Solvation structure and thermodynamics for Pr(III), Nd(III) and Dy(III) complexes in ionic liquids evaluated by Raman spectroscopy and DFT calculation

Abstract

The coordination states of trivalent praseodymium, neodymium, and dysprosium complexes in the ionic liquid, triethyl-n-pentylphosphonium bis(trifluoromethyl-sulfonyl) amide ([P2225][TFSA]) were investigated by Raman spectroscopy. The effect of the concentration of rare earth ions on the Raman spectra was investigated, ranging from 0.23 to 0.45 mol kg−1 of Pr(III), Nd(III), and Dy(III) in [P2225][TFSA]. Based on a conventional analysis, the solvation numbers, n, of Pr(III), Nd(III), and Dy(III) in [P2225][TFSA] were determined to be 4.99, 5.01, and 5.00 at 298 K and 5.04, 5.06, and 5.07 at 373 K, respectively.Thermodynamic properties such as ΔisoG, ΔisoH, and ΔisoS for the isomerism of [TFSA]− from trans- to cis-coordinated isomer in the bulk and the first solvation sphere of the central RE3+ (RE = Pr, Nd, and Dy) cation in [P2225][TFSA] were evaluated from the temperature dependence of the Raman bands, measured at temperatures ranging from 298 to 398 K. Regarding the bulk properties, ΔisoG(bulk), ΔisoH(bulk), and TΔisoS(bulk) at 298 K were found to be −1.06, 6.86, and 7.92 kJ mol−1, respectively. The trans-[TFSA]− was a dominant contributor to the enthalpy, as shown by the positive value of ΔisoH(bulk). The value of TΔisoS(bulk) was slightly larger than that of ΔisoH(bulk), and cis-[TFSA]− was, therefore, entropy-controlled in [P2225][TFSA]. In contrast, in the first solvation sphere of the RE3+ cation, ΔisoH(RE) became remarkably negative, suggesting that cis-[TFSA]− isomers were stabilized by enthalpic contributions. Furthermore, ΔisoH(RE) contributed to the remarkable decrease in ΔisoG(RE), and this result clearly indicates that cis-[TFSA]− conformers bound to RE3+ cations are the preferred coordination state of [RE(III)(cis-TFSA)5]2− in [P2225][TFSA].Moreover, optimized geometries and binding energies of [Pr(III)(cis-TFSA)5]2−, [Nd(III)(cis-TFSA)5]2−, and [Dy(III)(cis-TFSA)5]2− clusters were also investigated by DFT calculations using the ADF package. The bonding energy, ΔEb, was calculated as ΔEb = Etot(cluster) − Etot(RE3+) − nEtot([TFSA]−), and ΔEb ([Pr(III)(cis-TFSA)5]2−), ΔEb([Nd(III)(cis-TFSA)5]2−), and ΔEb([Dy(III)(cis-TFSA)5]2−) were calculated to be −4238.6 ± 6.8, −4362.3 ± 8.2, and −4284.2 ± 7.4 kJ mol−1, respectively. This series of structural results allows us to conclude that [Dy(III)(cis-TFSA)5]2− clusters are more stable state than the [Pr(III)(cis-TFSA)5]2− clusters in [P2225][TFSA]. Furthermore, the average atomic charges and the bond distances of these clusters were consistent with the thermodynamic properties.