Published on the web August 28, 2010
1069
Palladium Nanoparticles in Ionic Liquid by Sputter Deposition
as Catalysts for Suzuki-Miyaura Coupling in Water
Yoshiro Oda,1 Koji Hirano,1 Kazuki Yoshii,1 Susumu Kuwabata,1,2 Tsukasa Torimoto,2,3 and Masahiro Miura*1
1Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871
2Japan Science and Technology Agency, CREST, Kawaguchi, Saitama 332-0012
3Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University,
Chikusa-ku, Nagoya, Aichi 464-8603
(Received July 16, 2010; CL-100635; E-mail: miura@chem.eng.osaka-u.ac.jp)
Palladium nanoparticles in an ionic liquid prepared readily
by sputter deposition efficiently catalyze the Suzuki-Miyaura
coupling of hydrophobic as well as hydrophilic aryl halides in
water.
Palladium nanoparticles have received much attention in
modern synthetic chemistry since they have intrinsic potential to
show high performance as catalysts for carbon-carbon bond
20 nm
forming reactions.1 In particular, the Suzuki-Miyaura cross-
coupling, which is of genuine synthetic utility, has been
extensively studied with a variety of nanoparticles stabilized
or immobilized by ionic liquids,2 polymers,3 carbon,4 silica,5
and alumina.6 However, in most cases, their preparation
or generation protocols rely on the in situ reduction of
palladium(II) precursors with additional chemical species.
Recently, one of our groups has developed a highly clean
method to fabricate metal nanoparticles including Au, Ag, Au/
Ag, and Pt in ionic liquids at room temperature: the nonvolatility
Pd/BMI-PF6
Pd/BMI-TFSI
of ionic liquids enables the deposition of metals onto them under
high vacuum, which leads to the high dispersion of nanoparticles
with diameters of several nanometers and allows the inves-
tigation of inherent activity of nanoparticles in an ionic liquid
without the contamination of reducing reagents.7 Herein, we
report an application of the sputter methodology to the rapid and
facile preparation of waste-free palladium nanoparticles in ionic
liquids. The nanoparticles show good catalytic activity for the
Suzuki-Miyaura coupling in water and enable use of both water-
soluble and -insoluble aryl halides in the reaction without
employing excess ionic liquid,2,8 organic/aqueous mixed sol-
vent system,9 special hydrophilic polymer support,10 or addi-
tional phase transfer catalyst.11
Figure 1. TEM images and size distributions of Pd nano-
particles prepared by sputter deposition in BMI-PF6 (left) and
BMI-TFSI (right).
diameter. Thus, the size of nanoparticles depends on the counter
anion of ionic liquids employed as is anticipated.7 The nano-
particles were stable at least for 1 month when stored even under
ambient conditions.
With the palladium nanoparticles in BMI-PF6 (Pd/BMI-
PF6) and BMI-TFSI (Pd/BMI-TFSI) in hand, we began the
investigation of their catalytic activities for the Suzuki-Miyaura
coupling using iodobenzene (1a-I) and p-tolylboronic acid (2a)
as starting substrates (Table 1). The extensive screening of
reaction parameters with 0.2 mol % catalyst loading revealed that
a combination of i-Pr2NEt and water as base and solvent,
respectively, proved to be optimal for the Pd/BMI-PF6 catalyst
(Entries 1-9). On the other hand, in the case of Pd/BMI-TFSI,
an inorganic base, Cs2CO3, gave a better result (Entries 10 and
11).12,13 It is worth noting that due to the insolubility of 1a-I in
water, the reaction should proceed in a biphasic system, i.e.,
organic particles in water. The beneficial behavior of water
would arise from the hydrophobic effect of the substrate.14
We then conducted the Suzuki-Miyaura coupling of various
aryl iodides and bromides with arylboronic acids with the Pd/
BMI-TFSI catalyst (Table 2). In general, 0.3 mol % catalyst
loading and 4.0 equiv of Cs2CO3 gave good reproducibility. Not
only electron-rich but also electron-deficient aryl iodides 1b-I-
We chose 1-butyl-3-methylimidazolium hexafluorophos-
phate (BMI-PF6) and 1-butyl-3-methylimidazolium bis(trifluoro-
methanesulfonyl)imide (BMI-TFSI) as typical room-temperature
ionic liquids. A 0.20-cm3 aliquot of the degassed ionic liquid
was spread on a soda glass plate (2.5 cm © 2.5 cm), and the plate
was horizontally set in a Cressigngton 108 auto SE sputter
coater, which was located at a distance of 4.5 cm from a Pd foil
target (57 mm diameter). Sputter deposition was then carried out
at room temperature for 300 s with a 40 mA current under argon,
whose pressure did not exceed 8 Pa. The concentration of Pd
obtained in the ionic liquids was determined by ICP analysis:
the BMI-PF6 and BMI-TFSI contained ca. 0.011 and 0.030
¹1
¯mol mg as Pd atoms, respectively. Their TEM images and
size distributions are illustrated in Figure 1: Pd nanoparticles in
BMI-PF6 were generated with an average diameter of ca. 3.0 nm,
whereas the use of BMI-TFSI resulted in a ca. 2.2 nm average
Chem. Lett. 2010, 39, 1069-1071
© 2010 The Chemical Society of Japan