Salen complex of Cu(II) supported on superparamagnetic Fe3O 4@SiO2 nanoparticles: An efficient and recyclable catalyst for synthesis of 1- and 5-substituted 1H-tetrazoles

Article abstract of DOI:10.1016/j.jorganchem.2013.06.019

An efficient and general method has been developed for synthesis of 1- and 5-substituted 1H-tetrazoles from nitriles and amines using magnetite nanoparticles immobilized Salen Cu(II) as an efficient and recyclable catalyst. The structural and magnetic properties of functionalized magnetic silica are identified by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM) instruments. NMR, FT-IR, elemental analysis and XRD were also used for identification of these structures. Nanocatalyst can be easily recovered by a magnetic field and reused for subsequent reactions for at least 7 times with less deterioration in catalytic activity.

Full text of DOI:10.1016/j.jorganchem.2013.06.019

Journal of Organometallic Chemistry 743 (2013) 87e96
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Salen complex of Cu(II) supported on superparamagnetic Fe3O4@SiO2
nanoparticles: An efficient and recyclable catalyst for synthesis
of 1- and 5-substituted 1H-tetrazoles
*
Farzaneh Dehghani, Ali Reza Sardarian , Mohsen Esmaeilpour
Chemistry Department, College of Sciences, Shiraz University, Shiraz 71454, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient and general method has been developed for synthesis of 1- and 5-substituted 1H-tetrazoles
from nitriles and amines using magnetite nanoparticles immobilized Salen Cu(II) as an efficient and
recyclable catalyst. The structural and magnetic properties of functionalized magnetic silica are identified
by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and vibrating sample
magnetometer (VSM) instruments. NMR, FT-IR, elemental analysis and XRD were also used for identi-
fication of these structures.
Received 23 April 2013
Received in revised form
13 June 2013
Accepted 17 June 2013
Keywords:
Nanocatalyst can be easily recovered by a magnetic field and reused for subsequent reactions for at
least 7 times with less deterioration in catalytic activity.
1-Substituted 1H-tetrazole
5-Substituted 1H-tetrazole
Magnetite nanoparticles
Salen Cu(II)
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
substrate [9]. But, recycling homogeneous catalysts is often tedious
and time consuming and there is also product contamination
In recent years, magnetic nanoparticles (MNPs) have gained an
increasing interest because of their potential applications; exam-
ples include their uses for cell separation [1], magnetic resonance
imaging [2], drug delivery systems [3], protein separation [4] and
cancer treatments through hyperthermia [5].
observed when these catalysts are used.
Schiff base transition metal complexes have been extensively
studied because of their potential use as catalysts in a wide range of
reactions [10]. Moreover homogenous metal Schiff base complex
catalysts are deactivated easily through the formation of dimeric
Magnetite metal oxide nanoparticles, especially Fe3O4 nano-
particles have attracted increasing interest because of their unique
physical properties including the high surface area, super-
paramagnetism, low toxicity and their potential applications in
various fields [6]. Fe3O4 nanoparticles are easily prepared and
surface functionalized and they can be recycled from the solution
by external magnetic field. So, the catalyst supported on Fe3O4
nanoparticles can be easily separated from the reaction system and
reused [7].
Currently, much attention is focused on the synthesis of magnetic
coreeshell structures by coating a SiO2 shell around a preformed
nanoparticle [8].
Homogeneous catalysts show higher catalytic activities than
their heterogeneous counterparts because of their solubility in
reaction media, which increases catalytic site accessibility for the
peroxo and m-oxo species [11]. For overcoming the aforementioned
drawbacks, recently our group reported Schiff base complex of metal
ions supported on superparamagnetic Fe3O4@SiO2 nanoparticles
as an efficient, selective and recyclable catalyst for synthesis of
1,1-diacetates from aldehydes under solvent-free conditions [12].
Tetrazoles have attracted considerable interest in recent times
because of their wide applications [13]. Mainly with the roles
played by tetrazoles in coordination chemistry as ligands [14], in
medicinal chemistry as a more favorable pharmacokinetic profile
and a metabolically stable surrogate for carboxylic acid function-
alities [15]. Tetrazoles have been successfully used in various ma-
terial sciences and synthetic organic chemistry as analytical
reagents [16] and synthons [17]. So synthesis of this heterocyclic
nucleus is of much current importance.
Tetrazole rings can be prepared in several ways, 1-substituted
1H-tetrazoles are synthesized by reaction between hydrazoic acid
and isocyanides [18], acid-catalyzed cycloaddition between iso-
cyanides and trimethysilyl azide [19], cyclization between primary
amines, or their salts, with an orthocarboxylic acid ester and
* Corresponding author. Tel.: þ98 711 2287600; fax: þ98 711 2280926.
E-mail address: sardarian@chem.susc.ac.ir (A.R. Sardarian).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.06.019

88
F. Dehghani et al. / Journal of Organometallic Chemistry 743 (2013) 87e96
sodium azide [20] and cyclizations from an amine, triethyl ortho-
formate, and sodium azide using AcOH [21], PCl5 [22], In(OTf)3 [23],
Yb(OTf)3 [24], SSA [25], [HBIm]BF4 [26] and recently natrolite
zeolite as a catalyst [27].
comparison with literature values. The progress of the reaction was
monitored by TLC and purification was achieved by silica gel col-
umn chromatography.
The conventional method of synthesizing 5-substituted 1H-
tetrazoles is by addition of azide ions to organic nitriles. Different
homogenous and heterogeneous catalysts such as ZrOCl2 [28], AlCl3
[29], BF3$OEt2 [30], Pd(OAc)2/ZnBr [31], ZnO [32], ZnBr2 [33], ZnCl2/
tungstates [34], Zn/Al hydrotalcite [35], Zn(OTf)2 [36], Zn hy-
droxyapatite [37], ZnS [38], Cu2O [39a], nano CuFe2O4 [40], CdCl2
[41], natural zeolite [42], nano ZnO/Co3O4 [43], FeCl3eSiO2 [44],
Fe(OAc)2 [45], Fe(HSO4)3 [46], Zeolite and sulfated zirconia [47] and
2.1. General procedure
2.1.1. Preparation of Fe3O4 nanoparticles
The mixture of FeCl3$6H2O (1.3 g, 4.8 mmol) in water (15 mL)
was added to the solution of polyvinyl alcohol (PVA 15,000) (1 g), as
a surfactant, and FeCl2$4H2O (0.9 g, 4.5 mmol) in water (15 mL),
which was prepared by completely dissolving PVA in water fol-
lowed by addition of FeCl2$4H2O. The resultant solution was left to
be stirred for 0.5 h in 80 ꢀC. Then hexamethylenetetramine (HMTA)
(1.0 mol/L) was added drop by drop with vigorous stirring to make a
black solid product when reaction media reaches pH 10. The
resultant mixture was heated on water bath for 2 h at 60 ꢀC and the
black magnetite solid product was filtered and washed with
ethanol three times and was then dried at 80 ꢀC for 10 h [12].
g-Fe2O3 [48] have been reported for the promotion of reaction
between nitrile and NaN3 or TMS-N3. Recently, Ali Khalafi-Nezhad
et al. have reported nano CSMIL as a novel heterogeneous catalyst
for synthesis of tetrazole from nitrile [49].
Although most of these methods are worthwhile, many of them
have one or more of the following drawbacks: tedious workup of
the reaction mixture, difficulty in separation and recovery of the
catalyst, expensive moisture sensitive reaction conditions, toxic
metals, and hydrazoic acid which is highly toxic and explosive. So it
is of great practical importance to develop a more efficient and
environmentally benign method that avoids all these drawbacks.
In this work, our interest in this area led us to explore the Salen
Cu(II) complex immobilized onto the surface of magnetic nano-
particles, which can be sufficiently applied even for synthesis of 1-
and 5-substituted 1H-tetrazoles from nitriles and amines (Fig. 1).
The catalyst can be easily separated from the reaction mixture to
reuse.
2.1.2. Preparation of Fe3O4@SiO2 coreeshell
The coreeshell Fe3O4@SiO2 nanospheres were prepared by a
modified Stober method [50]. Briefly, Fe3O4 (0.50 g, 2.1 mmol) was
dispersed in the mixture of ethanol (50 mL), deionized water (5 mL)
and tetraethoxysilane (TEOS) (0.20 mL), followed by the addition of
5.0 mL of NaOH (10 wt%). This solution was stirred mechanically for
30 min at room temperature. Then the product, Fe3O4@SiO2, was
separated by an external magnet, and was washed with deionized
water and ethanol three times and dried at 80 ꢀC for 10 h. FT-IR
(KBr pellets, cmꢁ1): 3400 (OeH), 1000e1150 (SieOeSi) and 556
(FeeO) [12].
2. Experimental
2.1.3. General procedure for preparation of the salen ligand
To the solution of 3-aminopropyl (triethoxy) silane (1 mmol,
0.176 g) in 25 mL ethanol, the stoichiometric amount of salicy-
laldehyde (1 mmol, 0.122 g) in ethanol (25 mL) was added dropwise
Chemical materials were purchased from the Merck Chemical
Company in high purity. All the solvents were distilled, dried and
purified by standard procedures. Fourier transform infrared (FT-IR)
spectra were obtained using a Shimadzu FT-IR 8300 spectropho-
tometer. The conversions % was determined by GC on a Shimadzu
model GC-14A instrument. The NMR spectra were recorded on a
Bruker avance DPX 250 MHz spectrometer in chloroform (CDCl3)
and dimethylsulfoxide (DMSO) using tetramethylsilane (TMS) as an
internal reference. X-ray powder diffraction (XRD) spectra were
taken on a Bruker AXS D8-advance X-ray diffractometer with Cu K
a
radiation (
g
¼ 1.5418). Field emission scanning electron microscopy
(FE-SEM) images were obtained on HITACHI S-4160 and trans-
mission electron microscopy (TEM) images were obtained on a
Philips EM208 transmission electron microscope with an acceler-
ating voltage of 100 kV. Magnetic properties were obtained on a
BHV-55 vibrating sample magnetometer (VSM). Dynamic light
scattering (DLS) was recorded on a HORIBA-LB550. Tetrazoles were
characterized by their melting points, 1H NMR and 13C NMR and
Fig. 1. Synthesis of 1- and 5-substituted 1H-tetrazoles in the presence of Fe3O4@SiO2/
Fig. 2. Preparation process of salen complex of Cu(II) supported on superparamagnetic
Salen of Cu(II).
Fe3O4@SiO2 nanoparticles.

F. Dehghani et al. / Journal of Organometallic Chemistry 743 (2013) 87e96
89
to the yellow solution obtained, because of imine formation, then
the solution was stirred at room temperature for 6 h. The resulting
salen ligand, as the bright yellow precipitate, was separated by
filtration and washed with ethanol (5 mL) and then dried in vac-
uum. The crude product was recrystallized from ethanol to obtain
the pure product in 98% yield (0.271 g). Anal. found (%): C, 58.36; H,
8.52; N, 4.48. Calc. for C16H27NO4Si (%): C, 59.04; H, 8.36; N, 4.30. 1H
washed with ethanol. Then the product was purified by recrystal-
lization from ethanol and the resulting pure salen complex was
obtained. FT-IR spectrum of the complex showed the expected
bands, including a distinctive band due to C]N stretching, which is
lowered in frequency on complexation to metal ion.
2.1.5. General procedure for salen complex of metal ion
functionalized magnetite@silica nanoparticles
NMR (250 MHz, CDCl3):
d
¼ 0.7 (t, 2H, CH2, J ¼ 8.5 Hz); 1.22 (t, 9H,
CH3, J ¼ 7.0 Hz); 1.82 (m, 2H, CH2); 3.59 (t, 2H, CH2, J ¼ 6.5 Hz); 3.81
(q, 6H, CH2, J ¼ 7.0 Hz); 6.85e6.96 (m, 2H, CH aromatic); 7.21e7.29
(m, 2H, CH aromatic); 8.33 (s, 1H, CH); 13.59 (s, 1H, OH). 13C NMR
Fe3O4@SiO2 (2 g) was added to the solution of salen complex of
Cu(II) (1 mmol) in ethanol (10 mL) and the resultant mixture was
reflux for 12 h. Hot ethanol and water were added to the product,
Fe3O4@SiO2/Salen of Cu(II), and then nanocatalyst was separated by
an external magnet and dried at 80 ꢀC for 6 h.
(63 MHz, CDCl3):
118.81, 131.12, 132.05, 161.40 and 164.78.
d
¼ 7.94, 18.32, 24.37, 58.43, 62.05, 117.03, 118.38,
2.1.4. General procedure for the preparation of the salen complex of
Cu(II)
2.1.6. General procedure for the synthesis of 1-substituted-1H-
tetrazoles from amines
Cu(OAc)2 (0.182 g, 1 mmol) was added to the solution of the
salen ligand (0.651 g, 2 mmol) in ethanol (25 mL) and the mixture
was refluxed to complete the reaction. The progress of the reaction
was monitored by TLC. After the completion of complex formation,
a color change was observed. Resulting product was filtered and
A mixture of amine (1 mmol), sodium azide (1.2 mmol), triethyl
ortho-formate (1.2 mmol) and catalyst (0.02 g, contains 0.4 mol% of
Cu(II)) was taken in a round-bottomed flask and stirred at 100 ꢀC.
The progress of the reaction was followed by thin-layer chroma-
tography (TLC). After completion of the reaction, the reaction
mixture was cooled to room temperature and diluted with ethyl
acetate (3 ꢂ 20 mL). The catalyst was removed by using magnetic
field or filtration and then the resulting solution was washed with
water, dried over anhydrous Na2SO4 and was evaporated. The res-
idue was concentrated and recrystallized from EtOAcehexane
(1:9).
All products were characterized by 1H, 13C NMR, FT-IR, and
melting point which were in agreement with literature. We have
reported the spectral data of some aromatic and heteroaromatic
synthesized compounds.
Fig. 3. a) Fe3O4, b) Fe3O4@SiO2, c) Salen ligand, d) Salen complex of Cu(II), e)
Fig. 4. XRD patterns of (a) Fe3O4 [12], (b) Fe3O4@SiO2 [12] and (c) Fe3O4@SiO2/Salen of
Fe3O4@SiO2/Salen of Cu(II).
Cu(II).

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F. Dehghani et al. / Journal of Organometallic Chemistry 743 (2013) 87e96
2.1.6.1. 1-(4-Bromophenyl)-1H-tetrazole (Table 2, entry 4).
White solid (83% yield); m.p. 183e184 ꢀC. 1H NMR (CDCl3,
250 MHz):
d
¼ 7.00 (d, J ¼ 8.7 Hz, 2H), 7.54 (d, J ¼ 7.5 Hz, 2H), 8.07 (s,
1H) ppm. 13C NMR (CDCl3, 62.5 MHz):
d 153.5, 137.0, 128.9, 125.3,
118.0 ppm. FT-IR (KBr) (nmax cmꢁ1): 3532, 3167, 3058, 2863, 1675,
1591, 1575, 1488, 1415, 1319, 1284, 1235, 1200, 1153, 1100, 1098,
1013, 992, 834 cmꢁ1
.
2.1.6.2. 1-(4-Acetylphenyl)-1H-tetrazole (Table 2, entry 9).
Yellow solid (78% yield); m.p. 175e176 ꢀC. 1H NMR (CDCl3,
250 MHz):
d
¼ 2.76 (s, 3H), 7.78e7.97 (d, J ¼ 8.7 Hz, 2H), 8.12e8.31
(d, J ¼ 8.7 Hz, 2H), 9.32 (s, 1H) ppm. 13C NMR (CDCl3, 62.5 MHz):
d
26.58, 122.32, 131.12, 136.00, 138.67, 142.22, 194.11 ppm. FT-IR
(KBr) (nmax cmꢁ1): 3024, 1712, 1675, 1634, 1600, 1576, 1532, 1500,
1243, 1056, 976 cmꢁ1
.
2.1.6.3. 2-Methyl-6-(1H-tetrazol-1-yl) pyridine (Table 2, entry 7).
White solid (85% yield); m.p. 106e107 ꢀC. 1H NMR (250 MHz,
CDCl3):
(s, 1H) ppm. 13C NMR (62.5 MHz, CDCl3):
155.3 ppm. FT-IR (KBr) (nmax cmꢁ1): 3025, 1632, 1587, 1555, 1511,
1497, 1254, 1065, 973 cmꢁ1
d
¼ 2.90 (s, 3H), 7.21e7.43 (m, 1H), 7.84e8.02 (m, 2H), 9.32
Fig. 6. Magnetization curves at 300 K for Fe3O4 (a), Fe3O4@SiO2/Salen of Cu(II) (b). The
dispersion (c) and separation (d) process of magnetic Fe3O4@SiO2/Salen of Cu(II)
nanocatalyst.
d
123.4, 129.4, 130.2, 137.0,
.
Fig. 5. TEM images of Fe3O4 (a) and Fe3O4@SiO2 (b). FE-SEM image of Fe3O4@SiO2/Salen of Cu(II) (c). DLS of Fe3O4 (d), Fe3O4@SiO2 (e) and Fe3O4@SiO2/Salen of Cu(II) (f) respectively.

F. Dehghani et al. / Journal of Organometallic Chemistry 743 (2013) 87e96
91
Table 1
Effect of catalysts and solvents on formation of 1-substituted 1H-tetrazoles.a
N
N
N
Reaction Condition
N
NH2
NaN3
HC(OEt)3
Entry
Solvent/catalyst amount (g)
Temp. (ꢀC)
Time (min)
Yieldb (%)
1
2
3
4
4
5
6
7
8
Neat/0.005
Neat/0.01
Neat/0.015
Neat/0.02
Neat/0.025
DMF/0.02
DMSO/0.02
EtOH/0.02
MeOH/0.02
100
100
100
100
90
90
90
75
75
100
100
120
100
60
73
85
92
92
78
75
40
50
100
Reflux
Reflux
Reflux
Reflux
a
Conditions: aniline (1 mmol), triethyl orthoformate (1.2 mmol), and sodium azide (1 mmol).
Yields refer to isolated pure product.
b
Table 2
Preparation of 1-substituted 1H-tetrazoles in the presence of Fe3O4@SiO2/Salen Cu(II).a
Entry
Substrate
Product
Time (h)
1
Yieldb (%)
Mp. (ꢀC) (Lit.)c
Ref.
[49]
N
N
1
92
75
85
87
96
90
63e64 (64)
NH2
N
N
N
N
N
2
3
4
5
6
3
200e201 (200)
152e155 (153)
168e170 (170)
115 (115)
[49]
[49]
[49]
[49]
[49]
O2N
Cl
NH2
O2N
Cl
N
N
N
1.5
1.5
1
NH2
N
N
N
N
N
Br
NH2
Br
N
N
N
N
MeO
Me
NH2
MeO
Me
N
N
N
1
91e101 (92)
NH2
N
N
N
N
N
NH2
N
7
3
80
165e166 (166)
[49]
N
N
Me
Me
N
N
N
NH2
8
9
2.5
3
80
77
77 (77)
[49]
[49]
N
N
O
N
O
N
N
N
175e177 (175)
N
NH2
N
N
N
NH2
10
3
83
75
128e130 (130)
142e144 (144)
[49]
[49]
N
N
N
11
N
3.5
NH2
N
a
Conditions: amines (1 mmol), triethyl orthoformate (1.2 mmol), and sodium azide (1 mmol).
Yields refer to isolated pure product.
Melting points reported in the parenthesis refer to the literature melting points.
b
c

92
F. Dehghani et al. / Journal of Organometallic Chemistry 743 (2013) 87e96
Table 3
2.1.7.3. 5-(Naphthalen-1-yl)-1H-Tetrazole (Table 5, entry 12).
Comparison of various catalysts in synthesis of 1-substituted 1H-tetrazoles.a
White solid (90% yield); m.p. 263 ꢀC. 1H NMR (250 MHz, DMSO-d6):
Entry Catalyst
Solvent T (ꢀC) Yieldb (%) Time (h) Ref.
d
¼ 7.66e7.81 (m, 3H), 7.91e8.03 (m, 1H), 8.09e8.12 (m, 1H), 8.19e
8.23 (m, 1H), 8.62e8.72 (m, 1H) ppm. 13C NMR (62.5 MHz, DMSO-
d6): 125.9, 126.7, 127.9, 128.0, 128.8, 131.3, 132.8, 169.0 ppm. FT-IR
(KBr) (nmax cmꢁ1): 3429, 3061, 2817, 2720, 1628, 1601, 1523, 1491,
1385, 1358, 1252, 1128, 1100, 958, 869 cmꢁ1
1
2
3
4
5
Natrolite zeolite
In(OTf)3
Silica sulfuric acid
[HBIm]BF4
Fe3O4@SiO2/Salen Cu(II) Neat
Neat
Neat
Neat
Neat
120
100
120
100
100
82
89
95
91
96
4
1.5
5
0.33
1.0
[27]
[23]
[25]
[26]
e
d
.
a
Conditions: p-methoxy aniline (1 mmol), triethyl orthoformate (1.2 mmol), and
2.1.7.4. 5-(Phenanthren-9-yl)-1H-tetrazole (Table 5, entry 11).
sodium azide (1 mmol), solvent free, 100 ꢀC.
White solid (83% yield); m.p. 213e215 ꢀC. 1H NMR (250 MHz,
b
Yields refer to isolated pure product.
DMSO-d6):
d
¼ 7.50e7.70 (m, 4H), 7.99 (d, J ¼ 8.3 Hz, 2H), 8.43 (d,
J ¼ 8.2 Hz, 2H), 8.52 (s, 1H) ppm. 13C NMR (62.5 MHz, DMSO-d6):
d
125.3, 126.8, 128.2, 130.9, 131.8, 133.6, 134.0, 134.9, 161.0 ppm. FT-
IR (KBr) (nmax cmꢁ1): 3448, 3063, 2823, 2743, 1622,1594, 1511, 1490,
2.1.7. General procedure for the synthesis of 5-substituted-1H-
tetrazoles
1381, 1361, 1253, 1125, 1100, 969, 867 cmꢁ1
.
A mixture of nitrile (1 mmol), sodium azide (1.5 mmol) and
catalyst (0.02 g, contains 0.4 mol% of Cu(II)) in DMF (3 mL) was
taken in a round-bottomed flask and stirred at 120 ꢀC. The
progress of the reaction was followed by thin-layer chromatog-
raphy (TLC). After completion of the reaction, the reaction
mixture was cooled to room temperature and diluted with ethyl
acetate (3 ꢂ 20 mL). The catalyst was removed by using magnetic
field or filtration and then the resulting solution was washed
with 1 N HCl, dried over anhydrous Na2SO4 and then was evap-
orated. The crude products were obtained in excellent yields. All
products were characterized by 1H, 13C NMR, FT-IR, and melting
point which were in agreement with literature. We have re-
ported the spectral data of some aromatic and heteroaromatic
synthesized compounds.
3. Result and discussions
Salen complex of Cu(II) was prepared by refluxing stoichio-
metric amounts of Schiff base ligand and copper acetate in ethanol.
The complexes were insoluble in water but soluble in most organic
solvents (Fig. 2).
Determination of Cu content was performed by Inductively
Coupled Plasma (ICP) analyzer. According to the ICP analysis, the Cu
content in the magnetic nanocatalyst was determined which
revealed the presence of 0.21 mmol/g for this catalyst.
The IR spectra of complex show important bands from the free
salen ligand. The free ligand exhibits a
n (C]N) stretch at
1634 cmꢁ1 while in the complex, this band shifts to lower fre-
quency and appears at 1622 cmꢁ1 because of coordination of the
nitrogen with Cu(II). Vibrations in the range of 1480e1600 cmꢁ1 are
attributed to the aromatic ring. The presence of vibration bands in
559e588,1100 and 3400 cmꢁ1, which are caused by FeeO, SieOeSi,
and OH respectively, demonstrates the existence of Fe3O4 and SiO2
components in the synthesized complex. The presence of several
bands with medium intensity in 2750e3000 cmꢁ1 region is allo-
cated to CeH stretching of methylene group. The presence of two or
three bands in the low frequency region between 420 and
550 cmꢁ1 indicates the coordination of phenolic oxygen in addition
to azomethine nitrogen. CeO stretching vibrations shows a peak
2.1.7.1. 2-(1H-tetrazol-5-yl) pyridine (Table 5, entry 9). White solid
(90% yield); m.p. 210e211 ꢀC. 1H NMR (250 MHz, DMSO-d6):
d
¼ 7.69 (m, 1H), 8.23 (m, 1H), 8.21 (d, J ¼ 8.2 Hz, 1H), 8.78 (d,
J ¼ 5.0 Hz, 1H) ppm. 13C NMR (62.5 MHz, DMSO-d6):
d 120.4, 138.4,
137.8, 150.6 ppm. FT-IR (KBr) (nmax cmꢁ1): 3434, 2865, 2716, 1635,
1596, 1510, 1383, 1367, 1045, 867 cmꢁ1
.
2.1.7.2. 5-(4-Chlorophenyl)-1H-tetrazole (Table 5, entry 6).
White solid (84% yield); m.p. 261e263 ꢀC. 1H NMR (250 MHz,
DMSO-d6):
13C NMR (62.5 MHz, DMSO-d6):
156.4 ppm. FT-IR (KBr) (nmax cmꢁ1): 3423, 2931, 2822, 2745, 1612,
1468, 1445, 1398, 1356, 1173, 1089, 1043, 878, 773 cmꢁ1
d
¼ 7.72 (d, J ¼ 8.6 Hz, 2H), 8.12 (d, J ¼ 8.6 Hz, 2H) ppm.
centered at 1200e1320 cmꢁ1. The OH stretching vibration,
n(OeH)
d
123.5, 129.3, 130.2, 136.3,
found as a medium band at 3410 cmꢁ1 in free Schiff base ligand,
disappears in the spectra of the complex (Fig. 3).
.
Table 4
Effect of catalysts and solvents on forming 5-substituted 1H-tetrazoles.a
N
Reaction Condition
N
N
O2N
NaN3
O2N
CN
N
H
Entry
Solvent/catalyst amount (g)
Temp. (ꢀC)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
EtOH/0.02
MeOH/0.02
DMSO/0.02
Toluene/0.02
Neat/0.02
THF/0.02
DMF/0.005
DMF/0.01
DMF/0.015
DMF/0.02
DMF/0.025
90
100
120
120
120
100
120
120
120
120
120
24
24
10
12
12
12
12
12
8
40
55
87
65
70
70
75
82
87
92
92
9
10
11
6
6
a
Conditions: p-nitro benzonitrile (1 mmol) and NaN3 (1 mmol).
Yields refer to isolated pure product.
b

F. Dehghani et al. / Journal of Organometallic Chemistry 743 (2013) 87e96
93
Table 5
Preparation of 5-substituted 1H-tetrazoles in the presence of Fe3O4@SiO2/Salen complex of Cu(II).a
Entry
Substrate
Product
Time (h)
Yieldb (%)
90
Mp. (ꢀC) (Lit.)c
215 (215)
Ref.
N
N
1
7
[49]
[39]
[49]
[49]
[39]
[39]
[39]
CN
N
N
H
N
N
N
O2N
Cl
2
3
4
5
6
7
6
6
92
88
88
85
85
92
219 (219e220)
O2N
Cl
CN
N
H
N
N
261e263 (262)
265e266 (266)
CN
N
N
H
N
N
N
Br
6.5
12
9
Br
CN
N
H
N
N
N
MeO
Me
NC
233 (231e233)
MeO
Me
CN
N
H
N
N
248e249 (248e249)
CN
CN
N
N
H
N
N
N
6
256 (255)
NC
NC
N
H
CN
N
N
N
8
7
90
209e210 (208e210)
[39]
N
H
NC
N
N
N
CN
CN
9
6
6
90
90
210e211 (211)
239 (238e240)
[49]
[47]
N
H
N
N
N
N
N
10
N
H
N
N
CN
11
12
12
80
213e215 (215)
[49]
N
N
N
HN
N
CN
8
9
85
83
263 (264)
[49]
[39]
N
N
N
H
N
N
N
CN
13
123 (123e124)
NH
a
Reaction conditions: nitrile compounds (1 mmol), NaN3 (1 mmol), Fe3O4@SiO2/Salen Cu(II) (0.02 g), DMF, 120 ꢀC.
Isolated yield.
Melting points reported in the parenthesis refer to the literature melting points.
b
c

94
F. Dehghani et al. / Journal of Organometallic Chemistry 743 (2013) 87e96
The crystalline structures of the Fe3O4 particles, Fe3O4@SiO2 and
best choice for these reactions. Other organic solvents like DMF,
DMSO, MeOH and EtOH afforded the desired product in lower
yields.
Fe3O4@SiO2/Salen Cu(II) were determined by powder X-ray
diffraction (XRD). As shown in Fig. 4, it can be seen that the Fe3O4
obtained has highly crystalline cubic spinel structure which agrees
with the standard Fe3O4 (cubic phase) XRD spectrum (PDF#88-
After optimizing the reaction conditions, we next investigated
the generality of this condition using triethyl orthoformate, sodium
azide, and several amines. The results are summarized in Table 2. A
wide range of anilines containing electron-withdrawing and
electron-donating groups such as, chloro, bromo, methyl, methoxy,
acetyl and nitro underwent condensation in short reaction times
with excellent isolated yields (Table 2). The catalytic system also
worked well with heterocyclic amine such as amino pyridines
(Table 2, entries 7, 8) to generate the corresponding tetrazoles.
Also for aliphatic amines such as benzyl amine and n-butylamine
(Table 2, entry 10, 11) a good yield of desired product was obtained.
To show the advantage of Fe3O4@SiO2/Salen Cu(II) over some of
the reported catalysts in the literature, we showed a reaction of p-
methoxy aniline with triethyl orthoformate, and sodium azide in
the presence of 0.02 g Fe3O4@SiO2/Salen Cu(II) (Table 3). In com-
parison with the other reported catalysts in literature, we observed
that the Fe3O4@SiO2/Salen Cu(II) gives better yield in shorter re-
action time and lower temperature than SSA and natrolite zeolite.
Also this catalyst is comparable with [HBIm]BF4 and In(OTf)3.
The first step for the 5-substituted 1H-tetrazoles synthetic
approach involved optimization of reaction conditions and exploring
the catalytic activity of Fe3O4@SiO2/Salen complex of Cu(II). The re-
action of p-nitro benzonitrile (1 mmol) and NaN3 (1 mmol), was
investigated in the presence of Fe3O4@SiO2/Salen Cu(II) as a catalyst
in various solvents and temperatures in present of various amount of
catalyst. The results were summarized in Table 4.
0866). The patterns indicate a crystallized structure at 2q
: 30.1ꢀ,
35.4ꢀ, 43.1ꢀ, 53.4ꢀ, 57ꢀ and 62.6ꢀ, which are assigned to the (220),
(311), (400), (422), (511) and (440) crystallographic faces of
magnetite (reference JCPDS card no. 19-629). The XRD pattern of
Fe3O4@SiO2 prepared by the Stöber process, shows an obvious
diffusion peak at 2
q
¼ 15e25ꢀ that appeared because of the exis-
tence of amorphous silica. For Fe3O4@SiO2/Salen Cu(II) nano-
particles, the broad peak was transferred to lower angles due to the
synergetic effect of amorphous silica and salen complex of Cu(II).
According to the result calculated by Scherrer equation, it was
found that the diameter of Fe3O4 nanoparticles obtained was about
12 nm and Fe3O4@SiO2 microspheres were obtained with a diam-
eter of about 20 nm due to the agglomeration of Fe3O4 inside
nanospheres and surface growth of silica on the shell [51].
The morphology and sizes of (a) Fe3O4 and (b) Fe3O4@SiO2
particles were observed by transmission electron microscopy
(TEM) as shown in Fig. 5.
Fig. 5(b) displays the TEM images of Fe3O4 nanoparticles coated
with silica layers. The mesoporous silica shell on the surface
of Fe3O4 is quite homogeneous and exhibits good monodispersity
with estimated thickness of 8 nm. The morphology of Fe3O4@SiO2/
Salen of Cu(II) was also observed by FE-SEM (Fig. 5(c)).
In this study, Dynamic light scattering (DLS) was used for par-
ticle size analyzing of the catalyst. The average diameters of parti-
cles are evaluated to be about 12 nm for Fe3O4 Fig. 5(d), 20 nm for
Fe3O4@SiO2 Fig. 5(e) and 26 nm for Fe3O4@SiO2/Salen of Cu(II)
Fig. 5(f). The histogram was proposed according to the results ob-
tained from the XRD and TEM images.
As it was shown in Table 4, DMF as a solvent at 120 ꢀC with
0.02 g catalyst (contains 0.4 mol% Cu(II)) (Table 4, entry 10) is the
best choice for these reactions. Other organic solvents like DMSO,
THF, Toluene, EtOH, MeOH and neat condition afforded the desired
product in lower yields (Table 4, entries 1e4, 6).
The magnetic properties of the sample containing a magnetite
component were studied by a vibrating sample magnetometer
(VSM) at 300 K (Fig. 6).
After optimizing the reaction conditions, we next used different
nitriles as the substrates for this reaction. The results are summa-
rized in Table 5. As the entries in Table 5 show, the catalysis pro-
ceeded well for a wide variety of aryl nitriles, providing the
corresponding tetrazoles in high yields. The substituents on the
nitriles had a significant effect on the tetrazole formation reaction.
Reactions of electron poor aromatic and heteroaromatic nitriles,
such as 4-nitrobenzonitrile, 2-cyanopyridines, 3-cyanopyridines,
1,2-dicyanobenzene and 1,4-dicyanobenzene were completed
within a few hours (Table 5, entries 2, 7e10). Some electron rich
nitriles required longer reaction time (Table 5, entries 5, 6). The best
percentage conversions were observed for nitriles with electron
withdrawing substituents. Interestingly 1,4-dicyanobenzene and
1,2-dicyanobenzene (Table 5, entries 7, 8) afforded the mono-
addition product, though in the reaction between sodium azide
with 1,4-dicyanobenzene and 1,4-dicyanobenzene in the presence
of Zn(II) salts the double addition products were reported [33].
To show the advantage of Fe3O4@SiO2/Salen Cu(II) over some of
the reported catalysts in the literature, we showed a reaction of p-
nitro benzonitrile (1 mmol) and NaN3 (1 mmol) in the presence of
0.02 g Fe3O4@SiO2/Salen Cu(II) Table 6. In comparison with the
other reported catalysts in literature, we observed that the
Fe3O4@SiO2/Salen Cu(II) is comparable with some of these catalysts
such as nano ZnO/Co3O4 and Zn/Al-HT (Table 6, entries 2, 4) and
gives better yield in shorter reaction time than another ones.
The reusability of the catalyst is an important benefit especially
for commercial applications. So, the recovery and reusability of
nanocatalyst was investigated using the reaction of p-methoxy
aniline, triethyl orthoformate, and sodium azide in the presence of
Fe3O4@SiO2/Salen complex of Cu(II) under optimized conditions
(Fig. 7(a)). The catalyst was recovered by a magnetic field and was
Fig. 6 shows the absence of hysteresis phenomenon and in-
dicates that product has superparamagnetism at room tempera-
ture. The saturation magnetization values for Fe3O4 (a) and Fe3O4/
SiO2/Salen complexes of Cu(II) (b) were 64.8 and 35.2 emu/g,
respectively. These results indicated that the magnetization of
Fe3O4 decreased considerably with the increase of SiO2 and salen
complex of Cu(II). Nevertheless, the metal ion complex supported
on Fe3O4@SiO2 can still be separated from the solution by using an
external magnetic field on the sidewall of the reactor Fig. 6(c,d).
At the first stage for 1-substituted 1H-tetrazoles, the reaction of
aniline (1 mmol), triethyl orthoformate (1.2 mmol), and sodium
azide (1 mmol) was investigated in presence of Fe3O4@SiO2/Salen
Cu(II) as a catalyst in various solvents and under neat conditions in
the presence of various amount of catalyst. The results were sum-
marized in Table 1.
As it was shown in Table 1, solvent free condition at 100 ꢀC with
0.02 g catalyst (contains 0.4 mol% of Cu(II)) (Table 1, entry 4) is the
Table 6
Comparison of various catalysts in synthesis of 5-substituted 1H-tetrazoles.a
Entry Catalyst
Solvent T (ꢀC) Yieldb (%) Time (h) Ref.
1
2
3
4
5
Natural zeolite
Nano ZnO/Co3O4
ZnHAP
Zn/Al-HT
Fe3O4@SiO2/Salen Cu(II) DMF
DMF
DMF
DMF
DMF
120
120
120
120
120
90
90
78
84
92
14
12
12
12
6
[42]
[43]
[37]
[35]
e
a
Conditions: p-nitro benzonitrile (1 mmol) and NaN3 (1 mmol).
Yields refer to isolated pure product.
b

F. Dehghani et al. / Journal of Organometallic Chemistry 743 (2013) 87e96
95
Fig. 7. Reusability of the catalyst (a), reaction progress vs. conversion of the reaction of p-methoxy aniline, triethyl orthoformate, and sodium azide at different cycles of the reaction
under optimized conditions (b). (The conversion was determined by GC.)
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614e619.
reused for subsequent reactions for at least 7 times with less acti-
vation process.
We also decided to perform the kinetic studies to estimate the
reaction rates at different cycles according to some reported paper
in the literature [52]. For this purpose we selected the reaction of p-
methoxy aniline, triethyl orthoformate, and sodium azide in the
presence of Fe3O4@SiO2/Salen complex of Cu(II) under optimized
conditions to evaluate the reactivity of nanocatalyst at different
time at every cycle. The results were summarized in (Fig. 7(b)).
The amounts of Cu leaching into solution for the reaction was
detected by checking the Cu loading amount before and after each
reaction cycle through ICP. The amount of Cu leaching after the first
run was determined by ICP analysis to be only 0.2%, and after 7
repeated recycling was 5.4%.
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4. Conclusion
In this study, an efficient, and general method has been devel-
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and amines using magnetite nanoparticles immobilized Salen Cu(II)
as an efficient and recyclable catalyst. Nanocatalyst can be easily
recovered by a magnetic field and reused for subsequent reactions
for at least 7 times with less deterioration in catalytic activity.
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We are grateful to research council of Shiraz University for the
partial support of this work.
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