Voltammetric Characterization of the Ferrocene|Ferrocenium and Cobaltocenium|Cobaltocene Redox Couples in RTILs

Ferrocene, Fc, and cobaltocenium hexafluorophosphate, CcPF 6, 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 CcPF 6 is reported in eight commonly used RTILs; C 2 mimNTf 2 , C 4 mimNTf 2 , C 4 mimBF 4 , C 4 mimPF 6 , C 4 mimOTf, C 4 mimNO 3 , C 4 mpyrrNTf 2 , and P 14,6,6,6 FAP, where C n mim + = 1-butyl-3-methylimidazolium, NTf 2 - = bis(trifluoromethylsulfonyl)imide, BF 4 - = tetrafluoroborate, PF 6 - = hexafluorophosphate, OTf - = trifluoromethylsulfonate, NO 3 - = nitrate, C 4 mpyrr + = 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 CcPF 6 (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 CcPF 6 in all eight RTILs. Diffusion coefficients of Fc and CcPF 6 were calculated at a range of temperatures, and activation energies calculated. It was also determined that D for Fc and CcPF 6 does not change significantly with concentration. This supports the use of both Fc and CcPF 6 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.