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), {[Cd2(coh)2(dpt)2]·guests}n (JUK-20). Each subnetwork in JUK-20 is built of 2D coordination layers stacked by strong complementary C═O···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 (N2, 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.