Water-Soluble Girard T-Based Multidentate Ligands for Highly Selective Uranyl(VI) Binding

The development of chelating agents capable of selectively binding UO22+ in aqueous media remains a major challenge in uranium recovery and remediation, largely due to competitive complexation by vanadium and iron ions. Here, we introduce two water-soluble Girard-T-based acylhydrazone ligands, a planar pentadentate 2,2'-(((1E,1'E)-pyridine-2,6-diylbis(ethan-1-yl-1-ylidene))bis(hydrazin-1-yl-2-ylidene))bis(N,N,N-trimethyl-2-oxoethan-1-aminium) (H2dapGT2+) and tetradentate 2,2'-(((1E,2E)-ethane-1,2-diylidene)bis(hydrazin-1-yl-2-ylidene))bis(N,N,N-trimethyl-2-oxoethan-1-aminium) (H2glyxGT2+), designed to maximize equatorial coordination of the uranyl ion. The reaction of UVIO22+ and FeIII with H2dapGT2+ and H2glyxGT2+ led to the synthesis of [UVIO2(dapGT)(H2O)][ClO4]2 (1), [UVIO2(dapGT)MeOH][SbF6]2 (1'), [{UVIO2(dapGT)}2(μ-VV4O12)]·8H2O (2·8H2O), [UVIO2(glyxGT)(CH3COO)]ClO4 (3), [FeIII(HdapGT2+)Cl2][ClO4]2 (4) and [{FeIII(dapGT2+)}2(μ-O)(CH3OH)(H2O)][ClO4]4 (5). Single-crystal X-ray diffraction reveals that dapGT2+ occupies five from the six available sites of the uranyl equatorial plane to form highly stable hexagonal bipyramidal complexes, while the corresponding FeIII complexes adopt pentagonal bipyramidal geometries. Combined spectroscopic, thermodynamic, and theoretical studies show that dapGT2+ exhibits exceptional stability and pronounced selectivity for UVIO22+ over VVO2+, outperforming amidoxime-based chelators under competitive conditions. In contrast, the more flexible glyxGT2+ ligand forms weaker but still selective uranyl complexes. These findings establish ligand denticity and equatorial plane multidentate ligation as key design principles for achieving uranyl selectivity in water and provide a viable framework for next-generation actinide chelators relevant to uranium extraction, waste management, and actinide sequestration.