A nickel nanoparticle engineered CoFe2O4/SiO2-NH2@carboxamide composite as a novel scaffold for the oxidation of sulfides and oxidative coupling of thiols

Article abstract of DOI:10.1039/d1ra01592c

The purpose of this work was to prepare a new Ni-carboxamide complex supported on CoFe2O4 nanoparticles (CoFe2O4/SiO2-NH2@carboxamide-Ni). The carboxamide host material unit generated cavities that stabilized the nickel nanoparticles effectively and preve

Full text of DOI:10.1039/d1ra01592c

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A nickel nanoparticle engineered CoFe2O4/SiO2–
NH2@carboxamide composite as a novel scaffold
for the oxidation of sulfides and oxidative coupling
of thiols†
Cite this: RSC Adv., 2021, 11, 14717
*
Mina Zohrevandi, Roya Mozafari
and Mohammad Ghadermazi
The purpose of this work was to prepare a new Ni–carboxamide complex supported on CoFe2O4
nanoparticles (CoFe2O4/SiO2–NH2@carboxamide–Ni). The carboxamide host material unit generated
cavities that stabilized the nickel nanoparticles effectively and prevented the aggregation and separation
of these particles on the surface. This compound was appropriately characterized using FT-IR
spectroscopy, FE-SEM, ICP-OES, EDX, XRD, TGA analysis, VSM, and X-ray atomic mapping. The catalytic
oxidation of sulfides and oxidative coupling of thiols in the presence of the designed catalyst was
explored as a highly selective catalyst using hydrogen peroxide (H2O2) as a green oxidant. The easy
separation, simple workup, excellent stability of the nanocatalyst, short reaction times, non-explosive
materials as well as appropriate yields of the products are some outstanding advantages of this protocol.
Received 28th February 2021
Accepted 4th April 2021
DOI: 10.1039/d1ra01592c
rsc.li/rsc-advances
is not possible to separate nanoparticles by conventional
traditional methods such as ltration. Therefore, their separa-
1. Introduction
tion from the reaction mixture requires special techniques,
such as high-speed centrifugation,18,19 which is not technologi-
cally and economically feasible.20 Among these nanoparticles,
superparamagnetic cobalt ferrite nanoparticles, due to their
potential for catalyzing many organic reactions, have recently
attracted much attention. These nanoparticles can be easily
recovered from the reaction medium using an external
magnetic eld causing no need for ltration, and are reusable
catalytic systems. In this regard, it is also reported that selecting
the appropriate ligand groups is very important for strongly
immobilization and robust metal linkages to nanoparticle
cores.21 However, the proper bonding allows us to have stable
attachments of metals to magnetic nanoparticles, which makes
it easy to recover the complex catalysts for further use in these
procedures. Nanoscience has been enriched by the synthesis of
new compounds that have different applications, such as car-
boxamide22,23 derivatives that contain a ring junction nitrogen
and suitable compounds are listed as ligand groups.24,25 The
conventional method for selective formation of carboxamides is
by the reaction between an appropriate carboxylic acid and the
corresponding amines in an organic solvent, for instance,
pyridine. The synthesis of metal ions transitioning to organic
ligands is of considerable interest due to the unique structural
versatility and potential applications of various complexes
containing amide groups that are structurally and magnetically
interesting.26–28 In this direction, a complex derived from car-
boxamide was considered as a coating material for nano-
particles and it has been used as an efficient and direct catalyst
In recent years, catalysts have attracted great attention due to
their wide range of applications in different elds and has
generated interesting discussions in chemistry.1 Also, the
design and preparation of green catalysts2,3 have attracted much
attention in academia and industries,4–6 and hence, following
green chemistry7,8 is one of the important goals in synthetic
organic chemistry.9 Due to their application in human health
hazards and environmental activities as well as economic
importance, they have received exclusive attention.10,11 To
prepare heterogeneous catalysts, various supports have been
proposed. The heterogenization of catalysts on solid supports is
one of the appropriate methods to prevent wastage and can
provide opportunities to recycle them from the reaction envi-
ronment.12–14 The use of these catalysts is a well-established
method to control pollution. Thus, these catalysts have been
extensively used in organic synthesis compared to homogenous
catalysts because of economic and industrial aspects.14,15
Supports for nanometer-sized catalysts have recently attracted
great attention due to their appropriate surface area and
outstanding chemical stability, and excellent activity in the
liquid phase.16,17 Functionalized nanoparticles are used as
a feasible substitute for conventional materials as robust,
active, and high surface area catalyst supports. Nevertheless, it
Department of Chemistry, University of Kurdistan, P. O. Box 66135-416, Sanandaj,
Iran. E-mail: mghadermazi@yahoo.com; Fax: +98 87 3324133; Tel: +98 87 33624133
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra01592c
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in the synthesis of important products. The oxidation of NH2@carboxamide–Ni included SEM, EDX, XRD, VSM, FT-IR,
suldes to sulfoxides29,30 due to their applications in funda- TGA, and ICP-OES. The models of the apparatus are as
mental researches is one of the particularly important trans- following: the size distribution and the morphology of the
formations in organic compounds. Also, other extended usages nanocomposite were examined by using scanning electron
consist of the synthesis of suitable chemicals and biological microscopy (SEM) on an SEM TESCAN MIRA3 instrument. The
compounds, and as an intermediate for the production of elemental mapping and compositional analysis were performed
various chemical and biological molecules, as well as antioxi- using energy-dispersive X-ray spectroscopy (EDX) with a Keyex
dants and stabilizers in engine and oil fuels. They have Delta Class I equipped with the SEM instrument. Powder X-ray
important medicinal properties such as anti-inammatory and diffraction (XRD) was performed on a Philips diffractometer of
anti-cancer, and are also used in the treatment of diabetes and X'pert Company. Magnetic properties of fabricated nano-
Alzheimer's; due to the basic applications of these compounds, composites were studied using a vibrating sample magnetom-
they have received a lot of attention.31 Although different eter (VSM) provided by the Meghnatis Daghigh Kavier
methods have been developed for sulde oxidation, catalytic Company. The FT-IR spectrum was obtained through the KBr
oxidation switching to meet green chemistry has been estab- disc method on a VERTEX 70 model BRUKER FT-IR spectro-
lished, but it remains challenging.32–34 With the development of photometer. The thermal stability of the nanocatalyst was
nanocatalysts for organic transformation, and carboxamide in studied by thermo-gravimetric analysis (TGA), which was per-
the trapping and stabilizing of Ni nanoparticles, we present an formed on a Shimadzu DTG-60 instrument in the temperature
efficient and reusable nanocatalyst for the oxidation of suldes range of 20–700 ꢀC. The ICP-OES analyzer by PerkinElmer
through the oxidative coupling of thiols with H2O2 as a green Optima 8300, was used for measuring the Ni loading and was
oxidant.
a reliable solution for the elemental analysis of the catalyst.
NMR spectra were recorded on a BRUKER Advance 400 MHz
NMR spectrometer in CDCl3 and DMSO.
2. Experimental
2.1. Materials and methods
2.2. Preparation of magnetic CoFe2O4 nanoparticles
All materials were purchased from Sigma-Aldrich and Merck
chemical and used without any further purication. The First, Fe (NO3)3$9H2O (3.23 g, 8.0 mmol), Co (NO3)2$6H2O (1.2 g,
instruments used for the characterization of CoFe2O4/SiO2– 4.0 mmol) were dissolved in 30 mL distilled water at room
Scheme 1 The schematic pathway for preparing CoFe2O4/SiO2–NH2@carboxamide–Ni nanocomposite.
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temperature. A solution of sodium hydroxide (10%) was added
dropwise to the salt solution under constant stirring with
a magnetic stirrer until the stabilization of pH to 11. Then,
0.3 mL of olive oil as a surfactant was added and heated at 80 ꢀC
for 1 h. Aer cooling the mixture, nano CoFe2O4 (dark precipi-
tate) was separated by magnetic decantation and was washed
several times with distilled water–ethanol mixture solution. The
2.7. Procedure for the oxidation of suldes to sulfoxides
For the synthesis of sulfoxide from the desired sulde, in
a 50 mL round bottom ask equipped with a magnetic stirrer,
sulde (1 mmol), H2O2 (0.5 mL) (33%), and CoFe2O4/SiO2–
NH2@carboxamide–Ni catalyst (0.06 g) were added and the
ꢀ
mixture was magnetically stirred at room temperature (25 C).
The reaction was checked using thin-layer chromatography
(TLC), (using: hexane : ethyl acetate (4 : 1)).36 Finally, aer
completing the reaction, the product was extracted from the
reaction mixture with ethyl acetate. Then, the catalyst was
separated using an external magnet, washed several times with
ꢀ
synthesized sample was dried at 60 C for 12 h.
2.3. Preparation of 3-aminopropyl trimethoxysilane-coated
COFe2O4 (CoFe2O4/SiO2–NH2)
ꢀ
ethanol, and dried in a vacuum oven at 80 C for 5 h.
1 g of the prepared CoFe2O4 was well dispersed by ultrasonic
vibration in anhydrous toluene (25 mL) for 15 min. Then, the
linkage of 3-aminopropyl trimethoxysilane (APTMS) units was
obtained on the support surface. APTMS (1 mL) was added slowly
to the dispersion under vigorous stirring at reux conditions for
48 h. The suspended substances were collected using a magnet
and washed three times with ethanol/toluene to remove the
2.8. Procedure for the oxidation of thiol to disuldes
For the synthesis of disuldes, a mixture of thiol (1 mmol), and
0.4 mL H2O2 (33%) were stirred in the presence of CoFe2O4/
SiO2–NH2@carboxamide–Ni catalyst (0.05 g) in ethanol (2 mL).
The reaction was followed with TLC (using: hexane : ethyl
acetate (4 : 1)). Finally, CoFe2O4/SiO2–NH2@carboxamide–Ni
nanoparticles were magnetically separated and washed with
ethanol several times. These particles were dried for 6 h in
a vacuum oven at 70 ꢀC. The structures of the products were
ꢀ
remaining impurities and dried in a vacuum oven at 60 C for
12 h. The resulting product was denoted as CoFe2O4/SiO2–NH2.
2.4. Preparation of carboxamide
Carboxamide was obtained according to the previously reported
protocol.35 In this method, 4-chloropyridine-2,6-dicarboxylic
acid (0.183 g, 1 mmol) was dissolved in 20 mL of dry dichloro-
methane, then 2-amino-4-methyl phenol (0.246 g, 2 mmol) was
added to it slowly by stirring occasionally, the resulting brown
solution was stirred for 14 h at room temperature. The precip-
itated particles were formed and rinsed several times with
NaHCO3 (5%) to remove the remaining impurities. However, in
this case, carboxamide was used as the organic ligand.
1
recognized using their analytical and spectral (IR, H, and 13C
NMR) data outlined in ESI.†
2.5. Preparation of the CoFe2O4/SiO2–NH2@carboxamide
nanoparticles
In this step, the solution of CoFe2O4/SiO2–NH2 particles (1 g in
25 mL of distilled water), was prepared under constant stirring
ꢀ
at 40 C. At this time, carboxamide (0.6 g, 1.5 mmol) dissolved
in chloroform (25 mL), was added to the combination
mentioned above and the mixture solution was further stirred
in a ask for 24 h under reux. Aer cooling to room temper-
ature, the generated solid was isolated using an external
magnet, washed with water/ethanol, and dried at 60 ꢀC for 24 h
to afford a CoFe2O4/SiO2–NH2@carboxamide–Ni black solid.
2.6. Preparation of CoFe2O4/SiO2–NH2@carboxamide–Ni
nanoparticles
In the nal step, to prepare CoFe2O4/SiO2–NH2@carboxamide–
Ni, (1 g) of the synthesized nanocomposite CoFe2O4/SiO2–
NH2@carboxamide with Ni(NO3)2$6H2O (0.6 g 5.4 mmol) was
mixed in ethanol (30 mL) under reux condition at 80 ꢀC for 6 h.
Ultimately, the precipitates were isolated by using an external
magnet. It was washed with ethanol three times and dried in
a vacuum oven at 50 ꢀC for 4 h. The resulting material was
denoted as CoFe2O4/SiO2–NH2@carboxamide–Ni catalyst.
Fig. 1 FT-IR spectra of CoFe2O4 (A), CoFe2O4/SiO2–NH2 (B), car-
boxamide (C), CoFe2O4/SiO2–NH2@carboxamide (D), CoFe2O4/SiO2–
NH2@carboxamide–Ni (E).
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was separated as an efficient catalyst from the reaction mixture
using an isolated external magnet and dried at 70 C. The ob-
tained nanocatalyst was used for the next reaction under
optimal conditions. This procedure was repeated for eight runs,
without signicantly reducing on-target activity.
2.9. Selected spectral data
ꢀ
1
2.9.1. Methyl phenyl sulfoxide. HNMR (400 MHz, DMSO,
ppm): d 4.16 (s, 4H), 7.713–7.732 (m, 10H); 13CNMR (75 MHz,
DMSO, ppm): d 58.03, 129.6, 129.16, 128.81, 128.51, 127.5. IR
(KBr) (cmꢁ1): n (S]O): 1016–1072.
1
2.9.2. Benzyl phenyl sulfoxide. HNMR (400 MHz, DMSO,
ppm): d 4.68 (s, 2H), 7.48–7.16 (m, 4H), 7.73–7.53 (m, 4H).
13CNMR (75 MHz, DMSO, ppm): d 61.15, 138.83–134.29,
1131.45, 130.9, 131.1, 129.0. IR (KBr) (cmꢁ1): n (S]O): 1065.
2.9.3. 2,20-Disulfanediyldiethanol. 1HNMR (400 MHz,
CDCl3, ppm): d 2.262–2.798 (m, 4H), 3.217 (m, 4H); 13CNMR
(100 MHz, CDCl3, ppm): d 41.43, 61.61. IR (KBr) (cmꢁ1): n (S–S):
1058.
3. Results and discussions
The catalyst CoFe2O4/SiO2–NH2@carboxamide–Ni was prepared
by the concise route shown in Scheme 1. To perform efficient
immobilization of Ni on magnetic nanoparticles, initially,
naked CoFe2O4 was linked with coated-APTMS containing NH2
functional groups for better coordination. Subsequently, the
carboxamide was synthesized via the reaction of 4-
chloropyridine-2,6-dicarboxylic acid, and 2-amino-4-methyl
phenol. Then, carboxamide supported on CoFe2O4/SiO2–NH2
(CoFe2O4/SiO2–NH2@carboxamide) was synthesized by a cova-
lent linkage of Cl groups of the carboxamide with the amino
(–NH) groups of APTMS. Finally, the catalyst was synthesized by
the reaction of CoFe2O4/SiO2–NH2@carboxamide with Ni(NO3)2
2.9.4. 1,2-Diphenyldisulfane. 1HNMR (400 MHz, DMSO,
ppm): d 7.271–7.322 (m, 6H),7.325–7.382 (m, 4H); 13CNMR (100
MHz, DMSO, ppm): d 125.51, 127.28, 129.56, 136.83. IR (KBr)
(cmꢁ1): n (S–S): 1043.
2.10. Procedure for the recovery CoFe2O4/SiO2–
NH2@carboxamide nanocatalyst
To recycle the catalyst in the sulde oxidation reactions, the through a stable exchange interaction between the nickel and
synthesized CoFe2O4/SiO2–NH2@carboxamide nanocomposite carboxamide groups.
Fig. 2 EDX spectrum and elemental mapping of CoFe2O4/SiO2–NH2@carboxamide–Ni nanocomposite.
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3200 cmꢁ1 in carboxamide and the peak at 1760 cmꢁ1 was
assigned to C]O stretching vibrations in carboxamide. Also,
the stretching vibrations at 1680, 1540, and 1420 cmꢁ1 in this
compound are attributed to bonds in pyridine (Fig. 1C). The
peak at about 1590 cmꢁ1 and the peak at 1745 cmꢁ1 are
assigned to C–N vibrations and C]O stretching vibrations in
CoFe2O4/SiO2–NH2@carboxamide–Ni, respectively (Fig. 1D),
while for CoFe2O4/SiO2–NH2@carboxamide–Ni (Fig. 1E), this
peak is shied to 1710 cmꢁ1 due to the coordination of the
deprotonated oxygen and nitrogen carboxamide groups to the
nickel.
The EDX elemental analysis was used to check the elemental
composition in CoFe2O4/SiO2–NH2@carboxamide–Ni catalyst.
The existence of elements Ni, Co, Fe, N, C, Si, and O in the
catalyst conrmed the successful formation of CoFe2O4/SiO2–
NH2@carboxamide–Ni catalyst (Fig. 2). Also, the images
3.1. Catalyst characterization
The CoFe2O4/SiO2–NH2@carboxamide–Ni nanocomposite was
successfully synthesized and the chemical properties and
structure of the real catalyst were conrmed using XRD, FT-IR,
SEM, EDX, ICP-OES, TGA, and VSM methods. The FT-IR spectra
in the range of 400–4000 cmꢁ1 of CoFe2O4, CoFe2O4/SiO2–NH2,
carboxamide, CoFe2O4/SiO2–NH2@carboxamide, and CoFe2O4/
SiO2–NH2@carboxamide–Ni nanocomposite are shown in
Fig. 1. The presence of vibration bands at 470–530 cmꢁ1 is
assigned to the Fe–O bond in the CoFe2O4. The broad band at
3420 cmꢁ1 can be related to the characteristic –OH bands of
CoFe2O4 (Fig. 1A). The sharp peak at 1100 cmꢁ1 and the peak at
3200 cmꢁ1 are assigned to Si–O and NH2 in CoFe2O4/SiO2–NH2,
respectively (Fig. 1B). The stretching vibrations for the hydroxyl
functional group (O–H) on the surface of carboxamide were
recorded at 3000–3500 cmꢁ1, NH stretching band appeared at
Fig. 3 FE-SEM images of CoFe2O4 (a) and CoFe2O4/SiO2–NH2@carboxamide–Ni nanocomposite at 1 mm (b), 500 nm (c), and 200 nm (d)
magnifications.
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obtained from the elemental mapping showed an uniform
dispersion of Ni in the nanocomposite.
To identify the size and surface morphology, CoFe2O4/SiO2–
NH2@carboxamide–Ni nanocomposite was studied using FE-
SEM. As can be seen, spherical-shaped particles of CoFe2O4
have an average diameter of 4–5 nm (Fig. 3a) but the average
diameter of CoFe2O4/SiO2–NH2@carboxamide–Ni changes from
10–30 nm to a bigger diameter, indicating modication and
having a rather uniform coating of organic layers, and it can be
concluded that CoFe2O4/SiO2–NH2@carboxamide–Ni particles
were successfully synthesized (Fig. 3b–d).
Inductively denition ICP-OES analysis was used to deter-
mine the exact amount of nickel loaded on CoFe2O4/SiO2–
NH2@carboxamide–Ni nanocomposite, and it was found to be
0.43 mmol gꢁ1
.
The magnetic properties of CoFe2O4 and CoFe2O4/SiO2–
NH2@carboxamide–Ni nanoparticles were measured using
a vibrating sample magnetometer (VSM) at room temperature
(Fig. 4). VSM measurements for the uncoated CoFe2O4 (Fig. 4a)
and CoFe2O4/SiO2–NH2@carboxamide–Ni nanocomposite
(Fig. 4b) showed that in the same eld they were 68.7 and 27.3
emu gꢁ1, respectively. Also, a decrease in the magnetic prop-
erties of CoFe2O4/SiO2–NH2@carboxamide–Ni, compared to
CoFe2O4 nanoparticles is due to the coating of CoFe2O4 nano-
particles by organic layers and a nickel complex.
Fig. 5 shows the thermo-gravimetric analysis (TGA) curve of
the CoFe2O4/SiO2–NH2@carboxamide–Ni. The rst weight loss
(12.68%) between 50 and 250 ꢀC corresponds to the removal of
adsorbed water as well as the dehydration of the surface OH
groups. The second signicant weight loss (10.02%) from 260 to
360 ꢀC is due to the decomposition of carboxamide complex
Fig.
4
VSM curves of CoFe2O4 (a), CoFe2O4/SiO2–NH2@-
carboxamide–Ni nanocomposite (b).
ꢀ
residues. Some weight loss at around 370–530 C is due to the
disintegration of APTMS and carboxamide. In this way, the TGA
curves also conrm the successful graing of SiO2–NH2@-
carboxamide–Ni onto the surface of CoFe2O4.
XRD patterns of CoFe2O4 and CoFe2O4/SiO2–NH2@-
carboxamide–Ni catalyst are displayed in Fig. 6. The diffraction
peaks for CoFe2O4 were observed at 2q ¼ 21.17ꢀ, 30.09ꢀ, 41.45ꢀ,
50.45ꢀ, 62.69ꢀ, 67.37ꢀ, and 74.45ꢀ that corresponded to (2 2 0), (2
2 2), (3 1 1), (4 0 0) (4 2 2), (5 1 1), (4 4 0) and (5 3 3) planes of the
standard spinel structure of CoFe2O4 (JCPDS card no. 22-
1086).37 Also, no remarkable variation in the pattern of CoFe2O4/
SiO2–NH2@carboxamide–Ni (Fig. 6b) catalyst suggested that it
was retained aer graing of carboxamide on CoFe2O4 layers
and the crystalline structure of CoFe2O4 (Fig. 6a) was not
destroyed during the synthesis of the catalyst.
Fig. 5 The TGA curve of CoFe2O4/SiO2–NH2@carboxamide–Ni.
3.2. Catalytic activity
Due to the capability of CoFe2O4/SiO2–NH2@carboxamide–Ni as
a mild and efficient catalyst, it was decided to use this synthe-
sized catalyst for the oxidation of suldes into corresponding
sulfoxides (Scheme 2). Hence, methyl phenyl sulde was
investigated as a standard benchmark, and the inuence of
parameters such as the solvents, the amount of catalyst, H2O2,
temperature, and oxidation agent's conditions were checked
(Table 1). Initially, the effect of the catalyst amount on the
reaction yield was studied. The amount of catalyst between
0.02–0.05 g on the reaction was examined for the reaction (Table
1, entries 6–9); the highest activity was observed in the presence
of 0.04 g of the catalyst (Table 1, entry 12), and the oxidation
reaction was also tested without a catalyst and the desired
product was not obtained even aer several hours (Table 1,
entry 16). Then, the effects of the solvents on the model reaction
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Fig. 6 The XRD patterns of CoFe2O4 (a), CoFe2O4/SiO2–NH2@carboxamide–Ni (b).
reaction was performed at room temperature (Table 1, entry 12).
Moreover, the table also shows that in case of the absence of
H2O2, no reaction was performed (Table 1, entry 8), and
considering the amount of H2O2 (Table 1, entries 6, 7, 12),
increasing the amount of H2O2 proved to be the best choice for
the reaction (Table 1, entry 12). Furthermore, the reaction was
performed in the presence of CoFe2O4/SiO2–NH2@carboxamide
and CoFe2O4/SiO2–NH2 catalysts, however, it did not proceed
even aer a long time. The obtained result shows the impor-
tance of the role of nickel nanoparticles in the catalytic activity
for the synthesis of the product (Table 1, entries 17, 18).
Scheme 2 General scheme for the oxidation of methyl phenyl sulfide.
were evaluated, the solvent-free conditions and a variety of
solvents such as CH3CN, EtOH, EtOAc, CH2Cl2, and H2O were
examined. Finally, it was found that solvent-free conditions
gave the best results (Table 1, entry 12). The effect of tempera-
ture on the rate of the reaction was evaluated for the model
reaction and, the best result was observed when the model
Aer nding the results, however, to improve the reaction
conditions, we employed various substituted suldes for the
Table 1 Optimization of the oxidation of methyl phenyl sulfide by CoFe2O4/SiO2–NH2@carboxamide–Ni under different conditionsa
Catalyst
(g)
Temperature
(ꢀC)
Entry
Solvent (mL)
H2O2 (mL)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
CH3CN
EtOH
EtOAc
CH2Cl2
0.4
0.4
0.4
0.4
0.4
0.3
0.45
—
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.05
0.03
0.02
—
RT
RT
RT
RT
RT
RT
RT
RT
30
8 h
6 h
3 h
2 h
140
100
50
2 h
130
130
145
50
50
70
100
6 h
5 h
6 h
45
52
55
68
70
78
94
40
74
70
67
94
94
80
72
60
68
62
H2O
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
9
10
11
12
13
14
15
16
17
18
40
50
RT
RT
RT
RT
RT
RT
RT
CoFe2O4/SiO2–NH2@carboxamide
CoFe2O4/SiO2–NH2
a
Reaction conditions: methyl phenyl sulde (1 mmol), H2O2 (0.4 mL), catalyst (0.04 g), and solvent-free.
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synthesis of sulfoxides (Table 2). It is noteworthy that the rate (Table 2, entries 4 and 12). Furthermore, the oxidation of
reactions in the sulfoxide stage stop completely. For exploring suldes with sensitive functional groups such as hydroxyl did
the electronic effect on the performance, the range of para- not affect functional groups, indicating high chemical selec-
substituted methyl phenyl suldes was examined. The presence tion.38 (Table 2, entries 5, and 6). Interestingly, these systems
of the electron donor group increases the reaction process can also be used in the oxidation from aliphatic suldes, and
compared to that with the electron-withdrawing group, showing good to excellent performance yields were obtained in the
that the electronic nature of the suldes affected the reaction relevant sulfoxide systems (Table 2, entries 3, 8, and 10).
Table 2 Oxidation of various sulfides to sulfoxides in the presence of CoFe2O4/SiO2–NH2@carboxamide–Ni under optimization conditionsa
Entry
Substrate
Time (min)
Yield (%)
Mp (ꢀC)
1
50
85
137–140 (ref. 39)
2
65
75
124–128 (ref. 40)
3
4
60
45
40
70
Oil39
69–72 (ref. 41)
5
6
20
25
80
82
Oil42
Oil42
7
8
35
40
94
85
Oil39
70–73 (ref. 42)
9
40
30
90
80
Oil42
Oil40
10
11
73
70
45
47
Oil43
12
60–64 (ref. 43)
a
Reaction conditions: sulde (1 mmol), H2O2 (0.4 mL), catalyst (0.04 g), solvent-free, and room temperature conditions.
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of its O–OH bond to generate [Ni–O], and OH, radicals, where
the compound can be subsequently oxidized to the corre-
sponding sulfoxide by OH radicals. The reactivity can be linked
to the topology of the Ni–OOH catalyst due to the relative ease of
accessibility of the nucleophilic S atom on the sulde to the Ni
atom on the Ni–OOH active site. Sulfoxides are generated in
good selectivity at 25 ꢀC because the nucleophilic attack by
sulde is far superior to that of sulfoxide. Therefore, the present
catalytic system utilizes cheap and readily available agents as
a selective catalyst for clean oxidation of sulde products and
releases only innocuous water as the by-products.
Subsequently, we turned our attention to the oxidation
reaction of thiols to disuldes, with H2O2 under different
conditions. To optimize these reaction conditions and synthesis
routes, we surveyed the synthesize of 4-methyl benzene thiol as
the starting material under these different reaction conditions
(Scheme 4). The test results are summarized in Table 3. Initially,
the inuence of the catalyst amount shows that the corre-
sponding products were isolated in good to excellent yields with
0.03 g of CoFe2O4/SiO2–NH2@carboxamide–Ni as the most
suitable amount (Table 3, entry 12). The reaction was performed
without any catalyst, but it did not proceed for a long time
(Table 3, entry 16). The effect of H2O2 as an oxidant on the
oxidation of thiols was initially the reaction of the scientic
model with 0.4 mL of H2O2 (33%), which was the most appro-
priate amount (Table 3, entry 12). Also, the reaction was tested
in the absence of H2O2 and we found that the reaction did not
progress well (Table 3, entry 8). The reaction was performed in
different solvents in which the best result was obtained with
solvent-free conditions (Table 3, entry 12). To investigate the
effect of temperature on the reaction rate, this procedure was
studied at various temperatures and the best one was at room
temperature (Table 3, entry 12). More importantly, because of
Scheme
oxidation of sulphide.
3 CoFe2O4/SiO2–NH2@carboxamide–Ni catalyzed the
Scheme
methylbenzenethiol.
4 General scheme for the oxidation of 4-
A possible explanation for the mechanism of sulde using
H2O2 as the oxidizing agent over CoFe2O4/SiO2–NH2@-
carboxamide–Ni catalyst is proposed as shown in Scheme 3.
Based on the literature report,44 we propose that CoFe2O4/SiO2–
NH2@carboxamide–Ni can easily react with H2O2 to form a Ni–
OOH adduct. Then the Ni–OOH adduct may undergo cleavage
Table 3 Optimization of the 4-methylbenzenethiol by CoFe2O4/SiO2–NH2@carboxamide–Ni under different conditionsa
Catalyst
(g)
Temperature
(ꢀC)
Entry
Solvent (mL)
H2O2 (mL)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
CH3CN
EtOH
EtOAc
CH2Cl2
0.4
0.4
0.4
0.4
0.4
0.3
0.45
—
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.04
0.02
0.01
0.03
0.03
0.03
0.03
—
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
30
3 h
2 h
1 h
1 h
1 h
60
30
3 h
30
45
50
30
40
40
60
52
55
62
78
87
35
87
80
70
87
73
65
60
60
52
38
H2O
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
9
10
11
12
13
14
15
16
17
18
40
50
RT
RT
RT
45
55
5 h
7 h
7 h
CoFe2O4/SiO2–NH2@carboxamide
CoFe2O4/SiO2–NH2
a
Reaction conditions: 4-methylbenzenethiol (1 mmol), H2O2 (0.4 mL), catalyst (0.03 g), and solvent-free.
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Table 4 Oxidative 4-methylbenzenethiol by CoFe2O4/SiO2–NH2@carboxamide–Ni under optimization conditionsa
Entry
Substrate
Time (min)
Yield (%)
Mp (ꢀC)
1
60
91
91–95 (ref. 41)
2
3
30
25
93
92
102–107 (ref. 45)
132–136 (ref. 40)
4
36
84
280–283 (ref. 46)
5
6
15
25
91
94
102–106 (ref. 46)
Oil47
7
8
51
56
81
85
90–94 (ref. 40)
94–98 (ref. 46)
9
42
25
90
90
98–101 (ref. 48)
99–102 (ref. 40)
10
11
30
80
105–108 (ref. 40)
a
Reaction conditions: sulde (1 mmol), H2O2 (0.4 mL), catalyst (0.03 g), solvent-free, and room temperature conditions.
mild conditions of described heterogeneous systems, there is reaction times. The electronic nature of the compounds affected
no overoxidation to sulfone for the oxidation of suldes. Also, the reaction rate. From a view of the electronic effect, the
the reaction was performed in the presence of CoFe2O4/SiO2– presence of the electron-donating group enhanced the reaction
NH2@carboxamide and CoFe2O4/SiO2–NH2 as the catalyst, but (Table 4, entry 11). Then, the study was performed on substrates
it did not proceed aer a long time (Table 3, entries 17, 18).
containing electron-withdrawing groups (Table 4, entries 1, 10).
Also, continuing with the optimal conditions, the oxidation We found that they need much more time than most electron-
reaction conditions of this protocol were tried with a wide range donating groups. Also, to further extend this method to the
of different prepared thiol groups as shown in Table 4. Aer preparation of this catalytic system, aliphatic and aromatic
looking at the results, we found that, for the disulde synthesis, suldes were tested, and the acceptable yields of the corre-
the catalyst works well with good efficiency and with short sponding disuldes were obtained.
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Fig. 7 Reusability of the CoFe2O4/SiO2–NH2@carboxamide–Ni for
the oxidation of sulfides (a) and reusability of the CoFe2O4/SiO2–
NH2@carboxamide–Ni for oxidation coupling reaction of thiols (b).
Scheme 5 CoFe2O4/SiO2–NH2@carboxamide–Ni catalyzed oxidative
coupling of thiols to disulfides.
chemical structure of the catalyst aer the nal cycle, FT-IR,
TGA, and SEM analysis were performed and the results
showed that the chemical structure of the catalyst was preserved
without any change (Fig. 8).
The proposed mechanism for the oxidation of thiols into
disulde in the presence of CoFe2O4/SiO2–NH2@carboxamide–
Ni is shown in Scheme 5.49,50 The reaction of H2O2 with Ni-
catalyst leads to an intermediate A, which is converted to an
active oxidant B. Next, the nucleophilic attack of the thiol on
this intermediate gives cation C, which, in turn, produces the
corresponding product.
In this study, CoFe2O4/SiO2–NH2@carboxamide–Ni was
compared with the previously reported recoverable catalysts. As
shown in Table 5, the oxidation reaction of sulde (Table 5,
entries 1–5) and thiol (Table 5, entries 6–11) is shown. This
method is comparable to other reported catalysts in terms of
reaction time, yield, and ease of catalyst separation in the
presence of the synthesized catalyst.
3.4. ICP-OES technique
In the nal investigation, the number of atoms loaded on the
catalyst was measured using the ICP-OES technique. It was seen
that Ni atoms in the fresh catalyst and recycled one aer 8 times
were 0.41 mol gꢁ1 and 0.38 mol gꢁ1 respectively, indicating that
a minimum amount of Ni leaching happened in the catalytic
process, conrming the stability and efficiency of the catalyst.
3.5. Hot ltration test
Also, the oxidation of sulde in the presence of CoFe2O4/SiO2–
NH2@carboxamide–Ni was investigated by hot ltration to
determine whether the catalyst was heterogeneous or whether
Ni was removed from the solid catalyst into the solution. In this
3.3. Reusability of the CoFe2O4/SiO2–NH2@carboxamide–Ni
The reusability of CoFe2O4/SiO2–NH2@carboxamide–Ni for the experiment, under the optimized reaction conditions, we found
oxidation of suldes and the oxidation reaction of thiols was product yield at 62% reaction half-time. The reaction was then
checked. The nanocomposite structure remained unchanged repeated and during half of the reaction, the catalyst was
even aer eight consecutive applications runs for the oxidation removed from the reaction mixture by using a magnet. The
of sulde and thiol (Fig. 7). To investigate changes in the reaction yield at this stage was 68%, which was heterogeneous
Table 5 Comparison of CoFe2O4/SiO2–NH2@carboxamide–Ni for the oxidation of methyl phenyl sulfide (entries 1–5) and 4-methyl-
benzenethiol (entries 6–11) with the previously reported catalysts
Entry
Substrate
Catalyst
Solvent
Time
Yield (%)
Ref.
1
2
3
4
5
6
7
8
Methyl phenyl sulde
Methyl phenyl sulde
Methyl phenyl sulde
Methyl phenyl sulde
Methyl phenyl sulde
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
SBA-15/NH2–FeQ3
H2O
EtOH
EtOH
TBHP
Solvent-free
Solvent-free
Solvent-free
EtOH
EtOH
Solvent-free
Solvent-free
3h
2 h
2 h
10 h
60
60
40
15
70
87
99
92
98
87
96
97
96
97
97
92
48
51
52
53
Fe3O4@SiO2–APTES–FeL
Fe3O4@SiO2–APTES (Fe(acac)2)
WOx/SBA-15(20)
CoFe2O4/Pr–NH2 @carboxamide@Ni
Fe3O4@tryptophan@Ni
Cu-SBTU@MCM-41
Fe3O4@tryptophan–Cu
Fe3O4–adenine–Ni
MCM-41@leucine–Ce(IV)
CoFe2O4/Pr–NH2@carboxamide@Ni
This work
54
55
56
57
58
9
10
11
40
35
This work
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Fig. 8 TGA (a), SEM (b), and FT-IR (c) analyses of the reused CoFe2O4/SiO2–NH2@carboxamide–Ni after eight runs.
with a stable catalyst reaction and signicant leakage of the separated using an external magnet and reused several times
effective nickel metal.
with no detectable changes in its catalytic efficiency. We antic-
ipate that this protocol will be useful in the new research.
3.6. Catalyst poisoning test
Heterogeneity experiments were performed by the Hg poisoning
study. In this way, Hg(0) can poison the catalyst, either by using
a metal compound or by adsorption on the metal surface.59
Sulde oxidation was performed under the same conditions,
except for the addition of Hg (1 mmol) to the reaction mixture at
50% conversion of methylphenyl sulde to methylphenyl sulf-
oxide. Using this method, no product was obtained that is
strong evidence of the heterogeneity of the nature of the
catalyst.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors are grateful to the University of Kurdistan for
sponsoring this research project.
Notes and references
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