Trisaminomethane–cobalt complex supported on Fe3O4 magnetic nanoparticles as an efficient recoverable nanocatalyst for oxidation of sulfides and C–S coupling reactions

Article abstract of DOI:10.1002/aoc.5260

In this work, trisaminomethane–cobalt complex immobilized onto the surface of Fe₃O₄ magnetic nanoparticles was successfully prepared via a simple and inexpensive procedure. The prepared nanocatalyst was considered a robust and clean nanoreactor catalyst for the oxidation and synthesis of sulfides under green conditions. This ecofriendly heterogeneous catalyst was characterized by Fourier transform infrared spectroscopy, X-ray diffractometry, energy-dispersive X-ray spectroscopy, inductively coupled plasma-atomic emission spectroscopy, thermogravimetric analysis, vibrating sample magnetometry, X-ray mapping, scanning electron microscopy, and transmission electron microscopy techniques. Use of green medium, easy separation and workup, excellent reusability of the nanocatalyst, and short reaction time are some outstanding advantages of this method.

Full text of DOI:10.1002/aoc.5260

Received: 3 August 2019
DOI: 10.1002/aoc.5260
Revised: 29 August 2019
Accepted: 2 September 2019
F U L L P A P E R
Trisaminomethane–cobalt complex supported on Fe3O4
magnetic nanoparticles as an efficient recoverable
nanocatalyst for oxidation of sulfides and C–S coupling
reactions
Muhammad Aqeel Ashraf1,2 | Zhenling Liu3 | Wan‐Xi Peng1
1 School of Forestry, Henan Agricultural
University, Zhengzhou 450002, China
In this work, trisaminomethane–cobalt complex immobilized onto the surface
2 Department of Geology Faculty of
Science, University of Malaya, 50603
Kuala Lumpur, Malaysia
3 School of Management, Henan
University of Technology, Zhengzhou
450001, China
of Fe3O4 magnetic nanoparticles was successfully prepared via a simple and
inexpensive procedure. The prepared nanocatalyst was considered a robust
and clean nanoreactor catalyst for the oxidation and synthesis of sulfides under
green conditions. This ecofriendly heterogeneous catalyst was characterized by
Fourier transform infrared spectroscopy, X‐ray diffractometry, energy‐
dispersive X‐ray spectroscopy, inductively coupled plasma‐atomic emission
spectroscopy, thermogravimetric analysis, vibrating sample magnetometry, X‐
ray mapping, scanning electron microscopy, and transmission electron micros-
copy techniques. Use of green medium, easy separation and workup, excellent
reusability of the nanocatalyst, and short reaction time are some outstanding
advantages of this method.
Correspondence
Wan‐Xi Peng, School of Forestry, Henan
Agricultural University. Zhengzhou
450002, China.
Email: pengwanxi163@gmail.com,
pengwanxi@163.com
KEYWORDS
C–S coupling, Fe3O4@PTMS–Tris–Co, oxidation of sulfides, reusable catalyst, S‐arylation
1 | INTRODUCTION
attention because of their versatile physical surface and
catalytic properties, which can be attributed to their large
Heterogeneous catalysis is of paramount importance in
many catalysis fields.[1,2] In recent years, heterogeneously
catalyzed processes have been utilized as comparatively
more efficient methodologies because they allow facile
recovery and reusability of the catalysts.[3–6]] Preparing
heterogeneous catalysts by immobilizing the homogenous
precursors on a solid support is one of the most important
routes for developing novel and efficient heterogeneous
catalysts.[7–13]] Accordingly, stability of the catalyst and
even, occasionally, its activity will be improved when a
catalytic system is heterogeneously used.[14,15] By contrast
homogeneous catalysts are difficult to reuse and purifying
the products catalyzed by these chemicals leads to envi-
ronmental problems. Nanosupports have attracted much
surface area and active sites. However, many supports
such as silica nanoparticles,[16] SBA‐15,[7,8,17–21] SBA‐
16,[22] MCM‐41,[8,23–26] MCM‐48,[27] FSM‐16,[28–30] alu-
mina,[31] heteropoly acids,[32] graphene oxide,[[33,34]
Boehmite,[35] polymers,[36] peptide nanofibers,[37] or
CoFe2O4 ferrites nanoparticles[12] require high tempera-
ture for calcination or inert atmosphere. Furthermore,
their preparation times are long and involve tedious reac-
tion conditions. Besides, some of the previously men-
tioned supported metals and ligands such as
heteropolyacids,[38] ionic liquids[39] or some polymers[40–
42]
are more expensive. In this work, a simple and inex-
pensive procedure was utilized for immobilization of
trisaminomethane–cobalt (Tris–Co) complex onto the
Appl Organometal Chem. 2019;e5260.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5260

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ASHRAF ET AL.
surface of Fe3O4 magnetic nanoparticles (MNPs) using
commercial materials. Among various heterogeneous cat-
alysts, MNPs have emerged as an attractive method
because of their numerous applications in organic synthe-
sis.[43–46] MNPs can be regarded as one of the most widely
studied materials in catalysis research and have recently
emerged as ideal catalyst supports because of their large
surface area, straight forward and relatively low prepara-
tion cost, low toxicity, good stability, and facile separation
by magnetic forces.[6,47]
for the synthesis of these compounds represents a broad
area of organic chemistry. In recent years, C–S cross‐
coupling reactions have drawn significant attention
because of their potential in synthesizing numerous
bioactive natural products and pharmaceuticals.[68]
However, the related C–S bond formation reactions are
less studied as compared with C–N and C–O processes,
partially because organic sulfur compounds have a
tendency to bind to metals and act as metal
deactivators.[15,69,70]
Despite these important advantages, the most valuable
advantage of MNPs is their simple separation from the
reaction mixture using an external magnet, without the
need for filtration, centrifugation, or other tedious workup
processes.[10,11] In addition, most magnetic‐supported cat-
alysts can be reused multiple times with their initial activ-
ity level remaining intact.[48] Organometallic complexes,
which coordinate with organic base ligands, are mostly
known for their interaction with molecular oxygen to sub-
sequently form dioxygen adducts. Moreover, in recent
years, they have extensively been used as efficient and
selective homogeneous catalysts for a variety of organic
reactions.[49–51] However, the main drawback of these
homogeneous catalysts is the necessity of their separation
from the reaction mixture at the end of the reaction.
Immobilization of organometallic complexes onto the
MNP support by covalent attachment allows facile separa-
tion of the catalyst, accomplished using an external mag-
net, during the reaction.[52–54] In this sense, the magnetic
support offers high catalytic efficiency and better recycling
ability without having the inherent problems of
complex/ligand leaching during the reaction.[55–62]
Sulfoxides and sulfones are important intermediates
for the construction of high value‐added fine chemicals
and biologically active molecules such as pharmaceuti-
cals, agrochemicals, and chiral auxiliaries.[63–65] Thus,
the selective oxidation of sulfides to sulfoxides or sul-
fones, which is one of the most important transforma-
tions in organic synthesis, has attracted much
attention.[66] Besides, diaryl sulfides constitute an impor-
tant class of therapeutic agents in medicinal chemistry.[67]
Therefore, the development of efficient and mild methods
In this work, a Tris–Co complex supported on Fe3O4
MNPs (Fe3O4@PTMS–Tris–Co) was introduced as a new
and efficient reusable catalyst for the synthesis of diaryl
sulfides and selective oxidation of sulfides under green
reaction conditions. The results of this study differ from
the previously reported studies because of the natural ori-
gin of ligand; stability of the complex between ligand, Co,
and support; high yields of products; and excellent selec-
tivity, recoverability, and reusability of the catalyst. In
addition, water was used as a green solvent under aerobic
reaction conditions.
2 | EXPERIMENTAL
2.1 | General procedure for the
preparation of the Fe3O4@PTMS–Tris–Co
The Fe3O4 MNPs were prepared by the coprecipitation
technique as reported previously.[71] In the next step,
1 g of the prepared Fe3O4 nanoparticles was dispersed
in 20 ml of EtOH:water (1:1) by sonication for 15 min
and then 1.5 ml of 3‐chloropropyltrimethoxysilane
(CPTMS) was added to the reaction mixture. Next, the
reaction mixture was stirred under a nitrogen atmo-
sphere at 40 °C for 24 hr. Afterward, the product
obtained (Fe3O4@CPTMS MNPs) was separated by mag-
netic decantation, washed with ethanol, and dried at
90 °C in an oven for 4 hr. Subsequently, 1.5 mmol of
Tris was grafted on 1 g of Fe3O4@CPTMS MNPs in
water under a nitrogen atmosphere at reflux conditions
for 24 hr. In the next step, the reaction mixture was
SCHEME 1 Stepwise preparation of the Fe3O4@PTMS–Tris–Co catalyst. Tris, trisaminomethane

ASHRAF ET AL.
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TABLE 1 Optimization of oxidation of methyl phenyl sulfide for the synthesis of methyl phenyl sulfoxide as a model catalyzed by
Fe3O4@PTMS–Tris–Co under various conditionsa
Entry
Solvent
Temperature (°C)
Catalyst (mol%)
Time (min)
Yieldb (%)
1
Solvent free
Solvent free
Polyethylene glycol
H2O
Room temperature
Room temperature
Room temperature
Room temperature
Room temperature
Room temperature
Room temperature
Room temperature
Room temperature
Room temperature
Room temperature
Room temperature
Room temperature
Room temperature
Room temperature
0.30
0.30
0.30
0.30
0.30
0.30
0.30
0.30
0.30
0.30
0.25
0.20
0.15
0.10
0.05
2 days
15
Trace
99
2
3
15
89
4
15
Trace
78
5
Dimethylformamide
EtOAc
15
6
15
87
7
Acetonitrile
n‐Hexane
15
79
8
15
Trace
25
9
Dichloromethane
EtOH
15
10
11
12
13
14
15
15
79
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
15
91
15
87
15
81
15
57
15
31
aReaction conditions: Sulfide (1 mmol), H2O2 (0.4 ml), catalyst, and solvent (2 ml).
bIsolated yield.
TABLE 2 Oxidation of various sulfides using hydrogen peroxide for the synthesis of different sulfoxides catalyzed by the Fe3O4@PTMS–
Tris–Co nanocatalyst under optimized conditionsa
Melting point
Entry
Substrate
Product
Time (min)
Yield (%)a
Find
Reported
1
15
99
31–32
32–35[72]
2
3
10
20
98
99
131–132
130–133[72]
Oil
Oil[25]
4
25
100
Oil
Oil[25]
5
6
7
8
25
25
30
20
98
Oil
Oil[25]
98
Oil
Oil[25]
93
Oil
Oil[73]
100
62–64
63–65[72]
9
20
98
Oil
Oil[73]
aReaction conditions: Sulfide (1 mmol), H2O2 30% (0.4 ml), and Fe3O4@PTMS–Tris–Co nanocatalyst (0.30 mol%).
aIsolated yield.

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ASHRAF ET AL.
cooled down and Fe3O4@PTMS–Tris MNPs were sepa-
rated by magnetic decantation. Finally, to prepare the
Fe3O4@PTMS–Tris–Co organometallic complex, the
obtained Fe3O4@PTMS–Tris complex (0.1 g) was
dispersed in 25 ml ethanol by sonication for 30 min.
Afterward, Co(NO3)2·6H2O (0.73 g) was added to the
reaction mixture and stirred under reflux conditions
for 24 hr and washed with ethanol. Finally, the product
(Fe3O4@PTMS–Tris–Co) was separated using an
external magnet, washed with water and ethanol, and
then dried at 80 °C in an oven for 4 hr (Scheme 1
and Tables 1–3).
2.2 | General procedure for the oxidation
of sulfides
Sulfide (1 mmol), H2O2 30% (0.4 ml), and Fe3O4@PTMS–
Tris–Co nanocatalyst (0.30 mol%) were vigorously stirred
in a round‐bottomed flask at room temperature in
solvent‐free conditions for an appropriate time and the
progress of the reaction was monitored by thin‐layer
chromatography (TLC). When the reaction was finished,
the catalyst was separated using an external magnet,
and the products were extracted with H2O and CH2Cl2.
Afterward, the combined organics were dried over
TABLE 3 Optimization of reaction conditions for the cross‐coupling of iodobenzene with thiourea as a model reaction catalyzed by
Fe3O4@PTMS–Tris–Co under various conditions
Entry
Solvent
Catalyst (mg)
Base
Temperature (°C)
Time (min)
Yield (%)a
1
EtOH
—
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NaOH
KOH
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
100
2 days
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
NR
30
2
EtOH
0.10 mol%
0.15 mol%
0.20 mol%
0.25 mol%
0.30 mol%
0.35 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
0.40 mol%
3
EtOH
51
4
EtOH
68
5
EtOH
81
6
EtOH
87
7
EtOH
93
8
EtOH
100
73
9
H2O
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
H2O:EtOH(1:1)
Polyethylene glycol
Dimethyl sulfoxide
Dioxane
Dimethylformamide
Solvent free
EtOH
87
71
100
NR
48
Reflux
100
37
100
NR
69
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Rt.
EtOH
78
EtOH
Et3N
37
EtOH
K2CO3
Na2CO3
CS2CO3
K3PO4
KF/Al2O3
K2CO3
K2CO3
K2CO3
K2CO3
66
EtOH
60
EtOH
85
EtOH
Trace
59
EtOH
EtOH
NRb
EtOH
40
41
EtOH
60
83
EtOH
70
91
aIsolated yield.
bNo reaction(the reaction was not completed after 15 min).

ASHRAF ET AL.
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anhydrous Na2SO4. Evaporation of the solvent gave the
pure corresponding sulfoxides in good to high yield
(93%–100%). All of the products have been previously
reported and are well‐known (see Supporting information
Figures S1 and S2).
2.4.3 | Diphenyl sulfide
1H NMR (400 MHz, CDCl3): δH = 7.23–7.19 (m, 2H, Ar
H), 7.29–7.25 (m, 3H, Ar H), 7.34–7.31 (m, 5H, Ar H).
13C NMR (100 MHz, CDCl3): δH = 127.2 (C4), 129.4
(C3), 131.2 (C2), 135.9 (C1). See Table 4, Entry 1.
2.3 | General procedure for the S‐arylation
reaction
Thiourea (1.2 mmol), aryl halide (1 mmol), K2CO3
(3 mmol), and Fe3O4@PTMS–Tris–Co nanocatalyst
(0.40 mol%) in EtOH (2 ml) were stirred at reflux condi-
tions. The progress of the reaction was monitored by
TLC, using n‐hexane as eluent. After reaction completion,
the catalyst was separated using an external magnet and
the reaction mixture was treated with ethyl acetate
(10 ml) and water (3 ml). The organic layer was separated
and the aqueous layer was extracted with ethyl acetate
(3 × 5 ml). In addition, the combined organic solution
was washed with brine (3 × 5 ml). Drying by Na2SO4
and subsequent evaporation of the solvent provided a res-
idue that was purified on preparative TLC (silica gel)
using hexane. All of the products have been previously
reported and are well‐known (see Supporting information
Figures S3–S10).
2.4.4 | Bis(4‐methoxyphenyl)sulfane
1H NMR (400 MHz, CDCl3): δH = 3.78 [s, 6 H, CH3 (5)],
6.83 [d, J = 8.8 Hz, 4H, Ar‐H (3)], 7.27 [d, J = 8.8 Hz, 4H,
Ar‐H (2)]. 13C NMR (100 MHz, CDCl3) δH = 55.5 (C5),
114.9 (C3), 127.6 (C2), 132.9 (C1), 159.1 (C4). See Table 4,
Entry 4.
2.4.5 | Di‐(p‐tolyl)sulfane
1H NMR (400 MHz, CDCl3): δH = 2.32 [s, 6H, CH3 (5)],
7.10 [d, J = 7.6 Hz, 4H, ArH (3)], 7.23 [d, J = 8.4 Hz,
4H, Ar‐H (2)]. 13C NMR (100 MHz, CDCl3) δH = 21.3
(C5), 130.1(Ar C, C3), 131.2(Ar C, C2), 132.8(Ar C, C1),
137.1(C4). See Table 4, Entry 6.
2.4 | Selected spectral data
2.4.1 | Dodecyl methyl sulfoxide
1H nuclear magnetic resonance (NMR; 400 MHz,
dimethyl sulfoxide): δH = 3.08 [t, J = 8 Hz, 2H, CH2
(2)], 2.61[s, 3H, CH3 (1)], 1.22–1.70 [m, 20H, CH2 (3)],
0.86 [t, J = 6.8 Hz, 3H, CH3 (4)]. See Table 2, Entry 8.
2.4.6 | Bis(4‐nitrophenyl)sulfane
1H NMR (400 MHz, CDCl3): δH = 7.64 [dd, J = 8.4,
2.0 Hz, 4H, ArH (2)], 8.25 [dd, J = 8.4 Hz, 1.6 Hz, 4H,
ArH (3)]. 13C NMR (100 MHz, CDCl3) δH =124.8 (C3),
131.3 (C2), 142.2 (C1), 146.7 (C4). See Table 4, Entry 8.
3 | RESULTS AND DISCUSSION
3.1 | Catalyst preparation
2.4.2 | Dibenzyl sulfoxide
1H NMR (400 MHz, CDCl3): δH = 7.29–7.43 [m, 10H, Ar
H, (3–5)], 4.1 [S, 4H, CH2 (1)]. See Table 2, Entry 2.
This work describes a novel Tris–Co complex supported
on Fe3O4 as an efficient and green nanocatalyst for
organic reactions. Initially, the Fe3O4 nanoparticles,
which were modified by immobilization of CPTMS onto
the surface of Fe3O4@CPTMS MNPs, have been prepared

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ASHRAF ET AL.
TABLE 4 C–S coupling reaction for the synthesis of diaryl sulfides catalyzed by Fe3O4@PTMS–Tris–Co in EtOH at reflux conditionsa
Melting
point (°C)
Reported melting
point (°C)
Entry
Aryl >halide
Product
Time (min)
Yield (%)b
1
15
100
Oil
Oil[74]
2
3
30
30
98
94
Oil
Oil[74]
Oil
Oil
Oil[74]
4
45
99
Oil[74]
5
6
35
45
100
91
Oil
Oil
Oil[74]
Oil[74]
7
30
5
98
90
93
90
Oil
Oil[74]
8
157–159
Oil
157–160[23]
Oil[74]
9
20
40
10
Oil
Oil[74]
11
12
60
5
89
91
Oil
Oil[74]
157–159
157–160[23]
aReaction conditions: Thiourea (1.2 mmol), aryl halide (1 mmol), K2CO3 (3 mmol), and Fe3O4@PTMS–Tris–Co nanocatalyst (0.40 mol%) in EtOH (2 ml).
bIsolated yield.
according to the procedure reported previously.[71] To
prepare Fe3O4@PTMS–Tris MNPs, Tris has been
3.2 | Catalyst characterizations
The prepared Fe3O4@PTMS–Tris–Co MNPs catalyst was
also characterized by FT‐IR, XRD, EDS, ICP, TGA,
VSM, X‐ray mapping, SEM, and TEM.
grafted on Fe3O4@CPTMS MNPs via the substitution
reaction of NH2 with terminal Cl groups. Finally, the
catalyst was synthesized by the reaction of
Fe3O4@PTMS–Tris with Co(NO3)2·6H2O (Scheme 1).
This catalyst has been characterized by Fourier trans-
form infrared spectroscopy (FT‐IR), X‐ray diffractome-
try (XRD), energy‐dispersive X‐ray spectroscopy (EDS),
scanning electron microscopy (SEM), transmission elec-
tron microscopy (TEM), X‐ray mapping, thermogravi-
metric analysis (TGA), vibrating sample magnetometry
(VSM), and inductively coupled plasma (ICP)‐atomic
emission spectroscopy techniques.
Figure 1 shows the FT‐IR spectra of bare Fe3O4 MNPs
(a), Fe3O4@CPTMS (b), Fe3O4@PTMS–Tris (c), and
Fe3O4@PTMS–Tris–Co (d) within the range of 400–
4000 nm. The characteristic peak around 601 cm−1 is
related to the Fe–O bonds in Fe3O4 MNPs.[75] The charac-
teristic peaks around 3000–3500 cm−1 are related to the
surface of O–H groups (Figure 1, a).[75] The anchored
(3‐chloropropyl)trimethoxysilane group was confirmed
by the appearance of C–H stretching vibration at 2830–

ASHRAF ET AL.
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FIGURE 1 Fourier transform infrared spectroscopy spectra: Fe3O4 magnetic nanoparticles (a), Fe3O4@CPTMS (b), Fe3O4@PTMS–Tris
(c), and Fe3O4@PTMS–Tris–Co (d)
2960 cm−1 and the Si–O stretching vibration at around
950–1000 in the Fe3O4@CPTMS spectrum (Figure 1,
b).[76] As shown in the FT‐IR spectrum of
Fe3O4@PTMS–Tris (Figure 1, c), the loading of Tris
groups on the modified surface of MNPs was verified by
the presence of absorption bands in the range of 1500–
1650 and 1112 cm−1, which were assigned to C–N and
N–H vibration bands, respectively.[76] In addition, in the
FT‐IR spectrum of Fe3O4@PTMS–Tris–Co, the bending
vibration of the N–H group appeared at 1099 cm−1
(Figure 1, d). These results suggest that Co ions coordi-
nate on the surface of Fe3O4@PTMS–Tris.[76]
The XRD patterns of the Fe3O4@PTMS–Tris–Co
nanocatalyst are shown in Figure 2. It should be men-
tioned that the 2θ values at 35.27°, 41.52°, 50.56°,
63.24°, 67.49°, and 74.47° correspond to the (2 2 0), (3 1
1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) reflections, respec-
tively, as they are in agreement with the standard Fe3O4
with inverse spinel crystal structure in the XRD spec-
trum.[77–79] More importantly, the phase of Fe3O4 support
is not destroyed during the immobilization of PTMS–
Tris–Co organometallic complex on iron oxide layers.
Furthermore, the crystalline size of Fe3O4@PTMS–Tris–
Co MNPs in the nanocomposites was calculated from
XRD data using Scherer formula. The particle size
obtained from XRD data was found to be 17.75 nm.
EDS images of the synthesized Fe3O4@PTMS–Tris–Co
nanostructure showed the presence of Fe, O, Si, C, N, and
Co species in the catalyst. The results confirmed the
immobilization of Co complex onto the surface of
Fe3O4@PTMS–Tris (Figure 3).
Moreover, to extend the scope of catalyst characteriza-
tion, the exact amount of Co on Fe3O4@PTMS–Tris was
examined by ICP‐optical emission spectrometry. The Co
amount of the immobilized catalyst on MNPs was found
to be 0.94 mmol g−1
.
Figure 4 shows the TGA curves of Fe3O4 MNPs (a),
Fe3O4@CPTMS (b), and Fe3O4@PTMS–Tris–Co (c). The
FIGURE 2 X‐ray diffractometry curve of Fe3O4@PTMS–Tris–Co
catalyst
FIGURE 3 Energy‐dispersive X‐ray spectroscopy pattern of
Fe3O4@PTMS–Tris–Co catalyst

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ASHRAF ET AL.
The X‐ray mapping of Fe3O4@PTMS–Tris–Co
nanocatalyst is shown in Figure 6. The good dispersion
of Co on the surface of the catalyst was confirmed using
elemental map images.
Figure 7 illustrates the morphology of Fe3O4 MNPs
(a), Fe3O4@CPTMS (b), Fe3O4@PTMS–Tris (c),
Fe3O4@PTMS–Tris–Co
(d),
and
the
recovered
Fe3O4@PTMS–Tris–Co (e) catalyst as probed by
SEM. The SEM images show spherical morphology and
uniform nanometer size for most particles. It is worth
mentioning that the obtained morphology helps
maximize the simultaneous increase of the catalyst effi-
ciency and frequency of molecular interactions while
decreasing the activation energy of the rate‐determining
step.[61]
FIGURE 4 Thermogravimetric analysis (TGA) curves: Fe3O4
magnetic nanoparticles (a), Fe3O4@CPTMS (b), and
Fe3O4@PTMS–Tris–Co (c)
Therefore, the size and morphology of Fe3O4@PTMS–
Tris–Co (catalyst) were investigated using TEM. As
shown in Figure 8, most of the particles are quasi‐
spherical with an average diameter at about 10–20 nm,
which are in good agreement with the XRD results.
The Brunauer–Emmett–Teller (BET) surface areas of
the Fe3O4@PTMS–Tris–Co catalyst were investigated
using the nitrogen adsorption/desorption method. As
shown in Figure 9, the BET surface size and specific
surface area are 50.815 and 56.619 m2 g−1, respectively.
The high specific surface area may endow the material
stronger catalytic properties.
TGA curve of all samples shows a small amount of weight
loss before 200 °C, which results from the loss of the
adsorbed solvents and surface hydroxyl group.[80] In the
TGA curve of Fe3O4@CPTMS (Figure 4, b) and
Fe3O4@PTMS–Tris–Co (Figure 4, c), a weight loss at
about 27% and 53% from 200 to 500 °C was observed,
respectively, which may be due to the decomposition of
the organic structure grafted on the Fe3O4 surface.[81,82]
This result confirms the satisfactory grafting of organic
groups, including Co complex, onto Fe3O4.
The magnetic properties of Fe3O4 nanoparticles and
Fe3O4@PTMS–Tris–Co catalyst have been demonstrated
by VSM data (Figure 5). The magnetic saturation values
of Fe3O4 nanoparticles and Fe3O4@PTMS–Tris–Co are
77.2 and 38.2 emu g−1, respectively. The decrease in the
magnetization properties of bare Fe3O4 can be attributed
to the nonmagnetic PTMS–Tris–Co shell on the particle
surface.[72]
3.3 | Catalytic study
As a part of this research, it was decided to explore the
catalytic effect of Fe3O4@PTMS–Tris–Co on organic reac-
tions including the synthesis of oxidation of sulfides and
C–S coupling reactions.
To find the best reaction conditions for oxidation of
sulfides to their corresponding sulfoxide derivatives in
the presence of the Fe3O4@PTMS–Tris–Co catalyst, the
oxidation of methyl phenyl sulfide in the presence of
H2O2 as oxidant was selected as a model reaction and
the results are summarized in Table 1. To optimize the
reaction conditions, we examined different reaction
parameters including type of solvent and different
amounts of Fe3O4@PTMS–Tris–Co catalyst. As shown in
Table 1, the reaction did not proceed in the absence of
Fe3O4@PTMS–Tris–Co even after 2 days (Table 1,
Entry 1). In the next step, various types of solvents such
as polyethylene glycol, H2O, dimethylformamide, EtOAc,
acetonitrile, n‐hexane, dichloromethane, and EtOH, and
solvent‐free conditions were used (Table 1, Entries 2–
10), with the best results obtained in solvent‐free condi-
tions (Table 1, Entry 2). Moreover, the effect of different
amounts of Fe3O4@PTMS–Tris–Co catalyst were studied
FIGURE
Fe3O4@PTMS–Tris–Co catalyst (b)
5
Magnetization curves for Fe3O4 (a) and

ASHRAF ET AL.
9 of 15
FIGURE 6 X‐ray map analysis for the Fe3O4@PTMS–Tris–Co catalyst
for which inferior results were obtained using 0.5–
0.25 mol% of the catalyst on the basis of cobalt loading
(Table 1, Entries 10–13). However, when 0.030 mol% of
the catalyst was used, excellent sulfoxide yields were
obtained (Table 1, Entry 2). As shown in Table 1, sulfide
(1 mmol) in the presence of the catalytic amount of
Fe3O4@PTMS–Tris–Co (0.30 mol%) using H2O2 as oxi-
dant (0.4 ml) and in solvent‐free conditions was found
to be the ideal reaction condition for the formation of
the corresponding sulfoxide. In addition, the influence
of Co loading on the catalytic performance was investi-
gated. For this purpose, different amounts of Co(II)
nitrate (0.73, 0.50, 0.25, 0.15, and 0.10 g) were grafted on
the Fe3O4@PTMS–Tris support in ethanol. Their catalytic
performance was then examined under the obtained opti-
mized conditions. The results showed that the decrease in
Co loading led to an increase in the reaction time and a
decrease the performance of the reaction.
To explore the generality of this procedure, the cata-
lytic activity of Fe3O4@PTMS–Tris–Co in the oxidation
of a wide range of sulfides, bearing various functional
groups, was examined under the optimized conditions.
The results, which are summarized in Table 4 (Scheme 2),
show that the Fe3O4@PTMS–Tris–Co catalyst system
worked very well. It also revealed that different aromatic
and aliphatic sulfides showed good reaction activities for
the oxidation of the corresponding sulfoxides and pro-
vided the corresponding sulfoxide with pure products in
excellent yields.
The next part of the study focused on the utility of this
catalyst in S‐arylation reaction. To find the best reaction
conditions for C–S arylation in the presence of
Fe3O4@PTMS–Tris–Co catalyst, the coupling of thiourea
with iodobenzene was selected as the model reaction,
and the results are summarized in Table 3. To optimize
the reaction conditions, we examined different reaction
parameters, including type of solvent, different amounts
of Fe3O4@PTMS–Tris–Co catalyst, and effect of base and
temperature on the reaction efficiency. Initially, the
amount of the catalyst was optimized, and it was observed
that the best results were obtained with 0.40 mol% of the
catalyst. It is worth mentioning that the reaction did not
proceed in the absence of Fe3O4@PTMS–Tris–Co even
after 2 days (Table 3, Entry 1). Subsequently, the influ-
ence of different solvents was examined, with the highest
yield achieved using EtOH as solvent. The reactivity of
the catalyst in the presence of different bases was also
investigated. Among the tested bases (Et3N, Na2CO3,
K2CO3, KOH, and NaOH), K2CO3 was found to be the
best as it provided the highest yield of the product. Then,
the effect of various temperatures on the reaction was
investigated and the best result was obtained at reflux
conditions.
After the optimized reaction conditions were deter-
mined, we screened a range of commercially available
thiourea and phenol with a variety of aryl halides
(Scheme 3). The results are listed in Table 4. As shown in
Table 4, the target products were afforded in moderate to

10 of 15
ASHRAF ET AL.
FIGURE 7 Scanning electron microscopy images: Fe3O4 magnetic nanoparticles (a), Fe3O4@CPTMS (b), Fe3O4@PTMS–Tris
(c), Fe3O4@PTMS–Tris–Co (d), and the recovered Fe3O4@PTMS–Tris–Co catalyst (e)
FIGURE 8 Transmission electron microscopy images of the Fe3O4@PTMS–Tris–Co catalyst

ASHRAF ET AL.
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FIGURE 9 Nitrogen adsorption–desorption isotherms of the Fe3O4@PTMS–Tris–Co catalyst
excellent yields. The results clearly demonstrated that the
3.3.1 | Recyclability study
S‐arylation reactions of thiourea with p‐substituted aryl
halides were easier than those with o‐substituted and m‐
substituted aryl halides.
In summary, the results show that the catalytic system
is very efficient for both catalytic systems and affords the
corresponding products in high to excellent yields.
Reusability is one of the most important properties of this
catalyst. Therefore, the recyclability of Fe3O4@PTMS–
Tris–Co was investigated for the synthesis of (4‐
methoxyphenyl)(p‐tolyl)sulfane (Figure 10a) and oxida-
tion of methyl phenyl sulfide (Figure 10b). After

12 of 15
ASHRAF ET AL.
completion of the reactions, nanoparticles were magneti-
cally separated. The catalyst was washed with acetone
three to four times to ensure complete removal of any
organic residues and then dried under vacuum. It was
shown that the catalyst could be reused for five successive
runs without any appreciable loss of its catalytic activity
(Figure 10).
SCHEME 2 Fe3O4@PTMS–Tris–Co catalyzed the oxidation of
sulfides. RT, room temperature; Tris, trisaminomethane
3.4 | Hot filtration test
The heterogeneity of Fe3O4@PTMS–Tris–Co in the reac-
tion mixture was studied using the hot filtration test
which was performed during the synthesis of (4‐
methoxyphenyl)(p‐tolyl)sulfane and oxidation of methyl
SCHEME 3 Fe3O4@PTMS–Tris–Co catalyzed the coupling of
aryl halides with thiourea for the synthesis of diaryl sulfides. Tris,
trisaminomethane
FIGURE 10 Recyclability study
TABLE 5 Comparison of Fe3O4@PTMS–Tris–Co for the oxidation of methyl phenyl sulfide and C–S coupling of iodobenzene with previ-
ously reported procedures
Time
(min)
Yield
(%)a
Entry
Substrate
Catalyst
Ref
[11]
1
Methyl phenyl sulfide
Methyl phenyl sulfide
Methyl phenyl sulfide
Methyl phenyl sulfide
Methyl phenyl sulfide
Methyl phenyl sulfide
Methyl phenyl sulfide
Methyl phenyl sulfide
Diphenyl sulfide
Fe3O4–Adenine–Zn
CoFe2O4@glycine–Pr
Fe3O4–AMPD–Cu
Chiral (salen) Mn(III) complexes
SBA‐15@AMPD–Co
Fe3O4@Tryptophan–La
Cu@Fe3O4
75
98
92
96
54
94
98
97
99
85
97
95
95
93
85
100
[12]
[10]
[83]
[34]
[46]
[62]
2
55
3
10
4
180
35
5
6
10
7
45
8
Fe3O4@PTMS–Tris–Co
Ce(IV)–leucine@MCM‐41
CuFe2O4
15
This work
[8]
9
140
1230
240
180
258
15
[84]
[85]
[86]
[87]
[20]
10
11
12
13
14
15
Diphenyl sulfide
Diphenyl sulfide
Ni(II)‐modified‐GO
Cu(OAc)2
Diphenyl sulfide
Diphenyl sulfide
CuI
Diphenyl sulfide
SBA‐15@creatinine@Cu
Fe3O4@PTMS–Tris–Co
Diphenyl sulfide
15
This work
aIsolated yield.

ASHRAF ET AL.
13 of 15
phenyl sulfide under the optimal reaction conditions.
After half‐time, the reactions were terminated and the
corresponding products were obtained in 59% and 61%
of yields, respectively. Both reactions were repeated, and
at the half time the catalyst was separated from the reac-
tion mixture magnetically, but the reaction was allowed
to proceed further. Only a trace conversion (<2% and
3%, respectively) of the coupling and oxidation reactions
was achieved when heating the catalyst‐free solution for
the other half time of the reaction.
Engineering, Xuzhou University of Technology, Xuzhou,
221000, China.
CONFLICTS OF INTEREST
There are no conflicts to declare.
ORCID
Wan‐Xi Peng https://orcid.org/0000-0003-0229-8114
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How to cite this article: Ashraf MA, Liu Z, Peng
W‐X. Trisaminomethane–cobalt complex supported
on Fe3O4 magnetic nanoparticles as an efficient
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