The Role of H-Bonding in PCET: The Chemically Reversible One Electron, Three Acid Reduction of N-Methyl-4,4'-Bipyridine Radical in Acetonitrile

Abstract

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 are 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. Another means through which H-bonding can affect PCET is by stabilizing the products. In the PCET reaction to be discussed in this presentation, reduction of N-methyl-4,4'-bipyridinium (commonly called “monoquat” or MQ) in acetonitrile in the presence of acidic alcohols, it appears that both H-bond effects are present. MQ 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, CV studies show that addition of trifluoroethanol or dichloroethanol, both fairly acidic alcohols, result 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 begun to merge with the first reduction. What is 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− transformation involving three alcohols throughout the entire range. 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. The mechanism would involve initial 1 e−, 1H+ transfer occurring through the H-bond intermediate (wedge scheme), followed by rapid formation of the homoconjugate complex (RO−)(ROH)2. The results of current and future work on this system to further elucidate the mechanism, and either support or refute the wedge-scheme/homoconjugate hypothesis, will be presented. 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.