Formation of a Pt12 cluster by single-atom control that leads to enhanced reactivity: Hydrogenation of unreactive olefins

Article abstract of DOI:10.1002/anie.201302860

A platinum subnanocluster catalyst composed of 12 atoms was synthesized using a phenylazomethine dendrimer, which can assemble twelve PtCl₄ units by stepwise complexation, followed by reduction to Pt⁰. Unreactive olefins that were not activated by conventional 2 nm Pt nanoparticles were successfully hydrogenated by the subnanocluster. EWG=electron-withdrawing group. Copyright

Full text of DOI:10.1002/anie.201302860

Angewandte
Chemie
Communications
Nanoclusters
M. Takahashi, T. Imaoka, Y. Hongo,
K. Yamamoto*
&&&& — &&&&
Formation of a Pt12 Cluster by Single-
Atom Control That Leads to Enhanced
Reactivity: Hydrogenation of Unreactive
Olefins
0
A platinum subnanocluster catalyst
composed of 12 atoms was synthesized
using a phenylazomethine dendrimer,
reduction to Pt . Unreactive olefins that
were not activated by conventional 2 nm
Pt nanoparticles were successfully
hydrogenated by the subnanocluster.
EWG=electron-withdrawing group.
which can assemble twelve PtCl units by
4
stepwise complexation, followed by
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DOI: 10.1002/anie.201302860
Nanoclusters
Formation of a Pt Cluster by Single-Atom Control That Leads to
1
2
Enhanced Reactivity: Hydrogenation of Unreactive Olefins
Masaki Takahashi, Takane Imaoka, Yushi Hongo, and Kimihisa Yamamoto*
Metal nanoparticles (NPs) have been applied to organic
reactions as catalysts because of their large surface-to-volume
[
1]
ratio relative to those of bulk metals. In contrast, subna-
noclusters (SNCs) occasionally exhibit significantly different
[
2]
reactivities from those of the larger nanoparticles. There are
still few reports of the SNC catalysts in solution-phase
reactions because their application requires two contradictory
conditions: 1) suppression of the aggregation by a protection
of the surface; and 2) enough surface accessibility for the
reactants onto the catalytic active sites. An application of the
SNCs as the catalyst is one of the most important challenges
[
3]
in current catalytic chemistry. Although conventional sta-
bilizers, such as surfactants, polymers, and dendrimers have
been utilized for the size control of the NPs, the considerable
[
4]
size distribution still remains.
During the course of our studies, we have reported
excellent catalytic activity of precisely size-controlled mono-
disperse platinum NCs for oxygen reduction, and rhodium
NCs for hydrogenation reactions utilizing the phenylazome-
thine dendrimer (Figure 1). This dendrimer can act as
a template, which can define an exact number of metal
atoms based on an intramolecular basicity gradient of the
imines allowing their stepwise complexations with metal
Figure 1. Structures of phenylazomethine core G4 dendrimers (TPM
G4) used to control the number of platinum atoms of a cluster and to
stabilize a cluster.
[
5,6]
ions.
Herein, we describe the preparation of platinum SNCs
encapsulated in fourth-generation phenylazomethine den-
drimers with a tetraphenylmethane core (TPM G4 dendri-
mer) loaded onto graphitized mesoporous carbon (GMC),
which can provide both activity and further stability of the
clusters together with the advantage of facile handling as
[
7]
a solid catalyst (Scheme 1).
PtCl4 can quantitatively form complexes with imine
groups of the phenylazomethine dendrimers. The number of
metal atoms in one nanoparticle can be precisely controlled
by the stoichiometry of the PtCl to dendrimer ratio at 4, 12,
4
2
8, and 60 equiv, where each layer was filled up. The synthesis
of SNCs is shown in Scheme 1. These clusters were prepared
from the complex of PtCl4 with TPM G4 dendrimer by
a subsequent reduction using NaBH . It is noteworthy that the
4
Pt12 cluster prepared by this method has one-atom level
precision; it was characterized not only by TEM image (0.9 Æ
[8]
0
.1 nm) but also by ESI-TOF-MS spectrometry. The SNCs
Scheme 1. Representation of Pt @TPM G4/GMC using a phenylazo-
1
2
methine dendrimer. GMC=graphitized mesoporous carbon.
[*] M. Takahashi, Dr. T. Imaoka, Y. Hongo, Prof. K. Yamamoto
Chemical Resources Laboratory, Tokyo Institute of Technology
could be promptly supported onto the GMC by just adding
the GMC suspension to the solution of the dendrimer
encapsulating Pt SNCs. The filtrate after the supporting
became colorless (yellow or slight yellow in original color,
respectively), indicating complete deposition of the dendri-
4259 Nagatsuta, Midori-ku, Yokohama, 226-8503 (Japan)
E-mail: yamamoto@res.titech.ac.jp
Homepage: http://www.res.titech.ac.jp/~inorg/yamamoto/e/
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302860.
2
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Angewandte
Chemie
mer-encapsulated Pt SNCs onto the GMC, of which pore size
is 6.4 nm in the average diameter and this is suitable support
for the dendrimer molecules having 2.1–2.9 nm of the
Table 1: Turnover frequencies of olefin hydrogenation in the presence of
Pt12 or Pt (2.2Æ0.8 nm) catalysts.
[
9]
diameter observed by TEM and AFM.
A HAADF-STEM image of the catalyst in Figure 2a
shows monodispers SNCs supported on the GMC surface.
[a]
[a]
Entry Substrate
Pt @TPM G4 /
GMC
Pt (2.2Æ0.8 nm) /
1
2
The resulting Pt @TPM G4/GMC was highly stable against
1
2
GMC
their aggregation under air in a solution for over two weeks.
3
2
1
2
1.35ꢀ10
4.73ꢀ10
2
[b]
8.00ꢀ10
6.20ꢀ10
1.80ꢀ10
[
b]
[b]
3
0
3
3
4
5
5.85ꢀ10
2.03ꢀ10
4.80ꢀ10
2
[b]
[b]
1.30ꢀ10
3
2
6
1.69ꢀ10
8.89ꢀ10
À1 À1
[
a] The turnover frequencies [atom(Pt)
h
] were corrected for the total
Figure 2. HAADF-STEM images of a) Pt @TPM G4/GMC and b) Pt
1
2
metal content and were determined by GC analysis using anisole as an
internal standard. [b] The reaction was monitored after ten hours by GC
analysis.
(
2.2Æ0.8 nm) without dendrimer.
Despite such an excellent stability, the catalytic performance
of SNCs for hydrogenations of olefinic substrates was still
very high. The progress of the reaction under a hydrogen
atmosphere (1 atm) at room temperature was monitored as
a time-course of the conversion of the starting material based
on NMR and gas chromatograph (GC) analyses. The initial
activity of Pt @TPM G4/GMC in methanol was represented
as the turnover frequency (TOF), which corresponds to the
number of catalytic cycles within an hour. At initial inves-
tigation using the SNCs revealed a significantly higher turn-
(Table 1, entries 1 and 2). Moreover, the hydrogenation of p-
trifluoromethylstyrene was catalyzed only by Pt @TPM G4/
1
2
GMC catalysts (Table 1, entry 3). The hydrogenation of
cyclohexene, which is more difficult than 1-decene owing to
steric effects, also increased significantly upon the decrease in
particle size, while for the reaction of 1-decene it was not so
significant (Table 1, entries 4 and 5).
1
2
Hydrogenations of alkenes by the metal NPs have been
previously considered to be insensitive to the size of the
over-frequency (TOF) values for a hydrogenation reaction of
catalysts, although none of the previous reports demonstrated
À1 À1
[11,12]
styrene to be 1350 atom(Pt)
h
under atmospheric hydro-
the size effect on a subnanometer scale.
Our results
gen pressure at room temperature (258C). To the best of our
revealed for the first time that the reactivities of hydro-
genation of olefins by platinum cluster catalysts dramatically
changed in the size of NCs below 2 nm into subnanometer
scale, and SNCs can act as efficient catalysts even under mild
conditions for low-reactivity substrates that have an electron-
withdrawing group or steric hindrance.
knowledge, this catalytic activity is better than any previously
[10]
reported data under such mild conditions.
After the
reaction, the catalyst is able to be separated from reaction
mixture easily. TEM images of both as-prepared and used
Pt @TPM G4/GMC catalysts displayed no obvious aggrega-
1
2
tion of the SNCs during the reaction (see the Supporting
Information).
The scope of the reaction was extended to various olefins
under the same condition. To evaluate the particle-size effect
on the catalytic activities, NPs, of which average diameter is
In conclusion, we have developed an efficient SNC
catalyst encapsulated by TPM G4 dendrimer for the hydro-
genation of olefins. The catalytic activity of the SNC was
highest among the previously reported Pt catalysts. In
particular, this catalyst enabled the hydrogenations of low-
reactive olefins that have an electron-withdrawing group or
steric hindrance. These results demonstrate potential appli-
cation of the SNCs in catalyst chemistry. Further studies of the
origin of the size effect in hydrogenation reactions by
platinum clusters and applications of the catalysts to other
organic reactions are now undergoing.
2
.2 Æ 0.8 nm, were prepared at almost the same conditions
without dendrimers (Figure 2b) and compared to the SNCs.
The activities of the SNC catalyst are much higher than that of
the larger Pt NP catalyst for all of the substrates.
Of particular interest is that even for substrates that are
difficult to catalytically hydrogenate by larger NPs (entries 2,
3
, and 5) owing to the electronic or steric reasons could be
efficiently converted by the SNCs. For example, the TOF of
hydrogenation of p-chlorostyrene by the Pt12 cluster was
about 44 times higher than that of the larger Pt NPs. This
acceleration is much higher than non-substituted styrene
Experimental Section
Preparation of Pt nanoclusters: Pt @TPM G4/GMC: The procedure
1
2
was conducted under a dry nitrogen atmosphere. A solution of PtCl4
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Communications
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in acetonitrile (3 mmolL , 29 mL, 12 equiv for the TPMG4 dendri-
L. Li, J. C. Yang, A. I. Frenkel, B. R. Cuenya, J. Am. Chem. Soc.
2010, 132, 15714 – 15719.
À1
mer) was added to a solution of TPMG4 (2.4 mL, 3 mmolL ) in
dichloromethane–acetonitrile (1:1). After stirring for 30 min, sodium
borohydride solution in methanol (10 mL, 0.53 molL ; 61 equiv
relative to Pt) was added to the solution. Immediately after the
addition, the solution was added to a stirred suspension of GMC
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(1 mL, 1.67 mg) in dichloromethane–acetonitrile (1:1) in a sample
tube to afford a dispersion of Pt @TPMG4/GMC for the following
1
2
hydrogenation reactions.
Pt(2.2Æ0.8 nm)/GMC: The procedure was conducted under a dry
nitrogen atmosphere. A solution of PtCl in acetonitrile (29 mL,
4
À1
3
mmolL ) was added to a 0.45 mL solution of dichloromethane–
acetonitrile (1:1). After stirring for 30 min, a solution of sodium
borohydride in methanol (10 mL, 0.53 molL ; 61 equiv relative to Pt)
was added to the solution. After stirring for 1 hour, the solution was
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Typical procedure for Pt-catalyzed hydrogenation: The proce-
dure was conducted at room temperature under atmospheric
pressure. After removal of the solvent in the sample tube with the
catalyst solution (Pt @TPMG4/GMC or Pt/GMC) under reduced
1
2
pressure, the flask was charged with dry nitrogen. Styrene (45.5 mg,
.433 mmol) in methanol (1.5 mL) was then added into the flask.
0
Hydrogen gas was supplied from a balloon. The reaction progress was
monitored by GC after an hour without 1-decene. Because the
hydrogenation reaction of 1-decene is fast, the reaction was moni-
tored after half an hour. The samples for the GC measurement were
prepared by filtrations of the each reaction mixture in methanol
sampled through a short silica gel column.
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2
À
Reference [5a], and the ESI TOF MS peak of [Pt (CO) ] is
1
2
16
Received: April 6, 2013
shown in the Supporting Information of Reference [5a].
Published online: && &&, &&&&
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[
[
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480. It was mentioned that, in styrene hydrogenation, the TOF
Keywords: dendrimers · hydrogenation · nanoclusters · olefins ·
À1
.
value of their platinum catalysts calculated as 1333 h at 408C is
better than any previous reported data in mild condition. TOF
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room temperature (Table 1, entry 1).
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