Theoretical investigations of ferrocene/ferrocenium solvation in imidazolium-based room-temperature ionic liquids

The ferrocene/ferrocenium (Fc/Fc+) redox couple has been deemed one of the best candidates of the standard electrochemical reference for redox reactions in ionic liquids (ILs). To fully reveal the redox reaction mechanism in the IL condensed phase, solvation, as a prerequisite, apparently needs to be understood. As a preliminary attempt to study the solvation of Fc/Fc+ in imidazolium-based ILs, using model complexes, systematic investigations of the intrinsic pair-wise interactions between the solute and solvent ions in the gas phase are performed with B97-D, a dispersion-corrected density functional method. B97-D is carefully benchmarked with available experimental values and high-level ab initio quantum mechanical results for Fc/Fc+ alone and the solvated structures. Present calculations demonstrate dramatically different solvation features for Fc vs. Fc+, e.g., Fc forms direct stable interactions with both solvent cations and anions with binding energies of −11 to −14 kcal mol−1. As an interesting observation, no π-stacking interactions are captured between the imidazolium ring of the solvent cation and the cyclopentadienyl motif of the solute. However, due to the strong electrostatic interactions, Fc+ can only be solvated by solvent anions in the first solvation shell with much stronger binding energies of −72 to −86 kcal mol−1. Binding energies, structures, and electrostatic potentials are all characterized. The electronic structures are further analyzed with the Natural Bond Orbital method. The theoretical calculation results help explain several experimental observations obtained earlier or in the present work, including solubility, diffusion coefficient, and solution conductivity. Due to the involvement of the aromatic structures, the current study also sheds valuable light on π-interactions, in general.