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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(SiF 6 )(L) 2 ] n , (L=1,4-bis(1-imidazolyl)benzene, SiF 6 2- =SIFSIX) has been previously reported to exhibit reversible switching between closed and open phases, the activated phase of SIFSIX-23-Cu N , [Cu(SiF 6 )(L N ) 2 ] n (L N =2,5-bis(1-imidazolyl)pyridine), transformed to a kinetically stable porous phase with strong affinity for CO 2 . As-synthesized SIFSIX-23-Cu N , α, transformed to less open, γ, and closed, β, phases during activation. β did not adsorb N 2 (77 K), rather it reverted to α induced by CO 2 at 195, 273 and 298 K. CO 2 desorption resulted in α', a shape-memory phase which subsequently exhibited type-I isotherms for N 2 (77 K) and CO 2 as well as strong performance for separation of CO 2 /N 2 (15/85) at 298 K and 1 bar driven by strong binding (Q st =45-51 kJ/mol) and excellent CO 2 /N 2 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 ., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Coordination networks that reversibly switch between closed and open phases are of topical interest since their stepped isotherms can offer higher working capacities for gas-storage applications than the related rigid porous coordination networks. To be of practical utility, the pressures at which switching occurs, the gate-opening and gate-closing pressures, must lie between the storage and delivery pressures. Here we study the effect of linker substitution to fine-tune gate-opening and gate-closing pressure. Specifically, three variants of a previously reported pcu-topology MOF, X-pcu-5-Zn, have been prepared: X-pcu-6-Zn, 6=1,2-bis(4-pyridyl)ethane (bpe), X-pcu-7-Zn, 7=1,2-bis(4-pyridyl)acetylene (bpa), and X-pcu-8-Zn, 8=4,4'-azopyridine (apy). Each exhibited switching isotherms but at different gate-opening pressures. The N 2 , CO 2 , C 2 H 2 , and C 2 H 4 adsorption isotherms consistently indicated that the most flexible dipyridyl organic linker, 6, afforded lower gate-opening and gate-closing pressures. This simple design principle enables a rational control of the switching behavior in adsorbent materials., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)