Probing Induced-Fit Acetylene Adsorption and Separation in a Flexible Ultramicroporous Framework by Multinuclear SSNMR Spectroscopy: Insights Beyond Crystallography

This study investigates the molecular-level-induced-fit adsorption of acetylene (C₂H₂) and carbon dioxide (CO₂) in sql-SIFSIX-bpe-Zn, a flexible, anion-pillared ultramicroporous metal-organic framework (MOF). Sql-SIFSIX-bpe-Zn undergoes distinct pressure-dependent phase transitions, enabling a high selectivity for C₂H₂ over CO₂ and ethylene (C₂H₄). However, the unclear thermodynamic and kinetic mechanisms underlying this behavior, as well as limitations in models derived from single-crystal X-ray diffraction (SCXRD), necessitate further investigation. Using in situ multinuclear solid-state NMR (SSNMR) spectroscopy with isotopically enriched C₂D₂ and ¹³CO₂, we directly probed gas adsorption, framework dynamics, and competitive uptake in sql-SIFSIX-bpe-Zn across 153-298 K. Variable-temperature ²H SSNMR revealed a C₂D₂-induced pore opening between 253 and 233 K, marking a cooperative structural transformation process that maximizes acetylene adsorption. Complementary ¹³C and ¹⁹F SSNMR spectra show that both the SiF₆²⁻ pillars and bpe linkers participate in host-guest interactions through C-D···F hydrogen bonding and linker reorientation, while CO₂ remains bound near the linker region. In mixed-gas systems, CO₂ assists pore activation, but acetylene dominates adsorption, confirming its stronger affinity and selective uptake. These results provide a molecular-level mechanism for induced-fit adsorption and competitive selectivity in flexible hybrid ultramicroporous materials, guiding the design of next-generation adaptive MOF sorbents.