Urothermal synthesis of photoluminescent lanthanide–organic frameworks with unusual topologies

Abstract

As a newly developed synthetic method, urothermal synthesis was employed to construct a series of photoluminescent lanthanide–organic frameworks based on a flexible ligand. Eight new lanthanide compounds, namely Ln₂(obb)₃(e-urea)(H₂O) [Ln = Er (1), Yb (2), Ho (3); obb = 4,4′-oxybisbenzoic acid, e-urea = ethyleneurea], Ln₂(obb)₃(e-urea)(H₂O)₃ [Ln = Tb (4), Dy (5), Eu (6)], La₂(obb)₃(H₂O)_1.5(e-urea) (7) and [Sm₂(obb)₃(H₂O)₅]·(H₂O) (8), have been synthesized under urothermal reactions and structurally characterized by single-crystal X-ray diffraction. Compounds 1, 2 and 3 are isostructural and possess three-dimensional structures with (3,3,4,5,5)-connected new topology, while compounds 4, 5 and 6 are almost identical in structure with the space group P2₁/n and display an unusual (4,4,5)-connected topology. Compound 7 is a unique two-dimensional structure with the centrosymmetric space group Pbca and possesses a (4,4,7,7)-connected new topology. Different from the above-mentioned seven frameworks (1–7), compound 8 features a three-dimensional framework with unprecedented (3,4,4,5,6)-connected new topology. In these compounds, the structural variations were rationalized by the effect of lanthanide contraction, the effect of e-urea as the structure-directing agent and the different coordination modes of the flexible obb ligand. The luminescent properties of compounds 4 and 6 have been studied in the solid state at room temperature, and both of them display metal-based luminescence.