ELECTROCHEMICAL STUDIES OF A HEXAARYLBIIMIDAZOLE

The electrochemical behavior of the hexaarylbiimidazole derived from 2 -(2-chlorophenyl)-4,5-diphenylimidazole (o-Cl-HABI or L(2)) has been s tudied in acetonitrile. Reduction proceeds in an overall two-electron process that is presumed to occur by an ECE process in which the initi ally formed radical anion L(2)(-.) undergoes a fast cleavage reaction to give the corresponding anion of the triarylimidazole L(-) and the t riarylimidazoyl radical L(.), which is subsequently reduced to the ani on. The purported radical anion intermediate could not be detected by fast-scan (10(4) V s(-1)) cyclic voltammetry, putting a lower limit of 4 X 10(4) s(-1) on the rate constant for the cleavage reaction. The p roduct of controlled-potential reduction was L(-). On the positive-goi ng sweep of a cyclic voltammogram of L(2), the L(-) that was formed at the main cathodic peak is oxidized to L(.) that dimerizes to form the original o-Cl-HABI with a dimerization rate constant of 1.1 x 10(4) M (-1)s(-1) at 298 K. The reversible potential for the L(.)/L(-) couple was found to be -0.15 V vs. the ferrocene/ferrocenium potential. Analy sis of the voltammetric data from 0.02 to 10 V s(-1) allowed the estab lishment of limits for the parameters of the ECE reaction. Namely, the reversible potential for the L(2)/L(2)(-.) couple falls between -1.83 and -1.91 V vs. ferrocene, the standard heterogeneous electron transf er rate constant lies between 0.03 and 0.2 cm s(-1) and the rate const ant for cleavage of L(2)(-.) falls in the range from 4 x 10(4) to 1 x 10(7) s(-1).