Electrochemistry of Ferrocene-Functionalized Phosphonium Ionic Liquids

The synthesis of and voltammetry in undiluted form of several ferrocene-functionalized phosphonium ionic liquids is reported. Electron transport in the mixed valent diffusion layer around an electrode occurs primarily by Fc+/0 electron self-exchange reactions, as opposed to physical diffusion of the ferrocenated phosphonium species. The apparent diffusion coefficients for electron transport and for the counterions of the ionic liquid, and in particular their activation energy barriers, are similar to one another, evidence for the control of rates of electron transport by relaxation of the counterion atmosphere. For a net transport of electronic charge to accompany electron transfer, counterion displacement must occur to relax the charge displacement associated with the electron transfer. The ferrocenated phosphonium ionic liquids thus behave in a manner like that of previously investigated redox ionic liquids based on combinations of redox groupings with imidazolium and with short-chain polyether functionalities.