The targeted incorporation of defects into crystalline matter allows for the manipulation of many properties and has led to relevant discoveries for optimized and even novel technological applications of materials. It is therefore exciting to see that defects are now recognized to be similarly useful in tailoring properties of metal-organic frameworks (MOFs). For instance, heterogeneous catalysis crucially depends on the number of active catalytic sites as well as on diffusion limitations. By the incorporation of missing linker and missing node defects into MOFs, both parameters can be accessed, improving the catalytic properties. Furthermore, the creation of defects allows for adding properties such as electronic conductivity, which are inherently absent in the parent MOFs. Herein, progress of the rapidly evolving field of the past two years is overviewed, putting a focus on properties that are altered by the incorporation and even tailoring of defects in MOFs. A brief account is also given on the emerging quantitative understanding of defects and heterogeneity in MOFs based on scale-bridging computational modeling and simulations.

Flexible metal-organic frameworks (MOFs) are structurally flexible, porous, crystalline solids that show a structural transition in response to a stimulus. If MOF-based solid-state and microelectronic devices are to be capable of leveraging such structural flexibility, then the integration of MOF thin films into a device configuration is crucial. Here we report the targeted and precise anchoring of Cu-based alkylether-functionalised layered-pillared MOF crystallites onto substrates via stepwise liquid-phase epitaxy. The structural transformation during methanol sorption is monitored by in-situ grazing incidence X-ray diffraction. Interestingly, spatially-controlled anchoring of the flexible MOFs on the surface induces a distinct structural responsiveness which is different from the bulk powder and can be systematically controlled by varying the crystallite characteristics, for instance dimensions and orientation. This fundamental understanding of thin-film flexibility is of paramount importance for the rational design of MOF-based devices utilising the structural flexibility in specific applications such as selective sensors.

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By employing the mixed-component, solid-solution approach, various functionalized ditopic isophthalate (ip) defect-generating linkers denoted 5-X-ipH2 , where X=OH (1), H (2), NH2 (3), Br (4), were introduced into the mixed-valent ruthenium analogue of [Cu3 (btc)2 ]n (HKUST-1, btc=benzene-1,3,5-tricarboxylate) to yield Ru-DEMOFs (defect-engineered metal-organic frameworks) of the general empirical formula [Ru3 (btc)2-x (5-X-ip)x Yy ]n . Framework incorporation of 5-X-ip was confirmed by powder XRD, FTIR spectroscopy, ultrahigh-vacuum IR spectroscopy, thermogravimetric analysis, (1) H NMR spectroscopy, N2 sorption, and X-ray absorption near edge structure. Interestingly, Ru-DEMOF 1 c with 32 % framework incorporation of 5-OH-ip shows the highest BET surface area (≈1300 m(2)  g(-1) , N2 adsorption, 77 K) among all materials (including the parent framework [Ru3 (btc)2 Yy ]n ). The characterization data are consistent with two kinds of structural defects induced by framework incorporation of 5-X-ip: modified paddlewheel nodes featuring reduced ruthenium sites (Ru(δ+) , 0<δ<2, type A) and missing nodes leading to enhanced porosity (type B). Their relative abundances depend on the choice of the functional group X in the defect linkers. Defects A and B also appeared to play a key role in sorption of small molecules (i.e., CO2 , CO, H2 ) and the catalytic properties of the materials (i.e., ethylene dimerization and the Paal-Knorr reaction).

Host‐guest inclusion properties of UiO‐66 and UiO‐67 metal‐organic frameworks have been studied using ferrocene (FeCp2) as probe molecule. According to variable‐temperature solid‐state 1H and 13C CP‐MAS‐NMR, two different environments exist for adsorbed FeCp2 inside UiO‐66 and UiO‐67, which have been assigned to octahedral and tetrahedral cavities. At room temperature, a rapid exchange between these two adsorption sites occurs in UiO‐67, while at –80 °C the intracrystalline traffic of FeCp2 through the triangular windows is largely hindered. In UiO‐66, FeCp2 diffusion is already impeded at room temperature, in agreement with the smaller pore windows. Palladium nanoparticles (Pd NPs) encapsulated inside UiO‐66 and UiO‐67 have been prepared by chemical vapor infiltration of (allyl)Pd(Cp) followed by UV light irradiation. Infiltration must be carried out at low temperature (–10 °C) to avoid uncontrolled decomposition of the organometallic precursor and formation of Pd NPs at the external surface of the MOF. The resulting Pd‐MOFs are shape selective catalysts, as shown for the hydrogenation of carbonyl compounds with different steric hindrance.

By employing various synthetic conditions, such as Cu(NO 3 ) 2 ·3H 2 O and Cu(BF 4 ) 2 ·6H 2 O as copper ion sources and different solvents, defect-engineered analogues of metalorganic framework (MOF) [Cu 3 (BTC) 2 ] (HKUST-1; BTC = 1,3,5benzenetricarboxylate) with isophthalate (IP) incorporation (DEMOFs) were synthesised and characterised by powder XRD, SEM, IR spectroscopy, thermogravimetric analysis, NMR spectroscopy and N 2 sorption. The results show that the choice of counter ions impacts the properties of the samples especially [a]

In this work, we show that the stereoselectivity of a reaction can be controlled by directing groups of substrates, by network topology and by local cavity confinement of metal–organic framework (MOF) catalysts.

Three Zr-oxo-cluster node and porphyrin-linker based MOFs, MOF-525, PCN-222 and PCN-224 exhibiting different linker connectivities of 12, 8 and 6 and their porphyrin-linker metalated analogues, were synthesized and tested as catalysts for CO 2 fixation by using the cycloaddition of CO 2 and propylene oxide to propylene carbonate as the test reaction. In general, the catalytic activity correlates with the connectivity of the Zr-oxo nodes. The lowest connected PCN-224 (6-fold) exhibits a superior catalytic activity in this series, while higher connected PCN-222 (8-fold) and MOF-525 (12-fold) are less active. Interestingly, the catalytic activity of the higher connected MOFs significantly depends on defects. The (ideally) 12connected MOF-525, however exhibiting 16% of missing linker defects, features a higher catalytic activity compared to the 8connected PCN-222 with less defects. The overall catalytic activity is increased in dual site catalysts when the porphyrin linkers are metalated with Mn(III) and Zn(II) centers, which are acting as additional Lewis acid sites. Here, the metalated MOFs with higher connectivity exhibit the highest activity.