Controllable Synthesis and Catalytic Performance of Nanocrystals of Rare-Earth-Polyoxometalates

Article abstract of DOI:10.1021/acs.inorgchem.8b00763

Large-scale isolation of nanocrystals of rare-earth-polyoxometalates (RE-POMs) catalysts is important in fundamental research and applications. Here, we synthesized a family of monomeric RE-POMs by the self-assembly of Ta/W mixed-addendum POM {P₂

Full text of DOI:10.1021/acs.inorgchem.8b00763

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Controllable Synthesis and Catalytic Performance of Nanocrystals of
Rare-Earth-Polyoxometalates
Shujun Li, Yanfang Zhou, Qingpo Peng, Ruoya Wang, Xiaoge Feng, Shuxia Liu,* Xiaoming Ma,†
†
†
†
†
,§,⊥
†
,‡
Nana Ma, Jie Zhang,† Yi Chang, Zhiping Zheng,* and Xuenian Chen*
†
†
,†
†Henan Key Laboratory of Boron Chemistry and Advanced Energy Materials, School of Chemistry and Chemical Engineering, Henan
Normal University, Xinxiang, Henan 453007, China
‡
Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal University
Changchun, Jilin 130024, China
§
Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States
*
S Supporting Information
ABSTRACT: Large-scale isolation of nanocrystals of rare-earth-polyoxome-
talates (RE-POMs) catalysts is important in fundamental research and
applications. Here, we synthesized a family of monomeric RE-POMs by the
self-assembly of Ta/W mixed-addendum POM {P W Ta O } and rare-earth
2
15
3
62
(
RE) ions. These RE-POMs with molecular formulas of [RE(H O) ] -
2 7 3
P W Ta O ·nH O (RE = Y, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) are all
2
15
3
62
2
electroneutral molecular clusters, insoluble in water and common organic
solvents. The electronic structures, electrochemical properties, and catalytic
activities of them have been investigated by experimental and computational
methods. In particular, based on a mild and controllable synthetic process, a convenient and controllable approach to prepare
nanocrystals and self-organized aggregates of these monomers has been developed. They exhibit remarkable heterogeneous
catalytic activity for cyanosilylation. Both the increased Lewis acid strength of RE in the title compounds, as indicated by
theoretical calculations, and the decreased particle size contribute to their high catalytic performances.
INTRODUCTION
the structures of these compounds cannot be maintained in
aqueous solutions. Therefore, they can serve as valid catalysts in
neither homogeneous nor heterogeneous systems. It is highly
desirable to prepare the nanocrystals of RE-POMs and develop
heterogeneous catalytic systems with a high surface area and
■
Rare-earth-polyoxometalates (RE-POMs) have been attracting
ever-growing interest owing to their peculiar physicochemical
1
−3
properties and applications in the fields of luminescence,
4
−6
7,8
magnetochemistry,
catalysis.
electrochemistry,
and particularly
9
,21,22
9
,10
therefore enhanced catalytic activities.
Due to the Lewis acid nature of RE ions, RE-
However, it is very difficult to isolate RE-POMs catalysts in
the nanocrystal form utilizing the reaction of RE and lacunary
polyoxotungstates (POTs), which is the main synthetic strategy
POMs have shown remarkable performance in catalyzing
various organic reactions, such as Diels−Alder, Mukaiyama−
aldol, Mannich, and protein hydrolysis.
coexistence of both a Lewis acid (RE ions) and a Lewis base
POM), RE-POMs have been found to show great potential in
catalyzing cyanosilylation, from which cyanohydrinsimpor-
tant and otherwise hard to obtain intermediates in the synthesis
of α-hydroxy acids, β-aminoalcohols, and other biological
1
1−13
With the
23−27
to prepare RE-POMs currently.
Owing to the high
alkalinity of lacunary POTs and the high oxophilicity of RE
ions, the reactions between them and the crystallization course
of the producing RE-POMs are difficult to control. In most
cases, therefore, these RE-POMs can only be isolated as single
crystals in low yields.
(
1
4−18
compoundscan be facilely produced.
In this work, in considering its moderate reactivity (discussed
The above-mentioned RE-POMs catalytic systems reported
in the literature are mainly homogeneous, which are applicative
for the organic phase soluble and stable RE-POMs (generally
tetrabutylammonium salts of POMs), for example, monometal-
in “Synthesis and Reactivity”), Ta/W mixed-addendum POM
9
−29
[
P W Ta O ]
was selected to react with various RE salts
2
15
3
62
(
Y and the full range of lanthanide except for the radioactive
6
−
Pm) to construct nanostructured novel RE-POMs. Conse-
quently, a convenient and controllable approach was developed
to generate 15 RE-POMs (listed Table 1). Ten of these
obtained RE-POMs with the general formula of [RE(H O) ] -
substituted POMs [α-RE(H O) P W O ] and [α-RE-
2
4
2
17 61
4−
10,11
(
H O)PW O ] (RE = Y, La, Eu, Sm or Yb),
and
2
11 39
silicotungstate dimers [{RE(H O) (acetone)} (γ-
2
2
2
10−
14,15
SiW O ) ]
(RE = Y, Nd, Eu, Gd, Tb, or Dy).
But
2
7 3
1
0
36 2
most RE-POMs with novelty structures can only exist as
1
9,20
inorganic salts.
They cannot be dissolved in organic solvent
Received: March 22, 2018
and usually show a very small surface area. On the other hand,
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00763
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Synthesis of the Single Crystals of Eu-POM−Lu-POM. Eu-
POM−Lu-POM were prepared following the procedure descried for
Y-POM, but using respectively EuCl ·6H O, GdCl ·6H O, TbCl ·
Table 1. Labeling of the Title Compounds and the Average
RE-O(Ta) Bond Lengths, and Relevant Ionic Radii for the
RE
3
2
3
2
3
6
H O, DyCl ·6H O, HoCl ·6H O, ErCl ·6H O, TmCl ·6H O, YbCl ·
2 3 2 3 2 3 2 3 2 3
compounds
6H O, and LuCl ·6H O as the starting lanthanide salt. The final
2 3 2
products of sheet crystals were obtained with yields of 85%, 86%, 88%,
89%, 87%, 85%, 85%, 86%, 88%, and 84% respectively.
RE−O(Ta) bond lengths
RE ionic radii
32
monomer
Y-POM
chain
(Å)
(Å)
Synthesis of the Single Crystals of Sm-POM. [Sm (H O) ]-
3
2
19
2.252
2.421
2.385
2.353
2.362
2.305
2.289
2.306
2.253
2.244
Y (0.0893)
[
P W Ta O ]·18H O (Sm-POM) was prepared following the
2 15 3 62 2
30
3
0
La-POM
La (0.1061)
Ce (0.1034)
Pr (0.1013)
Nd (0.0995)
Sm (0.0964)
Eu (0.0950)
Gd (0.0938)
Tb (0.0923)
Dy (0.0908)
Ho (0.0894)
Er (0.0881)
Tm (0.0869)
Yb (0.0858)
Lu (0.0848)
procedure descried for La-POM, but using Sm(NO3)3 as rare-
earth reagent. The final yield for Sm-POM (tufted crystals) was 56%.
Preparation of the Aggregate of Y-POM. Step I: A sample of
K Na [P W O (TaO ) ]·17H O (0.20 g, 0.04 mmol) was dissolved
3
0
Ce-POM
3
0
3
Pr-POM
Nd-POM
Sm-POM
0
5
4
2
15 59
2
3
2
in 25 mL of deionized water at 75 °C. Solid NaHSO (0.04 g, 0.38
3
mmol) was added with stirring until the yellow solution became
Eu-POM
Gd-POM
Tb-POM
Dy-POM
Ho-POM
Er-POM
Tm-POM
Yb-POM
Lu-POM
colorless. Solid Y(NO ) ·6H O (0.1 g, 0.26 mmol) was added,
3
3
2
resulting in a colorless solution. The clear solution was acidified with
of HCl (aq. 1 M) and further stirred at 75 °C for 30 min.
Step II was performed in the following three different ways:
Step IIa: The clear solution was cooled to room temperature under
vigorous stirred for 6 h. The resulting suspension was collected on a
membrane filter and washed with 10 mL of deionized water and air-
3
3
1
0
2.198
2.229
2.201
2.195
2.133
dried (Y-POM-H O-6h isolated yield 33%).
2
Step IIb: After being cooled to room temperature, 2.1 mL of
ethanol (ethanol/H O = 1:12) was added under stirring. The resulting
2
suspension was stirred at 850 rpm for 4 h, and then collected on a
membrane filter and washed with 10 mL of deionized water and air-
dried (Y-POM-1:12−4h isolated yield 54%).
P W Ta O ·nH O [RE = Y (n = 27), Eu (n = 26), Gd (n =
2
15
3
62
2
2
5), Tb (n = 26), Dy (n = 25), Ho (n = 25), Er (n = 26), Tm
(n = 26), Yb (n = 26), Lu (n = 25)] (Figure 1 and Table 1)
Step IIc: After being cooled to room temperature, 25 mL of ethanol
(
ethanol/H O = 1:1) was added in 5 min under stirring to from a
2
turbid mixture (Y-POM-1:1−0h). Then the resulting turbid solution
was vigorous stirred for half an hour (Y-POM-1:1−0.5h), 1 h (Y-
POM-1:1−1h), 2 h (Y-POM-1:1−2h), and 4 h (Y-POM-1:1−4h,
isolated yield 91%) respectively. The final products were respectively
collected on a membrane filter and washed with deionized water and
ethanol and air-dried.
Large-scale synthesis of the aggregates of Y-POM-1:1−0.5h was
readily achieved by simply increasing all reactant by a factor of 10. The
product was obtained in about 80% yield.
The aggregates of other compounds (Eu-POM−Lu-POM) were
prepared by adopting the same procedures as that of Y-POM with the
use of the respective starting rare-earth salts.
Figure 1. Combined polyhedral/ball-and-stick representation of the
monomers (a, RE = Y, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) and
chain compound (b, Sm-C-2). Color scheme for polyhedra: WO6
Cyanosilylation Reaction. A typical cyanosilylation reaction was
performed as follows: 0.5 mmol aldehyde/ketone, 0.75 mmol
TMSCN, and 1 mol % catalyst was added to a 10 mL Schlenk vase.
The reaction mixture was stirred (600 rpm) at 25 °C under nitrogen.
The reactions progresses of aldehydes with TMSCN were monitored
(
teal), PO (pink), TaO (lime); for spheres: RE (yellow), Sm (dark
4 6
yellow), O (red).
show discrete monomeric structures. The synthetic approach of
these monomers, as the title compounds, was further optimized
for rapid preparation of nanocrystals and aggregates with
different morphologies in large scale. They have been shown to
effectively catalyze the cyanosilylation of a number of aldehydes
and ketones with trimethylsilyl cyanide (TMSCN).
1
1
by H NMR by tracing the decrease of the H resonance of CHO
hydrogen and the increase of the characteristic hydrogen in the
products. The reactions of ketones with TMSCN were monitored by
GC analysis. Conversions were determined by GC analysis using
naphthalene as internal standard. The products were confirmed by
1
comparison their GC retention times and H NMR spectra with those
of authentic samples. Conversions were determined by GC analysis
using naphthalene as the internal standard. After the reaction was
completed, the catalyst was recovered by centrifugation and purified by
washing with acetone and deionized water and then air-dried for the
next cycle. Before the catalytic test, the title catalysts were activated at
EXPERIMENTAL SECTION
■
(
Synthesis of the Single Crystals of [Y(H O) ] [P W Ta O ]·
2
7 3
2
15
3
62
28
2
7H O (Y-POM). A sample of K Na [P W O (TaO ) ]·17H O
2
5
4
2
15 59
2
3
2
0.20 g, 0.04 mmol) was dissolved in 25 mL of deionized water at 75
1
20 °C under a vacuum for 2 h.
°
C. Solid NaHSO (0.04 g, 0.38 mmol) was added with stirring until
3
the yellow solution became colorless, and then solid Y(NO ) ·6H O
0.05 g, 0.13 mmol) was added. The resulting clear solution was
3
3
2
RESULTS AND DISCUSSION
Structures and Properties. As shown in Table 1, the self-
(
■
acidified with 1.0 mL of HCl (aq. 1 M) or 2.0 mL of glacial acetic acid,
and further stirred at 75 °C for 30 min and then cooled to room
temperature. The reaction solution was kept in an open beaker for
crystallization at room temperature. Colorless sheet single crystals
began to appear within 3 days. Final yield: 0.18 g (82% based on
K Na [P W O (TaO ) ]·17H O). Anal. Calcd (%) for [Y(H O) ] -
III
assembly of {P W Ta } with RE (Y and the entire series of
2
15
III
3
III
lanthanide ions La −Lu except for the radioactive Pm), can
produce 15 compounds, which include two structural types:
monomers (Figure 1a) and one-dimensional chain compounds
5
4
2
15 59
2
3
2
2
7 3
(
Figure 1b). Four of the five chain compounds (La-POM, Ce-
[
1
9
P W Ta O ]·27H O: P 1.13, Y 4.86, Ta 9.90, W 50.27; found P
2 15 3 62 2
.23, Y 4.75, Ta 10.06, W 49.85. IR (KBr disks): 1622(w), 1090(m),
POM, Pr-POM, and Nd-POM) have been reported in our
−1
30
51(s), 908(s), 777(vs), 519(s) cm .
previous work. Eleven compounds, including the 10
B
DOI: 10.1021/acs.inorgchem.8b00763
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Inorganic Chemistry
Article
monomers (Y-POM, Eu-POM, Gd-POM, Tb-POM, Dy-
POM, Ho-POM, Er-POM, Tm-POM, Yb-POM, and Lu-
POM) and one chain compound (Sm-POM) are reported here
for the first time. Single crystal X-ray diffraction analysis and
TG curves (Figures S2−S4) revealed that the 10 monomers,
[
2
RE(H O) ] P W Ta O ·nH O [RE = Y (n = 27), Eu (n =
2 7 3 2 15 3 62 2
6), Gd (n = 25), Tb (n = 26), Dy (n = 25), Ho (n = 25), Er (n
=
26), Tm (n = 26), Yb (n = 26), and Lu (n = 25)], possess
similar structures with only slight differences in bond lengths
and angles and the numbers of lattice water molecules. The
structure of Y-POM is described below as an example. As
shown in Figure 1a, each {P W Ta } moiety in Y-POM is
2
15
3
connected with three Y through three Ta−O−Y bridges. Every
Y links to a {P W Ta } unit and seven aqua ligands, resulting
2
15
3
in a square-antiprismatic geometry with a total coordination
number of eight. The structure of Sm-POM Sm (H O) ]-
3
2
21
[
P W Ta O ]·18H O (Figure 1b) is similar to that of the
2 15 3 62 2
previously reported chain compounds (Re (H O) -
3
2
22
P W Ta O ·nH O [Re = La (n = 16), Ce (n = 16), Pr (n
2
15
3
62
2
30
=
16), Nd (n = 17)].
Notably, the formation of the RE-POMs in this manuscript is
controlled by the ionic radii of RE. As shown in Table 1, the
final structures are all monomers for RE ionic radii less than
Figure 2. (a) CVs of {P W }, {P W (TaO ) }, and Y-POM on
2
18
2
15
2 3
modified carbon paste electrodes, in 1 M Hac/NaAc buffer solution,
−1
0.095 Å, as for Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y. RE
scan rate: 20 mV·s the reference electrode was SCE. (b) LUMO
ions with ionic radii greater than 0.0964 Å (as for La, Ce, Pr,
Nd, and Sm) can link adjacent {La (H O) P W Ta O }
compositions of {P 18}, {P 15(TaO }, and Y-POM.
2
W
2
W
)
2 3
3
2
22
2
15
3
62
units to form chain complexes. However, analogous com-
III
and {P W (TaO ) }. Relatively, the lowest unoccupied orbital
2 15 2 3
pounds could not be obtained if RE ion was replaced by a
III
IV
IV
(LUMO, Figure 2b) of the one-electron reduction species of Y-
POM, {P W (TaO ) } and {P W } are all localized at the
smaller cation, such as Sc , Zr , and Hf . The average RE−O
bond lengths of the 15 compounds listed in Table 1 decrease
with atomic number, which is consistent with the trend in the
ionic radii of the lanthanide ions and follows the lanthanide
2
15
2 3
2
18
equatorial W centers, which clearly shows that the redox
35,36
centers of these species are at equatorial W positions.
32
Under the same conditions, Eu-POM and Yb-POM showed
similar CVs curves with that of Y-POM. While an addition
irreversible anode peak appears at +0.48 eV in the CVs curve of
Ce-POM (Figure S23) corresponds to the redox process of
contraction principle.
33
Bond valence sums (BVS) analyses of the title compounds
indicated the following oxidation states: W(VI), Ta(V), P(V),
and RE(III). The BVS values for O atoms in the POMs
skeleton are −2. The values for all the terminal O atoms of the
III
IV 37
Ce /Ce .
34
Synthesis and Reactivity. The moderate reactivity of Ta/
RE ions are less than 0.43, indicating their aqua ligand nature.
9−
W POM [P W Ta O ] is crucial for the controllable
2
15
3
62
These title compounds are electroneutral molecular clusters
with no discrete countercations (for example alkali metal ions)
in the lattice, which is unusual in RE-POMs. For this reason, all
of these compounds are almost insoluble in water and common
organic solvents, such as methanol, ethanol, acetone, chloro-
form, and acetonitrile. This property is beneficial for their use
in heterogeneous catalysis. The solid photoluminescence
behaviors of Sm-POM, Eu-POM, and Tb-POM were
investigated at room temperature (Figures S11−S14). They
exhibit the characteristic emissions of Sm, Eu, and Tb,
respectively.
synthesis of the title compounds and their nanocrystals and
V
VI
aggregates. First, the substitution of Ta for W can not only
increase the negative charge of the polyanion but also lead to
9
−
polarized charge distribution on [P W Ta O ] (Figure 3).
2
15
3
62
The {Ta } section becomes a nucleophilic center of the
3
Electronic Structure and Electrochemical Properties.
Cyclic voltammograms (CVs, Figure 2a) of Y-POM and
K Na [P W O (TaO ) ]·17H O ({P W (TaO ) }) both
5
4
2
15 59
2
3
2
2
15
2 3
showed three redox processes, while K [P W O ]
6
2
18 62
(
{P W }) showed two redox processes at the same condition.
2 18
It is worth noting that Y-POMs and {P W (TaO ) } possess
2
15
2 3
similar redox potential but obviously lower than that of
P W }. For instance, the first cathodic peak potentials of Y-
{
2
18
POM and {P W (TaO ) } appeared at −0.97 and −0.93 V
2
15
2 3
9−
Figure 3. Ball-and-stick representation of [P W Ta O ] fragment
(
(
2
15
3
62
respectively, which are obviously lower than that of {P W } at
2
18
a) and molecular electrostatic potentials (MEPs) of the 3D surface
b). The color of the electronic density isosurface (r 1/4 0.022 e/ua) is
a function of the MEP value. Red regions represent nucleophilic areas
where the electrostatic potential is negative, and blue regions represent
electrophilic areas where the electrostatic potential is less negative.
−
0.49 V. It can be concluded through the CVs comparisons
V
VI
that the substitution of Ta for W has remarkable influences
on the potentials, but the incorporating of the RE ions does not
cause obvious changes in the redox potential between Y-POM
C
DOI: 10.1021/acs.inorgchem.8b00763
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Inorganic Chemistry
Article
3
8
polyanion toward RE coordination; consequently, the reactivity
clear now, but there are two possible factors. First, the some
weak interactions, maybe coordination, may exist between
ethanol and the RE in RE-POM. Second, different ratios of
ethanol/water can adjust the polarity of the mixed solution,
which results in different aggregation speeds and different
morphologies.
9−
of [P W Ta O ] is superior to that of plenary POMs (for
2
15
3
62
6
−
example, [P W O ] ), which are generally inert toward RE,
2
18 62
because their terminal oxo ligands are coordinated to the metal
20
atoms with a considerable degree of π-bonding. Second, the
9−
more moderate reactivity of [P W Ta O ] toward RE than
2
15
3
62
lacunary POTs leads to the reaction more controllable. The
Time-dependent morphology changes were studied by
separating and examining the products at different time (0,
0.5, 1, 2, and 4 h) after the addition of ethanol (1:1). The newly
formed suspension particles after the addition of ethanol were
fine nanocrystals of Y-POM (Y-POM-1:1−0h, Figures 4a and
9−
direct addition of RE salts into the solution of [P W Ta O ]
2
15
3
62
does not cause any precipitation, which is unlike the high
alkalinity lacunary POTs. As a result, all the title compounds
can be synthesized through a simple reaction procedure under
undemanding conditions in the temperature range of 50−90 °C
and in the pH range of 2.0−6.5. The controllable synthesis
conditions of the title compounds provide favorable conditions
for the isolation of nanoscale RE-POMs.
Preparation of the Single Crystals, Nanoparticles, and
Aggregates. As shown in Scheme 1, the reaction of
Scheme 1. Synthetic Route of the Single Crystals and
Aggregates of Y-POM under Different Conditions and the
Corresponding Photograph of the Single Crystals of Y-POM
(
a) and the Scanning Electron Microscope Images of Y-
POM-H O-6h (b), Y-POM-1:12-4h (c), and Y-POM-1:1-4h
2
(d)
Figure 4. SEM images of the nanoparticles Y-POM-1:1−0h (a) and
aggregates: Y-POM-1:1−0.5h (b), Y-POM-1:1−1h (c), and Y-POM-
1
:1−2h (d).
S17). When the suspension solution was further stirred for half
an hour, these nanoparticles assemble into cauliflower-like
spherical aggregates with an average particle size of 159 nm (Y-
POM-1:1−0.5h, Figures 4b and S16). If the stirring time was
prolonged to 1 h, the micron-scale clusters aggregates were
observed (Y-POM-1:1−1h, Figure 4c). Plate aggregates were
observed after 2 h (Y-POM-1:1−2h, Figure 4d), which further
evolved into rhombic plate aggregates Y-POM-1:1−4h
(
Scheme 1d) 4 h later. The purity of these microcrystal and
nanocrystal particles were all confirmed by powder X-ray
diffraction (PXRD) patterns (Figure S8) and inductively
coupled plasma analysis.
9
−
3+
[
P W O (TaO ) ] , NaHSO , and Y in acidic solution
Catalytic Activities. The cyanosilylations of carbonyl
compounds with trialkylsilyl cyanide (TMSCN) are important
C−C bond-forming reactions in organic synthesis for
producing cyanohydrin derivatives, which are useful starting
materials for the synthesis of several biological active
compounds due to the presence of hydroxyl and nitrile
2
15 59
2 3
3
resulted in a clear solution. Sheet single crystals of the title
compounds grew from the reaction solution after standing for 3
days (Y-POM, Scheme 1a), with isolated yields of more than
0%. If the resulting solution was vigorously stirred for 6 h at
room temperature, an amorphous solid was obtained with low
8
18
isolated yield (33%) (Y-POM-H O-6h Scheme 1b). Micron-
functionalities. Many Lewis acid catalysts, that can electro-
philically activate carbanyl groups, and nucleophilic catalysts,
that can activate TMSCN, have been reported that can
2
sized irregular sheets could be observed (Y-POM-1:12−4h,
Scheme 1c), after adding a small quantity of ethanol to the
reaction solutions (ethanol/water = 1:12) and stirring for 4 h.
Increasing the ethanol/water ratio to 1:1, rhombic plate
aggregates formed after stirring for 4 h (Y-POM-1:1−4h,
Scheme 1d and Figures S15). Therefore, amorphous Y-POM
can form in water slowly, but this process is uncontrollable. The
addition of ethanol to the reaction solution could accelerate the
precipitation, increase the yield, and also greatly influence the
morphological structures of Y-POM. The assemble mechanism
and the effect of ethanol for the state of aggregation are not
39−45
effectively promote cyanosilylation.
It has been also
demonstrated that cyanosilylation can be catalyzed by some
RE-POMs that combine Lewis acid and Lewis base active sites
13−16,46
on the same POM molecules.
The coordinated water
molecules of the title compounds were gradually lost from
room temperature to about 300 °C (Figures S2 and S4)
resulting in open coordination sites of RE, which could behave
as Lewis acid sites. Simultaneously, the {P W Ta O }
2
15
3
62
skeletons possessing electronegative surfaces could act as
D
DOI: 10.1021/acs.inorgchem.8b00763
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Inorganic Chemistry
Article
Lewis base sites. Thus, the title compounds are expected to be
effective Lewis acid−base catalysts.
The cyanosilylation reactions of benzaldehyde with TMSCN
catalyzed by different RE-POMs (Table 2) and the reactions of
Table 3. Cyanosilylation of TMSCN and Aldehydes/Ketones
Catalyzed by Y-POM-1:1-0.5h
Table 2. Cyanosilylation of Benzaldehyde and TMSCN
a
Catalyzed by Different RE-POMs
b
TOF (h−1
)
c
entry
catalyst
time (h) yield (%)
1
2
3
4
5
6
7
8
9
Y-POM-1:1−0h
Y-POM-1:1−0.5h
Y-POM-1:1−1h
3.5
4
5
5
4
4
4
6
6
6
6
6
6
6
6
6
6
98
99
98
96
98
97
94
75
76
70
71
47
49
46
36
31
26
28.0
24.8
19.6
19.2
24.5
24.3
23.5
12.5
12.7
11.7
11.8
7.8
Y-POM-1:1−4h
Eu-POM-1:1−0.5h
Ho-POM-1:1−0.5h
Yb-POM-1:1−0.5h
d
Y-POM
Eu-POM
Tb-POM
Yb-POM
Sm-POM
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
3
0
La-POM
8.2
3
0
Pr-POM
7.6
a
Reaction conditions: TMSCN 0.75 mmol, benzaldehyde 0.5 mmol,
4
8,49
K [Y(α-1-P W O )]
6.0
b
7
2
17 61
catalysts 1 mol %, 25 °C under N . Conversions were determined by
GC analysis using naphthalene as the internal standard.
4
8,49
2
K [Yb(α-1-P W O )]
5.2
7
2
17 61
K Na [P W O (TaO ) ]
4.3
5
4
2
15 59
2 3
a
Reaction conditions: TMS(CN) 0.75 mmol, benzaldehyde 0.5 mmol,
b
catalysts (1 mol %), under N , 25 °C. Yields were determined by GC
analysis using naphthalene as the internal standard. TOF = converted
substrate (mol)/catalyst (mol)/time (h). The catalysts in Entries 8−
1
2
c
d
7 were ground samples of the as-prepared single crystal products.
different aldehydes/ketones with TMSCN catalyzed by Y-
POM-1:1−0.5h were examined (Table 3). As shown in Table
2
, the catalytic activities of the aggregates of the title
compounds (Entries 1−7) were apparently higher than that
of the single crystal products (Entries 8−11), probably because
of the increased surface areas (Table S2). As shown in Table 2
and Figure 5, it needs 3.5, 4, and 5 h to finish the reactions
catalyzed by Y-POM-1:1−0h (Entry 1), Y-POM-1:1−0.5h
(
Entry 2), and Y-POM-1:1−4h (Entry 4), respectively. And it
took about 9 h for the single crystal products of Y-POM.
However, the catalytic activity of Y-POM-1:1−0h was
markedly decreased after the first cycle (Figure S18). IR
spectra (Figure S21) of Y-POM-1:1−0h before and after the
catalytic reaction remained unchanged indicating the decrease
of the catalytic activity is not owing to the decomposition of
compounds. SEM images (Figure S22) showed that the small
particles aggregated into bulk sample after the catalytic reaction,
while the recovered catalyst of Y-POM-1:1−0.5h and Y-POM-
Figure 5. Conversion of cyanohydrin trimethylsilyl ether vs reaction
time catalyzed by Y-POM and the aggregates.
The title compounds (Entries 8−11) present a much higher
catalytic activity than that of the chain compounds previously
30
reported (Entries 12−14). This can be explained because the
title compounds feature a more open coordination space and
less steric hindrance around the RE ions. On the other hand,
there were no notable differences in the catalytic activity among
the title compounds with different RE (Entries 3−11), although
the RE ionic radii showed a wide variation. This may be
because the ionic radius has a negligible influence compared
with the open coordination sites around RE Lewis acid centers.
The catalytic efficiency of the title compounds Y-POM and
1
:1−4h could be reused for at least three cycles of the
cyanosilylation without an appreciable loss of catalytic perform-
ance (Figures S19 and S20). And their morphology did not
change significantly (Figure S22). The TOF of Y-POM-1:1−
0
.5h (24.8) is higher than that of crystal products of Y-POM
(
(
12.5) and the reported heterogeneous [Nd(H O) ] MoV O
2 5 2 2 26
47
14) and [Sm(H O) ][Sm(H O) ] (10), but obviously lower
2
5
2
7
than the homogeneous RE-POMs reported by the Mizuno
group (Table S3).
Yd-POM was remarkable higher than that of K [Y(α-1-
7
1
4,15
P W O )] (YP W , Entry 15) and K [Yb(α-1-P W O )]
2
17 61
2
17
7
2
17 61
E
DOI: 10.1021/acs.inorgchem.8b00763
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 6. Energy levels and d orbital contributions of Y-POM and YP W (green lines represent LUMOs containing d orbitals of Y).
2
17
(
YbP W , Entry 16), which are composed of the monovacant
smoothly with conversion higher than 90% in 4 h, while under
the same conditions, the catalytic performances for 4-tert-
butylbenzaldehyde (Entry 6) were relatively low. Cyclic
aliphatic ketones (Entries 7 and 8) can also smoothly react
with TMSCN to afford the corresponding products in high
yields. One exception is 4-tert-butylcyclohexanone (Entry 9),
which is similar to 4-tert-butylbenzaldehyde (Entry 6) in
aldehydes derivatives. Less reactive acetophenone derivatives
(Entries 10 and 11) are also converted into the corresponding
cyanohydrin trimethylsilyl ethers but in relatively low yields.
No signal was observed in the 31P NMR spectrum in the
catalytic process of Entries 1, 2, 3, and 8 in Table 2, indicating
that Y-POM was insoluble and no catalyst decomposition
occurred. After the cyanosilylation reactions, the catalysts could
be easily recovered quantitatively (>99.9%) by centrifugation
and purified by washing with acetone and deionized water and
could be reused in the next cycles (Figures S19 and 20). The
PXRD patterns and the IR spectrum of the recovered samples
were unchanged after cyanosilylation (Figures S9 and S21).
2
17
48,49
Dawson POMs and RE.
In lacunary RE-POMs, the RE ions
are deeply embedded in the lacuna of the negatively charged
POMs skeleton, which can weaken the electropositivity and
Lewis acidity of the RE and also decrease the available
coordination space. Conversely, in the title compounds all the
RE ions are located in the external polar region of the
{
P W Ta O } skeleton, and the anion cluster and cations in
2 15 3 62
the title compounds are relatively independent. Such a bonding
pattern should decrease the interaction between the RE and
{
may be enhanced. Furthermore, more coordination space of RE
is available for substrate molecules.
To further understand the difference between the title
compounds and the monovacant RE-POMs, theoretical
calculations were employed to determine their Lewis acid/
P W Ta O }. Hence, their Lewis acidity or Lewis alkalinity
2 15 3 62
base character. The calculated HOMO of Y-POM and YP W
2
17
are both composed of the p orbitals of O atoms (Figure 6). The
HOMO energy level of Y-POM is slightly lower than that of
YP W . Thus, the Lewis base character of the POMs section of
2
17
Y-POM may be slightly weaker than that of YP W . On the
other hand, the lowest energy levels of the unoccupied orbitals
2
17
CONCLUSIONS
■
{
By the reaction of Ta/W mixed-addendum POM
P W Ta O } and RE, a convenient and controllable
2 15 3 62
containing d orbital of Y atom for Y-POM and YP W are very
2
17
similar. But remarkably, the d orbitals of Y for Y-POM
synthetic method for a family of RE-POMs has been discovered
and further developed to facilely prepare the nanocrystals and
aggregates of these compounds for the first time. The
aggregates of five different morphologies were isolated by
controlling conditions. The remarkable catalytic activity of the
title catalysts for the cyanosilylation were elucidated by the
following factors. (i) In the crystal structures of the title
compounds, there is open coordination space around the RE
cations available for substrate molecules. (ii) The RE cations
contributed greatly to the LUMOs in the range from 22.3% to
9
8.1%. The Y atoms in YP W have only 1.7% d orbital
2 17
contributions for LUMOs. The electrons accepting ability of Y
in Y-POM is likely stronger than that of YP W . Hence, the
2
17
Lewis acid character of Y in Y-POM may be greater than that in
YP W . Thus, it can be concluded that the higher catalytic
2
17
activity of the title compounds mainly results from the higher
Lewis acid character.
The cyanosilylation reactions of different aldehydes/ketones
with TMSCN catalyzed by Y-POM-1:1−0.5h were examined
as shown in Table 3. The conversion efficiencies of aldehydes
are located in the external polar region of the {P W15Ta O62};
2 3
such a bonding pattern endows the RE with strong Lewis
acidity, as indicated by theoretical calculations. (iii) The
nanocrystals and aggregates possess increased surface areas
and surface atoms. The substitution reaction of the labile
terminal aqua ligands on the RE cations by organic ligands and
(
7
Entries 1−6) are generally higher than that of ketones (Entries
−11). Different substituents on the aromatic ring of
benzaldehyde derivatives had no notable effect on the reaction
yields among Entries 1, 2, 3, 4, and 5, which proceeded
F
DOI: 10.1021/acs.inorgchem.8b00763
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the assembly mechanism of the aggregates are currently under
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00763.
■
*
S
(
6) Li, Z.; Li, X. X.; Yang, T.; Cai, Z. W.; Zheng, S. T. Four-Shell
Polyoxometalates Featuring High-Nuclearity Ln26 Clusters: Structural
Transformations of Nanoclusters into Frameworks Triggered by
Transition-Metal Ions. Angew. Chem., Int. Ed. 2017, 56, 2664−2669.
Characterization methods and X-ray crystallography
analysis (crystallographic collection and refinement
details), PXRD, FTIR, TG, SEM and TEM images
BET surface areas, solid-state emission spectra, additional
catalytic experiments and computational details (PDF)
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Coordination-Driven Self-Assembly of a 2D Graphite-Like Framework
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via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
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Crystallographic Data Centre, 12 Union Road, Cambridge CB2
(
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AUTHOR INFORMATION
Corresponding Authors
(X.C.) E-mail: xnchen@htu.edu.cn.
(S.L.) E-mail: liusx@nenu.edu.cn.
(Z.Z.) E-mail: zhengzp@sustc.edu.cn.
■
2
(
*
*
*
́
N.;
̂
́
Polyoxocationic Complexes: Experimental and Theoretical Stability
ORCID
Studies and Lewis Acid Catalysis. Chem. - Eur. J. 2011, 17, 14129−
Shuxia Liu: 0000-0003-0235-8594
Xiaoming Ma: 0000-0002-3209-4182
Nana Ma: 0000-0003-3225-9554
Jie Zhang: 0000-0002-7693-6388
Xuenian Chen: 0000-0001-9029-1777
1
4138.
(13) Stroobants, K.; Moelants, E.; Ly, H. G.; Proost, P.; Bartik, K.;
Parac-Vogt, T. N. Polyoxometalates as a Novel Class of Artificial
Proteases: Selective Hydrolysis of Lysozyme under Physiological pH
and Temperature Promoted by a Cerium(IV) Keggin-Type Poly-
oxometalate. Chem. - Eur. J. 2013, 19, 2848−2858.
Present Address
(14) Kikukawa, Y.; Suzuki, K.; Sugawa, M.; Hirano, T.; Kamata, K.;
⊥
Shenzhen Grubbs Institute and Department of Chemistry,
Yamaguchi, K.; Mizuno, N. Cyanosilylation of Carbonyl Compounds
with Trimethylsilyl Cyanide Catalyzed by an Yttrium-Pillared
Silicotungstate Dimer. Angew. Chem., Int. Ed. 2012, 51, 3686−3690.
Southern University of Science and Technology, Shenzhen,
Guangdong 518000, China
(15) Suzuki, K.; Sugawa, M.; Kikukawa, Y.; Kamata, K.; Yamaguchi,
Notes
K.; Mizuno, N. Strategic Design and Refinement of Lewis Acid−Base
Catalysis by Rare-Earth-Metal-Containing Polyoxometalates. Inorg.
Chem. 2012, 51, 6953−6961.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (Grant Nos. 21401047, 21371029,
■
(16) An, H.; Wang, L.; Hu, Y.; Fei, F. Temperature-Induced Racemic
Compounds and Chiral Conglomerates Based on Polyoxometalates
and Lanthanides: Syntheses, Structures and Catalytic Properties.
CrystEngComm 2015, 17, 1531−1540.
2
1603062, and 21771057), the China Postdoctoral Science
Foundation (No. 2017M622350), the Open Research Fund of
Henan Key Laboratory of Polyoxometalate Chemistry,
(17) Fei, F.; An, H.; Meng, C.; Wang, L.; Wang, H. Lanthanide-
Supported Molybdenum−Vanadium Oxide Clusters: Syntheses,
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(
HNPOMKF1702), Key Program for High School of Henan
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Scientists Laboratory Project (C17213101). All calculations
were supported by the High Performance Computing Center of
Henan Normal University.
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DOI: 10.1021/acs.inorgchem.8b00763
Inorg. Chem. XXXX, XXX, XXX−XXX