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 ß (1:1)=5.94±0.02 and log ß (2:1)=6.33±0.01, respectively. The formation of the 1:1 complex was found to be enthalpy driven [ΔH (1:1)=(−71.5±10.0) kJ mol(−1); TΔS (1:1)=(−37.57±10.0) kJ mol(−1)], whereas the second complexation step was found to be endothermic and entropy driven [ΔH (2:1)=(32.8±4.0) kJ mol(−1); TΔS (2:1)=(68.97±4.0) kJ mol(−1)]. Moreover, the molecular structure of [UO(2)(H(6)L)(NO(3))](NO(3)) (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.