Kinetics of redox mediator tris(4-bromophenyl)amine in acetonitrile and ionic liquid [BMIm][PF6]: Oxidation of benzyl and cyclohexyl alcohols

A reversible one-electron oxidation of tris(4-bromophenyl)amine (TBPA) in acetonitrile or 1-butyl-3-methylimidazoliumhexafluorophosphate [BMIm][PF6] media unambiguously generates the cation-radical (TBPA^{{radical dot}+}), and its reduced form is catalytically regenerated in a follow-up chemical reaction with benzyl alcohol in the presence of 2,6-lutidine. The electron-transfer kinetics for these transformations has been investigated by a detailed comparison of the results obtained in both media. The apparent standard heterogeneous rate constant k_app⁰ for an interfacial single-electron transfer of the mediator in acetonitrile containing 0.1 M Et₄NClO₄ was estimated using cyclic voltammetry (CV) profiles within a wide range of sweep rates (20-1000 mV s^-1). The slightly higher k_app⁰ value for the TBPA/TBPA^{{radical dot}+} couple at a Pt electrode surface was found to be (2.1 ± 0.8) 10^-2 cm s^-1 compared to (1.4 ± 0.6) 10^-2 cm s^-1 at a glassy carbon (GC) surface. k_app⁰ values estimated in [BMIm][PF6] are comparatively two orders of magnitude lower (6.6 ± 0.5 × 10^-4 cm s^-1 and 4.9 ± 0.4 × 10^-4 cm s^-1 at Pt and GC, respectively). It has been found that the second-order homogeneous rate constant for the oxidation of benzyl alcohol by TBPA^{{radical dot}+} in acetonitrile (12.10 M^-1 s^-1) was estimated to be greater by one order of magnitude than that in ionic liquid [BMIm][PF6] (4.20 M^-1 s^-1). Similar experiments carried out for the oxidation of 2-chlorobenzyl alcohol, 4-tert-butylcyclo hexanol and 4-methylcyclohexanol by the TBPA^{{radical dot}+} showed no noticeable catalytic effect due to large potential difference of the mediator (E_p/2 = 0.74 V vs. Fc/Fc⁺), compared with those of the substrates.