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

Abstract

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.

Two mixed-ligand 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. Variable-temperature single-crystal X-ray diffraction studies show that both ligands of the isoreticular, isostructural, 2-periodic MOFs undergo significant conformational changes, depending on the external stimulus, indicative of the flexibility of the MOFs allowing significant carbon dioxide (195 K) and water vapor sorption (298 K).