Catalytic reduction of O2 by pyrazine derivatives

The catalytic reduction of molecular oxygen (O2) by four kinds of pyrazine derivatives2,3,5,6-tetramethylpyrazine, 2,3-dimethylquinoxaline, 2,3-diphenylquinoxaline, and phenazinehas 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 (E1/2) of the one-...