Framework materials assembled from magnesium carboxylate building units

Abstract

Four coordination polymers containing magnesium metal nodes and di- or tricarboxylic acid organic connectors have been synthesised and structurally characterised with the aid of single crystal X-ray diffraction. Mg₃(bdc)₃(DMA)₄ (1) and Mg₃(bdc)₃(EtOH)₂ (2) were prepared from the 1 : 1 reaction of 1,4-benzenedicarboxylic acid (H₂bdc) with Mg(NO₃)₂·6H₂O in dimethylacetamide (DMA) or EtOH respectively. Both 1 and 2 contain tri-metallic magnesium carboxylate units which act as six-coordinate nodes for network construction. These tri-magnesium nodes are joined by the bdc ligands in 1 to give a 2D layered structure with coordinated DMA solvent molecules between the layers, whereas in 2 they stack end-on to form a novel 3D network containing 1D Mg carboxylate chains. Both 1 and 2 are moderately hygroscopic, and powder X-ray diffraction and thermogravimetric analysis studies show them to be interconvertible with Mg(bdc) and the hydrated complex Mg(bdc)(H₂O)₂via a series of solid-state reactions. Reaction of magnesium nitrate with 4,4′-biphenyldicarboxylic acid (H₂bpdc) in DMA gives Mg₃(bpdc)₃(DMA)₄ (3), which in contrast to 2 forms a ‘squashed’ 3D cubic network as a result of the increased distance between the trinuclear nodes necessitated by the longer bpdc ligands. Reaction of magnesium nitrate with the triangular organic building block 1,3,5-tricarboxylic acid (H₃btc) in DMA gives crystals of Mg(H_1.5btc)_2/3(btc)_1/3(DMA)₂·(DMA)_1/3 (4). Compound 4 contains mono-metallic metal nodes which assemble with the btc ligands to give 2D 6³ tessellated layers with Mg-coordinated DMA molecules lying perpendicular to the layers. Some free DMA is also present in 4, sited in small pores between the layers.