Selective Oxidation of Sulfides Catalyzed by the Nanocluster Polyoxomolybdate (NH4)12[Mo36(NO)4O108(H2O)16]

Article abstract of DOI:10.1002/ejic.201500528

The (NH₄)₁₂[Mo₃₆(NO)₄O₁₀₈(H₂O)₁₆]·30.84H₂O ({Mo₃₆}) catalyst has been synthesized and successfully employed in the selective oxidation of various sulfides to sulfoxides with urea hydrogen peroxide as oxidant under mild reaction conditions with 84-99 % conversion and 58-99 % selectivity, with active functional groups such as the hydroxy group and C=C bonds tolerated in the oxidation. The {Mo₃₆} catalyst showed high catalytic activity for a high substrate/catalyst ratio (up to 30000:1) and is recyclable.

Full text of DOI:10.1002/ejic.201500528

FULL PAPER
DOI:10.1002/ejic.201500528
Selective Oxidation of Sulfides Catalyzed by the
Nanocluster Polyoxomolybdate
(NH4)12[Mo36(NO)4O108(H2O)16]
Mojtaba Amini,*[a] Hadi Naslhajian,[a,b] S. Morteza F. Farnia,*[b]
[c]
´
and Małgorzata Hołynska
Keywords: Cluster compounds / Polyoxometalates / Molybdenum / Sulfides / Oxidation
The (NH4)12[Mo36(NO)4O108(H2O)16]·30.84H2O ({Mo36}) cata- functional groups such as the hydroxy group and C=C bonds
lyst has been synthesized and successfully employed in the tolerated in the oxidation. The {Mo36} catalyst showed high
selective oxidation of various sulfides to sulfoxides with urea
hydrogen peroxide as oxidant under mild reaction conditions
with 84–99% conversion and 58–99% selectivity, with active
catalytic activity for a high substrate/catalyst ratio (up to
30000:1) and is recyclable.
bonds, potentially representing materials that could be
highly active towards selective redox chemistry.[25,26]
Introduction
Polyoxometalates (POMs) are a remarkable class of
metal–oxo clusters of the early transition metals charac-
terized by enormous structural variety resulting in different
dimensions, shapes, charge density, surface reactivity, and a
rich redox chemistry.[1–6] They have applications in areas
such as catalysis, magnetism, electrochemistry, molecular
electronics, energy conversion, and medicine.[7–10] The ap-
plications of POMs are primarily based on their redox
properties, photochemical response, magnetic properties,
ionic charge, conductivity, and ionic weights.[11–15]
As part of our ongoing study of the selective and green
synthesis of sulfoxides,[27–31] we report herein a highly ef-
ficient, recyclable, and green method for the selective oxid-
ation of a series of sulfides to the corresponding sulfoxides
in excellent yields and selectivities under mild reaction con-
ditions with the aid of the polyoxomolybdate nanocluster
(NH4)12[Mo36(NO)4O108(H2O)16] as an efficient catalyst
and urea hydrogen peroxide (UHP) as the oxidant.
Selective oxidation is an important process for the func-
tionalization of chemical feedstocks and there is a continual
need for the design of robust inorganic catalysts featuring
well-defined multinuclear active sites, contributing to the
development of sustainable and efficient oxidative processes
with environmentally benign O2 and H2O2 as oxi-
dants.[16–19] There has been particular interest in utilizing
catalysts containing molybdenum oxide, including the use
of heteropolyacids.[20–24] In the majority of these cases, the
molybdenum is usually present as MoV and MoVI and the
catalysts exhibit significant electron hopping between the
MoV and MoVI sites through MoV–O–MoVI bridging
Results and Discussion
Complex Characterization
The
compound
(NH4)12[Mo36(NO)4O108(H2O)16]·
30.84H2O ({Mo36}) was prepared analogously to a Mo36
polyoxometalate salt (1) reported by Müller et al.[32] This
product is a hydrate of the (NH4)12[Mo36(NO)4O108
-
(H2O)16] salt. Indeed, a polyoxomolybdate cluster of the
same molecular topology was confirmed to form by X-ray
diffraction studies. In this anion two characteristic {Mo17}
units linked by two {MoO2}2+ moieties and with four NO
substituents can be distinguished (Figure 1).
Similar anions have been reported in the form of salts
with different organic cations and substituents.[33,34] All
atomic positions except for water H atoms could be de-
tected. In this molecular structure, the atom labeling
scheme of Müller et al. was adopted. Selected geometric
parameters of {Mo36} are collected in Table 1. {Mo36} dis-
plays similar cell parameters to 1, but with significant dif-
ferences, especially concerning the a, b, and c constants [see
Table 1, compare with a = 16.742(3), b = 18.710(4), c =
24.638(5) Å, and β = 98.84(3)° reported by Müller et al. for
[a] Department of Chemistry, Faculty of Science, University of
Maragheh,
Golshahr, P. O. Box 55181-83111731, Maragheh, Iran
E-mail: mamini@maragheh.ac.ir
[b] Department of Chemistry, University of Tehran,
Tehran, Iran
E-mail: mfarnia@khayam.ut.ac
[c] Fachbereich Chemie and Wissenschaftliches Zentrum für
Materialwissenschaften (WZMW), Philipps-Universität
Marburg,
Hans-Meerwein-Str., 35032 Marburg, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500528.
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not be distinguished from the interstitial water molecules
based on our data and have been arbitrarily assigned. A
similar approach was adopted for 1. Nevertheless, the water
content of {Mo36} seems to be slightly different to that in
1 (30.84 vs. 33 in {Mo36} and 1, respectively).
The polyoxometalate anions form columns along [010]
with water molecules and ammonium cations distributed in
the voids between them (Figure 2).
Figure 1. Structure of the polyoxometalate anion in {Mo36}. Ther-
mal ellipsoids are plotted at the 20% probability level. The sym-
metry-independent part is labeled. Only the major-occupancy com-
ponent of the Mo7 atom is shown.
1
[32]]. Moreover, the data reported for 1 were collected at
Figure 2. Columnar arrangement of the polyoxometalate anions in
{Mo36}, projection along [010].
193(2) K, whereas for {Mo36} the data was collected at
100(2) K. We were unsuccessful in our attempts to refine
{Mo36} by using 1 as the starting model. However, chemi-
cally, {Mo36} seems to be equivalent to 1, even reproducing
the disorder of the Mo7 atom albeit with different occu-
pancies of the two components (0.7/0.3 and 0.6/0.4 for
{Mo36} and 1, respectively). In our model we also assumed
that there were 12 ammonium cations, however, these could
Catalytic Effects
First, the oxidation reaction of methyl phenyl sulfide
(MPS) as a standard substrate was first investigated by
using {Mo36} as a catalyst in different solvents at various
temperatures and with different amounts of oxidant or cat-
alyst. Different solvents and oxidants were added to a solu-
tion of 1 mmol MPS and 0.0001 mmol {Mo36} at room
temperature. The results of the MPS conversion and selec-
tivity can be found in Table 2 and show that the reaction
is sensitive to solvent. Polar solvents resulted in the best
conversions and selectivities, especially alcoholic solvents
(entries 2 and 3), whereas EtOAc, CH2Cl2, hexane, and tol-
uene only gave a moderate or low conversion due to the
poor solubility of the catalyst in these solvents. Among the
Table 1. Selected X-ray data for (NH4)12[Mo36(NO)4O108(H2O)16]·
30.84H2O.
{Mo36}
Formula
Mr
Temperature [K]
λ [Å]
Crystal system
Space group
a [Å]
12(NH4)·H32Mo36N4O128·30.84(H2O)
6362.25
100(2)
0.71073
monoclinic
P21/n
16.960(4)
18.384(4)
24.271(4)
99.28(3)
7468(3)
2, 2.829
3.04
6073
0.07ϫ0.06ϫ0.04
1.37–25.00
Table 2. Oxidation of methyl phenyl sulfide in different solvents
and with different oxidants.[a]
b [Å]
c [Å]
Entry Oxidant Solvent
Conv. [%][b] Selectivity [%][c]
β [°]
V [Å3]
1
2
3
4
5
6
7
8
UHP H2O
UHP EtOH
59
85
89
82
62
54
60
55
97
91
88
46
98
97
98
97
98
98
97
98
96
94
65
99
Z, ρcalcd. [gcm–3]
μ [mm–1]
F(000)
UHP MeOH
UHP MeCN
UHP CH2Cl2
UHP hexane
UHP toluene
UHP EtOAc
Crystal size [mm]
θ range [°]
Reflections, total/unique 47868/13163
R(int)
Absorption correction
Min., max. transmission 0.816, 0.907
factors
0.087
numerical
9
UHP MeOH/CH2Cl2
H2O2 MeOH/CH2Cl2
10
11
12
O2
MeOH/CH2Cl2
Data/restraints/param-
eters
13163/0/977
TBHP MeOH/CH2Cl2
[a] Reagents and conditions: 1 mL of solvent, 1 mmol of methyl
phenyl sulfide, 1 mmol oxidant, 0.0001 mmol (NH4)12[Mo36-
(NO)4O108(H2O)16], 15 min. [b] Determined by GC analysis of the
crude reaction mixture. [c] Selectivity to sulfoxide = [sulfoxi%/(sulf-
oxide% + sulfone%)]ϫ100.
GOF on F2
1.01
0.058
0.142
R1 [IϾ2σ(I)]
wR2 (all data)
Max., min. Δρelect. [eÅ–3] 2.64, –1.93
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solvents examined, a 1:1 mixture of CH3OH/CH2Cl2 was
the best solvent, providing the highest conversion (97%)
and selectivity (96%). Various oxidants, including UHP,
H2O2, tert-butyl hydroperoxide (TBHP), and molecular
oxygen were tested for the oxidation of MPS and we found
that UHP and H2O2 provided better results than TBHP
and molecular oxygen (entries 9–12), with the activity of the
catalytic reaction decreasing greatly with TBHP as oxidant
(entry 12). In addition to their oxidation performances,
UHP and H2O2 are also superior to TBHP in terms of its
environmentally benign properties.
The amount of catalyst also significantly affected the
conversion and selectivity for methyl phenyl sulfoxide (Fig-
ure 3). When the amount of catalyst was increased from
5ϫ10–5 to 1ϫ10–4 mmol, the conversion of MPS increased
dramatically from 70 to 97%. With a further increase in the
amount of catalyst, no significant changes in MPS conver-
sion and selectivity were observed. A blank reaction showed
very low conversion in the absence of the catalyst at room
temperature.
Figure 4. Influence of the substrate/catalyst ratio on the oxidation
of MPS . Reagents and conditions: MPS/UHP (mol/mol = 1:1),
0.0001 mmol {Mo36}, MeOH/CH2Cl2 (1:1), 15 min.
selectivity of the reaction (entries 8 and 9). The presence of
two phenyl rings in diphenyl sulfide required a longer reac-
tion time to complete the reaction (entry 3). Finally, in the
oxidation of sulfides bearing other active functional groups,
such as the hydroxy group and C=C bonds, the otherwise
easily oxidized functional groups were tolerated (entries 10
and 11).
Figure 5 (a–c) shows the IR spectra of the pure com-
pound {Mo36}, after its reaction with an excess of H2O2,
and after recycling of {Mo36}. Bands with wavenumbers
ranging from 884 to 624 cm–1 have been attributed to νMo=O
and νMo–O–Mo vibrations and the νNO vibrational frequency
at 1619 cm–1 is typical of a linear {MoNO} moiety. After
the addition of H2O2 to a solution of the compound
{Mo36} in acetonitrile, the solution changed from red to
yellow, indicating the formation of an active species. The
corresponding IR spectrum shows a new peroxo bond ab-
sorption ν(O–O) at 909 cm–1 and the band at 976 cm–1 has
been attributed to νMo=O arising from MoO(O2) species.
Figure 3. Influence of the amount of the catalyst on the oxidation
of MPS. Reagents and conditions: MPS/UHP (mol/mol = 1:1),
MeOH/CH2Cl2 (1:1), 15 min.
Because {Mo36} showed excellent catalytic activity and
efficiency in the oxidation of MPS, we investigated the in-
fluence of the substrate/catalyst ratio (Figure 4). When the
substrate/catalyst ratio was increased from 10000:1 to
30000:1, the conversion of MPS increased from 97 to 99%.
Further increases in the substrate/catalyst ratio above
30000:1 led to a decrease in the conversion of MPS from
99 to 86%.
Compound {Mo36} was also examined as a catalyst in
the oxidation of other sulfides, including dialkyl sulfides,
cyclic sulfides, benzyl alkyl sulfides, and aryl alkyl and di-
alkyl sulfides bearing different functional groups with UHP
as oxidant. The results are given in Table 3. Just like the
oxidation of MPS, excellent catalytic activities and selectivi-
ties were obtained for all the tested sulfides. Very good con-
versions of 84–99% (TON = 25200–29700), depending on
the nature of the sulfide, were obtained in all cases. In the
oxidation of benzyl phenyl sulfide and dibenzyl sulfide, no
oxidation was observed at the benzylic C–H bonds (en-
tries 4 and 5). In the oxidation of various phenyl-substi-
tuted MPSs to the corresponding sulfoxides, the electronic
Figure 5. IR spectra of a) pure compound {Mo36}, b) {Mo36} after
nature and position of the substituents had no effect on the reaction with H2O2, and c) recycled compound {Mo36}.
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Table 3. Oxidation of sulfides to sulfoxides with UHP catalyzed by (NH4)12[Mo36(NO)4O108(H2O)16].[a]
[a] Reagents and conditions: 3 mmol of sulfide, 3 mmol UHP, 0.0001 mmol (NH4)12[Mo36(NO)4O108(H2O)16], 1 mL of MeOH/CH2Cl2
(1:1). [b] Determined by GC analysis using an internal standard in the crude reaction mixture. [c] Selectivity to sulfoxide = [sulfoxide%/
(sulfoxide% + sulfone%)]ϫ100. [d] TON = (mmol of sulfoxide + mmol of sulfone)/mmol of catalyst.
The changes in the IR spectra of the catalyst before and ation of MPS with (NH4)12[Mo36(NO)4O108(H2O)16]
after addition of the oxidant indicate that the reaction starts as catalyst was complete, ethyl acetate was added to the
with nucleophilic attack of H2O2 to form dioxo–MoVI cen- reaction mixture and the catalyst precipitated from solu-
ters, which are converted into oxo–peroxo–MoVI centers.[27] tion. The catalyst was then filtered, washed with ethyl acet-
The attack of the uncoordinated sulfide on one of the oxy- ate, and dried in vacuo at room temperature. The catalyst
gen atoms of the peroxido ligand to yield sulfoxide and gen- was recycled three times following oxidation reactions that
erate the related dioxidomolybdenum(VI) centers completes were completed within 20 min with comparatively high
the catalytic cycle (Scheme 1).
conversion and selectivity. The IR spectrum of the
Because the life span of a catalyst is an important param- recycled compound {Mo36} (Figure 5, c) showed no vi-
eter in its evaluation, the reusability of the catalyst {Mo36} brational changes, which indicates a good stability of the
was also investigated (Figure 6). After the catalytic oxid- catalyst.
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Table 4. Oxidation of methyl phenyl sulfide (MPS) with different catalysts.
Entry Catalyst
Conditions
Conversion [%] Selectivity [%]
1
{Mo36}
0.0001 mmol catalyst, 3 mmol of MPS, 3 mmol UHP, 1 mL of MeOH/CH2Cl2
(1:1), 15 min
0.0001 mmol catalyst, 2.5 mmol of MPS, 2.5 mmol H2O2, 4 mL of MeOH,
10 min
0.0001 mmol catalyst, 1 mmol of MPS, 2 mmol H2O2, 1 mL of H2O, 45 min
99
97
2
{Mo8}[a]
99
98
[b]
3
4
{Mo132
}
92
96
100
92
{Mo72Fe30}[c] 0.00013 mmol catalyst, 0.1 mmol of MPS, 0.11 mmol H2O2, 1 mL of MeOH,
420 min
[a] See ref.[20] [b] See ref.[35] [c] See ref.[36]
various sulfides to sulfoxides catalyzed by (NH4)12-
[Mo36(NO)4O108(H2O)16] was achieved by using UHP as an
oxidant in a high substrate/catalyst ratio (up to 30000:1).
Short reaction times with excellent selectivities (58–99%)
were observed. Recycling experiments indicated that the
catalyst can be recycled and reused at least three times with-
out any significant loss of its activity, which indicates a
good stability of the catalyst.
Experimental Section
General: Chemicals and solvents were purchased from the Fluka
and Merck Chemical companies. The elemental analysis (CHN) of
{Mo36} was performed with a Carlo ERBA Model EA 1108 ana-
lyzer. FTIR spectra were obtained with a Unicam Matson 1000
FT-IR spectrophotometer using KBr disks at room temperature.
X-ray diffraction data were collected with an IPDS2t dif-
fractometer employing Mo-Kα radiation and an image plate detec-
tor (see Table 1 for selected crystallographic data). The oxidation
products were determined and analyzed with an HP Agilent 6890
gas chromatograph equipped with a HP-5 capillary column and a
flame-ionization detector.
Scheme 1. Proposed mechanism for the oxidation of sulfide cata-
lyzed by {Mo36}.
Synthesis of (NH4)12[Mo36(NO)4O108(H2O)16], {Mo36}: (NH4)12-
[Mo36(NO)4O108(H2O)16] was prepared according to a reported
procedure with some modification.[32] HCl (3.5%, 1.6 mL) was
added in a single portion to a solution of Na2MoO4 (1.2 g,
4.9 mmol), NH2OH·HCl (0.77 g, 1.11 mmol) and NH4Cl (0.44 g,
0.82 mmol) in H2O (30 mL) in a 100 mL Erlenmeyer flask. The
mixture was stirred slowly for 20 h in an oil bath at 70 °C. After
filtration of the hot solution the cooled red filtrate was stored for
crystallization at room temperature.
Figure 6. Recycling studies of the (NH4)12[Mo36(NO)4O108(H2O)16]
catalyst in the oxidation of sulfides.
General Procedure for the Oxidation of Sulfide: We used a standard
procedure for the sulfide oxidation experiments. UHP (0.4 mmol)
as oxidant was added to a solution of sulfide (0.2 mmol), chloro-
benzene (0.2 mmol) as internal standard and {Mo36}
(1ϫ10–4 mmol) in a 1:1 mixture of CH3OH/CH2Cl2 (1 mL). The
mixture was stirred at room temperature and the reaction progress
was monitored by GC. The products were assigned by comparison
with authentic samples.
To show the merit and efficiency of the present catalytic
system in comparison with other recently reported proto-
cols, we compared the results of the oxidation of methyl
phenyl sulfide in the presence of other polyoxomolybdates.
As shown in Table 4, in contrast to similar previously re-
ported systems, the catalytic system presented in this paper
does not suffer from harsh reaction conditions, such as the
use of a large amount of solvent (entry 2), long reaction
time (entries 3 and 4), high oxidant/substrate ratio (entry 3),
and low substrate/catalyst ratio (entries 2–4).
CCDC-1063764 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Conclusions
Acknowledgments
The nanocluster polyoxomolybdate (NH4)12[Mo36(NO)4-
O
108(H2O)16] has been prepared and characterized by X-
Partial funding was provided by the Research Council of the Uni-
ray crystallography and IR spectroscopy. The oxidation of versity of Maragheh (grant to M. A.).
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Received: May 15, 2015
Published Online: July 17, 2015
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