Shape-Memory Effect Enabled by Ligand Substitution and CO₂ Affinity in a Flexible SIFSIX Coordination Network

We report that linker ligand substitution involving just one atom induces a shape-memory effect in a flexible coordination network. Specifically, whereas SIFSIX-23-Cu, [Cu(SiF₆ )(L)₂ ]_n , (L=1,4-bis(1-imidazolyl)benzene, SiF₆ ^2- =SIFSIX) has been previously reported to exhibit reversible switching between closed and open phases, the activated phase of SIFSIX-23-Cu^N , [Cu(SiF₆ )(L^N )₂ ]_n (L^N =2,5-bis(1-imidazolyl)pyridine), transformed to a kinetically stable porous phase with strong affinity for CO₂ . As-synthesized SIFSIX-23-Cu^N , α, transformed to less open, γ, and closed, β, phases during activation. β did not adsorb N₂ (77 K), rather it reverted to α induced by CO₂ at 195, 273 and 298 K. CO₂ desorption resulted in α', a shape-memory phase which subsequently exhibited type-I isotherms for N₂ (77 K) and CO₂ as well as strong performance for separation of CO₂ /N₂ (15/85) at 298 K and 1 bar driven by strong binding (Q_st =45-51 kJ/mol) and excellent CO₂ /N₂ selectivity (up to 700). Interestingly, α' reverted to β after re-solvation/desolvation. Molecular simulations and density functional theory (DFT) calculations provide insight into the properties of SIFSIX-23-Cu^N .