Disorder and Sorption Preferences in a Highly Stable Fluoride-Containing Rare-Earth fcu-Type Metal-Organic Framework

Rare-earth (RE) metal-organic frameworks (MOFs) synthesized in the presence of fluorine-donating modulators or linkers are an important new subset of functional MOFs. However, the exact nature of the REX core of the molecular building block (MBB) of the MOF, where X is a μ_2 or 3-bridging group, remains unclear. Investigation of one of the archetypal members of this family with the stable fcu framework topology, Y-fum-fcu-MOF (1), using a combination of experimental techniques, including high-field (20 T) solid-state nuclear magnetic resonance spectroscopy, has determined two sources of framework disorder involving the μ₃-X face-capping group of the MBB and the fumarate (fum) linker. The core of the MBB of 1 is shown to contain a mixture of μ₃-F^- and (OH)^- groups with preferential occupation at the crystallographically different face-capping sites that result in different internally lined framework tetrahedral cages. The fum linker is also found to display a disordered arrangement involving bridging- or chelating-bridging bis-bidentate modes over the fum linker positions without influencing the MBB orientation. This linker disorder will, upon activation, result in the creation of Y³⁺ ions with potentially one or two additional uncoordinated sites possessing differing degrees of Lewis acidity. Crystallographically determined host-guest relationships for simple sorbates demonstrate the favored sorption sites for N₂, CO₂, and CS₂ molecules that reflect the chemical nature of both the framework and the sorbate species with the structural partitioning of the μ₃-groups apparent in determining the favored sorption site of CS₂. The two types of disorder found within 1 demonstrate the complexity of fluoride-containing RE-MOFs and highlight the possibility to tune this and other frameworks to contain different proportions and segregations of μ₃-face-capping groups and degrees of linker disorder for specifically tailored applications.