Chemical Properties of Water-soluble Porphyrins 3. The Reaction of Superoxide Radicals with Some Metalloporphyrins

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Abstract
The catalytic efficiency of some water soluble metalloporphyrins upon the disproportionation of the superoxide radicals was determined by observing the rate of decay of the radical at 254 nm. The Fe(III), Mn(III), Co(III), Ni(II), Cu(II) and Zn(II) derivatives of tetrakis-(4-N-methylpyridyl) porphyrin (TMPyP), tetra(4-N,N,N-trimethylanilinium) porphyrin (TAP), and tetra(4-sulfonatophenyl) porphyrin (TPPS4) were studied. The order of catalytic efficiency was Fe(III)TMPyP .mchgt. Mn(III)TMPyP > Co(III)TMPyP .simeq. Mn(TAP) > Fe(III)TPPS4. Other metalloporphyrins did not show catalytic activity. Reduction potentials of the catalytically active metalloporphyrins determined from cyclic voltamograms indicate the importance of having a second energetically accessible oxidation state available to the metal ion for catalysis. The catalytic action of the above metalloporphyrins is discussed in terms of inner and outer-sphere mechanisms. With the exception of Fe(III)TMPyP, which reacts via an inner-sphere mechanism, the others are probably governed by an outer-sphere mechanism.