Uranium(VI) Complexes with a Calix[4]arene-Based 8-Hydroxyquinoline Ligand: Thermodynamic and Structural Characterization Based on Calorimetry, Spectroscopy, and Liquid-Liquid Extraction.

The environmental aspects of ore processing and waste treatment call for an optimization of applied technologies. There, understanding of the structure and complexation mechanism on a molecular scale is indispensable. Here, the complexation of U with a calix[4]arene-based 8-hydroxyquinoline ligand was investigated by applying a wide range of complementary methods. In solution, the formation of two complex species was proven with stability constants of log =5.94±0.02 and log =6.33±0.01, respectively. The formation of the 1:1 complex was found to be enthalpy driven [Δ=(-71.5±10.0) kJ mol; Δ=(-37.57±10.0) kJ mol], whereas the second complexation step was found to be endothermic and entropy driven [Δ=(32.8±4.0) kJ mol; Δ=(68.97±4.0) kJ mol]. Moreover, the molecular structure of UO(HL)(NO) () was determined by single-crystal X-ray diffraction. Concluding, radiotoxic U was separated from a Eu-containing solution by the calix[4]arene-based ligand in solvent extractions.