Electron and mass transport in hybrid redox polyether melts: Co and Fe bipyridines with attached polyether chains

The coupling of electron self exchange reactions with physical diffusion has been used to measure electron transfer rate constants in a series of undiluted metal complex molten salts [M(bpy(CO₂MePEG)₂)₃](ClO₄)₂ where M = Co(II/I) and Fe(III/II) and MePEG is an oligomeric polyether of MW 150, 350, and 550. Physical self-diffusion rates in the melts vary with attached polyether chain length by over 10³-fold while the electron transfer rate constants show no strong systematic dependence. The electron transfer rates and activation parameters indicate that the metal complex cores move rapidly within their attached polyether 'solvent' shells relative to the rates of electron transfers, which are near adiabatic with large activation barriers reflecting the apparent inability of the attached polyether chains to act as a freely mobile 'solvent'. Ionic conductivities of the melts were measured in order to inspect for ionic and electronic migration effects which are present to minor degrees. Diffusion and heterogeneous transfer rates are also reported for dilute solutions of the cobalt complexes in a polyether solvent.