Spectroscopic and Electrochemical Studies of U(IV)−Hexachloro Complexes in Hydrophobic Room-Temperature Ionic Liquids [BuMeIm][Tf₂N] and [MeBu₃N][Tf₂N]

Abstract

The behavior of U(IV) octahedral complexes [cation]2[UCl6], where the [cation]+ is [BuMeIm]+ and [MeBu3N]+, is studied using UV/visible spectroscopy, cyclic staircase voltammetry, and rotating disk electrode voltammetry in hydrophobic room-temperature ionic liquids (RTILs) [BuMeIm][Tf2N] and [MeBu3N][Tf2N], where BuMeIm+ and MeBu3N+ are 1-butyl-3-methylimidazolium and tri-n-butylmethylammonium cations, respectively, and Tf2N- is the bis(trifluoromethylsulfonyl)imide anion. The absorption spectra of [cation]2[UCl6] complexes in the RTIL solutions are similar to the diffuse solid-state reflectance spectra of the corresponding solid species, indicating that the octahedral complex UCl6(2-) is the predominant chemical form of U(IV) in Tf2N--based hydrophobic ionic liquids. Hexachloro complexes of U(IV) are stable to hydrolysis in the studied RTILs. Voltammograms of UCl(6)2- at the glassy carbon electrode in both RTILs and at the potential range of -2.5 to +1.0 V versus Ag/Ag(I) reveal the following electrochemical couples: UCl6-/UCl6(2-) (quasi-reversible system), UCl(6)2-/UCl6(3-) (quasi-reversible system), and UCl(6)2-/UCl6(Tf2N)x-3+x (irreversible reduction). The voltammetric half-wave potential, Ep/2, of the U(V)/U(IV) couple in [BuMeIm][Tf2N] is positively shifted by 80 mV compared with that in [MeBu3N][Tf2N]. The positive shift in the Ep/2 value for the quasi-reversible U(IV)/U(III) couple is much greater (250 mV) in [BuMeIm][Tf2N]. Presumably, the potential shift is due to the specific interaction of BuMeIm+ with the uranium-hexachloro complex in ionic liquid. Scanning the negative potential to -3.5 V in [MeBu3N][Tf2N] solutions of UCl6(2-) reveals the presence of an irreversible cathodic process at the peak potential equal to -3.12 V (at 100 mV/s and 60 degrees C), which could be attributed to the reduction of U(III) to U(0).

Lanthanides and Actinides in Ionic Liquids

“…113 Also in studies on the mechanism of solvent extraction in ionic liquids, the absorption spectra of the uranyl ions were useful. 114 The intensities of the complex in ionic liquids were higher than those in acetonitrile, which was attributed to the stronger solvation in ionic liquids. The [UCl 6 ] 2-is stable against hydrolysis in the ionic liquids.…”

“…In contrast to its behavior in acidic chloroaluminate ionic liquids, the uranyl ion is stable in basic ionic liquids. 113 The uranium(VI) ion can be reduced electrolytically to uranium-(IV) with the formation of the [ 114 The study shows that the uranium redox potential is sensitive to the cation of the ionic liquid. The highest sensitivity was found for the uranium(IV)/uranium-(III) redox couple.…”

Luminescence Analysis of Actinides: Instrumentation, Applications, Quantification, Future Trends, and Quality Assurance

“…Uranium(V) occurs also as dioxo-cation [30]. In this oxidation state uranium disproportionates easily into its +6 and +4 states.…”

Cu(II) Extraction in Ionic Liquids and Chlorinated Solvents: Temperature Effect

“…Indeed, even water-immiscible RTILs are hygroscopic and usually contain some water. As regards hexafluorophosphate-containing ionic liquids, their hydrolysis instability has often been noted in literature [16][17][18] especially at high temperature and in the presence of strong acids. Hydrolysis of forms a mixture of products, including HF, POF 3 N] to the aqueous solution was achieved in 24 h, being the metal ion partition in the ionic liquid equal to 12% again.…”