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 Pyridine Bases in Acetonitrile
Authors: Pham Tammy Dung , Clare Laurie A. , Rafou Lily , Buenaventura Ayla , Arthurs Colin , 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.

This study focuses on the electrochemistry of p-tetramethylphenylenediamine, H 2 PD, in acetonitrile in the

presence of pyridine bases. In acetonitrile, this compound undergoes a reversible one electron oxidation

to the radical cation, H 2 PD + , followed by a second reversible oxidation to the strongly acidic quinoidal

dication, H 2 PD 2+ . With the addition of 1 equivalent of the pyridine base, no significant change in the first

oxidation is observed, but there is a large negative shift in the E 1/2 of the second oxidation with no loss in

reversibility. Subsequent additions show a smaller, incremental negative shift, still with no loss in

reversibility. We believe that this behavior signals proton transfer is occurring between the pyridine, pyr,

and the H 2 PD 2+ , so that the overall reaction occurring in the second oxidation corresponds to H 2 PD + + pyr

= HPD + + Hpyr + + e-. In this case, the observed E 1/2 should depend linearly on the pK a of the Hpyr + with a

slope of −60 mV/pH unit. To test this hypothesis, the voltammetry of H 2 PD was studied with different

pyridines that cover a range of pK a values. A plot of E 1/2 of the wave observed with 1 eq pyridine vs. pKa is

indeed linear with close to the predicted slope. Furthermore, it is found that the continued shift in E 1/2 of

the second oxidation 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 and proton transfer to occur through the H-bond complex formed

between H 2 PD + /pyr and HPD + /Hpyr + , the simulations nicely explain both the observed potential shifts and

the reversibility of the waves.

The next question is whether the electron and proton transfer within the H-bonding complex is concerted

or step wise. To test this, CV’s were run with 10:1 cyanopyridine:H2PD in 2% D 4 -methanol/acetonitrile or

2% H 4 -methanol/acetonitrile. If the second oxidation involved concerted electron-proton transfer, a

significant deuterium isotope effect would be expected, causing a larger ΔE p in the deuterated solvent

resulting from slower electron transfer. However, no significant difference was observed. Therefore,

there is no evidence that the electron-proton transfer is concerted. This result does not by any means

rule out the hypothesis that the proton-electron transfer is occurring within the H-bond complex, it merely

indicates that it is likely that the proton and electron transfer occur in a step-wise fashion within the H-

bond complex, and that facilitation of proton-electron transfer by H-bonding can happen even if the

process is not concerted.