Cooperative Agostic‐Type C─H···Cu, Quadrupolar Cu···CO 2 , and C─H···O/π Interactions in an Ultra‐Microporous MOF Enable One‐Step Methane Purification From Quaternary C 3 H 8 /C 2 H 6 /CO 2 /CH 4

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 CH 4 purification from a quaternary C 3 H 8 /C 2 H 6 /CO 2 /CH 4 mixture. Notably, LIFM‐XYY‐15 exhibits much higher C 3 H 8 /C 2 H 6 /CO 2 uptakes than CH 4 , showing C 3 H 8 /CH 4 , C 2 H 6 /CH 4 , and CO 2 /CH 4 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 (C 3 H 8 /CH 4 ), 20.34 (C 2 H 6 /CH 4 ), and 43.69 (CO 2 /CH 4 ) cm 3 cm −3 at 298 K and 1 bar, demonstrating excellent CH 4 purification ability from the quaternary mixture. Breakthrough experiments confirm the delivery of high‐purity CH 4 (≥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···‐CO 2 , and alkane C─H···O/π interactions, which collectively reinforce the affinity for C 2 –C 3 hydrocarbons and CO 2 while disfavoring CH 4 . This work discloses a crucial role of agostic‐type interactions as a powerful yet underexplored design principle for selective hydrocarbon separations.