Stepwise vs. concerted pathways in scandium ion-coupled electron transfer from superoxide ion to p-benzoquinone derivatives

Superoxide ion (O₂^-) forms a stable 1:1 complex with scandium hexamethylphosphoric triamide complex [Sc(HMPA)₃ ³⁺], which can be detected in solution by ESR spectroscopy. Electron transfer from O₂^--Sc(HMPA)₃³⁺ complex to a series of p-benzoquinone derivatives occurs, accompanied by binding of Sc(HMPA)₃³⁺ to the corresponding semiquinone radical anion complex to produce the semiquinone radical anion-Sc(HMPA)₃ ³⁺ complexes. The 1:1 and 1:2 complexes between semiquinone radical anions and Sc(HMPA)₃³⁺ depending on the type of semiquinone radical anions were detected by ESR measurements. This is defined as Sc(HMPA)₃³⁺-coupled electron transfer. There are two reaction pathways in the Sc(HMPA)₃³⁺-coupled electron transfer. One is a stepwise pathway in which the binding of Sc(HMPA) ₃³⁺ to semiquinone radical anions occurs after the electron transfer, when the rate of electron transfer remains constant with the change in concentration of Sc(HMPA)₃³⁺. The other is a concerted pathway in which electron transfer and the binding of Sc(HMPA) ₃³⁺ occurs in a concerted manner, when the rates of electron transfer exhibit first-order and second-order dependence on the concentration of Sc(HMPA)₃³⁺ depending the number of Sc(HMPA)₃³⁺ (one and two) bound to semiquinone radical anions. The contribution of two pathways changes depending on the substituents on p-benzoquinone derivatives. The present study provides the first example to clarify the kinetics and mechanism of metal ion-coupled electron-transfer reactions of the superoxide ion.