Uranium chloro complexes in hydrophobic room temperature ionic liquid: structural investigation and redox properties

U(IV) hexachloro complexes were studied in hydrophobic room temperature ionic liquids [BuMeIm][Tf2N] and [MeBu3N][Tf2N] to determine their structure, speciation and redox properties (BuMeIm+ and MeBu3N+ are 1-butyl-3-methyl-imidazolium and tri-n-butylmethylammonium respectively, and Tf2N- is bis(trifluoromethyl-sulfonyl)imide).
As the Tf2N- anion is known to be a weak ligand exhibiting oxygen coordination as 1-O or 2-O,O to actinides or lanthanides, the U(IV) chloro complex could be under the general chemical form UClx(Tf2N)y(4-x-y).
The diffuse UV-vis solid state reflectance of both U(IV) complexes agree closely with the absorption spectra obtained in ionic liquids. It is well known that U(IV) hexachloro complex has a centrosymmetric octahedral arrangement in the solid state as well as in acetonitrile solution. It can be concluded that octahedral anion UCl62- is the predominant form of U(IV) in Tf2N- based ionic liquids. EXAFS measurements were also performed to investigate the coordination sphere of U(IV) chloro complex in [BuMeIm][Tf2N]. The results confirmed that the octahedral complex UCl62- is the predominant chemical form of U(IV) in Tf2N- based hydrophobic ionic liquids and a competing reaction with Tf2N- can be excluded. The U-Cl distance measured in [BuMeIm][Tf2N] is 2.632(2) Å and is slightly longer than that in solid state. Probably, it can be related to some stronger H-bonding of UCl62- with BuMeIm+ cation in ionic liquid than that of solid.