A Lacunary Polyoxovanadate Precursor and Transition-Metal-Sandwiched Derivatives for Catalytic Oxidation of Sulfides

Article abstract of DOI:10.1002/chem.201905741

A SeO₃-centered lacunary Keggin-type heteropolyoxovanadate (hetero-POV) K₆H₂[SeV₁₀O₂₈(SeO₃)₃]?14 H₂O (1) was isolated by one-pot reaction of KVO₃ and SeO

Full text of DOI:10.1002/chem.201905741

A Journal of
Accepted Article
Title: A new lacunary polyoxovanadate precursor and TM-sandwiched
derivatives for catalytic oxidation of sulfides
Authors: Jing Yang Niu, Rong Wan, Peipei He, Zhen Liu, Xinyi Ma,
Pengtao Ma, Vikram Singh, Chao Zhang, Jingyang Niu, and
Jingping Wang
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To be cited as: Chem. Eur. J. 10.1002/chem.201905741
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A new lacunary polyoxovanadate precursor and TM-sandwiched
derivatives for catalytic oxidation of sulfides
Rong Wan, Peipei He, Zhen Liu, Xinyi Ma, Pengtao Ma, Vikram Singh, Chao Zhang, Jingyang Niu*
and Jingping Wang*[a]
[a]
R. Wan, P. He, Z. Liu, X. Ma, P. Ma, V. Singh, C. Zhang, Prof. J. Niu*, Prof. J. Wang*.
Henan Key Laboratory of Polyoxometalate Chemistry
College of Chemistry and Chemical Engineering, Henan University
Kaifeng, Henan, 475004, ( P. R. China), Fax: (+86)-371-23886876
E-mail: jyniu@henu.edu.cn, jpwang@henu.edu.cn
Abstract:
A
SeO3-centered
lacunary
Keggin-type
heteropolyoxovanadate (hetero-POV) K6H2[SeV10O28(SeO3)3]·14H2O
(1) was isolated by one-pot reaction of KVO3 and SeO2 under acidic
condition. X-ray studies revealed that it comprised a single {VO5}-
capped trivacant B-α-type Keggin ion [SeV9O33(VO)]14‒ with its
lacunary sites decorated by three {SeO3} pyramids. Interestingly, this
new basic hetero-POV building block was further used as precursor
to assembly with different transtion metal (TM) ions, yielding a series
of TM-sandwiched POVs K6H8[(SeV10O28(SeO3)3)2(M(H2O)4)]·24H2O
(M2+ = Mn2+ (2), Co2+ (3), Zn2+ (4)). All four compounds were
characterised by single-crystal X-ray structure analysis, IR, XPS, EPR,
and 51V NMR spectroscopy. Importantly, three TM-sandwiched
derivatives exhibited effectively catalytic activity for the
heterogeneous oxidative desulfurization of sulfides at room
temperature.
Figure 1. The two synthetic methods of compounds 1‒4. Combined
polyhedral/ball-and-stick representations of (a) a new type of hetero-POV
[SeV10O28(SeO3)3]8‒ (1a); (b) polyanions [(SeV10O28(SeO3)3)2(M(H2O)4)]14‒ (M =
Mn2+ (2a), Co2+ (3a), Zn2+ (4a)). Colour code: polyhedral VVO5 and VVO6, green;
Se, yellow ball; O, red ball, TMs, green ball.
Introduction
{X2nV18-n} (X = Si, Ge, As, Sb, n = 2‒4)8-11 (Scheme S1). Moreover,
some of these backbones could be further modified by organic
moieties and transition metal (TM) complexes, leading to a
continuous expansion of this subfamily.12 However, compared
with extensive works on hetero-POVs functionalized by TMs and
semimetals from group 14 and 15 elements (mostly Si, Ge, As,
and Sb), the reports regarding POVs encapsulating elements
located at group 16 on the periodic table, especially Se element,
has hardly any weight. To our knowledge, only two Se-substituted
POVs [SeV3O11]3− and [Se2V2O10]2− were isolated by Nakano and
co-workers in 2001,13 which were constructed from tetrahedral
VO4 and trigonal pyramidal SeO3 motifs to form ring-like
structures by sharing vertices (Figure S1). Such an apparent
blank in the investigation of Se-containing POVs have allowed us
to gain deep insights to study the mystery of assembling new type
of hetero-POVs together with their applications.
With compelling features especially chemical and functional
tunability, polyoxometalates (POMs) render
a
fascinating
structural and compositional diversity, and a multitude of
significant applications in academic and applied sciences, for
example catalysis, photo-/electrochemistry, and materials
chemistry.1 Traditionally, POMs are metal-oxide-based clusters,
therefore, based on their component transition metals,
polyoxotungstates (POTs), polyoxomolybdates (POMos), and
polyoxovanadates (POVs) are known as the most important and
vibrant subsets.2 However, being different from molybdenum and
tungsten which usually formed octahedral {MVIO6} units in their
cluster frameworks, vanadium in POVs is apt to display mixed-
valent states with flexible coordination geometries (normally
tetrahedral {VVO4}, square-pyramidal {VVO5} and {VIVO5},
octahedral {VVO6} and {VIVO6}).3 Due to such versatility in redox
behaviors, POVs can be opted as a promising candidate in
catalysis, magnetism and electrode materials, thus always being
preferred among researchers working in this area.4
Generally, the reported structures of POVs can be roughly
divided into two families, iso-POVs and hetero-POVs. So far, the
chemistry of hetero-POVs has achieved great success as the
introduction of heteroatoms tremendously broaden the scope of
hetero-POVs synthetic, structural and applied chemistry.3c,5-11 In
particular, the representative works were contributed by research
groups of Müller, Wang, Hu, Yang and so on, and the most
renowned findings in this subclass embraced but not limited to
{MV13/14} (M = Mn, Ni),4d,6 {XV14} (X = Al, P, As, Ge)7 and
In this work,
a
new type of hetero-POV precursor
K6H2[SeV10O28(SeO3)3]·14H2O (1) was isolated by reacting SeO2
with KVO3 (Figure 1a), featuring one {VO5}-capped trivacant
Keggin unit {B-α-SeV9O33} with its vacant sites being occupied by
three {SeO3} groups. Interestingly, by employing different TM2+
ions to assemble with this isolated vanadoselenite cluster or
mixture of KVO3 and SeO2, a family of vanadoselenite-based TM-
derivatives K6H8[(SeV10O28(SeO3)3)2(M(H2O)4)]·24H2O (M2+
=
Mn2+ (2), Co2+ (3), Zn2+ (4)) were prepared (Figure 1b), and each
of them existed a closely related structure of in-situ formed
vanadoselenite building block of {SeV10O28(SeO3)3} (aliased as
1
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a number of examples have been reported in the recent
investigations,15,16 classical hetero-POTs containing selenium(IV)
included TM-sandwiched structures [Cu3(H2O)3(α-SeW9O33)2]10−
,
[M4(H2O)3(α-SeW9O33)2](16−4n)− (M = Cr3+, Mn2+, Fe3+, Co2+, Ni2+,
Zn2+, Cd2+) and [Se2W21O69(H2O)]4‒ based on trivacant Keggin
{SeW9} unit,16a,16b dimeric [Mn4Se6W24O94Cl(H2O)6]13‒, trimeric
32‒
(Se2W12O46(WO(H2O))3]24‒ and tetrameric [Se8W48O176
]
assemblies constructed from hexavacant Dawson {Se2W12}
unit,16c,16d while researches on selenium(IV)-containing hetero-
POMos are dominated by a series of carboxylate ligands
functionalized POMos [SeMo6O21L3]4− (L = glycine, β-alanine,
acetic acid), [(SeMo6O21)2L3]10− (L = oxalic acid, succinic acid,
adipic acid, suberic acid, 1,3,5-tris(carboxymethoxy)benzene)
and [(SeMo6O21)6L10]32− (L = pyridine-2,6-dicarboxylic acid).16e−g
These units [SeW9O33]8−, [Se2W12O46]12− or [SeMo6O21]4− as-
mentioned above are always generated in-situ, and no simple
salts of them have been isolated and structurally characterized to
date. To our knowledge, few hetero-POV precursors have been
isolated and used to further construct their derivatives. Therefore,
the unique {Se4V10} cluster provides a new type of topology to the
limited family of hetero-POV clusters, which could apply as a
useful precursor in further synthetic study. A strong confirmation
is that the direct assembly of this isolated cluster {Se4V10} with
different TM2+ ions in the ratio of 2: 1 resulted in a family of TM-
Figure 2. The experimental and simulated PXRD patterns for the bulk products
of (a‒d) 1‒4.
{Se4V10}), which is also observed for the first time in POM
chemistry. These four vanadoselenites were also structurally
characterized by X-ray single crystal diffraction (Table S1), XRPD,
IR spectra (Figure S2), XPS, EPR and TGA analyses (Figure S3),
and then employed as heterogeneous catalysts toward the
catalytic oxidation of sulfides under mild conditions.
sandwiched
derivatives
K6H8[(SeV10O28(SeO3)3)2(M(H2O)4)]·24H2O (M2+ = Mn2+ (2), Co2+
(3) Zn2+ (4)).
Results and Discussion
Structural analysis elucidated that 2‒4 are crystallized in the
hexagonal space group P3221, occupying the isostructural
assemblies, in which there are two symmetrically-related subunits
[SeV10O28(SeO3)3]8‒, directly held together by one TM2+ ion (TM =
Mn, Co, Zn), generating the sandwiched architectures (Figure 1b
and Figure 3b). As shown in Figure 3, in-suit formed polyanion
[SeV10O28(SeO3)3]8‒ can also be regarded as one {VO5}-capped
trivacant Keggin ion [B-α-SeV9O33]17‒ decorated by three trigonal-
pyramidal {SeO3} moieties with their lone pairs directed at outside
but rather at the inside as polyanion 1a. The six-coordinated TM2+
ion resides in a distorted octahedron, in which the mid-plane is
occupied by three water molecules and one oxygen of the {SeO3}
moiety, whereas the axial sites are defined by a water ligand and
one oxygen from the other {SeO3} moiety in the second subunit
(Mn‒O: 2.120(3)–2.180(3) Å; Co‒O: 2.100(2)–2.120(19) Å; Zn‒
O: 2.084(14)‒2.133(13) Å) (Figure S5, Table S2). Thus, only a
patch of area was left in the sandwiched belt, which cannot afford
localization of any more sandwiched {M(H2O)4O2}2+ fragments,
even if increasing the dosage of TM salts in the synthesis system.
The observations mentioned above can led to following
conclusions with respect to the formation of 1‒4 in aqueous acidic
medium: (a) precursor 1 and its derivatives 2‒4 all comprise the
Crystal Structure
The experimental powder XRD (PXRD) patterns of the targeted
samples of 1−4 are in good agreement with their simulated PXRD
patterns derived from single-crystal X-ray diffraction, respectively,
revealing that the samples used for further characterizations and
catalytic tests are pure (Figure 2). It should be noted that
precursor 1 and its TM-sandwiched derivatives 2−4 exhibit
obviously different characteristic peaks in the same range of 2θ
angles because of space group difference, while powder XRD
patterns of samples 2−4 are nearly the same because they are
isostructural with negligible lattice parameter difference.
Single-crystal X-ray diffraction analysis shows that
1
crystallized in the monoclinic space group Cm, and comprised a
novel polyanion [SeV10O28(SeO3)3]8− (1a). In this new-found
vanadoselenite (Figure 1a and Figure 3a), three {V3O13} triads
combined together tightly in a corner-shared fashion with a {SeO3}
group as a center, leading to a trivicant B-a-Keggin polyanion
[SeV9O33]17‒ just like those trivicant Keggin-type POT cousins [B-
α-XW9O33]n‒ (X = As, Si, P, Ge, Se),14 and the capping penta-
coordinate {VO5} unit was just grafted to four µ2-oxo bridges at the
window of two adjacent {V3O13} triads. Note that the V6 atom is
found to be statistically half-occupancy disordered for two mirror-
symmetric positions, then, the structure of 1a was presented in
brief (Figure S4). In this way, such a unique vanadoselenite
cluster could be described as a single-capped trivacant B-α-type
Keggin ion [SeV9O33(VO)]14‒, of which the lacunary sites are
occupied by three {SeO3} trigonal pyramids. Here, it is worth
noting that {SeO3} pyramidal units were used as not only
templates for formation of new building block but also protective
agents that effectively stabilizing the highly negative-charged
[SeV9O33(VO)]14‒ unit. Indeed, the template effect of lone pair-
containing ions like SeO32‒ in POMs chemistry is well-known and
Figure 3. Combined polyhedral/ball-and-stick representation of polyanions (a)
[SeV10O28(SeO3)3]8‒ (1a), and (b) in-suit formed subunit [SeV10O28(SeO3)3]8‒ in
polyanions 2a‒4a.
2
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Figure 5. The relationships between the conversion/selectivity of oxidative
products (RR’SO: methyl phenyl sulfoxide; RR’SO2: methyl phenyl sulfone) and
(a) the dosage of catalyst 4, (b) the dosage of TBHP (oxidant agent), (c) reaction
temperature, and (d) solvent.
Figure 4. XPS spectra of V 2p (a) in 1‒4; (b) Solid state EPR spectra of 1 and
4 at room temperature; (c) Solution 51V NMR spectrum of 1 (H2O/D2O, 293 K);
(d) The mirror symmetry (light blue plane) in 1 resulted in six kinds of unique
vanadium atoms.
spectrum also indicated the presence of four main peaks at ‒
615.3, ‒698.1, ‒716.6, and ‒734.4 ppm as well as a very broad
signal between ‒680 and ‒600 ppm (Figure S7). By contrast,
though 1 and 4 showed very similar 51V NMR spectra owing to
their structrally-related building unit {Se4V10}, three 51V NMR
resonances in 4 was found slightly upfield while one signal
showed a little downfield relative to those normal positions in 1,
which may stem from that the introduction of Zn atom interfered
with the anisotropy of electron density of vanadium atoms, thus
resulting in the slight differences of the observed 51V NMR
cheimcal shifts.20
metastable hetero-POV cluster [B-a-SeV9O33]17‒ with high
negative charge, which can be trapped and stabilized by
electrophilic protecting groups like {SeO3} and {VO5} groups; (b)
a limited number (actually only one) of the {M(H2O)4O2}2+
fragment could be bound to the sandwiched belt of two trivicant
Keggin-type vanadoselenites because of steric hindrance; (c)
coordination with TM2+ ions turns the lone pairs of {SeO3} moieties
from inside to outside in 2‒4, that is, the coordination position flips
horizontally.
XPS, EPR and 51V NMR
Catalysis
Vanadium is a highly specific element, which is known for wide
range of oxidation states varying from +2 to +5, then X-Ray
photoelectron spectroscopy (XPS) was performed to characterize
their oxidation states. As shown in Figure 4a, the V 2p3/2 signals
of 1‒4 in signals all centered at a binding energy of approximately
517.3 eV, which are ascribed to VV centers.17 The results of the
EPR investigations shown in Figure 4b also confirmed this
expectation, because EPR spectra of 1 and 4 were all silent that
consistent with d0 electronic configuration in V(V) centers. In
contrast, the obvious paramagnetic signals of can be detected in
derivatives 2 and 3 owing to the presence of unpaired electrons
in Mn2+ (d5) and Co2+ (d7) ions, respectively (Figure S6).18
During the course of this study, we also investigated the
solution properties of diamagnetic 1 and 4 by 51V NMR
measurements in D2O/H2O at room temperature. The 51V NMR
spectrum of 1 exhibited only four obvious peaks (δ = ‒614.7, ‒
696.5, ‒714.6, and ‒738.4 ppm) rather than the expected six
peaks (Figure 4c and 4d). It is assumed that a broadening
resonance arising from the background between ‒680 and ‒600
ppm may correspond to the missing 51V NMR resonances, while
we could not assign them confidently. Possible reasons for this
phenomenon are 51V as a quadrupolar nucleus with significant
quadruple coupling constants, being highly sensitive to the
symmetry of the VOx polyhedron, which made absolute chemical
shift predictions more difficult.19 As for the dimagetic 4, 51V NMR
Considering the environmental protection and economic benefits
into account, it is crucial to remove sulfur species from sulfur-
containing compounds to transform them into valuable products,
for that oxidative desulfurization has been investigated as a
promising strategy.21 As was authenticated by previous studies,
POVs were found to be fruitful in the oxidation catalysis of sulfides
owing to their fast reversible multielectron transformation
activity.6c,22 Then, the oxidation of methyl phenyl sulfide (a
commonly used model substrate) was chosen as a benchmark
system to assess the catalytic activity of compounds 1‒4. Initially,
compound 4 was employed as a catalyst for preliminary
investigations on analyzing optimal reaction conditions, because
it is the most abundant of yield. After applying a series of
controllable tests, the optimized reaction condition for selective
oxidation of methyl phenyl sulfide to corresponding sulfoxide was
proved to be entry 4, of which 2 μmol catalyst loading with 3 mmol
tert-butyl hydrogen peroxide (TBHP) in methanol for one hour at
room temperature, allowed for achieving excellent conversion
(97.9%) and selectivity (100%) (Figure 5, Table S3). Kinetic
studies (Figure S8) stated that the catalytic reaction follow first-
order dependences over 25‒45 oC with apparent activation
energy Ea of about 0.226 KJ mol‒1.
Under afore-mentioned condition, a hot filtration experiment
was performed, of which sulfoxide product was no further
increased once catalyst 4 was removed from the reaction system
3
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Finally, the catalytic generality of 4 for sulfoxidation reaction
was examined by using various alkyl and aryl sulfides, especially
some derivatives of methyl phenyl sulfide with different electronic
and steric characters under optimized conditions, the results of
which are given in Table S5. It is worthy to mention that the
catalytic activity for dialkyl sulphides (entries 1 and 2) showed
high efficiency for a shortened reaction time of 25 minutes. On
basis of comparison with methyl phenyl sulphide, ethyl phenyl
sulfide (entry 3) afforded similar efficiencies of conversion but
lower selectivity under the similar conditions, while a higher
reaction temperature was required for diphenyl thioether (entry 4)
due to its large steric hindrance. Regarding substituted
derivatives of methyl phenyl sulphide (entries 5‒12), only a very
small difference in catalytic activity was observed when para-
position and meta-position of the aryl ring were substituted by
electron donor or acceptor groups, however, if a substituent was
introduced to the ortho-position of aryl ring, owing to larger steric
hindrance, it could only acquire a significant decreased catalytic
activity in this catalytic system. Thus, the steric hindrance effect
of aryl sulphides is suspected to be the main factor affecting the
yield of sulfoxide products.22c-e
Figure 6. (a) Hot filtration experiment for proving the nature of heterogeneous
catalyst 4; (b) Catalytic oxidative reaction of methyl phenyl sulfide by different
catalysts including compounds 1‒4; (c) Recycling of catalyst 4 for the catalytic
reaction; (d) XPRD patterns of catalyst 4 before and after the catalytic reaction.
Reaction conditions: methyl phenyl sulfide (1 mmol), catalyst (2 µmol), CH3OH
(2.5 mL), TBHP (3 mmol), 25 °C, 60 min.
Conclusion
after 10 min (Figure 6a), suggesting that this oxidation process is
heterogeneous. And not surprisingly, 2 and 3 as heterogeneous
catalysts also can catalyze the oxidation of methyl phenyl sulfide
to the corresponding sulfoxide with commendable conversion of
96.3%–98.8% and selectivity of 100% (Figure S9, Table S4).
From Figure 6b, the new-found hetero-POV 1 itself show
moderate catalytic activity (53.8%), while TM-derivatives 2‒4
exhibited significantly enhanced catalytic properties. It seemed
that both POV components and TM2+ ions (Mn2+, Co2+, Zn2+)
contributed to promoting catalytic reaction system efficiently. For
the purpose of probing the role of each active center of 4 in the
oxidation of sulfide, the activity of Zn(OAc)2·2H2O, SeO2 and
KVO3 were also studied. As shown in Table S4, the conversion of
sulfide is quite low or moderate when using Zn(OAc)2·2H2O
(7.7%), SeO2 (14.0%) or KVO3 (58.6%) as catalysts. However,
the mixture of Zn(OAc)2/KVO3 as homogeneous catalyst led to
obviously improved conversion of methyl phenyl sulfide (76.1%).
Similarly, the conversion was also increased (73.8%) when the
In
summary,
a
new
type
of
POV
{VO5}-capped
cluster
K6H2[SeV10O28(SeO3)3]·14H2O (1) featuring
a
trivacant Keggin-type {B-α-SeV9} cluster was proposed in this
work, and with the introduction of TM2+ ions on this base, a series
of sandwiched hetero-POVs [(SeV10O28(SeO3)3)2(M(H2O)4)]14‒
(M2+ = Mn2+, Co2+, Zn2+) (2‒4) were isolated based on structurally
relevant {VO5}-capped {B-α-SeV9} clusters. Preliminary results
indicated that 2‒4 showed excellent catalytic activities toward
selective oxidation reaction of sulfide to corresponding sulfoxide
under the mild condition. Specifically, compound 4 as a
representative can convert sulfides to corresponding sulfoxides
efficiently and can be reused at least four times. The isolation of
unique cluster [SeV10O28(SeO3)3]8‒ and the discovery of the in-situ
formed building block [SeV10O28(SeO3)3]8‒ provide two new types
of topology to the limited family of vanadium-based building
blocks, and inspire chemists to investigating their pertinent
synthetic and applied chemistry.
catalyst was
a
simple mechanical mixture of
1
and
Zn(OAc)2·2H2O with a molar ratio of 2 : 1. In the light of the above
results, it can be surmised that the “fusion” of TM2+ ions and POV
{Se4V10} units in 2‒4 might synergistically impact the catalytic
sulfoxidation process, which led to the improvement of the
resultant catalytic performance.4c,22a,22b
Experimental Section
Material and physical measurements
Except excellent performance, stability and generality are
another two vital criteria in the viability of a catalyst. Then,
compound 4 was also selected to examine the long-term stability
in a heterogeneous system. After each reaction completion, the
catalyst can be easily separated from the reaction mixture by
filtration and further reused directly in the subsequent
sulfoxidation reactions (Figure S10). After four consecutive uses
of catalyst 4, the conversion of methyl phenyl sulfide showed
negligible change in activity (Figure 6c). Moreover, IR spectra
(Figure S11), PXRD patterns (Figure 6d) and XPS spectra (Figure
S12) of catalyst 4 after four cycles were still identical with those of
the as-synthesized catalyst, indicating that it maintains its
structural integrity after catalytic experiments.
KVO3, SeO2, Zn(Ac)2·2H2O, Mn(Ac)2·4H2O, Co(Ac)2·4H2O and other
chemical reagents were acquired from commercial sources and used
directly. IR spectra were conducted on
a Bruker VERTEX 70 IR
spectrometer (KBr pellets) recording in the range of 4000–450 cm−1
.
Powder X-ray diffraction (PXRD) patterns were obtained by employing a
Bruker D8 ADVANCE diffractometer with Cu Kα radiation (the value of λ is
1.54056 Å). TG analyses were measured by a Perkin-Elmer TGA7
instrument under flowing N2 (heating rate, 10 °C min−1). The quantitative
analyses of Se, V, Mn, Co and Zn elements were achieved by PerkinElmer
Optima 2100 DV inductively coupled plasma optical-emission
spectrometer. Electron paramagnetic resonance (EPR) were all conducted
on a Bruker EMX-10/12 spectrometer in the X-band at room temperature.
X-ray photoelectron spectroscopy (XPS) were performed on an Axis Ultra
x-ray photoelectron spectroscope equipped with monochromatic Al Kα
(1486.7 eV) radiation. Solution 51V NMR spectrum was collected at the
4
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Bruker AVANCE III HD 500 MHz (11.7 T) liquid instrument with a BBO 500
S2 probe, and the pulse program is onepulse. The chemical shift is using
V2O5 as reference at −615 ppm.
Cambridge Crystallographic Data Center with CCDC numbers: 1923303‒
1923306. Crystal data and structure refinement parameters are detailed in
Table S1.
Preparation of K6H2[SeV10O28(SeO3)3]·14H2O (1): Under stirring, solid
SeO2 (0.132 g, 1.2 mmol) was added in a 10 mL aqueous solution
containing KVO3 (0.414 g, 3 mmol). The pH value of the solution was
maintained at about 3.3‒3.6 with glacial acetic acid, and followed by
addition of KCl solid (0.075 g, 1 mmol). The obtained red solution was
heated to 90 °C for 0.5 h. After cooling down to room temperature, filtration
and slow evaporation of the solution, dark red needle crystals were
collected after 3‒7 days. Yield: 0.153 g (29.9% based on V). In addition, a
few orange crystals of decavanadate would be obtained from the
remaining solution after several days. Elemental analysis calcd (%): Se,
16.52; V, 26.64. Found: Se, 16.67; V, 26.10. IR (KBr, cm‒1): 3432 (w), 1627
(m), 950 (s), 878 (s), 798 (sh), 738 (s), 621 (sh), 510 (m). TGA (weight
loss): the first step 25‒293 °C, 13.6%; the second step 293‒500 °C, 23%.
51V NMR (293 K, D2O/H2O) (chemical shifts, ppm): ‒614.7, ‒696.5, ‒714.6,
‒738.4.
General procedure for the selective oxidation of sulfides: The typical
experimental procedure for the catalytic oxidation of various sulfides were
carried out in a WP-TEC-102HC parallel from WATTCASTM (Figure S10).
Generally, 2 µmol of catalyst, 1 mmol of sulfides, 3 mmol of TBHP and 2.5
mL methanol were charged in the reaction tube (50 mL) at the room
temperature (25 oC) with constant stirring during the whole reactions. At
regular intervals, an aliquot of sample solution was taken directly from the
reaction mixture with a microsyringe and the liquid was analysed by gas
chromatography (GC) using naphthalene as the internal standard. As for
the recycling experiment, the POV catalyst was recovered by filtration at
the end of each cycle, and then washed thoroughly (at least three times)
by methanol, which was further dried at 60 oC in oven and reused for the
successive run under the identical reaction conditions.
Method 1 for syntheses of K6H8[(SeV10O28(SeO3)3)2(M(H2O)4)]·24H2O
(M = Mn2+ (2), Co2+ (3), Co2+ (4)): Under stirring, solid SeO2 (0.132 g, 1.2
mmol) was added in a 10 mL aqueous solution containing KVO3 (0.414 g,
3 mmol). A few minutes later, the pH value of the solution was first adjusted
to 3.0‒3.3 with glacial acetic acid. After the mixture being stirred for 10
minutes at room temperature, Mn(Ac)2·4H2O (0.049 g, 0.20 mmol) for 2 or
Co(Ac)2·4H2O (0.050 g, 0.20 mmol) for 3 or Zn(Ac)2·2H2O (0.048 g, 0.20
mmol) for 4 was added, respectively, and the pH value of the solution was
maintained at 3.3‒3.6 with 3.0 mol L–1 KOH solution. The obtained red
solution was heated to 90 °C for 0.5 h. After cooling down to room
temperature, filtration and slow evaporation of the solution, dark red block
crystals for 2‒4 were collected after 3‒7 days. Yield: 0.160 g (29.0% based
on V) for 2, 0.153 g (28.0% based on V) for 3, and 0.274 g (49.6% based
on V) for 4. Elemental analysis calcd (%) for 2: Se, 17.37; Mn, 1.51; V,
28.02; Found: Se, 17.84; Mn, 1.55; V, 27.83. Elemental analysis calcd (%)
for 3: Se, 17.35; Co, 1.62; V, 27.99. Found: Se, 16.95; Co, 1.65; V, 27.35.
Elemental analysis calcd (%) for 4: Se, 17.32; Zn, 1.79; V, 27.94. Found:
Se, 17.77; Zn, 1.85; V, 28.52. IR (KBr, cm‒1): 3434 (w), 1624 (m), 955 (s),
860 (s), 803 (s), 753 (s), 557 (m) for 2; 3440 (w), 1625 (m), 955 (s), 862
(s), 801 (s), 752 (s), 556 (m) for 3; 3470 (w), 1625 (m), 956 (s), 867 (s),
803 (s), 753 (s), 558 (m) for 4. TGA (weight loss): the first step 25‒293 °C,
11.8% for 2, 11.9% for 3, 12.0% for 4; the second step 293‒1000 °C ,
22.9% for 2, 23.5% for 3, 23.7% for 4. 51V NMR (293 K, D2O/H2O) for
dimagnetic 4 (chemical shifts, ppm): ‒615.3, ‒698.1, ‒716.6, ‒734.4.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21573056).
Keywords: Keggin • polyoxovanadate • vanadoselenite •
heterogeneous catalysts • oxidation of sulfides
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6
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Entry for the Table of Contents
New topology of polyoxovanadates: A rare example of lacunary Keggin-type vanadoselenite [SeV10O28(SeO3)3]8‒ ({Se4V10}) was
readily obtained, which shows sufficient reactivity with divalent transition metals (TMs) to form a new series of TM-sandwiched
complexes {(Se4V10)2M} (M = Mn, Co, Zn). Such new TM-derivatives can efficiently catalysis heterogeneous oxidation of sulfides in
high conversion and selectivity under mild conditions.
7
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