CL-140815
Received: August 30, 2014 | Accepted: September 29, 2014 | Web Released: October 3, 2014
Simple Homopolymer-incarcerated Gold Nanoclusters Prepared by Self-assembled Encapsulation
with Aluminum Reagents as Crosslinkers: Catalysts for Aerobic Oxidation Reactions
Tomohiro Yasukawa, Hiroyuki Miyamura, and Shū Kobayashi*
Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033
(
E-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp)
Simple homopolymer-incarcerated gold nanocluster cata-
lysts were developed using a self-assembled encapsulation
strategy. In this method, Red-Al acting as a reductant also
a) previous work
μ
x
y
z
played the role of an inter-crosslinker via the formation of
tetraalkoxyaluminate with the hydroxy groups in a homo-
polymer. Gold nanoclusters could be immobilized at high
loadings without aggregation, and high catalytic activities were
observed in several aerobic oxidation reactions.
heat
OH
4
x:y:z = 1:1:1
O
O
O
OH
internal crosslinker
HO
Na
O
b) this work
Heterogeneous metal nanoclusters as catalysts have attracted
much attention because of their robustness and unique activity.
Red-Al®
Aluminate as
inter crosslinker"
OR
OR
+
1
HO
RO
Al
O
Al
n
Al
HO
Na
O
O
In particular, gold nanoclusters have been shown to possess
remarkable catalytic activities for oxidation reactions and have
OR
"
O
RO
RO
Na
O
O
Al
O
RO
2
OR
Na
been widely studied. Our group has developed polystyrene-
based copolymers with crosslinking moieties, which incarcerate
gold nanocluster catalysts via microencapsulation and cross-
HO
OH
HO
OH
3
linking. In this method, nanoclusters are formed by the
Scheme 1. Polymers with crosslinkers.
reduction of gold salts with NaBH4 in a solution of the polymer,
and they can be stabilized by weak, but multiple, π interactions
between the benzene rings of the polymer and the nanocluster
surface. Addition of a poor solvent for the polymer to the
mixture precipitates the microencapsulated catalyst. Precipitates
containing gold nanoclusters prepared in this way are heated
under neat conditions to afford a solid catalyst: polymer-
incarcerated Au (PI-Au). Using PI-Au, we have demonstrated
various aerobic oxidation reactions.4 We then improved this
technique by introducing spherical carbon black with high
specific surface area as a second support to expand the surface
AuClPPh3
then Red-Al®
rt, THF
Et O
No solv.
n
2
filtration
150 °C, 4 h
No solv.
PIAL Au
wash (DCM, H O, THF)
170 °C, 5 h
2
OH
Scheme 2. Preparation of PIAL-Au catalyst.
addition of 1,3-dicarbonyl compounds to allylic alcohols using
5
area of the catalyst (PI/CB-M). Gold nanoclusters were highly
6
c
dispersed over the polymer matrix that was stabilized on the
surface of the carbon black and were immobilized at high
this catalyst. In this report, we found that tetraalkoxyborates,
which are catalysts for Michael addition reactions, formed from
the NaBH4 and hydroxy groups in the polymer backbone.
Although the borates could be flushed out by water, these results
implied the possibility of crosslinking from the formation of
metal alkoxides between reductants and polymers bearing
hydroxy groups (Scheme 1b). For example, it is known that
aluminum tetraalkoxide can be synthesized from alcoholysis
¹
1
loadings (ca. 0.28 mmol g ) without loss of catalytic activity.
In contrast, PI-Au could not maintain high catalytic activity with
3
a
such a high loading because of aggregation of the nanoclusters.
Using this method, we successfully demonstrated various
6
interesting metal nanocluster-catalyzed reactions. None of the
catalysts caused any significant metal leaching, and the catalysts
were easily recovered and reused.
7
of aluminum hydrides. Herein, we report a simple and readily
Although we have previously demonstrated a wide range
of applications of our method, the preparation of copolymers
from synthesized monomers is required. In our current strategy,
both epoxy and hydroxy groups as internal crosslinkers were
introduced into polymers to afford solid solvent-tolerant cata-
lysts through an attack of hydroxy groups to open epoxy rings to
form crosslinkages (Scheme 1a). Simplifying the structure of the
polymer, an alternative approach to using an internal cross-
linking strategy, such as an external addition of crosslinkers,
may be beneficial. Recently, we achieved coimmobilization of
a AuPd bimetallic catalyst and a boron catalyst using our PI
method and performed sequential aerobic oxidationMichael
prepared homopolymer-incarcerated gold nanocluster catalyst
using self-assembled encapsulation and an aluminum reagent as
a reductant and an inter-crosslinking agent.
We chose a (4-vinylphenyl)methanol-derived homopolymer
as the support and sodium bis(2-methoxyethoxy)aluminum
μ
hydride (Red-Al ) that has two roles: as a reductant to generate
a metal nanocluster from a metal salt and as an inter-crosslinking
reagent to form aluminates from the hydroxy groups in the
polymer (Scheme 2). A reddish purple solid appeared immedi-
μ
ately on addition of Red-Al to a THF solution of the polymer
and a gold salt. This indicated that reduction to form nano-
clusters occurred immediately and that crosslinking occurred
© 2015 The Chemical Society of Japan