The formation constants of the UO 2 2+ cation with the amidoximate ligands bzam (benzamidoxime) and acetam (acetamidoxime) are reported. These are of interest in light of their proposed use as the functional groups of extractants for uranium in seawater. The formation constants of bzam with UO 2 2+ were measured by monitoring the absorbance of the π→π* transitions in the UV spectrum of the bzam ligand in the presence of 1:1 UO 2 2+ as a function of pH. This yielded log K 1 = 12.4 for UO 2 2+ with bzam, and log K = 6.9 for the equilibrium UO 2 (bzam) + + OH-= UO 2 (bzam)OH at 25 o C and ionic strength zero. The bzam complexes were also studied monitoring the fluorescence of the UO 2 2+ system. Analysis of the intense fluorescence that occurs in 5 x 10-6 M UO 2 2+ solutions between pH 5 and 9 suggested that this was due to the [(UO 2) 3 O(OH) 3 ] + trimer. Monomeric species such as UO 2 2+ and [UO 2 (OH) 4 ] 2-, and dimers such as [(UO 2)(OH) 2 ] 2+ , fluoresce only weakly. Titration of such solutions with bzam supported the above log K values measured by absorbance, and with higher bzam concentrations yielded log  2 = 22.3. The acetam ligand does not have any absorbance, so that complexformation was monitored by fluorescence only. Formation constants measured by fluorescence may differ from those measured by other techniques such as absorbance. The agreement obtained between log K values measured by absorbance and fluorescence for the bzam complex of UO 2 2+ supported the log K values measured for the acetam complexes by florescence alone were reliable: log K 1 = 13.6, log  2 = 23.7, and log K UO 2 (acetam) + + OH-= UO 2 (acetam)OH = 6.8. The high log K values found for the bzam and acetam complexes of UO 2 2+ were analyzed using DFT calculations. These log K values are related to the ability of polymer-based extractants bearing bzam or acetam type functional groups to extract UO 2 2+ at a concentration of 1.3 x 10-8 M and in the competing 0.0025 M CO 3 2present in the oceans.

Studies of the complexation of new promising ligands with uranyl (UO) 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 ion binding affinity and selectivity. This concept is exemplified through the investigation of the complexes of UO, VO, and VO 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, of 16.5 for the UO/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 = 7.4, V(V) = 7.3). The structures of the corresponding UO, VO, and VO 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 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.