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

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 UCl₆^2- 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), UCl₆^2-/UCl₆^3- (quasi-reversible system), and UCl₆^2-/UCl₆(Tf₂N)_x^-(3+x) (irreversible reduction). The voltammetric half-wave potential, E_p/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 E_p/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 °C), which could be attributed to the reduction of U(III) to U(0).