Turning Flexibility into Rigidity: Stepwise Locking of Interpenetrating Networks in a MOF Crystal through Click Reaction

Post-synthetic modifications of metal-organic frameworks (MOFs) enable synthesis of materials with enhanced performance characteristics or those inaccessible by direct synthetic routes. In this work, for the first time, we utilize inverse-electron demand Diels-Alder (iEDDA) modification to control the structural flexibility and porosity of an open framework material. We selected a series of dienophiles with increasing bulkiness including ethyl vinyl ether (eve), cyclohexene (chx), norbornene (nor), and 5-norbornene-2-methanol (noh) to modify a tetrazine-based linker (3,6-dipyridyl-1,2,4,5-tetrazine, dpt) incorporated in a unique doubly interpenetrated 3D hybrid MOF-HOF porous material (HOF, hydrogen-bonded organic framework), {[Cd-2(coh)(2)(dpt)(2)]center dot guests}(n) (JUK-20). Each subnetwork in JUK-20 is built of 2D coordination layers stacked by strong complementary C O center dot center dot center dot H-N hydrogen bonds between carbohydrazide dibenzoate linkers (coh). By using the [4 + 2] click reactions of JUK-20, which proceed in a prominent single-crystal-to-single-crystal manner, we obtained a series of JUK-20-dienophile MOFs. The modifications lead to a stepwise decrease in structural flexibility of the JUK-20 platform until the highest rigidity and stability is reached for JUK-20-noh. Consequently, the adsorption capacity in the JUK-20-dienophile series increases, as revealed collectively by single-crystal X-ray diffraction, physisorption isotherms (N-2, CO2, and MeOH), and grand canonical Monte Carlo simulations. Our work demonstrates that post-synthetic iEDDA modification is a versatile and efficient tool for systematic functionalization of open framework materials under mild conditions.

Automated summary

Post-synthetic modifications of metal-organic frameworks (MOFs) using inverse-electron demand Diels-Alder (iEDDA) reactions were employed to control the structural flexibility and porosity of an open framework material. The modifications resulted in a series of MOFs with stepwise decrease in flexibility and increase in adsorption capacity, demonstrating the versatility and efficiency of iEDDA modification in functionalizing open framework materials under mild conditions.