BIOMIMETIC OXIDATIONS - OXYGENATION REACTIONS BY PHOTOEXCITED IRON PORPHYRINS

A pyrazine-bridged Fe(II) protoporphyrin polymeric species is observed to react with oxygen giving rise to the formation of a mixed Fe(II)-Fe(III) porphyrin polynuclear complex which is stable in air for several days in neutral or slightly acidic solution. The effect of the presence of pyridine or pyrazine on the photocatalytic properties of Fe(III) protoporphyrin or Fe(III) octaethylporphyrin is investigated. It is observed that the irradiation of these systems in the presence of benzene as a substrate leads to the formation of phenol, benzohydroquinones and benzoquinones. A similar behaviour is observed with phenol which, indeed, is also oxidized to quinones. No appreciable amounts of oxidation products are formed in the absence of pyrazine or pyridine and under conditions where pyrazine-bridged polymeric species are formed. This is taken as an evidence of a considerable trans effect of the nitrogen ligands, favouring the coordination and the activation of O2.