Complexation of U(VI) with BiPDA, DmBiPDA, and PhenDA: Comparison on Structures and Binding Strengths in Aqueous and DMSO/20%(v)H₂O Solutions

Abstract

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 UO2 2+ 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 UO2 2+ 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.

Complexation of Lanthanides with N,N,N′,N′-Tetramethylamide Derivatives of Bipyridinedicarboxylic Acid and Phenanthrolinedicarboxylic Acid: Thermodynamics and Coordination Modes

“…In our previous work, we investigated the complexation thermodynamics of BiPDA and PhenDA with U­(VI) as well as PhenDA with Np­(V) in solutions. In the present work, the stability constants and the enthalpy of the complexation between lanthanides (Nd and Eu) and N , N ,­ N ′, N ′-tetramethyl-2,2′-bipyridine-6,6′-dicarboxamide (TMBiPDA) and N , N ,­ N ′, N ′-tetramethyl-1,10-phenanthroline-2,9-dicarboxamide (TMPhenDA) in CH 3 OH/​10%(v)­H 2 O solutions were determined by thermodynamic measurements including spectrophotometry and microcalorimetry.…”

Theoretical insights into selective extraction of uranium from seawater with tetradentate N,O-mixed donor ligands

“…In the present work, the complexation mechanisms of PDA (L 1 ) with [UO 2 (CO 3 ) 3 ] 4− and H 2 VO 4 − , HVO 4 2− have been systematically studied by density functional theory (DFT). To explore the effect of the ligand backbone on the extraction behavior, two analogous tetradentate N,O-mixed donor ligands [2,2′-bipyridine]-6,6′-dicarboxylate acid 31,32 (L 2 ) and 5 H -cyclopenta[2,1- b :3,4- b ′]dipyridine-2,8-dicarboxylate acid 33,34 (L 3 ) were also investigated for comparative analysis (Fig. 1).…”

Role of O Substitution in Expanded Porphyrins on Uranyl Complexation: Orbital- and Density-Based Analyses

“…Following Hard-Soft-Acid-Base (HSAB) concept, it may be advisable to choose macrocyclic ligands with hard oxygen donor center for cation like U(VI), however, the extensive presence of lanthanides and other alkali metals within the nuclear waste often throws competition toward the efficiency of actinide extraction. Therefore, significant studies [19][20][21] have been devoted on exploring how other alternative donor center such as 'N', 'S' or mixed-donor center (N,O) based ligands perform in chelation.…”

“…However, the presence of lanthanides and other alkali metals in large excess within the nuclear waste often adversely affects actinide extraction. Therefore, significant studies − have been devoted to exploring how other alternative donor centers such as N, S, or mixed-donor center (N, O)-based ligands perform in chelation. Undoubtedly, the availability of sophisticated quantum-mechanical techniques such as density functional theory (DFT) and advancement of tools that utilize the orbital- and density-based information played a significant role in understanding the actinide complexation with macrocycles in depth. , Theoretical investigations on uranyl complexes with variants of hexaphyrin ligands indicated that the change in size and shape of a ligand cavity modulates the ionic/covalent character of metal–ligand bonds and subsequently alters the binding affinities …”