Synthesis, characterization, and catalytic application of Cr and Mn Schiff base complexes immobilized on modified nanoporous MCM-41

Article abstract of DOI:10.1007/s11164-015-2304-y

Immobilization of chromium and manganese Schiff base complexes by postgrafting of ligand then metal salts on the walls of mesoporous MCM-41 functionalized with (3-aminopropyl)triethoxysilane is described. Characterization of the resulting heterogeneous catalysts by Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, and Brunauer–Emmett–Teller (BET) techniques indicated successful grafting of these two complexes inside the nanochannels of MCM-41. The complexes were found to be efficient, selective catalysts for oxidation of various sulfides to sulfoxides and oxidative coupling of thiols to corresponding disulfides using H₂O₂ as oxidant at room temperature. The recycling results of these heterogeneous catalysts showed good recyclability without significant loss of activity or selectivity in successive runs, indicating that the Cr and Mn Schiff base complexes supported on nanoporous MCM-41 remained intact and the coordination environments were not altered during the reaction.

Full text of DOI:10.1007/s11164-015-2304-y

Res Chem Intermed
DOI 10.1007/s11164-015-2304-y
Synthesis, characterization, and catalytic application
of Cr and Mn Schiff base complexes immobilized
on modified nanoporous MCM-41
1
1
•
Mohsen Nikoorazm
1
Nourolah Noori
•
Arash Ghorbani-Choghamarani
Received: 24 June 2015 / Accepted: 28 September 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract Immobilization of chromium and manganese Schiff base complexes by
postgrafting of ligand then metal salts on the walls of mesoporous MCM-41
functionalized with (3-aminopropyl)triethoxysilane is described. Characterization of
the resulting heterogeneous catalysts by Fourier-transform infrared (FT-IR) spec-
troscopy, thermogravimetric analysis (TGA), transmission electron microscopy
(
TEM), X-ray diffraction (XRD) analysis, and Brunauer–Emmett–Teller (BET)
techniques indicated successful grafting of these two complexes inside the
nanochannels of MCM-41. The complexes were found to be efficient, selective
catalysts for oxidation of various sulfides to sulfoxides and oxidative coupling of
thiols to corresponding disulfides using H O as oxidant at room temperature. The
2
2
recycling results of these heterogeneous catalysts showed good recyclability without
significant loss of activity or selectivity in successive runs, indicating that the Cr and
Mn Schiff base complexes supported on nanoporous MCM-41 remained intact and
the coordination environments were not altered during the reaction.
Keywords MCM-41 Á Cr-MCM-41 Á Mn-MCM-41 Á Sulfoxide Á Disulfide Á Urea
hydrogen peroxide (UHP)
Introduction
Great interest has been shown in synthesis of Schiff bases and their transition-
metal (V, Cr, Mn, Fe, etc.) complexes because of various applications such as
metal chelating agents, heterogeneous and homogeneous catalysts, antimicrobial
agents, pesticides, antitumor agents, spectrophotometric and fluorimetric agents, in
&
Mohsen Nikoorazm
e_nikoorazm@yahoo.com
1
Department of Chemistry, Faculty of Science, Ilam University, P.O. Box 69315516, Ilam, Iran
123

M. Nikoorazm et al.
trace metal analysis, and in solar cells [1–5]. Among the above-mentioned
applications, heterogeneous catalysts play an important role in chemical processes
in industry [6, 7]. It is noted that application of homogeneous catalysts in
chemical reactions is greatly limited due to high cost of recovery and lack of
reusability. To heterogenize homogeneous catalysts, three general methods are
available: (1) encapsulation in nanocavities of mesoporous material, for instance
zeolites [8–10], (2) immobilization via covalent bonding with functionalized
polymers [11, 12], MCM-41, or SBA-15 materials [13, 14], and (3) polymeriza-
tion of the homogeneous catalyst itself [15–17], hence making them insoluble in
solvents. Among these support materials, mesoporous silica MCM-41 has been
employed as a useful and versatile solid support for preparation of heterogeneous
catalysts, enzyme immobilization, and drug delivery because of its large tunable
pore dimensions, high surface area, and great diversity in terms of surface
functionalization. In the last decade, the immobilization strategy used for linking
metal Schiff base complexes to supports to form heterogeneous catalysts has
received wide attention in the catalyst field and industry [18–22]. Immobilization
of homogeneous catalysts such as Schiff base complexes on MCM-41 yields
specific additional characteristics such as selectivity, thermal stability, and easy
separation from the reaction mixture and recyclability; For instance, copper and
cobalt salen compounds have been grafted onto the surface of MCM-41 by means
of aminosilane linkers via adduct formation, and these heterogeneous catalysts are
considered to be active for oxidation reactions of sulfides to sulfoxides and
conversion of thiols to disulfides under mild conditions in organic media [23, 24].
Considering the fact that thiols and sulfides are easily oxidizable and overoxi-
dized, many studies have been conducted to devise methods to control oxidation
of these compounds [25–28].
We report preparation and characterization of functionalized MCM-41 consisting
of chromium and manganese Schiff base complexes retained on the internal surface
of the nanoporous material using a postgrafting method. When using a direct co-
condensation method, the amino-organosilanes are introduced into the mother gel
with tetraethylorthosilicate (TEOS) simultaneously, leading to a homogeneous
distribution of amino-organic moieties in the silica framework, though the presence
of amino groups in the silica walls is unavoidable to some extent. On the other hand,
in the postsynthesis grafting method, all amino-organic moieties introduced into the
silica are anchored onto surface silanol groups via covalent bonds. This method may
be advantageous when the goal is to maximize the number of amino groups on the
surface. However, one disadvantage of the postsynthesis grafting method is that the
distribution of the organic moiety on the surface is uncontrollable [29]. These
immobilized complexes were used for oxidation reaction of various sulfides and
oxidative coupling of thiols as active and reusable catalysts with high stability at
room temperature. The effects of important reaction parameters such as the molar
ratio of UHP to substrate, the nature of the solvent, and the amount of catalyst on the
reactivity and selectivity were also examined. Finally, the reusability of the catalysts
was studied.
1
23

Synthesis, characterization, and catalytic application of…
Experimental
Techniques
Powder X-ray diffraction of Mn-MCM-41 and Cr-MCM-41 catalysts was carried
out using a Philips diffractometer with Cu K radiation at 40 kV and 30 mA. The
a
porosity and surface area of the samples were determine by the BET method using a
BELSORP-MINI instrument. Fourier-transform infrared (FT-IR) spectra were
recorded from KBr pellets using a Bruker VRTEX 70 FT-IR spectrometer.
Transmission electron microscopy (TEM) images were taken using a Philips
CM10 microscope at operating voltage of 200 kV. Ultraviolet–visible (UV–Vis)
spectra were recorded on a PerkinElmer Lambda 20 UV–Vis spectrophotometer
using barium sulfate as standard. Thermogravimetric analysis (TGA) of the samples
-
1
was carried out between 30 and 900 °C in air at heating rate of 10 °C min using a
Shimadzu DTG-60 automatic thermal analyzer.
Materials
Tetraethylorthosilicate (TEOS, 98 %), cationic surfactant cetyltrimethylammonium
bromide (CTAB, 98 %), chromium(III) chloride hexahydrate (CrCl Á6H O),
3
2
manganous(II) chloride dihydrate (MnCl Á2H O), 3,4-dihydroxybenzaldehyde, aryl
2
2
and alkyl sulfides, thiols, urea hydrogen peroxide (UHP), solvents, and other
reagents were purchased from Merck, Aldrich or Fluka chemical companies and
used as received without further purification.
Synthesis of MCM-41 modified with (3-aminopropyl)triethoxysilane (3-
APTES)
Synthesis of nanosized MCM-41 was carried out by the sol–gel method using
cetyltrimethylammonium bromide (CTAB) as template, tetraethylorthosilicate
(
TEOS) as Si source, and sodium hydroxide as pH control agent. In a typical
procedure, surfactant CTAB (2.74 mmol, 1 g) was added to a solution of deionized
water (480 mL) and NaOH (2 M, 3.5 mL) at 80 °C. When the solution became
homogeneous, TEOS (5 mL) was added dropwise to the solution under continuous
stirring at 80 °C, and the obtained gel was aged under reflux for 2 h. The mixture
was allowed to cool to room temperature, and the resulting product was filtered,
washed with distilled water, then Soxhlet-extracted with ethanol for 24 h and dried
in an oven at 60 °C. The obtained material was calcined at 550 °C in air for 5 h and
designated MCM-41. Postsynthesis organic modification of MCM-41 was per-
formed by stirring 1 g MCM-41 with a solution of 3-APTES in n-hexane (96 mL) at
80 °C for 24 h under nitrogen atmosphere. The resulting white solid of function-
alized MCM-41 (APTES@MCM-41) was filtered, washed with n-hexane, and dried
under vacuum [30].
123

M. Nikoorazm et al.
Preparation of heterogeneous catalyst Cr-Schiff base complex grafted
on mesoporous silica MCM-41 (Cr-MCM-41)
For grafting of the ligand to MCM-41-(SiCH CH CH NH ) , a mixture of freshly
2
2
2
2 x
dried APTES@MCM-41 (1 g) with 3,4-dihydroxybenzaldehyde (1 mmol, 0.134 g)
in ethanol was refluxed under N atmosphere for 3 h. Then, the resulting yellowish
2
solid was filtered, washed with ethanol for several times, and dried at room
temperature. Finally, a mixture of yellowish precipitate (1 g) and CrCl Á6H O
3
2
(
0.13 mmol, 0.5 g) in ethanol was refluxed and mechanically stirred for 24 h at
80 °C. The resulting Cr-MCM-41 catalyst was filtered, washed with ethanol, and
dried under vacuum (Scheme 1).
Preparation of heterogeneous catalyst Mn(II)-Schiff base complex grafted
on mesoporous silica (Mn-MCM-41)
To prepare silylated Mn-MCM-41, a mixture of the prepared yellowish precipitate
(
1 g) and MnCl Á2H O (1 mmol, 0.161 g) in ethanol was refluxed and mechanically
2
2
stirred at room temperature for 24 h. The resulting catalyst was filtered, washed
with ethanol, and dried under vacuum.
General procedure for oxidation of sulfides with UHP catalyzed by Cr-
MCM-41 and Mn-MCM-41
To examine the catalytic activity of Cr-MCM-41 and Mn-MCM-41, we studied
catalytic oxidation of several types of alkyl and aryl sulfides to corresponding
sulfoxides at room temperature (Scheme 2). A mixture of sulfides (1 mmol), UHP
(
5 mmol), and Cr- or Mn-MCM-41 (25 mg) was stirred in ethanol at room
temperature, and the progress of the reaction was monitored by thin-layer
chromatography (TLC). After completion of the reaction, the catalyst was separated
MCM-41
1
4
-
Si
M
C
N
M
1
OH
M
OH
OH
4
-
Si
M
C
N
M
OH
MCM-41
Scheme 1 Schematic representation of synthesis of heterogeneous catalyst M-MCM-41 (M = Cr or Mn)
1
23

Synthesis, characterization, and catalytic application of…
(A)
(B)
Scheme 2 Selectivity in a oxidation of sulfides and b oxidative coupling of thiols by present procedure
by simple filtration and washed with dichloromethane (3 9 10 mL). The results are
presented in Table 4.
General procedure for oxidative coupling of thiols to disulfides with UHP
catalyzed by Cr-MCM-41 and Mn-MCM-41
Thiols ((1 mmol), 5 mmol UHP, and 4 mL acetone as solvent along with 25 mg
catalyst were added to a 10-mL two-necked flask equipped with a stirrer. The
progress of the reaction was monitored by TLC. After completion of the reaction,
the catalyst was removed and the pure products were extracted with CH Cl
2
2
(
3 9 10 mL). The results are presented in Table 6.
Results and discussion
XRD analysis
The low-angle XRD patterns of MCM-41, MCM-41-NH , MCM-41-ligand, Cr-
2
MCM-41, and Mn-MCM-41 are shown in Fig. 1. The XRD pattern of MCM-41
shows a typical three-peak pattern with very strong reflection at 2h = 2.41° for d100
and also two other weaker reflections at 2h = 3.95° and 4.55° for d110 and d200
,
respectively, which can be attributed to formation of well-ordered mesoporous
material with hexagonal symmetry. All peaks are well resolved, indicating good-
quality material. Comparison of the XRD patterns for MCM-41 and Cr-MCM-41
shows that, upon postsynthetic grafting, the d110 and d200 reflections were no longer
observed and an overall decrease in the intensity of d100 was observed. This result
can be attributed to decrease of local order, such as variation in wall thickness, or
might be due to reduction of scattering between the channel walls of the silicate
framework and the Schiff base ligand present in Cr-MCM-41, as mentioned
previously [31]. The values of the unit cell parameter a = (2/H3)d
are 27.86
100
0
123

M. Nikoorazm et al.
Fig. 1 XRD patterns: a MCM-41, b MCM-41-NH , c MCM-41-ligand, d Cr-MCM-41, e Mn-MCM-41,
f recovered Cr-MCM-41, and g recovered Mn-MCM-41
2
˚
and 31.32 A for MCM-41 and Cr-MCM-41, respectively. The increase in the lattice
parameter of Cr-MCM-41 is due to the functionalization of MCM-41. Similarly,
Mn-MCM-41 and recycled Mn-MCM-41 prepared by the postsynthesis procedure
exhibited XRD patterns akin to those shown in Fig. 1. Obviously, upon postgrafting
of Cr and Mn Schiff base complexes, well-defined (100) reflections are observed in
the XRD patterns, but there are no well-resolved (110, 200) peaks, which are clearly
visible from corresponding MCM-41 analogs; the intensity of the peak at d100
decreased with increasing loading amount of Cr and Mn, indicating that the
hexagonal mesostructure was less ordered due to successful introduction of Cr and
Mn metal elements into the mesoporous channels of MCM-41. Also, the structural
stability of the samples was retained after grafting of the organometallic complexes.
N adsorption–desorption studies
2
The N adsorption–desorption isotherms of MCM-41, functionalized MCM-41, Cr-
2
MCM-41, and Mn-MCM-41 are shown in Fig. 2. According to the International
Union of Pure and Applied Chemistry (IUPAC) classification characteristics for
mesoporous structures, the isotherms of all samples show a type IV curve and a
hysteresis loop resulting from capillary condensation of N gas inside mesopores
2
[
32]. In comparison with the parent MCM-41, the relative pressure is transformed to
a lower value for the modified samples, leading to a reduction of the BET specific
area, pore volume, and pore diameter. These results are presented in Table 1. These
1
23

Synthesis, characterization, and catalytic application of…
2 2
Fig. 2 N adsorption–desorption isotherms for a MCM-41, b MCM-41-NH , and c Cr-MCM-41 and
Mn-MCM-41
Table 1 Surface properties of MCM-41, MCM-41-NH
2
, Cr-MCM-41, and Mn-MCM-41
2
3
Pore vol. (cm /g)
Sample
S
BET (m /g)
Pore diam. by BJH
method (nm)
Wall diam. (nm)
MCM-41
986.16
694.98
311.39
130.12
3.65
3.3
0.711
0.340
0.205
0.116
0.902
0.906
1.93
MCM-41-NH
Cr-MCM-41
Mn-MCM-41
2
1.2
1.1
2.02
results confirm good grafting of the transition-metal complexes into the MCM-41
channels. The N adsorption–desorption analysis of Cr-MCM-41 and Mn-MCM-41
2
gave typical type IV adsorption isotherms with a H1 hysteresis loop according to
the IUPAC classification, indicating presence of mesoporosity. As shown in
Table 1, the surface area and specific pore volume of Cr-MCM-41, Mn-MCM-41,
and functionalized MCM-41 with amino group decreased notably compared with
3
-1
MCM-41. The sorption experiment yielded a BET pore volume of 0.711 cm g
2
-1
and surface area of 986.16 m g for MCM-41, while postgrafting of amino group,
chromium and manganese Schiff base complexes remarkably reduced the surface
area compared with MCM-41. In addition, the pore diameter reduced from 3.56 nm
for pure silica MCM-41 to 1.2 and 1.1 nm for Cr-MCM-41 and Mn-MCM-41,
respectively. On the other hand, an increase of the wall thickness occurred, which is
in agreement with anchoring of functionalized groups onto MCM-41, based on
123

M. Nikoorazm et al.
which it can be concluded that the chromium and manganese Schiff base complexes
occur inside the channels.
Fourier-transform infrared (FT-IR) spectroscopy
Infrared spectroscopy was used as a key method to characterize the functional
groups of the products (Fig. 3). The FT-IR spectrum of MCM-41 shows three peaks
-
1
at 1083, 793, and 463 cm , corresponding to asymmetric stretching, symmetric
stretching, and bending Si–O–Si vibration, respectively. The presence of the peak at
-
1
around 1550 cm , mainly from NH2 symmetric bending vibration, indicates
successful grafting of organic amine onto the MCM-41 surface. The free ligand
-
1
exhibits a m(C=N) stretch at 1657 cm , while in Cr-MCM-41 and Mn -MCM-41
-
1
this band shifts to lower frequency and appears at 1638 and 1641 cm
,
respectively. The spectra for recycled Cr-MCM-41 and Mn-MCM-41 point out
that the catalysts were stable during the reaction.
TEM analysis
TEM micrographs showing top (A) and side (B) views of MCM-41 are presented in
Fig. 4. The regular arrangement of pores can be clearly observed for this material,
Fig. 3 FT-IR spectra of
a MCM-41, b MCM-41-NH
c MCM-41-ligand, d Cr-MCM-
1, e recycled Cr-MCM-41,
2
,
4
f Mn-MCM-41, and g recycled
Mn-MCM-41
1
23

Synthesis, characterization, and catalytic application of…
Fig. 4 TEM images of MCM-41: a top view, b side view, and c Cr-MCM-41 and Mn-MCM-41
and also the hexagonal pores remained intact. Also Fig. 4 shows TEM images for
Cr-MCM-41 and Mn-MCM-41. TEM observation of Cr-MCM-41 (Fig. 4c)
indicated well-ordered hexagonal arrays of mesopores and long-range mesoporous
architecture (50 nm). The Mn-MCM-41 sample also exhibited a similar type of
array of regular hexagonal arrangement in TEM micrographs (Fig. 4d). Therefore, it
can be concluded that both samples had a similar type of morphology.
SEM analysis
The morphologies of MCM-41, Cr-MCM-41, and Mn-MCM-41 are shown in
Fig. 5. As seen in the SEM images of Fig. 6, MCM-41, Cr-MCM-41, and Mn-
MCM-41 are agglomerations of small spherical regular particles. As can be seen,
123

M. Nikoorazm et al.
Fig. 5 SEM images: a MCM-41, b Cr-MCM-41, and c Mn-MCM-41
there is no difference in the particle surface morphology between MCM-41 and the
other materials, indicating that the Cr and Mn Schiff base complexes are supported
inside the channels of MCM-41.
Thermogravimetric analysis
The immobilization of the Schiff base complexes on the functionalized mesopores
and the percentage of chemisorbed organic functional groups on MCM-41 can be
studied based on TGA curves. The thermogravimetric (TG) analysis of the samples
is shown in Fig. 6. TGA of the synthesized MCM-41 indicated 4.16 % mass loss in
one step at 25–120 °C due to desorption of water [32]. The thermogram of amino-
functionalized MCM-41 indicated a slow weight loss up to 840 °C. The TGA curve
for MCM-41-(SiCH CH CH NH ) indicated four steps of weight loss (25–120,
2
2
2
2 x
1
20–340, 340–600, and[600 °C). The weight loss is 3 % in the first step, being due
1
23

Synthesis, characterization, and catalytic application of…
2
Fig. 6 TG profiles for MCM-41 (solid line), MCM-41-NH , (dotted line), MCM-41-ligand (dashed line),
Cr-MCM-41 (dashed dotted line), and Mn-MCM-41 (dashed double-dotted line)
Table 2 TGA data of catalysts
Catalyst
Temperature (°C)
Weight
loss (%)
Group lost
Cr-MCM-41
25–100
4.5
6
2
H O
2
50–350
50–600
25–150
80–350
50–600
Combustion of amine
4
7
Decomposition of Schiff base
Mn-MCM-41
6
2
H O
2
4
7
Combustion of amine
9
Decomposition of Schiff base
to removal of physically adsorbed water inside MCM-41 channels. The weight
losses in the second and third steps (8 %) are chiefly related to oxidative
decomposition of organic functional groups. In the final step, the third weight loss
(
1.67 %) is due to dehydroxylation of the silicate network, and the TGA curve of the
MCM-41-ligand shows 13 % weight loss above 350 °C, chiefly associated with
decomposition of organic groups. Based on elemental analysis, the amount of
-
1
functional groups of APTES and ligand were computed to be 1.2 and 0.5 mmol g
of MCM-41, respectively. The TGA data for the catalysts are presented in Table 2.
The TGA curve of Cr- or Mn-salen-MCM-41 shows three stages of weight loss,
confirming the formation of the Schiff base complexes.
123

M. Nikoorazm et al.
Fig. 7 UV–Vis diffuse-reflectance spectra of a MCM-41, b MCM-41-ligand, c Cr-MCM-41, d Cr-
MCM-41, e chromium Schiff base complex, and f manganese Schiff base complex
UV–Vis spectroscopy
Figure 7 shows the UV–Vis diffuse-reflectance spectra of the samples. The UV–Vis
spectra of the homogeneous complexes all displayed characteristic peak at 380–450 nm
typical of metal–ligand band, and one weak broad peak at about 530–610 nm associated
with d–d transitions. The UV–Vis spectra of all the immobilized complexes were quite
similar to those of their corresponding homogeneous complexes.
In continuation of our previous work on organic transformation reactions [25,
4–36], we also studied here the application of the Mn or Cr Schiff base complexes
3
supported on the modified nanoporous MCM-41 as heterogeneous and recoverable
catalysts in oxidation reactions of sulfides to sulfoxides and oxidative coupling of
thiols to corresponding disulfides using UHP as oxidizing agent (Scheme 2).
Oxidation of sulfides to sulfoxides with UHP catalyzed by Cr-MCM-41
and Mn-MCM-41
To determine the optimum reaction conditions, dibenzyl sulfide was selected as a
model substrate and oxidation of this substrate in various solvents was studied.
1
23

Synthesis, characterization, and catalytic application of…
Table 3 Oxidation of dibenzyl sulfide with UHP as oxidant in presence of Cr- or Mn-MCM-41 catalyst
in different solvents
Entry
Solvent
Time (h)
Yield (%)
Cr-MCM-41
Mn-MCM-41
2.5
Cr-MCM-41
Mn-MCM-41
1
2
3
4
5
6
7
Ethanol
2
95
91
93
95
91
90
87
93
–
a
Water
10
4.5
6
No reaction
n-Hexane
Acetonitrile
Dichloromethane
Ethyl acetate
Acetone
8
92
90
89
88
85
7
9
15
18
7.25
11
3.75
Reaction conditions: 1 mmol dibenzyl sulfide, 0.025 g catalyst, 5 mmol UHP in 4 mL solvent at room
temperature
a
After 24 h
Ethanol proved to be the optimum solvent (Table 3, entry 1) and was used in
subsequent optimization studies. When oxidation of dibenzyl sulfide (1 mmol) was
carried out using different concentrations of UHP, the reaction did not reach
completion when using less than 5 mmol UHP. However, when a higher
concentration (7 mmol) was employed, neither the conversion nor selectivity of
the reaction was improved.
We also examined oxidation of dibenzyl sulfide (1 mmol) using UHP (5 mmol)
in the absence of Cr- or Mn-MCM-41 catalyst at room temperature. It was observed
that the reaction was completed after 12 h. However, when similar oxidation was
conducted in the presence of the mentioned catalysts (25 mg) at room temperature
(Fig. 8), dibenzyl sulfoxide was formed in excellent yield in short period of time.
Based on the above observations, 1 mmol substrate, 5 mmol UHP, 25 mg catalyst,
9
6
1
00
94
Cr-MCM 41 Mn- MCM 41
8
9
88
8
6
8
4
7
9
7
5
76
7
1
7
5
2
5
0
5
5
1
4
8
3
8
12
15
20
25
Catalyst amount (mg)
Fig. 8 Effect of amount of catalysts in oxidation reaction of dibenzyl sulfide to sulfoxide
123

M. Nikoorazm et al.
1
23

Synthesis, characterization, and catalytic application of…
Fig. 9 Recycling experiment of Cr-MCM-41 and Mn-MCM-41 for oxidation of dibenzyl sulfide
Table 5 Oxidative coupling of 4-methylthiophenol using Cr- or Mn-MCM-41 and UHP in different
solvents
Entry
Solvent
Time (h)
Yield (%)
Cr-MCM-41
Mn-MCM-41
Cr-MCM-41
Mn-MCM-41
1
2
3
4
5
6
7
Ethanol
2.5
12
0.33
18
93
89
92
90
92
97
92
96
83
90
94
91
88
90
Water
n-Hexane
Acetonitrile
Dichloromethane
Acetone
4
9
17
1.25
15
6.25
0.6
3.45
5.5
6.5
Ethyl acetate
Reaction conditions: 1 mmol 4-methylthiophenol, 0.02 g catalyst, 5 mmol UHP in 4 mL solvent at room
temperature
and ethanol as solvent was the best combination for oxidation of sulfides to
sulfoxides. To determine the general scope of this method, a wide range of sulfides
were subjected to oxidation under the optimized condition. All the reactions
occurred with high selectivity for sulfoxide formation, and no overoxidation
products such as sulfone were detected in the reaction mixture (Table 4).
Finally, the reusability of Cr-MCM-41 and Mn-MCM-41 was investigated. After
the reaction reached completion, catalyst was easily separated from the product
using simple filtration, washed with ethanol and water, then reused in the next run.
Figure 9 shows the yield of six consecutive cycles of the oxidation reaction of
dibenzyl sulfide as model substrate.
123

M. Nikoorazm et al.
Fig. 10 Effect of catalyst loading in oxidative coupling of thiol to disulfide
Table 6 Oxidation of various thiols using UHP catalyzed by Cr-MCM-41 or Mn-MCM-41
R2SH?R2S2R)
(
a
Yield (%)
Entry
R
Time (min)
M.p. (°C)
Cr-
Mn-
Cr-
Mn-
Cr-
MCM-41 MCM-41 MCM-41 MCM-41 MCM-41 MCM-41
Mn-
1
2
3
4
5
6
7
8
9
p-MeC
6
H
4
–
6
36
35
32
24
4
20
15
60
24
18
10
75
16
38
27
27
96
93
92
89
91
94
90
93
91
95
94
94
93
93
95
92
91
90
94
92
94
89
41–43
Oil
42–43 [44]
Oil [44]
–CH
2-Naphthyl
2,6-DiClC
p-BrC
2-Pyridyl–
2
COOH
142–144
76–90
90–92
55–56
55–57
276–282
85–89
176–178
Oil
142–144 [44]
76–90 [44]
91–92 [45]
54–56 [46]
56–57 [42]
276–282 [44]
87–89 [44]
175–178 [44]
Oil [42]
6 3
H –
6
H
4
–
C
6
H
5
–
39
14
93
2-COOHC
6 4
H –
2-Benzoxazole
1
1
0
1
2-Benzothiazole 31
HOCH CH 21
2
2
–
Reaction conditions: substrate (1 mmol), UHP (5 mmol), 20 mg catalyst Cr-MCM-41 with acetone
solvent or Mn-MCM-41 with ethanol solvent
a
Isolated yield
Oxidative coupling of thiols to disulfides with UHP catalyzed by Cr-MCM-
4
1 and Mn-MCM-41
To indicate the role of Cr-MCM-41 and Mn-MCM-41 as catalyst in the oxidative
coupling of thiol reactions, the effects of various reaction parameters such as solvent,
1
23

Synthesis, characterization, and catalytic application of…
(A)
(B)
Scheme 3 Proposed mechanism of a oxidation of sulfide and b oxidative coupling of thiol in presence of
the synthesized catalysts
amount of catalyst, and substrate–oxidant ratio were studied to optimize the reaction
conditions. Optimization experiments were carried out using 4-methylthiophenol as
substrate. Among the different solvents listed in Table 5, ethanol and acetone were
chosen as the best solvent for Cr-MCM-41 and Mn-MCM-41, respectively.
We loaded various amounts of the chosen catalysts in this reaction. The resulting
optimized reaction condition for catalyst loading was 20 mg for Mn- or Cr-MCM-
41. When the amount of catalyst was increased from 10 to 20 mg, the conversion
increased from 64 to 95 % (Fig. 10).
123

M. Nikoorazm et al.
Fig. 11 Reusability of Cr-MCM-41 or Mn-MCM-41 as catalysts in oxidative coupling of
4-methylthiophenol
The reaction rate as well as product yield was also found to depend on the
oxidant concentration. It was seen that 5 mmol UHP was required to obtain disulfide
selectivity with 1 mmol of substrate. The system with Cr- or Mn-MCM-41 and UHP
as oxidizing agent was further extended to oxidation of various thiols (Table 6).
The active oxygen of UHP oxidizes RSH to RSOH, which reacts with another
RSH molecule to produce disulfide. The proposed mechanism of oxidation of
sulfide [47] and oxidative coupling [48] of thiol in presence of Cr-MCM-41 and
Mn-MCM-41 by UHP as oxidant is displayed in Scheme 3.
To study the stability of the immobilized catalysts, 4-methylthiophenol was
selected as a model substrate. The catalyst could be easily recovered after the reaction
by simple filtration. After recovery, the catalysts were thoroughly washed with ethanol
and then water, dried, and reused for subsequent experiments (six runs) by adding fresh
substrate and oxidant under similar reaction conditions. In all cases, reaction times and
yields of the desired product remained almost the same, establishing the recyclability
and reusability of the immobilized Mn- and Cr-Schiff base complexes (Fig. 11).
Conclusions
Chromium and manganese Schiff base complexes consisting of mesoporous
material were synthesized by means of a postgrafting procedure and characterized
using several techniques. Application of Cr-MCM-41 and Mn-MCM-41 as adequate
and heterogeneous catalysts for synthesis of several sulfoxides and disulfides under
mild conditions has been shown. The readiness of the catalyst synthesis,
environmental friendliness, mild reaction conditions, and efficient recycling are
among the other important benefits of this procedure.
1
23

Synthesis, characterization, and catalytic application of…
Acknowledgments This work was supported by the research facilities of Ilam University, Ilam, Iran.
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