Chemical Properties of Water-soluble Porphyrins. 4. The Reaction of A ‘Picket-fence-like’ Iron (III) Complex with the Superoxide Oxygen Couple

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Abstract
Solution properties of the iron-(III) ‘picket-fence-like’ porphyrin, Fe(III)-α,α,α,β-tetra-ortho (N-methyl-isonicotinamidophenyl) porphyrin, (Fe(III)PFP) were investigated. These were acid/base properties of the aquo complex with pKa of 3·9 and its aggregation (formation of dimer with K = 1 × 10−10 dm3 mol−1), complex formation with cyanide ions and 1-methyl imidazole (1-MeIm), spectral properties of the three iron complexes in their ferric and ferrous form and the one-electron reduction potential of these complexes. Knowing these properties, the reaction of the ferric complexes, aquo, dicyano and bis (1-MeIm), with the superoxide radical and other reducing radicals were studied using the pulse radiolysis technique. The second-order reaction rate constant of O−2 with the iron (III) aquo complex which governs the catalytic efficiency of the metalloporphyrin upon the disproportionation of the superoxide radical was 7·6 × 107 dm3 mol−1 s−1, two orders of magnitude faster when compared to the reaction of each of the other complexes. The reduction by other radicals with all iron (III) complexes had similar second-order rate constants (109 to 1010 dm3 mol−1 s−1). The reduction reaction in all cases produced Fe(II)PFP and no intermediate was found. The oxidation reaction of Fe(II)PFP by O−2 was one order of magnitude faster when compared to the reduction of Fe(III)PFP by the same radical. Since the reactivity of O−2 toward the three iron (III) porphyrin complexes follows their reduction potentials, it is suggesting the formation of a peroxo Fe(II) porphyrin as an intermediate. The reactions of the Fe(II)PFP complexes with dioxygen were also studied. The aquo complex was found to be first order in O2 and second order in Fe(II)PFP, suggesting the formation of a peroxo Fe(II) porphyrin as an intermediate. The intermediate formation was corroborated by evidence of the rapid CO binding reaction to the aquo complex of Fe(II)PFP. The two other complexes reacted very slowly with O2 as well as with CO.

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