Electroreduction mechanisms of meso-Tetraphenylphlorin cation in strongly acidic media of benzonitrile

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In nonaqueous media free-base porphyrins are easily protonated by strong proton donors to form the diprotonated porphyrins. The two-electron reduction of these species leads to phlorin cation species. To investigate the electroreduction products that follow the formation of these species, we evaluate the effect of increasing amounts of HClO4 on the electroreduction mechanism of the phlorin cation of meso-tetraphenyl porphyrin. We demonstrate that the phlorin cation undergoes a reversible protonation reaction to form a porphodimethene, which is further reduced to yield the chlorin-phlorin cation, a 4-electron reduced phlorin macrocycle with a hydrogenated pyrrolic Cβ = Cβ double bond. This undergoes further protonation and a 2-electron reduction to form a phlorin macrocycle, the bacteriochlorin-phlorin cation, wherein two pyrrole rings are hydrogenated. All these reduction processes occur at similar potential values, yet we managed to selectively form the 2-, 4- or 6- electron reduced product by a strict control of the number of protons in the medium. To the best of our knowledge, this is the first report of the electrochemical reduction of Cβ = Cβ bonds of pyrrolic rings. Thus, a new pathway for the electrochemical reduction of porphyrins is proposed.