Electrocatalytic reduction of molecular oxygen using non-planar cobalt tetrakis-(4-sulfonatophenyl)-beta-octabromoporphyrin

Synthesis of a new water soluble porphyrin, cobalt(II) tetrakis-(4-sulfonatophenyl)-beta-octabromo (Br8TPPS)Co and catalytic reduction of molecular oxygen to hydrogen peroxide using this porphyrin is described. The investigated porphyrin is highly non-planar and water soluble due to the presence of eight bromides on the beta-pyrrole positions and four negative charges of the sulfonatophenyl groups on the meso positions, The first reversible reduction of (Br8TPPS)Co-II is diffusion controlled one-electron transfer, located at E-1/2 = - 0.59 V vs. Ag\AgCl in a pH 8.9 solution and leads to the formation of a stable [(Br8TPPS)Co-I](-). The synthesized cobalt(II) porphyrin binds dioxygen and catalytically reduces it to peroxide with an over voltage for dioxygen reduction of nearly 250 mV, Cyclic and rotating ring-disc voltammetric techniques are used to examine the mechanistic aspects of the catalytic process. (C) 1997 Elsevier Science S.A.