Photochemistry of manganese porphyrins. Part 1.—Characterisation of some water soluble complexes

Manganese(III) forms many high spin d4 porphyrins, for which pyridyl, N-methylpyridyl, sulphonic acid and carboxylic acid and carboxylic acid functions act as water solubilising groups. Reduction with dithionite yields the corresponding manganese(II) porphyrin. Both oxidation states have been clearly identified by e.p.r. and magnetic susceptibility measurements. On the other hand, oxidation with persulphate gives an unstable manganese(III) porphyrin π-radical cation which appears to form a dimer in aqueous solution at pH ≈ 10. All the porphyrins undergo a series of acid/base transitions in alkaline solution due to axially coordinated water molecules. For manganese(III) porphyrins, there are two pK transitions, around 8.0 and 10.5, and electrophoresis has identified the total electronic charge of each of the acid/base forms.