Reference potential calibration and voltammetry at macrodisk electrodes of metallocene derivatives in the ionic liquid [bmim][PF6]

Abstract

Reference potential scales are not generally available in ionic liquids. Consequently, comparison of data with those obtained in conventional solvent (electrolyte) media is not possible. The process [Co(Cp)2](+/0) (Co(Cp)2 = cobaltocene) has been studied at gold, glassy carbon and platinum macrodisk electrodes to test the feasibility of using this redox couple as a voltammetric reference standard in the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]). A reversible, one-electron reduction process was observed, and the measured reversible potential versus a silver quasi-reference electrode was independent of the working electrode material, the concentration, and the scan rate. Ferrocene, the other traditionally used reference compound, is poorly soluble in this ionic liquid. However, the solution-phase voltammetry of ethylferrocene could be readily studied in [bmim][PF6], and a reversible oxidation process was observed. A reversible potential of +1285 +/- 5 mV versus the [Co(Cp)2](+/0) reference potential scale was obtained, and this value is comparable with that obtained in CH3CN (0.1 M Bu4NPF6) when referenced to the same potential scale. Ferrocene, decamethylferrocence, 1,1'-dimethylferrocene, 1,1'-diacetylferrocene, and ferrocenecarboxaldehyde were adhered to the working electrode surface and immersed in [bmim][PF6]. In each case, solid-state voltammetry provided well-defined, reversible one-electron oxidation processes that had the appearance of being diffusion controlled, with charge neutralization occurring via the ionic liquid. Reversible potentials of the solid-state processes referenced against the [Co(Cp)2](+/0) scale were similar to solution-phase values obtained in CH3CN (0.1 M Bu4NPF6).