Solvation of Co2+ ion in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid: a molecular dynamics and X-ray absorption study

The solvation of the Co2+ ion in the [C4mim][Tf2N] (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) room temperature ionic liquid (RTIL) has been studied from both a structural and thermodynamic point of view. Co K-edge X-ray absorption spectroscopy (XAS) data have been collected on a 0.1 M Co(Tf2N)2 solution in [C4mim][Tf2N] as well as on the metallic salt and classical MD simulations have been performed to obtain both structural and thermodynamic data. The analysis of the extended X-ray absorption fine structure (EXAFS) region of the spectrum of the liquid sample has been carried out with the aid of MD simulations showing that the [Co(Tf2N)6]4− complex is formed in solution. A different coordination is present in the solid compound, where Co2+ is coordinated by two bidentate and two monodentate anions to form a [Co(Tf2N)4]2− unit. Thermodynamic data obtained from free energy calculations provide a strongly negative solvation free energy (ΔGsolv) which is qualitatively similar to that previously found for the Zn2+ ion. Free energy calculations carried out at variable temperature provided negative values for both ΔΗsolv and ΔSsolv. Thermodynamic parameters for the water→[C4mim][Tf2N] ion transfer (ΔGtrans, ΔΗtrans and ΔStrans) have been also obtained. The positive ΔGtrans shows that Co2+ is preferentially solvated by water, in agreement with the spectral changes in the visible region occurring when water is added to a Co2+ solution in dry [C4mim][Tf2N] even at low H2O/metal ratios. At higher water concentrations (up to the saturation limit), the spectrum is compatible with the presence of the [Co(H2O)6]2+ species. Interestingly, the positive ΔGtrans results from a compensation between ΔΗtrans and ΔStrans, which are both negative. However, the ligand exchange energy calculated for the CoL6 complexes (L = [Tf2N]− and H2O) indicates that [Tf2N]− is a much weaker ligand than water. This evidence suggests that the energetic contributions to the overall solvation enthalpy due to outer sphere effects are markedly different in water and [C4mim][Tf2N], so that the final ΔHtrans results to be negative.