The oxidation of several nickel(II) porphyrins by various radicals has been studied by pulse radiolysis in different media (Cl{sub 2}{sup {sm bullet}{minus}} and Br{sub 2}{sup {sm bullet}{minus}} in aqueous systems, Br atoms in organic solvents, and peroxyl radicals in organic and aqueous/organic systems). Photochemical oxidation was also examined in some cases. The absorption spectrum of the oxidation product was monitored within several microseconds after the pulse. Two types of differential spectra were observed, a broad absorption at 640-700 nm ascribed to the {pi}-radical cation, or a sharp absorption at 560-580 nm ascribed to nickel(III) porphyrin. Ni{sup II}TPP (tetraphenylporphyrin) in several organic solvents, protic and aprotic, was oxidized to Ni{sup III}TPP. The addition of 10% water as cosolvent or 0.1 M of electrolyte changed the route of oxidation to give the radical cation Ni{sup II}TPP{sup {sm bullet}+}. On the other hand, Ni{sup II}TSPP (tetrakis(4-sulfonatophenyl)porphyrin), which has four negative charges, was oxidized on the porphyrin ligand by all the radicals examined, in water and in several organic solvents. Ni{sup II}bis(N-methyl-4-pyridyl)diphenylporphyrin, with a charge of +2, and Ni{sup II} tris(4-sulfonatophenyl)(N-methyl-4-pyridyl)porphyrin, with an overall charge of {minus}2, were oxidized on the ligand in aqueous solution but on the metal in organic solvents. These andmore » other results led to the conclusion that most radicals react with Ni{sup II}P by an inner-sphere mechanism to bind onto the metal and give the Ni{sup III}P form. However, when the porphyrin is sufficiently charged to repel the axially bound anion, and/or when the medium enhances the separation of this anion from the metal, the result is oxidation of the porphyrin {pi}-system. In all cases, however, the one-electron-oxidation products, whether Ni{sup II}P{sup {sm bullet}+} or Ni{sup III}P, decay to yield two-electron ring oxidation products.« less
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