Cooperative Agostic-Type C─H···Cu, Quadrupolar Cu···CO2, and C─H···O/π Interactions in an Ultra-Microporous MOF Enable One-Step Methane Purification From Quaternary C3H8/C2H6/CO2/CH4.

While agostic-type C─H···metal σ-interactions are central to organometallic catalysis, their role in gas separation remains elusive. In this study, a robust phosphine-functionalized ultra-microporous Cu(I/II)-organic framework, LIFM-XYY-15, featuring open metal sites (OMSs) and rich aromatic pore walls, is designed to enable one-step CH4 purification from a quaternary C3H8/C2H6/CO2/CH4 mixture. Notably, LIFM-XYY-15 exhibits much higher C3H8/C2H6/CO2 uptakes than CH4, showing C3H8/CH4, C2H6/CH4, and CO2/CH4 uptake ratios of 2.46, 2.30, and 3.92 at 298 K and 1 bar, respectively, which are significantly raised to 12.18, 8.59, and 9.89 at 0.1 bar. The separation potentials (∆q) reach 22.12 (C3H8/CH4), 20.34 (C2H6/CH4), and 43.69 (CO2/CH4) cm3 cm-3 at 298 K and 1 bar, demonstrating excellent CH4 purification ability from the quaternary mixture. Breakthrough experiments confirm the delivery of high-purity CH4 (≥99.9%) with a productivity of 1.39 mol L-1 with excellent recyclability. The in situ single-crystal x-ray diffraction (SCXRD), in situ FT-IR spectroscopy, and molecular simulations reveal that the exceptional selectivity arises from the synergistic interplay of agostic-type C─H···Cu, quadrupole-electrostatic Cu···-CO2, and alkane C─H···O/π interactions, which collectively reinforce the affinity for C2-C3 hydrocarbons and CO2 while disfavoring CH4. This work discloses a crucial role of agostic-type interactions as a powerful yet underexplored design principle for selective hydrocarbon separations.