Novel functional organic network containing quaternary phosphonium and tertiary phosphorus

Article abstract of DOI:10.1021/ma300278d

A quaternary phosphonium and tertiary phosphorus functionalized microporous polymer was obtained via nickel(0)-catalyzed Yamamoto-type cross-coupling reaction. The integration ratio of signals (quaternary phosphonium to tertiary phosphorus atoms) was close to 3:2. The polymer networks were stable toward water, base, and acid, and indeed no change in surface area was observed even after the material was treated with 10 M HCl. The pore size distribution calculated by the Horvath-Kawazoe method indicated the presence of micropores with a mean width of about 0.7 and 1.4 nm. Their apparent BET specific surface areas can be tuned (from 650 to 980 m² g^-1) by changing the counteranions (Br^- to F^-). It displays high intrinsic catalytic activity for the reaction between epoxide and CO₂ (yield: 98%, 1 atm). Pd nanoparticles supported on the polymer networks were also prepared, which exhibits high catalytic activity for cross-coupling reaction between 1-chlorobenzene and phenylboronic acid (yield: >95.8%).

Full text of DOI:10.1021/ma300278d

Article
pubs.acs.org/Macromolecules
Novel Functional Organic Network Containing Quaternary
Phosphonium and Tertiary Phosphorus
Qiang Zhang,†,‡ Suobo Zhang,*,† and Shenghai Li†
†Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun
130022, China
‡Graduate School of Chinese Academy of Sciences, Beijing 100049, China
S
* Supporting Information
ABSTRACT: A quaternary phosphonium and tertiary phosphorus functionalized
microporous polymer was obtained via nickel(0)-catalyzed Yamamoto-type cross-coupling
reaction. The integration ratio of signals (quaternary phosphonium to tertiary phosphorus
atoms) was close to 3:2. The polymer networks were stable toward water, base, and acid,
and indeed no change in surface area was observed even after the material was treated with
10 M HCl. The pore size distribution calculated by the Horvath−Kawazoe method
indicated the presence of micropores with a mean width of about 0.7 and 1.4 nm. Their
apparent BET specific surface areas can be tuned (from 650 to 980 m2 g−1) by changing the
counteranions (Br− to F−). It displays high intrinsic catalytic activity for the reaction
between epoxide and CO2 (yield: 98%, 1 atm). Pd nanoparticles supported on the polymer
networks were also prepared, which exhibits high catalytic activity for cross-coupling
reaction between 1-chlorobenzene and phenylboronic acid (yield: >95.8%).
rings, and hydroxyl groups exhibited excellent CO2 adsorption/
separation due to the microporous structures and interaction
between these polar groups and CO2.3,7 Noble metals have
been incorporated into MOPs containing functional units such
as bipyridine, metalloporphyrin, and triazine ring, which
displayed high catalytic activity for a host of reactions.8 For
example, a recent paper reported a porphyrin-derived MOP
that shows high catalytic activity for the oxidation of thiols.8b
The bipyridine-functionalized precursor networks were pre-
pared and treated with (DBPP)2Ir-(μ-Cl)2-Ir(DBPP)2, which
have been studied as heterogeneous catalysts for reductive
amination reaction.1c
A tetrahedral MOP based on a tetraphenylmethane unit has
been synthesized recently, which exhibits high surface area and
excellent stability.9 The high surface area can be attributed to
the default diamondoid framework topology imposed by the
tetrahedral monomer, which provides wide openings and
interconnected pores to eliminate efficiently “dead space”.10
Subsequently, the central carbon in tetrahedral networks was
replaced by other quadricovalent building centers such as
adamantane, silicon, and germanium atoms.11 In the present
work, the quaternary phosphonium cation has been selected to
be the center for the tetrahedral networks (Figure 1a). To the
best of our knowledge, this is the first report on ionic
microporous polymer prepared by homopolymerization. In
comparison with other tetrahedral MOPs containing centers
such as carbon, silicon, germanium, and adamantane, it
1. INTRODUCTION
The past decade has witnessed an immense growth in the
importance of microporous organic polymers (MOPs) with
pore size smaller than 2 nm.1 They have broad potential
applications in the area of molecular storage, separation,
delivery, or catalysis.1 Such applications often rely on the
presence of a large permanent surface area. In the past decades,
there has been an intense international effort to produce the
highly porous materials with large surface areas. However, it is
recognized that there is a pressing requirement for porous
materials containing functional groups to fulfill a certain task.2
For example, the incorporation of some functional groups will
play a major role in interacting more strongly with a certain
compound than others in a mixture or a solution, resulting in
excellent performance in the areas of ion exchange, purification,
and absorption or separation. For catalysts, catalytically active
centers or binding sites have to be incorporated in a porous,
heterogeneous material.2a
MOPs include polymers of intrinsic microporosity (PIMs),3
hypercrosslinked polymers (HCPs),4 covalent organic frame-
works (COFs),5 or in the case where fully aromatic
compositions are used, conjugated microporous polymers
(CMPs).6 Because most polymer skeletons do not incorporate
any functional groups or active sites, functionalization of MOPs
could open up second-generation porous materials with useful
combined chemical and physical properties.1c,2 Functional
group containing MOPs combines a robust functional frame-
work together with an open porous structure. The pores
provide space for the transmission of molecules, thus
promoting the performance of these materials. MOPs
functionalized with amines, carboxylic acids, tetrazole, triazine
Received: February 9, 2012
Revised: March 16, 2012
Published: March 29, 2012
© 2012 American Chemical Society
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Figure 1. (a) Structure of PP-Br polymer and (b) SEM image of microporous polymer.
possesses many unique properties, which has been applied in
conducting materials,12a,b catalyst transfer agents,12c ionic
liquids,12d and flame-proofing agents.12e Some of these
properties could be easily controlled by varying the counter-
ions. Also, the cationic centers could act as a catalyst in the
reaction of CO2 with epoxides yielding cyclic carbonates.
Quaternary phosphonium-containing polymers have been
applied as ion exchange materials, which exhibited excellent
stability and remarkable ion conductivity12a,b It also has
potential applications as an ion absorbent due to its high
surface area and ion exchange capacity (2.75 mmol/g) or as a
scaffold to immobilize noble metal nanoparticles for heteroge-
neous reactions.
89.6 Hz), 130.6 (8 C, d, J = 12.6 Hz), 136.6 (8 C, d, J = 10.2
Hz), 141.2 (4 C, d, J = 2.6 Hz). 31P NMR (202 MHz, CDCl3)
δ: 23.2. Anal. Calcd for C24H16Cl4PBr: C, 51.74; H, 2.89;
Found: C, 51.67; H, 2.64. MS (M+H)+ C24H16Cl4PBr:
557.0728, observed 557.0734.
Synthesis of PP-Br. The polymer networks are denoted as
PP−X, where X represents the counterions. Ni(cod)2 (2.25 g,
8.18 mmol) was added to a solution of 1,5-cyclooctadiene (1.05
mL, 8.32 mmol) and 2,2′-bipyridyl (1.28 g, 8.18 mmol) in
anhydrous DMF (100 mL), and the mixture was heated to 80
°C for 30 min. To the resulting solution was added tetrakis(4-
chlorophenyl)phosphonium bromide (0.874 g, 1.57 mmol) at
80 °C, and the mixture was stirred at that temperature for 10 h.
After cooling to room temperature, concentrated HCl (5 M, 20
mL) was added to the solution. The solid obtained was filtered,
and the residue was successively washed with DMF (5 × 30
mL), THF (5 × 30 mL), H2O (5 × 30 mL), and propan-2-one
(5 × 30 mL). The polymer was then dried in a vacuum oven at
120 °C (Yield: 83%).
2. EXPERIMENTAL SECTION
Materials. 1-Bromo-4-chlorobenzene, Pd(OAc)2, 1,5-cycloocta-
diene, Ni(cod)2, and 2,2′-bipyridyl were purchased from Aldrich and
used as received. Anhydrous THF was distilled from its sodium
naphthalenide solution under nitrogen. DMF was dried over CaH2 and
distilled under reduced pressure prior to use. Other chemicals were
used as received.
Gel fractions (GF = weight of insoluble dried sample/initial
sample weight) were determined gravimetrically from samples.
The GF content was estimated by weighing the insoluble
portion obtained after Soxhlet extraction in tetrahydrofuran
(THF) for 24 h.
Tris(p-chlorophenyl)phosphine. Tris(p-chlorophenyl)-
phosphine was prepared according to the previously reported
method.12f Yield: 65%; mp: 102.0−102.5 °C. H NMR (600 MHz;
1
DMSO) δ: 7.23−7.25 (6 H, t, J = 6.1 Hz), 7.48−7.50 (6 H, d, J = 12.0
Hz). 13C NMR (100 MHz, CDCl3) δ: 135.6 (s), 134.8 (d, JC−P = 21
Hz), 133.3 (s, JC−P = 11 Hz), 129.0 (d, JC−P = 7.2 Hz). 31P NMR (202
MHz, CDCl3) δ: −7.21. Anal. Calcd for C18H12Cl3P: C, 59.15; H,
3.21. Found: C, 59.17; H, 3.02. MS (M+H)+ C18H12Cl3P: calculated,
364.9814; observed, 364.9818.
Reaction of 2-Phenoxymethyloxirane with CO2. The
PP-Br (0.042 g) was added to a solution of 2-phenoxymethy-
loxirane (1.5 g, 10 mmol) in DMF (10 mL), and the mixture
was stirred at the given temperature under a slow stream (40
mL/min) of carbon dioxide gas. The catalyst was filtered, and
the filtrate was analyzed by GC−MS.
Tetrakis(4-chlorophenyl)phosphonium bromide. 1-
Bromo-4-chlorobenzene (1.15 g, 6 mmol), Pd(OAc)2 (0.013
g, 1 mol %) and tris(p-chloropheny1)phosphine (2.19 g, 6
mmol) were mixed together under an inert atmosphere of
argon in a dry 50 mL tube. To this mixture was added dry o-
xylene (2 mL, 3 M) and a magnetic stir bar. The reaction
mixture was heated to reflux for 5 h under argon. The
phosphonium salts precipitated as the reaction proceeded. After
5 h, the tube was cooled to room temperature, 20 mL of diethyl
ether was added, and the resulting suspension was stirred at
room temperature for 2 min. The precipitate was filtered and
washed with 50 mL of diethyl ether. Yield: 50%. mp: above
Preparation of PP−Pd0. The PP-Br polymer (0.1 g) was
soaked in aqueous H2PdCl4 (20 mL, 5.0 mmol L−1) for 24 h to
−
convert the Br− form into the PdCl4 form and finally washed
thoroughly with water (200 mL) to remove any excess
H2PdCl4. A 10-fold excess of aqueous NaBH4 was added with
vigorous stirring; then, the mixture was kept at RT for 12 h.
The resultant PP−Pd0 particles were collected by filtration,
washed with water several times, and dried under vacuum at 80
°C for 12 h.
Typical Procedure for the Suzuki Reaction. Phenyl-
boronic acid (1.2 mmol), K2CO3 (2.5 mmol), Pd catalyst (1
mol %), aryl halide (1.0 mmol), and H2O/EtOH (10 mL; 3:1
v/v) were added to a screw−capped vial with a side tube under
argon. The mixture was stirred at 80 °C for 6 h and then
1
degradation temperature. H NMR (600 MHz; DMSO) δ:
7.71−7.78 (8 H, q, J = 5.4 Hz), 7.84−7.88 (8 H, q, J = 16.8
Hz). 13C NMR (100 MHz, DMSO) δ: 115.4−116.3 (4 C, d, J =
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Scheme 1. Synthesis of Polymer Networks
cooled. The product was extracted from the reaction mixture
with diethyl ether, and the organic phase was analyzed by GC−
MS.
Ionic Exchange. The PP-Br polymer (0.1 g) was converted
to the PP−Cl or PP−F form by immersing the polymer in 1 M
NaCl (20 mL) or NaF (20 mL) at 80 °C for 12 h, followed by
immersion in deionized water for 24 h. This route was also
applied for the preparation of ionic exchange membranes.13
phosphine atoms) was close to 3:2. Figure S4 of the Supporting
Information gives the assignment of each carbon in polymer
networks. X-ray diffraction measurement of the material
indicated an amorphous structure (Figure S5 of the Supporting
Information), and scanning electron micrograph showed that
PP-Br consisted of relatively uniform spheres with a diameter of
100−200 nm (Figure 1b). The gel fraction of PP-Br was
examined by Soxhlet extraction in THF for 24 h. The weight
loss of PP-Br was negligible after Soxhlet extraction, indicating
that the gel fraction was 100%.
3. RESULTS AND DISCUSSION
The polymer was also stable toward water, base (NaOH 10
M, 1 h), and acid, and indeed no change in surface area was
observed even after the material was treated with 10 M HCl for
1 h. As shown in Figure S6 of the Supporting Information, PP-
Br displayed 5% weight loss at temperature of 360 °C. The
result reveals that it possesses a high thermal stability.
Synthesis and Characterization. We selected tetrakis(4-
chlorophenyl)phosphonium bromide as the building unit. The
monomer was obtained through the reaction of tris(p-
chlorophenyl)phosphine and 1-bromo-4-chlorobenzene in the
presence of Pd(OAc)2 as catalyst, and the synthetic route was
illustrated in Scheme 1. The yield of the reaction was 50%, and
1
Micropore Size and Surface Area. The porosity
parameters of the polymer were studied by sorption analysis
using nitrogen as the sorbate molecule. Nitrogen adsorption−
desorption isotherms measured at 77 K are shown in Figure 3.
The nitrogen isotherm for PP-Br exhibited a combination of
type I and II nitrogen sorption isotherms according to the
IUPAC classification.15 Applying the Brunauer−Emmett−
Teller (BET) model within the pressure range of P/Po =
0.05 to 0.3 resulted in an apparent surface area of 650 m2 g−1,
which makes PP-Br the first microporous polymer based on an
quaternary phosphonium salt. The high uptake at very low
pressures indicates a high microporosity. With increasing
pressure, the nitrogen uptake increases too. This indicates a
high external surface area due to the presence of very small
particles.16 The isotherm shows a hysteresis extending to low
relative pressure. The effect was interpreted as a result of
swelling in polymer framework. A further hint in this
interpretation is the high loss of volume of the sample, when
it is dried after synthesis. The pore size distribution calculated
by the Horvath−Kawazoe method indicated the presence of
micropores with a mean width of ∼0.7 and 1.4 nm (Figure 3b).
As the Br− anion is located inside the micropores, the surface
area can be fine-tuned by changing the anion size. For example,
their apparent BET specific surface areas increased from 650 to
750 and 980 m2 g−1, respectively, as the Br− anion was changed
to Cl− or F− by ionic exchange due to the decreasing ionic
radius of Cl− and F−. The amount of counterions was measured
by elemental analysis (Table S1 of the Supporting Informa-
tion), indicating that the Br− was completely replaced by other
counterions. In general, the surface area can only be controlled
on varying the monomer structure.17 We have thus developed a
novel approach for the variation of surface area that is more
facile than the traditional methods.
the proposed structure was confirmed by H NMR (Figure S1
of the Supporting Information). The signals at 7.86 and 7.74
ppm were assigned to the hydrogen atom in ortho and meta
position to quaternary phosphonium atom, respectively. The
integration ratio of the two signals is very close to 1:1, as
expected for the composition of tetrakis(4-chlorophenyl)-
phosphonium bromide. The 13C NMR spectrum also verified
the structure of tetrakis(4-chlorophenyl)phosphonium bromide
(Figure S2 of the Supporting Information).
The polymer networks were synthesized by the nickel(0)-
catalyzed Yamamoto-type cross-coupling reaction. This reac-
tion produced an off-white powdery compound, which had a
low density (0.0546 cm3 g−1) and was not soluble in any
common organic solvents. PP-Br was characterized by FT-IR
spectroscopy. The spectra clearly indicated the disappearance
of the C−Cl stretching resonance around 763 cm−1 (Figure S3
of the Supporting Information),14 thus indicating the
completion of the phenyl−phenyl coupling. To confirm further
the structure of the polymeric networks, the polymer was
characterized at the molecular level by solid-state 31P NMR
(Figure 2). Two signals at approximately 23 and −8.8 ppm
were observed, corresponding to quaternary phosphonium and
tertiary phosphine atoms, respectively. The resonance at −8.8
ppm corresponds to the tertiary phosphine atoms, resulting
from P−C-bond cleavage during Yamamoto polymerization.
The integration ratio of signals (quaternary phosphonium to
Catalytic Performances. Microporous polymers used in
heterogeneous catalysis systems usually contain various noble
metal complexes.1c,8a,18,19 However, microporous polymers
rarely possess intrinsic catalytic activity due to lack of active
sites.20 It is well known that the addition reaction of oxiranes
Figure 2. Solid-state 31P NMR spectrum of PP-Br polymer.
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Figure 3. (a) Nitrogen adsorption−desorption isotherms measured at 77 K (the adsorption and desorption branches are labeled with solid and open
symbols, respectively) and (b) pore size distribution of the polymer with different anions.
Table 1. PP-Br Catalyzed Conversion of 2-(Phenoxymethyl)oxirane to Cyclic Carbonate in the Presence of CO2 at 1 atm
a
entry
catalyst amount (%)
temperature (°C)
time (h)
yield (%)
1
2
1
2
3
4
2
2
2
2
2
2
90
90
30
30
30
30
70
50
40
24
19
20
29.5
41.3
46.2
50.0
80.4
75.3
78.7
76.9
84.2
98.5
3
90
4
90
5
90
6
100
110
120
130
140
7
8
9
10
a
Yield was determined by GC analysis (internal standard: dodecane).
with CO2 catalyzed by quaternary phosphonium salts gives the
corresponding cyclic carbonates in high yield.21 Recently,
polymeric nanoparticles supported by phosphonium salt were
prepared and applied as catalysts for cycloaddition of CO2 to
epoxides.21d An 83.7% yield for the cycloaddition reaction of
CO2 to styrene oxide was achieved when the reaction was
conducted at 160 °C and 3 MPa for 48 h. The reaction
mechanism was reported by Nishikubo and coworkers.21c In
this work, the PP-Br polymer was applied as a catalyst in the
conversion of CO2 to cyclic carbonates via reaction with
epoxides. Table 1 summarizes the catalytic activity of PP-Br in
the formation of cyclic carbonate from the reaction between 2-
(phenoxymethyl)oxirane and CO2 in DMF at 1 atm. The yield
of 4-(phenoxymethyl)-1,3-dioxolan-2-one increased gradually
with reaction time, temperature, and catalyst concentration, and
a yield of 98% was obtained at 140 °C after 20 h. According to
entries 1−10, it can be concluded that the reaction temperature
seems to be a relatively more critical factor than others. Under
these reaction conditions, no cyclic carbonate was formed in
the absence of PP-Br. Compared with the polymeric nano-
particles system, the high yield (98% within 24 h) of PP-Br
system resulted from the large surface area and porosity.21d
These studies demonstrate that PP-Br was active only during
the reaction and that the reaction proceeded on the
heterogeneous surface. The recovered PP-Br showed nearly
the same reactivity as the original (Table 2).
Table 2. Activity and Recycling of the PP-Br Catalyst in the
a
Reaction between 2-(Phenoxymethyl)oxirane and CO2
run
yield (%)
1
2
3
4
5
98.5
94.6
93.5
91.8
92.7
a
Reaction was carried out with 2 mol % PP-Br at 140 °C for 20 h. The
yield was determined by GC analysis (internal standard: dodecane).
Much interest has been directed toward noble-metal
nanoparticles captured in microporous materials for use in
heterogeneous catalytic systems.22 Budd and McKeown have
reported that microporous polymers act as scaffolds for
incorporating various metal nanoparticles such as Cu, Zn, Pd,
and Co.22d−f For modification with noble metals, several
methods have been developed, such as chemical vapor
deposition,23 ionic exchange,24 in situ generation,25 and so
on. In the present work, PP-Br has been used as a scaffold for
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Figure 4. (a) Schematic diagram of Pd0 nanoparticles immobilization inside PP-Br. TEM images of PP-Br (b) and PP-Pd0 (c).
Figure 5. (a) Nitrogen adsorption−desorption isotherms of PP-Pd measured at 77 K. (b) Pore size distribution of PP-Pd.
immobilizing palladium nanoparticles by ionic exchange. As
shown in Figure 4, tetrachloropalladate (PdCl42−) was
exchanged with bromide ion, followed by NaBH4 reduction
to obtain black PP−Pd0. Inductively coupled plasma (ICP)
analysis confirmed the loading of Pd on polymer networks to
be 0.96 mmol g−1; that is, the Pd/P molar ratio was found to be
1:2.2. The nitrogen-adsorption isotherms of PP-Pd0 were
collected at 77 K, and the results are shown in Figure 5. The
effect of the Pd on the surface area and pore size distribution
was investigated. As expected, the BET surface area decreases
from 650 m2 g−1 (PP-Br) to 114 m2 g−1 (PP-Pd). The decrease
in specific surface area can be ascribed both to partial pore
filling and to simple increase in mass. Also, no significant
change in the pore size was observed and the density of
micropores decreases before and after the loading of Pd (Figure
5b), which is similar to other works.18a Because the Pd0 was
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a
Table 3. PP−Pd0-Catalyzed Suzuki Cross-Coupling Reactions of Aryl Halides with Phenylboronic Acid
entry
X
Y
yield (%)
1
2
3
4
5
6
7
8
Br
Br
Br
Br
Br
I
CHO
COCH3
H
97.4
98.2
96.8
97.9
96.5
99.8
95.8
87.2
Me
OMe
H
Cl
H
F
H
a
Solution of aryl halide (1.0 mmol), phenylboronic acid (1.2 mmol), Pd0 (0.01 mmol), and K2CO3 (2.5 mmol) in H2O/EtOH (10 mL; 3:1 v/v) was
heated at 80 °C under N2 for 6 h. The yield was determined by GC analysis (internal standard: dodecane).
immobilized in polymeric networks by ionic exchange, it is
reasonable to assume that this synthesis route can be applied
for loading other classes of transition-metal nanoparticles (e.g.,
Au, Rh, Ru, and Ir). X-ray photoelectron spectroscopy (XPS)
analysis confirmed the presence of Pd0 nanoparticles within
polymer networks (Figure S7 of the Supporting Information).
The diffractogram of PP−Pd0 (Figure S5 of the Supporting
Information) showed a signal at 2θ = 39.0° confirming the
presence of Pd (1 1 1). The Pd (1 1 1) facets are generally
acknowledged to be the best surface for catalytic usage.25
Transmission electron microscopy (TEM) images were
obtained to check the morphology and measure the particle
size of Pd0 nanoparticles. The results presented in Figure 4c
show that the Pd0 particles are uniformly dispersed in the
polymer networks, and the average particle size was found to be
2.5 nm. No free Pd0 aggregates were observed in the PP−Pd0
even when the weight ratio of Pd to the polymeric scaffold was
increased to as high as 10%.
We carried out Suzuki cross-coupling reactions as a model
reaction to evaluate the catalytic ability and recyclability of the
PP−Pd0 catalyst. Because of environmental, economic, and
safety concerns, the Suzuki reactions were performed in
water.26 A suitable amount of EtOH was used as a cosolvent
to increase the solubility of the substrates. The cross-coupling
reactions were carried out with 1 mol % Pd catalyst in H2O/
EtOH (3:1 v/v) at 80 °C. After the reaction was complete, the
mass was extracted with diethyl ether (10 mL), and the yield of
the product was determined by performing GC−MS analysis of
the diethyl ether phase with dodecane as the internal standard.
Table 3 shows the catalytic activity of PP−Pd0 for the Suzuki−
Miyaura reaction with a range of aryl halides and phenylboronic
acid. The reactions proceeded efficiently and gave the desired
product in >95% conversion with aryl bromides or aryl
chlorides after 6 h. In general, relatively lower yields have been
reported for electron-rich aryl bromides. For example, Kato and
coworkers have recently shown that palladium nanoparticles
captured in polymeric networks gave an 82% conversion yield
for the reaction between 1-bromo-4-methoxybenzene and
phenylboronic acid.22a Using both electron-rich and electron-
deficient aryl bromides with the PP−Pd0 system, the desired
coupling products were obtained in high yield (>95%). More
importantly, a high yield of 95.8% for the reaction between
chlorobenzene and phenylboronic acid was obtained, which is
much higher than the 36% yield reported by our group with
another catalytic system.27 It is well known that catalytic C−C
cross-coupling reactions of fluoroderivatives are rare because of
the strong C−F bond.28 We obtained a yield of 87% in the
reaction between fluorobenzene and phenylboronic acid using
this PP−Pd0 catalyst. For comparison, the coupling of 1-fluoro-
4-methylbenzene with phenyl boronic acid was conducted in
the presence of Pd(PPh3)4, and no product was obtained.
We have also examined the recyclability and reusability of the
PP−Pd0 catalyst. The catalyst was recycled in the reaction
between 4-bromobenzaldehyde and phenylboronic acid, and
the catalytic activity of the preformed PP−Pd0 catalyst proved
to be almost constant with the initial yield of 97.4%, decreasing
to 93% after five runs (Table 4). The catalyst was separated
Table 4. Activity and Recycling of the PP−Pd0 Catalyst in
the Suzuki Coupling of 4-Bromobenzaldehyde and
a
Phenylboronic Acid
run
yield (%)
1
2
3
4
5
97.4
97.6
96.5
96.4
95.2
a
Solution of 4-bromobenzaldehyde (1.0 mmol), phenylboronic acid
(1.2 mmol), Pd0 (0.01 mmol), and K2CO3 (2.5 mmol) in H2O/EtOH
(10 mL; 3:1 v/v) was heated to 80 °C under N2 for 6 h. The yield was
determined by GC analysis (internal standard: dodecane).
from the solution by a simple filtration, and the resultant filtrate
was tested for palladium by ICP−MS. The result showed that
∼17 ppb of palladium leached into the solution, which
corresponds to an original metal loss of 0.00256% and was
thus found to be one hundred times lower than the
corresponding value (0.27%) reported for Kato’s system.22a
Therefore, PP−Pd0 was demonstrated to be a durable catalyst.
A possible explanation for the superior catalytic performance
of PP−Pd0 can be due to the structural features of the
composite system. The PP−Pd0 polymer can draw the reactants
into its pore channels by capillary condensation of the
micropores, where the catalyst particles are located. The
concentration of reagents around the Pd nanoparticles is
relatively higher than that in the bulk solution.
The micropores within the PP−Pd0 polymer can be
considered to act as nanoreactors, where they pose a space
confinement effect on the reactants, leading to a high catalytic
activity.22c Furthermore, the highly polar ionic environment
allows the electrostatic stabilization of preformed nanoparticles
(as for Bu4N+ ions in tetrabutylammonium bromide29) and
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could endow Pd0 with its high activity in the Suzuki cross-
coupling reactions. The Pd0 nanoparticles inside the networks
are very small, which can be another reason for the high activity
of the catalyst.
2131. (b) Martin, C. F.; Stockel, E.; Clowes, R.; Adams, D. J.; Cooper,
̈
A. I.; Pis, J. J.; Rubiera, F.; Pevida, C. J. Mater. Chem. 2011, 21, 5475.
(c) Moon, S. Y.; Bae, J. S.; Jeon, E.; Park, J. W. Angew. Chem., Int. Ed.
2010, 49, 9504.
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4. CONCLUSIONS
A quaternary phosphonium and tertiary phosphorus function-
alized microporous polymer was prepared by Yamamoto
coupling reaction. The content ratio of quaternary phospho-
nium to phosphine atoms was close to 3:2, determined by 31P
solid-state NMR. It was also stable toward water, base (1 M
NaOH, 24 h), and acid (1 M HCl, 24 h). The polymeric
networks display high surface area, and the surface area can be
tuned by changing the counteranions. Their apparent BET
specific surface areas increased from 650 to 750 and 980 m2 g−1
respectively, as the Br− anion was changed to Cl− or F− by ionic
exchange, due to the decreasing ionic radius of Cl− and F−. It
displays high intrinsic catalytic activity for the reaction between
epoxide and CO2. An 98% yield of the reaction between
epoxide and CO2 was obtained at 140 °C and 1 atom for 20 h.
Pd nanoparticles supported on PP were also prepared, which
are uniformly dispersed in the polymeric networks and exhibit
high catalytic activity for Suzuki reactions. The coupling
reactions between aryl chlorides and phenylboronic acid gave
the desired product in >95% conversion in the presence of PP-
Pd. More importantly, a yield of 87% was obtained when
fluorobenzene was used as reactant. In general, catalytic C−C
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ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra, TGA curves, IR, XPS, and WAXD
spectra for polymers and monomers. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sbzhang@ciac.jl.cn.
Notes
■
The authors declare no competing financial interest.
(15) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.;
ACKNOWLEDGMENTS
Pierotti, R. A.; Rouquer
1985, 57, 603.
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ol, J.; Siemieniewska, T. Pure Appl. Chem.
■
This research was financially supported by the National Basic
Research Program of China (nos. 2009CB623401,
2012CB932802) and the National Science Foundation of
China (nos. 51133008, 50825302, and 51021003).
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2008, 44, 2462.
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