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)·Et3NH] (1); [(UO2)2(C7H2NO5)2]·4H2O (2); [(UO2)2(C7H2NO5)(OH)2(H2O)2]·H2O (3); [(UO2)2(C7H2NO5)2(H2O)] (4); [(UO2)2(SmO)7(C7H2NO5)2(C7H4NO5)2(OH)4(NO3)(H2O)3]·2H2O (5); [(UO2)2(C7H2NO4Cl)2(H2O)3]·H2O (6); [(UO2)(C7H2NO4Cl)2(H2O)]·H2O (7); and Na2[(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.