Palladium/phosphorus-functionalized porous organic polymer with tunable surface wettability for water-mediated Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1039/c9ra06680b

A series of phosphorus-functionalized porous organic polymers supported palladium catalysts with tunable surface wettability were successfully prepared using an easy copolymerization and successive immobilization method. The obtained polymers were carefully characterized by many physicochemical methods. Characterization results suggested that the prepared materials featured hierarchically porous structures, high pore volumes, tunable surface wettability and strong electron-donating ability towards palladium species. We demonstrated the use of these solid catalysts for water-mediated Suzuki-Miyaura coupling reactions. It was found that the surface wettability of the prepared catalysts has an important influence on their catalytic activities. The optimal catalyst, which has excellent amphipathicity and relatively high phosphorus concentration, displayed superior catalytic activity compared to the other catalysts. Under ambient conditions, a variety of aryl chlorides can be efficiently transformed to biaryls in high yields. Moreover, the catalyst could be easily recovered and reused at least six times.

Full text of DOI:10.1039/c9ra06680b

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PAPER
Palladium/phosphorus-functionalized porous
organic polymer with tunable surface wettability
for water-mediated Suzuki–Miyaura coupling
reaction†
Cite this: RSC Adv., 2019, 9, 36600
a
a
b
*
*
Zaifei Chen and Guangxing Li
Yizhu Lei,
A series of phosphorus-functionalized porous organic polymers supported palladium catalysts with tunable
surface wettability were successfully prepared using an easy copolymerization and successive
immobilization method. The obtained polymers were carefully characterized by many physicochemical
methods. Characterization results suggested that the prepared materials featured hierarchically porous
structures, high pore volumes, tunable surface wettability and strong electron-donating ability towards
palladium species. We demonstrated the use of these solid catalysts for water-mediated Suzuki–Miyaura
coupling reactions. It was found that the surface wettability of the prepared catalysts has an important
influence on their catalytic activities. The optimal catalyst, which has excellent amphipathicity and
relatively high phosphorus concentration, displayed superior catalytic activity compared to the other
catalysts. Under ambient conditions, a variety of aryl chlorides can be efficiently transformed to biaryls in
high yields. Moreover, the catalyst could be easily recovered and reused at least six times.
Received 24th August 2019
Accepted 1st November 2019
DOI: 10.1039/c9ra06680b
rsc.li/rsc-advances
applications.6 Heterogenization of
a homogeneous metal
Introduction
complex by immobilizing the metal complex onto a solid
support with the covalent bond has been expected to address
these problems,7 where organic polymers8 and porous silicas9
are the most widely used catalyst supports. Porous silicas
feature high surface area and stable porous structure, however,
their inert chemical nature limits their post-surface chemical
modication.10 Conventional polymers feature easy chemical
modication, nevertheless, the polymer-anchored molecular
catalysts oen suffer from poor stability, inhomogeneously
distributed active sites, and low efficiency in mass transport.11
Porous organic polymers (POPs), which feature high surface
areas, and designable chemical structure, have attracted
tremendous research interest as new catalyst supports recently
due to their potential to combine the best features of homoge-
neous and heterogeneous catalysts.12 In recent years, a number
of POPs containing PPh3 ligand and its derivatives, have been
successfully prepared and applied as the catalyst supports for
immobilizing transition metal complexes.5b,6,13 Due to the
strong interaction between transition metals and phosphine
ligands, transition metals supported on phosphine functional-
ized POPs usually exhibited long-term reusability, excellent
leaching resistant ability, and high catalytic activities.6,13,14
Some of them even outperformed the activities of their homo-
geneous analogues.6a,15 However, performing an organic reac-
tion in water over a solid catalyst inevitably suffers from poor
mass transfer efficiency of the hydrophobic organic substances,
and thus, leading to low catalytic efficiency.16 To address this
The increasing environmental concerns about harmful solvent
waste has led to a considerable interest of performing organic
transformations in water medium.1,2 As nature's chosen
medium, water not only offers the advantages of nontoxicity,
nonammability, ecological friendliness and low price,1,2 but
also accelerates some reactions' rates through the hydrophobic
or “on water” effects.3 On the other hand, heterogeneous
switching of homogeneous catalysis is considered as an
important strategy for realizing environmentally benign trans-
formations.4 Therefore, there is good reason to believe that
developing highly efficient solid catalysts for the aqueous and
heterogeneous switch of homogeneous organic reactions would
be an ideal pathway for green and sustainable organic
synthesis.
Triphenylphosphine (PPh3) and its derivatives are one kind
of the most important organic ligands that have been widely
used in homogeneous transition-metal catalysis.5 However, the
air-sensitivity and thermal instability of phosphorus ligands at
high temperature make the recycling/recovery of their metal
complexes difficult, and thus, restricting their more extensive
aSchool of Chemistry and Materials Engineering, Liupanshui Normal University,
Liupanshui, Guizhou 553004, PR China. E-mail: yzleiabc@126.com
bJingchu University of Technology, Jingmen, Hubei 448000, PR China. E-mail:
ligxabc@163.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra06680b
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problem, surface of the solid catalyst could be designed to be
The porous properties of PTVP-MBA-x and POL-PPh3 were
amphiphilic.17 Nevertheless, the overwhelming majority of measured by nitrogen adsorption–desorption isotherm
POPs, as well as those phosphine functionalized POPs, were measurements at 77 K. As shown in Fig. 1, all the samples
mainly composed of hydrophobic aromatic networks. The displayed the combined features of type I and type IV curves.
hydrophobic properties lead to their poor dispersion in water The two obvious steep steps of nitrogen gas uptake in the P/P0 <
and thus restricted their applications in water.18
0.01 and 0.80 < P/P0 < 1.0 regions indicated the presence of
Catalytic performance of a heterogeneous catalytic process is micropores and macropores, while the hysteresis loops at P/P0
usually affected by the following two parameters: (i) structure of in the range of 0.7–1.0 reected the presence of mesopores.
the active sites, (ii) the adsorption, desorption, and surface Consistently, pore size distribution curves (inset) based on the
transfer of the reactants and products.19 It is well known that NLDFT calculation method also conrmed the presence of
the catalyst wettability plays an important role in manipulation hierarchically porous structure. Such a hierarchical pore struc-
of the adsorption, desorption and surface transfer behaviour of ture has been found to be benecial for heterogeneous catalysts
the reactants and products.20 Therefore, designing heteroge- by accelerating the mass transport of reactants and products.23
neous catalysts with suitable wettability could improve the Specic surface areas and total pore volumes of the samples
catalytic performances.19–21 Recently, several investigations were also listed in Table 1. The results indicated that BET
about the importance of POPs' wettability in catalysis have been surface areas of the samples decreased from 1146 m2 gꢀ1 to 647
reported,21,22 however, studies of POPs with tunable wettability m2 gꢀ1 with the mass ratio (x) of TVP varied from 1.0 to 0.5.
and their catalytic performances in aqueous-phase catalysis Further decreasing the mass ratio (x) of TVP below 0.5, BET
have rarely been explored.
surface areas of the samples (PTVP-MBA-x, x ¼ 0.4, 0.3, 0.2)
In this study, a series of phosphorus-functionalized porous maintained at about 650 m2 gꢀ1. Meanwhile, with TVP mass
organic polymers (PTVP-MBA) supported palladium catalysts ratio (x) varied from 1.0 to 0.2, total pore volumes of the samples
were successfully prepared by a free-radical cross-linked copo- decreased previously from a maximum value (2.41 cm3 gꢀ1
,
lymerization of tris(4-vinylphenyl)phosphine (TVP) and N,N0- POL-PPh3) to a minimum value (0.93 cm3 gꢀ1, x ¼ 0.6) and then
methylenebisacrylamide (MBA), and successive immobilization increased later to a moderate value (1.68 cm3 gꢀ1, x ¼ 0.2).
of palladium species method. By simply varying the MBA/TVP
Selected SEM images of three representative samples (PTVP-
mass ratio, the palladium catalysts with varied surface wetta- MBA-x, x ¼ 0.2, 0.4, 0.6) were shown in Fig. 2A–C. These images
bility from hydrophobic to hydrophilic could be systematically showed that the copolymers had the similar sponge-like
obtained. The obtained palladium catalysts were tested in morphologies, which consisted of loosely packed and
Suzuki–Miyaura coupling reactions between aryl chlorides and
arylboronic acids. At room temperature, the optimal catalyst
exhibited excellent catalytic performance in water.
Results and discussion
Synthesis and characterization of the catalysts
Porous PTVP-MBA-x polymers (x refers to the mass ratio of TVP
to the total monomers) were directly synthesized through
a solvothermal, free-radical copolymerization reaction of TVP
and MBA (Scheme 1). For adjusting the wettability of PTVP-
MBA-x polymers, PTVP-MBA-x polymers with different phos-
phine contents (PTVP-MBA-x, x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.8) were
synthesized by varying the mass ratio of tris(4-vinylphenyl)
phosphine monomer. For comparison, TVP homopolymer
(POL-PPh3, x ¼ 1.0) was also synthesized.
Fig. 1 N2 absorption–desorption isotherms and pore size distribution
curves (inset) of PTVP-MBA-x samples at 77 K. (A) PTVP-MBA-0.2, (B)
PTVP-MBA-0.3, (C) PTVP-MBA-0.4, (D) PTVP-MBA-0.5, (E) PTVP-
Scheme 1 Synthetic scheme for the PTVP-MBA-x polymers.
MBA-0.6, (F) PTVP-MBA-0.8.
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Table 1 Textural properties
SBETa (m2 gꢀ1
)
Vpb (cm3 gꢀ1
)
Dave (nm)
c
Samples
PTVP-MBA-0.2
PTVP-MBA-0.3
PTVP-MBA-0.4
PTVP-MBA-0.5
PTVP-MBA-0.6
PTVP-MBA-0.8
POL-PPh3
663
646
675
647
760
834
1146
1.68
1.53
1.29
0.95
0.93
1.36
2.41
10.2
9.4
6.7
5.9
4.9
6.5
8.4
a
BET surface area. b Total pore volume. c Average pore size.
irregular-shape nanoparticles. SEM image of PdII@PTVP-MBA-
0.4 in Fig. 2D showed that the sponge-like morphology
remained unchanged aer coordination with Pd(OAc)2. TEM
image of PTVP-MBA-0.4 in Fig. S1 (ESI†) further conrmed the
loosely packed and sponge-like morphology.
Fig. 3 FT-IR spectra of the monomers and polymers: (a) TVP, (b) MBA,
(c) POL-PPh3, (d) PTVP-MBA-0.2, (e) PTVP-MBA-0.4, and (f) PTVP-
MBA-0.6.
Fig. 3 showed the FT-IR spectra of the monomers and
representative polymers. Peaks at 2930 and 2859 cmꢀ1 in
associated with the stretching vibration of –CH2 and –CH
groups could be clearly observed for all PTVP-MBA-x samples.
Peaks at 1661 cmꢀ1 assigned to the C]O bond of amide group,
were also clearly observed for PTVP-MBA-x samples, indicating
the presence of the MBA component. Peaks at 825 cmꢀ1 were
related to C–H out-plane exural vibration of 1,4-substituted
benzene ring, indicating the successful introduction of the TVP
component. Additionally, Fig. 3a and b showed a stretching
catalysts. As depicted in Fig. 4A, Pd 3d XPS spectra of
PdII@PTVP-MBA-x (x ¼ 0.2, 0.4, 0.6) samples revealed that Pd
species were present in the Pd(II) state. The binding energies of
Pd 3d5/2 were around 337.4 eV for PdII@PTVP-MBA-x (x ¼ 0.2,
0.4, 0.6) samples. These values are lower than that (338.4 eV) of
free Pd(OAc)2.24 Simultaneously, the P 2p binding energies of
PdII@PTVP-MBA-x (x ¼ 0.2, 0.4, 0.6) samples exhibited higher
values than that (132.0 eV) of PTVP-MBA-0.4. These results
suggested that there was a strong coordination interaction
between Pd species and PPh3 ligand in the frameworks. ICP-AES
results (Table S1, ESI†) showed that the palladium contents in
PdII@PTVP-MBA-x (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.8) and PdII@POL-
PPh3 catalysts were very close to the nominal amount (2 wt%),
suggesting the excellent palladium immobilization abilities of
the prepared polymers. In the following catalytic experiments,
nominal palladium content (2 wt%) was used for PdII@PTVP-
MBA-x (x ¼ 0.2, 0.3, 0.4, 0.5, 0.6, 0.8) and PdII@POL-PPh3
catalysts. Elemental components of the PTVP-MBA-x (x ¼ 0.2,
0.3, 0.4, 0.5, 0.6, 0.8) samples were also examined by elemental
analysis (Table S2, ESI†). The results suggested that the C, H,
O, N, and P contents of the samples were very close to theo-
retical values, with relative deviation lower than 5%. To study
the element dispersion of PdII@PTVP-MBA-x, SEM-mapping of
vibration band of terminal vinyl group at around 1627 cmꢀ1
,
and this band was disappeared aer polymerization reaction,
revealing that the polymerization reactions were nished.
To gain insight into the structural information and coordi-
nation states of Pd species, XPS studies have been performed on
the PTVP-MBA-0.4 support and three representative PdII@PTVP-
MBA-x (x ¼ 0.2, 0.4, 0.6) catalysts. In Fig. S2,† XPS full spectra of
PTVP-MBA-0.4 and PdII@P(TSP-MBA)-0.4 validated that C, N, P
and O elements were present in the porous polymer and
Fig. 4 (A) Pd 3d XPS spectra of PdII@PTVP-MBA-0.2 (a), PdII@PTVP-
MBA-0.4 (b) and PdII@PTVP-MBA-0.6 (c); (B) P 2p XPS spectra of
Fig. 2 SEM images for: (A) PTVP-MBA-0.2; (B) PTVP-MBA-0.4; (C) PTVP-MBA-0.4 (a), PdII@PTVP-MBA-0.2 (b), PdII@PTVP-MBA-0.4 (c)
PTVP-MBA-0.6 and (D) PdII@PTVP-MBA-0.4.
and PdII@PTVP-MBA-0.6 (d).
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PdII@PTVP-MBA-0.4 was examined (Fig. S3, ESI†). Obviously,
C, N, O, P, and Pd elements were well distributed with high
degrees of dispersion. Thermogravimetric analyses (TGA) of
PTVP-MBA-x (x ¼ 0.2, 0.4, 0.6) samples (Fig. S4, ESI†) showed
that the main weight loss occurred above 300 ꢁC, suggesting
that the prepared samples could be stable up to 300 ꢁC. A little
weight loss around 4 wt% could be also observed bellow 100 ꢁC;
this weight loss could be mainly ascribed to the removal of
trapped guest molecules, which is common for porous
materials.
The solid-state 13C CP/MAS spectra of PTVP-MBA-0.4 and
PdII@PTVP-MBA-0.4 were almost identical (Fig. S5, ESI†). The
bands at about 175 ppm were assignable to “C]O” carbon. The
bands ranged from 126 to 149 ppm could be attributed to the
aromatic carbons of the samples.13b While the bands ranged
from 30 to 43 ppm could be ascribed to the polymerized vinyl
groups.15a These results provided additional evidence for the
successful synthesis of the copolymer. The solid state 31P CP/
MAS spectra of PTVP-MBA-0.4 and PdII@PTVP-MBA-0.4 were
shown in Fig. S6 (ESI†). The 31P CP/MAS spectrum of PTVP-
MBA-0.4 displayed a main peak at 5.80 ppm and a small peak
at 27.52 ppm. The peak at ꢀ5.80 ppm could be attributed to
phosphorus in a tertiary state.13b,e While the peak at 27.52 ppm
was assignable to the oxidation state of phosphorus (P]O),13e
indicating partial oxidation of tertiary phosphorus occurred
Fig. 5 Contact angle measurements of the prepared catalysts for
water.
provides an efficient method for synthesizing amphiphilic
catalysts that enables the exible tuning of their water-
wettability at a molecular level.
Suzuki–Miyaura coupling reactions of aryl chlorides
during the polymerization reaction. The 31P CP/MAS spectrum With the expected materials in hand, Suzuki–Miyaura coupling
of PdII@PTVP-MBA-0.4 displayed two peaks at ꢀ6.36 ppm and of aryl halides, which is a typical coupling reaction, was
28.31 ppm. The peak at ꢀ6.36 ppm was assignable to the employed to investigate the catalytic performance of the ob-
uncoordinated tertiary phosphorus; while the peak at tained palladium catalysts.4c,5b,6a,13e To optimize the reaction
28.31 ppm could be ascribed to both the phosphorus atoms conditions, coupling reaction of phenylboronic acid with chlo-
coordination with palladium and the oxidation state of phos- robenzene was adopted for initial studies (Table 2). First the
phorus (P]O).13e This downeld shi of the resonance peak reaction was conducted with PdII@PTVP-MBA-0.5 as a catalyst
conrmed the electron-donating character of phosphine to screen the bases (entries 1–7). Under the described condi-
ꢁ
ligands. This result was consistent with XPS analyses.
tions at 25 C, K2CO3 afforded the best catalytic performance,
Wettability control of the solid catalyst is an important and and a 75% yield of biphenyl was gained in 8 h (entry 2). Then,
efficient strategy towards high catalytic performance, especially catalytic activities of the obtained PdII@PTVP-MBA-x were
for organic transformations in water.18 The MBA monomer is tested. Interestingly, catalytic activities of PdII@PTVP-MBA-x
hydrophilic, while TVP is hydrophobic. Therefore, PTVP-MBA-x catalysts changed obviously with the mass ratio (x) of TVP varied
samples with different content of MBA should exhibit different from 0.8 to 0.2, and reaching a maximum yield (83%) with
wettability properties. To test surface wettability of the obtained PdII@PTVP-MBA-0.4 as a catalyst (entries 9–13). Examination of
catalysts, the water contact angle measurement was conducted. entry 14 and 15 showed that no homocoupling product was
As shown in Fig. 5 and S7,† PdII@POL-PPh3 had the contact obtained in the absence of chlorobenzene or phenylboronic
angle of nearly 113ꢁ, indicating its water-repellent property. acid, suggesting that homocoupling reaction of phenylboronic
With increasement of the content of MBA, the water contact acid or chlorobenzene couldn't occur under the reaction
angles for the samples gradually decreased. PdII@PTVP-MBA- conditions. Thus, the signicant difference in yields over
0.8 and PdII@PTVP-MBA-0.6 had the contact angles of 59ꢁ and different PdII@PTVP-MBA-x catalysts should be mainly attrib-
16ꢁ for water droplets, respectively. When the mass ratio of MBA uted to their difference in the surface wettability. PdII@POL-
in the sample was no less than 0.5, the water droplets were PPh3, which had the highest PPh3 density, however, gave the
totally absorbed by the porous samples without any residue. lowest yield of biphenyl (entry 8). This low catalytic performance
Thus, the water contact angles of PdII@PTVP-MBA-x (x ¼ 0.2, could be attributed to its hydrophobic property, which led to its
0.3, 0.4, 0.5) were recorded as 0ꢁ. These superhydrophilic poor dispersion in water and low accessibility to the reactants.
features should be related to their hydrophilic and porous With mass ratio of hydrophobic TVP decreased from 1.0 to 0.4,
structures. Contact angle measurements between toluene and the surface wettability changed from hydrophobic to hydro-
PdII@PTVP-MBA-x were also conducted to test their lip- philic, and the yields of biphenyl were signicantly increased.
ophilicity. The results showed that toluene drops were totally However, further decreasing the x from 0.4 to 0.2, the yields
absorbed without any residue for all the PdII@PTVP-MBA-x and were decreased. This decrease could be attributed to the low
PdII@POL-PPh3 catalysts. Thus, this copolymerization strategy PPh3 concentrations in the PTVP-MBA-0.2 and PTVP-MBA-0.3
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Table
2
Effects of catalysts and bases on the Suzuki–Miyaura Table 3 Suzuki–Miyaura coupling of aryl chlorides and arylboronic
acids catalyzed by PdII@PTVP-MBA-0.4a
coupling of chlorobenzene and phenylboronic acida
Entry
Catalyst
Base
T (h)
Yieldb (mol%)
Entry
R1
R2
Time (h)
Yieldb (mol%)
1
2
3
4
5
6
7
8
PdII@PTVP-MBA-0.5
PdII@PTVP-MBA-0.5
PdII@PTVP-MBA-0.5
PdII@PTVP-MBA-0.5
PdII@PTVP-MBA-0.5
PdII@PTVP-MBA-0.5
PdII@PTVP-MBA-0.5
PdII@POL-PPh3
Na2CO3
K2CO3
K3PO4
DBU
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
71
75
55
21
14
23
11
12
29
57
88
81
72
—
—
—
21
<3
98
82
1
2
3
4
5
6
7
8
4-CH3
4-OCH3
4-NH2
4-OH
4-COCH3
4-F
H
H
H
H
H
H
H
H
H
H
H
4-CH3
4-C2H5
4-CF3
4-CN
4-CH3
4-CHO
12
14
15
10
12
12
16
12
12
24
10
12
12
12
12
12
11
93
94
87
99
96
96
81
97
92
78
98c
92
97
91
95
94
95
Et3N
NaOH
NaHCO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
4-Ph
4-NO2
3-NO2
2-NO2
2-NO2
4-OCH3
4-OCH3
4-OCH3
4-OCH3
4-CH3
4-CHO
9
PdII@PTVP-MBA-0.8
PdII@PTVP-MBA-0.6
PdII@PTVP-MBA-0.4
PdII@PTVP-MBA-0.3
PdII@PTVP-MBA-0.2
PdII@PTVP-MBA-0.4
PdII@PTVP-MBA-0.4
Pd/C
9
10
11
12
13
14c
15d
16
17e
18
19
20f
10
11
12
13
14
15
16
17
15
15
12
12
12
Pd/C
PdCl2(PPh3)2
a
Reaction conditions: PdII@PTVP-MBA-0.4 (Pd 1.0 mol%), aryl
PdII@PTVP-MBA-0.4
PdII@PTVP-MBA-0.4
chlorides 1.0 mmol, arylboronic acids 1.5 mmol, K2CO3 2.0 mmol,
b
c
water 3 mL, 25 ꢁC, 1000 rpm. Isolated yields. Reaction temperature
a
50 ꢁC.
Reaction conditions: Pd 1.0 mol%, chlorobenzene 1.0 mmol,
phenylboronic acid 1.5 mmol, base 2.0 mmol, water 3 mL, 25 ꢁC,
b
c
d
1000 rpm.
GC yields.
Without chlorobenzene.
Without
phenylboronic acid. e PPh3 2.0 mol%. f Pd 0.5 mol%.
corresponding biaryl in 99% aer only 10 h. This higher reac-
tion rate might be due to the good solubility of 4-chlorophenol
in water. Under ambient conditions, aryl chlorides bearing
electron-withdrawing groups at the para position also partici-
pated in the reactions readily, affording the corresponding
biaryls in moderate to excellent yields within 16 h (entries 5–8).
Sterically hindered substrates such as 3-nitrochlorobenzene
and 2-nitrochlorobenzene also gave good yields of biaryls, but
a prolonged reaction time or elevated reaction temperature was
needed (entries 9–11). The same protocol was also successfully
extended to several para-substituted arylboronic acids, and high
yields of desired biaryls were obtained (entries 12–15).
Symmetrical biaryls, 4,40-dimethyl and 4,40-dicarbaldehyde
biphenyls, could be gained in high yields (entries 16 and 17).
Rather surprisingly, although most of aryl chlorides in Table 3
are either solids or poorly soluble in water, good yields of biaryls
could be gained despite the fact that dissolution could limit
reaction efficiency. Thus, this study offers an active and
heterogeneous catalyst for the Suzuki–Miyaura coupling of aryl
chlorides and arylboronic acids in water.
networks. The importance of PPh3 ligand was further ascer-
tained by the catalytic performance of Pd/C under the optimal
conditions. Pd/C gave no target product, while a 21% yield was
gained in the presence of 2.0 mol% PPh3 (entries 16 and 17). For
comparison, PdCl2(PPh3)2, a traditional homogeneous catalyst
for this coupling reaction, was also tested. However, PdCl2(-
PPh3)2 gave the target product in trace yield (entry 18); this
could be attributed to the poor hydrophilicity and quick
formation of palladium black of PdCl2(PPh3)2. The effects of
reaction time and catalyst usage were also investigated (entries
19 and 20), and a 98% yield of biphenyl was obtained at 12 h
with 1 mol% of PdII@PTVP-MBA-0.4. Thus, an efficient and
water-compatible catalyst was obtained with ne-turning the
composition of the polymer support.
To investigate the scope of PdII@PTVP-MBA-0.4 catalyst in
Suzuki–Miyaura coupling reaction, aryl chlorides and arylbor-
onic acids bearing different functional groups were tested
(Table 3). The results showed that aryl chlorides bearing
electron-donating groups such as methyl, methoxy, and amino,
gave the biaryls as the sole product in high yields (entries 1–3),
but longer reaction times were needed for the latter two
substrates. This slower rate of reaction might be due to the
strong electron-donating effect of the substituent groups, which
resulted in stronger strength of C–Cl bond and lower rate of
oxidative addition step.25 Conversely, 4-chlorophenol, although
bearing an electron-donating hydroxy group, gave the
In addition to catalytic activity, reusability and metal leach-
ing of the catalyst are also important factors in the heteroge-
neous catalytic systems for commercial applications. The
reaction of chlorobenzene with phenylboronic acid was selected
to evaluate the recycling capacity of PdII@PTVP-MBA-0.4. Aer
each cycle, the yield of desired biaryl was analysed by GC, and
the Pd content in the ltrate was analysed by ICP-AES. As shown
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recovered in the 2nd and 6th recycle. As shown in Fig. 7, the
yields of biphenyl in the 2nd and 6th recycle were similar to those
of the fresh PdII@PTVP-MBA-0.4 catalyst. These results suggest
that the in situ formed palladium nanoparticles during the
recycles are also highly active for Suzuki–Miyaura coupling
reaction, and the catalyst could be recycled at least 6 times
without signicant loss of activity.
Conclusions
In conclusion, porous organic polymers supported palladium
catalysts with tunable surface wettability were successfully
prepared via an easy copolymerization and successive immo-
bilization method. The activity test in aqueous Suzuki–Miyaura
coupling reaction suggested that both the surface wettability
and phosphorus concentration of the catalyst have important
inuence on the catalytic activity. The catalyst, which featured
excellent amphipathicity and relatively high phosphorus
concentration, displayed the highest catalytic activity for water-
mediated Suzuki–Miyaura coupling of aryl chlorides. At
ambient conditions, a variety of aryl chlorides can be efficiently
transformed to biaryls in high yields. Additionally, the catalyst
can be easily recovered and reused several times without
signicant loss of catalytic activity. Thus, this protocol not only
provides an efficient heterogeneous catalyst for Suzuki–Miyaura
coupling reaction, but also some clues to prepare highly effi-
cient heterogeneous catalysts for aqueous organic reactions.
Fig. 6 (A) Reuse of the PdII@PTVP-MBA-0.4 for the Suzuki cross-
couplings of chlorobenzene with phenylboronic acid. The reaction
conditions were the same with that of entry 19 in Table 2; (B) TEM
image for the recycling PdII@PTVP-MBA-0.4 after the first run; (C) TEM
image for the recovered PdII@PTVP-MBA-0.4 after the 6th recycle; (D)
Pd 3d XPS spectra of recycling PdII@PTVP-MBA-0.4: (a) after the first
catalytic run, (b) after the 6th recycle.
in Fig. 6A, PdII@PTVP-MBA-0.4 exhibited excellent reusability
and could be effectively reused at least six times. Moreover, the
Pd leaching in the ltrate was undetectable (<10 ppb). The
morphology and composition of the recovered catalyst was
studied by TEM and XPS. Fig. 6B showed that the Pd nano-
particles with a mean diameter of about 2.5 nm were observed
aer the rst catalytic run (reuse number, 0). Interestingly, no
obvious aggregation of Pd nanoparticles occurred up to the 6th
recycle (Fig. 6C). Fig. 6D showed that Pd(0) and Pd(II) species
coexisted in the recovered catalyst. For the recovered catalyst
aer reusing 0 and 6 times, the ratios of Pd0/PdII were 1.7 and
2.1, respectively (estimated by the proportion of relative peak
areas). To investigate further the activity of the recovered cata-
lyst, reaction kinetics were also studied over the catalyst
Experimental
Materials
N,N0-Methylenebisacrylamide (99%) and palladium diacetate
(99%) were purchased from Energy Chemical and used as
received. 2,20-Azobis(2-methylpropionitrile) (AIBN) was got from
Aladdin Chemistry Co., Ltd. (Shanghai, China). Tris(4-
vinylphenyl)phosphine was prepared according to our
previous report.6d Aryl chlorides and arylboronic acids were
purchased from commercial suppliers and used without further
purication.
Synthesis of PTVP-MBA
Porous PTVP-MBA-x polymers (PTVP-MBA-x), where x refers to
the mass ratio of TVP to total monomers, were prepared by
a hydrothermal and free-radical cross-linked copolymerization
route, at 100 ꢁC for 24 h using dimethylformamide (DMF) as the
solvent and AIBN as the initiator. As a typical run, TVP (0.8 g),
MBA (1.2 g), DMF (20 mL) and AIBN (0.25 wt%, 50 mg) were
added into in a 100 mL Teon autoclave. Aer replacement of
the air in the autoclave with nitrogen, stirred at room temper-
ature for 2 h, the autoclave was heated statically in an oven at
100 ꢁC for 24 h. Aer cooling to room temperature, the resulting
monolith solid was extracted with ethanol and dried under
vacuum (60 ꢁC). The obtained white polymer was designated as
PTVP-MBA-0.4. Varying the mass ratio of TVP and keeping the
same total mass (2.0 g) of two monomers, other PTVP-MBA-x
Fig. 7 Reaction kinetics of the catalyst.
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polymers were prepared as the same synthetic procedures of
PTVP-MBA-0.4.
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Technology Fund Project of Guizhou Province (qian ke he ji chu
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