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.