PAMAM dendron-stabilised palladium nanoparticles: Effect of generation and peripheral groups on particle size and hydrogenation activity

Article abstract of DOI:10.1039/b710860e

Dendron stabilised Pd nanoparticles were prepared using the self-assembly of dendrons, which could catalyze a highly selective hydrogenation of dienes and acetylenes to monoenes. The Royal Society of Chemistry.

Full text of DOI:10.1039/b710860e

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
PAMAM dendron-stabilised palladium nanoparticles: effect of
generation and peripheral groups on particle size and hydrogenation
activity{
a
Tomoo Mizugaki, Makoto Murata, Sayaka Fukubayashi, Takato Mitsudome, Koichiro Jitsukawa and
a
a
a
a
ab
Kiyotomi Kaneda*
Received (in Cambridge, UK) 16th July 2007, Accepted 11th October 2007
First published as an Advance Article on the web 24th October 2007
DOI: 10.1039/b710860e
Dendron stabilised Pd nanoparticles were prepared using the
self-assembly of dendrons, which could catalyze a highly
selective hydrogenation of dienes and acetylenes to monoenes.
the dendron units provide multiple functionalities for the Pd
nanoparticles, e.g., templates for Pd nanoparticle formation,
ligands for active intermediates, and specific interactions with
substrates.
Dendrons, wedge-like molecules composed of a core, divergent
branching units, and, end groups, have generated interest because
of their great potential in tailoring highly ordered structures and
PAMAM dendrons with a pyridine core unit were synthesized
using a divergent method from the reaction of 4-picolylamine with
8b
methyl acrylate and ethylenediamine, and their terminal ester
1,2
functions. Self-assembly of dendrons can afford spherically
3
shaped architectures similar to micelles and vesicles. A variety of
moieties were alkylated by n-hexylamine [G Py–C (n = 1–3),
n
6
Fig. 1]. Similarly, using dodecylamine instead of hexylamine gave
1
subunits have been reported to create well-defined and finely tuned
capsule structures, which are expected to provide integrated
functional nano-materials in combination with metal species such
G
n
Py–C12. Their structures were characterized using H NMR
spectroscopy, dynamic light scattering (DLS) measurements and
TEM imaging. After complexation of [PdCl(C )] with the
above dendrons in dichloromethane, reduction of the palladium
complex with addition of 2 equivalents of LiB(Et) H gave a
homogeneous dark solution. Notably, the obtained palladium
nanoparticles [1a G Py–C Pd(0), 2a G Py–C Pd(0), and 3a G
Py–C Pd(0)] were stable for several months due to the
encapsulation effect of the globular assemblies of the dendrons.
H
5 2
3
4,5
as metal complexes and particles. The dendritic shell can act as a
stabilizer for active metal species in organic syntheses, where the
internal branch units function as solvents or additives to facilitate
catalytic reactions.
3
1
6
2
6
3
Metal nanoparticles have an extremely large surface area to
volume ratio, which can be highly tunable to create various
nanoscale objects. Applications of these metal nanoparticles are
widely expanding in areas such as catalysis, sensor devices, and
6
1
n 6 n
H NMR spectra of G Py–C and G Py–C12 exhibited broad
signals due to the internal methylenes at around d 3.2 ppm and the
pyridyl core at d 8.4 and 7.3 ppm, suggesting a reversed micelle-like
9
assembly for the dendrons. DLS and TEM imaging provided
6
nanoelectronics. Since their function strongly depends on the
metal particle size, preparation of size-regulated nanoparticles is a
7
crucial yet challenging problem. In particular, the precise control
information about the size of the dendritic Pd nanoparticles as
of particle sizes in the single nanometer (,10 nm) range is of
current interest in developing high performance catalysts for
organic synthesis. Although there have been many reports on the
stabilization of metal nanoparticles such as Pd, Pt and Au using
5
8
dendrimers and dendrons, the range of particle size has been
limited to between 1–3 nm. In the present paper, we describe
poly(amidoamine) (PAMAM) dendrons having a pyridine core
and alkyl end groups assembled into spherical micelle-like
structures. The size of Pd nanoparticles prepared in situ by these
dendrons can be controlled by chemical reduction. To our
knowledge, this is the first example of the controlled synthesis of
Pd nanoparticles with a wide size range between 3 to 10 nm using
dendron assemblies. The obtained dendritic Pd nanocomposites
catalyse selective hydrogenation of dienes and acetylenes, where
a
Department of Materials Engineering Science, Graduate School of
Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka,
Osaka, 560-8531, Japan. E-mail: kaneda@cheng.es.osaka-u.ac.jp;
Fax: +81-6-6850-6260; Tel: +81-6-6850-6260
Research Center for Solar Energy Chemistry, Osaka University, 1-3
Machikaneyama, Toyonaka, Osaka, 560-8531, Japan
Fig. 1 Synthesis of G
m = 6, 12) and the dendron-assembly encapsulated Pd nanoparticles G
Py–C Pd(0). (i) excess methyl acrylate, (ii) excess alkyl amine (x = 5, 11),
iii) excess ethylenediamine, (iv) [PdCl(C )] in CH Cl , (v) LiBEt H in
THF.
1 3 n m
to G amidoamine dendrons (G Py–C : n = 1–3,
b
n
m
(
3
H
5
2
2
2
3
{ Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b710860e
This journal is ß The Royal Society of Chemistry 2008
Chem. Commun., 2008, 241–243 | 241

View Article Online
shown in Table 1 and Fig. 2. DLS measurement of the dendrons
revealed the formation of globular assemblies in CH Cl with
2
2
average diameters of 10.8 ¡ 1.7, 10.9 ¡ 2.0, and 11.8 ¡ 1.7 nm
for G , G , and G Py–C (entries 1–3), respectively. Adding the
1
2
3
6
Pd complex to the dendrons did not alter their size distributions,
and even after reduction of the Pd complex, the size distributions
for 1a, 2a and 3a were similar to those of the parent dendron
assemblies (entries 4–6). From TEM images, the average Pd
nanoparticle sizes for G
.0(1a), 5.3 ¡ 1.7(2a), and 5.1 ¡ 1.9 nm(3a), respectively. By
changing the C alkyl chain to a C chain, G , G , and G Py–C
12
1 2 3 6
, G , and G Py–C dendrons were 6.8 ¡
2
6
12
1
2
3
dendrons gave smaller Pd nanoparticles of 4.6 ¡ 1.9(1b), 3.6 ¡
.3(2b) and 3.5 1.3(3b), respectively (entries 10–12).
Furthermore, the G Py–CO Me dendron without an alkyl chain
forms a large spherical assembly with an average diameter of
5.6 ¡ 2.0 nm, where the Pd nanoparticles of 10.6 ¡ 2.8 nm were
1
¡
2
2
1
Fig. 2 TEM images of Pd nanoparticles encapsulated within the G
2
accommodated within the dendron assembly (entries 13 and 14).
From these results, it is clear that the end group of the dendron
strongly influences and controls the size of Pd nanoparticles. Also,
increasing the generation of the dendron decreases the size of the
Pd nanoparticles. According to the results by DLS and TEM
measurements, the size distribution of the dendron assemblies
agreed well with the distribution derived from the combination of
the Pd nanoparticle and dendron sizes.{ These self-assembled
dendrons can thus form monolayer capsules with polar voids to act as
controllable templates for Pd nanoparticles with the wide size range
dendrons; (a) G
2
Py–CO Me Pd(0), (b) G
2
2
Py–C
6
Pd(0) (2a), (c) G
2
Py–
C12 Pd(0) (2b). and (d) the size distributions of 2a.
Table 2 Hydrogenation of conjugated dienes catalysed by dendron-
encapsulated Pd nanoparticle
a
8b
from 3 to 10 nm. This is the first example of the size regulation of
Pd nanoparticles by dendron assemblies.
2
TOF (mol H /mol Pd min)
Substrate
Dendron
G
1
G
2
G
3
In order to examine the catalytic activity of the dendron Pd
nanoparticles, the hydrogenation of olefins and acetylenes was
carried out. The dendron Pd catalysts show high selectivity for
hydrogenation of conjugated dienes to the corresponding mono-
enes. For example, using 1a–3a, 1,3-cyclooctadiene exclusively
gave cyclooctene (Table 2). The reactivity toward the monoene
was much lower than that toward the diene; cyclohexene was
barely hydrogenated compared to 1,3-cyclohexadiene. The initial
hydrogenation rates increased with increasing generation of the
n =1
n = 3
n = 3
a
Gn–C
Gn–C
Gn–C12
6
5.2
4.3
6.0
6.4
5.5
9.6
12.5
12.3
20.3
6
Reaction conditions: substrate 1.0 mmol, Pd 5.0 mmol, dendron
2 mmol, CH Cl 3 mL.
8
2
2
palladium active site; the increasing dendron generation provides
8e
a less congested particle surface. In the case of unimolecular
dendrimer encapsulated Pd nanoparticles, the surface active sites
were passivated by the highly congested surface and branching
dendrons: the G
3
dendron catalyst (3a) showed the highest
, G , and G . These findings are
catalytic activity among G
1
2
3
5f
units in the higher generation dendrimers. The above generation
effect of the dendrons was also observed with dendrons having C12
alkyl groups. The C12 dendron Pd nanoparticles of 1b–3b showed
interpreted in terms of accessibility of the substrate to the
Table 1 Average diameter of dendron assemblies and Pd nanopar-
ticles by DLS and TEM measurements
higher catalytic activities than 1a–3a having C alkyl groups, as
6
shown in Table 2, which might be due to the smaller Pd
nanoparticles. The above high monoene selectivities in the
hydrogenation of conjugated dienes were similar to the results
Average diameter/nm
a
b
Pd
Entry
Dendron
DLS
5f
by PPI dendrimer-encapsulated Pd nanoparticle catalysts.
1
2
3
4
5
6
7
8
9
G
G
G
G
G
G
G
G
G
G
G
G
G
G
1
2
3
1
2
3
1
2
3
1
2
3
2
2
Py–C
Py–C
Py–C
Py–C
Py–C
Py–C
Py–C12
Py–C12
Py–C12
Py–C12 Pd(0) 1b
Py–C12 Pd(0) 2b
Py–C12 Pd(0) 3b
Py–COOMe
Py–COOMe Pd(0)
b
6
6
6
6
6
6
10.8 ¡ 1.7
10.9 ¡ 2.0
11.8 ¡ 1.7
10.5 ¡ 1.6
11.0 ¡ 1.7
12.5 ¡ 2.6
9.4 ¡ 1.4
—
—
—
In the hydrogenation of 1-phenyl-1-propyne, catalyst 3a gave a
96% yield of 1-phenyl-1-propene (E/Z = 2/98) together with 4% of
Pd(0) 1a
Pd(0) 2a
Pd(0) 3a
6.8 ¡ 2.0
5.3 ¡ 1.7
5.1 ¡ 1.9
—
1-phenylpropane at 100% conversion of the substrate (Scheme 1).
The monoene selectivity is much superior to that of the Lindlar
catalyst under the same reaction conditions: 54% yield of 1-phenyl-
10.1 ¡ 1.6
11.2 ¡ 1.7
9.5 ¡ 1.6
10.0 ¡ 1.5
11.4 ¡ 1.8
15.6 ¡ 2.0
15.7 ¡ 3.3
—
—
1-propene (E/Z = 8/92) and 46% of propylbenzene were obtained
when the conversion of 1-phenyl-1-propyne reached 100% as
shown in Scheme 1. Also, cinnamyl alcohol (E/Z = 2/98) was
obtained in 97% yield together with 3% of 3-phenyl-1-propanol in
10
11
12
13
14
a
4.6 ¡ 1.9
3.6 ¡ 1.3
3.5 ¡ 1.3
—
10
the hydrogenation of 3-phenyl-1-propyne-1-ol. The above high
10.6 ¡ 2.8
selectivity for monoenes using the dendron stabilised Pd nanoparticle
11,12
is due to the ligand effect of the pyridine core of the dendrons.
2 2
Measured in CH Cl at 25 uC. Measured by TEM images.
§
2
42 | Chem. Commun., 2008, 241–243
This journal is ß The Royal Society of Chemistry 2008

View Article Online
(21COE) program ‘‘Creation of integrated EcoChemistry’’, Osaka
University.
Notes and references
{
The molecular sizes of G
Chem 3D are 1.8, 2.5, and 3.2 nm for C
respectively.
The Lindlar catalyst was obtained from N. E. ChemCat.
1
, G
2
, and G
3
Py–Cn dendrons calculated by
6
and 2.4, 3.1, and 3.8 nm for C12
,
§
Scheme 1 Hydrogenation of alkynes using 3a and Lindlar catalyst.
a
Reaction conditions are in similar to those in Table 2. Figures in
1 Dendrimers and Other Dendritic Polymers, ed. J. M. J. Fr e´ chet and D. A.
Tomalia, Wiley & Sons, New York, 2001.
parentheses are the selectivity of the (E)- and (Z)-olefins.
2
For excellent reviews on supramolecular dendritic catalysts, see: (a)
R. van Heerbeek, P. C. J. Kamer, P. W. N. M. van Leeuwen and
J. N. H. Reek, Chem. Rev., 2002, 102, 3717–3756; (b) S. M. Grayson
and J. M. J. Fr e´ chet, Chem. Rev., 2001, 101, 3819–3868; (c) D. Astruc
and F. Chardac, Chem. Rev., 2001, 101, 2991–3024.
3
4
S. Hecht and J. M. J. Fr e´ chet, Angew. Chem., Int. Ed., 2001, 40, 74–91.
For recent reports on dendritic metal complex catalysts, see: Pd: (a)
T. Iwasawa, M. Tokunaga, Y. Obora and Y. Tsuji, J. Am. Chem. Soc.,
2
004, 126, 6554–6555; (b) M. Ooe, M. Murata, T. Mizugaki, K. Ebitani
Scheme 2 Intermolecular competitive hydrogenation of 3-phenyl-2-
propyn-1-ol (ol) and 1-phenyl-1-propyne (yn) catalysed by 2a. Rol and
and K. Kaneda, J. Am. Chem. Soc., 2004, 126, 1604–1605; (c) K. Heuz e´ ,
D. M e´ y, D. Gauss and D. Astruc, Chem. Commun., 2003, 18,
Ryn are the initial hydrogenation rates of ol and yn, respectively.
2
274–2275Rh; (d) M. Q. Slagt, S.-E. Stiriba, H. Kautz, R. J. M. K.
Gebbink, H. Frey and G. van Koten, Organometallics, 2004, 23,
525–1532Rh; (e) B. Yi, Q.-H. Fan, G.-J. Deng, Y.-Mi. Li, L.-Q. Qiu
1
Furthermore, the effect of the surrounding dendron branches
and A. S. C. Chan, Org. Lett., 2004, 6, 1361–1364; (f) J. P. K. Reynhardt
and H. Alper, J. Org. Chem., 2003, 68, 8353–8360.
5 Recent examples of dendritic metal nanoparticle catalysts, see: (a) L. Wu,
B.-L. Li, Y.-Y. Huang, H.-F. Zhou, Y.-M. He and Q.-H. Fan, Org.
Lett., 2006, 8, 3605–3608; (b) Y. Jiang and Q. Guo, J. Am. Chem. Soc.,
was emphasized in the intermolecular competitive hydrogenation
between 3-phenyl-2-propyn-1-ol and 1-phenyl-1-propyne using 2a
as shown in Scheme 2. The initial hydrogenation rate for 3-phenyl-
2-propyn-1-ol (R ) was greater than that for 1-phenyl-1-propyne
ol
2006, 128, 716–717; (c) H. Ye, R. W. J. Scott and R. M. Crooks,
(R ) by a factor of 28, although the hydrogenation rate ratio for
yn
Langmuir, 2004, 20, 2915–2920; (d) K. Esumi, R. Isono and
T. Yoshimura, Langmuir, 2004, 20, 237–243; (e) R. W. J. Scott, H. Ye,
R. R. Henriquez and R. M. Crooks, Chem. Mater., 2003, 15,
3873–3878; (f) M. Ooe, M. Murata, T. Mizugaki, K. Ebitani and
K. Kaneda, Nano Lett., 2002, 2, 999–1002.
Recent reviews on metal nanoparticles, see: (a) C. Burda, X. Chen,
R. Narayana and M. A. El-Sayed, Chem. Rev., 2005, 105, 1025–1102;
(b) M.-C. Daniel and D. Astruc, Chem. Rev., 2004, 104, 293–346; (c)
H. B o¨ nnemann and R. M. Richards, Eur. J. Inorg. Chem., 2001,
2455–2480; (d) G. Schmid, M. B a¨ umle, M. Geerkens, I. Heim,
C. Osemann and T. Sawitowski, Chem. Soc. Rev., 1999, 28, 179–185.
individual hydrogenations of 3-phenyl-2-propyn-1-ol and 1-phe-
nyl-1-propyne was 2.1. Clearly, the hydrogenation of an acetylenic
compound is accelerated by the hydroxyl group of the substrate
through hydrogen bonding with the amide groups within the
6
13
dendron.
In conclusion, we demonstrated the preparation of supramole-
cular dendritic Pd(0) nanoparticles utilizing self-assembly of
dendrons. Fine tuning of the Pd nanoparticle size ranging from
3
nm to 10 nm was achieved by adjusting the alkyl chain length
7 O. M. Wilson, M. R. Knecht, J. C. Garcia-Martinez and R. M. Crooks,
J. Am. Chem. Soc., 2006, 128, 4510–4511.
and the generation of the dendrons. The dendron units provide
multiple functionalities to the Pd nanoparticles: (1) templates and
stabilizing agents for the palladium nanoparticles, (2) ligands of
active sites adjusting the catalytic activity and selectivity, and (3)
binding hosts by hydrogen bonding between the amido group of
the dendron and the oxygen group of the substrate. Further
investigations on the size controllability of these dendrons for high
performance metal nanoparticle catalysts are underway.
8
Typical examples of dendron protected metal nanopartcles: (a)
R. C. Advincula, Dalton Trans., 2006, 2778–2784; (b) M. Murata,
Y. Tanaka, T. Mizugaki, K. Ebitani and K. Kaneda, Chem. Lett., 2005,
34, 272–273; (c) G. Jiang, L. Wang, T. Chen, H. Yu and J. Wang,
Nanotechnology, 2004, 15, 1716–1719; (d) C. S. Love, V. Chechik,
D. K. Smith and C. Brennan, J. Mater. Chem., 2004, 14, 919–923; (e)
K. R. Gopidas, J. K. Whitesell and M. A. Fox, J. Am. Chem. Soc.,
2003, 125, 6491–6502.
I. Gistov and J. M. J. Frechet, Macromolecules, 1993, 26, 6536–6546.
9
This work was carried out with partial support from the Grant-
in-Aid for Scientific Research from the Ministry of Education,
Culture, Sports, Science, and Technology of Japan (17656263,
10 Highly selective hydrogenation of alkynes to monoenes was achieved
using nitrogen additives such as quinolines: R. L. Augustine, in
Heterogeneous Catalysis for the Synthetic Chemists, Marcel Dekker,
New York, 1996, ch. 16, p. 387.
18360389), and from the Grant-in-Aid for Scientific Research on
11 The phosphine dendron-stabilized Pd nanoparticle catalyst gave
Priority Areas (No.18065016, ‘‘Chemistry of Concerto Catalysis’’)
from the Ministry of Education, Culture, Sports, Science, and
Technology of Japan. TEM experiments were carried out by using
a facility in the Research Center for Ultrahigh Voltage Electron
Microscopy, Osaka University. We thank the Center of Excellence
quantitative formation of alkanes from alkynes. See ref. 5a.
2 The dendron catalyst is stable and no precipitation was observed after
complete conversion of the alkynes.
1
1
3 Dendrimer-encapsulated Pd nanoparticles showed similar selectivity
through an attractive hydrogen bonding interaction between the
dendrimers and the functional group of the substrate. See ref. 5f.
This journal is ß The Royal Society of Chemistry 2008
Chem. Commun., 2008, 241–243 | 243