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Dendrimers are three-dimensional, highly branched, monodispersed macromolecules, which are obtained by an iterative sequence of reaction steps, giving precisely defined branching structures. The possibility of designing these well-defined macromolecules (e.g., by choosing specific functional end groups, or certain multifunctional monomers) is a very powerful tool that enables the exploration and development of a wide variety of applications. One of the most promising applications of dendrimers is in catalysis. In general, organometallic dendrimers offer potentials in combining the advantages of both homogeneous and heterogeneous catalyses because of their structurally well-defined and specific number of active sites, as well as their advantage of facile recovery by nanofiltration or solvent precipitation.
Active sites can be introduced specifically at: a) the surface, b) the core, c) branches, and d) inner cavities of dendrimers in a controlled manner, although it is difficult to control the number and location of active metal species in traditionally supported metal catalysts using linear or cross-linked polymers. The active sites at the periphery of dendrimers are readily available for catalytic reactions because of their globular shapes. However, when catalytic species are incorporated at the core of dendrimers, these dendrimers are applied as shape-selective, size-selective, or enantioselective catalysts. Catalytic groups can be anchored to the branches of the interior, giving rise to catalysts with both high loading of catalytic sites and control of the nanoenvironment around the active species. Moreover, dendrimers serve as host-type “nanoreactors” for metal guest molecules. Encapsulation of catalytic sites into the cavities of dendrimers leads to both active site isolation and substrate size selectivity.
We describe here the unique catalytic properties of both periphery-functionalized dendrimer-bound Pd(II) and Pd(0) complexes, and dendrimer-encapsulated Pd(0) nanoparticles based on poly(propylene imine) dendrimers. The dendrimer-bound Pd(II) complex has higher catalytic activity for selective hydrogenation of conjugated dienes to monoenes, than those of the corresponding low-molecular-weight Pd complex. Moreover, the dendrimer-bound Pd(II) complex is easily recovered and reused without any loss of activity. The dendrimer-bound Pd(0) complex showed high stereoselectivity in the allylic substitution reaction. Facile recovery of dendritic Pd complexes can be achieved by the use of a thermomorphic system. The size-selective and substrate-specific hydrogenation of olefins is achieved by dendrimer-encapsulated Pd(0) nanoparticles. Unique catalysis is attributed to hydrogen bonding between the internal amino groups of the dendrimers and substrates.