Quinones as Electron Acceptors. X-Ray Structures, Spectral (EPR, UV−vis) Characteristics and Electron-Transfer Reactivities of Their Reduced Anion Radicals as Separated vs Contact Ion Pairs

Successful isolation of a series of pure (crystalline) salts of labile quinone anion radicals suitable for X-ray crystallographic analysis allows for the first time their rigorous structural distinction as “separated” ion pairs (SIPs) vs “contact” ion pairs (CIPs). The quantitative evaluation of the precise changes in the geometries of these quinones (Q) upon one-electron reduction to afford the anion radical (Q^-•) is viewed relative to the corresponding (two-electron) reduction to the hydroquinone (H₂Q) via the Pauling bond-length/bond-order paradigm. Structural consequences between such separated and contact ion pairs as defined in the solid state with those extant in solution are explored in the context of their spectral (EPR, UV−vis) properties and isomerization of tightly bound CIPs. Moreover, the SIP/CIP dichotomy is also examined in intermolecular interactions for rapid (self-exchange) electron transfer between Q^-• and Q with second-order rate constants of k_ET ≈ 10⁸ M^-1 s^-1, together with the spectral observation of the paramagnetic intermediates [Q,Q^-•] leading to 1:1 adducts, as established by X-ray crystallography.