Description
Proton-coupled electron transfer (PCET) reactions are essential to many of the fundamental chemical processes of life. In the Smith group we have studied a p-phenylenediamine-based urea, U(H)H, by applying PCET. In this study, the phenyl group in U(H)H is replaced by an imidazole group to form a three hydrogen bond array, UImH. Therefore, AAD arrays, such as APy, are needed as guest compounds to form three intermolecular non-covalent contacts. The initial hypothesis was that oxidation of the phenylenediamine of UImH should lead to stronger H-bonding with the guest compound, and this would make the oxidation easier leading to a negative shift in the E1/2 in the presence of the guest compound. The cyclic voltammetry (CV) of UImH has been examined in methylene chloride and acetonitrile with platinum (Pt) and glassy carbon (GC) electrodes. On both electrodes, CV shows two, closely-spaced, reversible waves of similar height, but a single wave is observed at the lowest concentration on the high scan rate (5.0 V/s). Interestingly, the single wave decreases in relative size as the concentration increases and appears to gradually split into the two smaller peaks seen at slow san rate. Based on the results of CV plus DFT calculations, it is likely that π dimerization is occurring during the electrochemical reaction. The newest mechanism hypothesizes two electrons per UImH with one intramolecular proton transfer at low concentrations and high scan rate and two sequential one electron transfers per two UImH at high concentration, producing a net one electron oxidation per UImH at high concentration. Addition of the guest, APy, results in a slight increase in the current of the CV waves of UImH. However, 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. The increase in current that is observed is most likely due to hydrogen bonding interfering with the π dimerization. Even though UImH did not show the expected behavior, the π dimerization is arguably far more interesting, and may have applications in supramolecular chemistry, including the design of smart materials.
