A series of three-dimensional lanthanide metal–organic frameworks with biphenylethene-4,4′-dicarboxylic acid: Hydrothermal syntheses and structures

Abstract

Biphenylethene-4,4′-dicarboxylic acid (H2bpea) reacts with lanthanide(III) oxide (Ln = Sm, Eu, Gd, Tb, Er) or nitrate salts of lanthanide(III) (Ln = Eu, La, Tb) under hydrothermal conditions to form a series of lanthanide complexes 1–8, which were characterized by elemental analysis, IR, PL TG and single-crystal X-ray diffraction. These eight complexes exhibit four different kinds of 3-D frameworks. The complexes [Ln(bpea)(formate)]n (Ln = Sm (1), Eu (2), Gd (3), Tb (4), Er (5)) were isomorphous and exhibited 3-D pillared layered frameworks. The eight-coordinated lanthanide ions in complexes 1–5 are bridged by the formate anions into 2-D covalent nets, which are further interconnected by the bpea ligands to produce the 3-D framework. While the formate anions are neglected, the new generated 3-D framework shows a four-connected SrAl2 net. The nine-coordinated europium ions in complex [Eu(bpea)(formate)]n (6) are also bridged by the formate anions into similar 2-D covalent nets, which are further interconnected by the bpea ligands to produce a different 3-D framework. Interestingly, complex 6 displays a 2-fold interpenetrating 3-D structure with Pts topology while neglecting the formate anions. The main reason is due primarily to the coordination geometry of the europium ion and the novel μ4:η6-bridging coordination mode of the bpea ligands. Complexes 7 and 8 have the same composition of [Ln(bpea)1.5(H2O)2]n (Ln = La (7) and Tb (8)), but crystallize in the P21/n and P space groups, respectively. In complex 7, the lanthanum centers are bridged by the bpea ligands in a common μ4:η4-bridging coordination mode into 2-D double layer motifs, which are further connected by the other μ4:η4-bridging bpea ligands to produce a 3-D hybrid framework with sixty-membered macro rings. By contrast, the terbium centers in complex 8 are bridged by the bpea ligands in an unprecedented μ4:η5-bridging coordination mode into 2-D layer architectures, which are further connected by the other μ2:η2-bridging bpea ligands to produce a 3-D hybrid framework.