Complexation of U(VI) with BiPDA, DmBiPDA, and PhenDA: Comparison on Structures and Binding Strengths in Aqueous and...

The stability constants (log β) of 1:1 uranyl complexes with three N,O-mixed donor ligands (L = 2,2'-dipyridyl-6,6'-dicarboxylate, 3,3'-dimethyl-2,2'-bipyridine-6,6'-dicarboxylate, and 1,10-phenanthroline-2,9-dicarboxylate, denoted as BiPDA, DmBiPDA, and PhenDA, respectively) in aqueous and DMSO/20%(v)H₂O solutions were determined by spectrophotometry in 0.1 M tetraethylammonium perchlorate. The effects of ligand preorganization, steric hindrance, and solvation on the binding strength of U(VI) with the three ligands were discussed. In aqueous solution, PhenDA forms stronger complexes with U(VI) than BiPDA due to its well-preorganized structure. In DMSO/20%(v)H₂O solution, in contrast, the strong solvation effect of DMSO on the ligands reduces the energy gap between the trans- and cis-conformations of BiPDA, resulting in log β(UO₂(BiPDA)) > log β(UO₂(PhenDA)). The steric hindrance of methyl groups on DmBiPDA makes the complex UO₂(DmBiPDA) of the lowest stability in both aqueous and DMSO/20%(v)H₂O solutions. Single-crystal structural data of U(VI) complexes with the three ligands indicate that the ligand coordinates with UO₂²⁺ via aromatic nitrogen atoms and carboxylate oxygen atoms. There is no clear correlation between the trend of the stability constants in solutions and the U-N/O bond lengths of the three crystal complexes. Nevertheless, DmBiPDA coordinates to UO₂²⁺ in a high-strain fashion as a result of the steric hindrance of methyl groups while BiPDA in a low-strain fashion, which is in accordance with the relative complexation strength of the two respective complexes. The results from this work help us understand the effect of ligand preorganization and solvation on the binding strength of actinides with multidentate N,O-mixed ligands in solid and solutions, which is of importance in designing ligands for the partitioning of actinides from nuclear wastes.