Structural and spectroscopic studies of the complex [BuMeIm] 2[UCl 6] in the solid state and in hydrophobic room temperature ionic liquid [BuMeIm][Tf 2N

Abstract

The complex [BuMeIm] 2[UCl 6] was characterized in the solid state and in a solution of [BuMeIm][Tf 2N] room temperature ionic liquid using single-crystal XRD, EXAFS, visible absorption spectroscopy and NMR techniques.The structure of solid [BuMeIm] 2[UCl 6] contains two crystallographically independent formula units (U(1) and U(2)).The U–Cl distances in UCl 6 2 - anions range from 2.576(6) to 2.638(4) Å, the average values being 2.621(6) and 2.601(8) Å for U(1) and U(2), respectively.The BuMeIm + cations have four different conformations of C 4H 9 chains.The shortest distances between the centres of the five-member rings of the BuMeIm + cations and U atoms of [UCl 6] 2− anions are in the range 5.53–4.90 Å.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 UCl 6 2 - octahedron, with the shortest distance Cl⋯H(C2) equal to 2.57 Å.Quantitative EXAFS measurements indicate that the octahedral complex UCl 6 2 - (R U–Cl = 2.632(2) Å) is the predominant chemical form of U(IV) in [BuMeIm][Tf 2N] solution.Visible absorption spectroscopy and 1H NMR spectroscopy confirm the existence of hydrogen bonding between the UCl 6 2 - anion and the acidic proton of the BuMeIm + cation in RTIL solution, similar to that in the solid state.