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)]n·n(DMF) (1) and [Mn3(ia)3(bppdo)(MeOH)]n·n(DMF) (2), have been synthesized, where ia = isophthalate, bppdo = 1,3-bis(4-pyridyl)propane-N,N′-dioxide, MeOH = methanol, and DMF = N,N′-dimethylformamide. Single-crystal X-ray diffraction studies reveal that 1 and 2, 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 (1′ and 2′) show a drastic reduction in their void spaces (3.7 and 5.7% for 1′ and 2′, respectively, at 298 K) as compared to their as-synthesized structures (potential void spaces of 21.6 and 22.3% for 1 and 2, 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 1′ and 2′, 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 1′ and 2′ 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.