A Series of Metal-Organic Framework Isomers Based on Pyridinedicarboxylate Ligands: Diversified Selective Gas Adsorption and the Positional Effect of Methyl Functionality.

Abstract

Solvothermal assembly of copper(II) cations and 5-(pyridine-3-yl)isophthalate linkers bearing different position-substituted methyl groups afforded four ligand-induced metal-organic framework (MOF) isomers as a platform for investigating diverse selective gas adsorption properties and understanding the positional effect of methyl functionality. Single-crystal X-ray diffraction (SCXRD) analyses showed that, when the methyl substituent is at the para position with respect to the pyridinic N atom, the resultant framework compound ZJNU-27 features an eea-type topology, while the other three solids possess an isoreticular structure with an rtl-type topology when the methyl group is situated at the other positions. As revealed by N2 physi-adsorption measurements at 77 K, they exhibit moderate specific surface areas ranging from 584 to 1182 m2 g-1 and distinct degrees of framework flexibility, which are heavily dependent on the methyl position. Comprehensive gas adsorption studies show that they are capable of effectively separating three pairs of binary gas mixtures including C2H2-CH4, CO2-CH4, and CO2-N2 couples. Moreover, their uptake capacities and adsorption selectivities can be tailored by altering the methyl position. In addition, their framework hydro-stability is also influenced by the methyl position. Compared to ZJNU-27 and ZJNU-28, ZJNU-26 and ZJNU-29 exhibit poorer stability against H2O, although the methyl group is more close to inorganic secondary building units (SBUs), which are believed to originate from the steric effect of the methyl group. Overall, the four MOFs display the methyl position-dependent network architectures, framework flexibilities, and selective gas adsorption properties as well as hydrostabilities. The findings observed in this work not only demonstrate the importance of the positional effect of the functional group but also highlight that engineering the substituent position is a potential strategy for achieving the modulation of MOF structures and properties.