Cyclic Voltammetry of N,N-Dimethyl-p-Phenylenediamine and its HCl-Salt in Acetonitrile with Pyridine as Proton Acceptor

Abstract

Summary The redox behaviour of N,N-dimethyl-p-phenylenediamine (PPD) and N,N-dimethyl-p-phenylenediamine · 2HCl (PPD · 2HCl) in acetonitrile was studied by means of cyclic voltammetry – both in the absence and in the presence of proton accepting pyridine. The oxidation occurs in two separate steps yielding first N,N-dimethyl-p-semiquinonediimine (SQDI+) and subsequently N,N-dimethyl-p-quinonediimine (QDI2+ or QDI+(H+)). In case of N,N-dimethyl-p-phenylenediamine the 1st oxidation step is fully reversible, while the 2nd oxidation step is slightly quasi-reversible. At 10°C the formal redox potentials of the PPD/SQDI+ and SQDI+/QDI2+ redox couples are −0.200 ± 0.005 V and 0.380 ± 0.010 V (versus Fc/Fc+), respectively. The synproportionation constant K syn = [SQDI+]2/[PPD][QDI2+] is 2.1 × 1010. Adding pyridine to the reaction mixture leads to a shift of the 2nd anodic wave to less positive potentials. This shift is primarily due to proton transfer from QDI2+ to pyridine to form QDI+, which overall renders the oxidation easier. The corresponding cathodic peak, on the other hand, disappears as the reduction of QDI+ necessitates a proton. The formal oxidation potential of the redox couple SQDI+/QDI+(H+) is about 0.190 V and, therefore, the synproportionation constant K syn = 1.7 × 106. N,N-dimethyl-p-phenylenediamine · 2HCl lacks completely the 1st anodic peak. The position of the anodic peak is given by the direct oxidation of PPDH+ to QDI2+(H+) and is similar to that of the SQDI+/QDI2+ redox couple. Upon adding pyridine the 2nd anodic peak shifts notably to less positive potentials and the redox process attributable to the PPD/SQDI+ redox couple is seen. As pyridine influences the potential of the Ag pseudo-reference electrode a reference electrode based on the ferrocene/ferrocenium redox couple was used.