A Crown-Shaped Ru-Substituted Arsenotungstate for Selective Oxidation of Sulfides with Hydrogen Peroxide

Article abstract of DOI:10.1002/chem.201800748

An acetate-bridged Ru-substituted arsenotungstate [H₂N(CH₃)₂]₁₄[As₄W₄₀O₁₄₀{Ru₂(CH₃COO)}₂]?22 H₂O (1) has been synthesized and structurally characterized. Four Ru atoms occupy the respective lacunary S2 sites of the crown-shaped polyanion [As₄W₄₀O₁₄₀]^28?, and each Ru atom is coordinated by one As atom and five μ₂-O atoms, comprising four from the S2 site and one from the acetate ligand. To the best of our knowledge, this coordination of the Ru atom, with an Ru?As bond length of 2.377(3)–2.387(3) ?, is unprecedented in polyoxometalate (POM) chemistry. Notably, 1 exhibits high efficiency, excellent selectivity, and good recyclability for the oxidation of sulfides with hydrogen peroxide (H₂O₂). Catalytic oxidation of various sulfides in the presence of 1 gives superior conversion and selectivity for sulfones in acetonitrile, whereas sulfoxides are obtained in methanol.

Full text of DOI:10.1002/chem.201800748

A Journal of
Accepted Article
Title: A Crown-Shaped Ru-Substituted Arsenotungstate for Selective
Oxidation of Sulfides with Hydrogen Peroxide
Authors: Jingyang Niu, Mengdan Han, Yanjun Niu, Rong Wan, Qiaofei
Xu, Jingkun Lu, Pengtao Ma, Chao Zhang, and Jingping
Wang
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To be cited as: Chem. Eur. J. 10.1002/chem.201800748
Link to VoR: http://dx.doi.org/10.1002/chem.201800748
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A Crown-Shaped Ru-Substituted Arsenotungstate for Selective
Oxidation of Sulfides with Hydrogen Peroxide
Mengdan Han, Yanjun Niu, Rong Wan, Qiaofei Xu, Jingkun Lu, Pengtao Ma, Chao Zhang, Jingyang
Niu* and Jingping Wang*[a]
Abstract: An acetate-bridged Ru-substituted arsenotungstate
[H2N(CH3)2]14[As4W40O140{Ru2(CH3COO)}2]·22H2O (1) has been
synthesized and structurally characterized, in which four Ru atoms
occupy all lacunary S2 sites of the crown-shaped polyanion
catalytic activity towards water oxidation reaction.[5b] Afterwards,
the di-Ru-substituted POM [Ru2IIIZn2(H2O)2(ZnW9O34)2]14
prepared by Shannon showed excellent electrocatalytic activity
for oxygen generation.[6] Therefore, the exploration of whole new
types of RSKPs especially with attractive catalytic properties is
scientifically significative and thus has been the subject of
intensive synthetic efforts.
28

[As4W40O140
]
and each Ru atom is coordinated by one As atom
and five 2-O atoms including four from S2 site and one from the
acetate ligand. Notably, the novel coordination pattern of Ru atom in
polyoxometalates (POMs) chemistry has been presented
unprecedentedly, which is the formation of RuAs bond with the
length of 2.377(3)2.387(3) Å. Particularly, 1 has exhibited high
efficiency, splendid selectivity and good recyclability for the oxidation
of sulfides with hydrogen peroxide (H2O2). It is worth mentioning that
catalytic oxidation of various sulfides in the presence of 1 gave
superior conversion and selectivity for sulfones in acetonitrile or
sulfoxides in methanol.
However, even after decades of research, the development of
RSKPs chemistry is still in its infancy. To date, the known
examples are mostly limited to the mono-di-Ru-substituted
species or their organo-ruthenium analogues, such as the mono-
Ru-substituted derivatives [XW11O39Ru(H2O)]5/4 (X = P, Si,
Ge)[4,710] and their dimethyl sulfoxide (dmso) analogues
[XW11O39Ru(dmso)]5/4
[XW11O39Ru(dmso)3(H2O)]6 (X
substituted derivatives {[(WZnRu2III(OH)(H2O)](ZnW9O34)2}11 [12]
[Ru2IIIZn2(H2O)2(ZnW9O34)2]14 [6]
the oxo-bridged derivatives
[{SiW11O39RuIV/III2}O]11 [13] [{PW11O39}2{(HO)RuIV-O-
(X
=
=
P,
Si,
Ge),[5,8,10]
SiIV, GeIV),[11] the di-Ru-
,
,
,
RuIV(OH)}]10 [14]
,
the
ruthenium-nitrido
derivatives
Introduction
[PW11O39RuVIN]4,[15] [γ-XW10O38{RuN}2]6 (X = Si, Ge),[16] and
the arene-Ru derivatives [{PW11O39{Ru(arene)}}2{WO2}]8 (arene
Polyoxometalates (POMs, predominantly polyoxotungstates and
polyoxomolybdates) have triggered widespread attention over
the past few decades not only because of their aesthetically
fantastic structures, diverse compositions and the inimitable
properties but also due to the potential applications in catalysis,
electrochemistry, magnetism, and material science etc.[1]
Amongst the multitudinous polyoxometalate structural motifs,
=
benzene, toluene, p-cymene, hexamethylbenzene),[17]
[XW11O39{Ru(II)(benzene)(H2O)}]6/5 (X = P, Si, Ge),[9] etc.
Nevertheless, it is still a significant challenge to synthesize the
multi-Ru-substituted polyoxotungstates.[1820] On the other hand,
the majority of reported RSKPs are composed of WO6 octahedra
and XO4 (X = PV, SiVI, GeVI, etc.) tetrahedra. In comparison, the
introduction of trigonal pyramidal XO3 (X = AsIII, SbIII, BiIII, SeVI,
TeVI) groups into Ru-substituted polyoxotungstates remains
largely unexplored. Up to now, only a handful of RSKPs based
on trigonal pyramidal XO3 (X = AsIII, SbIII, BiIII, SeVI, TeVI) groups
were addressed.[2023] When search for a new entry into RSKPs
chemistry, we first utilized the self-assembly strategy of sodium
tungstate, heteroatom ingredients such as sodium arsenite to
react with ruthenium ions aimed at discovering novel RSKPs for
the following reasons: a) The self-assembly strategy is an
effective method to construct POMs with diverse structures and
compositions. b) The stereochemically active lone pair on the
AsIII atom always precludes the formation of closed Keggin units,
which makes it possible that ruthenium ions can incorporate into
the lacunary sites, or more likely, integrate in situ generated
fragments together into large architectures.
ruthenium(Ru)-substituted
Keggin-type
polyoxotungstates
(RSKPs) endow POMs with unique catalytic properties due to
the redox and catalytic properties of ruthenium.[2] To our best
knowledge, it has been proved that the RSKPs own wonderful
catalysis towards the oxidation of various organic substrates, the
reduction of carbon dioxide and water oxidation reaction.[3] For
instance,
the
mono-Ru-substituted
silicotungstate
[SiW11O39RuIII(H2O)]5 first reported by Neumann could catalyze
not only the oxidation of alkanes, alkenes and alcohols but also
the photoreduction of carbon dioxide.[4] Bonchio obtained a
{Ru(dmso)}2+
substituted
phosphotungstate
[PW11O39RuII(dmso)]5 which was documented as an efficient
catalyst for the oxidation of cyclooctene.[5a] Subsequently,
Sadakane reported that [PW11O39RuII(dmso)]5 exhibited
Herein, we present a new acetate-bridged Ru-substituted
[a]
M. Han, Y. Niu, R. Wan, Q. Xu, J. Lu, Dr. P. Ma, Dr. C. Zhang, Prof.
J. Niu, Prof. J. Wang
arsenotungstate
[H2N(CH3)2]14[As4W40O140{Ru2(CH3COO)}2]·
22H2O (1), which was obtained successfully by the reaction of
Na2WO4·2H2O, NaAsO2 and RuCl3 in the presence of
dimethylamine hydrochloride in sodium acetate buffer solution.
The excellent catalytic activity of 1 was observed towards the
selective oxidation of sulfides with hydrogen peroxide (H2O2).
High selectivity and good conversions of sulfones or sulfoxides
could be obtained when the catalytic process was performed in
corresponding solvents (acetonitrile or methanol, respectively).
The results of recycling experiments implied that there were no
Key Laboratory of Polyoxometalate Chemistry of Henan Province
Institute of Molecular and Crystal Engineering
College of Chemistry and Chemical Engineering
Henan University, Kaifeng, Henan 475004 (P. R. China)
Fax: (+ 86)371-238-868-76
E-mail: jyniu@henu.edu.cn
jpwang@henu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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Crystal Structure
obvious changes in conversion and selectivity for the oxidation
of methyl phenyl sulfide to methyl phenyl sulfone in acetonitrile
across five runs, which showed the superior recyclability of 1.
Single-crystal structural analysis reveals that compound
1
crystallizes in the monoclinic space group P2/c and contains a
crown-shaped
Ru-substituted
arsenotungstate
[As4W40O140{Ru2(CH3COO)}2]14 (1a), fourteen monoprotonated
Results and Discussion
[H2N(CH3)2]+ counter cations and twenty-two lattice water
molecules. The polyanion [As4W40O140{Ru2(CH3COO)}2]14
Synthesis
28
comprises a cyclic unit [As4W40O140
]
({As4W40}) with two
[Ru2(CH3COO)]7+ ({Ru2(CH3COO)}) segments embedded in its
cavities (Figure 1c and Figure S1). The {As4W40} unit (Figure 1b)
is constructed from four trivacant [B-α-AsW9O33]9 ({AsW9})
fragments (Figure 1a) joined together by four additional WO6
octahedra, and is already confirmed as a multi-vacant cryptate
which contains four coordinative sites S2 and a central cryptate
site S1 (Figure S2).[24] Each WO6 octahedron links two adjacent
{AsW9} fragments by two 2-O atoms. The remaining oxygen
atoms of each WO6 octahedron together with two oxygen atoms
of each {AsW9} fragment define the S2 sites and the eight
remaining oxygen atoms of four WO6 octahedra define the
central site S1. In the polyanion 1a, all the lacunary S2 sites of
the {As4W40} are occupied by four Ru atoms whereas the central
site S1 remains vacant. A bidentate acetate ligand connects two
diametrically opposed Ru atoms in a (2-1:1) fashion, giving
rise to a {Ru2(CH3COO)} segment with a Ru···Ru distance of
5.9147(3) Å (Figure S3). As far as we know, such a crown-
shaped acetate-bridged Ru-substituted arsenotungstate is a first
report that supports the structural novelty of this rare compound.
Interestingly, there are two crystallographically independent Ru
atoms (Ru1 and Ru2) in the asymmetric unit which are hexa-
coordinated and adopt distorted octahedral geometries (Figure
2a-b). Both Ru atoms are defined by two 2-O atoms from the
{AsW9} fragment [RuO, 1.979(18)2.067(19) Å], two 2-O
atoms from two WO6 octahedra [RuO, 1.995(18)2.022(17) Å],
one 2-O atom from the {CH3COO} group [RuOCOO, 2.105(18)
The design and synthesis of RSKPs remain a great challenge
because of the intricacies in their synthetic mechanism. As a
result, it is quite difficult to search appropriate reaction
conditions that can allow access to novel RSKPs. With
unremitting efforts, we have now succeeded in preparing a new
crown-shaped
[H2N(CH3)2]14[As4W40O140{Ru2(CH3COO)}2]·22H2O
Ru-substituted
arsenotungstate
(1) by
reaction of Na2WO4·2H2O, NaAsO2 and RuCl3 with a molar ratio
of 7.7:1.1:1.0 in the sodium acetate buffer solution (0.5 mol L1,
pH = 4.3) in the presence of dimethylamine hydrochloride
(Scheme 1). Importantly, the sodium acetate buffer solution, as
a reaction medium, plays a vital role in the construction of 1. It
not only supplies a mild environment for Ru ions, preventing
them from hydrolyzing, but also provides the bidentate ligands
CH3COO that can coordinate to Ru ions. Besides, when the pH
value of the solution was adjusted to about 4.6, the quality of the
crystals was best. Another essential aspect in the formation of 1
is the counter cations. Initially, we selected Na+, K+, Rb+ and Cs+
as counter cations, however, no desired crystals were obtained.
In our subsequent exploration, dimethylamine hydrochloride was
introduced into our reaction system. We found that the solubility
of Ru ions was increased in the presence of dimethylamine
hydrochloride, which facilitates Ru ions incorporated into in situ
generated arsenotungstate fragments, giving birth to the title
compound.
9

Figure 1. a) Ball-and-stickpolyhedral representation of the [B-α-AsW9O33
]
fragment in 1a. b) Ball-and-stickpolyhedral representation of the cryptate
28
[As4W40O140]
. c) Ball-and-stick polyhedral representation of the polyanion
1a. Color code: WO6 octahedra, sea green; ruthenium, turquoise; arsenic,
yellow; oxygen, red; carbon, black, all the hydrogen atoms are omitted for
clarity.
Scheme 1. The schematic synthetic process of compound 1.
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9
Figure 3. a) The [B-α-AsW9O33
c) The dimeric
AsW9O33)2Ru2(CH3COO)]11. d) The polyanion 1a.
]
 fragment. b) The {Ru2(CH3COO)} segment.
mono-Ru-substituted
arsenotungstates
[(B-α-
mono-Ru-substituted arsenotungstate [(B-α-AsW9O33)2Ru2(CH3-
COO)]11 (Figure 3c). Two [(B-α-AsW9O33)2Ru2(CH3COO)]11
subunits in a vertical cross arrangement are linked by four
additional WO6 octahedra, resulting in a cyclic tetrameric motif
(Figure 3d). Notably, the linking mode of the acetate ligand in
[(B-α-AsW9O33)2Ru2(CH3COO)]11 subunits is different from that
Figure 2. a) The coordination environment of Ru1 ion. b) The coordination
environment
of
Ru2
ion.
c)
The
octa-nuclear
heterometallic
[Ru4W4O8(CH3COO)2]22+ core in 1a. d) The trigonal pyramid geometry of tetra-
nuclear Ru cluster. Atoms with A in their labels are symmetrically generated. A:
2x, y, 1.5z.
of
in
acetate-bridged
mono-lanthanide
substituted
polyoxotungstates
[{La(CH3COO)(H2O)2(α2-P2W17O61)}2]16 [26]
,
[{(α-SiW11O39)Ln(μ-COOCH3)(H2O)}2]12 (Ln = GdIII and YbIII)[27]
and [{(α-PW11O39)Ln(H2O)(η2,μ-1,1)-CH3COO}2]10 (Ln = SmIII,
EuIII, GdIII, TbIII, HoIII and ErIII),[28] where two Ln atoms are doubly
bridged by two acetate molecules in a (η2,μ-1,1) fashion (Figure
S5).
2.11(2) Å], and one As atom from the {AsW9} fragment [RuAs,
2.377(3)2.387(3) Å]. Herein, it is worth mentioning that the
RuAs bond in 1a has been observed for the first time in POMs
chemistry. Remarkably, irrespective of the {AsW9} fragments,
the remanent eight metals including four Ru and four W cations
are interconnected by eight 2-O and two acetate bridges,
Bond valence sums (BVS)[29] calculated for the two
crystallographically independent Ru atoms lie between 3.78 and
3.92, indicating that Ru atoms are +IV valence states (Table S1).
Additionally, the oxidation of RuIII in reaction solution to yield a
RuIV-containing POM has been reported.[30] The BVS values for
W and As atoms are 6.046.81 and 2.432.56, respectively
(Table S2 and Table S3), indicating that all the W and As are in
the +6 and +3 oxidation states, respectively. In addition, the BVS
values for the oxygen atoms including the oxygen atoms from
{CH3COO} groups are in the range of 1.682.20 (Table S4),
indicating that all the oxygen atoms are deprotonated.
resulting
in
an
octa-nuclear
heterometallic
core
[Ru4W4O8(CH3COO)2]22+ (Figure 2c), where four Ru cations
reside in a distorted tetrahedron (Figure 2d).
It should be pointed out that the polyanion 1a somewhat
resembles
to
[(H2O)10(CeIII2OH)2(B-α-AsW9O33)4(WO2)4]18–
previously reported by Pope et al, in which four S2 sites are
occupied by four Ce atoms (Figure S4a).[25] Nevertheless, there
are also obvious differences between them apart from
containing the same {As4W40} unit: (a) the former is an acetate
acid containing organic-inorganic hybrid cluster, while the latter
is a pure inorganic cluster; (b) the former consists of a tetra-
IR spectrum
The IR spectrum of 1 shows the characteristic vibrations in the
region of 1000650 cm1. The prominent peak at 962 cm1 is
attributed to the terminal WOt vibrations. Peaks at 891 cm1,
749 cm1 are derived from the two types of W-O-W stretching
vibrations and the peak at 705 cm1 is attributed to the WO(As)
vibration.[31] The strong peak at 834 cm1 is attributed to AsO
vibration. Additionally, the broad peaks at 3435 cm1 and 1623
cm1 are assigned to the stretching and bending modes of lattice
water molecules.[32] The weak bands at 3120 and 2926 cm1 are
derived from the NH and CH stretching vibrations,[33]
illustrating the existence of dimethylamine hydrochloride. In
addition, peaks appearing at 1623 cm1 and 1464 cm1 are
attributed to the asymmetric and symmetric stretching vibrations
of the CH3COO ligands, respectively, with the former band
overlapping with the bending vibration of the lattice water (Figure
S6).
ruthenium-containing
[Ru4W4O8(CH3COO)2]22+ (Figure 2c) while the latter includes a
tetra-cerium-containing bimetal–oxide core
heterometallic
core
[Ce4W4O8(OH)2(H2O)10]18+ (Figure S4b); (c) in the former, four
Ru atoms are hexa-coordinated and defined by one As atom
and five 2-O atoms, on the contrary, four Ce atoms in the latter
are all eight-coordinated and defined by five 2-O atoms and
three water molecules; (d) in the former, two pairs of opposite
Ru atoms are all linked together by a bidentate acetate bridge;
whereas in the latter, the opposite two of four Ce atoms are
joined together by a 2-O bridge; (e) the former was obtained

from the WO42, AsO2 and Ru3+ via the self-assembly strategy
whereas the latter was obtained from the cryptate precursor
28
[As4W40O140
]
.
From another perspective, the polyanion 1a can be perceived
as follows: two {AsW9} fragments (Figure 3a) are joined together
by a {Ru2(CH3COO)} segment (Figure 3b), forming a dimeric
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Powder X-ray diffraction
ESI-MS study
As shown in Figure S7, the powder X-ray diffraction peaks are in
good agreement with the simulated peaks originated from single-
crystal X-ray diffraction, indicating the good phase purity for the
targeted samples. The differences in intensity between the
experimental and simulated patterns may be attributed to the
variation in preferred orientation of the powder sample in the
process of collection of the experimental PXRD.
The negative-ion electrospray-ionization mass spectrometry
(ESI-MS) was performed to study the cluster species in solution.
Single-crystalline samples of 1 were dissolved in a mixed
solvent (V(H2O)/V(CH3CN) = 9:1, 4.5 mL H2O, CH3CN 0.5 mL).
As shown in Figure 5a, the ESI-MS spectrum displays three
dominant peaks corresponding to the intact cluster. These peaks
were assigned to charge values of 7, 6 and 5 at m/z of 1505.68
[(H2N(CH3)2)1H6As4W40Ru4O140(CH3COO)2(H2O)4]7 (simulated
UV-vis spectra
1505.69),
W40Ru4O140(CH3COO)2(H2O)4]6 (simulated 1756.81) and
2131.79
[(H2N(CH3)2)2H7As4W40Ru4O140(CH3COO)2(H2O)8]5
(simulated 2131.79), respectively (Figure 5b-d). Furthermore,
the simulated isotopic pattern at around m/z 2137.18
[(H2N(CH3)2)1H8As4W40Ru4O140(CH3COO)2(H2O)12]5 is provided
in the Supporting Information (Figure S14).
1756.80
[(H2N(CH3)2)1H7As4-
The UV-vis spectrum of 1 in the low concentrated aqueous
solution (1.0 × 106 mol L1) displays two characteristic peaks at
200 nm and 250 nm in the range of 400190 nm (Figure 4a).
The higher energy absorption at 200 nm can be ascribed to the
pπ–dπ charge-transfer transition of the Ot→W bands, whereas
the lower at 250 nm can be assigned to the pπ–dπ charge-
transfer transition of the Ob,c→W bands.[34] In the visible region,
a broad peak at around 478 nm is observable in more
concentrated solution (1.0 × 104 mol L1) (Figure 4b), which can
be ascribed to O→Ru charge transfer.[12,35]
The pH-varying absorption spectra of 1 in the visible region
were also investigated as POMs are sensitive to the pH value.
Diluted HCl solution and NaOH solution were used to adjust the
pH values in the acidic direction and in the alkaline direction,
respectively. The initial pH value of 1 in aqueous solution was
5.8. As shown in Figure 4c and 4d, as the pH value decreased,
the absorbance band at around 478 nm became weaker and
weaker. In contrast, the absorbance band is gradually red-
shifted at pH higher than 9.3. Besides, the on-going
spectroscopic measurements were also conducted in the
aqueous system (Figure S10). And 1 can stably exist in the
aqueous system within 1 hour at ambient temperature.
Figure 5. a) The ESI-MS spectrum corresponding to the intact cluster (zoom
in of the peaks at around 1505.68, 1756.80, 2131.79 were provided in
supporting information, Figure S11-S13). b), c) and d) The simulated (blue)
and experimental (black) patterns at around m/z 1505.68, 1756.80, 2131.79.
Catalysis
The selective oxidation of sulfides is a meaningful and important
process as the corresponding sulfones and sulfoxides are
extensively used in the synthesis of biological and
pharmaceutical compounds,[36] fine chemicals,[37] and as ligands
in asymmetric catalysis.[38] Here, catalytic reactions were
performed with hydrogen peroxide (H2O2) to investigate the
catalytic efficiency of 1 towards the heterogeneous oxidation of
sulfides. Initially, methyl phenyl sulfide (MPS) was used as the
benchmark substrate to study the catalytic oxidation of sulfides
in the presence of 1 in acetonitrile, which showed 99%
conversion and 53% selectivity for methyl phenyl sulfone at
50°C (Table S5, entry 5). In the subsequent catalytic reactions,
selectivity for methyl phenyl sulfone was promoted with
increasing the dosage of H2O2 and reached 100% until 2.5 mmol
H2O2 was used under otherwise identical conditions (Table 1,
Figure 4. The UV-vis spectra of 1 a) in the low concentrated solution
6
1
4
(1.010 mol L ). b) in more concentrated solution (1.010 mol L1). c) and
d) at different pH values.
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Table 1. Oxidation of methyl phenyl sulfide using
1
with H2O2 in
Table 3. Selective oxidation of sulfides to sulfoxides using 1 with H2O2 in
acetonitrile[a]
methanol[a]
Entry
Catalyst
(µmol)
H2O2
(mmol)
Temp.
(°C )
Time
(min)
Conv.[b]
(%)
Sel.[c]
(%)
Time
(min)
Conv.[b]
(%)
Sel.[c]
(%)
Entry
Substrate
Product
1
2
3
4
5
1.6
0.8
1.6
1.6
1.6
2.5
2.5
2.0
2.5
2.5
25
50
50
50
50
60
60
60
30
60
93
99
99
96
100
63
77
79
67
100
1
2
50
50
98.3
97.7
88.3
89
3
4
60
60
97.4
97
87
85
[a] Reaction conditions: substrate (1 mmol), 30% H2O2 (2.0 mmol, 2.5
mmol), catalyst (0.8 µmol, 1.6 µmol), acetonitrile (3 mL). [b] Conversions
were determined by GC-FID using an internal standard technique and are
based on substrates. [c] Selectivity for sulfone. The products were identified
by GC-MS.
5
6
7
60
98
84.3
79
60
96.6
48
Table 2. Selective oxidation of sulfides to sulfones using 1 with H2O2 in
120
96
acetonitrile[a]
Time
(min)
Conv.[b]
(%)
Sel.[c]
(%)
[a] Reaction conditions: substrate (1 mmol), 30% H2O2 (2.5 mmol), catalyst
(1.6 µmol), methanol (3 mL), 50 °C . [b] Conversions were determined by
GC-FID using an internal standard technique and are based on substrates.
[c] Selectivity for sulfoxides. The products were identified by GC-MS.
Product
Entry
Substrate
1
2
3
30
50
60
100
100
100
100
100
100
entries 3 and 5). The results indicated that 2.5 mmol H2O2 was
the optimum amount under these conditions. In addition, the
sulfone selectivity decreased dramatically when the loading of
catalyst was reduced to 0.8 µmol (Table 1, entry 2). Furthermore,
a series of experiments were also carried out to explore the
effects of the reaction time and temperature on the conversion
and selectivity of MPS. The corresponding sulfoxide and sulfone
products were verified by GC-MS (Figure S17-S18). The results
implied that the reaction time and temperature also played
crucial role in the conversion and selectivity of MPS (Table 1,
entries 1 and 4). On the basis of the above results, it is obvious
that the optimum temperature and reaction time are 50 °C and
60 min, respectively. As control experiments, the blank
experiment without 1 was performed in acetonitrile, with only
38% conversion and 54% selectivity observed (Table S5, entry
1). Besides, catalytic activity of Na2WO4·2H2O, RuCl3 and the
mixture of these two materials for the oxidation of MPS were
also independently investigated for comparison. As presented in
TableS5, the catalyst RuCl3 exhibited low conversion for the
reaction (Table S5, entry 2) while Na2WO4·2H2O and the mixture
of Na2WO4·2H2O and RuCl3 gave 89% and 90% conversion,
respectively (Table S5, entries 3 and 4). However, these two
synthetic materials were homogeneous catalysts in our reaction
system that couldn’t be separated and reused. Therefore,
catalyst 1, on the whole, was superior to the synthetic materials,
and these results indicated that the main catalytic center is the
POM unit. Under optimal conditions (1.6 mol catalyst, 1 mmol
substrate and 2.5 mmol 30% H2O2 in 3 mL acetonitrile at 50 °C),
MPS was converted to the expected sulfone with 100%
conversion and selectivity (Table 1, entry 5). Encouraged by the
4
5
60
60
100
100
100
100
6
7
8
60
60
90
100
100
99
100
100
97
9
120
180
69
52
100
10
98.5
[a] Reaction conditions: substrate (1 mmol), 30% H2O2 (2.5 mmol), catalyst
(1.6 µmol), acetonitrile (3 mL), 50 °C . [b] Conversions were determined by
GC-FID using an internal standard technique and are based on substrates.
[c] Selectivity for sulfones. The products were identified by GC-MS.
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10.1002/chem.201800748
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framework structure (Figure S20b). According to the ESI-MS
spectrum of the recovered catalyst after fifth-run, the skeleton of
the structure was still retained (Figure S21).
Conclusions
In summary, we have reported
a crown-shaped organic-
inorganic hybrid Ru-substituted arsenotungstate, to the best of
our knowledge, which represents the first example of acetate-
bridged Ru-substituted POMs and the Ru-As bond in 1 has been
reported for the first time in POMs chemistry. Importantly, 1
exhibits significant catalytic activity for the selective oxidation of
sulfides with hydrogen peroxide under mild conditions. Catalytic
reactions conducted in acetonitrile gave sulfone as the major
product with high selectivity while high sulfoxide selectivity with
excellent yields was obtained in methanol. In subsequent work,
continuing efforts will be devoted to constructing novel Ru-
substituted POMs with intriguing architectures and excellent
catalytic properties by adjusting different reaction components
such as different heteroatoms and carboxylates. We believe that
this work will enrich the structural diversity of POMs and
stimulate the development of POM chemistry.
Figure 6. Recycling of compound 1 catalytic system for the oxidation of methyl
phenyl sulfides to methyl phenyl sulfone.
catalytic activity of 1 towards the oxidation of MPS in acetonitrile,
a scope of experiments were also carried out under optimal
conditions to test the catalytic universality of 1, which was
presented detailly in Table 2. Gratifyingly, dialkyl sulfides (Table
2, entries 13) and some alkylaryl sulfides such as ethyl phenyl
sulfide, 4-methylthioanisole, 4-methoxythioanisole tested (Table
2, entries 57) were entirely converted with 100% selectivity for
the corresponding sulfones. However, for diphenyl thioether,
longer reaction time (90 min) was required to achieve
comparable conversion and selectivity, which might be ascribed
to the low electron density of the sulfur atom in this substrate
(Table 2, entry 8). Oxidation of refractory sulfur-containing
Experimental Section
Material and physical measurements
IR spectra were collected on a Bruker VERTEX 70 IR spectrometer using
compounds
including
dibenzothiophene
(DBT)
and
KBr pellets in the range of 4000400 cm1. UV-vis absorption spectra
benzothiophene (BT) were also studied, but reluctant
conversions were obtained (Table 2, entries 9 and 10). Notably,
the corresponding reactions which followed the optimum
conditions but performed in methanol gave sulfoxides as major
product with high conversions (96.6%98.3%) and good
selectivity (7989%) with the exception of diphenyl thioether
which reached 48% conversion and 96% selectivity for its
sulfoxide after 120 min (Table 3). In this regard, previous reports
have reported that the solvent has a significant influence on the
selectivity of sulfoxidation.[39]
The hot filtration experiment was conducted under optimal
conditions. 1 was removed at a reaction time of 15 min, and the
reaction was allowed to proceed with the filtrate. Only a very
small increase in conversion of MPS was observed, suggesting
that this oxidation process is heterogeneous (Figure S19).[40]
Besides, recycle reactions on the oxidation of MPS were
performed in acetonitrile to evaluate the recyclability of the
catalyst. After the oxidation reaction, the catalyst was separated
from the solventoxidantsubstrate system by filtration, washed
with acetonitrile, dried and reused for the next run. The recycling
experiments showed that 1 could be reused five runs without
distinguishable decrease in conversion and sulfone selectivity
(Figure 6).
were obtained on
a U-8100 spectrometer at room temperature.
Thermogravimetric analysis (TGA) was performed on a NETZSCH STA
449F5/QMS403D instrument under a nitrogen gas atmosphere with a
heating rate of 10 °C min1 from 30 to 900 °C. Elemental analyses of C,
H, and N were obtained on an Elementar Vario EL cube CHNS analyzer.
Inductively coupled plasma (ICP) spectra were conducted on a Perkin-
Elmer Optima 2000 ICP-OES. Powder X-ray diffraction (PXRD) were
collected on Bruker D8 Advance instrument with Cu K radiation ( =
1.5418 Å) in the angular range 2
= 5–50° at 293 K. ESI-MS
spectrometry was performed on an ABSCIEX Triple TOF 4600 mass
spectrometer. The ESI-MS spectrum of the catalyst after fifth-run was
obtained on a Q EXACTIVE mass spectrometry. The 1H NMR (400 MHz)
and 13C NMR (101MHz) spectra were detected on a Bruker AVANCE III
HD spectrometer with TMS as the internal standard. A freshly cleaned
glassy carbon disk elecreode (3 mm diameter) was used as a working
electrode, a platinum wire served as the counter electrode and an
Ag/AgCl electrode as the reference electrode.
Preparation of [H2N(CH3)2]14[As4W40O140{Ru2(CH3COO)}2]·22H2O:
Na2WO4·2H2O (2.210 g, 6.700 mmol) and dimethylamine hydrochloride
(0.510 g, 6.255 mmol) were dissolved in 20 mL sodium acetate buffer
solution (0.5 mol L1, pH = 4.3) with stirring. NaAsO2 (1 mol L1, 1 mL)
was dropwise added to the solution. Then, RuCl3 (0.180 g, 0.868 mmol)
was subsequently added into the reaction mixture. Immediately, lots of
precipitate was formed and the solution became turbid. After stirring for
30 min, the precipitate dissolved completely and the pH of the solution
was adjusted to about 4.6 with glacial acetic acid. On further stirring for
another 30 min, the reddish brown solution was sealed in a Teflon-lined
autoclave and heated at 150 °C for 24 h. After cooling to room
Althought noticeable increase of the intensity of the bonds at
834 cm1 and 749 cm1 are observable (Figure S20a), the IR
spectra of the recovered catalysts are almost identical to that of
fresh catalyst, all of which display the typical bands of the
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10.1002/chem.201800748
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anisotropically, while the O, C and N atoms were refined isotropically.
The partial DMA cations and lattice water molecules were located by
Fourier map; the remaining DMA cations and lattice water molecules
were determined by CHN element analysis and TGA result. The
hydrogen atoms of the organic groups were placed in calculated
positions and then refined using a riding model. The hydrogen atoms on
water molecules were directly included in the molecular formula.
Crystallographic data for the structure reported in this paper have been
deposited in the Cambridge Crystallographic Data Center with CCDC
Number: 1590368. The crystal data and structure refinement parameter
are listed in Table 4.
Table 4. Crystallographic data for 1.
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C32H162N14O166As4Ru4W40
11457.19
Monoclinic
P2/c
35.371(3)
20.1245(17)
30.087(3)
109.104(2)
20237(3)
b (Å)
c (Å)
Acknowledgements
βº
We gratefully acknowledge support from the NSFC (21571050)
and the Natural Science Foundation of Henan Province.
V (Å3)
Z
4
Keywords: polyoxometalates • ruthenium • arsenotungstate •
catalysis • sulfoxidation
3
Dcalcd (g cm
)
3.461
1
µ (mm
)
23.648
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F (000)
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-42 ≤ h ≤ 41; -21 ≤ k ≤ 24; -35 ≤ l ≤
35
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Entry for the Table of Contents
FULL PAPER
A crown-shaped Ru-substituted
arsenotungstate
Mengdan Han, Yanjun Niu, Rong Wan,
Qiaofei Xu, Jingkun Lu, Pengtao Ma,
Chao Zhang, Jingyang Niu* and
Jingping Wang*
[H2N(CH3)2]14[As4W40O140{Ru2(CH3C
OO)}2]·22H2O (1), has been prepared
by self-assembly strategy. The
catalytic activity of 1 towards the
selective oxidation of sulfides has
been systematically probed.
Page No. – Page No.
A Crown-Shaped Ru-Substituted
Arsenotungstate for Selectivite
Oxidation of Sulfides with Hydrogen
Peroxide
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