Voltammetric characterization of the ferrocene|ferrocenium and cobaltocenium|cobaltocene redox couples in RTILs

Abstract

Ferrocene, Fc, and cobaltocenium hexafluorophosphate, CcPF6, have been recommended for use as internal reference redox couples in room-temperature ionic liquids (RTILs), as well as in more conventional aprotic solvents. In this study, the electrochemical behavior of Fc and CcPF6 is reported in eight commonly used RTILs; [C2mim][NTf2], [C4mim][NTf2], [C4mim][BF4], [C4mim][PF6], [C4mim][OTf], [C4mim][NO3], [C4mpyrr][NTf2], and [P14,6,6,6][FAP], where [Cnmim]+ = 1-butyl-3-methylimidazolium, [NTf2]- = bis(trifluoromethylsulfonyl)imide, [BF4]- = tetrafluoroborate, [PF6]- = hexafluorophosphate, [OTf]- = trifluoromethylsulfonate, [NO3]- = nitrate, [C4mpyrr]+ = N-butyl-N-methylpyrrolidinium, [P14,6,6,6 ]+ = tris(n-hexyl)-tetradecylphosphonium and [FAP]- = trifluorotris(pentafluoroethyl)phosphate, over a range of concentrations and temperatures. Solubilities and diffusion coefficients, D, of both the charged and neutral species were determined using double potential-step chronoamperometry, and CcPF6 (36.5−450.0 mM) was found to be much more soluble than Fc (27.5−101.8 mM). It was observed that classical Stokes−Einstein diffusional behavior applies for Fc and CcPF6 in all eight RTILs. Diffusion coefficients of Fc and CcPF6 were calculated at a range of temperatures, and activation energies calculated. It was also determined that D for Fc and CcPF6 does not change significantly with concentration. This supports the use of both Fc and CcPF6 to provide a well-characterized and model redox couple for use as a voltammetric internal potential reference in RTILs contrary to previous literature reports in the former case.