Full Paper
Recently, hybrid nanocrystals containing two or more metals
have attracted much attention; the synthesis and applications
Results and Discussion
[13]
of such hybrid metal nanoparticles have been reported. The
properties of individual metal nanocrystals can be exploited in
hybrid nanocrystals, possibly leading to synergistic and en-
hanced properties. Among them, heterodimeric nanocrystals
have been synthesized from transition metals and metal
oxides and used for multifunctional applications including bio-
For mechanistic studies on the reactions using the magnetical-
ly recoverable PdÀFe O nanoparticle catalysts, two representa-
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tive reactions were selected: Suzuki coupling and Wacker oxi-
dation. Previously, we reported a series of representative cross-
coupling reactions based on the catalytic activity of the PdÀ
[15]
Fe O heterodimer nanocrystals. It was found that the PdÀ
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[
14]
medical sensing and catalytic reactions.
We reported a simple synthesis of PdÀFe O heterodimer
Fe O heterodimer nanocrystals have excellent catalytic activity
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for the Suzuki coupling reactions of arylboronic acids with vari-
ous aryl iodides, even though the catalyst showed slightly
lower activity towards less activated aryl bromides. After the
completion of the Suzuki reaction of phenylboronic acid with
iodobenzene, the PdÀFe O heterodimer nanocrystals could be
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nanocrystals by controlled one-pot thermal decomposition of
[
15]
metal acetylacetonate mixtures (Figure 1). The nascent nano-
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easily separated from the reaction mixture using an external
magnet, and the recovered nanocrystal catalyst could be recy-
cled for >10 times without any significant loss of the catalytic
activity. Successful Wacker oxidation was also carried out
through the iterative use of the PdÀFe O nanocrystal cata-
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[
19]
lyst. The PdÀFe O nanocrystal catalyst recovered using an
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external magnet retained catalytic activity for four times. At
the end of the recycling runs, the transmission electron micros-
copy (TEM) images, powder X-ray diffraction (XRD) patterns,
and inductively coupled plasma–atomic emission spectra (ICP–
AES) of the PdÀFe O heterodimer nanocrystals were com-
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pared to those before the recycling. No significant morphologi-
cal changes were observed compared to the morphology of
the original catalyst. However, these morphological results
cannot be considered as significantly contributing to the un-
derstanding of the mechanism of PdÀFe O nanocrystal cataly-
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Figure 1. TEM image of PdÀFe O heterodimeric nanocrystals.
crystals were easily recovered using an external magnet with-
out filtration or centrifugation. Using the magnetic heterodi-
meric nanoparticles, we reported several catalytic applications
of PdÀFe O heterodimeric nanocrystals, for example, Suzuki
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sis. Furthermore, X-ray photoelectron spectra (XPS) of fresh
and spent PdÀFe O samples were taken to examine any
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change in the oxidation state of the Pd metal. The oxidation
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[
15]
0
cross-coupling reactions, Heck reactions, Sonogashira reac-
state of the fresh catalyst was confirmed to be Pd (Figure S1,
[
16]
[17]
[18]
tions, direct catalytic CÀH arylation, polycondensation,
Supporting Information). After the Suzuki reaction was com-
plete, however, the XPS data indicated the presence of some
[19]
Wacker oxidation, and Wacker-type triple bond oxidation
II
0
(
Scheme 1). In almost all these reactions, the PdÀFe O hetero-
Pd as well as Pd species (Figure S2, Supporting Information).
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dimeric nanocrystals exhibited consistently effective catalytic
activities, even in the absence of ligands. This efficient and re-
usable nanocrystal catalyst system paves a way to green
chemistry, with great potential for industrial applications.
While studying various reactions using magnetic PdÀFe O
Kinetic Experiments
To gain insight into the mechanism of the reaction, kinetics of
the Suzuki reaction and Wacker oxidation were studied using
PdÀFe O . Kinetic studies are very valuable for checking the
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nanoparticles, we were intrigued by the true catalytic species
involved in the so-called “heterogeneous” reactions. Are these
reactions truly heterogeneous? Or is there a small amount of
soluble Pd species released in each recycling experiment that
is responsible for the iterative reactions? To answer these ques-
tions, detailed mechanistic investigations were carried out
using previously reported experimental methods for elucidat-
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presence of a precatalyst, which is characterized by a typical
induction period in the catalytic reaction. The reaction rates of
the PdÀFe O nanocrystal-catalyzed Suzuki coupling and
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Wacker oxidation reactions showed sigmoidal kinetic patterns
in both of the reactions. The substrate and product distribu-
tions were measured every hour by gas chromatography (GC).
As shown in Figure 2, a clear induction period (1 and 2 h for
Suzuki and Wacker oxidation reactions, respectively) was con-
firmed when the PdÀFe O nanocrystals were used. This obser-
[20]
ing the mechanism of heterogeneous catalytic reactions. Use
of Pd/C as a heterogeneous catalyst system that leaches out
soluble Pd species has been well documented; however, it has
been difficult to reuse the Pd/C system as a recyclable cata-
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vation strongly suggests that the PdÀFe O catalysis needs
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[
21]
lyst. Here, we report our findings on the mechanism of the
some time to generate an “active” catalytic species in the reac-
[22]
PdÀFe O nanoparticle-catalyzed Suzuki coupling reactions,
tion mixture as a presumably homogeneous Pd species. The
soluble Pd species may dissolve out from the nanocrystals at
high reaction temperatures.
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based on experiments such as kinetics studies, hot filtration
tests, and three-phase tests.
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Chem. Asian J. 2015, 00, 0 – 0
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Byun, Sangmoon
