Spectroscopic and electrochemical studies of U(IV)-hexachloro complexes in hydrophobic room-temperature ionic liquids [BuMelm][Tf2N] and [MeBU3N][Tf2N]

The behavior of U(IV) octahedral complexes [cation](2)[UCL6], where the [cation](+) is [BuMelm](+) and [MeBu3N](+), is studied using UV/visible spectroscopy, cyclic staircase voltammetry, and rotating disk electrode voltammetry in hydrophobic room-temperature ionic liquids (RTILs) [BuMelm][Tf2N] and [MeBu3N][Tf2N], where BuMelm(+) 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][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 UCl62- 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 UCl62- at the glassy carbon electrode in both RTILs and at the potential range of -2.5 to +1.0 V versus Ag/Ag(l) reveal the following electrochemical couples: UCl6-/UCl62- (quasi-reversible system), UCl62-/UCl62- (quasi-reversible system), and UCl62-/UCl6(Tf2N)(x)(-(3+x)) (irreversible reduction). The voltammetric half-wave potential, E-p/2, of the U(V)/U(IV) couple in [BuMelm][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 [BuMelm][Tf2N]. Presumably, the potential shift is due to the specific interaction of BuMelm+ with the uranium-hexachloro complex in ionic liquid. Scanning the negative potential to -3.5 V in [MeBu3N][Tf2N] solutions of UCl62- 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).