A paradigm shift in the room-temperature self-assembly of tunable metal–organic frameworks composed of flexible neutral linkers with six N-donor atoms and a curved dicarboxylate

Abstract

Understanding and predicting the pathways involved in the synthetic process of metal–organic frameworks (MOFs) composed of flexible counterparts has always been a challenging task. The complexity increases manifold when a three-component reaction is involved instead of the conventional two-component system. In this article, we shed light on the self-assembly process of MOFs from a three-component reaction at room temperature involving (a) flexible spanning bis(tridentate) linkers with six N-donor atoms, (b) a conformationally flexible curved dicarboxylate linker (4,4′-(1,3,4-oxadiazole-2,5-diyl)dibenzoate), oxdz²⁻, having a connecting angle of 144°, and (c) a divalent metal center preferring at least six-coordination geometry for generating 3D or 2D MOFs. Herein, we report one 3D MOF, {[Cd₂(oxdz)₂(tpbn)(H₂O)₂]·2H₂O·2C₂H₅OH}_n (1) and two 2D MOFs {[Cd₂(oxdz)₂(tphn)]·6H₂O}_n (2) and {[Cd₄(oxdz)₄(tpon)₂]·11H₂O·4CH₃OH}_n (3), where the spacer chain length in the bis(tridentate) linkers is varied, to establish the factors – flexibility and angularity of dual linkers – that govern their formation. This strategy is undoubtedly a paradigm shift, which has opened up a path of designing 3D MOFs comprising flexible counterparts. Exploiting the inherent polarizability of the oxadiazole moiety in 1–3, ultrafast and selective detection of mesityl oxide is reported at the ppb level. Interestingly, a crossover from turn-on to turn-off with respect to the ratio of methanol and water as solvent media is also noted for 2 and 3. Detailed mechanistic considerations with the help of experimental and computational data are provided to unfold the paths involved.