# Revisiting H‑Bond v. PT: The Role of Precursor and Successor Complexes in Intermolecular, Stepwise Proton-Coupled Electron Transfer

## TL;DR

The paper proposes a new mechanism for proton-coupled electron transfer in electrochemical reactions involving arylenediamines.

## Contribution

A new stepwise EECCC mechanism is proposed, reconciling previous thermochemical analyses and explaining chemically reversible electrochemical waves.

## Key findings

- A stepwise mechanism involving two electron transfer steps and three chemical steps explains E₁/₂ shifts in electrochemical reactions.
- Exergonic H-bond precursor complex formation and thermoneutral proton transfer are key steps in the mechanism.
- A reactivity continuum between H-bonding and proton transfer is demonstrated, contrasting previous views.

## Abstract

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 E
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 pK
a is varied via π-electron spacers. A thermochemical analysis,
based on equilibrium constants derived from biphasic E
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.