Dioxygen activation in the photochemistry of some oxo-metalloporphyrin complexes

Abstract

Ultraviolet irradiation of the peroxo-complexes [Ti(O₂)(tpp)] and [Mo(O₂)(tpp)](tpp = 5,10,15,20-tetraphenylporphyrinate) induces elimination of O₂ and generation of the corresponding oxometalloporphyrins [TiO(tpp)] and [MoO(tpp)]. Reductive elimination of O₂ from [{Fe(tpp)}₂O] also occurs upon irradiation of argon-purged solutions. Studies using spin traps in e.s.r. experiments, together with quenching experiments, indicate that photolysis of the peroxocomplexes produces O₂ in the ¹Δ_g excited state. For [{Fe(tpp)}₂O] the results differ with the conditions used. In aerated CH₂Cl₂ solutions, ¹O₂ is produced in the quenching process of the excited states by O₂. In degassed CH₂Cl₂ solutions apparently ¹O₂ is not formed in the photoinduced deoxygenation reaction, but rather, studies using spin traps for e.s.r. investigations have indicated the formation of the superoxide ion O₂^–. The attack on solvent by ¹O₂ and of photogenerated O₂^– involving radical formation, is also discussed.