Synthesis, spectroscopic and crystal structure investigation of [Cu(bzsmp)2Cl2];{bzsmp = 2-benzylsulfanyl-5-(2-methoxyphenyl)-1,3,4-oxadiazole}: cyclization of N2-[bis(benzylsulfanyl)methylene]-2-methoxybenzohydrazide to 2

Article abstract of DOI:10.1016/j.poly.2008.10.016

The ligand N²-[bis(benzylsulfanyl)methylene]-2-methoxybenzohydrazide (N²-bmbh) (1) on reaction with CuCl₂.2H₂O yielded the mononuclear complex [Cu(bzsmp)₂Cl₂] (2) (2-benzylsulfanyl-5-(2-met

Full text of DOI:10.1016/j.poly.2008.10.016

Polyhedron 28 (2009) 195–199
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, spectroscopic and crystal structure investigation of [Cu(bzsmp)2Cl2];
{bzsmp = 2-benzylsulfanyl-5-(2-methoxyphenyl)-1,3,4-oxadiazole}: cyclization
of N2-[bis(benzylsulfanyl)methylene]-2-methoxybenzohydrazide to
2-benzylsulfanyl-5-(2-methoxyphenyl)-1,3,4 -oxadiazole during complexation
M. Singh a, V. Aggarwal b, U.P. Singh b, N.K. Singh a,
*
a Department of Chemistry, Banaras Hindu University, Varanasi 221 005, Uttar Pradesh, India
b Department of Chemistry, Indian Institute of Technology, Roorkee 247 667, Uttarakhand, India
a r t i c l e i n f o
a b s t r a c t
The ligand N2-[bis(benzylsulfanyl)methylene]-2-methoxybenzohydrazide (N2-bmbh) (1) on reaction with
CuCl2.2H2O yielded the mononuclear complex [Cu(bzsmp)2Cl2] (2) (2-benzylsulfanyl-5-(2-methoxy-
phenyl)-1,3,4-oxadiazole, bzsmp). Complex 2 has been characterized by analytical, spectroscopic and
X-ray data. X-ray study of 2 reveals that the cyclized ligand 2-benzylsulfanyl-5-(2-methoxyphenyl)-
1,3,4-oxadiazole (bzsmp) acts as neutral bidentate ligand to form six membered chelate ring and the
Article history:
Received 30 July 2008
Accepted 16 October 2008
Available online 25 November 2008
Keywords:
Cu(II) complex
X-ray
presence of C–Hꢀ ꢀ ꢀ
p
(face to edge) and C–Hꢀ ꢀ ꢀS and two type of C–Hꢀ ꢀ ꢀCl weak interactions.
Ó 2008 Elsevier Ltd. All rights reserved.
1,3,4-Oxadiazole
C–Hꢀ ꢀ ꢀ
p interactions
Cyclization
1. Introduction
an excess ethylenediamine (en) [28,29]. A few papers are available
on the syntheses and X-ray studies of [bis(benzylsulfanyl)methy-
lene]-2-acidhydrazide [30,31] and (benzylsulfanyl)-1,3,4-oxadiaz-
ole [32,33], but for the first time we report here the metal
complex of 2-benzylsulfanyl-5-(2-methoxyphenyl)-1,3,4-oxadiaz-
ole obtained from the cyclization of N2-[bis(benzylsulfanyl)methy-
lene]benzohydrazide (N2-Hbmbh) during complexation. The
cyclization of N2-Hbmbh to 2-benzylsulfanyl-5-(2-methoxy-
phenyl)-1,3,4-oxadiazole has been performed under normal condi-
tion without the use of any other reagents viz. base or acid as
already described in the case of other oxadiazoles. The spectro-
scopic and single crystal X-ray studies of the resulting Cu(II)
complex obtained by the reaction of Cu(II)chloride and
N2-[bis(benzylsulfanyl)methylene]-2-methoxy-benzohydrazide
[N2-Hbmbh] is reported in the present paper.
The 1,3,4-oxadiazoles have found applications in several areas
[1,2] such as anti tuberculostatic, anti-inflammatory, analgesic,
antipyretic, anticonvulsant and medicine [3–6]. The syntheses of
this type of heterocyclic compounds have attracted considerable
attention and a number of methods have been used for their syn-
theses [7–9]. The widely used strategy involves the dehydrative
cyclization of diacylhydrazides usually with strong acids [10–13].
The cyclization of potassium salt of 3-benzoyldithiocarbazate into
the corresponding 5-phenyl-1,3,4-oxadiazole-2-thiol was first re-
ported by Hoggarth [14]. The cyclization of 3-acyldithiocarbazates
esters, N-aroyldithiocarbazates and their salts to 1,3,4-oxadiazoles
in the presence of a base is also reported in the literature [15–18].
Several other methods are reported for the synthesis of oxadiazoles
from acyclic precursor which includes oxidative cyclization of acyl-
hydrazones [19], acylthiourea [20–22] and acylthiosemicarbazides
[23–26]. Platinum assisted cyclization of S-methyl 3-acyl-2-meth-
yldithiocarbazates under mild conditions was reported by
Bergamini and co-workers [27]. Earlier we have reported the metal
assisted cyclization of N-benzoyldithiocarbazate to 5-phenyl-1,3,4-
oxadiazole-2-thiol and potassium[N0-(pyridine-3-carbonyl) hydra-
zine to 5-(3-pyridyl)-1,3,4-oxadiazole-2-thione in the presence of
2. Experimental
2.1. Materials and methods
Commercial reagents were used without further purification
and all experiments were carried out in open atmosphere. Methyl
2-methoxy benzoate (Sigma Aldrich, USA) and hydrazine hydrate
(S.D. Fine Chemicals, India) were used as such. All solvents
were purchased from Merck Chemicals, India and used after
purification.
* Corresponding author. Tel.: +91 542 2318529; fax: +91 542 2368127.
E-mail addresses: nksingh@bhu.ac.in, singhnk_bhu@yahoo.com (N.K. Singh).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.10.016

196
M. Singh et al. / Polyhedron 28 (2009) 195–199
2.2. Preparation of the ligand N2-Hbmbh (1)
The ligand N2-Hbmbh was synthesized as described previously
[30].
2.3. Preparation of the complex [Cu(bzsmp)2Cl2] (2)
A solution of N2-Hbmbh (1) (2 mmol) in MeOH (15 ml) was
added to a solution of CuCl2 ꢀ 2H2O (1 mmol) in 10 ml of MeOH
and the mixture was stirred for 2 h at room temperature. The
resulting green color solution was filtered off and kept for crystal-
lization. Green Crystals of 2 suitable for X-ray analyses were ob-
tained by slow evaporation of its methanolic solution over a
period of 21 days. Yield 64%; m.p. 145 °C. Anal. Calc. for
C32H28N4CuO4S2Cl2 (731.17): C, 52.57; H, 3.86; N, 7.66. Found: C,
52.42; H, 4.12; N, 7.60%. I.R. data (
m
m
cmꢁ, 1 KBr)
(C–S) 869 m;
m(Cu–Cl) 325.
m
(C@N) 1603 s;
(CH2) 2979;
m
m
(C–O–C) 1254 w;
m
(N–N) 1133 s;
m
(Cu–N) 478, (Cu–O) 420,
m
2.4. Physical measurements
Carbon, hydrogen and nitrogen contents were estimated on a
CHN Analyser Model CE-440. Magnetic susceptibility measure-
ment was performed at room temperature on a Cahn Faraday bal-
ance using Hg[Co(NCS)4] as the calibrant and electronic spectra
were recorded on UV-1700 Pharmaspec spectrophotometer. IR
spectra were recorded in the 4000–400 cmꢁ1 region as KBr pellets
on a Varian Excalibur 3100 FT-IR spectrophotometer.
3. Crystal structure determination
Fig. 1. Molecular structure of [Cu(bzsmp)2Cl2] with atomic numbering scheme.
Color code: C, grey; H, orange; N, blue; O, red; S, purple; Cl, green; Cu, cyan. (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
The X-ray data collection and processing for 2 were performed
on Bruker Kappa Apex-CCD diffractometer by using graphite
monochromated Mo Ka radiation (k = 0.71070 Å) at 273(2) K. Crys-
tal structure was solved by direct methods. Structure solution,
refinement and data output were carried out with the SHELXTL pro-
gram [34,35]. All non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms were placed in geometrically calculated
positions by using a riding model. Images were created with the
DIAMOND program [36]. Hydrogen bonding interactions in the crystal
lattice were calculated with both SHELXTL and MERCURY [37].
Table 1
Crystal refinement parameters of [Cu(bzsmp)2Cl2] (2).
Parameters
[Cu(bzsmp)2Cl2]
Color
Green
Crystal size (mm)
Empirical formula
Formula weight
Space group
Crystal system
T (K)
0.24 ꢂ 0.21 ꢂ 0.18
C32H30N4CuO4S2Cl2
733.19
P21/n
monoclinic
273(2)
4. Results and discussion
The [Cu(bzsmp)2Cl2] [bzsmp = 2-benzylsulfanyl-5-(2-methoxy-
phenyl)-1,3,4-oxadiazole] was obtained by stirring the methanolic
solution of N2-Hbmbh (1) and CuCl2 ꢀ 2H2O in a 2:1 molar ratio at
room temperature. A ring closure reaction takes place due to cleav-
age of C–S bond followed by elimination of PhCH2SH during com-
plexation resulting in the formation of complex 2 (Scheme 1). The
green solution obtained after the reaction was filtered off and on
k (Mo K
a
)
0.71073
a (Å)
b (Å)
c (Å)
13.0027(17)
8.2990(12)
15.634(2)
90
93.496(9)
90
1683.9(4)
16
1.442
a
(°)
b (°)
c
(°)
V (Å3)
Z
qcalc (Mg/m3)
V (mmꢁ1
F(000)
)
0.973
1344
S
S
S
h Range for data collection
Index ranges
1.98–40.01
ꢁ22 6 h 6 23,
ꢁ13 6 k 6 15,
ꢁ24 6 l 6 28
38153
10340
10340/0/206
0.817
0.0353, 0.1017
0.0736, 0.1367
0.533 and ꢁ0.714
CuCl2(H2O)2
O
N
Cl
O
N
MeOH, -HSCH2Ph
O
N
Cu
Cl
N
O
NH
O
N
Reflections collected
S
O
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
R1, wR2
R1, wR2, (all data)
Largest difference in peak and hole (e Åꢁ3
)
Scheme 1. Preparation of complex [Cu(bzsmp)2Cl2].

M. Singh et al. / Polyhedron 28 (2009) 195–199
197
slow evaporation of the solvent the resulting complex 2 crystal-
lized. Crystals of complex 2 were obtained after three weeks in
64% yield. Complex 2 has been structurally characterized by single
crystal X-ray. The molecular structure of 2 is shown in Fig. 1. The
physical and analytical data of compound 2 supports the X-ray
structure.
bands at 3252, 2969, 1658, 1071, 1598 and 877 which correspond
to
m
(NH), d(CH2),
m
(CO),
m
(N–N),
m
(C@N) and
(CO) are absent in 2 and a
(C–O–C) which suggests
m(C–S), respectively
[30]. The bands due to
m
(NH) and
m
new band appears at 1254 cmꢁ1 due to
m
cyclization [28,29] of acyclic {–C(O)–NH–N@C@} moiety by elimi-
nation of HSCH2C6H4 during complexation with Cu(II). Thus I.R.
data of 2 supports cyclization of 1 and formation of 1,3,4-oxadiaz-
4.1. IR spectra
ole moiety in the complex. A positive shift of 62 cmꢁ1 in
m(N–N) as
compared to the ligand suggests the involvement of this group as a
A comparative study of the IR spectral data of the complex with
ligand 1 gives meaningful information regarding bonding sites and
its cyclization during complexation. The IR spectrum of 1 shows
bonding site.
4.2. UV spectra
The electronic spectrum of the ligand N2-Hbmbh shows bands
at 337, 252 and 213 nm assigned to the n?p* and
p?
p* transi-
Table 2
tions. The magnetic moment of 1.82 B.M. for 2 is normal as ex-
pected for the presence of one unpaired electron. The complex
[Cu(bzsmp)2Cl2] displays electronic bands at 557, 316, 297,
257 nm. A Cu(II) complex with d9 configuration is expected to
Selected bond lengths for the complex [Cu(bzsmp)2Cl2] (Å) and angles (°).
Copper coordination sphere
Bond length
Cu(1)–N(1)
Cu(1)–Cl(1)
O(1)–C(3)
C(3)–C(4)
1.9999(10)
2.2830(4)
1.3612(16)
1.4018(18)
Cu(1)–O(1)
C(4)–C(19)
N(1)–C(19)
2.5080(4)
1.4532(18)
1.3013(15)
experience Jahn–Teller distortion which leads to further splitting
2
of the 2Eg and T2g levels and give rise to the 2B1g ? 2A1g
(m1),
(m m3) transitions which is expected to be close in energy
2B2g 2), 2Eg (
and generally appear as a broad band. Therefore the broad band at
557 nm is assigned to the envelope of 2B1g ? 2A1g 2B2g and 2Eg
Angles
O(1)–C(3)–C(4)
N(1)–C(19)–C(4)
C(3)–O(1)–Cu(1)
116.41(12)
131.88(10)
124.18(12)
C(19)–N(1)–Cu(1)
C(3)–C(4)–C(19)
132.65(8)
121.30(11)
,
transitions [38]. The strong absorptions found for complex 2 in
the range 316–257 nm is due to the ligand to copper(II) charge
transfer and intraligand transitions.
Oxadiazole ring
Bond lengths
O(1)–C(18)
N(1)–N(2)
N(1)–C(19)
S(1)–C(15)
1.4280(17)
1.3986(14)
1.3013(15)
1.7268(12)
O(2)–C(15)
O(2)–C(19)
N(2)–C(15)
S(1)–C(17)
1.3607(17)
1.3631(14)
1.2929(15)
1.8285(17)
4.3. Crystal structure of complex 2
The crystal structure refinement data related to complex 2 are
given in Table 1 and the main bond distances and bond angles of
2 are given in the Table 2. The hydrogen bonds are listed in Table
3. In the centrosymmetric mononuclear [Cu(bzsmp)2Cl2], each
Cu(II) ion is located at the inversion center. The six membered
[CuO1C3C4C19N1] chelate ring is not coplanar (rms deviation from
mean plane = 0.207 Å) but the two six membered chelate rings are
parallel to each other separating a distance of 0.532 Å. Two ligands
coordinate to Cu(II) center in meridonial fashion i.e. the carbonyl
oxygen and hydrazinic nitrogen are cis to each. The bite angle for
both C3NCuO chelate rings is 75.28°. Cu(II) is bonded octahedrally
with two methoxy oxygens, two oxadiazole nitrogens and two
chlorine atoms with Cu–N, Cu–O and Cu–Cl distances of
1.9999(10) Å, 2.5080(4) Å, 2.2830(4) Å}, respectively (Fig. 1). Thus
the ligand behaves as a neutral bidentate chelating ligand (Scheme
1). The Cu–N bond length in complex 2 agrees well with those
Angles
N(2)–C(15)–S(1)
C(15)–S(1)–C(17)
N(1)–Cu(1)–N(1)#1
Cl(1)–Cu(1)–Cl(1)#1
129.42(11)
99.99(7)
180.00(6)
180.00(12)
C(7)–C(17)–S(1)
O(2)–C(15)–S(1)
N(1)–Cu(1)–Cl(1)#1
N(1)#1–Cu(1)–Cl(1)#1
N(1)#1–Cu(1)–Cl(1)
114.95(11)
117.10(9)
89.31(3)
90.69(3)
89.31(3)
Symmetry transformations used to generate equivalent atoms: #1 ꢁx + 2, ꢁy + 1,
ꢁz + 1.
Table 3
Hydrogen bonds for Cu(bzsmp)2Cl2 (Å and °).
D–Hꢀ ꢀ ꢀA
d(D–H)
d(Hꢀ ꢀ ꢀA)
d(Dꢀ ꢀ ꢀA)
\(DHA)
C(6)–H(6)ꢀ ꢀ ꢀS(1)
0.93
0.97
2.970(3)
3.766(3)
144.57
141.70
C(17)–H(17A)ꢀ ꢀ ꢀCl(1)
2.900(12)
3.710(12)
Fig. 2. One dimensional chain formed by C–Hꢀ ꢀ ꢀ
p interaction (brown dotted) in the complex. Color code: C, grey; H, orange; N, blue; O, red; S, purple; Cl, green; Cu, cyan. (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

198
M. Singh et al. / Polyhedron 28 (2009) 195–199
found for compounds e.g. [Cu(2-PBZM)MoO4] {2-PBZM = 2-(2-pyr-
idyl)benzimidazole) [39] and [Cu2(EGTA)(Him)2]n, [Cu2(EGTA)
(Him)4] ꢀ 4H2O {Him = imidazole, EGTA = ethylenebis(oxyethylni-
trilo)tetraaceticacid)}. The Cu–O bond length is comparably shorter
than the sum of van der Waal’s radii of oxygen and copper atoms
(1.50 + 1.40 = 2.90 Å), indicating comparatively strong Cu–O bond-
ing [40]. The 2-methoxybenzene and oxadiazole rings make a dihe-
dral angle of 23.57°. Similarly, the dihedral angle between benzyl
and oxadiazole rings is 57.46°. The plane containing the oxadiazole
ring equally divides the 2-methoxy benzene ring in equal halves.
The crystal packing of 2 reveals the existence of C–Hꢀ ꢀ ꢀ
and C–Hꢀ ꢀ ꢀCl intermolecular interactions. There are two C–Hꢀ ꢀ ꢀ
interactions, one occurring between a hydrogen atom of the meth-
ylene group and the -electrons of the oxadiazole ring and the sec-
ond one between other hydrogen of the same methylene group and
the -electrons of benzyl group from the adjacent molecule with a
distance of 2.953(6) and 3.083(5) Å, respectively. These two C–
interactions provide one dimensional chain (Fig. 2). Addi-
p
, C–Hꢀ ꢀ ꢀS
p
p
p
Hꢀ ꢀ ꢀp
tionally, two molecules of the complex are also linked by C–Hꢀ ꢀ ꢀS
interaction (2.970(3) Å) formed between C–H of 2-methoxy phenyl
Fig. 3. One dimensional chain formed by C–Hꢀ ꢀ ꢀS interaction (orange dotted) in complex. Color code: C, grey; H, orange; N, blue; O, red; S, purple; Cl, green; Cu, cyan. (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Three-dimensional packing of complex [Cu(bzsmp)2Cl2] showing C–Hꢀ ꢀ ꢀCl interactions (green dotted). Color code: C, grey; H, orange; N, blue; O, red; S, purple; Cl,
green; Cu, cyan. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

M. Singh et al. / Polyhedron 28 (2009) 195–199
199
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ring and sulfur atom which extend in one dimensional chain
(Fig. 3). The three dimensional packing in the complex is also sta-
bilized by two types of C–Hꢀ ꢀ ꢀCl interactions (one between hydro-
gen atom of methoxy phenyl ring and chlorine atom and other
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5. Conclusion
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We have reported that acyldithocarbazates derivatives undergo
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for the cleavage of C–S bond leading to the elimination of
C6H5CH2SH and cyclization of the cleaved product. As a conclusion
of this work we describe a very facile route to obtain 2-ben-
zylsulfanyl-1,3,4-oxadiazole as a stable Cu(II) complex.
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Financial support [01(2152)07/EMR-II] from Council of Scien-
tific and Industrial Research (CSIR), New Delhi, India is gratefully
acknowledged. One of the authors (M. Singh) is thankful to CSIR,
New Delhi, India for the award of SRE, supported by Grant No. 9/
13(160) 2008/EMR-I and [01 (2152) 07/EMR-II].
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Appendix A. Supplementary data
CCDC 685385 contains the supplementary crystallographic data
for 1. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2008.10.016.
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