Magnetic core-shell nanoparticle-supported Sc (III): A novel and robust Lewis acid nanocatalyst for the selective oxidation of sulfides to sulfoxides by H2O2 under solvent-free conditions

Article abstract of DOI:10.1016/j.catcom.2019.02.021

For the first time scandium triflate was supported on modified Fe₃O₄ magnetic nanoparticles [MNPs-PhSO₃-Sc(OTf)₂] and characterized using scanning electron microscopy (SEM), inductively coupled plasma analysis (

Full text of DOI:10.1016/j.catcom.2019.02.021

Accepted Manuscript
Magnetic core-shell nanoparticle-supported Sc (III): A novel and
robust Lewis acid nanocatalyst for the selective oxidation of
sulfides to sulfoxides by H2O2 under solvent-free conditions
Donya Khaledian, Amin Rostami, Shamileh Rouhani
PII:
S1566-7367(19)30055-X
DOI:
https://doi.org/10.1016/j.catcom.2019.02.021
CATCOM 5632
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
14 October 2018
16 February 2019
21 February 2019
Please cite this article as: D. Khaledian, A. Rostami and S. Rouhani, Magnetic core-
shell nanoparticle-supported Sc (III): A novel and robust Lewis acid nanocatalyst for
the selective oxidation of sulfides to sulfoxides by H2O2 under solvent-free conditions,
Catalysis Communications, https://doi.org/10.1016/j.catcom.2019.02.021
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ACCEPTED MANUSCRIPT
Magnetic core-shell nanoparticle-supported Sc (III): A novel and robust
Lewis acid nanocatalyst for the selective oxidation of sulfides to sulfoxides by
H O under solvent-free conditions
2
2
a
a,*
a,b
Donya Khaledian , Amin Rostami a.rostami@uok.com, Shamileh Rouhani
a
Department of Chemistry, Faculty of Science, University of Kurdistan, 66177-15175 Sanandaj,
Iran
b
Nanotechnology and Water Sustainability Research (NanoWS) Unit, College of Science,
Engineering and Technology, University of South Africa, Johannesburg, South Africa, 1709,
South Africa

ACCEPTED MANUSCRIPT
Abstract
For the first time scandium triflate was supported on modified Fe O magnetic nanoparticles [MNPs-
3
4
PhSO -Sc(OTf) ] and characterized using scanning electron microscopy (SEM), inductively coupled
3
2
plasma analysis (ICP), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-
IR), energy dispersive X-ray spectroscopy (EDX) and vibrating sample magnetometer (VSM) techniques.
The MNPs-PhSO -Sc(OTf) was applied as a magnetically recyclable heterogeneous Lewis acid
3
2
nanocatalyst for the selective oxidation of a variety of sulfides to sulfoxides using hydrogen peroxide as a
green oxidant resulting in 90-98% yields under solvent-free conditions at room temperature. It could also
be recovered and reused for at least 15 reaction cycles without considerable loss of activity.
Keywords: Magnetic nanoparticles, Scandium triflate, Hydrogen peroxide, Sulfides, Sulfoxides.

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1
. Introduction
Lewis acid catalysts have been of great interest due to their unique activities and selectivities
in the synthesis of organic compounds under mild reaction conditions [1,2]. In this manner,
scandium triflate has been introduced as a promising mild, powerful and selective Lewis acid for
a variety of functional group transformations [2]. Furthermore, Sc(OTf) , in contrast to most
3
traditional Lewis acids which are disintegrated into their metal hydroxide in water or protic
solvents, and accordingly lose their catalytic activities, does not decompose or deactivate under
aqueous workup conditions, so its recovery is often possible. Nonetheless, the most popular
recycling methods for the recovery of Sc(OTf) are time-consuming and include the successive
3
extraction of organic reaction mixture using deionized water [3,4]. In order to facilitate catalyst
reuse and to preserve its catalytic activity, scandium has been immobilized on many different
types of supports such as silica [5-7], clays [8], polymer [9,10], zeolite [11] and metal-organic
frameworks (MOFs) [12]. Although these protocols represent considerable advances, they still
need the classical methods such as extraction, filtration or centrifugation to separate the
supported scandium (III) from the reaction solution.
Recently, the traditional supported matrices were replaced by magnetic nanomaterials that
have easy preparation and functionalization, large surface area ratio, facile recovery and
recyclability via magnetic force, as well as low toxicity and price [13,14]. To the best of our
knowledge, there are no examples of MNPs-based scandium (III) nanocatalyst. In continuation
of our studies on the preparation and application of magnetically recyclable nanocatalysts [15-
1
8], herein, we wish to present the design, synthesis and catalytic properties of a novel Sc(III)-
supported onto magnetic nanoparticles and use this catalyst as a novel, eco-friendly and
recyclable catalyst in the selective oxidation of sulfides to sulfoxides using hydrogen peroxide as

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a green oxidant at room temperature under solvent-free conditions (Scheme 1). The significant
novelties of the presence work are: (i) this is the first report of a magnetic nanoparticle-supported
scandium triflate for use as a robust heterogeneous Lewis acid catalyst, and (ii) it is the first time
that Sc(III) catalyst has been used for the chemical oxidation of sulfides to sulfoxides using
H O .
2
2
<
Scheme 1>
2
. Experimental
The magnetic nanoparticle-supported scandium (III) was prepared through a multi-step
functionalization procedure, details of which can be found in the provided Electronic Support
Information (ESI).
2
.1 General procedure for the oxidation of sulfides to sulfoxides
The sulfide (1 mmol) was added to a mixture of 30% H O (3.6 mmol) and MNPs-PhSO -
2
2
3
Sc(OTf) ) (25 mg), and the mixture was then stirred at room temperature for the time specified.
2
The progress of reaction was monitored by TLC (EtOAc/n-hexane, 3/10). After completion of
the reaction, the catalyst was separated from the reaction mixture by an external magnet and the
mixture was decanted. The product was extracted with Et O (2 × 5 mL) and the combined
2
organic phases were washed with brine (10 mL) and dried over anhydrous Na SO . The
2
4
evaporation of solvent under reduced pressure gave the pure products in 90–98% yields. All the
1
products were known and characterized by comparison of their H NMR spectra and physical
properties (melting point) with those of authentic samples [19-22].
3
. Results and discussion
3
.1. Preparation and characterization of MNPs-PhSO -Sc(OTf)2
3
Scandium triflate successfully was immobilized on MNPs by using the surface modification
strategy as descripted in Scheme 1.
<
Scheme 1>

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To illustrate the successful synthesis of the MNPs-PhSO -Sc(OTf) , we characterized its
3
2
chemical composition using different techniques. One indication of bond formation between the
nanoparticles and the catalyst can be inferred from TGA. The TGA curve of the MNPs-PhSO3-
Sc(OTf) shows the mass loss of the organic materials as they decompose upon heating (Fig. 1).
2
The weight loss at temperatures below 120˚C is due to the removal of physically adsorbed
solvent and surface hydroxyl groups [23,24]. Organic groups have been reported to desorb at
temperatures above 130˚C. The curve shows a weight loss  21% from 130 to 700˚C, resulting
from the decomposition of organic spacer grafting to the MNPs surface.
<
Fig. 1 >
The FT-IR spectra for the MNPs, MNPs-SiO , and MNPs-PhSO -Sc(OTf) samples are
2
3
2
-1
shown in Fig. 2. As can be seen from the FT-IR spectrum of MNPs, the strong band at 583 cm
-1
corresponds to Fe–O vibrations of the magnetite core, and the stretching vibration at 3421 cm is
attributed to the O–H bonds, which are attached to the surface iron atoms. The bond formation
-
between MNPs and TMOS is confirmed by the Fe–O–Si absorption band that appears at 631 cm
1
-1
-1
.
The band at 1099 cm represents Si–O bonds, and IR bands at 3381 and 1626 cm correspond
to the stretching and bending vibrations of Si–OH, respectively. In the FT-IR spectrum of the
−
1
MNPs-PhSO -Sc(OTf) (Fig. 2), the IR bands at  693 and 1101 cm are attributed to the
3
2
asymmetric bending absorption mode of CF groups and the asymmetric C-F stretching
3
absorption of the CF group, respectively. In addition, the sulfate stretching vibration absorption
3
−
1
IR band at 1409 cm could also be found in the MNPs-PhSO -Sc(OTf) sample.
3
2
<
Fig. 2 >

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Superparamagnetic particles are beneficial for magnetic separation. The magnetic property of
the MNPs-PhSO -Sc(OTf) has been compared with bare MNPs using the vibrating sample
3
2
−
1
magnetometer (VSM) technique (Fig. 3). VSM measurements for MNPs (74.3 emu g ) [25] is
−
1
higher than MNPs-PhSO -Sc(OTf) (27.71 emu g ). Decrease in magnetic property of MNPs-
3
2
PhSO -Sc(OTf) can originate from the grafting of organic layers and scandium triflate on Fe O
3
2
3
4
nanoparticles.
<
Fig. 3>
The SEM image of the MNPs-PhSO -Sc(OTf) confirmed that the catalyst was made up of
3
2
uniform nanometer-sized particles of less than 22 nm (Fig. S1). EDX microanalysis was
performed to provide qualitative determination of the elemental composition. The EDX spectrum
of MNPs-PhSO -Sc(OTf) confirmed the presence of Fe, Si, O, S, C and Sc (Fig. S2). The
3
2
loading of the scandium function in MNPs-PhSO -Sc(OTf) can be calculated from Inductively
3
2
Coupled Plasma (ICP) analysis, which confirmed a loading of  0.6 %.
3
.2. Catalytic application of MNPs-PhSO -Sc(OTf) in the oxidation of sulfides to sulfoxides
3 2
The catalytic activity of MNPs-PhSO -Sc(OTf) was investigated in the selective oxidation of
3
2
sulfides to sulfoxides using 30% H O as a green oxidant. To optimize the reaction conditions,
2
2
we evaluated the influence of different amounts of catalyst and hydrogen peroxide on the
oxidation of methyl phenyl sulfide as a model compound under solvent-free conditions at room
temperature. As shown in Table 1, the optimum amount of H O (3.6 mmol) in the presence of
2
2
MNPs-PhSO -Sc(OTf) (25 mg) is ideal for complete conversion of methyl phenyl sulfide to
3
2
methyl phenyl sulfoxide (Table 1, entry 5).
<
Table 1>

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The generality of this approach has been demonstrated by a facile oxidation of a wide variety of
aryl, benzylic, linear, cyclic and heterocyclic sulfides, as shown in Table 2. The reactions went
on well to afford products in excellent yields ranging from 90-98% in short times. The TON and
TOF values of the process confirm the catalytic efficiency of the method (Table 2). Meanwhile,
to show the chemoselectivity of this method, the sulfide containing oxidation-prone functional
group such as OH was subjected to the sulfoxidation reaction that this functional group remained
intact during the conversion of sulfide to sulfoxide (Table 2, entry 15).
<
Table 2>
The leaching of the catalyst was also checked in the oxidation of dibenzyl sulfide to dibenzyl
sulfoxide using hydrogen peroxide. The magnetic nanocatalyst was removed by an external
magnet from the reaction mixture after 30 min, and the mixture was allowed to react further for
6
0 min. The GC yield of the product after 30 min was 70%, whereas there was no significant
increase in the yield of the product after 60 min. This study clearly demonstrated that there was
no significant amount of leaching.
For practical purposes, the ability to easily recover and recycle the catalyst is highly
desirable. To investigate this issue, the recovery and reuse of MNPs-PhSO -Sc(OTf) was
3
2
examined for the oxidation of methyl phenyl sulfide to give methyl phenyl sulfoxide. We found
that this catalyst rapidly recovered and demonstrated remarkably excellent recyclability. In
particular, after the first run the catalyst was separated by an external magnet, washed
,
thoroughly with ether and dried under vacuum, and reused as such for subsequent experiments
under the above mentioned reaction conditions (general procedure). The catalyst was reusable
for up to 15 cycles without any significant loss of activity (Fig. S3). The SEM image of the

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reused MNPs-PhSO -Sc(OTf) catalyst (Fig. S4) also indicated that nanoparticles size has been
3
2
increased to 43 nm because of nanoparticles aggregation during the reaction and the recycling
stages. EDX analysis (Fig. S5) of the MNPs-PhSO -Sc(OTf) catalyst after 15 runs was used to
3
2
confirm the presence of Fe, O, S, C, Si and Sc elements in the reused magnetic nanocatalyst. The
ICP analysis of the reused magnetic nanocatalyst was performed to evaluate the remaining
quantity of the loaded scandium, illustrating  0.54% for scandium value. This implies that the
scandium quantity in the magnetic nanocatalyst after recycling is comparable with the fresh
magnetic nanocatalyst and the catalyst can be recovered and reused without any significant
leaching of scandium.
Based on previously reported mechanisms for the catalytic application of scandium triflate
as Lewis acid in organic synthesis [3,7], it is proposed that scandium (III) polarizes the O-O
bond in H O to produce the reactive oxygen transfer agent followed by the oxygen transfer to
2
2
the sulfide as shown in Scheme 2.
<
Scheme 2>
4
. Conclusions
The first magnetic core-shell nanoparticles-based scandium (III) nanocatalyst was
successfully synthesized by immobilizing scandium triflate into sodium benzenesulfonate-
functionalized magnetic nanoparticles. The present catalyst shows excellent activity in the
oxidation of sulfides to sulfoxide using H O . The system also couples the advantages of
2
2
homogeneous Sc(III) based systems (high activity and reproducibility) and heterogeneous (easy
separation and excellent reusability of up to 15 runs), which make it as a promising material for
practical and large-scale applications. The other merits of the protocol are the use of a

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commercially available, eco-friendly, cheap and chemically stable oxidant, the mild reaction
conditions, operational simplicity, practicability, short reaction times and high yields of products.
Acknowledgment
We are grateful for financial support from the University of Kurdistan.

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[
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[
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[
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Aldrich Chemical Company, Inc.: Milwaukee, WI, 1993, Vol. 1.
[
1
[
[
[
[
23] M. Z. Kassaee, H. Masrouri, F. Movahed, Appl. Catal. A Gen. 395 (2011) 28-33.
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Schemes
Scheme 1. The process for the preparation of MNPs-PhSO -Sc(OTf) .
3
2
MNPs-Sc(III)


H
O O
H
S
R1
R2
H O
2
2
MNPs-Sc(III)
OH2
MNPs-Sc(III)
H OH
O
O
S
S
R1
R2
R1
R2
Scheme 2. A possible mechanism for the oxidation of sulfides to sulfoxides using H O2
2
catalyzed by MNPs-PhSO -Sc(OTf) .
3
2

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Figures
Fig. 1. TGA curve of MNPs-PhSO -Sc(OTf) .
3
2
Fig. 2. FT-IR spectra of MNPs, MNPs-SiO and MNPs-PhSO -Sc(OTf)
2
2
3

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Fig. 3. Magnetization curve for MNPs-PhSO -Sc(OTf) recorded at room temperature.
3
2

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Tables
Table 1. The oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide under different conditions.
a
Entry
1
Catalyst (mg)
Catalyst-free
H O (mmol)
Time (min)
24h
Solvent
-
Yield (%)
40
2
2
3.6
2
3
4
5
6
7
8
9
Fe O MNPs (15)
3.6
3.6
3.6
3.6
3.6
2.4
4.8
3.6
3.6
2h
75
8
-
-
40
75
70
3
4
MNPs-PhSO Na (15)
3
MNPs-PhSO -Sc(OTf) (15)
3
2
b
MNPs-PhSO -Sc(OTf) (25)
8
-
-
-
-
98
3
2
MNPs-PhSO -Sc(OTf) (35)
8
98
80
98
50
45
3
2
MNPs-PhSO -Sc(OTf) (25)
8
3
2
MNPs-PhSO -Sc(OTf) (25)
8
3
2
MNPs-PhSO -Sc(OTf) (25)
8
CH CN
3
2
3
1
0
MNPs-PhSO -Sc(OTf) (25)
8
EtOH
3
2
a
b
Conversion of sulfide to sulfoxide was determined by GC. The bold represents the most effective conditions.
Table 2. MNPs-PhSO -Sc(OTf) (25 mg) catalyzed selective oxidation of sulfides (1 mmol) to
3
2
sulfoxides using 30% H O (3.6 mmol).
2
2
O
S
MNPs-PhSO -Sc(OTf) (Cat.), H O
2 2
S
3
2
R1
R2
Solvent-free, r.t.
R1
R2
a
-1
Entry
1
sulfide
Time (min)
3
Yield (%)
>98
TON
294
TOF(h )
5886
2143
1900
1425
S
S
CH3
2
3
4
8
9
>95
>95
>95
285
285
285
S
S
12

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S
Ph
5
6
30
90
91
270
273
540
137
S
Ph
120
S
Ph
7
8
5
>95
94
285
282
3434
282
60
Ph
S
Ph
S
9
65
92
276
255
7
1
0
1
2
3
CH (CH ) S (CH ) CH
CH (CH ) S (CH ) CH
3 2 7 2 5
15
15
12
5
92
93
>95
>95
276
279
285
285
1104
1116
1425
3434
3
2 7
2 2
3
3
3
1
1
1
CH (CH ) S (CH ) CH
3
2 3
2 2
CH S CH
3
3
1
4
S
10
95
285
1707
O
S
OH
1
5
5
>95
285
3434
a
Isolated yield.

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Highlights




The first MNPs-PhSO -Sc(OTf) was easily synthesized and characterized.
3
2
MNPs-PhSO -Sc(OTf) catalyzed highly efficient chemoselective oxidation of sulfides.
3
2
The catalyst allows for greener method while keeping high yields and low times.
The catalyst was reused up to 15 times without significant loss of activity.