Generation, spectral properties and stability of π-cation radicals of (5,10,15,20-tetraphenylporphyrinato)(chloro)rhodium(III)

Abstract

Earlier it was shown by us that for metal porphyrins in the presence of strong acids dissociation of axial/macrocyclic ligands or formation of H + -associates takes place. Recently one-electron oxidation of stable metal porphyrins in aerated sulphuric acid has been observed as well. This paper is devoted to kinetic description of the multistep one-electron oxidation of chloro(5,10,15,20-tetraphenylporphinato)rhodium(III) by aerated sulfuric acid with formation of a stable π-cation radical. The multistep mechanism involving two slow irreversible reactions and rapid preequilibrium has been suggested. For the first time, H+-associate of metalloporphyrin was found to coordinate molecular oxygen with formation of (O 2)RhTPP H + R This is followed by two slow consecutive reactions-(i) one-electron oxidation of macrocvcle in (O 2)RhTPP H + R by coordinated oxygen with an assistance of H + (k 298K 1 = 0.3 10 -5 M -2 s -1); (ii) coordination of HSO - 4 which is in large excess (k 298K 2 = 0.04 10 -5 M -1 s -1) by [RhTPP + ] +. The precise spectral picture of each consecutive reaction and their rate ratio allow us to study the kinetics of each stage separately. Spectral properties and stability of oxidized form of metal porphyrin are described. The identifcation and study of the stable oxidized forms of metal porphyrin complexes are needed to elucidate the mechanisms of their catalysis and to suggest the systems with the best catalytic activity. © ISUCT Publishing.