Ion atmosphere relaxation controlled electron transfers in cobaltocenium polyether molten salts

A room-temperature redox molten salt for the study of electron transfers in semisolid media, based on combining bis(cyclopentadienyl)cobalt with oligomeric polyether counterions, [Cp ₂Co](MePEG ₃₅₀SO ₃), is reported. The transport properties of the new molten salt can be varied (plasticized) by varying the polyether content. The charge transport rate during voltammetric reduction of the ionically conductive [Cp ₂Co] (MePEG ₃₅₀SO ₃) molten salt exceeds the actual physical diffusivity of [Cp ₂Co] ⁺ because of rapid [Cp ₂Co] ^+/0 electron self-exchanges. The measured [Cp ₂Co] _+/0 electron self-exchange rate constants (k _EX) are proportional to the diffusion coefficients (D _CION) of the counterions in the melt. The electron-transfer activation barrier energies are also close to those of ionic diffusion but are larger than those derived from optical intervalent charge-transfer results. Additionally, the [Cp ₂Co] _+/0 rate constant results are close to those of dissimilar redox moieties in molten salts where DCION values are similar. All of these characteristics are consistent with the rates of electron transfers of [Cp ₂Co] ^+/0 (and the other donor-acceptor pairs) being controlled not by the intrinsic electron-transfer rates but by the rate of relaxation of the ion atmosphere around the reacting pair. In the low driving force regime of mixed-valent concentration gradients, the ion atmosphere relaxation is competitive with electron transfer. The results support the generality of the recently proposed model of ionic atmosphere relaxation control of electron transfers in ionically conductive, semisolid materials.