In Situ Ligand-Transformation-Involved Synthesis of Inorganic-Organic Hybrid Polyoxovanadates as Efficient Heterogeneous Catalysts for the Selective Oxidation of Sulfides

Article abstract of DOI:10.1021/acs.inorgchem.0c02798

By intentionally involving in situ ligand transformation in the reaction system, two inorganic-organic hybrid polyoxovanadates (POVs), [Co(HDTBA)V2O6] (1) and [Ni(H2O)2(DTBA)2V2O4(OH)2]·4H2O (2), have been synthesized by using a hydrothermal method, where the 3,5-di[1,2,4]triazol-1-ylbenzoic acid (HDTBA) ligand originated from in situ hydrolysis of 3,5-di[1,2,4]triazol-1-ylbenzonitrile in the self-assembly process. The inorganic layers [Co2(V4O12)]n containing [V4O12]4- circle clusters were linked by HDTBA ligands to yield a 3D framework structure of compound 1. There existed a kind of binuclear [(DTBA)2V2O4(OH)2]2- vanadium cluster grafted directly by two DTBA ligands through the sharing of carboxyl oxygen atoms in compound 2, further extended into a 2D layer by nickel centers. The investigations on the catalytic properties indicated that compounds 1 and 2 as heterogeneous catalysts, especially 2, owned satisfying catalytic performances for catalyzing the selective oxidation of sulfides to sulfoxides in the presence of tert-butyl hydroperoxide as an oxidant, accompanied by excellent conversion of 100% and selectivity of above 99%, providing a promising way for developing inorganic-organic hybrid POVs as effective heterogeneous catalysts for catalyzing the selective oxidation of sulfides.

Full text of DOI:10.1021/acs.inorgchem.0c02798

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Article
In Situ Ligand-Transformation-Involved Synthesis of Inorganic−
Organic Hybrid Polyoxovanadates as Efficient Heterogeneous
Catalysts for the Selective Oxidation of Sulfides
Xiang Wang,* Tong Zhang, Yunhui Li, Jiafeng Lin, Huan Li, and Xiu-Li Wang*
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ABSTRACT: By intentionally involving in situ ligand transformation in the reaction
system, two inorganic−organic hybrid polyoxovanadates (POVs), [Co(HDTBA)V2O6]
(1) and [Ni(H2O)2(DTBA)2V2O4(OH)2]·4H2O (2), have been synthesized by using a
hydrothermal method, where the 3,5-di[1,2,4]triazol-1-ylbenzoic acid (HDTBA)
ligand originated from in situ hydrolysis of 3,5-di[1,2,4]triazol-1-ylbenzonitrile in the
self-assembly process. The inorganic layers [Co2(V4O12)]n containing [V4O12]4− circle
clusters were linked by HDTBA ligands to yield a 3D framework structure of
compound 1. There existed a kind of binuclear [(DTBA)2V2O4(OH)2]2− vanadium
cluster grafted directly by two DTBA ligands through the sharing of carboxyl oxygen
atoms in compound 2, further extended into a 2D layer by nickel centers. The
investigations on the catalytic properties indicated that compounds 1 and 2 as
heterogeneous catalysts, especially 2, owned satisfying catalytic performances for
catalyzing the selective oxidation of sulfides to sulfoxides in the presence of tert-butyl
hydroperoxide as an oxidant, accompanied by excellent conversion of 100% and
selectivity of above 99%, providing a promising way for developing inorganic−organic hybrid POVs as effective heterogeneous
catalysts for catalyzing the selective oxidation of sulfides.
more presynthesized organic ligands for exploiting POV-based
hybrids with various structures.
INTRODUCTION
■
More and more attention has been paid to the preparation of
inorganic−organic hybrids based on polyoxometalates
(POMs) because of their potential applications in the field
of catalysis,1−3 batteries,4 supercapacitors,5 and photochemis-
try and electrochemistry.6−8 As a remarkable branch of POMs,
polyoxovanadate (POV)-based inorganic−organic hybrids are
of considerable interest not only because of their remarkable
properties such as magnetism9 and catalysis and fluores-
cence10,11 but also because of their diverse architectures.12,13 In
the synthesis of POV-based hybrids with various structures and
outstanding properties, on the one hand, the selection and
design of organic ligands is very important. A universal
pathway is to introduce the organic ligands into the reaction
system to construct POV-based hybrids. Up to now, a large
number of presynthesized organic ligands have been involved
in the structures of POV-based hybrids, including not only
bidentate nitrogen-donor ligands such as 1,2-di(4-pyridyl)-
ethylene and 1,4-bis(imidazol-1-yl)butane as well as multi-
dentate such as 2,4,6-tri(4-pyridyl)-1,3,5-triazine and so on,
which have been summarized well in the review organized by
Arriortua in 2014,14 but also many carboxylic acid ligands,15−17
with the addition of those combining the nitrogen-donor group
and carboxylic acid.18,19 It should be pointed out that most
organic ligands in these works are presynthesized and used
directly, which prompts us to continue to design and develop
On the other hand, POV-based inorganic−organic hybrids
have been mentioned as a kind of efficient heterogeneous
catalyst for the oxidation of sulfide. For instance, not only can
the use of organic ligands, including 1,4-bis(1H-imidazoly-1-
yl)benzene,20 1,10-phenanthroline,21 resorcin[4]arene,22 etha-
nediamine, 1,2-cyclohexanediamine, and 1,2-diaminopro-
pane,23 generate charming architectures based on POVs, but
also these hybrids exhibited high efficiency and selectivity in
the oxidation of sulfides, suggesting that the continuous design
and synthesis of POV-based hybrids may be promising to
develop efficient catalysts used in the selective sulfoxidation of
sulfides.
In our group’s previous works, deliberately involving in situ
ligand transformation in the reaction system has been
considered to be a feasible pathway for the synthesis of
POM-based hybrids,24−26 where not only were the N-
bidentate ligands containing nitrile groups [3,5-di(1H-
Received: September 22, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c02798
© XXXX American Chemical Society
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A

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4.0 using a 1.0 M HCl solution, which was further placed in a 25 mL
Teflon-lined autoclave and heated at 160 °C for 4 days. Red block-
shaped crystals were isolated with a yield of 34% based on NH4VO3.
Anal. Calcd for C11H7CoN6O8V2 (512.04): C, 25.80; H, 1.28; N,
16.41. Found: C, 25.69; H, 1.42; N, 16.32. IR (solid KBr pellets,
cm−1): 1740 (s), 1688 (s), 1610 (s), 1521 (s), 944 (s), 903 (s), 848
(s), 794 (s).
imidazol-1-yl)benzonitrile (DICN), 3,5-di[1,2,4]triazol-1-yl-
benzonitrile (DTCN), and 3,5-di(benzimidazolyl-1-yl)-
benzonitrile] hydrolyzed in situ into new N-bidentate ligands
containing carboxylic acid in the final structures (Scheme 1),
Scheme 1. Representative In Situ Ligand Transformation
Synthesis of [Ni(H2O)2(DTBA)2V2O4(OH)2]·4H2O (2). The
synthesis process of complex 2 was similar to that of 1, except that
CoCl2·6H2O was replaced with NiCl2·6H2O (0.06 g, 0.25 mmol) or
DTCN (0.12 g, 0.5 mmol). Light-green block-shaped crystals were
obtained with a yield of 34% based on NH4VO3. Anal. Calcd for
C22H24N12NiO16V2 (873.07): C, 30.27; H, 2.77; N, 19.25. Found: C,
30.17; H, 2.91; N, 19.16. IR (solid KBr pellet, cm−1): 3431 (w), 1600
(s), 1538 (s), 1460 (s), 928 (s), 874 (s), 782 (s).
General Procedure for the Catalytic Oxidation of Sulfides.
Sulfide (0.5 mmol), TBHP (0.75 mmol), catalyst (3 μmol, 0.6 mol
%), and 2 mL of methanol were placed in a 5 mL round-bottom flask.
The catalytic reaction was operated at 50 °C for 15 min and
monitored by GC at various time intervals. The products were
analyzed by GC−MS.
X-ray Crystallographic Study. The data crystallographic data of
complexes 1 and 2 were acquired on a Bruker SMART APEX II with
Mo Kα (λ = 0.71073 Å) at 298 K. These structures were solved by
direct methods employing the SHELXT 2014 program packages.28,29
Refinement of the structure was finished by full-matrix least-squares
methods based on F2. The hydrogen atoms belonging to coordination
and lattice water molecules were not located in the structures but
embodied in the structure factor calculations. The detailed crystallo-
graphic data for complexes 1 and 2 are summarized in Table 1. Table
S1 involves the selected bond lengths and angles.
but also the generated ligands can participate in the formation
of POM-based hybrids, together with various structural
features. Inspired by the above results, in situ ligand
transformation was intentionally introduced into the reaction
system based on POVs that may be a hopeful pathway for the
preparation of POV-based inorganic−organic hybrids with
innovative architectures and catalytic properties.
Here, when DTCN was selected as the initial ligand, two
POV-based hybrids with the formulas of [Co(HDTBA)V2O6]
(1) and [Ni(H2O)(DTBA)2V2O4(OH)2]·4H2O (2) were
isolated by changing metal ions under hydrothermal
conditions, in which 3,5-di[1,2,4]triazol-1-ylbenzoic acid
(HDTBA) involved in the formation of hybrid structures
was derived from in situ hydrolysis of DTCN. Among them,
compound 1 was a 3D framework built from [Co2(V4O12)]n
inorganic layers consisting of [V4O12]4− circle clusters and
HDTBA ligands as linkers, and compound 2 possessed a 2D
layer structure composed of POV building units
[(DTBA)2V2O4(OH)2]2− modified directly by DTBA ligands.
Further, the studies on the catalytic performance using 1 and 2
as heterogeneous catalysts revealed that all compounds have
high activity and selectivity toward the oxidation of sulfide to
sulfoxide with tert-butyl hydroperoxide (TBHP) as the oxidant.
The magnetic behavior of compound 1 was also investigated
here.
Table 1. Crystal Data for Compounds 1 and 2
1
2
formula
fw
C11H7CoN6O8V2
512.04
C22H24N12NiO16V2
873.07
temperature/K
cryst syst
space group
a/Å
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
293(2)
triclinic
293(2)
triclinic
P1
̅
P1
̅
6.8675(8)
9.3105(11)
13.2286(16)
101.571(2)
93.950(2)
109.753(2)
771.27(16)
2
7.6969(7)
10.5154(9)
10.6644(9)
101.148(2)
101.397(2)
104.148(2)
793.49(12)
1
EXPERIMENTAL SECTION
■
Materials and Methods. Available reagents and solvents used in
this paper were purchased from commercial sources and used without
further purification. The DTCN ligand was prepared in accordance
with the method reported in the literature.27
Z
Dc/ g·cm−3
μ/mm−1
F(000)
2.205
2.313
504
0.0371, 0.0879
0.0501, 0.0941
1.024
1.806
1.259
432
0.0363, 0.0918
0.0505, 0.1002
Physical Measurements. The chemical composition of all
complexes was calculated from elemental analyses (carbon, hydrogen,
and nitrogen) on a PerkinElmer 2400C elemental analyzer. Fourier
transform infrared (FT-IR) spectra were recorded on a Scimitar 2000
near-FT-IR spectrometer with KBr pellets. Powder X-ray diffraction
(PXRD) data were collected on a Rigaku Ultima IV diffractometer at
room temperature. The catalytic reaction was analyzed by using a
Shimadzu Techcomp GC-7900 gas chromatograph with an flame
ionization detector equipped with a TM-5 Sil capillary column, where
naphthalene was selected as an internal standard substrate. Gas
chromatography (GC)−mass spectrometry (MS) spectra were
recorded on an Agilent 7890A-5975C spectrometer. The temper-
ature-dependent magnetic susceptibility data were collected on a
SQUID magnetometer in the temperature range of 2−300 K under a
1 kOe field.
a
b
final R1 , wR2 [I > 2σ(I)]
a
b
final R1 , wR2 (all data)
GOF of F2
1.025
a
b
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑w(Fo − Fc2)2/∑[w(Fo )2]1/2
.
RESULTS AND DISCUSSION
Synthesis. The feasibility of intentionally using in situ
■
ligand transformation to construct POM-based hybrids has
been confirmed in our previous works, where the cyano group
can be easily hydrolyzed in situ to a carboxylate group under
hydrothermal conditions, and a few of inorganic−organic
hybrids based on polymolybdates/polytungstates, including
[PMo12O40]3−, [SiW12O40]4−, [P2W18O62]6−, and [Mo8O26]4−,
have been isolated. On the basis of the idea of developing more
Synthesis of [Co(HDTBA)V2O6] (1). A mixture of NH4VO3 (0.06
g, 0.5 mmol), CoCl2·6H2O (0.06 g, 0.25 mmol), DTCN (0.06 g, 0.25
mmol), and 10 mL of deionized water was stirred at room
temperature for 1 h, and then the pH value was adjusted to about
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Figure 1. (a) View of the binuclear cobalt unit. (b) [V4O12]4− circular cluster constructed from two V2O62− polyoxoanions. (c) 2D layer structure
2−
built from V2O6 polyoxoanions and binuclear cobalt units. (d) 3D framework based on the POV of complex 1.
synthesis approaches for preparing POV-based hybrids, in situ
ligand transformation was deliberately introduced into the
reaction system to synthesize inorganic−organic hybrids based
on the POVs here. As expected, the common features were
that the DTCN ligand was hydrolyzed in situ to the HDTBA
ligand and two POV-based hybrids with different architectures
were isolated by tuning the metal ion and reaction conditions.
When the Co2+ ion was chosen together with the molar ratio of
2:1:1 for Co2+/NH4VO3/DTCN, the satisfactory red crystals
of 1 were collected by controlling the pH values of 4.0−4.5.
However, the cobalt vanadate mentioned by Ma’s group can be
generated at pH values above 4.530 and only an amorphous
solid for pH values of less than 4.0, while crystals of 2 were not
obtained under the same conditions as those of 1. When the
molar ratio of Ni2+/NH4VO3/DTCN was adjusted to 1:1:1,
light-green crystals of 2 were obtained, but no crystals
appeared under such a ratio for the Co2+ ion. In other
words, the pH and ratio of raw materials exhibited important
effects on the formation of crystalline 1 and 2.
In 1, one of the structural features is that the intentional in
situ transformation of DTCN to the HDTBA ligand has been
involved in constructing the POV-based hybrids as we
expected. The other one represents a new mode of
[V4O12]4− circular cluster coordination to metal ions. The
[V4O12]4− clusters accompanied by various coordination
modes with metal ions has generally been reported so far,
which is summarized in Scheme 2. The [V4O12]4− cluster can
Scheme 2. Diverse Linking Modes of the {V4O12}4− Cluster
Crystal Structure of 1. Compound 1 consists of one
cobalt(II) atom, one HDTBA ligand, and one [V2O6]2−
polyoxoanion. Bond-valence-sum calculations show that the
cobalt and vanadium atoms are in the II+ and V+ oxidation
states.31 The octahedral configuration of the Co2+ ion is
defined by four oxygen atoms in the equatorial position and
two nitrogen atoms from the HDTBA ligand in the apical
position. Two octahedral Co2+ ions are aggregated through an
edge-sharing mode to establish a binuclear cobalt unit (Figure
1a). Each of the crystallographically independent vanadium
centers in a [V2O6]2− polyoxoanion surrounding four oxygen
atoms possesses VO4 tetrahedral coordination geometry. Two
sets of [V2O6]2− polyoxoanions are linked by corner-sharing
oxygen atoms to result in a [V4O12]4− circular cluster (Figure
1b). These [V4O12]4− clusters are connected by binuclear
cobalt units to give rise to a 2D Co−O−V layer structure
(Figure 1c), where the remaining terminal oxygen atoms
belonging to the V1 center occupy the terminal and edge-
sharing oxygen atoms of adjacent binuclear cobalt units, and
only one terminal oxygen atom from the V2 center occupies
the terminal oxygen atom of the binuclear cobalt units. Finally,
the HDTBA ligands as bidentate linkers utilize two nitrogen
atoms of the triazolyl group to coordinate with cobalt atoms
from neighboring Co−O−V layers, yielding a 3D framework
based on the POV of complex 1 (Figure 1d).
display polydentate coordination modes, where the terminal
oxygen atom of the [V4O12]4− cluster is coordinated with the
metal center in either a one-to-one manner (types a−f) or an
occasional one-to-many manner (type g). Some mentioned
types have been expounded by Spodine and co-workers.32 The
other common bischelating modes, along with polydentate
coordination pattern mode, were also involved in the reported
cases of [{Cd(phen)2}2V4O12]·5H2O (type h),33 [{Zn-
(bipy)2}2V4O12],34 [{Co2(H2O)2(Bpe)2}(V4O12)]·4H2O·Bpe
(type j),35 and [Cu3(triazolate)2V4O12] (type k).36 In this
work, compound 1 shows a new type l mode, including two
chelate and four monodentate metal centers. The numbers of
metal centers around the [V4O12]4− cluster are the same as
those of types f and k, but the difference from type k is the
C
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Figure 2. (a) View of the binuclear vanadium cluster [V2O6(OH)2]4−. (b) Binuclear [(DTBA)2V2O4(OH)2]2− building unit modified by the
DTBA ligands. (c) 2D layer structure based on the POV of complex 2.
position of the oxygen atoms involved in the monodentate
coordination mode.
58.4%) belonging to decomposition of the water molecules
and organic ligands, respectively.
Crystal Structure of Compound 2. In 2, the crystallo-
graphically independent vanadium atom in the V+ oxidation
state is five-coordinated,31 completed by three oxygen atoms
and two hydroxy groups. A pair of vanadium atoms are each
gathered through sharing two μ2-OH groups, generating a
binuclear vanadium cluster [V2O6(OH)2]4− (Figure 2a).
Interestingly, the binuclear vanadium cluster [V2O6(OH)2]4−
is further decorated by two DTBA ligands, where two terminal
oxygen atoms of the vanadium cluster [V2O6(OH)2]4− are
shared by oxygen atoms of the carboxyl group (Figure 2b),
forming a [(DTBA)2V2O4(OH)2]2− building unit. In fact, a
series of isopolymolybdate-based hybrids modified by the
carboxyl group stemming from in situ ligand transformation of
the cyano group have been obtained in our previous works,
where DICN and DTCN were chosen as the initial ligands.
However, the POVs modified by the carboxyl group
originating from in situ ligand transformation of the cyano
group are scarce. These findings allow one to conclude that in
situ POMs could usually be easily grafted by in situ
ligands,25,26 but it rarely occurs in the reaction system based
on classical POMs, such as Keggin POMs [PMo12O40]3− and
[SiW12O40]4− and the Wells−Dawson POM [P2W18O62]6−,24
which suggests a potential approach for preparing the POM-
based complexes modified by in situ ligand transformation.
Further, the [(DTBA)2V2O4(OH)2]2− building units as
tetradentate linkages relying on four nitrogen atoms of the
imidazolyl group from two DTBA ligands are linked by nickel
centers to give a 2D layer structure of 2 (Figure 2c), and the
coordination of two water molecules compensates for the
octahedral configuration of each nickel center.
PXRD, FT-IR Spectra, and Thermogravimetric (TG)
Analyses. PXRD experiments of two compounds were carried
out to confirm the phase purities, as shown in Figure S1. The
experimental diffraction peaks of compounds 1 and 2 are in
accordance with the patterns simulated by single-crystal
analysis of 1 and 2, revealing that the phase purities of two
compounds are satisfactory. The FT-IR spectra of compounds
1 and 2 are present in Figure S2. The characteristic bands in
the region of 950−540 cm−1 should be derived from the
absorption peaks of VO and V−O−V.37 The bands in the
region of 1743−1400 cm−1 should be ascribed to the
characteristic peaks of the DTBA ligand.25 TG analyses of 1
and 2 were also provided to verify the composition of the
structure. Figure S3 embodies one step of the weight loss
process for 1, about 50.84% (calcd 49.8%) ascribed to the
decomposition of organic ligands, two steps of the weight loss
process for 2, about 13.1% (calcd 12.4%) and 57.6% (calcd
Magnetic Properties. The magnetic susceptibility depend-
ing on the temperature from 2 to 300 K of compound 1 was
investigated with a 1 kOe field, and the curves of χM versus T
−1
and χM versus T are demonstrated in Figure S4. The χmT
value of 4.22 cm3 K mol−1 at 300 K is much larger than the
expected χMT value of two isolated cobalt(II) ions (3.875 cm3
3
K mol−1 with g = 2 and S = /2), embodying the fact that the
orbital contribution derived from distorted octahedral cobalt-
(II) centers is important.38,39 The plot of χMT versus T
displays a continuous decrease with a change in the
temperature from 300 to 2 K, and the χMT value reaches
2.13 emu K mol−1 at 2 K. The magnetic susceptibility in the
range of 2−300 K of compound 1 obeys the Curie−Weiss
equation [χM = C/(T − θ)] with Weiss constants θ = −9.72 K
and C = 4.41 cm3 K mol−1, suggesting that an antiferromag-
netic coupling stemmed from the binuclear cobalt(II) unit in 1.
Catalytic Oxidation of Sulfides. Selective oxidation of
sulfides to sulfone and sulfoxide has drawn extensive attention
because of their widespread application in the preparation of
biological molecules,40 as auxiliaries in asymmetric and chiral
synthesis,41 where the hybrids based on POVs represented
kinds of very popular candidates as catalysts for the selective
oxidation of sulfides.42 Herein, the catalytic activities of
compounds 1 and 2 for oxidizing sulfide to sulfoxide were
investigated in detail. A typical modal reaction with methyl
phenyl sulfide (MPS) as the substrate was established to
evaluate the selective catalytic abilities of compounds 1 and 2
using TBHP as the oxidant (Scheme 3).
Scheme 3. Selective Oxidation of MPS to Sulfoxide
Catalyzed by Compounds 1 and 2
The heterogeneous catalytic efficiencies of 1 and 2 for the
selective oxidation of MPS to sulfoxide were assessed by
changes in the reaction conditions, including temperature,
dosage of the catalyst, oxidants, and reaction time, and the
catalytic results were analyzed by GC, as shown in Table 2.
The catalytic reactions with TBHP as the oxidant and 2 as the
catalyst were first carried out at room temperature and 50 and
60 °C for 15 min in methanol, and the dosages of the
substrate, catalyst, and TBHP were 0.5 mmol, 3 μmol, and 0.5
mmol (entry 3). The catalytic results revealed that the
D
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catalysts suggested that the catalytic reaction may focus on
organic−inorganic hybrid-based POVs.
Table 2. Conversion and Selectivity of the Oxidation of
MPS to Sulfoxide Analyzed by 2 with TBHP as the Oxidant
in Methanol
a
The catalytic activities of the raw material NH4VO3 and
corresponding complexes have been further studied to
demonstrate whether 1 and 2 as the representatives of a
combination of complexes and POVs has the advantage of
catalyzing the oxidation of MPS to sulfoxide. As shown in
Table 3, using NH4VO3 as the catalyst under optimum
catalyst
(μmol)
TBHP
(mmol)
time
(min)
temp
(°C)
convn
b
selectivity
c
entry
(%)
(%)
1
2
3
4
5
6
7
8
9
3
3
3
3
0.5
0.5
0.5
0.75
0.75
0.75
1
0.75
0.75
15
15
15
15
15
15
15
10
20
rt
<10
50
60
50
50
50
50
50
50
83
98
100
98
92
98
88
98
84
99
98
99
84
99
Table 3. Conversion and Selectivity of MPS to Sulfoxide by
d
3
a
Using Different Catalysts
4
3
3
3
catalyst
convn (%)
selectivity (%)
1
2
98
100
65
98
99
86
a
a
d
100 ,
100
89 , 94
d
NH4VO3
10
11
12
13
14
3
none
0.75
0.75
0.75
0.75
15
15
15
15
15
50
50
50
50
50
<1
<20
75
79
82
[Co(H2O)2(DTBA)2]·3H2O
[Ni(H2O)2(DTBA)2]·3H2O
<1
<13
none
3
3
3
e
98
98
99
a
f
Reaction conditions: MPS, 0.5 mmol; catalyst, 3 μmol (0.6 mol %);
g
TBHP, 0.75 mmol; methanol, 2 mL; 50 °C; 15 min.
a
Reaction conditions: substrate, 0.5 mmol; methanol, 2 mL;
b
compound 2, 3 μmol (0.6 mol %). The conversion and selectivity
to sulfoxides were confirmed by GC. Selectivity (%) = sulfoxide
conditions, a conversion of 65% and a selectivity of 86% were
obtained. The conversions were less than 1% and 13% when
complexes [Co(H2 O)2 (DTBA)2]·3H2 O and [Ni-
(H2O)2(DTBA)2]·3H2O were selected as the catalysts
(Scheme S1 and Figure S5). These results reveal that the
combination of complexes and POVs can generate heteroge-
neous catalysts (1 and 2) with excellent conversion and
selectivity for the oxidation of MPS to sulfoxide, which was in
accordance with the reported findings of the cation and
polyoxoanion parts enhancing the selectivity and conversion of
substrates.43,46 The catalytic mechanism speculated that the
POVs as catalytic active sites in the structures of complexes
were attacked by an oxidant during the catalytic process to
form metal peroxide intermediates, and the sulfide was then
oxidized to sulfoxide by the intermediate of the metal
peroxide.44,46,47
c
d
e
(mol)/sulfone (mol) × 100. Compound 1. Ethanol (2 mL).
f
g
Trichloromethane (2 mL). Acetonitrile (2 mL).
temperature of 60 °C led to a gratifying conversion of 98% but
only 84% for the selectivity of MPS to sulfoxide. The selectivity
was further promoted to 98% along with a reduction in the
reaction temperature to 50 °C; nevertheless, the conversion is
only 83% (entry 2). Subsequently, when the dosage of TBHP
was increased to 0.75 mmol, the conversion and selectivity of
100% and 99% were gratifying (entry 4), until the dosage of 1
mmol resulted in a declining selectivity of 84% (entry 7),
which is superior to the previously reported POV-based
inorganic−organic hybrids, such as [Cu(mIM)4]V2O,43 [Co2-
L0.5V4O12]·3DMF·5H2O,22 [(C2N2H8)4(CH3O)4VIV4VV O16]·
4
4CH3OH,44 K6H[VV
V
12(OH)4O60(OOC(CH2)4COO)8]·
Considering the best catalytic property of 2, several sulfides
with electron donor and acceptor groups, different steric effects
of 4-methoxythioanisole, 4-chlorothioanisole, diphenyl sulfide,
and alkyl thioethers such as dipropyl sulfide and 2-chloroethyl
ethyl sulfide, were chosen as substrates to evaluate further
sulfoxidation catalyzed by 2 under the optimum conditions. As
shown in Table 4, the corresponding obtained products were
analyzed by GC−MS (Figure S6). The introduction of the
electron donor and acceptor groups to MPS results in a slight
decrease of the catalytic oxidation conversion in comparison
with that of MPS (Table 4, entries 1−3); especially, a larger
steric resistance containing a substrate like diphenyl sulfide
exhibits obvious decreases of the conversion and selectivity
(Table 4, entry 4), similar to the reported results.21,48
Subsequently, the alkyl thioethers with less steric hindrance,
such as dipropyl sulfide and 2-chloroethyl ethyl sulfide, were
assigned as substrates; not only were satisfying conversion and
selectivity values comparable to that of MPS, but also less time
(10 min) and a low temperature (40 °C) were required (Table
4, entries 5 and 6). That is to say, the steric hindrance of the
substrate displays an important influence on the conversion of
sulfide to sulfoxide.
IV
17
nH2O,15 and so on but comparable to the aforementioned
example (en)[Cu3(ptz)4(H2O)4][Co2Mo10H4O38]·24H2O.45
Following continual attempts to enhance the selectivity in
the oxidation of MPS to sulfoxide by tuning the above factors,
including the dosage of the catalyst (entry 6), reaction time
(entries 8 and 9), solvent types (ethanol, trichloromethane,
and acetonitrile; entries 12−14), no more satisfying results
were obtained. Obviously, the optimum reaction time and
temperature for oxidizing MPS to sulfoxide with 2 as the
catalyst are 15 min and 50 °C in methanol, and the appropriate
mole ratio of nsubstrate:noxidate:ncatalyst was 1:1.5:0.6%. Besides, the
catalytic performance of 1 for the oxidation of MPS to
sulfoxide was also carried out under the above optimum
condition. The good conversion and selectivity of 98% were
obtained (entry 5), until 100% and 94% with increasing
reaction time to 20 min (entry 9), which are lower than that
using 2 as the catalyst, which may be caused by their different
structures, namely, a 3D framework for 1 but a 2D layer
network for 2, leading to the difference in exposed active
sites.43 The blank experiments without any analysts or oxidants
under optimum conditions were performed, and the effective
oxidation activity of MPS to sulfoxide was not observed.
Although the oxidation reaction can also occur in the presence
of the oxidant alone, the conversion of 20% is very low (entries
10 and 11). So, the excellent oxidation reaction with 1 and 2 as
The reutilization and stability are recognized as significant
factors for excellent heterogeneous catalysts. The recycle
sulfoxidation reactions of MPS to sulfoxide catalyzed by
compounds 1 and 2 were first carried out under optimum
E
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Inorg. Chem. XXXX, XXX, XXX−XXX

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pubs.acs.org/IC
Article
a
Table 4. Results of Selective Oxidation of Various Sulfides to Sulfoxides by Using 2 as the Catalyst
a
b
c
Reaction conditions: substrate, 0.5 mmol; methanol, 2 mL; TBHP, 0.75 mmol; compound 2, 3 μmol (0.6 mol %); 50 °C; 15 min. 25 min. 40
°C, 10 min.
Figure 3. Recycling of compounds 1 (left) and 2 (right) as catalysts for the sulfoxidation of MPS.
conditions for assessing the recyclability of the catalysts. After
the reaction finished, the catalysts filtered from the reaction
mixture were washed using methanol and reused to catalyze
the oxidation of MPS to sulfoxide, revealing that compounds 1
and 2 as the catalysts can be reused for at least three runs
without obvious loss of catalytic performance (Figure 3).
Moreover, the PXRD patterns of the recycled catalysts are
consistent with that simulated by the crystal data (Figure S7),
manifesting that the catalysts are stable after catalysis and
recycling.
strategy for preparing POV-based inorganic−organic hybrids
and exploiting effective heterogeneous catalysts toward the
selective oxidation of sulfides to sulfoxides.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02798.
■
sı
Table of bond lengths and angles, PXRD patterns, FT-
IR spectra, TG curves, structural figures, and MS spectra
(PDF)
CONCLUSIONS
■
Accession Codes
In a word, two inorganic−organic hybrid POVs have been
assembled depending on the in situ ligand transformation of
DTCN to HDTBA, which afforded not only a kind of
[V4O12]4− circle cluster with a new coordination mode in the
3D structure of 1 but also yielded a binuclear
[(DTBA)2V2O4(OH)2]2− vanadium cluster modified directly
by DTBA ligands in the 2D layer of 2. Magnetic investigations
reveal that compound 1 exhibits an antiferromagnetic behavior.
Furthermore, two compounds as heterogeneous catalysts,
especially 2, showed efficient catalysis for the selective
oxidation of sulfides to sulfoxides with excellent recyclability
and reusability. The above findings may provide a potential
CCDC 2027080 and 2027081 contain the supplementary
crystallographic 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 Authors
■
Xiang Wang − Liaoning Professional Technology Innovation
Center of Liaoning Province for Conversion Materials of Solar
F
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Article
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A. Anderson-Type Polyoxometalates Functionalized by Tetrathiaful-
valene Groups: Synthesis, Electrochemical Studies, and NLO
Properties. Inorg. Chem. 2018, 57, 3742−3752.
Cell, College of Chemistry and Materials Engineering, Bohai
University, Jinzhou 121000, P. R. China; orcid.org/0000-
0002-3793-6174; Email: xwang@bhu.edu.cn
Xiu-Li Wang − Liaoning Professional Technology Innovation
Center of Liaoning Province for Conversion Materials of Solar
Cell, College of Chemistry and Materials Engineering, Bohai
University, Jinzhou 121000, P. R. China; orcid.org/0000-
0002-0308-1403; Email: wangxiuli@bhu.edu.cn
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Electrochemical Behaviour of Composite Materials: Monovacant
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Acta 2013, 87, 853−859.
Authors
Tong Zhang − Liaoning Professional Technology Innovation
Center of Liaoning Province for Conversion Materials of
Solar Cell, College of Chemistry and Materials Engineering,
Bohai University, Jinzhou 121000, P. R. China
Yunhui Li − Liaoning Professional Technology Innovation
Center of Liaoning Province for Conversion Materials of
Solar Cell, College of Chemistry and Materials Engineering,
Bohai University, Jinzhou 121000, P. R. China
Jiafeng Lin − Liaoning Professional Technology Innovation
Center of Liaoning Province for Conversion Materials of
Solar Cell, College of Chemistry and Materials Engineering,
Bohai University, Jinzhou 121000, P. R. China
Huan Li − Liaoning Professional Technology Innovation
Center of Liaoning Province for Conversion Materials of
Solar Cell, College of Chemistry and Materials Engineering,
Bohai University, Jinzhou 121000, P. R. China
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Complete contact information is available at:
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Functionalised Polyoxovanadates. Chem. Soc. Rev. 2015, 44, 8443−
8483.
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Notes
The authors declare no competing financial interest.
(15) Wang, K.; Niu, Y. J.; Zhao, D. Y.; Zhao, Y. X.; Ma, P. T.; Zhang,
ACKNOWLEDGMENTS
■
D. D.; Wang, J. P.; Niu, J. Y. The Polyoxovanadate-Based Carboxylate
The National Natural Science Foundation of China (Grants
21771025, 21971024, 21671025, and 21501013), General
Program Fund for Education Department of Liaoning Province
(LJ2019004), and Liao Ning Revitalization Talents Program
(XLYC1902011) are gratefully acknowledged.
IV
Derivative K6H[VV
V
12(OH)4O60(OOC(CH2)4COO)8]·nH2O:
17
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