Unprecedented three-dimensional hydrogen-bonded hex topological chiral lanthanide–organic frameworks built from an achiral ligand

The design and preparation of chiral metal–organic frameworks (CMOFs) from achiral ligands are a big challenge. Using 3-nitro-4-(pyridin-4-yl)benzoic acid (HL) as a new linker, a total of eight chiral lanthanide–organic frameworks (LOFs), namely poly[diaquatris[μ₂-3-nitro-4-(pyridin-4-yl)benzoato-κ²O:O′]lanthanide(III)], l- and d-[Ln(C₁₂H₇N₂O₄)₃(H₂O)₂]_n [(1), Ln = Eu; (2), Ln = Gd; (3), Ln = Dy; (4), Ln = Tb], were hydrothermally synthesized without chiral reagents and determined by X-ray crystallography. Crystal structure analyses show that l-(1)–(4) crystallize in the hexagonal P6₅ space group and are isomorphous and isostructural, while the enantiomers d-(1)–(4) crystallize in the hexagonal P6₁ space group. All Ln^III ions are octacoordinated by six carboxyl O atoms of six 3-nitro-4-(pyridin-4-yl)benzoate ligands and two water molecules in a dodecahedral geometry. A one-dimensional neutral helical [Ln₂(CO₂)₃]_n chain is observed in (1)–(4) as a chiral origin. These helical chains are further interconnected via directional hydrogen-bonding interactions between pyridyl groups and water molecules to construct a three-dimensional (3D) homochiral network with hex topology. The present CMOF structure is the first chiral 3D hydrogen-bonded hex-net and shows good water stability. Solid-state circular dichroism (CD) signals revealed that (1)–(4) crystallized through spontaneous resolution. Furthermore, (1) and (4) display a strong red and green photoluminescence at room temperature, respectively, but their intensities reduce to almost half at 200 °C. Notably, upon excitation under visible light (463 nm), a circularly polarized luminescence (CPL) of (1) in the solid state is observed for the first time, with a g_lum value of 2.61 × 10^−2.