Breathing Metal–Organic\nFramework Based on\nFlexible Inorganic Building Units

Abstract

Five novel bismuth carboxylate coordination polymers\nwere synthesized\nfrom biphenyl-3,4′,5-tricarboxylic acid (H₃BPT)\nand [1,1′:4′,1′′]­terphenyl-3,3′′,5,5′′-tetracarboxylic\nacid (H₄TPTC). One of the phases, [Bi­(BPT)]·2MeOH\n(denoted SU-100, as synthesized), is the first example, to the best\nof our knowledge, of a reversibly flexible bismuth-based metal–organic\nframework. The material exhibits continuous changes to its unit cell\nparameters and pore shape depending on the solvent it is immersed\nin and the dryness of the sample. Typically, in breathing carboxylate-based\nMOFs, flexibility occurs through tilting of the organic linkers without\nsignificantly altering the coordination environment around the cation.\nIn contrast to this, the continuous breathing mechanism in SU-100\ninvolves significant changes to bond angles within the Bi₂O₁₂ inorganic building unit (IBU). The flexibility of\nthe IBU of SU-100 reflects the nondiscrete coordination geometry of\nthe bismuth cation. A disproportionate increase in the solvent accessible\nvoid volume was observed when compared to the expansion of the unit\ncell volume of SU-100. Additionally, activated SU-100 (SU-100-HT)\nexhibits a large increase in unit cell volume, yet has the smallest\nvoid volume of all the studied samples.