The fabrication of reactive hollow polysiloxane capsules and their application as a recyclable heterogeneous catalyst for the Heck reaction

Article abstract of DOI:10.1039/b612953f

The fabrication of hollow polysiloxane capsule-supported palladium complexes has been investigated. Capsule-supported cyano-palladium(0) complexes were firstly prepared and their catalytic performance in the Heck arylation of alkenes was studied. Generally the method involves the consecutive cocondensation of 4-(triethoxysilyl)butyronitrile and dimethyldimethoxysilane monomers onto a microemulsion of preformed polydimethylsiloxane and the subsequent removal of the templated polydimethylsiloxane by exposure to solvents, and then the above product is reacted with palladium acetate in anhydrous toluene and reduced with KBH₄ in ethanol to produce the hollow polysiloxane capsule-supported palladium complex. UV-VIS spectra and FT-IR spectra indicated that the capsules chelated with the palladium and the results of TEM and AFM measurements show that a hollow polysiloxane capsule-supported palladium complex has been prepared. These complex are highly active and stereoselective for the Heck reaction and can be easily separated from the reaction mixture. Moreover, these polysiloxane capsule-supported palladium(0) catalysts can be retrieved and reused with high catalytic stability. The Royal Society of Chemistry.

Full text of DOI:10.1039/b612953f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
The fabrication of reactive hollow polysiloxane capsules and their
application as a recyclable heterogeneous catalyst for the Heck reaction
a
Huigang Wang,* Xuming Zheng, Ping Chen* and Xiaoming Zheng
a
b
b
Received 7th September 2006, Accepted 5th October 2006
First published as an Advance Article on the web 23rd October 2006
DOI: 10.1039/b612953f
The fabrication of hollow polysiloxane capsule-supported palladium complexes has been
investigated. Capsule-supported cyano-palladium(0) complexes were firstly prepared and their
catalytic performance in the Heck arylation of alkenes was studied. Generally the method involves
the consecutive cocondensation of 4-(triethoxysilyl)butyronitrile and dimethyldimethoxysilane
monomers onto a microemulsion of preformed polydimethylsiloxane and the subsequent removal
of the templated polydimethylsiloxane by exposure to solvents, and then the above product is
4
reacted with palladium acetate in anhydrous toluene and reduced with KBH in ethanol to
produce the hollow polysiloxane capsule-supported palladium complex. UV-VIS spectra and
FT-IR spectra indicated that the capsules chelated with the palladium and the results of TEM and
AFM measurements show that a hollow polysiloxane capsule-supported palladium complex has
been prepared. These complex are highly active and stereoselective for the Heck reaction and can
be easily separated from the reaction mixture. Moreover, these polysiloxane capsule-supported
palladium(0) catalysts can be retrieved and reused with high catalytic stability.
1
1
Introduction
Based on our previous work towards the preparation of
organosilicon hollow spheres with controlled shell properties,
we report in this paper on the synthesis and characterization of
a hollow polysiloxane capsule-supported palladium complex
and the application of the supported palladium complex in the
Heck reaction.
In recent years, there has been intense interest surrounding the
fabrication of uniform hollow spheres with nanometer-to
micrometer dimensions that comprise either organic or
1
inorganic shells, Hollow spheres with defined properties in
their shells may lead to applications such as controlled release
4
Scheme 1 describes the synthetic route to the hollow
polysiloxane capsule-supported palladium complex and is
outlined in detail in the Experimental section.
2
capsules, artificial cells, microreactors and nanocontainers,
3
1
shape-selective catalysts etc. Various methods, including the
c
1a
5
layer-by-layer (LBL) method, the template approach, and
6
the emulsion polymerization approach, have been reported
Experimental
for the preparation of hollow spheres.
Hollow metallic nanospheres exhibit catalytic activities
different from their solid counterparts with the advantages of
Preparation of polydimethylsiloxane microemulsion
7
high surface area and low density. For example, Liang and
In a 500 mL three-necked flask, 3 g of the surfactant benzetho-
nium chloride (Sigma) was dissolved in saturated ammonium
hydroxide solution (125.0 g). Dimethoxydimethylsilane (11.0 g)
co-workers reported that hollow Pt spheres exhibit enhanced
electrocatalytic performance: the catalytic activity of the Pt
7
hollow nanospheres is twice that of solid Pt nanoclusters.
However, hollow metallic nanospheres often exhibit incom-
plete shells and readily disintegrate or aggregate to form
8
irregular clusters. This could be overcome with the develop-
ment of hollow capsule-supported metallic complexes with the
advantages of saving of materials, highly dispersed metallic
9
nanospheres and reduction of costs. The use of hollow
functionalized capsules, which serve as supports for catalysts,
in combination with reactants in a fluid phase yields the
advantages of easy purification of the desired product and the
1
0
ability to drive reactions to completion.
a
Department of Chemistry, Zhejiang SCI-TECH University, Hangzhou,
3
10018, China. E-mail: zdwhg@163.com; Tel: +86-571-86843224
b
Scheme 1 The procedure for the fabrication of the hollow poly-
Department of Chemistry, Zhejiang University, Hangzhou, 310028,
China. E-mail: c224@zju.edu.cn; Fax: +86-571-88273283;
Tel: +86-571-88273272
siloxane capsule-supported palladium complex. : surfactant, M
2
=
(CH Si(OCH , M = NC(CH Si(OCH ) , M = Me SiOCH .
1
3
)
2
3
)
2
3
)
3
3
3
3
3
3
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was added with vigorous stirring, at room temperature over
h. The opalescent dispersion was continuously stirred
Application as a catalyst for the Heck reaction
In a typical reaction, a mixture of acrylic acid (1.1 g, 15 mmol),
iodobenzene (2.05 g, 10 mmol), Et N (3.6 g, 35 mmol), 6 ml
DMF and the hollow polysiloxane–CN loaded palladium
complex (0.04 g, 0.01 mmol) was stirred under N in an oil
1
for more 12 h. Then the silanol end groups of the
polydimethylsiloxane were endcapped using methoxytri-
methylsilane (1.0 g).
3
2
bath at 70 uC. The reaction progress was monitored by thin
layer chromatography (TLC). After the reaction, the hollow
polysiloxane–CN loaded palladium complex was separated
from the mixture by filtration. The filtrate was poured into 2%
HCl solution. The white solid precipitate was filtered off,
washed with H O and dried in air to give trans-cinnamic acid.
2
The transmission electron microscopy (TEM) photographs
were measured with a JEM-200CX transmission electron
microscope. Each TEM sample was prepared by depositing a
drop of a 1% solution of the sample in THF on a carbon-
coated copper grid. The atomic force microscopy (AFM)
photographs were recorded with a SPI3800N probe station
(S II Seiko Instruments Inc.) operating in tapping mode at a
resonance frequency of about 280 kHz. One drop of a 1%
solution of hollow polysiloxane particles in THF was placed
on a freshly cleaved mica surface and spin cast.
Preparation of core–shell structured particles
Under vigorous stirring, a mixture of dimethoxydimethylsilane
(
4.0 g) and 4-(triethoxysilyl)butyronitrile (17.0 g) was added
slowly into the above-mentioned system within 1 h at room
temperature and the resultant opalescent dispersion was stirred
for a further 8 h. The active Si–OH groups were endcapped
using methoxytrimethylsilane (3.0 g), stirred for 3 h more then
endcapped with methoxytrimethylsilane (3.0 g) again. After
stirring for a further 12 h the dispersion was destabilized by the
addition of 400 mL of methanol and centrifuged to obtain the
precipitate. In order to remove the surfactant the precipitate
was washed 3 times with methanol. In order to completely
endcap the silanol groups, the above product was dissolved
in 50 mL toluene and 5.0 g of hexamethyldisilazane was
added. The reaction mixture was stirred at room temperature
for 12 h. The resulting product was precipitated with 300 mL
of methanol, then centrifuged to obtain core–shell type
microspheres.
Results and discussion
Transmission electron microscopy (TEM) and atomic force
microscopy (AFM) in the solid state confirmed the formation
of hollow capsules and allowed the determination of the sizes,
shapes, and structures of the materials.
Fig. 1A shows the polysiloxane particles with cyanopropyl
functionalized groups before infiltration (PDMS swelled
polysiloxane hollow capsules) and Fig. 1B shows the TEM
of hollow polysiloxane capsules after centrifugation. Infiltra-
tion and centrifuging of PDMS coated with polysiloxane
micronetworks (shown in Fig. 1A) resulted in hollow spheres
composed of polysiloxane (Fig. 1B). Samples A and B were
stained by 2.5% uranyl acetate and lead citrate to make the
particles stable and separated. The noticeable difference in
contrast of the spheres before (Fig. 1A) infiltration and after
Preparation of reactive hollow polysiloxane microcapsules
The above product was dissolved in THF, stirred for 12 h
and then centrifuged to obtain the precipitate. This
procedure was repeated to obtain the reactive hollow
polysiloxane microcapsules. The microcapsules were dissolved
in benzene, and freeze-dried overnight. Yield: 9.6 g of a white
powder.
Preparation of hollow polysiloxane–CN loaded palladium(II,0)
complex
2
To a solution of Pd(OAc) (0.08 g, 0.56 mmol) in anhydrous
toluene (80 ml) was added 1.0 g of microcapsules and the
mixture was stirred at room temperature for 5 h. The resulting
2
1
brown dispersion was centrifuged (4000 rev min ) and
washed with methanol; this procedure was repeated three
times to obtain the precipitate. The sticky residue was freeze-
dried overnight to obtain the hollow polysiloxane–CN loaded
palladium(II) complex. The above complex (0.7 g) was then
4
mixed with KBH (50 mg) in anhydrous ethanol (40 ml) and
stirred under N protection at room temperature for 1 h. The
2
resulting product was filtered, washed with anhydrous EtOH,
and dried under vacuum to give the grey palladium(0)
complex. The palladium content was 2.73 wt% and the
nitrogen content was 5.31 wt%.
Fig. 1 (A) TEM of PDMS chains coated with a polysiloxane shell.
(B) TEM of hollow polysiloxane capsules with cyanopropyl function-
alized groups.
4
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(
Fig. 1B) centrifugation confirmed that hollow microcapsules
beyond simple topographical mapping to differentiate regions
of high and low surface adhesion or hardness and to detect
different components in composite materials.
were produced by the extraction of the PDMS chains. From
Fig. 1A, the mean diameter of the uniformly formed core–shell
structured spheres was approximately 250 nm. The diameters
of the hollow capsules were significantly smaller than those of
the corresponding core–shell structured particles. From
Fig. 1B, the mean diameter of the hollow capsules was
approximately 205 nm.
Phase shifts were investigated for the present capsules which
were shown in Fig. 2B, the central domain of the hollow
microcapsule, where an increasing phase shift, i.e., larger
energy dissipation, represented as a darker gray zone in
Fig. 2B, indicated large tenderness, and large elastic response.
The brim of the hollow microcapsules and the mica surface
appear bright (Fig. 2B), which indicates the small phase shifts,
large hardness, and small elastic response, accordingly. All of
these cases are consistent with their hollow nature. The large
black dots correspond to the particles filled with silicon oil,
which clearly proves the soft elastic nature of the particles.
Fig. 3A shows the TEM of the hollow polysiloxane capsule-
supported palladium(0) complex particles. It can be seen that
the individual particles were composed of an empty core with a
uniform shell, and hollow capsules of the sample chelated with
the dispersed nano-palladium (dark spots in the figure). TEM
micrographs (Fig. 3A) of hollow polysiloxane capsule-
supported palladium(0) complex showed that the sphericity
of the capsule (Fig. 1B) was preserved, but there occurred
significant coalescence between the supported palladium(0)
complexes due to the enhanced surface energy of the highly
dispersed nano-palladium(0). The particle size distribution is
narrow and the mean diameter of the particles is 300 nm. The
average diameter of the hollow polysiloxane capsule-supported
palladium(0) complex was about 1.26 those of the corre-
sponding hollow capsules. This implies that upon chelation
with the dispersed nano-palladium, the capsules underwent
significant expansion.
Tapping-mode atomic force microscopy (AFM) images
provide information about the dry state collapsed polysiloxane
capsules, as well as their size and three-dimensional shape
when adsorbed onto the polar mica substrate (shown in
Fig. 2A). Statistical analysis of the diameter and height for one
hundred of capsules extracted from the AFM section analysis
is reported here. After deposition on substrates the hollow
capsules are completely collapsed, resulting in highly flattened
objects (their diameter : height ratios typically exceeded 20 : 1)
with a mean diameter R = 210 nm, which is consistent with
their hollow nature.
Besides the determination of the lateral and horizontal
dimensions of the capsules, AFM provides a qualitative
measure of the hardness of the investigated object. When
operated in the tapping mode, the phase shift of the vibrating
tip depends on the dissipation of energy caused by the
interaction of the tip with the surface. Phase imaging goes
The electron diffraction image of the hollow polysiloxane
capsule-supported palladium(0) complex particles, shown in
Fig. 3B, revealed that the palladium metal is highly dispersed.
The typical X-ray diffraction (XRD) pattern of the hollow
polysiloxane capsule-supported palladium(0) complex catalyst
shown in Fig. 4B is comparable to the diffraction pattern of
hollow polysiloxane capsules (Fig. 4A). The result shows that
the structure of the hollow polysiloxane capsules has not been
Fig. 2 (A) AFM topography of the hollow capsules. The brighter
parts describe the higher parts of the surface; the dark parts the deeper
one. The phase image (B) is obtained simultaneously with the
topographic image. Note the strong contrast between regions in the
above image. While the topographic image shows some corresponding
features, surface roughness hinders the identification of regions. The
phase image allows unambiguous resolution of the different materials’
hardness.
Fig. 3 (A) Transmission electron micrograph of hollow polysiloxane
capsule-supported palladium complex particles and (B) the corre-
sponding electron diffraction (ED) pattern.
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Fig. 6 The FT-IR spectra of polysiloxane capsules functionalized
with (a) cyanopropyl groups on the surface and (b) non-functionalized
groups.
Fig. 4 (A) XRD spectrum of hollow polysiloxane capsules. (B) XRD
spectrum of hollow polysiloxane capsule-supported palladium(0)
complexes.
this suggests that there is no interaction between the CMN
group and neighboring groups. There is another difference
between the two spectra, that is, on the low-energy side of the
damaged in the preparation of the solid catalysts. The XRD
pattern of palladium metal has major diffraction peaks at 2h =
21
spectrum of Fig. 6a, at 1185 cm , there is a low-intensity
4
0.1 (1 1 1) and 46.7 (2 0 0), which are not found in the XRD
absorption band related to a (CH ) CN stretching mode. The
2
3
pattern of hollow polysiloxane capsule-supported palladium
complex particles, indicating that the palladium metal is highly
dispersed.
above spectroscopic analysis attests to the presence of
cyanopropyl functions on the polysiloxane capsules.
The IR spectra of cyanopropyl functionalized polysiloxane
capsules (Fig. 7a) and templated core PDMS that dissolved
upon exposure to THF and infiltration from the core of
capsules (Fig. 7b) are shown in Fig. 7.
The efficiency of the complexation with palladium is
illustrated by UV-VIS diffuse reflectance spectroscopy. Fig. 5
shows the UV-VIS absorption spectra of hollow polysiloxane
capsules before and after Pd complexation. It can be seen in
Fig. 5a that there are two weak absorption peaks at about
21
A sharp band at 2243.6 cm for the CMN group stretch and
21
a weak peak at 1186 cm for the (CH ) CN stretch are found
2
3
1
93 nm and 292 nm for hollow polysiloxane capsules, the
in the IR spectrum of cyanopropyl functionalized polysiloxane
capsules (Fig. 7a) but no bands are present at equivalent
positions in the IR spectrum of templated core PDMS
positions of these peaks in the UV-VIS spectrum of hollow
polysiloxane capsule-supported palladium complex particles
(
Fig. 5b) shift to 207 nm and 341 nm respectively, and become
(
(
Fig. 7b). The IR spectrum of templated core PDMS
intense and significant, which revealed that charge transfer
occurs between the –CN and Pd after their complexation.
To investigate the chemical structure of the polysiloxane
capsules, a FT-IR spectroscopy comparison was made between
non-functionalized polysiloxane capsules (Fig. 6b) and cyano-
propyl functionalized polysiloxane capsules (Fig. 6a). The
bands for non-functionalized polysiloxane capsules and
cyanopropyl functionalized polysiloxane capsules are fairly
close in wavenumber. While polysiloxane capsules with non-
functionalized groups (Fig. 6b) shows no adsorption between
Fig. 7b) is characterized by strong absorption bands at
21
034–1098 cm , arising from the stretching mode of the D
1
type Si–O–Si groups, while the spectrum from the polysiloxane
capsules (Fig. 7a) shows a slight decrease in the Si–O–Si stretch
21
(
1026–1144 cm arising from the stretching mode of the T
type Si–O–Si groups) compared to the equivalent vibration of
the templated core PDMS, and the bands are broadened. The
IR contrast between the templated core and the polysiloxane
capsules provides further insight into the structure of the
polysiloxane capsules, and this also demonstrated that solvent
extraction is an effective way for removal of the templated
cores.
2
1
21
21
1
800 cm
and 2800 cm , a sharp band at 2243.6 cm
characteristic of the CMN group stretch is clearly identified in
the spectrum of Fig. 6a. The peak at 2243.6 cm is very sharp,
2
1
Fig. 5 The UV-VIS diffuse reflectance spectra of hollow polysiloxane
capsules (a) and the hollow polysiloxane capsule-supported palladium
complex particles (b).
Fig. 7 The FT-IR spectra of capsules functionalized with cyanopro-
pyl groups (a) and templated core PDMS (b).
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Table 1 Heck arylation of conjugated alkenes catalyzed by hollow
polysiloxane–CN loaded palladium(0) complex catalyst
the catalyst can be easily retrieved from the reaction vessel by
simple filtration.
Aryl iodide
Alkene
Product
Yield (%)
TOF
Conclusion
1
1
1
1
a
a
a
b
b
2a
2b
2a
2b
3a
3b
3c
3d
96
93
88
89
320
310
293
297
In summary, a new catalyst for the Heck reaction, consisting
of a hollow polysiloxane–CN loaded palladium(0) complex,
has been synthesized, the preparation of which is rather simple
and convenient. This complex not only has high activity and
stereoselectivity for the Heck reaction at 70 uC, but also offers
practical advantages such as easy handling, separation from
the product and reusability. Experiments directed at the
synthesis of hollow spheres with prochiral carbon are currently
in progress.
All reactions were carried out at 70 uC; 6 mL DMF, 35 mmol
Et N, 0.01 mmol Pd(0), 10 mmol iodobenzene and 15 mmol acrylic
3
acid were used. Isolated yield based on iodobenzene; TOF = mol
product per mol Pd per h.
The Heck arylation of conjugated alkenes was used as a test
reaction. The arylation reactions were carried out using
1
mol% Pd catalyst in DMF. All reactions were complete in
–5 h to give the corresponding trans products in high yields.
2
Acknowledgements
The trans selectivity was near quantitative and no cis product
was observed. The results are summarized in Table 1. The high
surface area of Pd spheres resulting from the nanoparticulate
nature of the shell is responsible for the high catalytic activity.
The financial support of the National Natural Science
Foundation of China (Grant No.20573095), Natural Science
Foundation of Zhejiang Province (Grant No.R405465), Key
Laboratory of Advanced Textile Materials and Manufacturing
Technology (Zhejiang Sci-Tech University), Ministry of
Education are gratefully acknowledged.
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1
2
one cycle, resulting in
a loss of catalytic activity.
Heterogeneous catalysts often suffer extensive leaching of the
active metal species during reactions and eventually lose their
1
2
catalytic activity. Two recycles were carried out for the
arylation of acrylic acid (2a) with iodobenzene (1a), and it was
found that the yield of trans-cinnamic acid (3a) deceased only
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resulted in 90% yield. As for the arylation reaction of acrylic
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4
5
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6
7
1
3
corresponding Heck reaction reported by Cai et al. Even
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a
1
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Activity (TOF)
1
6
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Initial complex
First recycle
Second recycle
a
96
92
90
320
307
300
1
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(
All reactions were carried out at 70 uC; 6 mL DMF, 7 mL Et
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were used. Isolated yield based on iodobenzene.
3
N,
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