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).