A spectroscopic study of the protonation of porphins and corroles

Abstract

The protonation behaviour of four porphyrins, two corroles, and aza-aetioporphyrin I in toluene–acetic acid mixtures has been studied by visible spectrophotometry. The macrocycles exist in the monoprotonated form in glacial acetic acid. The acidities of the seven monocations are compared. N-methyl substituted porphyrins are more basic than their unsubstituted analogues. The protonation in acetic acid–sulphuric acid mixtures of the monocations derived from five of the macrocycles have also been studied. Dications are formed at low sulphuric acid concentrations. The addition of a third proton at high sulphuric acid concentrations occurred for a corrole and aza-aetioporphyrin I.