This Account reports the synthesis and characterization of dendrimer-encapsulated metal nanoparticles and their applications to catalysis. These materials are prepared by sequestering metal ions within dendrimers followed by chemical reduction to yield the corresponding zerovalent metal nanoparticle. The size of such particles depends on the number of metal ions initially loaded into the dendrimer. Intradendrimer hydrogenation and carbon-carbon coupling reactions in water, organic solvents, biphasic fluorous/ organic solvents, and supercritical CO 2 are also described.

Nearly monodisperse (1.7 ( 0.2 nm) palladium nanoparticles were prepared within the interiors of three different generations of hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimers. These dendrimerencapsulated catalysts (DECs) were used to hydrogenate allyl alcohol and four R-substituted derivatives in a 4:1 methanol/water mixture. The results indicate that steric crowding on the dendrimer periphery, which increases with dendrimer generation, can act as an adjustable-mesh nanofilter. That is, by controlling the packing density on the dendrimer periphery, it is possible to control access of substrates to the encapsulated catalytic nanoparticle. In general, higher-generation DECs or larger substrates resulted in lower turnover frequencies (although some interesting exceptions were noted). Although the main products of the olefin hydrogenation reactions were the corresponding alkanes, ketones were also obtained when monosubstituted R-olefins were used as substrates. NMR spectroscopy was used to measure the size selectivity of DECs for the competitive hydrogenation of allyl alcohol and 3-methyl-1-penten-3-ol. The effect on catalytic rate as a function of nanoparticle size is also briefly discussed.

Palladium nanoparticles (2−3 nm in diameter) have been prepared within covalently functionalized poly(propylene imine) (PPI) dendrimers, and the resulting composite materials are shown to be effective for Heck coupling reactions. Two novel concepts are demonstrated in this report. The first concept involves the incorporation of Pd 0 nanoparticles into PPI dendrimers covalently functionalized with perfluorinated polyether chains on their periphery. The second concept involves the first example of a carbon−carbon coupling reaction catalyzed by a dendrimer-templated nanomaterial, specifically, the catalytic heterocoupling between nonactivated aryl halides and n-butylacrylate mediated by the dendrimer-encapsulated catalysts. These reactions were carried out in a homogeneous fluorous/organic reaction phase at elevated temperature, and the catalyst was recovered by cooling to room temperature and concomitant phase separation. The catalyst was found to be catalytically active at a reaction temperature of only 90 °C in the absence of toxic phosphines, and it was 100% selective for the production of n-butyl-trans-formylcinnamate with unoptimized isolated yields up to 70%. The recovered catalysts retained a significant fraction of the original activity, comparable to coordination complex catalysts that use a similar catalyst recovery system.

This chapter describes composite materials composed of dendrimers and metals or semiconductors. Three types of dendrimer/metal-ion composites are discussed: dendrimers containing structural metal ions, nonstructural exterior metal ions, and nonstructural interior metal ions. Nonstructural interior metal ions can be reduced to yield dendrimer-encapsulated metal and semiconductor nanoparticles. These materials are the principal focus of this chapter. Poly(amidoamine) (PAMAM) and poly(propylene imine) dendrimers, which are the two commercially available families of dendrimers, are in many cases monodisperse in size. Accordingly, they have a generation-dependent number of interior tertiary amines. These are able to complex a range of metal ions including Cu 2+ , Pd 2+ , and Pt 2+ . The maximum number of metal ions that can be sorbed within the dendrimer interior depends on the metal ion, the dendrimer type, and the dendrimer generation. For example, a generation six PAMAM dendrimer can contain up to 64 Cu 2+ ions. Nonstructural interior ions can be chemically reduced to yield dendrimer-encapsulated metal nanoparticles. Because each dendrimer contains a specific number of ions, the resulting metal nanoparticles are in many cases of nearly monodisperse size. Nanoparticles within dendrimers are stabilized by the dendrimer framework; that is, the dendrimer first acts as a molecular template to prepare the metal nanoparticles and then as a stabilizer to prevent agglomeration. These composites are useful for a range of catalytic applications including hydrogenations and Heck chemistry. The unique properties of the interior dendrimer microenvironment can result in formation of products not observed in the absence of the dendrimer. Moreover the exterior dendrimer branches act as a selective gate that controls access to the interior nanoparticle, which results in selective catalysis. In addition to single-metal nanoparticles, it is also possible to prepare bimetallic nanoclusters and dendrimer-encapsulated semiconductor nanoparticles, such as CdS, using this same general approach.

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