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

Abstract

Oxidation or reduction of organic redox couples typically leads to large changes in acidity or basicity, withthe result that proton transfer often accompanies electron transfer, particularly in aqueous solution. In lesspolar organic solvents, H-bonding also can play an important role. While it is generally appreciated thatproton transfer will have a greater effect on the overall reaction than H-bonding, it is not always straightforward 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 uponaddition 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 thepresence of pyridine bases. In acetonitrile, this compound undergoes a reversible one electron oxidationto the radical cation, H 2 PD + , followed by a second reversible oxidation to the strongly acidic quinoidaldication, H 2 PD 2+ . With the addition of 1 equivalent of the pyridine base, no significant change in the firstoxidation is observed, but there is a large negative shift in the E 1/2 of the second oxidation with no loss inreversibility. Subsequent additions show a smaller, incremental negative shift, still with no loss inreversibility. 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 aslope of −60 mV/pH unit. To test this hypothesis, the voltammetry of H 2 PD was studied with differentpyridines that cover a range of pK a values. A plot of E 1/2 of the wave observed with 1 eq pyridine vs. pKa isindeed linear with close to the predicted slope. Furthermore, it is found that the continued shift in E 1/2 ofthe second oxidation with increasing concentrations of pyridine is well-accounted for simply by applyingthe Nernst equation to the overall reaction. However, while proton transfer can explain the potentials ofthe CV waves in the presence of added pyridine, simulations of the voltammetry show that proton transferby itself cannot explain the observed reversibility of the second oxidation wave in the presence ofincreasing amounts of added pyridine. This is where H-bonding can play a role. By including H-bondingsteps, and allowing electron transfer and proton transfer to occur through the H-bond complex formedbetween H 2 PD + /pyr and HPD + /Hpyr + , the simulations nicely explain both the observed potential shifts andthe reversibility of the waves. The next question is whether the electron and proton transfer within the H-bonding complex is concertedor step wise. To test this, CV’s were run with 10:1 cyanopyridine:H2PD in 2% D 4 -methanol/acetonitrile or2% H 4 -methanol/acetonitrile. If the second oxidation involved concerted electron-proton transfer, asignificant deuterium isotope effect would be expected, causing a larger ΔE p in the deuterated solventresulting 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 meansrule out the hypothesis that the proton-electron transfer is occurring within the H-bond complex, it merelyindicates 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 theprocess is not concerted.