Poly(amidoamine)-Dendrimer-Stabilized Pd(0) Nanoparticles as a Catalyst for the Suzuki Reaction Michael Pittelkow, † Kasper Moth-Poulsen, †,‡ Ulrik Boas, § and Jørn B. Christensen* ,† Chemical Laboratory II, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark, The Nano-Science Center, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark, and The Danish Veterinary Institute, Bu ¨ lowsvej 27, DK-1790 Copenhagen, Denmark Received May 22, 2003 Introduction Nanoparticles and polymer-stabilized nanoobjects are promising candidates for applications in catalysis and nanotechnology. 1,2 An important issue when dealing with nanoparticles is to achieve protection of the particles against aggregation without disturbing the desired prop- erties. In traditional catalysis, this is done by immobilizing the particles on a suitable inert surface. In homogeneous catalysis, this can be achieved by stabilization with micelles or vesicles made of polymers or surfactants. Common for both types of stabilization is that the possibilities for controlling or designing the microenvi- ronment surrounding the particles are limited. A simple way to get well-defined cavities for the protection of nanoparticles is to use a well-defined macromolecule as a host. This will also allow for the design of different types of nanoenvironments and open up for an investigation of the interplay between the properties of the nanoparticle and the surrounding microenvironment. We have chosen to study dendrimers as hosts for nanoparticles from a purely synthetic point of view. Dendrimers are highly branched, well-defined, synthetic macromolecules available in nanometer dimensions. One of the properties of dendrimers is the presence of cavities inside the molecules. This was first demonstrated with the so-called “dendritic box” 3 and has since been a major topic in the field of dendrimer science. 4 Encapsulation of metal nanoparticles inside dendrimers was originally shown by the groups of Tomalia 5 and Crooks. 6 Cu(0) clusters were formed inside hydroxy- terminated poly(amidoamine) (PAMAM) dendrimers, and it was shown that the dendrimer host stabilizes the cluster against aggregation and prevents precipitation of the metal. More recently, the preparation of Pt(0) and Pd(0) dendrimer-stabilized metal clusters was reported. 7 These complexes have the interesting property that the metal clusters are formed inside the dendrimer as a result of the affinity of the metal to the amino groups inside the dendrimer, and the complexes are water soluble as a result of the hydroxy groups on the dendrimer surface. The catalytic properties of dendrimer-encapsulated Pd(0) nanoparticles in homogeneous catalysis have only been subject to preliminary investigations. The reduction of olefins within the nanoparticles was demonstrated by Zhao and Crooks in aqueous solution 7 and by Kaneda et al. in organic solvents, 8 and size selectivity was observed. Previous work from our group has showed catalysis of the Heck reaction using PAMAM-dendrimer-encapsulated Pd(0) nanoparticles with a very low amount of catalyst. 9 The Crooks group showed fluorous-phase catalysis of the Heck reaction using a modified poly(propylene imine)- dendrimer-encapsulated Pd(0) nanoparticle. 10 One of the most important palladium-catalyzed reactions is the Suzuki cross-coupling reaction between aryl halides (or triflates) and boronic acid derivatives 11 (Figure 1). The classical conditions for performing the Suzuki reaction involve the use of a Pd(0) complex or a Pd(II) salt and a phosphine ligand, which stabilizes Pd(0) as PdL 4 or PdL 3 complexes. 12 Pd-catalyzed reactions under phos- phine-free conditions is a topic of considerable interest because of both economic as well as environmental reasons. Suzuki cross-couplings with dendrimer-encapsulated Pd(0) nanoparticles have previously been reported by Li and El-Sayed. 2e However, their findings on the preparation and use of small Pd(0) nanoparticles (Pd 10 ) differ so much from our own findings on the preparation and use of Pd 60 nanoparticles that we report on our results. Experimental Section Measurements. Transmission electron microscopy (TEM) pictures were obtained with a Philips CM20 at 200 kV and a magnification of 150 000×. Chemicals. PAMAM G4-OH dendrimer was purchased from Aldrich Chemical Company. p-Tolylboronic acid was prepared * To whom correspondence should be addressed. E-mail: jbc@kiku.dk. Fax: +45 35320212. † Chemical Laboratory II, University of Copenhagen. ‡ The Nano-Science Center, University of Copenhagen. § The Danish Veterinary Institute. (1) Lewis, L. N. Chem. Rev. 1993, 93, 2693. (2) For a representative collection of recent publications, see: (a) Strimbu, L.; Liu, J.; Kaifer, A. E. Langmuir 2003, 19, 483. (b) Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M. A. Org. Lett. 2000, 15, 2385. (c) Li, Y.; Boone, E.; El-Sayed, M. A. Langmuir 2002, 18, 4921. (d) Reetz, M. T.; Westermann, E. Angew. Chem., Int. Ed. 2000, 39, 165. (e) Li, Y.; El-Sayed, M. A. J. Phys. Chem. B 2001, 105, 8938. (f) Kim, S.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 26, 7642. (g) Jansson, A. M.; Grøtli, M.; Halkes, K. M.; Meldal, M. Org. Lett. 2002, 1, 27. (h) Bergbreiter, D. E.; Osburn, P. L.; Wilson, A.; Sink, E. M. J. Am. Chem. Soc. 2000, 122, 9058. (i) Yamada, Y. M. A.; Takeda, K.; Takahashi, H.; Ikegami, S. Org. Lett. 2002, 20, 3371. 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Chem. 1999, 147, 576. (b) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457. (c) Miyaura, N. Top. Curr. Chem. 2002, 219, 11. (c) Martin, A. R.; Yang, Y. Acta Chem. Scand. 1993, 47, 221. (12) Hegedus, L. S. Transition Metals in the Synthesis of Complex Organic Molecules; University Science Books, 1999. Figure 1. Suzuki cross-coupling reaction. 7682 Langmuir 2003, 19, 7682-7684 10.1021/la0348822 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/16/2003