A novel silver(I)-Keggin-polyoxometalate inorganic-organic hybrid: A Lewis acid catalyst for cyanosilylation reaction

Article abstract of DOI:10.1039/c5ce00953g

A novel polyoxometalate (POM) hybrid material based on a Keggin polyanion and silver(I)-organic sheet, namely, [Ag₄(apym)₄(SiW₁₂O₄₀)]_n (1), where apym = 2-aminopyrimidine, has been synthesized under h

Full text of DOI:10.1039/c5ce00953g

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A novel silverIJI)-Keggin-polyoxometalate
inorganic–organic hybrid: a Lewis acid catalyst for
cyanosilylation reaction†
Cite this: DOI: 10.1039/c5ce00953g
Tuo-Ping Hu,ab Ya-Qin Zhao,a Kai Mei,a Shu-Jie Lin,a Xing-Po Wanga and
a
*
Di Sun
A novel polyoxometalate (POM) hybrid material based on a Keggin polyanion and silverIJI)–organic sheet,
namely, ijAg4IJapym)4IJSiW12O40)]n (1), where apym = 2-aminopyrimidine, has been synthesized under
hydrothermal conditions and characterized by elemental analysis, IR spectroscopy, thermal gravimetric
analysis, powder X-ray diffraction and single-crystal X-ray diffraction. In the structure of 1, polyanions link
2D 3-connected 4·82-fes AgI–organic sheets to generate a 3D complicated framework. Moreover, the cat-
alytic performance of 1 towards the cyanosilylation of carbonyl compounds has been tested under mild
conditions, showing a high turnover number and turnover frequency.
Received 18th May 2015,
Accepted 24th June 2015
DOI: 10.1039/c5ce00953g
www.rsc.org/crystengcomm
catalyze chemical reactions. The combination of two species
into one hybrid may bring a new structure as well as function-
Introduction
As a large family of inorganic materials, polyoxometalates
(POMs) are anionic metal oxide clusters of early transition
metals such as Mo, W, V, and Nb in their highest oxidation
states.1 POMs are excellent candidates for secondary building
units (SBUs) because of their several significant characteris-
tics including high stability and ease of functionalization.2
Hybrid inorganic–organic POMs constructed from inorganic
POM building blocks with various organic and/or transition
metal complex moieties can bring novel structural motifs and
functionalities into one entity.3–8 So, the design and develop-
ment of functional POM-based hybrids is a hot research field
aiming at potential applications in areas like biology, mate-
rials chemistry, catalysis, medicine, electronics, photo-
catalysis, electrochemistry, and so on,9 which, in turn, makes
the deliberate assembly strategies for the synthesis of new
POM-based hybrids come forth. Many POM-based hybrids
with variable dimensionalities and topologies have been
constructed under hydrothermal conditions.10 The silver ion
could exhibit versatile coordination numbers (2–8) in the con-
struction of interesting motifs.11 When a low coordination
number of the Ag atom is incorporated, it exhibits unsatu-
rated coordination sites, which may act as Lewis acid sites to
alities such as enhanced chemical catalysis performance. As a
very simple but versatile ligand, 2-aminopyrimidine (apym)
has found its niche in silverIJI) coordination chemistry,12 but
to date, incorporation of silverIJI)-apym motifs into POMs has
not been achieved.
Until now, the systematic synthesis and utilization of sat-
isfying POM catalysts has been a crucial challenge for chem-
ists because of some inherent drawbacks of these materials,
like high solubility in aqueous solution, small specific sur-
face area and low stability under catalytic conditions.13 These
drawbacks widely reduce the catalytic efficiency and recovery
rate as well as restrict their applications. On the other hand,
the characteristics of nanosized space and functional surface
of metal–organic frameworks (MOFs) make them to be effec-
tive host materials.14,15 Especially the internal restricted and
functionalized channels of MOFs provide a distinct chemical
environment to capture molecules and facilitate catalytic
reactions.16 Hence, utilizing POMs to construct MOFs is
intriguing and has been reported in succession.17,18 Never-
theless, there are only a few articles reported on the use of
Keggin polyoxoanions as templates to direct the construction
of catalytic MOFs.19
Based on the above consideration and our previous
works, in this study, we achieved the goal of obtaining a
Keggin-POM based organic–inorganic hybrid material,
a Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of
Chemistry and Chemical Engineering, Shandong University, Jinan, 250100,
PR China. E-mail: dsun@sdu.edu.cn
b Department of Chemistry, North University of China, Taiyuan, Shanxi 030051,
People's Republic of China
† Electronic supplementary information (ESI) available: IR, TGA PXRD and 1H
NMR. CCDC number 1059648. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c5ce00953g
ijAg4IJapym)4IJSiW12O40)]n (1, apym
= 2-aminopyrimidine)
constructed from a Keggin polyoxoanion and AgIJI)-apym motifs.
This novel silverIJI)-Keggin-POM inorganic–organic hybrid
exhibits high catalytic efficiency for the cyanosilylation of
carbonyl compounds.
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Results and discussion
Crystal structure description
for an ideal trigonal bipyramid).20 The coordination mode of
the apym ligand in compound 1 is uniform μ2-N1:N1 (Fig. 1b)
4−
(Ag–N: 2.15IJ4)–2.24IJ4) Å). The Keggin-type SiW12O40 anion
The single crystal X-ray study reveals that 1 crystallizes in the
monoclinic space group Pn. The structure of 1 is a 3D frame-
work constructed by 2D silverIJI)–organic sheets and
acts as an octadentate inorganic ligand that coordinates to
eight AgIJI) ions completely with Ag–O bond distances ranging
from 2.57IJ4)–2.88IJ2) Å (Fig. 1c). A better insight into the
structural nature of 1 can be achieved by the application of
topological analysis; that is, simplifying the structure into
node and linker nets. According to the simplification princi-
ple, each AgIJI) atom can be considered as a 3-connected
node, while coordinated apym ligands and μ2-O atoms serve
as linkers. Therefore, the Ag-apym motifs can be described as
2D 3-connected fes (4·82) sheets, as illustrated in the Fig. 1d.
On account of the oxygen atoms which bond to the AgIJI)
4−
SiW12O40 building blocks. The asymmetric unit of 1 is com-
posed of four AgIJI) ions, four apym ligands, and one
4−
SiW12O40 anion. As shown in Fig. 1a, Ag1, Ag3 and Ag4
atoms are all four-coordinated by two nitrogen atoms from
two different apym ligands and two oxygen atoms from two
4−
different SiW12O40 anions, showing a distorted AgN2O2
tetrahedral coordination geometry. The Ag2 atom binds
to two nitrogen atoms from two different apym ligands
and three oxygen atoms from three different SiW12O40
anions, displaying a five-coordinated AgN2O3 distorted square
pyramidal geometry with a τ5 parameter of 0.198 (τ5 = Addi-
son parameter, τ5 = 0 for an ideal square pyramid and τ5 = 1
4−
4−
4−
atom from SiW12O40
anions, SiW12O40
anions, AgIJI)
atoms, and organic apym ligands finally form a complicated
3D network (Fig. 1e).
Catalytic activity
To evaluate the catalytic activity of compound 1, we used the
cyanosilylation of carbonyl compounds catalyzed by Lewis
acids as a test reaction. The reaction of carbonyls with tri-
methylsilyl cyanide (TMSCN) in the presence of a catalyst pro-
duces the corresponding cyanohydrin trimethylsilyl ethers,
which are industrially valuable and important intermediates
in the synthesis of cyanohydrins, α-hydroxy acids, β-amino
alcohols and other biological compounds.21 Before the cata-
lytic test, compound 1 was activated at 50 °C under vacuum
for 20 min. The cyanosilylation was carried out in the pres-
ence of 1 with a 1 : 2 molar ratio of the selected carbonyls
and TMSCN for 4 h at room temperature under nitrogen. The
conversions were calculated based on 1H NMR spectroscopy
(see the ESI†). In the course of studies of the catalytic proper-
ties of 1, we found that 1 with 0.1 mol% loading is an active
and selective catalyst for cyanosilylation of various types of
carbonyl compounds (Table 1). The reaction of benzaldehyde
with TMSCN proceeds with high conversion and the corre-
sponding product is obtained in up to 96.2% yield, in a short
reaction time (4 h) and under mild conditions. This high
conversion indicates that 1 is a quite efficient catalyst for this
reaction. For ketones, the activity is much lower due to the
low reactivity of ketones compared with aldehydes.
The plot of yield versus time for the cyanosilylation reac-
tion of benzaldehyde and TMSCN with different loadings of
compound 1 is presented in Fig. 2. With the extension of
reaction time from 1 h to 6 h, an obvious increase in conver-
sion was detected in the presence of 0.1 mol% loading of 1
(entries 1–5, Table 2). The same experimental phenomenon
was obtained when the loading of catalyst 1 was 1 mol%
(entries 7–12, Table 2). The 0.5 mol% loading of 1 was also
tested, giving a conversion of 93.4% (entry 6, Table 2).
Furthermore, when the reaction was performed with 0.1
mol% of the catalyst, the turnover number (TON) can reach
up to 565 for the cyanosilylation of benzaldehyde in 1 h, and
the corresponding turnover frequency (TOF) is 565 h−1, which
Fig. 1 (a) Polyhedral and ball-and-stick representation of the coordi-
nation environments of Ag atoms in 1. (b) Coordination mode of apym
4−
found in 1. (c) Polyhedral representation of the octadentate SiW12O40
anion in 1. (d) 3-Connected (4·82)-fes 2D Ag-apym sheet. (e) Polyhedral
and ball-and-stick representation of the 3D framework of 1 (colour
code: purple = Ag, red = O, blue = N, gray = C).
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Table 1 Results for the catalytic cyanosilylation of aldehydes in the pres-
ence of 1a
Entry
1
Aldehyde/Ketone
Conversion (%)
96.2
2
3
4
99.7
99.5
97.1
Fig. 2 Plots of cyanohydrin conversion vs. time for the cyanosilylation
reaction of benzaldehyde with TMSCN at 0.1 mol% loading of 1 (blue)
and at 1 mol% loading of 1 (red).
5
99.5
99.0
99.5
2.6
6
amounts of residual Ag and W atoms (~0.007% and ~0.005%,
respectively), which may come from some unknown silver or
W species or nanoparticles that are difficult to remove by fil-
tration. These results clearly prove that the cyanosilylation
reaction could mainly proceed in a heterogeneous manner
and the catalytic effect from leached active species in the
solution, if any, was insignificant.
7
8
We also checked the stability and recyclability of 1 in
cyanosilylation reaction. Benzaldehyde is still selected as the
reaction substrate (1% loading, 30 min). The PXRD of recov-
ered catalyst 1 confirmed the structural integrity of 1 after 2
h of catalytic reaction (Fig. S1 in the ESI†), thus verifying the
excellent stability of 1 during the cyanosilylation reaction.
Also importantly, for 1, the recycling experiments (1% load-
ing, 30 min) presented in Fig. S5† suggest a decrease in activ-
ity from 97% to 85% during the first cycle, but the following
four cycles only show a very slight decrease in activity from
85% to 78%. However, if the reaction time was elongated to 2
h, the yield could recover to 83%.
9
3.6
10
~0
a
Reaction conditions: aldehyde/ketone, 0.5 mmol; TMSCN, 1 mmol;
catalyst 1, 5 × 10−4 mmol (0.1%, based on the formula weight of 1);
at room temperature under N2 for 4 h.
is much higher than those of several previously reported
MOFs (see Table S2 in the ESI†).22–26 Under other similar
reaction conditions, the respective TONs for 2 h, 3 h, 4 h,
and 6 h are 735, 909, 962, and 999. Additionally, upon raising
the catalyst amount from 0.1 to 0.5 mol%, the TON was 187
under 2 h of reaction time. Finally, as shown in Table 2, the
TONs of 1 mol% loading for compound 1 were 11, 45, 97, 97,
99, and 99 with increasing reaction time, respectively.
A series of experiments such hot filtration test and induc-
tively coupled plasma (ICP) analysis were performed to verify
the heterogeneous nature of catalyst 1. When the catalytic
reaction was stopped after 0.5 hour (benzaldehyde as sub-
strate), the supernatant obtained by high-speed centrifugation,
then filtration through a regular filter paper did not afford
any additional product by further reaction for another 1
hour. ICP analysis of the supernatants gave only trace
Experimental
Methods
All chemicals and solvents used in the syntheses were of ana-
lytical grade and used without further purification. IR spectra
were recorded on a Nicolet AVATAT FT-IR360 spectrometer as
KBr pellets in the frequency range of 4000–400 cm−1. Elemen-
tal analyses (C, H, N contents) were carried out on a Vario EL
III analyzer. Elemental analyses of Si, W, and Ag were
performed by a Leaman inductively coupled plasma (ICP)
spectrometer. Powder X-ray diffraction (PXRD) data were col-
lected on a Philips X'Pert Pro MPD X-ray diffractometer with
MoKα radiation equipped with an X'Celerator detector.
Thermogravimetric analyses (TGA) were performed on a
Netzsch STA 449C thermal analyzer from room temperature
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Table 2 Optimization of the parameters of the cyanosilylation reaction
between benzaldehyde and TMSCN with compound 1 as the catalyst
Table 3 Crystal data for compound 1
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
C16H20Ag4N12O40SiW12
3686.21
298(2)
Monoclinic
Pn
12.476(4)
12.003(4)
17.582(5)
90
Entry Catalyst (%mol) Time
Conversion (%) TON TOF (h−1
)
1
2
3
4
5
6
7
8
0.1
0.1
0.1
0.1
0.1
0.5
1
1
1
1
1
1 h
2 h
3 h
4 h
6 h
2 h
56.5
73.5
90.9
96.2
99.9
93.4
11.5
565
735
909
962
999
187
11
45
97
97
99
565
368
303
241
167
93
132
270
194
97
5 min
10 min 45.0
30 min 97.1
1 h
2 h
3 h
β/°
γ/°
94.066(4)
90
2626.4(13)
2
Volume/Å3
Z
9
10
11
12
97.1
99.0
99.0
ρcalc/mg mm−3
4.661
50
33
μ/mm−1
27.740
3220.0
3.394 to 50°
1
99
FIJ000)
TON = %conversion (mmol of substrate/mmol of catalyst); TOF =
%conversion (mmol of substrate/mmol of catalyst per hour).
2θ range for data
collection
Index ranges
−13 ≤ h ≤ 14, −7 ≤ k ≤ 14, −15 ≤ l ≤
20
Reflections collected
11 357
Independent reflections
7563 ijRIJint) = 0.0476]
to 800 °C under nitrogen atmosphere at a heating rate of 10
°C min−1. 1H NMR spectra were measured on a Bruker
AVANCE-300 NMR Spectrometer.
Data/restraints/parameters 7563/206/797
Goodness-of-fit on F2
0.959
Final R indexes [I ≥ 2σIJI)] R1 = 0.0561, wR2 = 0.1464
Final R indexes [all data] R1 = 0.0579, wR2 = 0.1487
Synthesis of ijAg4IJapym)4IJSiW12O40)]n (1)
A mixture of AgNO3 (27.2 mg, 0.16 mmol), apym (4.75 mg,
0.05 mmol), H4SiW12O40 (60 mg, 0.021 mmol) and 5 mL of
H2O was adjusted to pH = 5.0 with 3d 1 M NaOH and stirred
for 10 min at room temperature. The resulting mixture was
sealed in a 25 mL Teflon-lined high pressure bomb, then
heated to 120 °C for 10 hours and kept at 120 °C for 50
hours, then slowly cooled to 30 °C for 13 hours. Yellow block
crystals of 1 were isolated by filtration, washed with alcohol,
atoms were placed in calculated positions and included as
riding atoms with isotropic displacement parameters 1.2–1.5
times Ueq of the attached C atoms. All structures were exam-
ined using the Addsym subroutine of PLATON30 to assure
that no additional symmetry could be applied to the models.
Crystal data for compound 1 is given in Table 3. Selected
bond lengths and angles are collated in Table S1 (ESI).†
and dried in air. Elemental analysis calcd (%) for
1
IJC16H20Ag4N12O40SiW12): C, 5.21; H, 0.55; N, 4.56; Si, 0.76; W,
59.85; Ag, 11.71. Found: C, 5.15; H, 0.62; N, 4.46; Si, 0.67; W,
59.75; Ag, 11.68. Selected IR peaks (cm−1): 3438(w), 3334(w),
1626(m), 1580(m), 1478(w), 924(s), 784(s), 526(w).
Catalytic studies
In a typical experiment, to a Schlenk tube was added a mix-
ture of aldehyde and IJCH3)3SiCN (TMSCN), and then solid
MOF-catalyst 1 was rapidly added into the reactor. The cyano-
silylation reactions of aldehydes were carried out at room
temperature under N2 atmosphere, and aliquots were taken
X-ray crystallography
Single crystals of compound 1 with appropriate dimensions
were analyzed under an optical microscope and quickly
coated with high vacuum grease (Dow Corning Corporation)
before being mounted on a glass fiber for data collection.
Data for them were collected on a Bruker Apex II CCD diffrac-
tometer with a graphite-monochromated Mo Kα radiation
source (λ = 0.71073 Å). Cell parameters were retrieved using
SMART software and refined with SAINT on all observed
reflections.27 Data reduction was performed with the SAINT
software and corrected for Lorentz and polarization effects.
Absorption corrections were applied with the program
SADABS.27 In all cases, the highest possible space group was
chosen. All structures were solved by direct methods using
SHELXS-97 (ref. 28) and refined on F2 by full-matrix least-
squares procedures with SHELXL-97.29 Atoms were located
from iterative examination of difference F-maps following
least squares refinements of the earlier models. Hydrogen
1
and analyzed by H NMR spectroscopy at given time intervals
to evaluate the yield of aldehydes or ketones.
Conclusions
In this work, we have successfully constructed one POM-
supported hybrid material based on a Keggin and silverIJI)-
apym framework under hydrothermal condition. Compound
1
shows a fascinating 3D framework incorporating 2D
3-connected fes (4·82) ijAgIJapym)]n sheets. In the presence of
1, the cyanosilylation of several structurally diverse aldehydes
and ketones with TMSCN selectively proceeded to produce
the corresponding cyanohydrin trimethylsilyl ethers. Espe-
cially, the catalytic performance for aldehydes was very con-
siderable; for example, TON = 565 and TOF = 565 h−1 were
observed for benzaldehyde. In consequence, this approach
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provides a new strategy for the design of a highly efficient
catalyst for Lewis acid-catalyzed reactions.
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Acknowledgements
This work was supported by the NSFC (grant no. 21201110),
Research Award Fund for Outstanding Middle-aged and
Young Scientist of Shandong Province (BS2013CL010), Inter-
national Scientific and Technological Cooperation Projects of
Shanxi Province (no: 2015081043), and “131” Leading Talents
Project in Colleges and Universities, Science and Technology
Innovation Project of Shanxi Province (no. 2014101002).
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