Schiff base complex of Mo supported on iron oxide magnetic nanoparticles (Fe3O4) as recoverable nanocatalyst for the selective oxidation of sulfides

Article abstract of DOI:10.1007/s13738-017-1046-8

In this work, a new tridentate Schiff base dioxo-molybdenum(VI) complex immobilized on silica-coated magnetic nanoparticles (MoO₂5CML–Fe₃O₄@SiO₂) has been synthesized and characterized using different techniques such as FTIR, TGA, AAS, ICP–AES, XRD, VSM, EDX and SEM analyses. The catalytic activity of synthesized complex was examined in the oxidation of various sulfides in the presence of H₂O₂ as cheap, green and eco-friendly oxidant. This catalytic system provides high conversion and selectivity toward either sulfoxides or sulfones under different conditions. Also, the nanocatalyst could be easily separated and regenerated from reaction media by external magnet and could be reused for ten times without significant loss of the activity and selectivity.

Full text of DOI:10.1007/s13738-017-1046-8

J IRAN CHEM SOC
DOI 10.1007/s13738-017-1046-8
ORIGINAL PAPER
Schiff base complex of Mo supported on iron oxide magnetic
nanoparticles (Fe O ) as recoverable nanocatalyst for the selective
3
4
oxidation of sulfides
Milad Aghajani · Niaz Monadi1
1
Received: 20 August 2016 / Accepted: 2 January 2017
©
Iranian Chemical Society 2017
Abstract In this work, a new tridentate Schiff base dioxo-
molybdenum(VI) complex immobilized on silica-coated
magnetic nanoparticles (MoO 5CML–Fe O @SiO ) has
and environment [1–3]. The recent advancement in green
nanotechnology led to potential applications of magnetic
nanoparticles for the development of various applications
including nanocatalysis [4], sensors [5], biomedicine [6],
energy storage [7] and drug delivery [8]. In the field of catal-
ysis, core–shell iron oxide magnetic nanoparticles (Fe O ) as
2
3
4
2
been synthesized and characterized using different tech-
niques such as FTIR, TGA, AAS, ICP–AES, XRD, VSM,
EDX and SEM analyses. The catalytic activity of synthe-
sized complex was examined in the oxidation of various
sulfides in the presence of H O as cheap, green and eco-
3
4
a support have found with applications due to their unique
magnetic properties such as immense surface area, easy sep-
aration from the reaction mixture by external magnetic field
rather than filtration or centrifugation, enhancing the contact
between reactants and catalyst and finally recoverable with-
out losing their activity and selectivity [9–11]. Magnetic
nanocatalysts have been used in many organic reactions
[12–17] that one of which involves the oxidation of sulfides.
The selective oxidation of organic sulfides as environmen-
tal pollutants into the corresponding sulfoxides has been an
attractive and important challenge in synthetic organic chem-
istry, since sulfoxides as biologically significant molecules
are important intermediates in the synthesis of pharmaceu-
ticals, agrochemicals and other fine chemicals [18–20].
Moreover, a basic obstacle during the oxidation of sulfides
is overoxidation of the sulfoxides to their corresponding sul-
fones. Therefore, it is very important that the catalyst has a
high selectivity toward sulfoxide or sulfone. According to
green chemistry protocol, catalytic oxidation of sulfides with
environmentally friendly oxidant has become significantly
important [21–24]. Hydrogen peroxide (H O ) is a desired
2
2
friendly oxidant. This catalytic system provides high con-
version and selectivity toward either sulfoxides or sulfones
under different conditions. Also, the nanocatalyst could be
easily separated and regenerated from reaction media by
external magnet and could be reused for ten times without
significant loss of the activity and selectivity.
Keywords Molybdenum complex · Magnetic Fe O ·
3
4
Schiff base · Sulfoxide and sulfone · Hydrogen peroxide
Introduction
Green nanotechnology is a bridge between green chemistry
and nanotechnology that refers to the use of nanotechnol-
ogy in order to enhance the eco-friendly processes producing
negative externalities. This nanoscience has caused a funda-
mental change due to its direct implications on human health
2
2
oxidant because of high-efficient oxygen content, safety with
low cost, readily available reagent, being eco-friendly, and
also it produces water as the only by-product [25]. A variety
of homogeneous transition metal catalysts are well known
for the oxidation of organic sulfides [26–30], but the vast
majority of these catalysts suffer from drawbacks such as
difficult separation of the product from the reaction medium,
deactivation, difficult recovery and recycling of the catalyst.
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-017-1046-8) contains supplementary
material, which is available to authorized users.
*
Niaz Monadi
Nimonadi@umz.ac.ir
1
Department of Inorganic Chemistry, Faculty of Chemistry,
University of Mazandaran, Babolsar, Iran
1
3

J IRAN CHEM SOC
Scheme 1 Schematic model for the preparation of catalyst
A feasible strategy to overcome these problems is to hetero-
genize homogeneous catalysts by anchoring them onto the
convenient insoluble supports such as iron oxide magnetic
nanoparticles. In this paper, a stepwise procedure is reported
for the synthesis of molybdenum Schiff base complex sup-
ported on iron oxide magnetic nanoparticles modified with
silica coating (MoO 5CML–Fe O @SiO ) and as an effi-
The organic composition of materials and catalyst was
determined by thermogravimetric analysis (TGA) and
differential thermoanalysis (DTG), which was heated
from 25 to 800 °C under nitrogen flow using a STA 409
PC analyzer (Netzsch). The products of the oxidation of
the sulfide were determined and analyzed using a Varian
CP-3800 gas chromatograph equipped with a capillary
column and a flame ionization detector. The metal con-
tent of the catalysts was measured by inductively coupled
plasma atomic emission analysis (ICP–AES, Varian com-
pany VISTA-PRO model).
2
3
4
2
cient, selective and recyclable catalyst for the selective oxi-
dation of different sulfides with H O .
2
2
Experimental
Synthesis
Materials and methods
The synthesis of MoO 5CML–Fe O @SiO is depicted in
2
3
4
2
All reagents and solvents were procured from the commer-
cial sources. Fourier transform infrared spectroscopy on
KBr pellets of the compounds was recorded on a Bruker
Vector 22 in the range of 400 and 4000 cm . Elemen-
tal analyses (C, H, N) were carried out by the Elementar,
Vario EL III. Mo atomic absorption spectroscopy (AAS)
was performed on an Atomic Absorption Spectrometer
Varian Spectra AA 110. Scanning electron microscopy
Scheme 1 and is described as follows.
Preparation of iron oxide magnetic nanoparticles
(Fe O )
−
1
3
4
Fe O was synthesized according to the literature with
3
4
a chemical co-precipitation method [9, 10, 31]. Briefly,
FeCl ·6H O (21.6 mmol, 5.83 g) and FeCl ·4H O
3
2
2
2
(
SEM) equipped with energy-dispersive X-ray spec-
(10.8 mmol, 2.147 g) were dissolved in 150 mL deion-
ized water under constant magnetic stirring and refluxed at
85 °C under N atmosphere. Then, 10–15 mL of NH ·H O
troscopy (EDX) was carried out with a KYKY-EM3200
model. Magnetic properties of the prepared materials were
measured using a homemade vibrating sample magnetom-
eter (Meghnatis Daghigh Kavir Company, Iran) at room
temperature from −10,000 to +10,000 Oe. The X-ray
powder patterns (XRD) were recorded with a Bruker, D8
ADVANCE (Germany) diffractometer (CuKa radiation).
2
3
2
(25% w/w) was added dropwise. After continuously stir-
ring for 2 h, the magnetite precipitates were washed by
deionized water and ethanol several times. Black precipi-
tates were collected with an external magnet and dried at
50 °C in vacuum for 12 h.
1
3

J IRAN CHEM SOC
O
S
Preparation of silica‑coated Fe O magnetic
3
4
nanoparticles (Fe O @SiO )
3
4
2
R
2
.
t
R1
Sulfoxide
r
l)
,
o
Fe O @SiO was synthesized according to the reported
m
ree
3
4
2
m
(
2
t- f
O
H
2
2
ven
procedures [32, 33]. In a typical procedure, 1 g of Fe O
was adequately dispersed in a mixture of ethanol (70 mL)
and deionized water (50 mL) for 60-min sonication at
3
4
Sol
S
Catalyst
H
R1
R2
2
O
2
(3
m
Sulfide
m
o
C
H
l)
,
3
3
C
5
o
room temperature. Then, NH OH (2.4 mL) was added to
N
4
C
O
O
the suspension at room temperature followed by dropwise
addition of tetraethyl orthosilicate (TEOS) (2 mL). After
S
R1
R
2
stirring at 50 °C for 2 h, the black precipitates (Fe O @
3
4
Sulfone
SiO ) were magnetically separated, washed with etha-
2
nol and deionized water for three times and dried in a
vacuum.
Scheme 2 General procedure for green oxidation of sulfides
Preparation of functionalized Fe O @SiO with amine
Preparation of the nanocatalyst (MoO 5CML@Fe O @
2 3 4
3
4
2
groups (Fe O @SiO –NH )
SiO2)
3
4
2
2
The modification of Fe O @SiO nanoparticles with
5CML@Fe O @SiO (0.5 g) was dispersed in ethanol
3
4
2
3
4
2
3
-aminopropyltriethoxysilane (APTES) was performed
in accordance with the literature in ethanol solution [34,
5]. Typically, Fe O @SiO (1 g) was dispersed in 50 mL
(50 mL), and a solution of MoO (acac) (1.5 mmol, 0.49 g)
in ethanol (20 mL) was added to this mixture and refluxed
2 2
3
under N atmosphere for 24 h. The prepared nanocatalyst
3
4
2
2
ethanol using an ultrasonic bath for 30 min. Next, APTES
2 mL) was added dropwise into the mixture under vigor-
was separated by magnetic decantation. To remove the
unreacted MoO (acac) , Soxhlet extraction was carried
(
2
2
ous stirring and was refluxed for 12 h under N flow. The
out with ethanol, and the resulting nanocatalyst was dried
under vacuum.
2
precipitate (Fe O @SiO –NH ) was collected by mag-
3
4
2
2
netic decantation and washed several times with ethanol to
remove the unreacted silylating agent and finally dried at
General procedure for oxidation of sulfides
6
0 °C for 6 h.
The oxidation of sulfides was performed in a round-bot-
tom flask containing a mixture of sulfide (1 mmol), H O
Preparation of Fe O @SiO –NH ‑tethered 5CML
3
4
2
2
2
2
(
5CML–Fe O @SiO )
(2 mmol) and catalyst (1 mol%, 17 mg based on AAS) at
room temperature (Scheme 2). Progress of the reaction
was monitored by TLC. After completion of the reaction,
the catalyst was separated by external magnet, washed with
water/ethanol and reused for subsequent recycling runs.
The products were extracted by CH Cl . Evaporation of
3
4
2
5
-Chloromethyl-2-hydroxybenzaldehyde was synthesized
with regard to the literature from salicylaldehyde by the
classical chloromethylation method [36]. The tridentate
Schiff base ligand was synthesized by addition of 5-chlo-
romethyl-2-hydroxybenzaldehyde (5 mmol, 0.85 g) in
CH Cl (5 mL) to 1-amino-2-propanol (5 mmol, 0.41 g)
2
2
CH Cl under vacuum gave corresponding sulfoxides or
2
2
sulfones.
2
2
in CH Cl (5 mL). The reaction mixture was stirred for
2
2
2
h at room temperature, giving yellow oil. The oil prod-
uct washed several times with CH Cl and ethyl acetate.
Results and discussion
Catalyst characterization
FTIR
2
2
The yellow oil was denoted as 5CML. Then, Fe O @
3
4
SiO –NH (1 g) was dispersed in 80 mL ethanol using an
2
2
ultrasonic bath for 30 min. 5CML ligand (5 mmol, 1.2 g)
was added to above mixture and stirring at room tempera-
ture for 12 h under N flow. The residual solid was filtered
2
and washed with CH Cl and ethanol and then dried in
The FTIR spectra of prepared materials are shown in
Fig. 1. In the FTIR spectrum of Fe O , an intense broad
2
2
vacuum.
FTIR of 5CML: 3362 (OH); 2974, 2892 (Aliph-H);
3
4
−1
peak at 577 cm is assigned as Fe–O–Fe band of Fe O
3
4
−
1
−1
1
646 (C=N); 1259 (CH –Cl); 764 (C–Cl) cm .
and a broadband around 3386 cm
is due to O–H
2
1
3

J IRAN CHEM SOC
Fig. 1 FTIR spectra for Fe O
(e)
(d)
3
4
(
a), Fe O @SiO (b), Fe O @
3 4 2 3 4
Aliphatic (C-H)
Aromatic (C-H)
945
SiO –NH (c), 5CML–Fe O @
1636
(C=N)
591
(Fe-O)
2
2
3
4
2
(MoO )
SiO (d) and MoO 5CML–
2
2
Si-O-Si
Fe O @SiO (e)
3
4
2
Aliphatic (C-H)
Aromatic (C-H)
3
412
1645
588
(
OH)
(C=N)
(Fe-O)
Si-O-Si
Si-O-Si
(c)
1629
2
366
5
73
-NH
2
(
-CH
2
)
(
Fe-O)
(b)
5
74
(
Fe-O)
1
091
(a)
Si-O-Si
3
368
(
OH)
5
77
(
Fe-O)
-1
Wavenumber (cm )
(
a)
stretching of adsorbed water (Fig. 1a). The FTIR spec-
trum of Fe O @SiO exhibits significant reduction of the
100
3
4
2
90
80
70
intensity of the Fe–O–Fe stretching and new peaks appear
−
1
at 1091 and 800 cm corresponding to Si–O–Si and Si–
OH bands, which could reveal the encapsulation of the
Fe O surface with the silica shell (Fig. 1b). The FTIR
(
b)
3
4
spectrum of Fe O @SiO –NH exhibits the peaks at 2924
3
4
2
2
60
−
1
(
c)
and 2366 cm , which is ascribed to the –CH stretching
2
5
0
0
vibration aminopropyl group. Moreover, a strong band at
−
629 cm corresponding to vibration mode of –NH indi-
2
1
1
4
cates that the aminopropyl group have successfully bonded
to the surface of Fe O @SiO (Fig. 1c).
0
200
400
Temperature, ºC
600
800
3
4
2
Spectrum of 5CML–Fe O @SiO (Fig. 1d) exhibits
3
4
2
−
1
new absorption peaks in the 1645 cm region, which are
Fig. 2 TGA analysis of Fe O (a), Fe O @SiO –NH (b) and
3 4 3 4 2 2
MoO 5CML–Fe O @SiO (c)
assigned to the stretching of C=N, and some weak bands at
2
3
4
2
−1
1
550–1450 cm assigned to stretching vibrations of aro-
matic rings of Schiff base ligand that were not present in
Fe O @SiO –NH spectrum, confirming successful immo-
The TGA curve of Fe O shows a small weight loss
3
4
2
2
3
4
bilization of ligand on Fe O @SiO –NH . In FTIR spec-
(~2%) below 100 °C, which corresponds to the evapora-
tion of the physically adsorbed water. As can be seen, there
is no significant weight loss in the range of 100–800 °C
(Fig. 2a). The TGA pattern of Fe O @SiO –NH shows
3
4
2
2
trum of MoO 5CML–Fe O @SiO (Fig. 1e), C=N stretch-
2
3
4
2
−
1
ing vibration (1645 cm ) is shifted toward the lower
−
1
frequency and appeared at 1634 cm , indicating coordi-
nation of C=N group of supported ligand with molybde-
num center [13, 14, 36]. Also, the appearance of peak at
3
4
2
2
three weight losses at temperatures ranging from 25 to
800 °C (Fig. 2b). The first region, which is below 150 °C
(~5%), is related to the evaporation of physically adsorbed
solvent or trapped water. The second weight loss observed
in the range of 250–450 °C (~12%) is due to decomposition
of aminopropyl moieties. The third weight loss at higher
temperature (above 700 °C) is resulted from the decom-
position of silica shell [37]. Based on TGA analysis, the
amount of loaded aminopropyl on the surface of Fe O @
− 2+
1
9
45 cm (stretching vibration of MoO2 ) is evidence of
the successful grafting of metal complex on the surface of
the magnetite [33].
TGA and loading results
The TGA curves of Fe O , Fe O @SiO –NH and
3
4
3
4
2
2
3
4
−
SiO is 1.98 mmol g .
2
1
MoO 5CML–Fe O @SiO are shown in Fig. 2.
2
3
4
2
1
3

J IRAN CHEM SOC
1
00
value. The molybdenum content of nanocatalyst was found
−1
to be 0.515 mmol g based on ICP–AES analysis. The
amount of loaded Mo was also measured by atomic absorp-
8
6
4
2
0
0
0
0
0
(a)
−
1
tion spectrophotometer (AAS) to be 0.497 mmol g ,
which is close to ICP–AES analysis.
(b)
(c)
(d)
VSM studies
The VSM curves of Fe O , Fe O @SiO and MoO 5CML–
3
4
3
4
2
2
Fe O @SiO are depicted in Fig. 3. As shown in Fig. 3, the
3
4
2
-
10000
-5000
0
5000
10000
magnetization curves exhibit no obvious remanence effect,
which clearly indicates the superparamagnetic nature of the
-
-
-
-
20
40
60
80
materials. The magnetization saturation values for Fe O ,
3
4
Fe O @SiO and MoO 5CML–Fe O @SiO are 80.1, 58.3
3
4
2
2
3
4
2
−1
and 36.4 emu g , respectively. Compared with Fe O ,
3
4
decrease in magnetization saturation values of Fe O @
3
4
SiO and MoO 5CML–Fe O @SiO is due to the coating
2
2
3
4
2
of SiO shell and grafting the Schiff base metal complex
2
-
100
on the surface of Fe O [35]. The catalyst could be easily
3
4
Applied field (Oe)
separated using an external magnet while in the absence of
external magnetic field it can be well dispersed by slightly
shaking, indicating that the synthesized catalyst possesses
good redispersibility and magnetic responsivity (Fig. 4).
Fig. 3 Magnetization curves obtained by VSM at room temperature
for Fe O (a), Fe O @SiO (b), MoO 5CML–Fe O @SiO (c) and
recycled catalyst after 10th run (d)
3
4
3
4
2
2
3
4
2
XRD studies
The TGA curve of MoO 5CML–Fe O @SiO is shown
2
3
4
2
The XRD pattern of Fe O (Fig. 5a) shows six character-
two weight losses (Fig. 2c). The first weight loss occurs
near 150 °C that is related to the loss of trapped solvent and
physically adsorbed water in complex. The second mass
loss occurs at temperature range of 330–544 °C (~28%)
that is corresponding to the thermal decomposition of
Schiff base complex, confirming the successful grafting of
complex onto the surface of Fe O @SiO –NH . The TGA
3
4
istic peaks at 2θ = 30.2, 35.5, 43.2, 53.6, 57.1 and 62.7°,
corresponding to (220), (311), (400), (422), (511) and
(
440) Bragg reflections, respectively, which agrees with the
standard in JCPDS card number (19-0629). The XRD pat-
tern of Fe O nanoparticles shows an inverse cubic spinel
3
4
structure without any impurity. The XRD patterns of Fe O
3
4
3
4
2
2
(
Fig. 5a) and Fe O @SiO (Fig. 5b) show a new broad peak
analysis indicates that the nanocatalyst is thermally stable
up to about 330 °C. The amount of loaded Schiff base com-
3 4 2
at 2θ = 23°–24° due to the existence of amorphous silica
38]. Moreover, the peak intensities in XRD pattern of the
nanocatalyst (Fig. 5c) slightly decreased in comparison
−
1
[
plex is 0.618 mmol g based on the weight losses, which
is in good agreement with the ICP–AES measurements
Fig. 4 Easy separation of cata-
lyst using an external magnet
from reaction mixture (a) and
redispersibility by slight shak-
ing (b)
1
3

J IRAN CHEM SOC
Fig. 5 XRD pattern of the
Fe O (a), Fe O @SiO (b) and
3
4
3
4
2
MoO 5CML–Fe O @SiO (c)
2
3
4
2
(c)
(b)
SiO
2
(
311)
(
220)
(440)
(
511)
(
400)
(
422)
(a)
Position [º2Theta]
Fig. 6 EDX analysis of
MoO 5CML–Fe O @SiO
2
3
4
2
with magnetic nanoparticles (Fig. 5a), which could be
assigned to the shielding effect of silica on the surface of
Fe O . It should be noted that the coating process did not
that the reaction between Schiff base complex and amine
groups on the surface of the Fe O nanoparticles occurred
successfully.
3
4
3
4
induce any phase change of Fe O during the preparation
3
4
procedure [39].
SEM images
EDX studies
The morphology, core–shell features and diameter of the
precursors and catalyst were examined by SEM (Fig. 7)
[14]. As shown in Fig. 7a, most of Fe O nanoparticles are
EDX analysis of MoO 5CML–Fe O @SiO nanocata-
2
3
4
2
3
4
lyst is depicted in Fig. 6, demonstrating the presence of
C, O, N, Mo, Fe and Si. The results obviously confirm
approximately spherical in morphology with a diameter
of 30–40 nm, which is in good agreement with calculated
1
3

J IRAN CHEM SOC
Fig. 7 SEM images of the Fe O (a), Fe O @SiO (b), MoO 5CML–Fe O @SiO (c) and recycled nanocatalyst after 10th run (d)
3
4
3
4
2
2
3
4
2
value by Debye–Scherrer equation (36.3 nm). After encap-
sulation of SiO through the sol–gel approach, Fe O @
end, we selected the oxidation of thioanisole as representa-
tive for initial studies in order to obtain the optimization of
various parameters including solvent, amounts of oxidant,
reaction time, amounts of catalyst and temperature.
The effect of different solvents on oxidation of thio-
anisole reaction is presented in Fig. 8a. Significant yield
and selectivity improvement were observed when solvent-
free conditions was employed, which might be due to the
blocking of active sites by solvent molecules. In general,
in solvents with higher dielectric constants such as meth-
anol, ethanol and acetonitrile, higher yield of the product
is obtained in compared with nonpolar solvents (dichlo-
romethane and toluene).
2
3
4
SiO exhibited a uniform diameter of 50–60 nm (Fig. 7b).
2
After anchoring Schiff base complex on the surface of
Fe O , the supported catalyst still exhibits the spherical
3
4
morphology with slightly larger particle size of 90–100 nm
Fig. 7c). It is obvious that coating process and covalent
(
grafting have not significantly influenced the spherical
morphology.
Catalytic performance
Oxidation of sulfides to sulfoxides
The amounts of H O as oxidant in the oxidation of thio-
2
2
anisole at room temperature were tested, and the highest
catalytic performance was observed using 2 mmol of H O
2
The catalytic activity of the MoO 5CML–Fe O @SiO for
2
3
4
2
2
the selective oxidation of sulfides in the presence of H O
(Fig. 8b).
2
2
as oxidant was investigated at room temperature, and the
results are given in Tables 1 and 2. Selective oxidation of
sulfur-containing organic molecules, such as sulfides, is
one of the most important organic transformations. To this
The effect of catalyst concentrations on the reaction
is shown in Fig. 8c. As it is clear in this Fig., 1 mol% of
catalyst was chosen as optimum with respect to reaction
time and selectivity. Increasing the catalyst loading above
1
3

J IRAN CHEM SOC
Table 1 Oxidation of different
O
S
O
S
sulfides by MoO 5CML–
2
C a t. (1 m o l% )
(2 m m o l), R .T
Fe O @SiO under solvent-free
S
3
4
2
R 1
R
2
R 1
R
2
H
2
O
2
R 1
R 2
condition
O
S u lfid e
S u lfo x id e (A )
S u lfo n e (B )
a
a
Entry
1
Sulfide
Time (h)
A (%)
B (%)
Conversion
%)
(
3:15
100
100
100
0
S
S
2
3
4
3:30
3:45
100
0
S
100
100
96
91
4
9
4
:30
S
5
6
2:45
100
100
99
1
0
S
S
b
3
:30
:45
100
Br
b
S
3
100
100
100
100
0
7
8
2
O N
b, c
S
1
0:15
83
17
13
S
b, c
9
9:30
87
HO
OH
Reaction conditions: sulfide (1 mmol), H O (2 mmol), catalyst (1 mol%), under solvent-free conditions,
2
2
room temperature
a
Determined by GC yield
b
Determined by isolated yield
c
For this case, solvent is ethanol
1
mol% decreases the reaction time and selectivity, and
oxone (TBAO) were employed in the oxidation reaction,
and the results are presented in Fig. 8d. As it is obvious
in this figure, H O as eco-friendly oxidant exhibits high-
est selectivity and reaction conversion compared with other
examined oxidants.
mixture of sulfoxide and sulfone is obtained.
To investigate the effect of the oxidizing agent, different
oxidants such as H O , urea hydrogen peroxide (UHP), tert-
2
2
2
2
butyl hydroperoxide (TBHP) and tetra-n-butylammonium
1
3

J IRAN CHEM SOC
Table 2 Oxidation of different
sulfides to sulfones by
O
S
C a t. (2 m o l% )
MoO 5CML–Fe O @SiO
S
2
3
4
2
H 2 O 2 (3 m m o l), 3 5 o
C
R 1
R
2
R 1
R
2
O
S u lfid e
S u lfo n e
Entry
Sulfide
Time (h)
1:30
Conversion
%)
Selectivity to
sulfone (%)
a
(
S
1
100
100
S
S
2
3
2:15
2:30
100
100
100
100
100
95
3:15
S
4
5
1:20
100
100
S
S
b
6
7
2:30
3:20
100
100
98
Br
S
b
99
2
O N
S
b
1
00
100
8
9
8:00
7:20
S
b, c
100
100
HO
OH
Reaction conditions: sulfide (1 mmol), H O (3 mmol), catalyst (2 mol%), CH CN (1 mL), 35 °C
2
2
3
a
Determined by GC yield
b
Determined by isolated yield
In the next step, the effect of temperature on the activ-
ity of the catalyst was studied (Fig. 8e). Room temperature
was selected as optimized temperature with respect to reac-
tion conversion and selectivity. Increasing the temperature
to 40 and 60 °C, the selectivity between sulfoxides and sul-
fones is decreased.
To investigate the effects of catalytic active centers,
background reactions were performed and the results are
shown in Fig. 9. The oxidation reaction did not proceed
using Fe O , Fe O @SiO , and Fe O @SiO –NH as cat-
3
4
3
4
2
3
4
2
2
alyst while if MoO 5CML was used as catalyst oxidation
2
reaction proceeded only in 55% conversion. Furthermore in
1
3

J IRAN CHEM SOC
Fig. 8 Optimization of the solvent nature (a), H O (b), amounts of catalyst (c), different oxidant (d) and temperature (e) in the oxidation of
2
2
thioanisole to the corresponding sulfoxide
the presence of MoO 5CML–Fe O @SiO , the oxidation
H O : 2 mmol, reaction time: 3:15 h at room temperature),
2 2
2
3
4
2
reaction occurred smoothly in 100% conversion with excel-
lent selectivity. These results clearly show the role of Mo
center in the oxidation reaction.
In order to find the general applicability of this catalytic
system, after optimization of different parameters in oxida-
tion of thioanisole (substrate: 1 mmol, catalyst: 1 mol%,
the catalytic activity of nanocatalyst was examined in the
oxidation of various aromatic and aliphatic sulfides under
the optimized reaction conditions, and the results are sum-
marized in Table 1.
As it is clear in this table, phenylalkyl sulfides were
oxidized selectively to their corresponding sulfoxides in
1
3

J IRAN CHEM SOC
100
100
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
8
7
5
5
0
5
Conversion (%)
Selec vity (%)
40
2
1
-
5
0
5 0
30
60
90
120
150
180
210
240
Time (h)
Fig. 9 Background reaction in the oxidation of thioanisole
Fig. 10 Hot filtration test for study of catalytic performance
excellent yields (Table 1, entries 1–4). Similarly, arylme-
thyl sulfides were oxidized smoothly to their corresponding
sulfoxides (Table 1, entries 6 and 7).
1
00
90
80
70
Diaryl sulfides are shown lower reactivity in oxidation
to their sulfides under optimal reaction conditions with
slightly lower selectivity (Table 1, entries 8 and 9). This
could be due to the steric hindrance of the diaryl groups.
Dibutyl sulfide as a representative for dialkyl sulfides dis-
played excellent yield and selectivity to dibutyl sulfoxide
6
50
4
3
2
10
0
0
Conversion (%)
Selecꢀvity (%)
0
0
0
10
9
8
7
6
5
4
3
(
Table 1, entry 5).
2
1
In order to study the reaction mechanism, a catalytic run
with optimized condition was performed in the presence
of 2,6-di-tert-butyl-p-cresol (1 mmol) as a radical scaven-
ger. The results showed that the conversion was reduced
from 100 to 39% after 3:15 h (Table S1, entries 1 and 2 in
supporting information) and reduced to almost 48% after
Fig. 11 Reusability of catalyst for study of catalytic performance
as catalyst; 2 mol%, H O as oxidant; 3 mmol, reaction
2
2
time; 1:30 h, reaction temperature; 35 °C, CH CN as sol-
3
1
2:15 h (Table S1, entries 3–5). These results confirm that
a radical pathway is more likely in the oxidation process
12]. The oxidation reaction was also carried out in the
vent; 1 mL (Fig. S2(a–c)). The scope of the reaction was
evaluated by employing different aromatic and aliphatic
sulfides under the optimized reaction conditions, and the
results are listed in Table 2. As it is clear in Table 2, various
aromatic and aliphatic sulfides were efficiently oxidized to
corresponding sulfones with good to excellent selectivity.
It is interesting to mention that the presented catalytic sys-
tem could selectively oxidize sulfides to their correspond-
ing sulfoxides (Table 1) or sulfone (Table 2) under different
reaction conditions.
[
presence of imidazole (Table S1, entry 6), and conversa-
tion was reduced from 100 to 32%. It is probably due to the
competition between oxidant and imidazole or the binding
of imidazole as ligand to molybdenum metal center mak-
ing the nanocatalyst inactive. The presence of unoccupied
coordination sites on the molybdenum metal center of the
catalyst is vital for its catalytic performance.
Oxidation of sulfides to sulfones
Hot filtration test
Oxidation of thioanisole was chosen as a model reaction
to investigate the effect of solvent, oxidant, catalyst load-
ing and temperature on the reaction yield and selectiv-
ity. The results are summarized in Fig. S2 in supporting
information. As it can be seen in this figure, optimization
of reaction conditions for selectivity oxidation of thioani-
sole to the corresponding sulfone led to optimal condi-
tions as: thioanisole; 1 mmol, MoO 5CML–Fe O @SiO
To verify the nature of heterogeneity or homogeneity of
the catalyst, hot filtration test was carried out in the oxi-
dation of thioanisole to its sulfoxide under optimal reac-
tion conditions, and the results are presented in Fig. 10.
Typically, after 90 min, the nanocatalyst was separated
by external magnet and the filtrate was allowed to stir for
additional 4 h and the reaction conversion was monitored
by GC. The reaction conversion was 53% after 90 min and
2
3
4
2
1
3

J IRAN CHEM SOC
Table 3 Comparison of MoO 5CML–Fe O @SiO with various heterogeneous catalysts for selective oxidation of sulfides
2
3
4
2
Entry Catalyst (amount)
Oxidant
Solvent
Temperature Time
Conversion
%)
Selectivity to
sulfoxide (%)
Refs.
(
1
2
3
4
5
6
7
8
9
Mo complexes sup-
ported on silica (II)
H O (2 mmol)
CH CN/MeOH
(1:1)
−10 °C
12 h
1.5 h
3 h
100
99
92
[40]
2
2
3
Silica-based tungstate H O (3 mmol)
CH Cl /MeOH
82
92
99
95
94
–
[41]
2
2
2
2
(
2 mol%)
Fe(salen) complex
2 mol%)
Na WO , C H PO H , H O (10 mmol)
(1:1)
H O (1.5 mmol)
H O
20 °C
35 °C
r.t
100
100
99
[28]
2
2
2
(
Solvent-free
18 h
6 h
[42]
2
4
6
5
3
2
2
2
PTC (0.01 mmol)
Fe O @SiO @
UHP
CH Cl /MeOH
[12]
3
4
2
2
2
VO(salen) (10 mg)
(1:1)
Au/CTN-silica
H O (3 mL)
MeOH
60 °C
60 °C
45 °C
r.t
2 h
100
83
[43]
2
2
(20 mg)
Fe O /salen of Cu(II) H O (0.5 mL)
EtOH
3 h
[16]
3
4
2
2
(50 mg)
Co@SiO2@[Mn(II)
SBC] (40 mg)
H O (1.17 mmol) Ethyl acetate
40 min 100
3:15 h 100
92
100
[44]
2
2
MoO 5CML–
H O (2 mmol)
Solvent-free
This work
2
2
2
Fe O @SiO
3
4
2
(1 mol% = 17 mg)
5
9% after additional 4 h. This observation revealed that the
used nanocatalyst after ten cycles showed superparamag-
netic behavior and magnetization saturation value is about
leaching of the catalyst into the reaction mixture should be
negligible, and the catalytic system in this reaction is truly
heterogeneous.
−1
33 emu g (Fig. 3d), which shows that there is no consid-
erable change in its magnetic property. These observations
reveal that MoO 5CML–Fe O @SiO can be used as an
2
3
4
2
Reusability of the catalyst
efficient recoverable and recyclable nanomagnetic catalyst
in the selective oxidation of sulfides to their corresponding
sulfoxides and sulfones.
The stability and reusability of the catalyst are of great
importance, especially for green chemistry and commer-
cial applications. Hence, the reusability of the catalyst
was investigated under optimal conditions for oxidation of
thioanisole to the corresponding sulfoxide. After comple-
tion of the reaction, the nanocatalyst was easily separated
from the reaction mixture by an external magnet, washed
with ethanol/CH Cl 3 × 10 mL, dried in vacuum and
The amount of molybdenum leaching was also investi-
gated by ICP–AES analysis. The molybdenum leaching in
the first and 10th runs was measured to be 0.5 and 3.3%,
respectively (Table S2). It can be concluded from these
results that molybdenum Schiff base complex is strongly
bonded to the surface of Fe O @SiO –NH nanoparticles
3
4
2
2
and nanocatalyst is stable in the catalytic process because
negligible metal leaching is happened.
2
2
reused directly in the subsequent runs (Fig. 11). The results
depicted in Fig. 11 indicate that the nanocatalyst could be
reused for at least 10 cycles without losing its catalytic
activity and selectivity significantly.
In order to evaluate the performance and efficiency of
our method, the catalytic activity of MoO 5CML–Fe O @
2
3
4
SiO for selective oxidation of sulfides was compared with
2
The structure and morphology of the recycled nano-
catalyst after ten cycles were examined by FTIR, SEM and
VSM. FTIR spectrum of the recycled nanocatalyst after
the several earlier reported heterogeneous catalysts in the
literature (Table 3, entries 1–9). It is obvious that presented
procedure in this work shows higher catalytic activity in
terms of reaction time, reaction conversion, selectivity
and temperature compared with other reported methods
(Table 3, entries 1–8 and entry 9). The attractive features of
the catalytic system in this method, such as stability, non-
toxicity, low cost and simple recyclability and easy sepa-
ration by external magnet, make it particularly suitable for
oxidation reactions.
1
0th run was similar to that of fresh catalyst and showed
expected Fe–O, Si–O–Fe, Mo–O, C=N, aromatic C–H
and aliphatic C–H vibration bands (Fig. S1). SEM images
of the recycled catalyst after ten runs were very similar to
those of fresh catalyst Fig. 7c, d, confirming that the mor-
phology and structure of the nanocatalyst have been main-
tained during the recycling reactions. Furthermore, the
1
3

J IRAN CHEM SOC
1
2. M. Bagherzadeh, M.M. Haghdoost, A. Shahbazirad, J. Coord.
Chem. 65, 591 (2012)
Conclusion
1
3. M. Esmaeilpour, A.R. Sardarian, J. Javidi, Appl. Catal. A Gen.
In conclusion, we have developed a molybdenum Schiff
base complex supported on Fe O modified with silica as
4
45–446, 359 (2012)
14. R.K. Sharma, Y. Monga, A. Puri, G. Gaba, Green Chem. 15,
800 (2013)
1
1
3
4
2
novel, efficient, recoverable and reusable nanocatalyst for
the selective oxidation of various sulfides to their corre-
sponding sulfides or sulfones under mild and green condi-
tions. Oxidation of sulfides to sulfoxides was performed
under solvent-free conditions at room temperature. How-
ever, oxidation of sulfides to sulfones was performed in
5. L. Chen, B. Li, D. Liu, Catal. Lett. 144, 1053 (2014)
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3
20. P. Zhang, Y. Wang, H. Li, M. Antonietti, Green Chem. 14, 1904
Both sulfoxides and sulfones could be achieved with excel-
lent selectivity by changing conditions in our catalytic sys-
tem. The fresh and recycled catalyst after 10th cycles was
characterized by different spectroscopic and microscopic
techniques. The morphology and structure of catalyst
were maintained after 10 recycles without any leaching of
molybdenum confirming the stability of the nanocatalyst.
The notable advantages of this work are use of green, cheap
and nontoxic materials, good stability of nanocatalyst, high
selectivity and facile reusability.
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