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Closed-to-open structural transformations in flexible coordination networks are of potential utility in gas storage and separation. Herein, we report the first example of a flexible SiF 6 2- -pillared square grid material, [Cu(SiF 6 )(L) 2 ] n (L = 1,4-bis(1-imidazolyl)benzene), SIFSIX-23-Cu . SIFSIX-23-Cu exhibits reversible switching between nonporous ( β1 ) and several porous ( α , γ1 , γ2 , and γ3 ) phases triggered by exposure to N 2 , CO 2 , or H 2 O. In addition, heating β1 to 433 K resulted in irreversible transformation to a closed polymorph, β2 . Single-crystal X-ray diffraction studies revealed that the phase transformations are enabled by rotation and geometrical contortion of L. Density functional theory calculations indicated that L exhibits a low barrier to rotation (as low as 8 kJmol -1 ) and a rather flat energy surface. In situ neutron powder diffraction studies provided further insight into these sorbate-induced phase changes. SIFSIX-23-Cu combines stability in water for over a year, high CO 2 uptake (ca. 216 cm 3 /g at 195 K), and good thermal stability.