Chiral Lanthanum Metal-Organic Framework with Gated CO Sorption and Concerted Framework Flexibility.

A metal-organic framework (MOF) based on lanthanum(III) and the conformationally chiral linker 1,2,3,4,5,6-hexakis(4-carboxyphenyl)benzene, cpb: [La(cpb)]·1.5dmf was prepared by the solvothermal method in dimethylformamide (dmf) and characterized by variable-temperature X-ray powder diffraction (VTPXRD), variable-temperature X-ray single-crystal diffraction (SCXRD), and thermogravimetric analysis (TGA). is a rod-MOF with new topology . It has high-temperature stability with Sohncke space groups 622/622 at 90 K and 622 at 300 and 500 K, all phases characterized with SCXRD and at 293 K also with three-dimensional (3D) electron diffraction. VTPXRD indicates a third phase appearing after 620 K and stable up to 770 K. Gas sorption isotherms with N indicate a modest surface area of 231 m g for , roughly in agreement with the crystal structure. Carbon dioxide sorption reveals a gate-opening effect of where the structure opens up when the loading of CO reaches approximately ∼0.45 mmol g or 1 molecule per unit cell. Based on the SCXRD data, this is interpreted as flexibility based on the concerted movements of the propeller-like hexatopic cpb linkers, the movement intramolecularly transmitted by the π-π stacking of the cpb linkers and helped by the fluidity of the LaO coordination sphere. This was corroborated by density functional theory (DFT) calculations yielding the chiral phase (622) as the energy minimum and a completely racemic phase (6/), with symmetric cpb linkers representing a saddle point in a racemization process.