Heptanuclear Fluorido-Bridged Cobalt Metal-Organic Frameworks Containing BF4- Counteranions for Highly Selective C2H2/CO2 Separation.

The separation of C2H2 and CO2 is highly challenging owing to their similar molecular sizes and physicochemical properties. Herein, we report two isostructural metal-organic frameworks, LIFM-712 and LIFM-713, constructed from fluorido-bridged heptanuclear [Co7F12]2+ clusters and tritopic pyridyl ligands, which feature a pyr topology. Confined BF4- counteranions synergize with bridging fluorido ligands to generate fluorine-rich pore environments, thus providing abundant C-H···F recognition sites for C2H2. LIFM-712 exhibits stronger C2H2 binding (Qst = 35.2 kJ mol-1), whereas LIFM-713 balances affinity and specific surface area, achieving a higher C2H2 uptake of 149.8 cm3 g-1 at 298 K and 1 bar. For LIFM-713, ideal adsorbed solution theory calculations afford a C2H2/CO2 selectivity up to 4.5, and breakthrough experiments demonstrate a clear separation with a 35 min g-1 separation window at 298 K. For the two Co-MOFs, theoretical simulations and in situ infrared spectra reveal that cooperative interactions between cluster-bound fluorido sites and pore-confined counteranions govern the selective adsorption of C2H2. This work highlights a synergistic fluorine-functionalization strategy for designing efficient adsorbents for challenging gas separations.