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, SiF₆²⁻=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.