Solvation structure and thermodynamics for lanthanide complexes in phosphonium-based ionic liquid evaluated by Raman spectroscopy and density functional theory

Abstract Coordination states of the divalent and trivalent lanthanide complexes in an ionic liquid (IL), triethyl-n-pentylphosphonium bis(trifluoromethyl-sulfonyl) amide [P2225][NTf2], were investigated by Raman spectroscopy and density functional theory (DFT) with the Amsterdam density functional package (ADF). Thermodynamic properties (ΔisoG, ΔisoH, and ΔisoS) for the trans-to-cis isomerism of [NTf2] in bulk and the first solvation sphere of the [Ln3+] cation in the IL were evaluated from their temperature dependence in the range 298–398 K. The ΔisoG(bulk), ΔisoH(bulk), and TΔisoS(bulk) values at 298 K were −1.06, 6.86, and 7.92 kJ mol−1, respectively. The trans-[NTf2] isomer had the highest enthalpy due to the positive value of ΔisoH(bulk); TΔisoS(bulk) was slightly higher than ΔisoH(bulk), and hence, cis-[NTf2] was entropy-controlled in the IL. In the first solvation sphere of the [Nd3+] cation, the negative value of ΔisoH (Nd) (−47.39 kJ mol−1) remarkably increased, which implied that the cis-[NTf2] isomers were stabilized for enthalpy. This result revealed that cis-[NTf2] bound to the [Ln3+] cation was preferred and the coordination state of [Ln(III)(cis-NTf2)5]2− was stable in the IL. The optimized geometries and the bonding energies of the [Ln(II)(cis-NTf2)4]2− and [Ln(III)(cis-NTf2)5]2− clusters were also investigated using the ADF. The bonding energy, ΔEb, was calculated from ΔEb = Etot(cluster) − Etot(Ln2,3+) − nEtot([NTf2]−). The ΔEb([Nd(II)(cis-NTf2)4]2−), ΔEb([Nd(III)(cis-NTf2)5]2−), ΔEb([Dy(II)(cis-NTf2)4]2−), and ΔEb([Dy(III)(cis-NTf2)5]2−) values were −2241.6, −4362.3, −2135.4, and −4284.2 kJ mol−1, respectively. This result showed that the [Ln(III)(cis-NTf2)5]2− cluster formed stronger coordination bonds than the [Ln(II)(cis-NTf2)4]2− cluster.