Chemical properties of water-soluble porphyrins. 5. Reactions of some manganese (III) porphyrins with the superoxide and other reducing radicals.

Abstract

Solution properties of three manganese porphyrins, in monomeric form, were investigated. These were the 'picket-fence-like' porphyrin Mn(III)-alpha,alpha,alpha,beta- tetra-ortho(N-methylisonicotinamidophenyl)porphyrin (Mn(III)PFP) and two 'planar unhindered' porphyrins, the Mn(III)TMPyP (tetrakis (4-N-methylpyridyl)porphyrin) and Mn(III)TAP (tetra(4-N,N,N-trimethylanilinium)porphyrin). The porphyrin properties studied were: the absorption spectra in their manganic and manganous forms; acid/base properties of the aquo complexes; the effect of potential axial ligands (up to a concentration of 0.1 mol dm-3) and their one electron reduction potentials. Knowing these properties, the reaction of the Mn(III) porphyrins with the superoxide radical and other reducing radicals were studied using the pulse radiolysis technique. The second-order reaction rate constant of O2- with the Mn(III) porphyrins, which governs the catalytic efficiency of the metalloporphyrins upon the disproportionation of the superoxide radical, was 5.1 X 10(7) to 4.0 X 10(5) dm3 mol-1 s-1, depending on the pH and the nature of the metalloporphyrin. These values are at least one order of magnitude lower than found for Fe(III)TMPyP. One electron reduction of the three Mn(III) porphyrins by eaq-, CO2-, CH2OH and (CH3)2COH had similar second-order rate constants (10(9)-10(10) dm3 mol-1 s-1). That for (CH3)2(CH2)COH was about 10(5) dm3 mol-1 s-1. Reduction in all cases produced the corresponding Mn(II) porphyrin and no intermediate was found. The oxidation reaction of the Mn(II) porphyrins by O2- was approximately two orders of magnitude faster when compared to the reduction of Mn(III) porphyrins with the same radical. Since the reactivities of O2- towards the three manganese (III) compounds follow their reduction potentials, it is suggested that these reactions are governed by an outer-sphere mechanism. This suggestion is corroborated by the finding that water molecules acting as axial ligands, in these aqueous solution systems, are not replaced by another potential ligand when the latter is in the concentration range of 100 mM or less.