the use of protective ligands such as thiol,3a,b amine,1f,3c-e
and phosphine3f-j has been extensively studied.
the dendrimer acted as a stabilizer, a recycling vehicle, as
well as a ligand in catalysis.
Encapsulation of metal nanoparticles inside dendrimers
was originally demonstrated by the groups of Tomalia5 and
Crooks,6 and has been attracting considerable attention.1b,7
Polyamidoamines (PAMAMs)-encapsulated metal particles
have been demonstrated to be effective catalysts for olefin
hydrogenations7a-d and C-C coupling reactions,7e-i and the
dendrimers acted as both templates and porous nanoreactors.
Alternatively, a dendron with a coordinating group at the
focal point can be used as a capping ligand for the preparation
and stabilization of metal nanoparticles.8 In sharp contrast
to the small molecule ligand-stabilized nanoparticles, limited
dendritic ligands could be bound to the metallic core due to
the sterically demanding dendritic structure. Thus, a sub-
stantial fraction of the surface area of the core metal particle
is activated and available for participation in catalytic
reactions. These properties could promote the activity of the
nanoparticle catalysts. However, the successful examples of
such metal nanoparticles reported in catalysis are rather
limited.8e Recently, Fox et al. reported the first example of
thiol dendrimer-stabilized Pd nanoparticles using a third-
generation Fre´chet-type dendrimer with a disulfide group at
the core. Although it could efficiently catalyze Heck and
Suzuki coupling reactions, no activity was observed for the
hydrogenation reaction. As part of our continuing interest
in the synthesis of dendritic phosphines and their applications
in catalysis,9 we report here the first example of phosphine
dendrimer-stabilized Pd nanoparticles and their applications
in the Suzuki coupling reaction and hydrogenation, in which
Fre´chet-type polyaryl ether dendrons were chosen for this
study owing to their chemical inertness and inability to
coordinate palladium.10 The three different generation phos-
phine ligands GnDenP (n ) 1-3) were synthesized by
reaction of the corresponding dendritic bromides with KPPh2
according to the reported procedure with some modifica-
tions.11 The synthetic route to the phosphine dendrimer-
stabilized Pd nanoparticles (referenced as GnDenP-Pd, n )
1-3) was outlined in Figure 1. Reduction of Pd(acac)2 with
Figure 1. Synthesis and TEM images of GnDenP-Pd catalysts.
(4) (a) Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M. A. Org. Lett.
2000, 2, 2385. (b) Ramarao, C.; Ley, S. V.; Smith, S. C.; Shirley, I. M.;
DeAlmeida, N. Chem. Commun. 2002, 1132. (c) Narayanan, R.; El-Sayed,
M. A. J. Am. Chem. Soc. 2003, 125, 8340. (d) Chauhan, B. P. S.; Rathore,
J. S.; Bandoo, T. J. Am. Chem. Soc. 2004, 126, 8493. (e) Liu, Y. B.;
Khemtong, C.; Hu, J. Chem. Commun. 2004, 398. (f) Kim, S. W.; Kim, S.;
Tracy, J. B.; Jasanoff, A.; Bawendi, M. G. J. Am. Chem. Soc. 2005, 127,
4556. (g) Nishio, R.; Sugiura, M.; Kobayashi, S. Org. Lett. 2005, 7, 4831.
(5) Balogh, L.; Tomalia, D. A. J. Am. Chem. Soc. 1998, 120, 7355.
(6) Zhao, M. Q.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120,
4877.
(7) For selected recent examples, see: (a) Niu, Y.; Yeung, L. K.; Crooks,
R. M. J. Am. Chem. Soc. 2001, 123, 6840. (b) Ooe, M.; Murata, M.;
Mizugaki, T.; Ebitani, K.; Kaneda, K. Nano Lett. 2002, 2, 999. (c) Scott,
R. W. J.; Datye, A. K.; Crooks, R. M. J. Am. Chem. Soc. 2003, 125, 3708.
(d) Jiang, Y.; Gao, Q. J. Am. Chem. Soc. 2006, 128, 716. (e) Rahim, E. H.;
Kamounah, F. S.; Frederiksen, J.; Christensen, J. B. Nano Lett. 2001. 1,
499. (f) Li, Y.; El-Sayed, M. A. J. Phys. Chem. B 2001, 105, 8938. (g)
Pittelkow, M.; Moth-Poulsen, K.; Boas, U.; Christensen, J. B. Langmuir
2003, 19, 7682. (h) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2004,
108, 8572. (i) Garcia-Martinez, J. C.; Lezutekong, R.; Crooks, R. M. J.
Am. Chem. Soc. 2005, 127, 5097.
(8) For selected examples, see: (a) Chechik, V.; Crooks, R. M. Langmuir
1999, 15, 6364. (b) Wang, R.; Yang, J.; Zheng, Z.; Carducci, M. D.; Jiao,
J.; Seraphin, S. Angew. Chem., Int. Ed. 2001, 40, 549. (c) Kim, M. K.;
Jeon, Y. M.; Jeon, W. S.; Kim, H. J.; Hong, S. G.; Park, C. G.; Kim. K.
Chem. Commun. 2001, 667. (d) Gopidas, K. R.; Whitesell, J. K.; Fox, M.
A. J. Am. Chem. Soc. 2003, 125, 6491. (e) Gopidas, K. R.; Whitesell, J.
K.; Fox, M. A. Nano Lett. 2003, 3, 1757. (f) Guo, W.; Li, J. J.; Wang, Y.
A.; Peng, X. J. Am. Chem. Soc. 2003, 125, 3901. (g) Love, C. S.; Chechik,
V.; Smith, D. K.; Brennan, C. J. Mater. Chem. 2004, 14, 919.
(9) (a) Fan, Q. H.; Li, Y. M.; Chan, A. S. C. Chem. ReV. 2002, 102,
3385. (b) Fan, Q. H.; Chen, Y. M.; Chen, X. M. Chem. Commun. 2000,
789. (c) Deng, G. J.; Fan, Q. H.; Chen, X. M. Chem. Commun. 2002, 1570.
(d) Yi, B.; Fan, Q. H.; Deng, G. J. Org. Lett. 2004, 6, 1361. (e) Deng, G.
J.; Li, G. R.; Zhu, L. Y.; Zhou, H. F.; He, Y. M.; Fan, Q. H.; Shuai, Z. G.
J. Mol. Catal. A: Chem. 2006, 244, 118.
hydrogen was successfully carried out in THF in the presence
of the dendritic ligands, giving GnDenP-Pd as black pow-
ders.12 The formation of Pd nanoparticles was confirmed by
transmission electron microscopy (TEM) and 31P NMR
spectra.13 The diameter and size distribution of the nanosized
catalysts were examined by TEM, which ranged from 5.0
( 0.4 to 4.6 ( 0.5 to 3.2 ( 0.5 nm, decreasing with increase
of the generation of the dendritic ligands. These dendrimer
catalysts are soluble in common nonprotic organic solvents
such as toluene, THF, and dichloromethane, but insoluble
in methanol.
The catalytic activity of GnDenP-Pd was examined in the
Suzuki coupling reactions. This choice was based on the fact
that such reaction provides a powerful tool for the synthesis
of biaryls, which are found in many natural and synthetic
(10) Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638.
(11) Catalano, V. J.; Parodi, N. Inorg. Chem. 1997, 36, 537.
(12) GnDenP-Pd dendrimers were synthesized as follows: A 25-mL
glass-lined stainless autoclave with a magnetic stirring bar was charged
with 20 mg of Pd(acac)2 (0.066 mmol), 2 equiv of GnDenP, and 10 mL of
dried, degassed THF. The autoclave was pressurized with hydrogen to 25
atm, and the mixture was stirred at 60 °C for 18 h. After the H2 was carefully
released, the solvent was removed under reduced pressure. The resulting
black powders were further purified by sonication in ethanol more than 20
times. The Pd content of GnDenP-Pd was determined by ICP-XRF as
follows: G1DenP-Pd, 6.46%; G2DenP-Pd, 5.96%; G3DenP-Pd, 5.15%.
(13) For details, see the Supporting Information.
3606
Org. Lett., Vol. 8, No. 16, 2006