A uniform bimetallic rhodium/iron nanoparticle catalyst for the hydrogenation of olefins and nitroarenes

Article abstract of DOI:10.1002/anie.201102836

Mix and more than match: Relative to the catalytic activity of pure Rh nanoparticles in a dendrimer cage, Rh/Fe bimetallic nanoparticles in dendrimers have improved catalytic activity towards the hydrogenation of olefins, and unlike Wilkinson catalyst could catalyze nitroarene hydrogenation (see scheme, G4=4th generation dendrimer).

Full text of DOI:10.1002/anie.201102836

Communications
DOI: 10.1002/anie.201102836
Hydrogenation
A Uniform Bimetallic Rhodium/Iron Nanoparticle
Catalyst for the Hydrogenation of Olefins and
Nitroarenes**
Ikuse Nakamula, Yoshinori Yamanoi, Takane Imaoka, Kimihisa Yamamoto,* and
Hiroshi Nishihara*
Angewandte
Chemie
5
830
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 5830 –5833

Dendrimer-stabilized metal nanoparticles
have attracted much attention recently as a
new research direction in the field of catal-
[1]
ysis. Dendrimer matrices serve as support-
ing materials to prevent nanoparticles from
aggregating and to provide a desired chem-
ical interface between the nanoparticles and
the reaction media. We recently reported
that fourth-generation (G4) phenylazome-
thine dendrimers (TPP-DPA G4) are useful
for synthesizing and stabilizing monometallic rhodium-nano-
Scheme 1. The preparation of Rh Fe @TPP-DPA G4.
3
2
28
The formation of Rh/Fe bimetallic nanoparticle was
confirmed by TEM, X-ray fluorescence (XRF), and X-ray
photoelectron spectroscopy (XPS). TEM images revealed
small uniform nanoparticles with an average particle size of
(1.1 Æ 0.2) nm (Figure 1a,b). Figure 1c shows the XRF spec-
[
2–4]
particles with a very narrow size distribution.
The catalyst
was prepared by NaBH reduction of RhCl in a mixture of
4
3
toluene and methanol with dendrimer as stabilizer. Bimetallic
nanoparticle catalysts are widely applicable to heterogeneous
transformations because the properties of these materials
overcome several disadvantages of single-component nano-
[
5,6]
particles.
Increasingly, the shape, size, composition, and
architecture of a nanoparticle are being recognized as
important control parameters for tailoring new bimetallic
nanoparticle systems. Few reports have described the prep-
aration of almost completely uniform (standard variation: ꢀ
Æ 0.2 nm) bimetallic nanoparticles. Herein, we describe the
preparation of uniform bimetallic nanoparticles in a phenyl-
azomethine dendrimer, which provides improved catalytic
reactivity compared with monometallic Rh nanoparticles in a
dendrimer cage.
Iron drew our attention for its low cost, nontoxicity, and
environmentally benign properties. FeCl can quantitatively
3
complex with the imine groups of phenylazomethine den-
[
7]
drimers. The synthesis of bimetallic Rh/Fe nanoparticles
shown in Scheme 1 was conducted using a slightly modified
version of the procedure previously reported for the synthesis
Figure 1. Characterization of Rh32Fe28@TPP-DPA G4. a) TEM image of
Rh/Fe nanoparticles on a carbon-coated copper grid. A few of the
nanoparticles are circled. b) Size distribution (n: frequency, d: particle
size). c) XRF spectrum.
[
2]
of monometallic Rh nanoparticles. These materials were
3
prepared by the stepwise complexation of RhCl and FeCl3
with TPP-DPA G4 in a controlled stoichiometry, and subse-
quent reduction with NaBH . The resulting bimetallic nano-
4
particles were stable in solution under Ar, and no precip-
itation or color changes occurred over several weeks.
trum taken from the sample shown in Figure 1a, which
indicated that the nanoparticles were composed of Rh and Fe
in a ratio of 32:28. These values agreed well with the mol% of
RhCl and FeCl used in the synthetic mixture. The XPS result
3
3
[
*] I. Nakamula, Dr. Y. Yamanoi, Prof. H. Nishihara
Department of Chemistry, School of Science
The University of Tokyo
revealed information about the surfaces of the nanoparticles.
Medium intensity Rh 3d5/2 and Rh 3d3/2 signals with binding
energies of 307.5 eV and 312.4 eV, characteristic of metallic
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 (Japan)
[8]
Rh, were observed. A weak Fe 2p3/2 signal with a binding
energy of 709.4 eV was detected. This peak corresponded to a
Fax: (+81)3-5841-8063
E-mail: nisihara@chem.s.u-tokyo.ac.jp
[
9]
low-valent Fe species (FeCl : 711.3 eV, FeCl : 710.7 eV). No
Dr. T. Imaoka, Prof. K. Yamamoto
Chemical Resources Laboratory
Tokyo Institute of Technology (Japan)
Fax: (+81)45-924-5260
3
2
oxidized Rh or Fe was detected in this sample.
The catalytic performance of Rh/Fe bimetallic nano-
particles was initially tested for the reduction of olefinic
substrates under a hydrogen atmosphere (1 atm). The prog-
ress of the reaction was monitored by conversion of the
starting material as a function of time. The initial activities of
Rh @TPP-DPA G4 and Rh Fe @TPP-DPA G4 in MeOH
E-mail: yamamoto@res.titech.ac.jp
[**] This work was supported by Grant-in-Aids for Scientific Research on
Innovative Areas “Coordination Programming” (area 2107, No.
21108002) and the Global COE Program for “Chemistry Innovation
6
0
32
28
through the Cooperation of Science and Engineering”, from the
Ministry of Education, Culture, Sports, Science, and Technology
(
represented as the turnover frequency (TOF), the number of
conversions within 30 min) toward olefin hydrogenation, and
of Wilkinson complex ([RhCl(PPh ) ]) were measured. As
(
Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102836.
3 3
shown in Table 1 (entries 1–3), the Rh/Fe nanoparticles
Angew. Chem. Int. Ed. 2011, 50, 5830 –5833
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5831

Communications
À1
Table 1: Turnover frequencies (TOFs [h ]) of the hydrogenation of
olefins and nitroarenes in the presence of various Rh catalysts.
Entry
Substrate
Wilkinson
Rh @
TPP-DPA
Rh Fe @
32 28
TPP-DPA
6
0
[
c]
[c]
[
a]
a]
1
2
3
8
12
57
173 (10380)
292 (17520)
78 (4680)
510 (30600)
500 (30000)
139 (8340)
[
[
a]
[
[
[
b]
b]
b]
4
5
6
0
0
0
2 (120)
2 (120)
5 (300)
11 (660)
17 (1020)
11 (660)
[a] 1.75 mmol olefinic substrate, 0.3 mol% catalyst (based on metal),
5
0
mL MeOH under H (1 atm). [b] 0.7 mmol aromatic nitro compound,
.3 mol% catalyst, 5 mL MeOH under H2 (1 atm). [c] The TOF in
2
parentheses is corrected for the total metal content (calculated as
0 atoms/cluster). Highest values highlighted in bold.
6
showed enhanced catalytic activity over the Rh nanoparticles
or Rh complex. The scope of the reaction was extended by
examining the hydrogenation of various nitroarenes under
similar conditions. Although hydrogenation of nitroarenes
has been recognized as an important chemical transformation
for obtaining aniline derivatives, Wilkinson catalyst showed
essentially no reactivity for this transformation. Hydrogena-
tion of the nitroarenes was observed using nanoparticles as
Figure 2. Dependence of conversion (X) on the reaction time (t) in the
rhodium-catalyzed hydrogenation of a) styrene and b) 4-nitroaniline.
Wilkinson catalyst (blue), Rh @TPP-DPA G4 (red), and Rh Fe @TPP-
6
0
32
28
DPA G4 (green).
[
10]
catalyst.
As shown in Table 1 (entries 4–6), the TOF
measured using a bimetallic catalyst was up to 8.5-times
higher than that measured using the Rh nanoparticle catalyst.
Comparison of hydrogenation activities of the different Rh
catalysts clearly indicated that the catalysis environment
played the dominant role in determining the activity of the
catalyst (Figure 2).
particle catalysts in dendrimer cages, we found that the
bimetallic Rh/Fe nanoparticles showed improved catalytic
ability toward these hydrogenation reactions. These results
also provide a means for preparing almost completely uni-
form bimetallic metal catalysts under easily controlled con-
ditions. This observation provides new critical experimental
parameters for the design and optimization of durable
nanoparticle catalysts. Forthcoming reports will describe
bimetallic nanoparticles prepared from different metal com-
binations.
TEM studies of both fresh and used catalysts were
conducted to understand the shape and size of the particles.
No obvious aggregation of nanoparticles was observed by
TEM during the reaction. To determine if the nanoparticles
were just a physical mixture of Rh and Fe nanoparticles or are
bimetallic, we also prepared Fe @TPP-DPA G4 (1.3 Æ
6
0
[11]
0
.4 nm) and examined the catalytic activity.
Fe @TPP-
60
Experimental Section
DPA G4 did not show catalytic activity at all and the physical
mixture of Rh and Fe nanoparticles in a ratio of 53:47 (ca.
Fe @TPP-DPA G4: All processes were conducted under a dry
6
0
nitrogen atmosphere in the dark. Iron(III) chloride (0.90 mg,
5.3 mmol, 60 equiv relative to the dendrimer) in N,N-dimethylform-
amide (DMF, 1.5 mL) was added to the TPP-DPA G4 (1.0 mg,
3
2:28) showed lower reactivity than the bimetallic nano-
particles. The higher TOFs observed for Rh/Fe bimetallic
nanoparticles were thought to arise from synergistic elec-
tronic effect.
In conclusion, we developed an efficient nanoparticle
catalyst system for the hydrogenation of olefins and nitro-
arenes under relatively mild conditions. In a comparison with
previously reported hydrogenation reactions using Rh nano-
0.088 mmol) solution in toluene (3 mL). After stirring for 30 min,
sodium borohydride (1.0 mg, 26 mmol, 5 equiv relative to the iron) in
methanol (0.5 mL) was added dropwise to the solution. This solution
was stirred overnight to afford a dispersion of dendrimer-stabilized Fe
nanoparticles.
Rh Fe @TPP-DPA G4: All processes were conducted under a
3
2
28
dry nitrogen atmosphere in the dark. Iron(III) chloride (0.40 mg,
5
832
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2
.5 mmol, 28 equiv relative to dendrimer) in DMF (0.5 mL) was added
[3] For reports describing the phenylazomethine dendrimer, see:
a) N. Satoh, T. Nakashima, K. Kamikura, K. Yamamoto, Nat.
Nanotechnol. 2008, 3, 106; b) K. Yamamoto, Y. Kawana, M.
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Takenaga, A. Sonoi, Nat. Chem. 2009, 1, 397.
to the TPP-DPA G4 (1.0 mg, 0.088 mmol) solution in toluene (3 mL).
After stirring this mixture for 30 min, rhodium(III) chloride trihy-
drate (0.80 mg, 2.8 mmol, 32 equiv relative to dendrimer) in DMF
(1 mL) was added and stirred for another 30 min. Then, sodium
borohydride (1.0 mg, 26 mmol, 5 equiv relative to the sum of iron and
rhodium) in methanol (0.5 mL) was added dropwise to the solution.
This solution was stirred overnight to afford a dispersion of reddish
black dendrimer-stabilized Rh/Fe nanoparticles.
Typical procedure for Rh Fe @TPP-DPA G4 catalyzed hydro-
3
2
28
genation: All processes were conducted at room temperature under
atmospheric pressure in the dark. A carefully dried Schlenk flask
equipped with a magnetic stir bar was charged with dry nitrogen.
Butyl acrylate (0.25 mL, 1.75 mmol) in methanol (5 mL) was then
added into the flask. A solution of Rh Fe @TPP-DPA (0.3 mol%
[4] The chemical formula of TPP-DPA G4 is provided in the
Supporting Information.
3
2
28
metal relative to the olefin) was added to this olefin solution.
Hydrogen gas was supplied from a balloon. The reaction was
monitored by GC-MS. The samples for GC-MS were prepared by
filtering a drop of the reaction mixture in methanol through a small
amount of silica gel at each time point.
[5] For a discussion of dendrimer-encapsulated bimetallic nano-
particles with enhanced catalytic activity, see: a) R. W. J. Scott,
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Received: April 25, 2011
Published online: June 1, 2011
37, 1619. See also Ref. [1h].
[
6] For other representative examples of bimetallic nanoparticle
catalysts, see: a) S. Alayoglu, A. U. Nilekar, M. Mavrikakis, B.
Eichhorn, Nat. Mater. 2008, 7, 333; b) J. He, I. Ichinose, T.
Kunitake, A. Nakao, Y. Shiraishi, N. Toshima, J. Am. Chem. Soc.
Keywords: dendrimers · hydrogenation · iron · nanoparticles ·
.
rhodium
2003, 125, 11034.
[
[
[
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6
0
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[
2] I. Nakamula, Y. Yamanoi, T. Yonezawa, T. Imaoka, K. Yama-
moto, H. Nishihara, Chem. Commun. 2008, 5716.
Angew. Chem. Int. Ed. 2011, 50, 5830 –5833
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5833