ELECTROCHEMISTRY OF THE MANGANESE(III)-MANGANESE(IV) 5,10,15,20-TETRAKIS(PARA-TRIMETHYLAMMONIOPHENYL)PORPHYRINATE COUPLE

The redox behaviour of the [ Mnlll(tmap)]-[MnlV(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 [MnIV(tmap)] complex is quite stable at high pH values (half-life ca. 20 min at pH 13.4) and converts back to the MnlIl state with a rate which gradually increases when the solution is acidified. The decomposition of water is described in terms of an electrochemicalLchemical catalytic cycle which involves the [ MnIV(tmap)] complex.