Orchestrating the Semiquinone Stability for Catalytic Proton-Coupled Electron Transfer

Semiquinone radicals are indispensable intermediates in biological proton-coupled electron transfer (PCET) yet remain elusive in synthetic systems owing to their intrinsic instability. Here, we showed that semiquinone stability can be engineered by combining metal-assisted radical delocalization with reversible covalent masking, thereby rendering semiquinones catalytically competent for PCET. Coordination to a high-valent Ti(IV) center weakens the hydroquinone O-H bond and stabilizes the resulting semiquinone via radical delocalization. The Lewis-acid-stabilized semiquinone can be covalently captured by a persistent carboradical, affording an isolable, air-stable semiquinone synthon. Crucially, the masking process is reversible under reductive conditions through an unusual negative hyperconjugation effect. This dual stabilization enables interconversion between hydroquinone and semiquinone states, affording a recyclable PCET mediator. The ability to access, manipulate, and recycle this short-lived, one-electron redox intermediate opens new avenues for PCET chemistry.