Nanocage catalysts - Rhodium nanoclusters encapsulated with dendrimers as accessible and stable catalysts for olefin and nitroarene hydrogenations

Article abstract of DOI:10.1039/b813649a

The phenylazomethine dendrimer generation 4 (TPP-DPA G4) and polyamidoamine dendrimer (PAMAM G4-OH) encapsulating rhodium nanocluster were found to be highly effective for olefin and nitroarene hydrogenations, affording high TOF (up to 17520 h^-1); the important feature of the nanocage catalyst is that substrates can pass through the branches of the protecting groups of nanoclusters without releasing nanoclusters from the dendrimer. The Royal Society of Chemistry.

Full text of DOI:10.1039/b813649a

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Nanocage catalysts—rhodium nanoclusters encapsulated with dendrimers
as accessible and stable catalysts for olefin and nitroarene
hydrogenationsw
Ikuse Nakamula,a Yoshinori Yamanoi,a Tetsu Yonezawa,a Takane Imaoka,b
Kimihisa Yamamoto*b and Hiroshi Nishihara*a
Received (in Cambridge, UK) 5th August 2008, Accepted 1st September 2008
First published as an Advance Article on the web 30th September 2008
DOI: 10.1039/b813649a
The phenylazomethine dendrimer generation 4 (TPP-DPA G4)
and polyamidoamine dendrimer (PAMAM G4-OH) encapsulat-
ing rhodium nanocluster were found to be highly effective for
olefin and nitroarene hydrogenations, affording high TOF (up to
17520 hÀ1); the important feature of the nanocage catalyst is
that substrates can pass through the branches of the protecting
groups of nanoclusters without releasing nanoclusters from the
dendrimer.
methine dendrimer generation 4 (TPP-DPA G4) as well as
flexible and hydrophilic ployamidoamine dendrimer genera-
tion 4 with a terminal hydroxy group (PAMAM G4-OH)5 as
an effective catalyst for hydrogenation.6 These dendrimers
have coordination sites at the branching points working as
good electron-donating groups for Lewis acids. Consequently,
it is expected that nanoclusters prepared using these dendri-
mers are well-controlled in size with a narrow size distribution.
Scheme 1 shows the preparation procedure of phenylazo-
methine dendrimer-encapsulated rhodium nanoclusters as an
example. Initially, the complex 1 was obtained by coordina-
tion of RhCl3 to TPP-DPA G4. Reduction of the rhodium
complex with sodium tetrahydroborate then gave rhodium
cluster stabilized with TPP-DPA G4 (Rh60@TPP-DPA G4).
The formation of rhodium nanoclusters was confirmed by
transmission electron microscopy (TEM), matrix assisted laser
desorption ionization-time of flight (MALDI-TOF)-MS, and
X-ray photoelectron spectroscopy (XPS). The TEM study
indicated that the average diameter was 1.2 Æ 0.3 nm
(Fig. 1(a)). This diameter corresponds to 64 rhodium atoms
in a cluster, on the assumption of a face-centered cubic (fcc)
close-packed structure. The distribution of the cluster
diameter is very narrow, and these nanoclusters are almost
uniform. Furthermore, the peak around 17 000 in the
MALDI-TOF-MS spectrum shows that approximately 58
rhodium atoms are encapsulated in a single dendrimer mole-
cule.7 Both values obtained from TEM and MALDI-TOF-MS
measurements are in accord with the quantity of rhodium
added initially, and the error is primarily due to the ambiguity
of the TEM images and the MALDI-TOF-MS peaks. The
XPS spectra showed the surface rhodium atoms of these
nanoclusters to be zero-valent (lit.8 Rh(0): 3d5/2 307.0 eV,
3d3/2 311.8 eV, found: 3d5/2 307.0 eV, 3d3/2 311.5 eV). One of
the significant features of encapsulation with TPP-DPA G4 is
the exact match in the number of rhodium atoms before and
after cluster formation. While TPP-DPA G4 could exactly
produce the Rh60 cluster in the dendrimer template
(Rh60@TPP-DPA G4), a well-known dendrimer template
based on polyamidoamine architecture (PAMAM G4-OH)
did not afford such a finely regulated rhodium nanocluster
by the same in-solution-phase synthesis. The average diameter
was 1.8 Æ 0.4 nm by the TEM study (Fig. 1(b)). The averaged
number of rhodium atoms calculated from the cluster size with
a broader distribution was approximately 200 on the assump-
tion of an fcc structure. However, the electronic status
observed by the XPS spectra (3d5/2 306.9 eV, 3d3/2 311.6 eV)
Recently, colloidal nanoclusters of transition metals have been
used as quasi-homogeneous catalysts in organic transforma-
tion.1 Although the bulk metals have little catalytic activity,
metal nanoclusters show high catalytic efficiency due to the
large surface-to-volume ratio. In order to prepare the metal
nanoclusters, stabilizers such as surfactants, polymers and
dendrimers are utilized for the suppression of aggregation
and for the control of cluster size. The most serious problem
in using metal clusters as a catalyst is the negative relationship
between stability and catalytic activity, i.e. stabilization of
clusters brings about a decrease in substrate accessibility.
During the course of our study of dendrimers,2 we have been
interested in the development of accessible and stable metal
nanocluster catalysts encapsulated inside dendrimers.3 We
report here phenylazomethine dendrimer- and polyamido-
amine dendrimer-stabilized rhodium nanoclusters and their
applications to olefin and nitroarene hydrogenations. In these
clusters, the dendrimer acts as a stabilizer as well as a
substrate-capturing nanoreactor.
Rhodium complexes have been used as a catalyst for
hydrogenation reactions.4 Wilkinson’s complex (RhCl(PPh3)3)
is the representative complex catalyst enabling catalytic hy-
drogenation of alkenes and other unsaturated compounds in
homogeneous solution. Although Wilkinson’s complex can be
used for hydrogenation of various kinds of substrates under
relatively mild conditions, it is unstable against oxygen in
solution. To improve this defect, we designed rhodium na-
noclusters protected by rigid and hydrophobic phenylazo-
a Department of Chemistry, School of Science, The University of
Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-003, Japan.
E-mail: nisihara@chem.s.u-tokyo.ac.jp; Fax: (+81) 3-5841-8063
b Department of Chemistry, Faculty of Science and Technology,
Keio University, Hiyoshi, 3-14-1, Kouhoku-ku, Yokohama,
Kanagawa, 223-0061, Japan. E-mail: yamamoto@chem.keio.ac.jp;
Fax: (+81) 45-566-1718
w Electronic supplementary information (ESI) available: Analytical
data for all products. See DOI: 10.1039/b813649a
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Scheme 1 Synthetic procedure of Rh60@TPP-DPA G4.
Fig. 1 TEM images and the particle size distributions of (a) Rh60@TPP-DPA G4 and (b) Rh@PAMAM G4-OH.
was the same as that obtained with TPP-DPA G4. These
results strongly suggest that the character of dendritic encap-
sulation between the rigid TPP-DPA G4 and flexible PAMAM
G4-OH differ from each other. Observations of the catalytic
activity, as shown below, also reflect the structural and
dynamic aspects of these chemical encapsulations.
of the reasons for the relatively higher activity because the
effective surface area for one rhodium atom calculated for the
observed cluster size (7.54 [A2 atomÀ1] for 1.2 nm) is greater
than that of PAMAM G4-OH (4.63 [A2 atomÀ1] for 1.8 nm).
However, this result is not sufficient to explain the difference in
the TOFs. The primary reason would be based on the differ-
ence in dendritic encapsulations for each catalytic system that
act as stabilizers for these unprotected rhodium clusters.
Functionalized aniline derivatives are important intermedi-
ates in the production of pharmaceuticals.9,10 We next carried
out a reduction of nitroarenes using these rhodium catalysts
under hydrogen atmosphere (1 atm) to expand the scope of
this method, because catalytic hydrogenation of nitroarenes is
a simple method for the production of aminoaromatics. The
results are shown in Table 2. Although there was no reactivity
The catalytic activities of these rhodium nanoclusters were
examined by olefin hydrogenation. Table 1 represents the
results of hydrogenation of some substrates under mild con-
ditions catalyzed by the rhodium cluster in TPP-DPA G4,
PAMAM G4-OH, and Wilkinson’s complex. The turnover
frequencies (TOFs) were calculated using the reaction rate at
the final stage. Rh60@TPP-DPA G4 shows up to 1460 times
higher catalytic activity per rhodium atom than other rhodium
catalysts. The smaller size of the cluster in TPP-DPA G4 is one
Table 1 TOFs (hÀ1)/atom (TOFs (hÀ1)/cluster) of olefin hydrogenation
Entry
Substrate
TPP-DPA G4
PAMAM G4-OH
RhCl(PPh3)3
1
2
3
4
5
6
a
Butyl acrylate
6-Phenyl-1-hexene
1-Decene
292 (17 520)a
210 (12 600)a
199 (11940) a
173 (10 380)a
17 (1020)a
4 (800)b
3 (600)b
1 (200)b
3 (600)b
3 (600)b
1 (200)b
12
12
24
8
11
9
Styrene
1,5-Cyclooctadiene
1,1-Diphenylethylene
b
11 (660)a
Calculated as Rh60@TPP-DPA G4. Calculated as Rh200@PAMAM G4-OH.
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Table 2 TOFs (hÀ1)/atom (TOFs (hÀ1)/cluster) of nitroarene hydro-
genation
dendrimers. Additional developments in this area will be
reported in due course in our laboratories.
Notes and references
1 For reviews, see: (a) D. Astruc, F. Lu and J. R. Aranzaes, Angew.
Chem., Int. Ed., 2005, 44, 7852; (b) M. Moreno-Manas and R.
Pleixats, Acc. Chem. Res., 2003, 36, 638; (c) A. T. Bell, Science,
2003, 299, 1688; (d) A. Roucoux, J. Schulz and H. Patin, Chem.
Rev., 2002, 102, 3757; (e) M. A. El-Sayed, Acc. Chem. Res., 2001,
34, 257; (f) R. M. Crooks, M. Q. Zhao, L. Sun, V. Chechik and L.
K. Yeung, Acc. Chem. Res., 2001, 34, 181.
2 For our previous reports on dendrimers, see: (a) N. Satoh, T.
Nakashima, K. Kamikura and K. Yamamoto, Nat. Nanotechnol.,
2008, 3, 106; (b) K. Yamamoto, Y. Kawana, M. Tsuji, M. Hayashi
and T. Imaoka, J. Am. Chem. Soc., 2007, 125, 9256; (c) T. Imaoka,
S. Tanaka, M. Arimoto, M. Sakai, M. Fujii and K. Yamamoto, J.
Am. Chem. Soc., 2005, 127, 13896; (d) R. Nakajima, M. Tsuruta,
M. Higuchi and K. Yamamoto, J. Am. Chem. Soc., 2004, 126,
1630; (e) T. Imaoka, H. Horiguchi and K. Yamamoto, J. Am.
Chem. Soc., 2003, 125, 340; (f) K. Yamamoto, M. Higuchi, S.
Shiki, M. Tsuruta and H. Chiba, Nature, 2002, 415, 509.
Entry
R
H
TPP-DPA G4 PAMAM G4-OH RhCl(PPh3)3
1
2
3
4
5
a
1 (60)a
22 (4400)b
48 (9600)b
33 (6600)b
17 (3400)b
19 (3800)b
b
0
0
0
0
0
MeOCO 18 (1080)a
MeCO
H2N
MeO
13 (780)a
2 (120)a
5 (300)a
Calculated as Rh60@TPP-DPA G4. Calculated as Rh200@PAMAM
G4-OH.
3 For other examples of nanoclusters stabilized with dendrimers, see:
(a) C. Ornelas, J. R. Aranzaes, L. Salmon and D. Astruc,
Chem.–Eur. J., 2008, 14, 50; (b) K. Gopidas, J. Whitesell and M.
Fox, J. Am. Chem. Soc., 2003, 125, 6491; (c) O. Wilson, R. Scott, J.
Gracia-Martinez and R. Crooks, J. Am. Chem. Soc., 2005, 127,
1015; (d) H. Lang, R. May, B. Iversen and B. Chandler, J. Am.
Chem. Soc., 2003, 125, 14832; (e) L. Balogh and D. Tomalia, J.
Am. Chem. Soc., 1998, 120, 7355; (f) D. Volkmer, B. Bredenkotter,
¨
¨
J. Tellenbroker, P. Kogerler, D. Kurth, P. Lehmann, H.
¨
Schnablegger, D. Schwahn, M. Piepenbrink and B. Krebs, J.
Am. Chem. Soc., 2003, 125, 14832; (g) L. Wu, B.-L. Li, Y.-Y.
Huang, H.-F. Zhou, Y.-M. He and Q.-H. Fan, Org. Lett., 2006, 8,
3605, and see also ref. 1f.
4 (a) G. C. Bond, in Catalysis by Metals, Academic Press, London,
1962; (b) P. N. Rylander, in Catalytic Hydrogenation in Organic
Syntheses, Academic Press, New York, 1979; (c) B. R. James, in
Homogeneous Hydrogenation, Wiley, New York, 1973.
5 This dendrimer is available from Sigma-Aldrich.
6 For the examples of rhodium nanoclusters as catalysts, see: (a) K.
H. Park, K. Jang, H. J. Kim and S. U. Son, Angew. Chem., Int. Ed.,
2007, 46, 1152; (b) A. J. Bruss, M. A. Gelesky, G. Machado and J.
Dupont, J. Mol. Catal. A: Chem., 2006, 252, 212; (c) I. S. Park, M.
S. Kwon, N. Kim, J. S. Lee, K. Y. Kang and J. Park, Chem.
Commun., 2005, 5667; (d) B. Yoon and C. M. Wai, J. Am. Chem.
Soc., 2005, 127, 17174.
Fig. 2 Back folding images of TPP-DPA G42b (a) and PAMAM
G4-OH12 (b).
upon nitroarene hydrogenation in the presence of Wilkinson’s
complex, the nitro group was reduced to the amino moiety in
the presence of rhodium nanoclusters. In particular, PAMAM
G4-OH-stabilized rhodium nanoclusters were more effective in
this transformation. In addition, no reduction of ester or
ketone groups was observed during the hydrogenation
(entries 2 and 3).
7 See ESIw for the compound characterization data.
It should be noted that no significant difference was observed
by the comparison of the average diameters of rhodium na-
noclusters using TEM analysis during both the olefin and
nitroarene hydrogenations for both dendrimer-stabilized nano-
particles. From the perspective of catalytic activity, the solid
shell of TPP-DPA G4 with a sufficient inner-cavity (Fig. 2(a))
allows a wide variety of hydrophobic substrates to access the
rhodium cluster surface. In contrast, the liquid shell of PAMAM
G4-OH swelled with polar solvents (Fig. 2(b)) refuses hydro-
phobic substrates approaching the interior, whereas it assists in
the transmission of polar substrates such as nitroarenes.11
In conclusion, rhodium nanoclusters stabilized with phenyl-
azomethine and polyamidoamine dendrimers were prepared
with a narrow size distribution. These nanoclusters proved to
be an effective catalyst for hydrogenation of olefins and
nitroarenes, demonstrating that the catalytic activity of Rh
nanoclusters can be altered by the type of encapsulating
8 C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder and G. E.
Muilenberg, in Handbook of X-Ray Photoelectron Spectroscopy,
Perkin-Elmer, Minnesota, 1978.
9 For reviews, see: (a) A. M. Tafesh and J. Weiguny, Chem. Rev.,
1996, 96, 2035; (b) N. Ono, The Nitro Group in Organic Synthesis,
Wiley-VCH, New York, 2001.
10 Recently, several groups have reported nitroarene hydrogenation
with Pt, Pd, Co and Ni-Pd nanoclusters: (a) M. Takasaki, Y.
Motoyama, K. Higashi, S.-H. Yoon, I. Mochida and H.
Nagashima, Org. Lett., 2008, 10, 1601; (b) P. Yang, W. Zhang,
Y. Du and X. Wang, J. Mol. Catal. A: Chem., 2006, 260, 4; (c) R.
Raja, V. B. Golvko, J. M. Thomas, A. Berenguer-Marcia, W.
Zhou, S. Xie and B. F. G. Johnson, Chem. Commun., 2005, 2026.
11 The difference of reactivities can be interpreted by the value of
permittivity. Judging from the polarity on the terminal moieties,
DPP-TPA G4 has a low permittivity (benzene: e = 2.27), and
PAMAM G4-OH has a higher permittivity (ethanol: e = 24.3).
The dielectric constants of styrene and nitrobenzene are 2.43 and
34.8, respectively.
12 P. K. Maiti, T. Cagin, G. Wang and W. A. Goddard III,
Macromolecules, 2004, 37, 6236.
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5718 | Chem. Commun., 2008, 5716–5718