Thermodynamic and Kinetic Effects of Ion Pairing in Class II/III Mixed-Valent Systems

Electrostatic effects of ion pairing on the kinetic, thermodynamic, and electronic properties of a pyrazine-bridged triruthenium class II/III mixed-valent (MV) dimer anion are described. The MV state of the dimer was electrochemically generated in the presence of potassium [K⁺], tetramethylammonium [TMA⁺], potassium 18-crown-6 [K'rown⁺], and tetraoctylammonium [TOA⁺] hexafluorophosphate [PF₆^-] supporting electrolyte in acetonitrile. Using cyclic voltammetry, electronic absorption spectroelectrochemistry, and infrared spectroelectrochemistry, we quantify the influence of ion pairing across a series of supporting electrolytes. Half-wave potential splittings correlate strongly with the overall electrostatic potential of the paired ions, indicating that weakly interacting counter cations enhance MV-state stability. Intervalence charge transfer (IVCT) band intensities similarly decrease with increasing counter ion interaction, consistent with local-field effects that damp electronic transitions without significantly altering electronic coupling. Infrared measurements show no change in ν(CO) coalescence or electron-transfer rates, reflecting kinetics too fast to be perturbed by ion migration. Electrostatic and resonance contributions dominate the overall comproportionation energetics, with paired-ion interactions accounting for the observed thermodynamic trends. These results underscore the importance of ion-pair electrostatics in highly delocalized MV systems and suggest that stronger dependencies will emerge as systems approach greater valence localization.