The Role of Intermolecular Hydrogen Bonding and Proton Transfer in Proton-Coupled Electron Transfer

An example of proton-coupled electron transfer (PCET) comprised by the electrochemical oxidation of 1,4-hydroquinone (1,4-H(2)Q) in acetonitrile was studied in the presence of Bronsted bases in acetonitrile. Of the two types of bases studied, the negatively, charged carboxylates, trifluoroacetate (TFAC(-)), benzoate (BZ(-)), and acetate (AC(-)), showed hydrogen bonding with 1,4-H(2)Q,whereas the neutral amines, pyridine (PY) and N, N'-diisopropylethylamine (DIPEA), did not. This difference allowed a unique investigation of the effect of proton transfer on PCET with and without the influence of hydrogen bonding using two bases (TFAC(-) and PY) with approximately the same pK(a) (similar to 12). The study revealed that hydrogen bonding of 1,4-H(2)Q with the base TFAC(-) made the half wave redox potential of 1,4-H(2)Q more negative (easier to oxidize) by 0.186 V with respect to the oxidation in the presence of the same concentration of added PY, which does not hydrogen bond with 1,4-H(2)Q, Both types of bases studied, carboxylates and amines, showed a combination of kinetic and thermodynamic effects in the oxidation voltammerty of 1,4-H(2)Q(i) however, no evidence of concerted pathways was found at the conditions studied as indicated by HID kinetic isotope experiments. The mechanism from fitted digital simulations for all the bases supports a stepwise PCET, even in the presence of hydrogen bonding, implying that the latter does not prevail in the transition state nor is rate determining. Hydrogen bonding was verified by H-1 NMR spectroscopy, while the electrochemical studies were carried out by cyclic voltammetry. The hydrogen bonding constants and diffusion coefficients determined by H-1 NMR were used in digital simulations that were fitted to experimental voltammograms.