Electrochemistry of the manganese(III)-manganese(IV) 5,10,15,20-tetrakis(p-trimethylammoniophenyl)porphyrinate couple

The redox behaviour of the [Mn^III(tmap)]-[Mn^IV(tmap)] couple [tmap = 5,10,15,20-tetrakis(p-trimethylammoniophenyl)porphyrinate] has been studied in aqueous solutions using cyclic voltammetry, optical transparent electrode thin-layer spectroelectrochemistry, a rotating ring-disc electrode, and differential pulse voltammetry. The couple shows reversible behaviour at pH > 8.5 with a half-wave potential of +0.04 V at pH 13.4. The potential is shifted in a positive direction by 120 mV per decrease of pH unit. The [Mn^IV(tmap)] complex is quite stable at high pH values (half-life ca. 20 min at pH 13.4) and converts back to the Mn^III state with a rate which gradually increases when the solution is acidified. The decomposition of water is described in terms of an electrochemical - chemical catalytic cycle which involves the [Mn^IV(tmap)] complex.