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.

A new layered mixed-linker metal-organic framework [Zn(iso)(pcih)] (MOF) built from isophthalate ions (iso) and 4-pyridinecarbaldehyde isonicotinoyl hydrazone (pcih) was prepared using both solution and mechanochemical methods. By use of the latter, the 2D MOF is obtained either in a one-mortar three-component grinding or on the way of a two-step mechanosynthesis. Tuning of mechanochemical synthetic conditions allowed us to identify both necessary and favorable factors for the solid-state formation of the MOF. Single-crystal X-ray diffraction reveals the presence of interdigitated layers in the ABAB arrangement and interlayer 0D cavities filled with guest molecules. Upon thermal activation, the dynamic framework exhibits stepwise and selective adsorption of CO over N as well as high-pressure H adsorption reaching maximum excess of 1.15 wt% at 77 K. The mechanochemical synthetic protocol is expanded to a few other interdigitated structures.

A series of mixed-linker metal–organic frameworks [Zn2(Xiso)2(pcih)2] n containing substituted isophthalate linkers (Xiso2–; X = OH or CH3 or NH2 or H) and 4-pyridinecarbaldehyde isonicotinoyl hydrazone pillars (pcih) have been prepared by using both solution and mechanochemical methods. Single-crystal X-ray diffraction reveals their interdigitated two-dimensional structures with different arrangements of layers, dependent on hydrogen bonding and CH···π interactions involving the substituents and/or linkers. These supramolecular interactions are responsible for the formation of interlayer pores of various volumes, shapes, and dimensionality. All materials exhibit selective gas adsorption of CO2 over N2 with diverse profiles. Polar groups (OH and NH2) of the isophthalate linkers increase chemical affinity to carbon dioxide as well as hydrolytic and thermal stability of the frameworks.