Solvation structure for Fe(II), Co(II) and Ni(II) complexes in [P2225][NTf2] ionic liquids investigated by Raman spectroscopy and DFT calculation
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Graduate School of Environment and Information Sciences, Yokohama National University
Graduate School of Environment and Information Sciences, Yokohama National University
Department of Applied Chemistry and Biochemistry, National Institute of Technology, Wakayama College
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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.
Solvation structure for Fe(II), Co(II) and Ni(II) complexes in [P2225][NTf2] ionic liquids investigated by Raman spectroscopy and DFT calculation
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