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-CuN, [Cu(SiF6)(LN)2]n (LN = 2,5-bis(1-imidazolyl)pyridine), transformed to a kinetically stable porous phase with strong affinity for CO2. As-synthesized SIFSIX-23-CuN, 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 (Qst = 45-51 kJ/mol), high CO2 capacity (209 cm3/g) and excellent CO2/N2 selectivity (up to 700). Interestingly, α' reverted to β after resolvation/desolvation. Density functional theory (DFT) calculations provide insight into the properties of SIFSIX-23-CuN.