of complex 4 (Fig. 1) obtained at a
Pyrolytic Graphite Electrode (PGE) in DMSO medium using 0.2 M
KCl as the supporting electrolyte in the potential range +1.5 V to
ꢀ1.5 V with a scan rate 50 mV s - 1 exhibits a cathodic peak at
Epc = 0.891 V and the associated anodic peak at Epa = 1.192 V with
respect to the Ag/AgCl reference. The separation between the
6.2. Crystal structure descriptions
The molecular structures of compounds 1, 2, 3 and 4 were
cathodic and anodic peak potentials (
D
Ep = Epa ꢀ Epc) of 0.301 V
determined by single crystal X-ray diffraction. The details of data
Table 1
Crystallographic data and structure refinement for 1, 2, 3 and 4.
Compound
1
2
3
4
Empirical formula
Formula weight
T (K)
C
13H10NS
C28H26HgN10S2
767.32
293(2)
C16H16HgN8O2S2
617.08
295(2)
C16H22N8NiO2S4
545.37
295(2)
254.32
296(2)
K
(Mo K
a
) (Å)
0.71073
0.71073
0.71073
0.71073
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/n
monoclinic
P21/c
monoclinic
P121/c1
monoclinic
P21/n
6.0573(4)
18.3976(13)
11.4326(8)
105.021(4)
1230.51(15)
4
1.373
0.249
528.0
13.638(9)
9.066(6)
23.664(17)
96.624(6)
2906(3)
12.9479(11)
14.1593(9)
11.5205(9)
110.924(10)
1972.8(3)
4
1.487
8.045
1184
11.9679(4)
8.8841(3)
12.0806(4)
113.588(4)
1177.14(7)
2
1.539
1.209
564
b (°)
V (Å3)
Z
4
Dcalc (mg mꢀ3
)
1.754
5.477
1504
l
(mmꢀ1
F(000)
)
h range (°)
Index ranges
2.15–28.80
ꢀ8 6 h 6 8
ꢀ24 6 k 6 24
ꢀ15 6 l 6 15
2509
3.29–29.19
ꢀ17 6 h 6 18
ꢀ10 6 k 6 12
ꢀ32 6 l 6 18
7884
5.12–25.50
ꢀ15 6 h 6 15
ꢀ17 6 k 6 17
ꢀ13 6 l 6 13
5743
5.00–32.68
ꢀ14 6 h 6 17
ꢀ13 6 k 6 12
ꢀ16 6 l 6 17
3912
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
1426
2509/0/163
0.795
0.0463, 0.1216
0.0997, 0.1554
0.155, ꢀ0.159
4812
7884/0/371
0.911
0.0731, 0.0929
0.0422, 0.0783
0.116, ꢀ0.760
4098
5743/0/263
1.067
0.0708, 0.1877
0.1047, 0.2084
1.487, ꢀ1.755
2930
3912/10/158
1.034
0.0379, 0.0774
0.0587, 0.0881
0.288, ꢀ0.277
Final R indices wR2b[I > 2
r(I)](Rint
)
Final Ra indices (all data)
Largest difference in peak/hole (e Åꢀ3
)
a
R1
R2 = [
=
R
R
||Fo| ꢀ |Fc||
R|Fo|.
b
w(|Fo2| ꢀ |Fc2|)2/
R .
w|Fo2|2]1/2

A. Bharti et al. / Polyhedron 50 (2013) 582–591
587
Table 3
Selected molecular dimensions (Å and °) in [Hg(en)(pptt)2] (2).
Bond lengths (Å)
Bond angles (°)
Hg(1)–N(10)
Hg(1)–N(9)
Hg(1)–S(1)
Hg(1)–S(2)
S(1)–C(1)
S(2)–C(14)
N(2)–C(2)
N(2)–N(1)
N(1)–C(1)
N(5)–N(6)
N(5)–C(14)
2.323(5)
2.354(4)
2.451(13)
2.484(14)
1.725(5)
1.732(6)
1.306(6)
1.378(6)
1.322(6)
1.395(6)
1.304(6)
N(10)–Hg(1)–N(9)
N(10)–Hg(1)–S(1)
N(9)–Hg(1)–S(1)
N(10)–Hg(1)–S(2)
N(9)–Hg(1)–S(2)
S(1)–Hg(1)–S(2)
C(1)–S(1)–Hg(1)
C(14)–S(2)–Hg(1)
C(28)–N(10)–Hg(1)
C(27)–N(9)–Hg(1)
N(1)–C(1)–S(1)
75.45(17)
116.44(12)
118.32(12)
111.26(12)
105.38(12)
120.98(5)
97.43(16)
99.13(17)
110.0(4)
108.0(4)
127.1(4)
Table 4
Selected molecular dimensions (Å and °) in [Hg(en)(4-pot)2] (3).
Fig. 4. ORTEP diagram of the ligand Hpptt (1).
Bond length (Å)
Bond angle (°)
Hg(1)–N(2)
Hg(1)–N(1)
Hg(1)–S(1B)
Hg(1)–S(1A)
S(1A)–C(1A)
S(1B)–C(1B)
N(1)–C(1)
C(2)–N(2)
C(2)–C(1)
O(1A)–C(1A)
2.274(15)
2.368(12)
2.428(5)
2.449(4)
1.687(16)
1.749(16)
1.45(2)
1.45(2)
1.49(3)
1.359(17)
N(2)–Hg(1)–N(1)
N(2)–Hg(1)–S(1B)
N(1)–Hg(1)–S(1B)
N(2)–Hg(1)–S(1A)
N(1)–Hg(1)–S(1A)
S(1B)–Hg(1)–S(1A)
C(1A)–S(1A)–Hg(1)
C(1B)–S(1B)–Hg(1)
C(1)–N(1)–Hg(1)
C(2)–N(2)–Hg(1)
75.9(5)
Table 2
Selected molecular dimensions (Å and °) in Hpptt (1).
114.8(3)
116.9(4)
119.8(3)
104.8(3)
117.06(14)
101.3(5)
103.6(5)
108.6(10)
109.3(11)
Bond lengths (Å)
Bond angles (°)
S(1)–C(1)
N(1)–C(2)
N(1)–N(2)
N(2)–C(1)
N(3)–C(1)
N(3)–C(2)
N(3)–C(8)
1.677(2)
1.301(3)
1.366(3)
1.341(3)
1.377(3)
1.391(3)
1.440(3)
C(2)–N(1)–N(2)
C(1)–N(2)–N(1)
C(1)–N(3)–C(2)
C(1)–N(3)–C(8)
C(2)–N(3)–C(8)
N(2)–C(1)–N(3)
N(2)–C(1)–S(1)
104.47(18)
113.83(18)
107.57(17)
125.12(17)
127.29(17)
103.43(18)
128.28(17)
Table 5
Selected bond lengths (Å) and bond angles (°) in [Ni(en)2(5-thot)2] (4).
collection, structure solution and refinement are listed in Table 1.
ORTEP diagrams of compounds 1, 2, 3 and 4 with atom numbering
schemes are shown in Figs. 4, 6, 8 and 10 respectively. Selected
bond lengths and angles are included in Tables 2–5. Hydrogen
bond parameters for complexes 3 and 4 are given in Tables 6 and 7.
Bond length (Å)
Bond angles (°)
Ni–N(12)
Ni–N(11)
Ni–N(1)
S(1)–C(1)
O–C(2)
2.098(14)
2.103(16)
2.137(14)
1.678(18)
1.359(2)
1.389(2)
1.320(2)
1.406(2)
1.284(2)
N(12)–Ni(1)–N(11)
N(12)–Ni(1)–N(11)
N(12)–Ni(1)–N(1)
N(12)–Ni(1)–N(1)
N(11)–Ni(1)–N(1)
N(11)–Ni(1)–N(1)
N(1)–Ni(1)–N(1)
C(2)–O–C(1)
82.62(6)
97.38(6)
87.81(6)
92.19(6)
90.42(6)
89.58(6)
180.0
O–C(1)
6.2.1. Crystal structure description of Hpptt (1)
N(1)–C(1)
N(1)–N(2)
N(2)–C(2)
Fig. 4 shows the molecular structure of Hpptt (1), which is sta-
104.27(27)
108.30(13)
C(1)–N(1)–N(2)
bilized by
pꢁ ꢁ ꢁ
p
interactions and intermolecular C–Hꢁ ꢁ ꢁN and N–
Hꢁ ꢁ ꢁS hydrogen bonding, producing a supramolecular structure
(Fig. 5). The N–Hꢁ ꢁ ꢁS hydrogen bonds are formed between the tri-
azole nitrogen and thione sulfur. In addition to this, there is a C–
Hꢁ ꢁ ꢁS weak interaction between the thione sulfur and phenyl ring
hydrogen. The dihedral angle between the triazole and pyridine
rings is found to be 27.55°, indicating that both rings are tilted with
respect to each other, whereas the dihedral angle between the tri-
azole and phenyl rings is 70.73°, suggesting that the phenyl ring is
almost perpendicular to the triazole ring. The bond lengths and an-
gles for the pyridine, phenyl and triazole rings are normal. The
S(1)–C(1) bond length [1.677(2) Å] is found to be in the range for
carbon sulfur double-bonds [S@C (1.56 Å)] [22] and indicates that
the ligand is present in the thione form. This is further supported
by the adjacent N(1)–C(2) bond length of [1.301(3) Å], which
comes in the range for a C–N single bond [1.310 Å] [23]. The two
triazole rings are tilted (\ACB = 69.23°) and displaced with respect
to each other (Fig. 5). The displacement, as measured by the angle
formed between the ring centroids AB and the ring normal BC to
the triazole plane, is found to be 0.364 Å with a displacement angle
of 16.38°, and the ring centroids contact (Cgꢁ ꢁ ꢁCg) of 3.973 Å is well
within the reported range [24].
two thiolato sulfur atoms of the triazole ligand. The exocyclic
C(1)–S(1) bond distance of 1.726(8) Å is longer as compared to
Hpptt (1) (1.677 Å), indicating the conversion of the thione form
of the ligand to the thiol form and subsequent bonding of the thio-
lato sulfur to Hg(II). The exocyclic C(1)–S(1) bond distance of
1.726(8) Å is smaller as compared to an earlier reported value of
1.82 Å, showing strong Hg–S bonding [25]. The Hg–S distances
are in the range 2.454–2.472 Å (Table 3) which is normal, as re-
ported for Hg–S bonding [26]. The bonding parameters within
the en molecule agree with those of other en complexes of Hg(II)
[27]. The mercury, in a tetrahedral geometry, is bonded to two
nitrogen atoms of an en ligand which is involved in hydrogen
bonding. The elements of the structure are linked together in the
crystal packing via intramolecular N–Hꢁ ꢁ ꢁN interactions between
the pyridine nitrogen atom and NH2 of en. C–Hꢁ ꢁ ꢁS intramolecular
interactions occur between the phenyl ring carbon and thiolato
sulfur, leading to the formation of a supramolecular network
(Fig. 7).
6.2.2. Crystal structure description of [Hg(en)(4-pptt)2] (2)
6.2.3. Crystal structure description of [Hg(en)(4-pot)2] (3)
The molecular structure of complex 2 shows that in the com-
plex unit of [Hg(en)(4-pptt)2] (2), the metal ion is four coordinate
tetrahedral, bonding through two nitrogens of en ligands and
In complex 3, the ligand 5-(4-pyridyl)-1,3,4-oxadiazole-2-thi-
one is situated on an inversion center and Hg is on a two-fold axis.
The C@S bond distance in the complex is larger (1.695 Å) than that

588
A. Bharti et al. / Polyhedron 50 (2013) 582–591
Table 6
Hydrogen bonds for [Hg(en)(4-pot)2] (Å and °) (3).
D–Hꢁ ꢁ ꢁA
D–H (Å)
Hꢁ ꢁ ꢁA (Å)
Dꢁ ꢁ ꢁA (Å)
\D–Hꢁ ꢁ ꢁA (°)
Symmetry equivalent operators
N(1)–H(1C). . .N(3A) #1
N(1)–H(1D). . .N(3B) #2
0.90
0.90
2.14
2.37
3.02(2)
3.221(19)
165.5
158.6
#1 x ꢀ 1, ꢀy + 3/2, z ꢀ 1/2
#2 x + 1, ꢀy + 3/2, z + 1/2
Table 7
Hydrogen bonds for [Ni(en)2(5-thot)2] (Å and °) (4).
D–Hꢁ ꢁ ꢁA
D–H (Å)
Hꢁ ꢁ ꢁA (Å)
Dꢁ ꢁ ꢁA (Å)
\D–Hꢁ ꢁ ꢁA (°)
Symmetry equivalent operators
N(11)–H(11A). . .S(1) #2
N(11)–H(11B). . .O #3
N(12)–H(12A). . .N(2) #1
N(12)–H(12B). . .S(1)
0.90
0.90
0.90
0.90
2.80
2.51
2.40
2.53
3.611(16)
3.311(19)
3.024(2)
149.9
149.2
126.7
142.7
#1 ꢀx + 1, ꢀy + 1, ꢀz + 2
#2 ꢀx + 1/2, y ꢀ 1/2, ꢀz + 3/2
#3 x + 1/2, ꢀy + 1/2, z + 1/2
3.290(16)
Fig. 5. Showing
pꢁ ꢁ ꢁp stacking and the relative shift between two triazole ring centroids and C–Hꢁ ꢁ ꢁN and N–Hꢁ ꢁ ꢁS hydrogen bonding leading to the supramolecular structure.
of the ligand (1.661 Å) (Table 2) because of weakening of the C@S
bond due to its involvement in intramolecular hydrogen bonding
between thione sulfur and the NH2 hydrogen of en. This is in accor-
rings suggests that the extended coplanar ring system of the two
ligands around Hg(II) approaches symmetrically and coordinates
to the metal. In the solid state the complex is stabilized via inter-
molecular N–Hꢁ ꢁ ꢁN interactions between oxadiazole nitrogen
atoms and NH2 hydrogen atoms of the en molecule, leading to
the formation of a symmetrical linear arrangement (Fig. 9). The
pyridine and oxadiazole rings are almost parallel (\ACB = 82.69°),
but area displaced with respect to each other (Fig. 10). The dis-
placement, as measured by the angle formed between the ring cen-
troids AB and the ring normal BC to the oxadiazole plane, is found
to be 0.103 Å, with a displacement angle of 22.88° and the ring cen-
troids contact (Cgꢁ ꢁ ꢁCg) of 3.553 Å is well within the reported
dance with a decrease in m(C@S) in the IR spectrum of the complex,
as discussed earlier. The deviation in bond angle as compared to
the expected value for a regular tetrahedron represents a minor
deviation from the tetrahedral geometry (Table 4). The separation
between the two planes formed by the oxadiazole rings in the
same complex is found to be 1.779 Å. In complex 3 the chelate
and phenyl rings lie nearly in the same plane. The phenyl ring thus
makes an extended coplanar system with the chelate ring. The
dihedral angle of 2.28° formed between the oxadiazole and phenyl
Fig. 6. ORTEP diagram of [Hg(en)(pptt)2] (2).

A. Bharti et al. / Polyhedron 50 (2013) 582–591
589
Fig. 7. N–Hꢁ ꢁ ꢁN interaction between the pyridine nitrogen and NH2 of an en molecule.
Fig. 8. ORTEP diagram of [Hg(en)(4-pot)2] (3).
Fig. 9. Linear arrangement of molecules via N–Hꢁ ꢁ ꢁN interactions.
range [24]. Similarly the two pyridine rings are also almost parallel
(\DFE = 97.72°) but displaced by 0.433 Å with respect to each
other, as measured by the angle formed between the ring centroids
DE and the ring normal EF to the pyridine plane, with a displace-
ment angle of 20.45°. The ring centroids contact (Cgꢁ ꢁ ꢁCg) of
6.2.4. Crystal structure description of [Ni(en)2(5-thot)2] (4)
The molecular structure of 4 shows that in the centro symmetric
unit of [Ni(en)2(5-thot)2] (4) the metal ion is six coordinated, four
equatorial nitrogens of two en and two axial nitrogens of 5-
(thiophen-2-yl)-1,3,4-oxadiazole-2-thiolato anions (Fig. 11). The
elements of the structure are linked together in the crystal packing
via intramolecular N–Hꢁ ꢁ ꢁS interactions between the thione
sulfur and NH2 hydrogen atoms of the en ligand (Fig. 12). The
arrangement of the monomeric [Ni(en)2(5-thot)2] units in the two
3.917 Å is found within the limits of
p p interactions. The values
ꢁ ꢁ ꢁ
of the
pꢁ ꢁ ꢁp interactions suggest stronger interactions between the
pyridine and oxadiazole rings as compared to the pyridine–pyri-
dine rings.

590
A. Bharti et al. / Polyhedron 50 (2013) 582–591
Fig. 10. Showing
p
ꢁ ꢁ ꢁp
stacking and the relative shift between two pyridine–pyridine and pyridine–oxadiazole ring centroids.
Fig. 11. ORTEP diagram of the [Ni(en)2(5-thot)2] complex (4).
Fig. 12. Wave-like arrangement formed by N–Hꢁ ꢁ ꢁO and N–Hꢁ ꢁ ꢁS interactions in [Ni(en)2(5-thot)2] (4).
dimensional architecture along the a axis provide a supramolecular
network (Fig. 12). The dihedral angle between the plane formed by
the oxadiazole and the thiophen ring is 7.35°, indicating that both
rings are almost coplanar. Within the chelate rings formed by two
en ligands, the Ni–N(11) bond distance is 2.1031(16) Å and that
of Ni–N(12) is 2.0981(14) Å (Table 2). The complex involves two
five membered chelate rings, C2N2Ni, with
a bite angle of
82.62(6)°, which represent a minor deviation from the octahedral

A. Bharti et al. / Polyhedron 50 (2013) 582–591
591
[4] (a) G. Turan-Zitouni, Z.A. Kaplancikli, M.T. Yildiz, P. Chevallet, D. Kaya, Eur. J.
Med. Chem. 40 (2005) 607;
geometry. The geometry and bonding parameters within the en
molecule agree with those of other en complexes [28]. The nickel
centre, in D4h symmetry, is bonded to four nitrogen atoms of two
en ligands which offer interesting hydrogen bonding. Weak inter-
molecular N–Hꢁ ꢁ ꢁO and N–Hꢁ ꢁ ꢁS interactions occurring between
the oxadiazole oxygen/thione sulfur and NH2 hydrogen atoms of
en stabilize the structure of complex 4. The two Nꢁ ꢁ ꢁS distances of
3.611(16) and 3.290(16) Å are close to the mean Nꢁ ꢁ ꢁS distance re-
ported for NHꢁ ꢁ ꢁS hydrogen bonds by Shrinivasan and Chacko [29].
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This paper reports on the syntheses and crystal structures of
Hpptt (1) and three new Hg(II) and Ni(II) complexes of 5-(3-pyri-
dyl)-1,2,4-triazole-3-thione, 5-(4-pyridyl)-1,3,4-oxadiazole-2-thi-
one and 5-(thiophene-2-yl)-1,3,4-oxadiazole-2-thione, containing
ethylene diamine as a co-ligand. In [Hg(en)(4-pptt)2] (2) and
[Hg(en)(4-pot)2] (3), the Hg(II) centre has a four coordinate tetra-
hedral arrangement involving two nitrogen atoms of en ligands
and two covalently bonded sulfur atoms from triazole-3-thiolato/
oxadiazole-2-thiolato anions. In [Ni(en)2(5-thot)2] (4), the Ni(II)
centre has a six coordinated octahedral arrangement involving four
nitrogen atoms of en ligands and two nitrogen atoms of oxadiaz-
ole-2-thione anions. The crystal structures of the complexes are
stabilized by various intermolecular and intramolecular hydrogen
bondings. [Ni(en)2(5-thot)2] (4) shows quasi-reversible redox
behavior assignable to a Ni2+/Ni3+ one electron transfer. The photo-
luminescence properties indicate that the complexes are fluores-
cent materials with maximum emissions at 433, 376 and 465 nm
for complexes 2, 3 and 4 at an excitation wavelength of 293, 273
and 327 nm, respectively. All the complexes contain extended
hydrogen bonding, providing supramolecular frameworks.
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One of the authors (R.J.B.) acknowledges the NSF-MRI program
(Grant No. CHE0619278) for funds to purchase the X-ray diffrac-
tometer. Thank are also due to Prof. S.K. Gupta, visiting from School
of Studies in Chemistry, Jiwaji University, Gwalior-474 011, Gwal-
ior, MP, India as a Fulbright-Nehru Fellow in the laboratory of R.J.B.,
who worked on the structure.
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Appendix A. Supplementary data
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Chem. 51 (2012) 370.
CCDC 875784, 864052, 841681 and 841506 contains the sup-
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(2), [Hg(en)(4-pot)2] (3) and [Ni(en)2(5-thot)2] (4), respectively.
These data can be obtained free of charge via http://www.ccdc.ca-
m.ac.uk/conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
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