Oxo-vanadium(IV) unsymmetrical Schiff base complex immobilized on γ-Fe2O3 nanoparticles: A novel and magnetically recoverable nanocatalyst for selective oxidation of sulfides and oxidative coupling of thiols

Article abstract of DOI:10.1002/aoc.6170

In this research, an unsymmetrical salen-type oxo-vanadium(IV) complex, [VO(salenac-OH)] (salenac-OH = [9-(2′,4′-dihydroxyphenyl)-5,8-diaza-4-methylnona-2,4,8-trienato](-2)), was synthesized and covalently immobilized on the surface of magnetic γ-Fe₂

Full text of DOI:10.1002/aoc.6170

Received: 19 September 2020
DOI: 10.1002/aoc.6170
Revised: 21 December 2020
Accepted: 6 January 2021
F U L L P A P E R
Oxo-vanadium(IV) unsymmetrical Schiff base complex
immobilized on γ-Fe O nanoparticles: A novel and
2
3
magnetically recoverable nanocatalyst for selective
oxidation of sulfides and oxidative coupling of thiols
Abolfazl Mahdian
Samira Saeednia
| Mehdi Hatefi Ardakani
| Esmaeil Heydari-Bafrooei
|
Department of Chemistry, Faculty of
Science, Vali-e-Asr University of
Rafsanjan, Rafsanjan, Iran
In this research, an unsymmetrical salen-type oxo-vanadium(IV) complex,
0 0
[
VO(salenac-OH)] (salenac-OH
-methylnona-2,4,8-trienato](-2)), was synthesized and covalently immobilized
on the surface of magnetic γ-Fe nanoparticles. The resulting γ-Fe @[VO
salenac-OH)] nanoparticles were characterized by several techniques includ-
=
[9-(2 ,4 -dihydroxyphenyl)-5,8-diaza-
4
Correspondence
Mehdi Hatefi Ardakani, Department of
Chemistry, Faculty of Science, Vali-e-Asr
University of Rafsanjan, Rafsanjan
2
O
3
O
2 3
(
ing Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD),
scanning and transmission electron microscopies (SEM and TEM), energy-
dispersive X-ray (EDX) spectroscopy, vibrating sample magnetometry (VSM),
thermal gravimetric analysis (TGA), inductively coupled plasma (ICP), and
elemental analysis. The prepared γ-Fe O @[VO(salenac-OH)] nanoparticle
7
7188-97111, Iran.
Email: m.hatefi@vru.ac.ir;
m_hatefi_chem@yahoo.com
Funding information
Vali-e-Asr University of Rafsanjan
2
3
was utilized as an efficient catalyst for selective oxidation of sulfides to sulfox-
ides using 30% H O as oxidant and oxidative coupling of thiols into disulfides
2
2
with urea/H O (UHP) as an oxidizing reagent. The products were achieved
2
2
with good to excellent yields at room temperature with no over-oxidation of
sulfoxides and disulfides to unexpected by-products. This catalyst can be mag-
netically recovered by applying an external magnet and reused for five contin-
uous cycles in both oxidation reactions without a significant loss in its catalytic
activity. Furthermore, the FT-IR spectrum and XRD pattern of the recovered
catalyst showed no critical change to those of the fresh one.
K E Y W O R D S
2 3
magnetic nanocatalyst, oxo-vanadium(IV) complex, sulfide, thiol, γ-Fe O nanoparticle
1
| INTRODUCTION
removal of sulfur compounds such as dibenzothiophene
DBT) from fuels is a necessary action in the petrochemi-
(
The selective oxidation of sulfides into the corresponding
sulfoxides has taken up a great part in modern synthetic
cal processes. The main reason for S removal is, however,
that a few parts per million of sulfur are enough to poison
the catalysts used for the purification of the exhaust gases
of diesel cars. In this regard, for removing these com-
pounds from fuels, the oxidation of sulfur compounds
coupled with the extraction of the resulting sulfone by
[1]
organic chemistry. Organic sulfoxides play an essential
role in the synthesis of chemically useful and biologically
active compounds including drugs, flavors, germicides,
[2]
and catabolism regulators. On the other hand, the
Appl Organomet Chem. 2021;35:e6170.
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© 2021 John Wiley & Sons, Ltd.
1 of 16
https://doi.org/10.1002/aoc.6170

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MAHDIAN ET AL.
[
3]
means of a solvent has been reported. Likewise, the con-
version of thiols to disulfides is essential in medical, bio-
logical, and industrial areas. Disulfides are used to
stabilize peptides in proteins, design of rechargeable lith-
ium batteries, sulfonylation of enolates, and as vulcanizing
Homogeneous metal-based Schiff base complexes
(notably, the expensive metal ones) have limited catalytic
applications because of complications in their separation
and reusability. Moreover, deactivation of homogeneous
catalysts occurs by the formation of μ-oxo and dimeric
[4]
[21]
agent for rubber and elastomers. Unfortunately, some
methods based on the oxidation of sulfides and thiols
require hazardous and toxic reagents or include over-
peroxo-species.
These problems can be overcome by
immobilization of metal Schiff base complexes on suit-
able solid supports.
[
5]
oxidation of sulfides and thiols to undesired by-products.
Nowadays,
nanoparticles,
specially
magnetic
Hence, further progress in eco-friendly selective oxidation
of sulfides to sulfoxides and thiols to disulfides using less
poisonous catalysts, oxidants, and solvents is still needed.
nanoparticles (MNPs), are employed as catalyst supports
because of their unique properties, including ease of
separation by a magnet, high surface area, high disper-
sion, low toxicity, appropriate recycling of catalyst, and
Recently, a series of Fe/Fe C-containing nitrogen-doped
3
[
22–26]
porous carbon microspindles (Fe/Fe C@NC-x, where x
remarkable stability.
Whereas most heterogeneous
3
ꢀ
ꢀ
represents the pyrolysis temperature from 500 C to 800 C)
have been constructed by the pyrolisis of a metal–organic
framework precursor as highly selective catalysts for the
oxidation of sulfides to sulfoxides under mild reaction con-
systems require filtration, centrifugation, or tiresome
workup to recover the catalysts from the final reaction,
the magnetic nanocatalysts can be magnetically sepa-
rated from the reaction medium with no need for filtra-
tion or centrifugation steps. Furthermore, the MNPs
can be prepared from inexpensive materials and can be
simply modified through appropriate surface modifica-
[6]
ditions. Also, Xu et al. developed a visible-light-induced
strategy for the selective aerobic oxidation of thiols to dis-
ulfides and oxidative synthesis of asymmetric disulfides on
[
27]
anatase TiO as a nontoxicity, low cost, and high availabil-
tions.
Recently, the combination of MNPs and lay-
2
[7]
ity photocatalyst.
ered double hydroxide (LDH) as magnetic-LDH has
been provided to be applied in a wide range of syn-
thetic methods because the achieved nanoparticles pos-
sess a high surface to volume ratio, which increases
Azomethines or Schiff base ligands significantly con-
tribute to the extension of transition metal coordination
chemistry due to their facile synthesis, structural variety,
biological and catalytic properties, and chemical and
[
28]
their activity and selectivity.
Among the many avail-
[8,9]
thermal stability.
Unsymmetrical Schiff base ligands
able MNPs, the maghemite (γ-Fe O ) nanoparticles
2
3
are of great importance because of their optoelectronic,
have been specially interesting due to their excellent
[10]
electrochemical, and magnetic properties.
Also, the
properties. The γ-Fe O3 nanoparticles are super para-
2
existence of bonds between the central metal ions and
unsymmetrical Schiff base ligands in natural systems has
been reported.
magnetic materials used in catalysis, sensors, drug
delivery systems, magnetic resonance imaging (MRI),
[11]
[29]
and biology.
Moreover, the γ-Fe O nanocomposites
2
3
In recent years, special attention has been paid to oxo-
vanadium(IV) Schiff base complexes due to their catalytic
applications, biological roles in biochemical processes, and
with tin oxide, graphene oxide, silica, and starch find
applications in catalysis, optical device, and selective
[30]
removal of heavy metal ions from aqueous solution.
Recently, Abbasi et al. and Amini et al. synthesized Ag
supported on hydroxyapatite-core-shell γ-Fe O
nanoparticles (γ-Fe O @Hpa-Ag NPs) as an efficient
[12]
interesting structural features. The vanadium's ability to
quickly change its oxidation states between +IV and +V
makes this element well-suited elements for catalytic oxi-
2
3
2
3
[13]
dation reactions.
In the catalysts field, the immobilized
and magnetically recyclable catalyst for the synthesis of
oxo-vanadium complexes catalyze various reactions such
as oxidation of alcohols, epoxidation of alkenes and allyl
alcohols, conversion of sulfides to sulfoxides, and oxidative
coumarin derivatives and regioselective ring opening of
[
31,32]
2+
epoxides.
and Ni
Also, preparation and application of Fe
immobilized on hydroxyapatite-core-shell
2+
[14–18]
coupling of thiols to disulfides.
Recently, Qi and co-
γ-Fe O nanoparticles as efficient catalysts for the vari-
2
3
[
33–36]
workers prepared novel N-incorporated carbon-
a
ous organic reactions have been reported.
supported vanadium catalyst (V@CN-750) via in situ
pyrolysis of chitin and the VO(acac)2 complex. The
V@CN-750 catalyst exhibited excellent activity, selectivity,
and recyclability in the selective oxidation of sulfides to
In this work, an oxo-vanadium(IV) complex con-
taining an unsymmetrical salen-type Schiff base was syn-
thesized and immobilized covalently on γ-Fe O MNPs.
2
3
The prepared γ-Fe O @[VO(salenac-OH)] nanoparticles
2
3
[19]
sulfoxides under mild conditions.
Moreover, the tita-
were fully characterized by various physicochemical
methods and successfully used for selective oxidation of
sulfides to sulfoxides and oxidative coupling of thiols to
disulfides at room temperature.
nium dioxide doped with vanadium(V-TiO ) showed high
2
activity in the photocatalytic oxidation of sulfide to sulfox-
[20]
ide under UV irradiation.

MAHDIAN ET AL.
3 of 16
2
2
| EXPERIMENTAL
.1 | Materials and methods
2.3 | Synthesis of γ-Fe O MNPs
2
3
The γ-Fe O MNPs were synthesized via a chemical co-
2
3
[38]
precipitation method described in the literature.
All the materials used were of commercial reagent grade
acquired from Merck or Fluka chemical companies. The
Fourier transform infrared (FT-IR) spectra were
recorded as KBr disks by a Thermo Scientific Nicolet
iS10 FT-IR spectrophotometer. The UV–vis spectrum
was recorded using a T80 UV–vis spectrophotometer.
An ICP-SpectroCiros CCD spectrometer was used to
evaluate the metal content of the catalyst. The X-ray dif-
fraction (XRD) patterns of the powder samples were
recorded at room temperature by a Panalytical X'pert
Pro diffractometer with Cu-Kα radiation. The scanning
electron microscopy (SEM) and energy-dispersive X-ray
Briefly, 1.99 g of FeCl ꢁ4H O and 3.25 g of FeCl ꢁ6H O
2
2
3
2
were dissolved in 30 ml of deionized water under N
2
atmosphere at room temperature. Then 200 ml of a 0.6-M
NH OH solution was added drop by drop to the mixture
4
under continuous stirring at room temperature to set the
reaction pH of 11. The resultant black dispersion was
stirred continuously for 1 h at room temperature and sub-
sequently refluxed for 1 h to obtain a brownish disper-
sion. Afterward, the nanoparticles were magnetically
collected and washed repeatedly with deionized water.
ꢀ
Finally, the synthesized particles were heated at 250 C
for 3 h to give a reddish-brown powder.
(
5
(
EDX) analysis were performed via a Zeiss Sigma VP-
00 system. The transmission electron microscope
TEM) images were taken using a Zeiss-EM10C instru-
2.4 | Synthesis of chloro-functionalized
ment. The magnetic measurements were carried out by
an LBKFB Meghnatis Daghigh Kavir vibrating sample
magnetometer (VSM). The thermal gravimetric analysis
γ-Fe O3
2
The prepared γ-Fe O nanoparticles (1.8 g) were soni-
2
3
(
1
TGA) was conducted on a Rheometric Scientific STA
500 system. The elemental analysis was performed by
cated in 50 ml of dry toluene for 30 min. Then 2 ml of
3-chloropropyltrimethoxysilane was added to the dis-
persed γ-Fe O nanoparticles under mechanical stirring.
a Vario EL III CHNS analyzer. The reaction progress
was checked through the thin-layer chromatography
2
3
The resultant mixture was refluxed under N atmosphere
2
(TLC) of silica-gel polygram SILG/UV 254 plates. The
for 48 h. Then the obtained chloro-functionalized
gas chromatography (GC) analyses were carried out by
a Philips GC-PU 4600 instrument equipped with a flame
ionization detector (FID).
γ-Fe O3 nanoparticles were magnetically collected,
2
washed with toluene, deionized water, and ethanol in
[
38]
turn, and finally dried under vacuum.
2
.2 | Synthesis of [VO(salenac-OH)]
2.5 | Preparation of γ-Fe2O3@[VO
complex
(salenac-OH)] nanocatalyst
Unsymmetrical tetradentate Schiff base ligand,
The synthesized chloro-functionalized γ-Fe O (2 g) was
2
3
0
0
salenac-OH
-methylnona-2,4,8-trienato](-2), was synthesized in good
yield based on the procedure described in the litera-
=
[9-(2 ,4 -dihydroxyphenyl)-5,8-diaza-
dispersed in 50 ml of dry acetone by sonication for
30 min. Afterward, 0.5 g of the synthesized [VO(salenac-
OH)] complex was added to the above stirring mixture.
The resultant suspension was refluxed for 24 h. The solid
product was then collected with a permanent magnet
and washed with acetone three times. Finally, the
4
[37]
ture.
To prepare oxo-vanadium(IV) Schiff base com-
plex, [VO(salenac-OH)], 1 mmol of VO(acac) , that is,
2
0
.265 g, was dissolved in 25-ml warmed methanol, and
ꢀ
the obtained solution was added dropwise to the stirred
solution of 1 mmol (0.262 g) of Schiff base ligand in
obtained solid was dried in an oven at 90 C for 4 h to fur-
nish γ-Fe O @[VO(salenac-OH)] nanocatalyst.
2
3
2
4
5-ml methanol. After refluxing the reaction mixture for
h, it was allowed to be cooled to room temperature.
The resultant green solid product was collected and
washed several times with methanol and then purified
from hot methanol (yield: 65%). Anal calc. for
C H N O V (%): C, 51.36; H, 4.94; N, 8.56, found: C,
2.6 | General procedure for the oxidation
of sulfides to sulfoxides with 30% H O
2
2
catalyzed by γ-Fe O @[VO(salenac-OH)]
2
3
14
16 2 4
−
1
5
1.27; H, 5.12; N, 8.45. FT-IR (KBr pellet, cm ): 3450
OH), 1621 (C N), 1595 (C N), 1447 (C C, aromatic),
88 (V O). UV–vis (CH CN, λmax/nm): 235, 273, 365.
In a typical procedure, 25 mg (0.70 mol%) of the prepared
(
γ-Fe O @[VO(salenac-OH)] nanocatalyst, 1 mmol of sul-
2
3
9
fide, and 3 mmol 30% H O were mixed in 4 ml of
2 2
3

4
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MAHDIAN ET AL.
1
1
,2-dichloroethane inside a test tube. This mixture was
Methylphenyl sulfoxide (Table 2, entry 4), H NMR
magnetically stirred at room temperature, and the reac-
tion progress was checked by TLC or GC. After comple-
tion of the reaction, the catalyst was magnetically
(500 MHz, CDCl ) δ (ppm): 7.45 (m, 5H), 2.63 (m, 3H).
3
13
C NMR (500 MHz, CDCl ) δ (ppm): 145.83, 131.06,
3
129.32, 123.54, 43.72.
1
collected, washed with Et O, and dried in an oven of
Bis(4-methylphenyl) disulfide (Table 4, entry 1), H
2
ꢀ
8
0 C. Lastly, to achieve the pure product, it was extracted
NMR (500 MHz, CDCl ) δ (ppm): 7.50 (d, 4H), 7.18 (d,
3
13
with CH Cl (3 × 10 ml) and purified on a silica-gel
4H), 2.38 (s, 6H). C NMR (500 MHz, CDCl ) δ (ppm):
2
2
3
1
column. Some of the products were characterized by H
134.94, 134.06, 133.51, 132.16, 131.97, 127.67, 20.54.
13
1
NMR,
C
NMR, and GC spectra (Supporting
Diphenyl disulfide (Table 4, entry 3), H NMR
Information).
(500 MHz, CDCl ) δ (ppm): 7.52–7.45 (m, 4H), 7.32–7.19
3
13
(
m, 6H). C NMR (500 MHz, CDCl ) δ (ppm): 135.15,
3
132.45, 130.79, 130.19.
2
.7 | General procedure for the oxidative
coupling of thiols to disulfides with UHP
catalyzed by γ-Fe O @[VO(salenac-OH)]
3 | RESULTS AND DISCUSSION
2
3
To a mixture of thiol (1 mmol) and urea/H O (UHP)
3.1 | Characterization of [VO(salenac-
OH)] complex
2
2
(5 mmol) in ethanol (4 ml), 25 mg (0.70 mol%) of
γ-Fe O @[VO(salenac-OH)] was added. Then the mix-
2
3
ture was stirred at room temperature, and the progress of
reaction was monitored using the TLC technique. At the
end of the reaction, the catalyst was separated with a
The synthesized [VO(salenac-OH)] complex was charac-
terized by elemental analysis (CHN), FT-IR, and UV–vis
spectra. In the FT-IR spectrum of [VO(salenac-OH)] com-
plex (Figure 1), two bands appeared at 1621 and
magnet, rinsed several times with Et O, and dried in the
2
ꢀ
−1
oven at 80 C. Finally, the corresponding disulfide was
1595 cm due to the two different C N bonds in the
extracted with CH Cl (3 × 10 ml) and refined by a silica-
unsymmetrical Schiff base ligand. The observed bands at
1447 and 3450 cm can be assigned to the aromatic
2
1
2
13
−1
gel column. The H NMR, C NMR, and GC spectra of
some of the products are given in the Supporting
Information.
C C and O H stretching vibrations, respectively. Simi-
larly, the appeared band at 988 cm− was attributed to
the V O stretching vibration frequencies.
1
Figure 2 shows the UV–vis spectrum of the [VO
(salenac-OH)] complex. Three bands were seen in the
spectrum at 235, 273, and 365 nm. The two bands in the
higher energy region (235 and 273 nm) were attributed to
2
S8)
.8 | Selected spectral data (Figures S1–
1
*
Dibenzyl sulfoxide (Table 2, entry 1), H NMR (500 MHz,
the π ! π transitions, and the band at 365 nm was
*
CDCl ) δ (ppm): 7.49–7.44 (m, 6H), 7.43–7.35 (m, 4H),
assigned to the n ! π transitions. Also, a band with very
3
13
3
.90 (m, 4H). C NMR (500 MHz, CDCl ) δ (ppm):
low intensity appeared at 597 nm, which was attributed
3
1
29.62, 128.83, 128.23, 127.18, 56.78.
to the d–d transition.
FIGURE 1 Fourier transform infrared
spectrum of [VO(salenac-OH)] complex

MAHDIAN ET AL.
5 of 16
FIGURE 2 UV–vis spectrum of [VO
(
3
salenac-OH)] complex (10–4 M in CH CN)
3
.2 | Preparation and characterization of
inductively coupled plasma (ICP), and elemental analysis.
The ICP analysis showed that 0.28 mmol of vanadium was
anchored on 1.0 g of the supported catalyst.
γ-Fe O @[VO(salenac-OH)] nanocatalyst
2
3
The preparation procedure of γ-Fe O @[VO(salenac-OH)]
FT-IR spectra of γ-Fe O3 nanoparticles, chloro-
2
3
2
heterogeneous nanocatalyst is shown in Scheme 1. First,
functionalized γ-Fe O , γ-Fe O @[VO(salenac-OH)]
2
3
2 3
the γ-Fe O3 MNPs were synthesized by chemical co-
nanocatalyst, and the recovered γ-Fe O @[VO(salenac-
2
2 3
precipitation of ferric and ferrous ions in the alkali solu-
OH)] are shown in Figure 3. In all the spectra, the band
−1
tion. Then the prepared γ-Fe O3 nanoparticles were
revealed around 550–640 cm
is attributed to the
stretching vibrations of the Fe O bond in these com-
2
allowed to react with 3-chloropropyltrimethoxysilane to
[
39]
give chloro-functionalized γ-Fe O . Finally, γ-Fe O @[VO
pounds.
In Figure 3b,c, the band positioned at around
2
3
2 3
2920 cm− may be related to the C H bond's stretching
1
(salenac-OH)] nanocatalyst was synthesized via covalent
[
40]
grafting of [VO(salenac-OH)] complex on the surface of
vibrations in methylene groups.
In the FT-IR spectrum
the chloro-functionalized γ-Fe O nanoparticles.
of γ-Fe O @[VO(salenac-OH)] (Figure 3c), two bands
2
3
2 3
appeared at 1628 and 1591 cm− are assigned to the
vibrations of two different azomethine (C N) groups of
the unsymmetrical Schiff base ligand in the immobilized
1
The prepared γ-Fe O @[VO(salenac-OH)] nanocatalyst
2
3
was characterized by FT-IR and EDX spectra, XRD pat-
terns, SEM and TEM images, VSM measurements, TGA,
SCHEME 1 Preparation process of γ-Fe
salenac-OH)] nanocatalyst
2 3
O @[VO
(

6
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MAHDIAN ET AL.
VO(salenac-OH)] complex. Also, the band at 1450 cm−
1
[
can be assigned to the aromatic C C stretching vibra-
tions of the immobilized [VO(salenac-OH)] complex.
Moreover, the band that appeared at 983 cm− in the FT-
IR spectrum of γ-Fe O @[VO(salenac-OH)] is attributed
1
2
3
[
41]
to the V O stretching vibrations.
These results
indicate that the [VO(salenac-OH)] complex has been
successfully grafted onto the γ-Fe O MNPs.
2
3
The XRD pattern of the prepared γ-Fe O @[VO
2
3
(
salenac-OH)] catalyst is shown in Figure 4a. The diffrac-
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
tion peaks around 2θ = 30.4 , 35.7 , 43.4 , 53.7 , 57.4 ,
and 63.0 correspond to Bragg reflections from (2 2 0),
ꢀ
(
3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) planes of face-
centered cubic structure of γ-Fe O , respectively (JCPDS
2
3
File No. 04-0755). The average crystallite size of the
prepared catalyst determined using the Debye–Scherrer
formula is about 34.8 nm.
SEM image of the γ-Fe O @[VO(salenac-OH)] cata-
2
3
lyst is shown in Figure 5. As seen, it presents a uniform
and spherical morphology of the MNPs. Furthermore, its
EDX spectrum (Figure 6) confirms the expected C, N, O,
Fe, and V species in the prepared catalyst.
The particle size and structure of the prepared
γ-Fe O @[VO(salenac-OH)] nanocatalyst were further
2
3
determined through the TEM images (Figure 7). They
show the approximately spherical shape of nanoparticles
with an average diameter of 40 nm.
FIGURE 3 FT-IR spectrum of (a) γ-Fe
2
O
3
nanoparticles,
@[VO(salenac-OH)]
2 3
nanocatalyst, and (d) recovered γ-Fe O @[VO(salenac-OH)] after
The magnetic properties of the synthesized γ-Fe O @
[VO(salenac-OH)] nanocatalyst were measured using a
2
3
(
b) chloro-functionalized γ-Fe , (c) γ-Fe
2
O
3
2
O
3
VSM at room temperature. The saturation magnetization
[38]
five uses
of bare γ-Fe O nanoparticles has been reported
as
2
3
FIGURE 4 X-ray diffraction pattern of
(
(
2 3
a) fresh and (b) recovered γ-Fe O @[VO
salenac-OH)] catalyst

MAHDIAN ET AL.
7 of 16
2 3
FIGURE 5 Scanning electron microscope image of γ-Fe O @
[
VO(salenac-OH)]
−
1
6
8.5 emu g , and the saturation magnetization of the
−
1
prepared catalyst is 54.5 emu g (Figure 8). The decrease
observed in the magnetization of γ-Fe O @[VO(salenac-
2
3
OH)] with respect to γ-Fe O is caused by the grafting of
2
3
organic layers, including oxo-vanadium complex on the
surface of γ-Fe O3 nanoparticles. However, γ-Fe O @
2
2 3
[
VO(salenac-OH)] nanocatalyst, despite its decreased
magnetization compared with γ-Fe O , can be quickly
2
3
separated from the reaction mixture by an external
magnet.
FIGURE 7 Transmission electron microscope images of
γ-Fe @[VO(salenac-OH)]
2 3
O
The thermal stability of γ-Fe O @[VO(salenac-OH)]
2
3
nanocatalyst was studied by TGA. The TGA curve of
[
42]
γ-Fe O @[VO(salenac-OH)] (Figure 9) shows two weight
the support.
In comparison, the second one (about
2
3
ꢀ
ꢀ
loss steps. The first one (about 2.5%) around 120 C is
9.4%) in the 250–800 C range may be related to the
decomposition of organic component attached to the
associated with the loss of water molecules adsorbed on
FIGURE 6 Energy-dispersive X-ray
2 3
spectrum of γ-Fe O @[VO(salenac-OH)]

8
of 16
MAHDIAN ET AL.
the studied solvents, 1,2-dichloroethane (C H Cl ) was
2
4
2
found to be the best, and the highest sulfoxide yield was
achieved in this solvent (Table 1, entries 1–8). Subse-
quently, different amounts of γ-Fe O @[VO(salenac-OH)]
2
3
catalyst were used in the model reaction, and the superior
sulfoxide yield was obtained with 0.70 mol% (25 mg) of the
catalyst (Table 1, entries 8 and 12–16). It is notable that
with no catalyst, the oxidation of dibenzyl sulfide was
incomplete even after 24 h (Table 1, entry 12). Addition-
ally, the ability of different oxidants including 30% H O ,
2
2
UHP, tert-BuOOH (TBHP, 70% aqueous solution), and
NaIO was examined in the oxidation of dibenzyl sulfide
4
as a model substrate, and 30% H O gave the best sulfoxide
2
2
yield (Table 1, entries 8 and 17–20). It is worth mentioning
that H O is a cheap, easy-to-handle, and environmentally
2
2
acceptable oxidant. The various molar ratios of H O /
2
2
2 3
FIGURE 8 Magnetization curve of γ-Fe O @[VO(salenac-OH)]
sulfide were also examined for the model oxidation reac-
tion, and the optimum molar ratio of H O to substrate of
2
2
3:1 mmol was found to be ideal for this system (Table 1,
entries 8–11). In addition, when excess amount of H O
2
2
(
4 mmol) was employed, the sulfoxide yield was decreased
due to the consecutive reaction of the transformation of
sulfoxide to sulfone (Table 1, entry 9).
The catalytic ability of the γ-Fe O @[VO(salenac-OH)]
2
3
nanocatalyst was studied in the oxidation of various sul-
fides in the optimized conditions (Scheme 2). As summa-
rized in Table 2, a wide variety of sulfides were
successfully oxidized to their corresponding sulfoxides
with good to excellent yields. In this protocol, the oxidation
of diphenyl sulfide as sterically hindered and diethyl sul-
fide as linear sulfides completed with good yields (Table 2,
entries 6 and 8). The chemo-selectivity of this method was
also notable. The conversion of sulfides containing
hydroxyl group (Table 2, entry 7) and C C bonds (Table 2,
entries 9 and 10) into their corresponding sulfoxides is
accompanied by no carbonyl and epoxide by-products.
FIGURE 9 Thermal gravimetric analysis curve of γ-Fe
VO(salenac-OH)]
2 3
O @
[
support. According to the TGA profile, the amount of
[
43]
organic compounds supported on the γ-Fe O3
Based on the literature,
a possible mechanism for
2
−
1
nanoparticles is estimated to be 0.26 mmol g . These
the oxidation of sulfides to sulfoxides with H O in the
2
2
results are in good agreement with the elemental analysis
presence of γ-Fe O @[VO(salenac-OH)] catalyst is pro-
2
3
(N = 0.76%) and ICP data.
posed (Scheme 3). At first, γ-Fe O @[VO(salenac-OH)]
2
3
reacts with H O2 to form intermediate A, which is
2
converted to the active oxidant B. Next, nucleophilic
attack of the sulfide on this intermediate gives cation C,
which produces the corresponding sulfoxide and regener-
ates the catalyst for the subsequent catalytic cycle.
3
.3 | Catalytic performance
3
.3.1 | Oxidation of sulfides to sulfoxides
catalyzed by γ-Fe O @[VO(salenac-OH)]
2
3
Following the characterization of the synthesized
γ-Fe O @[VO(salenac-OH)], its catalytic activity was stud-
ied in the oxidation of various sulfides at room
temperature.
3.3.2 | Oxidative coupling of thiols to
disulfides catalyzed by γ-Fe O @[VO
2
3
2
3
(salenac-OH)]
To explore the optimum reaction conditions, different
solvents and solvent-free conditions were applied in the
oxidation of dibenzyl sulfide as a model substrate. Among
In the second part of our study, the prepared γ-Fe O @
[VO(salenac-OH)] nanoparticles were used as the catalyst
2
3

MAHDIAN ET AL.
9 of 16
TABLE 1 Optimization of the
Entry
Solvent
Catalyst (mol%)
0.70
Oxidant
Isolated yield (%)
reaction parameters in the oxidation of
1
2
3
4
5
6
7
8
Solvent-free
30% H
30% H
30% H
30% H
30% H
30% H
30% H
2
2
2
2
2
2
2
O
O
O
O
O
O
O
2
2
2
2
2
2
2
23
15
27
18
35
43
67
95
85
57
25
0
dibenzyl sulfide catalyzed by γ-Fe
2 3
O @
[
VO(salenac-OH)] at room temperature
CH
3
OH
0.70
EtOH
0.70
H
2
O
0.70
CH
CH
3
3
CN
0.70
COCH
3
0.70
CHCl
3
0.70
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
H
4
Cl
2
0.70
2 2
30% H O
a
9
H
H
H
H
H
H
H
H
H
H
H
H
4
4
4
4
4
4
4
4
4
4
4
4
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
2
0.70
30% H
30% H
30% H
30% H
30% H
30% H
30% H
30% H
2
O
O
O
O
O
O
O
O
2
b
10
11
12
13
14
15
16
17
18
19
20
2
0.70
2
2
2
2
2
2
2
2
2
2
2
2
2
c
2
2
2
2
2
2
2
2
2
2
0.70
d
0
0.14
24
45
63
75
55
23
12
0
0.28
0.42
0.56
2
0.70
Urea/H
2
O
2
(UHP)
0.70
tert-BuOOH
NaIO
No oxidant
0.70
4
0.70
Note: Reaction conditions: dibenzyl sulfide (1 mmol), solvent (4 ml), oxidant (3 mmol), catalyst, reaction
time (1 h). Bold emphasis is used due to contrast and comparison with other data.
a
H
2
O
2
= 4 mmol.
= 2 mmol.
2 2
H O = 1 mmol.
b
2 2
H O
c
d
Reaction time = 24 h.
for the oxidative coupling of thiols into disulfides at room
temperature. To find the best reaction conditions,
corresponding disulfides with excellent yields (Scheme 4,
Table 4). This oxidizing system allowed the chemo-
selective oxidation of 2-mercaptoethanol to the
corresponding disulfide (Table 4, entry 8). Interestingly,
the hydroxyl group in this substrate remained intact
during the oxidation reaction.
4
-methyl thiophenol was selected as the model substrate,
and the influence of the nature of solvent (Table 3,
entries 1–7), amount of catalyst (Table 3, entries 2 and
8–12), and kind of oxidant (Table 3, entries 2 and 13–16)
were investigated for this oxidation reaction. The results
Table 3) indicate that the optimum conditions for the
completion of the reaction are catalyst (0.70 mol%,
5 mg), UHP (5 mmol) as oxidant, and EtOH as the
solvent.
The generality of this protocol has been demonstrated
A possible mechanism for the oxidative coupling of
thiols to the corresponding disulfides with UHP as the
oxidant in the presence of γ-Fe O @[VO(salenac-OH)]
(
2
44]
3
[
2
catalyst is outlined in Scheme 5.
To show that the γ-Fe O @[VO(salenac-OH)] catalyst
2
3
is truly heterogeneous, a hot filtration test was carried
out in the oxidation of 4-methyl thiophenol under
by the successful conversion of different thiols to the
SCHEME 2 Oxidation of sulfides with 30%
H
2
O
2
2 3
catalyzed by γ-Fe O @[VO(salenac-OH)]

10 of 16
MAHDIAN ET AL.
TABLE 2 Oxidation of sulfides to sulfoxides with 30% H
2
O
2
catalyzed by γ-Fe
2
O
3
@[VO(salenac-OH)] at room temperature
a
Entry
Sulfide
Product
Time (h)
Yield (%)
1
1
95
92
85
2
3
1
1
4
5
1
1
93
90
6
1
1
87
90
b
7
b
8
1.5
1.5
90
86
b
9
b
10
1.5
83
Note: Reaction conditions: sulfide (1 mmol), C
2
H
4
Cl
2
(4 ml), 30% H
O
2 2
(3 mmol), catalyst (0.70 mol%).
a
Isolated yields.
b
Yields determined by gas chromatography.

MAHDIAN ET AL.
11 of 16
SCHEME 3 Proposed mechanism for
2 2
oxidation of sulfides with 30% H O catalyzed by
γ-Fe @[VO(salenac-OH)]
2 3
O
TABLE 3 Optimization of the
Entry
Solvent
CH OH
EtOH
Catalyst (mol%)
Oxidant
UHP
UHP
UHP
UHP
UHP
UHP
UHP
UHP
UHP
UHP
UHP
UHP
Isolated yield (%)
reaction conditions in the oxidative
coupling of 4-methyl thiophenol
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
3
0.70
0.70
0.70
0.70
0.70
0.70
0.70
0
43
95
27
35
47
25
18
0
catalyzed by γ-Fe
2 3
O @[VO(salenac-
OH)] at room temperature
2
H O
CH
3
3
CN
CH
COCH
3
C
2
H
4
Cl
2
CHCl
3
a
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
0.14
0.28
0.42
0.56
0.70
0.70
0.70
0.70
23
45
62
75
58
35
15
0
0
1
2
3
4
5
6
30% H
tert-BuOOH
NaIO
No oxidant
2
O
2
4
Note: Reaction conditions: 4-methyl thiophenol (1 mmol), solvent (4 ml), oxidant (5 mmol), catalyst,
reaction time (45 min).
Abbreviation: UHP, urea/H
2
O
2
. Bold emphasis is used due to contrast and comparison with other data.
a
Reaction time = 24 h.

12 of 16
MAHDIAN ET AL.
SCHEME 4 Oxidative coupling of thiols
with urea/H (UHP) catalyzed by γ-Fe
VO(salenac-OH)]
2
O
2
2 3
O @
[
a
TABLE 4 Oxidative coupling of
thiols into disulfides using urea/H
catalyzed by γ-Fe @[VO(salenac-
OH)] at room temperature
Entry
Thiol
Product
Time
Yield (%)
2 2
O
1
45 min
95
2 3
O
2
45 min
90
3
4
45 min
45 min
93
90
5
6
7
50 min
92
90
85
87
1 h
1 h
b
8
1 h
Note: Reaction conditions: thiol (1 mmol), EtOH (4 ml), urea/H
2
O
2
(5 mmol), catalyst (0.70 mol%).
a
Isolated yields.
b
Yield determined by gas chromatography.
optimal reaction conditions. Typically, after 25 min, the
catalyst was separated by an external magnet, and the fil-
trate was allowed to stir for an additional 45 min. The
reaction yield was 55% after 25 min and 60% after an
additional 45 min. This observation showed that the
leaching of the immobilized Schiff base complex into the
reaction mixture should be negligible, and the γ-Fe O @
2
3
[VO(salenac-OH)] catalyst acts heterogeneously.

MAHDIAN ET AL.
13 of 16
SCHEME 5 Proposed mechanism for
oxidative coupling of thiols with urea/H
2 2
O
(
UHP) catalyzed by γ-Fe @[VO(salenac-OH)]
2 3
O
2 3
TABLE 5 Comparison of catalytic activity of γ-Fe O @[VO(salenac-OH)] with the reported catalysts in the oxidation of methyl phenyl
sulfide and oxidative coupling of 4-methyl thiophenol
Time
(min)
Yield
(%)
Entry Substrate
Methyl phenyl
Catalyst
Conditions
Ref.
[
44]
1
2
3
4
5
6
7
8
9
1
1
1
V(IV)O-MCM-41
EtOH/UHP, r.t.
480
95
83
100
99
83
93
95
96
94
99
83
95
Nikoorazm et al.
sulfide
[45]
Methyl phenyl
sulfide
Fe
3
O
4
@SiO
2
@VO(salen)
CH
3
OH/CH
2
Cl
2
/
360
Bagherzadeh et al.
Bagherzadeh et al.
UHP, r.t.
ꢀ
[46]
Methyl phenyl
sulfide
VO(acac)@Sb@SMNPs
C
2
H
4
Cl
2
/TBHP, 85 C 60
3
ꢀ
[47]
Methyl phenyl
sulfide
Co@SiO
Mn(III)
Fe /salen of Cu(II)
2
[(EtO)
3
Si-L ]/
CH
EtOH/H
Cl /H
3
CN/H
2
O
2
, 65 C
50
Ghahri-Saremi et al.
ꢀ
Methyl phenyl
sulfide
3
O
4
2
O
2
, 60 C
180
60
Ghorbani-Choghamarani
[
48]
et al.
This work
Methyl phenyl
sulfide
γ-Fe
2
O
3
@[VO(salenac-
C
2
H
4
2
2 2
O , r.t.
OH)]
ꢀ
[49]
4-Methyl
thiophenol
Co(phcy)(SO
Co-Salophen
3
Na)
4
H
2
O/air, 60 C
360
180
80
Dou et al.
ꢀ
[50]
4-Methyl
thiophenol
EtOH/air, 60 C
Chai et al.
4-Methyl
thiophenol
Ni-SMTU@boehmite
EtOH/H
2
O
2
, r.t.
Ghorbani-Choghamarani
et al.
[51]
[
52]
0
1
2
4-Methyl
thiophenol
Fe
3
O
4
-Adenine-Zn
@SiO -NH @Mn
@[VO(salenac-
EtOAc/H
2
O
2
, r.t.
90
Tamoradi et al.
[53]
4-Methyl
thiophenol
Fe
3
O
4
2
2
CH
UHP, r.t.
EtOH/UHP, r.t.
2
Cl
2
/CH
3
OH,
120
45
Bagherzadeh et al.
This work
(III)
γ-Fe
OH)]
4-Methyl
2
O
3
thiophenol
2 2
Abbreviations: MNPs, magnetic nanoparticles; r.t., room temperature; UHP, urea/H O .

14 of 16
MAHDIAN ET AL.
TABLE 6 Reusability of the γ-Fe
catalyst in the oxidation of dibenzyl sulfide with 30% H
2
O
3
@[VO(salenac-OH)]
advantages, namely, high yields of products, short reac-
tion times, and convenient catalyst recyclability by an
applied magnetic field.
2
O
2
at room
temperature
Run
Yield (%)
TON
135.7
132.8
128.6
128.6
124.3
TOF (h−
1 a
)
1
2
3
4
5
95
93
90
90
87
135.7
3.4 | Catalyst reusability
132.8
128.6
The recovery and reusability of catalysts are of great con-
cern from economic, environmental, and industrial per-
spectives. In this regard, the reusability of the
synthesized γ-Fe O @[VO(salenac-OH)] catalyst was
128.6
124.3
2
3
Note: Reaction conditions: dibenzyl sulfide (1 mmol), C
2 4 2
H Cl (4 ml), 30%
investigated in the oxidation of dibenzyl sulfide and oxi-
dative coupling of 4-methyl thiophenol under the opti-
mum reaction conditions. Following each reaction, the
catalyst was collected magnetically from the reaction
2 2
H O
(3 mmol), catalyst (0.70 mol%), reaction time (1 h).
a
TOF = TON/time (h).
TABLE 7 Reusability of the γ-Fe
2
O
3
@[VO(salenac-OH)]
mixture and used in the next run after washing with
ꢀ
catalyst in the oxidative coupling of 4-methyl thiophenol with urea/
at room temperature
Et O and drying in the oven at 80 C. The results in
2
2 2
H O
Tables 6 and 7 show that the catalyst can be reused five
times in both oxidation reactions with no considerable
loss in its initial activity.
TOF (h−
1 a
Run
Yield (%)
TON
135.7
131.4
128.6
124.3
124.3
)
1
2
3
4
5
95
92
90
87
87
180.9
175.2
171.4
165.7
165.7
The recovered catalyst has also been characterized via
FT-IR and XRD techniques. The FT-IR spectrum of the
recovered catalyst after five runs is shown in Figure 3d.
−
1
The band around 550–640 cm corresponds to Fe O
vibrations, and the ones at 2919 and 1447 cm− are asso-
ciated with aliphatic CH2 groups and aromatic C C
stretching vibrations, respectively. Furthermore, the two
1
Note: Reaction conditions: 4-methyl thiophenol (1 mmol), EtOH (4 ml),
urea/H (5 mmol), catalyst (0.70 mol%), reaction time (45 min).
TOF = TON/time (h).
2 2
O
a
−1
bands at 1625 and 1593 cm are assigned to two differ-
ent C N stretching vibrations of the Schiff base ligand.
−
1
Also, the recovered catalyst exhibits a band at 985 cm
The
activity
of
γ-Fe O @[VO(salenac-OH)]
corresponding to the V O stretching vibrations. All these
IR bands emerge for the fresh catalyst (Figure 3c) as well.
The XRD pattern of the recovered catalyst is shown in
Figure 4b. As seen, it is in good agreement with that of
the fresh catalyst (Figure 4a). Consequently, the
2
3
nanocatalyst in the catalytic oxidation of methyl phenyl
sulfide and oxidative coupling of 4-methyl thiophenol
was compared with the performance of some other cata-
lysts (Table 5). Accordingly, these protocols have several
FIGURE 10 Leaching test of γ-Fe
salenac-OH)] catalyst in the oxidation of
dibenzyl sulfide
2 3
O @[VO
(

MAHDIAN ET AL.
15 of 16
synthesized γ-Fe O @[VO(salenac-OH)] catalyst has
investigation; visualization. Samira Saeednia: Concep-
2
3
been stable during the present oxidation reactions.
tualization; formal analysis; investigation; visualization.
DATA AVAILABILITY STATEMENT
The data that support the finding of this study are openly
available in references.
3.5 | Leaching test
To verify that the vanadium leaching did not occur dur-
ing the reaction, oxidation of dibenzyl sulfide was carried
out in the optimized conditions. The catalyst was sepa-
rated from the catalytic system when the yield was about
ORCID
Abolfazl Mahdian https://orcid.org/0000-0001-6445-
0930
5
7% after 0.5 h. Subsequently, the filtrate was stirred at
Mehdi Hatefi Ardakani https://orcid.org/0000-0002-
room temperature for an additional 1 h. No further yield
of the reaction was observed after the removal of the
catalyst (Figure 10). Also, the vanadium content in the
filtrate solution was analyzed by atomic absorption spec-
troscopy (AAS), and it was found that only 1.5% of initial
vanadium dissolved in the filtrate solution. These results
show that leaching of the [VO(salenac-OH)] complex
from the catalyst sample during the oxidation reaction
was negligible, and the catalyst was stable.
2216-6840
Esmaeil Heydari-Bafrooei https://orcid.org/0000-0001-
9652-4041
Samira Saeednia https://orcid.org/0000-0003-1058-
2246
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