Chiral Lanthanum Metal-Organic Framework with Gated CO 2 Sorption and Concerted Framework Flexibility.

A metal-organic framework (MOF) CTH-17 based on lanthanum(III) and the conformationally chiral linker 1,2,3,4,5,6-hexakis(4-carboxyphenyl)benzene, cpb 6- : [La 2 (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). CTH-17 is a rod-MOF with new topology och . It has high-temperature stability with Sohncke space groups P 6 1 22/ P 6 5 22 at 90 K and P 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 2 indicate a modest surface area of 231 m 2 g -1 for CTH-17 , roughly in agreement with the crystal structure. Carbon dioxide sorption reveals a gate-opening effect of CTH-17 where the structure opens up when the loading of CO 2 reaches approximately ∼0.45 mmol g -1 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 6 coordination sphere. This was corroborated by density functional theory (DFT) calculations yielding the chiral phase ( P 622) as the energy minimum and a completely racemic phase ( P 6/ mmm ), with symmetric cpb linkers representing a saddle point in a racemization process.