The Effect of H-Bonding and Proton Transfer on the Voltammetry of 2,3,5,6-Tetramethyl-p-phenylenediamine in Acetonitrile. An Unexpectedly Complex Mechanism for a Simple Redox Couple

The voltammetry of 2,3,5,6-tetramethyl- p -phenylenediamine, H 2 PD, has been studied and compared to that of its isomer N, N, N ′, N ′-tetramethyl- p -phenylenediamine, Me 2 PD. Both undergo two reversible electron transfer processes in acetonitrile that nominally correspond to 1 e− oxidation to the radical cations, Me 2 PD + and H 2 PD +, and a second 1 e− oxidation at more positive potentials to the quinonediimine dications, Me 2 PD 2+ and H 2 PD 2+ . While the voltammetry of Me 2 PD agrees with this simple mechanism, that of H 2 PD does not. The second voltammetric wave is too small. UV−vis spectroelectrochemical experiments indicate that the second wave does correspond to oxidation of H 2 PD + to H 2 PD 2+ in solution. The fact that the second wave is not present at all at the lowest concentrations (5 μM), and that it increases at longer times and higher concentrations, indicates that H 2 PD + is not the initial solution product of the first oxidation. A number of lines of evidence suggest instead that the initial product is a mixed valent, H-bonded dimer between one H 2 PD in the full reduced, fully protonated state, H 4 PD 2+, and another in the fully oxidized, fully deprotonated state, PD. A mechanism is proposed in which this dimer is formed on the electrode surface through proton transfer and H-bonding. Once desorbed into solution, it breaks apart via reaction with other H 2 PD’s, to give 2 H 2 PD +, which is the thermodynamically favored species in solution.