Effect of position of polyether attachment on the electron self-exchange activation barrier energies of redox polyether hybrid molten salts

We have synthesized highly viscous, room temperature molten salts by associating [M(bpy)₃]²⁺ cations (M = Ru or Co, bpy = 4,4′-bipyridine) with polyether-tailed 2-sulfobenzoate anions. Microelectrode voltammetry in the undiluted melts yields, on the basis of the coupling of electron hopping with physical diffusion, electron self exchange rate constants for the Co^II/I and Ru^III/II couples. Activation studies show that homogeneous electron transfers in the melts have large activation barriers (34-39 kJ/mol) that are similar to those of melts in which the polyether chains are covalently attached to the [M(bpy)₃]²⁺ cations via 4,4′-bipyridine ester linkages (26-29 kJ/mol) and the counterion is perchlorate. The similarity of the activation barriers indicates that the large activation barriers do not result from a peculiar inner-sphere reorganizational barrier term caused by covalent linkage of the "solvent" (the polyether chains) to the bipyridine ligands. Also reported are physical diffusion constants, ionic conductivities, and viscosities. The number of ethylene oxide segments in the polyether tails seems to be an important determinant of physical diffusion rates and viscosity.