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

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Abstract
The redox behaviour of the [MnIII(tmap)]–[MnIV(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 MnIII 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 [MnIV(tmap)] complex.

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