Exploring New Assembly Modes of Uranyl Terephthalate: Templated Syntheses and Structural Regulation of a Series of Rare 2D → 3D Polycatenated Frameworks

The reaction of uranyl nitrate with terephthalic acid (H2TP) 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. [UO2(TP)1.5](H2BPP)0.5·2H2O (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, [(UO2)2(μ-OH)2(TP)2]2(H2BPP)2·4.5H2O (2). Another new polycatenated framework with a monomeric uranyl unit, [(UO2)2(TP)3](H2BPP) (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, [UO2(TP)1.5](H2BPP)0.5 (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 H2TP 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.