Green and highly efficient approach for the reductive coupling of nitroarenes to azoxyarenes using the new mesoporous Fe3O4@SiO2@Co–Zr–Sb catalyst

Article abstract of DOI:10.1007/s11164-020-04126-7

Efficient, green, simple and environmentally friendly approach for the straightforward reductive coupling of nitroarenes to the corresponding azoxyarenes has been developed in the presence of Fe₃O₄@SiO₂@Co–Zr–Sb as a novel recyclable nanocatalyst. The Co–Zr–Sb trimetallic nanoparticles immobilized on silica-layered magnetite have been prepared by the co-precipitation method. The mesoporous catalyst has been characterized by FT-IR, SEM, EDX, VSM, TEM and XRD analyses. The chemoselective hydrogenation of nitrobenzenes was carried out successfully in refluxing water to afford the corresponding azoxybenzenes within 2–10?min in good to high yields. The reusability of the heterogeneous nanocatalyst has also been studied using the FT-IR and SEM analyses. The catalyst was utilized four times in sequential runs without significant loss of activity. The current research includes remarkable advantages of short reaction times, absence of hazardous organic solvents, mild reaction conditions, high yields, using water as a green solvent and the ability to utilize the recyclable nanomagnetic catalyst.

Full text of DOI:10.1007/s11164-020-04126-7

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-020-04126-7
Green and highly efcient approach for the reductive
coupling of nitroarenes to azoxyarenes using the new
mesoporous Fe3O4@SiO2@Co–Zr–Sb catalyst
Behzad Zeynizadeh1 · Masumeh Gilanizadeh1
Received: 18 December 2019 / Accepted: 10 March 2020
© Springer Nature B.V. 2020
Abstract
Efcient, green, simple and environmentally friendly approach for the straightfor-
ward reductive coupling of nitroarenes to the corresponding azoxyarenes has been
developed in the presence of Fe3O4@SiO2@Co–Zr–Sb as a novel recyclable nano-
catalyst. The Co–Zr–Sb trimetallic nanoparticles immobilized on silica-layered
magnetite have been prepared by the co-precipitation method. The mesoporous cata-
lyst has been characterized by FT-IR, SEM, EDX, VSM, TEM and XRD analyses.
The chemoselective hydrogenation of nitrobenzenes was carried out successfully
in refuxing water to aford the corresponding azoxybenzenes within 2–10 min in
good to high yields. The reusability of the heterogeneous nanocatalyst has also been
studied using the FT-IR and SEM analyses. The catalyst was utilized four times in
sequential runs without signifcant loss of activity. The current research includes
remarkable advantages of short reaction times, absence of hazardous organic sol-
vents, mild reaction conditions, high yields, using water as a green solvent and the
ability to utilize the recyclable nanomagnetic catalyst.
Keywords Fe3O4@SiO2@Co–Zr–Sb · Azoxybenzenes · Nanocatalyst · Nitroarenes ·
Reductive coupling
Introduction
Azoxybenzenes or diphenyldiazene oxides have been considered as one of the
important and prominent intermediates in material science, pharmaceuticals and
agrochemicals [1]. These materials, as 1,3-dipolar compounds, are composed of
an unusual oxygen–nitrogen–nitrogen bond in their structure [2]. Azoxyarenes
have been utilized as polymerization inhibitors, therapeutic medicines, chemical
* Masumeh Gilanizadeh
masumehgilanizadeh@gmail.com
1
Faculty of Chemistry, Urmia University, Urmia 5756151818, Iran
Vol.:(0123456789)
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B. Zeynizadeh, M. Gilanizadeh
stabilizers, reducing agents, dyes, analytical reagents and also key building blocks
for electronic devices [3–8]. Commonly, these types of compounds are prepared by
the oxidation of anilines and reduction of nitroarenes. Anilines can be oxidized using
the heterogeneous catalysts and oxidants to produce azoxybenzenes [9–11]. How-
ever, this procedure provides the products with low selectivity and by-products in a
large amount. On the other hand, nitroarenes can be reduced by using several types
of methods [12–18]. But because of the complexity of multiple reduction steps, pro-
ducing the highly selective azoxybenzenes often requires unique catalysts or com-
plex reduction systems [19]. As well, azoxyarenes can be prepared from nitrosohy-
droxylamine ammonium salts [20], hydroxylamines [21], nitroso [22, 23] and azo
compounds [24, 25]. As outlined in Scheme 1, types of intermediates and products
are obtained during the reduction of nitroarenes. Accordingly, the use of appropriate
reaction conditions is required to acquire a selective intermediate or product.
Currently, heterogeneous catalysts have been developed as a major part in various
organic syntheses to improve and facilitate diverse synthetic reactions [26–40]. To
date, the utilization of magnetic nanoparticles (MNPs) has attracted a lot of atten-
tion due to their easy separation by using an external magnetic feld instead of usual
fltration methods [41, 42]. Also, the application of magnetite core–shell nanocom-
posites as efective catalysts has been widely considered [43–52].
In addition, the use of trimetallic catalysts has been considerably attracted due to
the better technological and scientifc properties of this type of nanoparticles. The
trimetallic NPs have been synthesized by various methods such as selective catalytic
reduction, microwave, micro-emulsion, hydrothermal and co-precipitation. The sur-
face of trimetallic nanoparticles is relatively unstable, so it easily precipitates from
their solution and eventually reduces their catalytic activity. Thus, the trimetallic
nanocomposites can be stabilized and synthesized with organic and inorganic com-
pounds [53].
In recent years, the development of simple approaches for the synthesis of dif-
ferent chemical compounds, especially natural products and fundamental materials
Scheme 1 Reaction pathways during the reduction of nitroarenes
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Green and highly efcient approach for the reductive coupling…
Scheme 2 One-pot reduction of nitroarenes to azoxyarenes with Fe3O4@SiO2@Co–Zr–Sb/NaBH4/H2O
system
Scheme 3 Synthesis of nanomagnetic Fe3O4@SiO2@Co–Zr–Sb NPs
under green conditions, is desirable. In this regard, the use of heterogeneous mag-
netic trimetallic nanocomposites based on green chemistry under mild reaction con-
ditions is required for efcient synthesis of azoxybenzenes.
In continuation of our research on the successful synthesis of layered double
hydroxides (LDHs) with a combination of three cations on the surface of nano-
magnetic cores [40], herein, we would like to report the novel recoverable trime-
tallic Fe3O4@SiO2@Co–Zr–Sb nanocatalyst for the chemoselective reduction of
nitroarenes to azoxyarenes in the presence of NaBH4 as a hydrogen donor in water,
with a simple and by-product free approach (Scheme 2).
Results and discussion
Synthesis and characterizations of Fe3O4@SiO2@Co–Zr–Sb NPs
The magnetic nanocatalyst was prepared with an easy co-precipitation method using
a three-step process: (i) preparation of magnetite NPs; (ii) synthesis of Fe3O4@SiO2
NPs at ambient temperature; and (iii) growth of Co–Zr–Sb trimetallic nanoparti-
cles on the surface of SiO2-layered Fe3O4 by an in situ growth method via aqueous
solutions of Co2+, Zr4+ and Sb3+ salts (Scheme 3). Also, Fe3O4@SiO2@Co–Zr–Sb
nanocatalyst was characterized using various methods such as Fourier transform
infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), energy-dis-
persive X-ray (EDX), vibrating sample magnetometer (VSM), transmission electron
microscopy (TEM) and X-ray difraction (XRD).
Firstly, the functional groups in the magnetic nanoparticles of Fe3O4, Fe3O4@
SiO2 and Fe3O4@SiO2@Co–Zr–Sb were identifed by the Fourier transform
infrared spectroscopy. The strong absorption peak at low wavenumber (579 cm−1)
1 3

B. Zeynizadeh, M. Gilanizadeh
in the FT-IR spectrum of magnetite (Fig. 1a) comes from the vibration of Fe–O
bonds in iron oxide. The absorption bands at around 3400 and 1625 cm−1 corre-
spond to the O–H stretching and deforming modes in the adsorbed water, respec-
tively. FT-IR spectrum of silica-layered magnetite (Fig. 1b) shows additional
peaks compared to the Fe3O4 one. The broad bands at 1090 and 791 cm−1 are
referred to the asymmetrical and symmetrical stretching vibrations of Si–O–Si,
which confrm the presence of SiO2 layers in the magnetic nanoparticles. Fig-
ure 1c exhibits the FT-IR spectrum of Fe3O4@SiO2@Co–Zr–Sb MNPs. The
characteristic peaks of 3500 cm−1 (stretching band of metal hydroxyl groups
and hydrogen-bonded water molecules), 1600 cm−1 (deformation absorption of
2−
water), 1385 cm−1 (stretching of interlayer CO3 ions) and 500–1000 cm−1 (lat-
tice vibration peaks of M–OH and M–O) indicate the structure in the core–shell
nanocomposite.
The scanning electron microscopy (SEM) provides more detailed informa-
tion on the morphology of the surface and size distribution of the synthesized
magnetic nanomaterials (Fig. 2). The Fe3O4 nanoparticles show roughly granular
and spherical shape in the range of 23–43 nm (Fig. 2a). As well, the microscopy
image of Fe3O4@SiO2 NPs represents uniform particles in the range of 22–30 nm
(Fig. 2b). In the case of Fe3O4@SiO2@Co–Zr–Sb NPs with diferent magnif-
cations, SEM images indicate that the surface of nanoparticles is modifed and
the size distribution is about 18–23 nm (Fig. 2c,d). Accordingly, the structure of
nanocatalyst can be identifed by SEM images.
The energy-dispersive X-ray spectroscopy is a convenient method for detect-
ing elements with weight and atomic percentage in a material. EDX analysis of
Fe3O4@SiO2@Co–Zr–Sb (Fig. 3) shows the presence of O, Si, Fe, Co, Zr and
Sb elements in the structure of magnetic nanocomposite. This technique clearly
Fig. 1 FT-IR spectrum of a Fe3O4, b Fe3O4@SiO2, and c Fe3O4@SiO2@Co–Zr–Sb NPs
1 3

Green and highly efcient approach for the reductive coupling…
Fig. 2 SEM images of a Fe3O4, b Fe3O4@SiO2, and c, d Fe3O4@SiO2@Co–Zr–Sb NPs
shows that Co–Zr–Sb species have been successfully immobilized on the surface
of Fe3O4@SiO2 NPs.
Magnetic properties were studied by the vibrating sample magnetometer
(VSM). Figure 4 illustrates the magnetic characterization of the synthesized sam-
ples. The curves clearly show a decrease in the amount of magnetism during the
immobilization of a layer around the other. The saturation magnetization (Ms)
value of Fe3O4 (Fig. 4a), Fe3O4@SiO2 (Fig. 4b) and Fe3O4@SiO2@Co–Zr–Sb
1 3

B. Zeynizadeh, M. Gilanizadeh
O Kα
6000
5000
4000
3000
2000
1000
0
ZrLα
CoKα
FeLα
CoLα
SbLα
SiKα
SbLβ
FeKα
ZrLβ
CoKβ
FeKβ
keV
0
10.00
Element
Weight percent
Atom percent
(%)
(%)
31.46
5.31
O
Si
65.53
6.30
Fe
Co
Zr
Sb
4.40
2.62
14.84
7.17
3.54
100.00
26.25
19.63
12.95
100.00
Fig. 3 EDX spectrum and elemental composition of Fe3O4@SiO2@Co–Zr–Sb NPs
NPs (Fig. 4c) is around 70, 30 and 4 emu g−1, respectively. Therefore, the amount
of Ms is reduced by coating one layer on another.
The transmission electron microscopy (TEM) provides more detailed informa-
tion on the particle morphology of the as-prepared nanocomposite. TEM images of
Fe3O4@SiO2@Co–Zr–Sb NPs indicate the dark cores surrounded by the other shells
(Fig. 5).
The XRD pattern of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2@Co–Zr–Sb NPs was
measured by the X-ray difraction (Fig. 6). Figure 6a represents the difraction peaks
at 2θ values of 30.2°, 35.5°, 43.3°, 53.7°, 57.2° and 62.9° correspond to (022), (113),
(004), (224), (115) and (044) crystal planes of nanomagnetite. Fe3O4 has crystalline
cubic spinel structure which agrees with the standard Fe3O4 XRD spectrum (JCPDS
No. 65-3107). The same patterns of characteristic peaks were observed for Fe3O4@
SiO2 NPs (Fig. 6b), demonstrating that the embedded magnetite cores keep their
1 3

Green and highly efcient approach for the reductive coupling…
80
a
60
40
20
b
c
Applied Field (oe)
0
-10000 -8000 -6000 -4000 -2000
0
2000 4000 6000 8000 10000
-20
-40
-60
-80
Magneꢀzaꢀon (emu/g)
Fig. 4 Magnetization curve of (a) Fe3O4, (b) Fe3O4@SiO2 and (c) Fe3O4@SiO2@Co–Zr–Sb NPs
crystalline phase after SiO2 covering. Finally, the XRD pattern of Fe3O4@SiO2@
Co–Zr–Sb (Fig. 6c,d) was investigated to determine the crystal structure. The X-ray
difraction pattern showed amorphous states and phases. To be sure, the XRD was
repeated by another device and the results were the same. Accordingly, the synthe-
sized nanocomposite has an amorphous structure.
Reductive coupling of nitroarenes to azoxyarenes
Initially, the trimetallic nanocomposite was prepared according to our previous
research on the successful synthesis of LDHs with a combination of three cations
on the surface of nanomagnetic cores. The main reason of choosing these metals
is to provide three efcient cations for the synthesis of LDH-like nanocomposites
and to observe their catalytic activity in various organic reactions. Due to this, the
novel nanomagnetic Fe3O4@SiO2@Co–Zr–Sb catalyst has been successfully syn-
thesized and characterized. The catalytic activity of prepared core–shell nanoparti-
cles was investigated by the reductive coupling of nitrobenzenes to azoxybenzenes.
This research was started with the primary aim of using the nanocatalyst to reduce
nitroarenes to the corresponding anilines with sodium borohydride. Therefore, the
reduction of nitrobenzene with reducing agent was selected as a model reaction in
the presence of diferent catalytic amounts of nanocatalyst. At frst, it was amazing
to get orange red solution instead of aniline as an expected product. During fur-
ther studies, the instrumental analysis confrmed the formation of azoxybenzene
as a single product. The summarized results in Table 1 show the efect of some
1 3

B. Zeynizadeh, M. Gilanizadeh
Fig. 5 TEM images of nanomagnetic-Fe3O4@SiO2@Co–Zr–Sb NPs
parameters on the model reaction. In the absence of the catalyst, the reaction showed
no improvement. In order to study the efect of solvent, the model reaction is car-
ried out in various solvents in the presence of heterogeneous nanocatalyst. Investiga-
tions revealed the optimal conditions for the synthesis of azoxybenzene. The start-
ing compound was completely consumed using NaBH4/H2O system under refux
conditions in the presence of the Fe3O4@SiO2@Co–Zr–Sb MNPs (Table 1, entry
11). The examinations exhibited that using nitrobenzene (1 mmol), Fe3O4@SiO2@
Co–Zr–Sb MNPs (20 mg) and NaBH4 (2 mmol) in refuxing water as a green sol-
vent performs azoxybenzene within 3 min in 93% yield (Table 2, entry 1).
In the next step, the usability and generality of this study were investigated by the
reduction of structurally diverse nitroarenes under optimized reaction conditions.
The results show that in all cases, reductive coupling of nitroarenes occurred and
the corresponding products were obtained in high yields (Table 2). The examina-
tions revealed that nitroaromatic compounds (1 mmol) can be reduced using NaBH4
(2 mmol) and Fe3O4@SiO2@Co–Zr–Sb MNPs (20 mg) in refuxing water within
2–10 min to provide azoxybenzenes in 83–93% yields.
1 3

Green and highly efcient approach for the reductive coupling…
Fig. 6 XRD patterns of (a) Fe3O4, (b) Fe3O4@SiO2 and (c), (d) Fe3O4@SiO2@Co–Zr–Sb
The series of control experiments on the model reaction were investigated
with Fe3O4, SiO2, Co–Zr–Sb, Fe3O4@SiO2 and SiO2@Co–Zr–Sb to illustrate
that which part is the main catalytic site. The results revealed that the Co–Zr–Sb
NPs immobilized on silica-layered magnetite is the most efcient catalyst and the
other parts can react in longer time than the original reaction and of course the
synthesis of azoxybenzenes cannot progress as well as the reported catalyst.
In addition, the synthesis of magnetic trimetallic catalyst was performed
using the co-precipitation method. The molar ratios of Co/Zr/Sb were suggested
in accordance with the proposed procedures for the synthesis of layered dou-
ble hydroxides (LDHs) [32, 38]. Due to our experience with LDHs, we selected
the appropriate molar ratio of cations in the catalyst formation. The synthesized
nanocatalyst has three layers, each layer is coated on the other, and the analysis
techniques can confrm this.
In order to study the suitability of this approach, the reductive coupling of
nitrobenzene to azoxybenzene by NaBH4/Fe3O4@SiO2@Co–Zr–Sb MNPs sys-
tem was compared with previously reported methods (Table 3). The current
investigation shows advantages in terms of recoverability and reusability of the
1 3

B. Zeynizadeh, M. Gilanizadeh
Table 1 Optimization experiments for reductive coupling of nitrobenzene to azoxybenzene with NaBH4/
Fe3O4@SiO2@Co–Zr–Sb NPs system under diferent reaction conditions
Entry
NaBH4
(mmol)
Catalyst (g)
Solvent
Condition
Time (min)
Conversion (%)
1
2
3
4
5
6
7
8
2
2
–
2
2
2
2
2
2
2
2
2
2
–
H2O
H2O
H2O
H2O
60–70 °C
r.t.
Refux
300
90
180
90
40
40
40
40
40
40
3
–
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.01
0.03
Trace
0
20
95
70
40
50
0
Trace
100
100
100
50–60 °C
70–80 °C
70–80 °C
70–80 °C
70–80 °C
70–80 °C
70–80 °C
Refux
H2O
EtOH
EtOH/H2O
MeOH
CH3CN
DMF
H2O
9
10
11
12
13
H2O
H2O
Refux
Refux
7
2
All reactions were carried out with 1 mmol of nitrobenzene in 2 mL of solvent
nanocatalyst, green reaction conditions, selective product formation, high yields
and short reaction times.
Although the exact mechanism of this synthetic method is unclear, we propose
the following plausible pathway for the one-pot reductive coupling of nitroarenes
(Scheme 4). Reduction of nitrobenzenes to nitrosobenzenes (1) and phenylhydroxy-
lamines (2) was taken place by the absorption of hydrogen on the surface of nano-
catalyst, selectively. In the next step and through the activation of intermediates, the
condensation of nitrosoarenes and arylhydroxyamines produced the corresponding
azoxybenzenes with water loss.
Recycling of Fe3O4@SiO2@Co–Zr–Sb catalyst
The economic and green aspect of this procedure was examined by the reusability of
Fe3O4@SiO2@Co–Zr–Sb MNPs in reductive coupling of nitroarene to azoxyarene
under the optimized reaction conditions (Table 1, entry 11). After completion of
the reaction, the nanocatalyst was magnetically separated from the reaction mix-
ture, washed with EtOAc and then dried to use in the next runs. The catalyst can be
reused for four consecutive cycles without the remarkable loss of catalytic activity
(Fig. 7).
1 3

Green and highly efcient approach for the reductive coupling…
Table 2 Chemoselective hydrogenation of nitroarenes with NaBH4/Fe3O4@SiO2@Co–Zr–Sb/H2O sys-
tem
Molar ratio
Subs./NaBH4
Time
(min)
Yielda
(%)
M.p. (°C)
(lit.)
Entry
Substrate
Product
References
[22]
O
NO2
N+
N
1
1:2
1:2
1:2
1:3
3
8
6
5
93
87
90
83
< 35 (< 35)
< 35 (16)
67–69 (69)
109–111
O
Me
NO2
Me
N+
N
2
3
4
[14]
[3]
–
Me
Me
O
NO2
N+
N
Me
Me
O
OHC
NO2
HOH2C
N+
N
CH2OH
CH2OH
O
NO2
N+
N
5
6
7
8
1:3
1:2
1:3
1:2
2
4
5
6
85
88
83
91
193–195
–
[14]
–
OHC
HOH2C
Cl
O
NO2
N+
N
97–99
(81–83)
Cl
Cl
Cl
O
OHC
NO2
Cl
HOH2C
N+
N
147–149
CH2OH
OMe
Cl
O
NO2
N+
N
113–115
(118, 135)
[3]
MeO
MeO
CH(OH)CH3
O
NO2
N+
N
9
1:3
10
87
105–107
–
H3COC
H3CH(OH)C
All reactions were carried out with Fe3O4@SiO2@Co–Zr–Sb (20 mg) and H2O (2 mL) under refux con-
ditions
aYields refer to isolated pure products
In addition, to study the reusability and durability of Fe3O4@SiO2@Co–Zr–Sb
MNPs, FT-IR spectrum (Fig. 8) and SEM image (Fig. 9) of the recycled nanomag-
netic catalyst was investigated. The results revealed that the structure of nanocatalyst
is almost constant.
Experimental section
All chemical substrates were purchased from chemical companies and were used
1
without further purifcation. H NMR spectrum was measured by Bruker Avance
spectrometer (300 MHz), and FT-IR spectra were obtained by Thermo Nicolet
Nexus 670 apparatus. Mass spectra of the samples were obtained by mass spec-
trometer (Agilent, 5973 N, 20–70 eV). Morphology and size distribution of the
1 3

B. Zeynizadeh, M. Gilanizadeh
Table 3 Comparison of the current protocol for the reductive coupling of nitrobenzene to azoxybenzene
with other reported systems
Entry Catalyst system
Condition
Time (h) Yield (%) Reusability References
a
1
NaBH4/Fe3O4@ H2O/refux
0.05
93
4
SiO2@Co–
Zr–Sb
2
3
4
5
6
7
NaN(SiMe3)2
InBr3/Et3SiH
KBH4/PEG-400 KOH/H2O/refux
NaBH4
CuI/Ligand
THF/150 °C
THF/60 °C
12
8
4
6
8
21
96
61
55
85
89
–
–
–
–
–
–
[18]
[19]
[14]
[13]
[51]
[52]
Diglyme/90–100 °C/refux
NaOH/DMSO/H2O/80 °C
EtOH/NaOH/THF/60 °C
Ru/C
18
aPresent work
Scheme 4 A plausible mechanism for reductive coupling of nitrobenzenes to azoxybenzenes
particles were examined by SEM using FESEM-TESCAN and FESEM-ZEISS.
Elemental composition of the sample was determined using EDX analysis. Mag-
netic properties of nanoparticles were measured by a vibration sample mag-
netometer (VSM, model MDKFT). TEM images were captured on a microscope
(Zeiss EM10C) at accelerating voltage of 100 kV. The crystalline structure of the
nanocatalyst was surveyed by X-ray difraction on a Philips PANalytical X’Pert
Pro difractometer in a range of Bragg’s angle (2θ = 5°–80°). TLC was utilized for
reaction monitoring over silica gel 60 F254 aluminum sheet.
1 3

Green and highly efcient approach for the reductive coupling…
100
80
93
91
93
89
60
40
20
0
1
2
3
4
Run number
Fig. 7 Recycling of nanomagnetic-Fe3O4@SiO2@Co–Zr–Sb NPs in chemoselective hydrogenation of
nitrobenzene
Fig. 8 FT-IR spectrum of nanomagnetic-Fe3O4@SiO2@Co–Zr–Sb after the second recovery
Synthesis of Fe3O4 NPs
Magnetite nanoparticles were synthesized by a chemical co-precipitation of chlo-
ride salts [54]. Commonly, a solution of 2.147 g of FeCl2·4H2O (0.0108 mol) and
5.838 g of FeCl3·6H2O (0.0216 mol) was prepared in H2O (100 mL). The solu-
tion was stirred at 85 °C under N2 atmosphere for 10 min. Next, by the addition of
aqueous ammonia (25 wt%, 10 mL), the nanoparticles of Fe3O4 were immediately
precipitated. The resulting mixture was stirred at 85 °C under N2 atmosphere for
30 min. The mixture was cooled to the ambient temperature and nanoparticles of
1 3

B. Zeynizadeh, M. Gilanizadeh
Fig. 9 SEM image of nanomagnetic-Fe3O4@SiO2@Co–Zr–Sb after the second recovery
Fe3O4 were magnetically separated. Magnetite was washed twice with a solution of
NaCl and H2O.
Synthesis of Fe3O4@SiO2 NPs
SiO2 layer was covered on the surface of Fe3O4 core according to the reported
method [55]. Magnetite (1.5 g) was dispersed in water (20 mL); the suspension
was added to 2-propanol (200 mL) and then homogenized by ultrasonic for 30 min.
Under continuous stirring, polyethylene glycol-400 (5.36 g), H2O (20 mL), aqueous
ammonia (28 wt%, 10 mL) and tetraethyl orthosilicate (2 mL) were added sequen-
tially into the suspension. The mixture was stirred for 28 h at ambient temperature.
After completion of reaction, the product was magnetically separated and washed
with H2O and EtOH.
Synthesis of immobilized Co–Zr–Sb NPs on silica‑coated magnetite (Fe3O4@SiO2@
Co–Zr–Sb NPs)
Co–Zr–Sb trimetallic nanoparticles immobilized on SiO2-layered Fe3O4 have been
prepared by the in situ growth method. Typically, 0.160 g of sodium hydroxide
(0.004 mol), 1.060 g of sodium carbonate (0.01 mol) and 0.25 g of Fe3O4@SiO2
were dispersed in 40 mL of H2O. Another solution of Co(NO3)2·6H2O (4.365 g,
0.015 mol), ZrCl4 (0.349 g, 0.0015 mol) and SbF3 (0.625 g, 0.0035 mol) in 40 mL
of H2O was prepared. Both of suspensions and solutions were sonicated for 60 min.
Then, in a mean time and drop-wisely, they were added to a round-bottomed fask
containing magnetic stirrer and water (40 mL). The resulting slurry was stirred
for further 30 min at ambient temperature and subsequently was stored for 24 h at
1 3

Green and highly efcient approach for the reductive coupling…
80 °C. The mixture was cooled to the ambient temperature and then fltered. Next,
the product was dried at 50 °C to aford Fe3O4@SiO2@Co–Zr–Sb MNPs as a solid
material in 3.15 g.
A typical procedure for the chemoselective hydrogenation of nitrobenzene
A mixture of 20 mg of Fe3O4@SiO2@Co–Zr–Sb MNPs and 0.123 g of nitrobenzene
(1 mmol) in 2 mL of water was stirred for a minute. Next, NaBH4 (0.076 g, 2 mmol)
was added and the mixture was stirred under refux conditions for 3 min. The forma-
tion of azoxybenzene was determined by changing the color of the mixture to orange
red. TLC indicated the progress of reaction (EtOAc/n-hexane: 2/5). Next, the mix-
ture was cooled to the room temperature. The nanocatalyst was magnetically sepa-
rated, and the reaction mixture was then extracted with ethyl acetate (2×5 mL). The
combined organic layers were dried over anhydrous sodium sulfate. Evaporation of
solvent provided pure azoxybenzene in 93% yield (Table 2, entry 1).
Conclusions
In conclusion, the novel efcient Fe3O4@SiO2@Co–Zr–Sb catalyst has been devel-
oped for the chemoselective hydrogenation of nitroarenes to azoxyarenes under
green conditions using NaBH4 as a hydrogen donor. The recoverable heterogene-
ous nanocatalyst has been characterized using FT-IR, SEM, EDX, VSM, TEM and
XRD analyses. Reductive coupling of nitrobenzenes was carried out successfully
within 2–10 min to give products in good to high yields. Undoubtedly, this prac-
tical approach can open a new way for proper and green reduction of nitroarenes
to azoxyarenes. This synthetic procedure ofers several advantages in terms of mild
reaction conditions, short reaction times, absence of hazardous organic solvents,
using water as a green solvent and reusability of the magnetic nanocatalyst.
Acknowledgements The authors gratefully acknowledge the fnancial support of this work by the
research council of Urmia University.
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