Discovery of a New Family of Polyoxometalate-Based Hybrids with Improved Catalytic Performances for Selective Sulfoxidation: The Synergy between Classic Heptamolybdate Anions and Complex Cations

Article abstract of DOI:10.1021/acs.inorgchem.9b02601

A series of functional cation-regulated isopolymolybdate-based organic-inorganic hybrid compounds, Na₂H₂[Mo₄O₁₂(C₈H₁₇O₅N)₂]·10H₂O (1), Na₂[M(Bis-tris)(H₂O)]₂[Mo₇O₂₄]·10H₂O [M = Cu, 2; Ni, 3; Co, 4; Zn, 5; Bis-tris = 2,2-Bis(hydroxymethyl)-2,2′,2″-nitrilotriethanol], and (NH₄)₂[M(Bis-tris)(H₂O)]₂[Mo₇O₂₄]·6H₂O (M = Zn, 6; Cu, 7), were synthesized and characterized toward advanced molecular catalyst design. Compound 1 is a covalently bonded adduct, and its self-assembly process can be probed by electrospray ionization mass spectrometry (ESI-MS). Compounds 2-7 are polyoxometalate (POM)-based hybrids containing classic heptamolybdate anions and complex cations with Bis-tris ligands. All of these compounds showed remarkable catalytic effects for selective sulfide oxidation. To the best of our knowledge, compound 5 presents the best catalytic activity so far among the reported hybrid materials with common easily synthesized small-molecule POM clusters and also exhibits outstanding reliability. The conclusion of the catalytic effect is drawn from the results that Zn-based compounds have better catalytic effects than other transition-metal-containing compounds and the compound constructed by Na⁺ has higher catalytic activity than that constructed by NH₄ ⁺. The mechanism studies show that the improvements of the catalytic performance are caused by the synergy between classic heptamolybdate anions and complex cations. ESI-MS data and UV-vis spectra revealed that the POM anions can form intermediate peroxomolybdenum units during catalytic reaction. Further, the combination of the substrate thioanisole with complex cations was characterized by NMR experiments and UV-vis spectra. Thus, a new synergistic mechanism of anions and cations is proposed in which the activated thioanisole is used as a nucleophile to attack the peroxomolybdenum bonds, and this provides a new strategy in the design of reliable POM-based catalysts.

Full text of DOI:10.1021/acs.inorgchem.9b02601

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Discovery of a New Family of Polyoxometalate-Based Hybrids with
Improved Catalytic Performances for Selective Sulfoxidation: The
Synergy between Classic Heptamolybdate Anions and Complex
Cations
†
,†
†
†
,†,‡
Yin Zhang, Wei-Dong Yu,* Bin Li, Zheng-Fan Chen, and Jun Yan*
†
‡
School of Chemistry and Chemical Engineering and Hunan Provincial Key Laboratory of Efficient and Clean Utilization of
Manganese Resources, Central South University, Changsha 410083, P. R. China
*
S Supporting Information
ABSTRACT: A series of functional cation-regulated isopolymolybdate-
b a s e d o r g a n i c − i n o r g a n i c h y b r i d c o m p o u n d s ,
Na H [Mo O (C H O N) ]·10H O (1), Na [M(Bis-tris)-
2
2
4
12
8
17
5
2
2
2
(
2
H O)] [Mo O ]·10H O [M = Cu, 2; Ni, 3; Co, 4; Zn, 5; Bis-tris =
2 2 7 24 2
,2-Bis(hydroxymethyl)-2,2′,2″-nitrilotriethanol], and (NH ) [M(Bis-
4
2
tris)(H O)] [Mo O ]·6H O (M = Zn, 6; Cu, 7), were synthesized
and characterized toward advanced molecular catalyst design. Com-
2
2
7
24
2
pound 1 is a covalently bonded adduct, and its self-assembly process can
be probed by electrospray ionization mass spectrometry (ESI-MS).
Compounds 2−7 are polyoxometalate (POM)-based hybrids containing
classic heptamolybdate anions and complex cations with Bis-tris ligands.
All of these compounds showed remarkable catalytic effects for selective
sulfide oxidation. To the best of our knowledge, compound 5 presents the best catalytic activity so far among the reported
hybrid materials with common easily synthesized small-molecule POM clusters and also exhibits outstanding reliability. The
conclusion of the catalytic effect is drawn from the results that Zn-based compounds have better catalytic effects than other
+
transition-metal-containing compounds and the compound constructed by Na has higher catalytic activity than that
+
4
constructed by NH . The mechanism studies show that the improvements of the catalytic performance are caused by the
synergy between classic heptamolybdate anions and complex cations. ESI-MS data and UV−vis spectra revealed that the POM
anions can form intermediate peroxomolybdenum units during catalytic reaction. Further, the combination of the substrate
thioanisole with complex cations was characterized by NMR experiments and UV−vis spectra. Thus, a new synergistic
mechanism of anions and cations is proposed in which the activated thioanisole is used as a nucleophile to attack the
peroxomolybdenum bonds, and this provides a new strategy in the design of reliable POM-based catalysts.
INTRODUCTION
interacted with them by covalent or hydrogen bonding,
which could stabilize the functional POM core or tune the
solubility of the material, and this type of molecule is classified
■
Polyoxometalates (POMs) have been studied for decades since
their discovery with the structural feature constructed by MOx
x = 5 or 6, M = Mo, W, V, etc.) and have been applied in
10
as a POM-based organic−inorganic hybrid material.
(
1
Structurally, there are three ways to get functional hybrid
POMs. One is to combine POMs with organic bases, such as
metal−organic frameworks (MOFs), graphene, cyclodextrin,
many fields, such as catalysis, optical materials, medicine, etc.
Considering their outstanding catalytic properties, chemists
have made great efforts in the design and synthesis of POM-
based catalysts. Among the hundreds of reports, tuning the
self-assemble process of a POM framework has proven to be an
effective way to generate new clusters with better catalytic
11−14
etc.
POM units were located in the cavities of these
materials, and these kinds of POM materials are normally
15,16
harsh in synthesis and complicated in process.
Another
2
,3
way is to combine organic parts with anion clusters of POMs
performances. However, stability is still the major challenge
for these new POM-based materials because the tough
synthetic conditions normally decrease the stability of the
cluster in POM chemistry. Most of the catalytic performance
tests are limited to the first few rounds for these new clusters,
1
7
by covalent bonds, but only a few reaction types of
18−23
modification of the POMs have been developed so far.
The last approach is to develop an organic−inorganic hybrid
by introducing functionalized cations. This approach not only
ensures the stability and reliability of the material but also
4
,5
and their further applications are constrained. To balance
the activities and stabilities of POM units, chemists introduced
different organic units in classic inorganic POM synthesis.
The organic parts located around the POM units and
6
−9
Received: August 29, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02601
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
introduces the functional type and diversity of organic
Scheme 1. Process for the Synthesis of Compounds 1−7
(TM = Transition Metal)
2
4−28
parts.
Toward the design of a POM hybrid with improved
catalytic performance, the synergy has been extensively studied
in the field of POM catalysts, while the most reported studies
were focused on the synergy between POMs and substrates,
2
9,30
such as MOFs,
and the synergy between the same metals
31−35
in different valence states.
However, when the complex
cations were utilized in POM synthesis, most of the previous
studies focused on structural decentralization of the POM
units, and some attention has been paid to the synergistic effect
36−40
between POM anions and cations.
Although the intrinsic
interaction between the POM anions and complex cations is
very similar to that in the POM−MOF system, the latter could
also contribute to the catalytic performance improvements and
provide a simpler model in catalytic mechanism studies.
Herein, we reported the design and synthesis of POM-based
materials with the synergistic effect to enhance their catalytic
activities by using classic isopolymolybdate clusters and simple
complex cations. 2,2-Bis(hydroxymethyl)-2,2′,2″-nitrilotrietha-
nol (Bis-tris) was selected to modify the isopolymolybdate. As
corresponding transition-metal salts in H O at room temper-
2
ature. Bis-tris could coordinate with transition metals and
combine the complex cations to form compounds with
isopolymolybdate anions. Interestingly, a similar structure (6
and 7) can be obtained by using Anderson-type POMs
({ZnMo } and {CuMo }) and Bis-tris to react under
a
r e s u l t , b o t h t h e h y b r i d c o m p o u n d
Na H [(C H O N) Mo O ]·10H O (1) modified by cova-
2
2
8
17
3
2
4
16
2
6
6
lent bonds and the heptamolybdate-based hybrid compounds
Na [M(Bis-tris)(H O)] [Mo O ]·10H O (M = Cu, 2; Ni, 3;
hydrothermal conditions. However, because of the difference
in counter cations, the arrangement of POM anions and
complex cations is different from that of 2−5.
Crystal Structures. Single-crystal X-ray analysis reveals
that compound 1 contains a centrosymmetric rhomboid of the
2
2
2
7
24
2
Co, 4; Zn, 5) and (NH ) [M(Bis-tris)(H O)] [Mo O ]·
4
2
2
2
7
24
6
H O (M = Zn, 6; Cu, 7) were obtained and fully
2
characterized by chemical analysis. According to electrospray
ionization mass spectrometry (ESI-MS) data, the self-assembly
process of compound 1 was proposed. Most importantly, a
sulfoxidation reaction was used to investigate the catalytic
activity of these compounds, and the results indicated that the
introduction of complex cations significantly improved the
catalytic activity of the POM-based materials. Further, the
metal center had a noteworthy influence on the catalytic
processes, and compound 5 exhibited the best catalytic effect
among all seven compounds and other hybrid materials with
classic POM clusters. In addition, the synergistic catalytic
mechanism was proposed according to the catalytically active
{
Mo } core in the monoclinic system with space group P21/c.
4
As shown in Figure 1, two O atoms of the hydroxymethyl
connect to molybdenum to form the anion of
4−
[
Mo O (C H O N) ] . The adjacent two anions are
4
12
8
17
5
2
+
connected by Na to form a 1D chain structure, and each
Na is a six-coordinated deformed octahedral structure. The
+
uncoordinated H O molecules are evenly distributed around
2
the 1D chain.
Different from compound 1, compound 2 crystallizes in the
monoclinic system with space group C2/c and contains
6−
2+
[
Mo O ] anions, complex cations [Cu(Bis-tris)(H O)] ,
7
2
4
2
1
intermediate detected by ESI-MS, H NMR, and UV spectra.
+
Na and lattice H O molecules. As shown in Figure 2A, Bis-tris
2
can provide five coordination atoms when coordinated with
RESULTS AND DISCUSSION
2+
■
Cu , four of which are O atoms of the hydroxyl groups and
2
+
Synthesis Strategy. Isopolymolybdate has favorable
one of which is a N atom. The other coordination site of Cu
is occupied by the solvent H O molecule. As shown in Figure
41
effects in some catalytic fields, and heptamolybdate, a
commercially available and cheap isopolymolybdate, is often
used as a molybdenum source to synthesize new hetero-
2
6
−
+
2D, adjacent [Mo O ] anions are connected by Na to form
a zigzag-like 1D chain structure, and each Na is coordinated
7
24
+
4
2
polymolybdates with wide applications. However, the
catalytic property of heptamolybdate itself is rarely inves-
with two end-group O atoms and four H O molecules in two
2
6−
adjacent [Mo O ] anions to form a deformed octahedral
7
24
4
3−45
tigated.
Besides, Bis-tris is a commercially available
structure. Because there are large numbers of H atoms in the
complex cations, the cations are evenly distributed around each
1D chain linked by hydrogen bonds with an average O−H···O
distance of 2.702(5) Å (as shown in Figure 2C). This close-
packing mode gives a strong interaction between the complex
cations and POM anions, which may enhance its properties by
multidentate ligand that can “wrap” the metals with small
radii such as the first transition metal and hardly “wrap” the
metals with large radii such as molybdenum.
4
6−48
This
coordination feature allows Bis-tris to chelate with a transition
metal with an additional exchangeable coordinated solvent
ligand on the metal ion, which is a general structural feature for
a molecular catalyst. Thus, Bis-tris was chosen as the organic
part to form organic−inorganic hybrid complex cations, and
Na MoO was used to selectively self-assemble to form
synergy. Furthermore, the uncoordinated H O molecules are
2
also evenly embedded in the voids between each 1D chain.
The structures of compounds 3−5 are similar to that of
compound 2 and are shown in Figures S1−S3.
2
4
6
−
[
1
Mo O ] by regulating the pH value. As shown in Scheme
In addition, it is noticed that all compounds 1−5 contain a
7
24
+
, compound 1 was prepared by reacting Bis-tris with
few Na ions in the final structure. To verify that the catalytic
Na MoO at room temperature with pH 5 and showed a
properties may be affected by different structures, we attempt
2
4
2
+
hybrid structure in which the organic and inorganic parts were
connected by covalent bonds. Unlike 1, compounds 2−5 were
obtained by reacting Bis-tris and Na MoO with the
to synthesize hybrids that contain only [M(Bis-tris)(H O)]
2
6
−
and [Mo O ] units. No crystal materials have been obtained
7
24
so far, but compound 6 was isolated when
2
4
B
DOI: 10.1021/acs.inorgchem.9b02601
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Inorganic Chemistry
Article
4−
+
Figure 1. Structure view of compound 1: (A) anion of [Mo O (C H O N) ] ; (B) 1D chain connected by Na . Partial H atoms are removed
4
12
8
17
5
2
for clarity.
have similar POM building blocks. As shown in Figures S8−
S13, in addition to the common {Mo }, {Mo }, {Mo }, and
2
3
4
{
Mo } units, which are formed by decomposition of the
5
isopolymolybdate anions, the fragments of {(Bis-tris)Mo } (x
x
=
1 or 2) are also found. Thus, a possible self-assembly process
for compound 1 is proposed, as shown in Figure 4. First, one
Mo coordinates with Bis-tris to form two types of {(Bis-
−
tris)Mo} anions, [(C H O N)MoO ] and [(C H O N)-
8
16
5
2
8
18
5
−
MoO ] . Second, another Mo was added to form {(Bis-
3
−
tris)Mo } anions, [(C H O N)Mo O ] and [(C H O N)-
2
8
16
5
2
5
8
18
5
−
Mo O ] . Last, {(Bis-tris)Mo } dimerized to form {(Bis-
2
6
2
tris)Mo } as compound 1.
2
2
Nevertheless, compounds 2−7 did not form similar
structures, indicating that transition metals and Bis-tris were
coordinated to form more stable complex cations to prevent
the binding of Bis-tris with Mo. Besides, an integral peak at m/
z 691.12 is found in the ESI-MS spectrum of compound 5
Figure 2. Structure view of compound 2: (A) cation of [Cu(Bis-
2+
6−
tris)(H O)] ; (B) anion of [Mo O ] ; (C) two 1D chains and the
2
7
24
+
complex cations in between; (D) 1D chain connected by Na . Partial
H atoms are removed for clarity.
(
[
Figure 5), which can be assigned to {H [Zn(Bis-tris)(H O)]-
(
NH ) [ZnMo O H ] was used as the metal source in the
2 2
4
4
6
24
6
2−
Mo O ](CH OH)} (simulation, m/z 691.16). The pres-
synthesis. Compound 6 was obtained in the orthorhombic
system with space group Pnma. The cations in the structure
changed from Na to (NH ) , which causes the POM anions
7
24
3
ence of this peak indicates that the interaction forces between
6−
2+
+
+
[Mo O ] and [Zn(Bis-tris)(H O)] are strong enough to
7 24 2
4
be detected in the gas phase, and according to single-crystal
XRD analysis, the interaction forces are not only the
electrostatic forces between the POM anions and complex
cations but also the hydrogen-bonding force. Besides, other
to no longer form a 1D chain (Figure 3). Similar to 5, complex
−
peaks such as m/z 614.58 can be assigned to {Na[Mo O ]}
4
13
(
simulation, m/z 614.55), which is generated from disassembly
6−
of the [Mo O ] unit.
7
24
The IR spectra of compounds 1−7 were recorded from 400
−
1
to 2000 cm , as shown in Figure S6. Because of the different
structures of the isopolymolybdate parts, compound 1 shows
−
1
different peaks from other compounds in the 400−1000 cm
region, which can be assigned to Mo−O−Mo or MoO
stretching vibrations. Considering the similar POM anions
6−
[
Mo O ] of compounds 2−7, similar peaks are found in the
range from 400 to 1000 cm . Powder X-ray diffraction
PXRD) analysis is used to characterize the phase purity of the
7
24
−1
(
samples. Compared with the PXRD spectrum simulated from
single-crystal XRD, the peaks are matched undifferentiated,
which can verify the purity of the compounds. In addition,
similar structures of compounds 2−7 lead to similar PXRD
patterns, as shown in Figure S5. Thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) analysis
are investigated under an Ar atmosphere of 20−800 °C.
Because of the similar structures of compounds 2−7, the TGA
and DSC curves show similar weight loss stages. The first
Figure 3. Ball-and-stick representations of compounds 5 (A) and 6
B). Partial H atoms are removed for clarity.
(
cations link with the heptamolybdates by hydrogen bonds with
an average O−H···O distance of 2.732(4) Å, but the packing
modes of the complex cations and POM anions are evidently
different. The structure of 7 is similar to that of 6; only the
transition metal is changed from Zn to Cu, and the structural
diagram is shown in Figure S4.
weight loss stage corresponds to the loss of H O molecules.
2
Characterization. ESI-MS is used to detect the solution
The rest of the weight loss stages correspond to the departure
of Bis-tris and the retention of C from Bis-tris, which can only
be slowly removed from the solid residue. This phenomenon is
also observed in other POM-based organic−inorganic hybrid
behaviors and to investigate the self-assembly process of these
49
compounds in solution. Several similar kinds of peaks are
found in the spectra of 1−6 in the low m/z region, which
4−
6−
50
indicated that both [Mo O (C H O N) ] and [Mo O ]
compounds. As for compound 1, the first weight loss stage of
4
12
8
17
5
2
7
24
C
DOI: 10.1021/acs.inorgchem.9b02601
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Inorganic Chemistry
Article
Figure 4. Illustration of the self-assembly process of compound 1. Color scheme: C, black; N, orange; O, red; Mo, blue. H atoms are removed for
clarity.
Table 1. Investigation of the Catalyst Conditions for
a
Sulfoxidation of Thioanisole to Sulfoxide
T
conv.
sel.
time
TOF
−1
d
Figure 5. ESI-MS spectrum of 5 in N,N-dimethylformamide/
b
c
entry oxidant
solvent
(°C) (%)
(%)
(min) (h )
CH OH. Inset: Signal of the peak centered at m/z 691.12, which
3
2
−
can be assigned to {H [Zn(Bis-tris)(H O)][Mo O ](CH OH)} .
1
2
3
4
5
6
7
8
9
1
1
1
H
TBHP
O2
2
O
2
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
THF
50
50
50
50
70
35
10
50
50
50
50
50
99
<1
<1
<1
99
20
10.4
<10
99
99
99
99
95
15
90
90
90
13
15
15
90
30
17
16
20
2800
2
2
7
24
3
e
none
H O
1
5.0% also corresponds to the loss of solvent H O molecules
2
88
99
99
3231
560
291
2
2
2
2
2
2
2
2
2
(
calcd 14.8%). For compounds 2−7, the loss of solvent
H O
2
numbers are 8.7%, 8.9%, 10.6%, 9.6%, 5.9%, and 5.9%,
respectively.
H O
2
H O
2
Catalyst Properties. Sulfide oxidation catalysis has
received extensive attention in recent years because sulfoxide,
as one of the sulfide oxidation products, is an important
H O
2
CHCl3
90
94
92
91
1400
2471
2625
2100
0
1
2
H O
2
acetonitrile
methanol
toluene
H O
2
51,52
intermediate in the synthesis of many industrial products.
In addition, sulfoxidation reaction is also an important model
H O
2
a
53,36
Reaction conditions: solvent (5 mL), substrate = thioanisole (82 μL,
reaction to investigate chemical warfare agents.
Although
b
0
.7 mmol), oxidant (105 μL), catalyst = compound 5 (1 μmol). The
the selective oxidation of sulfides to sulfoxides rather than
sulfones or other byproducts has been widely studied, low O-
atom efficiency, general economic benefits, and the cost of
1
c
conversion was determined by H NMR. Selectivity = sulfoxide/
d
(
sulfoxide + sulfone) × 100%. TOF = moles of sulfide consumed per
mole of catalyst per hour. TBHP = tert-butyl hydroperoxide.
e
54−57
catalysts still restrict this reaction for industrialization.
Hence, sulfoxidation of thioanisole to sulfoxide still has
research value and is chosen as a model reaction for
investigating the catalytic performance of the compounds.
To optimize the reaction conditions, compound 5 is chosen
as the catalyst. As shown in Table 1, when using tert-butyl
optimum conditions were to use H O as the oxidant, ethanol
2
2
as the reaction solvent, and 50 °C as the reaction temperature.
As shown in Table 2, different compounds were used to
catalyze the sulfoxidation reaction at optimum conditions, and
the contents in the molar ratio versus time profiles of
sulfoxidation using compounds 1−6 as catalysts are shown in
Figure S17. When no catalyst was added to the reaction
system, the reaction could not proceed. Compared with
compound 1, compound 5 showed better catalytic effect and
selectivity, which verified that the Bis-tris ligand had little effect
on the catalysis and the added transition metal will promote
the catalysis. Compared to compounds 2−4, compound 5
showed the best catalytic effect, which verified that different
transition-metal ions had different effects on the reaction, and
the catalyst of the Zn-based compound showed a better
hydroperoxide (TBHP) or O as the oxidant, the sulfoxidation
2
reaction cannot proceed. However, when using hydrogen
peroxide (H O ) as the oxidant, the catalytic reactions
2
2
achieved a good convention of thioanisole and high selectivity
to afford sulfoxide. Compared to H O , TBHP had a bigger
2
2
size, which made it difficult to combine with the catalyst of
compound 5 for oxidation. As for O , its weaker oxidizability
2
compared to H O2 led to low conversion. The reaction
2
temperature also affected the catalytic performance in this
catalytic system. With an increase of the temperature to 70 °C,
the reaction time decreased to 13 min, but the selectivity of
sulfoxide dropped to 88%. This may due to the overoxidation
at high temperature. Various solvents were also used to
optimize the catalyst conditions. In acetonitrile, methanol
58
catalytic effect, which was known from previous reports. This
conclusion can also be seen from the catalytic effects of 6 and
7. Meanwhile, (NH ) [Mo O ] and [Zn(Bis-tris)]Cl were
4
6
7
24
2
(
CH OH), and toluene, the conversion and selectivity were
used as the catalysts to investigate the catalytic effects of anions
3
close to those in ethanol. However, in tetrahydrofuran (THF),
the conversion was much lower. Considering environmental
protection, ethanol was chosen as the solvent. Therefore, the
and cations, respectively. The results indicated that [Zn(Bis-
2
+
tris)(H O)] had a poor catalytic effect, but when cocrystal-
2
6
−
lized with [Mo O ] as the organic−inorganic compound 5,
7
24
D
DOI: 10.1021/acs.inorgchem.9b02601
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Inorganic Chemistry
Article
Table 2. Selected Catalysts and Their Catalytic Efficiency
for Sulfoxidation of Thioanisole to Sulfoxide
PXRD and IR spectra of 5 showed no obvious changes after
the catalytic reaction. Hot filtration is used as a test for
leaching to verify the heterogeneity of the catalytic reaction.
a
58c
After 7 min of reaction, the solution was hot-filtered to remove
the catalyst and reaction of the filtrate was continued at 50 °C
1
to 15 min. According to H NMR (Figure S42), the yield was
only 54.5%, indicating only traces of the catalyst had leached
and verifying the heterogeneous nature of the reaction.
Encouraged by the excellent catalytic activity of these
compounds toward the oxidation of thioanisole, a series of
substrates were further carried out at optimal conditions. As
shown in Table S4, gratifyingly, when thioanisole was replaced
by other substrates, compound 5 also showed the best catalytic
effect.
conv.
b
sel.
time
TOF
−1
d
c
entry
catalyst
(%)
(%)
(min)
(h )
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
97
99
99
97
99
99
99
99
99
9
99
98
93
95
98
98
95
89
99
99
25
20
25
20
15
25
25
25
20
15
30
1630
2100
1680
2037
2800
1680
1680
1680
2100
252
Reaction Mechanism. Because the catalytic effect of
(NH ) [ZnMo O H ]
(NH ) [Mo O ] is much better than that of [Zn(Bis-
4
6
7
24
4
4
6
24
6
(NH ) [Mo O ]
tris)]Cl , the active site is considered to be on POM anions
4
6
7
24
2
1
1
0
1
[Zn(Bis-tris)]Cl2
none
rather than on complex cations. In order to find the synergistic
effect of the complex cations on the catalytic reaction,
<1
a
Reaction conditions: ethanol (5 mL), substrate = thioanisole (82 μL,
.7 mmol), H O (105 μL), temperature (50 °C), catalysis (1 μmol).
The conversion was determined by H NMR. Selectivity =
sulfoxide/(sulfoxide + sulfone) × 100%. TOF = moles of sulfide
consumed per mole of catalyst per hour.
thioanisole was titrated into the [Zn(Bis-tris)]Cl solution
2
1
0
and characterized by H NMR, which can exclude the effect of
2
2
b
1
c
POM anions. As shown in Figure S18, the peak at about 2.43
d
ppm, assigned to CH − of thioanisole, shifts to high magnetic
3
field along with increasing thioanisole. Besides, when [Zn(Bis-
tris)]Cl2 was added into the solution of thioanisole, the
absorption peak at 225 nm red-shifted to about 230 nm
(Figure S22). Hence, cations and substrates can combine in a
weak interaction to activate the substrate and make it more
easily oxidized. On the other hand, this weak interaction
prevents the complex cations from forming H O as the
2
+
a synergistic effect between [Zn(Bis-tris)(H O)] and
Mo O ] can enhance the catalytic performance compared
2
6
−
[
7 24
2+
6−
to both cations [Zn(Bis-tris)(H O)] and anions [Mo O ] .
2
7
24
M o r e o v e r ,
(
a
c l a s s i c A n d e r s o n - t y p e P O M ,
NH ) [ZnMo O H ], was used to compare the synergy
4
4
6
24
6
2
2
between Zn and Mo, which showed that the synergy between
cations and anions is much better. In addition, a comparison of
the catalytic effects of compounds 2 and 5−7 indicated that
different structures have an influence on the catalytic effect,
which provides research ideas in the design and synthesis of
molecular catalytic materials.
Although there have been many reports on the sulfoxidation
of thioanisole by using POM-based catalysts, complex 5
showed the best catalytic effect among the common easily
synthesized small-molecule POM catalysts. As can be seen in
Table S5, the TOF of our work was much higher than those of
other works and showed the best economic utility. It is worth
mentioning that the synergistic effect had rarely been studied
in previous reports. To evaluate the reusability of these
compounds, catalytic reactions were carried at the optimized
conditions five times. In addition, IR and PXRD analyses were
used to confirm the stability of the catalysts. As shown in
Figure 6, compound 5 was reused four times without
significant loss of catalytic activity and selectivity. Moreover,
intermediate, so that [Zn(Bis-tris)]Cl cannot rapidly catalyze
2
the oxidation of thioanisole. Besides, when thioanisole is
titrated into the NaCl solution, no shift is observed, as shown
+
−
in Figure S19, indicating that Na and Cl have no influence
on the substrate. Fortunately, peroxomolybdenum was found
by using ESI-MS and UV−vis spectra. As shown in Figure S20,
two peaks at m/z 1383.40 and 1423.36 were found and can be
assigned to {H [Zn(Bis-tris)(H O)][Mo O ](OH)-
3
2
7
24
−
(
(
CH OH)} and {NaH [Zn(Bis-tris)(H O)][Mo O ](OH)-
3 2 2 7 24
−
CH OH)} , respectively. Besides, the changes in the
3
absorbance of 5 when H O was added also prove the
2
2
5
9
formation of peroxomolybdenum (Figure S21). As a
consequence, the complex cations interact with thioanisole to
make it more susceptible to oxidation by POM anion
peroxides, thereby playing a synergistic role. The catalytic
effect of compounds 5 and 6 verified that different arrange-
ments of the anions and cations also influence their catalytic
effect. In order to investigate whether the catalytic reaction is a
radical reaction or not, the radical scavenger 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO) was added during
the reaction. The results (Figure S40) showed that the
catalytic yield after 15 min decreased to 7.4%, which proved
that the reaction process produced radicals to promote the
60,61
reaction. Thus, on the basis of previous reports
and
experimental results, a possible synergistic mechanism has
been proposed in Figure 7. First, H O is added to compound
2
2
5
to form a peroxomolybdenum, and thioanisole is activated by
the complex cations. Second, nucleophilic attack by the S atom
of thioanisole affords the sulfoxide product and H O. The
specific form and type of the intermediate state was not
captured in the experiment.
Figure 6. (A) Reusability of compound 5 to catalyze sulfoxidation of
thioanisole to sulfoxide: (black) conversion; (red) selectivity. (B)
PXRD spectra of compound 5 before and after reaction.
2
E
DOI: 10.1021/acs.inorgchem.9b02601
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
918005, 1918009, 1918011, 1918032, and 1945229 contain the
supplementary crystallographic data for this paper.
H Na [Mo O (C H O N) ]·10H O (1). Na MoO (1 g, 4.86
2
2
4
12
8
17
5
2
2
2
4
mmol) and Bis-tris (0.5 g, 2.42 mmol) were added to 20 mL of
deionized water. Dilute hydrochloric acid was added to adjust the pH
to 5−6. After the solution was stirred at room temperature for 6 h, the
mixture was filtered, and white block-shaped crystals were obtained in
−
1
a few days. Yield (based on Mo): 40%. IR (KBr, cm ): 638 (w), 755
w), 851 (m), 899 (m), 1063 (w), 1098 (s), 1414 (m), 1648 (s),
(
2
3
697 (w), 2938 (w), 3398 (s). ICP-OES (%): Mo, 32.08 (calcd
1.56). Elem anal. Found (calcd): C, 15.73 (15.76); H, 4.50 (4.60);
N, 2.31 (2.30).
Na [Cu(C H O N)(H O)] [Mo O ]·10H O (2). Bis-tris (0.5 g,
2
8
19
5
2
2
7
24
2
2
.42 mmol) was added to 20 mL of deionized water containing
Na MoO (1 g, 4.86 mmol) and CuCl ·2H O (0.5 g, 2.93 mmol).
2
4
2
2
Dilute hydrochloric acid was added to adjust the pH to 5. After the
solution was stirred at room temperature for 6 h, the mixture was
filtered, and blue block-shaped crystals were obtained in a few days.
Figure 7. Proposed pathway for the formation of sulfoxide.
−
1
Yield (based on Mo): 46%. IR (KBr, cm ): 462 (w), 563 (w), 620
(s), 827 (w), 892 (s), 1049 (m), 1444 (m), 1637 (m), 3327 (s). ICP-
OES (%): Mo, 36.34 (calcd 36.04); Cu, 6.90 (calcd 6.82). Elem anal.
Found (calcd): C, 10.14 (10.30); H, 3.14 (3.33); N, 1.34 (1.50).
Na [Ni(C H O N)(H O)] [Mo O ]·10H O (3). Bis-tris (0.5 g,
CONCLUSIONS
■
In summary, seven new functional cation-regulated POM-
based organic−inorganic hybrid compounds were synthesized.
Single-crystal XRD and other chemical analyses were used to
2
8
19
5
2
2
7
24
2
2
.42 mmol) was added to 20 mL of deionized water containing
characterize the structures of these compounds. Compounds
Na MoO (1 g, 4.86 mmol) and NiCl ·6H O (0.5 g, 2.1 mmol). A
2
4
2
2
+
1
−5 featured 1D chain structures linked by Na , while
total of 0.5 mL of hydrochloric acid (12 mol/L) was added to adjust
the pH. After the solution was stirred at room temperature for 6 h, the
mixture was filtered, and green block-shaped crystals were obtained in
compounds 6 and 7 exhibited structures different from that of
by changing the Mo source and showed a 0D structure. ESI-
5
−
1
a few days. Yield (based on Mo): 52%. IR (KBr, cm ): 471 (w), 557
MS also revealed the self-assembly process of compound 1,
(
(
6
3
w), 621 (s), 829 (w), 879 (s), 1022 (m), 1351 (m), 1452 (m), 1639
m), 3155 (s), 3443 (s). ICP-OES (%): Mo, 36.62 (calcd 36.23); Ni,
.31 (calcd 6.33). Elem anal. Found (calcd): C, 10.24 (10.36), H,
.18 (3.34), N, 1.43 (1.51).
which first forms {(Bis-tris)Mo } units and then dimerizes to
2
compound 1. Sulfoxidation of thioanisole to sulfoxide was used
as the reaction model to study the catalytic effect of these
compounds. According to the results, compound 5 exhibited
the best catalytic effect among all six compounds and other
common POM catalysts, indicating that Zn-based compounds
have a better catalytic effect than other transition-metal-based
Na [Co(C H O N)(H O)] [Mo O ]·10H O (4). The procedure
2
8
19
5
2
2
7
24
2
was similar to the synthesis of compound 3. Bis-tris (0.5 g, 2.42
mmol) was added to 20 mL of deionized water containing Na MoO
2
4
(1 g, 4.86 mmol) and Co(CH COO) ·4H O (0.5 g, 2 mmol). A total
3
2
2
+
compounds and the activity of the compounds containing Na
of 0.5 mL of hydrochloric acid (12 mol/L) was added to adjust the
pH. After the solution was stirred at room temperature for 6 h, the
mixture was filtered, and red block-shaped crystals were obtained in a
+
is better than that of cationic NH . In addition, the
4
mechanism of synergistic catalysis of cations and anions was
−
1
1
few days. Yield (based on Mo): 58%. IR (KBr, cm ): 465 (w), 560
speculated according to H NMR and ESI-MS, in which the
(
(
6
3
w), 622 (s), 832 (w), 881 (s), 1023 (m), 1347 (m), 1454 (m), 1640
m), 3158 (s), 3435 (s). ICP-OES (%): Mo, 36.89 (calcd 36.22); Co,
.39 (calcd 6.36). Elem anal. Found (calcd): C, 10.39 (10.36); H,
.19 (3.34); N, 1.48 (1.51).
sulfide that activated by the complex cations acted as a
nucleophile and attacked the peroxomolybdenum bond.
Further studies of their catalytic synergy are in progress.
Na [Zn(C H O N)(H O)] [Mo O ]·10H O (5). The procedure
2
8
19
5
2
2
7
24
2
EXPERIMENTAL SECTION
was similar to the synthesis of compound 3. Bis-tris (0.5 g, 2.42
■
Materials and Physical Measurement. All reagents were
mmol) was added to 20 mL of deionized water containing Na
2 4
MoO
purchased from commercial sources and used without further
(1 g, 4.86 mmol) and Zn(CH COO) ·2H O (0.5 g, 2.27 mmol). A
3
2
2
1
purification except those noted specifically. The H NMR spectra
total of 0.5 mL of hydrochloric acid (12 mol/L) was added to adjust
the pH. After the solution was stirred at room temperature for 6 h, the
mixture was filtered, and pale-pink block-shaped crystals were
obtained in a few days. Yield (based on Mo): 62%. IR (KBr,
were performed on a Bruker Avance 400 MHz spectrometer in a
deuterium generation reagent with tetramethylsilane as the inner
standard. Fourier transform infrared (FT-IR) spectra were obtained
on an Alpha Centauri FT-IR spectrometer in the 400−4000 cm
region with a KBr pellet. TGA/DSC was performed on a PerkinElmer
−
1
−1
cm ): 453 (w), 564 (w), 625 (s), 833 (w), 888 (s), 1055 (m), 1356
(w), 1463 (w), 1640 (s), 3419 (s). ICP-OES (%): Mo, 35.31 (calcd
35.97); Zn, 6.97 (calcd 7.01). Elem anal. Found (calcd): C, 10.38
(10.28); H, 3.18 (3.32); N, 1.40 (1.50).
TGA7 instrument under flowing Ar, and the temperature was set from
4
0 to 800 °C under a heating rate of 10 °C/min with an Ar flow of 25
mL/min. ESI-MS spectra were recorded with a Bruker Compact
Quadrupole Time-of-Flight (QTOF) mass spectrometer with an
electrospray ionization detector, and data analysis was performed on
the Bruker Isotope Pattern. PXRD was employed using Cu Kα in the
range of 10−50° with a scanning rate of 10°/min.
( NH
(NH [ZnMo O
6
4
)
2
[ Z n ( B i s - t r i s ) ( H
2 2 7 2
O ) ] [ M o O 2 4 ] · 6 H O ( 6 ) .
)
24] (0.55 g, 0.5 mmol) and Bis-tris (0.104 g, 0.5
4
4
mmol) were dissolved in 15 mL of deionized water. The mixture was
sealed in a 30 mL Teflon-lined stainless steel autoclave heated at 140
°C for 24 h. After cooling to room temperature, the mixture was
filtered, and white block-shaped crystals were obtained in a few days.
X-ray Crystallography. The crystallographic data and structural
refinements are given in Table S1. Single-crystal X-ray analysis was
performed on a Bruker APEX-II diffractometer with graphite-
monochromated Mo Kα radiation at 296 K (λ = 0.71073 Å). A
multiscan technique was used for absorption correction. Using WingX,
the structure was solved with the ShelXT structure solution program
using intrinsic phasing and refined with the ShelXL refinement
package using least-squares minimization. CCDC 1917995, 1917999,
−
1
Yield (based on Mo): 42%. IR (KBr, cm ): 484 (w), 556 (w), 627
(s), 814 (w), 885 (s), 1035 (s), 1071 (m), 1415 (s), 1454 (m), 1370
(m), 3277 (s), 3492 (s). ICP-OES (%): Mo, 37.57 (calcd 37.20); Zn,
7.60 (calcd 7.25). Elem anal. Found (calcd): C, 10.59 (10.76); H,
3.48 (3.47); N, 2.95 (3.14).
(NH ) [Cu(Bis-tris)(H O)] [Mo O ]·6H O (7). The synthesis
4
2
2
2
7
24
2
process of compound 7 is similar to that of 6, except that
F
DOI: 10.1021/acs.inorgchem.9b02601
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
NH ) [ZnMo O ] is replaced by (NH ) [CuMo O ]. IR (KBr,
Advances on Polyoxometalates Intercalated Layered Double Hydrox-
ides: From Synthetic Approaches to Functional Material Applications.
Coord. Chem. Rev. 2014, 258−259, 58−71.
4
4
6
24
4
4
6
24
−
1
cm ): 484 (w), 556 (w), 627 (s), 814 (w), 885 (s), 1035 (s), 1071
(
(
m), 1415 (s), 1454 (m), 1370 (m), 3277 (s), 3492 (s). ICP-OES
%): Mo, 37.82 (calcd 37.69); Cu, 7.34 (calcd 7.13). Elem anal.
(2) (a) Gumerova, N. I.; Rompel, A. Synthesis, Structures and
Applications of Electron-Rich Polyoxometalates. Nat. Rev. Chem.
2018, 2, 112−132. (b) Yang, G. Y.; Wu, K. C. Two-Dimensional
Deep-Ultraviolet Beryllium-Free KBe BO F Family Nonlinear-
Found (calcd): C, 10.71 (10.78); H, 3.45 (3.3.48); N, 3.20 (3.14).
Zn(Bis-tris)]Cl . This compound was synthesized according to
[
2
4
8
previous literature. To the deionized water containing 0.02 mol of
Bis-tris was added 0.02 mol of ZnCl . The solution was stirred at 60
2
3 2
2
Optical Monolayer. Inorg. Chem. 2018, 57, 7503−7506.
°
C. The product was collected by evaporation of the solution under
reduced pressure. ESI-MS (m/z): 272.07, [(C H O N)Zn]
(3) (a) Ayass, W. W.; Fodor, T.; Farkas, E.; Lin, Z.; Qasim, H. M.;
Bhattacharya, S.; Mougharbel, A. S.; Abdallah, K.; Ullrich, M. S.; Zaib,
S.; Iqbal, J.; Harangi, S.; Szalontai, G.; Banyai, I.; Zekany, L.; Toth, I.;
+
8
18
5
+
(
2
theoretical, 272.05); 290.08, [(C H O N)Zn(H O)] (theoretical,
8 18 5 2
+
90.06); 308.04 [(C H O N)ZnCl] (theoretical, 308.02).
8 19 5
Kortz, U. Dithallium(III)-Containing 30-Tungsto-4-phosphate,
Catalytic Procedures. A mixture of sulfide (82 μL, 0.7 mmol)
16‑
[
Tl Na (H O) (P W O ) ] : Synthesis, Structural Characteriza-
2
2
2
2
2
15 56 2
and 30% H O (105 μL, 1 mmol of H O ) was dissolved in 5 mL of
2
2
2
2
tion, and Biological Studies. Inorg. Chem. 2018, 57, 7168−7179.
ethanol, and then 1 μmol (0.14 mol %) of the catalyst was added to
the solution. The catalyst was ground in advance. The mixture was
stirred at 50 °C until the sulfide was transformed to sulfoxide or
sulfone. The crude reaction mixture was purified by chromatography
on silica gel and then taken for NMR analysis. The productivity was
calculated by using H NMR. The catalyst was recycled by filtration
and washed with H O and ethanol several times.
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ASSOCIATED CONTENT
■
*
S
Supporting Information
IV
Y. An Oxalate-Functionalized Tetra-Zr -Substituted Sandwich-Type
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02601.
Silicotungstate: Hydrothermal Synthesis, Structural Characterization,
and Catalytic Oxidation of Thioethers. Eur. J. Inorg. Chem. 2019, 3−4,
417−422. (c) Huang, X. Q.; Li, J. K.; Shen, G. D.; Xin, N. N.; Lin, Z.
Synthesis and characterization details, including Figures
S1−S42 and Tables S1−S5 (PDF)
G.; Chi, Y. N.; Dou, J. M.; Li, D. C.; Hu, C. W. Three Pd-
Decavanadates with A Controllable Molar Ratio of Pd to
Decavanadate and Their Heterogeneous Aerobic Oxidation of
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Accession Codes
CCDC 1917995, 1917999, 1918005, 1918009, 1918011,
(5) Wang, K.; Niu, Y. J.; Zhao, D. Y.; Zhao, Y. X.; Ma, P. T.; Zhang,
1
918032, and 1945229 contain the supplementary crystallo-
D. D.; Wang, J. P.; Niu, J. Y. The Polyoxovanadate-Based Carboxylate
V
IV
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing 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.
Derivative K H[V 17V (OH) O (OOC(CH ) COO) ]·nH O:
6
12
4
60
2
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Synthesis, Crystal Structure, and Catalysis for Oxidation of Sulfides.
Inorg. Chem. 2017, 56, 14053−14059.
(6) (a) Yuan, L.; Qin, C.; Wang, X.; Li, Y.; Wang, E. A Series of
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Novel Organic-Inorganic Hybrids Based on α-[AlW O ] Poly-
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AUTHOR INFORMATION
■
Corresponding Authors
*
E-mail: fenly18@sina.com.
E-mail: yanjun@csu.edu.cn.
*
(c) An, H. Y.; Zhang, Y. M.; Hou, Y. J.; Hu, T.; Yang, W.; Chang, S.
ORCID
Z.; Zhang, J. J. Hybrid Dimers Based on Metal Substituted Keggin
Polyoxometalates (Metal = Ti, Ln) for Cyanosilylation Catalysis.
Dalton Trans 2018, 47, 9079−9089.
Wei-Dong Yu: 0000-0001-6937-6944
Jun Yan: 0000-0002-6158-0614
Notes
(
7) Jin, X. X.; Yu, W. D.; Nie, Y. M.; Liu, M. S.; Yan, J. Synthesis and
8
‑
Characterization of[(HPO ) Mo O (H O) ] : A New Redox
3
6
21 60
2
4
The authors declare no competing financial interest.
ActiveHeteropoly Blue Cluster with Layered Shape Containing A
Phosphite Template That Self-Assemblesunder Controlled Micro-
wave Irradiation. Dalton Trans 2016, 45, 3268−327.
ACKNOWLEDGMENTS
This work was supported by the Fundamental Research Funds
for the Central Universities of Central South University (Grant
■
(8) An, H. Y.; Wang, L.; Hu, Y.; Xu, T. Q.; Hou, Y. J. Spontaneous
Resolution of Evans-Showell-Type Polyoxometalates in Constructing
Chiral Inorganic-Organic Hybrid Architectures. Inorg. Chem. 2016,
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