Structural and spectroscopic studies of the complex [BuMeIm]2[UCl6] in the solid state and in hydrophobic room temperature ionic liquid [BuMeIm][Tf2N]

The complex [BuMeIm]2[UCl6] was characterized in the solid state and in a solution of [BuMeIm][Tf2N] room temperature ionic liquid using single-crystal XRD, EXAFS, visible absorption spectroscopy and NMR techniques.The structure of solid [BuMeIm]2[UCl6] contains two crystallographically independent formula units (U(1) and U(2)).The U–Cl distances in UCl6 2 anions range from 2.576(6) to 2.638(4) A ˚ , the average values being 2.621(6) and 2.601(8) A ˚ for U(1) and U(2), respectively.The BuMeIm+ cations have four different conformations of C4H9 chains.The shortest distances between the centres of the five-member rings of the BuMeIm+ cations and U atoms of [UCl6]2 anions are in the range 5.53–4.90 A ˚ .Single-crystal XRD data reveal the existence of a hydrogen bond between the most acidic H atom of the imidazolium ring (C2) and the chloride anion of the UCl6 2 octahedron, with the shortest distance Cl H(C2) equal to 2.57 A ˚ .Quantitative EXAFS measurements indicate that the octahedral complex UCl6 2(RU–Cl = 2.632(2) A ˚ ) is the predominant chemical form of U(IV) in [BuMeIm][Tf2N] solution.Visible absorption spectroscopy and 1H NMR spectroscopy confirm the existence of hydrogen bonding between theUCl6 2 anion and the acidic proton of the BuMeIm+ cation in RTIL solution, similar to that in the solid state.