An exploration of homo- and heterometallic UO22+ hybrid materials containing chelidamic acid: synthesis, structure, and luminescence studies

Six homometallic and two heterometallic UO22+/M (M = Na+, Sm3+) hybrid materials were synthesized hydrothermally and incorporate either chelidamic acid (4-hydroxy-2,6-pyridinedicarboxylic acid) or its chlorine substituted derivative: 4-chloro-2,6-pyridinedicarboxylic acid (4-chloro-2,6-pydc): [UO2(C7H2NO5)center dot Et3NH] (1); [(UO2)(2)(C7H2NO5)(2)]center dot 4H(2)O (2); [(UO2)(2)(C7H2NO5)(OH)(2)(H2O)(2)]center dot H2O (3); [(UO2)(2)(C7H2NO5)(2)(H2O)] (4); [(UO2)(2)(SmO)(7)(C7H2NO5)(2)(C7H4NO5)(2)(OH)(4)(NO3)(H2O)(3)]center dot 2H(2)O (5); [(UO2)(2)(C7H2NO4Cl)(2)(H2O)(3)]center dot H2O (6); [(UO2)(C7H2NO4Cl)(2)(H2O)]center dot H2O (7); and Na-2[(UO2)(3)(C7H2NO4Cl)(4)] (8). The two ligands afforded the opportunity to explore the different structural contributions of a hydroxyl versus a halogen functional group located on the para position of 2,6-pyridinedicarboxylic acid. Whereas the hydroxyl group provided an additional metal coordination site, the chlorine atom offered the foundation for halogen-based intermolecular interactions within the hybrid materials. Additional metal centers (Na+ and Sm3+) were incorporated into frameworks of 5 and 8, respectively and formed 3D architectures. Luminescence data were collected and typical uranyl emission was exhibited by 6 only. Heterometallic compound 5 was absent of any metal-based (UO22+ or Sm3+) emission, as well as ligand fluorescence, suggestive of a self-quenching mechanism.