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

摘要

A reversible one-electron oxidation of tris(4-bromophenyl)amine (TBPA) in acetonitrile or 1-butyl-3-methylimidazoliumhexaflurophosphate [BmIm][PF6] media unambiguously generates the cation-radical (TBPA(.+)). 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(app)(0) 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 mVs(-1)). The slightly higher k(app)(0) value for the TBPA/TBPA(.+) 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(app)(0) values estimated in [BMIm][PF6] are comparatively two orders of magnitude lower (6.6 +/- 0.5 x 10(-4) cm s(-1) and 4.9 +/- 0.4 x 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(.+) 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(.+) 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. (C) 2008 Elsevier B.V. All rights reserved.