Bipyridine-Directed Syntheses of Uranyl Compounds Containing Semirigid Dicarboxylate Linkers: Diversity and...

Bipyridine organic bases are beneficial to the synthesis of novel uranyl-organic hybrid materials, but the relationship between their molecular structures and specific roles as structure-directing agents, especially for the semirigid dicarboxylate systems, is still unclear. Here we demonstrate how the bipyridine ligands direct the coordination assembly of uranyl-organic compounds with a semirigid dicarboxylate linker, 4,4'-dicarboxybiphenyl sulfone (H₂dbsf), by utilizing a series of bipyridine ligands, 1,10-phenanthroline (phen), 2,2'-bipyridine (2,2'-bpy), 5,5'-dimethylbipyridine (5,5'-dmbpy), 4,4'-bipyridine (4,4'-bpy), or 1,3-di(4-pyridyl)propane (bpp). Under hydrothermal conditions, eight uranyl-organic coordination polymers (UCPs), four of which [[UO₂(dbsf)(phen)] (1), [UO₂(dbsf)(phen)]·H₂O (1'), [U₄O₁₀(dbsf)₃]₂[H₂bpp]₂ (6), and [U₄O₁₀(dbsf)₃]₂[H₂bpp] (6')] were reported previously, were synthesized and divided into two types based on the chelate or template effect of these bipyridine ligands. 1, 1', [UO₂(dbsf)(2,2'-bpy)] (2), and [(UO₂)₂(dbsf)₂(5,5'-dmbpy)₂] (3) are springlike triple helices with bipyridine ligands (phen, 2,2'-bpy, or 5,5'-dmbpy) as chelate ligands, while [U₄O₁₀(dbsf)₃][H₂(4,4'-bpy)] (4), [U₄O₁₀(dbsf)₃]₂[H(4,4'-bpy)]₂[Ni(H₂O)₆] (5), 6, and 6' are tetranuclear uranyl-mediated 2-fold-interpenetrating networks with 4,4'-bpy or bpp as template ligands and charge-balancing agents. The participation or not in uranyl coordination of different bipyridine ligands promotes not only diversity in uranyl speciation and final topological structures among different classes of organic bases but also consistency for the same types of bipyridine ligands, which thus endows the possibility of the rational design of UCPs based on semirigid dicarboxylate ligands with the aid of cautiously selected bipyridine ligands.