Synthesis and characterization of Co (II) and Fe (III) Schiff base complexes grafted onto mesoporous MCM-41: A heterogeneous and recyclable nanocatalysts for the selective oxidation of sulfides and oxidative coupling of thiols

Article abstract of DOI:10.1080/10426507.2016.1208200

Novel organic–inorganic hybrid heterogeneous catalysts containing cobalt(II) and iron(III) Schiff base complexes, grafted on the internal surface of MCM-41 pores were prepared by introducing a metal salt into a mesoporous silica functionalized with a Schiff base ligand. The chemical and physical properties of the catalysts were investigated by BET, TGA, XRD, FT-IR, and TEM techniques. These complexes were found to be efficient, selective catalysts for the oxidation of various sulfides to sulfoxides and oxidative coupling of thiols to their corresponding disulfides with urea hydrogen peroxide in excellent yield at room temperature. The designed catalytic system prevents effectively the overoxidation of sulfides and thiols to sulfoxides and sulfones, respectively. Also the heterogeneous catalysts can be recovered easily and reused many times without significant loss of activity and selectivity.

Full text of DOI:10.1080/10426507.2016.1208200

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Synthesis and characterization of Co (II) and Fe (III)
Schiff base complexes grafted onto mesoporous
MCM-4: A heterogeneous and recyclable
nanocatalysts for the selective oxidation of
sulfides and oxidative coupling of thiols
Nourolah Noori, Mohsen Nikoorazm & Arash Ghorbani-Choghamarani
To cite this article: Nourolah Noori, Mohsen Nikoorazm & Arash Ghorbani-Choghamarani
(2016): Synthesis and characterization of Co (II) and Fe (III) Schiff base complexes grafted onto
mesoporous MCM-4: A heterogeneous and recyclable nanocatalysts for the selective oxidation
of sulfides and oxidative coupling of thiols, Phosphorus, Sulfur, and Silicon and the Related
Elements, DOI: 10.1080/10426507.2016.1208200
To link to this article: http://dx.doi.org/10.1080/10426507.2016.1208200
Accepted author version posted online: 01
Jul 2016.
Published online: 01 Jul 2016.
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Synthesis and characterization of Co (II) and Fe (III) Schiff base complexes grafted onto
mesoporous MCM-4: A heterogeneous and recyclable nanocatalysts for the selective oxidation
of sulfides and oxidative coupling of thiols
*
Nourolah Noori, Mohsen Nikoorazm , Arash Ghorbani-Choghamarani
Department of Chemistry, Faculty of Science, Ilam University, P.O. Box 69315516, Ilam, Iran
*
Email: e_nikoorazm@yahoo.com
Abstract
Novel organic–inorganic hybrid heterogeneous catalysts containing cobalt(II) and
iron(III) Schiff base complexes, grafted on the internal surface of MCM-41 pores were
prepared by introducing a metal salt into a mesoporous silica functionalized with a
Schiff base ligand. The chemical and physical properties of the catalysts were
investigated by BET, TGA, XRD, FT-IR, and TEM techniques. These complexes were
found to be efficient, selective catalysts for the oxidation of various sulfides to
sulfoxides and oxidative coupling of thiols to their corresponding disulfides with urea
hydrogen peroxide (UHP) in excellent yield at room temperature. The designed
catalytic system prevents effectively the over-oxidation of sulfides and thiols to
sulfoxides and sulfones respectively. Also the heterogeneous catalysts can be recovered
easily and reused many times without significant loss of activity and selectivity.
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Key words
MCM-41, heterogeneous catalyst, urea hydrogen peroxide, oxidation, sulfides, thiols.
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INTRODUCTION
Catalysts have played a vital role in green chemistry because of their importance in fine
chemicals synthesis, organic reactions transformations, petrochemicals and have major impact on
economic progress as well as the quality of human life [1, 2]. Many homogeneous catalytic
systems cannot be commercialized, because most of homogeneous catalytic process widely
engaged in the manufacture of bulk as well as fine chemicals, producing large amount of side
waste material and imposing hazard impact on the surrounding environment and have difficulties
associated with separating the products from the catalyst [3]. To overcome these difficulties,
heterogeneous catalysts are developed, either by dispersing metal ions on porous solid supports,
or chemical binding of metal ions on functionalized solid supports. The second method of
synthesis of supported catalysts leads to simplification of reaction work-up and easy separation
of supported species from reaction mixture by filtration [4].
Since the synthesis of ordered mesoporous MCM-41 [5] a great affords have been made to
synthesis a new catalyst by immobilization of Schiff base complexes on MCM-41 [6-9]. The
important characteristics of mesoporous MCM-41 are its large surface area, hexagonal
°
arrangement of unidimensional pores in the range of 20-100 A , variable pore diameters, high
porosity and high-density surface silanol active sites, which can provide the immobilization of
active organic groups, metals and metal Schiff base complexes as valuable homogeneous
catalysts into MCM-41 framework [10-16]. Co@MCM-41 as well as Fe@MCM-41 is reported
to have catalytic activity in the oxidation of styrene [17, 18], cyclohexene oxidation reaction [18,
1
9] and they were also used as catalyst for oxidation of hydrocarbons [20]. Recently, a number
of reports have appeared describing the use of Schiff base complex as catalyst in the oxidation of
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sulfides. On the other hand, Sulfoxides are very important intermediates for the foods,
pharmaceuticals industry and for the construction of various biologically and chemically
significant molecules [21, 22] Sulfoxides are mostly prepared by the oxidation of their
corresponding sulfides. Moreover the controlled oxidative coupling of thiols to disulfides
without over-oxidation is important in organic synthesis, biological and chemical processes [23,
2
4] extensive studies have been carried out for their controlled oxidation with reagents such as
manganese(III) Schiff-base complex [25] , Fe(NO ) .9H O/Fe(HSO ) [26] iodine and
3
3
2
4 3
CeCl .7H O in graphite [23], N-phenyltriazolinedione [27] monochloro poly(styrenehydantoin)
3
2
beads [28] and Cu(II)–Schiff base complex-functionalized magnetic Fe O nanoparticles [29]
3
4
.
Although many methods have been developed for oxidation sulfides, and oxidative coupling of
thiols, the catalytic switching of oxidation to meet green chemical requirement still remains a
major challenge. Herein we report synthesis and characterization of Co (II) and Fe (III) Schiff
base complexes anchored on MCM-41. These compounds exhibit excellent selectivity and
catalytic activity in the oxidation of sulfides to sulfoxides and oxidative coupling of thiols into
disulfides using UHP as an oxidant at room temperature. Furthermore, catalysts are stable and
catalytic activity retains after reusability of the catalysts.
RESULT AND DISCUSSION
In continuation of our studies on application of new catalysts in organic transformations [30- 33],
we decided to prepare a novel heterogeneous catalyst. In this manner immobilized Co (II) and Fe
(
III) Schiff base complexes on MCM-41were synthesized and applied for the oxidation of
sulfides and oxidative coupling of thiols in the presence of urea hydrogen peroxide (UHP) as
oxidant.
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X-ray diffraction
Fig. 1 illustrates the low-angle powder X-ray diffraction patterns of Si- MCM-41, Co@MCM-41
and Fe@MCM-41 samples. The X-ray pattern of the Si-MCM-41 shows the presence of the
three reflection peaks corresponding to d1 0 0, d1 1 0 and d2 0 0 typically confirming the presence of
ordered hexagonal mesoporous structure of MCM-41. Upon post-synthetic grafting, the d110 and
d200 reflections were no longer observed and overall decrease in intensity of d100 was observed.
An overall decrease in the intensity of d100 was seen, this decrease in intensity were significant
for the Co@MCM-41 as well as Fe@MCM-41 samples. This is probably due to the difference in
the scattering contrast of the pores and the walls, and to the irregular immobilization grafting of
cobalt and iron Schiff base complexes on the nano channels of MCM-41 [34 and 35].
Nitrogen adsorption/desorption isotherms
Fig. 2 shows the N adsorption–desorption of MCM-41, MCM-41-NH ,Co@MCM-41 and
2
2
Fe@MCM-41. Analysis of N adsorption-desorption data exhibit a typical type-IV isotherm
2
(
defined by IUPAC). The relative pressure (P/P ) of the immobilized catalysts is shifted to a
0
lower value in comparison to the parent materials, indicating decrease in specific surface area
and pore volume which can attributed due to the incorporation of the cobalt and iron Schiff
base complex [36]. The textural properties of samples are listed in Table 1. The results
indicate modified samples possess lower pore size distribution compared to its parent's
o
material. With an increase in bulkiness inside the pores the hysteresis occurred at lower p/p
indicating the decrease of porous size. This is a clear indication of immobilization of (Co, Fe)
@
MCM-41inside the pore channel of mesoporous silica.
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FT-IR spectroscopy
FTIR spectra of the several materials prepared by post-grafting method are shown in Fig 3.
In the FTIR spectrum of calcined Si-MCM-41 (Figure S 1a Supplemental Materials) bands at
-1
-1
-1
-1
1
233 cm and 1079 cm (Si–O–Si stretching), 795 cm (Si–O–Si bending) and 450 cm (Si–O–
−
1
Si rocking) are observed. In the hydroxyl region (3200–3600cm ) a broad band is seen, which
can be attributed to the silanol groups inside the pores of Si-MCM-41. After the APTES grafting
-1
(
Fig. S 1b), a weak C-H stretching vibration at 2932 cm and also two band at 1642 and 1468
-1
cm (N-H group) were observed. The appearance of these bands in the IR spectra of MCM-41-
SiCH CH CH NH ) indicates the attachment of amino-propyl groups to the solid surface that
(
2
2
2
2 x
are absent in calcined Si-MCM-41. In the FT-IR spectrums of Co@MCM-41 and Fe@MCM-41,
-1
shifts to lower frequency appear at 1639 and 1641 cm respectively. This shift to lower
wavenumber is probably attributed to coordination of complex with lone electron pair of
nitrogen group of (C=N) as a sigma donor ligand. The FTIR results confirm the formation of the
Schiff base complexes and their immobilization within the pores of the parent MCM-41.
Thermogravimetric analysis
Thermo gravimetric (TG) analyses of the samples are given in Fig 4. A one-step weight loss with
approximate amount of 6% was observed for Si-MCM-41 (Figure S 2a Supplemental Materials)
which is due to absorbed water. The TGA curve of MCM-41-NH (Figure S 2b) shows two
2
º
distinct weight loss, the first weight loss (5.6%) in the region of 25-140 attributed to loss of
physisorbed water while the second weight loss(340-630°C) is due to the loss of organic
moieties. The TGA curve of MCM-41-Ligand (Fig .S2c) shows 13% weight loss at temperature
º
above 350 C that is mainly due to the decomposition of organic species. Based on TG analysis,
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about 1.2 and 0.5 mmol of amine groups and the ligand is grafted on the MCM-41 surface,
respectively. Also the curve of Fe@MCM-41 (Fig. S2d) shows three distinct regions of weight
º
º
º
loss at various temperature (below 100 C, 250-350 C and 450-600 C), the first weight loss (5%)
is due to adsorbed water, second weight loss (6%) is attributed to the loss of amino propyl
fragments and third weight loss (9%) is related to decomposition of Schiff base groups. The TG
spectra of the Co@MCM-41 (Fig. S2e) indicate three weight loss, first weight loss (3%) below
◦
1
3
00 C was the result of the physically adsorbed water, second step weight loss (5%) at 210-
00°C was related to the combustion of amine, and the third weight loss (4%) at 350-480 °C was
likely attributed to the decomposition of Schiff base groups.
TEM
The characteristic hexagonal channels of MCM-41 are clearly shown in Figure S 3
(Supplemental Materials). It is also observed a well-ordered hexagonal array of structures having
uniform diameters from 2 to 5 nm, which is confirmed with low angle XRD studies.
Oxidation of sulfides to the corresponding sulfoxides catalyzed by Co@MCM-41 or
Fe@MCM-41 with UHP as an oxidant at room temperature
The selective oxidation of sulfides to the corresponding sulfoxides requires the combination of
simplicity of method, rapid and selective reactions and high yields of products. In order to
achieve this goal, we disclosed a new heterogeneous catalytic protocol in the oxidation reactions
of different types of sulfide substrates. Herein, two heterogonous catalysts including Cobalt and
iron Schiff base complexes supported on modified MCM-41 were synthesized and catalytic
activity of these compounds were investigated in the selective oxidation of sulfides (Scheme 2,
Table 4).
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In order to optimize the reaction conditions, dibenzyl sulfide, was selected as model substrate
and it was subjected to different reaction conditions such as: solvent, amount of catalyst and
oxidant / substrate ratio. Initially, we screened different solvents for the oxidation of dibenzyl
sulfide with UHP in the presence of a catalytic amount of Co@MCM-41 or Fe@MCM-41. The
results recorded in Table 2 shows that dibenzyl sulfide was oxidized in ethanol faster than the
other solvents with high yield and short reaction time. Therefore, ethanol was proven to be an
effective solvent, and was used in subsequent optimization studies.
The optimum ratio of sulfide to UHP (1:6 mmol) in the presence of Co@MCM-41 or
Fe@MCM-41 (25 mg) in ethanol as solvent was found to be ideal for complete conversion of
sulfide to sulfoxide. For dibenzyl sulfide separate experiment showed the presence of the catalyst
is necessary, since oxidation with UHP alone occurs rather slowly.
Subsequently, the influence of the catalyst amount on the selective oxidation reaction of dibenzyl
sulfide (substrate: UHP molar ratio was 1:6 in ethanol as solvent) was carried out at room
temperature. The results show that, when amount of the catalyst was 0.025 g, oxidation of
dibenzyl sulfide was completed with good yield and short reaction time. The results are
summarized in Table 3.
Therefore, using the optimized reaction conditions, various types of sulfides were oxidized to the
corresponding sulfoxides (Table 4). In all the reactions no any by-products was observed, and all
products were obtained in high to excellent yields in the short reaction time.
The convenience of separation and reusability in the successive catalytic cycles is one of the
main advantages of heterogeneous catalysts. The reusability of the catalysts was studied in a
model reaction. Upon completion of the reaction, the catalyst was recovered, washed with
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ethanol and water then reused in the next cycle. The yield of the product was not substantially
changed after the fifth run (9% difference). The results are shown in Figure S 4 (Supplemental
Materials)
3
.7. Oxidative coupling of thiols to the corresponding disulfides catalyzed by Co@MCM-41
or Fe@MCM-41 with UHP oxidant at room temperature
Also we report here our efforts in developing new catalytic procedures for oxidative coupling of
thiols to disulfides using urea hydrogen peroxide (UHP) as oxidant (Scheme 3 and Table 7).
The influence of different reaction parameters on the selectivity and efficiency of oxidative
coupling of 4-methyl thiophenol as typical example in the presence catalytic amount of
Co@MCM-41 or Fe@MCM-41 was examined. Obviously, solvent plays an important role in the
organic reactions, in order to find appropriate solvent; we started our investigation with oxidative
coupling of 4-methyl thiophenol in different solvents. From the solvent screening (Table 5)
Ethanol was selected as the best solvent as lower catalytic activity was observed with other
solvents such as water, n-hexane, acetonitrile, dichloromethane, acetone and ethyl acetate.
To examine the effect of the catalyst loading in oxidative coupling of 4-methyl thiophenol,
systematic studies were carried out in the presence of different amounts of the catalyst (3, 5, 10,
1
5, 20, 22 mg) in ethanol as solvent (Table 6). Thus, the best yield was found in the presence of
just 20 mg of catalyst, and the higher amount of catalyst (22 mg) did not improve the results to
an appreciable extent (Table 6, entry 6).
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Subsequently with optimized experimental conditions for dibenzyl sulfide in hand, a series of
disulfides derivatives were prepared successfully with excellent yield. The results are illustrated
in Table 7.
The reusability of the catalyst system is an important advantage and makes them useful for
commercial applications, catalytic processes and for any catalytic reaction. Hence, the recovery
and reusability of Co@MCM-41 or Fe@MCM-41 were investigated in the oxidative coupling of
4
-methyl thiophenol. After the reaction reached completion, the catalyst was separated from
mixture by filtration, washed with ethanol and water then reused in the next cycle. As shown in
Figure S 5 (Supplemental Materials), the catalyst can be recycled up to 8 runs without any
significant decrease of their catalytic activity.
The possible mechanism for the oxidation of sulfides [47] and oxidative coupling thiols [48] to
the corresponding sulfoxides and disulfides using UHP as oxidant in the presence of Co@MCM1
or Fe@MCM-41 has been outlined in Scheme 4.
In order to investigate the efficiency of these nanocatalysts we compared our results on the
oxidation of methyl phenyl sulfide and oxidative coupling of 4-methylbenzenethiol with the
previously reported procedures in the literature (Table 8 entries). These results show the
efficiency of proposed methods in both reaction time and high yields of product as compared to
literature reports involving several heterogeneous systems under various conditions (Table 8).
CONCLUSIONS
The cobalt and iron Schiff base complexes were successfully anchored on MCM-41 via post-
grafting process. These mesoporous compounds catalyzed oxidation of sulfides to sulfoxides and
oxidative coupling of thiols into their corresponding disulfides in the presence of UHP. The main
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advantage of this catalytic system are easy catalyst preparation, mild reaction conditions, easy
workup procedure, high yields of products and finally these heterogeneous catalysts can be
recovered and re-used several times without significant loss of their catalytic activity and
selectivity.
EXPERIMENTAL
Materials
The tetraethylorthosilicate (TEOS, 98%), cationic surfactant cetyltrimethylammonium bromide
(CTAB, 98%), sodium hydroxide, organic sulfides, thiols, UHP, solvents, and other chemicals
were purchased from Merck, Fluka or Aldrich chemical Companies and were used as received.
Characterization techniques
Powder XRD patterns of samples were recorded on a Philips diffractometer of X’pert company
with monochromatized Cu Kα radiation under the conditions of 40 kV, λ = 1.5418 A and 30 mA.
FT-IR measurements were performed on a VRTEX 70 model Bruker FT-IR spectrometer, using
-1
KBr matrix in the range of 400-4000 cm . High resolution transmission electron microscopy
(HRTEM) was carried out on a Philips CM10 microscope at 200 kV. Thermal Gravimetric (TG)
analysis was performed by Shimadzu DTG-60 instrument. N adsorption–desorption isotherm
2
and BJH pore size distribution plot were recorded by (BELSORP-MINI). The content of Co and
Fe were measured using inductively coupled plasma-optical emission spectrometry (ICP-OES,
Perkin Elmer, optima 8000).
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Synthesis of MCM-41 mesoporous silica
CTAB (1.01 g) was dissolved in aqueous solution of NaOH (0.34 g in 30 mL deionized water).
After complete dissolution, of TEOS (5.78 g) was added slowly to the gel mixture and stirred
continuously for 1 h at room temperature and was transferred to a Teflon lined autoclave and was
statically heated at 383 k for 96 h. The resulting white solid was filtered, washed with deionized
water and dried at 353 K. The final MCM-41 mesoporous silica was received by calcination of
º
º
dried white solid at 550 C for 5h with a ramp of 2 C/min.
Synthesis of amino-functionalized MCM-41
Organic modification of MCM-41 with (3-aminopropyl) triethoxysilane (3-APTES) has been
performed by stirring MCM-41 (4.8 g) with a solution of 3-APTES in n-hexane (4.8 g in 96 mL)
°
at 70 C for 24 h under N atmosphere and reflux conditions. The white solid MCM41-
2
(
SiCH CH CH NH ) thus produced was filtered , washed with n-hexane and then dried under
2 2 2 2 x
vacuum (Scheme 1).
Synthesis of cobalt and iron Schiff base complexes grafted onto MCM-41 (Co@MCM-41
and Fe@ MCM-41)
APTES@MCM-41 (1.0 g) was refluxed with 3, 4-dihydroxybenzladehyde (1 mmol, 0.134g) in
ethanol (10 mL) under a N atmosphere for 3 h. Then the resulting yellowish solid (MCM-41-L)
2
was collected by filtration, washed with ethanol, and dried under vacuum. Eventually M@MCM-
4
1 (M= Co and Fe) were prepared by the stirring 1 g of above mentioned yellowish solid with of
Co (NO ) ·4H O or Fe (NO ) ·9H O (0.5 mmol in EtOH) at room temperature for 24 h. Finally,
3
2
2
3 3
2
the resulting solid was filtered, washed with ethanol and dried under vacuum (Scheme 1).
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Catalyst evaluation
Typical procedure for the synthesis of sulfoxides catalyzed by Co@MCM-41 or Fe@MCM-
4
1:As a typical procedure:
To a mixture of Sulfides (1 mmol) and the catalysts (25 mg) in EtOH (4 mL), UHP (5 mmol)
was added, and the resulting reaction mixture was stirred at room temperature for the specified
time (Table 4). After completion of the reaction, the catalyst was removed by simple filtration
and the resulting products were filtered and washed with dichloromethane (3×3mL).
Typical procedure for the synthesis of disulfides catalyzed by Co@MCM-41 or Fe@MCM-
4
1:As a typical procedure
A mixture of thiols (1 mmol), UHP (5 mmol) and Co or Fe@MCM-41 (20 mg) was stirred at
room temperature in ethanol and the progress of the reaction was monitored by TLC. After
completion of the reaction, the catalyst was removed and the products were extracted with
CH Cl (3×10 mL). The results are shown in Table. 7.
2
2
Selected spectral data
1
1
-(Propylsulfinyl) propane (Table 4, entry 6): H NMR (400 MHz, CDCl3): δ = 2.652.97(m,
H), 1.84-1.93 (m, 4H), 1.11 (t, J= 6 Hz, 6H) ppm.
4
1
Tetrahydrothiophene 1-oxide (Table 4, entry 11): H NMR (400MHz, CDCl ): δ =3.07 (t, J=
3
7
.6 Hz, 2H), 2.27 (t, J= 7.5 Hz, 2H) ppm.
1
Methyl (p-tolyl)sulfane (Table 4, entry 8): H NMR (400 MHz,CDCl3): δ = 2.22 (s, 3H), 2.57
(s, 3H), 7.19 (d, J= 3.2 Hz, 2H), 7.39 d, J= 3.2 Hz, 2H) ppm.
1
1
,2-di-p-tolyldisulfane (Table 7, entry 1): H NMR (400 MHz,CDCl ): δ = 7.43 (d, J= 7.89 Hz,
3
4
H), 7.12 (d, J= 7.9 Hz, 4H), 2.35 (s, 6H) ppm.
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1
1
2
1
,2-di(naphthalen-2-yl)disulfane (Table 7, entry 3): H NMR400 MHz, CDCl ): δ = 8.11 (s,
3
H), 7.80 (m, 4H), 7.74 (m, 2H), 7.63 (m, 2H), 7.49 (m, 4H) ppm.
1
,2-bis(4-bromophenyl)disulfane (Table 7, entry 5): H NMR 400 MHz, CDCl ): δ = 7.42 (t,
3
J= 8.2 Hz, 4H), 7.35 (d, J= 8.2 Hz, 4H) ppm.
1
1
,2-bis(benzo[d]thiazol-2-yl)disulfane (Table 7, entry 10): H NMR (400 MHz, CDCl ): δ =
3
7
.94 (d, J= 3.2 Hz, 2H) 7.80 (d, J= 3.2 Hz, 2H), 7.48 (t, J= 3.2 Hz, 2H), 7.36 (t, J= 3.2 Hz, 2H)
ppm.
1
2
,2'-Disulfanediyldiethanol (Table 7, entry 11): H NMR (400MHz, CDCl ): δ = 3.93 (t, J=5.6
3
Hz, 4H), 2.89 (t, J= 5.6 Hz, 4H), 2.60 (br, 2H) ppm.
ACKNOWLEDGMENTS
Authors thank the research facilities of Ilam University, Ilam, Iran, for financial support of this
research project.
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Table 1. Textural properties of MCM-41, MCM-41-NH , Co@MCM-41and Fe@MCM-41.
2
Sample
SBET
Pore diam by BJH
method (nm)
3.65
Pore vol
(cm /g)
0.711
0.340
wall diam
(nm)
0.902
2
3
(m /g)
MCM-41
MCM-41 -
NH2
986.16
694.98
3.3
0.906
Co@MCM-
100.19
150.22
1.5
1.8
0.185
0.206
1.66
1.44
4
1
Fe@MCM-
4
1
20
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a
Table 2 .The effect of different solvents on the oxidation of sulfide to sulfoxide.
Entry
Solvent
Time (h)
Yield %
Co@MCM41 Fe@MCM Co@MCM41 Fe@MCM
4
3
15
8
1
41
96
Trace
92
90
89
88
85
1
2
3
4
5
6
7
Ethanol
Water
n-hexane
Acetonitrile
Dichloromethane 10
2.5
10
4.5
4
98
91
93
95
91
90
87
5
12
7.5
4
Ethyl acetate
Acetone
8
5
a
The reactions condition: dibenzyl sulfide (1 mmol), catalyst (Co@MCM41( 0.4mol %),
Fe@MCM 41( 0.55 mol %)) and UHP (5 mmol) in 4 mL of solvent at room temperature.
21
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Table 3. Effect of amount of catalyst on selective oxidation of sulfides to sulfoxides with UHP
as oxidant.
Entry
catalyst mol %
Yield
Co@MCM-41
Fe@MCM-41 Co@MCM-41 Fe@MCM-41
1
2
3
4
5
6
-
0.05
0.08
0.16
0.24
0.07
0.11
0.22
0.33
0.44
0.55
68
76
83
85
90
98
60
75
80
82
88
96
0.32
0.4
7
a
The reactions condition: dibenzyl sulfide (1 mmol), UHP (5 mmol) in 4 mL of solvent at room
temperature.
22
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Table 4. Selective oxidation of sulfides to sulfoxides with UHP using Co@MCM-41or
a
Fe@MCM-41 as catalyst.
1
2
b
Ent R
ry
R
Yield %
Time (h)
Mp
Co@MC Fe@M Co@MC Fe@M Co@MC Fe@M
M41
CM-41 M41
CM-41 M41
CM-41
147-
1
48-
49
1
Ph
CH CH OH
93
90
18
20
[37]
2
2
1
149
1
33-
2
3
PhCH2
Ph
PhCH2
98
96
2.5
8 days
3
132-134
69-70
[37]
1
34
6
8-
Ph
93
95
97
90
95
96
12 days
[38]
7
0
1
20-
4
5
Ph
PhCH2
15
7
20
10
121-122
88-90
[37]
1
22
8
8-
C H
C H
12 25
[39]
1
2
25
9
0
1
02-
6
7
CH CH
CH CH
94
93
92
95
2
2.75
4.30
103-104
83-85
[38]
[40]
3
2
3
2
1
04
8
3-
Furan
Furan
2.25
8
5
[
41]
8
9
1
Furyl
PhCH2
Ph
CH3
CH3
CH3
95
96
95
93
95
92
0.75
1.5
1
3.5
5
Oil
Liq
Oil
[
42]
Liq
2
9-
0
1
2
6.25
30-31
[37]
3
1
Tetrahydr
othiophen
e
1
1
06-
1
-
97
92
96
91
1
0.75
4
106-109
[43]
09
CH CH CO
[42]
2
2
1
CH3
3.25
Liq
Liq
OCH3
a
The reactions condition: substrate (1mmol), UHP (6 mmol) and Co@MCM41( 0.4mol %),
Fe@MCM 41( 0.55 mol %)).
b
Isolated yield.
23
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Table 5. The effect of different solvents on the oxidative coupling of thiols to disulfides.
Entry
Solvent
Time (h)
Yield %
Co@MCM41 Fe@MCM Co@MCM41 Fe@MCM
4
1
7
3.5
4
5
4.25
4.75
1
41
97
90
93
91
92
95
96
1
2
3
4
5
6
7
Ethanol
Water
n-hexane
0.15
6
3
1.5
2
3
98
95
94
96
95
97
96
Acetonitrile
Dichloromethane
Acetone
Ethyl acetate
3.5
a
The reactions condition: 4-methyl thiophenol (1 mmol), catalyst (Co@MCM41( 0.4mol %),
Fe@MCM 41( 0.55 mol %))) and UHP (5 mmol) in 4 mL of solvent at room temperature
24
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Table 6. Effect of amount of catalyst on selective oxidative coupling of 4-methylthiophenol to
a
1
,2-di-p-tolyldisulfane with UHP as oxidant.
Entry
catalyst mol %
Yield
Co@MCM-41
Fe@MCM-41 Co@MCM-41 Fe@MCM-41
1
2
3
4
5
6
-
0.05
0.08
0.16
0.24
0.32
0.35
0.07
0.11
0.22
0.33
0.44
0.48
65
70
85
89
98
98
60
68
76
80
97
97
7
a
The reactions condition: 1 mmol of 4-methyl thiophenol, 5 mmol of UHP in 4 mL of solvent at
room temperature.
25
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Table 7. Selective oxidative coupling of thiols to disulfides with UHP using Co@MCM-41 or
a
Fe@MCM-41 as catalyst.
b
Entr
y
Yield
(%)
R
Time (min)
9
Mp (°C)
Co@MCM Fe@MCM
Fe@MCM Co@MCM Co@MCM
4
1
41
41
41
41-43
Oil
41
[
44]
1
p-MeC H
98
97
60
42-43
6
4-
[44]
2
3
-CH COOH
95
96
94
95
40
65
45
68
Oil
142-144
2
[
32]
[44]
2-Naphthyl
142-144
[
33]
[45]
4
5
2,6-diClC H -
94
95
95
93
38
26
36
21
76-90
76-90
91-92
6
3
[
45]
p-BrC H
90-92
55–56
55-57
6
4-
[
46]
5
4–56
6
2-Pyridylen-
C H -
94
92
15
12
[
42]
7
8
92
95
91
94
70
21
65
18
56-57
6
5
2
-
[44]
276–282
85–89
276–282
COOHC H -
6
4
2
-
[44]
9
93
97
95
96
70
36
75
41
87–89
Benzoxazole
2
-
[
44]
1
1
0
1
Benzothiaazol
e
176-178
Oil
175-178
[
42]
HOCH CH -
95
94
25
33
Oil
2
2
a
The reactions condition: substrate (1mmol), UHP (5mmol) and (Co@MCM41( 0.32mol %),
Fe@MCM 41( 0.44 mol %)) of catalyst (Co@MCM 41, acetone solvent ) and catalyst
(Mn@MCM 41, ethanol solvent )
b
Isolated yield
26
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Table 8. Comparison of Co@MCM41 and Fe@MCM41 for the oxidation of methyl phenyl
sulfide and oxidative coupling of 4-methylbenzenethiol with previously reported catalysts.
Time
a
Entry
Substrate
Catalyst
Yield %
Ref
(min)
4
-
1
2
3
4
5
6
7
8
9
Co@MCM-41
9
98
97
98
50
97
99
83.2
95
92
90
52
54
94
99
This work
Methylbenzenthiol
4
-
Fe@MCM-41
60
This work
Methylbenzenthiol
4
-
Al(NO ) ․9H O/SiO -
3
3
2
2
100
47
Methylbenzenthiol
OSO H
3
4
-
Monochloro
1440
720
120
360
15
48
Methylbenzenthiol poly(styrenehydantoin)
4
-
CuI/Cs CO /1,10-phen
49
2
3
Methylbenzenthiol
4
-
[bmim][SeO (OCH )]
50
2
3
Methylbenzenthiol
4
-
Mn–ZSM-5
51
Methylbenzenthiol
Methylphenyl
sulfide
Co@MCM-41
Fe@MCM-41
This work
Methylphenyl
sulfide
Methylphenyl
sulfide
Methylphenyl
sulfide
Methylphenyl
sulfide
Methylphenyl
sulfide
Methylphenyl
sulfide
50
This work
Polymer-anchored
Cu(II)
Copper(II)complex,TE
MPO
chiral (salen) Mn (III)
complexes
1
1
1
1
1
0
1
2
3
4
180
20 h
180
0.75
1.5
52
53
54
55
56
CuBr2
VO(acac) /Titania
2
a
Isolated yield.
27
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Fig 1. Small-angle XRD patterns of (a) Si-MCM-41, (b) Co@MCM-41and c) Fe@MCM-41
28
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Fig 2. N isotherms of MCM-41, MCM-41-NH , Fe@MCM-41 and Co@MCM-41
2
2
29
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