Homogeneous molecular catalysis of the electrochemical reduction of N(2)O to N(2): redox vs. chemical catalysis

Homogeneous electrochemical catalysis of N(2)O reduction to N(2) is investigated with a series of organic catalysts and rhenium and manganese bipyridyl carbonyl complexes. An activation-driving force correlation is revealed with the organic species characteristic of a redox catalysis involving an outer-sphere electron transfer from the radical anions or dianions of the reduced catalyst to N(2)O. Taking into account the previously estimated reorganization energy required to form the N(2)O radical anions leads to an estimation of the N(2)O/N(2)O˙(−) standard potential in acetonitrile electrolyte. The direct reduction of N(2)O at a glassy carbon electrode follows the same quadratic activation driving force relationship. Our analysis reveals that the catalytic effect of the mediators is due to a smaller reorganization energy of the homogeneous electron transfer than that of the heterogeneous one. The physical effect of “spreading” electrons in the electrolyte is shown to be unfavorable for the homogeneous reduction. Importantly, we show that the reduction of N(2)O by low valent rhenium and manganese bipyridyl carbonyl complexes is of a chemical nature, with an initial one-electron reduction process associated with a chemical reaction more efficient than the simple outer-sphere electron transfer process. This points to an inner-sphere mechanism possibly involving partial charge transfer from the low valent metal to the binding N(2)O and emphasizes the differences between chemical and redox catalytic processes.