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

Coordination states of the divalent and trivalent lanthanide complexes in an ionic liquid (IL), triethyl-n-pentylphosphonium bis(trifluoromethyl-sulfonyl) amide [P-2225][NTf2], were investigated by Raman spectroscopy and density functional theory (DFT) with the Amsterdam density functional package (ADF). Thermodynamic properties (Delta(iso)G, Delta H-iso, and Delta S-iso) for the trans-to-cis isomerism of [NTf2] in bulk and the first solvation sphere of the [Ln(3+)] cation in the IL were evaluated from their temperature dependence in the range 298-398 K. The Delta(iso)G(bulk), Delta H-iso/(bulk), and T Delta S-iso(bulk) values at 298 K were -0.71, 7.63, and 8.34 kJ mol(-1), respectively. The trans-[NTf2] isomer had the highest enthalpy due to the positive value of Delta H-iso(bulk); T Delta S-iso(bulk) was slightly higher than Delta H-iso(bulk), and hence, cis-[NTf2] was entropy-controlled in the IL In the first solvation sphere of the [Nd3+] cation, the negative value of Delta H-iso(Nd) (-47.79 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 [Ln(3+)] (Ln=Nd or Dy) 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, Delta E-b, was calculated from Delta E-b = E-tot (cluster) - E-tot(Ln(2,3+)) nE(tot)([NTf2](-)). The Delta E-b([Nd-(II)(cis-NTf2)(4)](2-)), Delta E-b([Nd-(III)(cis-NTF2)(5)](2-)). Delta E-b([Dy-(II)(cis-NTf2)(4)](2-)), and Delta E-b([Dy-(III)(cis-NTf2)(5)](2-)), values were -2219.2, -4340.5, -2118.3, and -4258.5 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. (C) 2020 Elsevier B.V. All rights reserved.