Transition metal–carbon bonds. Part X. Reactions between allylic palladium halides and tertiary phosphines, triphenylarsine, triphenylstibine, or carbon monoxide

Abstract

A series of complexes of the type [PdX(all)L](X = halogen, all = allylic ligand, and L = tertiary phosphine, triphenylarsine, or triphenylstibine) is described and the asymmetric bonding of the allylic ligand discussed. Several fast rate processes occur in solutions containing allylic palladium halides [Pd2X2(all)2] and neutral ligands L (e.g., phosphines) as the ratio L/Pd is varied from 0 to ca. 2 and these have been studied by n.m.r. spectroscopy at 34°. These fast rate processes involve the conversion of the π-allylic complexes into dynamic σ-allylic systems in which various rotations about carbon–carbon and palladium–carbon bonds and also exchange of the co-ordinated ligand L with free ligand occur. Co-ordination of dimethylphenylphosphine to the palladium atom of [Pd2Cl2(2-methylallyl)2] has been studied both by n.m.r. and by osmometry and at least two dimethylphenylphosphine ligands can become co-ordinated to the palladium, with the probable formation of [PdCl(σ-2-methylallyl)-(PMe2Ph)2] in solution. Carbon monoxide will also co-ordinate to the palladium atom of [Pd2Cl2(2-methylallyl)2], the n.m.r. spectrum in chloroform solution corresponding to that of a ‘dynamic’σ-2-methylallylic system.