Phosphine-metal carbonyl derivatives as free radical sources

Abstract

A study has been made of the free-radical producing activities of dinuclear derivatives of transition metal carbonyls containing phosphine substituents, in the presence of CCl4. As in earlier work with simple metal carbonyls the activities have been assessed on the basis of kinetic studies of the polymerization of methyl methacrylate initiated by these systems. Compounds of type A (see text) are particularly effective initiators. Measurements of the evolution of carbon monoxide under a variety of conditions suggest that the rate-determining process with group 6 metals is the rupture of a metal-phosphorus bond, without intervention of monomer or solvent. The cobalt derivative of type A appears to initiate as a result of scission of CO groups, probably by reaction with monomer. Derivatives of the group 6 metals of type B have activities which are generally closer to those of the parent carbonyls. In the only case which has been studied in detail, rupture of a metal-phosphorus bond is the predominant primary process. Both metal atoms in the molecules of these derivatives (types A and B) function as initiating centres. A few results have also been obtained with the mononuclear iron compounds Fe(CO)5, Fe(CO)4PPh3, Fe(CO)3(PPh3)2, which indicate that substitution of CO by PPh3 leads to progressively increased activity. A higher activity in the compounds mentioned, compared to the parent carbonyls, is considered to arise from the relative weakness of the metal-phosphorus bond. In special cases where structural features favour solvent intervention, a high activity may be brought about by CO displacement. So far, no example of the displacement of phosphine by solvent has been found.