Six uranyl-organic frameworks with naphthalene-dicarboxylic acid and bipyridyl-based spacers: syntheses, structures, and properties

Abstract

A new series of uranium coordination polymers have been hydrothermally synthesized by using 1,4-naphthalene dicarboxylic acid (H₂NDC), namely, (H₃O)₂[(UO₂)₂(NDC)₃]·H₂O (1), (H₂-bpp)[(UO₂)₂(NDC)₃]·EtOH·5H₂O (2), (H₂-bpe)_2/2[(UO₂)₂(NDC)₃]·EtOH (3), (H₂-bpp)[(UO₂)₂(NDC)₃]·5H₂O (4), (H₂-bpp)[(UO₂)(HNDC)(NDC)]₂·2H₂O (5), and (H₂-bpy)[(UO₂)(NDC)₂] (6) [bpp = 1,3-di(4-pyridyl) propane, bpe = 4,4′-vinylenedipyridine, bpy = 4,4′-bipyridine]. Single-crystal X-ray diffraction demonstrates that complex 1 represents the uranyl-organic polycatenated framework derived from a simple two-dimensional honeycomb grid network structure via a H₂NDC linker. Complexes 2–4 contain the dinuclear motifs of the two UO₇ pentagonal and one UO₈ hexagonal bipyramids which are linked by NDC²⁻ anions creating a (UO₂)₄(NDC)₂ unit, and further extend to a 2D layer through NDC²⁻ anions. Complex 5 displays a 1D zigzag double chain structure, in which the carboxylate groups of the NDC²⁻ anions adopt a chelate mode and further extends to a 2D framework via hydrogen bonds. The 1D structure of complex 6 is similar to the zigzag chain of complex 5. In addition, powder X-ray diffraction, elemental analysis, IR, thermal stability and luminescence properties of all complexes have also been investigated in this paper. The photocatalytic properties of the six complexes for the degradation of tetracycline hydrochloride (TC) under UV irradiation have been examined. Moreover, density functional theory (DFT) calculations were carried out to explore the electronic structural and bonding properties of the uranyl complexes 1–6.