Ligand Torsion Triggered Two Robust Fe-Tetratopic Carboxylate Frameworks with Enhanced Gas Uptake and Separation Performance.

By regulating the tetratopic carboxylate ligands, two robust Fe-MOFs (MOF=Metal-organic framework) comprising trigonal prismatic building blocks under a DMA/DMSO/HBF4 solvent system, namely, [(CH3 )2 NH2 ][FeII 3 (OH)(BPTC)1.5 (DMSO)3 ] (SNNU-60) and [FeIII FeII 2 (OH)(ABTC)1.5 (DMSO)3 ] (SNNU-61) (BPTC=3,3',5,5'-biphenyltetracarboxylic acid, ABTC=3,3',5,5'-azobenzenetetracarboxylic acid, SNNU=Shaanxi Normal University) have been successfully synthesized. The torsions between the benzene groups of the ligands result in two MOFs exhibiting completely different (4,6)-connected frameworks, which represent the only two MOF types constructed by [M3 (O/OH)(COO)6 ] trimeric building units and quadrilateral tetratopic carboxylate linkers until now. The robust Fe-MOFs SNNU-60 and SNNU-61 both exhibit high thermal/chemical stability, permanent microporosity, and excellent gas uptake capability for H2 , CO2 , C2 H2 , and C2 H4 under 1 bar. SNNU-60 in particular displays very high C2 H2 capture under low pressure (85 cm3  cm-3 at 0.15 bar and 298 K), which is among the top C2 H2 uptake MOF materials. Also, these two Fe-MOFs display high separation for CO2 and C2 -hydrocarbons over CH4 . Significantly, thanks to the high stability, suitable pore size, open Fe sites, and ion skeleton, SNNU-60 has extremely high C2 H2 /CH4 selectivity (83.6, 298 K), which surpasses most MOFs reported so far under the same conditions.