Synthesis, structure and magnetism of metal-organic framework materials with doubly pillared layers

Two novel coordination polymers with a 3D pillared-layer framework, [M ^II(Tdc)(4,4′-Bpy)]_n [M = Fe (1), Co (2); Tdc = thiophene-2,5-dicarboxylate; 4,4′-Bpy = 4,4′-bipyridine), have been hydrothermally synthesised and structurally characterised. A simultaneous reduction of iron(III) to iron(II) occurs under hydrothermal conditions in the self-assembly process of complex 1. The complexes are isostructural and, in each complex, the metal ions are linked by double μ₂-carboxylate bridges in a syn-syn mode, giving rise to dinuclear units that are connected to each other by chelating carboxylate groups from the Tdc ligands in an approximately perpendicular orientation to form 2D rectangle-grid layers. The adjacent layers are held together through 4,4′-Bpy molecules in a double-pillar support fashion to give a 3D network with three intersecting perpendicular channels. Variable-temperature magnetic susceptibility measurements revealed weak antiferromagnetic coupling interactions in both complexes and a field-induced magnetic transition can be observed in 2.