Copper based on diaminonaphthalene-coated magnetic nanoparticles as robust catalysts for catalytic oxidation reactions and C-S cross-coupling reactions

Article abstract of DOI:10.1039/d1ra01029h

In this work, the immobilization of copper(ii) on the surface of 1,8-diaminonaphthalene (DAN)-coated magnetic nanoparticles provides a highly active catalyst for the oxidation reaction of sulfides to sulfoxides and the oxidative coupling of thiols to disulfides using hydrogen peroxide (H₂O₂). This catalyst was also applied for the one-pot synthesis of symmetrical sulfidesviathe reaction of aryl halides with thiourea as the sulfur source in the presence of NaOH instead of former strongly basic and harsh reaction conditions. Under optimum conditions, the synthesis yields of sulfoxides, symmetrical sulfides, and disulfides were about 99%, 95%, and 96% respectively with highest selectivity. The heterogeneous copper-based catalyst has advantages such as the easy recyclability of the catalyst, the easy separation of the product and the less wastage of products during the separation of the catalyst. This heterogeneous nanocatalyst was characterized by FESEM, FT-IR, VSM, XRD, EDX, ICP and TGA. Furthermore, the recycled catalyst can be reused for several runs and is economically effective.

Full text of DOI:10.1039/d1ra01029h

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Copper based on diaminonaphthalene-coated
magnetic nanoparticles as robust catalysts for
catalytic oxidation reactions and C–S cross-
coupling reactions†
Cite this: RSC Adv., 2021, 11, 9366
Nasrin Yarmohammadi, Mohammad Ghadermazi * and Roya Mozafari
In this work, the immobilization of copper(II) on the surface of 1,8-diaminonaphthalene (DAN)-coated
magnetic nanoparticles provides a highly active catalyst for the oxidation reaction of sulfides to
2 2
sulfoxides and the oxidative coupling of thiols to disulfides using hydrogen peroxide (H O ). This catalyst
was also applied for the one-pot synthesis of symmetrical sulfides via the reaction of aryl halides with
thiourea as the sulfur source in the presence of NaOH instead of former strongly basic and harsh
reaction conditions. Under optimum conditions, the synthesis yields of sulfoxides, symmetrical sulfides,
and disulfides were about 99%, 95%, and 96% respectively with highest selectivity. The heterogeneous
copper-based catalyst has advantages such as the easy recyclability of the catalyst, the easy separation
of the product and the less wastage of products during the separation of the catalyst. This
heterogeneous nanocatalyst was characterized by FESEM, FT-IR, VSM, XRD, EDX, ICP and TGA.
Furthermore, the recycled catalyst can be reused for several runs and is economically effective.
Received 7th February 2021
Accepted 15th February 2021
DOI: 10.1039/d1ra01029h
rsc.li/rsc-advances
DAN has been widely utilized in the structure of inorganic
and organometallic complexes as a suitable ligand because of
having bidentate nucleophilic centers. This ligand has a unique
1
. Introduction
1
Catalysis is the master key to chemical transformations.
Almost all biological reactions and more than 90% of industrial structure and properties that make it an interesting choice for
2
processes need catalysts. It is known that homogeneous cata- a diverse range of applications over recent years. DAN complex
lysts, because of the solubility in the reaction medium, have has been used in various elds such as optical devices, biolog-
3
–5
14
good performance. However, these materials have disadvan- ical applications, conducting polymers, sensors, and elec-
6
15
tages such as the generation of highly toxic wastes and time- tronic science. This ligand has been recently considered
consuming catalyst separation. Additionally, metal catalysts because of its unique properties in heterocyclic synthesis.
7
16
are recycled with difficulty, and they produce high amounts of
Noble metals such as Pt, Cu, Au and Rh have been incor-
17–21
wastes. In industrial applications, homogeneous catalysts in porated into the structure of nanoparticles.
Among the
comparison with heterogeneous catalysts have a contribution of mentioned metals, copper nanoparticles have been paid atten-
3
less than 20%. From the standpoint of green chemistry, using tion due to their catalytic activity, suitable treating costs, and
22–24
heterogeneous catalysts to run procedures for making ne high conductivity, which make them useful in nanoscience.
chemical products has attracted remarkable attention from Cu-based materials, because of the several oxidation states of
industrial and academic researchers. However, the catalytic this metal, can catalyze many oxidative reactions via both one-
2,8,9
activity of heterogeneous catalysts is reduced over time.
and two-electron pathways. The feasible modication of the
Magnetic nanoparticle (MNP) catalysts can address the recovery properties of these materials via different synthetic policies and
and separation problems encountered in many catalytic reac- post-synthetic chemical treatments has engendered consider-
tions. Furthermore, these catalysts show not only high catalytic able interest in the eld of catalysis. Furthermore, Cu's high
activity, but also a high degree of chemical stability. MNP boiling point makes it a good candidate for reactions under
10–13
25
catalysts can be recycled with an external magnetic eld.
hard conditions such as high temperature and pressure.
It is clear that suldes are signicant precursors for the
synthesis of sulfoxides. Sulfoxides have been reported to
possess widespread pharmaceutical functions and biological
Department of Chemistry, University of Kurdistan, P.O. Box 66135-416, Sanandaj, properties such as anticancer, antifungal and anti-
26,27
Iran. E-mail: mghadermazi@yahoo.com; Fax: +98 87 3324133; Tel: +98 87 33624133 atherosclerotic activities.
Furthermore, they have been
†
Electronic supplementary information (ESI) available. See DOI:
1
0.1039/d1ra01029h
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extensively utilized for the preparation of carbon–carbon bond was gold-coated using a sputtering coater. FT-IR spectra of the
28,29
and rearrangement reactions.
prepared samples were recorded from a KBr disc using a VRTEX
Moreover, disuldes have important applications in biolog- 70 model BRUKER FT-IR spectrophotometer, in the range of 400
2
5
ꢀ1
1
ical and industrial elds such as protecting agents stabiliza- and 4000 cm . H NMR spectra were recorded using a Bruker
30
31,32
tion of protein structures, and vulcanizing entities.
The 400 MHz NMR AVANCE 300 spectrometer. TGA was performed
catalytic formation of the C–S bond is a basic transformation in using a Shimadzu DTG-60 instrument in the temperature range
ꢁ
synthetic reactions. Their importance is based on the applica- 25–800 C. Energy-dispersive X-ray spectroscopy (EDX) was used
tion in pharmaceutical industries and preparing a scaffold for for the elemental analysis. The copper content in the catalyst
33,34
aryl suldes as intermediates in synthetic organic chemistry.
was evaluated by inductively coupled plasma-optical emission
Although the sulde and disulde oxidation reactions have spectrometry (ICP-OES; 730-ES Varian). TEM analysis of the
35–44
been studied extensively,
it is necessary to introduce catalyst was performed using a Zeiss-EM10C transmission
procedures that are more simple, efficient and selective. Herein, electron microscope.
a magnetically separable heterogeneous catalyst was fabricated
by anchoring a Cu complex supported on functionalized
CoFe O nanoparticles. This catalyst exhibits efficient oxidation
2 4
2
.2 Preparation of CoFe O -DAN-Cu(II) nanocatalysts
2 4
First, the mixture of Fe(NO ) $6H O (16 mmol, 5.584 g) and
CO(NO ) $6H O (8 mmol, 2.328 g) was dissolved in deionized
water (20 mL) under vigorous stirring conditions until the
components were dissolved. A brown precipitate was produced
by adding sodium hydroxide (15 mL, 3 M) under magnetic
3
3
2
reactions of suldes to sulfoxides and oxidative coupling of
thiols to disuldes utilizing hydrogen peroxide. It is necessary
to introduce procedures that are more simple, efficient and
selective. Moreover, we used the prepared nanoparticle as
a catalyst for the odorless C–S cross-coupling reaction of aryl
halides under mild conditions. Furthermore, the catalyst is
separated from the reaction media via magnetic decantation
and the nanocatalyst can be recycled several times.
3
2
2
stirring. Aer adjusting the pH to 11, the solution was heated at
ꢁ
80 C for an hour. The obtained black magnetic nanoparticles
were separated, washed ve times with distilled water/ethanol
mixture solution, and dried in an oven at 60 C for 14 h. To
ꢁ
prepare CoFe O –Cl, 1 g of MNP powder was dispersed in 50 mL
2
4
of toluene under sonication for 30 min, then (1 mL) of 3-
chloropropyltrimethoxysilane (CPTMS) was added to the reac-
tion mixture and reuxed for 24 h. The obtained nanoparticles
2
. Experimental
2.1 Materials and methods
Chemical materials were supplied from Sigma-Aldrich and were separated by an external magnet, washed with ethanol/
ꢁ
Merck and used as received. The X-ray powder diffraction (XRD) water mixture solution and dried in an oven at 80 C. For the
data of the CoFe O -DAN-Cu(II) nanocatalyst were obtained functionalization of CoFe O –Cl with 1,8-diaminonaphthalene,
2
4
2 4
˚
using a Co radiation source with a wavelength of l ¼ 1.78897 A, 1 g of the suspension of CoFe O –Cl in 70 mL of acetonitrile was
2
4
4
0 kV. The morphology of the nanocatalyst was investigated by added to a mixture of KI (6 mmol, 0.996 g) and K CO (6 mmol,
2 3
FESEM-TESCAN MIRA3, operating at a voltage of 30 kV, which 0.828 g). The obtained mixture was stirred magnetically at room
Table 1 Optimization of conditions for oxidation of sulfides to sulfoxides in the presence of CoFe
2
O
4
-DAN-Cu(II)
ꢁ
a
Entry
Catalyst (mg)
Oxidation agent
Solvent
Temp. ( C)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
25
25
25
25
25
—
25
25
25
25
25
25
7
H
H
H
H
H
H
H
H
H
H
H
2
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
O
2
2
2
2
2
2
2
2
2
2
2
(0.5 mL)
(0.5 mL)
(0.5 mL)
(0.5 mL)
(0.5 mL)
(0.5 mL)
(0.55 mL)
(0.4 mL)
(0.3 mL)
(0.2 mL)
(0.1 mL)
Acetonitrile
n-Hexane
EtOAc
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
60
100
20
120
25
30
15
140
15
25
25
25
25
140
25
25
25
85
25
80
75
99
Trace
98
85
79
68
43
Trace
55
68
85
40
85
90
H
2
O
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
10
11
12
13
14
15
16
17
18
19
—
H
H
H
H
2
O
O
O
O
2
2
2
2
(0.5 mL)
(0.5 mL)
(0.5 mL)
(0.5 mL)
9
10
CuCl
25
25
2
2
2
2
$2H
2
O
65
NaIO
Oxone (0.3689 g, 0.6 mmol)
(2 MPa)
4
(2 mmol)
CH
3
CN/H
2
O
100
720
720
Ethanol
PEG
25
O
2
10
a
Isolated yields.
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2021 The Author(s). Published by the Royal Society of Chemistry
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temperature for 35 min. Aerwards, 1,8-diaminonaphthalene (2 g). The products were obtained by evaporating the organic
18 mmol, 2.84 g) was added to the solution and the mixture was solvent.
(
reuxed for 24 h. The resulting mixture was then ltered,
ꢁ
washed with 95% ethanol and dried at 60 C to obtain CoFe O -
2
4
2
.4 Typical procedure for the synthesis of derivative suldes
DAN. Finally, the CoFe
afforded by mixing CoFe
0 mL of ethanol and stirring under reux for 18 h. The ob-
2
O
4
-DAN-Cu(II) nanoparticles were
in the presence of CoFe -DAN-Cu(II)
2 4
O
2
O
4
-DAN (2 g) and CuCl $2H O (1 g) in
2
2
In a 25 mL round-bottom ask equipped with a magnetic stirrer
and heater, a mixture of the aryl halide (1 mmol), thiourea
(1 mmol, 0.076 g), NaOH (1 mmol, 0.04 g), and CoFe
Cu(II) (0.03 g) nanocatalyst were mixed under stirring at 130 C
2
tained product was then magnetically separated, washed with
ꢁ
2
O
4
-DAN-
ethanol and dried at 50 C.
ꢁ
in 3 mL DMSO for an appropriate time specied in Table 2.
Aer reaction completion (monitored by TLC), CH Cl2 was
added to the reaction mixture, the heterogeneous catalyst was
magnetically decanted. The ltered solution was washed with
2 2 4
H O and dehydrated over Na SO . Aer organic solvent vola-
2
2
.3 General procedure for oxidation of suldes to sulfoxides
with H in the presence of CoFe -DAN-Cu(II)
2
O
2
2 4
O
In a 25 mL round-bottom ask equipped with a magnetic stirrer
and heater, a suspension of 1 mmol sulde and 0.025 g catalyst
in ethanol (3 mL) was stirred for 2 min at room temperature.
tilization, the corresponding sulde was obtained.
Then, H
2
O
2
(0.5 mL, 4.90 mmol) was added dropwise to the
ꢁ
2.5 Typical procedure for oxidative coupling of thiols to
disuldes with H O in the presence of CoFe O -DAN-Cu(II)
2 2 2 4
reaction mixture and the obtained blend was stirred at 25 C for
appropriate time (Table 1). Aer reaction completion, ethyl
acetate (2 ꢂ 5 mL) was added to the mixture, a heterogeneous In a typical experiment, thiol (1 mmol), CoFe O -DAN-Cu(II)
2
4
catalyst was separated from the mixture using an external (0.025 g) and ethanol (3 mL) were taken in a 25 mL round-
magnet. Aer that, the ltered solution was dried over Na
2
SO
4
bottom ask, equipped with a magnetic stirrer and heater.
a
Table 2 Oxidation of sulfides with CoFe
2
O
4 2 2
-DAN-Cu(II) in the presence of H O
b
ꢁ
Mp ( C)
Entry
Substrate
Product
Time (min)
Yield (%)
5
1
2
1
2
15
15
99
95
Oil
Oil
5
53
3
2
93
Oil
5
4
3
4
5
27
6
95
94
Oil
Oil
5
6
7
20
85
93
88
112–114 (ref. 11)
130–133 (ref. 55)
8
100
105
87
85
117–119 (ref. 55)
47
9
Oil
a
ꢁ
b
Reaction conditions: catalyst (0.025 g), sulde (1 mmol), 30% H
368 | RSC Adv., 2021, 11, 9366–9380
2 2
O (0.5 mL) and solvent (3 mL) at 25 C. Isolated yields.
9
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2 4
Table 3 Optimization of conditions for sulfide synthesis in the presence of CoFe O -DAN-Cu(II)
ꢁ
a
Entry
Catalyst (mg)
Base
Thiourea (mmol)
Solvent
Temp. ( C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
30
30
30
30
30
—
30
30
8
10
20
30
30
30
30
30
30
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
KOH
1
1
1
1
1
1
0.5
0.8
1
1
1
1
1
1
1
1
1
H
PEG
DMF
2
O
130
130
130
130
130
130
130
130
130
130
130
45
65
85
100
130
130
130
4
4
3.5
2.5
3
N.R.
N.R.
52
95
35
Trace
67
82
37
52
DMSO
Toluene
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
4
0
1
2
3
4
5
6
7
8
79
40
65
75
85
Trace
Trace
Trace
Na
2
CO
3
CuCl
2
$2H
2
O
NaOH
1
a
Isolated yields.
ꢀ
1
The solution was stirred for 2 min at room temperature. Aer 139.16. IR (KBr) (cm ): 3333, 2854, 1625, 1425, 1303, 1073, 838,
that, H
2
O
2
(0.4 mL, 3.92 mmol) was added dropwise to the 697.
ꢁ
1
reaction mixture and the obtained blend was stirred at 25 C for
an appropriate time (Table 3). The reaction process was moni- d 3. 377 (s, 4H), 7.261–7.412 (m, 4H); C-NMR (100 MHz,
tored by TLC (ethyl acetate/n-hexane: 1 : 4). Upon completion of DMSO, ppm): d 35.15, 127.165, 128.69, 138.16. IR (KBr) (cm ): n
the reaction, the nanocatalyst was magnetically isolated and the (S–S): 1069.
2.6.7 Dibenzylsulfane. H-NMR (300 MHz, DMSO, ppm):
1
3
ꢀ1
product was extracted with ethyl acetate. The product was ob-
tained, dried over anhydrous Na
pure products.
2
SO and evaporated to yield 2.7 Procedure for the recovery of the CoFe O -DAN-Cu(II)
2 4
4
nanocatalyst
For recycling of the catalyst in the mentioned reactions, methyl
phenyl sulde, iodobenzene, and 4-methylthiophenol as model
substrates were used under optimized conditions. Aer
2
.6 Selected spectral data
1
2.6.1 Methyl phenyl sulfoxide. H-NMR (300 MHz, CDCl
3
,
1
3
ppm): d 2.98 (s, 3H), 7.46–7.87 (m, 5H); C-NMR (75 MHz, completion of the reaction, the CoFe
O -DAN-Cu(II) nano-
2 4
CDCl
(
3
, ppm): d 44.15, 124.44, 129.60, 131.45, 144.29. IR (KBr) catalyst was separated from the reaction mixture by a magnet,
ꢀ
1
cm ): n (S]O): 1079.
washed, and dried at 80 C. The obtained nanocatalyst was used
.6.2 Benzyl phenyl sulfoxide. H-NMR (400 MHz, DMSO, for the next reaction under optimum conditions. This proce-
1
2
1
3
ppm): d 4.68 (s, 2H), 7.14–7.34 (m, 4H), 7.53–7.75 (m, 4H). C- dure was repeated for nine runs. Selected spectral data using
1
NMR (75 MHz, DMSO, ppm): d 61.15, 128.51–128.81, 129.16,
29.60–131.45, 138.83. IR (KBr) (cm ): n (S]O): 1068.
.6.3 Dibenzyl sulfoxide. H-NMR (400 MHz, DMSO, ppm):
d 4.16 (S, 4H), 7.29–7.45 (m, 10H); C-NMR (75 MHz, DMSO,
H-NMR spectra for sulfoxides, symmetrical suldes, and
disuldes are given in the ESI.†
ꢀ
1
1
1
2
1
3
3
. Results and discussions
ꢀ
1
ppm): d 58.03, 127.57, 129.03, 130.18, 130.91. IR (KBr) (cm ): n
S]O): 1026–1089.
.6.4 Diphenyl sulde. H-NMR (400 MHz, DMSO, ppm): to a three-step procedure (Scheme 1). Cobalt ferrite was
(
CoFe O -DAN-Cu(II) nanoparticles were synthesized according
2 4
1
2
1
3
3+
2+
d 7.72–7.32 (m, 4H), 7.32–7.38 (m, 4H); C-NMR (75 MHz, prepared by the co-precipitation of aqueous Fe and Co salt
ꢀ
1
ꢁ
45
CDCl
3
, ppm): d 127.51, 129.66, 131.01, 134. 83. IR (KBr) (cm ): solutions in the presence of a base at 80 C. The functionali-
zation of the hydroxyl groups of CoFe with CPTMS yielded
.6.5 Chemical 4,4 -thiodiphenol. H-NMR (400 MHz, CoFe –Cl. Finally, CoFe -DAN was treated with CuCl
O to give CoFe -DAN-Cu(II) via an electrostatic interac-
, ppm): d 123.64, 128.81, 158.83. IR (KBr) tion between copper ions and functional groups of CoFe
cm ): 3421, 2822, 1625, 1498, 1117, 933, 797, 501. DAN. The catalytic performance of the synthesized heteroge-
.6.6 Di-p-tolylsulfane. H-NMR (400 MHz, DMSO, ppm): neous nanocatalyst was evaluated in the oxidation of suldes,
3
421, 3080, 1592, 1333, 1108, 1104, 838, 631.
2 4
O
0
1
2
2
O
4
2
O
4
2
-
1
3
CDCl
3
, ppm): d 6.63–6.68 (m, 4H), 7.713–7.732 (m, 4H); C- $2H
2
2 4
O
NMR (75 MHz, CDCl
3
O -
2 4
ꢀ1
(
1
2
1
3
d 2.43 (s, 6H), 7.26–7.32 (m, 4H), 7.33–7.49 (m, 4H); C-NMR thiols and C–S cross-coupling reactions of aryl halides (Scheme
400 MHz, CDCl , ppm): d 22.15, 131.46, 131.69, 132. 83, 1).
(
3
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Scheme 1 Preparation of CoFe
2
O
4
-DAN-Cu(II) nanocatalyst and its application in the oxidation of sulfides, thiols and C–S cross-coupling
reactions.
3.1 Catalyst characterization
mapping images indicate the uniform dispersion of copper in
the nanocatalyst. This has been further conrmed from the EDX
The obtained CoFe -DAN-Cu(II) nanocatalyst was character-
2 4
O
spectrum of the nanocatalyst. The ICP-OES technique was
studied for the measurement of Cu amounts loaded onto the
modied surface of the nanoparticles, from which the exact
ized by FESEM, FT-IR, EDX, TGA, VSM, XRD and ICP-OES
analysis. Fig. 1 depicts the particle shape and surface
morphology of CoFe O and CoFe O -DAN-Cu(II). According to
2
4
2 4
ꢀ
1
amount was found to be 0.43 mmol g
.
the SEM images, prepared nanoparticles were made up of
homogeneous and quite spherical particles ranging about
The magnetic properties of bare CoFe O (a) and CoFe O -
2
4
2 4
DAN-Cu(II) (b) were studied using a vibrating sample magne-
tometer (VSM) with a peak eld of 15 kOe (Fig. 2). The satura-
2
1 nm in size. Moreover, graing copper complex onto nano-
particles did not cause any signicant change in the
morphology and structure of CoFe -DAN-Cu(II) nanoparticles.
2 4
Transmission electron microscopic (TEM) study of the CoFe O -
tion magnetization value (M
to be 53.7 and 36.6 emu g respectively. As seen, the magne-
tization of the synthesized nanocatalyst is decreased because of
s
) of these nanoparticles was found
2 4
O
ꢀ
1
DAN-Cu(II) nanocatalyst is also shown in Fig. 1d. This image
2 4
the copper complex coated on CoFe O MNPs; however, this
reduction is insignicant for the separation of this nanocatalyst
using an external magnetic eld.
shows that CoFe O4 was encapsulated by copper complexes,
2
and the particles show an average diameter of about 21 nm, like
the sizes resulting from the SEM measurements.
The EDX spectrum can provide qualitative information
about the types of different chemical elements in the catalyst.
Fig. S2† shows the thermogravimetric analysis (TGA) curve of
ꢁ
CoFe
8
2
ꢁ
O
4
-DAN-Cu(II) at a temperature ranging from 25 C to
00 C. The rst weight loss was about 2 wt%, until the
temperature of 150 C, which is attributed to the removal of
adsorbed solvents and surface hydroxyl groups. In the range of
00–500 C, decomposition of the organic layer and Cu complex
2 4
Fig. S1† shows an EDX spectrum of CoFe O -DAN-Cu(II) and Fe,
ꢁ
Si, N, C, O, and Cu were detected. On the basis of these results, it
is demonstrated that copper(II) was immobilized on the surface
of diaminonaphthalene-coated MNPs. Moreover, the elemental
47
ꢁ
2
graed onto the surface of magnetite nanoparticles was shown.
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2 4 2 4 2 4
Fig. 1 SEM images of CoFe O at (a) 200 nm and CoFe O -DAN-Cu(II) at (b) 200 nm, (c) 20 mm, and TEM image of CoFe O -DAN-Cu(II) (d).
2 4 2 4
Fig. 2 VSM curves of CoFe O (a) and CoFe O -DAN-Cu(II) nanocatalyst (b).
ꢀ
1
On the basis of these results, CoFe
thermal stability that spreads its application for several types of characteristic –OH bands of CoFe
organic reactions.
2 4
O -DAN-Cu(II) has high respectively. The broad band at 3403 cm can be related to
48
2
O
4
(Fig. S3a†). The band
located at 2986 cm in the spectrum of CoFe –Cl could be
The FT-IR spectra shown in Fig. S3† exhibit a comparison of attributed to the stretching vibration of C–H (Fig. S3b†). The
ꢀ
1
2 4
O
ꢀ1
bare CoFe O , CoFe O Cl, CoFe O -DAN and CoFe O -DAN- sharp peak at 1101 cm is attributed to the Si–O stretching
2
4
2
4
2
4
2 4
Cu(II) nanoparticles. The pure nanoparticles exhibit bands at vibration, and these bands are assigned to the characteristic
ꢀ
1
4
30 and 586 cm characteristic of the Fe–O stretching vibra- absorptions of the linker CPTMS attached on the cobalt ferrite
49
ꢀ1
tion in the tetrahedral and octahedral sites of the CoFe
2
O
4
,
surface (Fig. S3b†). The peaks at about 1440–1660 cm are
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reaction, the oxidation reaction was performed in different
solvents. It was observed that polar aprotic solvents such as
acetonitrile, ethyl acetate, water and ethanol obtained 85%,
80%, 75% and 99% conversion of sulde respectively (Table 1,
entries 1 and 3–5). Moreover, the reaction was performed in
non-polar solvents like n-hexane that gave only 25% yield (Table
1, entry 2). These results indicated that ethanol is the appro-
priate solvent for this reaction. Moreover, it is seen that selec-
tivity remained the same in all solvents and sulfoxide is the only
product of this system. Then, different oxidation agents such as
NaIO , oxone (2KHSO $KHSO $K SO ) and O were studied
4
5
4
2
4
2
(
Table 1, entries 17–19). The oxidant H O showed higher
2 2
activity than other used oxidants, while the sulfoxide selectivity
was kept above 100%. The results also indicate that the reaction
did not perform in the absence of H O and very low yield was
2 2
obtained (Table 1, entry 12). The effect of changing catalyst
loaded (0.007, 0.009, 0.010 and 0.025 g) on methyl phenyl
sulde conversion was studied (Table 1, entries 13–15 and 5),
and it indicates that the existence of catalyst is vital for this
reaction. The highest yield was obtained in the presence of
0
.025 g CoFe
detectable product was observed when the reaction was carried
out in the absence of the CoFe -DAN-Cu(II) catalyst (Table 1,
entry 6). It should be mentioned that only 40% was obtained in
the presence of CuCl $2H O used as a catalyst (Table 1, entry
6). As shown, using ethanol as the solvent in the presence of
catalytic amounts of CoFe O -DAN-Cu(II) (0.025 g) and H O (0.5
2 4
O -DAN-Cu(II) nanocatalyst. It is shown that no
2 4
O
2
2
2 4 2 4
Fig. 3 XRD patterns of CoFe O (a) and CoFe O -DAN-Cu(II) (b)
nanocatalysts.
1
2
4
2 2
ꢁ
mL) at 25 C was realized to be the optimized conditions (Table
1
, entry 5).
due to the amine group bending vibration of the 1,8-dia-
ꢀ1
Because of the successful oxidation of methyl phenyl sulde,
minonaphthalene ring and also the band at 3400 cm
is
the reactions of different suldes under optimized conditions
were examined (Table 2). Sulfoxides were prepared at excellent
conversion in a short reaction time. It can be seen that phenyl
sulde with an electron-withdrawing group showed a longer
assigned to the amine stretching band of the synthesized
50
2 4
nanoparticles (Fig. S3c†). In the spectrum of CoFe O -DAN-
Cu(II), however, the shi absorption of amine to a lower wave
number occurs, which is attributed to a robust interaction
between the N group of the copper complex on CoFe O
2
4
(Fig. S3d†).
Fig. 3 represents the XRD patterns of CoFe O and CoFe O -
2
4
2 4
DAN-Cu(II). The diffraction peaks related to Bragg's reections
from the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), and (5 3 3)
planes correspond to the standard spinel structure of CoFe O
2 4
46
(
JCPDS 02-8517) (Fig. 3a). Furthermore, on the basis of the
Debye–Scherrer equation, the crystallite size of CoFe was
about 21 nm. The XRD pattern of CoFe O -DAN-Cu(II) exhibits
2 4
O
2
4
characteristic peaks whose relative intensities match well with
the standard XRD data of CoFe O . These results indicate that
2
4
the spinel structure of the CoFe
2 4
O framework was well main-
tained during the process of catalyst preparation.
3.2 Catalytic activity
Aer synthesis and characterization of the nanocatalyst, we
decided to study the catalytic activity in the oxidation of
suldes. Therefore, for optimizing the reaction conditions for
the oxidation of suldes, methyl phenyl sulde was used as
a model substrate and different amounts of the catalyst, H O
2
2
and various solvents were studied. The results are summed up
Scheme 2 Proposed mechanism for the oxidation of sulfides in the
2 4
in Table 1. In order to obtain the optimum solvents of the presence of CoFe O -DAN-Cu(II).
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reaction time and a lower yield (Table 2, entries 8 and 9). heterogeneous catalysts. The catalytic cycle for the oxidation of
Researches show that in this case, the resonance of the sulfur sulde by H O catalyzed by the CoFe O -DAN-Cu(II) catalyst is
2
2
2 4
electron pair with the aromatic ring is effective. Moreover, alkyl proposed in Scheme 2. The explanation for this transformation
suldes with longer alkyl chains proceeded in higher reaction is the formation of the intermediate A using the reaction of
times (Table 2, entries 4 and 6). These results are probably due CoFe O -DAN-Cu(II) with H O , followed by the conversion of
2 4 2 2
to the substrate's insolubility in ethanol that decreased the the intermediate A to the active oxidant. Then, nucleophile
substrate concentration in the reaction mixture and lowered the sulde attacks active oxidants and with heterolytic cleavage of
56,57
reaction rate. The selectivity of the catalyst is highly important the Cu–O bond yields sulfoxide.
in the industry and in the mentioned reaction, oxidation of In continuation of our experiment, the catalytic activity of
suldes with the optimal conditions is thoroughly selective and CoFe O -DAN-Cu(II) was examined for the synthesis of
2
4
the sulfone was not observed as a by-product.
symmetrical suldes. The effect of catalyst dosage, the nature of
Sulfoxide usually acts as an electrophile, while sulde is the solvents and bases, and the reaction temperature were
a nucleophilic reductant. This dual performance of the sulfur optimized (Table 3). Initially, the reaction of the coupling of
atom in the sulde and the sulfoxide makes it an appropriate iodobenzene (1 mmol), thiourea (1 mmol) as a sulfur source and
system to investigate nucleophilic behavior versus oxidant NaOH was chosen for a model reaction. In order to investigate
electrophilic behavior. H O as a mild oxidizing agent reacts the inuence of the solvents, we utilized DMF, toluene, water,
2 2
slowly and must be activated by homogeneous or DMSO and PEG as solvents (Table 3, entries 1–5). As shown, the
a
Table 4 Synthesis of symmetrical sulfides in the presence of CoFe
2
4
O -DAN-Cu(II) nanocatalysts
b
ꢁ
MP ( C)
Entry
1
Substrate
Product
Time (h)
Yield (%)
5
5
5
2.5
4
95
88
Oil
Oil
5
2
3
4
5
8
8
8
7
40
69
Oil
Oil
5
5
6
7
5.5
8
75
58
75
158–160 (ref. 56)
58
Oil
5
157–160 (ref. 58)
158–160 (ref. 59)
8
9
9.5
45
4.5
78
43
44–46 (ref. 56)
1
0
10.5
150–153 (ref. 26)
a
ꢁ
b
Reaction conditions: catalyst (0.030 g), aryl halide (1 mmol), CH
4
N
2
S (1 mmol), base (0.040 g) and solvent (3 mL) at 130 C. Isolated yields.
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2 4
Scheme 3 Plausible mechanism for the symmetrical sulfide synthesis by CoFe O -DAN-Cu(II) nanocatalysts.
2
reaction did not proceed in PEG, H O and yielded only 35% in amount was also checked (Table 3, entries 9–11 and 4), and the
toluene (Table 3, entries 1, 2 and 5). The sulde synthesis results show that with the increase in the amount of catalyst
reaction was accomplished in excellent yields in DMSO as the from 0.008 to 0.05, the yield also increased from 37% to 95%.
reaction solvent (Table 3, entry 4), whereas the product yield The control experiment showed that catalyst-free conditions
prepared in DMF under the same condition was 52% (Table 3, yielded trace amounts of products even aer 5 hours (Table 3,
entry 3). Additionally, different bases for the sulde synthesis entry 6). Furthermore, the reaction was carried out in the
were checked (Table 3, entries 16, 17 and 4), and 0.4 g of NaOH presence of CuCl $2H O as the catalyst, which afforded
2
2
was found to be the desired amount of the base (Table 3, entry diphenyl sulde in low yields (Table 3, entry 18).
). The reaction was carried out for catalytic amounts of thio- Several derivatives of aryl halides with electron-donating and
4
urea (Table 3, entries 7, 8 and 4). The highest yield of diphenyl electron-withdrawing groups were examined under optimal
sulde was achieved using 1 mmol of thiourea without conditions (Table 4). In the synthesis of symmetrical suldes,
increasing the catalyst loading (Table 3, entry 4). In order to gain used haloarenes follow ArI > ArBr > ArCl (Table 4, entries 1–3).
the best reaction temperature, the mentioned reaction was Aromatic haloarenes with electron-withdrawing groups are
investigated at several temperatures (Table 3, entries 12–15 and more reactive in comparison to electron-donating groups (Table
4
), and the results indicate that increasing temperature from 45 4, entries 4 and 7). Selectivity is one of the most notable
ꢁ
to 130 C increased the yield. Therefore, the optimized advantages of this system by which the coupling reaction of 1-
temperature was found to be 130 C. The effect of the catalyst bromo-4-nitrobenzene, 1-chloro-4-nitrobenzene, and 1-iodo-4-
ꢁ
Table 5 Optimization of conditions for oxidative coupling of thiols in the presence of CoFe
O
2 4
-DAN-Cu(II)
ꢁ
a
Entry
Catalyst (mg)
Oxidation agent
Solvent
Temp. ( C)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
25
25
25
25
25
25
—
25
25
25
25
25
8
10
20
CuCl
25
25
25
H
H
H
H
H
H
H
H
H
H
H
2
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
O
2
2
2
2
2
2
2
2
2
2
2
(0.4 mL)
(0.4 mL)
(0.4 mL)
(0.4 mL)
(0.4 mL)
(0.4 mL)
(0.4 mL)
(0.45 mL)
(0.3 mL)
(0.2 mL)
(0.1 mL)
Acetonitrile
n-Hexane
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
5
35
180
40
35
30
20
140
25
40
40
40
140
40
40
25
75
20
25
2
85
25
60
87
75
96
Trace
97
80
75
65
Trace
58
72
86
45
93
100
96
CH
2
Cl
2
EtOAc
H
2
O
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Methanol
10
11
12
13
14
15
16
17
18
19
—
H
H
H
H
2
O
2
O
2
O
2
O
2
(0.4 mL)
(0.4 mL)
(0.4 mL)
(0.4 mL)
2
2
2
2
$2H
2
O
NaClO
NaIO
DBDMH (0.2 eq.)
2
(1 mmol, 0.090 g)
4
(1 mmol, 0.213 g)
H
2
O
25
25
CHCl
3
a
Isolated yields.
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a
2 4
Table 6 Synthesis of disulfides in the presence of CoFe O -DAN-Cu(II) nanocatalysts
b
ꢁ
Entry
Substrate
Product
Time (min)
20
Yield (%)
MP ( C)
1
96
92
96
95
38–40 (ref. 26)
65–71 (ref. 55)
58–60 (ref. 53)
134–136 (ref. 26)
2
3
4
40
25
30
5
45
88
276–278 (ref. 26)
5
5
6
7
20
60
90
86
Oil
88–90 (ref. 26)
8
9
65
35
90
95
98–99 (ref. 55)
26
Oil
1
0
35
90
55–57 (ref. 55)
a
ꢁ
b
Reaction conditions: catalyst (0.025 g), thiol (1 mmol), 30% H O (0.4 mL) and solvent (3 mL) at 25 C. Isolated yields.
2 2
nitrobenzene resulted in the favorite product without extra
changes.
At last, the CoFe
ined for oxidative coupling of thiols to disuldes using 4-
2 4
O -DAN-Cu(II) catalytic system was exam-
A plausible mechanism for this transformation is outlined in methylthiophenol as the model substrate. To nd out the
the presence of the CoFe -DAN-Cu(II) nanocatalyst (Scheme optimized reaction conditions, the presence of various catalytic
). First, the oxidative addition of Cu nanoparticles to the aryl amounts of CoFe O -DAN-Cu(II) and several organic solvents
2 4
O
3
2
4
halide forms intermediate (I). Then, the coupling reaction of the using different amounts of H O was tested (Table 5). Initially,
2
2
aryl halide with thiourea generates intermediate (II), which is the model reaction was carried out in several solvents such as
transmitted to a thiol anion in the presence of NaOH. Finally, acetonitrile, n-hexane, EtOAc, CH Cl , EtOH and H O. Among
thiol anion reacts with intermediate (I) to produce the sulde them, acetonitrile (85%), EtOAc (87%) and EtOH (96%) showed
2
2
2
60
product as well as the initial catalyst.
better conversion for selective disulde synthesis. However,
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because of the nontoxic and availability properties, EtOH was reactions and sulde synthesis, respectively. FeNi
3
2
/SiO (Table
chosen as a green solvent (Table 5, entries 1–6). In order to study 7, entry 3) causes to use of m-CPBA instead of green oxidant
the effect of the oxidant nature, the reaction was also investi- H O . Accordingly, this catalyst affords an absolutely green
2
2
gated in the presence of several oxidants such as NaClO , NaIO
procedure, and these results clearly accentuate the efficiency of
and 1,3-dibromo-5,5-dimethyl-hydantoin (DBDMH) instead of this applied methodology.
(Table 5, entries 17–19). Compared with the other
oxidants, H showed higher activity. Then, the effect of the
2
4
2 2
H O
2 2
O
3.3 Catalyst recycling
catalyst amount (0.008, 0.010, 0.020 and 0.025 g) on 4-methyl-
thiophenol conversion was studied, and it was observed that the
activity of the catalyst was affected by the amount of the catalyst
Recycling as one of the main important properties of nano-
magnetic catalysts was also investigated by the synthesis of
methyl phenyl sulfoxide, diphenyl sulphide, and 1,2-di-p-tol-
yldisulfane under optimized conditions. Aer completion of the
(Table 5, entries 13–15 and 6). The highest yield was obtained in
the presence of 0.025 g of CoFe O -DAN-Cu(II). The blank
2
4
2 4
reactions, the CoFe O -DAN-Cu(II) nanocatalyst was removed
reaction showed that catalyst-free conditions produced trace
amounts of products even aer 140 minutes (Table 5, entry 7).
from the reaction mixture by a magnet and used in the next
cycle under mentioned optimum conditions. As shown in Fig. 4,
the catalyst can be recycled for nine runs without any consid-
erable loss of catalytic activity.
When the catalyst CoFe
2 4
O -DAN-Cu(II) was replaced with
CuCl $2H O, the desired oxidation product was obtained in
2
2
lower yields (45%) (Table 5, entry 16).
In order to clarify the changes in the chemical structure of
the prepared catalyst towards oxidation during the nine cycles,
FE-SEM and FT-IR analyses were performed. SEM and FT-IR
analyses of this catalyst aer nine runs show no signicant
changes during the reaction time (Fig. 5). Moreover, the amount
of copper on recovered CoFe O -DAN-Cu(II) was measured by
Utilizing the optimum reaction condition, a variety of
substituted thiols were selected for the synthesis of disuldes
(
Table 6). The results indicate that the CoFe O -DAN-Cu(II)
2 4
nanocatalyst exhibited efficient catalytic activity with good to
high yields in the reaction time. It was observed that aliphatic
thiols oxidized more slowly than aromatic ones (Table 6, entries
2
4
ICP-OES analysis for the conversion of methyl phenyl sulde to
methyl phenyl sulfoxide in the fresh catalyst and aer nine
1
and 9). It is also shown that aromatic thiols including
electron-donating groups are more reactive than thiols with
electron-withdrawing groups (Table 6, entries 1 and 7). This
system shows good chemoselectivity by which the hydroxyl
group of 2-mercaptoethanol remains constant, and a thiol
functional group of these molecules was oxidized to disulde.
Furthermore, selectivity is one of the main important features
of this system. Because of the selectivity of the mentioned
heterogeneous catalyst, there is no overoxidation to thio-
sulnates, disulfoxides, sulnyl sulfones, or disulfones.
ꢀ
1
consecutive cycles, it was found to be 0.43 mmol g
0
and
ꢀ
1
.32 mmol g , which indicates the minimum amount of Cu
leaching in the catalytic process. Furthermore, AAS analysis of
the reaction mixture aer catalyst removal showed no consid-
erable amount of copper. AAS analyses of the recycled CoFe O -
2
4
DAN-Cu(II) catalyst also exhibited no signicant (<0.001 mmol
ꢀ1
g
of Cu) differences compared with the fresh catalyst, pre-
senting that no considerable copper leaching occurred during
the catalytic processes. Moreover, regarding the heterogeneous
nature of CoFe O -DAN-Cu(II), the oxidation of methyl phenyl
2 4
sulde has been examined by a hot ltration experiment under
optimal reaction conditions. In this test, we found the yield of
A plausible mechanism for this transformation is shown in
23
Scheme 4, on the basis of the literature works. A comparison
for the efficiency of the catalytic activity of CoFe O -DAN-Cu(II)
2
4
for the oxidation of suldes to sulfoxides (Table 7, entries 1–7),
synthesis of symmetrical suldes (Table 7, entries 8–11), and
oxidative coupling of thiols to disuldes (Table 7, entries 12–15)
with several previously reported methods is presented.
2 3 3
Recently, the use of Co@SiO [(EtO) Si–L ]/Mn(III) (Table 7, entry
1
), FeNi /SiO (Table 7, entry 3), Mn(III)-binapthyl Schiff base
3
2
diamine-SBA-15 (Table 7, entry 4), K H [(SeV O (SeO ) ) (-
6
8
10 28
3 3 2
M(H O) )]$24H O (Table 7, entry 5), VO-TAPT-2,3-DHTA COF
2
4
2
(
Table 7, entry 6), CuFe
base@KIT-6 (Table 7, entry 13), and TiO(O
Table 7, entry 14) was reported for oxidation reactions. These
2
O
4
(Table 7, entry 12), Pd-isatin Schiff
2
CCF )/NaI/thiol
3
(
methods need toxic organic solvents with high reaction times.
The use of Mn(III)-binapthyl Schiff base diamine-SBA-15 (Table
7
, entry 4), K H [(SeV O (SeO ) ) (M(H O) )]$24H O (Table 7,
6 8 10 28 3 3 2 2 4 2
entry 5), VO-TAPT-2,3-DHTA COF (Table 7, entry 6), and Pd-
isatin Schiff base@KIT-6 (Table 7, entry 13) as the catalyst for
oxidation reactions was reported, requiring high reaction times.
Moreover, harsh conditions are found when using TiCl
SCF ) (Table 7, entry 14), TiO(O CCF (Table 7, entry 14),
CuFe (Table 7, entry 12), and ethyl 2-oxocyclohex-
3 3
(O -
3
2
3 2
)
2 4
O
Scheme 4 Plausible mechanism for the oxidative coupling of thiols by
2 4
anecarboxylate as catalysts (Table 7, entry 8) in oxidation CoFe O -DAN-Cu(II) nanocatalysts.
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a
2 4
Table 7 Comparison of CoFe O -DAN-Cu(II) in the catalytic oxidation reactions and synthesis of sulfide derivatives with other catalysts
b
Yield
(%)
Entry
1
Substrate
Catalyst
Time (min)
50
Catalyst loading
0.06 g
Condition
Ref.
61
1
Ph–S–CH
3
Co@SiO
Mn(III)
2
[(EtO)
3
Si–L
3
]/
Methyl phenyl sulde
(0.5 mmol),
99
acetonitrile, 90 mL H
and 65 C
2
O
2
ꢁ
2
2
3
4
5
Ph–S–CH
3
3
3
3
MNP@TA-IL/W
FeNi /SiO
60
0.4 mol%
0.04
H
2
O, 1.5 mmol H
2
O
2
99
62
63
64
65
ꢁ
and 25 C
DCM (2.0 mL), m-CPBA
3
Ph–S–CH
3
2
60
99
(2.0 mmol) and r.t
4
ꢁ
Ph–S–CH
Mn(III)-binapthyl Schiff
300
60
50 mg
2 mmol
CH
2
Cl
2
and 25 C
94
base diamine-SBA-15
5
2+
ꢁ
Ph–S–CH
M
-sandwiched POVs:
[(SeV10 28(SeO
(M(H O) )]$24H
VO-TAPT-2,3-DHTA COF
CoFe -DAN-Cu(II)
Ethyl 2-
oxocyclohexanecarboxylate
Fe @SBTU@Ni(II)
MNP-Si-NHC(Pyr)-Ni
CH
3
OH and 25 C
97.9
K
6
H
8
O
3
)
2
O
3)
2
2
4
6
ꢁ
6
7
8
Ph–S–CH
3
240
15
1200
20 mg
0.025 g
0.1 mmol
CH
3
CN and 25 C
95
99
96
66
ꢁ
Ph–S–CH
3
2
O
4
C
2
H
5
OH and 25 C
This work
67
8
ꢁ
Iodobenzene
80 C and under Ar
9
1
ꢁ
9
Iodobenzene
Iodobenzene
3
O
4
210
600
0.030
10 mol%
DMSO and 130 C
DMF, 100 C, base (2
94
92
60
68
0
ꢁ
1
0
mmol)
and thiol (1 mmol)
ꢁ
1
1
1
2
Iodobenzene
Thiophenol
CoFe
CuFe
2
O
4
-DAN-Cu(II)
150
24 h
0.030 g
10 mol%
DMSO and 130 C
95
95
This work
69
12
2
O
4
1.80 mmol of halide, 2.0
eq. of base,
5
mL of 1,4-dioxane and
under N
CH
5 mmol H
CH CN and under
reux conditions
2
atmosphere
CN, 25 C and
1
3
ꢁ
1
3
4
5
4-Methyl
Pd-isatin Schiff base@KIT-
6
30
0.025 g
1 mmol
3
95
70
thiophenol
2 2
O
14
1
Thiophenol
TiO(O
2
CCF
3
)
2
/NaI/thiol
150
20
3
100
96
71
ꢁ
1
Thiophenol
CoFe
2
O
4
-DAN-Cu(II)
0.025 g
C
2
H
5
OH and 25 C
This work
a
1
2
Abbreviations: Schiff-base Mn(III) and Co(II) complexes coated on Co nanoparticles, tungstate ions loaded onto triazine-based ionic liquid-
3
4
functionalized magnetic nanoparticle,
FeNi
3
nanoparticle conjugated tetraethyl orthosilicate;
chloro(S,S)(ꢀ)[N-3-tert-butyl-5-
0
0
0
0
0
chloromethylsalicylidene]-N -[3 ,5 -ditert-butylsalicylidene]1,1 -binapthyl-2,2 -diamine manganese(III) complex over modied surface of SBA-15,
5
6
transition metal-sandwiched heteropolyoxovanadate complexes, complex vanadium of Schiff base of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine–
,3-dihydroxyterephthaldehyde (TAPT–2,3-DHTA), ethyl 2-oxocyclohexanecarboxylate ligand in the presence of Cu
8
9
2
2
O as catalyst, nickel(II)
10
12
complex supported on modied surface of Fe
ferrite nanoparticles, Pd(II)-isatin Schiff base complex immobilized into three-dimensional mesoporous silica KIT-6, TiCl (O SCF ) and
3 3 3
TiO(O CCF ) as the catalyst. Isolated yields.
2 3 2
3
O
4
.
Magnetite/silica nanoparticles supported N-heterocyclic carbene nickel catalyst, copper
13
14
b
2 4
Fig. 4 Reusability of the CoFe O -DAN-Cu(II) nanocatalyst in the sulfide oxidation, sulfide synthesis and oxidative coupling of thiols.
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2 4
Fig. 5 FT-IR and SEM analyses of the CoFe O -DAN-Cu(II) nanocatalyst after nine runs.
product in a half time of the reaction to be 65%. Then, the
reaction was repeated and at half time of the reaction, the
Conflicts of interest
catalyst was separated using a magnet and allowed to react There are no conicts to declare.
further. The yield of the reaction in this stage was 69%, which
conrmed that the reaction proceeded heterogeneously and
aer hot ltration, the reaction of the residual mixture was Acknowledgements
completely stopped.
The authors wish to thank the nancial support from the
University of Kurdistan.
3.4 Catalyst poisoning test
Another heterogeneity test was performed with Hg poisoning.
Hg(0) can poison catalysts, either by amalgamating the applied
metal or adsorbing on the metal surface. The sulde oxidation
was conducted under the same mentioned conditions, except
the addition of elemental Hg (1 mmol) to the reaction mixture at
Notes and references
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