Air-stable and highly active dendritic phosphine oxide-stabilized palladium nanoparticles: Preparation, characterization and applications in the carbon-carbon bond formation and hydrogenation reactions

Dendrimer-stabilized palladium nanoparticles were formed in the reduction of palldium bis(acetylacetonate) [Pd(acac)(2)] in the presence of phosphine dendrimer ligands using hydrogen in tetrahydrofuran. The resulting Pd nanoparticles were characterized by TEM, P-31 NMR and P-31 MAS NMR. ne results indicated that the dendritic phosphine ligands were oxidized to phosphine oxides. These dendrimer-stabilized Pd nanoparticles were demonstrated to be efficient catalysts for Suzuki and Stille coupling reactions and hydrogenations. The dendritic wedges served as a stabilizer for keeping the nanoparticles from aggregating, and as a vehicle for facilitating the separation and/or the recycling of the Pd catalyst. In the case of the Suzuki coupling re-action, these Pd nanoparticles exhibited high catalytic efficiency (TON up to 65,000) and air stability as compared with the commonly used homogeneous catalyst tetrakis(triphenylphosphine)palladium [Pd(PPh3)(4)]. In addition, the results obtained from the bulky dendritic substrate suggest that the Pd nanoparticles might act as reservoir of catalytically active species, and that the reaction is actually catalyzed by the soluble Pd(0) and/or Pd(II) species leached from the nanoparticle surface.