Fe(III)-salen complex supported on dendrimer functionalized magnetite nanoparticles as a highly active and selective catalyst for the green oxidation of sulfides

Article abstract of DOI:10.1016/j.jpcs.2020.109642

In this study, we reported the preparation of polyamidoamine-modified magnetite nanoparticles and their use for the immobilization of Fe(III)-salen complex to form a novel magnetic catalyst. The prepared catalyst was characterized by some modern techniques i.e. Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), N₂ adsorption-desorption analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometry (VSM), and inductively couple plasma atomic emission spectroscopy (ICP-AES). The catalyst showed excellent activity and selectivity for the oxidation of sulfides to sulfoxides (conversion 87–100percent, selectivity 82–100percent) using H₂O₂ (30percent w/w) as oxidant in aqueous medium at 50 °C. Furthermore, the catalyst could be recovered in a facile manner from the reaction mixture by using a magnet and reused for five cycles with high catalytic stability.

Full text of DOI:10.1016/j.jpcs.2020.109642

Journal Pre-proof
Fe(III)-salen complex supported on dendrimer functionalized magnetite nanoparticles
as a highly active and selective catalyst for the green oxidation of sulfides
Mahsa Niakan, Zahra Asadi, Majid Masteri-Farahani
PII:
S0022-3697(20)31049-0
https://doi.org/10.1016/j.jpcs.2020.109642
PCS 109642
DOI:
Reference:
To appear in: Journal of Physics and Chemistry of Solids
Received Date: 19 April 2020
Revised Date: 7 June 2020
Accepted Date: 27 June 2020
Please cite this article as: M. Niakan, Z. Asadi, M. Masteri-Farahani, Fe(III)-salen complex supported on
dendrimer functionalized magnetite nanoparticles as a highly active and selective catalyst for the green
oxidation of sulfides, Journal of Physics and Chemistry of Solids (2020), doi: https://doi.org/10.1016/
j.jpcs.2020.109642.
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Fe(III)-salen complex supported on dendrimer functionalized magnetite
nanoparticles as a highly active and selective catalyst for the green
oxidation of sulfides
Mahsa Niakana, Zahra Asadia*, Majid Masteri-Farahanib
aDepartment of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran
bFaculty of Chemistry, Kharazmi University, Tehran, Iran
E-mail: zasadi@shirazu.ac.ir
Abstract
In this study, we reported the preparation of polyamidoamine-modified magnetite
nanoparticles and their use for the immobilization of Fe(III)-salen complex to form a novel
magnetic catalyst. The prepared catalyst was characterized by some modern techniques i.e.
Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), N2
adsorption-desorption analysis, X-ray diffraction (XRD), scanning electron microscopy
(SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy
(EDX), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometry (VSM),
and inductively couple plasma atomic emission spectroscopy (ICP-AES). The catalyst
showed excellent activity and selectivity for the oxidation of sulfides to sulfoxides
(conversion 87-100%, selectivity 82-100%) using H2O2 (30% w/w) as oxidant in aqueous
medium at 50 °C. Furthermore, the catalyst could be recovered in a facile manner from the
reaction mixture by using a magnet and reused for five cycles with high catalytic stability.
Keywords: Magnetite nanoparticles; Dendrimer; Immobilization; Fe(III)-salen complex;
Oxidation; Sulfide.
1

Introduction
Selective oxidation of sulfides to their corresponding sulfoxides is a transformation of
great interest as the sulfoxide products are important intermediates for the constructing of a
variety of chemically and biologically significant molecules such as drugs [1-5]. The most
popular and promising route for this transformation is to use transition metal-based catalysts.
In this context, manganese, titanium, vanadium, molybdenum, and iron complexes have been
applied. Among them, iron-based catalysts have attracted great attentions owing to the high
abundance, long-term availability, low cost, and low toxicity of iron [6-8]. Despite the
considerable achievements that have been made, these homogeneous catalytic systems are
unfortunately associated with many troublesome obstacles in practical applications, in terms
of catalyst separation and recycling. To solve these issues, the homogeneous catalysts have
been immobilized on various solid supports which improve their recovery and recycling
properties [9-13]. However, the catalytic activity and selectivity of most of these supported
catalysts decreases upon immobilization because of the low active sites availability [14,15].
Thus, it is highly desirable and necessary to develop new iron based catalytic systems that
include the advantages of both homogeneous and heterogeneous catalysts.
Dendrimers belong to a well-recognized class of precise macromolecules with highly
branched structure and globular shape that have been applied in several fields, one of the
most important being the catalysis [16-18]. Their advantages in catalysis are unique, such as
high dispersion, high loading level, high stability, good solubility, and controlled particle
sizes and morphologies of the catalyst [19-21]. However, the industrial application of
dendrimer-based catalysts has been restricted mainly because of the difficulty of their
separation and recycling. Therefore, there is a widespread interest in the recent years in
design and fabrication of these catalysts grown on solid supports to solve these problems.
[22,23].
2

Different types of supporting materials, such as carbon, mesoporous silica, amorphous
silica, magnetic nanoparticles, and titania have been used for the immobilization of
dendrimer-based catalysts [24-28]. In particular, magnetic nanoparticles have received the
considerable attention of researches due to their unique physical properties including high
surface to volume ratio, low toxicity, and excellent thermal and chemical stabilities [29-32].
Most importantly, the magnetic nanoparticles-supported catalysts could be easily separated
from the reaction medium by applying an external magnet without the necessity for filtration
or centrifugation [33,34].
Until now, there are some reports about the preparation of dendrimer-supported
transition metals using different solid supports and their applications as catalysts in various
oxidation reactions. For example, Moghadam and his group have reported silica dendrimer-
supported Mn, Ru, and Au catalysts in the oxidation of olefins and alcohols [35-37]. Also,
other silica dendrimer-supported Mn catalysts for the oxidation of olefins have been reported
[38,39]. Another research group, Hosseini-Monfared and co-workers, have developed
dendrimer functionalized magnetic nanoparticles to immobilize a Mn-tetraamide macrocycle
complex in order to prepare a heterogeneous catalyst for the oxidation of olefins [40]. In
2016, Vieira’s group reported the synthesis of two Mo complexes immobilized on dendrimer
functionalized silica, which was applied as catalysts for the oxidation of olefins [41]. All of
these catalysts exhibited excellent activity and recyclability in the oxidation reactions.
So, with the background of the above reports and considering the significant benefits of
dendrimer functionalized magnetic nanoparticles as solid support and iron complexes as
active catalysts, we reported on the utilization of the dendrimer functionalized magnetite
nanoparticles for immobilizing Fe(III)-salen complex to prepare a new heterogeneous
catalyst. The synthesized catalyst was shown to be a high-performance and reusable catalyst
for the selective oxidation of sulfides to sulfoxides using H2O2 (30% w/w) as oxidant in
3

aqueous medium at 50 °C. To the best of our knowledge, there was no report about the
immobilization of iron complexes on the dendrimer functionalized magnetite nanoparticles
for catalytic application purpose.
Experimental
The details of materials and methods section were reported in the supporting
information.
Preparation of G0-Fe3O4-SiO2
Silica-coated magnetic nanoparticles (Fe3O4-SiO2) were synthesized according to a
reported method [42]. To a homogeneous dispersed Fe3O4-SiO2 (1 g) in 30 mL dry toluene, 2
mL of 3-aminopropyltriethoxysilane was added. Then the mixture was refluxed for 24 h
under nitrogen atmosphere. The obtained product, nominated as G0-Fe3O4-SiO2 was
magnetically separated, washed thoroughly with ethanol, and dried at 80 °C overnight.
Preparation of dendrimer functionalized magnetic nanoparticles
1 g of the above synthesized G0-Fe3O4-SiO2 was dispersed in 20 mL methanol under
ultrasonication for 0.5 h. To this suspension, 0.42 g (5 mmol) of methyl acrylate was added
and the mixture was stirred at room temperature for 24 h. The resulting product (G0.5-
Fe3O4-SiO2) was isolated by an external magnet, washed with methanol to effectively remove
the unreacted reagents and then dried at 80 °C overnight. 1 g of the as-synthesized G0.5-
Fe3O4-SiO2 was suspended in 20 mL methanol, and after sonication for 0.5 h, 1.03 g of
diethylenetriamine (10 mmol) was added and refluxed for 6 h under nitrogen atmosphere.
After cooling to room temperature, the solid was separated using an external magnet, washed
with methanol, and dried at 80 °C overnight to give G1-Fe3O4-SiO2. G2-Fe3O4-SiO2 was
synthesized by repeating these two steps, the amount of reactants being adjusted as required.
4

Preparation of Fe3O4-SiO2-dendrimer-Fe catalyst
2 mmol of salicylaldehyde was added to a suspension of 1 g of G2-Fe3O4-SiO2 in 20 mL
absolute ethanol and the reaction mixture was then refluxed for 24 h under nitrogen
atmosphere. The resulting Schiff base immobilized on G2-Fe3O4-SiO2 was separated by
applying an external magnet, washed several times with ethanol and dichloromethane, and
dried at 80 °C overnight. Complexation of Schiff base with iron was done by stirring 2 mmol
of FeCl3.6H2O and 1 g of the synthesized Schiff base immobilized on G2-Fe3O4-SiO2 in 20
mL methanol at room temperature over a period of 24 h. The obtained solid was magnetically
separated, washed with methanol, and dried at 80 °C overnight. The resulting material was
denoted as Fe3O4-SiO2-dendrimer-Fe catalyst.
General procedure for the oxidation of sulfides to sulfoxides
A 10 mL round-bottom flask equipped with a magnetic stirrer was charged with sulfide
(1 mmol), H2O2 (30% w/w, 2 mmol), Fe3O4-SiO2-dendrimer-Fe catalyst (0.1 g, 2 mol%), and
water (3 mL). The mixture was stirred at 50 °C for an appropriate time. After completing the
reaction, the product was extracted from the reaction mixture with chloroform, evaporated on
the rotary evaporator at 40-50 °C and then analyzed by using gas chromatography (GC).
During the reusability assessment, the catalyst was separated from the reaction mixture with
an external magnet, washed several times with ethanol, dried in vacuum at 80 °C for 5 h, and
then used for the next reaction.
Results and discussion
Synthesis and characterization of the catalyst
Dendrimer functionalized magnetite nanoparticles were prepared (Scheme 1) and used as
support to anchor Fe(III)-salen complex to form Fe3O4-SiO2-dendrimer-Fe catalyst, as seen in
5

Scheme 2. The amount of the metal content in the catalyst was measured by inductively
couple plasma atomic emission spectroscopy (ICP-AES) analysis. The weight percentage of
Fe was determined approximately to be 0.2 mmol/g by calculating the difference between the
amount of Fe in G2-Fe3O4-SiO2 and Fe3O4-SiO2-dendrimer-Fe catalyst.
O
O
O
EtO
EtO Si
EtO
NH2
O
O
O
O
Si
NH2
O
Si
N
O
O
O
MeOH
Toluene
G0.5- Fe3O4-SiO2
Fe3O4-SiO2
G0-Fe3O4-SiO2
O
H
N
MeOH
NH2
H2N
O
O
H2N
O
O
N
O
O
O
N
O
O
N
N
O
H2N
O
N
NH2
O
N
O
O
O
O
O
O
O
O
Si
N
N
O
MeOH
O
Si
N
O
NH2
O
N
O
G1-Fe3O4-SiO2
G1.5-Fe3O4-SiO2
O
O
O
H
N
NH2
H2N
MeOH
H2N
NH2
N
N
O
H2N
H2N
N
NH2
O
O
N
NH2
N
O
H2N
H2N
N
O
NH2
N
NH2
N
N
N
H2N
O
O
NH2
NH2
O
O
Si
N
N
O
O
N
N
N
G2-Fe3O4-SiO2
H2N
O
NH2
O
H2N
Scheme 1. Preparation of dendrimer functionalized magnetite nanoparticles.
6

NH2
NH2
N
N
O
Fe Cl
O
O
HO
G2
G2
FeCl3
G2-Fe3O4-SiO2
Fe3O4-SiO2-dendrimer-Fe
Scheme 2. Preparation of Fe3O4-SiO2-dendrimer-Fe catalyst
.
FT-IR spectra of the products formed in each step of the synthesis of Fe3O4-SiO2-
dendrimer-Fe catalyst are shown in Figure 1. In all of the seven spectra, the bands around 800
and 1080 cm-1 were attributed to the asymmetric and symmetric stretching vibration of Si-O-
Si bond, and the band at about 590 cm-1 was assigned to the Fe-O-Fe stretching vibration.
Moreover, apart from the case of Fe3O4-SiO2, the C-H stretching vibrations of CH2 groups at
about 2800-3000 cm-1 were visible in all samples. The FT-IR spectra of G0.5-Fe3O4-SiO2 and
G1.5-Fe3O4-SiO2 exhibited a band around 1735 cm-1 which could be attributed to the
stretching vibrations of the ester groups and was disappeared in the case of G1-Fe3O4-SiO2
and G2-Fe3O4-SiO2 following the amidation reaction [43]. This clearly confirmed the growth
of the dendrimer on the surface of Fe3O4-SiO2. Importantly, the FT-IR spectrum of Fe3O4-
SiO2-dendrimer-Fe catalyst exhibited no obvious change compared to G2-Fe3O4-SiO2,
demonstrating that the structure of Fe3O4-SiO2 and dendrimer was intact after the
immobilization of Fe(III)-salen complex.
The growth of the dendrimer on the surface of Fe3O4-SiO2 was also confirmed by
thermogravimetric analysis (TGA). The TGA curves in Figure 2 revealed that the organic
moiety loaded on the surface of Fe3O4-SiO2 increased with each generation of growth.
7

Figure 1. FT-IR spectra of (a) Fe3O4-SiO2, (b) G0-Fe3O4-SiO2, (c) G0.5-Fe3O4-SiO2, (d) G1-Fe3O4-SiO2, (e)
G1.5-Fe3O4-SiO2, (f) G2-Fe3O4-SiO2, (g) Fe3O4-SiO2-dendrimer-Fe catalyst.
Figure 2. TGA curves of (a) G0-Fe3O4-SiO2, (b) G1-Fe3O4-SiO2, (c) G2-Fe3O4-SiO2.
Figure 3 exhibits the nitrogen adsorption-desorption isotherms for (a) G0-Fe3O4-SiO2,
(b) G1-Fe3O4-SiO2, (c) G2-Fe3O4-SiO2, and (d) Fe3O4-SiO2-dendrimer-Fe catalyst. All four
samples displayed typical type II isotherms. As the dendrimer generation increased, the
surface area systematically decreased from 68 in G0-Fe3O4-SiO2 to 63 and 58 m2 g-1 in G1-
Fe3O4-SiO2 and G2-Fe3O4-SiO2, respectively. The surface area further decreased to 54 m2 g-1
8

for Fe3O4-SiO2-dendrimer-Fe catalyst. Such decreases may be due to the more loading of
organic species on the surface of nanoparticles, indicating the successful immobilization of
the dendrimer and Fe(III)-salen complex on the surface of Fe3O4-SiO2.
Figure 3. N2 adsorption-desorption isotherms of (a) G0-Fe3O4-SiO2, (b) G1-Fe3O4-SiO2, (c) G2-Fe3O4-SiO2, (d)
Fe3O4-SiO2-Dendrimer-Fe catalyst.
Energy dispersive X-ray spectroscopy (EDX) analysis was employed to check the
elements presented in the prepared Fe3O4-SiO2-dendrimer-Fe catalyst. The EDX spectrum of
the catalyst (Figure 4) consisted of different peaks for C, N, O, Fe, Si, and Cl confirming the
successful formation of Fe3O4-SiO2-dendrimer-Fe catalyst. Furthermore, the elemental
mapping of the catalyst demonstrated that these elements were distributed homogeneously in
the texture of the catalyst (Figure 5).
9

Figure 4. EDX spectrum of Fe3O4-SiO2-dendrimer-Fe catalyst.
Figure 5. Elemental mapping of C, N, O, Fe, Si, and Cl.
Additionally, X-ray photoelectron spectroscopy (XPS) analysis on Fe3O4-SiO2-
dendrimer-Fe catalyst exhibited the characteristic peaks of C 1s, N 1s, O 1s, Cl 2p, Fe 2P, Si
2s, and Si 2p, as shown by the survey spectrum in Figure 6a. The high resolution XPS
spectrum of Fe 2p in Figure 6b revealed four peaks, among which the peaks with binding
10

energies of 710.1 (Fe, 2p3/2) and 723.7 eV (Fe, 2p1/2) could be indexed to the Fe (II) oxidation
state, whereas the peaks at 712.4 (Fe, 2p3/2) and 725.3 eV (Fe, 2p1/2) were assigned to the Fe
(III) oxidation state [44]. The results showed that the Fe ions had +3 and +2 oxidation states
in Fe3O4-SiO2-dendrimer-Fe catalyst.
Figure 6. (a) XPS survey spectrum of Fe3O4-SiO2-dendrimer-Fe catalyst and (b) corresponding high resolution
spectrum of Fe 2p.
The crystalline structures of Fe3O4-SiO2 and Fe3O4-SiO2-dendrimer-Fe catalyst were
disclosed by X-ray diffraction (XRD) patterns as presented in Figure 7. The diffraction peaks
for Fe3O4 at 30° (220), 35° (311), 43° (400), 53° (422), 56° (511), and 62° (440) were
indexed to the face-centered cubic structure of Fe3O4 (JCPDS Card no. 19-0629). Also, no
11

remarkable variation in the pattern of Fe3O4-SiO2-dendrimer-Fe catalyst suggested that the
crystalline structure of Fe3O4 did not destroyed during the synthesis of the catalyst.
Figure 7. XRD patterns of (a) Fe3O4-SiO2 and (b) Fe3O4-SiO2-dendrimer-Fe catalyst.
The structures and surface morphologies of Fe3O4-SiO2 and Fe3O4-SiO2-dendrimer-Fe
catalyst were investigated by scanning electron microscopy (SEM) and transmission electron
microscopy (TEM). The SEM images exhibited that both samples presented uniform
spherical-shape morphology (Figures 8a and 8b). The TEM image of Fe3O4-SiO2 shown in
Figure 8c further confirmed that Fe3O4-SiO2 consisted of spherical nanoparticles and a core-
shell structure, in which the condensation of SiO2 on the surfaces of Fe3O4 cores results in
microspheres with dark-colored Fe3O4 cores and gray-colored SiO2 shells. The magnetite
nanoparticles kept their core-shell structure after the immobilization of the dendrimer and
Fe(III)- salen complex (Figure 8d).
12

Figure 8. SEM images of (a) Fe3O4-SiO2 and (b) Fe3O4-SiO2-dendrimer-Fe catalyst and TEM images of (c)
Fe3O4-SiO2 and (d) Fe3O4-SiO2-dendrimer-Fe catalyst.
Magnetization curves of Fe3O4-SiO2 and Fe3O4-SiO2-dendrimer-Fe catalyst at room
temperature are given in Figure 9. As seen, no remanence or coercivity was detected in these
curves, suggesting their superparamagnetic character. The saturation magnetization of Fe3O4-
SiO2-dendrimer-Fe catalyst was 32 emu g-1, smaller than that of Fe3O4-SiO2 (47 emu g-1)
owing to the decrease in Fe3O4 content of the catalyst. Even with this reduction in the
saturation magnetization, Fe3O4-SiO2-dendrimer-Fe catalyst could be rapidly separated from
the mixture within 10 s by applying an external magnet (inset of Figure 9).
13

Figure 9. Room temperature magnetization of (a) Fe3O4-SiO2 and (b) Fe3O4-SiO2-dendrimer-Fe catalyst.
Catalytic performance of the catalyst in the selective oxidation of sulfides
After the detailed characterization of Fe3O4-SiO2-dendrimer-Fe catalyst, its catalytic
activity was checked in the oxidation of sulfides. Methyl phenyl sulfide was chosen as a
standard benchmark reactant to investigate the influence of various parameters on the
reaction and the results are shown in Table 1. The influence of the catalyst amount was
initially explored on the model reaction using H2O2 as oxidant at 50 °C in water. Different
amounts of catalyst between 1-4 mol% were examined for the reaction (Table 1, entries 1-3),
and 2 mol % of the catalyst was found to be the most suitable amount. Note that less than
30% conversions were achieved when the reactions were carried out without any catalyst and
in the presence of Fe3O4-SiO2 and G2-Fe3O4-SiO2 (Table 1, entries 4-6). The results
confirmed the poor catalytic role of these materials in this oxidation process. Next, the
influence of H2O2 to sulfide molar ratio on the model oxidation reaction was investigated
(Table 1, 7-9) which 2:1 molar ratio was found to be the best choice. The influence of
reaction temperature on the product yield was also studied and reactions were conducted at
oil bath temperatures of 30, 50, and 70 °C. The best result was obtained at 50 °C, while
14

decreasing the temperature to 30 °C led to a significant decrease in the product yield (Table
1, entry 10). On the other hand, further increasing the temperature to 70 °C did not lead to
significant improvements (Table 1, entry 11). Under these conditions, the effect of solvent on
the model oxidation reaction was monitored. It was observed that the reaction proceeded well
in different solvents such as toluene, ethanol, acetonitrile, dichloromethane, and water, and
water as a green solvent was preferred (Table 1, entries 12-15).
Table 1. Optimization of the oxidation of methyl phenyl sulfide.a
O
O
O
S
S
S
Catalyst
+
H2O2 (aq. 30%)
Solvent
A
B
Entry
Catalyst
(mol%)
H2O2
Solvent
Temperature
Time
(h)
Conversion
Selectivity
to A (%)c
(mmol)
(°C)
(%)b
1
2
3
4
5
1
2
4
-
2
2
2
2
2
H2O
H2O
H2O
H2O
H2O
50
50
50
50
50
2
2
2
8
8
67
100
100
14
93
100
100
93
Fe3O4-SiO2
(30 mg)
G2-Fe3O4-
SiO2
24
92
6
2
H2O
50
8
21
97
(30 mg)
2
7
8
1
3
4
2
2
H2O
H2O
H2O
H2O
H2O
50
50
50
30
70
4
2
2
2
2
86
100
100
93
92
100
100
95
2
2
2
2
9
10
11
100
100
15

12
13
14
15
2
2
2
2
2
2
2
2
toluene
EtOH
50
50
50
50
2
2
2
2
96
97
100
100
100
100
100
100
CH3CN
CH2Cl2
[a] Reaction conditions: methyl phenyl sulfide (1 mmol), solvent (3 mL).
[b] Conversions were determined by GC using an internal standard technique and are based on sulfides.
[c] Selectivity to sulfoxide = [sulfoxide %/(sulfoxide %+Sulfone %)]×100.
To exhibit the general applicability of this protocol, Fe3O4-SiO2-dendrimer-Fe catalyst
was further utilized for the selective oxidation of various sulfides to the corresponding
sulfoxides (Table 2). In all cases, the oxidations were completed within time periods between
2-6 h and gave the products in excellent yields. Notably, the reactions were observed to stop
completely at the sulfoxides stage, even on increasing the reaction time. For exploring the
electronic effect on the reaction performance, the para- substituted methyl phenyl sulfides
were tested. The presence of electron-donating bromide group enhanced the reaction in
comparison with electron-withdrawing nitro group, suggesting that electronic natures of the
sulfides affected the reaction rate (Table 2, entries 3 and 4). In a similar way, less reactive
substrates such as diaryl sulfides were successfully oxidized to the corresponding sulfoxides
with high yields and selectivities (Table 2, entries 5-7). Interestingly, the oxidation of more
challenging sulfides with other sensitive functional groups such as hydroxyl and olefin took
place without affecting the functional groups, indicating a high chemoselectivity (Table 2,
entries 6 and 8). To further extend the scope of this catalytic system, aliphatic sulfides were
also tested. Interestingly, this catalytic system also proved applicable to the oxidation of
aliphatic sulfides, and good to excellent yields of corresponding sulfoxides were achieved
(Table 2, entries 9 and10).
16

Table 2. Oxidation of various sulfides to the corresponding sulfoxides catalyzed by Fe3O4-SiO2-dendrimer-Fe
catalysta.
Entry
1
Sulfide
Time (h)
2
Sulfoxide (%)b
100
Selectivity (%)c
100
S
S
2
3
2.5
4.5
100
98
96
93
S
Br
S
4
5
6
7
6
6
4
6
93
87
91
92
97
95
O2N
S
S
>99
96
HO
OH
S
S
S
8
9
2
4
>99
96
98
95
82
10
4.5
95
S
[a] Reaction conditions: sulfide (1 mmol), 30% H2O2 (2 mmol), catalyst (2 mol%), and H2O (3 mL) at 50 (°C)
.
[b] Conversions were determined by GC using an internal standard technique and are based on sulfides.
17

[c] Selectivity to sulfoxide = [sulfoxide %/(sulfoxide %+Sulfone %)]×100.
We also compared the results achieved in this work with various recoverable catalysts
that have been reported in literature for the oxidation of methyl phenyl sulfide. The
comparison results summarized in Table 3 indicated that the catalyst was comparable or
better catalyst with respect to reaction condition, time, and yield than other catalytic systems.
Furthermore, like the catalysts immobilized on magnetic nanoparticles, the present catalyst
could be easily separated from the reaction mixture with the aid of an external magnetic field.
Table 3. Comparison of the activity of various catalysts in oxidation of methyl phenyl sulfide.
Entry
Catalyst
Reaction conditions
Conversion
(%)
Ref
1
Fe3O4-SiO2-dendrimer-Fe methyl phenyl sulfide (1 mmol),
100
This work
(2 mol%)
H2O (3 mL), H2O2(2 mmol), T =
50 °C, t = 2 h
2
3
Fe3O4@SiO2-APTES-FeL methyl phenyl sulfide (1 mmol),
>99
98
[45]
[46]
(2 mol%)
EtOH (1 mL), H2O2(3 equiv.),
RT, t = 2 h
WOx/SBA-15(20)
(0.05 g)
methyl phenyl sulfide (2 mmol),
CH3OH/CH2Cl2 (1:1, 10 mL),
TBHP (equiv.), RT, t = 10 h
4
5
SBA-15/NH2-FeQ3
(5 mol%)
methyl phenyl sulfide (0.5 mmol),
H2O (1 mL), H2O2(1.5 mmol),
RT, t = 3 h
87
92
[2]
[6]
Fe3O4@SiO2-APTES
(Fe(acac)2)
methyl phenyl sulfide (1 mmol),
EtOH (3 mL), H2O2(1.5 equiv.),
RT, t = 2 h
(2 mol%)
18

Recycling of the catalyst
The recyclability of Fe3O4-SiO2-dendrimer-Fe catalyst was investigated in the oxidation
of methyl phenyl sulfide under the optimized reaction conditions. At the end of each reaction,
the catalyst was readily separated from the reaction mixture by an external magnet, rinsed
with ethanol, dried in vacuum, and reused for subsequent reactions. As shown in Figure 10,
both the activity and selectivity of the catalyst remained unchanged even after 5 cycles.
Additionally, the stability of the catalyst was confirmed by FT-IR and TEM analyses of the
catalyst after five cycles. The similarity of the FT-IR spectrum (Figure S1) and the TEM
image (Figure S2) of fresh Fe3O4-SiO2-dendrimer-Fe catalyst and after five cycles clearly
demonstrated that the structure and morphology of the catalyst remained unaltered after five
recoveries.
Figure 10. Reusability of Fe3O4-SiO2-dendrimer-Fe catalyst.
Conclusion
In conclusion, we have designed and synthesized a novel heterogeneous catalyst by the
immobilization of Fe(III)-salen complex on dendrimer functionalized magnetic nanoparticles.
The obtained catalyst revealed excellent activity in green and selective oxidation of a variety
19

of sulfides to sulfoxides. Furthermore, the catalyst could be recovered and reused for at least
five times without any decrease in the catalytic activity and selectivity. Because of the
magnetic nature of the catalyst, it could be separated from the reaction mixture easily by
applying an external magnet, which eliminated the filtration and centrifugation process for
catalyst separation after completion of the reaction.
Acknowledgements
Financial support of this work by Iran National Science Foundation (Grant No. 97001228)
and Shiraz University is gratefully acknowledged.
Conflict of interest
The authors declare no conflict of interest.
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Highlights
A novel nanohybrid material based on salen iron complex immobilized on dendrimer
functionalized magnetite nanoparticles was synthesized
.
The synthesized nanohybrid material was successfully applied as a recyclable catalyst for the
oxidation of various sulfides.
The catalyst could be separated easily by applying an external magnet and recycled up to five
cycles without significant decrease in catalytic activity and selectivity.
1

Conflict of interest
The authors declare no conflict of interest.