One-pot synthesis of 1- and 5-substituted 1H-tetrazoles using 1,4-dihydroxyanthraquinone–copper(II) supported on superparamagnetic Fe3O4@SiO2 magnetic porous nanospheres as a recyclable catalyst

Article abstract of DOI:10.1002/aoc.3518

An effective one-pot, convenient process for the synthesis of 1- and 5-substituted 1H-tetrazoles from nitriles and amines is described using1,4-dihydroxyanthraquinone–copper(II) supported on Fe₃O₄@SiO₂ magnetic porous nanospheres as a novel recyclable catalyst. The application of this catalyst allows the synthesis of a variety of tetrazoles in good to excellent yields. The preparation of the magnetic nanocatalyst with core–shell structure is presented by using nano-Fe₃O₄ as the core, tetraethoxysilane as the silica source and poly(vinyl alcohol) as the surfactant, and then Fe₃O₄@SiO₂ was coated with 1,4-dihydroxyanthraquinone–copper(II) nanoparticles. The new catalyst was characterized using Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, field emission scanning electron microscopy, dynamic light scattering, thermogravimetric analysis, vibration sample magnetometry, X-ray photoelectron spectroscopy, nitrogen adsorption–desorption isotherm analysis and inductively coupled plasma analysis. This new procedure offers several advantages such as short reaction times, excellent yields, operational simplicity, practicability and applicability to various substrates and absence of any tedious workup or purification. In addition, the excellent catalytic performance, thermal stability and separation of the catalyst make it a good heterogeneous system and a useful alternative to other heterogeneous catalysts. Also, the catalyst could be magnetically separated and reused six times without significant loss of catalytic activity. Copyright

Full text of DOI:10.1002/aoc.3518

Full paper
Received: 7 March 2016
Revised: 11 April 2016
Accepted: 28 April 2016
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3518
One-pot synthesis of 1- and 5-substituted 1H-
tetrazoles using 1,4-dihydroxyanthraquinone–
copper(II) supported on superparamagnetic
Fe3O4@SiO2 magnetic porous nanospheres as a
recyclable catalyst
Mohsen Esmaeilpoura*, Jaber Javidib,c and Saeed Zahmatkeshd*
An effective one-pot, convenient process for the synthesis of 1- and 5-substituted 1H-tetrazoles from nitriles and amines is
described using1,4-dihydroxyanthraquinone–copper(II) supported on Fe3O4@SiO2 magnetic porous nanospheres as a novel recy-
clable catalyst. The application of this catalyst allows the synthesis of a variety of tetrazoles in good to excellent yields. The prep-
aration of the magnetic nanocatalyst with core–shell structure is presented by using nano-Fe3O4 as the core, tetraethoxysilane as
the silica source and poly(vinyl alcohol) as the surfactant, and then Fe3O4@SiO2 was coated with 1,4-dihydroxyanthraquinone–
copper(II) nanoparticles. The new catalyst was characterized using Fourier transform infrared spectroscopy, X-ray diffraction,
transmission electron microscopy, field emission scanning electron microscopy, dynamic light scattering, thermogravimetric anal-
ysis, vibration sample magnetometry, X-ray photoelectron spectroscopy, nitrogen adsorption–desorption isotherm analysis and
inductively coupled plasma analysis. This new procedure offers several advantages such as short reaction times, excellent yields,
operational simplicity, practicability and applicability to various substrates and absence of any tedious workup or purification. In
addition, the excellent catalytic performance, thermal stability and separation of the catalyst make it a good heterogeneous
system and a useful alternative to other heterogeneous catalysts. Also, the catalyst could be magnetically separated and reused
six times without significant loss of catalytic activity. Copyright © 2016 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: core–shell; magnetic nanocatalyst; magnetic separation; one-pot reactions; 1- and 5-substituted 1H-tetrazoles
Many methods for the synthesis of 5-substituted tetrazoles have
been proposed based on [3 + 2] cycloaddition of azide ion to corre-
Introduction
Tetrazoles are an increasingly popular functionality with a wide
range of applications such as in pharmaceuticals as lipophilic
spacers and metabolically stable surrogates for carboxylic acids in
medicinal chemistry, ligands in coordination chemistry, special
explosives in materials science, in photography and information
recording systems and valuable precursors to a variety of
nitrogen-containing heterocycles in organic synthesis.[1] They have
been found to be potential TNF alpha inhibitors,
P2X7-antagonists[2] and inhibitors of anandamide cellular uptake.[3]
Furthermore, tetrazoles are screened for various biological activities
such as anticonvulsant, antiulcer, anti-inflammatory, antifungal,
antiviral, antibacterial, antiulcer and antitubercular activities.[4–6]
Tetrazole rings can be prepared in several ways. The routes to
1-substituted tetrazoles include acid-catalysed cycloaddition
between hydrazoic acid and isocyanides,[7] acetic acid- or
trifluoroacetic acid-catalysed cyclization between primary amines,
or their salts, with an orthocarboxylic acid ester and sodium azide,[8]
acid-catalysed cycloaddition between isocyanides and trimethyl
azide[9] and cyclizations from an amine, triethyl orthoformate and so-
dium azide using PCl5,[10] Yb(OTf)3,[11] In(OTf)3,[12] Pd(OAc)2/ZnBr,[13]
natrolite zeolite[14] and Fe3O4@SiO2/ligand/Cu(II)[15] as catalysts.
sponding organic nitriles. The reactions were carried out using nu-
merous homogeneous catalysts such as Zn(OTf)2,[16] Cu2(OTf)2,[17]
Fe(OAc)2,[18] BF3–OEt2,[19] AlCl3 and AgNO3,[21] and using some
[20]
heterogeneous catalysts such as Zn/Al hydrotalcite,[22] COY
zeolites,[23] γ-Fe2O3,[24] nano-ZnO/Co3O4,[25] nano-CuFe2O4,[26]
AlPO-5 microporous molecular sieves[27] and Fe3O4@SiO2/Salen of
Cu(II).[28]
* Correspondence to: M. Esmaeilpour, Chemistry Department, College of Science,
University, Shiraz, Iran. E-mail: m1250m551085@yahoo.com
S. Zahmatkesh, Department of Science, Payame Noor University (PNU), Tehran,
Islamic Republic of Iran. E-mail: zahmatkesh1355@yahoo.com
a Chemistry Department, College of Science, Shiraz University, Shiraz, Iran
b Department of Pharmaceutics, School of Pharmacy, Shahid Beheshti University of
Medical Sciences, Tehran, Iran
c
Students Research Committee, School of Pharmacy, Shahid Beheshti University of
Medical Sciences, Tehran, Iran
d Department of Science, Payame Noor University (PNU), Tehran, Islamic Republic
of Iran
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.

Esmaeilpour M., Javidi J. and Zahmatkesh S.
Although most of these methods are worthwhile, many of them
have one or more of the following drawbacks: long reaction time,
elevated temperature conditions, low yields, expensive moisture-
sensitive reaction conditions, tedious workup of the reaction mix-
ture and difficulty in separation and recovery of the catalyst. In ad-
dition to this, some cases show formation of toxic hydrazoic acid
and involvement of alkyltin-based reagents which warrant the
safety of the procedure.[29] Thus, the choice of catalyst is one of
the most crucial steps for achieving good results.
Magnetite nanoparticles are attracting increasing interest due to
their unique magnetic responsivity, low toxicity, large surface-to-
volume ratio, biocompatibility and potential applications in several
fields.[30,31] Nanoscale magnetite (Fe3O4) has potential applications
in magnetic resonance imaging contrast agents, magnetic
bioseparations, treatment of cancer, drug delivery and
catalysis.[32,33] However, magnetic nanoparticles easily aggregate
because of anisotropic dipolar attraction and alter magnetic prop-
erties. Therefore, a protection layer is important to avoid such
limitations.[34]
Recently, magnetic core–shell materials have gained much atten-
tion and undergone intensive investigation for their unique poten-
tial applications in medicinal, low cytotoxicity, chemically
modifiable surface, magnetic responsivity, good stability, optical, bi-
ological, environmental and chemical areas.[35,36] Core–shell nano-
structure magnetic catalysts can be easily separated conveniently
and economically using an external magnetic field.[37,38]
Recently, surface-functionalized magnetic nanoparticles have
been developed in a range of organic transformations, and studies
of immobilization of organo-catalysts on core–shell nanoparticles
have been reported.[39,40]
Therefore, in this paper, we report the preparation of 1,4-
dihydroxyanthraquinone–copper(II)
immobilized
on
superparamagnetic Fe3O4@SiO2 nanoparticles as illustrated in
Scheme 1 and its use as a recyclable catalyst for the synthesis of
1- and 5-substituted 1H-tetrazoles from amines and nitriles. In addi-
tion, using the methodology described, catalyst separation be-
comes easier and cost effective with an external magnetic field.
Scheme 1. Preparation process of 1,4-dihydroxyanthraquinone–copper(II)
supported on superparamagnetic Fe3O4@SiO2 nanoparticles.
a vibrating sample magnetometer (BHV-55). Thermogravimetric
analysis (TGA) curves were recorded using a PerkinElmer device
manufactured by Thermal Sciences. X-ray photoelectron spectros-
copy (XPS) of the catalyst was performed with an XR3E2 (VG
Microtech) twin anode X-ray source using Al Kα radiation
(1486.6 eV). The amount of Cu nanoparticles supported on
Fe3O4@SiO2-DAQ was measured using inductively coupled plasma
(ICP) analysis (Varian, Vista-pro). The melting points of products
were determined with an Electrothermal type 9100 melting point
apparatus. Tetrazoles were characterized by their melting points
and 1H NMR and 13C NMR spectra and comparison with literature
values.
Experimental
Materials and physical measurements
All the solvents were distilled, dried and purified using standard
procedures. All the chemical reagents used in our experiments
were purchased from Merck Chemical Company and were of high
purity. Progress of reactions was monitored by TLC on silica gel
polygram STL G/UV 254 plates. NMR spectra were recorded with a
Bruker Avance DPX 250 MHz spectrometer in CDCl3 and deuterated
dimethylsulfoxide (DMSO-d6) as solvents in the presence of
tetramethylsilane as internal standard. A Shimadzu FT-IR 8300 spec-
trometer was used to record Fourier transform infrared (FT-IR) spec-
tra using KBr pellets. Large-angle X-ray diffraction (XRD) patterns
were recorded with a Bruker AXS D8-advance X-ray diffractometer
using Cu Kα radiation (λ = 1.5418). Transmission electron micros-
copy (TEM) observation was performed with a Philips EM208 micro-
scope operated at a 100 kV accelerating voltage. Particle
morphology was examined using field emission scanning electron
microscopy (FE-SEM; Hitachi S-4160 instrument). Dynamic light
scattering was performed with a HORIBA-LB550. Brunauer–
Emmett–Teller (BET) surface area and porosity of nanoparticles
were determined with a Micromeritics ASAP 2000 automatic
analyser. Magnetic properties of the samples were measured using
General procedures
Preparation of Fe3O4 nanoparticles
The Fe3O4 nanoparticles were synthesized by a facile co-
precipitation method.[41,42] Briefly, FeCl3Á6H2O (1.3 g, 4.8 mmol)
was dissolved in 30 ml of water, followed by addition of poly(vinyl
alcohol) (PVA 15000, 1 g) as a surfactant and 0.9 g (4.5 mmol) of
FeCl2Á4H2O. The mixture was stirred vigorously for 30 min at
80 °C. Then, to this mixture, hexamethylenetetramine (1.0 mol lÀ1
)
was slowly added under vigorous stirring to produce a black
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Fe3O4@SiO2-DAQ-Cu(II)-catalysed synthesis of tetrazole derivatives
precipitate and the solution pH was maintained at 10.0. The resul-
tant mixture was continuously stirred with nitrogen protection at
60 °C for 2 h and the obtained black magnetite particles were
collected with a permanent magnet, and sequentially washed with
ethanol and dried at 80 °C for 10 h.
General procedure for synthesis of 1-substituted 1H-tetrazole
derivatives in presence of Fe3O4@SiO2-DAQ-Cu(II) catalyst
A mixture of amine (1 mmol), triethylorthoformate (1.2 mmol) and
sodium azide (1 mmol) was stirred in the presence of
Fe3O4@SiO2-DAQ-Cu(II) (0.035 g, 0.8 mol% of Cu(II)) at 100 °C. After
completion of the reaction, as monitored by TLC using n-hexane–
ethyl acetate, the reaction mixture was diluted with ethyl acetate
(20 ml). The catalyst was collected using an external magnet, and
then the resulting solution was washed with water and dried over
anhydrous Na2SO4. The residue was concentrated and recrystal-
lized from EtOAc–hexane (1:10) to afford the pure product.
Preparation of Fe3O4@SiO2 core–shell nanospheres
Fe3O4@SiO2 core–shell nanospheres were prepared through a
modified Stöber method.[43–45] In a typical process, 0.5 g of Fe3O4
nanoparticles were dispersed in a mixture of ethanol (50 ml), deion-
ized water (5 ml) and 5.0 ml of NaOH (10 wt.%). Subsequently,
0.2 ml of tetraethoxysilane (TEOS) was added dropwise under vigor-
ous mechanical stirring. After stirring for 30 min, the products
(Fe3O4@SiO2) were collected using an external magnet and washed
with deionized water and ethanol three times and then dried under
vacuum at 80 °C for 10 h.
General procedure for synthesis of 5-substituted 1H-tetrazole
derivatives in presence of Fe3O4@SiO2-DAQ-Cu(II) catalyst
Fe3O4@SiO2-DAQ-Cu(II) (0.04 g, 0.9 mol% of Cu(II)) was added to a
mixture of nitrile (1 mmol), sodium azide (1.2 mmol) and DMF
(3 ml) and stirred at 100 °C. After completion of the reaction, the
resulting mixture was diluted with ethyl acetate (10 ml), the catalyst
was separated with an external magnet and washed with ethyl ac-
etate (10 ml) three times. The filtrate was treated with ethyl acetate
(10 ml) and 1 N HCl (10 ml) under vigorous stirring for 2 min. The
resultant organic layer was separated and the aqueous layer was
extracted with ethyl acetate (20 ml). The combined organic layer
was dried over anhydrous Na2SO4 and concentrated to give a crude
product. The crude product was purified by silica gel column chro-
matography using appropriate solvent mixture (hexane–EtOAc, 1:1)
to afford the pure product.
General procedure for the preparation of Fe3O4@SiO2 nanoparticles functional-
ized with 3-(triethoxysilyl)propylamine (Fe3O4@SiO2-NH2).
Fe3O4@SiO2 nanoparticles (1 g) were dispersed in ethanol (10 ml)
and sonicated for 20 min at room temperature. Then,
3-aminopropyl(triethoxy)silane (1 mmol, 0.176 g) was introduced,
and the mixture was stirred vigorously and refluxed for 12 h under
nitrogen atmosphere. After refluxing, the mixture was cooled to
room temperature, separated using an external magnet and
washed thoroughly with ethanol and water and dried in an oven
at 80 °C for 6 h to afford Fe3O4@SiO2-NH2.
Preparation of (4-hydroxy-9,10-dioxo-9,10-dihydroanthracen-1-yloxy)acetic
acid ethyl ester (2)
Results and discussion
FT-IR spectra of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2-NH2, ester 2,
Fe3O4@SiO2-DAQ and Fe3O4@SiO2-DAQ-Cu(II) are shown in Fig. 1.
In Fig. 1(a), the absorption band at 578 cmÀ1 is assigned to the
stretching vibration of the Fe-O bond of Fe3O4. Also, the peaks at
around 3400 and 1620 cmÀ1 in Fig. 1(a) are due to the adsorbed
water in the sample. The peaks at 1081 and 807 cmÀ1 are ascribable
asymmetric and symmetric stretching vibration of framework and
terminal Si-O-Si groups, suggesting the formation of silica layers
(Fig. 1(b)). Figure 1(c) shows the FT-IR spectrum of Fe3O4@SiO2-NH2
nanoparticles: the peaks at 577, 1000–1150, 1400–1410, 1546,
2810–2986 and 3165–3390 cmÀ1 are attributed to Fe-O
(stretching), Si-O-Si (asymmetric stretching), C-N (stretching), N-H
(bending) C-H (stretching) and N-H (stretching), respectively.
Figure 1(d) shows the FT-IR spectrum of ester 2. The bands at
2878–3031, 1734, 1665, 1476 and 1355 cmÀ1 are attributed to
C-H (stretching), C-O (ester stretching), C-O (stretching), CH2 (bend-
ing) and CH3 (bending), respectively. Also, the bands at 1038 and
1594 cmÀ1 are ascribed to the carbonyl group, hydrogen bonded
with the hydroxyl group positioned at position 1 in the anthraqui-
none molecule.[46] From the FT-IR spectrum of Fig. 1(e), the peaks
at 1645 and 1544 cmÀ1 belong to the stretching vibration mode
of C-O (amide and anthraquinone stretching) and C-O (hydrogen
bonded with hydroxyl group in position 1) bonds in
Fe3O4@SiO2-DAQ, and the absorption peak present at
1000–1150 cmÀ1 is due to stretching vibration of framework and
terminal Si-O-Si groups. The presence of vibration bands at 3400
(O-H stretching), 2880–3090 (C-H stretching), 1580 (C-O amide
stretching), 1531 (C-O non-coordinated stretching), 1436 (C-O coor-
dinated with copper), 1000–1150 (Si-O-Si asymmetric stretching)
and 675 cmÀ1 (Cu-O stretching) demonstrates the existence of
A mixture of 1,4-dihydroxyanthraquinone (1.0 mmol), ethyl
2-bromoacetate (0.5 mmol), t-BuOK (0.5 mmol) and
dimethylformamide (DMF; 10 ml) was stirred at room temperature
for 18 h. After completion of the reaction (monitored by TLC), 20 ml
of water was added, the mixture was filtered and the residue was
dried. The crude product (2) was purified via flash column chroma-
tography using hexane–EtOAc (1:1) as the eluent.
Preparation of Fe3O4@SiO2-DAQ nanoparticles
The modification of Fe3O4@SiO2-NH2 with 2) was carried out via
ligand formation between the amino group and the carbonyl group
of ester 2 as presented in Scheme 1. Fe3O4@SiO2-NH2 (0.5 g) and 2
(1 mmol, 0.325 g) were mixed with 10 ml of ethanol using mechan-
ical stirring to produce a homogeneous suspension. The reaction
was refluxed with continuous stirring under inert atmosphere for
24 h. Then, the modified Fe3O4@SiO2 nanoparticles (as
Fe3O4@SiO2-DAQ) were collected with a magnet, washed several
times with ethanol and distilled water, and dried in a vacuum des-
iccator at 70 °C.
Preparation of Fe3O4@SiO2-DAQ-Cu(II) nanoparticles
Fe3O4@SiO2-DAQ (0.5 g) was added to a solution of Cu(OAc)2
(1 mmol, 0.182 g) in ethanol (10 ml) and the reaction mixture
was refluxed for 12 h in nitrogen atmosphere. The resulting
Fe3O4@SiO2-DAQ-Cu(II) nanoparticles were collected using an
external magnetic field, then washed with deionized water
and ethanol three times and finally dried under vacuum at
80 °C for 10 h.
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Esmaeilpour M., Javidi J. and Zahmatkesh S.
Figure 2. XRD patterns: (a) Fe3O4; (b) Fe3O4@SiO2; (c) Fe3O4@SiO2-DAQ-Cu(II).
corresponding to a 10 nm thick SiO2 layer on the Fe3O4 (Fig. 3
(d)).[48] Fe3O4@SiO2 -DAQ-Cu(II) is also observed as having well
shaped spherical or ellipsoidal particles. The diameter of the nano-
particles is found to be approximately 50 nm (Fig. 3(e)).
Figure 1. FT-IR spectra: (a) Fe3O4; (b) Fe3O4@SiO2; (c) Fe3O4@SiO2-NH2; (d)
ester 2; (e) Fe3O4@SiO2-DAQ; (f) Fe3O4@SiO2-DAQ-Cu(II) nanoparticles.
Figures 3(f)–(h) show the particle size distributions of Fe3O4,
Fe3O4@SiO2 and Fe3O4@SiO2-DAQ-Cu(II) nanoparticles. This
size distribution is centred at a value of 12, 20 and 50 nm
for Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2-DAQ-Cu(II), respec-
tively. The result is in good agreement with the size of parti-
cles shown in TEM images.
1,4-dihydroxyanthraquinone–copper(II) supported on Fe3O4@SiO2
nanoparticles (Fig. 1(f)).[47]
The crystalline structures of the Fe3O4 nanoparticles, Fe3O4@SiO2
and Fe3O4@SiO2-DAQ-Cu(II) were determined using powder XRD.
As shown in Fig. 2(a), the XRD pattern of Fe3O4 is in agreement with
JCPDS card no. 19–0629 (cubic spinel structure).[48] As shown in
Figs 2(b) and (c), the peak positions of Fe3O4@SiO2 and
Fe3O4@SiO2-DAQ-Cu(II) do not show any changes. This confirms
that the surface modification and conjugation of the Fe3O4 nano-
particles do not lead to a phase change. The patterns of Fe3O4@SiO2
and Fe3O4@SiO2-DAQ-Cu(II) show an obvious diffusion peak around
2θ = 10–25° attributable to amorphous silica (Figs 2(b) and (c)). For
Fe3O4@SiO2-DAQ-Cu(II) nanoparticles, the broad peak is transferred
to lower angles due to the synergetic effect of amorphous silica and
ligand. Also, the XRD pattern of the Fe3O4@SiO2-DAQ-Cu(II)
nanocatalyst shows the peaks of both Fe3O4 and Cu, and the
diffraction peaks at 2θ around 28°, 65° and 78° are correlated to
the different copper phases of Cu(111), Cu(331) and Cu(220),
respectively (Fig. 2(c)).[47]
Magnetization
Fe3O4@SiO2-DAQ-Cu(II) magnetic samples show no hysteresis and
the saturationmagnetizationvalues are64.8, 40.3and 30.7emu gÀ1
curves
of
Fe3O4,
Fe3O4@SiO2
and
,
respectively, revealing their superparamagnetism (Fig. 4(A)). Obvi-
ously, the saturation magnetization of Fe3O4 gradually decreases
with the increase of SiO2 and ligand complex.
Nevertheless, the magnetism of Fe3O4@SiO2-DAQ-Cu(II) is still
high enough to allow it be magnetically separated by applying an
external permanent magnet, which can facilitate the separation of
catalyst from reaction solutions. With a magnet placed beside the
vial, Fe3O4@SiO2-DAQ-Cu(II) nanoparticles dispersed in DMF are
quickly attracted to the side of the vial within a few seconds with-
out the need for a filtration step, and then can be readily
redispersed with slight shake, which illustrates their magnetic na-
ture (Fig. 4(B)).
The morphology of Fe3O4 and Fe3O4@SiO2 particles was
observed using FE-SEM, as shown in Fig. 3. The FE-SEM images
indicate the successful coating of the magnetic Fe3O4 particles
(Figs 3(a) and (b)).
The morphologies and structures of the nanoparticles at various
synthetic steps were observed using TEM (Figs 3(c)–(e)). The TEM
image reveals that the diameter of the pure Fe3O4 is uniform at
about 10 nm (Fig. 3(c)). After coating with thin SiO2, the diameter
of the Fe3O4@SiO2 increases to approximately 20 nm,
Results of TGA analyses for Fe3O4@SiO2 and Fe3O4@SiO2-DAQ-Cu
(II) nanoparticles are shown in Fig. 4(C) obtained at a heating rate of
20 °C minÀ1 from 50 to 700 °C. According to Fig. 4(C) curve (a), the
Fe3O4@SiO2 nanoparticles exhibit a weight loss in the range
50–200 °C due to evaporation of surface-adsorbed water and etha-
nol, while weight loss between 200 and 560 °C is associated with
the release of the structure water.[49] As shown in Fig. 4(C) curve
(b), the TGA curve of Fe3O4@SiO2-DAQ-Cu(II) shows a weight loss
of 5.1% below 200 °C due to the loss of the surface hydroxyl groups.
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Fe3O4@SiO2-DAQ-Cu(II)-catalysed synthesis of tetrazole derivatives
Figure 4. (A) Magnetization curves at 300 K for (a) Fe3O4, (b) Fe3O4@SiO2
and (c) Fe3O4@SiO2-DAQ-Cu(II) nanoparticles. (B) Separation process of
Fe3O4@SiO2-DAQ-Cu(II) nanoparticles using a magnet. (C) TGA of (a)
Fe3O4@SiO2 and (b) Fe3O4@SiO2-DAQ-Cu(II) nanoparticles.
Figure 3. SEM images of (a) Fe3O4 and (b) Fe3O4@SiO2. TEM images of
(c) Fe3O4, (d) Fe3O4@SiO2 and (e) Fe3O4@SiO2-DAQ-Cu(II). Size
distributions of (f) Fe3O4, (g) Fe3O4@SiO2 and (h) Fe3O4@SiO2-DAQ-Cu(II).
The weight loss of about 24.8% in the range 200–700 °C is attrib-
uted to the decomposition of organic moieties on the surface.
Figure 5 shows the XPS spectrum of the synthesized Fe3O4@SiO2-
DAQ-Cu(II) particles. For a better understanding of the oxidation
state of copper, the binding energy obtained from the catalyst
was compared with the Cu2+ 2p3/2 and 2p1/2 peak positions. From
Fig. 5, it can be seen that the main peaks of Cu 2p3/2 and Cu 2p1/2
in Fe3O4@SiO2-DAQ-Cu(II) nanoparticles are about 934.61 eV and
942.67 eV indicating that Cu is in the formal +2 valance state.[50]
Determination of the Cu content was carried out using ICP anal-
ysis of the digested catalyst. The obtained results reveal that
0.23 mmol per gram of Cu is immobilized on Fe3O4@SiO2-DAQ.
The specific surface area and pore diameter were calculated
based on standard BET and Barrett–Joyner–Halenda methods,
respectively. According to the BET isotherms, the active surface area
of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2-DAQ-Cu(II) is estimated at
480, 430 and 362 m2 gÀ1, respectively (Table 1).
Figure 5. XPS spectrum of Fe3O4@SiO2-DAQ-Cu(II) nanoparticles.
In order to optimize the reaction conditions and performance of
Fe3O4@SiO2-DAQ-Cu(II) as a catalyst, the reaction between aniline
(1 mmol), triethylorthoformate (1.2 mmol) and sodium azide
(1 mmol) was selected as a model reaction (Table 2).
As a first stage, the reactions were conducted in EtOH, MeOH, Cl
(CH2)2Cl, CH3CN, DMF, DMSO and water as solvents and under
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Esmaeilpour M., Javidi J. and Zahmatkesh S.
Table 1. BET results for Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2-DAQ-Cu(II)
Table 3. Preparation of 1-substituted 1H-tetrazoles in the presence of
Fe3O4@SiO2-DAQ-Cu(II) catalysta
Sample
Specific surface Pore volume Average pore
Entry
area (m2 gÀ1
)
(cm3 gÀ1
)
radius (nm)
1
2
3
Fe3O4
480
430
362
0.803
0.755
0.730
1.254
1.787
1.833
Fe3O4@SiO2
Fe3O4@SiO2-
DAQ-Cu(II)
Entry
R
Product Time Yield
(h) (%)b
M.p. (lit.) (°C)c
1
C6H5
6a
6b
6c
1
1
96
91
61–62 (63–64)[28]
92–93 (92)[51]
2
4-MeC6H4
2,6-Cl2C6H3
4-MeOC6H4
4-BrC6H4
0.75 95 131–132 (133)[52]
0.75 93 114–116 (115)[52]
Table 2. Optimization of reaction conditions for the synthesis of 1-phe-
nyl-1H-tetrazolesa
3
4
6d
6e
6f
5
1.5
1.5
2
89 170–172 (170)[52]
86 150–152 (153)[52]
84 199–200 (200–201)[28]
85 178–179 (177)[15]
6
4-ClC6H4
7
4-NO2C6H4
4-MeCOC6H4
3-Pyridyl
6 g
6 h
6i
8
1.5
3
Entry
Catalyst amount
(mol%)/solvent
Temp.
(°C)
Time
(h)
Yield
(%)b
9
84
90 107–108 (106)[52]
77–79 (77)[28]
10
11
12
13
6-Methyl-2-pyridyl 6j
2
1.25 88 132–133 (130–132)[15]
1
0.8/EtOH
Reflux
Reflux
Reflux
Reflux
100
2
3
67
62
43
64
76
70
9
C6H5CH2
6 k
79 141–142 (143)[15]
85 168–169 (170)[52]
2
0.8/MeOH
CH3CH2CH2CH2
Cyclohexyl
6 l
2.5
3
3
0.8/ClCH2CH2Cl
0.8/CH3CN
3
6 m
4
4
aReaction conditions: amine compounds (1 mmol), NaN3 (1 mmol),
triethylorthoformate (1.2 mmol), Fe3O4@SiO2-DAQ-Cu(II) (0.8 mol%),
solvent-free, 100 °C.
5
0.8/DMF
3
6
0.8/DMSO
100
4
bIsolated yield.
7
0.8/H2O
Reflux
100
5
cMelting points reported in parentheses refer to literature melting points.
8
0.8/solvent-free
None/solvent-free
0.3/solvent-free
0.6/solvent-free
0.7/solvent-free
0.9/solvent-free
0.8/solvent-free
0.8/solvent-free
0.8/solvent-free
1
96
0
9
130
12
4
10
11
12
13
14
15
16
100
48
70
89
94
26
83
95
100
2
100
1
Table 4. Effect of Fe3O4@SiO2-DAQ-Cu(II) as catalyst, solvent and tem-
perature on the formation of 5-phenyl-1H-tetrazolesa
100
1
r.t.
6
80
2
110
1
aReaction conditions: aniline (1 mmol), triethylorthoformate (1.2 mmol)
and sodium azide (1 mmol).
bIsolated yield.
Entry
Catalyst amount
(mol%)/solvent
Temp.
(°C)
Time
(h)
Yield
(%)b
1
0.9/EtOH
0.9/MeOH
0.9/H2O
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
100
12
12
16
24
12
12
12
6
38
27
18
14
0
2
3
solvent-free conditions. As evident from Table 2, solvent-free condi-
tions at 100 °C with 0.8 mol% of catalyst are obviously the best
choices for this reaction (Table 2, entry 8).
4
0.9/CH3CN
0.9/CHCl3
0.9/CH2Cl2
0.9/1,4-dioxane
0.9/toluene
0.9/NMP
5
6
0
We also evaluated the amount of catalyst required for the reac-
tion. As evident from Table 2, with no catalyst, the reaction does
not proceed even at a high temperature (entry 9). The best result
is achieved by carrying out the reaction with the catalyst (0.8 mol
%), aniline (1 mmol), triethylorthoformate (1.2 mmol) and sodium
azide (1 mmol) under solvent-free conditions at 100°oC (Table 2,
entry 8). Decreasing the catalyst concentration results in lower
yields under the same conditions (Table 2, entries 10–12). Further
increase in catalyst concentration does not show any significant
effect on reaction time and yield of product (Table 2, entry 13).
Furthermore, the effect of temperature was also studied by carry-
ing out the model reaction. At room temperature a useful conver-
sion is not observed (Table 2, entry 14). However the productivity
increases on raising the temperature. Thus, the best results for this
reaction are obtained with 0.8 mol% of catalyst under solvent-free
conditions at 100 °C (Table 2, entry 8).
7
0
8
54
61
58
96
0
9
100
4
10
11
12
13
14
15
16
17
18
19
20
0.9/DMSO
0.9/DMF
100
8
100
2.5
24
6
None/DMF
0.3/DMF
140
100
27
71
82
85
94
Trace
63
95
0.6/DMF
100
6
0.7/DMF
100
3
0.8/DMF
100
3
1.0/DMF
100
3
0.9/DMF
r.t.
8
0.9/DMF
80
4
0.9/DMF
110
2.5
aReaction conditions: benzonitrile (1 mmol) and sodium azide (1.2 mmol).
bIsolated yield.
After optimization of the reaction conditions, the reaction of
triethylorthoformate and sodium azide with various amines was
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2016)

Fe3O4@SiO2-DAQ-Cu(II)-catalysed synthesis of tetrazole derivatives
carried out in the presence of 0.8 mol% Fe3O4@SiO2-DAQ-Cu(II). As
evident from Table 3, high yields of the relevant products are
obtained for the 1-substituted 1H-tetrazoles with electron-
withdrawing or electron-donating groups (e.g. Br, Cl, NO2, COMe,
Me, OMe; Table 3, entries 2–10). Heterocyclic compounds such as
aminopyridines efficiently react to give the corresponding
tetrazoles (Table 3, entries 9 and 10). Also, the reactions with
aliphatic amines such as benzylamine, n-butylamine and
cyclohexanamine give the desired products in good yields of 88,
79 and 85%, respectively (Table 3, entries 11–13).
Also, the catalytic activity of Fe3O4@SiO2-DAQ-Cu(II) was studied
in order to optimize the protocol for formation of 5-substituted
1H-tetrazoles using benzonitrile and NaN3 as model reaction. Our
optimization data are: 1 mmol of benzonitrile, 1.2 mmol of sodium
azide, 0.9 mol% of Fe3O4@SiO2-DAQ-Cu(II) catalyst and 3 ml of DMF
at 100 °C (Table 4, entry 11).
Table 6. Comparison of various catalysts used in synthesis of
5-substituted 1H-tetrazole from benzonitrile
Entry
Catalyst
Conditions
Time Yield
(h) (%)
Ref.
[54]
1
Cu–Zn alloy
DMF/120 °C
10
95
nanopowder
SiO2–H2SO4
[55]
[26]
[56]
[57]
[1]
2
DMF/130 °C
DMF/120 °C
DMF/120 °C
DMF/120 °C
DMF/120 °C
12
12
14
12
36
88
82
90
79
96
90
75
84
90
3
Nano-CuFe2O4
CoY zeolite
4
5
FeCl3–SiO2
6
ZnS nanospheres
Sulfated zirconia
ZnWO4
[58]
[23]
[22]
[25]
7
DMF/110–120 °C 24
DMF/120 °C 24
DMF/120–130 °C 12
DMF/120 °C 12
8
9
Zn/Al-HT
10
11
Nano-ZnO/Co3O4
Fe3O4@SiO2-DAQ-Cu(II) DMF/100 °C
2.5 96 This work
After optimizing the reaction conditions, we next examined the
generality of the reaction using sodium azide and various
benzonitriles. The results are summarized in Table 5. Aromatic ni-
triles containing both electron-donating and electron-withdrawing
groups undergo conversion smoothly in high to excellent yields.
The nature of the substituents on the benzonitriles had a significant
effect on the reaction time. Reactions of electron-poor aromatic
and heteroaromatic nitriles are completed within a few hours
(Table 5, entries 6–12). Nitriles having electron-donating substitu-
ents require longer reaction time (Table 5, entries 2–4). The
ortho-substituted nitriles render lower yields (Table 5, entry 8) due
to steric hindrance. Benzyl nitriles require longer reaction times
than benzonitriles because of absence of conjugated electron with-
drawing aromatic ring in benzyl nitrile (Table 5, entries 15 and 16).
A comparison among Fe3O4@SiO2-DAQ-Cu(II) and other catalysts
reported in the literature, in the synthesis of 5-phenyl-1H-tetrazoles,
reveals advantages of Fe3O4@SiO2-DAQ-Cu(II) over the most of
them in terms of higher yield and shorter reaction time (Table 6,
entry 11).
In order to confirm the reusability and stability of this magnetic
catalyst in the synthesis of 1- and 5-phenyl-1H-tetrazoles, it was
recovered by applying a magnetic field and reused six times with-
out observation of appreciable loss in its catalytic activity (Fig. 6(A)).
Also, to confirm if any aggregation of the nanoparticles occurs,
TEM analysis of the reused catalyst was performed (Fig. 6(B)). After
Table 5. Fe3O4@SiO2-DAQ-Cu(II)-catalysed synthesis of 5-substituted
1H-tetrazolesa
Entry
Substrate
Product Time Yield
(h) (%)b
M.p. (lit.) (°C)c
1
C6H5
8a
8b
8c
8d
8e
8f
2.5
4
96 215–217 (215)[52]
2
4-MeC6H4
4-MeOC6H4
4-OHC6H4
4-BrC6H4
4-ClC6H4
91 249–250 (248–249)[53]
87 230–231 (231–233)[53]
93 235–236 (234)[52]
3
5
4
5
5
4
93 266–268 (266–267)[15]
88 260–261 (261–263)[28]
95 217–218 (219–220)[51]
83 209–211 (208–210)[51]
92 210–212 (213)[52]
6
3
7
4-NO2C6H4
2-CNC6H4
3-CNC6H4
4-CNC6H4
2-Pyridyl
8 g
8 h
8i
2
8
4.5
3
9
10
11
12
13
14
15
16
8j
3
95
255 (254–256)[15]
8 k
8 l
3.5
4
94 211–212 (210–211)[28]
92 237–238 (239)[28]
95 261–262 (264)[52]
3-Pyridyl
1-Naphthalene 8 m
9-Phenanthrenyl 8n
3
6
88
216 (214–216)[15]
C6H5CH2
8o
8p
6
84 121–122 (123–124)[51]
90 181–183 (184)[52]
4-NO2C6H5CH2
4
aReaction carried out with nitrile (1.0 mmol), NaN3 (1.2 mmol) and
Fe3O4@SiO2-DAQ-Cu(II) (0.9 mol%) in 3 ml of DMF at 100 °C.
bIsolated yield.
Figure 6. (A) Recyclability of Fe3O4@SiO2-DAQ-Cu(II) in the synthesis of
(a) 1-phenyl-1H-tetrazole and (b) 5-phenyl-1H-tetrazole. (B) TEM image of
Fe3O4@SiO2-DAQ-Cu(II) nanoparticles after six reaction cycles.
cMelting points reported in parentheses refer to literature melting points.
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

Esmaeilpour M., Javidi J. and Zahmatkesh S.
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the morphology of the catalyst, but some nanoparticles might have
aggregated onto the surfaces of the matrices.
To further confirm the robustness of the catalyst and to measure
the extent of Cu leaching after the reactions, we carried out a hot
filtration experiment.[42] For this, we studied the condensation of
benzonitrile and sodium azide in the presence of Fe3O4@SiO2-
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was half complete, Fe3O4@SiO2-DAQ-Cu(II) nanoparticles were
removed in situ at the reaction temperature, and the reactants were
allowed to undergo further reaction in the solution. These results
indicate that after removal of the heterogeneous catalyst, the clear
filtrate was weakly active suggesting that only slight leaching had
occurred during the reaction.
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using the ICP technique shows that the leaching of Cu is negligible.
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Conclusions
In summary, we have reported that 1,4-dihydroxyanthraquinone–
copper(II) supported on superparamagnetic Fe3O4@SiO2 nanoparti-
cles can be an efficient and reusable catalyst for one-pot syntheses
of 1- and 5-substituted 1H-tetrazoles from nitriles and amines in
good to excellent yields. This method has notable advantages such
as thermal stability, heterogeneous nature, easy preparation and
easy separation of catalyst, short reaction time, clean and simple
procedure, excellent yields, easy product separation and purifica-
tion and lower loading of catalyst compared with other methods.
These factors make this method an alternative to the previous
methodologies for the scale-up of these reactions. In addition, the
catalyst could be successfully recovered and recycled at least six
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Acknowledgements
This work was supported by Council of Iran National Science
Foundation and University of Shiraz from the Iran Society for the
Promotion of Science.
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Supporting information
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Appl. Organometal. Chem. (2016)