N-Heterocyclic carbene palladium complex supported on ionic liquid-modified SBA-16: an efficient and highly recyclable catalyst for the Suzuki and Heck reactions

Article abstract of DOI:10.1039/b904136b

By grafting an N-heterocyclic carbene palladium complex and ionic liquid on the mesoporous cage-like material SBA-16, an efficient and highly recyclable heterogeneous catalyst for the Suzuki and Heck reactions was prepared. This catalyst afforded a fast c

Full text of DOI:10.1039/b904136b

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/greenchem | Green Chemistry
N-Heterocyclic carbene palladium complex supported on ionic
liquid-modified SBA-16: an efficient and highly recyclable catalyst for
the Suzuki and Heck reactions
Hengquan Yang,*a,b Xiaojing Han,a Guang Lia and Yunwei Wanga
Received 2nd March 2009, Accepted 23rd April 2009
First published as an Advance Article on the web 22nd May 2009
DOI: 10.1039/b904136b
By grafting an N-heterocyclic carbene palladium complex and ionic liquid on the mesoporous
cage-like material SBA-16, an efficient and highly recyclable heterogeneous catalyst for the Suzuki
and Heck reactions was prepared. This catalyst afforded a fast conversion of the unactivated
bromobenzene at a catalyst loading of 0.01 mol%, and a good yield was still achieved after the
catalyst was reused ten times. Its recyclability was much better than that of the catalyst prepared
from amorphous silica. The significantly enhanced recyclability could be attributed to the isolated
nanocages of SBA-16 which could efficiently prevent the aggregation and agglomeration of Pd
particles formed during the catalytic reaction into the less active large particles. The strategy for
stabilizing metal nanoparticles using the isolated nanocages may be applied to other catalytic
systems, in which the aggregation of metal nanoparticles is desired to be prevented.
reused catalysts. The recyclability of the heterogeneous catalysts
Introduction
is thus discounted.5
The Pd-catalyzed Heck and Suzuki reactions are well-known
Our previous investigation and other groups’ work revealed
as important methods to construct C–C bond in modern
that support structure and functional groups grafted on the
chemical transformations because of the usefulness and wide
support played an important role in preventing the aggregation
applicability to various substrates.1 Homogeneous catalysts for
of palladium nanoparticles into the less active large particles.6
the reactions such as phosphine, N-heterocyclic carbene and
Among various supports for immobilizing palladium catalysts,
palladacyclic complexes have been successfully established.2
the mesoporous materials are very promising because of their
These homogeneous catalysts, however, suffer from the practical
high surface area and regularly arranged pore structure. Differ-
problems such as catalyst separation, catalyst recycling and
ent to the mesoporous materials MCM-41 and SBA-15 which
product contamination. To address these problems, palladium
complexes or nanoparticles were immobilized on various sup-
are widely used as supports, the recently synthesized SBA-16
(cubic, Im3m) is one of new ordered mesoporous materials
ports such as silica, alumina, zeolite, organic polymers and
with cage-like structures.7 This mesoporous cage-like material
dendrimers to create heterogeneous catalysts because of the easy
has tunable cage size (4–10 nm) and tunable pore entrance
handling, recyclability and “green” process of the heterogeneous
size (generally less than 4 nm). The isolated nanocages are
systems.3 The extensive explorations of heterogeneous catalytic
three-dimensionally interconnected by pore entrances. Such a
systems are evidenced by a great number of publications and
nanocage of SBA-16 can not only accommodate metal particles
reviews in recent years. However, heterogeneous catalysts with
but also, in principle, limit the growth of metal particles by the
excellent recyclability, which is the key factor for the practical
spatial restrictions. The smaller pore entrances may inhibit the
applications, are still limited.
motion of metal nanoparticles, thereby preventing the undesired
One of the major reasons for poor recyclability is the
aggregation and agglomeration. At the same time, the ionic
aggregation and agglomeration of Pd naonoparticles into less
liquids are able to be used as not only “green” reaction media
active large particles (even bulk Pd) during the reaction due to
but also stabilizers for metallic particles due to their electrostatic
the high surface energy of nanoparticles. Even for these systems
interactions and the coordination with metal atoms on the
using molecular catalysts, the aggregation and agglomeration
surface of metal particles.8
still occurred because Pd nanoclusters were formed as the
In this work, the mesoporous cage-like material SBA-16
reaction proceeded.4 The aggregation and agglomeration of
Pd nanoclusters lead to the drastic decrease in activity of the
was chosen as a support and imidazolium ionic liquid was
employed to modify the surface of SBA-16. An N-heterocyclic
carbene palladium complex was used as a molecular catalyst for
immobilization because of easy synthesis from the imidazolium
ionic liquid.4b By grafting the N-heterocyclic carbene palladium
complex and ionic liquid on the internal surface of the nanocages
of SBA-16, we have successfully prepared an efficient heteroge-
neous catalyst for the Suzuki and Heck reactions. This catalyst
aSchool of Chemistry and Chemical Engineering, Shanxi University,
Taiyuan, 030006, China. E-mail: hqyang@sxu.edu.cn;
Fax: +86-351-7011688; Tel: +86-351-7018371
bFine Chemical Engineering Centre of Education Ministry, Shanxi
University, Taiyuan, 030006, China
1184 | Green Chem., 2009, 11, 1184–1193
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Scheme 1 Schematic descriptions for preparing the heterogeneous catalyst NHC-Pd/SBA-16-IL and the model for the reused catalyst.
afforded complete conversions of various aryl bromides even at
the catalyst loading of 0.01 mol%, and could be easily recovered
by a filtration. A satisfactory yield was obtained even after the
catalyst was reused ten times. Furthermore, its recyclability was
further highlighted by a comparison with that of the catalyst
prepared from amorphous silica.
liquid and NHC-Pd complex were first subjected to a derivation
with a trimethoxysilyl group, as described in the Experimental.
The trimethoxysilyl derivatives of NHC-Pd complex and ionic
liquid were then grafted on the internal surface of SBA-16 with
passivation through a one step silylation, eventually yielding the
catalyst NHC-Pd/SBA-16-IL.
The process for preparing the catalyst was monitored by N2
sorption. The N2 sorption isotherms and pore size distribution
of the corresponding samples are displayed in Fig. 1 and
Fig. 2, respectively. The textural parameters determined by
N2 sorption are listed in Table 1. SBA-16 exhibited a type-
IV isotherm pattern with an H2 hysteresis loop, which is
characteristic of the mesoporous cage-like structure. Similar
to the parent SBA-16, SBA-16 passivated with Ph2SiCl2 also
showed a type-IV isotherm with an H2 hysteresis loop, indicating
that the mesoporous cage-like structure was maintained after
passivation. It is worthwhile to note that the surface area and
pore volume of SBA-16 with passivation were slightly decreased.
The cage size decreased by only 0.1 nm and this value is much
Results and discussion
Catalyst preparation and characterization
The process for the preparation of the catalyst NHC-Pd/SBA-
16-IL is schematically described in Scheme 1. In order to graft
the N-heterocyclic carbene palladium complex (NHC-Pd) and
imidazolium ionic liquid (IL) on the internal surface of the
cages of SBA-16, the silanols on the external surface of SBA-16
were first passivated with Ph2SiCl2 according to the reported
method (this process is omitted in Scheme 1 for clarity).9 To
form the covalent linkage with the surface of SBA-16, the ionic
This journal is
The Royal Society of Chemistry 2009
Green Chem., 2009, 11, 1184–1193 | 1185
©

View Article Online
Table 1 Textural parameters of SBA-16, SBA-16 with passivation and
into the interior of SBA-16. However, the mesoporous cage-
like structure for NHC-Pd/SBA-16-IL was still maintained, as
evidenced by its type-IV isotherm with an H2 hysteresis loop.
The ordered mesoporous structure of NHC-Pd/SBA-16-IL was
further confirmed by the XRD pattern similar to that of the
parent material SBA-16 (in Fig. 3).
NHC-Pd/SBA-16-IL
Decrease
Cage
in cage
Samples
SBA-16
SBA-16 with passivation 604
NHC-Pd/SBA-16-IL 207
Sa/m2 g-1 Vb/cm3 g-1 sizec/nm sized/nm
763
0.80
0.68
0.22
6.7
6.6
6.1
—
0.1
0.6
a BET surface area. b Single point pore volume calculated at relative
pressure P/P0 of 0.99. c BJH method from adsorption branch. d The
decreased value of cage diameter, compared with the cage size of the
parent SBA-16.
Fig. 3 XRD patterns of SBA-16 and NHC-Pd/SBA-16-IL.
NHC-Pd/SBA-16-IL was further characterized with FT-IR
spectroscopy (Fig. 4) and solid state NMR (Fig. 5 and Fig. 6).
In Fig. 4, the FT-IR spectroscopy of NHC-Pd/SBA-16-IL
exhibited three new peaks around 1580, 2972 and 3078 cm-1,
compared with the FT-IR spectroscopy of SBA-16. These three
peaks corresponded to the ring vibration of the imidazole,
the stretching vibration of saturated C–H and the stretching
vibration of unsaturated C–H on the imidazole ring, respectively.
In Fig. 5, the carbon chemical shifts of the imidzaolium ionic
liquid measured by 13C CP-MAS NMR were in good agreement
with the theoretically predicted values. As displayed in Fig. 6, the
29Si MAS NMR spectroscopy of NHC-Pd/SBA-16-IL showed
a “T3” band at ca. -66 ppm, indicating that trimethoxysilyl
derivatives of imidzaolium ionic liquid and NHC-Pd complex
were immobilized on the SBA-16 through a condensation
Fig. 1 N2 sorption isotherms of samples: (a) SBA-16; (b) SBA-16 with
passivation; (c) NHC-Pd/SBA-16-IL.
Fig. 2 Pore size distribution of samples: (a) SBA-16; (b) SBA-16 with
passivation; (c) NHC-Pd/SBA-16-IL.
less than the molecular size of diphenyl silane (ca. 0.6 nm). This
result points to the fact that the most of diphenyl silane groups
were grafted on the external surface of SBA-16, as expected;
otherwise the decrease in cage size should be much larger than
0.1 nm. After further grafting of the NHC-Pd complex and ionic
liquid, the surface area and pore volume showed a significant
decrease, and the cage size decreased again by 0.5 nm, suggesting
that the NHC-Pd complex and ionic liquid were introduced
Fig. 4 FT-IR spectra of SBA-16 and NHC-Pd/SBA-16-IL.
1186 | Green Chem., 2009, 11, 1184–1193
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Table 2 The Suzuki reaction in the presence of NHC-Pd/SBA-16-IL
under aira
Entry
R
Reaction time/h
Isolated yield (%)
1
2
3
4
5
6
7
H
5
6
7
7
7
8
10
99
98
87
99
99
99
99
4-CHO
4-COCH3
4-CN
4-NO2
4-CH3
4-OCH3
a The reaction was carried out with 4 mmol of halide, 4.4 mmol of
phenylboronic acid, 4.4 mmol of K3PO4, 0.01 mol% Pd with respect to
aryl bromides, 4 mL of water and 4 mL of ethanol at 50 ◦C.
under such mild conditions. At a catalyst loading as low as
0.01 mol%, NHC-Pd/SBA-16-IL afforded a complete conver-
sion of bromobenzene within 5 hours, and formation of biphenyl
with 99% yield (Table 2, entry 1). For the aryl bromides bear-
ing electron-withdrawing groups, including –CHO, –COCH3,
–NO2 and –CN, complete conversions were observed and 87–
99% yields of the corresponding biaryl products could also be
achieved under mild reaction conditions (Table 2, entries 2–5).
For the aryl bromides bearing electron-donating groups, such
as –CH3 and –OCH3, NHC-Pd/SBA-16-IL still showed a high
activity, and 99% yield of the products were obtained within a
slightly longer time (Table 2, entries 6 and 7).
Fig. 5 13C CP-MAS NMR spectroscopy of NHC-Pd/SBA-16-IL. a, b,
c, d, e, f, g, h denote the carbon atoms and the corresponding peaks, the
peak denoted by i may be attributed to the carbon atoms of CH3COO-
in the NHC-Pd complex.
The recyclability of NHC-Pd/SBA-16-IL was further inves-
tigated because the recyclability of the heterogeneous catalyst
is one of the most important issues for practical applications.
Due to the unavoidable loss of solid catalyst during the recovery
and washing, the reaction scale was amplified 8 times to ensure
enough catalyst quality to perform the consecutive recycling
reactions, as described in the Experimental. The recycling results
of the catalyst NHC-Pd/SBA-16-IL are displayed in Fig. 7.
The catalyst NHC-Pd/SBA-16-IL gave a complete conversion
of bromobenzene, and 99% yield was achieved within 5 hours
Fig. 6 29Si CP-MAS NMR spectroscopy of NHC-Pd/SBA-16-IL.
reaction of the surface silanols and trimethoxysilyl derivatives.
The formation of the NHC-Pd complex was confirmed by XPS
spectroscopy in view of the fact that the Pd 3d binding energy
values changed form 338.7 eV to 336.5 eV after the ionic liquid
was reacted with Pd(OAc)2.
The above characterizations confirmed that the NHC-Pd
complex and ionic liquid were successfully grafted on the surface
of SBA-16.
Catalyst testing for the Suzuki reaction
The catalytic activity of NHC-Pd/SBA-16-IL was first tested
in the Suzuki reaction using various aryl bromides and phenyl-
boronic acid as substrates. All the reactions were carried out
at 50 ◦C under air using 50% aqueous ethanol as a reaction
medium. The reaction results are summarized in Table 2. NHC-
Pd/SBA-16-IL was very active for the Suzuki reaction even
Fig. 7 The recycling testing of NHC-Pd/SBA-16-IL, NHC-Pd/SBA-
15-IL and NHC-Pd/silica-IL. Reaction times from 5 to 49 h; for
the recycling test, the reaction time for NHC-Pd/SBA-16-IL, NHC-
Pd/SBA-15-IL and NHC-Pd/silica-IL was the same.
This journal is
The Royal Society of Chemistry 2009
Green Chem., 2009, 11, 1184–1193 | 1187
©

View Article Online
under the scale-up conditions. After the first reaction cycle, the
catalyst NHC-Pd/SBA-16-IL could be recovered by a simple
centrifugation and filtration. After being washed and dried,
the recovered catalyst was directly used for the next cycle. For
the second, third and fourth cycle of NHC-Pd/SBA-16-IL,
96%, 96% and 97% yield of biphenyl could still be obtained,
respectively. Furthermore, a satisfactory yield was still obtained
from the fifth cycle to the ninth cycle although a longer reaction
time was needed to complete the reaction. 90% yield could
be obtained even for the tenth cycle of NHC-Pd/SBA-16-IL
within 49 hours. The total TON (turnover numbers) for ten
cycles was up to 45 400. As far as we know, among the grafted
catalysts, this was one of the best catalysts for the Suzuki reaction
of unactivated bromobenzene in view of the high activity and
excellent recyclability at the low catalyst loading (0.02 mol%).8c–i
The high recyclability may be attributed to the mesoporous cage-
like structure of SBA-16.
In order to confirm the impact of the mesoporous cage-like
structure of SBA-16 on the recyclability, the NHC-Pd complex
and ionic liquid were immobilized on channel-like SBA-15
and amorphous silica through the same procedures, resulting
in NHC-Pd/SBA-15-IL and NHC-Pd/silica-IL, respectively.
Comparative tests for the recyclability were conducted. The
recycling results of NHC-Pd/SBA-15-IL and NHC-Pd/silica-
IL are also included in Fig. 7. Within the same reaction time
as NHC-Pd/SBA-16-IL, NHC-Pd/SBA-15-IL gave 99%, 93%,
80%, and 47% yields of biphenyl for the first, second, third
and fourth cycle. NHC-Pd/silica-IL gave 95%, 85%, 50% and
25% yields. The yield for the two latter catalysts dramatically
decreased upon recycling. Obviously, the recyclability of NHC-
Pd/SBA-16-IL was much better than that of NHC-Pd/SBA-15-
IL and NHC-Pd/silica-IL.
After being reused three times, the Pd 3d5/2 and Pd 3d3/2 binding
energies changed to 334.5 and 339.9 eV, corresponding to Pd(0)
binding energy. The XPS results showed that Pd(0) was formed
during the catalytic reaction. This was in an agreement with the
previous reported results that Pd(0) nanoparticles were evolved
during the catalytic reaction, although the molecular complex
was used as the initial catalyst.4
TEM images provided the further information about the
Pd particles. TEM images for the fresh NHC-Pd/SBA-16-IL
and fresh NHC-Pd/silica-IL, the NHC-Pd/SBA-16-IL reused
three times and NHC-Pd/silica-IL reused three times are
displayed in Fig. 9. For the fresh catalyst NHC-Pd/SBA-16-
IL (Fig. 9a), the (100) projection corresponding to a cubic Im3m
structure was clearly observed. After being reused three times,
the cage-like structure of the catalyst could be still observed
although the cage-like structure collapsed to some extent
(Fig. 9c). Interestingly, a portion of fine Pd particles with sizes of
less then 5 nm dispersed in the cages of SBA-16 were observed.
However, for the catalyst NHC-Pd/silica-IL after being reused
three times, it was observed that Pd particles were in the range
of 10–25 nm and not uniform in size (Fig. 9d). This observation
was broadly in agreement with the finding that Pd particles with
sizes of 10–40 nm were formed after the catalyst derived from
amorphous silica was used three times.4b Obviously, the sizes
are much larger than the sizes of Pd particles of the NHC-
Pd/SBA-16-IL reused three times. The larger Pd particles led
to the drastic decrease in activity because the Pd particles of
a few nanometres are active species. The size differences of Pd
particles between NHC-Pd/SBA-16-IL and NHC-Pd/silica-IL
accounted for their difference in recyclability.
The smaller Pd particles with a relatively uniform distribution
in size may be attributed to the mesoporous cage-like structure
of SBA-16. Once the catalytic reaction started, Pd(0) was formed
in situ and then evolved into Pd nanoclusters. Due to the
high surface energy, the unstable nanoclusters are prone to
aggregation and agglomeration into larger particles. Owing to
the spatial restriction of the isolated nanocages and smaller
pore entrances of SBA-16, the aggregation or agglomeration
of Pd clusters was efficiently prevented, as shown in Scheme 1.
Accordingly, the size of Pd particles was limited to be less than
the cage size of SBA-16.
To further understand the underlying reason for the signifi-
cant difference in recyclability, XPS and TEM were employed
to characterize the fresh and reused NHC-Pd/SBA-16-IL. The
XPS spectra are shown in Fig. 8. For the fresh NHC-Pd/SBA-
16-IL, the Pd 3d5/2 and Pd 3d3/2 binding energies were de-
termined to be 336.5 and 341.7 eV, respectively. These values
correspond to the Pd(II) binding energy of NHC-Pd(II) complex.
Catalyst testing for the Heck reaction
Encouraged by the achievements in the Suzuki reaction, we
expected to able to generalize the application of the catalyst
in the Heck reaction. Table 3 summarizes the results of the
Heck reaction in the presence of NHC-Pd/SBA-16-IL. Using
unactivated bromobenzene and styrene as substrates, NHC-
Pd/SBA-16-IL at a loading of 0.01 mol% Pd afforded a complete
conversion and 94% yield was obtained (Table 3, entry 1).
For bromobenzene with electron-withdrawing groups, including
–COCH3 and –CHO, –NO2 and –CN, the reaction proceed
relatively faster and 84%–95% yields of the corresponding
products were obtained. For bromobenzene with an electron-
donating group (–CH3) the reaction proceeded slower, and 82%
yield was obtained. Using substituted aryl bromides and methyl
acrylate as substrates, good yields could be also obtained.
Fig. 8 XPS spectra of the fresh NHC-Pd/SBA-16-IL and the reused
NHC-Pd/SBA-16-IL. a: the fresh NHC-Pd/SBA-16-IL; b: NHC-
Pd/SBA-16-IL reused three times.
1188 | Green Chem., 2009, 11, 1184–1193
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Fig. 9 TEM images for the fresh (or reused) NHC-Pd/SBA-16-IL and the fresh (or reused) NHC-Pd/silica-IL; the bar is 50 nm. a: the fresh
NHC-Pd/SBA-16-IL; b: the fresh NHC-Pd/silica-IL; c: the NHC-Pd/SBA-16-IL reused three times; d: NHC-Pd/silica-IL reused three times.
To determine the heterogeneity of the reaction, the activity of
the filtrate was tested. The solid catalyst was filtered out after
the reaction proceeded for 5.5 h and the determined conversion
was 69%. The obtained filtrate was continually stirred under
the reaction conditions. After 35 hours the conversion was
determined to still be 69%. This result indicated that the
catalytic reaction was caused by the solid catalyst instead of
leached Pd in filtrate. ICP-AES analysis of palladium content
in the filtrate after the first cycle was below 0.1 ppm. The
recyclability of NHC-Pd/SBA-16 was also tested with the
consecutive Heck reactions. Fig. 10 displays the results of the
Heck reaction between 4-bromobenzaldehyde and styrene. The
first cycle of the reaction afforded 99% yield. After the first
cycle reaction, NHC-Pd/SBA-16-IL was recovered by a simple
filtration, successively rinsed with NMP, distilled water (to
remove excess of base), ethanol and ether, and finally dried for
the next cycle. Although a longer reaction time was needed to
complete the reaction for the reused catalysts, a satisfactory yield
(above 89%) was still obtained. 87% yield could be obtained
even after the eighth cycle of NHC-Pd/SBA-16-IL. These
This journal is
The Royal Society of Chemistry 2009
Green Chem., 2009, 11, 1184–1193 | 1189
©

View Article Online
Table 3 The Heck reaction in the presence of NHC-Pd/SBA-16-ILa
even after the catalyst was reused ten times. Its recyclability was
much better than that of the catalyst prepared from amorphous
silica under the same conditions. The significantly enhanced
recyclability may be attributed to the isolated nanocages of
SBA-16 which could efficiently prevent the aggregation and
agglomeration of Pd particles formed during the catalytic
reaction into the less active large particles, as evidenced by
TEM. The strategy for stabilizing metal nanoparticles using
isolated nanocages may be applied to other catalytic systems,
in which the aggregation of metal nanoparticles is desired to be
prevented.
Isolated
Entry
R
H
Z
Temperature/◦C Time/h yield (%)
1
2
3
4
5
6
7
8
9
Ph
140
130
130
130
130
140
40
20
20
24
18
46
11.5
23
20
94
95
94
97
84
82
76
86
86
4-COMe Ph
4-CHO
4-CN
4-NO2
4-Me
Ph
Ph
Ph
Ph
4-COMe CO2Me 130
2-CN
4-NO2
CO2Me 130
CO2Me 130
Experimental
a The reaction was carried out with 4 mmol of halide, 4.2 mmol of styrene
(or methyl acrylate), 4.6 mmol of NaOAc, 0.01 mol% Pd with respect to
aryl bromides, and 3.5 mL of NMP under an N2 atmosphere.
Reagents and materials
Pluronic P123 copolymer (EO20PO70EO20), Pluronic copolymer
F127 (EO106PO70EO106) and Pd(OAc)2 were purchased from
Sigma Company. Tetraethyl orthosilicate, N-methylpyrrolin-2-
one (NMP, distilled before using), 1-methylimidazole, phenyl-
boronic acid, styrene, methyl acrylate and various aryl bromides
were obtained from Shanghai Chemical Reagent Company
of Chinese Medicine Group, (3-chloropropyl)trimethoxy silane
was from Jianghan Fine Chemical Company (China, distilled
before using). Amorphous silica gel with a specific surface area
of 328 m2 g-1 was obtained from Qidao Haiyang Chemical Plant.
Mesoporous material SBA-15 was synthesized according to the
reported procedure (specific surface area: 850 m2 g-1; BJH pore
diameter: 8.0 nm).10
results further confirmed the high recyclability of NHC-Pd/
SBA-16-IL.
Conclusions
By grafting the N-heterocyclic carbene palladium complex on
the surface of the nanocages modified with ionic liquids, an effi-
cient and highly recyclable heterogeneous catalyst for the Suzuki
and Heck reactions was successfully prepared. The developed
heterogeneous catalyst showed high activity for various aryl
bromides and afforded a complete conversion of the unactivated
bromobenzene even at a catalyst loading of 0.01 mol%. It could
be easily recovered by a filtration and a good yield was obtained
Mesoporous cage-like material SBA-16 was synthesized ac-
cording to the modified method.7 A mixture of Pluronic F127
(EO106PO70EO106, 7.42 g) and Pluronic P123 (EO20PO70EO20,
Fig. 10 Recycling test of NHC-Pd/SBA-16-IL with the Heck reaction, reaction times from 16 to 49 h.
1190 | Green Chem., 2009, 11, 1184–1193 This journal is The Royal Society of Chemistry 2009
©

View Article Online
1.19 g) was used as the templates. After the mixed templates were
completely dissolved in a solution of 300 mL of distilled water
and 52.5 g of concentrated hydrochloric acid (36%), the solution
was further stirred at 308 K. After 4 h, 28 mL of tetraethyl
orthosilicate was dropwise added to the solution. After being
stirred for 40 min, the resultant suspension was transferred into
autoclaves. The autoclaves were placed under static conditions
at 308 K for 24 h. Afterward, the temperature of the autoclaves
was raised up to 373 K and held for 32 h. After the hydrothermal
treatment, the precipitated solid was isolated by a filtration and
dried at 373 K for 24 h, yielding white solid powders. This powder
sample was then subjected to calcination at 825 K for 10 h
and the mesoporous cage-like material SBA-16 was eventually
obtained.
Immobilization of N-heterocyclic carbene palladium complex
and ionic liquid on SBA-16
The above solution of NHC-Pd complex in ionic liquid was
diluted with dry CHCl3 (13 mL). Into this solution 1.7 g of
SBA-16 with passivation was added under a N2 atmosphere.
The resulting mixture was refluxed for 32 h. After being cooled,
the solid material was isolated by a filtration and was repeatedly
washed with CHCl3, and then dried under vacuum overnight to
give N-heterocyclic carbene palladium complex and ionic liquid
immobilized on SBA-16. The resulting sample was denoted as
NHC-Pd/SBA-16-IL. The Pd content in NHC-Pd/SBA-16-IL
determined by ICP-AES was 0.23 wt%. The N and C content
in NHC-Pd/SBA-16-IL determined by elemental analysis was
2.65 wt% and 9.95 wt%, respectively.
For comparison, N-heterocyclic carbene palladium complex
and ionic liquid were immobilized on SBA-15 and amorphous
silica through the same procedures. The resultant catalysts were
denoted as NHC-Pd/SBA-15-IL and NHC-Pd/silica-IL. The
Pd contents in NHC-Pd/SBA-15-IL and NHC-Pd/silica-IL de-
termined by ICP-AES were 0.28 wt% and 0.13 wt%, respectively.
Preparation of N-3-(3-trimethoxysilylpropyl)-3-methyl
imidazolium chloride (ionic liquid)11
In a dry flask, (3-chloropropyl)trimethoxy silane (24.23 g) was
added to 1-methylimidazole (10.12 g), and then the system was
evacuated and purged with N2 five times. After being stirred at
90 ◦C for 48 h under N2 atmosphere, the resulting mixture was
allowed to cool down to room temperature, was washed with
dry ethyl acetate four times and dried under vacuum for 24 h
at room temperature. The finally obtained ionic liquid was a
kind of yellowish sticky liquids. The 1H NMR spectral data for
the ionic liquid is as follows: d = 10.1 (s, 1H), 7.3–7.6 (m, 2H),
3.8–3.9 (m, 5H), 3.59 (s, 9H), 1.81(m, 2H), 0.69 (m, 2H).
Typical procedures for the Suzuki reaction
A mixture of aryl bromides (4 mmol of halide), phenylboronic
acid (4.4 mmol), K3PO4·3H2O (4.4 mmol), ethanol (4 mL), H2O
(4 mL), and NHC-Pd/SBA-16-IL (0.01 mol% equiv. to aryl
bromide) was stirred at a given temperature for a given time. The
reaction was monitored by TLC. After the reaction, the mixture
was cooled down to room temperature and repeatedly extracted
with diethyl ether (4 mL ¥ 8). The combined organic layers were
concentrated and the resulting product was purified by column
chromatography on silica gel. The product was confirmed by 1H
NMR and melting point determination.
Preparation of a solution of N-heterocyclic carbene palladium
complex (NHC-Pd) in ionic liquid4b
In a well-dried flask, Pd(OAc)2 (0.0368 g) was added to the
synthesized ionic liquid (0.7350 g). The system was evacuated
and purged four times wit◦h N2. The mixture was stirred at 60 ◦C
for 12 h and then at 100 C for 4 h under N2 atmosphere. The
system was attached to the vacuum system for 2 hours to remove
the acetic acid and was then allowed to cool down to room
temperature, affording a pale green-yellow solution of NHC-Pd
complex in ionic liquid. In order to confirm the structure of
the N-heterocyclic carbene palladium complex, N-heterocyclic
carbene palladium complex was prepared from the ionic liquid
and Pd(OAc)2 (the molar ratio of them was 2 : 1) in DMSO. The
1H NMR spectral data for the complex is as follows: d = 7.3–7.4
(m, 2H); 3.4–4.0(m, 11H), 1.90 (m, 2H), 0.63 (m, 2H).
Recycling test for the Suzuki reaction was as follows: for the
first run, the mixture of 32 mmol of bromobenzene, 35.2 mmol
of phenylboronic acid, 35.2 mmol of K3PO4, 0.02 mol% Pd with
respect to bromobenzene, 32 mL of water and 32 mL ethanol
◦
was stirred at 50 C. The reaction was monitored by TLC. At
the end of the reaction the mixture was cooled down to room
temperature and repeatedly extracted with diethyl ether. The
obtained catalyst was washed with diethyl ether, water, methanol
and acetone in sequence and dried under vacuum. The recovered
catalyst was weighed again. The fresh solvent and substrates
were added, but the molar ratios of substrates and solvent to Pd
remained the same as the first run.
The passivation of the external surface silanols of SBA-16
Typical procedures for the Heck reaction
The passivation of the silanols on the external surface of SBA-
16 was performed according to the reported procedure with
some variations.9 3.0 g of SBA-16 (dried at 135 ◦C for 6 h) was
dispersed into 50 mL of dry tetrahydrofuran (THF), and then
0.2 mmol of diphenyldichloro silane (Ph2SiCl2) was added into
the system under N2 atmosphere. After refluxing for 20 min, the
solid was isolated by a filtration and washed with dry THF. The
obtained solid was again dispersed into 50 mL of dry THF, and
then 0.2 mmol of Ph2SiCl2 was added into the system under N2
atmosphere. After refluxing for 30 min, the solid was isolated by
a filtration and thoroughly washed with dry THF, resulting in
SBA-16 with passivation.
A mixture of aryl bromides (4 mmol of halide), styrene
(4.2 mmol), NaOAc (4.6 mmol), NMP (N-methylpyrrolidin-2-
one, 3.5 mL) and NHC-Pd/SBA-16-IL (0.01 mol% with respect
to aryl bromide) was stirred at a given temperature for a given
time under N2 atmosphere. After the reaction, the mixture
was cooled down to room temperature and the organic layer
was isolated by a centrifugation. The remaining residue was
washed with DMF and the organic layer was again isolated
by centrifugation. The combined organic layers were poured
into 100 mL of water, leading to a solid at the bottom of
vessel. The solid state organic product was isolated by filtration,
This journal is
The Royal Society of Chemistry 2009
Green Chem., 2009, 11, 1184–1193 | 1191
©

View Article Online
washed with water and dried. If necessary, the product was
purified by column chromatography on silica gel. The product
was confirmed by 1H NMR and melting point determination.
Recycling test for the Heck reaction was as follows: for the first
run, the mixture of 20 mmol of 4-bromobenzaldehyde, 21 mmol
of styrene, 23 mmol of NaOAc, 17.5 mL of NMP and 0.02 mol%
Pd with respect to bromobenzene was stirred at 130 ◦C. The
reaction was monitored by TLC. At the end of the reaction
the mixture was cooled down to room temperature and the
catalyst was isolated by a centrifugation. The resulting catalyst
was washed with DMF, methanol and acetone in sequence and
dried under vacuum. The recovered catalyst was weighed again.
The fresh solvent and substrates were added, but the molar ratios
of substrates and solvent to Pd remained the same as the first
run.
4-Methylbiphenyl (CDCl3, 300 MHz, ppm). 7.52 (d, 2H,
6 Hz); 7.43 (d, 2H, 6 Hz); 7.37 (t, 2H, 6 Hz); 7.27 (d, 1H, 6 Hz);
7.21 (d, 2H, 9 Hz); 2.34 (s, 3H).
4-Methoxybiphenyl (CDCl3, 300 MHz, ppm). 7.55–7.60 (m,
4H); 7.45 (t, 2H, J = 7.5 Hz); 7.33 (m, 1H); 7.00 (d, 2H, J = 9 Hz);
3.88 (s, 3H).
Stilbene (CDCl3, 300 MHz, ppm). 7.46–7.52 (m, 4H); 7.27–
7.39 (m, 6H); 7.16 (s, 2H).
4-Acetylstilbene (CDCl3, 300 MHz, ppm). 7.94 (d, 2H,
J = 9 Hz); 7.55 (m, 4H); 7.15–7.41 (m, 5H); 2.61 (s, 3H).
4-Formylstilbene (CDCl3, 300 MHz, ppm). 10.01 (s, 1H);
7.87 (d, 2H, J = 8.4Hz); 7.66 (d, 2H, 8.1 Hz); 7.55 (d, 2H,
J = 7.2 Hz); 7.13–7.43 (m, 5H).
4-Cyanostilbene (CDCl3, 300 MHz, ppm). 7.52–7.29 (m,
5H); 7.29–7.41 (m, 4H); 7.21 (s, 1H); 7.11 (s, 1H).
Characterization and analysis
Small-angle X-ray powder diffraction was performed on Rigaku
(Cu Ka, 40 kV, 30 mA). N2 physical adsorption was carried out
on a Micrometrics ASAP2020 volumetric adsorption analyzer
(before the measurements, samples were degassed at 393 K for
6 h). The Brunauer–Emmett–Teller (BET) surface area was
evaluated from data in the relative pressure range from 0.05 to
0.25. The total pore volume of each sample was estimated from
the amount adsorbed at the highest P/P0 (above 0.99). Pore
diameters were determined from the adsorption branch using
Barrett–Joyner–Halenda (BJH) method. FT-IR spectra were
collected on Thermo-Nicolet-Nexus 470 infrared spectrometer.
Pd content was analyzed by inductively coupled plasma-atomic
emission spectrometry (ICP-AES, AtomScan16, TJA Co.). C
and N content analysis were conducted on a Vario EL (Elemen-
tar). X-Ray photoelectron spectroscopy (XPS) was recorded on
a Perkin-Elmer 5400 ESCA and the C1S line at 284.9 eV was
used as the binding energy reference. TEM micrographs were
taken using a JEM-2000EX transmission electron microscope
at 120 kV. Solid state NMR spectra were obtained on an
Infinityplus 300 MHz spectrometer: for 13C CP-MAS NMR
experiments, 75.4 MHz resonant frequency, 4 kHz spin rate,
4 s pulse delay, 1.0 ms contact time, hexamethyl benzene
as a reference compound; for 13Si MAS NMR experiments,
79.6 MHz resonant frequency, 4 kHz spin rate, 4.0 s pulse delay,
TMS as a reference compound.
4-Nitrostilbene (CDCl3, 300 MHz, ppm). 8.21 (d, 2H,
J = 9 Hz); 7.63 (d, 2H, J = 9 Hz); 7.55 (d, 2H, J = 6 Hz);
7.12–7.43 (m, 5H).
4-Methylstilbene (CDCl3, 300 MHz, ppm). 7.52 (d, 2H,
J = 6 Hz); 7.35–7.43 (m, 4H); 7.25–7.29 (d, 1H, J = 6 Hz);
7.18 (d, 2H, J= 9 Hz); 7.10 (s, 2H); 2.39 (s, 3H).
Methyl-4-acetylcinnamate (CDCl3, 300 MHz, ppm). 7.96 (d,
2H, J = 8.0 Hz), 7.62–7.34 (m, 3H, J = 16.0 Hz), 6.54 (d,
1H, J = 16.0 Hz), 3.85 (s, 3H), 2.64 (s, 3H).
Methyl-2-cyanocinnamate (CDCl3, 300 MHz, ppm). 7.96 (d,
1H, J = 15 Hz), 7.49–7.74 (m, 4H); 6.59 (d, 1H, J = 15 Hz);
3.84 (s, 3H).
Methyl-4-nitrocinnamate (CDCl3, 300 MHz, ppm). 8.24 (d,
2H, J = 9 Hz), 7.69–7.74 (m, 3H); 6.54 (d, 1H, J = 15Hz), 3.84
(s, 3H).
Acknowledgements
We acknowledge New Teacher Foundation from Education
Ministry of China (200801081035).
References
1 (a) N. Miyaura, T. Yanagi and A. Suzuki, Synth. Commun., 1981, 11,
513; (b) A. Biffis, M. Zecca and M. Basato, J. Mol. Catal. A: Chem.,
2001, 173, 249; (c) L. Yin and J. Liebscher, Chem. Rev., 2007, 107,
133.
2 (a) I. P. Beletskaya and A. V. Cheprakov, Chem. Rev., 2000, 100, 3009;
(b) J. W. Ruan, O. Saidi, J. A. Iggo and J. L. Xiao, J. Am. Chem. Soc.,
2008, 130, 10510; (c) E. A. B. Kantchev, C. J. O’Brien and M. G.
Organ, Angew. Chem., Int. Ed., 2007, 46, 2768.
3 (a) A. Corma, H. Garc´ıa and A. Primo, J. Catal., 2007, 251, 39;
(b) B. M. Choudary, S. Madhi and N. S. Chowdari, J. Am. Chem.
Soc., 2002, 124, 14127; (c) G. Wei, W. Q. Zhang, F. Wen, Y. Wang
and M. C. Zhang, J. Phys. Chem. C, 2008, 112, 10827; (d) N. Jamwal,
M. Gupta and S. Paul, Green Chem., 2008, 10, 999; (e) L. Li, J. L. Shi
and J. N. Yan, Chem. Commun., 2004, 1990; (f) M. J. Gronnow, R.
Luque, D. J. Macquarrie and J. H. Clark, Green Chem., 2005, 7, 552.
4 (a) M. T. Reetz and J. G. de Vries, Chem. Commun., 2004, 1559; (b) B.
Karimi and D. Enders, Org. Lett., 2006, 8, 1237.
The 1H NMR data for the coupling products
Biphenyl (CDCl3, 300 MHz, ppm). 7.57 (d, 4H, J = 9 Hz);
7.41–7.46 (m, 4H); 7.34 (t, 2H, J = 7.5 Hz).
4-Biphenylcarbaldehyde (CDCl3, 300 MHz, ppm). 10.07 (s,
1H); 7.95 (d, 2H, J = 7.8 Hz); 7.75 (d, 2H, J = 7.8 Hz), 7.43–
7.66 (m, 5H).
4-Acetylbiphenyl (CDCl3, 300 MHz, ppm). 8.06 (d, 2H, J=
6 Hz); 7.39–7.68 (m, 7H); 2.67(s, 3H).
4-Cyanobiphenyl (CDCl3, 300 MHz, ppm). 7.53–7.64 (m,
4H); 7.50 (d, 2H, J = 7.8 Hz); 7.35–7.42 (m, 3H).
5 (a) N. T. S. Phan, M. V. D. Sluys and C. W. Jones, Adv. Synth.
Catal., 2006, 348, 609; (b) Y. Li, E. Boone and M. A. El-Sayed,
Langmuir, 2002, 18, 4921; (c) K. Shimizu, S. Koizumi, T. Hatamachi,
4-Nitrobiphenyl (CDCl3, 300 MHz, ppm). 8.29 (d, 2H,
J = 8.7 Hz); 7.73 (d, 2H, J = 8.7 Hz); 7.45–7.62 (m, 5H).
1192 | Green Chem., 2009, 11, 1184–1193
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
H. Yoshida, S. Komai, T. Kodama and Y. Kitayama, J. Catal., 2004,
228, 141.
Chem., 2005, 70, 6714; (e) H. L. Qiu, S. M. Sarkar, D. H. Lee and
M. J. Jin, Green Chem., 2008, 10, 37; (f) J. Schwarz, V. P. W. Bo¨hm,
M. G. Gardiner, M. Grosche, W. A. Herrmann, W. Hieringer and
G. Raudaschl-Sieber, Chem.–Eur. J., 2000, 6, 1773; (g) J. W. Byun
and Y. S. Lee, Tetrahedron Lett., 2004, 45, 1837; (h) S. Tandukar and
A. Sen, J. Mol. Catal. A: Chem., 2007, 268, 112; (i) M. J. Jin, A.
Taher, H. J. Kang, M. Choi and R. Ryoo, Green Chem., 2009, 11,
309.
6 (a) H. Q. Yang, G. Y. Zhang, X. L. Hong and Y. Y. Zhu, J. Mol.
Catal. A: Chem., 2004, 210, 143; (b) C. M. Crudden, M. Sateesh and
R. Lewis, J. Am. Chem. Soc., 2005, 127, 10045; (c) X. M. Ma, Y. X.
Zhou, J. C. Zhang, A. L. Zhu, T. Jiang and B. X. Han, Green Chem.,
2008, 10, 59; (d) G. Budroni, A. Corma, H. Garc´ıa and A. Primo,
J. Catal., 2007, 251, 345.
7 (a) H. Skaff and T. Emrick, Chem. Commun., 2003, 52; (b) T. W. Kim,
R. Ryoo, M. Kruk, K. P. Gierszal, M. Jaroniec, S. Kamiya and O.
Terasaki, J. Phys. Chem. B, 2004, 108, 11480.
8 (a) S. F. Liu and J. L. Xiao, J. Mol. Catal. A: Chem., 2007, 270,
1; (b) X. D. Mu, J. Q. Meng, Z. C. Li and Y. Kou, J. Am. Chem.
Soc., 2005, 127, 9694; (c) R. T. Tao, S. D. Miao, Z. M. Liu, Y. Xie,
B. X. Han, G. M. An and K. L. Ding, Green Chem., 2009, 11, 96;
(d) J. H. Kim, J. W. Kim, M. Shokouhimehr and Y. S. Lee, J. Org.
9 (a) P. Li and S. Kawi, J. Catal., 2008, 257, 23; (b) H. Q. Yang, J. Li, J.
Yang, Z. M. Liu, Q. H. Yang and C. Li, Chem. Commun., 2007, 1086;
(c) H. Q. Yang, G. Y. Zhang, X. L. Hong and Y. Y. Zhu, Micropor.
Mesopor. Mater., 2004, 68, 119.
10 D. Y. Zhao, Q. S. Huo, J. L. Feng, B. F. Chmelka and G. D. Stucky,
J. Am. Chem. Soc., 1998, 120, 6024.
11 C. Zhong, T. Sasaki, M. Tada and Y. Iwasawa, J. Catal., 2006, 242,
357.
This journal is
The Royal Society of Chemistry 2009
Green Chem., 2009, 11, 1184–1193 | 1193
©