Reduction and Alkylation of Cobalt(II) Tetrakis(4-sulfonatophenyl)porphyrin in Aqueous Solutions. A Kinetic Spectrophotometric Study

Article abstract of DOI:10.1021/j100232a011

The reactions of Co^IITPPS (TPPS = tetrakis(4-sulfonatophenyl)porphyrin) with alkyl and hydroxyalkyl radicals have been studied by pulse radiolysis.The alkyl radicals undergo axial addition with rate constants of (1.2-1.9) * 10⁹ M^-1 s^-1 to form R-Co^IIITPPS complexes which either are stable (R = CH3) or undergo decomposition (R = CH(CH3)2) to Co^ITPPS and an olefin via a β-proton elimination mechanism (k₁ = 0.3 s^-1 at pH 8 and 3 s^-1 at pH 13).The hydroxyalkyl radicals also form R-Co^IIITPPS with rate constants of (1.1-1.2) * 10⁹ M^-1 s^-1.These adducts undergo heterolytic cleavage of the cobalt-carbon bond to form Co^ITPPS and a carbonyl compound, with rate constants of 3.6 * 10² s^-1 (R = CH2OH) and 6.2 * 10³ s^-1 (R = C(OH)(CH3)2).The radicals (CH3)2C^.O^- and C^.O2^- reduce Co^IITPPS to Co^ITPPS by an outer-sphere mechanism with rate constants 7 * 10⁸ and 2 * 10⁸ M^-1 s^-1, respectively.Co^ITPPS is oxidized to CH3Co^IIITPPS by CH3I (k₂ = 3 * 10⁵ M^-1 s^-1) and to Co^IITPPS (probably through intermediate formation of Co^IIITPPS) by N2O (k₂ 2 M^-1 s^-1).The reactivities of the present systems have been compared with those of other models suggested for vitamin B₁₂ like cobaloximes, cobalamins, and other macrocyclic Co^II complexes.