Migration reactivities of a-bonded ligands of organoiron and organocobalt porphyrins depending on different high oxidation states

Migration reactivities of σ-bonded organo-iron and -cobalt porphyrins were examined as a function of the compound oxidation state. Migration rates were determined for both the one-electron and two-electron oxidized species produced in the electron-transfer oxidation with different oxidants in acetonitrile at 298 K. The investigated compounds are represented as [(OETPP)Fe(R)]ⁿ⁺, where n = 1 or 2, OETPP = the dianion of 2,3,7,8,12,13,-17,18-octaethyl-5,10,15,20-tetraphenylporphyrin, and R = C₆H₅, 3,5-C₆F₂H₃, or C₆F₅, and as [(TPP)Co(R)]ⁿ⁺, where n = 1 or 2, TPP = the dianion of 5,10,15,20-tetraphenylporphyrin, and R = CH₃ or C₆H₅. The rapid two-electron oxidation of (OETPP)Fe^III(R) occurs with [Ru(bpy)₃]³⁺ (bpy = 2,2′-bipyridine) to produce [(OETPP)-Fe^IV(R)]²⁺. The formation of this species is followed by a slow migration of the σ-bonded R group to a nitrogen of the porphyrin ring to give [(N-ROETPP)Fe^II]²⁺ and then by a rapid electron-transfer oxidation of the migrated product with [Ru(bpy)₃]³⁺ to yield [(N-ROETPP)Fe^III]³⁺ as a final product. When [Ru(bpy)₃]³⁺ is replaced by a much weaker oxidant such as ferricenium ion, only the one-electron oxidation of (OETPP)Fe(R) occurs to produce [(OETPP)Fe^IV(R)]⁺. A migration of the R group also occurs in the one-electron oxidized porphyrin species, [(OETPP)Fe^IV(R)]+, to produce [(N-ROETPP)Fe^II]⁺, which is rapidly oxidized by ferricenium ion to yield [(N-ROETPP)Fe^III]²⁺. The migration rate of the R group in [(OETPP)Fe^IV(R)]⁺ is about 10⁴ times slower than the migration rate of the corresponding two-electron oxidized species, [(OETPP)Fe^IV(R)]2+. The migration rate of the σ-bonded ligand of [(TPP)Co^IV(R)]⁺, produced by the one-electron oxidation of (TPP)Co^III(R) with [Fe-(phen)₃]³⁺ (phen = 1,10-phenanthroline) is also about 10⁴ times slower than the migration rate of the R group in the corresponding two-electron oxidized species, [(TPP)Co^IV(R)]²⁺, which is produced by the two-electron oxidation with [Ru(bpy)₃]³⁺. A comparison of the migration rates with the oxidation states of the porphyrins indicates that the migration occurs via an intramolecular electron transfer from the R group to the Fe(IV) or Co(IV) metal of the organometallic porphyrin.