Two Ce3+-Substituted Selenotungstates Regulated by N, N-Dimethylethanolamine and Dimethylamine Hydrochloride

Article abstract of DOI:10.1021/acs.inorgchem.9b00582

Two multi-Ce³⁺-substituted selenotungstates (STs), [HDMEA][H₂N(CH₃)₂]₄H₃Na₄[Ce₂(H₂O)₆(DMEA)W₄O₉(α-SeW₉O₃₃)₃]·26H₂O (1) and [H₂N(CH₃)₂]₁₀H₄Na₁₀[Ce₂W₄O₉(H₂O)₇(α-SeW₉O₃₃)₃]₂·63H₂O (2), were prepared by the one-pot approach of sodium tungstate, sodium selenite, and lanthanide nitrate in an acidic water solution in the presence of N,N-dimethylethanolamine (DMEA) or dimethylamine hydrochloride (DMAHC). 1 was obained in the presence of DMEA, whereas 2 was synthesized in the presence of DMAHC. The trimeric polyoxoanion of 1 contains an unusual V-shaped [Se₃W₂₉O₁₀₃]^20- group embracing a prominent heterometal oxide fragment, [Ce₂(H₂O)₆(DMEA)W₂O₅]⁸⁺, and the hexameric polyoxoanion of 2 is constructed from two equivalent trimeric [Ce₂W₄O₉(H₂O)₇(SeW₉O₃₃)₃]₂ ^24- subunits through two -O-W-O-Ce-O- linkages. The most worthy of attention is that the polyoxoanion of 1 can be approximatively viewed as a half of the polyoxoanion of 2 because of the coordination and blocking effect of DMEA. The stability of 1 and 2 in different water pH values was studied by electrospray ionization mass spectroscopy (ESI-MS), and the results manifest that 1 and 2 are stable in pH = 3.5-7.5 and 3.5-7.0, respectively. The oxidation reactions of aromatic sulfides catalyzed by H₂O₂ were studied when 1 or 2 worked as a catalyst, and experimental results reveal that 1 or 2 can serve as available catalysts for the oxidation of aromatic sulfides under mild conditions.

Full text of DOI:10.1021/acs.inorgchem.9b00582

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Two Ce3+-Substituted Selenotungstates Regulated by
N,N‑Dimethylethanolamine and Dimethylamine Hydrochloride
Hai-Lou Li,†,‡ Chen Lian,†,‡ Li-Juan Chen,*,† Jun-Wei Zhao,*,† and Guo-Yu Yang*,‡
†Henan Key Laboratory of Polyoxometalate Chemistry, College of Chemistry and Chemical Engineering, Henan University, Kaifeng,
Henan 475004, China
‡MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing
102488, China
S
* Supporting Information
ABSTRACT: Two multi-Ce3+-substituted selenotungstates (STs),
[HDMEA][H2N(CH3)2]4H3Na4[Ce2(H2O)6(DMEA)W4O9(α-
SeW9 O3 3 )3 ]·26H2 O (1) and [H2 N(CH3 )2 ]1 0 H4 Na1 0
-
[Ce2W4O9(H2O)7(α-SeW9O33)3]2·63H2O (2), were prepared by
the one-pot approach of sodium tungstate, sodium selenite, and
lanthanide nitrate in an acidic water solution in the presence of N,N-
dimethylethanolamine (DMEA) or dimethylamine hydrochloride
(DMAHC). 1 was obained in the presence of DMEA, whereas 2 was
synthesized in the presence of DMAHC. The trimeric polyoxoanion
20−
of 1 contains an unusual V-shaped [Se3W29O103
]
group
embracing a prominent heterometal oxide fragment,
[Ce2(H2O)6(DMEA)W2O5]8+, and the hexameric polyoxoanion of
2
is constructed from two equivalent tr imeric
24−
[Ce2W4O9(H2O)7(SeW9O33)3]2
subunits through two −O−W−O−Ce−O− linkages. The most worthy of attention is
that the polyoxoanion of 1 can be approximatively viewed as a half of the polyoxoanion of 2 because of the coordination and
blocking effect of DMEA. The stability of 1 and 2 in different water pH values was studied by electrospray ionization mass
spectroscopy (ESI-MS), and the results manifest that 1 and 2 are stable in pH = 3.5−7.5 and 3.5−7.0, respectively. The
oxidation reactions of aromatic sulfides catalyzed by H2O2 were studied when 1 or 2 worked as a catalyst, and experimental
results reveal that 1 or 2 can serve as available catalysts for the oxidation of aromatic sulfides under mild conditions.
It is apparent from the above results that research on ST
species is primarily focused on pure STs and TM-containing
STs (TMCSTs), and less work is involved in Ln-containing
STs (LCSTs); therefore, the continuous synthesis and
discovery of novel LCSTs with desirable properties is still a
challenging research field. In the past several years, our
laboratory has invested substantial effort in preparing neoteric
LCSTs by the one-pot approach of sodium tungstate,
lanthanide nitrate, sodium selenite, and organic solubilizers
and obtained some particular LCSTs.29−31 As a part of our
continuous exploration on LCSTs based on the one-pot
reaction approach, N,N-dimethylethanolamine (DMEA) or
dimethylamine hydrochloride (DMAHC) was introduced to
our reaction system because they can facilitate enhancement of
the solubility of Ln cations in the Se−O−W system. It can be
prudentially found from our previous results that dimethyl-
amine components from DMAHC are often protonated and
work as organocations to stabilize the desired LCSTs.
However, DMEA not only consists of an amino group that
can be protonated to act as a countercation but also consists of
1. INTRODUCTION
Polyoxometalates (POMs) serve as a well-known group of
multinuclear metal−oxo compounds and have aroused
extensive concern for their alluring architectures and
application diversities in molecular catalysis, gas sorption,
magnetism, medicine, electronics, and optical devices.1−12 In
terms of chemical components, POMs are usually assembled
by high-oxidation-state early transition metals (TMs), the
majority of which are tungsten(VI), molybdenum(VI),
vanadium(V), niobium(V), etc.13−18 In the aspect of
structures, POMs are formed by metal−oxo {MOn} polyhedra
combined via corner/edge/face-sharing modes. Within this
field, selenotungstates (STs) as a significant subclass have
attracted considerable concern. Great achievements on STs
have been made in the past decade. Cronin’s reported some
unprecedented and intriguing nanoscale poly(ST) aggregates
by the one-pot reaction approach in water and explored the
assembly rules of some poly(ST) clusters.19−23 In 2014,
Kalinina et al. discovered a multi-manganese-incorporated
dimeric Dawson ST.24 Moreover, Su’s group also addressed
some innovative STs encapsulating TMs, [(CH3)2Sn]2+, and
lanthanide (Ln) centers and probed the photocatalytic H2
evolution activity of some species.25−28
Received: February 27, 2019
© XXXX American Chemical Society
A
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an alcoholic group that can coordinate to the metal centers;
therefore, the utilization of DMEA may result in the structure
discrepancies of some desired LCSTs. Under the guidance of
these considerations, we performed exploration on the Ce−
Se−W−O system in the presence of DMEA or DMAHC. In
the present paper, we obtained two neoteric multi-Ce3+-
substituted STs, [HDMEA][H2N(CH3)2]4H3Na4[Ce2(H2O)6-
(DMEA)W4O9(α-SeW9O33)3]·26H2O (1) and [H2N-
(CH3)2]10H4Na10[Ce2W4O9(H2O)7(α-SeW9O33)3]2·63H2O
(2) (Table S1). It is fairly interesting that the structure
difference of 1 and 2 mainly stems from the utilization of
DMEA or DMAHC because of the fact that 1 was prepared in
the presence of DMEA while 2 was prepared in the presence of
DMAHC. The most worthy of attention is that the
polyoxoanion of 1 can be approximatively viewed as a half of
the polyoxoanion of 2 because of the coordination and
blocking effect of DMEA. The stability of 1 and 2 in water
under different pH values was investigated by virtue of
electrospray ionization mass spectroscopy (ESI-MS), indicat-
ing that 1 and 2 are stable in pH = 3.5−7.5 and 3.5−7.0,
respectively. Furthermore, the effects of 1 and 2 as catalysts on
the oxidation reactions of aromatic sulfides by H2O2 were
evaluated. Catalytic oxidation experimental results reveal the
good catalytic performances of 1 and 2 as catalysts toward the
oxidation of aromatic sulfides by H2O2.
organic components, which renders the preparation of
multinuclear LCSTs to be uneasy. To cope with the first
difficulty, we utilized the one-pot reaction approach of
commercial materials such as sodium tungstate, lanthanide
nitrate, and sodium selenite in an acidic water solution. To
solve the second and third difficulties, we introduced organic
components such as DMEA or DMAHC in the reaction to
improve the solubility of Ln ions; moreover, these organic
components can be protonated in an acidic environment and
function as larger countercations to promote crystallization of
target compounds and increase the stability of target
compounds. Thus, 1 and 2 were triumphantly made. Parallel
experimental results revealed that 1 and 2 were easily formed
in pH = 2.5−3.3. The pH of 3.0 led to the highest yield.
Besides, the effect of organic solubilizing agents on the
structures of products was taken into account. When there was
no DMEA or DMAHC in the reaction, the system was cloudy
and abundant amorphous precipitations appeared, resulting in
the preparation failure of 1 and 2. On the contrary, when
DMEA or DMAHC was used, the solution is clear in the
stirring precedure, 1 and 2 were successfully obtained. 1 was
obtained in the existence of DMEA, while 2 was made in the
presence of DMAHC. In addtion, when DMEA and DMAHC
were simultaneously utilized, only the trimer 1 was obtained
and no hexamer 2 was found.
Structure Description. The good phase purity of 1 and 2
was confirmed via powder X-ray diffraction patterns (Figure
S1). The structure unit of 1 comprises a unique organic−
inorganic hybrid trimeric entity of [Ce2(H2O)6(DMEA)W4O9-
(α-SeW9O33)3]12− (1a; Figures 1a and S2a), 3 protons, 1
[HDMEA]+, 4 [H2N(CH3)2]+, 4 Na+ ions, and 26 crystal
water molecules. Notably, [H2N(CH3)2]+ cations may be due
to the degradation of DMEA becuase dimethylamine was used
as a raw material in the preparation of DMEA. The bond
valences of 6+, 4+, and 3+ for W, Se, and Ce atoms in 1 can be
2. EXPERIMENTAL SECTION
Preparation of [HDMEA][H2N(CH3)2]4H3Na4[Ce2(H2O)6-
(DMEA)W4O9(α-SeW9O33)3]·26H2O (1). With stirring, Na2WO4·
2H2O (4.000 g, 12.127 mmol) and Na2SeO3 (0.200 g, 1.156 mmol)
were put in 40 mL of water, to which DMEA (2 mL, 1 moL·L−1) was
added. The pH of the solution was set at 3.0 by the addition of HCl
(6 mol·L−1) dropwise. Subsequently, Ce(NO3)3·6H2O (0.300 g,
0.691 mmol) was added to the solution, and the pH was readjusted to
3.0. Finally, the solution was filtered after stirring for 30 min. The
filtrate was left to evaporate at room temperature, and yellow block
crystals of 1 formed after 4 weeks. Yield: 0.54 g (17.40%) based on
Ce(NO3)3·6H2O. Elem anal. Calcd for 1: C, 2.14; H, 1.37; N, 0.94;
W, 63.47; Ce, 3.12; Se, 2.64; Na, 1.02. Found: C, 2.41; H, 1.55; N,
0.86; W, 63.55; Ce, 3.32; Se, 2.74; Na, 1.10. IR (cm−1): 3402, 3128,
2772, 1628, 1464, 968, 892, 856, 790 (Figure S3).
Preparation of [H2N(CH3)2]10H4Na10[Ce2W4O9(H2O)7(SeW9
O33)3]2·63H2O (2). The syntheic process of 2 is similar to that of 1
with DMAHC (2.002 g, 24.552 mmol) in place of DMEA. Yellow
block crystals for 2 formed after 3 weeks. Yield: 0.67 g (21.58%)
based on Ce(NO3)3·6H2O. Elem anal. Calcd for 2: C, 1.34; H, 1.33;
N, 0.78; W, 63.42; Ce, 3.12; Se, 2.64; Na, 1.28. Found: C, 1.53; H,
1.60; N, 0.89; W, 64.15; Ce, 3.67; Se, 2.79; Na, 1.34. IR (cm−1):
3415, 3147, 2792, 1632, 1471, 969, 889, 851, 795 (Figure S3).
3. RESULTS AND DISCUSSION
Synthetic Discussion. Over the past 10 years, some pure
STs, TMCSTs, and organotin-incorporated STs were reported;
nevertheless, little interest has been paid to multinuclear
LCSTs because there are some obstacles in the design and
preparation of neoteric multinuclear LCSTs. The difficulties
mainly lie in three aspects: First, in comparison with
arsenotungstates and tungstoantimonates, no available ST
precusors can be used as building blocks to make multinuclear
LCSTs. Second, highly oxyphilic Ln cations were directly
reacted with in situ formed ST fragments, which usually
resulted in amorphous precipitates rather than crystals. Third,
according to our experiments, it can be found that the
solubility of lanthanide nitrate in the in situ formed ST
reaction system is not good without the participation of
Figure 1. (a) View of 1a. (b) V-shaped [Se3W29O103]
20− cluster in 1a.
(c) Tetranuclear heterometal [Ce2(H2O)6(DMEA)W2O5]8+ cluster in
1a.
B
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calculated by bond valence sum (Table S2).32 The bond
valences of O53 and O87 atoms connected to W2 and W4
atoms are 0.407 and 0.315, which indicate that O53 and O87
are aqua ligands. The infrastructure of 1a could be described as
one tetranuclear heterometal [Ce2(H2O)6(DMEA)W2O5]8+
cluster (Figure 1c) encapsulated by a V-shaped
20−
[Se3W29O103
]
cluster (Figure 1b). The unprecedented
cluster is constructed from three
20−
[Se3W29O103
]
[SeW9O33]8− units linked by two {WO6} (W3 and W1)
20−
octabedra. The [Se3W29O103
]
cluster can also be imagined
as a fusion of two [Se2W19O68]14− units by sharing a trivacant
Keggin [α-SeW9O33]8− fragment; as far as we know, such a
configuration is quite rare. Although two [Se2W19O68]14− units
20−
in the [Se3W29O103
]
cluster are very similar, there is a
conspicuous difference between them: the intersection angle
(89.36°) formed by [α-Se1W9O33]8− and [α-Se2W9O33]8− in
[Se1Se2W19O68]14− is larger than that (67.87°) formed by [α-
8 −
8 −
Se1W9 O3 3
]
and [α-Se3W9 O3 3
]
in [Se1Se3-
W19O67(H2O)]14−, which may be mainly ascribed to the fact
that 3 W and 2 Ce centers are nipped in the pocket position of
[Se1Se2W19O68]14−, whereas 2 W and 2 Ce centers are
embedded in the vacancy of [Se1Se3W19O68]14−. In 1a, the V-
20−
shaped [Se3W29O103
]
cluster embraces the tetranuclear
heterometal [Ce2(H2O)6(DMEA)W2O5]8+ cluster through 12
μ2 -O atoms. In the tetranuclear heterometal
[Ce2(H2O)6(DMEA)W2O5]8+ cluster, the W2 atom joins the
[α-Se3W9O33]8− segment via 2 μ2-O atoms, 2 Ce3+ (Ce1 and
Ce2) ions via 2 μ2-O atoms, and the W4 atom through 1 μ2-O
atom and a terminal O atom and the W2−O distances range
from 1.747(11) to 2.250(14) Å. The W4 atom is combined
with the [α-Se2W9O33]8− segement via 2 μ2-O atoms, 1 Ce3+
ion via 1 μ2-O atom, the W2 atom through 1 μ2-O atom and
two terminal O atoms, and the W4−O distances are between
1.723(13) and 2.344(13) Å. Both Ce13+ and Ce23+ ions exhibit
distorted square-antiprismatic geometry, and the Ce−O
distances vary in the range of 2.408(12)−2.651(13) Å. The
coordination sphere of the Ce13+ ion is defined by 2 μ2-O
atoms from 1 [α-SeW9O33]8− unit, 3 μ2-O atoms from 3
{WO6} (W1, W2, and W3) groups, and 3 aqua ligands.
Differently, the coordination sphere of the Ce23+ ion is
constituted by 2 μ2-O atoms from one [α-Se3W9O33]8− unit, 4
μ2-O atoms from 4 different {WO6} (W1, W2, W3, and W4)
groups, 1 aqua ligand, and 1 μ2-O atom from DMEA.
Alternatively, 1a (Figure 2a) can be perceived as an
unprecedented hexanuclear heterometal [Ce2(H2O)6(DMEA)
W4O9]12+ cluster (Figure 2b) linking three [α-SeW9O33]8−
fragments (Figure 2c) in the triangular distribution motif by 16
μ2-O atoms from 3 [α-SeW9O33]8− fragments. Interestingly,
the hexanuclear heterometal [Ce2(H2O)6(DMEA)W4O9]12+
cluster shows a trigonal bipyramid defined by 3 W (W1, W2,
and W3) atoms situated at the basal plane and 2 Ce (Ce1 and
Ce2) atoms occupying two vertexes, and the W4 atom
connects W2 and Ce2 atoms through 2 μ2-O atoms (Figure
2d).
Figure 2. (a) View of 1a. (b) Hexanuclear heterometal
[Ce2(H2O)6(DMEA)W4O9]12+ cluster in 1a. (c) [α-SeW9O33]8−
fragment in 1a. (d) Simplified geometry of the [Ce2(H2O)6(DMEA)-
W4O9]12+ cluster. (e) Polyoxoanion of 3. (f) Tetranuclear heterometal
[Y3(H2O)3(CH3COO)3WO4)]4+ cluster in 3. (g) [α-SbW9O33]9−
f r a g m e n t i n 3 . ( h ) S i m p l i fi e d g e o m e t r y o f
[Y3(H2O)3(CH3COO)3WO4)]4+ cluster. Symmetry code: A, x, 0.5
− y, z.
3 trivacant Keggin POM fragments in 1a are bridged by a
hexanuclear heterometal [Ce2(H2O)6(DMEA)W4O9]12+ clus-
ter through W−O−W and Ce−O−W linkers, whereas 3
trivacant Keggin POM fragments in 3 are directly joined
t o g e t h e r b y
a
t e t r a n u c l e a r h e t e r o m e t a l
[Y3(CH3COO)3(H2O)3(WO4)]4+ cluster through Y−O−W
linkers (Figure 2f). (c) The coordination environment of W
atoms is different in the hexanuclear heterometal
[Ce2(H2O)6(DMEA)W4O9]12+ cluster and the tetranuclear
heterometal [Y3(CH3COO)3(H2O)3(WO4)]4+ cluster. (d)
The arrangement fashions of metal centers in the hexanuclear
{Ce2W4} cluster in 1a and the tetranuclear {Y3W} cluster in 3
are distinguishing: in the hexanuclear {Ce2W4} of 1, three
equatorial sites of the trigonal bipyramid are occupied by 3 W
centers, while two polar sites are occupied by 2 Ce centers and
1 W center links to one edge formed by a W center and a Ce
center via double μ2-oxo bridges (Figure 2d), whereas the
Bascal plane of the trigonal pyramid in 3 is constituted by 3 Y
centers and the apical site is occupied by 1 W center (Figure
2h). (e) Coordination modes of organic ligands are diverse: in
1, a DMEA ligand coordinates to the Ce3+ ion by the alcohol O
atom in the end-on mode; in contrast, each of 3 acetate ligands
It should be pointed out that 1a somewhat resembles the
[{Y(B-α-SbW9O31)(OH)2)(CH3COO)(H2O)}3(WO4)]17−
(3) cluster (Figure 2e) reported by Kortz et al. in 2011.33
Although 1a and 3 display the same rare-earth-substituted
triangular polyoxoanion skeletons, but significant discrepancies
can be found among them: (a) The trivacant Keggin POM
fragments are different; 1a contains the [α-SeW9O33]8−
fragment (Figure 2c), while 3 includes the [α-SbW9O33]9−
fragment (Figure 2g). (b) The connection modes are distinct;
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in 3 chelates with a Y3+ ion by carboxylic O atoms in the
bidentate mode.
Under the same synthetic conditions as those of 1, in which
DMEA was replaced by DMAHC, unexpectedly, the hexameric
LCST 2 was obtained. The structural unit of 2 comprises 1
hexameric [Ce2W4O9(H2O)7(SeW9O33)3]224− (2a) polyoxoan-
ion (Figure 3a,b), 4 protons, 10 [H2N(CH3)2]+ ions, 10 Na+
that 1a can be approximatively viewed as a half of 2a because
of the coordination and blocking effect of DMEA. It should be
noted that the hexanuclear heterometal [Ce2W4O9(H2O)7]12+
cluster in 2a rather resembles the hexanuclear heterometal
[Ce2(H2O)6(DMEA)W4O9]12+ cluster (Figure 3f) in 1a.
Notablely, albeit their structures are very analogous, 1a is
still somewhat disparate from 2a: (a) The prominent
divergence is that 1a contains a DMEA ligand and its alcoholic
hydroxyl O atom coordinating to the Ce23+ ion. (b) Their
other distinction is that Ce13+ and Ce23+ ions in 2a inhabit in
an eight/nine-coordinate square-antiprism/monocapped
square antiprism (Figure 3g,h), whereas Ce13+ and Ce23+
ions of 1a adopt an eight-coordinate square antiprism (Figure
3i,j). Alternatively, the hexameric 2a can be perceived as two
vacant Keggin [SeW10O36(H2O)]8− fragments acting as
bridges connecting two sandwich-type [(W3Ce2(H2O)7O6)-
(SeW9O33)2]4− subunits (Figure 4a−d). The alignment mode
of two sandwich-type [(W3Ce2(H2O)7O6)(SeW9O33)2]4−
moieties in 2a (Figure 4c) takes after that of two sandwich-
type [(W3Eu2(H2O)8AsO8(OH))(B-α-AsW9O33)2]8− (4a)
moieties in [(W3 Eu2 (H2 O)8 AsO8 (OH))(B-α-
AsW9O33)2]2 (4) (Figure 4e).34 It is easy to find that the
16−
sandwich-type [(W3Ce2(H2O)7O6)(SeW9O33)2]4− moiety
(Figure 4d) in 2a is also similar to the sandwich-type 4a
subunit (Figure 4f) in 4. The similarities between them are as
follows: (a) Both sandwich-type subunits are assembled from
two trilacunary POM segments sandwiching one pentanuclear
{W3Ln2} cluster (Figure 4d−f); in addition, the distances
bewteen two trilacunary POM segments are similar, 7.557 Å
for [(W3Ce2(H2O)7O6)(SeW9O33)2]4− in 2a and 7.564 Å for
4a in 4. (b) The pentanuclear {W3Ln2} clusters in 2a and 4
illustrate similar trigonal-bipyramidal distribution fashions. (c)
The polyoxoanions of 2a and 4 contain four Ln atoms.
Nevertheless, there still exist some differences bewteen them:
(a) The bridging fragments to connect two sandwich-type
subunits together are distinct; namely, the divacant Keggin
[SeW10O36(H2O)]8− fragment serves as the bridging fragment
in 2, while the triangular pyramidal {AsO2(OH)} works as the
bridging fragment in 4; in addition, the distances between two
sandwich-type subunits are discrepant, 13.746 Å for 2 and
12.584 Å for 4. (b) The connection modes of the bridging
fragments linking two sandwich-type subunits are different. In
2a, the vacant Keggin [SeW10O36(H2O)]8− fragment joins 1
sandwich-type subunit through 3 O−W−O and 1 O−Ce−O−
W−O linkers and the other through 1 O−Ce−O−W−O
linker. In 4, the {AsO2(OH)} group as a μ2-bridge connects 2
sandwich-type fragments through the W−O−As−O−W linker.
Solution Behavior. As is well-known, POMs are hyper-
sensitive to the pH of the medium. The ESI-MS technique can
be applied to measure the stability and integrity of POMs in
aqueous solution.35−45 Hence, the ESI-MS spectra of 1 and 2
dissolved in different pH water solutions were measured
(Figures S5 and S6). The pH of the water solution was
regulated using dilute sodium hydroxide and sulfuric acid.
After 1 or 2 was dissolved in water, the pH was 5.0. In the ESI-
MS spectrum of 1 collected in pH = 5.0 water, the strong peak
at m/z 1353.5743 is derived from the typical molecular
fragment [(C2H8N)H5Ce2(H2O)7W4O9(SeW9O33)3]6−, which
indicates that the polyoxoanion of 1 maintains a good integrity
in water. It can be explicitly seen from Figure S5 that the peak
at m/z 1353.5743 intensity is almost unchanged when the pH
gradually decreases to about 3.0. Upon further lowering of the
pH, this peak disappears, which implies that the polyoxoanion
Figure 3. (a and b) Representations of 2a viewed along different
directions. (c) Trimeric [Ce2W4O9(H2O)7(SeW9O33)3]12− cluster in
2a. (d) Trimeric 1a. (e) Hexanuclear heterometal
[Ce2W4O9(H2O)7]12+ cluster in 2a. (f) Hexanuclear heterometal
[Ce2(H2O)6(DMEA)W4O9]12+ cluster of 1a. (g and h) Coordination
geometry of Ce13+ and Ce23+ ions in 2a. (i and j) Coordination
geometry of Ce13+ and Ce23+ ions in 1a.
ions, and 63 crystal water molecules. The bond valences for W,
Se, and Ce atoms are calculated as 6+, 4+, and 3+, respectively
(Table S3).32 In addition, the bond valences of O56 and O60
atoms connected to W20 and W4 atoms are 0.450 and 0.351,
which indicate that O56 and O60 are aqua molecules. The
hexameric 2a polyoxoanion is formed by two trimeric entities
[Ce2W4O9H2O)7(SeW9O33)3]12− connected by double Ce−
O−W linkers. The trimeric entity of 2a (Figures 3c and S2b) is
constructed from a fascinating hexanuclear heterometal
[Ce2W4O9(H2O)7]12+ cluster (Figure 3e) in the center joining
3 trivacant Keggin [α-SeW9O33]8− fragments via 16 μ2-O
atoms from 3 [α-SeW9O33]8− fragments. The trimeric entity of
2a is very similar to the trimeric 1a (Figure 3d). When the
structures of 2a and 1a are compared, it can be clearly seen
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Figure 4. (a) View of 2a. (b) Vacant Keggin [(SeW10O36(H2O)]8− fragment in 2a. (c) Alignment mode of two sandwich-type
[(W3Ce2(H2O)7O6)(SeW9O33)2]4− moieties in 2a. (d) Sandwich-type [(W3Ce2(H2O)7O6)(SeW9O33)2]4− moiety in 2. (e) [(W3Eu2(H2O)8
16−
AsO8(OH))(B-α-AsW9O33)2]2 polyoxoanion of 4. (f) Sandwich-type moiety of 4a.
skeleton was destroyed. When the pH varies from 5.0 to 7.5,
the peak at m/z 1353.5743 can be clearly observed. When the
pH is higher than 7.5, this peak almost completely disappears,
suggesting that the polyoxoanion skeleton has been decom-
posed. The above results indicate that the polyoxoanion
skeleton of 1 in water can be stable between pH = 3.5 and 7.5.
Similarly, as indicated in Figure S6, the polyoxoanion skeleton
of 2 in water can be stable between pH = 3.5 and 7.0. In
addition, the stability of 1 and 2 in pH = 5 and 6 water
solutions at different times was also tested by ESI-MS
experiments, revealing that the polyoxoanion skeleton of 1 or
2 is still stable at least until 6 h (Figures S7−S10).
diphenyl sulfide (DPS), (4-methoxyphenyl)methyl sulfide, (4-
nitrophenyl)methyl sulfide, etc.; Figures S11−S13].
First, we explored the conversion and selectivity of catalytic
oxidation of 1 and 2 as catalysts for DPS by comparison with
Na2WO4·2H2O, Ce(NO3)3·6H2O, and Na2WO4·2H2O +
Ce(NO3)3·6H2O as catalysts as well as without any catalyst,
respectively. Obviously, in the absence of catalyst, the
conversion of DPS is only 8%, and extremely small amount
of diphenyl sulfone (DPSO2) is obtained. Additionally, the
comparison results of different catalysts are illustrated in Figure
5; it is easy to find that 1 and 2 as catalysts have extremely high
Catalytic Properties. Sulfones and sulfoxides have a wide
range of applications in biology, medicine, fine chemicals,
etc.,46,47 so the efficient conversion, precise selectivity, and
mild reaction conditions of thioethers are critical. POMs as
catalysts in the process of catalytic oxidation of organic species
have high active sites and excellent stability.48,49
It has been reported that POMs can effectively catalyze the
oxidation of aromatic sulfides in the presence of H2O2.50
Herein, 1 and 2 served as catalysts to investigate the
oxygenation reactions of aromatic sulfides at 40 °C with a
green oxidant H2O2 (Scheme 1). The oxygenation products of
aromatic sulfides were determined by the gas chromatography
(GC)−mass spectrometry (MS) technique. The selectivity and
conversion were quantified by the GC technique [such as
Scheme 1. Catalytic Oxidation of Aromatic Sulfides
Figure 5. Effect of different catalysts on the conversion of DPS and
the selectivity of DPSO2. Reaction conditions: 0.5 mmol of substrate,
O/S (oxidant/substrate) = 3, S/C (substrate/catalyst) = 500, 3 mL of
CH3CN.
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conversion and selectivity. Therefore, 1 and 2 may serve as
excellent catalysts in the aromatic thioether oxidation reaction.
Besides, under the same reaction conditions, as shown in
Table 1, we also conducted a series of catalytic oxidation
Table 1. Summary of Catalytic Oxidation of Different
a
Aromatic Sulfides Using 1 as a Catalyst.
Figure 6. Recycling test results of catalyst 1 upon DPS as the
substrate.
Table 2. Summary of Catalytic Oxidation of Different
a
Aromatic Sulfides Using 2 as a Catalyst.
a
Reaction conditions: substrate = 0.5 × 10−3 mol, O/S = 3, S/C =
500, CH3CN (3 mL).
experiments on 10 other aromatic thioethers in the presence of
1. Gratifyingly, these aromatic thioethers almost completely
disappear in the catalytic oxidation experiments (>98%
conversion), and the products are all sulfones (100%
selectivity). Especially, 2-bromothioanisole, benzothiophene,
and dibenzothiophene, for which catalyzing oxidation is
generally extremely difficult, also have satisfactory catalytic
results in the presence of 1. This is an unexpected result of
those catalytic thioether experiments in the presence of
polyoxotungstates. For example, recently, Niu et al. reported
a ruthenium-substituted arsenotungstate as a catalyst under
optimal experimental conditions, which only has 52% and 69%
for the catalytic oxidation conversion of benzothiophene and
dibenzothiophene, respectively.51
Moveover, under optimal reaction conditions, the recycling
tests of catalyst 1 were carried out when DPS was used as the
substrate. As shown in Figure 6, the catalytic activity of 1 is not
attenuated after five-cycle experiments (>99 conversion and
100% selectivity), which demonstrates that the catalytic
performance of 1 is very stable. Furthermore, the stability of
the catalyst was confirmed by IR spectra (Figure S14a). The
results reveal that 1 does not undergo structural change after
the catalytic reaction.
a
The reaction conditions are similar to those in Table 1.
under optimal conditions, especially in the catalytic oxidation
of 2-bromothioanisole and dibenzothiophene. In the presence
of the Zr24-cluster-substituted polytungstate, the catalytic
oxidation result of 2-bromothioanisole is not very ideal (69%
conversion and 10% selectivity) and there is almost no reaction
for dibenzothiophene. In addition, the optimal experimental
temperature of the Zr24-cluster-substituted polytungstate is
higher than that of this experiment, and the amount of catalyst
is more than 2.
In addition, we also investigated the recycling tests and
structural stability of catalyst 2 by recycling the catalytic
oxidation of DPS. As shown in Figure 7, in the cycle
experiments, the conversion of DPS is higher than 99% and the
selectivity of DPSO2 is 100%. Additionally, no obvious
discrepancy was observed in IR spectra before and after the
catalyst experiments (Figure S14b).
Similarly, we also conducted catalytic oxidation experiments
on 10 other aromatic thioethers in the presence of 2. As shown
in Table 2, the catalytic performance of 2 is also extremely
excellent. In 2014, our group reported a gigantic hexameric
Zr24-cluster-substituted polytungstate and investigated a series
of thioether oxidation catalyses.52 Although it is also a
hexamer, its catalytic performance is lower than that of 2
The catalytic oxidation mechanism of aromatic sulfides is
similar to those in refs 53−56. First, the oxidant H2O2 interacts
with the terminal bonds (WO) in the ST skeletons of 1 or 2
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: ljchen@henu.edu.cn.
*E-mail: zhaojunwei@henu.edu.cn.
*E-mail: ygy@bit.edu.cn.
ORCID
Jun-Wei Zhao: 0000-0002-7685-1309
Guo-Yu Yang: 0000-0002-0911-2805
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Figure 7. Recycling test results of catalyst 2 with DPS as the substrate.
This work was supported by the Natural Science Foundation
of China (Grants 21771052, 21671054, 21571048, and
21871077) and the 2014 Special Foundation for Scientific
Research Project of Henan University (Project
XXJC20140001).
to form active POM peroxides. Subsequently, the peroxide
species transfer the O atoms to aromatic sulfides, and the initial
terminal bonds are regenerated and can be used to restart the
catalytic cycle.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00582.
■
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Bond valences of some atoms, relevant figures, IR and
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