Modification of the Redox‐Properties of Tetraphenylporphyrin‐Complexes by Bases in Methylene‐Chloride Solution. The Equilibrium of Different Oxidation‐States with Added Base as Measured by Cyclic Voltammetry

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Abstract
The oxidation half‐wave potentials of Co2+ tetraphenylporphyrin, measured by cyclic voltammetry in methylenechloride solution, are changed by the addition of weak bases like methanol, acetone and dimethylformamide. The wave corresponding to the metal‐oxidation Co2+ ⇄ Co3+ is shifted to lower potential, while the wave, corresponding to the ligand oxidation Co3+(TPP) ⇄ Co3+(TPP)+. is shifted to higher potential. It can be shown, that this is due to a preferential interaction of Co3+(TPP) with the bases, which is stronger than that of the Co2+(TPP) as well as that of the Co3+(TPP)+. Because the shifts are proportional to the concentration of added base, they can be followed quantitatively and the equilibrium constants of the interactions between the different oxidation states and the bases can be calculated. The reduction wave of Co2+(TPP), corresponding to the Co3+ ⇄ Co1+ transition is not changed by the weak bases, but is shifted to lower potential by pyridine and the picolines. The shift is again proportional to the concentration of added base. The reduction wave of Fe3−(TPP)Cl− is not changed by the weak bases, but is shifted to lower potential by pyridine. While the reduction wave of the free Fe3+(TPP)Cl− is irreversible, that of the pyridine‐adduct is reversible. In this case the shift is not proportional to the amount of added base. The voltammetric curves of the Cu, Ni, and Zn tetraphenyl‐porphyrins are not changed by the addition of either the weak or the strong bases.