Amino Functionalization-Induced Coordination Geometry Switch Enables Pore Size Exclusion and Selective CHF3 Capture in Two Cu-Based Metal–Organic Frameworks

The coordination geometry of metal nodes is a critical yet overlooked variable in metal–organic framework (MOF) design. Here, we show that an amino substituent on the isonicotinic acid linker switches Cu(II) from five-coordinate square-pyramidal to four-coordinate square-planar, collapsing the framework from a 3D microporous network (Cu-ina, ina = isonicotinic acid) to a 2D layered structure (Cu-2ain, 2ain = 2-aminoisonicotinic acid). The amino group strengthens the Cu–N bond and eliminates one Cu–O contact, reducing the pore-limiting diameter to 3.27 Å and imposing complete size exclusion of both CHF3 (∼4.6 Å) and N2 (3.64 Å) in Cu-2ain. The 3D framework Cu-ina, free of this modification, achieves a CHF3 uptake of 50.1 cm3 g–1 at 298 K with an IAST selectivity of 46 for CHF3/N2, driven by cooperative C–H···F and C–H···O hydrogen bonding confirmed by GCMC simulations and DFT calculations. Five-cycle breakthrough experiments confirm excellent regenerability. This work demonstrates that ligand functionalization can inadvertently restructure metal coordination geometry and collapse framework dimensionality, overriding any intended surface-chemical benefit.