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

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 (β1) and several porous
(α, γ1, γ2, and γ3) phases triggered by exposure to N2, CO2, or H2O. 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 CO2 uptake (ca. 216 cm3/g at 195 K), and good thermal stability.