Determination of formal potentials and anion radical lifetimes for hexaarylbiimidazole derivatives

Abstract

Abstract The reduction of three hexaarylbiimidazole (HABl or L2) derivatives has been studied in acetonitrile by means of cyclic voltammetry. The reduction of o-Cl-HABl has previously been shown to occur by an ECE process. The radical anion initially formed in the first electron transfer (L2−) undergoes a fast cleavage reaction to produce the triarylimidazole anion (L−) and the radical (L−, the latter of which is reduced at the electrode to produce a second anion. The two other HABI derivatives investigated in this study show similar behavior. Homogeneous redox catalysis was used to determine the formal potential (E10) for the L2/L2− couple for each of the three HABIs and the rate constant of the cleavage reaction for o-Cl-HABI (kf = (5.6 ± 1.0) × 106s−1). The values of E10 were found to be −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 voltammetry was also used to determine the formal potential for the L/L− couple, E20, and the rate constant, kdim, for dimerization of the imidazolyl radicals. The results for E20are −0.15, −0.27 and −0.15 V vs. ferrocene and kdimis 1.1 × 104, 4.5 × 103and 4.2 × 103M−1s−1for o-Cl-HABI, o-EtO-HABI and TCTM-HABI, respectively.