Covalently stabilized Pd clusters in microporous polyphenylene: An efficient catalyst for Suzuki reactions under aerobic conditions

Article abstract of DOI:10.1002/smll.201301194

A novel catalyst composed of a microporous polyphenylene network and covalently stabilized Pd clusters (Pd/MPP) for highly efficient Suzuki-Miyaura coupling is synthesized with an in-situ one-pot chemical approach, through the catalytic trimerization of 1,3,5-triethynylbenzene. The unique Pd/MPP cluster exhibits very high catalytic activity for a broad scope of Suzuki-Miyaura reactions with short reaction time, good yield, and high turnover number and turnover frequency values, even in aqueous media under aerobic conditions. The strong covalent interaction between Pd and MPP network prevents the agglomeration or leaching of Pd clusters and enables the catalyst to remain highly active, even after a number of cycles. Copyright

Full text of DOI:10.1002/smll.201301194

Suzuki Catalysts
Covalently Stabilized Pd Clusters in Microporous
Polyphenylene: An Efficient Catalyst for Suzuki
Reactions Under Aerobic Conditions
Qi Song, Yuying Jia, Bin Luo, Haiyong He, and Linjie Zhi*
A
novel catalyst composed of a microporous polyphenylene network and covalently
stabilized Pd clusters (Pd/MPP) for highly efficient Suzuki–Miyaura coupling
is synthesized with an in-situ one-pot chemical approach, through the catalytic
trimerization of 1,3,5-triethynylbenzene. The unique Pd/MPP cluster exhibits very
high catalytic activity for a broad scope of Suzuki–Miyaura reactions with short
reaction time, good yield, and high turnover number and turnover frequency values,
even in aqueous media under aerobic conditions. The strong covalent interaction
between Pd and MPP network prevents the agglomeration or leaching of Pd clusters
and enables the catalyst to remain highly active, even after a number of cycles.
adjust their structures and properties by rational design of
1
. Introduction
different building blocks with a range of function groups.
Particularly, CMPs are more attractive since they hold highly
stable skeletons and amorphous structures which are reliable
in various catalytic reactions even in harsh conditions. Regret-
Suzuki–Miyaura coupling reaction is one of the most impor-
tant reactions for the synthesis of biaryl compounds, which
[
1]
has been widely used for various industry applications. Con-
ventionally, highly air-sensitive catalysts such as Pd(PPh3)4
and toxic solvents like benzene are used in the reaction.
As a result, restrict reaction conditions, such as air-free and
sealed systems, have to be used. In addition, the catalyst is
difficult to be separated and recycled. In order to overcome
these problems, Pd nanoparticles are combined with various
[24,25]
tably, the study of CMPs as catalyst supports is rare.
In general, supported palladium catalysts are prepared
by immersing supports in the solution of palladium salt fol-
lowed with reduction to deposit Pd particles into pores of the
supports. Unfortunately, this simple method is not suitable
for the supports based on microporous organic materials
since there are many drawbacks which significantly decline
the activity of the catalysts: 1) the Pd nanoparticles are easy
to agglomerate and to be washed away during reaction pro-
cessing and catalyst recycling, because the Pd-support inter-
action is not strong enough; 2) the pore size distribution and
the surface area of the support is changeable along with
[
2,3]
[4,5]
supports including porous silica dioxide,
metal oxides,
[
6,7]
[8,9]
[10,11]
carbon structures,
develop heterogeneous catalysts.
Microporous organic materials, including metal organic
polymers,
and dendrimers
to
[
12]
[13–16]
frameworks (MOFs),
covalent organic frameworks
[
17–20]
(COFs),
and conjugated microporous polymers
are one class of promising candidates for
[
21–23]
(CMPs),
[25–27]
[28]
the post treatment.
Recently, Cooper et al.
investi-
well-defined catalyst supports. A distinct advantage of these
microporous organic materials is the versatile potential to
gated the supercritical fluid processing technique to attach
palladium nanoparticles into CMPs so that the interaction
between Pd particles and the support was believed to be
stronger. Thomas et al. developed an efficient method to dis-
tribute uniform metal particles into CMPs by for instance
importing pre-synthesized Pd clusters into covalent triazine
Q. Song, Y. Jia, B. Luo, Dr. H. He, Prof. L. J. Zhi
National Center for Nanoscience and Technology
No. 11 Beiyitiao, Zhongguancun, Beijing
[29]
framework. Despite of these pioneering research works, it
100190, PR China
still appears to be rather challenging to develop controllable
methods which can introduce stabilized nanoparticles into
CMP networks and in the meantime keep the well-defined
E-mail: zhilj@nanoctr.cn
DOI: 10.1002/smll.201301194
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DOI: 10.1002/smll.201301194
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Q. Song et al.
the CMP networks, forming a highly stabilized Pd/CMP
catalyst which shows excellent catalytic activity and stability
for Suzuki reactions even in aerobic conditions.
2
. Results and Discussion
2.1. Synthetic Route
The synthetic route is shown in Figure 1. Being cata-
lyzed by Pd(PPh ) , the 1,3,5-triethynylbenzene (TEB)
3
4
is trimerizated into polyphenylene network by [2 + 2 + 2]
[
30]
homocyclotrimerization of alkynes.
And simultane-
ously, Pd(PPh3)4 is decomposed and aggregated into
alkyne-protected Pd clusters, which is similar with the Au
clusters stabilized by phenylacetylene reported by Maity
[
31]
etc. (Figure S1 and S2).
Immediately, Pd clusters are
stabilized and encapsulated by continuously extending
microporous polymer skeletons, resulting in palladium
cluster-functionalized conjugated microporous poly-
phenylene (Pd/MPP) in one step.
Figure 1. Schematic outline for the preparation of Pd/MPP clusters.
2.2. Characterization of Pd/MPP
porosity structure of CMPs. Herein, we report a novel in-
situ chemical approach with which the formation of CMP
support and the importation of Pd clusters are successfully The synthesis process is investigated by Fourier transform
realized in one step, resulting in a unique Pd/CMP hybrid infrared spectroscopy (FT-IR). As shown in Figure 2a,
−1
with well-defined porosity and monodispersed Pd clusters. after reaction, the peaks at 3280 cm ( ϵCϪH stretching)
−
1
More importantly, the Pd clusters are covalently linked with and 2109 cm (CϵC stretching) which can be attributed
Figure 2. FT-IR spectra of Pd/MPP and TEB (a), the solid 13C CP/MAS NMR result of Pd/MPP (b), XPS survey spectra of Pd/MPP (c), and XPS high-
resolution spectra of Pd3d in PdCl , Pd/C and Pd/MPP (d).
2
2
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Stabilized Pd Clusters in Microporous Polyphenylene
to the alkynyl groups are strongly attenuated. Meanwhile,
the distinct peak corresponding to the aromatic hydrogen
−
1
(
3100 cm ) is significantly enhanced. These results indi-
cate the successful alkynetrimerization and the formation
of benzene rings, which are further supported by the solid-
1
3
state
C cross-polarization magic-angle spinning (CP/
MAS) NMR of Pd/MPP, as shown in Figure 2b. Signals at
δ = 133.8 and 124.2 ppm are assigned to the different aro-
matic carbon atoms of the obtained MPP networks (marked
[
32]
‘
a’ and ‘b’ in Figure 2b). The peak splitting at 133.8 ppm
may attribute to the structural defects of the polyphenylene
network. Interesting information comes from the other three
peaks c, d, and e. Since the signal at 82.6 and 78.0 ppm cor-
responds to the alkynyl carbons connecting with phenyls and
hydrogen atoms, respectively, the weak peak at 91.3 ppm
Figure 3. The HR-TEM images and particle size distributions for Pd/MPP
a) and STEM images of Pd/MPP (b).
(
can be reasonably assigned to carbon atoms on alkyne being structures and the surface properties of MPP are reserved
[
33]
bounded to Pd clusters,
demonstrating the presence of in a high level. Pore size distribution calculation (nonlocal
both terminal alkynyl groups and their covalent bonding to density functional theory, NLDFT) demonstrates the exist-
Pd clusters. X-ray photoelectron spectroscopy (XPS) inves- ence of pores with a diameter of 0.7 nm, 1.2 nm, and a broad
tigation (Figure 2c and d) showed the Pd3d5/2 signal of Pd/ pore distribution in 1.2∼5.0 nm, respectively (Figure 4).
MPP is a broad peak at 336.6 eV as displayed in Figure 2d, Compared with most of the reported CMPs, COFs, or MOFs
which suggests the existence of both Pd(0) and Pd(II) in with rather small pores of less than 2 nm, the Pd/MPP holds
the product. Compared with the signals of PdCl and com- both micropores and mesopores which afford not only nano-
2
[
36]
mercial Pd/C, the broad peaks at 336.6 eV and 342.0 eV reactors but diffusion channels,
can reasonably been split into two sub-peaks: 336.1 eV and form as catalyst.
providing an ideal plat-
3
34.4 eV, corresponding to Pd3d5/2 and Pd3d3/2 of Pd(0),
respectively. Meanwhile the sub-peaks at 337.6 eV and in MPP, a critical issue for practical applications, can be fea-
42.8 eV for Pd3d5/2 and Pd3d3/2 of Pd(II) are observed as sibly adjusted by simply tuning the molar ratio of TEB and
well. It’s also notable that the signals of Pd(II) are slightly Pd(PPh ) , without any significant effect on the Pd cluster
It should be noted that the loading amount of Pd clusters
3
3
4
shifted to lower binding energy (about 0.4 eV) in comparison size distributions (Figure 5), for which other previously
with that of PdCl , which is mainly due to the relatively weak reported methods are difficult to achieve.
2
electron-withdrawing ability of terminal alkynes. Based on
the XPS analysis, the contents of Pd(II) species of Pd in Pd/
2
.3. Catalytic Activity tests of Pd/MPP
MPP is ca. 40.8%. These results disclose that some atoms of
Pd clusters form covalent bonds with terminal alkynes just
[
34,35]
like silver acetylide or other reports about metal-alkynes,
The catalytic activity of Pd/MPP is evaluated for catalyzing
while others still keep the state of zero-valence. These results Suzuki–Miyaura coupling reactions. Surprisingly, the Pd/
further indicate that the produced Pd clusters are covalently MPP catalyst shows very high catalytic activity for driving the
stabilized by the untrimerizated alkynes distributed in the
MPP networks.
The high resolution transmission electron microscopy
(HR-TEM) image and the scanning transmission elec-
tron microscopy (STEM) image of typical Pd/MPP sample
shows amorphous and highly porous structure in which the
Pd clusters with an average diameter of 1.7 ± 0.2 nm are
embedded and distributed homogeneously (Figure 3a and
b). The powder X-ray diffraction (XRD) pattern of Pd/MPP
(Figure S3) shows two broad bulges at a 2 Theta degree of
2
0 and 40, which can be assigned to amorphous MPP net-
works and the fine crystal particles of Pd(111), respectively.
The calculated average particle size of Pd clusters using
Scherrer formula is 1.8 nm, fitting well to the statistical results
of HR-TEM characterizations.
Nitrogen adsorption desorption analysis (Figure 4)
2
shows that Pd/MPP exhibits high surface areas of 659 m /g
3
with a total pore volume of 0.35 cm /g, which are also the
highest values reported in Pd@CMP materials. More impor-
tantly, the catalytic moieties are introduced in the synthesis
Figure 4. N adsorption-desorption analysis and pore size distribution
2
step rather than via post treatment, so that the porous of Pd/MPP.
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Q. Song et al.
Figure 5. HR-TEM images and particle size distributions for Pd/MPP synthesized by different reactant ratio. The molar ratio for TEB and Pd(PPh3)4
was 20:1, 10:1, and 5:1 for (a), (b), and (c), respectively. It clearly shows different loading amount of Pd clusters (2.21 wt%, 4.56 wt% and 11.6 wt%
for (a), (b), and (c), respectively, determined by ICP-OES analysis), but comparable particle size distributions: 2.0 ± 0.2 (d), 1.7 ± 0.2 (e) and
1.7 ± 0.2 (f) for (a), (b), and (c), respectively.
reaction even in aqueous media under aerobic conditions. reaction conditions in heterogeneous catalysis system; and
As summarized in Table 1, using only 0.2 mol% of Pd/ better results may be achieved by adjusting the reaction con-
MPP catalyst, the coupling reaction of phenylboronic acid ditions and processed under inert atmosphere.
with substituted aryl bromides can be finished in short time
As heterogeneous catalyst, Pd/MPP is feasible to be dis-
(0.25–1 h) with excellent yield (95–99%) at 80 °C in air. In persed in organic solvents, and is amenable to facile filter
fact, some of the aryl bromides with an electron-withdrawing separation and recycling. Recycle tests of Pd/MPP demonstrate
group can reach the end of the reaction in just a few minutes; that its catalytic activity and selectivity do not decrease even
and even 0.005 mol% of Pd/MPP is proved to be highly effi- after 5 cycles (Table 1, entry 10). HR-TEM characterizations
cient for this reaction just as Table 1, entry 8 shows. Taking demonstrate that the Pd clusters still hold quite small diameters
1
hour as the reasonable reaction time, both the calculated of 3.5 ± 1.5 nm with relatively narrow (1–5 nm) particle size
4
turnover number (TON, 1.92 × 10 ) and the turnover fre- distribution even after 5 cycles (Figure 6a). Statistic evaluation
4
−1
quency (TOF, 1.92 × 10 h ) are at a high level while main- shows that a large number of particles are still smaller than
taining a good yield, for which other frequently used catalysts 3.5 nm which are believed to be highly active for catalysis reac-
are difficult to achieve. It is worthwhile noted that either tions (Figure 6b). We also perform XPS analysis on Pd/MPP
electron-donating or electron-withdrawing substituent of after 5 cycles and compare the results with the XPS spectra of
aryl bromides has little influence on the catalysis, and no by- fresh Pd/MPP, just as Figure S5 shows. After 5 cycles, the Pd(II)
product such as the formation of homo-coupling products contents of Pd in Pd/MPP is dropped to 22.9% accroding to
from aryl bromides or phenylboronic acid has been detected XPS results, while the catalytic activity of Pd/MPP is still very
in the reaction. Meanwhile, we also compare the activity of high. Combined with the HR-TEM characterizations, it is
Pd/MPP with previously reported results (Table S1 in the indicating that Pd nanoparticles are slightly grown up but still
Supporting Information). Very high yield of the selected been covalently bonded to the support even after 5 cycles.
reaction has been achieved on Pd/MPP with much shorter Meanwhile, the liquid phase of the reaction mixture was col-
reaction time even in air, indicating the excellent catalytic lected by hot filtration after each cycle. Only very low amount
performance of Pd/MPP.
of dissolved palladium (less than 0.2% of the total palladium)
However, the Suzuki reactions for aryl chlorides can only is detected in the liquid by inductively coupled plasma optical
gave unsatisfied yield under similar conditions; the cross-cou- emission spectrometer (ICP-OES), which shows no catalysis
pling between phenylboronic acid and p-chloroacetophenone activity for a new reaction. Obviously, the excellent stability
gives less than 30% yield even after 24 h. (Table 1, entry 9) and high activity of Pd/MPP is in close relation to its unique
This may due to that the activation of aryl chlorides is much structure where Pd clusters is covalently stabilized and semi-
more difficult than aryl bromides and always requires harsher embedded in the three-dimensional network of MPP.
4
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DOI: 10.1002/smll.201301194

Stabilized Pd Clusters in Microporous Polyphenylene
T a b l e 1 . Catalytic activity test of Pd/MPP for Suzuki–Miyaura coupling trimerization of TEB. The unique Pd/MPP exhibits not only
reactions.
high surface area, gradient pore size distribution, and mono-
dispersed and highly stabilized Pd clusters, but very high
catalytic activity for a broad scope of Suzuki–Miyaura reac-
tions in aqueous media under aerobic conditions with short
Entry
1
R
X
Time
h]
Yield
[%]a)
reaction time, good yield, and high TON and TOF values.
The strong covalent interaction between Pd and MPP net-
work prevents the agglomeration or leaching of Pd clusters
and enable the Pd/MPP catalyst to remain highly active even
after a number of cycles. This work opens up a new window
for developing various MPP-based catalysts, in which the
MPP can act as a stable, robust, and versatile stabilizer to
host not only Pd but other noble metal clusters for catalyzing
a wide scope of reactions with high efficiency.
[
Br
0.5
98
97
2
3
4
Br
Br
Br
1
0.75
0.5
99
96
4. Experimental Section
5
Br
1
95
Typical procedure for Pd/MPP: TEB (150 mg, 1 mmol) and
Pd(PPh ) (115.5 mg, 0.1 mmol) were dissolved in THF (5.0 mL)
3
4
6
7
Br
Br
0.5
96
98
and kept stirring for 12 h at 65 °C under reflux at ambient atmos-
phere. The obtained solid was washed with THF, dichloromethane
and ethanol repeatedly. Further purification was carried out by
Soxhlet extraction from dichloromethane, and then dried at 80 °C
under vacuum to yield Pd/MPP as a brown powder almost quanti-
tatively. The Pd content was 4.56 wt% as determined by ICP-OES
analysis.
Catalytic Tests: These were carried out in a reaction tube
sealed with a Teflon screw cap. The tube was charged with boronic
acid (0.6 mmol, 1.2 equiv), sodium carbonate (106 mg, 1 mmol,
and 2 equiv), aryl bromide or aryl chloride (0.5 mmol, 1 equiv),
Pd/MPP (0.2 mol% Pd based on arylbromide), ethanol (2 mL)
and water (2 mL). The flask was sealed with air and stirred in a
preheated oil bath (80 °C) until reaction finished (monitored by
TLC). After cooling down to room temperature, the product was
extracted into dichloromethane and washed with water to remove
potassium carbonate. After further purification by column chroma-
tography over silica gel and removing the solvent under reduced
pressure, the desired products were obtained. In recycle tests, the
catalyst was washed with water and dried, and then used again
directly after each cycle.
<0.25
8b)
Br
Cl
Br
1
96
<30
98
9
24
1
0c)
<0.25
a)Isolated yield; b)0.005 mol% Pd/MPP was used; c)After 5 cycles.
Supporting Information
Supporting Information is available from the Wiley Online Library
Figure 6. The HR-TEM images and particle size distributions for Pd/MPP or from the author.
after 5 cycles (a) and STEM images of Pd/MPP after 5 cycles (b).
Acknowledgements
The authors would like to thank Yunfeng Yao and Long Hao for
their help and discussions and the finance supports from the
3
. Conclusion
In summary, a novel and high efficient catalyst Pd/MPP for National Natural Science Foundation of China(Grant no. 20973044,
Suzuki–Miyaura coupling reaction has been synthesized 21173057), the Ministry of Science and Technology of China
with an in-situ one-pot chemical approach via catalytic (no. 2012CB933400, no.2012CB933403), the Beijing Municipal
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Received: April 19, 2013
Revised: May 6, 2013
Published online:
6
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DOI: 10.1002/smll.201301194