Ionothermal Synthesis of an Antimonomolybdate Cluster, [Sb8MoVI13MoV5O66]5-, and Its Catalytic Behavior to the Reduction of Nitrobenzene

Article abstract of DOI:10.1021/acs.inorgchem.0c00825

The largest antimonomolybdate monomer, [Sb8MoVI13MoV5O66]5- (1-Sb8Mo18), has been isolated and displays a new breakthrough of polyoxometalates (POMs) with an ionothermal synthesis strategy. 1-Sb8Mo18 features the first hexanuclear sandwich-Type polymolybdate (POMo) with an unexpected metal ring {Sb6O12} to make its debut in Sb clusters. Furthermore, 1-Sb8Mo18 exhibits a prominent catalytic activity for reducing nitrobenzene to aniline with excellent sustainability.

Full text of DOI:10.1021/acs.inorgchem.0c00825

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Communication
Ionothermal Synthesis of an Antimonomolybdate Cluster,
[Sb8MoVI MoV O66]5−, and Its Catalytic Behavior to the Reduction of
13
5
Nitrobenzene
Shu-Kui Shi, Xin Li, Hui-li Guo, Yan-hua Fan, Hai-yan Li, Dong-Bin Dang,* and Yan Bai*
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ABSTRACT: The largest antimonomolybdate monomer, [Sb8MoVI13MoV O66]5− (1-Sb8Mo18), has been isolated and displays a
5
new breakthrough of polyoxometalates (POMs) with an ionothermal synthesis strategy. 1-Sb8Mo18 features the first hexanuclear
sandwich-type polymolybdate (POMo) with an unexpected metal ring {Sb6O12} to make its debut in Sb clusters. Furthermore, 1-
Sb8Mo18 exhibits a prominent catalytic activity for reducing nitrobenzene to aniline with excellent sustainability.
olyoxometalates (POMs) are discrete polyatomic ion
ionothermal method has been extended to the preparation of
Pclusters formed by O and early-transition-metal atoms in new POMs with novel structure, and some exciting results have
their high oxidation states.1−3 As a large and rapidly growing
been reported, for example, the high-nuclearity Fe-substituted
POM [WFe9O13(SiW9O34)3]23−, a high-nuclearity Ag70 shell
series of inorganic compounds, POMs have attracted great
9 −
encapsulating two lacunary [PW9 O3 4
]
cores
attention and been widely studied because of their unmatched
range of structural features, sizes, and chemical and physical
properties, which applies to diverse areas such as catalysis,
medicine, functional materials, etc.4−7 As an important branch
of POMs, lacunary POMs have a well-defined structure and
high reactive vacant sites that provide a lot of decent
opportunities to combine with organic ligands or metal
species.8−10 Therefore, lacunary POMs are considered to be
remarkable multidentate building blocks to construct POM-
based polynuclear clusters. According to this approach, a great
number of attractive POMs based on lacunary polyoxotung-
states (POTs) have been reported, but for lacunary
polymolybdates (POMos), there are only a few examples.
First, the separation of lacunary POMos is very difficult in the
aqueous solution because of their quite intricate equilibria
presence. Second, their extreme mutability in water can easily
lead to saturation or decomposition by a slight change of the
environment.11−13 Very recently, Suzuki and co-workers
utilized organic media to hold the stability of lacunary
POMo [A-α-PMo9O34]9− and produce three novel POM−
organic structures, [A-α-PMo9O31(py)3]3−, [(A-α-
PMo9O31)2(bpy)3]6−, and [(PMo9O31)4(tpyp)2]12− (py =
pyridine, bpy = 4,4′-bipyridine, and tpyp = 5,10,15,20-tetra-
4-pyridylporphyrin).14 This finding strongly confirms that
introducing a nonaqueous solvent to the reaction system
would be a facile strategy to stabilize lacunary POMos.
[Ag70(PW9O34)2(tBuCC)44(H2O)2]8+, and the quantum-
spin liquid candidate [V7O6F18]3−.22−24
Herein, using the ionothermal synthesis strategy, we
successfully prepare an unprecedented antimonomolybdate
structure, [EMIm]5[Sb8MoVI13MoV O66] (1-Sb8Mo18), with
5
the highest Sb/Mo ratio. The structure also represents a
breakthrough of polynuclear clusters based on lacunary
POMos, for there are only mono-, di-, and tetranuclear-
substituted sandwich-type POMo structures that have been
reported before (summarized in Table S2). It is worth
mentioning that the ring-shaped substitute unit [Sb6O12] also
has not been observed in Sb-containing compounds to the best
of our knowledge. It is necessary to emphasize that the self-
assembly process is very sensitive to the solvent environment.
The pure IL environment could give the only product of 1-
Sb8Mo18, while the existence of water in the system would
result in the generation of the other two products of
[EMIm]4[SiMo12Sb2O40] (2-SiMo12Sb2) and K[EMIm]3[β-
Mo8O26] (3-Mo8), respectively (Experimental Section).
A detailed crystallographic analysis reveals that the anion is
constituted by two trivacant Keggin {B-α-SbMo9O33} units
and a ringlike [Sb6O12]6− cluster (Figure 1a), which is
isostructural with the stannotungstate cluster
[Sn8W18O66]8−.25 The trivacant lacunary {B-α-SbMo9O33}
unit is a derivative of the Keggin structure by the removal of
three edge-sharing MoO6 octahedra. In each of the {B-α-
Ionic liquids (ILs) have been regarded as an ideal alternative
to the traditional molecular water or volatile organic solvents in
the process of synthesis because of their ion’s nature and
unique advantages such as negligible vapor pressure and high
chemical and thermal stability.15−19 The application of ILs in
inorganic synthesis, especially ionothermal synthesis, has been
rapidly developed since ILs were used to synthesize zeolitic
solids by Morris in 2004,20,21 In recent years, such an
Received: March 20, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c00825
© XXXX American Chemical Society
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Figure S13, the peaks of Mo 3d observed at 232.6 and 235.8
eV and 231.8 and 234.9 eV are attributed to those of the Mo
oxidation states for MoVI and MoV, respectively.35,36 The ratio
of MoV/MoVI directly obtained from the peak area is
approximately 5:13 (Table S8), drawing a conclusion that
there are 5 MoV and 13 MoVI atoms in the structure.
Furthermore, the status of the delocalized electrons in 1a has
also been studied by the solid-state magnetic measurements.
As shown in Figure S12, the effective magnetic moment of 1.75
μB for 1-Sb8Mo18 at room temperature is close enough for one
spin-only MoV ion (1.73 μB), indicating that only one of the
delocalized electrons is unpaired in the structure, while the
other four electrons are paired. Such a magnetic behavior has
been observed in other reduced POMs with an odd number of
MoV atoms.37−39
The direct reduction of aromatic nitro compounds is one of
the principal methods to obtain aromatic amines, and much
effort has been devoted to developing efficient catalysts with
cost-effectiveness for the reduction.40−44 Reductions catalyzed
by Lewis acid/base and ILs have been proven in previous
works.45−47 Very recently, Xu’s group employed mixed-valence
POMo as the catalyst to reduce the aromatic nitro compound
and receive an unexpected result of high catalytic perform-
ance.29,48,49 In view of the structure made up of a Lewis base
anion of a mixed-valence antimonomolybdate cluster and IL
cations of EMIm, an excellent catalytic activity to the reduction
should be anticipated for 1-Sb8Mo18.
The experiments of nitrobenzene reduction were employed
with a 1:5 mole ratio of ArNO2 and N2H4·H2O in C2H5OH at
80 °C in a heterogeneous manner. After 2 h, almost 100%
conversion of nitrobenzene to aniline was observed when 0.17
mol % 1-Sb8Mo18 was loaded as the catalyst (Table S10). In
the blank experiments in which the catalyst 1-Sb8Mo18 or
hydrazine was absent, almost no aniline or other intermediates
were detected. In addition, termination of the reduction after
moving out the catalyst at 40 min confirms a heterogeneous
catalytic behavior of 1-Sb8Mo18 (Figure S15). The optimum
catalytic condition was systematically researched by the
catalyst-dependent catalytic activity and solvent-dependent
catalytic performance (Table S12). The results show that the
optimum catalytic condition would be regarded as a catalyst
loading of 0.17 mol % in an alcohol solution. To the best of
our knowledge, this catalyst loading is lower than most of the
nitrobenzene reductions, which use N2H4·H2O as the hydro-
gen source (Table S9), showing the cost-effectiveness of such a
catalyst. Furthermore, various functionalized nitroarenes, such
as chloro-, bromo-, and iodo-substituted nitrobenzenes, 4-
nitrotoluene, and 2-nitrofluorene, were used to explore the
general applicability of the catalyst 1-Sb8Mo18. As shown in
Table S11, the excellent isolated yields of anticipated anilines
indicate that 1-Sb8Mo18 is a promising catalyst for nitro-
benzene reduction.
Figure 1. (a) Ball-and-stick representation of 1a (right) and (b) the
top view of the ring unit of [Sb6O12]6− (left). Average distances:
1.67(4) Å for MoOt (terminal oxygen), 1.92(3) Å for Mo−μ2-O
(doubly oxygen), 2.09(5) Å for Mo−μ3-O (triply oxygen), and 2.28 Å
for Mo−μ4-O (quadruply oxygen). Average Sb−O distances: 1.98(2)
and 2.09(2) Å for {SbO3} and {SbO4}, respectively.
SbMo9O33} hemispherical units, a crystallographically inde-
pendent Sb atom (Sb1 or Sb2) is centered in it and
coordinated by three O atoms to form the {SbO3} unit with
a distorted pyramidal geometry, which is centered in a
trivacant lacunary Keggin hemispherical unit and formed a
{B-α-SbMo9O33} unit. So, the bare lone electron pair of the Sb
atom point to the opposite position of the hemispherical unit
and effectively restrict the extra octahedron MoO6 to join into
the lacunary unit. As far as we know, this is the third case about
Sb-centered POMo, which is much rarer compared with that of
the common species POT.26−29 Two {SbMo9O33} units,
spinning approximately 60°, hold together in a face-to-face
mode by the bridge connection of six crystallographically
independent Sb atoms (from Sb3 to Sb8) to form the largest
antimonomolybdate cluster [Sb8Mo18O66]5− (1a) up to now.
For the six Sb atoms, they connect each other one by one by
the μ3-O bridges to result in a ringlike [Sb6O12]6− unit (Figure
1b), which has not been reported before in the Sb-containing
compounds. The distances of Mo−O and Sb−O are all located
in the range of those found in other related POMs.30−33 It is
interesting to note that the six Sb atoms almost lie in a plane
with a very small mean deviation of 0.03(5) Å, and the
adjacent Sb···Sb distance and Sb···Sb···Sb angle are about
3.40(3) Å and 120°, respectively. Further analysis shows that
[EMIm] cations are very important for enhancing the stability
of the structure because they not only balance the charge of 1a
as counterions but also form numerous C−H···O hydrogen
bonds with O atoms from anion clusters (Table S6).
A total of five [EMIm]+ counterions contained in one 1-
Sb8Mo18 unit suggest that the charge number of the 1a anion
would be 5−, which is confirmed by the results of charge
conservation, bond-valence-sum calculation,34 and X-ray
photoelectron spectroscopy (XPS) measurement. As shown
in Table S7, the bond valences of the Mo atoms are less than
the value of MoVI except those of Mo3, Mo11, Mo12, and
Mo16. The average value of 5.81 indicates that there are
delocalized electrons located in those atoms. The oxidation
states and chemometry of Mo atoms can be confirmed by the
XPS data in the binding energy regions of Mo 3d. As shown in
As is known, intermediates always are used to analyze the
mechanism of the catalytic reaction. The kinetic curves of the
substances depicted in Figure 2 show that the intermediates of
azoxybenzene and azobenzene can be detected after 20 min of
reaction. Their concentrations were both slowly increased to a
maximum at about 60 min and then faded away in the final
product decrease along the prolonged reaction time. There are
two pathways for the reduction of nitroarenes: direct and
indirect routes (Scheme S1).50−52 The detection of azox-
ybenzene and azobenzene suggests that an indirect route is
adopted in the process. To verify the process, the reduction
B
https://dx.doi.org/10.1021/acs.inorgchem.0c00825
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Communication
In summary, the first member of a hexanuclear sandwich-
type POMo compound (1-Sb8Mo18) containing the largest
antimonomolybdate monomer with a Sb/Mo ratio of 8:18 has
been directly synthesized by an ionothermal method and
structurally characterized by single-crystal X-ray diffraction.
More importantly, 1-Sb8Mo18 can be used as a sustainable
catalyst with high activity to the reduction of nitroarenes. The
kinetic study reveals that the reductions of the direct and
indirect routes coexist simultaneously, while the former
dominates the catalytic reaction process. In future work, we
will explore more novel polynuclear clusters based on lacunary
POMos with an ionothermal method as well as their catalytic
behavior to the reduction of aromatic nitro compounds.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00825.
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Figure 2. Kinetic curves for the reduction of ArNO2 to aniline under
80 °C, catalyst 1-Sb8Mo18 (7.5 mg, 0.17 mol %), and N2 atm.
Experimental details, crystallographic data and some
other tables, and figures of characterizations and catalysis
(PDF)
experiments starting from azobenzene and N-phneylhydroxyl-
amine under the same conditions were carried out. As shown
in Table S11, the yields of 4-aminobenzene are about 99% for
the former and 13% for the latter. A good conversion of
azobenzene (95%) was observed only when the catalyst
loading was increased to 1.7 mol %, indicating that it is not
dominated by such an indirect route. These results suggested
that both the direct and indirect routes should exist in the
reduction process and the direct route might be the major
one.53,54
Accession Codes
CCDC 1978727, 1992938, and 1992939 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
In order to further explore the catalytic behavior of 1-
Sb8Mo18, comparative experiments were carried out under the
same conditions using the related materials as the catalysts
instead of 1-Sb8Mo18. As shown in Table S10, the vastly
different conversions of 100%, 74%, and 16% for 1-Sb8Mo18,
2-SiMo12Sb2, and 3-Mo8 indicate that the strong Lewis basic
sites SbIII in POM play an important role for accelerating the
reduction reaction, which is consistent with the previous
reports.29 K4[β-Mo8O26], [EMIm]Br, the equal proportion
mixture of K4[β-Mo8O26] and [EMIm]Br, and 3-Mo8
exhibited catalytic activities with conversions of 8%, 1%,
18%, and 16%, respectively. The catalysts containing both
[EMIm]+ cations and [β-Mo8O26]4− anions showed better
activity. With the above results, we propose that the good
catalytic activity of 1-Sb8Mo18 should be mainly attributed to
the higher numbers of SbIII sites and their synergistic effect
with [EMIm]+ cations.
Sustainability is a vital indicator of heterogeneous catalysts.
As shown in Figure S16, almost no decrease was detected for
the catalytic activity of 1-Sb8Mo18 after six cycles. Also, powder
X-ray diffraction (PXRD), scanning electron microscopy
(SEM), and energy-dispersive X-ray spectroscopy (EDX) for
the fresh (POM isolated from the IL) and used samples were
measured to investigate the stability of 1-Sb8Mo18. For PXRD,
the main diffraction peaks of the used sample are all well
matched with those of the fresh one (Figure S17). In their
SEM images (Figure S18), the used 1-Sb8Mo18 presents the
same morphology of the micronanoblock as that of the ground
fresh one. More importantly, the EDX data give the same ratio
of Sb/Mo for the fresh and used samples (Table S13). All of
the evidence above suggest that 1-Sb8Mo18 could keep its
integrality during the recycling experiment.
■
Dong-Bin Dang − Henan Key Laboratory of Polyoxometalate
Chemistry, College of Chemistry and Chemical Engineering,
Henan University, Kaifeng, Henan 475004, P. R. China;
orcid.org/0000-0002-7042-2378; Email: dangdb@
henu.edu.cn
Yan Bai − Henan Key Laboratory of Polyoxometalate Chemistry,
College of Chemistry and Chemical Engineering, Henan
University, Kaifeng, Henan 475004, P. R. China;
Email: baiyan@henu.edu.cn
Authors
Shu-Kui Shi − Henan Key Laboratory of Polyoxometalate
Chemistry, College of Chemistry and Chemical Engineering,
Henan University, Kaifeng, Henan 475004, P. R. China
Xin Li − Henan Key Laboratory of Polyoxometalate Chemistry,
College of Chemistry and Chemical Engineering, Henan
University, Kaifeng, Henan 475004, P. R. China; Engineering
Technology Research Center of Henan Province for Solar
Catalysis, College of Chemistry and Pharmaceutical Engineering,
Nanyang Normal University, Nanyang, Henan 473061, P. R.
China
Hui-li Guo − Henan Key Laboratory of Polyoxometalate
Chemistry, College of Chemistry and Chemical Engineering,
Henan University, Kaifeng, Henan 475004, P. R. China
Yan-hua Fan − Henan Key Laboratory of Polyoxometalate
Chemistry, College of Chemistry and Chemical Engineering,
Henan University, Kaifeng, Henan 475004, P. R. China
Hai-yan Li − Henan Key Laboratory of Polyoxometalate
Chemistry, College of Chemistry and Chemical Engineering,
Henan University, Kaifeng, Henan 475004, P. R. China
C
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Complete contact information is available at:
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Notes
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ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Grant 21971055) and the Postdoctoral
Scientific Fund of Henan Province (in 2019, third aid).
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