Proton-coupled electron transfer (PCET) reactions are essential to many of the fundamental chemical processes of life. In the Smith group we are investigating the role of H-bonding intermediates in PCET by studying the voltammetry of compounds capable of multiple, strong H-bond interactions.  In particular we have studied a p-phenylenediamine-based urea, U(H)H, which contains two H donor sites making a DD array. In this study, the phenyl group in U(H)H is replaced by an imidazole group to form a three hydrogen bond array, UImH. In this array, the two major conformers both present a DDA H-bond motif (1 and 2 in Fig 1). Therefore, AAD arrays, such as APy, are needed as guest compounds to form three intermolecular non-covalent contacts.

The cyclic voltammetry of UImH has been examined in methylene chloride and acetonitrile with platinum (Pt) and glassy carbon (GC) electrodes. On both electrodes, CV’s show two, closely-spaced, reversible waves of similar height. We initially hypothesized that the first electron oxidation happens on the phenylenediamine unit, followed by rapid intramolecular proton transfer in conformer 2, Fig. 1. The second electron is removed at a slightly more positive potential due to the positive charge remaining on the array after the first oxidation. However, the voltammetry is observed to have a strong concentration dependence and this cannot be explained by this mechanism.  Based on the CVs we now propose the first wave is due to loss of 2e^- from UImH with an intramolecular PT to form UImH²⁺. Then UImH²⁺ undergoes an intermolecular PT with unreacted UImH to produce UIm⁺ and HUImH⁺.  The second wave is due to 2e- oxidation HUImH⁺ to form HUImH³⁺ at a more positive potential. Lastly, an intermolecular PT between HUImH³⁺and UIm⁺ gives the final product UImH²⁺.

Addition of the guest, APy, results in is a slight increase in the current of the CV waves of UImH. This could be due to preferential binding and stabilization of UImH which prevents decomposition and adsorption. Surprisingly, very little change in the potential of the CV wave is observed upon addition of the guest, indicating that oxidation does not change binding strength. Even though the H-donors in UImH²⁺ should be stronger than in UImH, the H-acceptor in UImH²⁺ is weaker than in UImH. So the two effects cancel each other.