Specific Interaction between Uranium Anionic Complexes and the Cations of Bis(trifluoromethylsulfonyl)imide Based Ionic Liquids

The redox properties of uranium­(IV) hexachloro complex have been investigated with a glassy carbon electrode in four bis­(trifluoromethylsulfonyl)­imide (Tf₂N^–) based ionic liquids (ILs): the 1-butyl-3-methylimidazolium ([BuMeIm]⁺), the 1-butyl-2,3-dimethylimidazolium ([BuMe₂Im]⁺), the <i>N</i>-butylmethylpyrrolidinium ([BuMePyr]⁺), and the tributyl-methylammonium ([MeBu₃N]⁺). The cyclic voltammetric analysis has revealed two main redox systems: U^VCl₆^–/U^IVCl₆^2– around 0.2 V and U^IVCl₆^2–/U^IIICl₆^3– around −2 V. The formation of U^VCl₆^–, a non-dioxo uranium­(V) species, can then be observed at the electrode in [Tf₂N]⁻ based ILs. This work also provides evidence of a specific interaction between the uranium anionic species and the IL cations because the standard potentials of both redox couples depend on the IL. The interaction extent has been evaluated by comparison of the IL cation number associated with the uranium anionic complex. For that purpose, the standard potentials of both systems have been measured in the less interacting medium [MeBu₃N]­[Tf₂N] as a function of the [BuMeIm]⁺, [BuMe₂Im]⁺, and [BuMePyr]⁺ concentration. Predominance diagrams for uranium hexachloro complexes, analogous to the Pourbaix diagram, have then been built in [MeBu₃N]­[Tf₂N] depending on the IL cations concentration. The exploitation of these diagrams leads to the conclusion that the interaction is function of the charge of the uranium hexachloro complex (U^VCl₆^– < U^IVCl₆^2– < U^IIICl₆^3–) and the IL cation ([BuMe₂Im]⁺ < [BuMePyr]⁺ < [BuMeIm]⁺). The influence of the IL cation could be correlated to the size and the electropositivity of the IL cation. The association would occur by H-bonding and electrostatic interaction. Ab initio calculations were also carried out to evaluate the strength of the interaction between the anionic uranium­(IV) chloro complex and the IL cations. The results show that [BuMeIm]⁺ interacts the most and [MeBu₃N]⁺ the least with U^IVCl₆^2–, and the magnitude of the interaction is comparable for [BuMe₂Im]⁺ and [BuMePyr]<sup>+