The Role of H-Bonding in Proton-Coupled Electron Transfer: The Reduction of N-Methyl-4,4’-Bipyridine Radical in Acetonitrile in the Presence of Acidic Alcohols

Abstract

Proton-coupled electron transfer reactions (PCET) play a significant role in many chemical and biological mechanisms. However, although these reactions have been known and studied for over a century, it is only within the last few decades that the importance of H-bonding intermediates in these reactions has come to be appreciated. In previous studies, we examined how explicit consideration of electron transfer and proton transfer within a H-bond intermediate (the “wedge” scheme) could explain the voltammetry of oxidation reactions involving phenylenediamines and bases in acetonitrile. The focus of this new study is to see if the wedge scheme can also explain the chemically reversible PCET reaction due to addition of acidic alcohols to the N-methyl-4,4’-bipyridinium redox couple (commonly called “monoquat” or MQ) in acetonitrile. MQ, which starts out as a +1 cation, undergoes two reversible reductions in acetonitrile, first to an uncharged radical and then to a quinoidal anion. For this study, different types of acidic alcohols, such as trifluoromethanol, methanol, and dichloroethanol, were added at different concentrations. The cyclic voltammetry shows a positive shift in the E1/2 of the second reduction after adding the acidic alcohol as a guest. This is likely due to protonation of the monoquat anion by the alcohol. The overall reaction was expected to be MQ + ROH + e- = MQH + RO-, which would show a 60 mV per log[ROH] change in E1/2. However, analysis of the collected data contradicts this hypothesis by giving a 180 mV per log[ROH] E1/2 shift, indicating that three alcohols are involved in the overall reaction. Since it is highly unlikely that the MQ anion would be triply protonated, our current hypothesis is that the product conjugate base of the alcohol forms a strongly H-bonded complex with two other alcohols, making the overall reaction, MQ + 3ROH + e- = MQH + (RO-)(ROH)2. Results of on-going research to either disprove or provide further support for this hypothesis will be reported.