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 SiF62--pillared square grid material, [Cu(SiF6)(L)(2)](n) (L = 1,4-bis(1-imidazolyl)benzene), SIFSIX-23-Cu. SIFSIX-23-Cu exhibits reversible switching between nonporous (beta 1) and several porous (alpha, gamma 1, gamma 2, and gamma 3) phases triggered by exposure to N-2, CO2, or H2O. In addition, heating beta 1 to 433 K resulted in irreversible transformation to a closed polymorph, beta 2. 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 CO2 uptake (ca. 216 cm(3)/g at 195 K), and good thermal stability.