Polyhedral Oligomeric Silsesquioxane-Encapsulating Amorphous Palladium Nanoclusters as Catalysts for Heck Reactions

Abstract

Palladium nanoparticles (Pd NPs) capped with a certain protective group possessing stable reactivity of Pd-catalyzed reactions has been quickly and easily synthesized. The thiol-Pd(2+) complexes of C(12)-Pd(2+) and POSS-Pd(2+) are prepared by the ligand exchange of acetate groups on Pd(OAc)(2) with 1-dodecanthiol and SH-POSS (polyhedral oligomeric silsesquioxane), respectively. After thermal treatment at 120 degrees C for 30 min, the C(12)-Pd(2+) and POSS-Pd(2+) turn into C(12)-Pd(0) and POSS-Pd(0) NPs through the reduction of palladium cations with thiol groups. Unlike the well-known metallic Pd NPS with constituent Pd atoms arranged in an orderly repeating pattern, the cores of these thial-Pd(0) NPs are composed of a disordered (amorphous) aggregation of Pd atoms due to the steric hindrance caused by the affinity between in situ Pd atoms and thial groups. The relatively larger gaps between two adjacent POSS moieties, formed by solvent permeation when one isobutyl group approaches another, can be employed as transportation channels for reactants such as iodobenzene (IB). Thus, methyl trans-cinnamate is produced as soon as the reactants of IB and methyl acrylate are added in the POSS-Pd(0) NP-catalyzed Heck coupling. In contrast, an activation time of similar to 1 h is needed for C(12)-Pd(0) NPs to replace these long-alkyl protective groups with iodobenzene.