Periodic mesoporous organometallic silicas with unary or binary organometals inside the channel walls as active and reusable catalysts in aqueous organic reactions

Article abstract of DOI:10.1021/ja909596a

(Figure Presented) A new type of heterogenized organometallic catalysts, periodic mesoporous organometallic silicas with organometals and organic groups embedded in silica walls have been developed using surfactant-directed co-condensation. These catalysts exhibit high efficiencies and strong durability in water-medium organic reactions. Moreover, a bifunctional catalyst containing both Pd(II) and Ru(II) organometals inside pore walls can catalyze the water-medium "one-pot" cascade reaction effectively. These catalysts can be conveniently recycled and reused without significant loss of catalytic activities.

Full text of DOI:10.1021/ja909596a

Published on Web 01/13/2010
Periodic Mesoporous Organometallic Silicas with Unary or Binary
Organometals inside the Channel Walls as Active and Reusable Catalysts in
Aqueous Organic Reactions
†
†
†
†
,‡
,†
Jianlin Huang, Fengxia Zhu, Wenhan He, Fang Zhang, Wei Wang,* and Hexing Li*
The Education Ministry Key Laboratory of Research Chemistry, Shanghai Normal UniVersity, Shanghai 200234,
China, and Department of Chemistry & Chemical Biology, UniVersity of New Mexico, Albuquerque,
New Mexico 87131-0001
Received November 11, 2009; E-mail: wwang@unm.edu; hexing-li@shnu.edu.cn
Organometallic catalysts are widely used in organic synthesis
owing to high activity and selectivity. Recently, they have also
2
Scheme 1. Preparation of M-PPh -PMO(R) by Surfactant-Directed
Co-Condensation
1
been explored in water-medium reactions, an important aspect of
2
green cehmistry. However, their applications in aqueous medium
are still limited by their reduced catalytic efficiency and difficulties
in recycling and subsequent reuse, which eventually increase costs
3
and cause heavy metal pollution.
Immobilization of organometals onto nanomaterials represents an
4
attractive strategy for catalyst recycling. Nevertheless, the heterog-
2
The FTIR and Raman spectra (Figures S3 and S4) of Pd-PPh -
enized catalyst usually exhibits lower activity and selectivity than the
homogeneous catalyst due to poor dispersion of active sites, the
enhanced steric hindrance, and the change of chemical microenviron-
PMO(Ph) samples displayed no significant absorbance characteristic
of a P123 molecule, indicating the almost complete removal of
7
5
surfactant template after extraction in ethanol. Compared with
ment of the active sites. Previously, we reported the immobilized
PMO(Ph), Pd-PPh
2
-PMO(Ph) shows three additional peaks around
organometallic catalysts on the mesoporous silica fucntionalized with
organic groups, which exhibited enhanced activity in water-medium
organic reactions owing to the high dispersion of active sites and the
enhanced surface hydrophobicity. However, the presence of organo-
metals and organic groups terminally bonded to the pore surface blocks
1
2
890, 2980, and 1527 cm- due to vibrations from C-H and C-P in
8
the CH
2
-CH
2 2
and PPh groups. The Raman peaks around 777, 1324,
-1
1
598, 1028, and 997 cm also confirm the presence of phenyl, Pd-P,
9
and P-C groups. These results demonstrate the successful incorpora-
6
tion of both Pd-PPh CH CH and the phenyl into silica walls, which
2
2
2
the pore channels and induces significant metal leaching. Herein, we
is further confirmed by solid NMR spectra (Figure S5). The 29Si CP-
wish to disclose new periodic mesoporous organometallic silicas
containing bridge-bonded organometals and organic groups directly
3
MAS NMR displays only three low-field peaks corresponding to T
2
1
(
δ ) -79 ppm), T (δ ) -73 ppm), and T (δ ) -63 ppm), indicating
integrated into the silica framework, denoted as M-PPh
2
-PMO(R),
to PPh CH CH
) groups. As demon-
10
13
2+
+
2+
+
that all Si species are covalently bound to carbon. The C CP-MAS
NMR displays three peaks at 13 and 58 ppm corresponding to C1,
C2′, and C1′ atoms and the other peak at 33 ppm to C2, which is
overlapped by the rotational band peak. An intense peak at 133 ppm
where M refers to Pd , Au , Ru , and Rh ions, PPh
ligands, and R to phenyl (Ph) or biphenyl (Ph
2
2
2
2
-
2
strated, these catalysts display comparable activities with the corre-
sponding homogeneous catalysts in water-medium organic reactions
and can be used repetitively without a significant loss of activity.
31
is assigned to the C atoms in the benzene ring. The P CP-MAS NMR
displays only one signal at 20.6 ppm, suggesting the unique Pd-PPh
2
A general approach for the preparation of M-PPh
illustrated in Scheme 1. First, organometal-bridged silanes are syn-
thesized by coordinating metallic ions with PPh CH CH Si(OEt) (see
2
-PMO(R) is
11
coordination bond.
2
2
2
3
the structural formula in Figure S1). Their compositions and purities
are determined by both ICP analysis and NMR spectra (Figure S2).
The organometal-bridged silanes are allowed to coassemble with
R-bridged silane and P123 surfactant. Removal of surfactant micelles
via extraction in ethanol solution leads to the M-PPh
samples. In parallel, the grafted M-PPh -PMO(R)-G are synthesized
for comparison studies by coordinating metallic ions with the PPh
2
-PMO(R)
2
2
-
ligands originally anchored on the PMO(Ph) via surfactant directed
co-condensation between bis(triethoxysilyl)benzene and 2-(diphe-
nylphosphino)ethyltriethoxysilane. Meanwhile, M-PPh
2
-SBA-15 and
Figure 1. N
2
sorption isotherm (a) and low-angle XRD pattern (b) of the
M-PPh -SBA-15(Ph) are prepared by co-condensation between orga-
2
Pd-PPh -PMO(Ph) sample. The inset is the TEM image.
2
nometal-bridged silane and tetraethylorthosilicate or phenyltrimethox-
ysilane in the presence of P123 (see Supporting Information for details).
Moreover, commercially available homogeneous organometal catalysts
are directly used for comparison studies.
As shown in Figure 1, Pd-PPh
adsorption-desorption isotherm with the H
angle XRD pattern exhibits a well resolved peak at 2θ ) 0.8°-1.0°, a
100) diffraction, indicating the ordered two-dimensional P6mm hexagonal
mesporous channels, which is further verified by the TEM image (see
Figure S6 for other samples). From Table 1, one can conclude that the
2
-PMO(Ph) displays a type IV nitrogen
2
hysteresis loop while the low-
(
12
†
Shanghai Normal University.
‡
University of New Mexico.
1
492 9 J. AM. CHEM. SOC. 2010, 132, 1492–1493
10.1021/ja909596a  2010 American Chemical Society

C O M M U N I C A T I O N S
Pd(II) complexes in Pd-PPh
rather than terminally bonded to the pore surface, which effectively
diminishes pore channel blockage, leading to higher SBET and D than
those of the grafted Pd-PPh -PMO(Ph)-G with an equivalent Pd loading.
Although Pd-PPh -PMO(Ph ) displays a similar mesoporous structure to
that of Pd-PPh -PMO(Ph) (see Figure S7), the wide-angle XRD patterns
Figure S8) demonstrate the crystal-like pore walls rather than the
amorphous pore walls, possibly as a result of the π-π stacking of biphenyl
2
-PMO(Ph) are embedded in the pore walls
P
2
2
2
2
(
13
groups.
As revealed in Table 1, Pd-PPh
and selectivity than corresponding Pd-PPh
5(Ph), and Pd-PPh -SBA-15 in water-medium Barbier reactions with the
same amount of Pd(II) used (see Scheme S1), obviously owing to the
higher SBET, the larger D , and the enhanced surface hydrophobicity
resulting from the Ph-modification, which facilitates the diffusion and
2
-PMO(Ph) exhibits much higher activity
-PMO(Ph)-G, Pd-PPh -SBA-
2
2
Figure 2. Recycling tests of the grafted Pd-PPh
2 2
-PMO(Ph), Pd-PPh -
1
2
PMO(Ph), and the Pd-PPh -PMO(Ph ) catalysts in Barbier reactions.
2
2
P
The assembly strategy has been successfully applied in the synthesis
of a novel bifunctional catalyst with Pd(II) and Ru(II) organometals
homogeneously inside ordered mesopore walls by co-condensation of
Pd(II) and Ru(II) organometal-bridged silanes with bis(triethoxysilyl)-
1
4
adsorption of organic reactants, especially in a water medium. Despite
the lower SBET and D , Pd-PPh -PMO(Ph ) exhibits similar activity to that
of Pd-PPh -PMO(Ph) as a result of the Ph -modified stronger surface
hydrophobicity. Remarkably, Pd-PPh -PMO(Ph) displays comparable
efficiencies in Barbier and Sonogashira reactions with the homogeneous
Pd(PPh Cl (Tables S1 and S2). Similar results are also observed for
Au-PPh -PMO(Ph), Rh-PPh -PMO(Ph), and Ru-PPh -PMO(Ph) catalysts
see Table 2). According to a standard procedure developed by Sheldon
P
2
2
2
2
benzene and P123 (see Figure S10). Impressively, such Pd@Ru-PPh
PMO(Ph) exhibits comparable efficiency with Pd(PPh Cl
Ru(PPh Cl in a water-medium “one-pot” Barbier-isomerization
2
-
2
3 2
)
2
/
3 3
)
2
3
)
2
2
cascade reaction and could be reused for at least 7 times (Figure S11).
In summary, we have developed a novel approach for the preparation
of ordered mesoporous organometallic catalysts with orgnometals embed-
ded in silica walls. Notably, they exhibit high catalytic efficiencies and
strong durability in water-medium organic reactions. Moreover, bifunc-
tional catalysts containing two kinds of organometals can also be
synthesized and explored in “one-pot” cascade reactions. The strategy
described in the study serves as a general approach for immobilization of
homogeneous catalysts with broad and practical applications.
2
2
2
(
15
2
et al., the Pd-PPh -PMO(Ph) catalyzed Barbier reaction is allowed to
proceed for 6 h with conversion exceeding 45%. After removing the solid
catalyst, the mother solution is allowed to react for another 12 h under
identical conditions. No significant change in either the conversion or the
product yield has been observed, indicating that the present catalysis indeed
is heterogeneous in nature rather than the dissolved Pd(II) species leached
2
from solid Pd-PPh -PMO(Ph).
Table 1. Structural Parameters and Catalytic Performances of
Organometallic Catalysts in Water-Medium Barbier Reaction
Acknowledgment. This work is supported by the National
Natural Science Foundation of China (20825724 and 50943048).
Pd loading
wt%)
S
BET
(m /g)
D
(nm)
P
Conv.
(%)
Select.
(%)
Catalyst
2
Supporting Information Available: Experimental procedures and
(
2 2
characterization of the Pd-PPh -PMO(Ph and Ph ) nanomaterials and
the results of water-medium reactions. This material is available free
of charge via the Internet at http://pubs.acs.org.
Pd-PPh
Pd-PPh
Pd-PPh
Pd-PPh
Pd-PPh
Pd(PPh
2
2
2
2
2
3
-SBA-15
0.85
0.54
1.7
1.7
3.9
/
389
145
438
654
444
/
3.9
1.7
3.6
5.7
3.3
/
56
61
78
93
92
96
71
88
91
96
95
94
-SBA-15(Ph)
-PMO(Ph)-G
-PMO(Ph)
-PMO(Ph
Cl
2
)
)
2
2
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Scheme S3
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Scheme S4
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2
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1
6
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-
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1
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JA909596A
J. AM. CHEM. SOC. 9 VOL. 132, NO. 5, 2010 1493