Solvation structure for Fe(II), Co(II) and Ni(II) complexes in [P2225][NTf2] ionic liquids investigated by Raman spectroscopy and DFT calculation

Abstract

The solvation structures of divalent iron, cobalt and nickel complexes in the ionic liquid, triethyl‑n‑pentylphosphonium bis(trifluoromethyl sulfonyl) amide ([P2225][NTf2]) were investigated by Raman spectroscopy. Based on a conventional analysis, the solvation numbers of Fe(II), Co(II), and Ni(II) in [P2225][NTf2] were determined to be 3.18, 3.21, and 3.14 at 298 K and 3.24, 3.32, and 3.37 at 373 K, respectively.From the temperature dependence of the Raman bands the isomerism of [NTf2]− from trans- to cis-coordinated isomer in the bulk and the first solvation sphere of the central M2+ (M = Fe, Co, and Ni) cation in [P2225][NTf2], thermodynamic properties such as ΔisoG, ΔisoH, and ΔisoS for the isomerism were evaluated. It was revealed that cis-[NTf2]− isomers were stabilized by enthalpic contribution, because ΔisoH(M) became remarkably negative in the first solvation sphere of the M2+ cation. Moreover, ΔisoH(M) contributed to the remarkable decrease in ΔisoG(M), and this result clearly indicates that cis-[NTf2]− conformers bound to M2+ cations are the preferred coordination state of [M(II)(cis-NTf2)3]− in [P2225][NTf2].The optimized geometries and the binding energies of [Fe(II)(cis-NTf2)3]−, [Co(II)(cis-NTf2)3]−, and [Ni(II)(cis-NTf2)3]− clusters were calculated by ADF simulations. The bonding energy, ΔEb, was calculated as ΔEb = Etot(cluster) − Etot(M2+) − nEtot([NTf2]−), and ΔEb ([Fe(II)(cis-NTf2)3]−), ΔEb([Co(II)(cis-NTf2)3]−), and ΔEb([Ni(II)(cis-NTf2)3]−) were calculated to be −2132.1 ± 6.4, −2254.6 ± 6.1, and −2283.4 ± 7.2 kJ mol−1, respectively. Furthermore, the bond distances of these clusters were consistent with the thermodynamic properties.