Redox Reaction of the Central Metal Ions Coordinated to Tetra(p-sulfophenyl)porphine(TPPS). I. Photoreduction of Co(III)TPPS by Methanol and 2-Propanol

Trivalent cobalt ion coordinated to 5,10,15,20-tetra(p-sulfophenyl)porphine (TPPS) in deaerated solution was reduced to the divalent ion on illumination of light with simultaneous oxidation of alcohols. The resulting divalent ion in the porphyrin was reoxidized by admission of dioxygen or air into the solution. It was found that the metalloporphyrin played a photocatalytic role in the autoxidation of alcohols. The kinetic study on the photoreduction revealed that the reduction rate of Co(III) followed the pseudo-first-order rate expression at 20 °C and pH 6.5 to 11.5 in the presence of excess methanol or 2-propanol. The observed rate constants were dependent on [alcohol] and/or pH. Possible mechanism for the photoreduction of Co(III)TPPS in the aqueous solution was discussed in terms of preequilibrium reactions of the axial ligation of substrates to the metalloporphyrin.