Catalytic oxidation of reduced sulfur compounds by homogeneous and heterogeneous Co(II) phthalocyanine complexes

An overview of the kinetics and mechanisms of the reactions of reduced sulfur compounds and other reductants with molecular oxygen as catalyzed by homogeneous and heterogeneous metal phthalocyanine complexes is presented. Catalysis of the autoxidation of S(IV) by Co(II) tetrasulfophthalocyanine was characterized in terms of an ordered ternary-complex mechanism in which S(IV) and oxygen are simultaneously bound to the Co(II)/Co(III) metal center. Hydrogen peroxide was identified as an intermediate reduction product of bound dioxygen. The reaction was examined by spectrophotometric and EPR methods and was found to proceed via a sulfito-cobalt(III)-superoxide complex that subsequently combines with a second sulfite molecule to produce sulfate and a sulfito-cobalt(III)-peroxide. The catalytic cycle is closed by the regeneration of the cobalt(II) complex via the production of H_2O_2. Direct evidence for he active sulfito-cobalt(III)-superoxide intermediate was obtained.