Nanopalladium immobilized on aminoethanol-functionalized poly(vinyl chloride): An easily prepared, air and moisture stable catalyst for Heck reactions

Article abstract of DOI:10.1007/s00706-008-0958-5

The Pd nanoparticles with narrow size distribution immobilized on the polymer surface of poly(vinyl chloride) supported 2-aminoethanol (PVC-AE) were achieved by a simple procedure applying the corresponding functionalized polymer and PdCl₂ in ethanol. The as-prepared catalyst (PVC-AE-Pd ⁰) was found to be air and moisture stable and exhibits significant catalytic activity for Heck reactions under mild operating conditions. Furthermore, the catalyst can be easily recovered by simple filtration and reused for at least 6 cycles without losing its activity.

Full text of DOI:10.1007/s00706-008-0958-5

Monatsh Chem 139, 1447–1451 (2008)
DOI 10.1007/s00706-008-0958-5
Printed in The Netherlands
Nanopalladium immobilized on aminoethanol-functionalized
poly(vinyl chloride): an easily prepared, air and moisture
stable catalyst for Heck reactions
Xu-Jiang Huang, Fen Dong, Lu Chen, Yi-Qun Li
Department of Chemistry, Jinan University, Guangzhou, China
Received 24 April 2008; Accepted 30 April 2008; Published online 9 July 2008
# Springer-Verlag 2008
Abstract The Pd nanoparticles with narrow size
distribution immobilized on the polymer surface of
poly(vinyl chloride) supported 2-aminoethanol (PVC-
AE) were achieved by a simple procedure applying
the corresponding functionalized polymer and PdCl2
in ethanol. The as-prepared catalyst (PVC-AE-Pd0)
was found to be air and moisture stable and exhibits
significant catalytic activity for Heck reactions under
mild operating conditions. Furthermore, the catalyst
can be easily recovered by simple filtration and reused
for at least 6 cycles without losing its activity.
plexes in the presence of phosphane ligands [3],
which play the role of stabilizers of in situ formed
soluble Pd0 in homogeneous solution in the presence
of base under an inert atmosphere. However, the
relatively high price of the palladium complex has
greatly limited the industrial application of homo-
geneous Heck reaction, and some of the palladium
phosphine complexes are sensitive to air and
moisture. On the other hand, phosphane-free cata-
lysts have received great interest recently as a less
complicated, environmentally friendly alternative to
phosphane-containing systems [4]. Significant ad-
vances have been made in the past few years in
developing such catalysts. These include the use of
N- [5] or S-based palladacycles [6], N,S-based car-
benes [7], and N-heterocyclic carbenes (NHC) [8] as
ligands in palladium-catalyzed Heck reactions.
However, all of these catalyst systems suffer from
drawbacks of one kind or another, such as the high
sensitivity of ligands towards air and moisture, the
problems of tedious separation, recycling, and deac-
tivation via the aggregation of Pd0 nanoparticles
formed in situ during the Heck reaction process. To
overcome these problems, many approaches to sep-
arate and recycle metal nanoparticles are to immobi-
lize them on solid supports such as organic polymer
matrix [9] and inorganic microsphere [10] (e.g., car-
bon, metal oxides, sol-gel, clays, and zeolites), mak-
ing them more stable and really easy and simple to
separate from the reaction products mixture. Among
Keywords Nanopalladium; Poly(vinyl chloride); Supported
catalyst; Heck reaction.
Introduction
The Pd-catalyzed arylation or vinylation of olefins,
universally referred to as the Heck reaction [1], is
one of the most important C–C coupling reactions
which provide a direct route to produce function-
alized olefin. They may be used for the synthesis
of multifunctional derivatives [2] including bioactive
compounds, natural products, pharmaceuticals, and
high-performance materials. The Heck reaction is
generally catalyzed by soluble Pd2þ or Pd0 com-
Correspondence: Yi-Qun Li, Department of Chemistry, Jinan
University, Guangzhou 510632, China. E-mail: tlyq@jnu.
edu.cn

1448
X.-J. Huang et al.
the preparations of nano-Pd0 particles, the chemical
reduction of palladium salt in an aqueous or organic
solution by suitable reducing agents such as hydrazine
[11], hydrogen [12], and sodium borohydride [13]
have been widely adopted in most cases, even though
there are questions regarding the toxicity of some of
these reducing agents. The employment of polymers
in these systems was mainly involved with the pro-
tection of the nanosized palladium from aggregation.
The report of other properties about the polymer such
as reducing properties was extremely rare. It would
still be desirable to develop a more robust, simple,
and cost-effective non-phosphane ligands catalyst sys-
tem for use in nano-Pd0 catalyzed Heck reactions.
It is well known that poly(vinyl chloride) (PVC) is
widely used, inexpensive, and easily modified by
functional groups via a displacement reaction di-
rectly without chloromethylation which makes it a
suitable polymeric support for the heterogeneous
catalyst. Recently, it has been discovered that amino
alcohols such as prolinol, trans-2-aminocyclohexa-
nol, and triethanolamine are effective ligands (O,N-
ligands) in the nickel-catalyzed Suzuki [14] and in the
palladium-catalyzed Heck reaction [15]. To develop
simple and reliable protocols for the immobilization
of catalytically active palladium nanoparticles on
functionalized PVC we attempted to prepare and
characterize the functionalized polymer of PVC sup-
ported 2-aminoethanol (e.g., PVC–NHCH2CH2OH,
abbreviated as PVC-AE) and further use it as support
to immobilize palladium nanoparticles. Herein, we
demonstrate that the as-prepared catalyst provides an
active and recyclable heterogeneous catalyst for the
Heck type carbon-carbon bond formation reaction in
the absence of phosphine ligands under air and mois-
ture with excellent yields.
Fig. 1 TEM image of PVC-AE-Pd0
3438 cmꢁ1 and the absorption of a double bond
(C¼C) at 1637 cmꢁ1. The amino group content was
found to be 2.87 mmol=g. The catalyst PVC-AE-Pd0
is directly prepared by simple in situ reduction of an
ethanolic solution of palladium chloride in the pres-
ence of PVC-AE support. Metal content in PVC-AE-
Pd0 catalyst was found to be 0.52 mmol=g by atomic
absorption spectroscopy. The TEM image shows the
formation of palladium nanoparticles in the range of
20–40 nm (Fig. 1).
The thermal stability of the PVC-AE has a great
effect on its catalytic activity and recyclability be-
cause the Heck reaction is usually carried out under
heat conditions. Figure 2 displays the TG curve of
PVC-AE-Pd0 at atmospheric conditions. TG analysis
shows that PVC-AE-Pd0 is stable up to 200ꢀC.
Preliminary testing for Heck reaction was carried
out by treating 4-iodoanisole with styrene in ethanol
catalyzed by PVC-AE-Pd0 with a yield of 90%.
Encouraged by this result, we studied other aryl
halides and olefins under the same nonoptimized
conditions. All reactions were performed at reflux
conditions by using 1.0 mol% of catalyst loading
(Scheme 2). The results are as shown in Table 1.
PVC-AE-Pd0 was used in Heck arylation of aryl
halide with styrene or acrylic acid using ethanol as a
solvent and triethylamine as base under atmospheric
conditions. The aryl olefins were obtained with
yields ranging from good to excellent and the struc-
Results and discussion
The preparation of the PVC-AE involved the addition
of an excess of 2-aminoethanol to commercially avail-
able PVC resin at 80ꢀC for the specific time to afford
the corresponding functionalized resin (Scheme 1).
The IR spectrum of the PVC-AE showed the char-
acteristic absorption of an amino (N–H) group at
1
tures of the products were confirmed by IR and H
NMR spectra. It can be seen that the Heck arylations
Scheme 1

Nanopalladium immobilized on aminoethanol-functionalized poly(vinyl chloride)
1449
tries 1 and 6), higher yields were obtained than for
those with electron-withdrawing groups (such as a
nitro group) on the aromatic ring (Table 1, entries 5
and 10).
One of the main aims of our study was to inves-
tigate the reuse and recycling of the PVC-AE-Pd0
catalyst. The catalyst was easily recovered from the
reaction mixture by simple filtration after the aryla-
tion, washed with 95% ethanol and diethyl ether, and
then dried in air. Reusabilities of the PVC-AE-Pd0
were examined by using the reaction of 4-chloro-1-
iodobenzene with acrylic acid as model. It can be
seen from Fig. 3 that the catalyst could be reused
for 6 runs while retaining the same activity and se-
lectivity as well as without loss of the activity.
The TEM analysis shows the presence of smaller
palladium nanoparticles along with some slight
agglomeration of the particles in the used catalyst
(Fig. 4). However, the consistent activity of the cat-
alyst from the recycling experiments reveals that
slight particle agglomeration has less influence on
the reactivity.
Fig. 2 TG curves of PVC-AE-Pd0
In conclusion, PVC-AE-Pd0 was prepared easily
by using an inexpensive support and exhibited high
activity in the Heck reaction of aryl iodides with
olefins in air. The catalyst can be easily separated
and recovered from the reaction mixture by filtra-
tion and reused at least for 6 times. The protocol
offers several advantages, such as mild reaction
condition, simple isolation procedure, cleaner reac-
tion profiles, high yields of products, as well as ease
of recovery and reuse of the novel supported nano-
Pd0 catalyst.
Scheme 2
of acrylic acid or styrene with aryl halides can be
efficiently carried out at 78ꢀC to afford the (E)-prod-
ucts in high yield, and the (Z)-products were not
observed. The electronic effects and the nature of
substituents on aryl halides ring did show obvious
effects on the yield of products. When an aromatic
cycle containing electron-donating groups (such
as a methoxy group) were employed (Table 1, en-
Table 1 PVC-AE-Pd0 catalyzed Heck reactions
Entry
R1
R2
Time=h
Yieldsa=%
Mp=ꢀC
Found
Ref. [16]
1
2
3
4
5
6
7
8
9
10
CH3O
CH3
H
Ph
Ph
Ph
Ph
5
5
5
5
5
4
4
4
4
4
90
81
77
83
66
80
68
71
82
62
134–136
118–120
123–125
126–128
156–158
171–173
198–200
130–132
247–249
285–287
134–135
118–119
124
127–128
156–157
173–174
200–201
131–132
249–250
286–287
Cl
NO2
CH3O
CH3
H
Cl
NO2
Ph
COOH
COOH
COOH
COOH
COOH
a
Isolated yields

1450
X.-J. Huang et al.
dried in air to give brown PVC-AE. The amino group content
was found to be 2.87 mmol=g by elemental microanalysis.
Preparation of nano-Pd immobilized on PVC-AE
(PVC-AE-Pd0)
The nano-Pd0 preparation process is ‘‘green’’ and very
straightforward. Specifically, 1 g PVC-AE was swollen in
20cm3 ethanol for 2 h by magnetic stirring, then 0.3 g PdCl2
(1.69 mmol) were added to the mixture and it was heated to
reflux. The resulting light yellow solution was further stirred
for 24h at the same temperature. 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 filtered off and washed with ethanol, acetone, and diethyl
ether. After drying, 2.2 g of dry PVC-AE immobilized nano-
Pd0 particles (abbreviated as PVC-AE-Pd0) were obtained. The
pallladium content was found to be 0.52mmol=g of dry resin
by AAS. The as-prepared nano-Pd0 can be preserved for
months under air without changes of properties.
Fig. 3 Reuse of the PVC-AE-Pd0 catalyst
General procedure for Heck reaction of aryl iodides
with olefins
To a mixture of 5.0 mmol aryl iodide, 12.0 mmol triethylamine,
and 6.0 mmol olefin in 1.0 cm3 95% ethanol, 0.1 g PVC-AE-
Pd0 catalyst (0.05 mmol) were added and stirred under reflux.
After a specified period, the reaction mixture was filtered
and the residue was washed with 95% ethanol. When a
styrene was used as substrate, the solvent was evaporated
under vacuum, diethyl ether was added, and it was washed
with distilled water. The organic layer was separated, dried
over Na2SO4, and the solvent was removed under vacuum.
The resultant mixture was purified by preparative TLC to get
the desired product; when acrylic acid was used as substrate,
the filtrate was poured into water, then neutralized with
diluted HCl to precipitate the product, which was further
purified by recrystallization. All of the products are known
and the data were found to be identical with those reported
in literature.
Fig. 4 TEM image of PVC-AE-Pd0 after six reaction cycles
Experimental
The chemicals were obtained from commercial sources
and used as received. Melting points were measured on an
Electrothemal X6 microscopy digital melting point appratus.
IR spectra were recorded on a Bruke Equinox-55 spectrometer
Acknowledgements
1
We are grateful to the National Natural Science Foundation of
China (No. 20672046) and the Guangdng Natural Science
Foundation (No. 04010458) for financial support.
using KBr pellets. H NMR spectra were performed with a
300 MHz Bruker Advance instrument using CDCl3 as a sol-
vent and TMS as an internal standard. The contents of elemen-
tal palladium in the polymeric catalyst were determined by
TAS-990 atomic absorption spectroscopy (AAS). The thermal
analysis was performed on a Perkin Elmer TGS-2 thermal
analysis system at a heating rate of 10ꢀC=min under air
atmosphere. Transmission electron microscopy (TEM) was
performed with a Philips Tecnai instrument operating at
40–100 kV.
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