Recently we introduced the wedge scheme as a convenient means to include H-bonded intermediates in an overall proton-coupled electron transfer (PCET) mechanism by showing that it can nicely explain the unusual solvent- and concentration-dependent voltammetry of a phenylenediamine-based urea, U(H)H. This compound undergoes an apparent 1 e(-) reversible oxidation in CH2Cl2 that actually corresponds to 2 e(-) oxidation of half of the ureas to the quinoidal cation accompanied by transfer of a H+ and deactivation of the other half of the ureas. The reversibility of the process is due to the second electron transfer and proton transfer proceeding through a H-bond complex between the oxidized urea and the dimethylamino group on another urea. In this study the effect of adding 1,8-naphthyridine, naph, which H-bonds to the U(H)H in its reduced state, is examined using cyclic voltammetry. Addition of naph causes both an increase in the current of the reversible U(H)H cyclic voltammetric wave as well as the appearance of a new oxidation peak at slightly more positive potentials that gradually merges with the main wave as the naph concentration increases. This behavior can be explained by a transition from a U(H)H-U(H)H wedge scheme to a U(H)H-naph wedge, with the new second oxidation peak being due to the oxidation of the UHH-+naph H-bond complex. This complex is harder to oxidize than the UHH+-UHH complex due to weaker H-bonding of naph to U(H)H2+. Overall this study demonstrates the utility of the wedge scheme in explaining complex PCET reactions in which multiple types of H-bonding intermediates are involved.
