2D Hybrid Architectures Constructed from Two Kinds of Polyoxovanadates as Efficient Heterogeneous Catalysts for Cyanosilylation and Knoevenagel Condensation

Article abstract of DOI:10.1021/acs.inorgchem.0c00999

The design of heterogeneous catalysts for highly efficient catalysis for cyanosilylation reactions and Knoevenagel condensations is greatly significant, due to the important application of their products in industry. Herein, five hybrid compounds construc

Full text of DOI:10.1021/acs.inorgchem.0c00999

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2D Hybrid Architectures Constructed from Two Kinds of
Polyoxovanadates as Efficient Heterogeneous Catalysts for
Cyanosilylation and Knoevenagel Condensation
Haiyan An,* Jie Zhang, Shenzhen Chang, Yujiao Hou, and Qingshan Zhu
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ABSTRACT: The design of heterogeneous catalysts for highly efficient catalysis
for cyanosilylation reactions and Knoevenagel condensations is greatly significant,
due to the important application of their products in industry. Herein, five hybrid
compounds constructed from two types of polyoxovanadates, [MV12O38]12− (M =
Ni, Mn) and [V10O28]6−, were successfully synthesized as heterogeneous catalysts
for both reactions, which are {(dpdo)[Ln2(H2O)9(dpdo)][Ln(H2O)5]2[Ln-
(H2O)4]2[V10O28][NiV12O38]·nH2O} (1, Ln = La, n = 23; 2, Ln = Ce, n = 27;
3, Ln = Pr, n = 27; dpdo = 4,4′-bipyridine N,N′-dioxide) and {(dpdo)-
[Ln2(H2O)9(dpdo)][Ln(H2O)5]2[Ln(H2O)4]2[V10O28][MnV12O38]·27H2O} (4,
Ln = La; 5, Ln = Pr). These compounds were characterized by IR spectroscopy,
elemental analysis, TG analysis and X-ray diffraction (single crystal and powder)
etc. Compounds 1−5 have similar 2D hybrid structures, in which isopolyvanadate
[V10O28]6− and heteropolyvanadate [MV12O38]12− are linked together by hydrated
Ln3+ cations and Ln-dpdo coordination complexes. Polyoxovanadates [V10O28]6−
and [MV12O38]12− both originate from the transformation of the [MV13O38]7− raw material. This kind of extended structure
containing isopolyvanadate and heteropolyvanadate has never been reported hitherto. Because of the Lewis acidity of the Ln3+ cation
and the Lewis basicity of the polyoxovanadates, all five compounds exhibit excellent catalytic performance in cyanosilylation and
Knoevengel condensation reactions. Especially, compound 3, with a Pr3+ cation, displayed the best catalytic results. Furthermore,
these catalysts exhibit a truly heterogeneous nature and good recyclability.
under mild conditions remains challenging but is highly
desirable.
INTRODUCTION
■
The cyanosilylation reaction of aldehydes/ketones with
trimethylsilyl cyanide (TMSCN) is one of the important
reactions for C−C bond formation, because this reaction
produces versatile cyanohydrins which can be used to produce
α-hydroxy acids, α-hydroxy aldehydes, β-amino alcohols, etc.1,2
Knoevenagel condensation is another crucial reaction for
forming C−C bonds, by adding aldehydes/ketones to active
methylene hydrogen compounds, which has been widely used
in the syntheses of fine chemicals and pharmaceutical
products.3,4 Thus, the exploration of efficient catalysts for
cyanosilylation reactions and Knoevenagel condensations is a
very significant topic. So far, some Lewis acid catalysts which
can activate the carbonyl substrates as electrophilic catalysts5−7
and Lewis base catalysts which can nucleophilically activate
TMSCN or methylene compounds8,9 have been developed for
both reactions. However, the reported catalysts have obvious
drawbacks: (i) homogeneous catalysts with high efficiency are
difficult to recover, (ii) heterogeneous catalysts have low
efficiency and yield, and (iii) high temperature and toxic
solvents are commonly used. Currently, the search for high-
efficiency heterogeneous catalysts which can promote
cyanosilylation reactions and Knoevenagel condensations
Polyoxometalates (POMs) are well-known anionic metal
oxides with Mo, V, W, etc. in their highest oxidation states, and
they have very attractive applications in catalysis owing to their
thermal and oxidative stability in comparison with organo-
catalysts and organometallic catalysts.10−12 A large number of
POMs have been used as redox catalysts for efficiently
catalyzing the oxidation of sulfides, water, and various olefins
or as acid catalysts for the hydrolysis of esters etc.13−17 In
contrast, there have been few examples of POMs catalyzing
cyanosilylation reactions and Knoevenagel condensations up to
now, which mainly focused on polyoxotungstates and
polyoxomolybdates.18−20 By virtue of the pioneering work of
Mizuno, Song, Niu, and other research groups, some lacunary
POMs with high negative charges were reported as Lewis base
Received: April 4, 2020
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catalysts to promote the cyanosilylation reaction or Knoeve-
nagel condensation.9,21−23 However, for these lacunary POMs,
the conversions of both catalytic reactions still need to be
improved. Then, lanthanide (Ln) or transition-metal cations,
with good Lewis acid ability, were combined with poly-
oxoanions to catalyze both reactions. For example, the Ln-
substituted lacunary Keggin POMs [{Y(H2O)2}2(γ-
SiW1 0 O3 6 )2 ]1 0 − and [{Ln(H2 O)2 (acetone)}2 (γ-
SiW10O36)2]10− (Ln = Nd3+, Eu3+, Gd3+, Tb3+, Dy3+, Y3+)
were first found by Mizuno and co-workers as homogeneous
catalysts for efficiently catalyzing the cyanosilylation reaction of
aldehydes/ketones.24,25 A novel hybrid architecture containing
lacunary [H2W11O38]8− linked by Cu-organic fragments,
[Cu2(bpy)(H2O)5.5]2[H2W11O38]·3H2O·0.5CH3CN, was pre-
pared by Niu’s group, showing good performance in
heterogeneously catalyzing the cyanosilylation reaction.22
Our group also reported some hybrid species based on
[Co2Mo10H4O38]6−, [PW10Ti2O40]7−, or [PW11LnO39]4− poly-
oxoanion with lanthanide cations as linkers, which exhibit good
catalytic activity for heterogeneously catalyzing the cyanosily-
lation reaction under solvent-free conditions.26−28 Taking
account of research status of POM-based catalysts, it is
necessary to explore new hybrid materials constructed from
highly negatively charged POMs linked by lanthanide groups,
which can heterogeneously and high-efficiently catalyze both
reactions under mild conditions.
Polyoxovanadates (POVs) seized our attention, not only
because vanadium atom has the flexible coordination
geometry, but also POVs commonly have good catalytic
ability and high negative charges such as isopolyvanadate
([V10O28]6−), and heteropolyvanadate with various central
heteroatoms([PV14O42]9−, [MnV13O38]7−). So far, some
success has been achieved in directly introducing lanthanides
into one kind of POV, including H[Ln(H2O)4]2[MnV13O38]·
9NMP·17H2O (Ln = La3+, Ce3+; NMP = N-methyl-2-
pyrrolidone), H[{Ln2(C6H5NO2)3(H2O)6}{[MnV13O38}]·
C6H5NO2·10H2O (Ln = La3+, Ce3+; C6H5NO2 = isonicotinic
acid), [{Ln(H2O)6}2As8V14O42(SO3)]·8H2O (Ln = La3+, Ce3+,
Sm3+), (TBA)2[Ln(V12O32(Cl))(H2O)2(CH3CN)2],
(Et4N)6[LnV9O27], {[V12O32(Cl)](LnPc)}4− (Pc = phthalo-
cyanine), etc.29−34 Two kinds of POV structural units in one
compound will lead to unique structures, which will produce
interesting catalytic properties. However, there is an enormous
challenge to build compounds containing two kinds of POM
structures, owing to the unclear formation mechanism of
POMs. Until now, very few compounds containing two kinds
of POM structures have been reported, which show excellent
catalytic performances in the decontamination of chemical
warfare agent simulants or photocatalytic hydrogen and oxygen
generation.17,35 To the best of our knowledge, no compounds
containing two kinds of POMs have been used to catalyze the
formation of C−C bonds to date, let alone POVs.
Consequently, the synthesis of compounds containing two
kinds of POVs and lanthanides is especially promising in
catalyzing cyanosilylation reactions and Knoevenagel con-
densations.
{ ( d p d o ) [ L n 2 ( H 2 O ) 9 ( d p d o ) ] [ L n ( H 2 O ) 5 ] 2 [ L n -
(H2O)4]2[V10O28][MnV12O38]·27H2O} (4, Ln = La; 5, Ln =
Pr). In these compounds, [V10O28]6− and [MV12O38]12−
polyoxoanions are joined together by lanthanide cations and
Ln-dpdo coordination fragments to yield a 2D hybrid network.
Interestingly, these compounds display high catalytic activities
for catalyzing the cyanosilylation reaction and Knoevenagel
condensation under mild conditions; in particular, compound
3 showed the best catalytic performance (99.6% yield for
cyanohydrin trimethysilyl ether within 3 h, 99.7% yield for 2-
benzylidenemalononitrile within 0.5 h). Furthermore, these
compounds are indeed heterogeneous catalysts and can retain
their high catalytic activities and stability of their structures
after recycling. To our knowledge, these compounds are the
first POVs that have been applied for cyanosilylation and
Knoevenagel condensation reactions, as well as the first
examples of extended structures containing isopolyvanadate
and heteropolyvanadate.
EXPERIMENTAL SECTION
■
Materials. K7MnV13O38·18H2O, K7NiV13O38·18H2O, and
K4Na2V10O28·10H2O were prepared following the literature meth-
ods36,37 and characterized by IR. All other reagents were purchased
directly from commercial sources and were no longer purified.
Physical Measurements. A PerkinElmer 2400 CHN elemental
analyzer was used to analyze the C, H, and N elements. A PLASMA-
SPEC (I) ICP atomic emission spectrometer was used to analyze the
metal ions of Ni, Mn, V, La, Ce, and Pr. IR spectra were measured on
an Alpha Centaur FT-IR spectrophotometer in a range from 400 to
4000 cm−1 with KBr pellets. PXRD patterns of the samples were
measured on a Rigaku Dmax 2000 X-ray instrument with Cu Kα
radiation (λ = 0.154 nm) with 2θ varying from 5 to 60° at 298 K. A
PerkinElmer TGA instrument was used to measure TG data in
flowing N2 from 30 to 800 °C. A Hitachi U-3900 spectrophotometer
was used to obtain UV−vis spectra at room temperature. The GC
analyses were performed on a Techcomp GC-7900 instrument with a
flame ionization detector (FID) equipped with an SE-54 column
(internal diameter 0.32 mm, length 30 m).
Synthesis. {(dpdo)[La2(H2O)9(dpdo)][La(H2O)5]2[La-
(H2O)4]2[V10O28][NiV12O38]·23H2O} (1). K7NiV13O38·18H2O (0.0577
g, 0.03 mmol) and La(NO3)3 (0.065 g, 0.2 mmol) were added to 10
mL of water in turn. A 1 M HNO3 solution was used to adjust the pH
value of the solution to 2.9. After the solution was heated for 1 h at 60
°C under water bath conditions, a 5 mL aqueous solution of dpdo
(0.1 mmol) was added. The solution was continuously heated for 3 h
at 60 °C. Then, red block crystals were obtained in 2 weeks after
filtration. The crystals were collected, washed with anhydrous ethanol,
and dried in air at room temperature before characterization and
catalytic testing. Yield: 25.7% (based on K7NiV13O38·18H2O). Anal.
Calcd for 1: H, 2.67; C, 5.52; N, 1.29; O, 44.18; Ni, 1.36; V, 25.82;
La, 19.18. Found: H, 2.73; C, 5.39; N, 1.41; O, 44.01; Ni, 1.44; V,
25.54; La, 18.95. FTIR data (cm−1): 3336 (s), 1470 (m), 1424.6 (w),
1223.7 (m), 1178 (m), 1110 (w), 906 (vs), 621.8 (s), 516.9 (w),
442.7 (m).
{(dpdo)[Ce2(H2O)9(dpdo)][Ce(H2O)5]2[Ce(H2O)4]2[V10O28]-
[NiV12O38]·27H2O} (2). The synthesis method of 2 was similar to that
of 1 except that Ce(NO3)3·6H2O (0.0868 g, 0.2 mmol) was used
instead of La(NO3)3 (0.065 g, 0.2 mmol). Red block crystals of 2
were harvested after 3 weeks of growth. Yield: 19.1% (based on
K7NiV13O38·18H2O). Anal. Calcd for 2: H, 2.80; C, 5.42; N, 1.27; O,
43.39; Ni, 1.33; V, 25.35; Ce, 18.98. Found: H, 2.63; C, 5.27; N, 1.46;
O, 43.12; Ni, 1.53; V, 25.22; Ce, 18.70. FTIR data (cm−1): 3340 (s),
1424.3 (w), 1224 (m), 1180 (m), 1111 (w), 908 (vs), 622.4 (s), 515
(w), 445.4 (m).
Herein, we obtain five new hybrid compounds containing
two types of polyoxovanadates, [V10O28]6− and [MV12O38]12−
(M = Ni4+, Mn4+), by carefully adjusting the reaction
conditions of the [MV13O38]7− mother solution: {(dpdo)-
[Ln2(H2O)9(dpdo)][Ln(H2O)5]2[Ln(H2O)4]2[V10O28]-
[NiV12O38]·nH2O} (1, Ln = La, n = 23; 2, Ln = Ce, n = 27; 3,
Ln = Pr, n = 27; dpdo = 4,4′-bipyridine N,N′-dioxide) and
{(dpdo)[Pr2(H2O)9(dpdo)][Pr(H2O)5]2[Pr(H2O)4]2[V10O28][NiV12O38]·
27H2O} (3). The synthesis method of 3 was similar to that of 1 except
that PrCl3 (0.0494 g, 0.2 mmol) was used instead of La(NO3)3 (0.065
g, 0.2 mmol). Red block crystals of 3 were harvested after 2 weeks of
B
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Scheme 1. Schematic Representation of the Formation Processes for Compounds 1−5
growth. Yield: 22.7% (based on K7NiV13O38·18H2O). Anal. Calcd for
3: H, 2.80; C, 5.42; N, 1.26; O, 44.78; Ni, 1.33; V, 25.33; Pr, 19.08.
Found: H, 2.69; C, 5.61; N, 1.47; O, 44.47; Ni, 1.55; V, 25.05; Pr,
18.88. FTIR data (cm−1): 3341.3 (s), 1473 (m), 1424.3 (w), 1224
(m), 1178.8 (m), 1114 (w), 910 (vs), 624 (s), 515.5 (w), 443.1 (m).
{(dpdo)[La2(H2O)9(dpdo)][La(H2O)5]2[La(H2O)4]2[V10O28]-
[MnV12O38]·27H2O} (4). The synthesis method of 4 was similar to that
of 1 except that K7MnV13O38·18H2O (0.0567 g, 0.03 mmol) was used
instead of K7NiV13O38·18H2O (0.0577 g, 0.03 mmol) and the pH
value of the mixture was about 2.7. Red block crystals of 4 were
obtained in 3 weeks after filtration. Yield: 18.4% (based on
K7MnV13O38·18H2O). Anal. Calcd for 4: H, 2.81; C, 5.44; N, 1.27;
O, 44.94; Mn, 1.24; V, 25.42; La, 18.88. Found: H, 2.63; C, 5.67; N,
1.45; O, 44.70; Mn, 1.48; V, 25.24; La, 18.66. FTIR data (cm−1):
3376.9 (s), 1469.5 (m), 1423.6 (w), 1216.7 (m), 1178.8 (m), 1116.8
(w), 912 (vs), 618 (s), 516.7 (w), 439 (m).
{(dpdo)[Pr2(H2O)9(dpdo)][Pr(H2O)5]2[Pr(H2O)4]2[V10O28]-
[MnV12O38]·27H2O} (5). The synthesis method of 5 was similar to that
of 4 except that PrCl3 (0.0494 g, 0.2 mmol) was used instead of
La(NO3)3 (0.065 g, 0.2 mmol) and the pH of the solution was 2.7.
Red block crystals of 5 were harvested after 3 weeks of growth. Yield:
19.6% (based on K7MnV13O38·18H2O). Anal. Calcd for 5: H, 2.80; C,
5.42; N, 1.27; O, 44.82; Mn, 1.24; V, 25.35; Pr, 19.10. Found: H,
2.44; C, 5.57; N, 1.39; O, 44.62; Mn, 1.41; V, 25.20; Pr, 18.90. FTIR
data (cm−1): 3400 (s), 1471.5 (m), 1424.6 (w), 1223.7 (m), 1180.1
(m), 1112.2 (w), 912.5 (vs), 621 (s), 516.5 (w), 440.4 (m).
X-ray Crystallography. The crystallographic data of compounds
1−5 were determined using a Bruker Smart CCD diffractometer by ω
and θ scan patterns with Mo Kα radiation (λ = 0.71073). The full-
matrix least-squares method on F2 using SHELXL-2014 software was
used to solve all structures.38,39 In compounds 1−5, except for the
partial water molecules of crystallization, all non-hydrogen atoms were
anisotropically refined. There is half of a disordered dpdo ligand in all
five compounds. In the disordered dpdo ligand, all carbon atoms
(C6−C15) and nitrogen atoms (N2, N3) are half-occupied. In 1−5,
occupancy factors of some water molecules of crystallization were set
to be less than 1 owing to disorder. Other restraints including “DFIX”
and “ISOR” were used in order to obtain reasonable atomic positions
and thermal parameters in the refinements. The crystallographic data
and structural determination details for 1−5 are provided in Table S1.
CCDC reference numbers: 1994401−1994405 for 1−5.
General Procedure for Cyanosilylation Reactions. A typical
method for the cyanosilylation of an aldehyde compound is as
follows: the substrate (0.5 mmol) and 1.0 mol % of the catalyst 1−5
(5 μmol) were added to trimethylsilyl cyanide (TMSCN) (1.5 mmol)
in a 1.5 mL resin vial without solvent and were stirred under 1 atm
and 298 K. The conversion of the product was determined by GC
analysis. After the reaction was complete, the catalyst was removed by
filtration and the reaction mixture was centrifuged. All products
(cyanohydrin trimethylsilyl ether) were confirmed by comparison of
GC retention times and GC/MS spectra with real data. Naphthalene
was the internal standard in the reaction process.
General Procedure for Knoevenagel Condensation. A typical
method for the Knoevenagel condensation of an aldehyde compound
is as follows: the substrate (0.5 mmol), 0.6 mol % of the catalyst (3
μmol), and DMF (0.2 mL) were placed in a resin vial in turn and the
reaction was started after adding malononitrile (1 mmol) to the
mixture. The structure of each component in the reaction system was
determined by GC/MS, and their contents were determined by the
mass concentration given by GC with naphthalene as a reference
standard. At the end of the reaction, the catalysts were recovered by
filtering, washing, and drying.
RESULTS AND DISCUSSION
■
Synthesis. The reactivity of K7MV13O38·18H2O (M = Ni4+,
Mn4+) and lanthanide cations in the presence of the dpdo
ligand has been investigated. The five isomorphic hybrid
frameworks 1−5 comprised of [V10O28]6− and [MV12O38]12−
polyoxoanions linked by hydrated Ln3+ cations and Ln-dpdo
coordination complexes are formed (see Scheme 1). They
represent the very rare examples of two kinds of polyoxoanions
in one structure. In these compounds, isopolyvanadate
[V10O28]6− and heteropolyvanadate [MV12O38]12− come from
the transformation of the raw material K7MV13O38·18H2O (M
= Ni4+, Mn4+). Numerous parallel experiments demonstrate
that the reaction temperature, pH value, heating time, metal
cations, and organic ligand are key synthetic parameters for the
isolation of compounds 1−5. First, the optimal synthetic
temperature for compounds 1−5 is 60 °C, meaning that the
coexistence of [MnV12O38]12− and [V10O28]6− only occurs at
about 60 °C. If the temperature is 80 or 40 °C, no crystals
were obtained. The group of Lu and Wang group has reported
some compounds containing [MnV13O38]7− and Ln3+/Ln-
organic coordination complexes, which can be obtained at a
temperature lower than 45 °C.40 Liu’s group has synthesized
five 2D inorganic structures constructed by the two kinds of
polyoxovanadates [MV12O38]12− (M = Ni4+, Mn4+) and
[MV13O38]7− and hydrated Ln3+ (Ln = La, Ce, Pr), which
were obtained at 80 °C.41 The above experimental results
reveal that the reaction temperature is a very important factor
to determine if the [MV13O38]7− polyoxoanion can be
transformed. Second, the quality of the crystals is obviously
affected by the pH values of the reaction solution. In the range
of pH 2.5−3.0, block crystals with proper size, regular shape,
and high yield could be obtained. If the pH value is higher than
3.0, needlelike crystals that could not be tested by single-crystal
C
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Figure 1. (a) ORTEP drawing of compound 1 with thermal ellipsoids at 50% probability. (b) Ball and stick representation of the [NiV12O38]12−
polyoxoanion. (c) Ball and stick representation of the [V10O28]6− polyoxoanion.
X-ray diffraction will be produced. Third, the effect of heating
time on the product structure has been studied. When the
heating time is shortened from 4 to 2 h, a new 2D hybrid
species (compound 6) constructed from only the
[MnV13O38]7− anion and La-dpdo coordination units was
synthesized (see Figure S7). This experimental fact shows that
the structural transformation of [MV13O38]7− anion to
[MnV12O38]12− and [V10O28]6− needs a longer heating time.
Fourth, metal cations can affect the structures of final products.
When transition-metal cations such as Co2+, Ni2+, Cu2+, and
Zn2+ were used, only amorphous precipitates could be
obtained. When other lanthanide cations such as Sm3+, Eu3+,
and Gd3+ were added to the system, crystals composed of
[V10O28]6− and lanthanide-organic complexes were produced.
Finally, the organic ligand dpdo also plays an important role in
inducing the successful synthesis of hybrid compounds with
two kinds of polyoxovanadates. When other O-donor or N-
donor organic ligands such as imidazole, 3-picolinic acid, 4-
picolinic acid, and malonic acid were used, no suitable crystals
were observed. Therefore, the coordination mode and size of
the organic ligand is another key factor in the formation of final
products. Obviously, there is still some challenging work that
needs to be further exploited, on the transformation
mechanism of [MV13O38]7− anion and the related synthetic
chemistry.
polyoxovanadates ([V10O28]6−, [MV12O38]12−) linked by Ln3+
cations and Ln-organic coordination complexes. Herein,
compound 1 is taken as an example to discuss the structure.
The asymmetric unit of 1 is composed of half of a
[NiV12O38]12− anion, half of a [V10O28]6− anion, three
crystallographically independent La3+ ions, and two halves of
a dpdo ligand (Figure 1a). The structure of [NiV12O38]12− with
Oh symmetry is composed of 13 edge-sharing MO6 octahedra
(M = Ni, V), in which Ni4+ is located on the inversion center.
The new type of polyoxoanion [NiV12O38]12− has rarely been
reported to date. Figure 1b shows the structure of the
[NiV12O38]12− polyoxoanion. The Ni−O bond lengths are
from 1.861(8) to 1.877(8) Å, and V−O bond lengths are from
1.631(9) to 2.264(8) Å, respectively. The O−Ni−O angles are
in the normal octahedral value range, whereas the O−V−O
angles range from 71.7(3) to 164.2(4)°, displaying consid-
erable distortion. The [V10O28]6− cluster is comprised of 10
edge-sharing VO6 groups, exhibiting a ball-shaped structure
(shown in Figure 1c). The V−O bond lengths are from
1.597(9) to 2.325(8) Å, and the O−V−O angles are from
74.3(3) to 175.2(4)°, which are similar to those of other
decavanadate clusters reported in the literature.42,43 The
[V10O28]6− polyoxoanion and [NiV12O38]12− anion are linked
together by La3+ cations.
There are three crystallographically independent La3+
cations in 1. All La3+ cations are nine-coordinated and have
a distorted monocapped square-antiprismatic geometry (see
Figure S2). The unique La(1) is linked to four water molecules
and five terminal oxygen atoms from two polyoxovanadates.
La(2) is defined by five water molecules and four terminal
Structural Description. Compounds 1−5 are isomor-
phous, as shown by single-crystal X-ray diffraction analyses.
They have similar unit cell parameters and volumes,
crystallizing in the triclinic space group P1
display interesting 2D structures constructed from two types of
̅
. All five compounds
D
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Figure 2. (a) Polyhedral and ball and stick view of the 1D chain in 1. (b) Polyhedral and ball and stick view of the 2D layer in 1. Color code: La,
yellow; V, purple; Ni, light blue; C, gray; N, dark blue; O, red.
Figure 3. (a) IR spectra for 1 (top), K7NiV13O38·18H2O (middle), and K4Na2V10O28·10H2O (bottom). (b) IR spectra for 4 (top), K7MnV13O38·
18H2O (middle), and K4Na2V10O28·10H2O (bottom).
oxygen atoms from one polyoxoanion. La(3) is linked to four
water molecules, four terminal oxygen atoms from one
polyoxoanion, and one oxygen atom from one dpdo molecule.
The La−O bond lengths range from 2.508(8) to 2.642(8) Å.
There are two half dpdo molecules. The half disordered dpdo
ligand acts as a bidentate ligand linking two adjacent La3+
cations together, and the other dpdo half is an isolated ligand.
In 1, [NiV12O38]12− and [V10O28]6− polyoxoanions are
alternately linked by hydrated La3+ cations to produce a 1D
linear chain (see Figure 2a). Neighboring parallel chains are
connected together by disordered dpdo ligands to form a 2D
layer with large windows (shown in Figure 2b). In the
structure, each [NiV12O38]12− polyoxoanion is connected with
two [V10O28]6− anions by two hydrated La3+ cations and with
two [NiV12O38]12− anions by two [La2(H2O)9(dpdo)]6+ units
and capped by two additional hydrated La3+ cations. Herein,
the new polyoxoanion [NiV12O38]12− is reported for the
second time, which cannot be separated as an isolated cluster
and need to be stabilized by six lanthanide cations surrounding
it (Figure S3), similar to the case in the literature.41 From the
view of topology, each [NiV12O38]12− as a 4-connected node
links to two La3+ cations in the chain and two
[La2(H2O)9(dpdo)]6+ clusters to form the 2D structure.
Thus, this 2D sheet is a common 4-connected 44 net with
[NiV12O38]12− ions as nodes and La3+ cations and
[V10O28]6−units as linkers (see Figure S4). In the 2D structure,
the size of each window is about 17.7 Å × 24.5 Å, which is
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a
Table 1. Cyanosilylation of Various Aldehydes Catalyzed by Compounds 1−5
a
b
Reaction conditions: 0.5 mmol of aldehydes, 1.5 mmol of TMSCN, 1.0 mol % of catalyst, without solvent, at room temperature for 3 h. Yields
c
were determined by GC using naphthalene as an internal standard. Moles of trimethylsilyl product per mole of catalyst.
large enough to accommodate other polyoxoanions to form a
oxidation states of Ni/Mn, V, Ln atoms are +4, +5, and +3,
2-fold interpenetrated 2D network (see Figure S5).
respectively.
The bond valence sum calculations (BVS)44 based on the
FT-IR Spectroscopy. The FT-IR spectra of 1−5 are
bond lengths observed in compounds 1−5 shows that the
displayed in Figure S8a−e. Their FT-IR spectra are very
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similar, which proves that these compounds are isomorphic.
Four characteristic peaks at positions of 906, 621.8, 516.9, and
442.7 cm−1 for 1, 908, 622.4, 515, and 445.4 cm−1 for 2, 910,
624, 516.5, and 443.1 cm−1 for 3, 912, 618, 516.7, and 439
cm−1 for 4, and 912.5, 621, 516.5, and 440.4 cm−1 for 5 are
attributed to the stretching vibrations of νas(VO), νas(V−
O−V), and νas(V−O−Ni/Mn). These characteristic peaks of
polyoxoanions are much different from those of the raw
material [MV13O38]7− (M= Ni, Mn), indicating that they have
different structures.
Cyanosilylation Study. The catalytic performances of
compounds 1−5 as heterogeneous catalysts were studied in the
cyanosilylation reactions of aldehydes with TMSCN under
solvent-free conditions. First, benzaldehyde was chosen as the
model substrate of cyanosilylation to estimate the catalytic
capabilities of all five compounds. The catalytic reaction was
run with benzaldehyde (0.5 mmol), TMSCN (1.5 mmol),
catalyst (5 μmol), and naphthalene (internal standard) at room
temperature (see Table 1). The results indicate that these five
compounds can effectively catalyze the cyanosilylation to give
the corresponding cyanohydrin trimethysilyl ether with 89.4−
99.6% yields within 3 h. Compound 3 with a Pr3+ cation has a
higher catalytic activity in comparison to the other isostructural
compounds. Thus, compound 3 was selected for a further
investigation of the cyanosilylation reaction of various
aldehyde compounds. When the substrate is 4-methylbenzal-
dehyde, 4-fluorobenzaldehyde, or 2-hydroxybenzaldehyde, the
yields of products are all over 99%, indicating that the
substituent group on the aromatic ring has no prominent effect
on the reaction. In general, for a substrate with a large
substituent, such as 1-naphthaldehyde, the steric hindrance
effect often leads to an unsatisfactory catalytic effect. However,
compound 3 exhibited an unusual catalytic ability for the
cyanosilylation reaction of 1-naphthaldehyde, and the yield can
reach 99.4% after 3 h. These results strongly demonstrate that
compound 3 is a highly efficient catalyst for the cyanosilylation
reaction of various substrates. This is the first time that
polyoxovanadates have exhibited remarkable performance as
catalysts in the cyanosilylation reaction.
Some control experiments for the cyanosilylation of
benzaldehyde were performed. Without a catalyst, it is difficult
to initiate the reaction to produce the target product (Table 1).
When the raw material K4Na2V10O28·10H2O, K7[NiV13O38]·
18H2O, or K7[MnV13O38]·18H2O is used as a catalyst, the
yields are respectively 70.3%, 44.2%, and 35.5% at 3 h, which
are far below those of the five compounds. When LaCl3·6H2O,
Ce(NO3)3·6H2O, or PrCl3 is used to catalyze the cyanosily-
lation of benzaldehyde under homogeneous conditions, the
yields are 98.5%, 92.7%, and 98.9%, respectively. Because of
the synergistic effect of the Lewis base (polyoxoanion) and the
Lewis acid (Ln3+ cations), compounds 1−5 exhibit better
catalytic activity. Furthermore, at an equivalent ratio,
compounds 1−3 containing [NiV12O38]12− display catalytic
results better than those for compounds 4 and 5 containing
[MnV12O38]12−, which proves that the composition of the
polyoxoanion has an obvious effect on the catalytic activity.
This may be due to the fact that the volume of [NiV12O38]12−
(87 Å3) is slightly smaller than that of [MnV12O38]12− (89 Å3),
so that the former polyoxoanion has relatively higher Lewis
basicity to promote the catalytic reaction. In addition,
experiments on the effect of catalyst content on the reaction
results were also conducted (Figure 4). When the catalyst
content is less than 1.0 mol %, the yield is lower. When the
catalyst content is more than 1.0 mol %, the yield is not
significantly increased, and so 1.0 mol % was selected as the
optimal amount.
In comparison with the IR data of [MV13O38]7− or
[V10O28]6− (Figure 3), the νas(VO) vibrational bands have
different levels of red shifts, perhaps because the strong
interactions between Ln3+ cations and terminal oxygen atoms
of polyoxovanadates reduce the VO bond force constant and
lead to a decrease in the VO vibration frequency.
Furthermore, owing to the vibrational coupling of
[NiV12O38]12− and [V10O28]6− polyoxoanions, the numbers
of characteristic peaks are reduced and the peaks become wide.
The peaks at 1470, 1424.6, 1223.7, 1178, and 1110 cm−1 for 1,
1424.3, 1224, 1180, and 1111 cm−1 for 2, 1473, 1424.3, 1224,
1178.8, and 1114 cm−1 for 3, 1469.5, 1423.6, 1216.7, 1178.8,
and 1116.8 cm−1 for 4, and 1471.5, 1424.6, 1223.7, 1180.1, and
1112.2 cm−1 for 5 come from the symmetric and asymmetric
stretching vibrations of the dpdo ligand.
TG Analysis. Five thermogravimetric curves of 1−5 are
given in Figure S9a−e, showing a multistep continuous weight
loss process. For compound 1, the first weight loss of 19.9%
from 30 to 200 °C corresponds to the removal of the lattice
water molecules and coordinated water molecules (theoretical
value: 20.7%). A plateau appears ranging from 200 to 270 °C.
Then, the next weight loss after 270 °C is assigned to removal
of the ligand molecules and decomposition of the framework.
The TG curves of compounds 2−5 display weight loss
processes similar to those of compound 1 (shown in Figure
S9b−e). For 2−5, the first weight losses of 20.9%, 20.6%,
21.1%, and 21.0% at 30−200 °C, respectively, agree with the
calculated values (21.9%, 21.9%, 22.0%, 22.0%, respectively),
due to the removal of lattice and coordinated water molecules.
Subsequently, the ligand molecule starts to be lost, and the
framework decomposes when the temperature is further
increased. Similar weight loss processes can be observed in
the reported POV-based hybrid compounds.45,46
PXRD Characterization. Five PXRD patterns of 1−5 are
displayed in Figure S10a−e. The simulated values of these
compounds are in agreement with the experimental results,
which proves that compounds 1−5 are pure phases. The
PXRD patterns of 1−5 are similar, indicating that they are
isostructural compounds, which is consistent with the results of
crystal structural data.
UV−Vis Diffuse Reflective Spectroscopy. The UV−vis
diffuse reflectance spectra of compounds 1−5 were measured
and displayed in Figure S11. To calculate the absorption (A)
data, the function A = log(1/R%) is utilized. In the function, R
represents the reflectivity at a given wavelength (λ). In the UV
region from 200 to 400 nm, there are two broad absorption
bands centered at 235 and 303 nm in 1, 229 and 296 nm in 2,
228 and 302 nm in 3, 226 and 301 nm in 4, and 231 and 295
nm in 5, which can be attributed to the O → V charge transfer
for polyoxoanions. For compounds 3 and 5, the curves display
an absorption band at 594 nm in the visible region from 400 to
800 nm, which can be assigned to the 3H4 → 1D2 transitions of
Pr3+ ions.47
The comparison of compounds 1−5 with other reported
POM-based catalysts for the cyanosilylation reaction of
aldehydes is shown in Table 2. When our compounds are
c o m p a r e d w i t h t h e h o m o g e n e o u s c a t a l y s t s
TBA8H2[Nd2(SiW10O36)2]·7H2O (benzaldehyde, 40 min,
yield 97%) and TBA8H2[(SiYW10O36)2] (benzaldehyde, 15
min, yield 94%), our compounds (compound 3, benzaldehyde,
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cyanosilylation to give the corresponding cyanohydrin
trimethysilyl ether with 72.4% yield within 3 h, which is far
lower than those of compounds 3 and 4.
In order to study the recyclability and stability of catalysts,
compound 3 after the catalytic reaction was filtered, washed,
dried, and tested for circulation using benzaldehyde as a model
reaction. No obvious reduction in the yield of the product after
five cycles (from 99.6% to 95.2%, seen Figure 5a) indicates the
excellent cycling stability of the catalyst. The structural stability
of the catalyst was tested by performing FT-IR and PXRD on
the catalyst before and after the catalytic reaction. PXRD
patterns demonstrate that the recovered catalyst is identical in
structure with the unused catalyst (Figure S12). FT-IR spectra
show that the recovered catalyst has the same characteristic
peaks as the unused catalyst (Figure S13), confirming that the
catalyst did not undergo structural changes during the catalytic
reaction. Furthermore, catalyst 3 was filtered after 30 min of
the catalytic reaction, and the filtrate was continued to be
stirred (Figure 5b). The product yield increased slightly from
50.4% to 52.9% after 3 h. These results demonstrate that these
compounds are genuine heterogeneous catalysts and are
capable of maintaining structural integrity. Furthermore,
cycle tests in the kinetic regime were performed to confirm
the cycling stability of the catalyst at less than 20% conversion.
No obvious reduction in the conversion of benzaldehyde in the
five cycles indicates the excellent cycling stability of the catalyst
(see Figure S18).
Additionally, the IR spectrum of compound 3 after
impregnation in the substrate benzaldehyde is displayed in
Figure 6a, showing the stretch at 1689.8 cm−1 of CO, which
may come from the red shift from 1701.4 cm−1 of
benzaldehyde, and further illustrating that there exists an
activation between the substrate and catalyst 3. Then, a
possible mechanism for the cyanosilylation reaction was
proposed and is shown in Figure 6b. First, the carbonyl
group of aldehyde interacts with the active Ln3+ sites, thereby
improving the electrophilic property of the carboxylic C atom.
At the same time, the Si atom of TMSCN interacts with the
highly charged POVs, increasing the nucleophilic property of
the CN− group. As a consequence, the nucleophilic attack from
the CN− group to the carboxylic C atom occurs. A C−C bond
is formed. Next, the Si atom of TMSCN further attacks the O
atom of the carbonyl group to produce the cyanosilylation
product. Meanwhile, the catalyst is recovered.
Figure 4. Effect of the amount of 3 on the cyanosilylation reaction.
Reaction conditions: 0.5 mmol of aldehydes, 1.5 mmol of TMSCN, 5
μmol of catalyst (1.0 mol %), without solvent, 3 h, room temperature.
3 h, yield 99.6%) display weaker catalytic activities. However,
the homogeneous catalysts have a great disadvantage in that
they are difficult to recycle. In comparison with heterogeneous
polyoxotungstate catalysts such as Cu2 (bpy)-
(H2O)5.5]2[H2W11O38]·3H2O·0.5CH3CN (benzaldehyde, 24
h, yield 98.1%) and {[Cu12(pbtz)2(Hpbtz)2(OH)4(H2O)16]-
[Na(H2O)P5W30O110]}·16H2O (benzaldehyde, 24 h, yield
61.7%),48 our compounds have better catalytic results as well
as a solvent-free advantage. In comparison with heterogeneous
polyoxomolybdate catalysts such as (NH4)3[Sm(H2O)6]-
[Co2Mo10H4O38]·6H2O (benzaldehyde, 5 h, yield 98.4%),
(C2N2H12)[Sr(H2O)5][Co2Mo10H4O38]·2H2O (benzalde-
hyde, 6 h, yield 99%)49 and [Nd(H2O)5]2Mo6V2O26·8H2O
(benzaldehyde, 7 h, yield 96.3%)50 under solvent-free
conditions, our five compounds show stronger catalytic
abilities in a shorter reaction time. The TON value of
compound 3 is 99.6, which is higher than those of most
heterogeneous catalysts based on POMs. It is speculated that
the better catalytic activities of 1−5 come from the inclusion of
two highly charged POV anions ([V10O28]6−, [MV12O38]12−
)
and lanthanide cations, and their unique 2D structures.
Furthermore, the catalytic performance of compound 6,
H[(dpdo)La(H2 O)3 ]2 [MnV1 3 O3 8]·14H2 O, with a
[MnV13O38]7− polyoxoanion has been studied. The catalytic
result is given in Table 2. Compound 6 can catalyze the
Table 2. Summary of the Catalytic Activity toward Cyanosilylation of Benzaldehyde over Various POMs
a
catalyst
temp (°C)
yield (%)
TON
time (h)
ref
3
4
25
25
25
25
25
rt
99.6
89.4
72.4
94.0
99.0
98.1
61.7
99.2
98.4
99.0
99.0
96.2
99.6
89.4
72.4
9400
990
49.05
61.7
49.6
49.2
49.5
49.5
96.2
3
3
3
0.25
0.03
24
24
4
5
6
7
7
this work
this work
this work
H[(dpdo)La(H2O)3]2[MnV13O38]·14H2O
TBA8H2[(SiYW10O36)2]·7H2O
24
25
22
48
28
26
49
27
50
TBA8H2[{Nd(H2O)2}2(SiW10 O36)2]·3H2O
{[Cu2(bpy)(H2O)5.5]2[H2W11O38]·3H2O·0.5CH3CN}
{[Cu12(pbtz)2(Hpbtz)2(OH)4(H2O)16][Na(H2O)P5W30O110]}
K[(H2O)4(Hpic)2Ce][(H2O)5 (pic)2Ce][PW10Ti2O40]·11H2O
(NH4)3[Sm(H2O)6][Co2Mo10H4O38] 6H2O
(C2N2H12)[Sr(H2O)5][Co2Mo10H4O38]·2H2O
[Zn2(H2O)2(bpe)3]H2[Co2Mo10H4O38]
rt
50
25
25
25
25
[Nd(H2O)5]2Mo6V2O26·8H2O
a
TON = product (mol)/catalyst (mol).
H
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Figure 5. (a) Recycling experiments for the cyanosilylation of benzaldehyde over catalyst 3. Reaction conditions: 0.5 mmol of aldehydes, 1.5 mmol
of TMSCN, 5 μmol of catalyst (1.0 mol %), without solvent, 3 h, room temperature. (b) Yields of product in the cyanosilylation reaction of
benzaldehyde catalyzed by 3 (black) and by the filtrate of 3 after 30 min of the reaction (red).
Figure 6. (a) IR spectra of 3 (bottom), benzaldehyde (top), and 3 obtained after the absorption of benzaldehyde (middle). (b) The possible
catalytic mechanism for the cyanosilylation of aldehydes.
Knoevenagel Condensation. Compounds 1−5 were
further used as catalysts to study their catalytic performances
for the Knoevenagel reaction. Benzaldehyde was first selected
as a model substrate to evaluate the catalytic activities of the
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a
Table 3. Knoevenagel Condensation Catalyzed by Compounds 1−5
a
Reaction conditions: 0.5 mmol of aldehydes, 1 mmol of malononitrile, 0.6 mol % of catalyst (based on substrate), 0.2 mL of solvent, room
b
c
temperature for 0.5 h. Yields were determined by GC using naphthalene as an internal standard. Moles of condensation product per mole of
catalyst per hour.
five compounds. The catalytic reaction was run with
benzaldehyde (0.5 mmol), malononitrile (1.0 mmol), catalyst
(3 μmol), DMF (0.2 mL), and a reaction time of 0.5 h at room
temperature (see Table 3). It can be seen that the catalytic
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Figure 7. (a) Recycling experiments for the Knoevenagel condensation reaction over catalyst 3. Reaction conditions: 0.5 mmol of aldehydes, 1
mmol of malononitrile, 3 μmol of catalyst (0.6 mol %), 0.2 mL of solvent, 0.5 h, room temperature. (b) Yields of product in the Knoevenagel
condensation of benzaldehyde catalyzed by 3 (black) and by the filtrate of 3 after 10 min of the reaction (red).
performance of compound 3 is better than those of other
isostructural compounds, which is also consistent with the
catalytic result drawn from the cyanosilylation reaction. With
compound 3 as the catalyst, the yield of 2-benzylidenemalo-
nonitrile can reach 99.7%, with a high turnover frequency
(TOF) of 332.3 h−1 after 0.5 h, which is faster than those for
the previously reported catalysts containing MOFs or POMs
(Table S2). In contrast, without a catalyst, benzaldehyde
reacted with malononitrile to give a yield of 40.9%, owing to
the weak basicity of the DMF solvent. When the raw material
K4 Na2 V1 0 O2 8 ·10H2 O, K7 [NiV1 3 O3 8 ]·18H2 O, or
K7[MnV13O38]·18H2O is used as a homogeneous catalyst,
the yields of the corresponding product are respectively 85.5%,
95.4%, and 81.4% within 0.5 h, which indicates that the raw
material has lower catalytic activity in comparison to the five
compounds 1−5. When LaCl3·6H2O, Ce(NO3)3·6H2O, or
PrCl3 is used to catalyze such a reaction under homogeneous
conditions, the yields are 96.6%, 90.2%, and 98.6%,
respectively. Thus, the catalysis efficiency for the Knoevenagel
condensation reaction can be improved due to the synergy
between POV anions and Ln3+ cations.
With compound 3 as the catalyst, catalytic experiments
under different substrate conditions were also carried out.
Whether benzaldehyde derivatives with electron-withdrawing
groups (−F) or with electron-donating groups (−CH3) are
used, the substrates are fully converted to the catalytic
products in excellent yields (98.9−99.5%) with high TOF
values (329.5−331.5 h−1), revealing that the substituent group
on the aromatic ring has no significant effect. When heptanal
and 1-naphthaldehyde are used as substrates, the yields of the
corresponding products are respectively 99.3% and 83.3%,
indicating that compound 3 is an excellent catalyst. The
catalytic conditions are optimized by changing the solvents
with benzaldehyde used as a model substrate. Table S3 gives
the catalytic effect of compound 3 on the reaction in different
solvents. When DMF and DMSO are used as solvents, the
conversion of benzaldehyde is over 99.5% in 0.5 h. However,
through an analysis of GC data and GC-MS data, the
selectivity of the product is only 44.84% in DMSO solvent,
which is far lower than that in DMF solvent (selectivity,
100%). Consequently, DMF is used as the solvent.
of the product after five cycles has no obvious decrease (from
99.7% to 95.3%, seen in Figure 7a), which reveals the excellent
cycling stability of the catalyst. After the catalyst 3 was
removed by filtration at 10 min, the reaction was continued.
The product yield increases only slightly from 60.9% to 61.6%
after 0.5 h, as shown in Figure 7b. This result further proves
the real heterogeneity of the catalyst with no ions leached into
the liquid phase. In addition, FT-IR spectra and PXRD
patterns of the catalyst 3 before and after the catalytic reactions
match well, confirming that the structure of the catalyst did not
change (Figures S15 and S16). Furthermore, cycle tests in the
kinetic regime were performed to confirm the cycling stability
of the catalyst at less than 20% conversion. No obvious
reduction in the conversion of benzaldehyde over the five
cycles indicates the excellent cycling stability of the catalyst
(Figure S18).
CONCLUSION
■
In summary, five new hybrid compounds composed of
[V10O28]6− and [MV12O38]12− (M = Ni, Mn) were successfully
synthesized, which represent the first examples where
isopolyvanadate and heteropolyvanadate coexist in one
extended structure. The five compounds utilizing polyoxova-
nadate anions as Lewis bases and lanthanide cations as Lewis
acids can significantly promote the cyanosilylation reaction and
Knoevenagel condensation under mild conditions. Compound
3 containing the Pr3+ cation displays the best catalytic
performance in both C−C formation reactions (yields up to
99.6%). In addition, catalysts 1−5 are genuine heterogeneous
catalysts and can keep their high catalytic activities after being
recycled five times. The successful synthesis of these hybrid
species with two kinds of polyoxovanadates with great catalytic
ability should stimulate researchers to enrich the research of
POMs in the field of catalyzing C−C bond formation.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00999.
■
sı
Structural comparison between [NiV13O38]7− and
[NiV12O38]12− polyoxoanions, the coordination modes
of three La3+ cations, the coordination environment of
The stability and recyclability of catalysts were also
investigated by using benzaldehyde as a substrate. The yield
K
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the [NiV12O38]12− polyoxoanion, the 2D topology
pubs.acs.org/IC
Article
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AUTHOR INFORMATION
Corresponding Author
Haiyan An − College of Chemistry, Dalian University of
Technology, Dalian 116023, People’s Republic of China;
orcid.org/0000-0003-3848-5210; Phone: +86-411-
84657675; Email: anhy@dlut.edu.cn
■
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progress on polyoxometalate-based transition-metal−rare-earth het-
erometallic derived materials: synthetic strategies, structural overview
and functional applications. Chem. Commun. 2016, 52, 4418−4445.
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advances in porous polyoxometalate-based metal−organic framework
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Dong, L. Z.; Qin, J. S.; Guan, W.; Liu, J.; Li, S. L.; Lu, M.; Lan, Y. Q.;
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Authors
Jie Zhang − College of Chemistry, Dalian University of
Technology, Dalian 116023, People’s Republic of China
Shenzhen Chang − College of Chemistry, Dalian University of
Technology, Dalian 116023, People’s Republic of China;
orcid.org/0000-0002-8983-9309
Yujiao Hou − College of Chemistry, Dalian University of
Technology, Dalian 116023, People’s Republic of China
Qingshan Zhu − College of Chemistry, Dalian University of
Technology, Dalian 116023, People’s Republic of China
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00999
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(15) (a) Zhang, J.; Hao, J.; Wei, Y. G.; Xiao, F. P.; Yin, P. C.; Wang,
L. S. Nanoscale chiral rod-like molecular triads assembled from achiral
polyoxometalates. J. Am. Chem. Soc. 2010, 132, 14−15. (b) Li, D. D.;
Xu, Q. F.; Li, Y. G.; Qiu, Y. T.; Ma, P. T.; Niu, J. Y.; Wang, J. P. A
Stable Polyoxometalate-Based Metal-Organic Framework as Highly
Efficient Heterogeneous Catalyst for Oxidation of Alcohols. Inorg.
Chem. 2019, 58, 4945−4953.
The authors appreciate the financial support from the National
Natural Science Foundation of China (21371027, 20901013),
the Natural Science Foundation of Liaoning Province
(2015020232) and the Fundamental Research Funds for the
Central Universities (DUT19LK01, DUT15LN18).
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