Influence of Linker Geometry on Uranyl Complexation by Rigidly Linked Bis(3-hydroxy-N-methyl-pyridin-2-one)

Abstract

A series of bis(3-hydroxy-N-methyl-pyridin-2-one) ligands was synthesized and their respective uranyl complexes were characterized by single crystal X-ray diffraction analyses. These structures were inspected for high-energy conformations and evaluated using a series of metrics to

The Influence of Linker Geometry in Bis(3‐hydroxy‐N‐methyl‐pyridin‐2‐one) Ligands on Solution Phase Uranyl Affinity

“…The UO 2 (L 9 )(DMSO) crystal structure displays the expected mononuclear speciation seen with other bis-Me-3,2-HOPO ligands, [35,36] with a pentagonal planar uranyl coordination geometry provided on four points by (L 9 ) 2-, and a fifth site occupied by a DMSO oxygen. Significantly, the propoxy substituents are situated away from the amide linkers, so it can be assumed that PEG groups in L 7 H 2 will not impart significant steric influence on the resultant uranyl complex.…”

“…[35,36] The Me-3,2-HOPO moiety is inert towards further substitution, relegating PEG substitution to the linker moieties. Due to the extended electronic conjugation in ligands containing 3,4-thiophene and o-phenylene linkers (L 6 H 2 and L 7 H 2 ), electronic effects on the Me-3,2-HOPO moiety are unavoidable upon substitution at any position on the linker moiety.…”

“…[35] Unfortunately, the tremendous insolubility imparted on bis-Me-3,2-HOPO ligands by the fluorene backbone cannot be undone by solubilizing group substitution due to synthetic impracticality and the natural electrophillic substitution behavior of fluorene and its structural analog Structural Consequence of PEG Substitution in L 7 H 2 . The geometric and electronic consequences of PEG substitution on the 3,4-thiophene and o-phenylene linkers must be addressed due to the extended conjugation possible through the aromatic linkers.…”

“…[34][35][36] The Me-3,2-HOPO moiety is a bidentate, structural analog to the catecholamide group which binds via hard Lewis basic oxygens and is thus ideal for chelating the strong Lewis-acidic actinide ions. While short linkers best preserve intramolecular hydrogen bonds typical of catecholamide ligands, [36] structural comparisons of the uranyl complexes against that with Pr-Me-3,2-HOPO (L 1 H, Figure 1) illustrated that the butyl-linked ligand 4Li-Me-3,2-HOPO provided the most favorable geometry for uranyl chelation of the linearlylinked, tetradentate ligands (nLi-Me-3,2-HOPO, where "n" is the number of carbon atoms in the linker and "Li" stands for "linear").…”

“…[36] Of the rigidly-linked ligands explored, the 3,4-thiophene, o-phenylene, α,α'-m-xylene, and 1,8-fluorene linkers ( Figure 1) showed the most favorable uranyl coordination geometries; the former two contain very short linkers, while the latter two contain the longest linkers investigated. [35] These results raise the question of whether favorable geometry (optimized in short and long linker lengths) or intramolecular hydrogen bonding (optimized in short linkers) is most important in determining uranyl affinity.…”