Electrochemical Evidence for Intermolecular Proton-Coupled Electron Transfer through a Hydrogen Bond Complex in a <i>p</i>‑Phenylenediamine-Based Urea. Introduction of the “Wedge Scheme” as a Useful Means To Describe Reactions of This Type

The electrochemistry of several <i>p</i>-phenylenediamine derivatives, in which one of the amino groups is part of an urea functional group, has been investigated in methylene chloride and acetonitrile. The ureas are abbreviated U­(R)­R′, where R′ indicates the substituent on the N that is part of the phenylenediamine redox couple and R indicates the substituent on the other urea N. Cyclic voltammetry and UV–vis spectroelectrochemical studies indicate that U­(Me)H and U­(H)H undergo an apparent 1e^– oxidation that actually corresponds to 2e^– oxidation of half the ureas to a quinoidal-diimine cation, U­(R)⁺. This is accompanied by proton transfer to the other half of the ureas to make the electroinactive cation HU­(R)­H⁺. This explains the observed irreversibility of the oxidation of U­(Me)H in both solvents and U­(H)­H in acetonitrile. However, the oxidation of U­(H)H in methylene chloride is reversible at higher concentrations and slower scan rates. Several lines of evidence suggest that the most likely reason for this is the accessibility of a H-bond complex between U­(H)⁺ and HU­(H)­H⁺ in methylene chloride. Reduction of the H-bond complex occurs at a less negative potential than that of U­(H)⁺, leading to reversible behavior. This conclusion is strongly supported by the appearance of a more negative reduction peak at lower concentrations and faster scan rates, conditions in which the H-bond complex is less favored. The overall reaction mechanism is conveniently described by a “wedge scheme”, which is a more general version of the square scheme typically used to describe redox processes in which proton transfer accompanies electron transfer