Variation in the Sorption Properties of a Pair of Highly Flexible, Isostructural MOFs Exhibiting Single-Crystal-to-Single-Crystal Transformations Rarely Observed in MOFs Based on a Linear Co₃/Mn₃ Trinuclear Cluster

Two mixed-ligand metal–organic frameworks (MOFs), [Co₃(ia)₃(bppdo)(MeOH)]_<i>n</i>·<i>n</i>(DMF) (<b>1</b>) and [Mn₃(ia)₃(bppdo)(MeOH)]_<i>n</i>·<i>n</i>(DMF) (<b>2</b>), have been synthesized, where ia = isophthalate, bppdo = 1,3-bis(4-pyridyl)propane-<i>N</i>,<i>N</i>′-dioxide, MeOH = methanol, and DMF = <i>N</i>,<i>N</i>′-dimethylformamide. Single-crystal X-ray diffraction studies reveal that <b>1</b> and <b>2</b>, based on a linear Co₃ and Mn₃ cluster, respectively, are isoreticular and isostructural and possess two-periodic frameworks consisting of linear trinuclear metal clusters. Variable-temperature single-crystal X-ray diffraction studies confirm single-crystal-to-single-crystal transformations (rarely observed for MOFs based on a linear Co₃/Mn₃ cluster) upon desolvation and subsequent hydration, despite significant structural changes occurring. The desolvated structures (<b>1′</b> and <b>2′</b>) show a drastic reduction in their void spaces (3.7 and 5.7% for <b>1′</b> and <b>2′</b>, respectively, at 298 K) as compared to their as-synthesized structures (potential void spaces of 21.6 and 22.3% for <b>1</b> and <b>2</b>, respectively, at 298 K). Despite the similar void spaces at 195 K that are also seemingly inaccessible, significant CO₂ sorption occurs at this temperature for <b>1′</b> and <b>2′</b>, indicating that structural changes had to occur in order to accommodate the adsorbed CO₂ molecules. Water vapor sorption at 298 K also induces structural changes in <b>1′</b> and <b>2′</b> as confirmed by single-crystal X-ray diffraction studies. The structural transformations that occur during desolvation and sorption processes emphasize the remarkable dynamic nature of these frameworks in responding to their environment.