The Role of H-Bonding in Pcet: The Chemically Reversible One Electron, Two Proton Reduction of N-Methyl-4,4'-Bipyridine Radical in Acetonitrile
Authors: Smith Diane K. , Pavlova Ksenia , Medina Alyza 
Published: February 2020

Oxidation and reduction of organic redox-couples typically leads to huge changes in their acidity/basicity, with the result that proton transfer (PT) often accompanies electron transfer (ET). These types of reactions have come to be known as proton-coupled-electron-transfer or PCET reactions. While the prevalence of PCET reactions in reversible organic redox transformations is something that has been recognized for over 100 years, it is only much more recently that it has become apparent that H-bonding intermediates often play a significant role in the mechanism of these reactions. One way in which H-bonding can do so is through what we have termed a “wedge-scheme” mechanism in which ET and PT occur through the H-bond intermediate formed between the redox-active acid or base and the added base or acid.1 This can provide an easier path for ET-PT in either direction and thus increase the reversibility of the overall reaction.
Previously we have demonstrated the utility of the wedge scheme in oxidation reactions involving phenylenediamines and bases.1,2 In new work, we are exploring the utility of the wedge scheme in explaining the chemically reversible PCET reaction occurring upon addition of acidic alcohols to the N-methyl-4,4'-bipyridinium redox couple (commonly called “monoquat” or MQ) in acetonitrile. This compound undergoes two reversible one electron reductions in acetonitrile, the first corresponding to reduction of the starting cation, MQ+, to an uncharged radical, MQ. This is followed by a second reduction at significantly more negative potentials of the radical to the anion, MQ-, which formally contains a negatively-charged nitrogen and is thus quite basic. Not surprisingly, cyclic voltammetry (CV) studies show that addition of trifluoroethanol (TFE), a fairly acidic alcohol, results in no change in E1/2 of the first reduction but a significant positive shift in the E1/2 of the second reduction, most likely due to protonation of the anion. The second reduction wave is slightly broad, but continues to shift positive and remain chemically reversible up to at least 400 equivalents of added alcohol, at which point it has shifted over 0.6 V and almost merged with the first reduction wave. What is perhaps surprising, in addition to the chemical reversibility, is that analysis of the observed E1/2 as a function of added alcohol concentration is consistent with a single 1e-, 2 H+ transformation (MQ + 2 HA + 1e- = MQH2+ + 2 A-) throughout the entire range. Qualitatively similar behavior observed with addition of alcohols to quinones in acetonitrile has commonly been attributed to strong H-bonding between the alcohol and the quinone dianion. However, a H-bonding only mechanism is not likely in this case both because of the expected basicity of MQ- and the uniformity of the reaction stoichiometry over the entire concentration range. (With H-bonding only, the stoichiometry would likely start 1:1, then go to 2:1 at higher concentrations.) On the other hand, a classic step-wise PCET mechanism, ET-PT-PT, can not explain the observed reversibility over the entire concentration range. What we believe can explain all of the above is initial 1 e-, 1H+ transfer occurring through the H-bond intermediate (wedge scheme), followed by the second 1H+ transfer. The results of current and future work on this system to further elucidate the mechanism, and either support or refute the wedge-scheme hypothesis, will be described in this presentation. These will include studies with other acids, CV simulations and analysis of deuterium isotope effects

1 L. A. Clare, A. T. Pham, F. Magdaleno, J. Acosta, J. E. Woods and D. K. Smith, Journal of the American Chemical Society
2013, 135, 18930-18941.

2B. T. Tamashiro, M. R. Cedano, A. T. Pham and D. K. Smith, Journal of Physical Chemistry C
2015, 119, 12865-12874.