Palladium nanoparticles captured in microporous polymers: A tailor-made catalyst for heterogeneous carbon cross-coupling reactions

Article abstract of DOI:10.1021/ja9062053

A new strategy based on polymerization-induced phase separation (PIPS) techniques was proposed for fabricating palladium nanoparticles (PdNPs) captured in a microporous network polymer. Pd(OAc)₂ was premixed with a monomer having a poly(amidoamine)-based dendrimer ligand, and subsequently this was thermally polymerized with an excess amount of ethylene glycol dimethacrylate under PIPS conditions. In this system, the formation of PdNPs occurred concurrently with the polymer synthesis in a one-pot process, even with no additional reducing reagent. The resultant microporous polymer was found to have a mesoporosity; the nitrogen sorption analysis gave a specific-surface area of 511 m² g^-1, an average pore diameter of 9.9 nm, and a total pore volume of 1.01 mL g^-1. The TEM images of the polymer revealed that the created PdNPs were very small with a diameter of mainly ca. 2.0 nm; the high-resolution images were lattice-resolvable, showing the crystalline nature of the PdNPs (Pd(111) facets). Catalytic performances of the PdNP-containing microporous polymers were investigated for a heterogeneous Suzuki-Miyaura reaction of 4--bromoacetophenone and phenylboronic acid in water. In the presence of 10^-2 molar equiv of the polymer, the reaction efficiently proceeded at 80 °C and gave the desired product, 4-acetylbiphenyl, in >90% yield after 2 h. On the basis of the ICP-AES analysis, the Pd content released into the solution phase was estimated to be only 0.27% of the initial charge. Thereby, this polymer was successfully recovered by simple filtration and reused with only a minimal loss of activity (yield >90% even at the eighth run). When the catalytic reaction was examined with a low amount of the polymer catalyst, the turnover number (TON) reached 8.5 - 10⁴ while maintaining a good yield. Finally, the dendrimer template effect of the polymer catalyst was discussed by referring to the catalytic performances of a control polymer prepared with nonintegrated ligand monomers.

Full text of DOI:10.1021/ja9062053

Published on Web 03/12/2010
Palladium Nanoparticles Captured in Microporous Polymers:
A Tailor-Made Catalyst for Heterogeneous Carbon
Cross-Coupling Reactions
Shin Ogasawara and Shinji Kato*
Kawamura Institute of Chemical Research, 631 Sakado, Sakura, Chiba 285-0078, Japan
Received July 23, 2009; E-mail: kato@kicr.or.jp
Abstract: A new strategy based on polymerization-induced phase separation (PIPS) techniques was
proposed for fabricating palladium nanoparticles (PdNPs) captured in a microporous network polymer.
Pd(OAc) was premixed with a monomer having a poly(amidoamine)-based dendrimer ligand, and
2
subsequently this was thermally polymerized with an excess amount of ethylene glycol dimethacrylate
under PIPS conditions. In this system, the formation of PdNPs occurred concurrently with the polymer
synthesis in a one-pot process, even with no additional reducing reagent. The resultant microporous polymer
2
was found to have a mesoporosity; the nitrogen sorption analysis gave a specific-surface area of 511 m
g
-
1
, an average pore diameter of 9.9 nm, and a total pore volume of 1.01 mL g-1. The TEM images of the
polymer revealed that the created PdNPs were very small with a diameter of mainly ca. 2.0 nm; the high-
resolution images were lattice-resolvable, showing the crystalline nature of the PdNPs (Pd(111) facets).
Catalytic performances of the PdNP-containing microporous polymers were investigated for a heterogeneous
Suzuki-Miyaura reaction of 4′-bromoacetophenone and phenylboronic acid in water. In the presence of
-
2
10
molar equiv of the polymer, the reaction efficiently proceeded at 80 °C and gave the desired product,
4-acetylbiphenyl, in >90% yield after 2 h. On the basis of the ICP-AES analysis, the Pd content released
into the solution phase was estimated to be only 0.27% of the initial charge. Thereby, this polymer was
successfully recovered by simple filtration and reused with only a minimal loss of activity (yield >90%
even at the eighth run). When the catalytic reaction was examined with a low amount of the polymer catalyst,
4
the turnover number (TON) reached 8.5 × 10 while maintaining a good yield. Finally, the dendrimer template
effect of the polymer catalyst was discussed by referring to the catalytic performances of a control polymer
prepared with nonintegrated ligand monomers.
7
8
9
Introduction
mina, and zeolites, and also polymers. However, it is
generally pointed out that the heterogeneous PdNP system has
a catalytic activity lower than that of its homogeneous coun-
terpart and the Pd atoms often leach out irreversibly from the
Palladium nanoparticles (PdNPs) supported on insoluble
solids are materials of considerable current interest in funda-
mental scientific fields as well as in environment-friendly
recyclable catalytic applications. This is based on the advan-
tages of PdNPs, which includes both the intrinsic versatility of
1
,5,10
1
support, preventing long-lived recyclable use.
many studies have been performed aiming to improve the above
drawbacks. Among these works, the capture approach for
immobilizing PdNPs in solid matrices is fascinating to us, since
Therefore,
1
1
2
Pd metals as a catalyst and the specific size effect of
nanoparticles: that is, a large surface-to-volume ratio. PdNPs
3
so far have been immobilized on a variety of solid supports,
such as charcoal, clay, metal oxides, including silica, alu-
4
5
6
(
(
5) Mitsudome, T.; Nose, K.; Mori, K.; Mizugaki, T.; Ebitani, K.;
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(
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(
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2
005, 105, 1025, and references therein. (b) Panigrahi, S.; Basu, S.;
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(
4
608 9 J. AM. CHEM. SOC. 2010, 132, 4608–4613
10.1021/ja9062053  2010 American Chemical Society

Pd Nanoparticles Captured in Microporous Polymers
A R T I C L E S
1
6
it is favorable to attain the long-term stability of Pd entrapment.
For example, Bradley et al. demonstrated that extensive cross-
linking of a commercial solid-phase synthesis resin (PS-PEG) was
compatible with precious-metal reduction. According to some
precedents, radicals at growing polymer chains are responsible
for the metal reductions. For example, when bulk polymerization
of methyl methacrylate or acrylonitrile was carried out in the
11a
an effective way for PdNP capture to suppress the Pd leaching.
presence of metal salts such as Pd2 and Ag , the corresponding
+
+
Highly durable catalysts were also obtained by the postcapturing
procedure for PdNPs on spherical silica particles, as shown by Lee’s
metal nanoparticles were obtained in polymer solid despite the
11c
17
group. These preceding works are interesting, but it should be
also pointed out that they are somewhat tedious, due to the stepwise
nature for construction of the capture frameworks. In this article,
we wish to propose a new capture strategy for PdNPs entrapped
in microporous network polymers. An interesting feature of this
method is that the PdNP formation concurrently occurs with
synthesis of polymer matrices. Consequently, this approach pro-
vides a one-pot convenient procedure to prepare captured PdNP
systems, which work as high-performance and durable heteroge-
neous catalysts.
absence of any specific reducing reagents. Moreover, it is of
interest that the radicals of growing polymers seem to be capable
of producing highly crystalline metal nanoparticles. This is
important because the crystallinity of metal nanoparticles is a
1
7c
18
crucial factor for determining its catalytic ability. We therefore
hypothesized that, if this reduction system is applied to the PIPS
setup of cross-linkable monomer/porogenic solvent mixtures,
crystalline PdNPs captured in microporous network polymers
would be produced in a convenient one-pot procedure.
In order to verify the hypothesis, a PIPS system of ethylene
glycol dimethacrylate (EGDMA) and dimethylformamide (DMF)/
diethylene glycol dimethyl ether (diglyme) mixed solvent was
arranged in the presence of Pd acetate (Pd(OAc)
initiator, 2,2′-azobis(isobutyronitrile) (AIBN). Poly(amidoamine)
PAMAM) dendrimers were selected as a ligand for both Pd
complexation and stabilization of the created PdNPs. PAMAM
and the related dendrimers are widely used for producing size-
tuned metal nanoparticles. Unlike the bulk polymerization cases
mentioned above, the addition of ligands should be crucial for the
present phase-separation system to make the Pd source more
distributed to the polymer phase rather than the solvent phase.
Microporous network polymers are highly designable solid
1
2
supports and have attracted increasing interest. The poly-
merization-induced phase separation (PIPS) technique is a
traditional method for preparing microporous network poly-
2
) and a radical
1
3
2+
(
mers. PIPS is typically carried out by polymerization of a
homogeneous mixture consisting of monomers and solvents,
where the solvents are miscible with the monomers but
immiscible with the generating polymers, so that a phase-
separated state can be obtained in the course of the polymeri-
zation. Subsequent removal of the solvents leaves pores inside
the network polymers, and the microporous structure thus
obtained usually shows a very high surface area. On the basis
of such interesting features, the microporous polymers prepared
by the PIPS method have been extensively utilized in a variety
of application fields. The molecular imprinting technique is a
unique example of microporous polymer usage, aiming toward
applications in molecular recognition, enzyme-mimic catalytic
19
Results and Discussion
Synthesis of Composite Microporous Polymers. The outline
of this study is depicted in Figure 1. A dendritic monomer
(G3-m) was initially synthesized by the equimolar reaction
between the terminal primary amines of generation-three PAMAM
dendrimer and 2-methacryloyloxyethyl isocyanate. G3-m contains
30 tertiary amines in the internal region and 32 peripheral
methacrylate functions tethered by urea linkages. After premixing
with Pd(OAc) in DMF, G3-m was copolymerized with large
excess of EGDMA (9-fold in weight ratio) in a PIPS setup at 70
°C to produce microporous network polymers. In the polymeri-
zation mixtures, excess Pd2 ions were introduced against the
tertiary amine of G3-m (Pd/N ratio 4/3), since the urea linkages
1
4
studies, and medicine. Interestingly, this technique strongly
depends on the rigid frameworks of the microporous polymers,
which result from the highly cross-linked polymer network
structure produced by the PIPS process. Moreover, recently
Kanamori et al. clearly proved the designable nature of PIPS.
According to their strategy, microporous network polymers with
well-defined bicontinuous morphologies were obtained in as-
sociation with living radical polymerization concepts through
2
+
1
2e,15
2+
11c,20
the characteristic spinodal decomposition.
are also expected to function as ligands for Pd capture.
The
On the basis of the aforementioned intriguing structural
features, we envisaged that the microporous polymers are
powerful tools for the capture of PdNPs as matrices. We also
focused on the fact that a radical polymerization process is
resulting polymer solids were sized to 75-300 µm and used for
6a,21
subsequent structural analysis and catalytic studies.
Several different amounts of AIBN were added to the
polymerization mixtures; the molar ratios to Pd(OAc) were
.046 (G3-p1), 0.092 (G3-p2), 0.18 (G3-p3), 0.55 (G3-p4), and
.1 (G3-p5), respectively. From a viewpoint of AIBN/vinyl unit
2
0
1
(
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(
13) (a) Inoue, T. Prog. Polym. Sci. 1995, 20, 119. (b) Kiefer, J.; Hedrick,
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(b) Rucker, T. G.; Logan, M. A.; Gentle, T. M.; Muetterties, E. L.;
Somorjai, G. A. J. Phys. Chem. 1986, 90, 2703.
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Phys. Chem. B 2005, 109, 692. (b) Astruc, D.; Ornelas, C.; Ruiz, J.
Acc. Chem. Res. 2008, 41, 841. (c) Andr e´ s, R.; de Jes u´ s, E.; Flores,
J. C. New J. Chem. 2007, 31, 1161.
(
15) (a) Kanamori, K.; Nakanishi, K.; Hanada, T. AdV. Mater. 2006, 18,
2
407. (b) Hasegawa, J.; Kanamori, K.; Nakanishi, K.; Hanada, T.;
Yamago, S. Macromolecules 2009, 42, 1270. (c) Hasegawa, J.;
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J. AM. CHEM. SOC. 9 VOL. 132, NO. 13, 2010 4609

A R T I C L E S
Ogasawara and Kato
Figure 1. Schematic outline of the preparation of microporous network polymers that contain PdNPs.
Figure 2. TEM images of created PdNPs in the microporous polymers: (a) G3-p1; (b) G3-p2; (c) G3-p3; (d) G3-p4; (e) G3-p5. Insets: appearances of each
sized polymer.
molar ratio, those correspond to 0.0017, 0.0033, 0.0066, 0.020,
and 0.040, respectively. For all of the cases, polymerization
reactions proceeded smoothly and finally insoluble polymer
solids were obtained through the PIPS process. Figure 2 shows
transmission electron microscopy (TEM) images of the poly-
mers, with insets displaying pictures of the sized polymers. It
is somewhat surprising that only a slight difference in color
was found among the polymers. The color of the polymers
should be closely related to the electronic state of the Pd atoms,
and therefore the fact that these polymers similarly displayed a
dark gray color certainly indicates that the Pd ions introduced
were reduced to a large extent despite the difference of AIBN
contents among the polymers. The TEM analysis supports this
presumption: that is, the created PdNPs were clearly found in
the images for all the polymer cases.
this criterion. The thermal spontaneous decomposition temper-
22
ature of Pd(OAc) is 200-260 °C, which is much higher than
2
the polymerization temperature of the present system. It is
therefore suggested that there have to be chemicals responsible
for Pd reduction in the polymerization process other than the
polymer radicals.
Currently, details regarding the present PdNP formation system
are still unclear. In addition to the radicals at growing polymer
chains mentioned above, an organic radical directly converted from
17c
23
the vinyl groups of EGDMA, the AIBN initiating radical itself,
2+
and the amino groups of G3-m can be also listed as possible
reducing reagents. However, a very important finding was obtained
herein: that is, the PIPS approach was proved to be a convenient
and rational method to fabricate a PdNP capture system, which
In the polymerization process, if the Pd reduction takes place
only by action of the radicals at growing polymer chains, two
(
21) For related composite materials having the dendrimer-stabilized metal
nanoparticles, see: (a) Scott, R. W. J.; Wilson, O. M.; Crooks, R. M.
Chem. Mater. 2004, 16, 5682. (b) Knecht, M. R.; Wright, D. W. Chem.
Mater. 2004, 16, 4890. (c) Huang, W.; Kuhn, J. N.; Tsung, C.-K.;
Zhang, Y.; Habas, S. E.; Yang, P.; Somorjai, G. A. Nano Lett. 2008,
2
+
17a
radicals are consumed to reduce a Pd ion.
An AIBN
molecule decomposes and generates two 2-cyano-2-propyl
2
+
radicals. Therefore, to reduce Pd ions stoichiometrically by
the growing polymer radicals that are initiated by the AIBN
decomposition, at least an equimolar amount of AIBN is
required against Pd2 ions. Among the polymerization mixtures,
only that having the highest amount of AIBN, G3-p5, satisfies
8
, 2027. (d) Kuhn, J. N.; Huang, W.; Tsung, C.-K.; Zhang, Y.;
Somorjai, G. A. J. Am. Chem. Soc. 2008, 130, 14026.
(22) (a) Gross, M. E.; Appelbaum, A.; Gallagher, P. K. J. Appl. Phys. 1987,
6
2
1, 1628. (b) Esumi, K.; Tano, T.; Meguro, K. Langmuir 1989, 5,
68. (c) Tano, T.; Esumi, K.; Meguro, K. J. Colloid Interface Sci.
+
1989, 133, 530.
4
610 J. AM. CHEM. SOC. 9 VOL. 132, NO. 13, 2010

Pd Nanoparticles Captured in Microporous Polymers
A R T I C L E S
Figure 4. Nitrogen adsorption (filled squares)-desorption (open squares)
isotherms of G3-p3. The inset shows the pore size distribution from the
desorption data.
Figure 3. FE-SEM image of the microporous polymer G3-p3.
involves the polymer synthesis and Pd reduction in one pot. Work
is now in progress to get a detailed insight into chemical events
occurring in the present approach.
Finally in this section, it is worthwhile to compare the present
system to that prepared with additional reducing reagents. When
4
tetrabutylammonium borohydride (TBABH ), a typical reagent
for metal reduction, was added to the PIPS process, the
polymerization was accompanied by the formation of Pd black
0
out of the microporous polymer. Additionally, the Pd species
in the polymer appear to be considerably large according to
the TEM observations (Figure S1). This apparently comes from
4
rapid Pd reduction with TBABH , probably due to the fact that
Pd reduction by additional reagents proceeds independently of
the polymerization reaction. In contrast, the Pd reduction without
TBABH proceeds competitively and cooperatively with the
4
polymer propagation, since all the possible reducing reagents
are inherent components for the PIPS process.
Figure 5. HR-TEM image of G3-p3: crystalline PdNPs with resolvable
atomic lattice.
As mentioned above, G3-p3 possesses very small PdNPs with
a diameter of around 2.0 nm, which was easily detected in the
image of Figure 5. The high-resolution image is lattice-
resolvable, showing the crystalline nature of the PdNPs. The
interplanar spacing of the 1D fringes is 0.22 nm, which agrees
well with the (111) lattice spacing of face-centered cubic (fcc)
Characterization of a Composite Microporous Polymer
(G3-p3). This section begins with a comparison among the TEM
images shown in Figure 2. It is of interest to note that G3-p3
prepared under the condition of a 0.18 AIBN/Pd ratio gave the
smallest PdNPs. The PdNPs in this polymer mainly consist of
small particles <2 and 3-5 nm in diameter, while other polymers
contain relatively large PdNPs of around 10 nm. To define the
reason why G3-p3 gave this specific result is not straightfor-
ward, but a possible speculation can be made that the amount
of AIBN radicals generated in G3-p3 at an early stage might
suit the controlled growth of PdNPs inside the dendrimers under
the competitive conditions between the polymer propagation
and Pd reduction. With reference to these results, G3-p3 was
used for the following studies.
24
Pd. The Pd(111) facets are generally acknowledged as the best
18
surface for catalytic usage.
These morphological features were consistent with the X-ray
scattering data of G3-p3. On the basis of the small-angle X-ray
scattering (SAXS) data of this polymer (Figure S2), the PdNPs
were calculated to consist of small particles with diameters of
1
0
.3 and 5.1 nm, where the ratios of numbers of particles were
.96 and 0.04, respectively. It is of interest that these values
for the particle size are very close to those evaluated from the
TEM results. Likewise, a peak that is assigned to the Pd(111)
facets was clearly observed in the wide-angle X-ray scattering
(
WAXS) pattern of G3-p3 (Figure S3), which supports the
Thermogravimetry/differential thermal analysis (TG/DTA) for
crystalline nature of the created PdNPs.
-
1
G3-p3 revealed that its Pd loading was 0.31 mmol g , which
Also of interest is the result of electron energy loss spec-
troscopy (EELS), in which elemental maps collected from G3-
p3 (Figure 6) reveal that Pd and N elements are located at nearly
same positions. Thus, this result supports that the PdNPs were
created in the dendrimer sites. In relation to this finding, when
a control polymer was prepared by the PIPS process in the
absence of G3-m, 48.8% of the added Pd2 was detected in the
porogenic solvent phase (ICP-AES). This strongly suggests
the crucial role of the dendrimer ligand in Pd2 capture.
Catalytic Performances of G3-p3. Catalytic performances of
G3-p3 were investigated for a heterogeneous Suzuki-Miyaura
2
+
indicates that 98% of the feed Pd ions are incorporated in the
polymer. This is in good accord with the result that no Pd species
was released to the solvent phase during the PIPS process.
Figure 3 shows a scanning electron microscopy (SEM) image
of G3-p3. This clearly exhibits a 3D bicontinuous structure that
consists of an interpenetrating network of polymer skeletons
and pores. On the basis of the nitrogen adsorption-desorption
analysis, the BET surface area of G3-p3 was very large, which
+
+
2
-1
was estimated to be 511 m g . Moreover, G3-p3 was found
to have a mesoporosity; BJH analysis gave a peak pore diameter
of 22.2 nm, an average pore diameter of 9.9 nm, and a total
pore volume of 1.01 mL g- (Figure 4). When G3-p3 is used
in catalytic studies, all these characteristics should benefit it in
making reaction substrate access to the Pd sites easier.
1
(
23) (a) Chen, C.-W.; Chen, M.-Q.; Serizawa, T.; Akashi, M. AdV. Mater.
1998, 10, 1122. (b) Chen, C.-W.; Serizawa, T.; Akashi, M. Langmuir
1
999, 15, 7998. (c) Chen, C.-W.; Serizawa, T.; Akashi, M. Chem. Mater.
002, 14, 2232. (d) Kong, H.; Jang, J. Chem. Commun. 2006, 3010.
2
To get an insight into more detailed structure of the PdNPs,
high-resolution (HR) TEM analysis was carried out for G3-p3.
(
24) Liu, Q.; Bauer, J. C.; Schaak, R. E.; Lunsford, J. H. Angew. Chem.,
Int. Ed. 2008, 47, 6221.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 13, 2010 4611

A R T I C L E S
Ogasawara and Kato
Figure 8. Recycle performances of the polymer catalysts: (a) G3-p3; (b)
Cont-p. Reaction conditions selected were identical with those shown in
Figure 7. The reaction yield was evaluated as an average value from at
least three independent experiments.
Figure 6. Spatially resolved elemental EELS maps of G3-p3: (a) zero-
energy loss image; (b) nitrogen element mapping; (c) palladium element
mapping. Note that the images given are not in identical positions due to
sample fluctuation but definitely present the information from the same area.
Figure 9. HR-TEM image of G3-p3 after recycle use for catalytic
Suzuki-Miyaura reactions: crystalline PdNPs with resolvable atomic lattice.
Table 1. G3-p3 Catalyzed Suzuki-Miyaura Reactions
a
G3-p3/mol equiv
time/h
yield/%
TON
0
0
0
.01
.0001
.00001
4
20
40
96
100
85
96
10000
85000
a
Turnover number.
loss of activity. Interestingly, it has been proved that the Pd
content in the reaction mixture after filtration corresponds to
only 0.27% of the initial charge on the basis of the ICP-AES
analysis. Thus, G3-p3 was demonstrated to be a durable catalyst
working under the successive recycle runs, which revealed a
high yield greater than 90% even at the eighth run (Figure 8a).
No noticeable change of the TEM image in size and lattice
pattern (Figure 9) and of the WAXS pattern (Figure S3) was
observed for the PdNPs after recycling. When the catalytic
reaction was examined with a low amount of G3-p3, the
Figure 7. Kinetic profile of the G3-p3 catalyzed Suzuki-Miyaura reaction
in water. Reaction conditions: 4′-bromoacetophenone (0.65 mmol), phe-
-2
nylboronic acid (0.86 mmol), potassium carbonate (2.0 mmol), G3-p3 (10
molar equiv), water (12 mL), 80 °C. The reaction yield was evaluated as
an average value from at least three independent experiments.
reaction of 4′-bromoacetophenone and phenylboronic acid in
4
water. The fabrication of water-compatible catalysts is a highly
turnover number (TON) reached 8.5 × 10 while a good yield
25
challenging subject in the field of green sustainable chemistry.
was maintained (Table 1).
Figure 7 shows a kinetic profile of the catalytic reaction. Under
the presence of 10- molar equiv of G3-p3, the reaction
efficiently proceeded and gave the desired product, 4-acetylbi-
phenyl, in >90% yield after 2 h. G3-p3 was successfully
recovered by simple filtration and reused with only a minimal
With regard to the catalytic high efficiency and recyclability
of G3-p3, the recent study of Astruc’s group is worthy of note.
They demonstrated that the very high catalytic activity of their
dendrimer-stabilized PdNPs system originated from the action
of detached Pd atoms in the solution phase, where the PdNP
2
26
(
25) For recent remarkable studies, see: (a) Ornelas, C.; Ruiz, J.; Salmon,
L.; Astruc, D. AdV. Synth. Catal. 2008, 350, 837. (b) Uozumi, Y.;
Yamada, Y. M. A. Chem. Rec. 2009, 9, 51. (c) Han, J.; Liu, Y.; Guo,
R. J. Am. Chem. Soc. 2009, 131, 2060.
(26) Diallo, A. K.; Ornelas, C.; Salmon, L.; Ruiz Aranzaes, J.; Astruc, D.
Angew. Chem., Int. Ed. 2007, 46, 8644. For related papers, see: Davies,
I. W.; Matty, L.; Hughes, D. L.; Reider, P. J. J. Am. Chem. Soc. 2001,
123, 10139. de Vries, J. G. Dalton Trans. 2006, 421.
4
612 J. AM. CHEM. SOC. 9 VOL. 132, NO. 13, 2010

Pd Nanoparticles Captured in Microporous Polymers
A R T I C L E S
Table 2. G3-p3 Catalyzed Suzuki-Miyaura Reactions of Various
Substrates
Figure 10. Chemical structures of the methacrylate monomers used for
the Cont-p synthesis.
entry
R
1
R
2
time/h
yield/%
1
2
3
4
5
CH
CN
3
CO
H
H
H
H
4
4
4
8
8
96
83
86
82
81
S4). However, Cont-p apparently gave different catalytic
durability regarding Suzuki-Miyaura reactions. In the early runs
with Cont-p, the desired biphenyl product was obtained in high
yield (ca. 90%). However, the reaction yield gradually decreased
with successive recycle runs and was only 60% at the sixth run
OH
CH
CH
3
O
O
3
3
CH O
a
Conditions: aryl bromide (0.65 mmol), arylboronic acid (0.86
-
2
mmol), potassium carbonate (2.0 mmol), G3-p3 (10 molar equiv of
aryl bromide), water (12 mL), 80 °C.
(
Figure 8b). The TEM analysis (Figure S5) indicates that the
catalytic use of Cont-p is likely to induce the size increase and/
or agglomeration of PdNPs. This is probably due to the lack of
an integrated coordination environment in Cont-p, which should
lead to Pd agglomeration under the provided dynamic equilib-
functions as a reservoir for delivering catalytically active Pd
atoms to the reaction field. On this basis, it is certainly suggested
that the catalytic high efficiency and recyclability of PdNPs are
compatible with the Pd leaching phenomena. In our system,
the loss of Pd was very small as mentioned above, thus probably
indicating that this is predominantly not the case for the solution-
phase catalysis. A possible explanation can be made that the
isolated Pd atom remains coordinated to the dendrimer ligands
2
6,27
rium mechanism between PdNPs and isolated Pd species.
Studies are now in progress to further investigate the dendrimer
template effect for the present PdNP system supported on
microporous polymers.
Conclusions
2
7
to a large extent, so that the Pd could subsequently return to
the mother PdNPs after the catalytic cycle. Alternatively, a
heterogeneous pathway that entirely operates the catalytic events
In the present study, we have demonstrated that the PIPS
technique is a convenient and useful tool to prepare microporous
polymer supports for PdNPs capture. In this system, the PIPS
plays two important roles: one is in producing microporous
platforms and the other in converting Pd ions to zerovalent
nanoparticles. Under the competitive conditions of the Pd
reduction and polymer propagation, the controlled growth of
PdNPs was achieved with the aid of the dendrimer ligands. The
resultant polymer was proved to have a mesoporosity with a
very high specific surface area, and the created PdNPs were
highly crystalline and very small with a diameter of mainly
around 2.0 nm. On the basis of the solidly built framework
structure, the loss of Pd was greatly suppressed during the
catalytic cycles. Thereby, the PdNP-containing microporous
polymer can function as an excellent catalyst for heterogeneous
aqueous carbon cross-coupling reactions, with the fascinating
nature of both high activity and durability.
2
8
at the surface of PdNPs is also a possible reason for explaining
the small loss of Pd during the reaction. At any rate, our capture
approach should have an advantage in suppressing the Pd
leaching, because of the plausible solidly built framework
structure that guards the PdNPs by the adjacent dendrimer
templates and the surrounding solid matrices of network
polymers. However, it should be noted that even a small loss
of the released Pd could contribute to some extent to the entire
2
9
catalytic performance of the polymer.
The catalytic activity of G3-p3 was then examined for the
aqueous Suzuki-Miyaura reactions with a range of aryl bromides
and arylboronic acids. Using both electron-rich and electron-
deficient reactants, the desired coupling products were obtained in
good yields. The results are summarized in Table 2.
Finally, it is of interest to refer to a preliminary study
investigating the dendrimer template effect of G3-p3. A control
polymer containing PdNPs (Cont-p) was prepared with both
tertiary amine bearing (TA-m) and urea linkage containing
The next important issue of our study is to elucidate the
detailed mechanism of the PIPS process involving Pd reduction
to lead to PdNP formation. Work is currently underway to get
a detailed insight into the present system and also to extend
this technique to other metals, ligands, and catalytic reactions.
(
(
(
UR-m) methacrylate monomers under identical PIPS conditions
Figure 10). Cont-p that contains amounts of the ligand sites
tertiary amines and urea linkages) identical with those in
Acknowledgment. Parts of the TEM images were obtained at
the Graduate School of Materials Science, Nara Institute of Science
and Technology, with the support of Kyoto-Advanced Nanotech-
nology Network. We thank Dr. Masafumi Uota for helpful
discussions and technical support. We also thank the Analysis
Center and Physics & Characterization Science Group, Corporate
R&D Department of DIC Corp., for analytical support.
G3-p3 was found to be highly porous and showed a large BET
2
-1
surface area of 345 m g . The porosity of Cont-p was
comparable with that of G3-p3, revealing that the 3D meso-
porous structures of these two polymers are nearly equal (Figure
(
(
27) Bernechea, M.; de Jes u´ s, E.; L o´ pez-Mardomingo, C.; Terreros, P.
Inorg. Chem. 2009, 48, 4491.
28) (a) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003, 125,
Supporting Information Available: Text and figures giving
experimental details, SAXS and WAXS profiles for the mi-
croporous polymers, and FE-SEM and TEM micrographs for
the control polymers. This material is available free of charge
via the Internet at http://pubs.acs.org.
8
340. (b) Mori, K.; Hara, T.; Mizugaki, T.; Ebitani, K.; Kaneda, K.
J. Am. Chem. Soc. 2004, 126, 10657.
(
29) For a reference, brief filtration tests were performed for G3-p3. The
polymer was placed in water at 80 °C with shaking for 4 h and then
filtered off. When the filtrate was used for a Suzuki-Miyaura reaction
(
for the conditions, see Figures 7 and 8), the catalytic yield obtained
was 3.1%, where the Pd content in the filtrate was estimated to be
.35% of the initial charge (ICP-AES).
0
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J. AM. CHEM. SOC. 9 VOL. 132, NO. 13, 2010 4613