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.