A Series of Lanthanide Secondary Building Units Based Metal−Organic Frameworks Constructed by Organic Pyridine-2,6-Dicarboxylate and Inorganic Sulfate

A series of novel lanthanide coordination polymers constructed from organic pyridine-2,6-dicarboxylate (2,6-pydc) and inorganic sulfate, namely, [Eu₃(2,6-pydc)₃(2,6-Hpydc)(SO₄)(H₂O)₃·(H₂O)₃] (1), [Ln₂(2,6-pydc)₂(SO₄)(H₂O)₂·(H₂O)₂] (Ln = Ce (2); Ln = Pr (3); Ln = Nd (4); Ln = Sm (5)), and [Ce₅(2,6-pydc)₆(SO₄)₂(H₂O)₃·(Me₂NH₂)] (6), have been synthesized under hydrothermal conditions. X-ray crystal structure analyses reveal that these compounds have a rich structural chemistry. Compound 1 presents a four-connected two-dimensional (2D) network based on hexanuclear {Eu₆} secondary building units (SBUs). The 2D layers are further linked into a three-dimensional (3D) supramolecular framework via strong π−π stacking interactions of the pyridyl rings. Compounds 2−5 are isostructural and adopt uninodal six-connected 3D framework of pcu topology constructed from square-planar tetranuclear {Ln₄} SBUs. Of particular interest is that compound 6 exhibits a protonated (Me₂NH₂)⁺ templated binodal (4,6)-connected 3D anionic framework with (4²5¹⁰7²8)(4²5⁴) topology, when the Ce(3) monomer and the planar tetranuclear {Ce₄} SBUs are regarded as four-connected and six-connected nodes, respectively. Notably, sulfate serves as an auxiliary supporting bridge to strengthen the {Ln₄} or {Ln₆} SBUs in these compounds. Compound 1 displays intense red luminescence and exhibits the characteristic transition of the Eu³⁺ ion. The magnetic properties of 1, 4, and 5 reveal the weak antiferromagnetic characters. Furthermore, infrared (IR), thermogravimetric analysis (TGA), elemental analyses (EA), and powder X-ray diffraction (PXRD) properties of these compounds are also studied.