Inorganic Chemistry
Article
(7 mL) were sealed in a 25 mL Teflon-lined stainless steel container,
which was heated to 120 °C and the temperature maintained under
autogenous pressure for 3 days. After cooling down to room
temperature, black block crystals were obtained. Yield: 85% (based
on Mo); IR (KBr pellet, v): 3122 (m), 2356 (w), 1658 (m), 1540
(m), 1421 (s), 1283 (w), 1226 (m), 1090 (m), 1046 (s), 930 (s), 893
(s), 767 (m), 661 (m), and 617 (m) cm−1; elemental analysis (%)
calcd for compound 1: Mo 23.28, P 0.94, V 12.36, C 23.32, H 3.21, N
13.60; found: Mo 22.94, P 0.98, V 12.89, C 23.37, H 2.96, N 13.61.
Synthesis of Compound 2. Compound 2 was synthesized
following a procedure similar to that for compound 1 except that 1-
ethylimidazole was used instead of 1-methylimidazole. Yield: 89%
(based on Mo); IR (KBr pellet, v): 3117 (m), 2978 (w), 1671 (m),
1514 (m), 1446 (m), 1401 (w), 1354 (m), 1286 (m), 1226 (s), 1094
(s), 961 (s), 933 (s), 775 (m), 665 (m), 624 (m) cm−1; elemental
analysis (%) calcd for compound 2: Mo 21.22, P 0.86, V 11.27, C
28.22, H 4.04, N 13.17; found: Mo 20.78, P 0.83, V 11.60, C 28.58, H
3.92, N 13.75.
3. RESULTS AND DISCUSSION
3.1. Crystal Structures of Compounds 1−3. As shown
in Scheme 2, compounds 1−3 were prepared by a facile
solvothermal method using H5PMo10V2O40, NH4VO3, VO-
(acac)2, and imidazole as starting materials. During the
formation of three clusters, imidazole derivatives play multiple
roles: solvents, functionalized ligands, and solvates. Control
experiments indicate that changing the length of alkyl chain on
imidazole have no effect on the assembly of POM skeleton.
Considering the structure similarity of compounds 1−3, only
the structure of compound 1 was described here.
̅
Compound 1 crystallizes in the P1 space group of a triclinic
system and consists of one [HPMo8V4O40(VO)2]4− anion, two
[(VO)(1-mIM)4]2+ groups, four lattice H2O, and eight free 1-
mIM molecules. As shown in Figure 1, the
Synthesis of Compound 3. Similarly, compound 3 was synthesized
by using 1-propyl imidazole (7 mL) as solvent. Yield: 86% (based on
Mo); IR (KBr pellet, v): 3118 (m), 2965 (w), 2873 (w), 1611 (w),
1521 (m), 1454 (w), 1401 (w), 1285 (w), 1233 (m), 1094 (s), 1050
(w), 928 (s), 760 (m), 658 (m), 618 (w) cm−1; elemental analysis
(%) calcd for compound 3: Mo 23.74, P 0.96, V 12.61, C 26.75, H
3.78, N 10.40; found: Mo 23.88, P 0.95, V 12.88, C 27.32, H 3.49, N
10.26.
Synthesis of Cs3PMo12O40. H3PMo12O40 (0.2 mmol) was dissolved
in 20 mL deionized water, and then Cs2CO3 (0.3 mmol) was added
with stirring and yellow precipitate was formed immediately. After 5
min, the yellow precipitate was collected by centrifugation, washed
with deionized water, and then dried at 40 °C for 24 h.
Synthesis of Cs5PMo10V2O40. Cs5PMo10V2O40 was prepared by a
similar method to that of Cs3PMo10O40 except that H5PMo10V2O40
was used instead of H3PMo12O40.
Figure 1. (a) Ball-and-stick representation of compound 1; (b)
polyhedra of {Mo4}; (c) polyhedra of {PMo8}; (d) structure of
pseudo-Keggin {PMo8V4(VO)2}; (e) coordination environment of
imidazole-functionalized V center.
2.3. X-ray Crystallography. The crystallographic data for 1−3
were obtained via Bruker APEX 2 DUO CCD single-crystal
diffractometer equipped with a sealed Mo tube and a graphite
monochromator (λ = 0.71073 Å). The software used for structure
solution and refinement was the SHELEXTL program package
(Bruker), and all structures were solved by direct methods and refined
by the full-matrix least-squared method.30−32 All heavy atoms were
refined with anisotropic thermal parameters. The crystallographic data
for 1−3 have been archived in the Cambridge Crystallographic Data
Center (CCDC), and the corresponding deposition numbers are
details of 1−3 are summarized in Table S3, and the selected bond
length and bond angles are given in Table S5. The final molecular
formulas of compounds 1−3 were defined by single-crystal X-ray
diffraction data combined with elemental analysis and TGA results.
2.4. Selective Oxidation of HMF to DFF. Before the reaction,
the sample of catalyst was ground and dried under vacuum at 100 °C
for 4 h. In a typical reaction, HMF (15 mg, 0.11 mmol), compound 1
(33 mg, 0.01 mmol) and toluene (1 mL) were added into a Shrek
tube equipped with an O2 balloon, which was heated at 100 °C for 4
h. After the reaction, the catalyst was separated by filtration and the
resultant solution was analyzed by GC. For the recycle test, the
collected catalyst was washed with ethyl acetate three times, and dried
under vacuum at 60 °C for 4 h and then used for the next cycle.
The oxidation of HMF under anaerobic and aerobic conditions was
performed as the following procedure. First, HMF (0.11 mmol),
compound 1 (0.01 mmol), toluene (1 mL), and naphthalene (48.8
μmol, internal standard) were added into a Shrek tube and the reactor
was cooled to 0 °C by an ice bath. Then, the air in reactor was
replaced by N2 via Schlenk Line and the Shrek tube was heated to 100
°C. After 2 h, O2 was purged into the Shrek tube by a O2 balloon and
the reaction was performed under otherwise identical conditions for
an additional 4 h. The catalytic data in this experiment are from 15
parallel experiments.
[HPMo8V4O40(VO)2]4− anion can be described as a pseudo-
Keggin {PMo8V4} cluster capped by two {VO} groups on the
square windows of {Mo4}. The pseudo-Keggin {PMo8V4O40}
cluster can be viewed as a combination of {PMo8} fragment
and four substituted {VO5} square pyramids. In this cluster,
four {MoO6} octahedra form a {Mo4} subunit (Figure 1b) by
sharing edge and corner O atoms, and two {Mo4} are
connected by a P atom to form {PMo8} (Figure 1c). Each Mo
center exhibits octahedral geometry with Mo−O distances in
the range of 1.660(7)−2.514(12) Å. The central P atom is
surrounded by eight half-occupied oxygen atoms. The {PMo8}
in our case can be derived from the tetravacant α-Keggin
cluster by a 90 °C rotation of one {Mo4} (Figure S1).
Interestingly, there are three types of V centers in compound
1: substituted V in pseudo-Keggin {PMo8V4} cluster, capped
{VO}, and organic-functionalized {(VO)(IM)4}. Four sub-
stituted V atoms lie on the equatorial plane of the pseudo-
Keggin cluster sandwiched by two {Mo4} subunits (Figure 1d).
Both the substituted and capped V centers are penta-
coordinated square pyramids. The substituted V atoms are
coordinated by four μ2-O (VO: 1.578(6)−1.924(8) Å) and
one terminal O atom (VOt: 1.654(6) Å), while the capped
V atoms are coordinated by four μ3-O atoms (VO:
1.654(6)−1.914(7) Å) and one μ2-O atom (VO:
1.663(6)−1.914(7) Å). The organic-functionalized V centers
are anchored on V caps by sharing corner O atoms with V
OV bond angles in the range of 178.7(4)−175.8(4)°. Two
functionized V are coordinated by four N atoms from 1-mIM
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Inorg. Chem. 2021, 60, 3909−3916