Structural variety in calcium metal–organic frameworks with a tetratopic carboxylate ligand†

The solvothermal synthesis of three new calcium-based metal–organic frameworks (MOFs) employing the tetratopic carboxylate ligand, 4,4′,4′′,4′′′-(naphthalene-2,3,6,7-tetrayl)tetrabenzoate (NTTB) is reported. 1: Ca(H₂NTTB)(DMA)₂, 2: Ca₅(H₂NTTB)(NTTB)₂(H₂O)₈ and 3: Ca₅(H₂NTTB)(NTTB)₂(H₂O)₅ (DMA = N,N-dimethylacetamide). These have distinct structures and compositions depending on the solvent used in synthesis. The crystal structures of the materials were elucidated through single-crystal X-ray diffraction (SC-XRD) analysis, along with a solvate of the acid form of the ligand. Sample purity was confirmed via powder XRD (PXRD), and thermal stability was assessed using thermogravimetric analysis (TGA). The resulting crystal architectures were analysed in detail with respect to the ligand binding modes, the connectivity of the calcium centres, the ligand conformation and hydrogen bonding. Notably, a solvent-dependent structural connectivity was observed: while the structures all contain six-coordinate Ca, substituting DMA with ethanol in synthesis induced a change in the MOF building units from discrete calcium nodes in 1 to infinite chains in 2 and interrupted chains in 3.