Water- and organo-dispersible gold nanoparticles supported by using ammonium salts of hyperbranched polystyrene: Preparation and catalysis

Article abstract of DOI:10.1002/asia.201300871

Gold nanoparticles (1 nm in size) stabilized by ammonium salts of hyperbranched polystyrene are prepared. Selection of the R groups provides access to both water- and organo-dispersible gold nanoparticles. The resulting gold nanoparticles are subjected to studies on catalysis in solution, which include reduction of 4-nitrophenol with sodium borohydride, aerobic oxidation of alcohols, and homocoupling of phenylboronic acid. In the reduction of 4-nitrophenol, the catalytic activity is clearly dependent on the size of the gold nanoparticles. For the aerobic oxidation of alcohols, two types of biphasic oxidation are achieved: one is the catalyst dispersing in the aqueous phase, whereas the other is in the organic phase. The catalysts are reusable more than four times without loss of the catalytic activity. Selective synthesis of biphenyl is achieved by the homocoupling of phenylboronic acid catalyzed by organo-dispersible gold nanoparticles. Copyright

Full text of DOI:10.1002/asia.201300871

FULL PAPER
DOI: 10.1002/asia.201300871
Water- and Organo-Dispersible Gold Nanoparticles Supported by Using
Ammonium Salts of Hyperbranched Polystyrene: Preparation and Catalysis
[
a]
[a]
[b]
[b]
Lei Gao, Takashi Nishikata, Keisuke Kojima, Katsumi Chikama, and
[
a]
Hideo Nagashima*
Abstract: Gold nanoparticles (1 nm in
size) stabilized by ammonium salts of
hyperbranched polystyrene are pre-
pared. Selection of the R groups pro-
vides access to both water- and organo-
dispersible gold nanoparticles. The re-
sulting gold nanoparticles are subjected
to studies on catalysis in solution,
which include reduction of 4-nitrophe-
nol with sodium borohydride, aerobic
oxidation of alcohols, and homocou-
pling of phenylboronic acid. In the re-
duction of 4-nitrophenol, the catalytic
activity is clearly dependent on the size
of the gold nanoparticles. For the aero-
bic oxidation of alcohols, two types of
biphasic oxidation are achieved: one is
the catalyst dispersing in the aqueous
phase, whereas the other is in the or-
ganic phase. The catalysts are reusable
more than four times without loss of
the catalytic activity. Selective synthesis
of biphenyl is achieved by the homo-
coupling of phenylboronic acid cata-
lyzed by organo-dispersible gold nano-
particles.
Keywords: biphasic
gold homogeneous catalysis
nanoparticles · polymers
catalysis
·
·
·
Introduction
Gold nanoparticles (Au NPs) have recently received much
attention from scientists in a broad range of fields owing to
[
1,2]
their unique properties,
tions of Au NPs to electronic and optical devices, sensor-
which suggest possible applica-
[1]
[
3a,b]
[3c–g]
s,
and medicines.
Among them, catalysis has been
one of the topics of nanogold chemistry since the discovery
by Haruta et al. of supported gold nanoclusters as a hetero-
Scheme 1. Preparation of water- and organo-dispersible Au NPs stabi-
+
ꢀ
3
lized by ammonium salts of hyperbranched polystyrene (HPS–NR Cl ).
THF=tetrahydrofuran.
[4]
geneous catalyst. It has long been recognized that bulk
gold is catalytically inactive; however, Haruta and co-work-
ers clearly demonstrated that a nanosized gold cluster be-
haved as a catalyst for CO oxidation. This is a starting point
for research on Au NP catalyzed reactions, and the number
of papers reporting related issues has increased rapidly, as
they tend to react with each other to cause a rapid increase
in the particle size. The sintering of NPs is particularly seri-
ous when studying the effect of the size of the Au NPs on
[4c]
catalysis.
other organosulfur compounds are frequently used as a sta-
To avoid the sintering of Au NPs, thiols and
[4c–g,5]
summarized in several reviews.
Although the discovery
[7]
by Haruta et al. involved active gold clusters for heterogene-
ous catalysis, research has now expanded to homogeneous
catalysis in aqueous and organic solution by preparing dis-
bilizer. However, these stabilizers are bound so tightly to
the surface of Au NPs that they often kill the catalytic activ-
ity. Dendrimers are another class of well-investigated stabil-
[5e,6]
[8]
persible Au NPs in these phases (Scheme 1).
izers for metal NPs, including Au NPs, in which metal NPs
In general, smaller metal particles have more reactive sur-
faces. Even if nanometer-sized metal particles are prepared,
are usually considered to be encapsulated in the inner space
of the dendrimers. Excellent properties of Au NPs stabilized
by dendrimers, which include applications to homogeneous
catalysis, have been demonstrated; however, it is a drawback
that tedious multistep procedures are necessary for prepara-
tion of the dendrimers.
[
a] Dr. L. Gao, Dr. T. Nishikata, Prof. Dr. H. Nagashima
Institute for Materials Chemistry and Engineering
Kyushu University, and CREST, JST
Kasuga, Fukuoka 816-8580 (Japan)
E-mail: nagasima@cm.kyushu-u.ac.jp
As stabilizers for catalytically active Au NPs, easily acces-
sible synthetic polymers of certain architecture have attract-
[
b] Dr. K. Kojima, Dr. K. Chikama
Chemical Research Laboratories
Nissan Chemical Industry, Ltd.,
Funabashi-shi, Chiba 274-8507 (Japan)
[9]
ed the attention of scientists. There are examples of Au
[
10]
[11]
NPs stabilized by homopolymers,
and hyperbranched polymers.
block copolymers,
[12]
These polymer stabilizers
are now more common than those of small organic mole-
cules. The most well-investigated polymer support for Au
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201300871.
Chem. Asian J. 2013, 8, 3152 – 3163
3152
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Hideo Nagashima et al.
[
13]
[10c,d]
NPs is polyvinylpyrrolidone (PVP) and its derivatives,
in which lactams in the PVP support are coordinated to the
surface of Au NPs. The groups of Tsukuda and Sakurai have
reported that careful reduction of gold precursors in the
presence of PVP led to Au NPs with uniform particle
water- and organo-dispersible Au NPs; this has not been
achieved if PVP is used as the support. In addition, electro-
+
ꢀ
static interactions of Au NPs with HPS–NR3 Cl may be
not as strong as those of Au NPs with thiols. Organic ammo-
ꢀ
ꢀ
nium salts with Br , Cl , and other counteranions are a rela-
[6,13]
[16]
sizes.
The Au NPs with several particle sizes were selec-
tively weak stabilizer for Au NPs, and are generally used
tively synthesized, and the effect of the size of the Au NPs
was demonstrated toward homogeneous catalytic aerobic
oxidation of alcohols, in which Au NPs about 1 nm in size
as a phase-transfer reagent for the preparation of robust
[17]
thiol-stabilized Au NPs in the Brust–Schiffrin method.
Herein, we wish to report that our challenge to synthesize
[13b,c]
+
ꢀ
showed the highest catalytic activity.
The advantage of
Au@HPS–NR3 Cl attained success through the reduction
+
ꢀ
PVP as the stabilizer for Au NPs is clear when considering
the size-selective preparation of Au NPs. In contrast, the
synthesis of Au NPs around 1 nm in size has not been ach-
ieved with other polymer supports, except dendrimers. Nev-
ertheless, PVP is not the best stabilizer for research into Au
NPs owing to the limited solubility of PVP in organic sol-
vents. In fact, Au NPs stabilized by PVP are generally used
as catalysts in aqueous media.
of a gold precursor in the presence of HPS–NnBu Cl or
3
+
ꢀ
+
ꢀ
HPS–NnOct3 Cl . Both of the Au@HPS–NR Cl synthe-
3
sized are thermally stable and can be stored for months in
the solid state without sintering of the Au NPs. Further stud-
+
ꢀ
ies on the preparation of Au@HPS–NR3 Cl and its appli-
cation to catalysis provided solutions for the following three
problems, which have frequently been seen in research into
the synthesis and catalysis of Au NPs. The first is the prepa-
ration of both water- and organo-dispersible Au NPs, as ach-
Recently, we have reported that several hyperbranched
+
ꢀ
+
ꢀ
+
ꢀ
polystyrenes with ammonium salts (HPS–NR3 Cl ) as func-
ieved by HPS–NnBu3 Cl and HPS–NnOct3 Cl , respec-
tively. The second is in the size control of Au NPs, which
has been accomplished by simply changing the reaction tem-
perature: well-dispersed, uniform, and size-controlled Au
NPs (1 nm in size) were successfully achieved by using
[
14a]
tional groups were easily prepared on a large scale.
A
+
ꢀ
characteristic feature of HPS–NR3 Cl is the existence of
large numbers of quaternary ammonium groups. It is known
that certain tetraalkyl ammonium salts are useful molecular
supports for transition-metal NPs, especially for platinum
+
ꢀ
+
ꢀ
either HPS–NnBu3 Cl or HPS–NnOct Cl as the stabiliz-
3
[15]
(
Pt) and palladium (Pd) NPs. Electrostatic and steric ef-
er. As described above, Au NPs around 1 nm showed the
best catalytic activity in Au@PVP. The small particle size of
fects of the ammonium groups contribute to the stabilization
+
ꢀ
+
ꢀ
of the metal NPs. As expected, HPS–NR3 Cl is a good
polymer support for Pt and Pd NPs of 2–3 nm in size with
Au NPs in Au@HPS–NR3 Cl actually solved the third
+
ꢀ
problem: good catalytic performance. Au@HPS–NR3 Cl of
around 1 nm shows high efficiency in the reduction of 4-ni-
+
ꢀ
narrow size distributions, and the Pd or Pt@HPS–NR3 Cl
species formed are good reusable catalysts for organic reac-
tions. It is important that both water- and organo-dispersible
Pd or Pt@HPS–NR3 Cl can be prepared through the judi-
trophenol by NaBH . In the biphasic aerobic oxidation of al-
4
+
ꢀ
cohols, Au@HPS–NR3 Cl can be recycled and reused. In
the homocoupling of phenylboronic acid, a high selectivity
of biphenyl formation is achieved through catalysis by
+
ꢀ
[
14]
cious choice of the R groups on the ammonium moieties.
+
ꢀ
+
ꢀ
These results stimulated us to investigate HPS–NR3 Cl as
a new polymer support for Au NPs. Studies on Pd@HPS–
NR3 Cl and Pt@HPS–NR3 Cl seemed to support the
idea that the use of an appropriate HPS–NR3 Cl as the
stabilizer for Au NPs could lead to the synthesis of both
Au@HPS–NnOct3 Cl .
+
ꢀ
+
ꢀ
+
ꢀ
Results and Discussion
+
ꢀ
Preparation and Characterization of Au@HPS–NR3 Cl
+
Ammonium salts of hyperbranched polystyrene (HPS–NR3
ꢀ
Cl ) were synthesized according to the procedure reported
Abstract in Japanese:
[14a]
+
in our previous paper.
The dispersibility of HPS–NR3
ꢀ
Cl is markedly dependent on the R groups. If R=alkyl, the
length of the alkyl chains (n) affects the dispersibility of
+
ꢀ
HPS–N
A
H
U
G
E
N
N
(C H2n+1)3 Cl ; those with nꢁ4 disperse well in
n
water, but not in hydrocarbons, whereas those with nꢂ6 dis-
perse in toluene, but not in water. Similar affinity to solvents
+
is observed for metal NPs supported by HPS–N ACHTUGNETRNN(UGN C H2n+1)3
n
ꢀ
+
ꢀ
Cl [M@N
ACTHGNUNTERNGNU( C H2n+1)3 Cl ]. It is known that NPs are stabi-
n
lized by both electrostatic and steric effects of the support.
+
ꢀ
All of HPS–N ACHTUGNETRNNUN(G C H2n+1)3 Cl are excellent for electrostatic
n
stabilization, but are not always good for steric stabilization.
In other experiments performed previously, we found that
the length of the R groups was important for the prepara-
tion of metal NPs of 1–4 nm with a uniform size; size con-
+
ꢀ
trol was difficult if we used HPS–N
A
H
U
G
E
N
N
(C H2n+1)3 Cl (nꢁ3) as
n
Chem. Asian J. 2013, 8, 3152 – 3163
3153
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Hideo Nagashima et al.
the support. If R=glycol unit, a different type of water-dis-
distribution ((1.1ꢃ0.2) nm) among the three samples. As
the temperature increased, Au NPs with larger particle sizes
were formed. The particle sizes of the Au NPs in 2a-2 and
2a-3 are (1.9ꢃ0.6) and (4.4ꢃ1.1) nm, respectively.
persible ammonium salts is available; however, the resulting
+
ꢀ
M@HPS–N
ACHTUNGTRENNUNG[ (CH CH O) Me] Cl is a viscous material at
2 2 2 3
room temperature; this gave us difficulties in handling. In
+
ꢀ
this context, we select two types of HPS–NR3 Cl with dif-
ferent R groups: n-butyl (water dispersible) and n-octyl (dis-
persible in nonpolar organic solvents) derivatives, HPS–
The UV/Vis spectra of these samples were also recorded
(Figure 2). For 2a-1 and 2a-2, no visible surface plasmon
can be observed. In contrast, a small peak is seen owing to
a typical broad surface plasmon signal between 500 and
+
ꢀ
+
ꢀ
NnBu3 Cl (1a) and HPS–NnOct3 Cl (1b), respectively,
which were prepared by treatment of HPSꢀCl with commer-
cially available NnBu and NnOct in good yields.
Water-dispersible Au NPs stabilized by HPS–NnBu3 Cl
were prepared with ease by the reduction of HAuCl ·4H O
3
3
+
ꢀ
4
2
+
ꢀ
with NaBH in water in the presence of HPS–NnBu Cl .
4
3
+
ꢀ
The resulting Au@HPS–NnBu3 Cl (2a) was isolated as
a dark brown solid after the removal of water. The particle
size of the Au NPs was tunable by changing the reaction
temperature. Reactions at 0, 25, and 508C gave three sam-
ples of Au@HPS–NnBu3 Cl (2a), 2a-1, 2a-2, and 2a-3, re-
spectively. All of 2a-1, 2a-2, and 2a-3 are dispersible in
+
ꢀ
water, methanol, and CHCl , whereas they are immiscible
3
with hydrocarbons. TEM images and the histograms are
shown in Figure 1. It can be seen that 2a-1 contained Au
NPs with the smallest particle size and the narrowest size
2
Figure 2. UV/Vis spectra of 2a-1, 2a-2, and 2a-3 (in H O, [Au]=0.3 mm).
6
00 nm for 2a-3; this suggests the existence of Au NPs with
particle sizes larger than 2 nm in 2a-3, which is consistent
with the TEM images. Powder XRD measurements of these
three samples show a broad peak around 2q=408, which
corresponds to the Au ACHUTNGRNENGU( 111) planes and an amorphous signal
of the hyperbranched polymer at 2q=258 (see Figure S6 in
the Supporting Information). It is known that sharp signals
corresponding to diffraction peaks of face-centered cubic
(
fcc) gold nanocrystals are observed for XRD results for
[18]
samples including relatively large Au NPs (>5 nm). The
broad signals of 2a are similar to those seen in Au NPs with
a size distribution of <2 nm in water-dispersible PVP, as re-
[13f]
ported by Tsukuda and co-workers;
this may be qualita-
tive support for 2a, in particular, for 2a-1, which contains
only Au NPs of <2 nm based on TEM analysis. In fact,
compound 2a-1 has the broadest Au ACHUTNGRNENUG( 111) peak in XRD;
this suggests that the particle size is the smallest of the three
samples.
When HAuCl ·4H O was reduced by NaBH in the pres-
4
2
4
+
ꢀ
ence of HPS–NnOct Cl , organo-dispersible Au NPs
Au@HPS–NnOct3 Cl , 2b) were prepared in good yields.
3
ꢀ
+
(
In this case, a mixed solvent of THF and water was a key
factor to obtain well-dispersed Au NPs. Optimization of the
reaction conditions showed that aqueous THF (THF/H O=
2
4
/1 (v/v)) gave the best results. A certain amount of water
was necessary for the reduction of the gold precursor by
NaBH , whereas THF was useful for dispersion of support
4
1
b. The resulting 2b was heterogeneous in water, but was
dispersible in organic solvents, such as THF, toluene, and
CHCl . Similar to water-dispersible 2a, compound 2b, which
3
contained Au NPs with different particle sizes, was prepared
by changing the temperature. Three samples were synthe-
sized in good yields at 0 (2b-1), 25 (2b-2), and 508C (2b-3).
The particle sizes obtained from TEM (Figure 3) analysis
Figure 1. TEM images and particle size distributions of A) 2a-1, B) 2a-2,
and C) 2a-3.
Chem. Asian J. 2013, 8, 3152 – 3163
3154
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Hideo Nagashima et al.
[2]
NaBH ) are those generally reported in the literature. It is
4
particularly important that Au NPs of around 1 nm size are
easily obtained through reaction at 08C with organo- and
+
ꢀ
water-dispersible HPS–NR3 Cl . In all cases, except for
PVP- and dendrimer-stabilized Au NPs, the reported Au
NPs supported by synthetic polymers have relatively broad
size distributions (from several nm to 100 nm) and the selec-
tive preparation of Au NPs of 1 nm in size has been not ach-
ieved. In particular, few of the organo-dispersible polymer
supports, except dendrimers, achieved the preparation of Au
NPs with a size of 1 nm. As mentioned above, dendrimers
require tedious synthetic procedures, which limit their appli-
cations.
Further discussion of the size control of Au NPs, in partic-
ular, the selective preparation of Au NPs of around 1 nm in
size by polymer supports is interesting for the comparison of
+
ꢀ
HPS–NR3 Cl with PVP. Tsukuda and co-workers reported
[13]
the size control of Au NPs stabilized by PVP in detail.
They synthesized Au NPs of around 1 nm through the re-
duction of HAuCl /PVP with NaBH by using either a batch
4
4
[13a]
[13e]
reactor
or a micromixer
to keep the temperature at
0
8C. Au NPs with larger particle sizes (2–10 nm) were pre-
[13c]
pared by a seed-mediated growth method.
Relatively
large amounts of PVP are needed for good stabilization of
Au NPs. For example, the method by Tsukuda and co-work-
ers required a ratio of [VP]/[Au]=100 (VP is a monomer
[13a]
unit of PVP) for typical reactions in a flask.
The use of
a smaller amount of the polymer support led to poor size
control of Au NPs. The microfluid reaction improved this;
however, a ratio of [VP]/[Au]=40 was necessary to achieve
[
13e]
Figure 3. TEM images and particle size distributions of A) 2b-1, B) 2b-2,
and C) 2b-3.
good control of the particle size.
correspond to weight percentages of Au NPs in PVP of only
.8 and 4.4 wt% for the batch and microfluid cases, respec-
The [VP]/[Au] values
1
+
ꢀ
tively. If HPS–NR3 Cl is used as the support, size control
of Au NPs is achieved by relatively small amounts of the
support. In a typical example, compound 1a keeps the Au
NPs monodisperse, if the Au NP loading in 2a increases to
9.7 wt%, which corresponds to a ratio of [monomer unit of
HPS–NR3 Cl ]/[Au]=6. This clearly demonstrates that this
HPS is an effective stabilizer for Au NPs. We searched the
literature for reports on other polymer stabilizers of Au
NPs, and found that larger amounts of the polymer supports
(Au%<5 wt%) were usually needed for effective stabiliza-
tion of Au NPs.
As described above, the synthesis of Au@HPS–NR3 Cl
with different particle sizes is possible by changing the reac-
tion temperature. This is reasonably explained by the differ-
ence in rate between nucleation and growth of Au NPs in
the reduction process. The initial step of NaBH -promoted
reduction of HAuCl ·4H O is the formation of unstable Au
species in the reaction medium, which aggregate to form
semistable small gold clusters. The gold clusters behave as
seed nuclei for the growth of gold particles; the particle size
of the gold clusters is increased through reaction with unsta-
ble Au species or their aggregation. If we assume that the
formation of seed nuclei and particle growth are kinetically
independent, the reason why small (1 nm size) gold particles
were (1.2ꢃ0.2), (2.3ꢃ0.8), and (3.2ꢃ1.2) nm for 2b-1, 2b-
2
, and 2b-3, respectively. Some aggregates with random
shapes are observed in the TEM images of 2b-3 (Fig-
ure 3C). UV/Vis spectra of these samples show no surface
plasmon band in 2b-1 and 2b-2, whereas it is visible in 2b-3
+
ꢀ
(
Figure S5 in the Supporting Information). The powder
XRD spectra are somewhat different from those of water-
dispersible 2a; the signals of 2b-1 and 2b-2 are broad, simi-
lar to those of 2a-1 and 2a-2, whereas peaks corresponding
[10–12]
to the Au
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(111), Au
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(200), and Au
A
H
U
G
R
N
U
G
+
ꢀ
visible for 2b-3 (Figure S6 in the Supporting Information).
The plasmon signal in the UV/Vis spectrum and observation
of sharp peaks in the XRD spectrum suggest that there are
relatively large Au NPs, of which the particle size is >2 nm
in 2b-3.
4
0
The present results clearly demonstrate the advantages of
4
2
+
ꢀ
HPS–NR3 Cl as the support for Au NPs over conventional
polymer supports. The first advantage is the possible prepa-
ration of both organo- and water-dispersible Au NPs
+
ꢀ
through the appropriate choice of HPS–NR3 Cl . The
second advantage is facile control of the particle size by
simply setting an appropriate reaction temperature, al-
though the preparative procedures of Au NPs (HAuCl4/
0
Chem. Asian J. 2013, 8, 3152 – 3163
3155
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Hideo Nagashima et al.
are selectively formed at lower
temperatures can be ascribed to
faster formation of the seed
nuclei than that of particle
growth. At low temperatures,
a large number of small gold
clusters are formed as seeds for
Au NPs and the resulting gold
clusters slowly increase the par-
ticle size. At higher tempera-
tures, particle growth is compet-
itive with the formation of seed
nuclei and the generation of
smaller numbers of seed nuclei
is followed by faster growth of
the particle size. Easy control
of Au NPs of different sizes al-
lowed studies on the rate of the
catalytic reactions with respect
to particle size, as discussed in
the next section.
Figure 4. Time-dependent UV/Vis spectra of the reaction mixture for the reduction of 4-nitrophenol with
NaBH catalyzed by 2a-2 (A) and 2b-2 (C). Plot of ln(A /A ) versus time for the reduction of 4-nitrophenol
catalyzed by 2a (B) and 2b (D).
Catalytic Reduction of 4-
Nitrophenol with NaBH4
4
A
H
U
G
E
N
N
t
0
The reduction of 4-nitrophenol
to 4-aminophenol by NaBH is
a model reaction for metal NPs, especially for Au NPs.
4
[19]
0.3) nm) under the same conditions. The k value of (4.1ꢃ
0.2)ꢁ10 for 2a-1 is much higher than that for
ꢀ3
ꢀ1
For water-dispersible 2a-1, 2a-2, and 2a-3, reactions were
carried out in a UV cell and UV/Vis spectra recorded in situ
at fixed intervals directly provided the relative rate among
the reactions (see the Experimental Section for details). A
typical change in UV/Vis spectra of the reactions catalyzed
by 2a-2 is shown in Figure 4A; the characteristic peak at
l=400 nm gradually decreases, whereas the peak at l=
s
ꢀ
3
ꢀ1
Au@PVP, k=(1.6ꢃ0.1)ꢁ10
s
(Figure S8 in the Support-
ing Information). Although a comparison of the catalytic ac-
tivity of Au NPs dispersed on several polymer supports was
[20a]
reported in the literature,
it was difficult to perform
quantitative analysis from the table of k because each ex-
periment was carried out by different research groups under
nonuniform reaction conditions. A qualitative comparison
of catalytic activity by using the turnover frequency (TOF)
based on the number of moles of gold charged would be
300 nm increases. This indicates the consumption of 4-nitro-
phenol and the formation of 4-aminophenol, respectively.
By using the absorption intensity at l=400 nm, a linear cor-
[
20b,c]
relation between ln
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(A /A ) and the reaction time at 258C is
more reasonable, as suggested by Kitaoka et al.
The
t
0
obtained, and displays a pseudo-first order, as shown in Fig-
TOF of the reactions with 2a-1, 2a-2, and 2a-3 described
above are estimated to be 185, 111, and 55 h , respectively.
ꢀ
1
ure 4B, in which A and A correspond to the absorbance at
t
0
different times and at the initial stage, respectively. From
the slope of the plot of ln(A /A ) versus time shown in Fig-
ure 4B, the reaction rate constants, k, were calculated to be
Because NaBH reduction of 4-nitrophenol is likely to take
4
A
H
U
G
R
N
U
G
place on the surface of the Au NPs, it is worthwhile to calcu-
late TOF normalized by the surface gold atoms
(TOFnormalized), according to the method reported by Tsukuda
t
0
ꢀ
3
ꢀ3
ꢀ3 ꢀ1
(
4.1ꢃ0.2)ꢁ10 , (3.3ꢃ0.4)ꢁ10 , and (1.2ꢃ0.1)ꢁ10
for 2a-1, 2a-2, and 2a-3, respectively. Because these reac-
tions have the same ratios of [Au]/[4-nitrophenol]/[NaBH ],
s
[13f]
and co-workers.
To simplify the calculation, it was as-
A
H
U
G
E
N
N
sumed that Au NPs were spherical in shape with a diameter
determined by TEM (dTEM), and TOFnormalized was calculated
by using Equation (1):
4
the results suggest that reaction rate is dependent on the
sizes of the particle. Upon decreasing the particle size of the
Au NPs, the reaction rate increases. As demonstrated above,
the order of the particle size is 2a-1<2a-2<2a-3, and the
reaction rate order is 2a-1>2a-2>2a-3. This is reasonable
upon considering that the rate of the surface reaction is gen-
erally governed by the surface area: smaller Au NPs have
a larger surface area than that of larger Au NPs.
3
TOFnormalized ¼ TOFðN
surface
total
Þꢀ ¼ TOF
ðdTEM
Þ
1
3
3
N
ðdTEMÞ ꢀ ðd
ꢀ0:576Þ
ð1Þ
TEM
It is of interest that the catalytic activity of 2a-1 (size=
in which Nsurface and Ntotal are numbers of gold atoms on the
(
1.1ꢃ0.2) nm) is higher than that of Au@PVP (size=(1.3ꢃ
surface and that in total, respectively, and the distance be-
Chem. Asian J. 2013, 8, 3152 – 3163
3156
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Hideo Nagashima et al.
tween gold atoms in the NPs is assumed to be 0.288 nm. The
of Au@PVP ((1.3ꢃ0.3) nm) reported by Tsukuda and co-
workers. For comparison, we prepared a sample of
[13b]
TOFnormalized values for 2a-1, 2a-2, and 2a-3 are calculated to
ꢀ
1
be 208, 167, and 160 h , respectively. Although the assump-
tion that the Au NPs are spherical and all of the gold atoms
on the surface are the catalytically active sites may be too
rough to discuss the catalytic activity, it is important that the
trend of TOFnormalized is 2a-1>2a-2>2a-3, which is similar
to that of TOF. These results indicate that smaller NPs show
higher catalytic activity. To the best of our knowledge, the
result achieved by 2a-1 is the highest among conventional
polymer-supported Au NPs for the reduction of 4-nitrophe-
Au@PVP, in which [VP]/[Au]=100/1, according to the re-
ported method, which was subjected to a study of the cata-
lytic oxidation of benzyl alcohol at 508C under air in the
presence of K CO . All benzyl alcohol was consumed after
2
3
6 h to give benzoic acid. The catalytic activity of Au@HPS–
+
ꢀ
NnBu3 Cl was dependent on the ratio of [monomer unit of
+
ꢀ
HPS–NnBu3 Cl ]/[Au]. Under the same reaction conditions
as those of the experiment with Au@PVP, 2a-1, in which
+
ꢀ
[monomer unit of HPS–NnBu3 Cl ]/[Au]=6/1, showed cat-
alytic activity; the conversion of benzyl alcohol was 58%
after 6 h and 100% after 24 h. In contrast, no reaction took
[20]
nol in homogeneous systems.
Different from 2a, compound 2b is not dispersible in
water. Although the reaction was sluggish, the reduction of
+
place with the sample of [monomer unit of HPS–NnBu3
ꢀ
4
-nitrophenol by NaBH occurred in a biphasic system of
Cl ]/[Au]=100/1. The results suggest that Au@HPS–
NnBu3 Cl , 1 nm in size, showed catalytic activity; however,
4
+
ꢀ
H O/CHCl , as reported in another organo-dispersible Au
2
3
[21]
NPs system. Because it was not possible to record UV/Vis
spectra in situ, we followed the method reported in the liter-
the reaction is disturbed when a large excess of HPS–
+
ꢀ
[23]
NnBu3 Cl is present in the reaction medium.
Further studies on the catalytic performance of
[21]
ature,
in which samples were taken from the aqueous
+
ꢀ
layer at fixed intervals, diluted, and subjected to UV/Vis
measurements (details are given in the Experimental Sec-
tion). Compared with the in situ measurement used for the
reaction in water with 2a, the experiments contained large
experimental error. Figure 4C shows the typical time-depen-
dent UV/Vis spectra of the reaction catalyzed by 2b-2 and
Au@HPS–NnBu3 Cl were performed in a biphasic system,
in which the Au NPs catalyst was in the aqueous phase and
the substrate was in the organic phase. Optimization of the
reaction showed that benzotrifluoride (BTF) was a better
solvent than toluene and base was needed to promote the
oxidation, otherwise only trace amount of product was ob-
served. The combination of a strong base, KOH, and pure
Figure 4D shows the plot of ln
ACHTUNGENTRNUNG( A /A ) versus time for 2b-1,
t 0
2
b-2, and 2b-3. The correlation between ln
A
H
U
G
R
N
N
(A /A ) and the
O as the oxidant gave a better catalytic activity than that of
2
t
0
reaction time is in a good linear relationship during the
early stage then the reaction become slower, which might be
weak base, K CO , and air. The biphasic system that gave
2
3
the best results we have examined so far was as follows:
[21]
+
ꢀ
due to the consumption of NaBH4.
From the slopes, k
Au@HPS–NnBu3 Cl dispersed in the aqueous phase
(2 atom% to alcohol) and substrate dissolved in BTF. The
reaction was carried out in the presence of KOH (3 equiv)
ꢀ
3
values are calculated to be (2.5ꢃ0.3)ꢁ10 , (1.4ꢃ0.2)ꢁ
ꢀ
3
ꢀ4 ꢀ1
1
2
2
2
0 , and (9.7ꢃ0.4)ꢁ10
s
for the reaction catalyzed by
b-1, 2b-2, and 2b-3, respectively. For the reactions of 2b-1,
b-2, and 2b-3, the TOF values are estimated to be 369,
46, and 148 h , whereas TOF
under 1 atm of O supplied from a balloon at 30 or 508C.
2
BTF is known to be a good solvent in aerobic oxidation,
ꢀ
1
[22]
values are calculated
owing to its affinity for O2.
normalized
ꢀ
1
to be 430, 425, and 330 h , respectively. Both k and TOF
values also show a trend in the size effect similar to that
seen in the water-dispersible Au NPs described above.
The results for the oxidation of several alcohols under the
conditions described above are summarized in Table 1.
1
13
H NMR, C NMR, and FTIR spectroscopy results of the
isolated products are shown in the Supporting Information.
The oxidation of 1-phenylethanol proceeded smoothly with
high conversion to form the corresponding acetophenone in
a good yield (Table 1, entry 1). Inductively coupled plasma-
mass spectrometry (ICP-MS) was used to check for the
leaching of gold in the BTF layer and confirmed that the
leaching of gold was negligible (see the Supporting Informa-
tion for details). Similarly, phenylethanols with either an
electron-withdrawing or -donating substituent on the ben-
zene ring can be selectively converted into the correspond-
ing acetophenone derivatives quantitatively (Table 1, en-
tries 2–5). As shown in entries 4 and 5 in Table 1, introduc-
tion of an electron-withdrawing substituent tended to slow
down the reaction rate. When benzyl alcohol was used as
substrate, benzoic acid was formed selectively (Table 1,
entry 6). Neither benzaldehyde nor benzyl benzoate were
detected as a byproduct. As shown in entry 7 in Table 1, 4-
methoxybenzyl alcohol was selectively converted into 4-me-
thoxybenzaldehyde without any further oxidation. The oxi-
Catalytic Aerobic Oxidation of Alcohols
Aerobic oxidation of alcohols by using O or air as the oxi-
2
dant also provides good evidence to check the catalytic per-
[6,22]
formance of Au NPs.
It is important that the rate of the
aerobic oxidation of alcohols is markedly dependent on the
particle size of Au NPs. A series of papers from Tsukuda
and co-workers, who used a homogeneous Au@PVP system
in aqueous media, demonstrated that the aerobic oxidation
of alcohols was best catalyzed by Au NPs of 1 nm in
[13b–f]
size;
in a typical example, benzyl alcohol was oxidized
to benzoic acid by Au@PVP (size=(1.3ꢃ0.3) nm) in the
presence of base (K CO or KOH) in water at 278C: TOF
2
ꢀ
3
1
ꢀ1 [13d]
reached 8.3 molatom h .
+
ꢀ
In contrast to the results that showed Au@HPS–NR3 Cl
was an efficient catalyst for the reduction of 4-nitrophenol,
compound 2a-1 (Au NPs=(1.1ꢃ0.2) nm) was somewhat
less efficient as the catalyst for alcohol oxidation than that
Chem. Asian J. 2013, 8, 3152 – 3163
3157
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Hideo Nagashima et al.
+
ꢀ
Table 1. Aerobic oxidation of various alcohols catalyzed by Au@HPS–NnBu
3
Cl .
catalyst was necessary for reuse
of the catalyst. Catalyst recy-
cling was successful for the first
five cycles: the reactions took
place without retardation of the
reaction rate and the product
was formed in quantitative
yield. The conversion started to
decrease from the sixth cycle,
as shown in Table 2.
[
a]
[b]
Entry
1
Substrate
Product
T [8C]
Conversion [%]
Yield [%]
92
30
>99
2
30
>99
>99
94
90
As described above, it is
a
characteristic feature of
+
ꢀ
Au@HPS–NR3 Cl that both
of water- and organo-dispersi-
ble Au NPs can be prepared
through the choice of the R
groups. In the experiments de-
scribed above, we used water
dispersible 2a-1 for the bipha-
sic system. This is important for
the oxidation of organo-disper-
sible reactants because the
aqueous layer containing the
catalyst species is easily sepa-
rated and can be subjected to
recycling experiments. In con-
trast, the advantage of using
organo-dispersible Au NPs for
catalyst recycling should be visi-
ble in the oxidation of water-
dispersible alcohols. As an ex-
3
4
30
30
92
>99
–
92
50
5
6
30
68
>99
–
90
50
30
50
>99
>99
94
90
7
8
[
c]
[c]
30
50
>99
>99
>99
[
d]
[c]
[c]
9
>99
1
[
a] Determined by H NMR spectroscopy. [b] Yield of product isolated. [c] Determined by GC analysis with
dibutyl ether as an internal standard. [d] 48 h.
+
ꢀ
Table 2. The recycling of Au@HPS–NnBu
3
Cl in biphasic aerobic oxi-
dation of cyclohexanol proceeded smoothly (Table 1,
entry 8), whereas that of 2-octanol required the application
of somewhat harsh conditions to complete the reaction
dation.
(
Table 1, entry 9). However, the oxidation of cinnamyl alco-
hol and 1-octanol under these conditions resulted in the re-
covery of the starting materials. Most of these results are
consistent with those obtained by using Au@PVP in water,
as reported by Tsukuda and co-workers, although the cata-
Run
Conversion [%]
1
2
3
4
5
6
7
8
9
>99
>99
>99
>99
>99
85
65
52
39
29
+
ꢀ
lytic activity of Au@HPS–NnBu Cl is somewhat low-
3
[
13b–f]
er.
The advantage of using a biphasic system is the easy sepa-
ration of the phase containing the product and reuse of the
catalyst phase. The oxidation of 1-phenylethanol was per-
formed as a typical example to check for the recyclability of
+
ꢀ
Au@HPS–NnBu3 Cl fixed in the aqueous phase. The re-
sults are shown in Table 2. For a biphasic system with BTF
as the phase containing both the reactant and the product
and water as the catalyst phase containing Au@HPS–
1
0
ample of a water-dispersible alcohol, we selected 1-(4-{2-[2-
(2-methoxyethoxy)ethoxy]ethoxy}phenyl) ethanol (3) and
carried out studies on aerobic oxidation with organo-disper-
sible 2b-1 (experimental details are shown in the Supporting
Information). Because the oxidation of 3 with the biphasic
catalyst system, in which the alcohol and KOH were dis-
solved in water, and 2b-1 was dispersible in BTF, was slow,
+
ꢀ
NnBu3 Cl , the catalyst recycle was examined 10 times.
After the first run of the reaction was carried out at 308C
for 24 h, the BTF phase was separated by decantation, from
which acetophenone was isolated quantitatively. The aque-
ous phase was subjected to further runs of oxidations under
the same reaction conditions. No activation of the recovered
Chem. Asian J. 2013, 8, 3152 – 3163
3158
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Hideo Nagashima et al.
we modified the reaction system as follows: substrate 3,
KOH, and catalyst 2b-1 (4 atom%) were placed in a flask
and BTF was added. The resulting mixture was heated
improvement in catalyst recycling by using thermosensitive
[24]
star polymers.
The recycling experiment described above shows the spe-
+
ꢀ
under O (1 atm) at 508C for 24 h. After the reaction, the
cial features of Au@HPS–NR3 Cl as a reusable catalyst in
homogeneous systems. Although the catalytic activity is
lower than that of Au@PVP, water-dispersible Au@HPS–
2
desired product and KOH were extracted with water. From
the aqueous solution, the product was isolated in good yield.
The organo-dispersible catalyst species was recovered as the
BTF phase, which was subjected to repeated experiments
without alteration. In each run, compound 3 and KOH were
added. The catalyst was reusable four times without any loss
in catalytic activity, and the product was isolated in a good
yield. After the fifth run, the catalytic activity gradually de-
creased, as shown in Table 3.
+
ꢀ
NnBu3 Cl can be applied to the biphasic system, in which
the catalyst is in the aqueous phase and the reactant and the
product are in the organic phase. Because many kinds of al-
cohols and the corresponding carbonyl products are organo-
soluble, this biphasic system is advantageous for the oxida-
tion of alcohols. Modification of the biphasic reaction condi-
tions was necessary for the oxidation of water-soluble alco-
+
ꢀ
hols by organo-dispersible Au@HPS–NnOct3 Cl ; however,
the reactions in BTF and extraction of the product and base
by water made it possible to recycle the catalyst species dis-
solved in BTF. In both cases, separation of the product from
the catalyst is easy; the recovered catalyst is reusable with-
out any catalyst reactivation process.
+
ꢀ
Table 3. The recycling of Au@HPS–NnOct
3
Cl in biphasic aerobic oxi-
dation.
Deactivation of the Au NPs catalyst in these catalyst-recy-
cling experiments was often discussed in relation to sintering
of the Au NPs. In a typical examples, the recycling of
Au@PVP resulted in some decrease in the catalytic activity,
and UV/Vis spectral analysis of recovered Au@PVP showed
peaks owing to the surface plasmon; this suggests an in-
[13d]
crease in the particle size of the Au NPs.
We investigated
Run
Conversion [%]
the UV/Vis spectra and TEM results for the recovered sam-
ples of 2a-1 (after the 10th run) and 2b-1 (after the 7th
run). In both cases, the results show an increase in the parti-
cle size of Au NPs, as shown in Figure 5 and Figure S12 in
Supporting Information. In a typical example, after 10 cycles
1
2
3
4
5
6
7
>99
>99
>99
>99
85
63
39
+
ꢀ
Au@HPS–NnBu3 Cl shows an evident surface plasmon
signal at l=529 nm in a UV/Vis spectrum (Figure 5C). This
There are several procedures for the recycling of Au NPs
catalysts in the aerobic oxidation of alcohols. Kobayashi and
co-workers reported Au NPs dispersed in cross-linked poly-
mers synthesized by their elegant polymer-incarcerated
[22]
methods. Because the resulting catalysts are insoluble in
organic solvents, the products and the catalyst are separable
by filtration. The catalytic activity was high in most cases
and the catalyst was reusable for more than five times. The
catalyst was robust, but the recovered catalyst required reac-
tivation by heating at 1508C for 5 h prior to use in another
run. The reactivation of the recovered catalyst is often
needed in metal NPs supported by insoluble solid materials.
Catalyst recycling in the oxidation of alcohols catalyzed by
Au NPs in homogeneous systems was examined by Tsukuda
and co-workers by using their size-controlled Au NPs stabi-
[13d]
lized by PVP.
The oxidation of 1-hydroxyindane was per-
formed in water and the product was extracted with ethyl
acetate after the reaction. The aqueous phase was filtered
and deionized by using a centrifugal ultrafiltration device
and subjected to another run of the catalyst. The catalyst
was reusable at least five times with a decrease in the cata-
+
ꢀ
Figure 5. TEM images of Au@HPS–NnBu
3
Cl : A) before the reaction
+
and B) after 10 catalytic cycles. C) UV/Vis spectra of Au@HPS–NnBu
Cl before the reaction and after 10 catalytic cycles.
3
[13d]
ꢀ
lytic activity.
Tsukuda and co-workers later reported an
Chem. Asian J. 2013, 8, 3152 – 3163
3159
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Hideo Nagashima et al.
is consistent with the TEM images, in which the Au NPs
size significantly increases to (6.1ꢃ1.8) nm (Figure 5B).
These results clearly demonstrate that the reason for the de-
crease in the catalytic activity can be attributed to the in-
crease in the size of the Au NPs. In other words, it is impor-
tant to synthesize small Au NPs (less than 2 nm), preferably
around 1 nm to achieve good catalytic activity of Au NPs.
shown in Table 4, biphenyl was obtained in 63% yield by
treatment of a mixture of phenylboronic acid with K CO in
2
3
a mixture of BTF and water at 508C for 24 h under aerobic
+
ꢀ
conditions with 1 mol% of Au@HPS–NnOct3 Cl (Table 4,
entry 7). After optimization of the reaction conditions, the
yield of biphenyl exceeded 94% in BTF (mixed with a trace
amount of water) in the presence of K CO at 508C for 24 h
2
3
under O (1 atm, balloon; Table 4, entry 8). There are sever-
2
al reports describing the homocoupling of phenylboronic
acid by heterogeneous catalysts to give biphenyl with high
selectivity; however, the selective preparation of biphenyl
without contamination from phenol has not been achieved
in a homogeneous system.
Homocoupling of Phenylboronic Acid
[25]
In addition to the reduction of 4-nitrophenol and the aero-
bic oxidation of alcohols, we also used Au@HPS–NR3 Cl
as a catalyst for the homocoupling of phenylboronic acid,
which was reported with Au@PVP as the catalyst in water.
According to the literature procedure, we performed the re-
action of phenylboronic acid in the presence of Au@PVP
+
ꢀ
Mechanistic Considerations
(
(1.3ꢃ0.3) nm, 1 atom%) in water under aerobic conditions
As briefly noted in the Introduction, organic ammonium
[16]
at 308C for 24 h; this resulted in the consumption of all of
the charged phenylboronic acid and the formation of bi-
phenyl and phenol in a ratio of 63:37, which was consistent
with the reported result.
place with 2a-1 as the catalyst under the same conditions in
salts are sometimes used as a stabilizer of Au NPs. Organ-
ic quaternary ammonium salts as stabilizers for transition-
metal NPs have been investigated in palladium, platinum,
rhodium, and iridium chemistry; the preparation and cataly-
sis of these ammonium salt stabilized NPs are well summar-
[13a]
In contrast, no reaction took
[
15]
+
water. Interestingly, the reaction occurred if organo-dispersi-
ble Au@HPS–NnOct3 Cl was used as the catalyst in organ-
ized in reviews. By using organic ammonium salts, NR4
+
ꢀ
ꢀ
X , in which R=C –C alkyl groups and X=Cl or Br, NPs
6
20
ic solvents, such as toluene, THF, or BTF. Because the prod-
of several nanometers in size can be prepared. All of these
+
ꢀ
+
ꢀ
uct and Au@HPS–NnOct3 Cl are in the same phase, the
removal of the catalyst was achieved by passing the mixture
through a short column of silica gel; this made recycling of
the catalyst difficult. Nevertheless, it is worthwhile highlight-
ing that an advantage lies in complete suppression of the
formation of byproduct (phenol).
As shown in Table 4, the reaction efficiently took place in
toluene or BTF (entries 1–9), but did not give the product in
good yields in MeOH, DMF, and THF (entries 10–12). In
toluene, a small amount of phenol as the byproduct was
formed (Table 4, entries 3 and 4). Among the reactions
NR4 X -stabilized metal NPs are organo-soluble except
+
those of betain-type ammonium salts such, as R N
3
ꢀ
[26]
CH CH CH SO , which are known to be water-soluble.
2
2
3
3
In other words, both organo- and water-soluble Au NP stabi-
lized organic ammonium salts can be prepared through the
appropriate choice of counteranion. However, the disad-
+
ꢀ
[16]
vantage of Au@NR4 X is the lack of long-term stability.
In a typical example, it was reported that the long-term sta-
+
ꢀ
+
ꢀ
bility of Au@NR4 Br was not good. Au@NR Br may be
4
stored in solution; however, removal of solvents from the
+
ꢀ
solution of Au@NR4 Br caused agglomeration of the parti-
[16h]
cles to form insoluble materials.
The advantage of Au@HPS–NR3 Cl over Au@NR4
+
ꢀ
+
ꢀ
Table 4. Homocoupling of phenylboronic acid catalyzed by Au@HPS–
X
+
ꢀ
NnOct
3
Cl .
is clearly demonstrated by their excellent stability; both 2a
and 2b are stable in solution state and in the solid state. In
fact, compounds 2a-1 and 2b-1, with Au NPs around 1 nm
in size, are stable in the absence of solvents and the solid
can be stored in a refrigerator over months. Upon redisper-
sion in solvent, no clear change in the particle size was ob-
served. It is also important that both water- and organo-dis-
persible Au NPs can by prepared through the choice of the
[
a]
[b]
Entry
Solvent
H
2
O
T
Yield [%]
[mg]
[8C]
3
4
[
c]
1
2
3
4
5
6
7
8
9
1
1
1
toluene
toluene
toluene
toluene
toluene
BTF
BTF
BTF
BTF
DMF
20
0
20
20
0
20
20
20
20
20
20
20
50
25
50
75
75
25
50
50
75
50
50
50
0
2
0
0
12
1
0
0
0
0
0
0
+
ꢀ
R group. The properties of Au@HPS–NR3 Cl are in sharp
contrast to that of typical organic ammoniums salt stabilized
58
60
43
trace
63
94
57
2
+
ꢀ
Au@NR3 X , both in stability and in dispersibility control.
+
ꢀ
These advantages of Au@HPS–NR3 Cl are due to the
+
ꢀ
special structural features of HPS–NR3 Cl . Based on our
previous research on Pt and Pd NPs stabilized by HPS–
[
d]
+
ꢀ
+
ꢀ
NR3 Cl , we propose a structure of Au@HPS–NR Cl in
3
0
1
2
+
ꢀ
which several molecules of HPS–NR3 Cl surround one Au
NP, as shown in Figure 6A. Part of the HPS–NR3 Cl
MeOH
THF
5
9
0
0
+
ꢀ
covers the surface of the Au NPs, in which large numbers of
ionic ammonium moieties electrostatically stabilize the Au
[
a] DMF=dimethylformamide. [b] Determined by GC with n-decane as
an internal standard. [c] Without catalyst. [d] Au=2 atom%, O
2
=1 atm.
Chem. Asian J. 2013, 8, 3152 – 3163
3160
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Hideo Nagashima et al.
alkyl group, such as nOct and n-dodecyl, the effect of hydro-
phobicity owing to the R groups becomes predominant and
the resulting Au NPs are organo-dispersible.
Conclusion
We have prepared Au NPs of 1 nm in size through the de-
velopment of new stabilizer, HPS–NR Cl . The resulting
+
ꢀ
Figure 6. Possible stabilization of Au NPs with A) HPS–NR
B) small molecules of ammonium salts.
3
Cl and
+
ꢀ
3
+
ꢀ
Au@HPS–NR3 Cl complexes could be dispersed in water
or organic solvents and the dispersibility was dependent on
the R moieties of the ammonium group. The resulting Au
NPs. Ionic ammonium moieties in another part of HPS–
NR Cl remote from gold help to disperse Au@HPS–NR
+
ꢀ
+
+
ꢀ
NPs stabilized by HPS–NR3 Cl showed unique catalytic
performances. In the reduction of 4-nitrophenol, the result-
ing Au NPs (1 nm in size) showed good catalytic activity,
and a clear size effect was confirmed for both water- and
organo-dispersible Au NPs. In aerobic oxidation of alcohols,
recycling of Au NPs was realized through a biphasic system
for both organo- and water-soluble alcohols. The homocou-
pling of phenylboronic acid catalyzed by organo-dispersible
Au NPs gave high selectivity. These results clearly demon-
strate the utility of HPS–NR3 Cl as a new and unique sta-
bilizer of Au NPs; this will stimulate studies on Au NPs not
only for catalysis, but also for their applications in various
scientific fields, in which PVP, dendrimers, and thiols have
3
3
ꢀ
Cl in the solvent.
+
ꢀ
One of the reasons why HPS–NR3 Cl is a better stabiliz-
er than organic ammonium salts lies in the polydispersity of
HPS–NR Cl , namely, the existence of a mixture of HPS–
NR3 Cl with different sizes; this contributes to better cov-
erage of the gold surface. Even under the circumstances, it
is likely that there are channels between the HPS–NR3 Cl
molecules, which deliver solvents and solutes to the gold sur-
face for catalytic transformation of the substrate. Another
reason is due to the existence of a large number of ammoni-
um groups on the surface of HPS–NR3 Cl , which effective-
ly bind to the polar charged surface of gold through multi
+
ꢀ
3
+
ꢀ
+
ꢀ
+
ꢀ
+
ꢀ
+
ꢀ
(
ionic ammonium salts as the ligand)–multi (polar sites of
been used in general, to date. We expect HPS–NR3 Cl to
be a versatile stabilizer not only for Au NPs, but also for
other transition-metal NPs. Related work to include the
effect of different counteranions is in progress.
the Au surface) interactions (Figure 6A). A single molecule
of a small ammonium salt can cover only a single polar site
of the gold surface; this is less effective for stabilization
(
Figure 6B).
The multi ACHTUNGTRENNUNGl i ACHTUNGTRENNUNGg and–multipolar site interactions also suppress
+
ꢀ
the sintering of Au NPs. In Au@NR4 X , once one pair of
interactions between the ammonium salt and the polar gold
surface is ’cut’, the molecules easily leave the surface of gold
to make the Au NPs unstable. The resulting ’bared’ gold sur-
face causes easy sintering of Au NPs. In contrast, Au@HPS–
Experimental Section
General
HPS-Cl was synthesized by Nissan Chemical Industry, as reported else-
[
14a]
where.
All other reagents were of analytical grade and used as re-
1
13
ceived. H and C NMR spectra were measured on ECA-400 (396 MHz)
or ECA-600 (600 MHz) spectrometers. TEM (JEOL JEM-2100F, V=
200 kV) and ICP-MS (Shimadzu ICP M-8500) analyses were performed
at the Analytical Center in Institute for Materials Chemistry and Engi-
neering, Kyushu University, or Nissan Chemical Industry. FTIR spectra
were measured on a JASCO FT/IR 4200 spectrometer. UV/Vis spectra
were recorded on a JASCO V-650DS spectrophotometer at ambient tem-
perature at a concentration of [Au]=0.3 mm. Powder XRD spectra were
measured on a Rigaku TTR III X-ray diffractometer with CuKa radiation
operating at 50 kV and 300 mA. GC analysis was performed on a Shimad-
zu GC-17A gas chromatograph with a TC-17 (30 m) column.
+
ꢀ
NR3 Cl , with plenty of interactions between the ammoni-
um groups of HPS–NR3 Cl and the polar sites of the gold
+
ꢀ
+
ꢀ
surface, eventually prevent HPS–NR3 Cl from opening the
bared surface for sintering. Even if one of these interactions
is weak and easily broken, many other ammonium groups of
HPS–NR3 Cl can still hold HPS–NR3 Cl on the surface
of Au NPs. Large numbers of ammonium groups also deter-
+
ꢀ
+
ꢀ
+
ꢀ
mine the dispersibility of Au@HPS–NR Cl . The ionic
3
+
ꢀ
nature of surface ꢀ-NR Cl groups provides hydrophilicity
3
+
ꢀ
+
+
ꢀ
Synthesis of HPS–NR
3
Cl
to HPS–NR Cl . In contrast, the core part of HPS–NR
3
3
ꢀ
Cl is composed of hydrophobic polystyrene and alkyl
groups of the surface ammonium groups help the hydropho-
bicity of HPS–NR3 Cl . The dispersibility of HPS–NR Cl
+
ꢀ
A procedure to synthesize HPS–NnBu
3
Cl was as follows: a solution of
tributylamine (4.08 g, 22 mmol) dissolved in isopropanol (10 g) was
added to a solution of HPS-Cl (3.04 g corresponding to 20 mmol of the
+
ꢀ
+
ꢀ
3
chloride) in CHCl
heated under reflux for 48 h. The solvents were removed (evaporator)
and the solid was redissolved in CHCl and precipitated in hexane with
3
(30 g) under a nitrogen atmosphere. The solution was
either in water or in organic solvent is determined by the
balance of the hydrophobicity/hydrophilicity properties,
3
[8]
which is similar to the case of dendrimers. It is important
that the existence of a large number of surface ammonium
groups eventually determines the dispersibility of Au@HPS–
stirring. Precipitation was repeated three times and the product was
dried at room temperature under vacuum (yield=90%). A slightly modi-
+
ꢀ
3
fied procedure was used for preparation of HPS–NnOct Cl , as de-
1
13
scribed in the Supporting Information. The H and C NMR spectra are
shown in the Supporting Information. The conversion of the chloride
groups in HPS-Cl to the ammonium moieties was calculated to be about
+
ꢀ
NR3 Cl : if R is a short hydrocarbon chain, such as nBu,
strong hydrophilicity of the ionic ammonium group makes
the resulting Au NPs water-dispersible. If R is a long-chain
1
3
[14a]
90 and 85% for 1a and 1b, respectively, from C NMR spectroscopy.
Chem. Asian J. 2013, 8, 3152 – 3163
3161
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Hideo Nagashima et al.
+
ꢀ
Preparation of Au@HPS–NR
3
Cl
Biphasic Aerobic Oxidation of 1-Phenylethanol: Catalyst Recycling
1-Phenylethanol (61.1 mg, 0.5 mmol), KOH (84.2 mg, 1.5 mmol),
+
ꢀ
A typical procedure for Au@HPS–NnBu
3
Cl : HAuCl
O (5 mL)) was added to an aqueous solution of HPS–
Cl (101.4 mg in H O (20 mL), 0.3 mmol of the monomer units).
4 2
·4H O (20.6 mg,
+
ꢀ
0
.05 mmol, in H
2
Au@HPS–NnBu
ide (BTF, 2.5 mL) were added to a 25 mL round-bottomed flask. The
mixture was stirred for 24 h under an O atmosphere (1 atm, balloon) at
3 2
Cl (Au=2 atom%), H O (2.5 mL), and benzotrifluor-
+
ꢀ
NnBu
3
2
The solution was cooled to 08C and stirred for 30 min. An aqueous solu-
tion of NaBH (0.5 mmol, 10 equiv to Au in 5 mL) was rapidly added to
2
1
4
308C. The conversion was determined by H NMR spectroscopy. The
BTF layer was separated by decantation, and filtered through a short pad
of silica gel. The solvents were removed in vacuo to give the desired
product in over 90% yield. The aqueous phase recovered was used for
another catalytic cycle without any further treatment.
the reaction mixture, during which the solution was stirred vigorously.
The color immediately turned to dark brown, which indicated the forma-
tion of small Au NPs. The reaction mixture was stirred for 1 h at 08C,
and then dialyzed to remove inorganic impurities by using a membrane
tube with a cutoff molecular weight of 30 kDa. The solution was either
diluted to 100 mL ([Au]=0.5 mm) or freeze-dried to obtain dark-brown
solids (90 mg, Au%=9.7 wt% by ICP-MS), which were kept at 48C. The
sample was named 2a-1. For the preparation of 2a-2 and 2a-3, the tem-
perature was kept at 25 and 508C respectively, and the other protocols
were the same as those described for 2a-1.
Aerobic Oxidation of 3: Catalyst Recycling
BTF (1 mL), 3 (56.9 mg, 0.2 mmol), KOH (33.7 mg, 0.6 mmol), and
+
ꢀ
Au@HPS–NnOct
3
Cl (Au=4 atom%) were placed in a 10 mL flask.
(balloon, 1 atm). After the re-
The mixture was stirred at 508C under O
action was over, the conversion was calculated from H NMR spectrosco-
py results. Then the product and KOH were extracted with water (3ꢁ
2
1
+
ꢀ
A typical procedure for Au@HPS–NnOct
THF/H O=4/1 was used in the following synthetic process. A solution of
HAuCl ·4H O (20.6 mg, 0.05 mmol in H O (5 mL)) was added with stir-
ring to a solution of HPS–NnOct
.3 mmol of the monomer units). The mixture was cooled to 08C and
stirred for 30 min. A solution of NaBH (18.9 mg, 0.5 mmol, 10 equiv to
Au in 5 mL) was rapidly added to the vigorously stirred reaction mixture.
The color immediately turned to dark brown, which indicated the forma-
tion of small Au NPs. The reaction mixture was stirred at 08C for 1 h.
After the reaction was over, THF was removed by evaporation. The
aqueous layer was extracted with CHCl
extracts were dried over anhydrous Na
evaporation. After vacuum drying, Au@HPS–NnOct
as a dark-brown solid (150 mg, Au%=3.8 wt% by ICP-MS). This sample
was named 2b-1. For the preparation of 2b-2 and 2b-3, the temperature
was kept at 25 and 508C, respectively, and other protocols were the same
as those described for 2b-1.
3
Cl : A mixed solvent of
2
5 mL). The BTF layer containing Au NPs was dried over Na
another run of the catalytic reaction. The aqueous phase was extracted
with CHCl and the extracts combined were dried over Na SO and then
2 4
SO before
4
2
2
+
ꢀ
3
Cl (151.9 mg in H
2
O (20 mL),
3
2
4
0
concentrated. The corresponding ketone was obtained in >95% yield.
4
Homocoupling of Phenylboronic Acid
In
a typical procedure, phenylboronic acid (61.0 mg, 0.5 mmol),
+
ꢀ
Au@HPS–NnOct
3
2 3
Cl (2b-1, 1 atom%), K CO (207.3 mg, 1.5 mmol),
and the solvent (5 mL) indicated in Table 4 were placed in a 25 mL flask.
The reaction mixture was stirred vigorously at room temperature under
aerobic conditions. After 24 h, the Au catalyst was removed by passing
the mixture through a short column of silica gel. The yield of biphenyl
was determined by GC with n-decane as an internal standard.
3
(3ꢁ10 mL). After the combined
SO
2
4
, CHCl
3
was removed by
+
ꢀ
3
Cl was obtained
Catalytic Reduction of 4-Nitrophenol with NaBH
4
Acknowledgements
+
ꢀ
The catalytic performance of Au@HPS–NnBu
3
Cl for the reduction of
was examined as follows: In
Cl (0.4 mL, 0.02 mm in H O) and
O) were mixed at room temperature.
Then, a freshly prepared aqueous solution of NaBH (3.2 mL, 5 mm) was
added. The initial molar ratio of [Au]/[4-nitrophenol]/[NaBH ] was 1/20/
000. UV/Vis spectra were measured at fixed intervals. The reaction rate
constant, k, was determined through the change in the absorbance at l=
00 nm as a function of time.
The reduction of 4-nitrophenol with NaBH
This work was supported by the Core Research Evolutional Science and
Technology (CREST) Program of the Japan Science and Technology
Agency (JST). We are thankful to Prof. Tatsuya Tsukuda (Tokyo Univer-
sity) for his helpful suggestions on experimental techniques. We are
grateful to Yuki Nohara, Shun Fujita (Nissan Chemical Co. Ltd.), Dr.
Liwei Zhao, and Prof. Shigeto Okada (Kyushu University) for TEM and
XPS measurements.
4
-nitrophenol to 4-aminophenol with NaBH
4
+
ꢀ
a UV cell (4 mL), Au@HPS–NnBu
3
2
4
-nitrophenol (0.4 mL, 0.4 mm in H
2
4
A
H
U
G
R
N
U
G
4
2
4
4
catalyzed by Au@HPS–
biphasic system. A disper-
Cl ([Au]=0.039 mm in CHCl ) was mixed
with an aqueous solution of 4-nitrophenol (3.0 mm, 16 mL). The mixture
was stirred at 328C, to which an aqueous solution of NaBH (4 mL,
.5m) was added. During the reaction, the mixture was stirred vigorously.
[
1] For general reviews of Au NPs, see: a) G. Schmid, B. Corain, Eur. J.
Inorg. Chem. 2003, 3081–3098; b) R. Sardar, A. M. Funston, P. Mul-
vaney, R. W. Murray, Langmuir 2009, 25, 13840–13851; c) M. C.
Daniel, D. Astruc, Chem. Rev. 2004, 104, 293–346.
+
ꢀ
NnOct
3
Cl was carried out in a H O/CHCl
2
3
+
ꢀ
sion of Au@HPS–NnOct
3
3
4
[
2] For the synthesis of Au NPs, see: P. X. Zhao, N. Li, D. Astruc,
Coord. Chem. Rev. 2013, 257, 638–665.
1
The yellow color of the aqueous phase gradually faded and disappeared
within 1 h. Periodically, samples (1.0 mL) were taken from the aqueous
[
3] For applications of Au NPs, see: a) J. M. Pingarrꢂn, P. YꢃÇez-
SedeÇo, A. Gonzꢃlez-Cortꢄs, Electrochim. Acta 2008, 53, 5848–
layer and diluted with H
were then recorded.
2
O (4.0 mL). UV/Vis spectra of the samples
5
866; b) K. Saha, S. S. Agasti, C. Kim, X. N. Li, V. M. Rotello,
Chem. Rev. 2012, 112, 2739–2779; c) D. A. Giljohann, D. S. Seferos,
W. L. Daniel, M. D. Massich, P. C. Patel, C. A. Mirkin, Angew.
Chem. 2010, 122, 3352–3366; Angew. Chem. Int. Ed. 2010, 49, 3280–
3294; d) R. A. Sperling, P. Rivera Gil, F. Zhang, M. Zanella, W. J.
Parak, Chem. Soc. Rev. 2008, 37, 1896–1908; e) L. C. Kennedy,
L. R. Bickford, N. A. Lewinski, A. J. Coughlin, Y. Hu, E. S. Day,
J. L. West, R. A. Drezek, Small 2011, 7, 169–183; f) A. Llevot, D.
Astruc, Chem. Soc. Rev. 2012, 41, 242–257; g) E. C. Dreaden, A. M.
Alkilany, X. Huang, C. J. Murphy, M. A. El-Sayed, Chem. Soc. Rev.
2012, 41, 2740–2779.
Catalytic Aerobic Oxidation of Alcohols
In typical procedure, benzyl alcohol (54.1 mg, 0.5 mmol), KOH
a
+
ꢀ
(
(
84.2 mg, 1.5 mmol), Au@HPS–NnBu
2.5 mL), and benzotrifluoride (BTF, 2.5 mL) were added to a 25 mL
at-
mosphere (1 atm, balloon). After 24 h, the reaction was quenched with
m HCl. The BTF layer was separated by decantation and filtered
3
Cl
(Au=2 atom%),
2
H O
round-bottomed flask. The mixture was stirred at 308C under an O
2
1
through a short pad of silica gel. The solvents were removed in vacuo to
give the desired product.
[4] a) M. Haruta, T. Kobayashi, H. Sano, N. Yamada, Chem. Lett. 1987,
6, 405–406; b) M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J.
1
Catal. 1989, 115, 301–309; for reviews, see: c) M. Haruta, Catal.
Today 1997, 36, 153–166; d) M. Haruta, M. Date, Appl. Catal. A
2
001, 222, 427–437; e) A. Haruta, Chem. Rec. 2003, 3, 75–87; f) M.
Chem. Asian J. 2013, 8, 3152 – 3163
3162
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Hideo Nagashima et al.
Haruta, Gold Bull. 2004, 37, 27–36; g) T. Ishida, M. Haruta, Angew.
Chem. 2007, 119, 7288–7290; Angew. Chem. Int. Ed. 2007, 46, 7154–
Appl. Organomet. Chem. 2008, 22, 288–299; e) N. Yan, C. Xiao, Y.
Kou, Coord. Chem. Rev. 2010, 254, 1179–1218.
7
156.
5] a) D. Astruc, F. Lu, J. R. Aranzaes, Angew. Chem. 2005, 117, 8062–
083; Angew. Chem. Int. Ed. 2005, 44, 7852–7872; b) B. K. Min,
[16] a) J. Fink, C. J. Kiely, D. Bethell, D. J. Schiffrin, Chem. Mater. 1998,
10, 922–926; b) K. G. Thomas, P. V. Kamat, J. Am. Chem. Soc. 2000,
122, 2655–2656; c) K. G. Thomas, J. Zajicek, P. V. Kamat, Langmuir
2002, 18, 3722–3727; d) W. Cheng, S. Dong, E. Wang, Langmuir
2003, 19, 9434–9439; e) S. R. Isaacs, E. C. Cutler, J. S. Park, T. R.
Lee, Y. S. Shon, Langmuir 2005, 21, 5689–5692; f) H. Choo, S. R.
Isaacs, A. Small, S. Parmley, Y. S. Shon, J. Colloid Interface Sci.
2007, 316, 66–71; g) H. Kawasaki, K. Nishimura, R. Arakawa, J.
Phys.Chem. C 2007, 111, 2683–2690; h) Y. S. Shon, S. Chuc, P.
Voundi, Colloids Surf. A 2009, 352, 12–17; i) M. Kasture, M. Sastry,
B. L. V. Prasad, Chem. Phys. Lett. 2010, 484, 271–275; j) Y. Leng, Y.
Li, A. Gong, Z. Shen, L. Chen, A. Wu, Langmuir 2013, 29, 7591–
7599.
[17] a) M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, R. Whyman, J.
Chem. Soc. Chem. Commun. 1994, 801–802; b) M. J. Hostetler, J. E.
Wingate, C. J. Zhong, J. E. Harris, R. W. Vachet, M. R. Clark, J. D.
Londono, S. J. Green, J. J. Stokes, G. D. Wignall, G. L. Glish, M. D.
Porter, N. D. Evans, R. W. Murray, Langmuir 1998, 14, 17–30.
[18] D. V. Leff, L. Brandt, J. R. Heath, Langmuir 1996, 12, 4723–4730.
[19] P. Hervꢄs, M. Pꢄrez-Lorenzo, L. M. Liz-Marzꢄn, J. Dzubiella, Y. Lu,
M. Ballauff, Chem. Soc. Rev. 2012, 41, 5577–5587.
[
[
8
C. M. Friend, Chem. Rev. 2007, 107, 2709–2724; c) J. C. Fierro-Gon-
zalez, B. C. Gates, Chem. Soc. Rev. 2008, 37, 2127–2134; d) M. Stra-
takis, H. Garcia, Chem. Rev. 2012, 112, 4469–4506; e) Y. Zhang,
X. J. Cui, F. Shi, Y. Q. Deng, Chem. Rev. 2012, 112, 2467–2505.
6] a) T. Tsukuda, H. Tsunoyama, H. Sakurai, Chem. Asian J. 2011, 6,
7
36–748; b) H. Tsunoyama, Y. M. Liu, T. Akita, N. Ichikuni, H. Sa-
kurai, S. H. Xie, T. Tsukuda, Catal. Surv. Asia 2011, 15, 230–239;
c) T. Tsukuda, Bull. Chem. Soc. Jpn. 2012, 85, 151–168.
[
7] a) R. C. Jin, Nanoscale 2010, 2, 343–362; b) E. Pensa, E. Cortꢄs, G.
Corthey, P. Carro, C. Vericat, M. H. Fonticelli, G. Benꢅtez, A. A.
Rubert, R. C. Salvarezza, Acc. Chem. Res. 2012, 45, 1183–1192.
8] a) R. M. Crooks, M. Zhao, L. Sun, V. Chechik, L. K. Yeung, Acc.
Chem. Res. 2001, 34, 181–190; b) R. van Heerbeek, P. C. J. Kamer,
P. W. N. M. van Leeuwen, J. N. H. Reek, Chem. Rev. 2002, 102,
[
3
717–3756; c) R. W. J. Scott, O. M. Wilson, R. M. Crooks, J. Phys.
Chem. B 2005, 109, 692–704; d) L. M. Bronstein, Z. B. Shifrina,
Chem. Rev. 2011, 111, 5301–5344.
9] a) J. Shan, H. Tenhu, Chem. Commun. 2007, 4580–4598; b) Y. Ofir,
B. Samanta, V. M. Rotello, Chem. Soc. Rev. 2008, 37, 1814–1825.
[
[20] a) K. Kuroda, T. Ishida, M. Haruta, J. Mol. Catal. A: Chem. 2009,
298, 7–11; b) H. Koga, E. Tokunaga, M. Hidaka, Y. Umemura, T.
Saito, A. Isogai, T. Kitaoka, Chem. Commun. 2010, 46, 8567–8569;
c) A. Azetsu, H. Koga, A. Isogai, T. Kitaoka, Catalysts 2011, 1, 83–
96.
[
10] a) H. Chen, Y. Wang, Y. Wang, S. Dong, E. Wang, Polymer 2006, 47,
63–766; b) S. Li, Y. Wu, J. Wang, Q. Zhang, Y. Kou, S. Zhang, J.
7
Mater. Chem. 2010, 20, 4379–4384; c) N. Yan, J. Zhang, Y. Yuan,
G. T. Chen, P. J. Dyson, Z. C. Li, Y. Kou, Chem. Commun. 2010, 46,
1
1
2
631–1633; d) Y. Yuan, N. Yan, P. J. Dyson, Inorg. Chem. 2011, 50,
1069–11074; e) I. Biondi, G. Laurenczy, P. J. Dyson, Inorg. Chem.
011, 50, 8038–8045; f) M. Beija, J. D. Marty, M. Destarac, Chem.
[21] a) N. Hu, J. Y. Yin, Q. Tang, Y. Chen, J. Polym. Sci. Part A 2011, 49,
3826–3834; b) Y. Liu, Y. Fan, Y. Yuan, Y. Chen, F. Cheng, S.-C.
Jiang, J. Mater. Chem. 2012, 22, 21173–21182.
Commun. 2011, 47, 2826–2828; g) S. K. Bae, S. Y. Lee, S. C. Hong,
React. Funct. Polym. 2011, 71, 187–194; h) S. Wen, F. Zheng, M.
Shen, X. Shi, Colloids Surf. A 2013, 419, 80–86.
[22] a) S. Kobayashi, H. Miyamura, Chem. Rec. 2010, 10, 271–290; b) H.
Miyamura, R. Matsubara, Y. Miyazaki, S. Kobayashi, Angew. Chem.
2007, 119, 4229–4232; Angew. Chem. Int. Ed. 2007, 46, 4151–4154;
c) C. Lucchesi, T. Inasaki, H. Miyamura, R. Matsubara, S. Kobaya-
shi, Adv. Synth. Catal. 2008, 350, 1996–2000.
[
11] a) S. Chen, C. Guo, G. H. Hu, J. Wang, J. H. Ma, X.-F. Liang, L.
Zheng, H. Z. Liu, Langmuir 2006, 22, 9704–9711; b) D. Miyamoto,
M. Oishi, K. Kojima, K. Yoshimoto, Y. Nagasaki, Langmuir 2008,
[23] One of the reviewers suggested that the electronic properties of Au
+
ꢀ
2
4, 5010–5017; c) S. Goy-Lꢂpez, E. Castro, P. Taboada, V. Mos-
3
NPs may explain the lower reactivity of Au@HPS–NR Cl in the
quera, Langmuir 2008, 24, 13186–13196; d) T. Azzam, L. Bronstein,
A. Eisenberg, Langmuir 2008, 24, 6521–6529; e) N. Ali, S.-Y. Park,
Langmuir 2008, 24, 9279–9285; f) W. L. Leong, P. S. Lee, A. Lohani,
Y. M. Lam, T. Chen, S. Zhang, A. Dodabalapur, S. G. Mhaisalkar,
Adv. Mater. 2008, 20, 2325–2331; g) P. Khullar, V. Singh, A. Mahal,
H. Kaur, V. Singh, T. S. Banipal, G. Kaur, M. S. Bakshi, J. Phys.
Chem. C 2011, 115, 10442–10454.
aerobic oxidation of alcohols than that of Au@PVP, which may be
estimated by X-ray photoelectron (XP) spectroscopy. We measured
XP spectra for 2a-1, 2b-1, and Au@PVP. The binding energies of
Au 4 f7/2 were determined to be 82.9, 83.3, and 82.5 eV for 2a-1, 2b-
1, and Au@PVP, respectively. For comparison, the value of bulk
gold is 84.0 eV. The somewhat negative Au 4f7/2 value of Au@HPS–
+
ꢀ
3
NR Cl compared with that of Au@PVP may be one of the rea-
[
[
12] a) C. Y. Bao, M. Jin, R. Lu, T. R. Zhang, Y. Y. Zhao, Mater. Chem.
Phys. 2003, 82, 812–817; b) N. Pꢄrignon, J. D. Marty, A. F. Mingo-
taud, M. Dumont, I. Rico-Lattes, C. Mingotaud, Macromolecules
sons for the low catalytic activity in aerobic oxidations; however,
the catalytic activity is apparently affected by the coordination be-
+
ꢀ
havior of HPS–NR
3
Cl , as described in the text. We are now inves-
2
007, 40, 3034–3041; c) Y. Shen, M. Kuang, Z. Shen, J. Nieberle, H.
tigating the relationship between the structural and electronic prop-
+
ꢀ
Duan, H. Frey, Angew. Chem. 2008, 120, 2259–2262; Angew. Chem.
Int. Ed. 2008, 47, 2227–2230.
3
erties and catalytic activity by using a series of M@HPS–NR X
with different metals and X. A detailed discussion will be given in
the near future.
13] a) H. Tsunoyama, H. Sakurai, N. Ichikuni, Y. Negishi, T. Tsukuda,
Langmuir 2004, 20, 11293–11296; b) H. Tsunoyama, H. Sakurai, Y.
Negishi, T. Tsukuda, J. Am. Chem. Soc. 2005, 127, 9374–9375; c) H.
Tsunoyama, H. Sakurai, T. Tsukuda, Chem. Phys. Lett. 2006, 429,
[24] S. Kanaoka, N. Yagi, Y. Fukuyama, S. Aoshima, H. Tsunoyama, T.
Tsukuda, H. Sakurai, J. Am. Chem. Soc. 2007, 129, 12060–12061.
[25] a) S. Carrettin, J. Guzman, A. Corma, Angew. Chem. 2005, 117,
2282–2285; Angew. Chem. Int. Ed. 2005, 44, 2242–2245; b) N. G.
Willis, J. Guzman, Appl. Catal. A 2008, 339, 68–75; c) A. Primo, F.
Quignard, Chem. Commun. 2010, 46, 5593–5595; d) J. Zheng, S.
Lin, X. Zhu, B. Jiang, Z. Yang, Z. Pan, Chem. Commun. 2012, 48,
6235–6237; e) L. Wang, W. Zhang, D. S. Su, X. Meng, F. S. Xiao,
Chem. Commun. 2012, 48, 5476–5478.
5
2
28–532; d) H. Tsunoyama, T. Tsukuda, H. Sakurai, Chem. Lett.
007, 36, 212–213; e) H. Tsunoyama, N. Ichikuni, T. Tsukuda, Lang-
muir 2008, 24, 11327–11330; f) H. Tsunoyama, N. Ichikuni, H. Sa-
kurai, T. Tsukuda, J. Am. Chem. Soc. 2009, 131, 7086–7093.
[
14] a) K. Kojima, K. Chikama, M. Ishikawa, A. Tanaka, T. Nishikata, H.
Tsutsumi, K. Igawa, H. Nagashima, Chem. Commun. 2012, 48,
1
0666–10668; b) T. Nishikata, H. Tsutsumi, L. Gao, K. Kojima, K.
[26] a) M. T. Reetz, W. Helbig, J. Am. Chem. Soc. 1994, 116, 7401–7402;
b) M. T. Reetz, S. A. Quaiser, Angew. Chem. 1995, 107, 2461–2463;
Angew. Chem. Int. Ed. Engl. 1995, 34, 2240–2241.
Chikama, H. Nagashima, unpublished results.
[
15] a) H. Bçnnemann, W. Brijoux, R. Brinkmann, R. Fretzen, T. Jous-
sen, R. Kçppler, B. Korall, P. Neiteler, J. Richter, J. Mol. Catal.
1
994, 86, 129–177; b) H. Bçnnemann, R. M. Richards, Eur. J. Inorg.
Received: July 1, 2013
Revised: August 19, 2013
Published online: September 23, 2013
Chem. 2001, 10, 2455–2480; c) A. Roucoux, J. Schulz, H. Patin,
Chem. Rev. 2002, 102, 3757–3778; d) L. D. Pachꢂn, G. Rothenberg,
Chem. Asian J. 2013, 8, 3152 – 3163
3163
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim