PALLADIUM CLUSTERS - STOICHIOMETRIC AND CATALYTIC REACTIONS

The mechanistic aspects of organic reactions catalyzed with palladium clusters and stoichiometric reactions of carbonyl and carbene clusters are discussed. Palladium carbonyl carboxylates Pd4(CO)4(OCOR)4 (R = Me, CMe3, Ph, CF3, CCl3) undergo thermolysis above 110-130-degrees-C, giving rise to CO2, CO and diacyls. In solutions of aromatic compounds the insertion of carbon dioxide into the aromatic C-H bond or activated C-H bond of alkylaromatic compounds was observed in the course of the thermolysis. The decomposition of palladium carbene carboxylate cluster Pd4(Ph2C)4(OAc)4 at 80-degrees-C has been found to involve inner sphere carbene oxidation during which an oxygen atom is transferred from the carboxylate group to the carbene ligand. Analogously, the reaction of [Pd(OAc)2PPh3]2 with formic acid, a reaction involving intermediate cluster formation, includes the transfer of an oxygen atom from the formate droup to the P atom of a phosphorus containing ligand, supposedly a diphenylphosphido bridging group. Positional and geometric alpha-alkene isomerization in aqueous PdCl42- solution has been found to be calalyzed by palladium (I) complexes of type Pd2Cl42-. Colloidal clusters containing more than 500 palladium atoms in the metal core, which are soluble in polar organic solvents, have been found to catalyze the oxidative reactions of alkenes, toluene, alcohols and formic acid. Alcohols bearing at least one hydrogen atom in alpha-position undergo dehydration under mild conditions in non-acidic solution containing a Pd, Mo octanuclear anionic cluster [Pd4Mo4(CO)12Cp]2- . The reaction of benzyl alcohol gives rise to trans-stilbene. All these catalytic reactions can be rationalized within a scheme including the oxidative addition of the substrates across cluster metal-metal bond as a key step.