Unexpected H-Bond Dimers in Organic Electrochemistry: The Case of Phenylenediamines and Pyridines in Acetonitrile
Authors: Smith Diane K. , Clare Laurie A. , Pham Tammy Dung , Rafou Lily 
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. For one thing, oxidation and reduction can strengthen or weaken H-bonding interactions in the absence of proton transfer. It can also result in strong interactions between the products of the electron transfer/proton transfer, which can have a significant effect on the mechanism of the overall reaction. A case in point is the electrochemistry of p-phenylenediamines in acetonitrile. In general two reversible or quasi-reversible voltammetric waves are observed that correspond to the one electron oxidation to a stable radical cation, followed at more positive potential by the removal of a second electron to give the highly acidic quinoidal dication. However, as we showed a number of years ago (J. Phys. Chem C 2010, 114, 8938-49), the actual mechanism if NH’s are present is far more complex. The second CV wave decreases substantially in size relative to the first as the phenylenediamine concentration is decreased and/or the scan rate increased. In fact, at low enough concentration and fast enough scan rate, it is not present at all. The best explanation that we could come up with was that at low concentration the current was dominated by a surface process in which the effective high concentrations lead to dimerization of the initially formed radical cations, followed by proton transfer and disproportionation to give a mixed-valent, H-bonded dimer that can not be further oxidized. This dimer is stable at the high concentrations on the surface, but not at the low concentrations in solution. So once desorbed, it breaks apart with reverse electron transfer/proton transfer to give the radical cation, which is then oxidized to the quinoidal dication in the second voltammetric wave. New evidence supporting this hypothesis comes from close examination of the electrochemistry of the phenylenediamines in the presence of pyridines. Previously we had observed that addition of 1 equivalent of pyridine leads to a large negative shift in the potential of the second oxidation with maintenance of reversibility, followed by much smaller shifts in potential upon further additions. Unpublished UV-vis spectroelectrochemical data indicate the formation of pyridinium, suggesting that the large shift in potential upon addition of one equivalent is due to proton transfer. The fact that the wave stays quasi-reversible, and does not lead to an immediate jump in height of the first wave (as would be expected if the uncharged radical was formed) further suggests that the pyridinium remains H-bonded to the oxidized phenylenediamine. In new work, CV’s with sub-stoichiometric amounts of pyridine have been examined. These studies show that the negatively shifted second oxidation wave is actually fully formed with only half equivalent of pyridine added. This lends strong support to the dimer hypothesis, since the only simple explanation of this stoichiometry is that pyridine removes one proton per two phenylenediamines.