Controlling topological diversity and magnetic properties in three Tb(iii) metal–organic frameworks through amide ligand variation

Abstract

Three three-dimensional (3D) Tb(III) metal–organic frameworks (Tb-MOFs), {[Tb₂(DMF)₄(H₂dobdc)₄]·DMF}_n (Tb-MOF1), {[Tb₂(DMA)₂(H₂O)₂(H₂dobdc)₆]·2DMA}_n (Tb-MOF2), and {Tb₂(DEF)₆(H₂dobdc)₃}_n (Tb-MOF3, H₄dobdc = 2,5-di-hydroxy-benzene-1,4-di-carboxylic acid; DMF = N,N-dimethylformamide, DEF = N,N-diethylformamide, and DMA = dimethylacetamide), have been synthesized using a hydroxy-functionalized dicarboxybenzene and different amide solvents to investigate the influence of amides on their structures and magnetic properties. Single-crystal X-ray crystallographic analysis revealed that these Tb-MOFs all consist of Tb₂ nodes bridged by H₂dobdc²⁻ linkers with distinct 3D frameworks. By changing the amide solvents, the coordination geometry of the Tb³⁺ ions and the connectivities of the Tb₂ node can be simultaneously tuned, resulting in three types of structures: pcu (Tb-MOF1), bsn (Tb-MOF2), and sxb (Tb-MOF3). Detailed structural analyses uncover the crucial role that the amide ligands played in accessing varied structures by the different steric effects of the amides. Due to a variety of pore structures and coordination geometry of the Tb³⁺ ions (square antiprism for Tb-MOF1; muffin and square antiprism for Tb-MOF2; biaugmented trigonal prism for Tb-MOF3), the resulting Tb-MOFs show drastically different magnetic behaviors. Tb-MOF1 exhibits slow magnetic relaxation under an applied magnetic field through the Orbach and Raman mechanisms, being a field-induced single-molecule magnet (SMM) MOF, while Tb-MOF2 and Tb-MOF3 display simple paramagnetic behaviors. This work establishes solvent-directed steric engineering as a powerful strategy to diversify topology and magnetic properties within Ln-MOF systems derived from a single functionalized scaffold.