Enhanced Thermodynamic Stability of UO 2 2+ Complex Through Structure Preorganization of N 3 O 2 ‐Pentadentate Planar Ligand for Uranium Harvesting from Seawater

ABSTRACT Uranium harvesting from seawater is recently attracting special attention as a promising and sustainable resource option of this primary nuclear energy source. Through appropriate preorganization of the ligand backbone, we have achieved a remarkable enhancement in the coordination affinity of an N 3 O 2 ‐pentadentate planar ligand with UO 2 2+ by more than 5 orders of magnitude compared to bis(2‐hydroxyphenylmethylaminoethyl)amine (H 2 saldian) recognized as the strongest U‐chelator under seawater conditions. 2,6‐Bis(2‐hydroxyphenylmethylaminomethyl)pyridine (H 2 saldamp) was confirmed to form a 1:1 complex, UO 2 (saldamp), in a simulated seawater. Its thermodynamic stability (log β U = 33.60 ± 0.01) is superior not only to UO 2 (saldian) (log β U = 28.05), but also to any UO 2 2+ complexes bearing amidoxime‐based/‐inspired ligands reported so far. H 2 saldamp also exhibits notable separability of UO 2 2+ from other concomitant ions, including VO 2 + , Al 3+ , Ni 2+ , Cu 2+ , and Zn 2+ as pronounced by separation factors of >10 12 . While contamination of U by V is one of the issues to be resolved in U harvesting from seawater, their perfect separation can be anticipated using H 2 saldamp due to its negligible affinity for VO 2 + . Further preorganization by replacing the H 2 saldian backbone with bis(2‐aminophenyl)amine (H 2 saldiphan) and 1,9‐diaminophenazine (H 2 salphenazine) proved unsuccessful, implying that Lewis‐basicity of all coordinating atoms and equatorial planarity around UO 2 2+ must also be taken into consideration.