Novel isopolyoxotungstate [H2W11O38] 8- based metal organic framework: As lewis acid catalyst for cyanosilylation of aromatic aldehydes

Article abstract of DOI:10.1021/ic5004879

A novel polyoxometalate-based metal organic framework (POMOF) constructed from isolated isopolyoxotungstate [H₂W₁₁O ₃₈]^8- cluster, {[Cu₂(bpy)(H₂O) _5.5]₂[H₂W₁₁O₃₈] ·3H₂O·0.5CH₃CN} (1, where bpy = 4,4′-bpydine), has been synthesized under solvothermal conditions and charaterized by elemental analysis, infrared spectroscopy, and single-crystal X-ray diffraction. In 1, {W₁₁} clusters are alternately linked by two [Cu(2)(H₂O)_1.5(O_t)₃(N)] ²⁺ cations in an unexpected end-to-end fashion leading to a one-dimensional (1D) chain. Adjacent 1D chains are linked through Cu(1)-bpy-Cu(2) in an opposite direction to form a two-dimensional (2D) wavelike sheet along the ab plane. These 2D sheets are further stacked in a parallel fashion giving rise to the 1D channels with copper(II) cations aligned in the channels. The resulting POMOF acted as a Lewis acid catalyst through a heterogeneous manner to prompt cyanosilylation with excellent efficiency.

Full text of DOI:10.1021/ic5004879

Article
pubs.acs.org/IC
Novel Isopolyoxotungstate [H2W11O38]8− Based Metal Organic
Framework: As Lewis Acid Catalyst for Cyanosilylation of Aromatic
Aldehydes
Qiuxia Han,*,† Xueping Sun,‡ Jie Li,† Pengtao Ma,† and Jingyang Niu*,†
†Institute of Molecular and Crystal Engineering, School of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004,
People’s Republic of China
‡Department of Environmental Engineering and Chemisty, Luoyang Institute of Science and Technology, Luoyang 471023, People’s
Republic of China
S
* Supporting Information
ABSTRACT: A novel polyoxometalate-based metal organic framework (POMOF)
constructed from isolated isopolyoxotungstate [H2W11O38]8− cluster, {[Cu2(bpy)-
(H2O)5.5]2[H2W11O38]·3H2O·0.5CH3CN} (1, where bpy = 4,4′-bpydine), has been
synthesized under solvothermal conditions and charaterized by elemental analysis,
infrared spectroscopy, and single-crystal X-ray diffraction. In 1, {W11} clusters are
alternately linked by two [Cu(2)(H2O)1.5(Ot)3(N)]2+ cations in an unexpected end-to-
end fashion leading to a one-dimensional (1D) chain. Adjacent 1D chains are linked
through Cu(1)−bpy−Cu(2) in an opposite direction to form a two-dimensional (2D)
wavelike sheet along the ab plane. These 2D sheets are further stacked in a parallel
fashion giving rise to the 1D channels with copper(II) cations aligned in the channels.
The resulting POMOF acted as a Lewis acid catalyst through a heterogeneous manner to
prompt cyanosilylation with excellent efficiency.
[XW12O40]n− and Wells−Dawson [XpW18Oy]n− (p = 1 or 2, m
INTRODUCTION
■
= 60 or 62) anions, have gone through extensive development
Polyoxometalate-based metal organic frameworks (POMOFs),
with infinite networks constructed from polyoxometalates
(POMs) that act as either templates or inorganic connecting
and were used to construct POMOFs with excellent catalytic
efficiency in a heterogeneous manner.10−16 Compared to the
popular hetero-POT-based POMOFs, only a few examples of
nodes, and metal organic frameworks (MOFs), have attracted
POMOFs consisting of iso-POTs with the general formula
increasing attention and developed rapidly as a new class of
[WqOy]n− were reported,17,18 which presents a great oppor-
organic−inorganic hybrid solids over recent years.1,2 The
unique physical and chemical properties and diverse coordina-
tunity to launch explorations on their possibility to be reliably
utilized as a set of transferable building blocks in the formation
tion modes of POMs together with the diversity of organic
of new materials.
linkers provide a stimulus to the synthesis of multifunctional
POMOFs. Recently, POMOFs have revealed broad prospects
Among the reported iso-POTs, [W11O38]10− (abbreviated as
{W11}) is a fundamental building block because it has the
in applications as solid acids and oxidation catalysts and
potential to be degraded to a [H2W8O32]14− cluster or be
become attractive candidates as heterogeneous catalysts
aggregated to larger clusters.19−22 Cronin and co-workers
because of their size- and shape-selective restriction abilities
through elaborately fine-tuned channels or pores.3,4 In
reported several appealing iso-POT clusters generated from
pure iso-POT {W11} clusters, such as [H4W22O74]12−
,
particular, the introduction of POMs into MOFs at the
molecular scale will contribute to the complexity, stability, and
corresponding functionality of the resulting POMOFs, thus
endowing them with new features and multiple functionalities
distinctively different from the POMs or MOFs alone.5,6
Polyoxotungstates (POTs) are a large class of well-known
polynuclear tungstate−oxo clusters with an unmatched
structure variety and excellent thermal and oxidative stability
properties, representing one of the diverse building blocks for
the construction of functional POMOFs.7−9 Two main types of
POTs are known: the hetero-POTs and the iso-POTs. Hetero-
POTs with the general formula [XpWqOy]n− (where X = PV,
AsV, SiIV, GeIV), especially the archetypal and lacunary Keggin
[H10W34O116
sisted of two {W11} subunits fused via two μ2-oxo bridges in a
] ]
18−, and [H12W36O120 12−. [H4W22O74]12− con-
18−
trans fashion; [H10W34O116
]
was built from two {W11}
clusters connecting with one [H2W12O42]10− {W12} cluster
through two μ2-oxo bridges in a trans fashion, giving an
expanded §-like architecture;20 and [H12W36O120
]
displayed
12−
an overall triangular shaped cluster constructed from three
{W11} clusters linking together with three {W1} bridges sharing
four bridging oxo ligands in the equatorial plane.21 Then, they
Received: March 1, 2014
© XXXX American Chemical Society
A
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Inorganic Chemistry
reported quadrangular shaped superstructure
Article
a
cluster first exists as an isolated unit rather than an aggregation
and simultaneously coordinates with six Cu complex ions to
display a new 2D framework. These 2D sheets are further
stacked in a parallel fashion giving rise to the 1D channels with
copper(II) ions aligned in the channels, which ensures the
resulting well-defined POMOF 1 can act as a Lewis acid
catalyst to prompt the cyanosilylation with excellent efficiency
by interacting with guest molecules getting into the channels.
Besides, catalyst 1 can be easily separated from product and
reusable at least three times with only moderate loss of activity,
which are considered advantages of this heterogeneous catalyst.
[H10TeVI2W58O198 26−{W58} cluster that consisted of two
]
tellurium-centered Dawson clusters and two {W11} clusters.22
This fact clearly indicated that the {W11} cluster is very labile
and more inclined to assemble by W−O−W bonds. In addition,
Kortz and co-workers further extended the structural motif and
discovered a new member of the iso-POT family based on the
{W22} architecture, such as [Ln2(H2O)10W22O71(OH)2]8− (Ln
= lanthanide), in which the {W22} cluster acted as a tridentate
ligand coordinated to two Ln3+ ions.23 Yao also reported a
similar structure Ca2(H2O)12{Ca4(H2O)18[H2W11O37]2}·
20H2O, in which the {W22} decorated six calcium ions.24
Subsequently, Kortz reported the first example of polyanion,
[Ln2(H2O)10W28O93(OH)2]14−, which was constructed by two
{Ln(H2O)n}3+ groups linking two isolated {W11} clusters and a
hexatungstate {W6} fragment.25
EXPERIMENTAL SECTION
■
General Methods and Materials. All reagents were commercially
purchased and used without further purification. (TBA)4W10O32
(where TBA = [(C4H9)4N]+) was prepared according to the literature
and characterized by IR spectroscopy.26 Elemental analyses (EA) of C,
H, and N were performed on a Vario ELIII elemental analyzer.
Inductively coupled plasma (ICP) of Cu and W analyses were
performed on a Jarrel-Ash Model J-A1100 spectrometer. The infrared
spectra (IR) were recorded from a sample powder palletized with KBr
on a Nicolet170 SXFT-IR spectrometer over a range of 400−4000
cm−1. Powder X-ray diffraction (PXRD) data were obtained on a
Rigaku Model D/Max-2400 X-ray diffractometer with a sealed copper
As illustrated previously, most of the reported iso-POTs
based on {W11} clusters were purely inorganic aggregations and
synthesized in the conventional aqueous solution. However, the
organic−inorganic hybrids containing isolated {W11} cluster
have not been reported. Because the {W11} cluster possesses
high charges, strongly basic oxygen surfaces and much more
lacunary-like positions, we envisioned that the {W11} cluster
may be beneficial for coordination with more transition metal
ions as multidentate ligand in the presence of organic ligand to
form new POMOFs with multifunctionalities. Therefore, the
present work is based on excellent past studies of the POMOFs
in catalysis aspect, introducing a new strategy to broaden the
application of POMOFs as heterogeneous catalysts for
cyanosilylation of aromatic aldehydes. Using [W10O32]4− as
starting material to react with Cu(II) cation and bpy under
hydrothermal conditions resulted in a novel POMOF based on
iso-POT {W11} {[Cu2(bpy)(H2O)5.5]2[H2W11O38]·3H2O·
0.5CH3CN} (1; see Scheme 1). Unexpectedly, the {W11}
1
tube (λ = 1.54178 Å). H NMR was measured on a Varian INOVA-
400 spectrometer with chemical shifts reported in parts per million
(ppm; in CDCl3; tetramethylsilane (TMS) as internal standard).
Synthesis of {[Cu2(bpy)(H2O)5.5]2[H2W11O38]·3H2O·0.5CH3CN} (1).
Compound 1 was synthesized by the self-assembly approach under
hydrothermal conditions: (TBA)4W10O32 (65 mg, 0.02 mmol),
Cu(NO3)2·6H2O (57.5 mg, 0.19 mmol), and bpy (8 mg, 0.05
mmol) were mixed in 3 mL of water and 3 mL of acetonitrile solution
at pH 3.7. The resultant mixture was stirred for 6 h, then sealed in a 25
mL Teflon-lined autoclave, and maintained at 130 °C for 3 days. After
cooling the autoclave to room temperature, green cubic single crystals
were separated, washed with water, and air-dried. The crystals were
obtained with a moderate yield of 40% based on (TBA)4W10O32. EA
and ICP (%). Calcd for C21H47.5Cu4N4.5O52W11: C 7.27, H 1.38, N
1.82, Cu 7.32, W 58.25;. Found: C 7.30, H 1.32, N 1.88, Cu 7.37, W
58.34. IR (KBr; ν): 3423 (br, s), 2927 (w), 2862 (w), 2281 (w) 1606
(s), 1542 (w), 1488 (w), 1412 (m), 1221 (w), 935 (s, WOt), 823 (s,
W−O−W) cm−1.
Scheme 1. Synthetic Procedure of 1, Showing the
Rearrangement of Precursor [W10O32]4− and the 2D
Framework Constructed by [H2W11O38]8− Anions and
a
Copper(II) Complexes.
X-ray Crystallographic Analysis. The crystallographic data of 1
were collected on a Bruker SMART APEX CCD diffractometer with
graphite-monochromated Mo Kα (λ = 0.71073 Å) using the SMART
and SAINT programs.27,28 The intensity data were corrected for
Lorentz and polarization effects as well as for multiscan absorption.
The structure was solved by direct methods and refined by full-matrix
least-squares on F2 using the SHELXTL-97 program package.29 All of
the non-hydrogen atoms except for partial crystal water and CH3CN
molecules were refined anisotropically in 1. Positions of the hydrogen
atoms attached to the carbon atoms were refined isotropically as a
riding mode using the default SHELXTL parameters. The hydrogen
atoms attached to water molecules were not located in 1.
Crystallographic data for 1 were summarized in Table 1. Selected
bond lengths and angles of 1 are listed in Table S1 in the Supporting
Information.
RESULTS AND DISCUSSION
■
Synthesis. Compound 1 was solvothermally prepared by
reaction of (TBA)4W10O32, Cu(NO3)2·6H2O, and bpy in mixed
water and acetonitrile with a yield of 40%. As we know, the
formation of various structure types of POMs mainly depends
on the pH value and the reaction temperature. The results
showed that the pH value of the system varied over a range of
3.7−3.9, which was helpful for the formation of 1. When at pH
4.3, a new organic−inorganic hybrid [Zn(bpy-NH2)4(H3O)2]-
a
Color legend: green, Cu2+; gray, C; blue, N; red, O; teal, W. For the
sake of clarity, the H atoms and coordinated H2O molecules are
omitted for clarity.
B
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Inorganic Chemistry
Article
[H2W11O38]8− or hetero-POT [XW12O40]n− (X = transition
metal) during the course of the reaction, which benefited from
the produced high temperature and pressure under solvother-
mal conditions that can make the reaction shift from the
thermodynamic to the kinetic. In addition, the introduction of
the rigid organic ligand bpy into this system is conducive to the
stability of the formed POMOF. Therefore, the successful
synthesis of 1 showed that this simple but very efficient strategy
can offer an effective way to make novel multidimensional iso-
POT frameworks.
Table 1. Crystallographic Data and Structure Refinement for
1
parameter
1
empirical formula
mol wt, M (g mol−1
crystal system
space group
a (Å)
C21H47.5Cu4N4.5O52W11
3471.53
)
monoclinic
P2(1)/m
10.008(1)
21.721(3)
14.825(2)
108.029(2)
3064.5(7)
1
b (Å)
c (Å)
Structural Description. Single-crystal structural determi-
nation reveals that the compound 1 crystallizes in a space group
P2(1)/m and consists of an iso-POT [H2W11O38]8−, four
hexacoordinated copper cations ([Cu(1)(H2O)4(Ot)(N)]2+
and [Cu(2)(H2O)1.5(Ot)3(N)]2+), three water molecules of
crystallization, and a half molecule of CH3CN (see Figure 1).
The bond-valence sum (BVS) model clearly indicates that all of
the tungsten atoms are in the +6 oxidation state and the copper
atoms are in the +2 oxidation state, respectively.32 Both of the
two crystallographically independent Cu(II) ions in 1 adopt the
distorted octahedral geometries. In [Cu(1)(H2O)4(Ot)(N)]2+,
the Cu(1) atom is defined by four oxygen atoms from
coordinated water [Cu(1)−Ow, 1.986(6)−2.424(5) Å], one
bridging oxygen atom from {W11} [Cu(1)−O(13), 1.971(4)
Å], and one nitrogen atom from bpy [Cu(1)−N(1), 1.992(6)
Å]. O(1W) and O(2W) occupy the axial positions of the
elongated octahedron; other atoms build the equatorial plane.
In [Cu(2)(H2O)1.5(OPOT)3(N)]2+, the Cu(2) atom is defined
by 1.5 water oxygen atoms (sof (site occupancy fraction) of
O(9W) is 0.5) [Cu(2)−Ow, 1.866(13)−2.274(5) Å], three
oxygen atoms from {W11}[Cu(2)−OPOT, 1.912(4)−1.982(4)
Å], and one nitrogen atom from bpy [Cu(2)−N(1), 1.983(5)
Å].
β (deg)
V (Å3)
Z
Dcalcd (g cm−3
)
3.729
T (K)
296(2)
reflcns collected/unique
14120/5522 [R(int) = 0.0433]
22.016
μ (mm−1
)
F(000)
3042
GOF
1.009
a
R1 (I > 2σ(I))
0.0411
b
R2w (I > 2σ(I))
0.1111
a
R1 (all data)
0.0604
b
R2w (all data)
0.1187
diff peak and hole, eÅ−3
2.986/−2.249
a
b
R1 = ∑∥Fo| − |Fc∥/∑|Fo|. R2w = [∑w(Fo − Fc2)2/∑w(Fo )2]1/2; w
2
2
= 1/[σ2(Fo ) + (xP)2 + yP], P = (Fo2 + 2Fc2)/3, where x = 0.0700 and
2
y = 6.9018 for 1.
[W10O32]·2H2O was obtained, in which the [W10O32]4−
structure was maintained.30 When at pH 3.6, [W10O32]4−
reacted with Zn(NO3)2 in the presence of bpy and formed a
new POMOF, in which [W10O32]4− was reorganized into
Keggin-type [ZnW12O40]6−.31 These results indicate the
precursor [W10O32]4− tends to transform to iso-POT
Similar to the framework geometry of the [H4W11O38]6−
cluster reported by Lehmann and Fuchs,33 the {W11} subunit in
Figure 1. (a) Polyhedron representation of the {W9} trivacant lacunary Keggin anion and the {W11} adduct. (the heteroanion is omitted for clarity).
To make a comparison between the iso-POT and hetero-POT, the different parts of the two structures are highlighted. (b) Ball-and-stick
representations of the coordinated mode of {W11} and coordination environments of copper(II) ions. (c) Crystal structure of 1 showing the stacking
pattern of the grid-like sheets and the 1D channels along the a axis. The H atoms and free H2O molecules are omitted for clarity. Color legend: gray,
C; blue, N; red, O; green, Cu2+; teal, polyhedra {W11}.
C
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Article
1 is composed of four distinct fundamental groups: one
{W4O21} group formed from four edge-shared WO6 octahedra
connected through a μ4-hydroxo, one {W3O16} group formed
from three edge-shared WO6 octahedra connected through a
μ3-hydroxo bridge, and two {W2O10} fragments, each formed
from two edge-shared WO6 which share corners on one side.
The structure of {W11} can be seen as the derivative from the
trivacant Keggin anion A-[XW9O34]n‑,34 in which a tetrameric
unit {W4O16} displaces a dimeric unit {W2O10} introducing
asymmetry to the {W11} cluster (Figure 1a). All W centers have
the distorted WO6 octahedral coordination geometries, with
one terminal Ot (average bond lengths of W−Ot, 1.728 Å)
extending away from the cluster. Relevant W−Oa (central)
bond distances vary from 2.182(3) to 2.318(3) Å (but both
W(2)−O(3) and W(2)−O(3A) are 1.866(4) Å). The W−Ob,c
(bridging) bond distances range from 1.784(4) to 2.130(4) Å.
Within the {W11} moiety, the four inner μ−O positions span a
nearly regular {O4} tetrahedron (O···O distances, 2.754−2.996
Å). The μ4-O and μ3-O display the highest negative partial
charges (BVS for the sites are 1.3 for O(2) and 1.5 for O(14),
respectively.). Most interestingly, each {W11} cluster acts as a
hexadentate ligand to coordinate with six copper complex
cations nearly in a plane by the terminal or bridging oxygen
atoms of a ring of six basal W positions. The {W11} fragment
possesses much more negative charges, which may be beneficial
for coordination with more TM ions as the multidentate ligand
to form a new organic−inorganic framework. It is fact that the
WOt bonds participated in coordination with copper were
0.026 Å (on average) longer than the other WOt bonds.
While one or two metal centers coordinated with oxygen atoms
of POM is a well-established structural paradigm for POMs,35 a
higher coordination number for a POM anion is very unusual.
In 2003, the first example that a Keggin-type [PV2Mo10O40]5−
was bound to six [Cu(H2O)4]2+ by the terminal oxygen atoms
of a ring of six basal W positions was reported by Hill et al.36
Then, Arumuganathan and Das reported a POV cluster anion,
[As8V14O42(SO3)]6−, with six surrounding lanthanum−aqua
complex cations.37
As can be seen from Figure 1b, {W11} clusters are alternately
linked by two [Cu(2)(H2O)1.5(OPOT)3(N)]2+ cations in an
unprecedented end-to-end fashion to form a 1D chain with the
linkage mode of (-{W11}−Cu(2)−{W11}-). Adjacent 1D chains
are linked through Cu(1)−bpy−Cu(2) in an opposite direction
to form a 2D wave-like network along the ab plane (see Figure
S3 in the Supporting Information). These 2D sheets are further
stacked in a parallel fashion through the interactions between
{W11} and the copper(II) centers giving rise to the 1D channels
along the a axis with a cross-section of about 5.6 × 6.6 Å2 (see
Figure 1c). However, there are a few of the free H2O and
CH3CN molecules occupying the pores of the channels. As we
know, the solvents contained in MOFs can be removed by two
common treatments. One is thermally assisted evacuation, and
the other is liquid exchange of the encapsulated solvents such as
dimethylformamide (DMF) and dimethylacetamide (DMAC)
for lower boiling point solvents such as MeOH and
tetrahydrofuran (THF) followed by a degas process or pore
evacuation at moderate temperature.38,39 Herein, the first
treatment was adopted to extract the water and CH3CN from
the pores. As a result, channels of 1 were theoretically enlarged
with the accessible pores from 160.4 to 429.6 Å3 (from 5.2% to
14.0% of the unit cell volume), as calculated from PLATON
analysis.40 The water molecules occupy several coordination
sites of copper(II) cations and potentially act as removable
labile ligands, allowing for copper(II) to serve as the
catalytically active sites interacting with the guest molecules
that get into the channels.41,42
Although hetero-POTs have been extensively exploited to
develop the organic−inorganic architectures, no organic−
inorganic hybrids based on isolated {W11} clusters and
transition metal complexes have been reported to date. To
the best of our knowledge, except for a handful of pure
inorganic iso-POT clusters related {W11} assembled by W−O−
W bonds,20−22 only one lanthanide-containing iso-POT
[Ln2(H2O)10W28O93(OH)2]14− constructed by isolated {W11}
clusters with {Ln(H2O)n}3+ ions has been reported.23 In 1,
{W11} acts as a lacunary POT cluster possessing four naked
bridging oxygen connected with two copper complexes,
resulting in a nontypical sandwich structure. It is particularly
worth mentioning that the {W11} clusters are linked by two
copper(II) complexes in an end-to-end fashion, while the
reported dinuclear sandwiched hetero-POM are usually
connected in a face-to-face way.43 For example, [Cu-
(en)2H2O]2{[Cu(en)2][Cu(en)2AsIIIAsVMo9O34]}2·4H2O is
constructed from two {[Cu(en)2][AsIIIAsVMo9O34]}4− units
and two [Cu(en)2]2+ complex cations via two O−Cu−O
bridges in a trans fashion, which is analogous to
20
[H4W22O74]12−
.
The successful synthesis of 1 showed that
{W11} cluster can act as a versatile inorganic unit for
construction of POMOFs displaying diverse structural motifs
and properties. Therefore, the systematic exploration of the
{W11} system still remains a great challenge and further study is
needed.
Powder XRD Characterization and IR Spectra. The
powder XRD pattern for compound 1 is presented in Figure S1
in the Supporting Information. The very good correspondence
between the calculated and the experimental patterns suggests
the high purity of the bulk sample. This conclusion is in
agreement with the results of the single-crystal X-ray analysis.
The IR spectrum of compound 1 displays characteristic
stretching vibrations related to those seen in other species
25
containing [W11O38]10− (see Figure S2 in the Supporting
Information). The IR spectrum of 1 shows two very strong
absorption peaks near 935 and 823 cm−1 that are assigned to
the vibration of WOt and W−O−W of the iso-POT {W11}
cluster, respectively. The broad peak at 3423 cm−1 is attributed
to the characteristic vibration peak of H2O. In addition, the
vibration peaks from 1221 to 1606 cm−1 are indicative of the
bpy ligand. The occurrence of these resonance signals confirms
the presence of bpy and water molecules, which is in good
agreement with the single-crystal structural analysis.
Catalysis. The cyanosilylation was carried out in the
presence of POMOF 1 with a 1:2.4 mol ratio of the selected
aromatic aldehydes and cyanotrimethylsilane in CH3CN for 24
h at room temperature through a heterogeneous manner. The
results summarized in Table 2 showed that the 2 mol % mole
ratio loading of catalyst 1 (0.01 mmol) caused significant
catalytic efficiency with conversion of up to about 98% of the 2-
phenyl-2-(trimethylsilyloxy)acetonitrile corresponding to ben-
zaldehyde. In addition, the removal of 1 by filtration after 12 h
terminated the reaction, and the filtrate afforded only 9%
additional conversion after being stirred at temperature for
another 12 h. Solids of 1 could be easily isolated from the
reaction suspension by a simple filtration alone and reused at
least three times with moderate loss of activity (from 98.2% to
93.8% conversion; see Table S2 in the Supporting Informa-
tion). The index of the PXRD pattern of the bulky sample that
D
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Inorganic Chemistry
Article
the reported face-to-face fashion. The resulting POMOF 1 has
been successfully applied to prompt the cyanosilylation with
excellent conversion effciency through a heterogeneous
manner. The study provides a basis for further design of
other iso-POT-based POMOFs with distinct structural features
and properties.
Table 2. Results for the Catalytic Cyanosilylation of
Aldehydes in the Presence of 1
a
b
entry
Ar−
yield (%)
1
2
3
4
5
phenyl
98.1
89.3
87.8
88.0
52.4
ASSOCIATED CONTENT
* Supporting Information
4-methoxyphenyl
1-naphthyl
2-naphthyl
■
S
Tables listing selected bond lengths and angles for 1 and
recycling results for catalyst 1, text describing the location of
protons by means of BVS calculations and catalysis details,
figures showing PXRD patterns, IR spectra, and the crystal
structure of 1 and HPLC chromatograms of the cyanosilylation
of aldehyde products in CDCl3, and crystal data in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
3-formyl-1-phenylene-(3,5-di-tert-butylbenoate)
a
Reaction conditions: (CH3)3SiCN, 1.2 mmol; aldehyde, 0.5 mmol;
catalyst 1, 0.01 mmol (2 mol %); CH3CN, 2 mL; at room temperature
under N2 for 24 h. The conversions were determined by H NMR
spectroscopy of crude products.
b
1
was collected after three runs of the catalytic reaction evidenced
the maintenance of the crystallinity (see Figure S1 in the
Supporting Information). These observations suggested that 1
is a true heterogeneous catalyst. The use of such a catalyst can
be extended to other aromatic aldehyde substrates with
comparable activity. Infrared spectroscopy of the catalyst 1
impregnated with a CH3CN solution of benzaldehyde exhibited
one broad C−O stretch at 1693.6 cm−1. The v(C−O) stretch
had a red shift of 12.9 cm−1 from 1706.5 cm−1 of the free
benzaldehyde (see Figure S2 in the Supporting Information).
This experiment unambiguously demonstrated the absorbance
of benzaldehyde and possible activation of the substrate by the
unsaturated Cu(II) in the channels of 1.
In contrast to the smooth reaction of substrates 1−4, the
cyanosilylation catalytic reaction in the presence of bulky
aldehyde 3-formyl-1-phenylene-(3,5-di-tert-butylbenzoate) gave
less than 52% of conversion under the same reaction
conditions. Further adsorption experiments by immersing
solids of 1 into a solution of substrate 5 also confirmed that
substrate 5 was too large to be adsorbed in the channels of the
POMOF.44 The size selectivity of the substrate suggested that
cyanosilylation indeed occurs in the channels of the POMOF,
not on the external surfaces. The control experiments for
cyanosilylation of benzaldehyde with free Cu(NO3)2 and
(TBA)12[H4W22O74] (prepared according to literature meth-
ods20) in a homogeneous manner gave 45.4% and 11%
conversion, respectively, which are far lower than that of 1 in
the heterogeneous manner. The higher conversion with 1 is
possibly attributed to the suitable distribution of copper(II) and
{W11} in the POMOF that provides effective contacts with
substrates at the same time.
AUTHOR INFORMATION
Corresponding Authors
*(Q.H.) Tel./Fax: (+86)-378-3886876. E-mail:hdhqx@henu.
edu.cn.
■
*(J.N.) Tel./Fax: (+86)-378-3886876. E-mail:jyniu@henu.edu.
cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support from the
National Natural Science Foundation of China (NSFC; Grant
Nos. 21101055 and U1304201).
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■
In summary, a novel polyoxometalate-based metal organic
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