A simple catalyst for aqueous phase Suzuki reactions based on palladium nanoparticles immobilized on an ionic polymer

Article abstract of DOI:10.1007/s11426-015-5542-3

Palladium nanoparticles immobilized on a cross-linked imidazolium-containing polymer were evaluated as a catalyst for Suzuki carbon-carbon cross-coupling reactions using water as the solvent. The nanocatalysts show good catalytic activities for aryl iodides and aryl bromides and moderate activity with aryl chloride substrates. Coupling of sterically hindered substrates could also be achieved in reasonable yields. The heterogeneous catalyst is stable, can be stored without precautions to exclude air or moisture, and can be easily recycled and reused.

Full text of DOI:10.1007/s11426-015-5542-3

SCIENCE CHINA
Chemistry
April 2016 Vol.59 No.4: 482–486
doi: 10.1007/s11426-015-5542-3
• ARTICLES •
A simple catalyst for aqueous phase Suzuki reactions based on
palladium nanoparticles immobilized on an ionic polymer
Saeideh Ghazali-Esfahani1,2, Emilia Păunescu1, Mojtaba Bagherzadeh2, Zhaofu Fei1,
Gabor Laurenczy1 & Paul J. Dyson1*
1Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland
2Chemistry Department, Sharif University of Technology, Tehran 11155-3615, Iran
Received July 31, 2015; accepted August 26, 2015; published online December 21, 2015
Palladium nanoparticles immobilized on a cross-linked imidazolium-containing polymer were evaluated as a catalyst for Suzu-
ki carbon-carbon cross-coupling reactions using water as the solvent. The nanocatalysts show good catalytic activities for aryl
iodides and aryl bromides and moderate activity with aryl chloride substrates. Coupling of sterically hindered substrates could
also be achieved in reasonable yields. The heterogeneous catalyst is stable, can be stored without precautions to exclude air or
moisture, and can be easily recycled and reused.
catalysis, biphasic catalysis, polymeric ionic liquids, nanoparticles, aqueous phase catalysis, Suzuki reaction
Citation:
Ghazali-Esfahani S, Păunescu E, Bagherzadeh M, Fei ZF, Laurenczy G, Dyson PJ. A simple catalyst for aqueous phase Suzuki reactions based on
palladium nanoparticles immobilized on an ionic polymer. Sci China Chem, 2016, 59: 482–486, doi: 10.1007/s11426-015-5542-3
of efficient heterogeneous catalysts for the Suzuki reaction
1 Introduction
attracts much interest, as they facilitate product purification
and enable facile catalyst recovery and recycling. Palladium
nanoparticles immobilized on many different types of sup-
ports, including zeolites [7,8], silica [9,10], dendrimers
[11,12], magnetic beads [13,14] and other materials [15–17]
have been reported. In addition to the development of het-
erogeneous catalysts, the use of alternative solvents to the
volatile organic solvents typically used for the Suzuki reac-
tion has also been considered. Notably, ionic liquids have
attracted a lot of attention as reaction solvents as they are
stable, non-volatile and can facilitate product separation and
catalyst recycling [18–20]. A number of functionalized ion-
ic liquids have been developed for cross-coupling reactions
although most are used together with water [21–24]. Water
has also been extensively used as a solvent for Suzuki and
other cross-coupling reactions [25–29].
The Suzuki-Miyaura cross-coupling reaction of aryl halides
with aryl boronic acids is one of the most versatile synthetic
routes employed in organic chemistry for the generation of
biaryl structures [1,2]. In general, molecular catalysts al-
most exclusively based on palladium(0), e.g. Pd(PPh3)4 or
palladium(0) catalysts generated in situ from palladium(II)
salts, e.g. Pd(PPh3)2Cl2, in donor solvents such as tetrahy-
drofuran (THF) or dioxane [3–5] are used as pre-catalysts in
this reaction. Increasingly, palladium nanoparticles are also
employed for these coupling reactions, although it is gener-
ally accepted that the nanoparticles act as catalyst reservoirs
affording palladium(0) single atom species that are the ac-
tual active catalysts [6].
Despite this mechanism involving an interplay between
heterogeneous and homogeneous entities, the development
Previously, we reported a polymeric ionic liquid derived
from a styrene-functionalized imidazolium salt, i.e. 1-(4-
*Corresponding author (email: paul.dyson@epfl.ch)
© Science China Press and Springer-Verlag Berlin Heidelberg 2015
chem.scichina.com link.springer.com

Ghazali-Esfahani et al. Sci China Chem April (2016) Vol.59 No.4
483
vinylbenzyl)-3-methylimidazolium chloride, [vbim]Cl (Fig-
ure 1), termed poly[vbim]Cl, able to stabilize gold nanopar-
ticles in both ionic liquids [30] and in aqueous media [31].
The solubility properties of the polymeric ionic liquid may
be modulated by exchange of the counter-anion. For exam-
ple, with the chloride counter-ion the polymer and the cor-
responding polymer coated metal nanoparticles are hydro-
philic, whereas with bis(trifluoromethylsulfonyl)imide,
[Tf2N], they become hydrophobic. Palladium nanoparticles
coated with the latter polymer, i.e. poly[vbim][Tf2N], im-
mobilized in an ionic liquid were shown to catalyze Suzuki
reactions under relatively mild conditions [32].
By attaching a second vinyl-group to the imidazolium
monomer, i.e. 1,3-bis(4-vinylbenzyl)imidazolium chloride,
[bvbim]Cl (Figure 1) [33], a cross-linked polymer termed
poly[bvbim]Cl is obtained [34]. This polymer is insoluble in
most common solvents and it is highly thermal stable and
was employed as an organocatalyst for the cycloaddition of
carbon dioxide to epoxides.
palladium in the Pd-NP-PIL catalyst. Samples were digested
in concentrated nitric acid at 80 °C for 16 h and then water
was added to a total volume of 7 mL. Indium was added as
an internal standard at a concentration of 0.5 ppb. Determi-
nation of palladium content was achieved on a Perkin Elmer
Elan DRC II ICP-MS instrument (USA) equipped with a
Meinhard nebulizer and a cyclonic spray chamber. The
ICP-MS instrument was tuned using a solution provided by
the manufacturer containing 1 ppb of each element Mg, In,
Ce, Ba, Pb and U. External standards were prepared gravi-
metrically in identical matrix to the samples (with regard to
internal standard and nitric acid) with single element stand-
ards obtained from CPI International (Netherlands).
2.2 Preparation of the Pd-NP-PIL catalyst
A mixture of PdCl2 (47.4 mg, 0.268 mmol) and NaCl (313.2
mg, 5.36 mmol) in deionized water (7 mL) (Milli-Q, Milli-
pore, USA) was heated at 80 °C until the entire quantity of
PdCl2 reacted to give a red solution of Na2[PdCl4]. The
mixture was cooled to r.t. and poly[bvbim]Cl (0.250 g,
0.744 mmol based on the monomer) was added. The sus-
pension was stirred for 20 h at r.t., filtered, and the precipi-
tate was washed with water (3×5 mL) and dried for 16 h
under vacuum at r.t.. The resulting orange powder was sus-
pended in ethanol (7 mL) and a solution of NaBH4 (70 mg,
1.87 mmol) in ethanol (7 mL) was added dropwise to afford
a black suspension that was stirred at r.t. for a further 6 h.
The suspension was filtered, the precipitate was washed
with water (3×5 mL) and dried for 16 h under vacuum at r.t.
The palladium content was determined by ICP-MS (0.0087
mmol Pd per 10 mg Pd-NP- PIL catalyst).
Herein, we describe the preparation and characterization
of palladium nanoparticles immobilized on poly[bvbim]Cl.
The resulting material was investigated as a heterogeneous
catalyst in Suzuki cross-coupling reactions in aqueous me-
dia.
2 Experimental
2.1 General
Chemicals were obtained from commercial sources and
used without further purification. The polymer ionic liquid,
poly[bvbim]Cl, was prepared according to a literature pro-
cedure [33]. Transmission electron microscopy (TEM) was
performed on a Philips CM-12 TEM instrument (Nether-
lands) with a LaB6 source operated at 120 kV. Samples for
TEM analysis were prepared by adding the Pd-NP-PIL cat-
alyst suspension to isopropanol, followed by ultrasonication
for 30 min. A few drops of the suspension were then placed
onto a carbon film coated Cu grid which was then dried for
1 h under an infrared lamp. X-ray photoelectron spectros-
copy (XPS) measurements were performed on a Per-
kin-Elmer PHI 5000CESCA (USA) system with a base
pressure of 109 Torr. Inductively coupled plasma mass
spectrometry (ICP-MS) was used to quantify the amount of
2.3 General procedure for the Suzuki coupling reac-
tions
A mixture of aryl halide (0.5 mmol, 1.0 equiv.), phenyl-
boronic acid (0.6 mmol, 1.2 equiv.), K2CO3 (2 mmol, 4
equiv.), the Pd-NP-PIL catalyst (1.7 mol% based on the aryl
halide substrate) in water (3 mL) was heated to 100 °C un-
der nitrogen with vigorous stirring for 4 h. The reaction
mixture was then cooled at r.t. and the product was extract-
ed with diethyl ether (3×3 mL). The combined extracts were
washed with brine (15 mL), dried over anhydrous MgSO4,
filtered and the solution was analyzed by GC. The structure
1
of the products was also confirmed by H NMR spectros-
copy.
2.4 Recycling studies
After each reaction the catalyst was separated from the re-
action mixture by centrifugation, washed with water (3×3
mL), diethyl ether (3×3 mL) and acetone (3×3 mL) and
further dried under vacuum for 18 h and used for another
catalytic cycle as described in Section 2.3.
Figure 1 Styrene functionalized imidazolium chloride monomers. (a)
1-(4-vinylbenzyl)-3-methylimidazolium chloride ([vbim]Cl); (b) 1,3-bis(4-
vinylbenzyl)imidazolium chloride ([bvbim]Cl).

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Ghazali-Esfahani et al. Sci China Chem April (2016) Vol.59 No.4
3 Results and discussion
The polymer ionic liquid poly[bvbim]Cl was reacted with
Na2[PdCl4] (prepared in situ by the addition of NaCl to an
aqueous solution of PdCl2) leading to the exchange of the
chloride counter-anions in the polymer by the [PdCl4]2
anions. The resulting poly[bvbim][PdCl4] material was re-
acted with Na[BH4] to afford palladium nanoparticles
trapped on the polymer surface, termed Pd-NP-PIL. The
transmission electron microscopy (TEM) image of poly
[bvbim]Cl shows a characteristic spherical (aggregated)
shape (Figure 2). The TEM of the Pd-NP-PIL material
shows palladium nanoparticles adhered to the surface of the
polymer with diameters ranging from 2 to 5 nm (the spheri-
cal shape of the polymer is retained).
The catalyst (Pd-NP-PIL) was also characterized by
X-ray photoelectron spectroscopy (XPS) (Figure 3). Peaks
corresponding to both Pd(II) and Pd(0) species were de-
tected (Figure 3(b)). The binding energies of 335.6 eV (Pd
3d5/2) and 340.7 eV (Pd 3d3/2) are related to fully reduced Pd
nanoparticles, whereas the peaks at 337.7 eV (Pd 3d5/2) and
343 eV (Pd 3d3/2) suggest that a number of unreduced Pd2+
ions are present in the catalyst. The results are in accord-
ance with previous reports which show the palladium na-
noparticles supported by polymers contain surface Pd2+ spe-
cies [35–37].
The Pd-NP-PIL material is insoluble in common solvents
and is stable up to 350 °C and, consequently, was evaluated
as a heterogeneous catalyst in Suzuki coupling reactions in
Figure 3 (a) XPS survey spectrum of Pd-NP-PIL; (b) XPS spectrum of
palladium 3d (color online).
o
aqueous solution at 100 C using various aryl halide sub-
strates (Table 1).
The Pd-NP-PIL catalyst efficiently converts aryl iodides
and aryl bromides to biaryl products in the presence of
phenylboronic acid in high yield using short reaction times
(4 h) at low catalyst loadings (10 mg, 1.7 mol%). The
Pd-NP-PIL catalyst in water tends to be superior to many
catalysts that operate in ionic liquids [38,39]. Moreover, the
Pd-NP-PIL catalyst is also able to couple aryl chloride sub-
strates in modest yield (16%–23%). It is noteworthy that
under comparable conditions most heterogeneous catalysts
tend to give <5% yield [22,23,32].
Table 1 Suzuki cross-coupling reactions between phenylboronic acid and
various aryl halides catalyzed by Pd-NP-PIL a)
Entry
1
R, X
H, I
Yield b) (%)
98
93
99
99
97
70
99
92
98
23
17
16
2
Me, I
3
MeO, I
NO2, I
CN, I
4
5
The Pd-NP-PIL catalyst was also evaluated in the cross-
coupling of more sterically hindered substrates (Table 2).
6
Me, Br
MeO, Br
NO2, Br
CN, Br
Me, Cl
MeO, Cl
CN, Cl
7
8
9
10
11
12
a) Reaction conditions: aryl halide/phenylboronic acid/K2CO3 1:1.2:4,
catalyst Pd-NP-PIL (10 mg, 1.7 mol%), water (3 mL), T=100 oC, t=4 h; b)
yield determined by GC.
Good yields were obtained for the methyl-substituted biaryl
product whereas the dimethyl-product was obtained in only
moderate yield. Nevertheless, under the conditions employed
Figure 2 Transmission electron microscopy images of poly[bvbim]Cl (a)
and the Pd-NP-PIL material (b).

Ghazali-Esfahani et al. Sci China Chem April (2016) Vol.59 No.4
485
Table 2 Suzuki cross-coupling of sterically hindered substrates catalyzed
by Pd-NP-PIL a)
Aryl halide
Boronic acid
Entry
Time (h) Yield b) (%)
R′
R, X
1
2
3
4
Me, I
H
Me
H
4
24
4
86
46
69
41
Me, I
Me, Br
Me, Br
Me
24
a) Reaction conditions: aryl halide/boronic acid/K2CO3 1:1.2:4,
Pd-NP-PIL (10 mg, 1.7 mol%), water (3 mL), T=100 °C, t=4 h; b) yield
determined by GC.
Figure 5 The Pd-NP-PIL catalyst after 5 catalytic runs from the reaction
of iodobenzene and phenylboronic acid.
and in the presence of a cheap and inexpensive base the
yields obtained are comparable or even superior to other
heterogeneous catalysts [24,32,40].
4 Conclusions
In conclusion, we have prepared a highly robust and active
heterogeneous catalyst for Suzuki cross-coupling reactions
that operates in water in the presence of an inexpensive
base. The catalyst is simple to prepare, stable for prolonged
periods, and easy to use and recycle.
The ability to reuse the Pd-NP-PIL catalyst was investi-
gated using two different aryl iodide substrates. For both
substrates good recyclability was observed over 5 catalytic
runs with >90% of the original activity retained (Figure 4).
The amount of palladium leached from catalyst was deter-
mined by ICP analysis after each catalytic run employing
iodobenzene as the substrate. The leaching tests revealed
that the amount of palladium in solution is <0.3% of the
total Pd content.
The structure of the Pd-NP-PIL catalyst after 5 batches
from the reaction of iodobenzene and phenylboronic acid
was studied by TEM (Figure 5). Compared to the freshly-
prepared catalyst (Figure 2) the structures are essentially the
same, although the size of the palladium nanoparticles is
slightly larger, demonstrating the high stability of the cata-
lyst. Such stability is unusual, especially given the likeli-
hood that the active catalyst corresponds to mononuclear
palladium species extracted from the palladium nanoparti-
cles, and hence it is not unreasonable that as the palladium
species reattach to the nanoparticle surface after reaction
agglomeration is observed.
Acknowledgments This work was supported by the Ecole Polytechnique
Fédérale de Lausanne and the Iranian Ministry of Science, Research and
Technology (to S. G.-E.). Besides, we thank the staff at the EPFL Centre
Interdisciplinaire de Microscopie Electronique (CIME) for assistance with
the TEM.
Conflict of interest The authors declare that they have no conflict of
interest.
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