Influence of Counterions on the Hydration Structure of Lanthanide Ions in Dilute Aqueous Solutions

A synergic approach combining molecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) has been used to investigate diluted (0.1 M) aqueous solutions of two lanthanide ions (Ln³⁺), namely, La³⁺ and Dy³⁺, with triflate, nitrate, and bis(trifluoromethylsulfonyl)imide (Tf₂N^-) as counterions. The different complexing ability of the three anions has been highlighted by the analysis of the MD simulations: Tf₂N^- does not form inner-sphere complexes, while a small amount of triflate coordinates both the La³⁺ and Dy³⁺ cations in their first solvation shell. On the other hand, the nitrate ion is almost absent in the La³⁺ first coordination sphere, while forming contact ion pairs with Dy³⁺. Both lanthanide ions are found to preferentially interact with the water molecules, and the total number of oxygen atoms coordinating the Ln³⁺ cations in their first solvation sphere is the same in all of the solutions, regardless of whether they belong to water molecules or to the counterion. The presence of counterions in the cation first or second shell changes neither the first shell distance nor the symmetry of the hydration complex formed in solution. The MD results have been confirmed by comparison with the Ln K-edge XAS experimental data, and the quantitative analysis of the extended X-ray absorption fine structure (EXAFS) spectra of the three salt solutions has provided a definite proof of the accuracy of the force field employed in the simulations and of the MD structural result. The anion-water and water-water hydrogen bond lifetimes have been analyzed highlighting the slow down effect of the triflate, nitrate, and Tf₂N^- anions on the hydrogen bond dynamics in the Ln³⁺ first solvation shell, with the effect being stronger in the Dy³⁺ solutions, due to the higher charge density of the Dy³⁺ ion as compared to La³⁺.