Nanoparticle supported, magnetically separable vanadium complex as catalyst for selective oxidation of sulfides

Article abstract of DOI:10.1080/00958972.2012.657188

A magnetically recyclable vanadium(V) catalyst was synthesized by covalent anchoring of VO(salen)Cl on silica-coated Fe₃O₄ nanoparticles. This straightforward preparation yields magnetically separable Fe₃O₄@SiO₂@VO(salen) nanoparticles with high vanadium loading. These nanoparticles were efficient catalysts for selective oxidation of sulfides to corresponding sulfoxides with urea hydrogen peroxide in excellent yields. Leaching and recycling experiments revealed that the nanocatalyst can be applied for nearly complete oxidation of sulfides for at least five successive cycles.

Full text of DOI:10.1080/00958972.2012.657188

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Nanoparticle supported, magnetically
separable vanadium complex as
catalyst for selective oxidation of
sulfides
a
a
Mojtaba Bagherzadeh , Mohammad Mehdi Haghdoost & Ayoob
a
Shahbazirad
a
Chemistry Department, Sharif University of Technology, P.O. Box
11155-3615, Tehran, Iran
Version of record first published: 02 Feb 2012.
To cite this article: Mojtaba Bagherzadeh, Mohammad Mehdi Haghdoost & Ayoob Shahbazirad
(2012): Nanoparticle supported, magnetically separable vanadium complex as catalyst for selective
oxidation of sulfides, Journal of Coordination Chemistry, 65:4, 591-601
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Journal of Coordination Chemistry
Vol. 65, No. 4, 20 February 2012, 591–601
Nanoparticle supported, magnetically separable vanadium
complex as catalyst for selective oxidation of sulfides
MOJTABA BAGHERZADEH*, MOHAMMAD MEHDI HAGHDOOST
and AYOOB SHAHBAZIRAD
Chemistry Department, Sharif University of Technology,
P.O. Box 11155-3615, Tehran, Iran
(
Received 20 October 2011; in final form 23 December 2011)
A magnetically recyclable vanadium(V) catalyst was synthesized by covalent anchoring of
VO(salen)Cl on silica-coated Fe
3
O
4
nanoparticles. This straightforward preparation yields
@VO(salen) nanoparticles with high vanadium loading.
magnetically separable Fe O @SiO
4
3
2
These nanoparticles were efficient catalysts for selective oxidation of sulfides to corresponding
sulfoxides with urea hydrogen peroxide in excellent yields. Leaching and recycling experiments
revealed that the nanocatalyst can be applied for nearly complete oxidation of sulfides for at
least five successive cycles.
Keywords: Magnetic separation; Nanocatalyst; Vanadium(V); Oxidation of sulfides; Surface
functionalization
1. Introduction
In many cases, application of the homogeneous catalysts on industrial scale has been
prevented due to difficult separation and regeneration of the catalyst from the reaction
mixture. Using heterogenized homogeneous catalysts is an attractive option to facile
separation of catalyst, simplified product work-up and continuity of catalytic system
[
1–3]. However, immobilization of a homogeneous catalyst on solid supports somewhat
facilitates its separation but normally results in a decrease in the active surface area and
the reactivity of the catalyst [4]. Rational improvement of the catalytic systems will
require advanced materials that have a high reactivity toward the chemical reactions
and can be recycled and reused through simple separation and regeneration methods
[
5]. Homogeneous catalysts immobilized on magnetic nanoparticles (MNPs) surface
occupy a unique position among these advanced materials due to combining the
advantages of both homogeneous and heterogeneous catalysts [4–10]. Because of large
surface area, nanoparticles can carry a very large payload of catalytically active species
and therefore exhibit high catalytic activity and selectivity [5]. In addition, super-
paramagnetic nature of MNPs enable very simple separation of the immobilized
*
Corresponding author. Email: bagherzadeh@sharif.edu
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.657188

5
92
M. Bagherzadeh et al.
catalysts from the reaction mixture using an external magnet, which is typically more
effective than filtration or centrifugation [8].
For this purpose and also to continue our study on the catalytic activity of metal
complexes for synthesis of organic compounds [11–15] we herein report that an Fe O4
3
nanoparticle-supported vanadium(V) catalyst (Fe O @SiO @VO(salen)) efficiently
3
4
2
catalyzes selective oxidation of sulfides to sulfoxides using urea hydrogen peroxide
UHP) as oxidant. This catalytic system features two major novelties: (1) this is the first
(
time that a magnetically recyclable nanocatalyst has been used in catalytic oxidation of
sulfides and (2) this is the first report of a magnetically separable vanadium complex.
The present catalyst also was easily recovered and reused for more than five times
without any loss of catalytic activity or selectivity.
2
2
. Experimental
.1. Methods
Gas chromatographic (GC) analyses were performed on an Agilent Technologies
6
890 N, 19019 J-413 HP-5, 5% phenyl methyl siloxane, capillary 60 m ꢀ 250 mm ꢀ 1 mm.
1
13
H NMR and C NMR spectra were recorded at room temperature with a Bruker FT
NMR 500 (500 MHZ) spectrophotometer using CDCl as solvent and chemical shifts
3
are reported in ppm with tetramethylsilane as an internal standard. Elemental analyses
(
(
C, H, and N) were performed using a Heraeus Elemental Analyzer CHN-O-Rapid
Elementar-Analysesysteme, GmbH). IR spectra were recorded as KBr pellets using an
ABB FT-IR spectrophotometer. Measures of pH were carried out by a Mettler Toledo
TM
S40 SevenMulti
pH-meter. Dynamic light scattering (DLS) analysis was conducted
ꢁ
using a NanoZS (red badge)-ZEN3600 light scatter from Malvern instrument at 25 C.
The chemical analysis of compounds was carried out in an ICP-AES Varian
equipment. X-ray diffraction (XRD) pattern was obtained by a D4 ENDEAVOR
diffractometer (Bruker AXS Inc.) with Cu-Ka as a radiation source; the scan range (2ꢀ)
ꢁ
ꢁ
was from 5 to 70 .
2
.2. Synthesis of VO(salen)Cl complex
0
H salen (bis(N,N -disalicylalethylenediamine)) was prepared according to the reported
2
procedure [16]. 1.73 g (4 mmol) of H Salen was loaded into a 100 mL Schlenk flask and
2
filled with 20 mL dry CH Cl . To this stirring solution, 0.20 mL (0.70 g, 4 mmol) of
2
2
freshly prepared VOCl was added via syringe. After stirring under argon at room
3
temperature for 2 h, the solvent was removed under vacuum, yielding dark green
powder of VO(salen)Cl.
H salen: Yield: 97%; Anal. Calcd for C H N O (%): C, 71.62; H, 6.01; N, 10.44.
2
2
16 16
2
ꢂ1
Found: C, 71.52; H, 5.97; N, 10.49. IR (KBr, cm ): 1634, 3141, 1196, 739, 1583;
1
H NMR (500 MHz, CDCl , 21 C): 3.9 ppm (s, 2H, CH ), 6.9 ppm (d, 1H, CH),
3
ꢁ
2
7
N(CH)), 13.4 ppm (s, 1H, OH); C NMR (500 MHz, CDCl , 21 C): 69.6, 117.3, 119.4,
.2 ppm (d, 1H, CH), 7.3 ppm (m, 2H, CH), 7.4 ppm (m, 2H, CH), 8.6 ppm (s, 1H,
1
3
ꢁ
3
ꢁ
1
32.5, 133.2, 161.4, 167.8 ppm; Melting point: 128 C; Mol. Wt.: 268.31.

Nanoparticle supported vanadium
593
VO(salen)Cl: Yield: 80%; Anal. Calcd for C H ClN O V (%): C, 52.12; H, 3.83; N,
14
1
6
2
3
ꢂ1
7
1
7
.60. Found: C, 52.24; H, 3.83; N, 7.57. IR (KBr, cm ): 654, 763, 1619, 970, 1264,
1
550; H NMR (500 MHz, CDCl , 21 C): 4.4 ppm (s, 2H, CH ), 6.9 ppm (d, 1H, CH),
ꢁ
3
2
1
.2 ppm (d, 2H, CH), 7.6 ppm (m, 2H, CH), 8.6 ppm (s, 2H, N(CH)); C NMR
3
ꢁ
(500 MHz, CDCl , 21 C): 166, 165.4, 137.9, 135.5, 123.2, 122.2, 115.3, 61.41 ppm; Mol.
Wt.: 368.69.
3
2
.3. Synthesis of Fe O nanoparticles
3 4
Fe O nanoparticles were synthesized according to the procedure described previ-
3
4
ously [17]. In brief, under N2 atmosphere, 5.2 g (19.3 mmol) of FeCl ꢃ 6H O, 2 g
3
2
(
10.0 mmol) of FeCl ꢃ 4H O and 0.85 mL concentrated HCl were dissolved in 25 mL
2
2
degassed water. This solution was added dropwise at room temperature to 250 mL
ꢂ1
of NaOH solution (1.5 mol L ) under N . The reaction mixture was vigorously
2
stirred for 30 min (1300 rpm). The formed black precipitates were separated using a
strong magnetic field (0.5 T magnet) and washed several times with degassed water.
Finally, for storage, Fe O nanoparticles were dispersed in 200 mL degassed water
3
4
under N . The density of the resulting solution was found to be approximately
2
ꢂ1
1
1.2 mg mL
.
2
.4. Synthesis of Fe O @SiO nanoparticles
3 4 2
In the next step, Fe O MNPs were coated with a thin layer of silica [18]. One gram of
3
4
freshly prepared Fe O nanoparticles was added into 30 mL of an aqueous solution of
3
4
ꢂ1
citric acid (0.02 g mL ), then the pH was adjusted to 5.2 using ammonia, and the
ꢁ
mixture was heated to 80–90 C for 1.5 h. After heating, the pH of the reaction mixture
was increased with ammonia to pH ¼ 11 and 1.25 mL of tetraethylorthosilicate (TEOS)
dissolved in ethanol (12.5 mL) was added dropwise into the suspension of particles. The
mixture was stirred at room temperature for 24 h to let the base-catalyzed hydrolysis
and condensation of TEOS monomers on the nanoparticle surface go to completion.
Finally, the dark brown Fe O @SiO nanoparticles were separated using a 0.5 T
3
4
2
magnet and were washed with distilled (not degassed) water (5 times, 150 mL) and
ethanol (5 times, 150 mL).
2
.5. Synthesis of Fe O @SiO @VO(salen) nanoparticles
3 4 2
In a subsequent process, Fe O @SiO nanoparticles were functionalized with a
2
3
4
vanadium(V) complex. A total of 0.7 g of the Fe O @SiO nanoparticles was suspended
3
4
2
in 100 mL of dry dichloromethane (DCM) and then 0.1 g (0.27 mmol) of VO(salen)Cl
was added to the suspension of nanoparticles and the reaction mixture was refluxed for
1
0 h. After cooling, nanoparticles were separated using a strong magnet, washed with
dry DCM several times, and then dried in an oven for 24 h, yielding a green-brown
powder.

5
94
M. Bagherzadeh et al.
2.6. General conditions for catalytic oxidation of sulfides
A typical reaction using the Fe O @SiO @VO(salen) nanoparticles as catalyst and
3
4
2
sulfides as substrate is described as follows. To a solution of sulfide (0.4 mmol),
chlorobenzene (40 mL, 0.4 mmol) as the internal standard and catalyst (0.01 g,
containing 0.031 mmol of vanadium complex) in a (1 : 1) mixture of CH OH–CH Cl
2
3
2
(
1 mL), was added UHP (0.042 g, 0.45 mmol) as the oxidant. The mixture was stirred at
room temperature for 6 h. The catalyst particles were then collected at the bottom of the
test tube using a magnet, supernatant carefully decanted and formation of products was
monitored by GC. To establish the identity of the products clearly, the retention times
and spectral data of products were compared with those of commercially available
sulfoxides and sulfones. After separation of catalyst nanoparticles, the supernatant was
ꢁ
dried under vacuum at 40 C to yield the corresponding sulfoxide compound. Isolated
1
products were weighted and also analyzed by H NMR. Washing several times with
methanol and DCM, the catalyst particles were dried in vacuum and could be reused.
3
. Results and discussion
Magnetically separable vanadium(V) catalyst was synthesized by the route schemat-
ically demonstrated in scheme 1. In the first step, Fe O nanoparticles were prepared
using the co-precipitation method and characterized with XRD (figure 1). Six
3
4
ꢁ
characteristic peaks for Fe O (2ꢀ ¼ 32.1, 35.3, 43.1, 51.0, 54.9, and 64.1 ), which
3
4
correspond to (220), (311), (400), (422), (511), and (440) Bragg reflections, respectively,
were observed. These peaks are consistent with the database in JCPDS file
(PCPDFWIN v.2.02, PDF No. 85-1436) and reveal that the resultant nanoparticles
were pure Fe O with a cubic spinel structure without impurity phases.
3
4
The silica-coated nanoparticles (2) were prepared by hydrolysis and condensation of
TEOS on the surface of the Fe O nanoparticles (1). Figure 2 shows FT-IR spectra of
the Fe O MNP before and after being supported with a silica layer. The FT-IR
3
4
3
4
ꢂ1
spectrum of Fe O nanoparticles exhibits significant absorption band at 597 cm
3
,
4
ꢂ1
while absorption band at 1055 cm was observed for the silica-coated nanoparticles
which could be assigned to vibration of Si–O–Si bond [19]. As a result, FT-IR spectrum
indicates that the surface of Fe O nanoparticles has been coated by a silica layer.
3
4
FT-IR spectrum of Fe O MNP shows the presence of trace amount of water.
4
3
3 4 2
Scheme 1. Step-by-step synthesis of the Fe O @SiO @VO(salen) catalyst.

Nanoparticle supported vanadium
595
The results of the previous experiments show that base-catalyzed hydrolysis and
condensation of TEOS provides the resulting particles with negative surface charges due
ꢂ
to the surface formation of (ꢄSiO ) [20]. It is expected that surface negative oxygen
atoms (hard base) could replace the chloride at the axial position of VO(salen)Cl
complex and link to the hard vanadium(V) cation. An interesting example of grafting
vanadium(V) complex onto the silica surface via displacement of the chloride by free
ꢂ
surface SiO groups was recently reported [21]. With this idea in mind, we treated
Fe O @SiO nanoparticles with VO(salen)Cl in CH Cl solvent to synthesize
3
4
2
2
2
Fe O @SiO @VO(salen) nanoparticles (3). During optimization of this reaction, we
3
4
2
observed that the Fe O @SiO @VO(salen) nanoparticles synthesis does not proceed in
3
4
2
methanol and acetonitrile solvents. Comparing FT-IR spectra of 2 and 3 (figure 3), IR
spectrum of Fe O @SiO @VO(salen) nanoparticles shows new peaks at 659, 765, 916,
3
4
2
ꢂ1
971, 1013, 1039, 1087, 1255, 1277, 1402, and 1509 cm , also observed in FT-IR
spectrum of VO(salen)Cl. CHN analysis of the Fe O @SiO @VO(salen) nanoparticles
gave the percentages of C, H, and N to be 7.52%, 2.24%, and 4.45%, respectively.
3
4
2
3 4
Figure 1. XRD patterns of the as-synthesized Fe O nanoparticles.
3 4 3 4 2
Figure 2. IR spectra of Fe O (up) and Fe O @SiO (down) nanoparticles.

5
96
M. Bagherzadeh et al.
Figure 3. IR spectra of (a) VO(salen)Cl complex, (b) Fe
c) Fe @SiO @VO(salen) nanoparticles.
3
O
4
@SiO
2
nanoparticles, and
(
3
O
4
2
The FT-IR spectrum, coupled with the elemental analyses, provide evidence for
anchoring of vanadium(V) complex on silica-coated Fe O nanoparticles. The loading
3
4
ꢂ1
of vanadium(V)-salen complex was 3.1 mmol g , determined by ICP-AES test of
vanadium after acid digestion. To the best of our knowledge, this loading value is much
higher than that previously reported for similar systems [7, 10].
The average size and the size distribution of nanoparticles have been determined
using DLS measurements. As can be seen in figure 4, for the synthesis of
Fe O @SiO @VO(salen) nanoparticles (3), we started with Fe O nanoparticles (1)
3
4
2
3
4
of approximately 25 nm average diameter. These nanoparticles have narrow size
distribution (12–32 nm). However, the DLS study of Fe O @SiO @VO(salen) nano-
3
4
2
particles (figure 4b) confirmed that the size distributions lie in the range 14–50 nm with
0 nm average diameters and 50–115 nm with 103 nm average sizes, corresponding to
3
the uncoated and coated particles, respectively. These results also demonstrate that the
Fe O @SiO @VO(salen) catalyst has particles well in the nanosize range.
3
4
2
Oxidation of sulfides to sulfoxides or sulfones is a fundamental chemical reaction and
has been intensively studied due to economic and environmental benefits [11, 22–25].
0
The capability of d metal complexes to activate peroxides led us to test the
Fe O @SiO @VO(salen) nanoparticles as a heterogeneous catalyst for selective
4
3
2
oxidation of sulfides (table 1). During the optimization of the catalytic conditions, it
was observed that stirring Fe O @SiO @VO(salen) nanoparticles and 4 in CH Cl /
3
4
2
2
2
MeOH solvent in the presence of UHP for 6 h afforded 5 in 499% yield and 95%
selectivity (table 1, entry 5). To evaluate the effect of solvent, oxidation of sulfide 4 also
was carried out in methanol, DCM, and acetonitrile (table 1, entries 6–8). Sulfoxide 5
was obtained in 93% conversion in methanol, but in DCM and acetonitrile, the reaction
proceeded slowly and afforded lower conversion of 4. Compared with other oxidants
tested, UHP exhibited the highest conversion and selectivity; tert-butyl hydroperoxide
(TBHP) also resulted in relatively high yield of product (table 1, entries 9–10) and
cumene hydroperoxide (CHP) showed low activity (table 1, entry 11). In the presence of
a radical scavenger such as 2,6-di-tert-butyl-p-cresol, the conversion reduced to 47% and
remained almost constant (table 1, entries 12–13), suggesting a radical pathway. Control
experiments indicated that no oxidation of sulfide 4 occurred in the absence of either
Fe O @SiO @VO(salen) nanoparticles or oxidants (table 1, entries 1–2). In addition,
3
4
2

Nanoparticle supported vanadium
597
Figure 4. Size distribution of (a) bare Fe
3
O
4
nanoparticles and (b) Fe
3
O
4
@SiO
2
@VO(salen) nanoparticles.
the use of Fe O or Fe O @SiO nanoparticles in place of Fe O @SiO @VO(salen)
3
3
4
3
4
2
4
2
nanoparticles did not promote oxidation (table 1, entries 3–4).
The reaction scope for Fe O @SiO @VO(salen) nanoparticles in the selective
oxidation of sulfides is summarized in table 2. Fe O @SiO @VO(salen)/UHP system
3
4
2
3
4
2
could oxygenate a wide range of sulfides, affording the corresponding sulfoxides in
excellent conversion and selectivity. For example, arylalkyl sulfides (table 2, entries
1
–2), diaryl sulfides (table 2, entries 3–5), and dialkyl sulfides (table 2, entries 6–7) were
successfully employed. Conversions of diaryl sulfides (table 1, entries 3–5) were lower
than others, as a result of the electronic and steric effect of the aryl groups. The highest
yields were obtained for methylphenyl (table 2, entry 1) and diethyl (table 2, entry 7)
sulfides. In the case of dioctyl sulfide (table 2, entry 6), the conversion of the substrate
was also reduced by the steric effect of two octyl groups.
Catalyst stability and reusability are equally important issues for industrial
applications of heterogeneous catalyst systems. The Fe O @SiO @VO(salen) nanoca-
3
4
2
talyst can be reused more than five times, by simple magnetic separation (figure 5),
without significant loss in selectivity and activity (table 3). Fe O @SiO @VO(salen)
nanoparticles retained their magnetic properties even after six successive usages.
3
4
2

5
98
M. Bagherzadeh et al.
a
3 4 2
Table 1. Oxidation of methylphenylsulfide by Fe O @SiO @VO(salen) catalyst.
O
S
S
Fe O @SiO @VO(salen)/oxidant
3
4
2
room temperature
4
5
b,c
Conversion
(%)
Selectivity
to 5 (%)
d
Entry
Catalyst
–
@SiO @VO(salen)
Oxidant
Solvent
t (h)
1
2
3
4
5
6
7
8
9
UHP
–
CH
CH
CH
CH
CH
2
2
2
2
2
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
/MeOH
/MeOH
/MeOH
/MeOH
/MeOH
24
24
24
24
6
6
6
6
6
6
6
6
24
1
Trace
Trace
Trace
Trace
499 (83)
499 (81)
64
–
–
–
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
3
3
3
3
3
3
3
3
3
3
3
3
O
O
O
O
O
O
O
O
O
O
O
O
4
4
4
4
4
4
4
4
4
4
4
4
2
UHP
UHP
UHP
UHP
UHP
UHP
TBHP
TBHP
CHP
UHP
UHP
UHP
@SiO
@SiO
@SiO
@SiO
@SiO
@SiO
@SiO
@SiO
@SiO
@SiO
2
2
2
2
2
2
2
2
2
2
–
@VO(salen)
@VO(salen)
@VO(salen)
@VO(salen)
@VO(salen)
@VO(salen)
@VO(salen)
@VO(salen)
@VO(salen)
95
93
75
100
91
100
90
92
91
100
MeOH
CH
CH
CH
CH
CH
CH
CH
CH
3
2
2
3
2
2
2
2
CN
Cl
Cl
2
2
13
91
/MeOH
10
11
12
13
14
CN
75
42
Cl
Cl
Cl
Cl
2
2
2
2
/MeOH
/MeOH
/MeOH
/MeOH
e
47
48
e
VO(salen)Cl
499
a
Reaction condition: sulfide (0.4 mmol), UHP (0.45 mmol), Fe
room temperature.
3
O
4
2
@SiO @VO(salen) nanoparticles (0.01 g), 1 mL solvent,
b
Determined by GC using chlorobenzene as an internal standard.
Values in parentheses are the isolated yields.
c
d
Determined by GC, selectivity to sulfoxide ¼ [sulfoxide %/(sulfoxide % þ sulfone %)] ꢀ 100.
e
In the presence of 2,6-di-tert-butyl-p-cresol (0.25 equiv.).
a
Table 2. Oxidation of sulfides by Fe
3
O
4
@SiO
2
@VO(salen) nanoparticles in the presence of UHP.
O
O
O
Fe O @SiO @VO(salen)/UHP
S
3
4
2
S
+
S
8
R1
R2
CH Cl /MeOH
R1
R2
2
2
R1
R2
room temperature
6
7
b,c
Conversion (%)
d
Selectivity to 7 (%)
Entry
Substrate
Time (h)
1
2
3
4
5
6
7
Methylphenyl sulfide
Ethylphenyl sulfide
Diphenyl sulfide
Diphenyl sulfide
Diphenyl sulfide
Dioctyl sulfide
6
6
6
30
24
6
499 (83)
84
95
89
100
100
76
9
37
80 (54)
60
e
100
100
Diethyl sulfide
4
499 (91)
a
Reaction condition: sulfide (0.4 mmol), UHP (0.45 mmol), Fe
.5 mL MeOH, room temperature.
Determined by GC using chlorobenzene as an internal standard.
3 4 2 2 2
O @SiO @VO(salen) nanoparticles (0.01 g), 0.5 mL CH Cl ,
0
b
c
Values in parentheses are the isolated yields.
Determined by GC, selectivity to sulfoxide ¼ [sulfoxide %/(sulfoxide % þ sulfone %)] ꢀ 100.
d
e
8
mmol of UHP were employed.

Nanoparticle supported vanadium
599
Figure 5. (a) Fe
(b) Fe @SiO @VO(salen) nanocatalyst adsorbed on the magnet.
3 4 2
O @SiO @VO(salen) nanocatalyst dispersion in reaction mixture and
3
O
4
2
a
Table 3. Results of catalyst reuse experiments in the oxidation of various sulfides.
b,c
d
Entry
1
Substrate
Product
Cycles Conversion
Selectivity
95
1
499 (83)
S
O
S
2
3
4
5
6
7
2
3
4
5
6
1
499
499
499 (93)
499
499 (89)
499 (91)
92
97
93
97
91
100
S
S
O
S
8
9
0
2
3
1
499
499 (90)e
80 (54)
100
100
76
1
O
S
e
11
2
78
77
a
Reaction condition: sulfide (0.4 mmol), UHP (0.45 mmol), Fe
.5 mL CH Cl , 0.5 mL MeOH, room temperature.
3
O
4
2
@SiO @VO(salen) nanoparticles (0.01 g),
0
2
2
b
Determined by GC using chlorobenzene as an internal standard.
Values in parentheses are the isolated yields.
c
d
Determined by GC, selectivity to sulfoxide ¼ [sulfoxide%/(sulfoxide% þ sulfone%)] ꢀ 100.
e
8
mmol of UHP were employed.
After each catalytic reaction, the catalyst particles were collected at the bottom of the
test tube using a magnet, supernatant carefully decanted and analyzed by ICP-AES
analysis to determine the amount of vanadium leaching. ICP-AES analysis did not
show any leaching of vanadium species into the solution (50.01 ppm). In addition,

6
00
M. Bagherzadeh et al.
Figure 6. FT-IR spectrum of (a) freshly prepared Fe
catalyst after six reuses.
3 4 2
O @SiO @VO(salen) nanoparticles and (b) same
FT-IR pattern of catalyst was the same as freshly prepared Fe O @SiO @VO(salen)
3
4
2
nanoparticles (figure 6), which indicated that decomposition of surface vanadium
complexes did not occur. These results imply high stability of the
Fe O @SiO @VO(salen) catalyst.
3
4
2
4
. Conclusion
Fe O @SiO @VO(salen) nanoparticles are efficient, easily recoverable catalysts for
3
4
2
selective oxidation of sulfides. With this catalytic system, various sulfides were
transformed to corresponding sulfoxide with the following advantages: (1) high
conversions and selectivities; (2) applicability to a wide range of sulfides; (3) high
loading value, and (4) successful recycle of catalyst by simple magnetic separation with
stable conversion efficiency of 100%. We believe that such magnetically separable
nanocatalysts for oxidation can be applied not only to sulfides but also to alkenes,
alkanes, and alcohols. Further study on the wider applications of this efficient strategy
is under exploration in our group.
Supplementary material
1
Diphenyl sulfoxide: H NMR (500 MHz, CDCl , 21 C): 7.44–7.50 ppm (m, 3H), 7.67–
ꢁ
3
ꢁ
7
.69 ppm (m, 2H); Melting point: 68–70 C.
1
Methylphenyl sulfoxide: H NMR (500 MHz, CDCl , 21 C): 2.88 (d, 3H), 7.54 ppm
ꢁ
3
ꢁ
(
(
s, 5H); Melting point: 25–27 C.
1
Diethyl sulfoxide: H NMR (500 MHz, CDCl , 21 C): 1.46 ppm (t, 3H), 2.78 ppm
ꢁ
3
q, 2H).

Nanoparticle supported vanadium
601
Acknowledgments
We acknowledge the Research Council of Sharif University of Technology for research
funding of this project. We also thank Dr M.R. Hormozi-Nezhad for useful discussions.
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