Hexadentate Terephthalamide(bis-hydroxypyridinone) Ligands for Uranyl Chelation: Structural and Thermodynamic Consequences of Ligand Variation

Several linear, hexa- and tetradentate ligands incorporating a combination of 2,3-dihydroxy-terephthalamide (TAM) and hydroxypyridinone-amide (HOPO) moieties have been developed as uranyl chelating agents. Crystallographic analysis of several {UO2[TAM(HOPO)2]}2– complexes revealed a variable and crowded coordination geometry about the uranyl center. The TAM moiety dominates the bonding in hexadenate complexes, with linker rigidity dictating the equality of equatorial U–O bonding. Hexadentate TAM(HOPO)2 ligands demonstrated slow binding kinetics with uranyl affinities on average 6 orders of magnitude greater than those of similarly linked bis-HOPO ligands. Study of tetradentate TAM(HOPO) ligands revealed that the high uranyl affinity stems primarily from the presence of the TAM moiety and only marginally from increased ligand denticity. Uranyl affinities of TAM(HOPO)2 ligands were within experimental error, with TAM(o-phen-1,2-HOPO)2 exhibiting the most consistent uranyl affinity at variable pH.