We have observed a remarkable stabilization of a number of aromatic amine radical cations in our studies of organic electrode reactions in aluminum chloridealkali metal chloride melts. Our evidence for the stabilization of amine radical cations in A1C13-containing melts is derived from cyclic voltammetric studies* of several aromatic amines. Figure shows a cyclic voltammogram (0.5 V s-1), of triphenylamine at a tungsten electrode in a 50 : 50 mole % A1C13 : NaC1 melt at 175 ยฐ C vs. an A1 reference. The significant featureof this voltammogram when compared to that reported 2 for triphenylamine in acetonitrile at 25ยฐC is the presence of a large cathodic peak on reverse sweep. The ratio of the cathodic peak current to the anodic peak current calculated by the method of Nicholson 3 was one. When the sweep rate, v, was varied over three orders of magnitude (0.1-100 V s-1), the anodic peak potential and ip/V ~ values remained constant as did the anodic and cathodic peak separation which was 90-100 mV, close to the theoretical value of 85 mV for 175ยฐC. It should be pointed out that the pure melts are characterized by large background currents which may have an effect on the peak separations observed. These observations are consistent with triphenylamine undergoing a one-electron oxidation to a stable radical cation which is reduced on reverse sweep. It appears that the radical cation is enormously stabilized in the A1C13-containing solvent ; in acetonitrile at temperatures 150 ยฐ lower, the radical cation dimerizes 4 with a rate constant of 1.2 x 103 1 mol 1 s-1.
Although we believe that the above observations offer strong evidence for a relatively stable triphenylamine radical cation, this species does undergo further reaction. Controlled potential electrolysis of a colorless triphenylamine solution gives rise to a blue solution which slowly changes to a blue-green color. The triphenylamine radical cation in several different media is reported 2 to have a u.v.-visible spectrum with 2 max between 640 and 660 nm. An ill-defined cyclic voltammogram of the electrolyzed solution indicated the presence of at least one other electroactive species. Also, at higher temperature and concentration after many cycles the cyclic voltammogram for triphenylamine shows signs of another wave at potentials cathodic * Electrochemical measurements were made on a multipurpose instrument utilizing operational amplifier circuitry 1. Either a Hewlett-Packard model 7030 AM X-Y recorder or a Tektronix type 564 oscilloscope equipped with type 3A3 and 2A63 plug-ins and a Polaroid camera were used for recording the cyclic voltammograms. All the experiments were carried out in a dry box with closed cells.