Homogeneous Catalysis of the Electrochemical Reduction of Dioxygen by a Macrocyclic Cobalt(III) Complex

Article abstract of DOI:10.1021/ja00415a016

The rotating ring-disk electrode was used to examine the reduction of dioxygen to hydrogen peroxide as catalyzed by the macrocyclic cobalt complex, trans-aneN4)(OH2)2>³⁺.Two different mechanisms were identified that depended on the ratio of dioxygen to cobalt.With excess cobalt, a well-characterized μ-peroxo complex, trans-<(Co(<14>aneN4)(OH2))2O2>⁴⁺, is formed at potentials where the initial Co(III) complex is reduced.At more negative potentials the μ-peroxo complex is reduced to H2O2 and aneN4)(OH2)2>²⁺ which re-enters the catalytic cycle.With excess dioxygen, the initial product of the reaction between the Co(II) complex and dioxygen is very rapidly further reduced to form a new complex thought to be trans-aneN4)(OH2)(O2H)>²⁺, a new, end-bonded, hydroperoxide complex.The hydroperoxide complex is further reduced at more negative potentials to yield H2O2 and aneN4)(OH2)2>²⁺.The rate of reaction of aneN4)(OH2)2>²⁺ with dioxygen proved too fast to measure with the rotating ring-disk electrode.The related complex, 4,11-diene N4)(OH2)2>³⁺, also catalyzes the reduction of dioxygen and at a much more positive potential but at a somewhat lower rate.