Synthesis of immobilized nanopalladium on polymer-supported Schiff base, and study of its catalytic activity in the Suzuki-Miyaura reaction

Article abstract of DOI:10.1007/s00706-009-0198-3

Nanopalladium particles immobilized on a matrix of poly(vinyl chloride) (PVC)-supported Schiff base were prepared from PVC with sequential attachment of ethylenediamine and salicylaldehyde, followed by treatment with an ethanolic solution of palladium chloride. The as-prepared catalyst was found to be air and moisture-stable and to have significant catalytic activity in Suzuki-Miyaura reactions under mild operating conditions. Various phenyl halides were coupled with phenylboronic acid in aqueous ethanol, under air, to afford the corresponding cross-coupled products in good yields. Furthermore, the catalyst can be easily recovered by simple filtration and reused for up to five cycles without losing its activity.

Full text of DOI:10.1007/s00706-009-0198-3

Monatsh Chem (2009) 140:1425–1429
DOI 10.1007/s00706-009-0198-3
ORIGINAL PAPER
Synthesis of immobilized nanopalladium on polymer-supported
Schiff base, and study of its catalytic activity
in the Suzuki–Miyaura reaction
•
Jie Liu Yi-Qun Li Wen-Jie Zheng
•
Received: 28 January 2009 / Accepted: 11 October 2009 / Published online: 7 November 2009
Ó Springer-Verlag 2009
Abstract Nanopalladium particles immobilized on a
matrix of poly(vinyl chloride) (PVC)-supported Schiff
base were prepared from PVC with sequential attachment
of ethylenediamine and salicylaldehyde, followed by
treatment with an ethanolic solution of palladium chloride.
The as-prepared catalyst was found to be air and moisture-
stable and to have significant catalytic activity in Suzuki–
Miyaura reactions under mild operating conditions. Various
phenyl halides were coupled with phenylboronic acid in
aqueous ethanol, under air, to afford the corresponding cross-
coupled products in good yields. Furthermore, the catalyst can
be easily recovered by simple filtration and reused for up to
five cycles without losing its activity.
attempts have been made to develop effective palladium
complexes, which can act as highly active catalysts for this
homogeneous reaction. Among these, various ligands for
example phosphorus ligands [7–10], N-heterocyclic carb-
enes [11–13], P,O-based ligands [14], bis(thiourea) ligands
[15], and thiosemicarbazone [16], etc., have been used to
stabilize the catalytic palladium species. Although homo-
geneous catalysts often have greater activity in such
transformations, most are expensive, not easily available,
and require air-free handling to prevent oxidation; the main
drawback of these catalysts is, however, the need to sep-
arate them from the reaction mixture at the end of the
reaction. The heterogenization of these catalysts on poly-
meric supports therefore constitutes a logical approach to
combining their homogeneous catalytic properties with the
advantages of heterogeneous catalysts, for example facile
separation from the reaction mixture and recyclability.
Therefore, the development of an insoluble polymer-sup-
ported palladium catalyst has attracted much attention in
organic synthesis. Palladium complexes immobilized on
solid supports, for example organic polymeric matrices
(e.g. PS–PEG resin [17], cellulose [18], and polymer-sup-
ported phosphine [19, 20], etc.) and inorganic microspheres
(e.g. carbon [21], metal oxides [22], and zeolites [23], etc.),
have been reported as catalysts in Suzuki–Miyaura reac-
tions. Nevertheless, there is still a need for an efficient
heterogeneous catalyst that is air and moisture stable and
applicable to a wide range of Suzuki–Miyaura reactions.
It is well known that poly(vinyl chloride) (PVC) is
widely used, inexpensive, and easily modified by func-
tional groups via displacement reaction directly without
chloromethylation, which makes it a suitable polymeric
support for heterogeneous catalysts. Palladium is well
known to form stable complexes with a wide variety of
organic ligands with P, N, O, and S atoms, because of its
Keywords Poly(vinyl chloride) Á Schiff base Á
Nanopalladium Á Heterogeneous catalysis Á
Suzuki–Miyaura reaction Á Biaryls
Introduction
Transition metal-catalyzed cross-coupling is a versatile and
highly useful transformation, which yields a variety of
organic compounds. In particular, the Suzuki–Miyaura
cross-coupling reaction, which is the palladium-catalyzed
cross-coupling of organic halides with organoboron com-
pounds, is one of the most important methods of forming
sp2–sp2 carbon–carbon bonds in synthetic chemistry, and in
industrial applications [1–6]. In the past few years, many
J. Liu Á Y.-Q. Li (&) Á W.-J. Zheng
Department of Chemistry, Jinan University,
510632 Guangzhou, China
e-mail: tlyq@jnu.edu.cn
123

1426
J. Liu et al.
pronounced coordination properties [7–17]. In a continua-
tion of our work on the development of simple and reliable
procedures for immobilization of catalytically active pal-
ladium nanoparticles on functionalized PVC, we have
attempted to prepare and characterize the functionalized
polymer of PVC-supported Schiff base and, further, to use
it as carrier to immobilize palladium nanoparticles. In this
paper, we report that the as-prepared catalyst had high
activity as a recyclable heterogeneous catalyst for the
Suzuki–Miyaura type carbon–carbon bond formation in
ambient atmosphere, with excellent yields.
directly prepared by simple in-situ reduction of of palla-
dium chloride, in ethanolic solution, with heterogenized
ligands of PVC–EDA–SA. The synthesis of nanopalladium
immobilized on PVC-supported Schiff base is shown in
Scheme 1.
The IR spectra of PVC–EDA–SA and PVC–EDA–SA–
Pd0 catalysts were recorded. The IR spectrum of PVC–
EDA–SA contained absorption bands at 1,640.7 and
1,278.6 cm-1 for (C=N azomethine) and C–O (phenolic)
bonds. These bands are negatively shifted to 1,630.5 cm-1
(C=N) and to 1,261.3 cm-1 (C–O) in the PVC–EDA–SA–
Pd0, indicating coordination of the nitrogen and oxygen
atoms with the palladium in the complexes. Additionally,
the IR spectrum of the PVC–EDA–SA–Pd0 contains
absorption at 551.0 cm-1, which was assigned to the Pd–N
bond, confirming the formation of a palladium complex on
the surface of the polymer.
Results and discussion
Preparation and characterization of PVC–EDA,
PVC–EDA–SA, and PVC–EDA–SA–Pd0
The morphology of PVC–EDA–SA–Pd0 and the
polymer support PVC–EDA–SA was studied by scanning
electron microscopy (SEM) and transmission electron
microscopy (TEM). A clear change in morphology is
observed after anchoring palladium on the polymer support
(Fig. 1). TEM images showed the presence of palladium
nanoparticles of about 40 nm size distributed on the
surface of the polymer matrix (Fig. 2, left).
The preparation of the PVC–EDA involved addition of
excess ethylenediamine to commercially available PVC
resin at 80 °C for a specific time to afford the corre-
sponding functionalized resin. The synthesized PVC–EDA
resin was then treated with salicylaldehyde in ethanol
solution to produce the PVC-supported Schiff base resin
(PVC–EDA–SA). The catalyst PVC–EDA–SA–Pd0 was
Scheme 1
HO
H
2-HO-C6H4CHO
N
H2NCH2CH2NH2
Cl
NH2
NH N CH
PVC-EDA-SA
95% EtOH
PVC-EDA
PVC
nano-Pd
H
O
PdCl2
NH N CH
95% EtOH
PVC-EDA-SA-Pd0
Fig. 1 SEM images of the
polymer-supported PVC–EDA–
SA (left) and the fresh catalyst
PVC–EDA–SA–Pd0 (right)
123

Synthesis of immobilized nanopalladium on polymer-supported Schiff base
1427
Fig. 2 TEM images of the fresh
catalyst (left) and the reused
catalyst (right) at the same
magnification
than with K2CO3, with comparable yields (Table 1, entries
4 and 5). K2CO3 was thus chosen as base in the Suzuki–
Miyaura reaction. Yields of up to 95% were obtained in
reaction times less than 30 min. Turnover numbers (TON)
Table 1 Effect of the base on catalyst performance
Entry Base
Time (h) Yielda (%) TONb TOFc (h-1
)
1
2
3
4
5
6
7
8
No base
12
2
0
99
95
99
99
93
65
72
0
198
190
198
198
186
130
144
0
99
and turnover frequencies (TOF) of up to 190 and 380 h-1
,
Cs2CO3
K2CO3
Na2CO3
respectively, were observed (Table 1, entry 3).
0.5
4
380
50
Suzuki–Miyaura reaction of aryl halides with aryl
boronic acids catalyzed by PVC–EDA–SA–Pd0
NaHCO3 1.25
158
42
Na3PO4
NaOH
KOH
4.5
1.25
1.5
104
96
To explore the scope of the Suzuki–Miyaura reaction
catalyzed by PVC–EDA–SA–Pd0, a variety of substituted
aryl halides and substituted aryl boronic acids were further
evaluated (Scheme 2). The results are listed in Table 2.
Better results were obtained with the relatively reactive
aryl iodides than with the aryl bromides. In particular, aryl
iodides with electron-withdrawing groups, for example
p-NO2 and p-Cl, were more smoothly converted to the
products in high yields with higher TON and TOF values
(Table 2, entries 2, 3, and 6–9). Aryl bromides with elec-
tron-withdrawing groups, for example p-NO2, p-CO2Me,
and m-NO2, also resulted in moderate yields with slightly
higher TON and TOF values (entries 14–17). In addition,
the electronic nature of substituted arylboronic acids has no
obvious effect on yields (entries 6–13, 15, and 17).
Reaction conditions: 4-nitrophenyl iodide (1.0 mmol), phenylboronic
acid (1.0 mmol), base (2.0 mmol), PVC–EDA–SA–Pd0 (0.05 g,
0.01 mmol of Pd), and 10.0 cm3 95% EtOH at reflux under air
a
Isolated yield based on 4-nitrophenyl iodide
b
TON (turnover number) = product (mol)/catalyst (mol)
c
TOF (turnover frequency) = TON/reaction time (h)
Effect of the base on catalytic performance
For practical purpose, ethanol was used as the solvent of
choice because it had better properties for both swelling of
the catalyst and dissolution of the substrate. We first
screened several different bases in the model reaction of
p-nitrophenyl iodide with phenylboronic acid in the pres-
ence of 1.0 mol% Pd loading in 95% aqueous ethanol
under air. The results are listed in Table 1.
Reusability of the PVC–EDA–SA–Pd0 catalyst
It was shown that the base was required for Suzuki–
Miyaura cross-coupling reaction (Table 1, entry 1). Among
the bases screened, although Cs2CO3 worked best, it was
not chosen owing to its high price (Table 1, entry 2). Using
NaHCO3 and Na2CO3 the reaction time is much longer
One of the main objectives of our study was to investigate
the recycling and reuse of the catalyst. We explored the
reusability of the PVC–EDA–SA–Pd0 catalyst using the
reaction of p-nitrophenyl iodide with phenylboronic acid as
Scheme 2
0
PVC-EDA-SA-Pd
X
+
(HO) B
R
2
R
2
2
R
R
1
1
3
1
2
123

1428
J. Liu et al.
Table 2 The Suzuki–Miyaura coupling reactions catalyzed by PVC–EDA–SA–Pd0
Entry
Aryl halides 1
Arylboronic acid 2
Time (min)
Product 3
Yieldsa (%)
TONb
TOFc (h-1
)
1
C6H5I
C6H5B(OH)2
240
30
3a
3b
3c
3d
3e
3f
84
95
88
54
76
99
99
98
91
74
58
40
61
53
82
51
84
10
Trace
168
190
176
108
152
198
198
182
196
148
116
80
42
380
352
108
228
594
594
546
588
148
116
40
2
4-NO2–C6H4I
4-Cl–C6H4I
C6H5B(OH)2
3
C6H5B(OH)2
30
4
4-Me–C6H4I
C6H5B(OH)2
60
5
4-MeO–C6H4I
4-NO2–C6H4I
4-NO2–C6H4I
4-NO2–C6H4I
4-NO2–C6H4I
4-MeO–C6H4I
4-MeO–C6H4I
4-MeO–C6H4I
4-MeO–C6H4I
4-NO2–C6H4Br
4-NO2–C6H4Br
4-MeO2C–C6H4Br
3-NO2–C6H4Br
4-Br–C6H5Br
4-NO2–C6H4Cl
C6H5B(OH)2
40
6
4-MeO–C6H4B(OH)2
4-Me–C6H4B(OH)2
4-Cl–C6H4B(OH)2
4-F–C6H4B(OH)2
4-Me–C6H4B(OH)2
4-MeCO–C6H4B(OH)2
4-F–C6H4B(OH)2
4-MeO–C6H4B(OH)2
C6H5B(OH)2
20
7
20
3g
3h
3i
8
20
9
20
10
11
12
13
14
15
16
17
18
19
60
3j
60
3k
3l
60
30
3m
3b
3h
3n
3o
3p
3b
122
106
164
102
168
20
244
35
180
240
270
180
260
24 h
4-Cl–C6H4B(OH)2
C6H5B(OH)2
41
23
4-Cl–C6H4B(OH)2
C6H5B(OH)2
56
4
C6H5B(OH)2
–
–
Reactions were carried out with aryl halide (1.0 mmol), aryl boronic acid (2.0 mmol), PVC–EDA–SA–Pd0 (0.1 g, 0.01 mmol of palladium), and
K2CO3 (1.0 mmol) in 2 cm-3 95% ethanol
a
Yield of isolated product 3 based on the aryl halide
b
TON (turnover number) = product (mol)/catalyst (mol)
c
TOF (turnover frequency) = TON/reaction time (h)
model reaction. After the first run, the catalyst (1.0 mol%
Pd) was filtered and extensively washed with EtOH and
Et2O and dried in vacuo. It was then directly reused under
similar conditions. The product was obtained in 95, 95, 95,
98, 95, and 80% yields in consecutive runs, with the TOF
decreasing from 380 to 46 h-1. It can be seen that the
catalyst could be reused up to five times while retaining its
catalytic activity. Characterization of the fresh and the
reused catalysts by transmission electron microscopy
(TEM) showed that the deactivation could be correlated
with an increase in the average size of the crystallites from
40 nm to [200 nm (Fig. 2).
example mild reaction conditions, simple isolation pro-
cedure, cleaner reaction profiles, high yields of products,
and a novel efficient catalyst for the Suzuki–Miyaura
reaction.
Experimental
Melting points were measured on an Electrothermal X6
microscopic digital melting-point apparatus. IR spectra
were recorded on a Bruker Equinox-55 spectrometer using
KBr pellets. 1H NMR spectra were obtained with a
300 MHz Bruker Avance instrument using CDCl3 as sol-
vent and TMS as internal standard. Elemental analyses
were performed on a Perkin–Elmer EA2400II elemental
analyzer. The elemental palladium content of the poly-
meric catalysts was determined by Perkin–Elmer Optima
2000DV inductively coupled plasma (ICP) spectroscopy.
Scanning electron microscopy (SEM) was performed with
a Philips XL 30ESEM instrument. Transmission electron
microscopy (TEM) was performed with a Philips Tecnai
instrument operating at 40–100 kV. The chemicals were
obtained from commercial sources and used as received.
Conclusion
Nanopalladium immobilized on the surface of PVC-sup-
ported Schiff base had high catalytic activity in Suzuki–
Miyaura cross coupling reactions in 95% aqueous ethanol
under atmospheric conditions. The catalyst can be easily
separated and recovered from the reaction mixture by
filtration and reused up to five times without noticeable
lose of activity. The procedure has several advantages, for
123

Synthesis of immobilized nanopalladium on polymer-supported Schiff base
1429
Preparation of PVC supported ethylenediamine
(PVC–EDA)
was stirred under reflux. After the reaction was judged to
be complete, by TLC analysis, the solution was cooled to
room temperature and the liquid was filtered through a
Bu¨chner funnel to remove the resin, which was washed
with 95% ethanol (3 9 2 cm3). The combined organic
fractions were then concentrated on a rotary evaporator to
obtain the desired biaryls in excellent yield. These were
further purified by recrystallization. All the products were
known and were characterized by melting point, and IR
and 1H NMR spectroscopy; the data were found to be
identical with those reported in the literature [24–30].
Poly(vinyl chloride) (20.0 g) was added to 80 cm3 ethy-
lenediamine in a round-bottomed flask and the mixture was
stirred at 80 °C, in air, for 48 h. After being cooled to room
temperature, the reaction mixture was filtered and the solid
was washed with a large volume of deionized water, eth-
anol (3 9 20 cm3), and diethyl ether (3 9 20 cm3), and
dried at 60 °C in vacuo for 12 h to give brown PVC–EDA.
The amino group content was found to be 8.95 mmol g-1
by elemental microanalysis.
Acknowledgments We are grateful to the National Natural Science
Foundation of China (20672046) and the Guangdong Natural Science
Foundation (8151063201000016) for financial support.
Preparation of ethylenediamine functionalized
PVC-supported Schiff base (PVC–EDA–SA)
The ethylenediamine-functionalized PVC (10.0 g) was
allowed to swell in 50 cm3 ethanol for 2 h. Salicylaldehyde
(89.5 mmol) was then added and the solution and stirred at
60 °C for 8 h. After being cooled to room temperature, the
brown colored polymer was isolated by filtration, washed
thoroughly with water, ethanol (3 9 20 cm3), and diethyl
ether (3 9 20 cm3), and dried in vacuo at 60 °C for 12 h.
The IR spectrum showed the characteristic absorption of an
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Preparation of nano-Pd immobilized on PVC-supported
Schiff base (PVC–EDA–SA–Pd0)
PVC–EDA–SA (2.0 g) was swollen in 15 cm3 ethanol for
2 h, with magnetic stirring, then 0.3 g PdCl2 (1.69 mmol)
was added and the mixture was heated at reflux for 48 h.
During this process the color of the solution turned from
light yellow to colorless, indicating the formation of nano-
Pd0. The supported Pd0 nanoparticles were isolated by
filtration and washed with ethanol (3 9 20 cm3), acetone
(3 9 20 cm3), and diethyl ether (3 9 20 cm3). After dry-
ing, 2.02 g dry polymer immobilized nano-Pd0 particles
(PVC–EDA–SA–Pd0) were obtained. The IR spectrum
showed the characteristic absorption of an azomethine
(C=N) group at 1,630.5 cm-1. Metal content was found to
be 0.107 mmol g-1 by ICP.
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123