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 [C mim][NTf], [Cmim][NTf], [C mpyrr][NTf], [Cmim][BF], and [Cmim][PF] (where [Cmim] = 1-alkyl-3-methylimidazolium, [Cmpyrr] = N-butyl-N-methylpyrrolidinium, [NTf] = bis(trifluoromethylsulfonyl)-imide, [BF] = tetrafluoroborate, and [PF] = hexafluorophosphate). Diffusion coefficients, D, of 4.87, 3.32, 2.05, 1.74, and 1.34 × 10 m s and heterogeneous electron-transfer rate constants, k, of 0.0109, 0.0103, 0.0079, 0.0066, and 0.0059 cm s were calculated for TMPD in [Cmim][NTf], [Cmim][NTf], [Cmpyrr][NTf], [Cmim][BF], and [Cmim][PF], respectively, at 303 K. The oxidation of TMPD in [Cmim] [PF ] was also carried out at increasing temperatures from 303 to 343 K, with an activation energy for diffusion of 32.3 kJ mol. k was found to increase systematically with increasing temperature, and an activation energy of 31.4 kJ mol was calculated. The study was extended to six other p-phenylene-diamines with alkyl/phenyl group substitutions. D and k values were calculated for these compounds in [Cmim][NTf], and it was found that k showed no obvious relationship with the hydrodynamic radius, r.