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-VI 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 beta(1:1)=5.94 +/- 0.02 and log beta(2:1)=6.33 +/- 0.01, respectively. The formation of the 1:1 complex was found to be enthalpy driven [Delta H-1:1=(-71.5 +/- 10.0) kJ mol(-1); T Delta S-1:1=(-37.57 +/- 10.0) kJ mol(-1)], whereas the second complexation step was found to be endothermic and entropy driven [Delta H-2:1=(32.8 +/- 4.0) kJ mol(-1); T Delta S-2:1=(68.97 +/- 4.0) kJ mol(-1)]. Moreover, the molecular structure of [UO2(H6L)(NO3)](NO3) (1) was determined by single-crystal X-ray diffraction. Concluding, radiotoxic U-VI was separated from a Eu-III-containing solution by the calix[4]arene-based ligand in solvent extractions.