Revisiting H-Bond <i>v</i> . PT: The Role of Precursor and Successor Complexes in Intermolecular, Stepwise Proton-Coupled Electron Transfer

Multiple mechanisms have been proposed to explain the electrochemical proton-coupled electron transfer (PCET) of arylenediamines upon weak base addition in aprotic media. Similar to quinone electrochemistry, endergonic deprotonation to form freely diffusing products is a central criterion used to exclude proton transfer. However, the second oxidation wave of arylenediamines shows large, chemically reversible <i>E</i> _1/2 shiftsoften exceeding hundreds of millivoltsupon base addition. To explain these effects, proposed mechanisms invoke strong H-bonding or H-bonding followed by nonconcerted PCET. Here, we elucidate a new mechanism using cyclic voltammetry of a new arylenediamine series where the p<i>K</i> _a is varied via π-electron spacers. A thermochemical analysis, based on equilibrium constants derived from biphasic <i>E</i> _1/2 shifts observed from substoichiometric to excess base concentrations, supports a stepwise mechanism. This mechanism involves two sequential heterogeneous electron transfer steps (EE) followed by three homogeneous chemical steps (CCC), constituting an overall EECCC electrochemical mechanism. The CCC coupled equilibria correspond to exergonic H-bond precursor complex formation, thermoneutral proton transfer (PT), and endergonic successor complex dissociation. The energy well formed by the CCC coupled equilibria reconciles previous thermochemical analyses and provides a new explanation for chemically reversible electrochemical waves. Furthermore, a reactivity continuum between H-bonding and PT is demonstrated, which contrasts with the prevailing view, where either H-bonding or PT dominates the mechanism.