Formation of Higher Chloride Complexes of Np(IV) and Pu(IV) in Water-Stable Room-Temperature Ionic Liquid [BuMeIm][Tf2N]

A UV/vis/near-IR spectroscopic study shows that in [BuMeIm][(CF3SO2)2N] hydrophobic room-temperature ionic liquid solutions, [BuMeIm]2[AnCl6] complexes, where BuMeIm+ is 1-n-butyl-3-methylimidazolium and An(IV) is Np(IV) or Pu(IV), have an octahedral An(IV) environment similar to that observed in solid complexes. Water has no influence on the absorption spectra of AnCl6(2-) complexes, indicating their stability to hydrolysis in ionic liquid. Adding [BuMeIm]Cl modifies the UV/vis/near-IR absorption spectra of An(IV) in the ionic liquid and causes solids to precipitate. The solid-state reflectance spectra of the precipitates reveal considerable differences from the corresponding An(IV) hexachloro complexes. A voltammetric study indicates that AnCl6(2-) complexes are electrochemically inert in [BuMeIm][(CF3SO2)2N] at the glassy carbon working electrode. By contrast, quasi-reversible electrochemical reduction An(IV)/An(III) and An(IV) oxidation are observed in ionic liquids in the presence of [BuMeIm]Cl. The oxidation wave of noncoordinated chloride ions interferes with the An(IV) oxidation waves. The spectroscopic and voltammetric data clearly indicate the formation of nonoctahedral actinide(IV) chloride complexes with a Cl-/An(IV) ratio exceeding 6/1 in [BuMeIm][(CF3SO2)2N] in excess chloride ions.