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, alpha, transformed to less open, gamma, and closed, beta, phases during activation. beta did not adsorb N-2 (77 K), rather it reverted to alpha induced by CO2 at 195, 273 and 298 K. CO2 desorption resulted in alpha ', a shape-memory phase which subsequently exhibited type-I isotherms for N-2 (77 K) and CO2 as well as strong performance for separation of CO2/N-2 (15/85) at 298 K and 1 bar driven by strong binding (Q(st)=45-51 kJ/mol) and excellent CO2/N-2 selectivity (up to 700). Interestingly, alpha ' reverted to beta after re-solvation/desolvation. Molecular simulations and density functional theory (DFT) calculations provide insight into the properties of SIFSIX-23-Cu-N.