Kinetics and mechanism of the autoxidation of tris[biacetyl bis(methylimine)]iron(II)

Abstract

Tris[biacetyl bis(methylimine)]iron(II) undergoes autoxidation in acidic aqueous media through a free-radical process. Spectrophotometric and n.m.r. measurements show that molecular oxygen attacks the co-ordinated ligands without cleavage of the metal–ligand bond. The main product of the reaction is due to oxidation of a C-methyl group to a hydroperoxide group. Kinetic measurements show that the reaction is first order with respect to the complex and half order with respect to the oxygen concentration, and suggest that protonation is involved in a pre-equilibriumstep. Iron(III) promotes the reaction, while iron(II) ions strongly inhibit it. A mechanism is proposed which involves an intramolecular electron transfer followed by a free-radical chain process.