Polymer-supported macrocyclic Schiff base palladium complex: An efficient and reusable catalyst for Suzuki cross-coupling reaction under ambient condition

Article abstract of DOI:10.1016/j.catcom.2010.12.017

Polymer-supported macrocyclic Schiff base palladium complex was prepared and characterized. The catalyst exhibits excellent catalytic activity and stability for Suzuki cross-coupling reaction under ambient condition. Various aryl bromides were coupled with aryl boronic acids in DMF/H₂O, under air, in the presence of 0.1 mol% of the catalyst to afford corresponding cross-coupled products in high yields within 20-30 min. Furthermore, the heterogeneous catalyst can be readily recovered by simple filtration and reused several times without significant loss in its activity.

Full text of DOI:10.1016/j.catcom.2010.12.017

Catalysis Communications 12 (2011) 678–683
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Catalysis Communications
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Short Communication
Polymer-supported macrocyclic Schiff base palladium complex: An efficient and
reusable catalyst for Suzuki cross-coupling reaction under ambient condition
⁎
Ying He, Chun Cai
Chemical Engineering College, Nanjing University of Science & Technology, Nanjing 210094, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 October 2010
Received in revised form 10 December 2010
Accepted 13 December 2010
Available online 20 December 2010
Polymer-supported macrocyclic Schiff base palladium complex was prepared and characterized. The catalyst
exhibits excellent catalytic activity and stability for Suzuki cross-coupling reaction under ambient condition.
Various aryl bromides were coupled with aryl boronic acids in DMF/H2O, under air, in the presence of 0.1 mol%
of the catalyst to afford corresponding cross-coupled products in high yields within 20–30 min. Furthermore,
the heterogeneous catalyst can be readily recovered by simple filtration and reused several times without
significant loss in its activity.
Keywords:
© 2010 Elsevier B.V. All rights reserved.
Polymer-supported palladium complex
Macrocyclic Schiff base
Heterogeneous catalysis
Suzuki cross-coupling reaction
1. Introduction
species, which have been demonstrated to be inactive in various
catalytic reactions [27–29]. Moreover, homogeneous processes suffer
Transition metal-catalyzed cross-coupling reactions have been
recognized as powerful synthetic tools and a major area of interest in
multiple organic transformations for academic and industrial pro-
cesses [1]. In particular, the Suzuki cross-coupling reaction, which is
the palladium-catalyzed cross-coupling of organic halides with aryl
boronic acids, is one of the most important methods of forming C–C
bonds in synthetic chemistry, and finds important industrial applica-
tions [2–8].
In recent years, great efforts in catalysis research have been
devoted to the introduction and application of effective and safe
heterogeneous catalysts. Polymers play a significant role in this area
offering different ways of metal attachment to the polymer matrix via
covalent or non-covalent bonding, through hydrogen bridges, as well
as through ionic, hydrophobic or fluorous interactions [9]. Corre-
spondingly, palladium complexes anchored on polymers with Schiff
bases [10,11], N-heterocyclic carbene groups [12–15], and dendrimers
[16,17] have been described recently. In addition, a few efficient
heterogeneous Pd catalysts, which are of general use for this reaction
of aryl chlorides, have also been reported [18–25].
from the problems concerning separation from reaction mixture,
reuse of expensive metal catalysts and metal contamination in the
products. In order to overcome these drawbacks, immobilization via
covalent bond on the support is more advantageous over conven-
tional impregnation in terms of minimizing catalyst leaching and
improving the long-term stability of the solid catalyst. As for
generation of the C–C bonds, Li and co-workers have reported the
use of poly (vinyl chloride) supported Schiff base nano-palladium
complex for Suzuki cross-coupling reaction efficiently [30]. Homoge-
neous palladium non-symmetrical salen-type Schiff base complexes
have been prepared and shown to be effective in the heterogeneous
catalysis of C–C cross-coupling reactions [31]. However, most of the
supported Schiff base palladium complexes used as promoter in the
Suzuki reaction is either longer reaction time or heating required. In
continuation of our studies in developing new functional supported
catalysts, herein we reported an efficient and widely applicable
approach for the preparation of chloromethylated polystyrene resin
supported macrocyclic Schiff base palladium complex and their
application as heterogeneous catalyst in Suzuki cross-coupling
reaction under ambient condition (Scheme 1).
Schiff bases are an important class of ligands in coordination
chemistry and have studied extensively [26] as they are selective and
sensitive toward various metal ions. However, the use of metal Schiff
base complexes as catalyst in homogeneous solution often suffers
from deactivation due to easy formation of dimeric peroxo- and μ-oxo
2. Experimental
2.1. General remarks
All chemicals were reagent grade and used as purchased.
Chloromethylated polystyrene resin (1% divinylbenzene, 1.0 mmol/g
of Cl, grain size range: 100–200 mesh) was obtained from GL Biochem
⁎
Corresponding author. Tel.: +86 25 84315514; fax: +86 25 84315030.
E-mail address: c.cai@mail.njust.edu.cn (C. Cai).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.12.017

Y. He, C. Cai / Catalysis Communications 12 (2011) 678–683
679
polystyrene resin 1.0 g (1.0 mmol Cl) was allowed to swell in DMF
(15 ml) for 24 h. Then, 2-[3-(2-formylphenoxy)-2-hydroxypropoxy]
benzaldehyde (1.5 mmol, 0.45 g), K2CO3 (0.276 g, 2.0 mmol), and 18-
crown-6 (0.053 g, 0.2 mmol) were added in the mixture. The mixture
was stirred at 100 °C for 24 h. The polymer beads were filtered, washed
with distilled water, dichloromethane and ethanol and then dried in
vacuo. After that, the beads obtained were swollen in methanol (5 ml)
for 2 h, and a solution of o-phenylene diamine (1.0 mmol, 0.108 g) and
a drop of concentrated HCl were added. The contents were refluxed
for 24 h. The color of the polymer beads changed from white to pale
yellow, indicating attachment of o-phenylene diamine. The beads
were filtered, washed with distilled water, dichloromethane and
methanol, and then dried in vacuo. The formation of macrocycle Schiff
base of the polymer resin was confirmed using analytical and infrared
(IR) spectral data. The macrocycle Schiff base content of the functional
beads was 0.68 mmol/g according to the result of elemental analyses
based on N elemental.
Scheme 1. Polymer-supported macrocyclic Schiff base palladium complex for Suzuki
cross-coupling reaction.
(Shanghai) Ltd. (Shanghai, China). IR spectra were recorded in KBr
disks with a SHIMADZU IRPrestige-21 FT-IR spectrometer. Elemental
analyses were performed on a Vario ELIII recorder. Scanning electron
microscopy (SEM) analyses were performed with JEOL JSM-6380LV
instrument. Transmission electron microscopy (TEM) images were
performed with a JEM-2100 instrument.1H NMR spectra were
measured with a Bruker Avance III 500 analyzer. GC–MS analyses
were performed on a Saturn 2000GC/MS instrument. Palladium
content of the catalyst was measured by inductively coupled plasma
(ICP) on PE5300DV analyzer.
2.2.3. Preparation of the catalyst
The functional beads (0.35 g) were swollen in toluene (10 ml) for
about 2 h. To this was added Pd(OAc)2 (0.25 mmol, 0.056 g) and the
mixture was stirred at 90 °C for about 12 h. Then the resulting beads
were filtrated and washed with methanol, dried at 100 °C under
vacuum overnight. The content of palladium was 5.8% as determined
by ICP.
2.3. General procedure for Suzuki cross-coupling reaction
Under air atmosphere, around-bottomed flask was charged with
aryl halide (1.0 mmol), phenylboronic acid (1.5 mmol), K2CO3
(2.0 mmol), DMF/H2O (1:1) (6 ml) and catalyst (0.1 mol%). The
mixture was reacted for a certain time (monitored by GC). After the
reaction completed, water (10 ml) and ether (20 ml) were added. The
catalyst was separated by filtration, washed with ether and water, and
dried under vacuum for the next cycle. The organic phase of filtrate
was separated, dried over Na2SO4, and evaporated. The crude product
obtained was purified by flash chromatography with n-hexane/EtOAc
as eluent to afford the corresponding products.
2.2. Preparation of catalyst
2.2.1. Preparation of 2-[3-(2-formylphenoxy)-2-hydroxypropoxy]
benzaldehyde
2-[3-(2-Formylphenoxy)-2-hydroxypropoxy] benzaldehyde was
performed according to the literature method [32]. Salicylaldehyde
(0.11 mol, 11.36 g) was added to 100 ml of aqueous sodium hydroxide
(0.11 mol, 4.4 g) and heated to 60 °C under nitrogen atmosphere.
Then, epichlorohydrin (0.05 mol, 4.36 g) was added dropwise within
2 h. The mixture was stirred continuously for another 4 h. After
cooling to room-temperature, the yellow precipitate was filtrated off,
washed with water for several times, and dried under vacuum. It was
recrystallized with methanol/water (8:1, v/v) affording yellow oil
solid, and then several portion of water was added to afford the desired
product to yield of 34%.
3. Results and discussion
3.1. Synthesis and characterization of the palladium catalyst
The complex has a macrocyclic structure with palladium as the
central metal atom with the corners being occupied by two nitrogen
atoms and two oxygen atoms, and it was designed by the sequence
of reactions given in Scheme 2. In order to ascertain the functiona-
lized polymer and its corresponding Pd complex, IR spectra were
2.2.2. Preparation of polymer supported macrocycle Schiff base
The preparation of the polymer supported Schiff base was modified
from the literature methods [32,33]. Pre-washed chloromethylated
Scheme 2. Preparation of polymer-supported macrocyclic Schiff base palladium complex.

680
Y. He, C. Cai / Catalysis Communications 12 (2011) 678–683
recorded separately at different stage of preparation. It can be seen
from Fig. 1, the spectrum of chloromethylated polystyrene resin
shows an absorption band at 600 and 1265 cm− 1, which is
attributed to the C–Cl bond, was weakened after the introduction
of 2-[3-(2-formylphenoxy)-2-hydroxypropoxy] benzaldehyde. Cor-
respondingly, a strong band at 1689 cm− 1 in curve B was assigned
to the C_O vibration. Moreover, a bond at 1689 cm−1 disappeared
in curve C after the introduction of o-phenylene diamine. The
range of 1600–1500 cm−1 corresponds to the υ(C_C) and υ(C_N)
stretch of aromatic rings [11]. In the polymer-supported palladium
complexes, both υAr–O–C and υC_N undergo a slight positive shift
indicating that the palladium is chelated with the nitrogen and
oxygen atom.
D
C
B
A
1689
1265
1600
Then, scanning electron micrograph (SEM) was also recorded to
understand morphology of the surface of the support and catalyst. It
can be easily seen from Fig. 2; the resin beads have different size
and roughness. The presence of Pd caused changes, demonstrated
by polymer particle size decrease and roughness of the surface
(Fig. 2).
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers/cm-1
Fig. 1. IR spectra of chloromethylated polystyrene resin (A), polymer-supported 2-[3-
(2-formylphenoxy)-2-hydroxypropoxy] benzaldehyde (B), polymer-supported mac-
rocyclic Schiff base (C), and polymer-supported macrocyclic Schiff base palladium
complex (D).
Fig. 2. SEM images of (A) polymer-supported macrocyclic Schiff base; and (B) the fresh palladium catalyst.
Table 1
Effect of solvents for the Suzuki reactiona.
Cat.
H3CO
Br
(HO)2B
H3CO
K2CO3, solvent, 25°C
Entry
Solvent
Time (h)
Yield (%)b
TON
TOF (h−1
)
1
2
3
4
DMF
1
0.5
1
1
1
28
62
6
32
3
280
620
60
320
30
280
1240
60
320
30
EtOH
CH3CN
Toluene
THF
5
6
7
8
9
10
11
12
H2O
6
56
93
98
57
78
83
80
560
930
980
570
780
830
800
93.3
1860
1960
1140
1560
1660
1600
EtOH/H2O (v/v=1:1)
DMF/H2O (v/v=1:1)
EtOH/H2O (v/v=1:2)
EtOH/H2O (v/v=2:1)
DMF/H2O (v/v=1:2)
DMF/H2O (v/v=2:1)
0.5
0.5
0.5
0.5
0.5
0.5
a
Reaction conditions: p-bromoanisole (1.0 mmol), phenylboronic acid (1.5 mmol), K2CO3 (2.0 mmol); catalyst (0.1 mol%) in 6 ml solvent at room-temperature in air.
Isolated yield.
b

Y. He, C. Cai / Catalysis Communications 12 (2011) 678–683
681
3.2. Catalytic activity of the catalyst in Suzuki cross-coupling reaction
100
80
60
40
20
0
To check the potency of chloromethylated polystyrene resin
supported macrocyclic Schiff base palladium catalyst, it was used in
Suzuki cross-coupling reaction. The coupling between p-bromoani-
sole and phenylboronic acid was chosen as model reaction. Initially,
the single solvent such as DMF, EtOH, CH3CN, toluene, THF, and H2O
was studied. As could be seen in Table 1, the single solvents gave low
to moderate yields for the reaction at the room-temperature (Table 1,
entries 1–6). However, when we adopted the organic/aqueous co-
solvent, high yields of 93% to 98% were obtained with high turnover
number (TON) and turnover frequency (TOF) (Table 1, entries 7
and 8). The merit of the co-solvent may be attributed to the good
solubility of the organic reactants and the inorganic base. Then, we
tested the influence of different volume ratios of DMF/H2O and EtOH/
H2O as a solvent under the Suzuki reaction at room-temperature.
Yields of 57–83% were obtained as demonstrated in Table 1, entries
9–12.
98
97
3
96
2
94
4
90
5
83
6
1
Run
Fig. 3. Recycling experiment.
entries 1–6). As shown in Table 2, K2CO3 was the best base for the
reaction with high TON 980 and TOF 1960 h−1. Also, the organic base
NEt3 was studied but unsatisfied yield was obtained (Table 2, entry 7).
Then, different catalyst loadings were tested for the reaction. As
illustrated in Table 2, 0.01 mol% of catalyst gave rise to extremely high
TON 8600 and TOF 17,200 h−1 (Table 2, entry 9) but lower yield
Next, we examined the effects of bases on the room-temperature
Suzuki reaction in DMF/H2O. The inorganic bases including K3PO4,
K2CO3, Na2CO3, NaHCO3, NaOAc and KF were investigated (Table 2,
Table 2
Effect of bases and catalyst loadings for the Suzuki cross-coupling reactiona.
Cat.
H CO
Br
(HO) B
H CO
3
3
2
base, DMF/H O, 25°C
2
Entry
Catalyst loading (mol%)
Base
Time (h)
Yield (%)b
TON
TOF (h−1
)
1
2
3
4
5
6
7
8
9
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.5
0.01
K3PO4
K2CO3
Na2CO3
NaOAc
NaHCO3
KF
NEt3
K2CO3
K2CO3
0.5
0.5
0.5
0.5
0.5
0.5
1
96
98
91
18
51
45
26
98
86
960
980
910
180
510
450
260
196
8600
1920
1960
1820
360
1020
900
260
392
17,200
0.5
0.5
a
Reaction conditions: p-bromoanisole (1.0 mmol), phenylboronic acid (1.5 mmol), base (2.0 mmol); catalyst in 6 ml DMF/H2O (1:1) at room-temperature in air.
Isolated yield.
b
Table 3
Suzuki cross-coupling reaction of aryl halides with aryl boronic acidsa.
Cat. (0.1 mol% Pd)
R
2
R
2
X
(HO) B
2
K CO , DMF/H O, 25°C
R
1
R
2
3
2
1
Entry
X
R1
R2
Time (min)
Yield (%)b
TON
TOF (h−1
)
1
2
3
4
5
6
7
8
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
H
p-NO2
p-CH3
p-CH3O
p-CF3
p-CH3CO
p-CHO
m-CF3
o-CH3O
H
H
H
H
p-NO2
p-CF3
H
H
H
H
H
H
H
H
30
20
30
30
20
20
20
20
30
30
30
30
30
180
180
99
99
97
98
99
98
99
96
90
95
98
93
92
16c
9c
990
990
970
980
990
980
990
960
900
950
980
930
920
160
90
1980
2970
1940
1960
2970
2940
2970
2880
1800
1900
1960
1860
1840
53.3
9
H
p-Cl
p-CF3
p-CH3
p-CH3O
H
10
11
12
13
14
15
Cl
H
30
a
Reaction conditions: aryl halides (1.0 mmol), aryl boronic acids (1.5 mmol), K2CO3 (2.0 mmol); catalyst (0.1 mol%) in 6 ml DMF/H2O (1:1) at room-temperature in air.
Isolated yield.
At 50 °C for 180 min.
b
c

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Y. He, C. Cai / Catalysis Communications 12 (2011) 678–683
Fig. 4. TEM images of (A) recovered catalyst after first run for the Suzuki reaction; and (B) recovered catalyst after sixth run for the Suzuki reaction.
obtained. Thus, we selected K2CO3 as the base, DMF/H2O in ratio of 1:1
as solvent, and 0.1 mol% of catalyst as the optimal conditions for the
reaction.
4. Conclusions
In summary, we have successfully prepared polymer supported
macrocyclic Schiff base palladium complex which was used as
heterogeneous catalyst for the room-temperature Suzuki cross-
coupling reaction. The catalyst has showed highly catalytic activities
for the reaction affording a diverse range of biphenyls in excellent
yields within 20–30 min, and could be easily recovered by simple
filtration and reused for 5 times without significant loss in its activity.
The excellent catalytic efficiency as well as the recyclability made
them an attractive alternative to the large number of heterogeneous
palladium catalysts reported to date.
Encouraged by the efficiency of the reaction protocol described
above, we investigated the substrate scope. As shown in Table 3, a
wide range of functional groups have also been tolerated in the
reaction. The coupling between aryl bromides and phenylboronic
acid, which contained electron-donating as well as electron-with-
drawing groups, proceeded readily to afford the corresponding
products in 90%–99% yields with high TOF (up to 2970 h−1). Mean-
while, the coupling reaction could be efficiently executed of aryl
boronic acids with electron-withdrawing or electron-donating groups.
Then, we tried to examine whether aryl chlorides were active for the
Suzuki reaction. However, poor yields were obtained even for a
prolonged time at 50 °C (Table 3, entries 14 and 15).
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