Reactivities of 1,2-, 1,3-, and 1,4- Dihydroxynaphthalenes toward Electrogenerated Superoxide in N,N-Dimethylformamide through Proton-coupled Electron Transfer

We have carried out an electrochemical and theoretical study on the reactivity of 1,2-, 1,3-, and 1,4-dihydroxynaphthalenes (1nH₂NQ, n = 2, 3, 4) toward electrogenerated superoxide radical anion (O₂^•−) in N,N-dimethylformamide. Cyclicvoltammetry and in situ electrolytic electron spin resonance measurements revealed that the quinone–hydroquinone π-conjugation plays an important role in a successful O₂^•− scavenging by 12H₂NQ and 14H₂NQ through proton-coupled electron transfer (PCET) reaction. The reactivities of 12H₂NQ and 14H₂NQ toward O₂^•− were mediated by the ortho- (catechol) or para-diphenol (hydroquinone) moieties, as experimentally confirmed in comparative analyses with catechol, hydroquinone, and 13H₂NQ, aided by density functional theory (DFT) calculations. The electrochemical and DFT results suggested that a concerted PCET mechanism involving two-proton transfers and one-electron transfer proceeds with a superior kinetic, demonstrating a successful O₂^•− scavenging by 12H₂NQ and 14H₂NQ. Furthermore, a subsequent electron transfer between molecular dioxygen and product-naphthoquinone-radicals was observed, where O₂^•− was generated. The DFT analysis suggested that the spin distribution on the planar naphthalene ring embodies the superior kinetics of the PCET and the subsequent generation of O₂^•− from dioxygen demonstrated in the electrochemical results.