Two organic-inorganic hybrid polyoxovanadates as reusable catalysts for Knoevenagel condensation

Article abstract of DOI:10.1039/C8NJ06460A

Two novel organic-inorganic hybrid polyoxovanadates, i.e., [Ni(1-mIM)₄(H₂O)₂][Ni(H₂O)₅]₂V₁₀O₂₈·5.5H₂O (compound 1) and [V(O)(1-vIM)₄]₂

Full text of DOI:10.1039/C8NJ06460A

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Two Organic-Inorganic Hybrid Polyoxovanadates as
Reusable Catalysts for Knoevenagel Condensation
Chunxia Li,a Dandan Zhong,a Xianqiang Huang,*a Guodong Shen,a Qiang Li,a Jiyuan Du,a Qianli Lia,
Received 00th January 20xx,
Accepted 00th January 20xx
Suna Wang,a Jikun Li*b and Jianmin Dou*a
DOI: 10.1039/x0xx00000x
Two novel organic-inorganic hybrid polyoxovanadates, i.e., [Ni(1-mIM)4(H2O)2][Ni(H2O)5]2V10O28·5.5H2O (compound 1) and
[V(O)(1-vIM)4]2V4O12·H2O (compound 2) (1-mIM = 1-methylimidazole; 1-vIM = 1-vinylimidazole) were synthesized under
routine conditions. The two polyoxovanadates hybrids were further unequivocally characterized by usual physicochemical
techniques, such as single crystal X-ray diffraction (SCXRD), fourier transform infrared spectroscopy (FT-IR), powder X-ray
diffraction (PXRD), thermogravimetric analyses (TG) and elemental analyses. SCXRD analyses demonstrate that the three
Ni2+ atoms of compound 1 present the distorted octahedral geometry, and one of Ni2+ atoms is coordinated with four 1-mIM
molecules and two coordinated water molecules, exhibiting a [NiN4O2] binding set. On the other hand, the other two Ni2+
atoms are coordinated with five water molecules and one decavanadates polyanion, presenting a [NiO6] coordination
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environment. The structure of 2 exhibits the central [V4O12]
4- unit attached on opposite sides of two [V(O)(1-vIM)4]2 groups,
in which the V center displays a distorted octahedron configuration. More importantly, two organic-inorganic hybrid
polyoxovanadates further were studied in the Knoevenagel condensation, and found to display efficient heterogeneous
catalytic activities (yield up to 99%) and the catalytic performance of the compound 1 did not decrease after three runs
catalytic cycle.
date, these transition-metal polyoxovanadates hybrids involved with
base catalytic reactions are rather rare. Consequently, the
development of constructing highly effective transition-metal POVs
hybrids catalysts for such transformation remains an attractive
research field.
Introduction
Polyoxometalates (POMs) are early transition-metal-oxygen
clusters with well-defined structure and high anionic charge, and
their unique physical and chemical functionalities can be precisely
tuned by choosing corresponding Mo, W, V, Nb and Ta, etc,
elements, and therefore they are of increasing interest in wide fields
including catalysis, medical science and materials science, etc.1-2 In
the territory of POMs chemistry, polyoxovanadates (POVs) are of
great importance due to their attractive magnetic and electronic
performance, along with their long-promised potential as oxidative
catalysts.3-4 As for pure POVs, they have been generally applied in the
field of oxidation catalysis due to vanadium has strong reversible
redox properties.5-6 In addition, it is generally accepted that the
combination of transition-metal complexes with Lewis bases ligands
and POVs anions play a key role in the synthesis process, which affect
not only the diversity of POVs structure, but also their catalytic
activities in the oxidative and acid-base reactions.7 Recently, some
transition-metal POVs hybrids have been investigated in the catalytic
oxidation systems of organic compounds with molecular oxygen or
tert-butyl hydroperoxide as the oxidant.8-10 However, as reported to
Knoevenagel condensation of aromatic carbonyl compounds
with active methylene derivatives, as one of the significant reactions
for carbon-carbon double bonds formation, has been widely applied
in the production of pharmaceutical intermediates and fine
chemicals.11-12 In the last few years, the application of Knoevenagel
condensation ranged from polymers to pharmaceutical industry
have been widely reported.13-14 Recently, a large variety of catalysts
have been proven to significantly improve Knoevenagel
condensation, such as zinc(II) organic-inorganic hybrid
[Zn(L)(H2O)2]n·n(DMF) (L = 5-acetamidoisophthalate; DMF = N-
methylformamide),15
tricarboxytriphenyl
[Zn2(TCA)(BIB)2.5](NO3)
amine, BIB
(H3TCA
=
=
1,3-bis(imidazol-1-
ylmethyl)benzene)16 and some Ni coordination supramolecular
complexes17, etc. Thereby, the development of suitable catalysts for
Knoevenagel condensation is still an increasing demand.
Owing to facile tunability at atom level, polyoxometalates might
be ideal catalysts for Knoevenagel condensation. Mizuno reported
highly negatively charged γ-Keggin germanodecatungstate and
TBA8[α-Si2W18O62]·3H2O as efficient Knoevenagel condensation
catalysts, however, these POMs catalysts are homogeneous in the
case.18-19 Compared with homogeneous catalysts, the
heterogeneous catalysts mainly provided significant advantages,
such as succinctly recovery, efficient recyclability, least possible
a. Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell
Technology, School of Chemistry & Chemical Engineering, Liaocheng University,
Liaocheng, Shandong 252059, China. E-mail: hxqqxh2008@163.com
b. College of Chemistry and Chemical Engineering, Taishan University, Tai’an,
271000, P. R. China. E-mail: lijk0212@163.com
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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contamination by the reaction medium and superior stability. anions, i.e. the {[Ni(H2O)5]2[V10O28]}2- subunits in 1 are all connected
Importantly, few POVs demonstrated as fine catalysts for to the adjacent one through various intermolecular O-H···O
Knoevenagel condensation are rare, and thus developing synthetic hydrogen-bonding actions [O3···O8 (-x+1/2, -y+1/2, -z+2): 2.741(7) Å;
methods of transition-metal POVs hybrids for the condensation with O10··· O4 (-x-1, -y, -z): 2.713(7) Å; O5···O11(-x+1/2, -y+1/2, -z+2):
View Article Online
DOI: 10.1039/C8NJ06460A
mild and green methodologies is highly desirable.
2.720(6) Å] (Table S2) forming a 2D network along the ab plane (Fig.
2).
In the present study, and in continuation to develop our
investigation in the base catalytic field of POVs, we controllably Structure Analysis of Compound (2) [V(O)(1-vIM)4]2V4O12·H2O
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synthesized two organic-inorganic polyoxovanadates hybrids [Ni(1-
mIM)4(H2O)2][Ni(H2O)5]2V10O28·5.5H2O (compound 1) and [V(O)(1-
vIM)4]2V4O12·H2O (compound 2) by controlling different solvents,
SCXRD analysis displayed that the asymmetric unit of compound
2-
2 comprises one VO2+ cation, one V2O6 anion and four 1-vIM N-
ligands (Fig. 3). The V atom of VO2+ unit is six-coordinated and
base environment and amount of transition metal salts. The new
exhibited in a distorted VN4O2 octahedral geometry, and the V
organic-inorganic polyoxovanadate hybrids as highly efficient
coordination sphere is completed by four nitrogen atoms (N1, N3, N5
heterogeneous catalysts were further used in the Knoevenagel
and N7) from 1-vIM N-ligands, one bridging oxygen atom (O1) from
condensation of various aromatic aldehydes with malononitrile
VO4 tetrahedron and one terminal oxygen atom (O7) (Fig. 4: Top).
under solvent-free conditions.
Results and discussion
Syntheses
In the structure database of organic-inorganic hybrid vanadates,
there exist a host of fascinating structures. In continuation to explore
and develop the formation of the various architectures of hybrid
vanadates with potential applications, we chose the simple
vanadium source V2O5 or VO(acac)2 as the precursors along with 1-
mIM to react with NiCl2·6H2O. At first, water was used as eco-friendly
solvent in the synthetic procedure, compound 1 was obtained with
1-mIM as the ligand. When we used 1-vIM only to replace water as
the solvent and ligand, remaining V2O5 as the vanadium source, to
react with NiCl2·6H2O, no crystal was obtained due to the poor
solubility of these materials. Then, we further used VO(acac)2 instead
of the NiCl2·6H2O to enhance the vanadium species solubility,
compound 2 was obtained fortunately.
Fig. 1 The crystal structure of compound 1.
Structure
Analysis
of
Compound
(1)
[Ni(1-
mIM)4(H2O)2][Ni(H2O)5]2[V10O28]·5.5H2O
As presented in Fig. 1, the result of SCXRD analysis reveals that
compound 1 contains a typical [V10O28]6-polyanion, two five water-
coordinated [Ni(H2O)5]2+ along with
a six-coordinated [Ni(1-
mIM)4(H2O)2]2+ counteraction part, and the two terminal oxygen
atoms of [V10O28]6- polyanion are coordinated with the two five
water-coordinated Ni2+ cations to form the (Ni(H2O)5)2[V10O28]2- unit.
The [V10O28]6- of 1 has a crystallographic inversion center on the
midpoint of the O12 and O12A and is made up of an ten distorted
VO6 octahedron including with highly condensed ten vanadium
atoms and twenty-eight different types of oxygen atoms. The bond
distances and bond angles of the [V10O28]6- anion (Table S1) are
similar with those of reported [V10O28]6- units.20-22 For the [V10O28]6-
cluster in 1, the terminal oxygen atom O2 and O2A form the bridges
to two outer-shell [Ni(H2O)5]2+ cations. Additionally, the bond
distance (V4-O2) is 1.642(6) Å, which is usual for the vanadyl oxygen
double bonds, however, the bond length (Ni2-O2) is 2.076(6) Å,
which is longer than that of V=O bonds as a result of being the
relatively weak coordination bonds.
Fig. 2 The crystal structure of compound 1.
Fig. 3 The crystal structure of compound 2.
In addition, the supramolecular structure of 1 was stabilized by
intermolecular electrostatic interactions and strong hydrogen-
bonding interactions between [Ni(H2O)5]2+ cations and [V10O28]6-
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IR analysis
View Article Online
DOI: 10.1039/C8NJ06460A
In generally, the IR can afford some valuable information for
investigation of polyoxovanadates. Especially, the 400-1000 cm-1
region is worthy of carefully discriminating, in where M-Ot (terminal
oxygen atoms) and M-Ob (bridging oxygen atoms) that can be found.
The stretching bands of V=Ot in 1 appear at 964 cm−1, 942 cm−1 for 2.
Absorption bands at 836, 742 and 665 cm−1 for compound 1 (Fig. S1),
and 879, 763 and 642 cm−1 for compound 2 (Fig. S2) can be
attributable to the V–O–V or Ni–O–V stretching vibrations.27 The
apparent stretch absorption bands from 1650 cm−1 to 1510 cm−1
region are assigned to C=C and C=N (1-methylimidazole/1-
ethylimidazole ligands).28 Moreover, absorption bands of
coordinated water in the compounds 1-2 could be positioned at
around 3300–3500 cm-1.29
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PXRD analysis
To investigate the repeatability and purity of the two organic-
inorganic hybrid polyoxovanadates, the obtained compounds 1-2
were ground to suitable powder for PXRD analysis. The results of
PXRD detection endorsed us to acknowledge that the two
synthesized samples were single phase materials, respectively. As
shown in Fig. S3-4. These peaks positions of the simulated PXRD
patterns of compounds 1-2 were in well agreement with those of as-
synthesized PXRD patterns, which revealed that the phase purity of
compounds 1-2.
Fig. 4 Top: The coordination environment of V3 atom; Bottom: The
polyhedral schematic diagram of compound 2.
2-
In the asymmetric V2O6 cluster of 2, both V1 and V2 atoms
possess a distorted tetrahedral geometry. The tetrahedron of V(1)O4
is occupied by one terminal oxygen (O2) and three bridging oxygen
(two oxygen atoms (O3, O4) from the two adjacent VO4 tetrahedrons
and one (O1) from the VN4O2 octahedron. The V2 atom possesses a
distorted V(2)O4 tetrahedron and coordinated by two terminal
oxygen (O5, O6), two bridging oxygen (O3, O4) from the two adjacent
VO4 tetrahedrons. In addition, the adjacent VO4 tetrahedrons are
further linked by the corner-sharing oxygen atoms and formed an
Thermogravimetric analyses
Compounds 1-2 were investigated by TG analysis, which was carried
out in a flowing N2 atmosphere with a heating rate of 5 °C min−1 from
30 to 800 °C (Fig. S5-6). For 1, the TG curve shows two-step weight
losses. The first-step weight losses of 11.42 % at 85–271 °C is related
to the loss of coordinated water molecules and free water molecules.
Then, there is sharp weight loss to 800 °C, which means that the
compound’s skeleton begins to collapse. For 2, the skeleton of
compound 2 begins to collapse at the start of heating, indicating poor
thermal stability.
4-
4-
intact V4O12 anion cluster. In the V4O12 anion, the V-O and V=O
4-
bond lengths are comparable with those of reported V4O12
inorganic-organic hybrid vanadates.23-26 As showed the Fig. 4
bottom, it was obvious that the V4O124- subunit coordinate with two
VN4O2 octahedrons by the terminal oxygen atoms on the either side
4-
of the V4O12 unit and formed the final V2N4O2V4O12 skeleton.
Additionally, strong intermolecular hydrogen bonds exist in the
supramolecular structure of compound 2 due to the lattice water
molecules with the distances of 2.815(10) Å and 2.906(11) Å
corresponding to O8···O2 (-x+3/2, y-1/2, -z+1/2) and O8···O5 (x-1/2,
-y+3/2, z-1/2) (Table S2), respectively, which further stabilize the
supramolecular structure of 2 (Fig. 5).
Knoevenagel Condensation Reaction
Initially, benzaldedyde and malononitrile were used as model
substrates to investigate the catalytic performance of the two
catalysts (Scheme 1). In a model experiment, benzaldedyde (0.5
mmol) and malononitrile (0.8 mmol) along with the compound 1 (0.1
mol%) were mixed uniformly without solvent in a flask and reacted
at 40 °C for 1 h, achieving only a product 2-benzylidenemalononitrile
in 99% yield (Fig. 6g). Whereas, the blank experiment indicates that
only trace conversion was achieved in the absence of catalysts (Fig.
6a). To evaluate the catalytic performance of cations and anion of Ni-
Bond Valence Sum (BVS) calculations of compound 2 show the
oxidation states of six V atoms are mixed-valence VIV/VV(four
vanadium atoms in the V4O124- unit are VV and the rest two are VIV),
which also be similar with the reported compounds (Table S3).5
POVs on the Knoevenagel conversion,
a series of designed
experiments were carried out. The results presented that poor yields
of 2-benzylidenemalononitrile was achieved with NiCl2·6H2O (Fig.
6b). Although 1-methylimidazole as homogeneous catalyst can
produce 2-benzylidenemalononitrile (Fig. 6e), the yield (34.6%) is
much lower than those of compounds 1-2. When the Knoevenagel
reaction was completed using the mixture of NiCl2·6H2O, V2O5 and 1-
methylimidazole as catalyst under the same conditions, the yield was
47% (Fig. 6d). Those control experiments obviously suggested that
Fig. 5 The hydrogen bonds interaction of compound 2.
compound
1
played an important role in the Knoevenagel
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condensation. Indeed, catalytic performance of compound 1 in the
Knoevenagel condensation was superior to many reported
heterogeneous catalysts, i.e. Ni-induced fabrication of ZIF-8
nanoframes,30 interface-active ZIF-8,31 etc.
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DOI: 10.1039/C8NJ06460A
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Additionally, the catalytic recycle of the catalyst was further
investigated and compound 1 was simply filtrated after the reaction
was completed and then washed by methanol; the recovered
compound 1 can be used for the successive three runs recycles.
Remarkably, the catalytic performance of compound 1 was kept at
high level even after three recycles [Yields of products: 99% (First
run), 97% (Second run), 98 % (Third run)] (Fig. 7) in the Knoevenagel
condensation. Importantly, the recovered compound 1 remained its
structural integrity after three runs recycles and they were confirmed
by the PXRD patterns (Fig. 8).
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Fig. 8 The PXRD patterns of compound 1 after three runs recycles
In the case, to explore the substrate scope and generality of
aromatic aldehyde, various substituted benzaldehyde derivatives
were expanded under optimal conditions and the corresponding
results were shown in Table 1. First, various functional groups with
bearing electron-withdrawing groups and on the para-position of
the benzaldehyde were scanned (-Cl, -Br, -NO2). Para-chloro-,
bromo- or nitro- substituted benzaldehyde lead to the desired
products (Table 1, entries 2-4) in similar yield as that of entry 1. The
yields with halogen on the para-position of benzaldehyde are slightly
decreased from 96.4 to 98.3% by changing from methyl- to methoxyl-
(Table 1, entries 5-6). In addition, sterically hindered 2-
bromobenzaldehyde also gives the 96.2% yield (Table 1, entry 7).
Based on the above results, it was obvious that highly selective and
efficient Knoevenagel condensation of various substituted aldehydes
Scheme 1. Knoevenagel condensation of benzaldedyde and
malononitrile.
can
be
realized
by
applying
[Ni(1-
mIM)4(H2O)2][Ni(H2O)5]2V10O28·5.5H2O as catalyst and the results
were similar with those of in the reported literature.32
Table 1. Compound 1-Catalyzed Knoevenagel Condensation [a]
Fig. 6 Knoevenagel condensation of benzaldehyde with malononitrile
with different catalysts. Reaction conditions: benzaldehyde (0.5
mmol), malononitrile (0.8 mmol), catalysts (0.1 mol%), 40 °C, 1 h; (a)
Blank; (b) NiCl2·6H2O; (c) V2O5; (d) NiCl2·6H2O and 1-mIM; (e) 1-mIM;
(f) NiCl2·6H2O,V2O5 and 1-mIM; (g) compound 1; (h) compound 1 (0.1
mol%) (h) compound 1 (0.05 mol%); (h) compound 1 (0.075 mol%);
(h) compound 2.
Entry
1
Substrates
Products
Yieldb(%)
>99
Fig. 7 The reaction results of Knoevenagel condensation catalyzed by
2
>99
compound
1 in recycle experiments. Reaction conditions:
benzaldehyde (0.5 mmol), malononitrile (0.8 mmol), compound 1
(0.1 mol%), 40 °C, 1 h.
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Z
2
2
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DOI: 10.1039/C8NJ06460A
3
4
5
6
7
>99
Dcalc.(g cm-3)
2.172
1.077
F(000)
1744
1320
R1[I>2σ(I)]
0.0713
0.1905
0.0908
0.2063
0.993
0.0448
0.1050
0.0869
0.1267
1.008
>99
wR2[I>2σ(I)]
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R1(all data)
wR2(all data)
98.3
GOOF
CCDC No.
1875917
1875916
96.4
96.2
[a] Reaction performed at 40 °C under solvent-free conditions, 0.5
mmol aldehyde, 0.8mmol malononitrile, 0.1 mol% compound 1,
1h; [b] Yield of products were determined by GC.
Table 2.Crystallographic data for compounds 1 and 2
Compound 1
C16H57N8Ni3O45.5V10
1775.22
Compound 2
C40H50N16O15V6
1300.60
Scheme 2. A proposed plausible mechanism for the reaction of
Formula
Mr
benzaldehyde with malononitrile
Although the detailed mechanism of this Knoevenagel
condensation catalyzed by Ni-POVs remains to be elucidated, a
tentative mechanism on the basis of reported work is proposed and
Crystal system
Space group
Temperature
a (Å)
Monoclinic
C2/m
Monoclinic
P2(1)/n
presented in Scheme 2.17,
In our coordination Ni-POVs, first,
33
dissociation of coordinated water molecules or imidazole molecules
of Ni-POVs occurs under reaction conditions, the carbonyl group of
benzaldehyde interacts with the active nickel center, achieving a
polarization of the carbonyl group and improving the electrophilic
property of the carboxylic carbon atom, which was favored the
nucleophilic attack from the malononitrile. Meanwhile, the
interaction of nitrogen atom of cyano group with nickel increases the
acidity of the methylene group, which increases the deprotonation
of methylene group of malononitrile. As a consequence, the basic
sites (carboxylate-O) can abstract the proton from the methylenic
group to generate the corresponding nucleophilic species and the
species further attacks the carbonyl group of benzaldehyde, and
forms the C−C bond via aldol condensation. Next, aldol condensation
intermediate is dehydrated to form the Knoevenagel products D and
reacts with guest molecules to form A again.34
298(2) K
298(2) K
19.5903(17)
9.3317(8)
15.8463(12)
90
14.4908(13)
12.5546(12)
15.4384(14)
90
b (Å)
c (Å)
α (deg)
β (deg)
110.430(2)
90
106.662(2)
90
γ (deg)
V (Å3)
2714.7(4)
2690.7(4)
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Catalytic Knoevenagel condensation of aromatic aVldieewhAyrtdicelesOanlninde
malononitrile by organic-inorganic polyoxoDvaOnI:a1d0a.1t0e3s9/C8NJ06460A
Experimental
Materials and reagents
The catalytic reaction was conducted in a 10 mL reaction tube
using aromatic malononitrile (0.8 mmol), aldehydes (0.5 mmol) and
catalyst (0.1 mol%). The reaction mixture was slowly heated to 40 °C
in a Wattecs Parallel Reactor for 1 h. When the reaction was finished,
catalyst was simply retrieved by filtration, washed with methanol (ca.
3 * 5 mL), and the reaction mixtures were analyzed using an
Shimadzu 2014C GC equipped with a flame ionization detector. All
the products were analyzed over Bruker 500 MHz spectrometer.
V2O5, VO(acac)2, 1-vinylimidazole, 1-methylimidazole, 25
tetramethylammonium hydroxide, 25
hydroxide, nickel(II) chloride hexahydrate and other solvents and
reagents purchased from Aladdin Co., Ltd.
%
% tetraethylammonium
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The FT-IR spectra of compounds 1-2 were recorded on Nicolet 170
SXFT/IR spectrometer in the range 4000–400 cm-1. PXRD data was
obtained by using a Rigaku D/max-2550 diffractometer with Cu-Kα
radiation. After the each catalytic reaction was completed, the result
of product was evaluated by GC (Shimadzu GC-2014C) and GC-MS
(Agilent 7890A-5975C) using naphthalene as internal standard
substrate. The elemental analyses measurements were done on
Perkin-Elmer 240C elemental analyzer. H and 13C NMR spectra were
measured on Bruker 500 MHz spectrometer by using
tetramethylsilane (TMS) as the internal standard.
Conclusions
In summary, we report the facile synthesis of two new organic-
inorganic hybrid polyoxovanadate, by employing various
solvent, imidazole ligand and base environment, etc. These Ni-
POVs with highly catalytic active centers show distinctive
coordination environments with organic ligands. Importantly,
compound 1 demonstrated efficient catalytic activities (up to
yield 99%) in the Knoevenagel condensation under solvent-free
conditions, especially, the novel catalyst was heterogeneous
and can be simply recovered and recycled without losing its
catalytic activity. Work is underway to explore the synthesis of
catalytic active new organic-inorganic polyoxovanadates and
expand this approach to other potential catalytic reactions for
these catalysts.
1
Synthesis
Synthesis of [Ni(1-mIM)4(H2O)2][Ni(H2O)5]2V10O28·5.5H2O (1):V2O5
(2.75 mmol, 500mg) and 25 wt% tetraethylammonium hydroxide
(100 mg) were slowly added to 20 mL water in a clean beaker. The
above solution was stirred intensely at 25 °C for 60 min, and to this
were added nickel(II) chloride hexahydrate (119 mg, 0.5 mmol), 1-
mIM (300 mg, 3.65 mmol), respectively. The reaction mixture of
earthy yellow was stirred for another 24h and followed to heat at 50
°C for 40 min. Finally, the resulting mixture was filtrated and the
resulted filtrate stood for seven days, yellow crystals, suitable for X-
ray diffraction, were obtained. Yield: 60.8%. Anal. Calcd. (found) for
C16H57N8Ni3O45.5V10: C, 10.83 (10.93); H, 3.24 (3.02); N, 6.31 (6.52).
FT-IR spectrum, ν (cm-1): 3338 (s), 3147 (w), 2926 (w), 1637 (s), 1531
(s),1384 (s), 1244 (w), 1114 (w),1098 (w), 964 (s), 836 (s),742 (s), 665
(w),598 (s), 453 (w).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Synthesis of [V(O)(1-vIM)4]2V4O12·H2O (2):V2O5 (2.75 mmol,
500mg) and 25 wt% tetramethylammonium hydroxide (100 mg)
were added to 3 mL 1-vinylimidazole. The resulting mixture of earthy
The work was financially supported by the National Natural
Science Foundation of China (21871125, 21802062, 21671093
21571092 and 21401094) and the Natural Science Foundation
of Shandong Province (ZR2017LB002).
yellow was stirred at ambient temperature for
1 h, next
vanadiumoxyacetylacetonate (67 mg, 0.25 mmol) was added. The
obtained mixture was stirred for another 24h and then heated at 50
°C for 40 min. After the reaction was completed, the mixture was
filtrated and the resulted filtrate stood for seven days, green crystals
suitable for X-ray diffraction were achieved. Yield: 56.5%. Anal. Calcd.
(found) for C40H50N16O15V6: C, 36.94 (36.82); H, 3.88 (4.01); N, 17.23
(17.12). FT-IR spectrum, ν (cm-1): 3421 (O-H, s), 3115 (m), 2929 (w),
1639 (s), 1504(s), 1389 (s), 1224(m), 1096 (s), 942 (s), 879 (w), 763(s),
642(m), 591(m), 508 (m).
Notes and references
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DOI: 10.1039/C8NJ06460A
Graphic Abstract
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Two organic-inorganic hybrid polyoxovanadates as heterogeneous catalyst have been
controllable synthesized and fully characterized. Importantly, compound 1 can
effectively catalyze Knoevenagel condensation reaction (yields up to 99%) under
solvent-free and mild conditions in heterogeneous system and its activity is basically
maintained after three cycles.