Breathing Metal-Organic Framework Based on Flexible Inorganic Building Units

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