Reversible Switching between Nonporous and Porous Phases of a New SIFSIX Coordination Network Induced by a Flexible Linker Ligand

Abstract

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 SiF₆^2--pillared square grid material, [Cu(SiF₆)(L)₂]_<i>n</i> (L = 1,4-bis(1-imidazolyl)benzene), <b>SIFSIX-23-Cu</b>. <b>SIFSIX-23-Cu</b> exhibits reversible switching between nonporous (<b>β1</b>) and several porous (<b>α</b>, <b>γ1</b>, <b>γ2</b>, and <b>γ3</b>) phases triggered by exposure to N₂, CO₂, or H₂O. In addition, heating <b>β1</b> to 433 K resulted in irreversible transformation to a closed polymorph, <b>β2</b>. 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. <b>SIFSIX-23-Cu</b> combines stability in water for over a year, high CO₂ uptake (ca. 216 cm³/g at 195 K), and good thermal stability.