The reaction of uranyl nitrate with terephthalic acid (HTP) under hydrothermal conditions in the presence of an organic base, 1,3-(4,4'-bispyridyl)propane (BPP) or 4,4'-bipyridine (BPY), provided four uranyl terephthalate compounds with different entangled structures by a pH-tuning method. [UO(TP)](HBPP)·2HO (1) obtained in a relatively acidic solution (final aqueous pH, 4.28) crystallizes in the form of a noninterpenetrated honeycomb-like two-dimensional network structure. An elevation of the solution pH (final pH, 5.21) promotes the formation of a dimeric uranyl-mediated polycatenated framework, [(UO)(μ-OH)(TP)](HBPP)·4.5HO (2). Another new polycatenated framework with a monomeric uranyl unit, [(UO)(TP)](HBPP) (3), begins to emerge as a minor accompanying product of 2 when the pH is increased up to 6.61, and turns out to be a significant product at pH 7.00. When more rigid but small-size BPY molecules replace BPP molecules, [UO(TP)](HBPP) (4) with a polycatenated framework similar to 3 was obtained in a relatively acidic solution (final pH, 4.81). The successful preparation of 2-4 represents the first report of uranyl-organic polycatenated frameworks derived from a simple HTP linker. A direct comparison between these polycatenated frameworks and previously reported uranyl terephthalate compounds suggests that the template and cavity-filling effects of organic bases (such as BPP or BPY), in combination with specific hydrothermal conditions, promote the formation of uranyl terephthalate polycatenated frameworks.

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Novel uranyl coordination polymers, UO2(bqdc)­(phen)·H2O (1), [UO2(μ-OH)­(bqdc)­(H2bpy)0.5(H2O)] (2), Na­[(UO2)2(bqdc)3Na­(H2O)2] (3), and [Na­(bqdc)0.5(bpp)­(H2O)] (4) (H2bqdc = 2,2′-biquinoline-4,4′-dicarboxylic acid; phen = 1,10-phenanthroline; bpy = 4,4′-bipyridine; bpp = 1,3-di­(4-pyridyl)­propane), with bqdc2– ligands have been successfully synthesized by hydrothermal reactions and characterized by single-crystal X-ray diffraction, Infrared spectroscopy (IR), thermogravimetric analysis (TGA), and powder X-ray diffraction (PXRD). The topological structures feature 1D chain to 3D framework by altering N-donor ancillary ligands. Compound 1 shows a 1D wave-shaped zigzag chain structure and further extends to a two-dimensional (2D) layer through π···π interactions between the quinoline ring of bqdc2– ligand and benzene ring of phen ligand. The uranium adopts an approximate hexagonal bipyramidal coordination geometry with the equatorial plane warped to the unusual chair conformation. Compound 2 features rectangular-shaped units with space range of 12.28(2) Å × 7.16(3) Å, exhibiting an intriguing 2D uranyl double layered motif formed by 1D ladder chains. The protonated bpy molecules provide space filling and form hydrogen bonds with the layers. Compound 3 is based upon 3D heterometallic frameworks constructed from UO2 2+, Na+, and bqdc2– ligands. The most striking feature of compound 3 is that one sodium ion is located in the middle of two adjacent uranyl ions, forming the trinuclear heterometal clusters (U2Na), which are further connected by bqdc2– ligands to generate UOFs with the cavity size of 10.07(0) Å × 13.86(2) Å. The local 1D structure of compound 3 is similar to the zigzag chain of compound 1. Compound 4 displays 1D chain structure and further extends to 3D framework via hydrogen bond and π···π interactions. Moreover, the electronic structural and bonding properties of the uranyl compounds 1–3 have been systematically explored by density functional theory (DFT) calculations.