Electron transfer reactions of cobalt(III) and ruthenium(III) ammines with europium(III), ytterbium(II), and samarium(II) in aqueous solutions

Chemical properties of water-soluble porphyrins. 4. The reaction of a 'picket-fence-like' iron (III) complex with the superoxide oxygen couple

Solution properties of the iron-(III) 'picket-fence-like' porphyrin, Fe(III)-alpha,alpha,alpha, beta-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 X 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 O2- with the iron (III) aquo complex which governs the catalytic efficiency of the metalloporphyrin upon the disproportionation of the superoxide radical was 7.6 X 10(7) 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 (10(9) to 10(10) dm3 mol-1 s-1). The reduction reaction in all cases produced Fe(II)PEP and no intermediate was found. The oxidation reaction of Fe(II)PEP by O2- was one order of magnitude faster when compared to the reduction of Fe(III)PFP by the same radical. Since the reactivity of O2- 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.

Chemical properties of water-soluble porphyrins 3. The reaction of superoxide radicals with some metalloporphyrins

The catalytic efficiency of some water soluble metalloporphyrins upon the disproportionation of the superoxide radicals has been 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-sulphonatophenyl) porphyrin (TPPS4) were studied. The order of catalytic efficiency found was Fe(III)TMPyP much greater than Mn(III)TMPyP greater than Co(III)TMPyP approximately equal to Mn(TAP) greater than 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. It is suggested that with the exception of Fe(III)TMPyP which reacts via an inner-sphere mechanism, the others are governed by an outer-sphere mechanism.