Analysis of coordination states for Dy(II) and Dy(III) complexes in ionic liquids by Raman spectroscopy and DFT calculation

Abstract

The coordination states of the divalent and trivalent dysprosium complexes in ionic liquid, triethyl-pentyl-phosphonium bis(trifluoromethylsulfonyl) amide [P2225][TFSA] were investigated by Raman spectroscopy and DFT calculations. The concentration dependences of the deconvoluted Raman spectra were investigated for 0.23–0.45molkg−1 of pure Dy(III) and the mixed sample of Dy(II)/Dy(III)=1/3 at the molar ratio in [P2225][TFSA]. According to the conventional analysis, the solvation number; n of Dy(II) and Dy(III) in [P2225][TFSA] were determined to be n=4.12 and 5.00, respectively.Then, the thermodynamic properties such as ΔisoG, ΔisoH and ΔisoS for the isomerism of [TFSA]− from trans- to cis-isomer in bulk and the first solvation sphere of the centered [Dy3+] cation in [P2225][TFSA] were evaluated from the temperature dependence in the range of 298–398K. As for the bulk condition, ΔisoG(bulk), ΔisoH(bulk) and TΔisoS(bulk) at 298K were −1.06, 6.86, and 7.92kJmol−1, respectively. The trans-[TFSA]− was dominant in the enthalpy due to the positive value of ΔisoH(bulk) and TΔisoS(bulk) was slightly larger than ΔisoH(bulk), so that cis-[TFSA]− was revealed to be an entropy-controlled in [P2225][TFSA]. On the other hand, in the first solvation sphere of [Dy3+] cation, ΔisoH(Dy) (−26.92kJmol−1) increased to the negative value remarkably and implied that the cis-[TFSA]− isomers were stabilized for enthalpy. ΔisoH(Dy) contributed to the remarkable decrease in the ΔisoG(Dy) and this result clearly indicated that the cis-[TFSA]− bound to the [Dy3+] cation was preferred and the coordination state of [Dy(III)(cis-TFSA)5]2− was stable in [P2225][TFSA].Moreover, the optimized geometries and the bonding energies of [Dy(II)(cis-TFSA)4]2− and [Dy(III)(cis-TFSA)5]2− clusters were also investigated from DFT calculations with ADF package. The bonding energy; ΔEb was calculated from ΔEb=Etot(cluster)−Etot(Dy2,3+)−nEtot([TFSA]−), so that ΔEb ([Dy(II)(cis-TFSA)4]2−) and ΔEb([Dy(III)(cis-TFSA)5]2−) were −2135.37 and −4284.21kJmol−1, respectively. This result revealed that [Dy(III)(cis-TFSA)5]2− cluster formed stronger coordination bonds than [Dy(II)(cis-TFSA)4]2− cluster. The average atomic charges and the bond distances of these clusters were consistent with the thermodynamic properties.