Porous polymer supported palladium catalyst for cross coupling reactions with high activity and recyclability

Article abstract of DOI:10.1007/s11426-011-4491-8

Porous polymer supported palladium catalyst for cross coupling reactions with high activity has been successfully prepared by coordination of Pd2+ species with Schiff bases functionalized porous polymer. The catalyst has been systemically investigated by a series of characterizations such as TEM, N2 adsorption, NMR, IR, XPS, etc. TEM and N2 isotherms show that the sample maintains the nanoporous structure after the modification and coordination. XPS results show that chemical state of palladium species in the catalyst is mainly +2. More importantly, the catalyst shows very high activities and excellent recyclability in a series of coupling reactions including Suzuki, Sonogashira, and Heck reactions. Hot filtration and poison of catalysts experiments have also been performed and the results indicate that soluble active species (mainly Pd(0) species) in-situ generated from the catalyst under the reaction conditions are the active intermediates, which would redeposit to the supporter after the reactions. Science China Press and Springer-Verlag Berlin Heidelberg 2012.

Full text of DOI:10.1007/s11426-011-4491-8

SCIENCE CHINA
Chemistry
October 2012 Vol.55 No.10: 2095–2103
doi: 10.1007/s11426-011-4491-8
• ARTICLES •
Porous polymer supported palladium catalyst for cross coupling
reactions with high activity and recyclability
SUN Qi1, ZHU LongFeng2, SUN ZhenHua3, MENG XiangJu1* & XIAO Feng-Shou1*
1Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University (XiXi Campus), Hangzhou 310028, China
2State Key Laboratory of Inorganic Synthesis and Preparative Chemistry; Jilin University, Changchun 130012, China
3Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Received November 6, 2011; accepted December 9, 2011; published online January 11, 2012
Porous polymer supported palladium catalyst for cross coupling reactions with high activity has been successfully prepared
by coordination of Pd2+ species with Schiff bases functionalized porous polymer. The catalyst has been systemically investi-
gated by a series of characterizations such as TEM, N2 adsorption, NMR, IR, XPS, etc. TEM and N2 isotherms show that the
sample maintains the nanoporous structure after the modification and coordination. XPS results show that chemical state of
palladium species in the catalyst is mainly +2. More importantly, the catalyst shows very high activities and excellent recycla-
bility in a series of coupling reactions including Suzuki, Sonogashira, and Heck reactions. Hot filtration and poison of catalysts
experiments have also been performed and the results indicate that soluble active species (mainly Pd(0) species) in-situ gener-
ated from the catalyst under the reaction conditions are the active intermediates, which would redeposit to the supporter after
the reactions.
heterogenous catalysis, Pd, coupling reactions, nanoporous polymers
pared with conventional organic supports, inorganic sup-
1 Introduction
ports with nanopores can provide high surface area, large
pore volume, and 3-dimesional pore architecture, which are
favorable for mass transfer in catalysis. However, the dis-
advantages of inorganic supports including relatively low
stability in alkaline media and complex functionalization of
organic groups are a challenge for preparation of heteroge-
neous catalysts with high activity and excellent recyclability.
For example, Si–O–Si bonds in ordered mesoporous silicas
are not stable in alkaline media, and immobilization of pal-
ladium species on silica surface generally requires unique
organic linkers [41, 42].
Recently, novel porous polymers (polydivinylbenzene,
PDVB) with good stabilities in both acidic and alkaline me-
dia as well as controllable and simple functionalization of
organic groups have been reported [43, 44]. We demon-
strate here a successful coordination of palladium species
with Schiff bases functionalized porous PDVB. As hetero-
geneous catalysts, they exhibit both high activities and ex-
Cross-coupling reactions of Suzuki, Heck, Negishi, and
Sonogashira are greatly important for the creation of new
C–C bonds [1, 2]. Normally, these reactions are performed
in the homogeneous catalytic process, but the difficulty for
separating or removing the homogeneous palladium cata-
lysts from the products strongly hinders their wide applica-
tions in large scale due to the high cost of palladium cata-
lysts coupled with toxic effects associated with environ-
mental and economic concern [3–7]. To solve this problem,
immobilization of palladium catalysts onto supports has
been employed, and typical supports are block polymers
[8–10], ligand-incorporated polymers [11–15], zeolites
[16–23], carbons [24–28], hydroxides [29–31], mesoporous
silicas [32–37], and mesoporous zeolites [38–40]. Com-
*Corresponding authors (email: mengxj@zju.edu.cn; fsxiao@mail.jlu.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2012
chem.scichina.com www.springerlink.com

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Sun Q, et al. Sci China Chem October (2012) Vol.55 No.10
cellent recyclability in Suzuki, Heck, and Sonogashira cou-
pling reactions.
1.2 mmol of phenylboric acid (vinylic substrate or phenyla-
cetylene), 1.5 mmol of K3PO4·3H2O, 0.01 g (0.36% mol)
catalyst, 1 mmol of dodecane (internal standard), and 5 mL
of solvent were stirred at required temperature. After the
reaction for various time, the catalyst was taken out from
the system by centrifufation and the liquid can be analyzed
by gas chromatography (GC-1690 Kexiao Co., flame
ionization detector) with a flexible quartz capillary column
coated with HP-INNOWAX.
2 Experimental section
2.1 Catalyst preparation
2.1.1 Porous copolymer of divinylbenzene with meth-
acrylic acid (1)
At the same time, many products were also isolated and
purified by column chromatography on silica gel (200–300
mesh), followed by futher identification of 1H NMR
technique (Supporting Inforamtion). As a typical run for
isolation, the catalyst was removed by centrifugation and
washing with acetone. Then, the filtrate was collected. After
evaporation of the solvent, a crude product was obtained.
The pure product was obtained from further treatment of the
crude product by column chromatography on silica gel with
a mixed solvent (ethyl acetate with petroleum ether, volume
ratio of 1/10) as eluent.
1 was synthesized according to refs. [43, 44]. As a typical
run, 2 g of divinylbenzene (DVB) and 0.44 g of methacrylic
acid (MA) monomers (molar ratio of DVB to MA at 3)
were dissolved in 27 mL of ethyl acetate, followed by addi-
tion of 0.067 g of azobisisobutyronitrile (AIBN). After stir-
ring for 3 h at room temperature, the mixture was trans-
ferred into an autoclave for 24 h at 100 C. After evapora-
tion of solvent, a solid product (1) was obtained.
2.1.2 Transformation of carboxyl groups into acyl chlo-
rides (2)
2 was also synthesized according to the refs. [43, 44]. As a
typical run, 1 g of 1 was refluxed at 80 C in thionyl chlo-
ride solution for 12 h, followed by evaporation of the thio-
nyl chloride and by-product HCl.
2.3 Characterization
Nitrogen isotherms at the temperature of liquid nitrogen
were measured using Micromeritics ASAP 3020M and
Tristar system. The samples were outgassed for 10 h at
100 C before the measurements. Transmission electron
microscope (TEM) experiments were performed on a
Philips Tecnai F30 electron microscope (FEI Company)
with an acceleration voltage of 300 kV. The loading of Pd
was determined by inductively coupled plasma (ICP) with a
Perkin-Elmer plasma 40 emission spectrometer. The 13C
MAS NMR spectra were recorded on a Bruker Avance-
2.1.3 Functionalization with amino groups (3)
3 was synthesized by acylamidation 2 with 1,6-hexanedia-
mine in CH2Cl2. As a typical run, 1 g of 2 was mixed into
30 mL of CH2Cl2, followed by addition of 1 g of
1,6-hexane-diamine and 1 g of triethylamine dissolved in 5
mL of CH2Cl2. The mixture was stirred at room temperature
for 24 h. After filtrating, washing with excess ethanol and
water (until the pH at 7), and drying at 90 C overnight, 3
was finally obtained.
1
400WB spectrometer. H NMR spectra were recorded on a
Bruker Avance-400 (400 MHz) spectrometer. Chemical
shifts are expressed in ppm downfield from TMS at  0
ppm, and J values are given in Hz. FTIR spectra were per-
formed on an IFS 66 V (Bruker) IR spectrometer in the
range of 400–4000 cm–1. XPS spectra were performed on a
Thermo ESCALAB 250 with Al K irradiation at  = 90
for X-ray sources, and the binding energies were calibrated
using the C1s peak at 284.9 eV.
2.1.4 Functionalization with Shiff bases (4)
The functionalization of Shiff bases on the supports was
synthesized as described in refs. [45–47]. After reaction of
pyridine-2-aldehyde (the same mol to MA in the synthesis
of 1) with dried 3 in ethanol at room temperature for 24 h,
filtration, washing with ethanol, and drying at 90 C over-
night, 4 was finally obtained.
2.1.5 Supporting palladium species on 4 (5)
3 Results and discussion
Palladium species supported on 4 sample was carried out by
treatment of Pd(OAc)2 with 4. As a typical run, 0.1 g of
Pd(OAc)2 was introduced into 50 mL of acetone, followed
by addition of 1 g 4. After stirring at room temperature for
24 h, a solid sample with brown color (5) was obtained by
filtrating, washing with acetone and toluene, and drying at
90 °C overnight.
As shown in Scheme 1, PDVB with carboxyl groups (1)
was prepared via copolymerization of divinylbenzene (DVB)
and methacrylic acid (MA). After refluxing in thionyl chlo-
ride, acyl chloride was successfully grafted on PDVB (2).
After acylamidation of 2 with 1,6-hexanediamine in CH2Cl2,
amido-functionalized PDVB (3) was obtained. Reaction of
3 with pyridine-2-aldehyde in ethanol resulted in the for-
mation of Schiff base-functionalized PDVB (4). After
treatment of 4 with Pd(OAc)2 in acetone, palladium species
2.2 Catalytic tests
As a typical run for coupling reactions, 1 mmol of halide,

Sun Q, et al. Sci China Chem October (2012) Vol.55 No.10
2097
Scheme 1 Preparation of porous polymer supported palladium catalyst.
coordinated with Schiff bases functionalized PDVB (5)
were finally obtained [45–47]. The loading of Pd in the cat-
alysts was 0.36 mmol/g, detected by inductively coupled
plasma (ICP).
addition, elemental mapping shows obvious signals
associated Pd and N elements in 5 (Figure 1(E, F)), con-
firming the presence of Pd and N elements in the sample.
Moreover, high resolution TEM image shows that it is dif-
ficult to observe Pd clusters or nanoparticles (Figure 1(G))
in the sample.
Figure 1(A) shows N2 isotherms of 1 and 5. Both sam-
ples show typical type-IV isotherms with a hysteresis loop
at 0.70–0.98, indicating the presence of large porosity in the
samples. BET surface area and pore volume are 544 and
313 m2/g as well as 0.89 and 0.65 cm3/g for 1 and 5, respec-
tively. Transmission electron microscope (TEM) images of
1 and 5 in a large area (Figure 1(B–D)) clearly show the
presence of abundant large pores (20–100 nm). Notably, the
pore sizes of 1 and 5 are similar (Figure 1(B–D)), suggest-
ing that the pore structure in the samples is basically stable
under the chemical treatments as shown in Scheme 1. In
Figure 2(A) shows 13C spectra of 1 and 4. 1 gives a peak
at 185 ppm associated with characteristic peak of carboxyl
groups [48], indicating that MA is copolymerized with
DVB. Interestingly, in the spectrum of 4, a strong peak ap-
peared at 175 ppm, assigned to acylamide species [49].
Figure 2(B) shows IR spectra of 1, 4, and 5. 1 shows a band
at 1700 cm–1, which is associated with carboxyl groups [50].
4 shows very broad and strong bands at 1663 cm–1, which is
due to overlapping of signals for amide groups and conju-

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Sun Q, et al. Sci China Chem October (2012) Vol.55 No.10
would provide great potential opportunity for this sample as
a popular efficient catalyst in a wide range of Suzuki reac-
tions for the synthesis of fine chemicals, because aryl io-
dides are more expensive than aryl bromides while aryl
bromides are much less active than aryl iodides in conven-
tional Suzuki reactions [58]. Compared with aryl bromides
and aryl iodines, aryl chlorides give very low activities in
Suzuki reactions (entry 9–11, Table 1). For example, when
the reaction is carried out at 120 C for 120 min, the yield is
only 37%. Therefore, it is still a challenge for synthesis of
highly active heterogeneous catalysts for the coupling of
aryl chlorides with phenylboronic acid. Additionally, when
aryl halides are changed with electron donating and with-
drawing groups, the catalyst is still very active (95%–99%
yield, entry 2–8, Table 1), suggesting a good suitability for
this catalyst in Suzuki reaction. It is remarkable that high
reactivity could be observed at a very low catalyst loading
of 0.1–0.01 mol% (entry 12 and 13, Table 1). The turnover
frequency (TOF) almost achieves 2500 h–1 for the coupling.
Furthermore, the effect on solvent and base has also been
investigated. The results show that a solvent of toluene or a
mixture of water and ethanol as well as a base of
K3PO4·3H2O is suitable for this reaction (Table S1). Con-
sidering the economical and environmental concerns, wa-
ter-ethanol solvent is more preferable [59–62].
Normally, homogeneous Sonogashira coupling requires
the presence of CuI as a co-catalyst [63–67]. However, 5 is
very active for Sonogashira reaction even in the absence of
CuI (entry 17–21, Table 2), giving the yields higher than
Figure 1 (A) N2 isotherms of (a) 1 and (b) 5; large-area TEM images of
(B) 1 and (C) 5; (D) STEM image of 5; (E) Pd and (F) N mapping of 5
corresponding to the red rectangle area in STEM image (D); (G) HR TEM
image of 5.
Table 1 Catalytic data in Suzuki reaction over 5 a)
R1
X
R1
+
B(OH)2
gated C–N bonds [51, 52]. After interaction of Pd2+ species
with 4, 5 gives an additional band at 1648 cm–1, a shift to
low frequency ca. 15 cm–1, suggesting the coordination
between Pd2+ species with Schiff bases in the sample [53].
Figure 2(C) shows N1s spectra of 4 and 5. Compared with
4 (401.0 eV), 5 shows relatively low binding energy (399.4
eV). This result confirms the coordination between Pd2+
species with Schiff bases in the sample (Figure 2(C)) [53].
Figure 2(D) shows Pd3d spectrum of 5, giving Pd3d5/2
and Pd3d3/2 at 337.8 and 343.1 eV, respectively. These
results suggest that chemical state of palladium species
is mainly +2 [55]. However, compared with Pd(OAc)2
(338.6 eV for Pd3d5/2), the binding energy of Pd2+ in 5 is
negatively shifted to 337.8 eV, which is also good in
agreement with that the Schiff base could donate electrons
to Pd2+ species [56, 57].
Entry
1
R1
H
X
Time (min)
15
Yields (%) b)
>99(98)
>98(96)
>99(96)
>99(97)
95(93)
97(94)
98(95)
98(96)
37
I
2
CH3
CH3O
H
I
15
3
I
15
4
Br
Br
Br
Br
Br
Cl
Cl
Cl
Br
Br
Br
Br
Br
20
5
CH3
CH3O
CN
20
6
20
7
15
8
CH3CO
H
15
9
120
120
120
30
10
11
12 c)
13 d)
14 e)
15 f)
16 g)
NO2
COCH3
CN
59(53)
53
>99
CN
240
15
>99
CN
>99
CN
15
98
CN
15
92
Table 1 presents catalytic activities of 5 in Suzuki reac-
tions under mild conditions. In Suzuki reactions, both aryl
iodines and aryl bromides could be completely coupled with
phenylboronic acid in 95%–99% yield within a very short
reaction time (15–20 min, entry 1–8, Table 1). This feature
a) Reaction conditions: aryl halides (1 mmol), phenylboronic acid (1.2
mmol), K3PO4·3H2O (1.5 mmol), 5 (0.36 mol%), ethanol-water (2 mL/3
mL), 80 C; b) GC yields (isolated yields); c) 0.1 mol% 5 was used as
catalyst; d) 0.01 mol% 5 was used as catalyst; e) reuse; f) recycles for 10
times; g) 0.36 mol% Pd(OAc)2 was used as catalyst.

Sun Q, et al. Sci China Chem October (2012) Vol.55 No.10
2099
Figure 2 (A) 13C MAS NMR spectra, (B) IR spectra, (C) N1s spectra, and (D) Pd3d spectra of (a) 1, (b) 4, and (c) 5.
Table 2 Catalytic data in Sonogashira reaction over 5 a)
99%. The CuI-free in the Sonogashira reaction completely
avoids the formation of 1,4-diphenyl-1,3-butadiyne by-
product, which results from side-reaction of phenylacety-
lene homocoupling. When aryl iodines are replaced by aryl
bromides, 5 still gives very high yields (higher than 99%,
entry 20 and 21, Table 2) by increasing the temperature
(120 C) and time (60–100 min). Additionlly, 5 still exhibits
high conversion even at very low catalyst loading of
0.1–0.01 mol% (entry 22 and 23, Table 2), but takes longer
time. Catalytic tests show that alcohols such as butanol and
ethanol are good solvents, and suitable bases include
K3PO4·3H2O, NaOH, and KOH (Table S2).
In Heck reaction, 5 is also very active (entry 27–33, Ta-
ble 3). For example, aryl iodines with electron donating
could be fully coupled with various vinylic substrates in a
short time (20 min), giving the yields at 99% (entry 27–30,
Table 3). Particularly, when aryl bromide is used, the yields
are still 99% (entry 32 and 33, Table 3). Similarly, the stud-
ies on solvents and bases show that N-methy-2-pyrroli-
1
1
+
X
R
R
Entry
17
R1
H
X
I
Time (min)
Yields (%) b)
>99(96)
>99(97)
>99(96)
>99(92)
>99(93)
94
30
30
18
CH3
CH3O
CN
I
19
I
30
20
Br
Br
I
60
21
CH3CO
100
90
22 c)
23 d)
24 e)
25 f)
26 g)
H
H
H
H
H
I
720
30
99
I
98
I
30
92
I
30
74
a) Sonogashira reaction conditions: ary hailide (1 mmol), phenylacety-
lene (1.2 mmol), K3PO4·3H2O (1.5 mmol), 5 (0.36 mol%), ethanol (5 mL),
80 C; b) GC yields (isolated yields); c) 0.1 mol% 5 was used as catalyst;
d) 0.01 mol% 5 was used as catalyst; e) reuse; f) recycles for 5 times; g)
0.36 mol% Pd(OAc)2 was used as catalyst.

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Sun Q, et al. Sci China Chem October (2012) Vol.55 No.10
Table 3 Catalytic data in Heck reactions over 5 a)
2
R
1
1
+
R
X
R
2
R
Entry
27
R1
H
R2
X
I
Time (min) Yields (%) b)
CH3COO
CH3COO
CH3COO
C2H5COO
Ph
20
20
20
20
60
40
40
60
20
20
20
>99(98)
>98(96)
>99(96)
>99(97)
95(93)
98(95)
98(96)
99
28
CH3
I
29
CH3O
I
30
H
H
I
31
I
32
CN
CH3CO
H
CH3COO
CH3COO
CH3COO
CH3COO
CH3COO
CH3COO
Br
Br
I
33
34 c)
35 d)
36 e)
37 f)
H
I
>99
H
I
98
H
I
91
Figure 3 Reaction progress as a function of time on (A) Suzuki, (B)
Sonogashira and (C) Heck reactions by using 5 under normal conditions
(■), in the presence of 400 equiv of Hg(0) (◄), in the presence of 400
equiv of PVP (●) and in the presence of 400 equiv of SBA-propyl-SH (►).
a) Heck reaction conditions: ary hailide (1 mmol), vinylic substrates
(1.2 mmol), K3PO4·3H2O (1.5 mmol), 5 (0.36 mol%), 1-methy-2-pyrroli-
dinone (5 mL), 130 C; b) GC yields (isolated yields); c) 0.1 mol% 5 was
used as catalyst; d) reuse; e) recycles for 5 times; f) 0.36 mol% Pd(OAc)2
was used as catalyst.
dinone (NMP), N,N-dimethylformamide (DMF), and dime-
thyl sulfoxide (DMSO) are good solvents, and K3PO4·3
H2O is a suitable base (Table S3).
To determine the nature of the active species, the poison
of catalysts has also been investigated. And three poison
reagents (about 400 equivalent to total palladium content),
including insoluble cross-linked poly(4-vinylpyridine),
thiol-modified mesoporous materials and Hg(0), are em-
ployed, working as an excellent scavenger for soluble or
naked Pd [68–70]. All the reactions almost quenched in the
presence of poison reagents (Figure 3), suggesting that the
reactions occurred in homogeneous solutions [79–83]. Ad-
ditionally, hot filtration technique has also been employed,
and 14% yields in Suzuki reaction can be achieved, con-
firming the existence of active species (soluble Pd (0)) in
the hot filtrate. However, if the solution is cooled to room
temperature before filtration, the cold filtrate is completely
inactive (Figure S1), which suggests that the soluble active
species redeposit to the support and are undetectable in the
filtrate [84–86]. XPS Pd3d spectrum of reused 5 for Suzuki
reaction still shows that palladium species are still +2
(Figure S2). Additionally, ICP analysis of the solution in-
dicated that no Pd was detected in the solution after reac-
tion. These results suggest that Pd2+ species in 5 are stable
and almost leaching-free, which is responsible for the recy-
clability in Suzuki, Sonogashira, and Heck reactions as
shown in Tables 1–3 (entry 14–15, 24–25, 35–36). For
example, after reuse or even 10 recycles, the yield in
Suzuki reaction was higher than 98% (entry 15, Figure 4).
On the contrary, the same ligands with Pd grafted on inor-
ganic mesoporous MCM-41 showed low recyclability and
lost half activities after 5 recycles (Figure S3), attributed to
the low stability of inorganic framework in alkaline medi-
um [41, 42].
Figure 4 Recycle of 5 in Suzuki reaction using 4-bromobenzenitrile with
phenylboronic acid.
Thus, the catalysis reaction process over the nanoporous
polymers supported Pd(II) catalyst can be summarized as
follows: the catalyst itself is insoluble in the solvent; then
under the reactions conditions, it can in-situ generate solu-
ble active species (mainly Pd(0) species) that subsequently
react with reactants to form the products [75]; finally, when
the reactions are over and temperature cools down, the sol-
uble Pd species redeposit to the support and return to its
original structure, which can then be easily separated and
reused [75]. Compared with homogeneous catalysts, heter-
ogeneous catalysts are excellently recyclable and almost
leaching-free, which is very important for wide applications
of industrial processes at a large scale. Combining the ad-
vantages of homogeneous catalysis (high activity) and het-
erogeneous catalysis (easy separation and recycle), such
nanoporous polymers supported Pd catalyst thus provides a
green approach for a series of homogeneous catalytic reac-
tions [86–91].

Sun Q, et al. Sci China Chem October (2012) Vol.55 No.10
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2010, 12: 65–69
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We have prepared an efficient catalyst of palladium species
coordinated with Schiff bases functionalized porous poly-
mer, which is highly active and excellently recyclable in a
series of cross couplings including Suzuki, Sonogashira,
and Heck reactions. Combining the advantages of homoge-
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(easy separation and recycle), such nanoporous polymers
supported Pd catalyst thus provides a green approach for a
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This work was supported by the National Natural Science Foundation of
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(2009CB623507), and Fundamental Research Funds for the Central Uni-
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