Introducing Phosphate Ester into DAPhen by Propyl Enhanced the Selectivity for UO₂²⁺ over Th⁴⁺

A new type of phenanthroline carboxamide(DAPhen)-phosphate ester ligand (<b>L1</b>/<b>L2</b>) was synthesized for the selective separation of U(VI) over Th(IV). Liquid-liquid extraction experiments showed that the introduction of phosphate ester could increase the extraction ability of ligands for U(VI), especially <b>L2</b>, which showed high selectivity for the separation of U(VI) over Th(IV). The slope analysis indicated that <b>L1</b> could form 1:1 and 1:2 complexes with U(VI) and 1:1 complexes with Th(IV). NMR titration revealed that the DAPhen unit of ligands combined with one U(VI) to form 1:1 complexes, and then the phosphate ester unit of the 1:1 complexes further combined with another U(VI) to form 1:2 complexes. Ligands provide only the DAPhen unit to Th(IV) to form 1:1 complexes. The crystal structures found 1:2 complexes of <b>L1</b> and U(VI), 1:1 complexes of <b>L2</b> and U(VI), and 1:1 complexes of <b>L1</b> and Th(IV). The larger stability constant (log β) of the 1:1 complexes of <b>L2</b> with U(VI) than that of the 1:1 complexes of <b>L1</b> with U(VI) showed that the binding ability of U(VI) with the DAPhen unit of <b>L2</b> is stronger than that of U(VI) with the DAPhen unit of <b>L1</b>. This study provides new ideas for designing extractants with excellent properties.