# Enhanced Thermodynamic Stability of UO2 2+ Complex Through Structure Preorganization of N3O2‐Pentadentate Planar Ligand for Uranium Harvesting from Seawater

## TL;DR

A new ligand design significantly improves uranium extraction from seawater by forming stable complexes and separating uranium from other ions.

## Contribution

The study introduces an N3O2-pentadentate planar ligand with preorganized structure that achieves the highest coordination affinity for UO2 2+ reported to date.

## Key findings

- The H2saldamp ligand forms a stable 1:1 complex with UO2 2+ (log β U = 33.60 ± 0.01) in simulated seawater.
- H2saldamp shows excellent separation of UO2 2+ from other metal ions like VO2 +, Al3+, and Cu2+ with separation factors exceeding 10^12.
- Replacing the H2saldian backbone with other structures failed to improve performance, highlighting the importance of ligand planarity and Lewis-basicity.

## 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 N3O2‐pentadentate planar ligand with UO2
2+ by more than 5 orders of magnitude compared to bis(2‐hydroxyphenylmethylaminoethyl)amine (H2saldian) recognized as the strongest U‐chelator under seawater conditions. 2,6‐Bis(2‐hydroxyphenylmethylaminomethyl)pyridine (H2saldamp) was confirmed to form a 1:1 complex, UO2(saldamp), in a simulated seawater. Its thermodynamic stability (log β
U = 33.60 ± 0.01) is superior not only to UO2(saldian) (log β
U = 28.05), but also to any UO2
2+ complexes bearing amidoxime‐based/‐inspired ligands reported so far. H2saldamp also exhibits notable separability of UO2
2+ from other concomitant ions, including VO2
+, Al3+, Ni2+, Cu2+, and Zn2+ as pronounced by separation factors of >1012. 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 H2saldamp due to its negligible affinity for VO2
+. Further preorganization by replacing the H2saldian backbone with bis(2‐aminophenyl)amine (H2saldiphan) and 1,9‐diaminophenazine (H2salphenazine) proved unsuccessful, implying that Lewis‐basicity of all coordinating atoms and equatorial planarity around UO2
2+ must also be taken into consideration.

Thanks to the appropriate structure preorganization of an N3O2‐pentadentate planar ligand, 2,6‐bis(2‐hydroxyphenylmethylaminomethyl)pyridine (H2saldamp) exhibits the strongest coordination affinity to UO2
2+ (log β
U = 33.60) compared to ligands developed for uranium harvesting from seawater to date, as well as good separability of UO2
2+ from other metal ions concomitantly present in seawater.