Solvation structure of iron group metal ion in TFSA-based ionic liquids investigated by Raman spectroscopy and DFT calculations

Solvation structures of the iron group metal ion (M(II) = Fe(II), Co(II) and Ni(II)) in bis(trifluoromethyl-sulfonyl)amide type ILs have been researched by UV–Vis, Raman spectra and DFT calculations. In order to investigate the coordination structure, UV–Vis spectra of divalent iron group metal ions in TFSA-based ILs compared with corresponding aqua complexes of each metal ion. Raman spectra of Fe(II), Co(II) and Ni(II) on the concentration range of 0.23–0.59 mol kg−1 in TFSA-based ILs were measured at room-temperature and 150 °C. Moreover, the geometries and SCF energies of [Fe(cis-TFSA)3]− and [Fe(trans-TFSA)3]− were also examined using the theoretical DFT calculations because it was known that the TFSA− anion involves two conformers of cis and trans symmetries in equilibrium. The results enabled us to indicate that three TFSA− anions were bidentate ligand by two O atoms to the M2+ ion in TFSA-based ILs. Furthermore, DFT calculations were revealed that the trans-conformer was preferred to the cis-conformer in the vicinity of the Fe2+ ion.