Anchoring Ni (II) on Fe3O4@tryptophan: A recyclable, green and extremely efficient magnetic nanocatalyst for one-pot synthesis of 5-substituted 1H-tetrazoles and chemoselective oxidation of sulfides and thiols

Article abstract of DOI:10.1002/aoc.4445

A green, novel and extremely efficient nanocatalyst was successfully synthesized by the immobilization of Ni as a transition metal on Fe₃O₄ nanoparticles coated with tryptophan. This nanostructured material was characterized using Fourier transform infrared spectroscopy, transmission electron microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, thermogravimetric analysis, inductively coupled plasma optical emission spectroscopy, vibrating sample magnetometry and X-ray diffraction. The prepared nanocatalyst was applied for the oxidation of sulfides, oxidative coupling of thiols and synthesis of 5-substituted 1H-tetrazoles. The use of non-toxic, green and inexpensive materials, easy separation of magnetic nanoparticles from a reaction mixture using a magnetic field, efficient and one-pot synthesis, and high yields of products are the most important advantages of this nanocatalyst.

Full text of DOI:10.1002/aoc.4445

Received: 7 March 2018
DOI: 10.1002/aoc.4445
Revised: 2 May 2018
Accepted: 6 May 2018
F U L L P A P E R
Anchoring Ni (II) on Fe O @tryptophan: A recyclable, green
3
4
and extremely efficient magnetic nanocatalyst for one‐pot
synthesis of 5‐substituted 1H‐tetrazoles and chemoselective
oxidation of sulfides and thiols
1
1
1
Nazanin Moeini
| Taiebeh Tamoradi
| Mohammad Ghadermazi
|
2
Arash Ghorbani‐Choghamarani
1
Department of Chemistry, Faculty of
A green, novel and extremely efficient nanocatalyst was successfully
synthesized by the immobilization of Ni as a transition metal on Fe O
Science, University of Kurdistan,
Sanandaj, Iran
3
4
2
Department of Chemistry, Faculty of
nanoparticles coated with tryptophan. This nanostructured material was
characterized using Fourier transform infrared spectroscopy, transmission
electron microscopy, scanning electron microscopy, energy‐dispersive X‐ray
spectroscopy, thermogravimetric analysis, inductively coupled plasma optical
emission spectroscopy, vibrating sample magnetometry and X‐ray diffraction.
The prepared nanocatalyst was applied for the oxidation of sulfides, oxidative
coupling of thiols and synthesis of 5‐substituted 1H‐tetrazoles. The use of
non‐toxic, green and inexpensive materials, easy separation of magnetic
nanoparticles from a reaction mixture using a magnetic field, efficient and
one‐pot synthesis, and high yields of products are the most important
advantages of this nanocatalyst.
Science, Ilam University, PO Box
69315516, Ilam, Iran
Correspondence
Mohammad Ghadermazi, Department of
Chemistry, Faculty of Science, University
of Kurdistan, Sanandaj, Iran.
Email: mghadermazi@yahoo.com
KEYWORDS
3 4
5‐substituted 1H‐tetrazoles, Fe O nanoparticles, nickel, sulfides, thiols
1
| INTRODUCTION
physical and chemical properties. Superparamagnetism,
high magnetic susceptibility and low Curie temperature
are some unique properties of MNPs. Fe O nanoparti-
3 4
[
3]
Catalysis plays a crucial role in chemical processes,
academic and industrial laboratories and various organic
transformations.
cles are a good candidate as a support material for hetero-
geneous catalysts because of their great properties such as
the abundance of unique activities, low toxicity and price,
simple synthesis and functionalization, large surface area
Recently,
the
applications
of
homogeneous catalysts in organic transformations have
been limited because of difficulties in their recovery and
reusability. For this reason, heterogeneous versions of
catalysts can be used. However, the activity of heteroge-
neous catalysts is less than that of their homogeneous
counterparts. The solution to this problem is the use of
the nanomaterials. Nowadays, nanoscale materials are
used widely in sciences like chemistry, biology and
[
4–6]
and easy separation with magnetic field.
Fe O has a
3 4
[1]
[3]
cubic inverse spinel structure. Magnetic nanocatalysts
are used to accelerate various organic reactions.
[7]
Examples are the syntheses of compounds such as sulfox-
ides and disulfides, which are obtained from the
oxidation of sulfides and oxidative coupling of thiols,
respectively. Sulfoxides play a major role in the synthesis
[2]
physics. Magnetic nanoparticles (MNPs) have attractive
Appl Organometal Chem. 2018;e4445.
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© 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4445

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MOEINI ET AL.
of intermediates in drug metabolism, the production of
2 | EXPERIMENTAL
biological compounds, industrial processes, C─ C bond
[8–10]
formation, etc.
The products of oxidative coupling
2.1 | Materials and Instrumentation
of thiols are disulfides, having many valuable
applications in organic synthetic chemistry as synthetic
intermediates and are essential moieties in peptides and
Chemicals and solvents used in this work were obtained
from Sigma‐Aldrich and Merck. They were used without
further purification. The particle size and morphology
were examined using scanning electron microscopy
(SEM) with a FESEM‐TESCAN MIRA3. Fourier trans-
form infrared (FT‐IR) spectra of samples were recorded
as KBr pellets with a Bruker Vertex 70 FT‐IR spectropho-
[11]
proteins.
Also, they are widely used in industry as
vulcanizing agents and oils for rubber and
[12,13]
elastomers.
Tetrazoles are types of polyazaheterocyclic com-
[
14]
pounds.
They have widespread applications in medi-
1
[
15]
tometer. H NMR spectra were recorded with a Bruker
cine, materials science and coordination chemistry.
400 MHz NMR spectrometer. Transmission electron
The [2 + 3] cycloaddition of azides to the corresponding
nitriles in the presence of an appropriate catalyst is the
best method for the synthesis of 5‐substituted
microscopy (TEM) was carried out with a Zeiss EM10C
instrument. Powder X‐ray diffraction (XRD) measure-
ments were performed using a Philips X'pert diffractome-
ter (Cu K αradiation, λ= 1.54060 Å). The content of Ni
was measured using inductively coupled plasma optical
emission spectrometry (ICP‐OES). Thermogravimetric
analysis (TGA) curves were obtained with a Shimadzu
DTG‐60 instrument. The supermagnetic properties of
the catalyst were measured with vibrating sample magne-
tometry (VSM; MDKFD). Melting points were obtained
usingan Electrothermal 9100 apparatus.
[16]
1H‐tetrazoles.
Transition metals play an important
role as catalysts in synthesis of organic molecules. For
instance, copper catalysts have been applied in the
synthesis sulfoxides, disulfides and 5‐substituted
[
17–19]
tetrazoles.
dation of thiols to disulfides using gold nanoparticles.
Other reagents and catalysts are available for the oxida-
Corma and co‐workers catalysed the oxi-
[
20]
[
21]
tion of sulfides, such as PhIO/Mn (III) complexes,
vanadium (IV) complexes of dibromo‐ and diiodo‐
functionalized chiral Schiff bases
[22]
and magnesium
[23]
monoperoxyphthalate.
But low selectivity which pro-
2
.2 | Preparation of Fe O MNPs
3 4
duces side products (sulfones), low yields, long reaction
times and high cost of the reagents and catalysts are
For preparation of Fe O MNPs, 5.838 g of FeCl ⋅6H O
3
4
3
2
[
24]
[25]
(0.022 mol) and 2.147 g of FeCl ⋅4H O (0.011 mol) were
2
2
major disadvantages of these methods.
Fe (OAc)2,
and ZnS nano-
spheres are several new catalysts that have been investi-
[26]
[27]
[28]
dissolved in 100 ml of deionized water under a nitrogen
BF –OEt ,
FeCl3,
β‐cyclodextrin
3
2
atmosphere. Then, 10 ml of NH was added to the reac-
3
[
29]
tion mixture and was stirred for 30 min at 80 °C. Finally,
the black precipitate was isolated using an external mag-
net, washed several times with water and ethanol and
gated for the synthesis of 5‐substituted 1H‐tetrazoles.
Nickel is considered as one of the most effective tran-
sition metals for catalytic purposes in many synthetic
transformations. Nickel is in group 10 of the periodic
table, above palladium. It has chemical properties similar
to those of palladium and platinum metals, with the
advantages that it is cheaper, highly available, shows fac-
[
34]
dried at 50 °C.
2
.3 | Preparation of Fe O @tryptophan
3 4
[
30]
An amount of 1 g of obtained Fe O MNPs was dispersed
ile oxidative addition and is environmentally friendly.
3 4
in 250 ml of distilled water for 25 min. Then 2 mmol of
tryptophan was added to the mixture and was refluxed
Nickel‐based catalysts are used in a large number of
reactions: reductive coupling, cross‐coupling, etc. The
chemoselective oxidative coupling reaction of aliphatic
thiols was reported as being catalysed by Ni nanoparti-
for 24
h
at 90 °C. The nanoparticle product
(
Fe O @tryptophan) was isolated by magnetic decanta-
3
4
[31–33]
tion, washed twice with deionized water and dried at
cles.
nanohybrid catalyst containing nickel metal was synthe-
sized. We report facile method to synthesize
Fe O @tryptophan@Ni as a heterogeneous and novel
In this regard, a magnetically recoverable
[
35]
room temperature.
a
3
4
2
.4 | Preparation of
nanocatalyst for the synthesis of 5‐substituted
H‐tetrazoles, the oxidative coupling of thiols and the
Fe O @tryptophan@Ni
3
4
1
oxidation of sulfides under mild conditions. The magnetic
nanocatalyst was separated from reaction media using
magnetic decantation. Also, the efficiency and reusability
of the catalyst were evaluated.
An amount of 1 g of Fe O @tryptophan was dispersed in
3 4
25 ml of ethanol for 20 min. Subsequently, Ni
(NO ) ⋅6H O (2 mmol) was added to the suspension.
3
2
2
The resultant mixture was refluxed for 15 h. The

MOEINI ET AL.
3 of 12
1
synthesized Fe O @tryptophan@Ni was decanted from
2,2′‐Disulfanediyldisuccinic acid. H NMR (400 MHz,
3
4
the reaction mixture using a magnetic field, washed with
ethanol and dried at room temperature.
DMSO, δ , ppm): 2.59 (s, 4H), 6.89 (br, 4H). FT‐IR
H
−
1
(KBr, cm ): ν (S─ S): 1043.
1
1
,2‐Di‐p‐tolyldisulfane. H NMR (400 MHz, DMSO,
δ , ppm): 2.30 (s, 6H), 6.74–6.81 (m, 4H), 7.74–7.98
H
2
.5 | Procedure for Oxidation of Sulfides
−1
(
m, 4H). FT‐IR (KBr, cm ): ν (S─ S): 1034.
to Sulfoxides
1
2
‐(1H‐Tetrazol‐5‐yl)phenol. H NMR (400 MHz,
DMSO, δ , ppm): 6.99–7.03 (t, J = 7.4, 1H), 7.09
H
To a mixture of 1 mmol of sulfide and 0.4 ml of H O2
2
(
d, J = 8, 1H), 7.39–7.43 (t, J = 7.6, 1H), 8.01 (d, J = 8,
−
(
33%), 0.004 g of nanocatalyst was added (amount of
1
−
1
1H), 13.05 (br, 1H). FT‐IR (KBr, νmax, cm ): 3233,
anchored Ni in 4 mg of catalyst: 0.0006 mmol g ) and
stirred at room temperature for a specified time. TLC
was used to evaluate the reaction progress.
Fe O @tryptophan@Ni nanocatalyst was isolated using
3029, 2923, 2710, 2520, 2400, 1600, 1555, 1465, 1392,
1
355, 1275, 1250, 1110, 1056, 899, 810, 732, 685, 550.
1
4
‐(1H‐Tetrazol‐5‐yl)benzonitrile. H NMR (400 MHz,
3
4
DMSO, δ , ppm): 8.09–8.21 (m, 2H), 8.21–8.23 (m, 2H).
H
an external magnet when reaction had completed and
the resulting mixture was washed with ethyl acetate.
Finally, pure sulfoxides were obtained in high yields
−
1
FT.IR (KBr, νmax, cm ): 3394, 3095, 3050, 2877, 2859,
737, 2624, 2565, 2480, 2230, 1618, 1566, 1494, 1457,
1353, 1227, 1115, 1071, 997, 886, 757, 698, 559.
2
[36]
(80–99%) after solvent evaporation.
3
| RESULTS AND DISCUSSION
2.6 | Procedure for Oxidative Coupling of
Thiol to Disulfide
The synthesis of the new MNPs was realized by attaching
tryptophan to the Fe O nanomagnetic substrate. Ulti-
mately, the Fe O @tryptophan@Ni nanocatalyst was pre-
3 4
2
A mixture containing 1 mmol of thiol, 0.4 ml of H O2
3
4
(33%), 0.004
g
of Fe O @tryptophan@Ni catalyst
3 4
(
0
amount of anchored Ni in
.0006 mmol g ) and 2 ml of ethanol as a solvent was
4
mg of catalyst:
pared using a stable interaction between the N atom
−
1
2+
(
primary amine) of tryptophan and Ni (Scheme 1).
stirred at room temperature. The reaction process was
monitored by TLC. After the reaction was complete, the
magnetic nanocatalyst was removed by magnetic separa-
tion and the product was washed with ethyl acetate. After
evaporation of ethyl acetate, the corresponding pure
3.1 | Catalyst Characterization
The
FT‐IR
spectra
confirm
the
successful
functionalization of the MNPs. Figure 1 shows the FT‐
[37]
disulfides were obtained in high yields (85–99%).
IR spectra for Fe O4 MNPs, Fe O @tryptophan and
3
3 4
−
1
Fe O @tryptophan@Ni. A strong peak near 578 cm is
3
4
seen in Figure 1(a), which verifies the existence of the
2
.7 | Procedure for Synthesis of 5‐
Fe─ O bond and is evidence for the synthesis of Fe O
3
4
Substituted 1H‐Tetrazole Derivatives
[
39]
MNPs.
The FT‐IR spectra of Fe O @tryptophan
3
4
A mixture of 1 mmol of nitrile, 1 mmol of sodium azide
and 0.08 g of the nanocatalyst (amount of anchored Ni
(
Figure 1b) and Fe O @tryptophan@Ni (Figure 1c) show
3
4
−1
the band of the bending vibration of NH near 1400 cm
2
−
1
in 0.08 g of catalyst: 0.0122 mmol g ) in 1 ml of poly
ethylene glycol) (as green solvent) was stirred at
20 °C. TLC was used for indicating completion of the
this band is shifted to lower wavenumber in the spectrum
of Fe O @tryptophan@Ni because of coordination of
(
1
3
4
[40]
nitrogen atom of the amino group to nickel.
The FT‐
reaction. After the reaction was cooled, the nanocatalyst
was separated from the resultant material using a mag-
netic field. Then, 10 ml of hydrochloric acid (4 N) was
added and ethyl acetate was used for extracting the tetra-
zole. The organic part was separated from the water part
IR spectrum of Fe O @tryptophan shows an NH
3
4
2
−1
stretching band near 3250 cm
while in the
Fe O @tryptophan@Ni spectrum this is shifted to
3
4
−1 [41]
3
236 cm .
All of these bands indicate that the surface
of Fe O MNPs is successfully modified with tryptophan
3
4
[38]
and concentrated to form crude crystalline solid.
and Ni (II) immobilized on Fe O @tryptophan.
3
4
The thermal stability of Fe O @tryptophan@Ni nano-
3
4
particles was investigated using TGA from 35 to 635 °C.
2
.8 | Selected Spectral Data
The overlaid TGA curves of Fe O4 nanoparticles,
3
1
Methylphenylsulfoxide. H NMR (400 MHz, CDCl , δ ,
ppm): 2.74 (s, 3H), 7.00–7.99 (m, 5H). FT‐IR (KBr, cm ):
ν (S═ O): 1026–1089.
Fe O @tryptophan and Fe O @tryptophan@Ni are
3
H
1
3
4
3 4
−
shown in Figure 2. The removal of solvent, physically
adsorbed water and surface hydroxyl groups was detected

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MOEINI ET AL.
SCHEME 1 Synthesis of
Fe @tryptophan@Ni
3 4
O
FIGURE
2
TGA curves of (a) Fe
3
O
4
nanoparticles, (b)
Fe @tryptophan and (c) Fe O @tryptophan@Ni
3
O
4
3 4
analysed using XRD (Figure 3). The pattern of Fe O MNPs
3
4
indicates a crystallized structure with peaks at 2θ= 30.37°,
5.82°, 43.25°, 53.92°, 57.39° and 62.77°, assigned
to the crystallographic faces (2 2 0), (3 1 1), (4 0 0), (4 2 2),
3
(
5 1 1) and (4 4 0), which is indicative of an inverse cubic
[
43]
spinel structure.
The Fe O @tryptophan@Ni phase
3 4
was identified by the peak positions at 2θvalues of 29.99°,
5.76°, 43.25°, 53.89°, 57.15° and 62.61°. The XRD patterns
of Fe O @tryptophan and Fe O @tryptophan@Ni clearly
3
3
4
3 4
showed that these modifications did not cause a phase
change during the synthesis of the nanocatalyst and the
structure was retained.
As shown in Figure 4, the size and morphology of
Fe O @tryptophan@Ni nanocatalyst were investigated
3 4
FIGURE 1 FT‐IR spectra of (a) Fe
3 4
O nanoparticles, (b)
Fe @tryptophan and (c) Fe @tryptophan@Ni
3
O
4
3 4
O
[
42]
by the decrease in the mass in the region below 200 °C.
A
weight decrease at 250–635 °C corresponds to the decom-
position of the organic substance on the substrate of MNPs.
The crystalline structures of synthesized Fe O MNPs,
using SEM. According to the SEM images, spherical mor-
phology was observed for most particles. A TEM image of
the Fe
O @tryptophan@Ni catalyst is shown in Figure 5.
3 4
3
4
Fe O @tryptophan and Fe O @tryptophan@Ni were
The particle size was measured to be about 15 nm and
3
4
3 4

MOEINI ET AL.
5 of 12
3 4
FIGURE 3 XRD patterns of (a) Fe O
MNPs, (b) Fe
3 4
O @tryptophan and (c)
3 4
Fe O @tryptophan@Ni MNPs
FIGURE 4 SEM images of Fe
3
O
4
@tryptophan@Ni at different magnifications
FIGURE
6
Energy‐dispersive
X‐ray
spectrum
of
3 4
Fe O @tryptophan@Ni
3 4
FIGURE 5 TEM image of Fe O @tryptophan@Ni catalyst
spherical morphology was confirmed. The results of TEM
and SEM were in agreement.
The magnetic properties of Fe O , Fe O @tryptophan
3 4 3 4
and Fe O @tryptophan@Ni were evaluated using VSM at
3
4
The components of Fe O @tryptophan@Ni were
room temperature (Figure 7). The saturation magnetiza-
3
4
investigated using energy‐dispersive X‐ray spectroscopy
Figure 6). The results showed that the main elements
tion value (M ) of the Fe O MNPs was found to be
s
3 4
−
1
(
66.87 emu g
and those for Fe O @tryptophan and
3 4
were C, N, O, Fe and Ni, which established that trypto-
phan was well attached to the Fe O substrate.
Fe O @tryptophan@Ni composite were 47.83 and
42.95 emu g , respectively. As shown in Figure 7, the
3
4
−
1
3
4

6
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MOEINI ET AL.
3 4
FIGURE 7 VSM curves of (a) Fe O , (b)
Fe
Fe
3
O
O
4
4
@tryptophan and (c)
3
@tryptophan@Ni MNPs
decline in the magnetization of the nanocatalyst relative
to the Fe O MNPs is related to the coating of the Fe O
TABLE 1 Optimization of oxidation of sulfides catalysed by
Fe O @tryptophan@Ni under various conditions
3 4
3
4
3
4
MNPs with the organic layers (tryptophan) and coordina-
H
(ml)
2
O
2
Catalyst
(mg)
Time
(min)
Yield
(%)
[38]
tion with nickel.
Nonetheless, this magnetic property
Entry
Solvent
is sufficient and the Fe O @tryptophan and
3
4
1
2
3
4
5
6
7
8
9
10
Solvent‐free
Solvent‐free
Solvent‐free
Solvent‐free
Solvent‐free
Solvent‐free
Solvent‐free
EtOH
0.4
0.4
0.4
0.4
0.2
0.1
0.6
0.4
0.4
0.4
0
3
4
6
4
4
4
4
4
4
120
24
10
80
98
98
90
52
95
75
45
20
Fe O @tryptophan@Ni
nanoparticles
could
be
3
4
completely, efficiently and rapidly separated using a mag-
net from reaction mixtures.
20
The amount of anchored Ni on the nanoparticles was
15
−
1
determined using ICP‐OES. The value was 0.152 mmol g .
40
100
20
3.2 | Catalytic Studies
135
960
1800
Application of Fe O @tryptophan@Ni nanocatalyst in the
Ethyl acetate
Acetonitrile
3
4
oxidation of sulfides to sulfoxides, in the oxidative cou-
pling of thiols to disulfides and also in the synthesis of tet-
razole derivatives was studied. Different amounts of the
of Fe O @tryptophan@Ni at room temperature (Table 1,
entry 3).
Therefore, after finding the optimal conditions, vari-
3
4
catalyst, H O2 and various solvents were investigated
2
using methylphenylsulfide (as a model compound) to
obtain the optimized conditions for oxidation of sulfides
ous derivatives of sulfides were evaluated. The results
are presented in Table 2. The best yields were obtained
in short times in the presence of the nanocatalyst for oxi-
dation of various sulfides.
All corresponding sulfoxides were obtained in excel-
lent yields within a relatively short period of time. Selec-
tivity is one of the most important features in catalytic
systems. The reaction of the oxidation of sulfides with
the optimized conditions described previously is
completely selective. Therefore in these reactions, sulfone
was not seen as a by‐product. A possible reaction mecha-
nism for the oxidation of sulfides in the presence of the
(Scheme 2). The results are summarized in Table 1. In
the absence of the catalyst, even after 120 min, the desired
product was not obtained. Therefore, this showed evi-
dence of the efficiency of the synthesized nanocatalyst in
this reaction. The optimized conditions were found to be
0
.4 ml of hydrogen peroxide, solvent‐free condition, 4 mg
SCHEME 2 Oxidation of sulfides to sulfoxides

MOEINI ET AL.
7 of 12
TABLE 2 Oxidation of sulfides to sulfoxides in the presence of
Fe O @tryptophan@Ni
3 4
Time Yield
(min) (%)
SCHEME 4 Oxidative coupling of thiols to disulfides
Entry Substrate
M.p. (°C)
[
27]
1
2
3
4
5
6
7
8
9
1
2‐(Phenylthio)ethanol
5
6
99
92
94
80
82
94
85
98
98
96
Oil
TABLE 3 Optimization of oxidation of thiols catalysed by
[
34]
Dibenzylsulfide
140–143
Fe O @tryptophan@Ni under various conditions
3
4
[
27]
2‐(Methylthio)ethanol
Dodecylmethylsulfide
Allyl sulfide
5
Oil
H
(ml)
2
O
2
Catalyst
(mg)
Time
(min)
Yield
(%)
[
41]
5
60–63
Entry
Solvent
[
42]
5
Oil
1
2
3
4
5
6
7
8
9
EtOH
0.4
0.4
0.4
0.4
0.2
0.6
0.1
0.4
0.4
0.4
0
3
4
6
4
4
4
4
4
4
60
5
14
92
97
99
90
98
49
88
90
77
3,3′‐Thiodipropionic acid
Dipropylsulfide
5
205–210
EtOH
[
[
42]
27]
5
Oil
Oil
EtOH
5
Methylphenylsulfide
Thiodiglycolic acid
Ethylphenylsulfide
20
6
EtOH
4
160–175
EtOH
14
5
[
27]
0
25
Oil
EtOH
EtOH
55
20
13
110
Solvent‐free
Ethyl acetate
Acetonitrile
catalyst is shown in Scheme 3. The reaction of hydrogen
peroxide with the catalyst produces intermediate A,
which converts to active oxidant B. Subsequently, sulfide
with a nucleophilic attack on intermediate C leads to the
1
0
[44]
production of sulfoxide.
Also, we decided to examine the oxidative coupling of
thiols to corresponding disulfides (Scheme 4).
Mercaptosuccinic acid was selected as a model to opti-
mize the reaction conditions. Oxidation reactions were
performed in several solvents, various amounts of
Fe O @tryptophan@Ni catalyst and H O . Amounts of
the catalyst show the positive effect of the catalyst on
the reaction.
Then, ten substrate thiols were converted to the
desired disulfides under the optimal conditions. As evi-
dent from Table 4, the oxidative coupling of thiols in
the presence of the nanoparticles occurred in the desired
times and with good yields. A plausible mechanism for
3
4
2 2
4
mg of the catalyst and 0.4 ml of H O in 2 ml of ethanol
2 2
[
48]
were optimal conditions for the reaction (Table 3). The
reaction was investigated in the absence of the catalyst.
Low yield and a long reaction time in the absence of
this process is shown in Scheme 5.
The chemoselectivity of substrates containing a
hydroxyl group (2‐(phenylthio) ethanol, 2‐(methylthio)
SCHEME 3 Possible reaction
mechanism for oxidation of sulfides in the
presence of the catalyst

8
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MOEINI ET AL.
3 4
TABLE 4 Oxidative coupling of thiols catalysed by Fe O @tryptophan@Ni under various conditions
Entry
Substrate
Time (min)
Yield (%)
M.p. (°C)
[
41]
1
2
3
4
5
6
7
8
9
2‐Mercaptoethanol
5
5
98
99
98
86
87
96
88
85
96
85
Oil
[
[
[
17]
41]
45]
Mercaptosuccinic acid
2‐Mercaptobenzoic acid
2‐Mercaptobenzimidazole
Thiophenol
105–110
270–272
195–197
15
60
65
55
50
180
60
10
[
46]
55–57
[47]
Benzylmercaptan
7072
[
34]
Benzo [d]thiazole‐2‐thiol
Benzo [d]oxazole‐2‐thiol
4‐Methylbenzenethiol
2‐Mercaptoacetic acid
169–170
[
[
34]
34]
98–99
38–40
[41]
1
0
Oil
SCHEME 5 Plausible mechanism for
oxidative coupling of thiols to
corresponding disulfides
SCHEME 6 Chemoselective
sulfoxidation of 2‐(phenylthio) ethanol
and 2‐(methylthio)ethanol
SCHEME 7 Chemoselective
sulfoxidation of 2‐mercaptoethanol

MOEINI ET AL.
9 of 12
ethanol and 2‐mercaptoethanol) in the oxidation of sul-
fide and thiol was investigated. In these oxidation reac-
tions, as shown in Schemes 6 and 7, hydroxyl group
remained intact and functional groups of sulfide and
thiol were oxidized to sulfoxide and disulfide,
respectively.
(DMF) at 120 °C (were obtained. Also, the reaction pro-
ceeds in the absence of the catalyst.
Various derivatives of nitriles were evaluated under
the optimized conditions. The results are presented in
Table 6.
Finally, the catalytic effect of Fe O @tryptophan@Ni
3
4
3
.3 | Reusability and Leaching of Catalyst
on the synthesis of tetrazoles was investigated (Scheme 8).
Initially, to optimize the reaction conditions,
malononitrile was selected and studied in the presence
of various amounts of Fe O @tryptophan@Ni, several
Reusability is one of the most important advantages of
catalysts that makes them economically efficient. The
recyclability of Fe O @tryptophan@Ni was assessed. We
3
4
3 4
solvents and at different temperatures. As evident from
Table 5, the appropriate outcomes) 0.08 of
Fe O @tryptophan@Ni and poly (ethylene glycol)
used the separated nanocatalyst for nine, seven and five
runs in the oxidative coupling of thiols, oxidation of sul-
fides and synthesis of 5‐substituted 1H‐tetrazoles, respec-
tively, without a significant decrease in activity of
subsequent runs, as shown in Figure 8.
g
3
4
(PEG) as a green solvent instead of dimethylformamide
In order to investigate the leaching of nickel from
Fe O @tryptophan@Ni
in
the
synthesis
of
3
4
methylphenylsulfoxide, the exact amount of Ni loaded
on modified MNPs was analysed using the ICP‐OES tech-
nique for recycled and fresh nanocatalyst. The amounts
SCHEME 8 Synthesis of tetrazoles
3 4
TABLE 5 Optimization of formation of 5‐(4‐chlorophenyl)‐1H‐tetrazole catalysed by Fe O @tryptophan@Ni under various conditions
Entry
Solvent
Temperature (°C)
Catalyst (g)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
EtOH
DMF
DMSO
PEG
80
0.08
0.08
0.08
0.08
0.08
0.08
0.06
0.05
0
1440
150
180
20
22
60
55
97
85
70
85
72
16
120
120
120
100
80
PEG
150
195
30
PEG
PEG
120
120
120
PEG
90
PEG
300
3 4
TABLE 6 Synthesis of 5‐substituted 1H‐tetrazoles in the presence of Fe O @tryptophan@Ni in PEG
Entry
Substrate
Time (min)
Yield (%)
M.p. (°C)
[
[
[
[
[
[
[
[
[
[
49]
49]
49]
49]
49]
49]
49]
50]
49]
51]
1
2
3
4
5
6
7
8
9
Phthalonitrile
5
97
89
91
97
97
98
88
97
98
91
209–213
141–145
175–180
110–112
215–218
210–215
122–125
220–223
265–268
195–200
3‐Nitrobenzonitrile
2‐Chlorobenzonitrile
Malononitrile
480
327
20
25
15
20
10
5
4‐Nitrobenzonitrile
Benzonitrile
Benzyl cyanide
2‐Hydroxybenzonitrile
3‐Chlorobenzonitrile
Terephthalonitrile
1
0
30

10 of 12
MOEINI ET AL.
FIGURE 8 Reuse of the nanocatalyst in
oxidative coupling of thiols, oxidation of
sulfides and synthesis of 5‐substituted 1H‐
tetrazoles
3 4
TABLE 7 Comparison of Fe O @tryptophan@Ni for oxidation of methylphenylsulfide with previously reported catalysts
Entry
Substrate
Catalyst
Time (min)
Yield (%)
[
[
[
[
[
29]
43]
41]
44]
47]
1
2
3
4
5
6
7
8
9
Ph–SCH
Ph–SCH
Ph–SCH
Ph–SCH
Ph–SCH
Ph–SCH
3
Cu‐SPATB/Fe
3
O
4
95
98
95
98
90
98
3
3
3
3
3
Ni–salen–MCM‐41
Fe –adenine–Ni
156
55
3 4
O
Polymer‐anchored Cu (II)
Cd–salen–MCM‐41
180
150
20
Fe
3
O
4
@tryptophan@Ni
98 (this work)
[
[
[
52]
38]
41]
4‐Methylbenzenthiol
4‐Methylbenzenthiol
4‐Methylbenzenthiol
4‐Methylbenzenthiol
Benzonitrile
Ni‐SMTU@boehmite
93
94
99
97
Fe
3
Fe
3
Fe
3
O
4
O
4
O
4
‐adenine‐Zn
90
‐adenine‐Ni
70
10
11
12
13
14
15
@tryptophan@Ni
60
96 (this work)
[
[
[
[
49]
53]
54]
55]
Ni–SMTU@boehmite
Fe @SiO2@L‐histidine
93
94
90
94
98
Benzonitrile
3
O
4
180
180
20
Benzonitrile
Pd‐2A3HP‐MCM‐41
Benzonitrile
Cuttlebone
Benzonitrile
Fe
3
O
4
@tryptophan@Ni
15
98 (this work)
of nickel in the recycled and fresh catalysts were 0.10 and
available materials, high yields and short reaction times
are several advantages of this synthesized nanomagnetic
catalyst.
−
1
0
.152 mmol g , respectively.
3
.4 | Comparison of Catalysts
4
| CONCLUSIONS
The catalytic activity of Fe O @tryptophan@Ni was
3
4
compared with that of other materials. The results
summarized in Table 7 show that oxidations of
Ni (II) immobilized on Fe O nanoparticles coated with
3 4
tryptophan was prepared as a novel, green, inexpensive
and efficient magnetic nanocatalyst. The nanostructure
was characterized using TGA, ICP‐OES, FT‐IR, energy‐
dispersive X‐ray spectroscopy, XRD, TEM and SEM.
Furthermore, the catalytic activity of the catalyst was
investigated for the oxidation of sulfides, oxidative cou-
pling of thiols and synthesis of 5‐substituted 1H‐
methylphenylsulfide,
benzonitrile occur
4‐methylbenzenthiol
in the presence
and
of
Fe O @tryptophan@Ni in a shorter time period and with
3
4
higher yields than in the presence of the other catalysts.
Non‐toxicity of ligand and metal in the catalyst, magnetic
and easy separation of nanohybrid catalyst, commercially

MOEINI ET AL.
11 of 12
tetrazoles. The Fe O @tryptophan@Ni nanocatalyst can
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Appl. Organometal. Chem. 2015, 29, 170.
3
4
be separated easily with an external magnet, and also it
can be reused for several runs with little loss of activity.
Excellent yields, easy preparation, low cost, eco‐friendli-
ness and chemical stability are the other advantages of
the synthesized nanocatalyst.
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ACKNOWLEDGEMENTS
The authors are deeply grateful to Erfan Ghadermazi and
the University of Kurdistan for financial support of this
research project.
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Nazanin Moeini http://orcid.org/0000-0001-7022-3734
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