Differences of behaviour in electrooxidation of 3,3′-dimethoxybenzidine and its N,N,N′,N′-tetraalkylated derivatives: a further example of potential inversion

The anodic oxidation of 3,3′-dimethoxybenzidine and its derivatives were studied by cyclic voltammetry, voltammetry at a rotating platinum electrode, chronopotentiometry and constant potential coulometry. The exhaustive oxidation of 3,3′-dimethoxybenzidine led at the potential of the first oxidation peak and second oxidation peak, respectively, to a stable cation radical salt (characterized by ESR spectroscopy) and to a quinone-diimine. Whereas, 3,3′-dimethoxy-N,N,N′,N′-tetramethylbenzidine afforded directly a dication salt owing to potential inversion 3,3′-dimethoxy-N,N,N′,N′-tetraethylbenzidine gave the corresponding soluble cation radical salt.