Oxidation of Several p-Phenylenediamines in Room Temperature Ionic Liquids:  Estimation of Transport and Electrode Kinetic Parameters

The electrochemical oxidation of N,N,N‘,N‘-tetramethyl-p-phenylenediamine (TMPD) has been studied by cyclic voltammetry and potential step chronoamperometry at 303 K in five ionic liquids, namely [C₂mim][NTf₂], [C₄mim][NTf₂], [C₄mpyrr][NTf₂], [C₄mim][BF₄], and [C₄mim][PF₆] (where [C_nmim]⁺ = 1-alkyl-3-methylimidazolium, [C₄mpyrr]⁺ = N-butyl-N-methylpyrrolidinium, [NTf₂]^- = bis(trifluoromethylsulfonyl)imide, [BF₄]^- = tetrafluoroborate, and [PF₆]^- = hexafluorophosphate). Diffusion coefficients, D, of 4.87, 3.32, 2.05, 1.74, and 1.34 × 10^-11 m² s^-1 and heterogeneous electron-transfer rate constants, k₀, of 0.0109, 0.0103, 0.0079, 0.0066, and 0.0059 cm s^-1 were calculated for TMPD in [C₂mim][NTf₂], [C₄mim][NTf₂], [C₄mpyrr][NTf₂], [C₄mim][BF₄], and [C₄mim][PF₆], respectively, at 303 K. The oxidation of TMPD in [C₄mim][PF₆] was also carried out at increasing temperatures from 303 to 343 K, with an activation energy for diffusion of 32.3 kJ mol^-1. k₀ was found to increase systematically with increasing temperature, and an activation energy of 31.4 kJ mol^-1 was calculated. The study was extended to six other p-phenylene-diamines with alkyl/phenyl group substitutions. D and k₀ values were calculated for these compounds in [C₂mim][NTf₂], and it was found that k₀ showed no obvious relationship with the hydrodynamic radius, r.