Squarate-Based Isoreticular Analogue of CALF-20: Fact or Fiction?

The hypothetical squarate-based isoreticular analogue of benchmark metal-organic framework (MOF) CALF-20 was recently predicted by computations to outperform the parent material as a sorbent for CO₂ capture. In this work, we performed a synthetic screening aimed at synthesizing this hypothetical MOF, which failed to afford the desired product and led to the discovery of two new nonporous coordination polymers, named UdP-40 and UdP-41, with the same 2:2:1 Zn/1,2,4-triazolate/squarate stoichiometry expected for a CALF-20 analogue, but different crystal structure. UdP-40 and UdP-41 were characterized by elemental analysis, thermogravimetric analysis, variable temperature powder X-ray diffraction,infrared spectroscopy and CO₂ sorption analysis, and their structures were solved from powder and single crystal X-ray diffraction, respectively. Both compounds crystallize in orthorhombic space groups and display the same peculiar tetradentate coordination mode for squarate, involving two monodentate O atoms and one bridging O atom, while leaving one free O atom, leading to the formation of previously unreported topologies. UdP-40 features one coordinated water molecule per formula unit, whose removal at temperatures above 250 °C leads to complete amorphization.