Flexibility and sorption selectivity in rigid metal-organic frameworks: The impact of ether-functionalised linkers

The functionalisation of well-known rigid metal-organic frameworks (namely, [Zn₄O(bdc)₃]_n, MOF-5, IRMOF-1 and [Zn ₂(bdc)₂(dabco)]_n; bdc=1,4-benzene dicarboxylate, dabco=diazabicyclo[2.2.2]octane) with additional alkyl ether groups of the type -O-(CH₂)_n-O-CH₃ (n = 2-4) initiates unexpected structural flexibility, as well as high sorption selectivity towards CO₂ over N₂ and CH₄ in the porous materials. These novel materials respond to the presence/absence of guest molecules with structural transformations. We found that the chain length of the alkyl ether groups and the substitution pattern of the bdc-type linker have a major impact on structural flexibility and sorption selectivity. Remarkably, our results show that a high crystalline order of the activated material is not a prerequisite to achieve significant porosity and high sorption selectivity. Just breathe! The implementation of linkers functionalised with flexible ether groups in known, but rigid metal-organic frameworks (MOFs), yields isoreticular frameworks that exhibit unexpected responsiveness towards small molecules and high sorption selectivity for CO₂ (see figure).