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

Abstract

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.