Amino Functionalization-Induced Coordination Geometry Switch Enables Pore Size Exclusion and Selective CHF₃ Capture...

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.