Dendrimer-like core cross-linked micelle stabilized ultra-small gold nanoclusters as a robust catalyst for aerobic oxidation of α-hydroxy ketones in water

Article abstract of DOI:10.1039/c6gc00010j

As one of the most general and promising stabilizers, dendrimers have been widely used to prepare ultra-small gold nanoclusters. However, the complex synthesis of dendrimers hinders the further application of protected nanoclusters. Here we report a facile strategy to prepare an alternative material via core cross-linking of self-assembled micelles. The resulting dendrimer-like core cross-linked micelles (DCCMs) retain the main characteristics of dendrimers and avoid complex chemical synthesis. As expected, the DCCMs could easily encapsulate gold nanoparticles within their cores. The ultra-small clusters of Au₅ were prepared without the participation of external reductants. Importantly, the DCCM stabilized noble gold clusters furnish excellent catalytic activity and perfect reusability for aerobic oxidation of α-hydroxy ketones in water. Only in open air the oxidation could be repeated up to 48 times with negligible turn-over frequency change. The total turnover number (TON) of the reaction reached unexpectedly >48 000, the highest TON for metal catalysed oxidation of hydroxy ketones so far. The further mechanism study hints that the carboxylic group of substrates might be involved in the catalytic process. The simple catalyst preparation, the environmentally benign reaction conditions, and the excellent catalytic performance and durability make the novel DCCM protected gold nanocluster a green catalyst.

Full text of DOI:10.1039/c6gc00010j

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DOI: 10.1039/C6GC00010J
Green Chemistry
PAPER
Received 00th January
20xx,
Dendrimer-like core cross-linked micelle stabilized ultra-small
gold nanocluster as robust catalyst for aerobic oxidation of α-
hydroxy ketones in water
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
a
a
a
b
a
Yangyang Yu, Chenlu Lin, Bing Li, Pengxiang Zhao and Shiyong Zhang*
As one of the most general and promising stabilizers, dendrimers have been widely used to prepare ultra-small gold
nanoclusters. However, the complex synthesis of dendrimers hinders the further application of protected nanoclusters.
Here we report a facile strategy to prepare an alternative material via core cross-linking of self-assembled micelles. The
resulting dendrimer-like core cross-linked micelles (DCCMs) keep the main characteristic of dendrimers and avoid complex
chemical synthesis. As expected, the DCCMs could easily encapsulate gold nanoparticles within their cores. The ultra-small
clusters of Au were prepared without the participation of external reductants. Importantly, the DCCM stabilized noble
5
gold clusters furnish excellent catalytic activity and perfect reusability for aerobic oxidation of α-hydroxy ketones in water.
Only under open air the oxidation could be repeated up to 48 times with negligible turn-over frequency change. The total
turnover number (TON) of the reaction reached unexpected >48,000, a highest TON for metal catalysed oxidation of
hydroxy ketones so far. The further mechanism study hints that the carboxylic group of substrate might be involved in the
catalytic process. The simple catalyst preparation, the environmentally benign reaction condition, and the excellent
catalytic performance and durability make the novel DCCM protected gold nanoculster a green catalyst.
stabilizers suffer from drawbacks. For example, most dendrimers
require complex chemical synthesis, especially for high-generation
compounds; even the increase of one generation, one has to pay
Introduction
Ultra-small noble-metal nanoclusters (NCs), for example Au, are of
fundamental interest and technological importance in the field of
catalysis due to their size dependent catalytic activity.1 The
syntheses to these particles usually include chemical or
electrochemical reduction of metal salts in the presence of
much more efforts than that of lower ones. The high cost largely
10
hinders their industrialization and commercialization. In this
regard, development of new stabilizers with dendrimer-like
characteristic but much easier preparation method would be highly
desirable.
2
stabilizers. The purpose of the stabilizers is to control particle size
and enhance resistance to agglomeration and deactivation. In past
The self-assembly utilizes non-covalent interactions to build
varied nanostructures and avoids the complex chemical synthesis,
which might be a good protocol to develop new stabilizers of metal
3
4
11
years, some innovative stabilizers such as dendrimers, thiols,
5
6
7
8
polymers, peptide complexes, surfactants and supported matrix
have been developed for preparation of ultra-small gold nanoclusters. For instance, the surfactant micelles driven by
nanoclusters. Among them, dendrimers probably stand as the most amphiphilic interaction represent dendrimer-like spherical
general and promising ones. Indeed, the fairly spherical dendrimer structure and have been served as template for metal
a
12
features controllable size, functionalized exterior, and highly nanoparticles. However, this kind of particles could hardly be used
chemical stability, which make it particularly well-suited hosting in catalysis since the rapid dissociation equilibrium of the
9
metal nanoparticles for catalysis. Nevertheless, the dendrimer noncovalent structure would tend to precipitate the metal catalyst
out during the reaction process and, the hard separation between
the stabilizer and product would also make the catalysts reusability
difficult. These drawbacks are fatal in modern catalysis because
a.National Engineering Research Centre for Biomaterials, Sichuan University,
Chengdu, 610064, China. E-mail: szhang@scu.edu.cn. Phone: +86-28-85411109.
Fax: +86-28-85411109.
13
the development of sustainable catalysts is becoming more urgent
than ever in face of the stringent environmental standards and
b.
Science and Technology on Surface Physics and Chemistry Laboratory, Mianyang
6
21907, Sichuan, China.
1
4
economic pressures.
Electronic Supplementary Information (ESI) available: Synthetic procedures of
chemicals, Measurement of critical micelle concentrations of 1b-d, additional
Figures, Tables, NMR and MS data. See DOI: 10.1039/x0xx00000x
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Green Chemistry
Results and discussion
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DOI: 10.1039/C6GC00010J
Design, synthesis and characterization of dendrimer-like core
cross-linked micelles. Compounds 1a-d were simply prepared in
four steps by alkylation of methyl gallate with 6-bromohexene,
reduction of the ester with lithium aluminium hydride,
bromination of the resulting alcohol with phosphorus
tribromide, and nucleophilic displacement of the bromide with
polyethylene glycol (PEG) (See Supporting Information for
details). With a hydrophobic alkyl head and a hydrophilic PEG
tail, these neutral surfactants were expected to form micelles
in water. However, as compound 1a was added into deionized
water, a white suspension, which would further precipitate out
of solution over time, was obtained. This phenomenon
happened possibly because the hydrophilic tail of 1a could be
not long enough to protect the relatively large hydrophobic
head, consequently leading to the formation of meta-stable
molecular aggregates rather than micellar self-assemblies.
Scheme 1. Preparation of DCCMs by thiol-ene addition and
synthesis of ultra-small gold nanoclusters.
Indeed, compounds 1b-d with longer hydrophilic parts were
readily soluble in water and gave rise to the stable pale blue
emulsions at room temperature, which is characteristic of the
self-assembly taking place. The initial dynamic light scattering
Inspired by our long-term effort in developing stable
7,15
materials from various self-assemblies,
we hypothesized
that the dynamic micelles might be covalently captured to get
a solid media for metal catalysts. Although cross-linking of
surfactant micelles has been reported as early as in the 1970s,
there is no mention of their application in stabilizing metal
(
DLS) measurement of aqueous emulsion from 1b revealed
nanoparticles with a hydrodynamic diameter of ~9.0 nm
formed. With continuously increasing the hydrophilic chain to
PEG-1000 and PEG-2000 (1c and 1d), larger nanoparticles with
sizes of ~28.0 and ~75.0 nm were obtained (Fig. 1). Further
analysis indicated that the critical micelle concentrations (CMC)
16
nanoparticles for catalysis. In this contribution, we employ
for the first time the highly efficient thiol-ene “click” reaction
to cross-link the core of neutral micelles formed by readily of 1b
-
1d were approximately 1.60, 1.18 and 0.60 µmol/L,
17
available surfactants 1a
-1d (Scheme 1). The resulting core respectively (Fig. 1S).
cross-linked micelle demonstrates dendrimer-like properties
such as spherical shape, controllable size, hydrophobic interior
and highly chemical stability, and is proved to be outstanding
template for gold nanoparticles. The ultra-small clusters of Au
5
were prepared without the participation of any external
reducing reagents. Importantly, the novel dendrimer-like core
cross-linked micelle (DCCM) protected gold nanocluster
(Au@DCCM) was found to be excellent catalyst for aerobic
oxidation of α-hydroxy ketones in water. The oxidation of
alcohols to corresponding carbonyl compounds is ubiquitous
importance in production of fine chemicals and
18
intermediates. Traditional methods based on the use of a
stoichiometric amount of oxidants are often toxic and release
19
considerable by-products. As a consequence of the ever-
growing concerns over green chemistry and chemical
processes, the gold particle catalyst combined with molecular
Fig. 1 Distribution of the hydrodynamic diameters of micelles
oxygen represents an emerging alternative to the traditional (black line) and DCCMs (red line) formed by 1b
-
1d in aqueous
1
procedures. However, the previous gold particle based solution determined by DLS ([
catalytic systems need additionally bubbled air or oxygen to
1] = 2.6 mM).
The design of surfactants puts numerous alkenyl groups at
the core of micelles, which are readily cross-linked by UV
irradiation in the presence of dithiothreitol (DTT) –a cross-linker
20
push the reaction, which increases the economic burden to
some extent. Besides, most of the reported systems suffer
from the harsh synthesis and cumbersome reusability of
and 2,2’-dimethoxy-2-phenylacetophenone (DMPA)
–a
20,21
catalyst.
In contrast, the colloidal Au@DCCM system
photoinitiator to form a covalent captured 3D spherical
material (Scheme 1 and see Experimental for details). The
reported here features the easy catalyst preparation, the
environmentally benign and economic reaction condition
1
cross-linking process could be easily monitored by H NMR
(open air as oxygen source, water as the sole solvent, and as
spectroscopy. Taking 1d as example, upon core cross-linking,
low as 0.1% catalyst loading), the excellent reaction
performance, and the outstanding catalyst reusability (cycle up
to 48 time!). It thus would be severed as a real “green” catalyst
for oxidation of α-hydroxy ketones.
1
the sharp H NMR signals of the surfactant were replaced by
broad peaks and the protons belonging to alkenyl groups
disappeared almost (Fig. 2S). The successful cross-linking was
2
| J. Name., 2012, 00, 1-3
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also confirmed by DLS measurement except a little bit size of external reductants less than 1.0 nm in sizeView(FAirgti.cle4Ocn,ldin)e.
Noticeably, from the TEM one can find
D
th
distributed in the whole DCCM rather than in a special space
Fig. 4b,d). This observation suggests that the PEG chains, which
O
a
I:
t
10th.1
e
03
A
9/
u
C6
N
G
P
C
s
00
w
0
e
10re
J
increase from prior to cross-linking ~9.0, 28.0 and 75.0 nm to
after cross-linking ~12.0, 40.0 and 90.0 nm, respectively (Fig. 1).
In addition, the resulting DCCMs were further characterized by
transmission electron microscopy (TEM). As shown in Fig. 2a, a
micrograph obtained from the DCCM of 1b very clearly
revealed the formation of distinct spherical aggregates with a
uniform diameter of around 20.0 nm. Further analysis revealed
that the particles increased in size with the increase of
(
occupy the most space of DCCM, play an important role in
stabilizing the Au NPs. The previous reports have shown that
the PEG units could protect Au NPs through the multiple but
weak interaction between Au and the pseudocrown ethers. In the
present case, the thick PEG layers of DCCM would likely do the
same. It should be noted that although the thioethers existed
24
hydrophilic fraction, which is consistent with the trend in the DCCM could also provide weak coordination with gold
observed in DLS assays. For example, the DCCM from 1c gives nanoparticles, this interaction could not be the main factor for
a larger size in the micrograph, averaging about 35.0 nm in the stabilization since all thioethers centred in the core of
25
DCCMs (Scheme 1).
diameter (Fig. 2b); the particles from 1d are much larger, with
their sizes mostly ranging from 80.0 to 100.0 nm (Fig. 2c).
Fig. 2 TEM micrographs of DCCMs formed by a) 1b, b) 1c and c) 1d
in aqueous solution (2.6 mM) on the carbon-coated copper grids.
All samples were stained with an aqueous solution of 2%
phosphotungstic acid.
As expected, the stability of micelles is highly enhanced via
the covalent capture. After dialysis in water and evaporation of
the solvent, the DCCMs could not only re-dissolve in water but
also in most polar organic solvents (e.g. methanol, chloroform
and tetrahydrofuran, etc.) without decomposition of the
spherical structure determined by DLS (data not shown).
Besides, the cross-linked materials could be stably maintained
over a year at ambient temperature with a neglectable size
decrease. It is particularly worthy to note that the DCCMs
could even tolerate high temperature treatment and survive
from a broad range of pH conditions (1-14).
Fig. 3 UV-Vis spectra of Au@DCCMs formed with and without
-
4 4
externally added reducing agent NaBH . [AuCl ]/[1d] = 0.5, [1d] =
-4
5
.2 × 10 M.
Synthesis of DCCMs stabilized gold nanoparticles and
nanoculsters. With the super stable DCCMs in hand, we
directed toward the template synthesis of gold nanoparticles
-
(NPs), which are carried out by sequestering AuCl
4
within
DCCMs followed by chemical reduction (See Supporting
Information for details). Different reducing conditions had an
enormous impact on the nanoparticle production. Addition of
-
NaBH
4
to the water solution formed by 0.5 of [AuCl
4
]/[DCCM-
1
d
] immediately turned its colour from light yellow to dark
purple. The UV-Vis spectrum shows a plasmon band at 520 nm
Fig. 3, red). This means that the Au NPs formed have a
(
2
2
diameter larger than 2.0 nm. Surprisingly, when the aurate-
containing DCCMs were allowed to sit at room temperature
4
without any NaBH added, the solution also turned from
yellow (the distinctive colour of aurate) to light purple in a few
hours whereas the surface plasmon absorption band at 520
nm disappeared, indicating the formation of ultra-small (< 1.0
nm) nanoclusters (Fig. 3, blue).23 This conclusion was
confirmed by TEM measurement showing that the Au NPs
Fig. 4 TEM images of Au@DCCMs formed with (a, b) and without (c,
d) the addition of NaBH , ([HAuCl ]/[1d] = 1:1). The gold
nanoparticles appear as dark spots due to their high electron
density.
4
4
4
formed in the presence of NaBH averaged about 4.0 nm in
diameter (Fig. 4a,b) and nanoclusters obtained in the absence
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Green Chemistry
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DOI: 10.1039/C6GC00010J
Fig. 5 (a) Photographs of Au nanoclusters prepared in DCCM templates with and without NaBH
4
under a hand-held UV lamp (365 nm).
-
(
b) Emission spectra of Au@DCCMs obtained with and without NaBH
4
. λex = 330 nm; [AuCl
4
]/[1d] = 0.5, [1d] = 1.3 mM. (c) Comparison
of emission spectra of Au nanoclusters formed in DCCM templates with different amphiphiles 1b (λex = 390 nm), 1c (λex = 380 nm) and
-
1
d (λex = 330 nm). [AuCl
4
]/[1] = 0.5, [1] = 1.3 mM. The gold nanoclusters were formed without externally added reducing reagents. (d)
-
Comparison of emission spectra of Au@DCCMs formed with different ratios of [AuCl
mM. The gold nanoclusters were formed without externally added reducing reagents.
4
]/[1d] (0.02, 0.1 and 0.5 respectively), [1d] = 1.3
The ultra-small Au nanoclusters obtained in the absence of
external reducing reagents were further investigated by
photoluminescence study. As we know, when their size
approaches the Fermi wavelength of electrons, gold
nanoclusters would display molecule-like emission. Indeed,
under a hand-held UV lamp, the Au-DCCMs prepared without
the external reductant gave a bright blue light but the
materials obtained by NaBH reduction showed no signs of
4
luminescence (Fig. 5a). The fluorescence spectroscopy
confirmed this trend. As shown in Fig. 5b, the Au@DCCMs
DCCM protected gold nanoculsters (Au@DCCMs) as
green” catalyst for aerobic oxidation of α-hydroxy ketones
“
in water. As mentioned in the beginning, the catalytic activity
of a metal catalyst is highly sensitive to its surface property.
The gold catalyst obtained by spontaneous reduction gives
smaller particle size compared that by reduction with external
reducing reagents, which is expected to afford higher reactivity
since the smaller particle size can provide more contacting
surface area between catalyst and substrates.[
26
1a,1g,29]
prepared by spontaneous reduction demonstrated the intense Additionally, the spontaneous reduction avoids the
fluorescence emission while the sample obtained by NaBH
incorporation of exotic impurities and is more in line with the
4
reduction kept almost silent. From the excitation/emission requirements of green synthesis. Based on above
spectra peaks of 330 nm (3.76 eV)/387 nm (3.20 eV),27 the gold
considerations, the gold NCs stabilized by DCCM from 1d
without the participation of external reducing reagents were
nanocluster size could be tentatively ascribed to 5 atoms
according to the jellium model described by Zheng et al.,28
chosen to test the catalytic activity of Au@DCCMs.
although the theoretical datum (3.22 eV) is slightly larger than
Optimization of catalytic conditions was performed with the
that observed in our experiment (Fig. 3S, □). Interestingly, the
aerobic oxidation of benzoin as the model reaction. Firstly,
size of the gold NCs could be further adjusted by the size of
-
temperatures were examined, and it was found that the
4
DCCMs. For example, in 0.5 of [AuCl ]/[1] samples, the larger
o
complete conversion of benzoin was observed at 50 C after 14
the DCCM size (from 1b to 1c to 1d), the shorter the
fluorescence emission, indicating the formation of smaller Au
h (Table 1S, entry 3). With this promising observation, we then
clusters (Fig. 5c). Moreover, the cluster size could also be turned our attention to the aurate loading in the DCCMs. A
controlled by the amount of gold precursor loaded in the number of loadings from 2%-100% were tried and the
-
-
materials. For instance, in the Au@DCCM samples with [AuCl
/[1d] from 0.02 to 0.1 to 0.5, the excitation/emission
4
optimized result was obtained in 50% of [AuCl ]/[1d], which
4
]
30
gave 33.1% yield within 6 h (Table 1S, entries 4-12). An ideal
wavelengths changed from 300/358 to 309/374 to 330/387
nm, respectively (Fig. 5d).
catalysis would also be a system where the catalyst used as
little as possible. In this context, we tried to reduce the
amount of the catalyst under above optimized reaction
4
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condition. To our delight, even the catalyst decreased as low
as 0.1%, the quantitative yield was still obtained in despite of a
little bit longer reaction time needed (Table 1S, entry 15). It
should be noted that under the identical condition the
reaction run very slowly in the absence of Au (Table 1S, entry
View Article Online
5
36
>98
DOI: 10.1039/C6GC00010J
6
7
48
48
>98
>98
1
7).
With the established optimal reaction condition, we then
managed to investigate the scope of this novel Au@DCCMs
catalysed oxidation reaction and the results are summarized in
Table 1. To our delight, the aryl α-hydroxy ketones with either
electron donating or withdrawing substituting groups on the
aromatic ring all performed excellent reactivity (entries 1–8);
even the sterically hindered ortho-substituted benzoins
likewise got quantitative yields (entries 4, 6, and 8). Hetero-
aryl groups also afforded the desired oxidised products in
excellent yields. The reaction conditions were mild enough to
tolerate furan, thiophene and pyridine rings (entries 9-11).
Particularly, it was surprising that the same excellent reactivity
was obtained for alkyl substituted α-hydroxy ketones as well,
although the reactions needed to be carried out at a little bit
higher temperature (entries 12,13). The reaction condition
used for alkyl substituted α-hydroxy ketones was further
employed to try the oxidation of benzylic alcohols. AS shown in
Table 2S, the benzylic alcohol with electron withdrawing group
on the benzene ring also gave excellent reactivity (Table 2S,
entry 1). Unfortunately, the naked benzylic alcohol, or benzylic
alcohol with electron donating group, or even benzylic alcohol
with electron withdrawing group at the α-position could only
get a moderate yield (Table 2S, entries 2-4). Interestingly, once
a carbonyl benzylic alcohol was employed to do the oxidation,
8
9
1
60
3.5
48
8
>98
>98
>98
>98
0
11
1
2c
36
36
>98
>98
3c
1
a)
Reaction conditions: α-hydroxyketone (0.077 mmol), Au@
CO (32 mg, 0.23 mmol), H
2 mL), open air, 50 C. Yields were determined by H NMR
DCCM-1d (0.077 µmol, for Au), K
(
2
3
2
O
o
b)
1
c)
o
analysis. This reaction was performed at 80 C.
although the carbonyl groups was located at the β-position,
quantitative yield was obtained again (Table 2S, entry 5). This
result hints that the carbonyl group might play an important
3
1
role in the catalytic process. It needs to point out that the all
isolated products were fully characterized from their spectral
data and by direct comparison with the reported data.
Table 1. Au@DCCMs catalysed aerobic oxidation of α-hydroxy
a
ketones in water.
Fig. 6 Recycle of Au@DCCMs in the aerobic oxidation of α-hydroxy
ketone (thenoin) in water. The reaction detail was reported in
experimental section.
Entry
1
α-Hydroxyketone
t (h)
48
Yield (%)b
>98
Reusability of the Au@DCCMs catalyst system. The results
so far are quite promising whereas the largest superiority of
the DCCM stabilized gold catalyst is not reflected in that but its
ulta-high reusability. In a typical experiment, substrate 2-
hydroxy-1,2-di(thiophen-2-yl)ethanone was employed to test
the recycle of the gold catalyst (See Supporting Information for
details). At the end of each cycle, the catalyst was simply
recovered by extracting the aqueous phase with ethyl ether,
2
24
>98
3
4
8
>98
>98
1
and the organic layer was concentrated and analyzed by H
12
NMR spectroscopy. To our surprise, the catalyst could be
reused as high as 48 times with negligible turn-over frequency
(TOF) change (Fig. 6), and even at the 49th cycle the yield still
remained quite good (81%). The total turnover number (TON)
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Green Chemistry
of the reaction reached unexpected over 48,000,32 which In summary, a very simple method to prepare deVinewdrAirmticleerO-nlilkinee
constitutes so far, from this point of view, the highest efficient materials has been developed via coreDOcrI:o1s0s.1-0li3n9k/iCn6gGCo0f0s0e10lfJ-
catalytic system in metal catalysed oxidation of hydroxy assembled surfactant micelles. The resulting dendrimer-like
ketones. It should be mentioned that the reaction mixture core cross-linked micelle keeps the main characteristic of
stayed completely clear at the end of the reaction, showing no dendrimers such as fairly spherical structure, functionalized
formation of gold black (Fig. 4S). The further spectroscopic exterior, size regulation and especially highly chemical stability.
study at the molecular lever showed there is no obvious Similar to dendrimers, the DCCMs were proved to be excellent
difference before and after reaction, suggesting the recycled templates for the synthesis of gold nanoparticles. Different
catalyst keeping the same property with that before catalysis reducing conditions had an enormous impact on the
(Fig. 5S). Considering so many times treatment between the nanoparticle production. The ultra-small gold clusters of Au
5
reactions, the gold catalyst was extremely robust when were prepared by spontaneous reduction without the
protected in the DCCMs.
participation of any external reducing reagents. The size of
gold clusters could be further adjusted by simply using DCCMs
with different sizes or the amount of gold precursor loaded in
the materials.
Proposed mechanism for the aerobic oxidation of alcohols by
Au@DCCMs. Based on the previous studies
performance, a mechanism of O activated by anionic Au cores is
1g,21a,33
and the catalytic
2
Importantly, the DCCM protected gold nanocluster is an
outstanding catalyst for aerobic oxidation of α-hydroxy
ketones in water. The mechanism study suggests that the
carboxylic groups of substrates are involved in the catalytic
process. In particular, the gold catalyst system is highly
reusable and could be reused as high as 48 times with
negligible TOF change. The total turn-over number of the
reaction attained unexpected >48,000, a highest TON for metal
catalysed oxidation of hydroxy ketones so far. By the way, the
DCCM stabilized gold catalyst system reported here is totally
environmentally benign and economic, as it uses readily
available stabilizer, it avoids the participation of external
reducing reagents during its synthesis, it uses water as the sole
solvent in catalysis, it utilizes open air as the oxygen resource
without bubbled air or oxygen, it needs only 0.1% for
efficiently catalytic results, it tolerates a wide range of
substrates, it separates easily with products, and it reuses so
many times. The next work is following the attractive direction
to investigate the applicable scope of other metal particles and
nanoalloys stabilized by the novel DCCMs, and some
interesting outcomes have actually been obtained and will be
reported in due course.
suggested (Fig. 7). As we above-mentioned, the gold clusters were
encapsulated in the cavities formed by the thick and constrained
pseudocrown ethers of PEG coils (Fig. 7, I). During the catalysis,
parts of the weakly coordinated pseudocrown ethers were “pushed
away” from the Au NCs and replaced by the substrate with stronger
coordination sites (carbonyl group and oxy anion here) (Fig. 7, II).34
This replacement on one hand exposed more portions of the cluster
surfaces, on the other hand enhanced the cluster’s surface charges
2
further. Both of them would facilitate the attraction of O on Au
NPs in the form of superoxo or peroxo species (Fig. 7, III). The
n-
resulting O
2
species finally abstract hydrogen from alcohol to
squeeze out the ketone product and the Au NPs were quickly re-
protected by the surrounding PEG chains to complete the catalytic
cycle. From the mechanism, the substrate’s carbonyl group is
involved in the coordination with Au clusters in such a way that the
substrate molecule could push away the thick PEG layers around
the Au surface easier. This involvement might partially explain the
high reactivity of carbonyl group-containing substrates. Although
the thick PEG layers of DCCMs hindered the contact between the
substrate and Au surface to some extent, it is on the other hand,
that the tight arrangement of PEG layers could avoid the possible
aggregation of Au clusters during catalysis, which would be
responsible for the excellent reusability of the catalyst.
Experimental
General Method. Routine NMR spectra were obtained on a
1
Bruker AV II-400. The H NMR chemical shifts were measured
relative to CDCl
.26 ppm; D O-d
were given using CDCl
7.16 ppm). Mass spectrometry was performed on a Waters
3
or D
: δ = 4.79 ppm). The 13C NMR chemical shifts
as the internal standard (CDCl : δ =
2 2 3
O-d as the internal reference (CDCl : δ =
7
2
2
3
3
7
Q-Tof premier instrument. UV-Vis spectrometry was
monitored on a Puxin TU-1901 instrument. Fluorescence
spectra were obtained using a Hitachi F-7000 fluorescence
spectrometer. DLS experiments were recorded using a
Malvern Zetasizer Nano ZS particle analyser instrument. TEM
studies were carried out using a Tecnai G2F20S-TWIN
instrument, operating at 120 kV. The TEM specimens were
prepared by gently placing a carbon-coated copper grid on the
surface of the sample.
Fig. 7 Proposed mechanism for aerobic oxidation of α-hydroxy
ketones by Au@DCCMs based on activation of molecular oxygen.
Chemicals. Unless otherwise noted, all reagents were
obtained from commercial suppliers and used without further
Conclusions
6
| J. Name., 2012, 00, 1-3
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Green Chemistry
PAPER
2
005, 44, 7852-7872; (b) H. Miyamura, R. MVieawtsAurtbicalerOa,nlinYe.
purification. All solvents for reactions were freshly distilled
prior to use. Millipore water was used in all aqueous
experiments.
Miyazaki, and S. Kobayashi, Angew. Chem. Int. Ed., 2007, 46,
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4
151-4154; (c) A. S. K. Hashmi and G. J. Hutchings, Angew.
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Somorjai and P. Yang, Angew. Chem. Int. Ed., 2006, 45, 7824-
Typical preparation of dendrimer-like core cross-linked
micelles. The cross-linker DL-Dithiothreitol (DTT, 20 mg, 0.13
mmol) and photoinitiator dimethylolbutanoic acid (DMPA, 170
μL of 0.01 mg/mL MeOH solution, 1.72 μmol) were added to a
micellar solution of surfactant 1d (96 mg, 0.043 mmol) in
millipore water (4 mL). The reaction mixture was stirred slowly
and under UV light irradiation. After 4 h, the mixture was
dialyzed against deionized water using a 5000 Da molecular
weight cut-off tubing.
7
828; (f) R. Sardar, A. M. Funston, P. Mulvaney and Royce W.
Murray, Langmuir, 2009, 25, 13840-13851; (g) H. Tsunoyama,
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1
31, 7086-7093; (h) A. Abad, P. Concepción, A. Corma and H.
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3
88.
L. Shang, S. Dong and G. U. Nienhaus, Nano Today, 2011, 6,
01-418.
2
.
.
Typical preparation of Au@DCCMs with NaBH
reducing agent. A 10 mM aqueous solution of HAuCl
was added to a 10 mM solution of DCCM from 1d in H
4
as the
(0.5 mL)
O (1
4
4
3
(a) W. I. Lee, Y. Bae and A. J. Bard, J. Am. Chem. Soc., 2004,
126, 8358-8359; (b) M. L. Tran, A. V. Zvyagin and T. Plakhotnik,
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(a) Y. Shichibu, Y. Negishi, H. Tsunoyama, M. Kanehara, T.
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T. Huang, R. W. Murray, L. Menard and R. G. Nuzzo, J. Am.
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Hollander, G. C. Schatz and R. Jin, J. Am. Chem. Soc., 2008, 130,
2
mL). A freshly prepared aqueous solution of sodium
borohydride (50 mM, 0.2 mL) was slowly added to the
vigorously stirred reaction mixture, and a dark purple solution
of gold nanoparticles was thus obtained with seconds.
4
.
.
Typical preparation of Au@DCCMs without externally
added reducing agents. A 10 mM aqueous solution of HAuCl
4
(0.5 mL) was added to a 10 mM solution of DCCM from 1d in
2
H O (1 mL) under stirring. Upon sitting at room temperature
5
883-5885; (d) Y. Negishi, N. K. Chaki, Y. Shichibu, R. L.
overnight, a light purple solution of gold nanoclusters was
obtained.
Typical procedure for aerobic oxidation of α-hydroxy
ketones in water. The appropriate amounts of catalyst
Whetten and T. Tsukuda, J. Am. Chem. Soc., 2007, 129, 11322-
11323.
5
(a) S. Kanaoka, N. Yagi, Y. Fukuyama, S. Aoshima, H.
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29, 12060-12061; (b) J. Han, Y. Liu and R. Guo, J. Am. Chem.
2 3
Au@DCCMs, benzoin, K CO , and water were added into a 20
Soc., 2009, 131, 2060-2061; (c) H. Duan and S. Nie, J. Am.
Chem. Soc., 2007, 129, 2412-2413.
mL glass vial at room temperature. The reaction mixture was
o
then heated at 50 C under the open air condition. The 6.
progress of the reaction was monitored by TLC. After
completion of the reaction, the reaction mixture was extracted
with diethyl ether (3 × 10 mL), The organic layers were
combined and concentrated in vacuo. The yields of the
(a) A. G. Tkachenko, H. Xie, D. Coleman, W. Glomm, J. Ryan, M.
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1
reactions were determined by H NMR spectroscopy.
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phase (3 mL), where the catalyst was, was extracted with
diethyl ether (3 × 3 mL) and left standing at 50 °C for 2 min to
evaporate the residual diethyl ether. Another batch of thenoin
was added and the next cycle of the oxidation was performed.
Extraction and solvent evaporation were performed in air
without special protection.
8
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This journal is © The Royal Society of Chemistry 2015
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Table of contents entry
-
------------------------------------------------------------------------------------------------------------------------------------------------------
Dendrimer-like core cross-linked micelle stabilized ultra-small gold nanoclusters as
robust catalyst for aerobic oxidation of α-hydroxy ketones in water
Yangyang Yu, Chenlu Ling, Bing Li, Pengxiang Zhao and Shiyong Zhang*
This journal is © The Royal Society of Chemistry 2015
Green Chem., 2015, 00, 1-3 | 9
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-------------------------------------------------------------------------------------------------------------------------------------------------------
Dendrimer-like core cross-linked micelle stabilized ultra-small gold nanoclusters as
robust catalyst for aerobic oxidation of α-hydroxy ketones in water
a
a
a
b
a
Yangyang Yu, Chenlu Lin, Bing Li, Pengxiang Zhao and Shiyong Zhang*
a
National Engineering Research Centre for Biomaterials, Sichuan University, Chengdu, 610064, China.
Science and Technology on Surface Physics and Chemistry Laboratory, Mianyang 621907, Sichuan, China.
b
*
To whom correspondence should be addressed. S. Zhang, E-mail: szhang@scu.edu.cn.
Graphical Abstract. Dendrimer-like core cross-linked micelle stabilized gold clusters furnish excellent activity and
reusability for aerobic oxidation of α-hydroxy ketones in water.