Disclosing the mechanism of uranium(VI) solvent extraction by polydentate ligands in a polar solvent: The role of ion pairs

[Display omitted] •Structure of U(VI) complexes with tri- and tetra-dentate ligands are studied.•Tetradentate ligands coordinate U(VI), forming[UO2LNO3]+[UO2(NO3)3]− complex.•Tridentate ligands form a mixture of complexes with U(VI) with different stoichiometry.•Both extractant exhibit high U(VI) loading capacity. Uranium is a key element in nuclear power. Developing new extraction systems for its purification and concentration is an important task for improving the nuclear fuel cycle. In this work solvent extraction and uranyl complexes with tri-dentate pyridine-based and tetra-dentate phenanthroline-based diphosphonates were studied in thorough detail, both in the organic phase and the solid state. A combination of experimental methods (loading isotherms and conductivity measurements), spectroscopic techniques (UV–vis, EXAFS, 31P NMR, and Raman spectroscopy) and theoretical calculations were used to disclose the mechanism of U(VI) solvent extraction. We demonstrated that with tetradentate phenanthroline-based ligands, tight ion pairs ([UO2LNO3]+[UO2(NO3)3]−) are formed. With tridentate ligands, U(VI) forms a mixture of complexes with different stoichiometries. Significant differences in the structure of complexes with tridentate ligands in the solid state compared to complexes in the organic solution were also shown.