Three Zr(IV)-Substituted Polyoxotungstate Aggregates: Structural Transformation from Tungstoantimonate to Tungstophosphate Induced by pH

Article abstract of DOI:10.1021/acs.inorgchem.0c01910

Three novel Zr-substituted polyoxotungstate aggregates [H2N(CH3)2]7NaH2[Zr2Sb2O3(A-α-PW9O34)2]·16H2O (1), [H2N(CH3)2]6H12[ZrSb4(OH)O2(A-α-PW8O32)(A-α-PW9O34)]2·33H2O (2), and [H2N(CH3)2]4Na11.5H4.5[Zr4W8Sb4P5O49(OH)5(B-α-SbW9O33)2]·53H2O (3) have been made in hydrothermal reactions of the [B-α-SbW9O33]9- precursor with Zr4+ cations and PO43- anions in the presence of dimethylamine hydrochloride and sodium acetate buffer (pH = 4.8) and structurally characterized. Different pH values induce structural transformation from tungstoantimonate (TA) to tungstophosphate (TP). 1 is a di-Zr-substituted sandwich-type TP, the tetranuclear heterometallic [Zr2Sb2O3]8+ entity sandwiched by two [A-α-PW9O34]9- moieties. 2 is a double sandwich-type structure, which can be perceived as two equivalent sandwiched [Sb3(PW8O32)(PW9O34)]11- further sandwiching one [Sb2Zr2(OH)2O4]4+ core to form a novel large-size sandwich-type architecture. Different from 1 and 2, 3 is a tetra-Zr-substituted sandwiched configuration, in which two [B-α-SbW9O33]9- fragments sandwich a unique 21-core Sb-P-W-Zr oxo cluster ({Zr4W8Sb4P5}). Furthermore, the catalytic oxidation of aromatic thioethers by 3 as the heterogeneous catalyst has been investigated, showing high conversion and remarkable selectivity as well as excellent recyclability.

Full text of DOI:10.1021/acs.inorgchem.0c01910

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Article
Three Zr(IV)-Substituted Polyoxotungstate Aggregates: Structural
Transformation from Tungstoantimonate to Tungstophosphate
Induced by pH
Hai-Lou Li, Chen Lian, Da-Peng Yin, and Guo-Yu Yang*
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ABSTRACT: Three novel Zr-substituted polyoxotungstate aggregates [H2N-
(CH3)2]7NaH2[Zr2Sb2O3(A-α-PW9O34)2]·16H2O (1), [H2N(CH3)2]6H12[ZrSb4(OH)-
O2(A-α-PW8O32)(A-α-PW9O34)]2·33H2O (2), and [H2N(CH3)2]4Na11.5H4.5
[Zr4W8Sb4P5O49(OH)5(B-α-SbW9O33)2]·53H2O (3) have been made in hydrothermal
reactions of the [B-α-SbW9O33]9− precursor with Zr4+ cations and PO4 anions in the
3−
presence of dimethylamine hydrochloride and sodium acetate buffer (pH = 4.8) and
structurally characterized. Different pH values induce structural transformation from
tungstoantimonate (TA) to tungstophosphate (TP). 1 is a di-Zr-substituted sandwich-
type TP, the tetranuclear heterometallic [Zr2Sb2O3]8+ entity sandwiched by two [A-α-
PW9O34]9− moieties. 2 is a double sandwich-type structure, which can be perceived as
two equivalent sandwiched [Sb3(PW8O32)(PW9O34)]11− further sandwiching one
[Sb2Zr2(OH)2O4]4+ core to form a novel large-size sandwich-type architecture. Different
from 1 and 2, 3 is a tetra-Zr-substituted sandwiched configuration, in which two [B-α-SbW9O33]9− fragments sandwich a unique 21-
core Sb−P−W−Zr oxo cluster ({Zr4W8Sb4P5}). Furthermore, the catalytic oxidation of aromatic thioethers by 3 as the
heterogeneous catalyst has been investigated, showing high conversion and remarkable selectivity as well as excellent recyclability.
product, [H2N(CH3)2]6Na6H8{Zr2[SbP2W4(OH)2O21][α2-
PW10O38]}2·50 H2O,37 has been obtained under the weak
acidic conditions, in which the conversion from TA to TP was
observed. On the basis of the occurrence of this configuration
transformation and considering that the pH value is always a
vital factor in making POMs as part of our ongoing synthesis
work, we attempt to explore whether the pH affects the
configuration transformation between TA and TP by
elaborately regulating the pH in the subsequent experiments.
Fortunately, three types of Zr-containing POTs [H2N-
(CH3)2]7NaH2[Zr2Sb2O3(A-α-PW9O34)2]·16H2O (1), [H2N-
(CH3)2]6H12[ZrSb4(OH)O2(A-α-PW8O32)(A-α-PW9O34)]2·
INTRODUCTION
■
The polyoxometalates (POMs), a well-known large class of
metal-oxo clusters, has existed for nearly two hundred years.1−5
Great efforts have been devoted to POMs and led to a rapid
growth due to their structural diversity and extensive
applications in various fields, such as magnetism, catalysis,
medicine, and photophysics.6−11 In POMs, two important
subfamilies of tungstoantimonates (TAs) and tungstophos-
phates (TPs) with unique structures and excellent properties
have attracted a lot of attention in the past decades, especially
the functionalization, such as catalytic and magnetic properties,
by transition metal (TM) ingredients, resulting in many TM-
containing TAs and TPs having been made.12−20 Nevertheless,
although Zr-substituted polyoxotungstates (POTs) possess
good catalytic performance,21,22 only a few reports have been
covered so far.23−27
The vacant Keggin precursor linked functional units (such as
TMs, lanthanides, organometallic entities)28−31 represents one
of the largest and most active candidates widely used for
making novel POT architectures. There have been a few
studies on the conversion between two types of precur-
sors;32−35 however, more possibilities to generate new building
blocks and obtain heteroatom bridged structures during the
course of transformation exist.
3 3 H
O
( 2 ) ,
a n d
[ H N -
2
2
(CH3)2]4Na11.5H4.5[Zr4W8Sb4P5O49(OH)5(B-α-SbW9O33)2]·
53H2O (3, Scheme 1) have been made. 1 is a di-Zr-substituted
TP dimer built by two [A-α-PW9O34]9− ({PW9}) fragments
encapsulating a {Zr2Sb2} ([Zr2Sb2O3]8+) core. 2 is a double
sandwich-type TP, in which two equivalent sandwiched
subunits [Sb3(PW8O32)(PW9O34)]11− are further sandwiched
by a [Sb2Zr2(OH)2O4]4+ core. Different from 1 and 2, 3 is a
Received: June 28, 2020
Recently, a novel Zr-substituted TA, [H2N(CH3)2]7Na3H5
[Zr3(OAc)W7(H2O)O25][B-α-SbW9O33]2·22H2O,36 has been
3−
reported, and then by introducing PO4 ion, a new TP
https://dx.doi.org/10.1021/acs.inorgchem.0c01910
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Scheme 1. Schematic Synthetic Processes of Clusters 1, 2, 3,
EXPERIMENTAL SECTION
■
a
37
and 4 ({Zr2[SbP2W4(OH)2O21][α2-PW10O38]}2
)
Preparation of [H2N(CH3)2]7NaH2[Zr2Sb2O3(A-α-PW9O34)2]·
16H2O (1). Na9[B-α-SbW9O33]·19.5H2O (1.205 g, 0.421 mmol),
Na2CO3(0.201 g, 1.896 mmol), Na3PO4·12H2O (0.501 g, 1.317
mmol), dimethylamine hydrochloride (0.402 g, 4.930 mmol), and
ZrOCl2·8H2O (0.256 g, 0.794 mmol) were dissolved in 10 mL of 0.5
mol·L−1 sodium acetate buffer (pH = 4.8). While stirring for 10 min,
the pH of the solution was adjusted to 5.0 by using HCl (4 mol·L−1),
and after another 10 min, it was readjusted to 5.0. Then, the mixture
was sealed in a 25 mL Teflon-lined bomb at 120 °C for 5 days, cooled
to room temperature, and filtered. Evaporation of the solvent at room
temperature resulted in faint yellow quadrangular prism crystals of 1
after several days. Yield: 82 mg (7.01% based on Na9[B-α-SbW9O33]·
19.5H2O).
Preparation of [H2N(CH3)2]6H12[ZrSb4(OH)O2(A-α-PW8O32)-
(A-α-PW9O34)]2·33H2O (2). Na9[B-α-SbW9O33]·19.5H2O (1.401 g,
0.489 mmol), Na2CO3(0.198 g, 1.868 mmol), Na3PO4·12H2O (0.504
g, 1.326 mmol), dimethylamine hydrochloride (0.401 g, 4.920 mmol),
and ZrOCl2·8H2O (0.252 g, 0.782 mmol), were dissolved in 10 mL of
0.5 mol·L−1 sodium acetate buffer (pH = 4.8). The following
synthesis process was similar to 1, except for keeping the pH of the
solution at 5.7, yielding yellow strip crystals of 2. Yield: 74 mg (5.70%
based on Na9[B-α-SbW9O33]·19.5H2O).
Preparation of [H2N(CH3)2]4Na11.5H4.5[Zr4W8Sb4P5O49
(OH)5(B-α-SbW9O33)2]·53H2O (3). Na9[B-α-SbW9O33]·19.5H2O
(1.703 g, 0.595 mmol), Na2CO3 (0.405 g, 3.821 mmol),
Na3PO412H2O (0.496 g, 1.310 mmol), dimethylamine hydrochloride
(0.398 g, 4.881 mmol), and ZrOCl2·8H2O (0.258 g, 0.801 mmol)
were dissolved in 10 mL of 0.5 mol·L−1 sodium acetate buffer (pH =
4.8). When the pH was kept at 7.5 by the introduction of HCl (4 mol·
L−1) during stirring, the mixture was sealed in a 25 mL Teflon-lined
bomb at 100 °C for 5 days, then cooled to room temperature, and
filtered. Evaporation of the solvent at room temperature resulted in
colorless strip crystals of 3 after several days. Yield: 217 mg (11.57%
based on ZrOCl2·8H2O).
RESULTS AND DISCUSSION
■
Synthesis. With our continuous work on the conversion
between TA and TP configurations to make novel Zr-
containing POT aggregates, 1, 2, and 3 were all made by
reacting Na3PO4·12H2O, {SbW9}, Na2CO3, and dimethyl-
amine hydrochloride with ZrOCl2·8H2O under the weak acidic
or neutral conditions by hydrothermal synthesis. During the
course of the exploration, we gave priority to the influence of
pH under similar conditions; when in different intervals,
unexpectedly, several types of structures were obtained. These
results of the parallel experiments revealed that pH played a
crucial role in the reaction process. As pH ranges from 4.8 to
5.2, 1 was found and with the highest yield at pH 5.0.
However, when pH varied from 5.3 to 5.8, 2 was obtained, and
the most advantageous pH was at 5.7. Interestingly, at pH 5.7,
the mixed phase of 2 and 4 ({Zr2[SbP2W4(OH)2O21][α2-
PW10O38]}237) appeared (the colors of crystals 2 and 4 were
yellow and colorless, respectively; therefore, they could be
mechanically separated and identified by powder X-ray
diffraction patterns). Besides, experimental results indicated
that pH in the range of 5.6−6.2 was all beneficial for the
formation of 4, and the yield and purity were the highest when
the value was close to 6.0 while keeping the other conditions
unchanged. Moreover, no crystals were achieved if pH was
distributed between 6.3 and 7.0. However, 3 was available at
pH 7.0−7.8, and the highest yield was at pH 7.5, which was Zr-
substituted TA.
a
Color codes: WO6 octahedra, red; ZrOX polyhedra, yellow; PO4
tetrahedra, pink; Sb atoms, bright green; O atoms, red.
tetra-Zr-substituted sandwiched TA, showing two [B-α-
SbW9O33]9− ({SbW9}) fragments sandwiching a unique 21-
core Sb−P−W−Zr oxo cluster [Zr4W8Sb4P5O49(OH)5]2−
({Zr4W8Sb4P5}). Notably, during the synthesis process, we
find that, under weak acid or acidic conditions, TA is easily
converted to TP, whereas under neutral or alkaline conditions,
this conversion is relatively difficult; it may be perceived that,
under weak acid or acidic conditions, the binding of PO4 and
W-oxide units to polymerize into TP is more efficient than that
of SbO3 and W-oxide units to obtain TA, while this combined
capacity is flattened under neutral and alkaline conditions.
Thus, the conversion is not completed, and it only aggregates
into TA. This discovery may furnish an important reference for
future synthesis. Furthermore, the catalytic oxidation of
thioethers by 3 as the heterogeneous catalyst has been
investigated in detail, showing high conversion and prominent
selectivity in the catalytic oxidation with H2O2 as well as good
stability and excellent recyclability.
Otherwise, under the optimal pH, we also paid attention to
the influences of the usage amount of raw materials and
reaction temperature on the products as well. We found that 1,
B
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Table 1. Crystallographic Data and Structure Refinements for 1−3
1
2
3
formula
Fw
crystal system
space group
a, Å
b, Å
c, Å
α, deg
β, deg
γ, deg
C14H90N7NaO87P2Sb2W18Zr2
5569.10
triclinic
C12H118N6O171P4Sb8W34Zr2
10624.42
monoclinic
P21/n
26.3763(12)
13.6682(5)
28.3444(16)
90
101.175(5)
90
C8H147.5N4Na11.5O173P5Sb6W26Zr4
9363.51
triclinic
P1
̅
P1
̅
13.5951(5)
17.9815(7)
21.8864(7)
74.320(3)
75.362(3)
69.381(3)
4747.0(3)
2
21.2608(5)
26.4779(7)
37.4815(9)
98.953(2)
95.250(2)
111.103(2)
19194.2(8)
4
V, Å−3
Z
10024.9(8)
2
μ, mm−1
F(000)
T, K
22.629
4948
293(2)
20.705
9348
293(2)
16.723
16796
293(2)
limiting indices
−16 ≤ h ≤ 16
−21 ≤ k ≤ 21
−26 ≤ l ≤ 25
49802
16685
0.0585
−30 ≤ h ≤ 31
−15 ≤ k ≤ 16
−33 ≤ l ≤ 33
94273
17576
0.1015
−25 ≤ h ≤ 25
−30 ≤ k ≤ 31
−44 ≤ l ≤ 44
189254
67382
0.0971
no. of reflns collected
no. of indep reflns
Rint
data/restrains/params
GOF on F2
16685/15/1030
1.003
17576/30/947
1.062
67382/391/3649
1.021
final R indices [I > 2σ(I)]
R1 = 0.0428
wR2 = 0.0824
R1 = 0.0782
wR2 = 0.0912
R1 = 0.0592
wR2 = 0.1363
R1 = 0.1091
wR2 = 0.1580
R1 = 0.0723
wR2 = 0.1608
R1 = 0.1519
wR2 = 0.1961
R indices (all data)
2, and 3 could be obtained with the amount of {SbW9} ranging
from 1.200 to 1.800 g. Besides, when the usage of ZrOCl2·
8H2O changed from 0.200 to 0.300 g, 1, 2, and 3 could be
afforded, and the yields were the highest with the usage of
about 0.250 g. If the dosage of Zr4+ cations was overused
(>0.400 g), it was easy to get jelly. Conversely, once the
amount of Zr4+ cations was too small (<0.100 g), it was
difficult to obtain crystals. Finally, the temperatures between
100 and 130 °C were helpful for the formation of 1 and 2, with
the most suitable temperature being 120 °C. Different from 1
and 2, the temperatures conducive to the formation of 3 were
80 to 120 °C, and the most suitable temperature was 100 °C.
Structure Description. Single-crystal X-ray diffraction
̅
shows that 1 crystallizes in triclinic space group P1 (Table
1) and consists of a sandwiched polyoxoanion [Zr2Sb2O3(A-α-
PW9O34)2]10− ({Zr2Sb2}@{PW9}2, Figure 1a). Bond valence
sum (BVS) calculations38 of 1 indicate that the oxidation states
of all the W, P, Sb, and Zr centers are +6, +5, +3, and +4,
respectively (Table S1). The polyoxoanion is a sandwiched
TP, in which a U-shaped tetranuclear {Zr2Sb2} unit (Figure
1b) bridges two {PW9} fragments (Figure 1c) by −O−Sb−
O−/−O−Zr−O− linkers. In the {Zr2Sb2} entity, two
independent Zr4+ cations with the slightly distorted hexa-
coordinate triangular prism geometry (Figure 1d) are defined
by 4 μ-O atoms from 2 {PW9} fragments, 1 μ-O atom of a
SbO3, and 1 μ-O atom of a ZrO6. In addition, each SbO3 group
links to 2 {PW9} fragments and 1 ZrO6. Notice that the tetra-
nuclear {Zr2Sb2} acting as a bridge to link two {PW9}
fragments has not been observed in POM chemistry.
Figure 1. (a) The polyoxoanion of 1. (b) The tetranuclear
heterometallic [Zr2Sb2O3]8+ unit. (c) The {PW9} fragment. (d)
The hexa-coordinate geometry of Zr14+ and Zr24+.
Figure 2. (a) The {SbW9} precursor. (b) The W7O28 intermediate.
(c) The trivacant Keggin {PW9} fragment.
Meaningfully, the {PW9} fragment can be considered to be
derived from the reassembly of the {SbW9} precursor and PO4
tetrahedron (Figure 2a−c): one corner-sharing W2O11 group
and SbO3 unit are removed from the {SbW9} precursor to
create a W7O28 intermediate that is then linked to one edge-
C
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sharing W2O10 group via 4 μ-O atoms; a PO4 tetrahedron
occupies the center vacancy of the W7O28 intermediate via 1
μ4-O and 3 μ3-O atoms, consequently forming the {PW9}
fragment.
coordination numbers of 3 and 7 for two outer Sb atoms and
two inner Zr4+ cations in 2c are distinct from that of 8 and 7
for two outer Zr4+ and two inner Zr4+ cations in the
[Zr4O2(OH)2(H2O)4]10+ cluster in 6.
Additionally, the formation of the {PW9} segment in 2 is
similar to that in 1. The {PW8} segment in 2 can also be
perceived as a derivative originating from decomposition and
reassembly of the {SbW9} precursor: one corner-sharing
W2O11 group and heteroatom are taken away from {SbW9}
to create the W7O28 intermediate, and then, one WO6 links to
the W7O28 intermediate via 2 μ-O atoms,; the PO4 unit
occupies the center vacancy of the W7O28 intermediate via 1
μ4-O, 2 μ3-O, and 1 μ-O atoms to make the {PW8} segment
(Figure S4).
Different from 1, 2 crystallizes in the monoclinic space group
P21/n (Table 1) and contains a polyoxoanion [ZrSb4(OH)-
8−
O2(A-α-PW8O32)(A-α-PW9O34)]2 (2a, Figure 3a), 6 [H2N-
When the pH was increased to 7.5, 3 was surprisingly
isolated. Different from 1 and 2, 3 is a sandwiched TA and
̅
crystallizes in the triclinic space group P1, which contains 8
[H2N(CH3)2]+, 23 Na+, 106 lattice water molecules, and two
[Zr4W8Sb4P5O49(OH)5(B-α-SbW9O33)2]20−({Zr4W8Sb4P5}
@{SbW9}2, 3a) polyoxoanions (Figure 4a). BVS calculations
Figure 3. (a) The sandwich-type structure of 2b. (b) The
quadrilateral 2c fragment. (c) The double sandwich-type representa-
tion of 2a. A: 2 − x, −y, −z.
(CH3)2]+, and 33 lattice water molecules. BVS calculations of
2 indicate that the oxidation states of all the W, P, Sb, and Zr
centers are +6, +5, +3 and +4, respectively (Table S2). In
addition, the BVS value of the terminal O56 atom connected
to Sb4 is 1.08, suggesting that O56 is monoprotonated.
Figure 4. (a) The polyoxoanion 3a. (b, c) The 21-core Sb−P−W−Zr
oxo cluster [Zr4W8Sb4P5O49(OH)5]2−.
indicate that all the W, Sb, P, and Zr centers are +6, +3, +5,
and +4, respectively (Table S3). The terminal oxygens O133,
O61, O97, O65, O111, O233, O192, O10, O56, and O231 on
P1−P10 are calculated to be 1.39, 1.45, 1.36, 1.25, 1.15, 1.38,
1.23, 1.34, 1.41, and 1.20, respectively, suggesting that they are
monoprotonated.
Interestingly, 2a can be regarded as a double sandwiched
configuration, i.e., {Sb2Zr2}@[{3Sb}@({PW8}{PW9})]2, in
which {PW9} and {PW8} fragments sandwich 3 Sb3+ atoms
to form a small-size sandwich-type cluster {3Sb}@({PW8}-
{PW9}) (2b), and then, two 2b are further sandwiched by a
[Sb2Zr2(OH)2O4]4+ ({Sb2Zr2}, 2c) core, resulting in a large-
size sandwich-type cluster 2a. In fact, 2b is similar to
[Sb3(SiW9O34)2]11− (5),39 but several remarkable differences
are as follows: (a) different chemical components of POM
fragments, the TP in 2b (Figure S1a) is dissimilar to the
silicotungstate in 5 (Figure S1b); (b) discrepant vacant POM
fragments, 2b contains tri/tetra-vacant {PW9}/{PW8} seg-
ments (Figure S1c), whereas there are two identical trivacant
{SiW9} in 5 (Figure S1d); (c) distinct linking modes between
vacant POM fragments, 3 Sb3+ atoms in 2b connect {PW9}
and {PW8} via 8 Sb−O−W linkages, while 9 Sb−O−W
linkages exist in 5 joining two {SiW9} fragments. Furthermore,
the Zr4+ cation in 2c shows heptacoordinate monocapped
trigonal prism geometry (Figure S2). In 2006, Kortz et al.
published a dimeric [Zr4O2(OH)2(H2O)4(β-SiW10O37)2]10−
(6)40 (Figure S3b) where two divacant [β-SiW10O37]10−
segments sandwiched a [Zr4O2(OH)2(H2O)4]10+ cluster
(Figure S3d). Despite showing similar link modes in
tetranuclear metal clusters, the most conspicuous imparities
are that 2 is heterometallic and 6 is monometallic. Besides, the
Futhermore, two {SbW9} fragments in 3a sandwich a unique
21-core cluster {Zr4W8Sb4P5} (3b, Figure 4b) via W−O−Zr
linkers. Notably, the 3b cluster contains 4 Zr, 5 P, 4 Sb, and 8
W atoms and can be perceived as the fusion of two
{ZrW4Sb2P2} units joined by {Zr2P} via Zr−O−P/Sb linkers
(Figure 4c). All Zr4+ cations are heptacoordinate and exhibit
distorted monocapped trigonal prism geometry (Figure S5).
Moreover, the 3b cluster includes two W4O18 ({W4})
fragments from the decomposition of the precursors. So far,
similar {W4} fragments have been covered in a 20-Ni
substituted POT.41,42 The [Ni(enMe)2 ]3 [H6Ni2 0
P4W34(OH)4O136(enMe)8(H2O)6]·12H2O (7) contains a
W4O16 fragment that somewhat resembles {W4} in the 3b
cluster (Figure 4b). However, apart from different number of
O atoms, the distribution motifs of four W atoms in the tetra-
W fragment are distinguishing. In 3b, four W atoms display a
tilted trigonal pyramid geometry by standing on four vertexes
(Figure S6a,c), which differs from the parallelogram geometry
in 7 (Figure S6b,d). Furthermore, the P/Sb atoms in 3b
simultaneously act as bridging atoms. Four tricoordinate Sb
D
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atoms without any terminal O atom are divided into two
groups of Sb2O5 units. Except for the P5 atom containing two
terminal O atoms, the other 4 P atoms have only one terminal
O atom, respectively.
Catalytic Oxidation. Since the oxygenation products
(sulfoxides and sulfones) of sulfides own multiple applications
in the fields of synthetic chemistry, functional materials, and
biomedicine, the selective oxidation of sulfides has attracted a
widespread and lasting attention.43,44 The POMs due to their
unique chemical nature (redox properties, flexible acidity, high
stability, etc.) are widely accepted as a good candidate for
catalysis.45,46 Potential application prospects derive us to
explore the catalytic performance, and here, 3 has been
investigated as a heterogeneous catalyst for catalytic oxidation
of various aromatic thioethers by using H2O2 oxidant in the
acetonitrile solvent (Scheme S1). When one used GC-MS to
estimate the catalytic products, the selectivity of sulfoxide and
sulfone as well as the conversion of thioethers were quantified
by GC (Figure S8).
Figure 6. Conversion of MPS and the selectivity of MPSO2 with
different amounts of catalyst 3.
Our group has reported a Zr24-containing POM as the
catalyst to catalyze organic thioethers to sulfones and
sulfoxides. Referring to previous catalytic conditions, we first
conducted the parallel tests of methylphenyl sulfide (MPS) as
a substrate, under the conditions of 0.5 mmol of substrate, 3
O/S molar ratio, 100 S/C molar ratio, 3 mL of acetonitrile, 60
°C, and a 1 h reaction time. It is easy to see from Figure 5 that,
Figure 7. Conversion of MPS and the selectivity of MPSO2 at
different amounts of oxidizer.
converts to MPSO2 and the efficiency of H2O2 also reaches
over 90%. While the O/S of 2.4 leads to 100% conversion of
MPS and 100% selectivity of MPSO2, exhibiting more excellent
catalytic performance, it was chosen for the following
experiments.
The reaction temperature and time are significant criteria for
evaluating catalysts in the catalytic oxidation process. The
results of the parallel experiments reveal that the conversion of
MPS and the selectivity of MPSO2 are both 100% at the
relatively mild 45 °C, which is superior to that of the Zr24-
catalyzed oxidation of thioethers previously reported; hence,
45 °C is selected for the optimal experimental temperature
(Figure 8). Additionally, we have attempted to shorten the
reaction time to 30 min, but the selectivity of MPSO2 is greatly
reduced; so, 60 min is still the choice (Figure 9).
Furthermore, the cyclic experiments have been conducted
for MPS to evaluate the catalyst performance of 3 under the
optimal conditions. In the heterogeneous catalytic cycle
experiments, the catalyst at the end of each experiment was
collected by filtration and then reused for the next test. As
shown in Figure 10, the conversion and selectivity are 100% in
the first three cycles, and in the next two cycles, 3 still
maintains excellent performance with the conversion of 100%
in spite of a slight reduction of the selectivity. Moreover, the
catalytic oxidation of diphenyl sulfide (DPS) has also been
systematically studied while keeping other conditions constant
(Figures S9−S12). Obviously, temperature is more vital than
catalytic time during the catalytic oxidation of DPS. At 45 °C
for 60 min, 100% conversion and 89% selectivity are obtained.
However, at 60 °C, 100% conversion and 96% selectivity can
be achieved in only 15 min. Besides, under the optimal
Figure 5. Conversion of MPS and the selectivity of MPSO2 with
different catalysts.
when there is no catalyst, the conversion of MPS is 26%, and
the selectivity of the methyl phenyl sulfone (MPSO2) is 3%;
when ZrOCl2·8H2O and {SbW9} are the catalysts, the
conversion of MPS is 94% and 100%, and the selectivity of
MPSO2 is 4% and 69%. However, when 3 is the catalyst, both
the conversion of MPS and the selectivity of MPSO2 are 100%,
indicating 3 possesses a desirable performance.
The explorations of the amount of 3 as catalyst are shown in
Figure 6; the conversion of MPS and the selectivity of MPSO2
are not attenuated as the catalyst dosage decreases within a
certain range (100% conversion, 100% selectivity). Therefore,
1000 S/C is adopted to reduce cost.
Additionally, the correlation of the amount of oxidant has
been studied at various O/S molar ratios of 1.0, 2.0, 2.2, 2.4,
2.6, 2.8, and 3.0. Theoretically, the catalytic oxidation has a
complete reaction when the O/S is 2 under the premise that
the effective utilization rate of the oxidant is 100%, but it may
be difficult to achieve due to some unavoidable factors such as
partial decomposition of H2O2 during the reaction. As shown
in Figure 7, when the O/S is 2.2, almost all MPS completely
E
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Table 2. Selective Oxidation of Various Aromatic
Thioethers to Sulfones
Figure 8. Conversion of MPS and the selectivity of MPSO2 at
different reaction temperatures.
Figure 9. Conversion of MPS and the selectivity of MPSO2 at
different reaction times.
2-bromophenyl methyl sulfone can be promoted from 18% to
39% (entry 9). Besides, at 60 °C, we obtain an extremely
desirable result for the catalytic oxidation of dibenzothiophene
(entry 10), while the conversion of benzothiophene is still
relatively low at 60 °C. Sequentially, when the reaction time is
extended from 1 to 2 h, the conversion of benzothiophene is
improved from 48% to 64% and the selectivity increases from
93% to 99% (entry 11).
CONCLUSIONS
■
In summary, three novel Zr-substituted POT aggregates have
been made under hydrothermal reactions. 1 is a mono-
sandwich-type dimer {Zr2Sb2}@{PW9}2, while 2 is a double
sandwiched structure, in which the small-size sandwiched
{3Sb}@({PW8}{PW9}) cluster contains two different frag-
ments of {PW8} and {PW9}, and two such sandwich-type units
are further sandwiched by one {Sb2Zr2} core to form a novel
large-size sandwich-type cluster aggregate {Sb2Zr2}@[{3Sb}
@({PW8}{PW9})]2. 3 is a double cluster aggregate, in which
each aggregate contains two {SbW9} fragments and one unique
21-core {Zr4W8Sb4P5} cluster: two {SbW9} fragments are
further sandwiched by one 21-core {Zr4W8Sb4P5} cluster to
produce a sandwich-type {Zr4W8Sb4P5}@{SbW9}2 cluster
aggregate. The thermal stability of 3 has been investigated by
recording infrared spectra at different temperatures, suggesting
that 3 is stable to 350 °C. Furthermore, the catalytic oxidation
of thioethers by 3 as the heterogeneous catalyst has been
explored in detail, showing that 3 possesses high conversion
and remarkable selectivity in the catalytic oxidation of aromatic
thioethers with H2O2 and exhibits excellent recyclability.
Figure 10. Recycling of the catalytic system for the oxidation of MPS
at 45 °C and 2.4 O/S.
conditions, the results of the cycle experiments for DPS also
demonstrate the excellent catalytic performance of 3. In
addition, the stability of 3 was determined by comparing the
PXRD patterns and IR spectra before and after the catalytic
reaction (Figure S13).
Moreover, the conversion and selectivity of most tested
aromatic thioethers are higher than 98% (Table 2, entries 1,
3−8) under the following conditions: 0.5 mmol of substrate,
2.4 O/S, 1000 S/C, 3 mL of acetonitrile, 45 °C, and 1 h. The
conversion and selectivity of 2-bromothioanisole, benzothio-
phene, and dibenzothiophene, which are generally extremely
difficult to catalyze oxidation, are enhanced by increasing
temperature and prolonging reaction time (entry 9−11). When
the temperature is elevated from 45 to 60 °C, the selectivity of
F
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01910.
■
sı
BVS calculations of W, Zr, P, and Sb atoms in 1−3;
related structural figures; IR spectra; TG curves of 1−3
(PDF)
Accession Codes
CCDC 1882473−1882475 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
Guo-Yu Yang − MOE Key Laboratory of Cluster Science, School
of Chemistry and Chemical Engineering, Beijing Institute of
Technology, Beijing 102488, China; orcid.org/0000-0002-
0911-2805; Email: ygy@bit.edu.cn
■
(13) Zhao, J.-W.; Zhang, J.; Zheng, S.-T.; Yang, G.-Y. Combination
of lacunary polyoxometalates and high-nuclear transition-metal
clusters under hydrothermal conditions. 5. A novel tetrameric cluster
of [{FeIIFeIII12(μ3-OH)12(μ4-PO4)4}(B-α-PW9O34)4]22‑. Inorg. Chem.
2007, 46, 10944−10946.
Authors
Hai-Lou Li − MOE Key Laboratory of Cluster Science, School of
Chemistry and Chemical Engineering, Beijing Institute of
Technology, Beijing 102488, China
Chen Lian − MOE Key Laboratory of Cluster Science, School of
Chemistry and Chemical Engineering, Beijing Institute of
Technology, Beijing 102488, China
Da-Peng Yin − MOE Key Laboratory of Cluster Science, School
of Chemistry and Chemical Engineering, Beijing Institute of
Technology, Beijing 102488, China
(14) Clemente-Juan, J. M.; Coronado, E.; Forment-Aliaga, A.;
́
́
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hepta-nuclear cobalt(II) cluster encapsulated in a novel hetero-
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Powell, A. K.; Kortz, U. Hexadecacobalt(II)-containing polyoxome-
talate-based single-molecule magnet. Angew. Chem., Int. Ed. 2011, 50,
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01910
́
(16) Goberna-Ferron, S.; Vigara, L.; Soriano-Lopez, J.; Galan-
II
́
Mascaros, J. R. Identification of a nonanuclear {Co } polyoxome-
9
Notes
talate cluster as a homogeneous catalyst for water oxidation. Inorg.
The authors declare no competing financial interest.
Chem. 2012, 51, 11707−11715.
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ACKNOWLEDGMENTS
■
This work was supported by the NSFC (nos. 21831001,
21571016, and 91122028) and the NSFC for Distinguished
Young scholars (no. 20725101).
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