Oxo-vanadium(IV) Schiff base complex supported on modified MCM-41: A reusable and efficient catalyst for the oxidation of sulfides and oxidative S-S coupling of thiols

Article abstract of DOI:10.1002/aoc.3295

Oxo-vanadium(IV) Schiff base complex supported on MCM-41 as an organic-inorganic hybrid heterogeneous catalyst was synthesized with post-grafting of MCM-41 with 3-aminoropropyltrimethoxysilane and subsequent reaction with 3,4-dihydroxybenzaldehyde and then complexation with oxo-vanadium acetylacetonate salt. The catalyst was analysed using a series of characterization techniques such as Fourier transform infrared spectroscopy, small-angle X-ray diffraction, nitrogen absorption isotherm, transmission electron microscopy and thermogravimetric analysis. The data collected provided evidence that the vanadium complex was anchored onto MCM-41. High catalytic activity of this catalyst was observed in the oxidation of various sulfides and thiols (into sulfoxides and disulfides, respectively) with urea hydrogen peroxide as oxidant in high to excellent yields and selectivity under mild conditions. The heterogeneous catalyst could be recovered easily and reused several times without significant loss in catalytic activity and selectivity.

Full text of DOI:10.1002/aoc.3295

Full Paper
Received: 13 December 2014
Revised: 16 January 2015
Accepted: 21 January 2015
Published online in Wiley Online Library: 21 March 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3295
Oxo-vanadium(IV) Schiff base complex
supported on modified MCM-41: a reusable and
efficient catalyst for the oxidation of sulfides
and oxidative S–S coupling of thiols
Mohsen Nikoorazm*, Arash Ghorbani-Choghamarani and Nourolah Noori
Oxo-vanadium(IV) Schiff base complex supported on MCM-41 as an organic–inorganic hybrid heterogeneous catalyst was synthe-
sized with post-grafting of MCM-41 with 3-aminoropropyltrimethoxysilane and subsequent reaction with 3,4-
dihydroxybenzaldehyde and then complexation with oxo-vanadium acetylacetonate salt. The catalyst was analysed using a series
of characterization techniques such as Fourier transform infrared spectroscopy, small-angle X-ray diffraction, nitrogen absorption
isotherm, transmission electron microscopy and thermogravimetric analysis. The data collected provided evidence that the vana-
dium complex was anchored onto MCM-41. High catalytic activity of this catalyst was observed in the oxidation of various sulfides
and thiols (into sulfoxides and disulfides, respectively) with urea hydrogen peroxide as oxidant in high to excellent yields and se-
lectivity under mild conditions. The heterogeneous catalyst could be recovered easily and reused several times without significant
loss in catalytic activity and selectivity. Copyright © 2015 John Wiley & Sons, Ltd.
Keywords: MCM-41; VO(IV)–MCM-41; heterogeneous; sulfoxide; disulfides; urea hydrogen peroxide (UHP)
synthetic intermediates for the construction of various chemically
Introduction
and biologically crucial molecules,[17] particularly drugs and natural
products.[18] Likewise, oxidative coupling of thiols to disulfides is im-
Homogeneous conventional oxidation catalysts (i.e. transition
portant in biological processes and synthetic organic chemistry.[19]
metal complexes and soluble transition metal salts) have been
widely employed for the oxidation of organic substrates such as
sulfides, thiols, cyclohexene, styrene, etc., because of their high cat-
alytic performance.[1–3] However, due to the high costs of recover-
ing and recycling these catalysts at the end of the reaction, their
applications are seriously constrained. The irreversible deactivation
of active metal complex moieties due to the formation of several di-
meric and polymeric species, particularly when oxidant is added, is
also a limitation.[4] To make the catalytic process green and sustain-
able, several methods and strategies have been employed to im-
mobilize homogeneous catalysts using various insoluble supports
such as organic polymers, polystyrene or resins,[5] mesoporous
silica (e.g. MCM-41),[6] clays,[7] zeolite Y,[8] etc.
In biological systems, thiols are oxidized by flavins, cytochromes
and dehydroascorbic acid; due to this process the cellular redox po-
tential is controlled and oxidative damage is prevented..[20,21] Also,
disulfides are important reagents in organic synthesis and can be
used to prepared sulfinyl and sulfenyl compounds.[22]
In continuation of our studies of the application of new
heterogeneous systems in organic reactions, herein we report
the synthesis and characterization of oxo-vanadium(IV)
grafted/immobilized onto MCM-41 (VO(IV)–MCM-41). The
heterogenized complex was used in catalytic studies of
sulfoxidation and oxidative coupling of thiols in the presence
of urea hydrogen peroxide (UHP) as oxidant.
Recently, the focus has shifted towards mesoporous MCM-41 as a
major support material because of the particular features that it
possesses such as large surface area, variable pore diameter and
high-density surface silanol sites.[9] Mesoporous MCM-41 provides
an anchoring for valuable homogeneous catalysts/active organic
moieties or the incorporation of redox metal ions into framework
positions in a quite simple way, and thus increases their intrinsic
catalytic activity as a result of the amorphous nature of the pore
walls.[10–13] For example, MCM-41-supported oxo-vanadium shows
high selectivity for organic reactions such as epoxidation of olefins
and bromination of hydroxyl aromatic compounds and
alcohols.[14–16] Furthermore, the selective oxidation reaction of sul-
fides to sulfoxides is very significant because of critical applications
in organic synthetic chemistry, and also sulfoxides are beneficial
Experimental
Materials and Physical Techniques
The cationic surfactant cetyltrimethylammonium bromide (CTAB;
98%), 3,4-dihydroxybenzaldehyde, tetraethylorthosilicate (TEOS),
*
Correspondence to: Mohsen Nikoorazm, Department of Chemistry, Faculty of
Science, Ilam University, PO Box 69315516, Ilam, Iran.
E-mail: e_nikoorazm@yahoo.com
Department of Chemistry, Faculty of Science, Ilam University, PO Box 69315516,
Ilam, Iran
Appl. Organometal. Chem. 2015, 29, 328–333
Copyright © 2015 John Wiley & Sons, Ltd.

Synthesis of oxo-vanadium(IV) complex on modified MCM-41
3-aminopropyltrimethoxysilane (APTES), sodium hydroxide, VO
(acac)2, organic sulfides, UHP and solvents were purchased from
Merck, Fluka and Aldrich. The physicochemical characteristics were
determined using powder X-ray diffraction (XRD) with a Philips
X’pert diffractometer with monochromatized Cu Kα radiation under
the conditions of 40 kV, λ = 1.5418Å and 30 mA. FT-IR spectra
of samples were recorded using KBr matrix in the range
400–4000 cmꢀ1 with a Bruker VRTEX 70 FT-IR spectrometer. Nitro-
gen adsorption–desorption isotherm and Barrett–Joyner–Halenda
(BJH) pore size distribution were recorded with a BELSORP-MINI.
TGA of the samples was carried out using a Shimadzu DTG-60 auto-
matic thermal analyser in the temperature range 30–900 °C at a
heating rate of 10 °Cminꢀ1 in air.
Preparation of Mesoporous Silica MCM-41
Parent silica MCM-41 was prepared using
a
standard
procedure.[23] The source of silicon was TEOS and the
structure-directing agent was CTAB. A typical synthesis gel was
prepared by adding gradually 5.78 g of TEOS to an aqueous so-
lution containing 1.01 g of CTAB, 0.34 g of NaOH and 30 ml of
deionized water. After stirring for about 1 h at room tempera-
ture, the resulting homogeneous mixture was crystallized under
static hydrothermal conditions at 383 K in a Teflon-lined auto-
clave for 96h. After cooling to room temperature the resultant
solid was recovered by filtration, washed with deionized water
and dried in air at 353 K. The collected product was calcined
at 813 K for 5 h with a ramp of 2 °Cminꢀ1 to remove the CTAB
surfactant. Finally, we obtained mesoporous MCM-41.
Scheme 1. Preparation of VO(IV)–MCM-41.
Organic Modification of MCM-41 with APTES
Silica MCM-41 (4.8 g) with 4.8 g of APTES in n-hexane (96 ml) was
stirred under reflux conditions and nitrogen atmosphere for 24h.
After completion of the reaction, the white solid of APTES-
functionalized MCM-41 (APTES@MCM-41) was filtered, washed with
n-hexane and dried under vacuum (Scheme 1).
Scheme 2. Oxidation of sulfides and oxidative coupling of thiols catalysed
by VO(IV)–MCM-41.
Preparation of VO(IV)–MCM-41
MCM-41-supported vanadium catalysts were prepared from the re-
action of APTES@MCM-41 (1mmol, 1 g) with 3,4-
dihydroxybenzladehyde (1mmol, 0.134 g) in ethanol under reflux
conditions and nitrogen atmosphere for 3 h. The yellowish solid
was collected by filtration, washed with ethanol and dried under
vacuum. VO(IV)–MCM-41 was prepared by dissolving (0.25 mmol,
0.066 g) of VO(acac)2 and the yellowish solid obtained as described
above in chloroform (20 ml). This mixture was stirred for 24 h at
60 °C. The resulting light greenish solid was recovered by vacuum
filtration, washed with chloroform using Soxhlet extraction for
12 h to remove any unanchored vanadyl species and dried under
vacuum (Scheme 1).
(3× 10 ml) and was dried over anhydrous Na2SO3. Simple filtration
followed by evaporation of the solvent from the filtrate gave the
pure product. Oxidation products were obtained in very high iso-
lated yields.
General Procedure for Oxidative Coupling of
Thiols to Disulfides
A 25ml round-bottom flask was charged with thiol (1mmol), UHP
(5mmol), VO(IV)–MCM-41 (20 mg) and acetonitrile (4ml) as solvent
(Scheme 2). The reaction mixture was stirred at room temperature,
and the progress of the reaction was monitored using TLC. After
completion of the reaction, the corresponding disulfides were eas-
ily obtained by extraction with CH2Cl2.
Oxidation of Sulfides into Sulfoxides using Catalytic Amounts
of VO(IV)–MCM-41 and UHP
The catalytic activity of VO(IV)–MCM-41 in the oxidation reactions of
various sulfides was investigated using the following procedure. A
mixture of sulfide (1mmol), UHP (5mmol) and VO(IV)–MCM-41
(20 mg) was added to ethanol (4ml; Scheme 2), and the mixture
was stirred magnetically at room temperature for the appropriate
time. The reaction progress was monitored using TLC. After com-
pletion of the reaction, the product was extracted with CH2Cl2
Results and Discussion
Small-Angle XRD Patterns
The powder XRD patterns for the parent MCM-41 and samples de-
rived by post-synthetic grafting are shown in Fig. 1. After post-
synthetic grafting, the position of d100 (2θ = 2.21°) reflection
Appl. Organometal. Chem. 2015, 29, 328–333
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

M. Nikoorazm et al.
and 1468 cmꢀ1. The band at 1657 cmꢀ1 is assigned to –C¼N
stretching vibration of the imine group. The FT-IR spectrum of
VO(IV)–MCM-41 displays
a
peak for C¼N stretching at
1644 cmꢀ1. This shift to lower wavenumber is probably attributed
to coordination of the complex with lone electron pair of nitrogen
group of (C¼N) as a sigma donor ligand. This result is evidence for
the successful coordination of vanadium ion to functionalized
MCM-41.
BET Measurements
Figure 3 shows the nitrogen adsorption–desorption isotherms of
MCM-41, MCM-41-NH2 and VO(IV)–MCM-41. Isotherm data ex-
hibit typical irreversible type IV sorption isotherms as defined
by IUPAC with a small hysteresis loop in the P/P0 range 0.5 to
0.8 for MCM-41, MCM-41-NH2 and VO(IV)–MCM-41. The nitrogen
sorption analysis indicates that the surface areas of MCM-41 and
VO(IV)–MCM-41 are 986.16 and 258.10 m2 gꢀ1, respectively. The
average pore diameter is calculated to be 3.65 using the BJH
method. The pore diameter and pore volume of the mesopores
indicate a marked decrease as a result of grafting vanadium
Schiff base complex on the surface of the functionalized MCM-
41 material. The results indicate that due to the presence of
the large organic moieties within the pore channels of mesopo-
rous silica the vanadium complex loading appears as a thin layer
on the inner surface of MCM-41. The results are summarized in
Table 1.
Figure 1. X-ray diffractgrams of (top to bottom) MCM-41, MCM 41-NH2,
ligand, VO(IV)–MCM-41 and recovered VO(IV)–MCM-41.
remains virtually constant in all samples, suggesting that the hexag-
onal pore arrangement of MCM-41 is intact. Comparison of powder
XRD patterns of MCM-41 and VO(IV)–MCM-41shows that after an-
choring VO(IV) through complex formation into MCM-41, the d110
and d200 reflections are no longer observed. A similar type of behav-
iour was observed by Joseph et al.[24] Furthermore an overall de-
crease in the intensity of d100 is seen, this decrease in intensity
being significant for the VO(IV)–MCM-41sample. This is probably
due to the difference in the scattering contrast of the pores and
the walls, and to the irregular immobilization grafting of vanadium
complex on the nano-channels of MCM-41.
Transmission Electron Microscopy (TEM)
FT-IR Spectroscopy
A TEM image of MCM-41 sample (Fig. 4) shows an arrangement of
well-ordered hexagonal arrays of mesopores (two-dimensional
channels) and long-range mesoporous architecture (50 nm). This
is in good agreement with the XRD and BET results discussed as
above.
The FT-IR spectra of the samples in the range 400–4000 cmꢀ1 are
depicted in Fig. 2. The absorption band around 1100–1200 cmꢀ1
is due to Si–O asymmetric stretching vibrations of Si–O–Si bridges
which indicates the presence of silica in MCM-41. The spectrum of
functionalized MCM-41 shows C–H stretching vibration at
2932 cmꢀ1 and also two bands (N–H group) appearing at 1642
Thermogravimetric Analysis (TGA)
The TGA results of MCM-41, MCM-41-NH2 and VO(IV)–MCM-41 are
shown in Fig. 5. TGA patterns of MCM-41 indicate a sharp weight
Figure 2. FT-IR spectra of (a) MCM-41, (b) MCM-41-NH2, (c) ligand, (d) VO
(IV)–MCM-41 and (e) fifth-stage recovery of VO(IV)–MCM-41.
Figure 3. Nitrogen adsorption–desorption isotherms of (top to bottom)
MCM-41, MCM-41-NH2 and VO(IV)–MCM-41.
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Appl. Organometal. Chem. 2015, 29, 328–333

Synthesis of oxo-vanadium(IV) complex on modified MCM-41
Table 1. Texture parameters obtained from nitrogen sorption studies
weight loss above 350 °C that is mainly related to the decomposition
of organic groups. Based on TGA, about 1.2 and 0.5 mmol of amine
groups and the ligand are grafted on the MCM-41 surface, respec-
tively. The TGA pattern of VO(IV)–MCM-41 indicates three weight
loss steps: the first (4 wt%) below 120 °C is the result of physically
adsorbed water, the second (5%) at 250–370 °C is attributed mostly
to the combustion of amine and the third (4 wt%) at 450–650 °C is
likely related to the decomposition of Schiff base groups.
Sample
SBET
Pore diameter
by BJH
method (nm)
Pore
Wall
diameter
(nm)
(m2 gꢀ1
)
volume
(cm3 gꢀ1
)
MCM-41
986.16
694.98
258.10
3.65
3.3
0.711
0.340
0.191
0.902
0.906
1.42
NH2-MCM-41
VO(IV)–MCM-
41
1.34
Oxidation of Sulfides and Oxidative Coupling of Thiols with
UHP Catalysed by VO(IV)–MCM-41
The catalytic activity of VO(IV)–MCM-41 was evaluated for the oxi-
dation of sulfides and oxidative coupling of thiols with UHP under
mild conditions (Scheme 2). To achieve optimum reaction condi-
tions, various parameters, namely the nature of solvent, amount
of catalyst and UHP, were studied.
Dibenzylsulfide and 4-methylthiophenol were selected as
model substrates to optimize reaction conditions. In order to
find an appropriate solvent for the oxidation of sulfides and ox-
idative coupling of thiols, we designed typical reactions for these
transformations in various organic solvents. Ethanol was chosen
as reaction medium for the oxidation of sulfides while for oxida-
tive coupling of thiols, acetonitrile was selected based on reactiv-
ity and selectivity (Table 2).
After optimizing the solvent, catalyst amounts were varied
from 0.003 to 0.025g for reactions carried out at room temper-
ature. The results are shown in Fig. 6. There is a general trend
of increasing conversion of thiol and sulfide to the oxidized
products (blank substrates) with increasing amount of catalyst.
These results demonstrate clearly that VO(IV)–MCM-41 catalyst
is very active in this reactions system, and that even a small
amount (20 mg) can lead to significant conversion.
Finally the variation of substrate/UHP molar ratio was inves-
tigated. The optimum ratio of blank substrate to UHP of
1:6 mmol in the presence of catalyst is found to be ideal for
oxidative coupling of thiols and oxidation of sulfides. It is ob-
served that for a fixed amount of substrate (1 mmol), 20 mg of
catalyst and 6 mmol of UHP in 4 ml of solvent are found to be
the best conditions under which to achieve 100% conversion
of substrates (sulfides and thiols). The results are summarized
in Tables 3 and 4. An examination of the data in Table 3 indi-
cates that various types of aryl- and alkylsulfides are
converted successfully to sulfoxides in good yields at room
temperature. All the reactions take place with total selectivity
Figure 4. TEM image of MCM-41.
loss below 100 °C corresponding to the loss of physisorbed water,
and the TGA pattern of MCM-41-NH2 indicates a three-step weight
loss at temperatures ranging from 30 to 800 °C. The weight loss
(3 wt%) below 120 °C is the result of physically adsorbed water, the
8% weight loss at 200–300 °C typically is related to the elimination
of crystallization and the desorption of physisorbed water, whereas
the third weight loss is related to the decomposition of organic
groups. The TGA pattern of ligand-grafted MCM-41 indicates 13%
Table 2. Effect of various solvents on the oxidation of sulfides and ox-
idative coupling of thiols
Entry
Solvent
Yield (%)a Yield (%)b Time (h)a Time (h)b
1
2
3
4
5
6
Ethanol
98
85
91
87
88
90
83
80
94
83
89
87
1.75
5
0.66
4.25
0.33
8
Chloroform
Acetonitrile
Water
3.25
10
7.5
11
Dichloromethane
n-Hexane
12
7
aReaction conditions: dibenzylsulfide (1 mmol), UHP (5 mmol), catalyst
(20 mg), 4 ml solvent.
bReaction conditions: 4-methylthiophenol (1 mmol), UHP (5 mmol),
catalyst (20 mg), 4 ml solvent.
Figure 5. TGA curves of (a) MCM-41, (b) MCM-41-NH2, (c) MCM-41-ligand
and (d) VO(IV)–MCM-41.
Appl. Organometal. Chem. 2015, 29, 328–333
Copyright © 2015 John Wiley & Sons, Ltd.
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M. Nikoorazm et al.
Table 4. Oxidation of various thiols using UHP, catalysed by VO(IV)–
MCM-41 in acetonitrilea R–SH → R–S–S–R
Entry
R
Yield (%)b
Time (min)
M.p. (°C)
1
p-MeC6H4–
–CH2COOH
2-Naphthyl
2,6-Di-ClC6H3–
p-BrC6H4-
96
92
90
94
91
95
30
89
94
93
90
20
11
50
21
20
11
87
85
18
24
25
41–43[32]
Oil[32]
2
3
142–144[32]
76–90[33]
90–92[33]
55–56[34]
55–57[30]
85–89[32]
175–178[32]
Oil[30]
4
5
6
2-Pyridyl–
Figure 6. Effect of amount of catalyst on the oxidation of dibenzylsulfide
and oxidative coupling of 4-methylthiophenol.
7
C6H5–
8
2-Benzoxazole
2-Benzothiazole
HOCH2CH2–
2-COOHC6H4–
9
10
11
for sulfoxide formation; no over-oxidation products such as
276–282[32]
sulfones are found in the reaction mixtures.
A similar
aReactions were run under air at room temperature and in the presence
of substrate (1 mmol), UHP (5 mmol) and 20 mg of catalyst.
bIsolated yield.
mesporous catalyst has previously been reported by Gao
and co-workers[13] with hydrogen peroxide at higher temper-
ature with lower yields; therefore our procedure shows more
activity and efficiency compared with similar previously re-
ported procedures.
The data in Table 4 also demonstrate that all oxidative cou-
pling reactions are conducted under totally heterogeneous
conditions at room temperature and they are successfully
converted into disulfide with good to excellent yields (except
entry 12).
A possible mechanism for the oxidation of sulfides and
thiols to the corresponding sulfoxides and disulfides using
UHP as oxidant in the presence of VO(IV)–MCM-41 is outlined
in Scheme 3.
Finally, the reusability of the VO(IV)–MCM-41 catalyst was in-
vestigated. Fig. 7 shows the yield of four consecutive cycles for
the oxidation of dibenzylsulfide and 4-methylthiphenol. After
the reaction reached completion, the catalyst was recovered,
washed with ethanol and water then reused in the next cycle.
The results show only an 11% decrease in the conversion after
four cycles. Therefore these results demonstrate that the reus-
ability of the catalyst satifactory.
Table 3. Oxidation of various sulfides by UHP catalysed by V(IV)–MCM-
41 in ethanola R1–S–R2 → R1–(S¼O)–R2
Entry
R1
R2
Yield Time
(%)b (h)
M.p. (°C)
1
2
3
4
5
6
7
8
9
Ph
CH2CH2OH
PhCH2
Ph
93 20
147–149[25]
PhCH2
Ph
98
89 12 days 68–70[26]
95 18
120–122[25]
8.25 88–90[27]
103–104[26]
1.75 131–134[25]
Ph
PhCH2
C12H25
CH3CH2
Furan
CH3
C12H25
CH3CH2
Furan
94
95
94
93
96
95
92
1
3.25 83–85[28]
Scheme 3. A possible mechanism path for (A) oxidation of sulfides to
sulfoxides and (B) oxidative coupling of thiols to disulfides.
Furyl–CH2
1
Oil[29]
Liquid[30]
29–31[25]
PhCH2
CH3
1.5
8
10 Ph
CH3
11 Tetrahydrothiophene
12 CH3
—
0.75 106–109[31]
Conclusions
CH2CH2COOCH3 89
PhCHCOOH
1.75 Liquid[30]
13 Ph
90 10
73–81
Oil[31]
We have succeeded in preparing a heterogeneous catalyst by
post-grafting VO(acac)2 to form a Schiff base complex into func-
tionalized MCM-41. VO(IV)–MCM-41showed excellent catalytic ac-
tivity in the oxidation reactions of various sulfides and thiols
with good conversion and 100% selectivity using UHP as the ox-
idant. In addition, the catalyst can be recycled several times
14 CH3
CH2CH2CHO
93
1.33
aReactions were run under air at room temperature and in the presence
of substrate (1 mmol), UHP (5 mmol) and 20 mg of catalyst.
bIsolated yield.
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Appl. Organometal. Chem. 2015, 29, 328–333

Synthesis of oxo-vanadium(IV) complex on modified MCM-41
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Acknowledgments
The authors thank the research facilities of Ilam University, Ilam,
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