Effect of the aqueous-organic solvent structure on the cobalticenium-cobaltocene redox potential: The redox couple as a basis for determination of the single ion transfer energies

The redox potentials of the cobalticenium-cobaltocene couple have been determined voltammetrically in several aqueous-organic mixed solvents. The cosolvents studied were dimethylformamide, acetone, 1,4-dioxane (all three disrupting the 3D network of the water's hydrogen bonds), propylenecarbonate (at low content structure-making, at higher - structure-breaking), and glycerol that incorporates into the 3D network. Cobalticenium spectra demonstrate the absence of ion pairs and solute-cosolvent complexes. The liquid junction potentials water-mixed solvent in the range of the cosolvent concentrations studied are small. At equal volume fractions of structure-breaking cosolvents the shifts of redox potential relative to water are similar. The potential shifts are more than by order of magnitude higher than those corresponding to the change of the mixed solvent's dielectric constant. This is explained as due to disruption of the water hydrogen bonded structure, and disappearance therefore of the water anomalous dielectric response. The cobalticenium redox potential in the hypothetic “structureless” water was estimated as being ∼70 mV more positive than in real water. This is the potential that should be compared directly with the data for practically unstructured aprotic solvents.