Giant palladium clusters: synthesis and characterization

Abstract

A series of palladium clusters containing from four to several hundred Pd atoms in the metal skeleton has been prepared and characterized with respect to structure and chemical properties, including catalytic activity. For smaller clusters good agreement was observed. between single-crystal X-ray and EXAFS structural data. Giant clusters approximating to Pd₅₆₁L₆₀(OCOCH₃)₁₈₀(L = phen, bipy) and Pd₅₆₁phen₆₀O₆₀X₆₀(X = PF₆, ClO₄, BF₄, CF₃CO₂), were characterized with TEM, SAXS, EXAFS, NMR and magnetic succeptibility data. These clusters contain a closepacked metal core and ligands L and X that are located at the periphery of a cluster. They are very soluble in water and polar organic solvents and can be considered as a bridge between low molecular clusters and particles of colloidal metals. Giant Pd clusters were found to be active homogeneous catalysts for various organic reactions, e.g. oxidative acetoxylation of alkenes and alkylarenes; oxidation of alkenes, formic acid and alcohols; dehydration of alcohols and formation of acetals. The kinetics and mechanism of the homogeneous oxidation of alkenes and HCO₂H in solutions of giant clusters were elucidated.