A Family of UO₂²⁺–5-Nitro-1,3-dicarboxylate Hybrid Materials: Structural Variation As a Function of pH and Structure Directing Species

Four new UO22+ compounds were hydrothermally synthesized and incorporate 5-nitro-1,3-benzenedicarboxylic acid (n-bzdc): [(UO2)­(C8N1O6H3)­(H2O)]·H2O (1), [(UO2)2(C8N1O6H3)2H2O]·TEA (2); [(UO2)2(OH)­(C8N1O6H3)2]·(TEAH)­(H2O) (3); [(UO2)­(C8N1O6H3)­(C8N1O6H4)]·TMAH (4). Compound 1 consists of pentagonal bipyramidal uranyl monomers bridged via n-bzdc linkers to form a double-stranded chain, a structural motif that also serves as the foundation for the 2-D sheet of 2 and the 3-D framework of 3. The increasing dimensionality displayed throughout compounds 1–3 is a function of pH and the structure directing influence of TEA (triethylamine) in the case of 2 and 3. Compounds 1 and 2 exhibit similar 2-D configurations, yet each compound offers a different route toward structural assembly: supramolecular noncovalent interactions (1) and direct metal-to-ligand coordination (2). The role of TEA as a template in 2 and 3 was investigated by substituting TEA with alternative species that can potentially serve as structure directing agents: TMA (trimethylamine), TPA (tripropylamine), NaOH(aq), and CsOH(aq). The formation of compound 4 was observed in the presence of TMA and revealed a different chain motif compared with that observed in 1–3. Luminescence spectra were collected in order to ascertain whether any relationship existed between structural variation and uranyl emission.