Design of Pore-Space-Partitioned Metal-Organic Frameworks Using Spiro Ligand Chemistry

An emerging direction in the design of crystalline porous materials is the application of bioisosteric replacement strategy that replaces benzene ring with an aliphatic core; however, maintaining isoreticular chemistry upon such replacement is elusive. While bicyclic cores have been employed for this purpose, few studies are known that use spiro ligands for designing metal-organic frameworks for gas separation applications. As a benzene bioisostere, spiro ligands are more challenging than other bioisosteres because they are more flexible and deviate significantly from that of para-benzene-based ligands. In this work, we show that the use of a pore-space-partition strategy generates a multimodule system in which it is possible to exert a high level of control over the positioning, orientation, and alignment of carboxylate groups in a spiro ligand. Here, we report a family of isoreticular MOFs on a partitioned acs (pacs) structure with spiro[3.3]heptane-2,6-dicarboxylic acid as the framework-forming module, together with three different pore-partition modules based on tripyridyl ligands. The isoreticular chemistry can be extended to both homometallic (Fe) and heterometallic (CoV and CoFe) compositions. These materials exhibit excellent sorption properties such as high uptake capacity for CO₂ (61.9 cm³/g) and small hydrocarbon gases (e.g., 116.3 cm³/g for C₂H₂ and 101.4 cm³/g for C₂H₄), inverse C₂H₆/C₂H₄ selectivity, and promising separation performance for C₂H₂/CO₂ (selectivity up to 5.5), C₃H₈/CH₄ (selectivity up to 279), and C₂H₆/CH₄ (selectivity up to 22.7) gas mixtures.