Heterogeneous Pd/POSS Nanocatalysts for C-C Cross-Coupling Reactions

An insoluble solid support of N-heterocyclic imidazolium-functionalized cage-like silsesquioxane (SQ); Bim-SQ, was synthesized by nucleophilic substitution of octakis(3-chloropropyl)octasilsesquioxane with an excess bis-(imidazol-1-yl)methane and investigated as a new organic-inorganic hybrid support. In the presence of Bim-SQ, Pd(II) from [PdCl4](2-) can be stabilized via both electrostatic interactions and imidazole-N-coordination, giving an amorphous material of Pd(II)@Bim-SQ. Subsequent insitu reduction of Pd(II)@Bim-SQ during Suzuki-Miyaura cross-coupling reactions afforded monodispersed Pd nanoparticles (2.33 +/- 1.50 nm) stablized on Bim-SQ support, PdNp@Bim-SQ with Pd loading up to 14% w/w. Catalytic studies revealed that the pre-catalyst Pd(II)@Bim-SQ was active heterogeneous catalyst for Suzuki-Miyaura cross-coupling reactions, having the maximum TOFs of 5400 h(-1) under mild conditions and in aqueous ethanol medium. Furthermore, PdNp@Bim-SQ was also shown to be an efficient catalyst for Heck reactions. Simple catalyst recovery and reusability of PdNp@Bim-SQ for at least 5 catalytic cycles without loss of activity were also demonstrated.