DETERMINATION OF FORMAL POTENTIALS AND ANION-RADICAL LIFETIMES FOR HEXAARYLBIIMIDAZOLE DERIVATIVES

The reduction of three hexaarylbiimidazole (HABI or L-2) derivatives h as been studied in acetonitrile by means of cyclic voltammetry. The re duction of o-Cl-HABI has previously been shown to occur by an ECE proc ess. The radical anion initially formed in the first electron transfer (L-2(radical anion)) undergoes a fast cleavage reaction to produce th e triarylimidazole anion (L-) and the radical (L-.), the latter of whi ch is reduced at the electrode to produce a second anion. The two othe r HABI derivatives investigated in this study show similar behavior. H omogeneous redox catalysis was used to determine the formal potential (E-1(0)) for the L-2/L-2(radical anion) couple for each of the three H ABIs and the rate constant of the cleavage reaction for o-Cl-HABI (k(f ) = (5.6 +/- 1.0) x 10(6) s(-1)). The values of E-1(0) were found to b e -1.85 +/- 0.01 V vs. the ferrocene/ferrocenium formal potential for o-Cl-HABI and TCTM-HABI, and -2.01 V for o-EtO-HABI. Cyclic voltammetr y was also used to determine the formal potential for the L-./L- coupl e, E-2(0), and the rate constant, k(dim), for dimerization of the imid azolyl radicals. The results for E-2(0) are -0.15, -0.27 and -0.15 V v s. ferrocene and k(dim) is 1.1 x 10(4), 4.5 x 10(3) and 4.2 x 10(3) M- 1 s(-1) for o-Cl-HABI, o-EtO-HABI and TCTM-HABI, respectively. (C) 199 7 Elsevier Science S.A.