Pd nanoparticles on a porous ionic copolymer: A highly active and recyclable catalyst for Suzuki-Miyaura reaction under air in water

Article abstract of DOI:10.1039/c0cc04498a

A porous copolymer of an IL with divinylbenzene was prepared and applied as a support for Pd nanoparticles. The supported Pd nanocatalyst was found to be extremely active for Suzuki-Miyaura reaction of aryl bromides and chlorides with phenylboronic acid even with 10 ppm Pd loading under air in water.

Full text of DOI:10.1039/c0cc04498a

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Pd nanoparticles on a porous ionic copolymer: a highly active and
recyclable catalyst for Suzuki–Miyaura reaction under air in waterw
a
ab
a
a
a
a
Yanpeng Yu, Tongjie Hu, Xiaorong Chen, Kunling Xu, Junli Zhang and Jun Huang*
Received 19th October 2010, Accepted 20th January 2011
DOI: 10.1039/c0cc04498a
A porous copolymer of an IL with divinylbenzene was prepared
and applied as a support for Pd nanoparticles. The supported Pd
nanocatalyst was found to be extremely active for Suzuki–
Miyaura reaction of aryl bromides and chlorides with phenyl-
boronic acid even with 10 ppm Pd loading under air in water.
bromides is much more difficult than aryl iodides, thus the
development of heterogeneous nanocatalysts that can activate
aryl chlorides and bromides with high efficiency and durability
was highly desirable. We have developed a general and
efficient Pd catalyst system for C–O coupling of aryl bromides
1
0
and chlorides with phenols recently, and we are interested in
the development of practical Pd catalysts that can be used in
laboratory as well as industrial scale. Based on the Pd nano-
Ionic liquids (ILs) have received great attention in various fields
due to their unusual properties, such as negligible vapor pressure
1
and high thermal stability. Imidazolium-based ILs exhibit
11
catalysts in ILs, we attempt to develop a stable and active
dynamic locally heterogeneous environments, which allow their
use as media for preparation of nanoparticles and for synthesis of
2
other nanostructured materials. Pd nanocatalysts in ILs were
heterogeneous nanocatalyst for the coupling reactions.
Recently, polymeric ILs were developed and applied in
12
transition metal catalysis with good performance. However,
investigated extensively for hydrogenation and Heck reactions,
and generally carbene ligands played an important role in the
these catalyst systems are limited with the problem of the mass
transfer since the polymeric ILs are nonporous and insoluble in
the reaction systems. Accordingly, porous polymeric ILs will be
favorable and much more efficient for the related reactions. And
herein we demonstrate a facile one-step synthetic strategy to a
porous ionic copolymer (PIC) via radical copolymerization of
an IL with divinylbenzene (DVB). With the immobilized IL
material, we designed a heterogeneous Pd nanocatalyst system
for Suzuki–Miyaura reaction, which showed extremely high
activity and durability under air in aqueous media. The PIC
was prepared easily (50 g), and can be used directly with
3
reaction systems. Pd catalyzed Suzuki–Miyaura reaction is of
4
great importance in modern organic chemistry. Although
various Pd/ligand systems have been reported for Suzuki–
4
Miyaura reaction, industrial applications of the reaction remain
challenging as the Pd/ligand systems are expensive and difficult to
be recycled. Heterogeneous palladium catalysts are a promising
option. There have been numerous supported Pd nanocatalysts
reported for the Suzuki–Miyaura reaction, such as Pd nano-
5
6
particles on carbon, mesoporous zeolites, metal oxides and
7
polymers. Nevertheless, the heterogeneous Pd catalyst has
Pd(OAc) for Suzuki–Miyaura reaction.
2
generally lower activity than that of its homogeneous counter-
part. Moreover, the Pd atoms often leach out into solutions
which limited the applications in practical use. To improve the
activity and recyclability of heterogeneous Pd catalysts, some
The PIC was prepared as shown in Scheme 1, and the IL
(
VIA-HBr) was prepared by reaction of 1-vinylimidazole and
bromoacetic acid. And VIA-HBr was then copolymerized with
DVB in ethanol using azobisisobutyronitrile (AIBN) as the
6
,8
studies have been performed. But from a practical point of
view, these Pd catalyst materials were not easy to be produced,
especially in a large scale. Therefore, it is highly desirable to
develop cheap and easy-produced heterogeneous Pd nano-
catalysts with high durability and activity for Suzuki–Miyaura
reaction.
2 3
initiator. The copolymer was then treated with Na CO to
remove HBr to give the ionic copolymer PIC. The detailed
experimental section was added as ESI.w
The PIC was characterized by thermogravimetric analysis
(
see Fig. S1 in ESIw) to detect its thermal stability. The small
weight loss before 250 1C resulted from the loss of the
adsorbed water. The PIC was stable until 390 1C and the
further weight loss above 390 1C was attributed to the
decomposition of the PIC. The SEM image showed (Fig. 1)
that the material has a sponge-cake structure. On the basis of
The use of aryl bromides and chlorides was preferable as
they are inexpensive and readily available for most cross-
9
coupling reactions. The activation of aryl chlorides and
a
State Key Laboratory of Materials-Oriented Chemical Engineering,
College of Chemistry and Chemical Engineering,
Nanjing University of Technology, Nanjing 210009, China.
E-mail: junhuang@njut.edu.cn; Fax: +86 25-83172261
College of Chemistry and Chemical Engineering,
b
Jiangsu University, Zhenjiang, Jiangsu, 212013, China
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c0cc04498a
Scheme 1 Preparation of the porous ionic copolymer.
This journal is c The Royal Society of Chemistry 2011
3
592 Chem. Commun., 2011, 47, 3592–3594

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Table 1 Pd catalyzed Suzuki coupling of 4-bromoanisole with
phenylboronic acid
a
b
Yield (%)
Entry
Time/h
Base
[Pd] (mol%)
c,d
1
16
16
16
16
16
2
2
2
40
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
1
1
1
1
62
99
99
87
99
99
99
99
95
c
2
3
4
e
5
6
0.1
0.1
0.01
0.01
0.001
f
7
f
Fig. 1 The SEM image of the PIC.
8
2 3
Na CO
g
9
NaOH
the nitrogen adsorption–desorption analysis, the BET surface
a
Reaction conditions: 4-bromoanisole, 0.5 mmol; phenylboronic acid,
.75 mmol; base, 1.0 mmol; water, 1 mL; TBAB, 1.0 equiv.; Pd
2
À1
area of PIC was 397 m g (Fig. S3 in ESIw) after vacuum
pretreatment at 300 1C, which indicated that the PIC was a
stable porous material. The PIC was found to be an irregular
0
catalyst, 1.0 mol%; under air. Yields were determined by GC
b
c
analysis, hexadecane used as internal standard. Under argon.
d
e
f
porous material with macropores, mesopores and micropores,
À1
Without TBAB. 80 1C. 4-Bromoanisole, 2.5 mmol; phenyl-
g
boronic acid, 3.0 mmol; base, 5.0 mmol, H
.5 mmol; phenylboronic acid, 9.0 mmol; base, 15.0 mmol; H
TBAB = tetrabutylammonium bromide.
2
O, 5 mL. 4-Bromoanisole,
and BJH analysis gave a total pore volume of 0.29 mL g
.
7
2
O, 15 mL.
Large surface and large pore volume of the PIC are highly
beneficial to the reactions in catalytic studies.
The catalyst system can be prepared by supporting
Pd(OAc)
prepared as follows. The PIC was immerged into Pd(OAc)
solution in acetone, and then acetone was removed by
evaporation to give the Pd catalyst Pd(OAc) /PIC (see ESIw).
The Pd/PIC nanocatalyst was formed in situ in the reaction
mixture (Pd catalyzed coupling reaction of 4-bromoanisole
with phenylboronic acid under air in water) with Pd(OAc)2/
2
on the PIC directly. Briefly, the Pd catalyst was
the phase transfer agent TBAB enhanced the yield significantly
(Table 1, entries 1–3). Since TBAB was a well known stabilizer
for nanoparticles, the TBAB presumably played a role also for
the stabilization of the Pd nanocatalyst. Interestingly, the
coupling reaction of 4-bromoanisole with phenylboronic acid
gave 4-methoxybiphenyl in 99% yield with 0.1 mol% or
0.01 mol% loading of Pd in 2 hours (Table 1, entries 4–7).
And even with 10 ppm (0.001 mol%) loading of Pd, high yield
(95%) was obtained, which showed extremely high activity of the
Pd catalyst system (with low Pd loadings, the reaction was scaled
up to 2.5–7.5 mmol of 4-bromoanisole) (Table 1, entry 9). A
weak base (Na CO ) was also suitable for the reaction and the
2
2
PIC, and the Pd(OAc) presumably reduced by phenylboronic
2
6
,13
acid in the reaction mixture.
Pd/PIC can be separated easily
by filtration after one cycle of the reaction, and the Pd/PIC
nanocatalyst was recyclable (see Fig. S6, S7 and S8 in ESIw)
without loss of efficiency. The TEM image of the catalyst after
2
3
5
times reaction (Fig. 2, right) indicates that the mean diameter
yield was nearly the same as NaOH used (Table 1, entry 8). It is
noteworthy that the products were separated by simple organic
extraction and purified by crystallization or by column.
of the nanoparticles is around 2–5 nm, which is similar to the
catalyst separated from the reaction mixture firstly (Fig. 2, left).
After removing the Pd/PIC nanocatalyst by filtration, no Pd was
detected in the filtrate analyzed by ICP-OES (below the detection
limitation 7 ppb) and the reaction did not proceed any more in
Next, several aryl bromides were coupled with phenyl-
boronic acid with 0.01 mol% Pd loading (Pd(OAc) /PIC)
2
under the optimized reaction conditions. To improve the
the filtrate, which showed that Pd(OAc) /PIC was turned into a
2
functional group tolerance, Na CO was used as the base.
3
2
heterogeneous catalyst after one reaction cycle.
As shown in Table 2, the catalyst showed high efficiency and
good functional group tolerance. The coupling reactions of all
the aryl bromides with phenylboronic acid proceeded in
excellent yields with low Pd loadings (0.01 mol% Pd in most
cases). Functional groups, including methoxyl, nitriles, nitro,
aldehydes and ketones, were well tolerated under the reaction
conditions, and the corresponding products were obtained in
high yields (Table 2, entries 1–8, and 11–15). The coupling of
The coupling of 4-bromoanisole with phenylboronic acid
2
was performed as the model reaction with Pd(OAc) /PIC in
water. As shown in Table 1, high yield of 4-methoxybiphenyl
was obtained in water under air or argon and the addition of
1
-naphthyl and 2-naphthyl bromides gave corresponding
products in high yields also (Table 2, entries 9 and 10), and
ortho-substituents in the aryl bromides did not hinder the
coupling reactions (Table 2, entries 2, and 12–15).
Afterwards, several aryl chlorides were tested also for the
coupling reaction (Table 3). The coupling reaction of electron-
poor aryl chlorides afforded the corresponding biphenyl
compounds in excellent yields at 120 1C in 10 hours with
0.01 mol% to 1% Pd loadings (Table 3, entries 1–3, and 5–8).
The deactivated aryl chloride, 4-chloroanisole, was also
Fig. 2 TEM images of Pd/PIC after 1 reaction cycle (left, scale bar 50
nm); Pd/PIC after 5 reaction cycles (right, scale bar 50 nm).
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3592–3594 3593

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a
Table 2 Suzuki coupling of aryl bromides and phenylboronic acid
In summary, we demonstrated a new type of organic porous
materials (PIC) by copolymerization of an IL structure with
DVB. Application of the PIC was performed as an example
and the Pd nanocatalyst was prepared and employed as a
highly efficient catalyst for Suzuki–Miyaura reaction. The Pd
b
Yield (%)
Entry
R
nanocatalyst (Pd/PIC) was formed in situ with Pd(OAc) /PIC
2
1
2
3
4
5
6
7
8
9
4-MeO
2-MeO
4-tert-Butyl
4-HCO
4-MeCO
4-CN
99
98
97
97
99
98
97
97
95
97
97
96
97
94
99
with a diameter of mainly around 2–5 nm in the Suzuki–
Miyaura reaction. Pd/PIC was a heterogeneous catalyst
for the coupling reaction with high activity and excellent
recyclability. Moreover, Pd/PIC was widely applicable for
the coupling reaction of aryl bromides and chlorides, and
functional groups, such as methoxyl, nitriles, nitro, aldehydes,
ketones and ester groups, were well tolerated under the
reaction conditions.
4-Me
4-NO
2
1-Naphthyl
2-Naphthyl
3-Me
2-CN
2-HCO
1
1
1
1
1
1
0
1
2
3
4
5
This work was supported by the National Natural Science
of Foundation of China (grant No. 20802008, 20636020).
c
2-Me-5-NO
2-Me-4-NO
2
2
Note and references
a
Reaction conditions: aryl bromides, 2.5 mmol; phenylboronic acid,
.0 mmol; [Pd] catalyst (Pd(OAc) /PIC), 0.01 mol%; Na CO
.0 mmol; TBAB, 2.5 mmol; H O, 5 mL; at 100 1C; 2 h; under
1
2
3
(a) P. Wasserscheid and W. Keim, Angew. Chem., Int. Ed., 2000,
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3
5
2
2
3
,
3
2
b
c
air. Isolated yield. [Pd] catalyst (0.1 mol%).
2
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103, 831; (b) X. Huang, C. J. Margulis, Y. Li and B. J. Berne,
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Table 3 Suzuki coupling of aryl chlorides and phenylboronic acid
b
Yield (%)
Entry
1
2
3
4
5
6
7
8
R
4-HCO
4-CN
4-NO
2
4-MeO
2-CN
2-HCO
4-MeCO
4-MeOOC
93 (95)
96 (97)
95 (97)
85 (87)
93 (95)
92 (93)
95 (96)
96
c
d
(
e) C. C. Cassol, A. P. Umpierre, G. Machado, S. I. Wolke and
e
J. Dupont, J. Am. Chem. Soc., 2005, 127, 3298; (f) M. Scariot,
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M. A. Novak, G. Ebeling and J. Dupont, Angew. Chem., Int.
Ed., 2008, 47, 9075.
c
c,f
4
(a) N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457;
a
(
b) R. Martin and S. L. Buchwald, Acc. Chem. Res., 2008, 41,
461 and references therein.
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b) Y. Kitamura, S. Sako, T. Udzu, A. Tsutsui, T. Maegawa,
Reaction conditions: aryl chlorides, 0.5 mmol; phenylboronic acid,
.6 mmol; [Pd] catalyst (Pd(OAc) /PIC), 1.0 mol%; NaOH, 1.0 mmol;
TBAB, 0.5 mmol; H O, 1.0 mL; at 120 1C, 10 h under air. Isolated
yields (GC yields in parenthesis, hexadecane used as internal
1
0
2
b
2
(
Y. Monguchi and H. Sajiki, Chem. Commun., 2007, 5069.
M. Choi, D. H. Lee, K. Na, B. W. Yu and R. Ryoo, Angew. Chem.,
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c
d
e
2 3
standard). Na CO , 1.0 mmol. [Pd] catalyst, 0.01 mol%. [Pd]
6
f
catalyst, 3.0 mol%; KOH used instead of NaOH, 1.0 mmol. [Pd]
catalyst, 0.1 mol%.
7 (a) B. Karimi and P. F. Akhavan, Chem. Commun., 2009, 3750;
(
(
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coupled with phenylboronic acid in good yield but with higher
Pd loading (Table 3, entry 4). The protocol was widely
applicable, and aryl chlorides with functional groups, including
methoxyl, nitriles, nitro, aldehydes, ketones and ester groups,
were coupled with phenylboronic acid smoothly to obtain
corresponding biphenyl compounds.
Catal., 2009, 351, 2147.
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The PIC has an IL structure (vinylimidazoliumylacetate)
with a potential carbene group and a carboxylic group, which
is important for the formation and stabilization of Pd nano-
particles. The catalytic activity of Pd is highly dependent on
the size of the Pd nanoparticles, and the PIC stabilized the Pd
nanoparticles well in about 2–5 nm. Benefited from the use
of the PIC for Suzuki–Miyaura reaction, no aggregation
of Pd nanoparticles was observed during the reaction and no
leaching of Pd occurred on separation of the heterogeneous
nanocatalyst. In addition, the PIC material was porous, which
was highly profitable for the heterogeneous catalysis.
1
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594 Chem. Commun., 2011, 47, 3592–3594
This journal is c The Royal Society of Chemistry 2011