Catalytic reduction of O₂ by pyrazine derivatives

The catalytic reduction of molecular oxygen (O₂) by four kinds of pyrazine derivatives-2,3,5,6-tetramethylpyrazine, 2,3-dimethylquinoxaline, 2,3-diphenylquinoxaline, and phenazine-has been studied by cyclic and rotating disk voltammetry and molecular orbital calculations using the MOPAC PM5 method. In sufficiently acidic solution (pH ≈ 1), the pyrazine derivatives underwent two successive one-electron reductions. Therefore, it was possible to compare the reactivity of both one-electron reduced forms (the radicals) and two-electron reduced forms of the derivatives toward O₂ in this acidic solution. Theoretical calculations showed that all of the redox forms of the derivatives are protonated to be cations in this solution and that all of the reduced forms, except for that of phenazine, react with O₂. The rate constants (k₁) of the second-order reactions of the reduced forms with O₂ were estimated by rotating disk voltammetry. A linear relationship between log(k₁) and the half-wave potential (E_1/2) of the one-electron and two-electron reduced forms was observed, indicating that the catalytic reduction of O₂ proceeds by a common reaction mechanism and that its rate constant depends on its thermodynamic driving force.