Electrochemical and digital simulation analyses of two-proton-coupled electron transfer between superoxide and hydroquinone

Electrochemical reduction of dioxygen (O2) in the presence of benzene-1,4-diol (hydroquinone) in N,N-dimethylformamide (DMF) was investigated using cyclic voltammetry and digital simulation techniques. Along the electrochemical reduction, a quasireversible cyclic voltammogram (CV) of O2 forming superoxide radical anion (O2•−) was modified in the presence of hydroquinone, where electrogenerated O2•− was scavenged by hydroquinone forming quinone radical anion and hydroperoxide with superior reaction kinetics via two-proton-coupled electron transfer (2PCET). Digital simulations of the CVs were conducted to clarify the details of 2PCET, where heterogeneous electrochemical reactions and homogeneous solution reactions between electrogenerated O2•− and hydroquinone are involved. As a result of the simulation analyses, thermodynamic and kinetic parameters for the 2PCET were obtained, which involved three elementary steps, (i) formation of the prereactive complex from the free reactants, (ii) intra-complex 2PCET forming the product complex via a transition state, and (iii) dissociation of the product complex yielding the free products. The obtained parameters provide fundamental and prerequisite information for the development of artificial electron transfer catalysts and electron transfer carriers using H2Q derivatives.