l-Arginine complex of copper on modified core–shell magnetic nanoparticles as reusable and organic–inorganic hybrid nanocatalyst for the chemoselective oxidation of organosulfur compounds

Article abstract of DOI:10.1007/s13738-020-02040-8

In this paper, we report the fabrication and characterization of a stable heterogeneous nanostructure catalyst of copper immobilized on Fe₃O₄@SiO₂@l-Arginine, for the oxidation of sulfides and oxidative coupling of thiols. The prepared nanocatalyst has been characterized by different techniques such as FTIR, XRD, SEM, TEM and TGA. These nanoparticles were the effective catalyst for selective oxidation of sulfides and oxidative coupling of thiols using 30% H₂O₂. The suggested method offers several prominent advantages such as mild condition, use of magnetically reusable catalyst, simple workup procedure, good to high yields of products and great selectivity.

Full text of DOI:10.1007/s13738-020-02040-8

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-020-02040-8
ORIGINAL PAPER
l‑Arginine complex of copper on modiꢀed core–shell magnetic
nanoparticles as reusable and organic–inorganic hybrid nanocatalyst
for the chemoselective oxidation of organosulfur compounds
Mohsen Nikoorazm1 · Parisa Moradi · Nourolah Noori · Gouhar Azadi
1
1
1
Received: 19 January 2020 / Accepted: 14 August 2020
©
Iranian Chemical Society 2020
Abstract
In this paper, we report the fabrication and characterization of a stable heterogeneous nanostructure catalyst of copper immo-
bilized on Fe O @SiO @l-Arginine, for the oxidation of sulꢀdes and oxidative coupling of thiols. The prepared nanocatalyst
3
4
2
has been characterized by diꢁerent techniques such as FTIR, XRD, SEM, TEM and TGA. These nanoparticles were the
eꢁective catalyst for selective oxidation of sulꢀdes and oxidative coupling of thiols using 30% H O . The suggested method
2
2
oꢁers several prominent advantages such as mild condition, use of magnetically reusable catalyst, simple workup procedure,
good to high yields of products and great selectivity.
Keywords Magnetic nanoparticles · Copper · Oxidation reactions · Sulꢀde · Thiols
Introduction
organic transformations [14, 15]. Organic molecules are nor-
mally bounded or adsorbed onto the nanoparticle surfaces
in order to design their surface properties, e.g., hydrophilic-
ity/hydrophobicity and the availability of speciꢀc functional
groups [16, 17]. Selective oxidation of sulꢀdes to sulfoxides
and oxidative coupling of thiols to disulꢀdes is one of the
consequential and most important chemical processes in
both chemistry and biology [18–27]. Thus in continuation
of our recent success in the development of novel and green
method for the oxidation reactions [28], herein we report a
simple and environmentally benign method for the selec-
tive oxidation of sulꢀdes and oxidative coupling of thiols
catalyzed by Fe O @SiO @l-Arginine@Cu. The green
The development of noble metal nanoparticles (MNPs) has
become an important subject for wide-ranging scientist and
researchers. In the last decade, MNPs have reported the
noteworthy potential for various applications in the ꢀeld of
chemical industry and catalytic process due to their strange
and worthy properties, when compared to their bulk coun-
terparts [1–5]. Generally, the surface properties of nanopar-
ticles have to be tuned to meet the requirements for diꢁerent
applications. Fe O NPs are considered as ideal supports for
3
4
the heterogenization of homogeneous catalysts. Metal nano-
particles (MNPs) are of signiꢀcant interest for technological
applications in several areas of science and industry, espe-
cially in catalysis due to their high activity [6–9]. Due to the
simple and eꢁortless separation of magnetic nanoparticles
from the ꢀnal products using a magnetic ꢀeld, metal nano-
particles (MNPs) have been extensively studied [10–13].
Among the various metal complexes, the transition metals
bearing N, O-donor ligands are a great deal of interest, and
in the past decades, quantiꢀer has been reported for diꢁerent
3
4
2
nanocatalyst was easily synthesized compared with many
reported procedures [29–32], and importantly, in this cata-
lytic system l-Arginine was used as a ligand that is environ-
mentally friendly, commercially available, inexpensive and
does not use expensive and high-priced silane compounds
such as 3-aminopropyltriethoxysilane or (3-chloropropyl)
triethoxysilane for functionalization of catalyst.
*
Mohsen Nikoorazm
e_nikoorazm@yahoo.com
1
Department of Chemistry, Faculty of Science, Ilam
University, P.O. Box 69315516, Ilam, Iran
Vol.:(0
1
123456789)
3

Journal of the Iranian Chemical Society
Experimental
General procedure for the oxidation of sulꢀdes
to sulfoxides
Materials
To a mixture of sulꢀde (1 mmol), and 30% H O (0.5 mL),
2
2
The reagents and materials were used in this research
paper obtained from Sigma-Aldrich, Fluka or Merck
chemical companies and utilized without further puriꢀ-
cation. The particles size and morphology were investi-
gated by a JEOL JEM-2010 scanning electron microscopy
Cu immobilized on Fe O @SiO @l-Arginine as catalyst
3 4 2
(0.01 g, 8.2 mol%) was added and the mixture was stirred
at room temperature in solvent-free conditions. At the end
of the reaction which was monitored by TLC in the mix-
ture of n-hexane and acetone (8:2), the reaction mixture was
diluted with ethyl acetate and the catalyst separated using
a magnetic device. Then, water (15 mL) was added to the
mixture, and the product was extracted in the organic phase.
The organic phase dried over anhydrous Na SO and then
(
SEM), on an accelerating voltage of 200 kV. The catalyst
was characterized by XRD patterns, which were collected
on a Rigaku-Dmax 2500 diꢁractometer with nickel-ꢀltered
Cu Kα radiation (λ = 1.5418°A, 40 kV).
2
4
Na SO was removed by ꢀltration. The ꢀltered solvent was
2
4
evaporated to give the corresponding pure sulfoxide.
Synthesis of the Cu immobilized on Fe O @
3
4
SiO @l‑Arginine nanoparticles
General procedure for the oxidative coupling
of thiols to disulꢀdes
2
Fe O @SiO @l-Arginine nanoparticles were readily syn-
3
4
2
thesized similar to a previously reported work [33]. In the
A solution of thiol (1 mmol), H O (30%) and Cu immobi-
2
2
next step, the prepared Fe O @SiO @l-Arginine (1 g) was
lized on Fe O @SiO @l-Arginine (0.005 g, 4.1 mol%) in
3
4
2
3 4 2
dispersed in ethanol (50 mL) by ultrasonic bath for 30 min.
Subsequently, CuCl (0.5 g) was added and the mixture
was stirred at reꢂux conditions for 24 h. Then, the reac-
tion mixture was cooled to room temperature and the ꢀnal
product was separated by a magnetic device and repeatedly
washed with ethanol several times to remove the remaining
impurities (Scheme 1).
ethyl acetate was stirred at room temperature. The reaction
progress was monitored by TLC in the mixture of n-hexane
and acetone (8:2). Then, the catalyst was separated using a
magnetic ꢀeld. Then, water (15 mL) was added to the reac-
tion mixture and the product was extracted in the organic
phase. Finally, the excess of solvent was removed under
reduced pressure to give the corresponding pure disulꢀdes
with excellent yield.
Scheme 1 Synthesis of Cu immobilized on Fe O @SiO @l-Arginine
3
4
2
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Journal of the Iranian Chemical Society
−
1
Results and discussion
the catalyst appear at 1629 cm which is lower than C=N
stretching vibrations in the FTIR spectrum of Fe O @
3
4
The catalyst synthesized using the procedure is shown in
SiO2@l-Arginine due to the formation of metal–ligand
bonds.
Scheme 1. Copper immobilized on Fe O @SiO @l-Argi-
3
4
2
nine was fully characterized by FTIR, XRD, SEM, TGA
The structure and phase purity of the Cu immobilized
on Fe3O4@SiO2@l-Arginine were investigated by X-ray
diꢁractometry (XRD). All diꢁraction peaks observed at
2θ=30.4°, 35.7°, 43.2°, 53.7°, 57.3° and 63.1° which can
be assigned to the (220), (311), (400), (422), (511) and (440)
planes of Fe O , respectively [36, 37]. Also, the XRD pat-
and AAS techniques.
Characterization of Cu immobilized on Fe O @
3
4
SiO @l‑Arginine
2
3
4
To understand the nature of the interaction between l-Argi-
nine molecule and the magnetic nanoparticles, the FTIR
technique was used (Fig. 1). Peaks appearing at 2924 and
tern of the catalyst (Fig. 2) contains a sequence of particular
diꢁraction peaks, which are indexed to Cu indicating the
presence of Cu in the prepared nanocatalyst [38, 39]. Other
crystalline impurities did not show in XRD pattern, which
revealed that the compositions of the catalyst are Fe3O4 and
Cu metal.
−
1
2
856 cm are related to the asymmetric and symmetric
stretching vibrations of methylene groups (CH ) in all sam-
2
ples [34]. Stretching vibration of Fe–O and C=O bonds is
−
1
presented at 583 and 1633 cm , respectively, which is con-
The average size of the nanocatalyst particles was calcu-
lated to be 25 ± 2 nm from the XRD results by Scherrer’s
equation: D=kλ/β cosθ.
ꢀ
rmed that the l-Arginine was successfully supported on
−
1
the magnetite surface [8]. The strong peak at 1070 cm in
all spectra is related to stretching vibrations of Si–O [35].
Also, C=N stretching vibrations in the FTIR spectrum of
The particle size distribution of Cu immobilized on
Fe3O4@SiO2@l-Arginine was evaluated using transmis-
sion electron microscopy (TEM) (Fig. 3). This study showed
Fig. 1 FTIR spectra of pure
l-Arginine (a), Fe O @SiO
3
4
2
(
b), Fe O @SiO @l-Arginine
3 4 2
(
c) and Cu immobilized on
Fe O @SiO @l-Arginine (d)
3
4
2
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Journal of the Iranian Chemical Society
diameters of approximately 100–120 nm for the magnetic
nanoparticles.
Selected images of scanning electron microscopy (SEM)
have been used to study the structure and morphology of the
catalyst. SEM images revealed that the catalyst presented
the uniform particles with spherical particle morphology
(
Fig. 4).
The element’s content of Cu immobilized on Fe O @
3
4
SiO @l-Arginine was determined by energy-dispersive
2
X-ray spectroscopy (EDS) analysis. The EDS diagram of
this catalyst is shown in Fig. 5. As depicted, the EDS result
of this catalyst shows the presence of iron, oxygen, silica,
carbon, nitrogen and also copper species. X-ray mapping
Fig. 2 XRD pattern of the Cu immobilized on Fe O @SiO @l-Argi-
3
4
2
nine
(
WDX) is an analytical tool used to non-destructively deter-
mine the elemental analysis and chemical composition of the
samples. As shown in Fig. 6, WDX analysis of Cu immobi-
lized on Fe O @SiO @l-Arginine conꢀrmed homogeneous
3
4
2
distributions of all elements (Fe, O, Si, C, N and Cu) in the
structure of this catalyst. Also, the exact amount of loaded
copper chloride on Fe O @SiO @l-Arginine surface was
3
4
2
also calculated by atomic absorption spectrophotometer
−
3
(
AAS) that was found to be 8.2×10 mol/g.
To indicate the bond formation between the Fe O
3
4
MNPs with the organic spacer group and to obtain infor-
mation on the thermal stability, the thermogravimetric
analysis (TGA) was performed (Fig. 7). The TGA/DTA/
DTG diagrams of Cu immobilized on Fe O @SiO @l-
3
4
2
Arginine are shown three mass loss over the temperature
range of 27–800 °C by heating rates at 10 °C/min and
in the air atmosphere. The weight loss of ~ 5% in the
Fig. 3 TEM images of Cu immobilized on Fe O @SiO @l-Arginine
3
4
2
Fig. 4 SEM images of the Cu immobilized on Fe O @SiO @l-Arginine
3
4
2
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Journal of the Iranian Chemical Society
Fig. 5 EDS diagram of Cu
immobilized on Fe O @
3
4
SiO @l-Arginine
2
Fig. 6 Elemental mapping of a Fe, b O, c Si, d C, e N and f Cu for Ni-Cytosine@MCM-41
temperature range of 100–250 °C attributed to the removal
of physically adsorbed solvent and water [40, 41]. Besides,
the analysis showed another decreasing peak at a tempera-
ture between 250 and 600 °C due to the decomposition of
the organic spacer group [42, 43]. The ꢀnal weight loss
(about 2%) indicated above 600 °C which is corresponded
to the condensation of the silanol groups [40, 44].
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Journal of the Iranian Chemical Society
Fig. 7 TGA/DTA/DTG analysis
of Cu immobilized on Fe O @
3
4
SiO @l-Arginine
2
Catalytic study
2-mercaptobenzoxazole (Scheme 2) has been selected as
model reactions. Initially, the reactions were performed in
diꢁerent solvents, which the results indicated solvent-free
conditions and ethyl acetate were preferable mediums for
the oxidation of sulꢀdes and oxidative coupling of thiols,
respectively (Table 1, entries 4, 7). Then, the reaction was
evaluated by diꢁerent amounts of catalyst. As can be seen
from Table 1, the yield of products and the rate of the reac-
tion were also found to be dependent on the concentration
of the catalyst. Therefore, 0.01 g of catalyst for oxidation of
The oxidation of sulꢀdes and oxidative coupling of thiols
are the most important functional group transformations in
organic synthesis, as they are highly useful intermediates
and building blocks in the preparation of numerous chemi-
cally and biologically active compounds. Thus, these reac-
tions were considered to study the catalytic activity of the
prepared catalyst. In order to optimize the reaction condi-
tion, the oxidation reaction of methyl phenyl sulꢀde and
Table 1 Oxidation reactions
Yield (%)a
Entry
Substrate
Solvent
EtOH
Catalyst (mg)
Time (min)
of methyl phenyl sulꢀde and
2
-mercaptobenzoxazole in the
1
2
3
4
5
6
7
8
9
0
1
2
3
4
Methyl phenyl sulꢀde
Methyl phenyl sulꢀde
Methyl phenyl sulꢀde
Methyl phenyl sulꢀde
Methyl phenyl sulꢀde
Methyl phenyl sulꢀde
2-Mercaptobenzoxazole
2-Mercaptobenzoxazole
2-Mercaptobenzoxazole
2-Mercaptobenzoxazole
2-Mercaptobenzoxazole
2-Mercaptobenzoxazole
2-Mercaptobenzoxazole
2-Mercaptobenzoxazole
10
10
10
10
8
5
5
3
2
5
5
120
120
120
120
120
120
30
30
30
30
30
N.R.
N.R.
N.R.
96
90
80
98
81
65
60
89
80
54
70
presence of Cu immobilized on
CH CN
3
Fe O @SiO @l-Arginine
3
4
2
EtOAc
Solvent free
Solvent free
Solvent free
EtOAc
EtOAc
EtOAc
1
1
1
1
1
Solvent free
EtOH
CH CN
5
5
5
30
30
30
3
CH Cl
2
2
Acetone
Reaction condition: sulꢀde or thiol (1 mmol), H O 30% (0.5 mL), solvent (5 mL), room temperature
2
2
a
Isolated yield
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Journal of the Iranian Chemical Society
sulꢀdes and 0.005 g of catalyst for oxidation of thiols were
found to be optimum amounts at room temperature, respec-
tively (Table 1, entries 4, 7).
fresh substrate and subjected to the next. As shown in Fig. 8,
the change in the product yield was negligible in a trial of
ꢀve recycling experiments, which demonstrates the practical
recyclability of this catalyst. The atomic absorption tech-
nique was used to determine the amount of copper leach-
ing after recovery. Thus, using the atomic absorption tech-
nique, the amount of copper found in the ꢀnal catalyst was
measured. It was found that the amount of copper before the
Then, we have extended the procedure using structurally
diverse sulꢀdes and thiols under optimized reaction condi-
tions (Scheme 3). As shown in Table 2, all the examined
substrates obtained the corresponding products in good to
excellent yields compared with many reported procedures
and the catalytic systems performed in a green medium.
This catalyst is shown a good selectivity for the oxidation of
sulꢀdes to sulfoxides without overoxidation to formation of
sulfone as by-product. More importantly, this catalytic sys-
tem is chemoselective. When we used the substrates includ-
−
3
reaction was 8.2×10 mol/g and after the reaction became
−
3
7.9 × 10 mol/g, indicating a very slight ad measurable
change in the amount of copper during the reaction that
indicates the heterogeneity of the catalytic reaction.
To consider the heterogeneity of catalyst, the catalyst was
separated by applying a magnetic ꢀeld after half of the nor-
mal reaction time in the oxidation of methyl phenyl sulꢀde.
The ꢀltrate was then allowed to react further as normal.
After catalyst separation, the progress of the reaction was
increased only 5%, which shows the heterogeneity of this
catalytic system.
ing COOCH , OH, COOH and carbon–carbon double bond
3
functional groups, these substrates selectively underwent
oxidation at the sulfur atom without extra structural changes
in their functional group that is an attractive feature of these
catalytic systems (Scheme 4).
Catalyst recovery
The XRD pattern of the recovered catalyst (Fig. 9)
includes several peaks which are indexed to copper and
Fe O nanoparticles. The XRD pattern of the recovered cata-
The most important advantage of the applied catalyst is
recoverability and reusability. It is important to note that
the magnetic property of this catalyst facilitates its eꢃ-
cient recovery from the ꢀnal products. In this regard, the
reusability of the nanostructure catalysts for subsequent
catalytic cycles was examined using dipropyl sulꢀde and
3
4
lyst is shown good agreement with fresh catalyst. As shown
in Fig. 9, the XRD pattern of recovered catalyst showed a
good stability of the catalyst after recycling.
Comparison of the catalysts
2
-mercaptobenzoxazole as the substrate. To recover the
catalyst after completion of the reaction, the catalyst was
isolated by applying an external magnetic ꢀeld and then
washed with acetone to remove residual product and dried
at vacuum. Then, the reaction vessel was charged with the
To establish the high catalytic activity of the synthesized
catalyst and greenness of this method, we compared our
results on the oxidation of methyl phenyl sulꢀde and oxida-
tive coupling of 4-methylbenzenethiol with data from the
literature (Table 1). As can be seen from Table 3, this cata-
lytic system is greener. More importantly at the end of the
reaction, the catalyst was easily separated using an external
magnet and does not need any ꢀltration.
Conclusion
In conclusion, we introduced a simple method for the syn-
thesis of Cu immobilized on Fe O @SiO @l-Arginine
3
4
2
as heterogeneous nanostructure catalyst. The catalyst was
applied for the oxidation of sulꢀdes and oxidative coupling
of thiols. The prominent features of this method are the use
of non-toxic, commercially available, inexpensive and green
substrate for the synthesis of catalyst, simple experimental
procedure, applicability to various substrates, waste-free,
green and eꢃcient synthetic entry to excellent yield. Fur-
thermore, nanocatalyst can be easily recovered by a mag-
netic ꢀeld and reused for subsequent reactions with high
generation eꢃciency.
Scheme 2 Oxidation of methyl phenyl sulꢀde and 2-mercaptobenzo-
xazole in the presence of Cu immobilized on Fe O @SiO @l-Argi-
3
4
2
nine
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Journal of the Iranian Chemical Society
a
Table 2 Oxidation of sulꢀdes
and oxidative coupling of
thiols in the presence of Cu
immobilized on Fe O @
Entry
1
Substrate
Product
Time (min)
120
Yield (%)
96
3
4
SiO @l-Arginine
2
2
3
5
2
90
95
4
5
45
1
95
92
6
7
200
10
95
98
95
8
9
30
30
30
91
90
85
1
1
0
1
180
1
1
1
2
3
4
15
12
30
97
95
98
1
1
5
6
80
60
96
94
1
1
7
8
15
75
95
65
1
2
2
9
0
1
90
25
45
70
98
97
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Journal of the Iranian Chemical Society
Table 2 (continued)
Reaction condition for oxidation of sulꢀdes/H O /Cat. (1 mmol:0.5 mL:10 mg) and thiols/H O /Cat.
2
2
2
2
(
1 mmol:0.5 mL:5 mg)
Isolated product
b
Recovery
a b
100
90
80
70
60
50
40
30
20
10
0
1
2
3
4
5
Run
Fig. 8 The recycling experiment of Cu immobilized on Fe O @
3
4
SiO @l-Arginine in the oxidation reaction of dipropyl sulꢀde (col-
2
umn a) and 2-mercaptobenzoxazole (column b)
1
200
000
Scheme 3 Oxidation of sulꢀdes and oxidative coupling of thiols in
the presence of Cu immobilized on Fe O @SiO @l-Arginine
Cu
3
4
2
1
8
6
4
2
00
00
00
00
0
Cu
Cu
20
30
40
50
Posiꢀon [2 Theta]
60
70
80
Fig. 9 XRD pattern of the recovered Cu immobilized on Fe O @
3
4
SiO @l-Arginine
2
Scheme 4 Chemoselective sulfoxidation of sulꢀdes in the presence
of Cu immobilized on Fe O @SiO @l-Arginine
3
4
2
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Journal of the Iranian Chemical Society
Table 3 Comparison of
Entry
Substrate
Catalyst
Time (h)
Yield (%)
Fe O @SiO @l-Arginine
3
4
2
with other reported catalysts
for oxidation of methyl phenyl
sulꢀde and oxidative coupling
of 4-methylbenzenethiol
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
Methyl phenyl sulꢀde
Methyl phenyl sulꢀde
Methyl phenyl sulꢀde
Methyl phenyl sulꢀde
Methyl phenyl sulꢀde
Methyl phenyl sulꢀde
Methyl phenyl sulꢀde
Methyl phenyl sulꢀde
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-methylbenzenethiol
4-Methylbenzenethiol
Methyl phenyl sulꢀde
VO F(dmpz)
5
6
0.5
6
3
4.5
2
2
0.5
1.7
2
95 [45]
85 [46]
92 [47]
98 [48]
90 [49]
93 [50]
93.4 [51]
93 [52]
99 [48]
98 [53]
98 [54]
98 [54]
95 [55]
97 [55]
95 [this work]
96 [this work]
2
2
ZnBr2
SSA
Zr-oxide@MCM-41
Polymer-anchored Cu(II)
NBS
Nb O /AC
2
5
[VO(TPPABr)]CBr3
Zr-oxide@MCM-41
Silica sulfuric acid
Zr(IV)-ninhydrin supported MCM-41
Zr(IV)-ninhydrin supported MCM-48
Ni-salen-MCM-41
1
1
1
1
1
1
1
3
0.42
0.33
0.2
2
Cd-salen-MCM-41
Fe O @SiO @l-Arginine
3
4
2
Fe O @SiO @l-Arginine
3
4
2
Acknowledgements This work was supported by the research facilities
8. V. Polshettiwar, R. Luque, A. Fihri, H. Zhu, M. Bouhrara, J.M.
Basset, Magnetically recoverable nanocatalysts. Chem. Rev.
of Ilam University, Ilam, Iran.
1
11, 3036–3075 (2011). https://doi.org/10.1021/cr100230z
9
.
J. Wang, B. Xu, H. Sun, G. Song, Palladium nanoparticles sup-
ported on functional ionic liquid modiꢀed magnetic nanoparti-
cles as recyclable catalyst for room temperature Suzuki reaction.
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