Speciation of uranium bis(trifluoromethylsulfonyl)imide in a room temperature ionic liquid

The speciation of uranium bis(trifluoromethylsulfonyl)imide (TFSI) species formed from UI₃(THF)₄ in butyl-methylpiperidinium bis(trifluoromethylsulfonyl)imide ([BMPip][TFSI]) and [BMPip][TFSI]/HTFSI was investigated by X-ray absorption fine structure (XAFS) spectroscopy. Following the dissolution of UI₃(THF)₄ in the room temperature ionic liquid (RTIL), X-ray absorption near edge spectroscopy (XANES) indicates that U(III) did fully oxidize to U(VI) in [BMPip][TFSI] while a U(VI)/U(IV) mixture was obtained in [BMPip][TFSI]/HTFSI media. Extended X-ray absorption fine structure (EXAFS) results are consistent with the formation of UO₂(TFSI)₂(THF)₃ in [BMPip][TFSI]. The U-S(TFSI) distance (i.e., 3.81(4) Å) is consistent with the presence of TFSI ligands coordinated to the UO₂²⁺ unit in monodentate mode. In [BMPip][TFSI])/HTFSI, EXAFS indicates the presence of UO₂(TFSI)₂(THF)₃ (∼35%) and of a U(IV) species coordinated to THF and TFSI ligands. The number of TFSI ligands coordinated to U(IV) (i.e., ∼0.5) is consistent with the presence of an outer-sphere U(IV)-TFSI complex. (author)