Unprecedented formation of a μ-oxobridged polymeric copper(II) complex: Evaluation of catalytic activity in synthesis of 5-substituted 1H-tetrazoles

Article abstract of DOI:10.1016/j.jorganchem.2018.06.004

The reaction of CuCl₂·2H₂O with Schiff base ligand, 4-[(2-hydroxy-3-methoxybenzylidene)amino-1,5-dimethyl-2-phenyl-1H-pyrazol-3-one] (hmdpH), in 1:1 molar ratio led to a novel and unprecedented oxo-bridged polymeric copper (II) complex, [Cu(μ-O) (hmdp)]_n. The complex was isolated as crystalline solid and characterized by FTIR, UV–visible and EPR spectroscopic techniques. The molecular structure of the complex was also determined by single crystal X-ray diffraction studies. The formation of the complex is unique and unprecedented in the sense that one of the CH₃ group of the 4-aminoantipyrene unit of the Schiff base ligand is oxidized in situ to CH₂OH, during the complex formation. The catalytic potential of the complex has been demonstrated in the synthesis of a series of 5-substituted 1H-tetrazoles via [3 + 2]-cycloaddition reactions of substituted benzonitriles and sodium azide in ethylene glycol.

Full text of DOI:10.1016/j.jorganchem.2018.06.004

Accepted Manuscript
Unprecedented formation of a μ-oxobridged polymeric copper(II) complex: Evaluation
of catalytic activity in synthesis of 5-substituted 1H-tetrazoles
Samaresh Layek, Rakesh Ganguly, Devendra D. Pathak
PII:
S0022-328X(18)30319-X
10.1016/j.jorganchem.2018.06.004
JOM 20463
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 30 April 2018
Revised Date: 31 May 2018
Accepted Date: 4 June 2018
Please cite this article as: S. Layek, R. Ganguly, D.D. Pathak, Unprecedented formation of a μ-
oxobridged polymeric copper(II) complex: Evaluation of catalytic activity in synthesis of 5-substituted 1H-
tetrazoles, Journal of Organometallic Chemistry (2018), doi: 10.1016/j.jorganchem.2018.06.004.
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ACCEPTED MANUSCRIPT
Graphical Abstract:

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Unprecedented formation of a
µ
-oxobridged polymeric copper(II) complex:
Evaluation of catalytic activity in synthesis of 5-substituted 1H-tetrazoles
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Samaresh Layeka, Rakesh Gangulyb, Devendra D. Pathaka*
a Department of Applied Chemistry, Indian Institute of Technology (ISM), Dhanbad-826004, India
b Division of Chemistry & Biological Chemistry, Nanyang Technological University, Singapore-639798
Email: ddpathak@yahoo.com
*Phone number: +91 9431126250
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Abstract
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The
methoxybenzylidene)amino-1,5-dimethyl-2-phenyl-1H-pyrazol-3-one] (hmdpH), in 1:1 molar
ratio led to a novel and unprecedented oxo-bridged polymeric copper(II) complex, [Cu(
reaction
of
CuCl2.2H2O
with
Schiff
base
ligand,
4-[(2-hydroxy-3-
µ-
O)(hmdp)]n. The complex was isolated as crystalline solid and characterized by FTIR, UV-
visible and EPR spectroscopic techniques. The molecular structure of the complex was also
determined by single crystal X-ray diffraction studies. The formation of the complex is unique
and unprecedented in the sense that one of the CH3 group of the 4-aminoantipyrene unit of the
Schiff base ligand is oxidized in situ to CH2OH, during the complex formation. The catalytic
potential of the complex has been demonstrated in the synthesis of a series of 5-substituted 1H-
tetrazoles via [3+2]-cycloaddition reactions of substituted benzonitriles and sodium azide in
ethylene glycol.
Keywords
Schiff base; polymeric copper(II) complex; crystal structure; [3+2]-cycloaddition reaction; 5-
substituted 1H-tetrazoles
1. Introduction
Among various N-heterocycles, tetrazoles are the most stable five membered ring comprising of
four nitrogen atoms. In general, heterocyclic compounds have been used as excellent ligands in
coordination chemistry [1]. Majority of heterocycles also exhibit interesting
biological/pharmaceutical activity [2]. Owing to lower toxicity and higher lipophilicity of
tetrazoles, they have been used as isosteric replacement in drugs [3], antidiabetics [4], antibiotics
[5] and HIV-inhibitors [6].
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Because of the widespread range of applications of tetrazole derivatives, several preparative
methods for synthesis of tetrazole derivatives are described in the literature [7]. The most
common method to synthesize a tetrazole is the [3+2] cycloaddition reaction between an organic
nitrile and an azide in presence of an acid [8]. However, during the last few years, the synthesis
of 5-substituted 1-H terazoles have been reported using a large number of homo- and
heterogeneous catalysts, such as ZnO [9], ZnBr2 [10] Zn(OTf)2 [11], Cu2O [12], Fe(OAc)2 [13],
InCl3 [14], AgNO3 [15], Cu(OTf)2 [16], Pd(PPh3)4 [17], CuSO4.5H2O [18], Cu(NO3)2. 3H2O [19],
In(OTf)3 [20], silica-supported ZnS nanospheres [21], Cu-Zn alloy nanopowder [22], Zn/Al
hydrotalcite [23], Mw-Pd/Co Nanoparticles [24], zinc hydroxyapatite [25], nano-ZnO/Co2O3
[26a], ZnO nanoflakes [26b], CoY zeolite [27], Pt NPs@AC [28a], CAES [28b], CuFe2O4
nanoparticles [29], and Ln(OTf)3-SiO2 [30], [Pd-SMTU@boehmite] [31a], [Pd-SBT@MCM-41]
nanoparticles [31b], monodisperse carbon black decorated Pt nanoparticles [32a], and Pt
NPs@rGO [32b] etc. from an organic cyanide and sodium azide. Although most of these
methods are worthwhile, many of the prevailing methods suffer from one or more of the
drawbacks such as long reaction time, elevated temperature, low yield, harsh reaction conditions,
tedious workup and use of hazardous solvents. In some cases, where alkyltin-based reagents are
used as co-catalysts, toxic hydrazoic acid is formed [33] as by product which is a major concern
from green chemistry point of view. In this respect, a more environmental-friendly, stable, and
easy to handle cheap catalytic systems are desirable.
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By virtue of good thermal and chemical stability, simple and easy synthesis, Schiff bases
are recognized as suitable ligands to develop new catalysts with various metals [34].
Consequently, Schiff base metal complexes have played a pivotal role in catalysis and find
miscellaneous industrial applications [35]. Motivated by these results and our on-going research
interest in developing air- and moisture-stable transition metal-based catalysts for various
organic transformations [36], we report herein the synthesis and characterization of a novel and
an unprecedented oxo-bridged polymeric copper(II) complex [Cu(µ-O)(hmdp)]n, and its catalytic
application in synthesis of a wide range of 5-substituted 1H-tetrazoles by the reaction of various
substituted benzonitriles and sodium azide involving a [3+2]-cycloaddition.
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2. Results and Discussion
2.1. Synthesis and characterization
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The Schiff base ligand (hmdpH) was synthesized in 81% yield by the reported method [37] and
isolated as yellow crystalline solid, m.p. 205-207oC. The drop-wise addition of a clear solution of
CuCl2.2H2O (0.086 mg, 0.50 mmol) in 5 mL methanol to methanolic suspension of the ligand
hmdpH (0.168 g, 0.50 mmol) at room temperature resulted in the precipitation of reddish brown
powder (Scheme 1). The complex was found to be insoluble in benzene, toluene,
dichloromethane and water but soluble in methanol, DMF and DMSO. Initially, the complex was
characterized by FTIR, UV-visible, and EPR spectroscopy. Due to ambiguity about the structure,
single crystals of the complex, suitable for X-ray crystallography were grown from a saturated
solution in DMF on prolonged standing over three weeks. Moreover, the molecular structure of
the complex was resolved by single crystal X-ray crystallography which confirmed the formation
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of a
µ-oxo bridged polymeric complex. The formation of the complex is unique and
unprecedented in the sense that one of the CH3 group of the 4-aminoantipyrene unit of the Schiff
base ligand is oxidized in situ to CH2OH, during the complex formation. At present, the exact
mechanism of this structural change in the ligand in presence of Cu2+ in solution, is ambiguous
but the source of oxygen is likely to be the atmospheric air [38].
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Scheme 1. Synthesis of polymeric copper (II) complex
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The FTIR spectrum of the Schiff base ligand exhibited three prominent peaks at 1665
cm-1, 1593 cm-1 and 1242 cm-1 corresponding to ν (C=O), ν (C=N) of azomethine group and OH
in plane bending, respectively (Fig. S1). The data is consistent with the literature [37]. The FTIR
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spectrum of the complex exhibited two prominent peaks at 1659 cm-1 and 1579 cm-1 due to
ν (C=O), and ν (C=N), respectively. A comparison of the FTIR spectra of the complex and the
ligand clearly indicated that the ν (C=N) stretching frequency was shifted to lower wavenumber
by 14 cm-1 (i.e. from 1593 cm-1 in free ligand to 1579 cm-1 in the complex) which confirmed the
coordination of the nitrogen atom of the azomethine group of the ligand to the metal center [39].
The C=O stretching frequency of the pyrazole ring of the Schiff base ligand was shifted to lower
wavenumber by a magnitude of 06 cm-1 only, upon complexation. This indicated the
coordination of the C=O group of pyrazole ring of the Schiff base ligand to metal ion to form a
polymeric chain. The disappearance of OH (in plane bending) peak of the ligand in the FTIR
spectrum of the complex also substantiated the deprotonation of the hydroxyl group and the
emergence of a new peak at 547 cm-1 due to ν Cu-O indicated the coordination of the phenolic
oxygen to the copper [40].
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The UV-vis absorption spectra of the ligand and copper(II) complex measured in
methanol at room temperature in the range of 200-1000 nm are depicted in Fig. S2. In the UV-
vis spectrum of Schiff base ligand, four distinct absorption bands were observed at 235, 284, 335
and 414 nm. The bands at 235 and 284 nm were attributed due to π-π transition of the aromatic
ring and the bands at lower energies (at 335 and 413 nm) were associated with the n-π* of the
azomethine group [41]. Upon coordination to Cu(II), all bands were shifted to higher
wavelengths due to LMCT which confirmed that the nitrogen of the azomethine was involved in
the coordination with Cu(II) ion [41-42]. The presence of an additional broad band at 695 nm in
the UV spectrum of the complex may be ascribed to the forbidden d-d transitions of copper and
indicated the distorted square pyramidal geometry of the complex. The d-d transitions in copper
(II) complexes have been reported to be of low energy due to the Jahn-Teller distortion [42].
The EPR spectrum of complex, recorded at room temperature, is shown in Fig. 1. The
EPR spectrum of the powdered complex revealed an isotropic behavior with sharp signal and
without any hyperfine splitting. The giso value was calculated to be 2.14 which indicated a
distorted square pyramidal geometry of the complex [43].
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Fig 1. EPR spectrum of copper(II) complex
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2.2. Description of the crystal structure of [Cu(µ-O)(hmdp)]n
Diffraction quality crystals of the complex were grown by slow evaporations of DMF solution of
the complex and structure of the complex was further confirmed by X-ray crystallography. An
ORTEP drawing of the complex is shown in Fig. 2. A summary of the crystallographic and
refinement data of complex is given in Table 1 and selected bond lengths and bond angles are
given in Table S1. X-ray analysis reveals that complex crystallizes in monoclinic system, having
space group P 1 21/c 1. The structure analysis of the complex shows that complex is a one
dimensional polymer built up from asymmetric monomer units in which Cu(II) ion is surrounded
by five donor atom. Copper atoms centrally bound through two bridging oxygen atoms with Cu-
O bond length of 1.938(2) Å. The asymmetric units are further bridged by oxygen atom of C=O
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group of pyrazole ring of the Schiff base ligand, forming the polymeric chain in copper complex
.
Interestingly, the CH3 group of 4-aminoantipyrene analogue of Schiff base ligand has been
oxidized to CH2OH, which bridges with two copper atoms via deprotonation. In the complex, the
azomethine nitrogen atom and oxygen of orthovanilate are coordinating to the copper atoms
forming two six-membered chelate ring, having Cu1-O1 and Cu1-N1 bond length 1.884(2) Å
and 1.947(3) Å, respectively. The inter-atomic Cu1-Cu1A distance and Cu1-O301-Cu1 bridge
angle in complex are 3.0101(8) Å and 102.83(10)o, respectively, which are in close agreement
with the values reported for other similar type of copper complexes containing two and one-atom
bridging ligands [44]. The four-membered Cu2(µ-O)2 unit is co-planar with a Cu1-O301-Cu1-
O301 torsion angle of 0.00(8)o. Thus, each copper center in polymeric chain, is ligated by one
azomethine nitrogen atom, two bridging oxygen atoms, one oxygen atom of orthovaniline
moities and oxygen atom of C=O group of pyrazole ring forming five coordinated distorted
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square-pyramidal geometry around each of the copper atoms. The coordination geometry around
the Cu(II) atom is a distorted square pyramidal as structure index τ value of 0.12 as defined by
Addison et al. [45] (The structure index is defined as τ = (β-α)/60, where α and β are the largest
coordination angles; τ = 0 for an ideal square pyramidal and τ = 1 for a trigonal bipyramidal).
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Fig. 2. ORTEP diagram of the polymeric copper chain (top); binuclear unit of polymeric
copper(II) complex (bottom right side) and asymmetric unit of the polymeric complex (bottom
left side). Non-H atoms are shown with displacement ellipsoids at the 50% probability level (red:
oxygen; blue: nitrogen; magenta: copper; grey: carbon)
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Table 1. Crystal data and refinement parameters of copper(II) complex
Crystal data
Chemical formula
Formula weight
C19H17CuN3O4
414.90
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Temperature
153(2) K
Wavelength
0.71073 Å
Crystal size
0.08 x 0.12 x 0.22 mm
Crystal system
Space group
monoclinic
P 1 21/c 1
Unit cell dimensions
a = 7.8548(5) Å;
b = 12.6281(5) Å
c = 17.1032(8) Å;
α
β
= 90°;
= 97.088(2)°;
= 90°
γ
Volume
1683.52(16) Å3
Z
4
Density (calculated)
Absorption coefficient
F(000)
1.637 Mg/cm3
1.330 mm-1
852
Data collection and structure refinement
Theta range for data collection
Index ranges
2.40 to 26.50°
-9<=h<=9, -15<=k<=14, -18<=l<=21
11996
Reflections collected
Independent reflections
Function minimized
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices
3495 [R(int) = 0.0948]
2
Σ
w(Fo - Fc2)2
3495 / 6 / 252
1.010
2556 data; I>2
σ(I)
R.M.S. deviation from mean
0.119 eÅ-3
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Catalytic activity of copper(II) complex in the synthesis of 5-substituted 1H tetrazole:
After synthesis and characterization of the copper(II) polymeric complex, its catalytic activities
was examined in the synthesis of 5-substituted 1H tetrazole via [3+2]-cycloaddition reaction of
nitriles with sodium azide (Table 2 & Table 3).
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In order to optimize the appropriate reaction condition, a model reaction of benzonitrile
(1.0 mmol) and sodium azide (1.2 mmol) was carried out and various reaction parameters such
as, catalyst loading, solvents, temperature and time were studied. The results are summarized in
Table 2. When the reaction was carried out in the absence of the catalyst in ethylene glycol at
120oC for 24 h, no product was obtained (Table 2, entry 1). After that, the reaction was carried
out in presence of 0.003 g copper(II) complex at 120oC for 3 h, product was obtained in 79%
yields (Table 2, entry 2). Further screening of the catalyst loading showed that, when the reaction
was carried out with 0.005 g copper(II) complex, the desired product was obtained in 91% yield
within 3 h at 120oC (Table 2, entry 5). Increasing the amount of catalyst (0.008 g) had no
beneficial effect on the yield of the product (Table 2, entry 7). Thus, it was concluded that 0.005
g loading of the catalyst was optimal (Table 2, entry 5). After optimization of the catalyst
loading, the effect of solvents in the reaction was also investigated by conducted in various
solvents such as water, THF, DMF, DMSO, ethanol, methanol etc. (Table 2, entries 8-13).
Though DMF and DMSO gave satisfied yield of the product (Table 2, entries 8-9), considering
the green chemistry approach, the all reactions were carried out in environmentally benign
ethylene glycol as green solvent and it gave 91% yield of the product (Table 2, entries 5-7). To
investigate the effect of temperature, the reaction was performed at 90oC, 120oC and 140oC, the
best result was obtained at 120oC (Table 2, entries 3-6). Thus, the optimal reaction conditions
and best yield (91%) was achieved in the presence of 0.005 g of copper(II) catalyst in ethylene
glycol as solvent at 120oC within 3 h.
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Table 2. Optimization of reaction parameters for the reaction of benzonitrile with sodium azide
Entry Catalyst loading (g)
Solvent
Temp
(oC)
120
120
90
Time (h)
Yield
(%)
-
1
2
3
4
------
0.003
0.003
0.003
Ethylene glycol
Ethylene glycol
Ethylene glycol
Ethylene glycol
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3
79
3
43
120
3
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0.005
0.005
0.008
0.005
0.005
0.005
0.005
0.005
0.005
Ethylene glycol
Ethylene glycol
Ethylene glycol
DMSO
120
140
120
120
120
65
3
3
91
90
6
7
3
91
8
3
89
9
DMF
3
84
10
11
12
13
CH3OH
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12
12
12
53
CH3CH2OH
H2O
80
50
100
100
trace
15
THF
1
2
After optimizing the reaction conditions, the scope and applicability of the catalyst was
examined to various substituted benzonitrile with sodium azide. The results are summarized in
Table 3. As variety of benzonitrile bearing electron-donating and electron-withdrawing
substituents were successfully employed and the corresponding 5-substituted 1H-tetrazole
derivatives are obtained in good to excellent yields (Table 3, entries 2-10). But benzonitrile
having electron-donating substituents require longer reaction time than benzonitrile comprising
electron-withdrawing groups. For instance, -NO2, -Br, -Cl and also -COCH3, that incorporate
electron-withdrawing communities (Table 3, entries 3-7 & 8), react more quickly in better yields
compared to the nitriles made up of electron-releasing groups like -OCH3, -OH (Table 3, entries
2, 8 & 9) and these observations may be due to the electrophilic nature of the benzonitrile i.e. the
presence of electron withdrawing groups increases the polarity of the cyanide group inductively
[46]. The substituent at o-position (Table 3, entry 9) and substituent at m-position (Table 3, entry
5) gave lower yield than p-position (Table 3, entries 4 & 8), presumably due to steric effect. The
products were obtained as white to off-white crystalline solid and fully characterized by melting
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point, FTIR and H NMR (Fig. S3-S12). Furthermore, the catalytic activity of the synthesized
complex was also compared with some of the previously reported catalyst [15,19,26b,27,31a].
The results show that our catalytic system exhibits a better catalytic activity than some of the
earlier reported catalysts in terms of yields, catalyst loading, reactions time, TON and TOF
(Table 4).
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Table 3. Copper(II)-catalyzed synthesis of 5-substituted 1H-tetrazoles a
Entry
Nitriles
Product
Time (h) Yield (%) b TON c TOF (h-1) d
1
2
3
91
94
74.72
77.48
76.58
77.50
76.77
24.90
22.14
21.88
31.00
30.70
3.5
3
3.5
2.5
93
94
4
5
2.5
93
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7
2.5
2.5
95
92
78.50
75.72
31.40
30.28
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9
3.5
3.5
92
90
76.01
73.97
21.72
21.13
10
3
93
76.50
25.50
a Reaction condition: Nitriles (1.0 mmol), NaN3 (1.2 mmol)
b Isolated yields after column chromatography
c TON= moles of product formed per mole catalyst
d TOF= TON/time (h) of the reaction.
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Table 4. A comparison study of the synthesized polymeric copper(II) complex with some of the
reported catalysts
Entry
Catalyst
Conditions
TON TOF (h-1) Yields % Ref.
Solvent/Temp./Time
DMF/120 oC/5h
1
2
3
10 mol% (17 mg)
AgNO3
10 mol% (24 mg)
CuNO3. 3H2O
100 mg ZnO
83
92
87
[15]
[19]
DMF/ 120 oC/16h
DMF/ 125 oC/14h
DMF/ 120 oC/14h
1.77
0.13
[26b]
nanoflakes
20 mg CoY zeolite
4
5
36.96
2.64
90
95
[27]
20 mg [Pd-
SMTU@boehmite]
5 mg
PEG-400/ 120 oC/2.5h 31.65
12.66
[31a]
6
Ethylene glycol/
120 oC/3h
74.72
24.90
91
This-
work
[Cu(µ-O)(hmdp)]n
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A plausible reaction mechanism for the synthesis of 5-substituted 1H-tetrazoles using
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synthesized copper(II) complex as catalyst, is proposed in Scheme 2 based on earlier reports [16,
19]. Initially, coordination of nitrogen atoms of nitrile with Cu(II) takes place and forms complex
B. After that, the [3 + 2]-cycloaddition between the pre coordination of the nitrogen atom of the
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C≡N bond with azide ion takes place and readily to form the intermediate D, which accelerate
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the cyclization step. This idea is supported by performing the reaction in the absence of
copper(II) complex as catalyst. Without any catalyst, cycloaddition reaction is not completed
even after a long period of time (Table 2, entry 1). Protonolysis of intermediate D by 2 N HCl to
affords the 5-substituted 1H-tetrazole as white solid.
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Scheme 2. Plausible reaction mechanism for the synthesis of 5-substituted 1H-tetrazoles
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3. Conclusion
In summary, an unprecedented oxo-bridged polymeric copper(II) complex [Cu(µ-O)(hmdp)]n
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has been synthesized and characterized by FTIR, UV-visible, EPR and single crystal X-ray
diffraction study. The oxidation of one of the CH3 group of the 4-aminoantipyrene unit of the
Schiff base ligand to CH2OH during the complex formation in air is unprecedented. However,
the exact mechanism of oxidation of CH3 group to CH2OH is unambiguous at present. The future
studies will focus on to unravel the mechanistic aspect of this transformation. The complex
exhibited better catalytic activity for the synthesis of a wide range of 5-substituted 1H-tetrazoles
as compared to some of the previously reported catalysts in terms of yields, catalyst loading,
reactions time, TON and TOF. The ease of synthesis, air-stability, and good catalytic activity are
some of the key characteristics of the synthesized complex to make it as an interesting and active
catalyst for organic synthesis.
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4. Experiment
4.1. Materials and Instrumentations:
All reagents and solvents for the synthesis and analysis were commercially available and used as
received without further purification.
FT-IR spectra were recorded using KBr pellets on a Perkin Elmer Spectrometer in the range of
4000-400 cm-1. Electronic absorption spectral analysis was recorded on a Shimadzu UV-1800
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Spectrophotometer in the wave length range of 200-1100 nm. H NMR spectra of the isolated
products were recorded on a Bruker Avance-II HD-400 MHz spectrometer in (CD3)2SO using
TMS as the internal Standard. The EPR spectra were recorded at room temperature on JES-
FA200 ESR spectrometer (JEOL-Japan).
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4.2. Synthesis of Schiff base ligand 4-[(2-hydroxy-3-methoxybenzylidene) amino-1,5-dimethyl-
2-phenyl-1H-pyrazol-3-one] (hmdpH):
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The Schiff base ligand (hmdpH) was synthesized by the reported method [37]. A solution of 4-
aminoantipyrene (0.204 g, 1 mmol) in ethanol (5 mL) was added drop wise to an ethanolic
solution (5 mL) of ortho-vaniline (0.1522 g, 1 mmol). The resultant solution was heated to reflux
for a time of 6 h to produce yellow solid. The reaction mixture was cooled to room temperature
and the precipitate was filtered, washed with cold ethanol and dried in vacuum over anhydrous
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o
CaCl2. Yield: 0.233 g, 81%. M.P. 205 C. Selected FT-IR (KBr), cm-1: 1665 (C=O str., Ar.),
1593 (C=N str.), 1460 (C=C str.), 1242 (OH in plane bending), 1137(C-O-C str.) UV-Vis [
max(nm)]: 235, 284, 335 and 414.
λ
4.3. Synthesis of complex [Cu(µ-O)(hmdp)]n:
A methanolic solution of CuCl2. 2H2O (0.086 g, 0.50 mmol) was added drop-wise to a
suspension of the Schiff base ligand (hmdpH) (0.168 g, 0.50 mmol) in methanol (5 mL). The
reaction mixture was stirred for 12 h at room temperature to give a brownish precipitate which
was filtered, washed with cold methanol and dried vacuum over anhydrous CaCl2. The
precipitate was re-crystallized from DMF. Suitable single crystals for X-ray crystallography were
grown over a period of weeks from a concentrated solution of the complex in DMF.
Selected FT-IR (KBr), cm-1: 1,659 (C=O str., Ar.), 1579 (C=N str.), 1,460 (C=C str.), 547 (Cu-O
str.), 499 (Cu-N str.). UV-Vis [λ max(nm)]: 270, 338, 418, 514, 695.
4.4. General procedure for the synthesis 5-substituted 1-H tetrazoles:
In 25 mL round-bottomed flask, sodium azide (0.076 g, 1.2 mmol) and polymeric copper(II)
complex (0.005 g) were added to a solution of benzonitrile (0.103 g, 1 mmol) in ethylene glycol
(3 mL) with stirring at room temperature. The reaction temperature was raised up to 120 °C for 3
h. The reaction was monitored by TLC at regular intervals. After completion of the reaction, the
reaction mixture was cooled to room temperature and treated with 10 mL HCl (2N) and extracted
with 10 mL ethyl acetate. The resulted organic layers were separated and washed with 2x10 mL
distilled water, dried over anhydrous sodium sulphate and evaporated under reduced pressure.
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The residue was then purified by column chromatography on silica gel (100-200 mesh) to afford
the corresponding products.
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4.5. X-ray structure determination:
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The X-ray diffraction data were collected on a Bruker Kappa di
equipped with a CCD detector, employing Mo K radiation ( = 0.71073 Å), with the SMART
suite of programs [47]. All data were processed and corrected for Lorentz and polarization
ects with SAINT and for absorption e ects with SADABS [48]. Structural solution and
ffractometer at 153(2) K
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α
λ
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eff
ff
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refinement were carried out with the SHELXTL suite of programs [49]. The structures were
refined (weighted least squares refinement on F2) to convergence. All the non-hydrogen atoms in
all the compounds were refined anisotropically by full-matrix least-squares refinement. Precise
unit-cell parameters were determined by a least-squares fit of 2556 reflections of the highest
intensity, chosen from the whole experiment.
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Acknowledgments
We are thankful to the CRF IIT (ISM), Dhanbad; SAIF IIT, Madras and SAIF Panjab University,
Chandigarh for providing help in the analysis of the samples and we are also grateful to NTU,
Singapore for the single crystal X-ray analysis. Samaresh Layek acknowledges the receipt of IIT
(ISM) research fellowship.
Supplementary data
CCDC 1578086 contains the supplementary crystallographic data for this paper. This 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 mail. Supporting information also includes experimental details and
characterization data for synthesized compounds.
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Highlights:
‹ Synthesis and characterization of Schiff base ligand derived an unprecedented polymeric
copper(II) complex
‹ Single crystal X-ray diffraction study confirmed a distorted square pyramidal geometry
around the Cu(II) ion
‹ The complex has been found to be a very good catalyst for the synthesis of 5-substituted
1H-tetrazoles
‹ Ethylene glycol used as green solvent for the synthesis of 5-substituted 1H-tetrazoles