Core-shell structured magnetite silica-supported hexatungstate: A novel and powerful nanocatalyst for the synthesis of biologically active pyrazole derivatives

Article abstract of DOI:10.1002/aoc.6409

This article describes the synthesis of a novel core-shell structured magnetic silica-supported hexatungstate (Fe₃O₄@SiO₂-NH₃[W₆O₁₉]) nanocatalyst through immobilization of hexatungstate on

Full text of DOI:10.1002/aoc.6409

Received: 23 February 2021
DOI: 10.1002/aoc.6409
Revised: 22 July 2021
Accepted: 30 July 2021
F U L L P A P E R
Core-shell structured magnetite silica-supported
hexatungstate: A novel and powerful nanocatalyst for the
synthesis of biologically active pyrazole derivatives
Ramin Nemati
| Dawood Elhamifar
| Ali Zarnegaryan
|
Masoumeh Shaker
Department of Chemistry, Yasouj
University, Yasouj, Iran
Abstract
This article describes the synthesis of a novel core-shell structured magnetic
silica-supported hexatungstate (Fe3O4@SiO2-NH3[W6O19]) nanocatalyst
through immobilization of hexatungstate on amine-modified magnetic silica
nanoparticles. The physicochemical properties of Fe3O4@SiO2-NH3[W6O19]
were investigated by using Fourier transform infrared (FT-IR) spectroscopy,
powder X-ray diffraction, vibrating sample magnetometer, wide-angle PXRD,
scanning electron microscopy, transmission electron microscopy, and energy-
dispersive X-ray spectroscopy analyses. The Fe3O4@SiO2-NH3[W6O19]
nanocatalyst was used as a powerful and heterogeneous catalyst in the
synthesis of pyrazole derivatives. This novel catalyst was reused at least seven
times without loss of its activity.
Correspondence
Dawood Elhamifar, Department of
Chemistry, Yasouj University, Yasouj
75918-74831, Iran.
Email: d.elhamifar@yu.ac.ir
Funding information
Iran National Science Foundation; Yasouj
University
K E Y W O R D S
core-shell, nanocatalyst, polyoxometalate, pyrazole
1 | INTRODUCTION
Polyoxometalates are a class of metals with highest
oxidation state containing oxo-ligands. Due to the unique
Pyrazole and its derivatives are among the five-
membered heterocycles constituting a class of com-
pounds particularly effective in organic synthesis.
These have received considerable attention due to their
wide range of medicinal properties and biological
activity.[1,2] Some biological activities of pyrazoles are
antioxidant,[3] antibacterial,[4,5] analgesic,[6] antitumor,[7,8]
cardiovascular,[9] anticonvulsant,[10] and so on. Although
many studies have been reported for the synthesis of
pyrazole products using homogeneous and heteroge-
neous catalysts, most of these reports suffer from restric-
tions of high temperature and high pressure and the use
of environmentally harmful organic solvents.[11,12] There-
fore, it is very important to develop a clean catalytic pro-
cess for the efficient synthesis of pyrazoles.
properties of polyoxometalates such as diversity in
structure, high thermal stability, high surface charge
distribution, and resistance toward oxidative conditions,
they have been thoroughly considered as catalyst either
under heterogeneous or homogeneous conditions.[13–16]
Among different structures of polyoxoanions, Lindqvist
one with formula of [M6O19]nÀ (M = W, Mo, Ta, V, and
Nb) is more considered.[17] As an individual metal oxide
cluster, the Lindqvist hexatungstate is a very good
candidate for the construction of hybrid materials.
Facing with the requirement of sustainable
expansion, many efforts have been made to design and
preparation of polyoxometalate-based hybrid materials,
not only due to their appealing architectures[18,19] but
also owing to their potential applications in the catalysis,
Appl Organomet Chem. 2021;e6409.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6409

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NEMATI ET AL.
surface and interface science, magnetic chemistry,
materials science, and analytical chemistry.[15,16,20–23]
On the other hand, in recent years, nanocomposites
have attracted a lot of attention due to their wide applica-
tions in various fields such as photocatalysts[24,25] and
catalysts,[26–33] lithium batteries,[34] biomedical,[35] solar
cells,[36] and sensors.[37] Magnetic core-shell structured
nanomaterials, as a special class of nanocomposites, are
particularly important between researchers. Among
these, the Fe2O3@SiO2 NPs with Fe3O4 core and silica
SCHEME 1 Preparation of
the Fe3O4@SiO2-NH3[W6O19]
nanocatalyst
FIGURE 1 FT-IR spectra of (a) [Bu4N]
2[W6O19], (b) Fe3O4, (c) Fe3O4@SiO2,
(d) Fe3O4@SiO2-NH3Cl, and (e) Fe3O4@SiO2-
NH3[W6O19]

NEMATI ET AL.
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shell has attracted more attention.[38–40] It is also
important to note that the Fe3O4 nanoparticles (NPs),
prepared from easily available precursors and with
tunable properties, have drawn much attention owing to
their widespread applications in material and chemical
processes.[41–43] Also, silica has received much
attention due to its poor chemical permeability, non-
toxicity, high chemical and thermal stability, and high
availability of silanol groups on its surface for any
2.2 | Synthesis of Fe3O4 NPs
The Fe3O4 NPs were prepared according to our previous
procedure.[64] For this, firstly, FeCl3Á6H2O (3 g) and
FeCl2Á4H2O (2 g) were dissolved in deionized water
(25 ml). Then, NH3 (20 ml, 25% wt) was added dropwise
in the reaction mixture while stirring. The resulted
combination was stirred at room temperature for 30 min.
modification.[26,44]
Recently,
different
magnetic
nanoparticles (MNPs) have been used as support for
homogeneous catalysts. The most fascinating feature of
MNP-supported catalysts is the easy recycling controlled
by simple magnetically separation.[45–52] As example, a
number of MNP-supported polyoxometalates have been
prepared and applied as recoverable heterogeneous
catalysts in organic reactions.[53,54] Some of the recent
reports in this matter are γ-Fe2O3@SiO2-PW,[55]
magPOM-SILPs,[56] Fe3O4@SiO2-HPW,[57] BeW12O40-
ILSCCFNPs,[58]
Fe@PILPW-AM,[19]
and
Fe3O4@SiO2@HPW.[59]
In view of the above and in continuation of our recent
studies in the fields of MNP-supported catalysts,[60–62]
a
Lindqvist-type polyoxometalate ([Bu4N]2[W6O19]) is
immobilized on amine-modified Fe3O4@SiO2 MNPs to
deliver a powerful and highly recoverable catalyst for the
synthesis of pyrazole derivatives.
2 | EXPERIMENTAL
2.1 | Synthesis of tetrabutylammonium
hexatungstate salt ([Bu4N]2[W6O19])
Lindqvist-type polyoxometalate ([Bu4N]2[W6O19]) was
prepared according to a previous method.[63] For this,
sodium tungstate dihydrate (Na2WO4.2H2O, 1 g) was
mixed with dimethylformamide (DMF, 0.9 ml) and
acetic anhydride (1.2 ml) while incubating at 100ꢀC for
4 h to give a white cream-like product. Then, HCl
(0.6 ml, 12 N) and acetic anhydride (0.6 ml) and DMF
(1.5 ml) were added to the reaction vessel. After
10 min, the mixture was filtered, washed with
methanol, and allowed to cool to room temperature.
Then, methanol (1.5 ml) and tetrabutylammonium
bromide (C16H36BrN, 0.5 g) were added to the resulted
mixture, and it was stirred for 10 min. The resulting
precipitate was separated and washed with diethyl ether
and methanol. Next, this material was added to
dimethylsulfoxide (DMSO, 5 ml) and kept at 25ꢀC for
1 day. Finally, the resulted mixture was filtered and
dried under the vacuum condition. The resulted prod-
uct was called [Bu4N]2[W6O19].
FIGURE 2 The wide-angle PXRD pattern of (a) Fe3O4,
(b) Fe3O4@SiO2, (c) Fe3O4@SiO2-NH3Cl, and (d) Fe3O4@SiO2-
NH3[W6O19]

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NEMATI ET AL.
FIGURE 3 SEM image of (a) Fe3O4, (b) Fe3O4@SiO2, (c) Fe3O4@SiO2-NH3Cl, and (d) Fe3O4@SiO2-NH3[W6O19]
FIGURE 4 The EDX analysis of
Fe3O4@SiO2-NH3[W6O19]

NEMATI ET AL.
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After that, the product was separated using an
external magnet and washed with deionized water. The
final material was dried at 70ꢀC for 12 h and called
Fe3O4 NPs.
2.6 | Procedure for the synthesis of
pyrazole derivatives using Fe3O4@SiO2-
NH3[W6O19]
For this purpose, Fe3O4@SiO2-NH3[W6O19] (5 mg) was
added to a flask containing malononitrile (1 mmol), alde-
hyde (1 mmol), phenylhydrazine (1 mmol), and ethanol
(10 ml), and the resulted mixture was magnetically
stirred at room temperature. The reaction progress was
monitored by thin-layer chromatography (TLC). After
completion of the process and removal of the catalyst,
hot ethanol (8 ml) was added. Some ice species were
finally added to the reaction vessel to precipitate the
desired pyrazole products.
2.3 | Synthesis of Fe3O4@SiO2
For this purpose, the Fe3O4 NPs (0.5 g) were
completely dispersed in a solution containing water
(10 ml) and ethanol (40 ml) at room temperature for
20 min. After that, ammonia (4 ml, 25% wt) was
added to the reaction vessel. Then, TEOS (0.5 ml) was
added dropwise, and the resulted mixture was stirred
at room temperature for 16 h. Next, the product was
collected using a magnetic field and washed several
times with ethanol and deionized water. The final
material was dried at 70ꢀC for 6 h and denoted as
Fe3O4@SiO2.
2.4 | Synthesis of Fe3O4@SiO2-NH3Cl
For this, 1 g of Fe3O4@SiO2 was completely dispersed in
toluene (30 ml) under ultrasonication. After 15 min,
3-chloropropyltrimethoxysilane (CPTMS, 1 mmol) was
added in the vessel reaction, and this combination
was refluxed for 24 h. Then, the product was collected
using an external magnet, washed with ethanol and
deionized water, dried at 70ꢀC, and denoted as
Fe3O4@SiO2-PrCl. Subsequently, the Fe3O4@SiO2-PrCl
(1 g) was dispersed in toluene (40 ml). After adding of
N-(aminomethyl)methanediamine (1.5 mmol), the
mixture was refluxed for 16 h. Next, the resulted material
was collected using a magnetic field and washed with
ethanol and deionized water. Finally, the product was
acidified with dilute HCl solution (40 ml, 0.5 M) to give
Fe3O4@SiO2-NH3Cl.
FIGURE 5 The TEM image of Fe3O4@SiO2-NH3[W6O19]
2.5 | Immobilization of [Bu4N]2[W6O19]
on Fe3O4@SiO2-NH3Cl
For this, Fe3O4@SiO2-NH3Cl (1 g) was dispersed in
DMSO
(15 ml)
under
ultrasonication.
Then,
[Bu4N]2[W6O19] (0.4 g) was added, and the resulted mix-
ture was stirred at room temperature for 24 h. Next, the
mixture was heated to 100ꢀC and stirred at for 4 h.
The final material was collected using a magnetic field
and washed with ethanol and deionized water and dried
in at 70ꢀC for 7 h. The product was denoted as
Fe3O4@SiO2-NH3[W6O19].
FIGURE 6 The VSM analysis of the Fe3O4@SiO2-NH3[W6O19]

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NEMATI ET AL.
3 | RESULTS AND DISCUSSION
The wide-angle PXRD (WAPXRD) analysis of Fe3O4,
Fe3O4@SiO2, Fe3O4@SiO2-NH3Cl, and Fe3O4@SiO2-
NH3[W6O19] nanomaterials showed eight signals at 2θ of
30.49ꢀ, 35.78ꢀ, 43.39ꢀ, 53.87ꢀ, 57.38ꢀ, 62.91ꢀ, 74.63ꢀ, and
75.20ꢀ, respectively, corresponding to the Miller index
values (hkl) of (220), (311), (400), (422), (511), (440),
(533), and (620) (Figure 2).[60,67] These peaks are related
to the crystalline structure of magnetic iron oxides, prov-
ing high stability and durability of these magnetite NPs
during catalyst preparation steps.
Scanning electron microscopy (SEM) was performed
to investigate the particle morphology at different steps
of catalyst preparation. This clearly showed the presence
of particles with spherical morphology and uniform size
for the materials at all steps (Figure 3). In particular, the
SEM of the Fe3O4@SiO2-NH3[W6O19] clearly showed
the presence of spherical particles with an average size of
68 nm (Figure 3d). These types of core-shell structured
spherical particles are very good candidates for the
adsorption and catalysis applications.
The chemical composition of Fe3O4@SiO2-
NH3[W6O19] was studied by energy-dispersive X-ray
(EDX) spectroscopy (Figure 4). This analysis showed the
presence of Fe (13.5%), W (1.9%), N (2.8%), Si (10.8%), C
(9.1%), and O (61.6%) wt% elements in the sample, con-
firming the well incorporation/immobilization of
expected species into/onto the material framework.
Figure 5 provides the transmission electron micros-
copy (TEM) image of the Fe3O4@SiO2-NH3 [W6O19] cata-
lyst. According to this analysis, a core-shell structure
The preparation of Fe3O4@SiO2-NH3[W6O19] is illus-
trated in Scheme 1. As shown, a silica shell was first
coated on the magnetite NPs through a sol–gel process to
give Fe3O4@SiO2. The Fe3O4@SiO2 NPs were then
modified with CPTMS and N-(aminomethyl)met-
hanediamine (C2H9N3) to deliver Fe3O4@SiO2-NH2. The
Fe3O4@SiO2-NH2 material was acidified using a dilute
HCl solution (2 M) to give Fe3O4@SiO2-NH3Cl. Finally,
the Fe3O4@SiO2-NH3[W6O19] was obtained via
treatment of Fe3O4@SiO2-NH3Cl with [Bu4N]2[W6O19]
polyoxometalate.
The functional groups of the synthesized magnetic
materials were determined using Fourier transform
infrared (FT-IR) spectroscopy (Figure 1). The peak
appeared at 530 cmÀ1 is related to Fe–O bond
(Figure 1b–e).[44] The peak at 1655 cmÀ1 is assigned to
bending and stretching vibrations of the N–H groups
(Figure 1d,e). The peak observed at 3422 cmÀ1 is due to
O–H bond (Figure 1a–e). The signals at 2825 and
2924 cmÀ1 are related to the vibrations of the C–H bonds
of the propyl group (Figure 1d,e). The sharp peaks at
1070 and 823 cmÀ1 corresponded to the symmetric and
asymmetric vibrations of Si–O–Si bonds of SiO2 moieties
(Figure 1c–e).[65] Also, the sharp peak at 800 cmÀ1 is
related to the W-O-W vibrations (Figure 1a,e).[66] These
results confirm successful incorporation/immobilization
of magnetite core, silica moieties, and polyoxometalate
complex into/onto material framework.
TABLE 1 Screening various
parameters in the synthesis of pyrazoles
catalyzed by Fe3O4@SiO2-NH3[W6O19]a
Entry
Solvent
-
Catalyst (mg)
Yield (%)a
1
2
3
4
5
6
7
8
9
5
5
5
5
5
3
4
6
7
42
79
68
85
94
80
88
91
92
Acetonitrile
Toluene
H2O
EtOH
EtOH
EtOH
EtOH
EtOH
aAll reactions were performed at room temperature for 25 min.

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TABLE 2 Preparation of pyrazole derivatives in the presence of Fe3O4@SiO2-NH3[W6O19] nanocatalysta
Entry
R
Time (min)
Yield (%)
M. P. (ꢀC)
M. P. reported
Product
1
25
94
157–160
158–159[70]
2
3
4
5
6
25
15
25
30
27
95
95
88
85
86
128–129
165–168
106–109
207–209
119–121
127–129[70]
161–162[70]
107–108[70]
208–210[71]
117–118[70]
(Continues)

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NEMATI ET AL.
TABLE 2 (Continued)
Entry
R
Time (min)
Yield (%)
M. P. (ꢀC)
M. P. reported
Product
7
35
82
140–142
141–144[72]
8b
40
74
128–130
128–129[73]
aReaction conditions: aldehyde (1 mmol), malononitrile (1 mmol), phenylhydrazine (1 mmol), EtOH (5 ml), Fe3O4@SiO2-NH3[W6O19] (5 mg), room
temperature.
b2 mmol of aldehyde was used.
FIGURE 7 Reusability of Fe3O4@SiO2-
NH3[W6O19]
with black core (magnetite NPs) and gray shell (modified
silica layer) was confirmed for the designed catalyst.
magnetization of about 38 emu/g is observed for the
designed catalyst, confirming very good super-
paramagnetic behavior of the material that is a very
important characteristic, especially in the field of cata-
lytic application.[68,69]
The
NH3[W6O19] were also studied using vibrating sample
magnetometer (VSM) (Figure 6). As shown,
magnetic
properties
of
Fe3O4@SiO2-
a

NEMATI ET AL.
9 of 13
After characterization of Fe3O4@SiO2NH3[W6O19], its
to these results, the use of 5 mg of catalyst and ethanol as
an environmentally friendly solvent at room temperature
were selected as optimum conditions.
Under optimal conditions in hand, the efficiency of
the designed catalyst in the synthesis of different deriva-
tives of pyrazole was investigated (Table 2). As shown, all
aromatic aldehydes bearing both electron-donating and
catalytic activity was studied in a multicomponent reac-
tion to prepare pyrazole derivatives. The optimized condi-
tion was obtained in the reaction of benzaldehyde,
phenylhydrazine, and malononitrile as a test model. For
this, the effect of solvent and amount of catalyst were
examined. Under solvent-free condition, only a low yield
of the desired product was obtained (Table 1, Entry 1). In
acetonitrile, toluene, and H2O, respectively, 79%, 68%,
and 85% yields were resulted (Table 1, Entries 2–4). The
highest yield and conversion were obtained in ethanol
(Table 1, Entry 5). It was also found that by increasing
the catalyst amount from 3 to 5 mg, the yield is increased,
and the further amount of catalyst has no notable effect
on the reaction progress (Table 1, Entries 5–9). According
electron-withdrawing
substituents
gave
the
corresponding pyrazole products in high yields (Table 2,
Entries 1–6). The heteroaromatic substrates such as
thiophene-2-carbaldehyde also delivered the pyrazole
adduct in high yield (Table 2, Entry 7). The aliphatic
aldehydes such as ethanal also gave corresponding
pyrazole in good yield (Table 2, Entry 8). These results
confirm high performance of the designed catalyst to
FIGURE 8 FT-IR spectrum of the recovered
Fe3O4@SiO2-NH3[W6O19] catalyst
FIGURE 9 PXRD pattern of the recovered
Fe3O4@SiO2-NH3[W6O19] catalyst

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NEMATI ET AL.
prepare a wide range of biologically active pyrazole
derivatives.
The SEM image of the recovered catalyst also showed
a uniform spherical structure the same as its fresh parent,
confirming high stability of the material structure during
applied conditions (Figure 11).
The TEM image of the recovered catalyst was also
performed and compared with that of fresh catalyst
(Figure 12). As shown, the same as fresh catalyst, the
TEM image of the recovered catalyst also shows a core-
shell structure, proving the high stability of the material
structure during applied conditions.
After that, the reusability and recoverability of the
catalytic system were investigated. To do this, the con-
densation between malononitrile, benzaldehyde, and
phenylhydrazine in the presence of the Fe3O4@SiO2-
NH3[W6O19] nanocatalyst was selected as a test model.
For this, after the end of each run, the catalyst was sepa-
rated using a magnetic field and reused in another cycle
under conditions the same as the first run. The results
showed that the Fe3O4@SiO2-NH3[W6O19] catalyst can
be reused and recovered at least seven times without a
significant loss in its performance (Figure 7). This con-
firms high durability and high stability of the designed
catalyst under applied conditions.
In the next step, a leaching test was carried out to
study the heterogeneous behavior of the designed
To confirm the chemical and structural stability of
the nanocatalyst during eight runs of use, the recovered
catalyst was characterized by the FT-IR, powder X-ray
diffraction (PXRD), VSM, SEM, and TEM analyses. The
FT-IR pattern of the recovered catalyst was the same as
the FT-IR spectrum of the fresh catalyst, proving the high
chemical stability of the Fe3O4@SiO2-NH3[W6O19] cata-
lyst under applied conditions (Figure 8).
As shown in Figure 9, the pattern of PXRD analysis
of the recovered catalyst is also the same as the PXRD
pattern of the fresh catalyst, confirming high stability of
the crystalline structure of the Fe3O4 NPs during the
recovery times.
The magnetic properties of the recovered catalyst
were investigated by VSM analysis (Figure 10). The result
showed that the recovered catalyst has the same super-
paramagnetic behavior as the fresh catalyst and its mag-
netic saturation did not change significantly during the
reaction conditions.
FIGURE 11 The SEM image of the recovered Fe3O4@SiO2-
NH3[W6O19] catalyst
FIGURE 10 VSM analysis of the recovered Fe3O4@SiO2-
FIGURE 12 The TEM image of the recovered Fe3O4@SiO2-
NH3[W6O19] catalyst
NH3[W6O19] catalyst

NEMATI ET AL.
11 of 13
SCHEME 2 Proposed mechanism for the synthesis of 5-amino-1,3-diphenyl-1H-pyrazole-4-carbonitrile derivatives using Fe3O4@SiO2-
NH3[W6O19] catalyst
TABLE 3 A comparison study between performance of Fe3O4@SiO2-NH3[W6O19] and former catalysts in the synthesis of pyrazoles
Entry
Catalyst
Conditions
H2O, r.t
Time (min)
Recovery times
Ref.
[74]
1
2
3
4
5
Fe3O4
60
3
6
5
4
4
7
[70]
[75]
[72]
Alumina-silica-MnO2/SDBS
Glu.@Fe3O4
Distilled water, 35ꢀC
H2O, r.t.
8
Fe3O4@CQDs@Si (CH2)3NH2@CC@EDA@Cu
Fe3O4@SiO2-N[W6O19]
Solvent-free, r.t.
EtOH, r.t.
5
25
This work
catalyst. For this, after completion of about 40% of the
condensation process, the catalyst was removed using a
magnet, and the progress of the residue mixture was
monitored. Interestingly, after 50 min, no significant pro-
gress was observed in the reaction, implying the effective-
ness of the Fe3O4@SiO2-NH3Cl support in the well
immobilization of active hexatungstate species.
A mechanism for the synthesis of 5-amino-1,-
3-diphenyl-1H-pyrazole-4-carbonitrile derivatives using
Fe3O4@SiO2-NH3[W6O19] catalyst is proposed in
Scheme 2. In the first step, the acidic proton of
malononitrile is deprotonated by Lewis basic oxygen
groups of the catalyst, and the corresponding anion is
produced. Then, this anion is condensed with the W-

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NEMATI ET AL.
activated aldehyde through a Knoevenagel process to
deliver intermediate 1. In the next step, the intermediate
1, as a Michael acceptor, reacts with the phenylhydrazine
to give intermediate 2. After that, intermediate 2 is
converted to intermediate 3 through an intramolecular
cyclocondensation. Finally, the desired product 4 is
formed through oxidation and releasing a hydrogen mol-
ecule (–H2).[70,71]
Finally, the catalytic performance of our designed cat-
alyst was compared with some of the catalysts recently
used in the preparation of pyrazoles. This study showed
that present catalyst is better than of the previous cata-
lysts in recycling times and reaction time. Also, this study
confirmed the higher efficiency and stability of the pre-
sent catalyst compared with previous catalytic systems
(Table 3).
ORCID
Ramin Nemati https://orcid.org/0000-0002-0800-1433
Dawood Elhamifar https://orcid.org/0000-0003-4889-
9505
Ali Zarnegaryan https://orcid.org/0000-0001-8630-0335
Masoumeh Shaker https://orcid.org/0000-0001-8216-
2238
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A novel core-shell structured magnetite silica-supported
hexatungstate (Fe3O4@SiO2-NH3[W6O19]) nanocatalyst
was prepared, and its properties were investigated. The
FT-IR and EDX analyses of the Fe3O4@SiO2-
NH3[W6O19]
proved
the
well
immobilization/
incorporation of hexatungstate ions and silica shell on
the surface of the Fe3O4 NPs. The SEM and TEM
images showed spherical particles with core-shell
structure for the catalyst. The PXRD and VSM analyses
confirmed
good
magnetic
properties
of
the
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D. Elhamifar, A. Zarnegaryan, M. Shaker, Appl
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