Interaction of phthalocyaninatoiron(II) with molecular oxygen. Kinetics and mechanism of the reaction in dimethyl sulphoxide

Abstract

The reaction of phthalocyaninatoiron(II), FePc, with dioxygen in dimethyl sulphoxide solution has been studied kinetically by measuring the visible spectral changes occurring under high concentration conditions (h.c.c.: [FePc] = 1 X 10 −4 mol dm −3 or higher) and low concentration conditions (l.c.c.: 1 X 10 −5 mol dm −3 or lower). At h.c.c. the observed spectral variations (a decrease in intensity of the band at 653 nm and a parallel increase of a band at 620 nm) develop with an autocatalytic-like pattern and can be reversed either by nitrogen bubbling or dilution. The insoluble μ-oxo species (FePc) 2O is the final reaction product. Under l.c.c. the spectral changes (a decrease in intensity of the spectrum at all wavelengths within the range 550–750 nm) are associated with an irreversible degradation of the complex. This spectral trend is also observed under h.c.c. when use is made of a free radical inhibitor (2,6-di(tert-butyl)-4-methylphenol). The whole experimental picture is satisfactorily explained by a mechanism in which an intermediate oxenic species FePcO can either react with FePc, to give reversibly the μ-oxo compound, or can oxidize the solvent (or a species derived from the solvent). In the h.c.c. reaction FePc behaves as an O-atom transfer catalyst.