Introducing\nPhosphate Ester into DAPhen by Propyl\nEnhanced the Selectivity for UO<sub>2</sub><sup>2+</sup> over Th<sup>4+</sup>

A new type of phenanthroline carboxamide(DAPhen)-phosphate\nester\nligand (<b>L1</b>/<b>L2</b>) was synthesized for the selective\nseparation of U(VI) over Th(IV). Liquid–liquid extraction experiments\nshowed that the introduction of phosphate ester could increase the\nextraction ability of ligands for U(VI), especially <b>L2</b>, which showed high selectivity for the separation of U(VI) over\nTh(IV). The slope analysis indicated that <b>L1</b> could form\n1:1 and 1:2 complexes with U(VI) and 1:1 complexes with Th(IV). NMR\ntitration revealed that the DAPhen unit of ligands combined with one\nU(VI) to form 1:1 complexes, and then the phosphate ester unit of\nthe 1:1 complexes further combined with another U(VI) to form 1:2\ncomplexes. Ligands provide only the DAPhen unit to Th(IV) to form\n1:1 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),\nand 1:1 complexes of <b>L1</b> and Th(IV). The larger stability\nconstant (log β) of the 1:1 complexes of <b>L2</b> with\nU(VI) than that of the 1:1 complexes of <b>L1</b> with U(VI)\nshowed 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\nwith excellent properties.