Evaluating the Roles of Proton Transfer and H-Bonding in the Electron Transfer Reactions of Organic Redox Couples in Non-Aqueous Solvents: Oxidation of Phenylenediamines in the Presence of Added Bases in Acetonitrile
Authors: Pham Tammy Dung , Smith Diane K. 
Published: February 2020

Oxidation or reduction of organic redox couples typically leads to large changes in acidity or basicity, with the result that proton transfer often accompanies electron transfer, particularly in aqueous solution. In less polar organic solvents, H-bonding also can play an important role. While it is generally appreciated that proton transfer will have a greater effect on the overall reaction than H-bonding, it is not always straight forward to distinguish between the two, and, despite a considerable amount of research, a complete, quantitative understanding of the relative roles that the two play in the voltammetry that is observed upon addition of acids or bases to organic redox couples in non-aqueous solution remains elusive. Most of the research in this area has been done with quinones in the presence of H-donors. In contrast, this study focuses on p-tetramethylphenylenediamine, H2PD, in the presence of H-acceptors. While quinones undergo two successive reductions to form the increasingly basic radical anion, then dianion, phenylenediamines, which are weakly basic to begin with, undergo two successive oxidations to form an increasingly acidic radical cation, then quinoidal dication. Upon addition of DMF, a weak H-acceptor, to H2PD in acetonitrile, small negative shifts in potential of the second oxidation are observed in the cyclic voltammetry (CV), with little change in wave shape. This is consistent with H-bonding of the DMF to the quinoidal dication, H2PD2+. Somewhat similar behavior is observed when slightly more basic guests are added such as cyanopyridine or trifluoromethylpyridine, but, unlike with DMF, with just 1 equivalent pyridine guest, a significant shift in the potential of the second oxidation is observed, followed by smaller shifts with additional equivalents. We believe that this behavior signals proton transfer between the pyridine, pyr, and the H2PD2+, so that the overall reaction occurring in the second oxidation corresponds to H2PD+ + pyr = HPD+ + Hpyr+ + e- . In this case, the observed E1/2 should depend on the pKa of the Hpyr+. To test this hypothesis, the voltammetry of H2PD is currently being studied with different pyridines that cover a range of pKa values. If correct, the explanation for the continued shift in potential with increasing concentrations of pyridine is well-accounted for simply by applying the Nernst equation to the overall reaction. However, while proton transfer can explain the potentials of the CV waves in the presence of added pyridine, simulations of the voltammetry show that proton transfer by itself cannot explain the observed reversibility of the second oxidation wave in the presence of increasing amounts of added pyridine. This is where H-bonding can play a role. By including H-bonding steps, and allowing electron transfer to occur through the H-bond complex formed between H2PD2+ and pyr (the intermediate in the proton transfer), the simulations can nicely explain both the observed potential shifts and the reversibility of the waves.