Discovery of two predictable (3,18)-connected topologies based on Wells–Dawson type cages for the design of porous metal phosphonocarboxylate frameworks

Abstract

Highly connected molecular building blocks (MBBs) have been demonstrated to play a crucial role in reticular chemistry, particularly in predicting the topologies of metal–organic frameworks. Metal phosphonate clusters exhibit considerable advantages in constructing high-connectivity MBBs, owing to the multiple coordination modes offered by phosphonic ligands. Herein, four metal (M = Co^II, Mn^II) phosphonocarboxylate frameworks (CoPCF-1,2 and MnPCF-1,2) were successfully prepared under solvothermal conditions by utilizing the phosphonocarboxylic ligand, 4′-phosphonobiphenyl-3,5-dicarboxylic acid (H₄pbpdc), and their structural characterization was performed using single-crystal X-ray diffraction (SCXRD). The structures feature a duodenary nuclear M₁₂(µ₃-OH)₂(CO₂)₁₂(PO₃)₆(DMF)₆/(CH₃COO)_4.5 cluster, bearing resemblance to the well-known Wells–Dawson ion from polyoxometallate chemistry. It is the first time a Wells–Dawson type cage has served as an 18-connected molecular building block, forming two kinds of porous metal phosphonocarboxylate frameworks with novel (3,18)-connected gez and gea topologies. Their permanent porosities were confirmed through N₂ adsorption studies. Notably, the MBB Co₁₂ cluster-based CoPCF-1 shows a loss and recovery process of µ₃-OH through single-crystal-to-single-crystal (SCSC) transformation. The magnetic properties of the four compounds exhibit antiferromagnetic behavior.