Highly Preorganized Ligand 1,10-Phenanthroline-2,9-dicarboxylic Acid for the Selective Recovery of Uranium from Seawater in the Presence of Competing Vanadium Species

Studies of the complexation of new promising ligands with uranyl (UO 2 2+ ) and other seawater cations can aid the development of more efficient, selective, and robust sorbents for the recovery of uranium from seawater. In this work, we propose that the ligand design principles based on structural preorganization can be successfully applied to obtain a dramatic enhancement in UO 2 2+ ion binding affinity and selectivity. This concept is exemplified through the investigation of the complexes of UO 2 2+ , VO 2 + , and VO 2+ with the highly preorganized ligand 1,10-phenanthroline-2,9-dicarboxylic acid (PDA) using a combination of fluorescence and absorbance techniques, along with density functional theory (DFT) calculations. The measured stability constant value, log K 1 , of 16.5 for the UO 2 2+ /PDA complex is very high compared to uranyl complexes with other dicarboxylic ligands. Moreover, PDA exhibits strong selectivity for uranyl over vanadium ions, since the determined stability constant values of the PDA complexes of the vanadium ions are quite low (V(IV) log K 1 = 7.4, V(V) = 7.3). The structures of the corresponding UO 2 2+ , VO 2 + , and VO 2+ complexes with PDA were identified by systematic DFT calculations and helped to interpret the stronger binding affinity for uranium over the vanadium ions. Because of its high chemical stability, selectivity, and structural preorganization for UO 2 2+ complexation, PDA is a very promising candidate that can be potentially used in the development of novel adsorbent materials for the selective extraction of uranium from seawater.