Molybdenum liquid salts immobilized on ionic liquid-modified silica as efficient heterogeneous catalysts for sulfoxide reduction

Article abstract of DOI:10.1016/j.tetlet.2013.05.046

Two room temperature liquid salts containing molybdenum(VI), [BMIM] ₂[MoO₂(NCS)₄] (1) and [DMIM] ₂[MoO₂(NCS)₄] (2), were immobilized on ionic liquid-modified silica leading to materials whi

Full text of DOI:10.1016/j.tetlet.2013.05.046

Tetrahedron Letters xxx (2013) xxx–xxx
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Tetrahedron Letters
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Molybdenum liquid salts immobilized on ionic liquid-modified silica
as efficient heterogeneous catalysts for sulfoxide reduction
⇑
Mojtaba Bagherzadeh , Saeideh Ghazali-Esfahani
Chemistry Department, Sharif University of Technology, PO Box 11155-3615, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Two room temperature liquid salts containing molybdenum(VI), [BMIM] [MoO (NCS) ] (1) and
2
2
4
Received 9 January 2013
Revised 20 April 2013
Accepted 3 May 2013
Available online xxxx
[
DMIM]
showed notable catalytic activity for the reduction of sulfoxides into sulfides with triphenylphosphine
PPh ). The molybdenum liquid salts immobilized on the supported ionic liquid phase could be reused
at least for three catalytic cycles without significant loss of activity.
2 2 4
[MoO (NCS) ] (2), were immobilized on ionic liquid-modified silica leading to materials which
(
3
Ó 2013 Published by Elsevier Ltd.
Keywords:
Supported ionic liquid phase
Heterogeneous catalyst
Molybdenum
Reduction
Catalytic reactions using ionic liquids (ILs) as bulk solvents usu-
ally present advantages of high catalytic activity and good selectiv-
ity. Nevertheless, using ILs as bulk solvents in conventional
hydrogenations,7 Ru-catalyzed ring-closing olefin metathesis,8
and Pd-catalyzed carbon–carbon coupling reactions have been re-
ported using supported ionic liquid phase catalysis.
9
1
biphasic catalysis normally needs a large amount of ILs that are
still considered as expensive solvents. In addition, the high viscos-
ity of ILs often causes mass transfer limitations. Thus, a large pro-
portion of the catalytically active species is not able to take part in
the catalytic process. The concept of supported ionic liquid phase
Despite various studies on molybdenum catalysts in ionic liq-
uids,10 there are only a few reports on molybdenum catalysts
1
1
immobilized in supported ionic liquids. Among various oxo-
molybdenum(VI) compounds studied extensively as catalysts for
2
À
oxygen atom transfer, [MoO
2
4
(NCS) ]
has been reported as a sim-
12
(
SILP) catalysis was introduced by immobilization of transition me-
ple active oxo-transfer catalyst or efficient oxidizing agent. In
continuation of our studies on molybdenum compounds as cata-
lysts, we previously reported the synthesis and catalytic applica-
tal catalysts immersed in a thin film of ionic liquid on a solid sup-
2
13
port by Mehnert et al. Dissolving catalytic species in a supported
2
À
film of ionic liquid was introduced as a promising strategy for
immobilizing catalysts.3 The ionic liquid layer acts as a phase for
the catalytic reaction combining both the advantages of an ionic li-
tions of several new ionic liquids containing the [MoO
2 4
(NCS) ]
anion for the deoxygenation of sulfoxides in an ionic liquid/organic
1
4
biphasic system. In the present work, the immobilization of
[BMIM] [MoO (NCS) ] (1) (BMIM = 1-butyl-3-methylimidazolium)
and [DMIM] [MoO (NCS) ] (2) (DMIM = 1-n-decyl-3-methylimida-
4
quid and heterogeneous catalytic systems. Various catalytic reac-
2
2
4
tions such as Friedel–Crafts,5 Rh-catalyzed hydroformylations,
6
2
2
4
2 2 4 2 2 4
Scheme 1. [BMIM] [MoO (NCS) ] (1) and [DMIM] [MoO (NCS) ] (2) as molybdenum containing ionic liquids.
E-mail address: bagherzadeh@sharif.edu (M. Bagherzadeh).
0
040-4039/$ - see front matter Ó 2013 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tetlet.2013.05.046
Please cite this article in press as: Bagherzadeh, M. ; Ghazali-Esfahani, S. Tetrahedron Lett. (2013), http://dx.doi.org/10.1016/
j.tetlet.2013.05.046

2
M. Bagherzadeh, S. Ghazali-Esfahani / Tetrahedron Letters xxx (2013) xxx–xxx
Scheme 2. Synthesis of the ionic liquid-modified silica.
zolium) (Scheme 1) on ionic liquid-modified silica is reported. Both
molybdenum based supported catalysts proved to be efficient for
oxo-transfer reactions.
to obtain the catalysts Mo1/SILP and Mo2/SILP.16 The loading of
molybdenum in Mo1/SILP and Mo2/SILP was shown to be 0.064
and 0.057 mmol/g, respectively, according to inductively coupled
plasma-atomic emission spectroscopy. The color of both molybde-
num supported catalysts was orange-red before taking part in oxo-
transfer catalytic process.
The preparation of the ionic liquid-modified silica is shown in
Scheme 2. 1-Methyl-3-(3-trimethoxysilylpropyl)-imidazolium
chloride was synthesized by the reaction of (3-chloropropyl)tri-
methoxysilane and 1-methylimidazole (see Supplementary
The reduction of sulfoxides using Mo1/SILP and Mo2/SILP as
1
5
17
data). The obtained ionic liquid was treated with potassium
hexafluorophosphate in acetone to give 1-methyl-3-(3-trimeth-
oxysilylpropyl)-imidazolium hexafluorophosphate. Next, dried sil-
catalysts was performed as shown in Scheme 3. The conversion
increased remarkably on raising the temperature from room tem-
perature to 50 °C, using methyl phenyl sulfoxide as model sub-
strate (Table 1). The catalyst amount, referenced to the number
of mmols of the supported molybdenum complex, was changed
in the range of 0–5 mol % (Fig. S2). In the absence of catalyst and
under the same reaction conditions, the conversion of methyl phe-
nyl sulfoxide was negligible. Sulfoxide conversion increased to 87%
with an increase in the catalyst amount to 2.5 mol %. Further
increasing the catalyst amount did not cause a significant enhance-
ment in the conversion. Comparing this SILP catalysis with a bipha-
2
ica gel (with a surface area of 550 m /g and particle size of 230–
4
00 mesh) was heated with 1-methyl-3-(3-trimethoxysilylpro-
pyl)-imidazolium hexafluorophosphate in refluxing chloroform to
give the modified support materials.2
,11
Elemental analysis (C
and N quantification) showed that 0.714 mmol of 1-methyl-3-(3-
trimethoxysilylpropyl)-imidazolium hexafluorophosphate ionic li-
quid was anchored to 1.0 g silica. Comparing the IR spectrum of
bare silica with that of silica modified with 1-methyl-3-(trimeth-
oxysilylpropyl)imidazolium hexafluorophosphate confirmed the
presence of the ionic liquid anchored to the silica surface
2 2 4
sic catalytic system using [BMIM] [MoO (NCS) ] as catalytic
1
4
species, it was observed that the mol % of the molybdenum cat-
alyst required for catalytic reduction was reduced by 50% in this
SILP catalysis. This can be explained by the fact that a large part
of the catalytic species remains in the bulk ionic liquid phase in bi-
phasic systems, which cannot take part in the catalytic process. It
seems that immobilizing the molybdenum salts into the ionic li-
quid layer on the surface of silica decreased the amount of molyb-
denum catalyst required for the reaction. This catalytic system was
used for reducing other sulfoxide substrates, that is, diethyl sulfox-
ide and diphenyl sulfoxide, into their corresponding sulfides. It was
observed that the Mo1/SILP catalyst was also an efficient catalyst
À1
À1
(
Fig. S1). The bands around 1100 cm and 1635 cm are associ-
ated with the stretching of the siloxane bonds („Si–O) on the silica
and bending of the water molecules, respectively. The broad band
À1
at 3430 cm is due to stretching of the O–H bonds of the silanols
and water molecules in silica. The IR spectrum of silica modified
À1
with the ionic liquid additionally showed a band at 1575 cm
due to the aromatic C@C stretching of the imidazolium ring and
À1
the C–H stretching vibration of imidazolium at 3180 cm which
demonstrated that the ionic liquid had been attached to silica.
[
BMIM]
synthesized as previously reported.
were immobilized into the ionic liquid layer on the surface of ionic
liquid-modified silica by adding [BMIM][PF ] as second ionic liquid
2
[MoO
2
(NCS)
4
] (1) and [DMIM]
2
[MoO
2
(NCS)
4
] (2) were
1
2c,14
Molybdenum complexes
for oxo-transfer from these sulfoxide substrates to PPh
The recyclability of Mo1/SILP and Mo2/SILP was investigated in
the reduction of methyl phenyl sulfoxide in the presence of PPh
Table 3). After each catalytic cycle, the separated catalyst was re-
3
(Table 2).
6
3
(
Table 1
The effect of the temperature on the catalytic reduction of methyl phenyl sulfoxide by
a
molybdenum ionic liquid supported phase catalysts
Entry
1
Catalyst
—
Temperature
RT
Conversionb (%)
0
0
50 °C
2
3
Mo1/SILP
Mo2/SILP
RT
40
87
50 °C
RT
30
85
50 °C
a
Reaction conditions: solvent: toluene (1 ml), catalyst/methyl phenyl sulfoxide/
ratio = 1:40:40.
Conversion (%) determined by GC after 28 h using chlorobenzene as the internal
PPh
3
b
Scheme 3. Reduction of sulfoxides by molybdenum supported ionic liquid phase
catalysts.
standard.
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M. Bagherzadeh, S. Ghazali-Esfahani / Tetrahedron Letters xxx (2013) xxx–xxx
3
Table 2
Table 4
a
Reduction of different sulfoxides by the Mo1/SILP catalyst
Study of the catalytic activity and recyclability of the molybdenum supported
catalysts synthesized by the impregnation method using non-modified silica as the
support
Entry
Substrate
Conversionb (%)
87 (0)c
a,b
1
2
2
Methyl phenyl sulfoxide
Diphenyl sulfoxide
Diethyl sulfoxide
Conversionb (%)
c
Run
1
Catalyst
90 (20)
c
98 (0)
Mo1/Silica
Mo2/Silica
88
75
a
Catalyst/substrate/PPh
Conversion (%) determined by GC after 28 h using chlorobenzene as the internal
standard.
3
ratio = 1:40:40, T = 50 °C.
b
2
3
4
Mo1/Silica
Mo2/Silica
70
60
c
Conversion without catalyst.
Mo1/Silica
Mo2/Silica
43
52
Mo1/Silica
Mo2/Silica
42
45
Table 3
a
Catalyst recycling studies for the reduction of methyl phenyl sulfoxide
a
Reaction conditions: solvent: toluene (1 ml), reductant: PPh
phenyl sulfoxide, catalyst/substrate/PPh
3
, substrate: methyl
ratio=1:40:40, T = 50 °C.
Conversion (%) determined by GC after 28 h.
Run
1
Catalyst
Conversionb (%)
Mo leachedc (wt.%)
3
b
Mo1/SILP
Mo2/SILP
87
85
0.035
0.025
2
3
4
5
Mo1/SILP
Mo2/SILP
80
82
0.022
0.015
was also observed in the UV/vis spectra of [BMIM]
2 2 4
[MoO (NCS) ]
and [DMIM] [MoO (NCS) solutions in dichloromethane
2
2
4
]
Mo1/SILP
Mo2/SILP
65
79
0.014
0.008
1
4
(Fig. S3). This band, which is related to Mo(VI), disappeared after
five catalytic cycles and a new band at 530 nm appeared in the UV/
vis diffuse reflectance spectra of both SILP catalysts (Fig. 1b). The
new absorption band at 530 nm was due to a new molybdenum
species, which was responsible for the purple color. According to
Mo1/SILP
Mo2/SILP
60
68
0.009
0.008
Mo1/SILP
Mo2/SILP
35
44
—
—
1
4
18
our previous paper and other letters, this purple color can be
a
Reaction conditions: solvent: toluene (1 ml), catalyst/methyl phenyl sulfoxide/
ratio=1:40:40, T = 50 °C.
Conversion (%) determined by GC after 28 h using chlorobenzene as the internal
4
À
attributed to Mo(V) complexes such as [Mo
2
O
3
(NCS)
8
] , which
PPh
3
b
formed in the ionic liquid phase after adding PPh
to the reaction
3
standard.
mixture. Interestingly and contrary to that observed in dichloro-
methane; the purple molybdenum species is stable in the ionic li-
quid phase.
c
Determined by ICP.
In order to make an activity and recyclability comparison, the
2 2 4
two ionic liquid–molybdenum complexes, [BMIM] [MoO (NCS) ]
and [DMIM] [MoO (NCS) ], were supported on the surface of
2
2
4
non-modified silica by physisorption using the conventional
wet impregnation method, without the addition of extra ionic li-
quid. The use of Mo1/Silica and Mo2/Silica as catalysts for the
reduction of methyl phenyl sulfoxide showed that the catalytic
activity of Mo1/Silica was similar to that of Mo1/SILP in the first
run (Table 4, Entry 1). Nevertheless, during the next runs, this
catalyst did not show similar recyclability as was observed for
Mo1/SILP (Table 4). Hence, it seems that in spite of the ionic li-
quid nature of these molybdenum complexes, the covalently an-
6
chored ionic liquid on the silica surface and [BMIM]PF , added as
an extra ionic liquid, had an essential role in reducing the leach-
ing of the molybdenum species. Surprisingly, Mo2/Silica showed
low catalytic activity in comparison with Mo2/SILP. Both Mo1/
Silica and Mo2/Silica remained orange-red in color after four cat-
alytic cycles. This proves that the Mo(V) species are stable in the
ionic liquid phase. Hence, even if the purple compound had been
formed during the catalytic oxo-transfer reaction in the absence
of the ionic liquid phase, it was not stable enough to be
observed.
Figure 1. UV/vis diffuse reflectance spectra of (a) Mo1/SILP before use, (b) Mo1/SILP
after 5 catalytic cycles.
In conclusion, the Mo(VI) liquid salts immobilized into ionic li-
quid-modified silica exhibited remarkable activity as catalysts for
the reduction of sulfoxides into sulfides. These catalysts can be
recycled easily at least three times without any notable loss in
the catalytic activity. The UV/vis diffuse reflectance spectra of the
catalyst before and after the catalytic cycles showed that Mo(V)
compounds formed during the catalytic reaction.
used after washing with diethyl ether (3 Â 3 mL) and drying at
5
0 °C. The results for 5 catalytic cycles are shown in Table 3. Leach-
ing of the Mo species into the reaction solution was detected by ICP
analysis. During the three first cycles, the conversion did not show
any considerable change using Mo2/SILP as the catalyst. The de-
crease in catalytic activity for Mo1/SILP was more noticeable after
two catalytic cycles and can be attributed to the higher Mo leach-
ing observed for Mo1/SILP.
The color of the catalysts changed to purple after the first cata-
lytic cycle and remained purple during the subsequent runs. The
UV/vis diffuse reflectance spectra of both supported catalysts
showed a strong absorption peak at about 480 nm (Fig. 1a), which
Acknowledgment
Financial support of this work by the Research Council of Sharif
University of Technology is greatly appreciated.
Please cite this article in press as: Bagherzadeh, M. ; Ghazali-Esfahani, S. Tetrahedron Lett. (2013), http://dx.doi.org/10.1016/
j.tetlet.2013.05.046

4
M. Bagherzadeh, S. Ghazali-Esfahani / Tetrahedron Letters xxx (2013) xxx–xxx
1
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1
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6. Preparation of the SILP catalyst; typical procedure: [BMIM]
2
2 4
[MoO (NCS) ]
(
0.1 mmol, 0.638 g) was dissolved in 0.5 g of 1-butyl-3-methylimidazolium
2
6
hexafluorophosphate ([BMIM][PF ]) and then 10 mL MeCN was added. After
dissolving the ionic liquid phase in MeCN, ionic liquid-modified silica (1 g) was
added and the mixture was stirred for 2 h. After evaporation of the solvent
under vacuum, an orange-red solid was obtained.
4
5
6
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.
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7. General procedure for the catalytic reduction: the molybdenum based supported
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2
Please cite this article in press as: Bagherzadeh, M. ; Ghazali-Esfahani, S. Tetrahedron Lett. (2013), http://dx.doi.org/10.1016/
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