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

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], [C ₄mim][PF₆], [C₄mim][OTf], [C ₄mim][NO₃], [C₄mpyrr][NTf₂], and [P_14,6,6,6][FAP], where [C_nmim]⁺ = 1-butyl-3-methylimidazolium, [NTf₂] = bis-(trifluoromethylsulfonyl) imide, [BF₄]- = tetrafluoroborate, [PF₆] = hexafluorophosphate, [OTf] = trifluoromethylsuifonate, [NO₃] ^- = nitrate, [C₄mpyrr]⁺ = N-butyl-N- methylpyrrolidinium, [P_14,6,6,6]⁺ = 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.