Structural study on uranyl ion in 1-butyl-3-methylimidazolium nonafluorobutanesulfonate ionic liquid

The structure of uranyl ion in 1-butyl-3-methylimidazolium nonafluorobutanesulfonate ionic liquid (BMINfO) has been studied with ¹H- and ³⁵Cl-NMR, Raman, and UV-visible spectroscopy. In the ¹H-NMR spectrum of the BMINfO solution prepared by dissolving UO₂(ClO₄)₂·5 6H₂O, the signal of H₂O coordinated to UO₂²⁺ was observed at 6.64 ppm at 50°C (free H₂O in BMINfO: 3.1 ppm at 50°C), suggesting that the uranyl species exists as the aquo complex, UO₂(H₂O)_n]²⁺. The signal of the coordinated H₂O disappears with heating at 120°C for 3 h under vacuum. This indicates the dehydration from UO₂(H₂O)_n]²⁺. On the other hand, the ³⁵Cl-NMR signal of ClO₄^? as the counter anion of UO₂²⁺ was observed at 1011 ppm (vs. Cl^? in D₂O) regardless of heating. This indicates that no ClO₄^? ion is in the first coordination sphere of UO₂²⁺. Furthermore, the UV-visible absorption spectra showed that the characteristic absorption bands due to UO₂²⁺ were sharpened with the dehydration. This means the simplification of the structure around UO₂²⁺. These results described above suggest that UO₂²⁺ in BMINfO has no ligand in its equatorial plane after the dehydration, i.e. UO₂²⁺ exists as a bare cation in this system.