A new type of C₂H₂ binding site in a cis-bridging hexafluorosilicate ultramicroporous material that offers trace C₂H₂ capture

Hybrid ultramicroporous materials (HUMs) comprising hexafluorosilicate (SiF₆²⁻, SIFSIX) and their variants are promising physisorbents for trace acetylene (C₂H₂) capture and separation, where the inorganic anions serve as trans-bridging pillars. Herein, for the first time, we report a strategy of fluorine binding engineering in these HUMs via switching the coordination mode of SIFSIX from traditional trans to rarely explored cis. The first example of a rigid HUM involving cis-bridging SIFSIX, SIFSIX-bidmb-Cu (bidmb = 1,4-bis(1-imidazolyl)-2,5-dimethylbenzene), is reported. The resulting self-interpenetrated network is found to be water stable and exhibits strong binding to C₂H₂ but weak binding to C₂H₄ and CO₂, affording a high Q_st of 55.7 kJ mol⁻¹ for C₂H₂, a high C₂H₂ uptake of 1.86 mmol g⁻¹ at 0.01 bar and high ΔQ_st values. Breakthrough experiments comprehensively demonstrate that SIFSIX-bidmb-Cu can efficiently capture and recover C₂H₂ from 50/50 or 1/99 C₂H₂/CO₂ and C₂H₂/C₂H₄ binary mixtures. In situ single crystal X-ray diffraction (SCXRD) combined with dispersion-corrected density functional theory (DFT-D) calculations reveals that the C₂H₂ binding site involves two cis-SiF₆²⁻ anions in close proximity (F⋯F distance of 7.16 Å), creating a new type of molecular trap that affords six uncoordinated fluoro moieties to chelate each C₂H₂via sixfold C-H⋯F hydrogen bonds. This work therefore provides a new strategy for binding site engineering with selective C₂H₂ affinity to enable trace C₂H₂ capture.