Guests Like Gear Levers: Donor Binding to Coordinatively Unsaturated Metal Sites in MIL-101 Controls the Linker′s Rotation

We present investigation of the effect of electron-donor guests on framework mobility in the metal–organic framework (MOF) MIL-101(Cr) monitored by solid state ²H NMR spectroscopy. In a guest-free material, the mobile phenylene fragments of the terephthalate (TP) linkers populate two fractions with notably different kinetic parameters for torsional motion. Two fractions of rotational motion are indicative of non-equivalence of TP linker binding to the Cr₃O trimer, the primary building unit of the MIL-101 framework. It is established that the interaction of the guest molecules with coordinatively unsaturated metal sites (CUS) of the MOF dramatically decreases torsional barriers for the linker motions, enhancing the rotation rate. This result is opposite to a more conventional slowing down effect on the linker rotation of the guests not selectively interacting with the adsorption sites inside the framework of the MOFs. The effect of coordination on both the torsional barrier and the rotation rate depends notably on the particular guest interacting with the CUS. The found effects of the guest on the rotational motion represent a basis for developing the strategy for ruling and controlling the linker rotation in MOFs with CUS. It is shown that if water occupies CUS, another guest (tert-butanol, cyclohexanone) fails to competitively coordinate to the site.     

Rapid Generation of Hierarchically Porous Metal–Organic Frameworks through Laser Photolysis

Hierarchically porous metal–organic frameworks (HP‐MOFs) facilitate mass transfer due to mesoporosity while preserving the advantage of microporosity. This unique feature endows HP‐MOFs with remarkable application potential in multiple fields. Recently, new methods such as linker labilization for the construction of HP‐MOFs have emerged. To further enrich the synthetic toolkit of MOFs, we report a controlled photolytic removal of linkers to create mesopores within microporous MOFs at tens of milliseconds. Ultraviolet (UV) laser has been applied to eliminate “photolabile” linkers without affecting the overall crystallinity and integrity of the original framework. Presumably, the creation of mesopores can be attributed to the missing‐cluster defects, which can be tuned through varying the time of laser exposure and ratio of photolabile/robust linkers. Upon laser exposure, MOF crystals shrank while metal oxide nanoparticles formed giving rise to the HP‐MOFs. In addition, photolysis can also be utilized for the fabrication of complicated patterns with high precision, paving the way towards MOF lithography, which has enormous potential in sensing and catalysis.     

Pore Aperture Control Toward Size-Exclusion-Based Hydrocarbon Separations

Metal–organic frameworks (MOFs) have been proposed as a promising material for non-thermal chemical separations owing to their high structural diversity and tunability. Here, we report the synthesis of a zinc-based MOF containing a three-dimensional (3D) linker, bicyclo[2.2.2]octane-1,4-dicarboxylic acid, with high thermal stability towards the separation of hexane isomers. The incorporation of the 3D linker enhances the structural stability and provides well-defined pore apertures/channels with sub-Ångstrom precision. This precision allowed for the separation of similarly sized hexane isomers based on subtle differences in their kinetic diameters. Multi-component liquid phase batch experiments confirmed the separation of hexanes mixture into linear, monobranched, and dibranched isomers. This work represents a significant milestone in the construction of stable Zn-based MOFs and the incorporation of 3D linkers as a potential solution to challenging separations.     

Pore‐Environment Engineering with Multiple Metal Sites in Rare‐Earth Porphyrinic Metal–Organic Frameworks

Multi‐component metal–organic frameworks (MOFs) with precisely controlled pore environments are highly desired owing to their potential applications in gas adsorption, separation, cooperative catalysis, and biomimetics. A series of multi‐component MOFs, namely PCN‐900(RE), were constructed from a combination of tetratopic porphyrinic linkers, linear linkers, and rare‐earth hexanuclear clusters (RE₆) under the guidance of thermodynamics. These MOFs exhibit high surface areas (up to 2523 cm² g^?1) and unlimited tunability by modification of metal nodes and/or linker components. Post‐synthetic exchange of linear linkers and metalation of two organic linkers were realized, allowing the incorporation of a wide range of functional moieties. Two different metal sites were sequentially placed on the linear linker and the tetratopic porphyrinic linker, respectively, giving rise to an ideal platform for heterogeneous catalysis.     

Assessing Chemical Heterogeneity at the Nanoscale in Mixed‐Ligand Metal–Organic Frameworks with the PTIR Technique

Recently, the use of mixtures of organic‐building‐block linkers has given chemists an additional degree of freedom for engineering metal–organic frameworks (MOFs) with specific properties; however, the poor characterization of the chemical complexity of such MixMOF structures by conventional techniques hinders the verification of rational design. Herein, we describe the application of a technique known as photothermal induced resonance to individual MixMOF microcrystals to elucidate their chemical composition with nanoscale resolution. Results show that MixMOFs isoreticular to In‐MIL‐68, obtained either directly from solution or by postsynthetic linker exchange, are homogeneous down to approximately 100 nm. Additionally, we report a novel in situ process that enables the engineering of anisotropic domains in MOF crystals with submicron linker‐concentration gradients.     

Atomically Dispersed Metal Sites in MOF‐Based Materials for Electrocatalytic and Photocatalytic Energy Conversion

Metal sites play an essential role in both electrocatalytic and photocatalytic energy conversion. The highly ordered arrangements of the organic linkers and metal nodes as well as the well‐defined pore structures of metal‐organic frameworks (MOFs) make them ideal substrates to support atomically dispersed metal sites (ADMSs) located in their metal nodes, linkers, and pores. Porous carbon materials doped with ADMSs can be derived from these ADMS‐incorporating MOF precursors through controlled treatments. These ADMSs incorporated in pristine MOFs and MOF‐derived carbon materials possess unique advantages over molecular or bulk metal‐based catalysts and bridge the gap between homogeneous and heterogeneous catalysts for energy‐conversion applications. This Review presents recent progress in the design and incorporation of ADMSs in MOFs and MOF‐derived materials for energy‐conversion applications.     

Rapid Generation of Hierarchically Porous Metal–Organic Frameworks through Laser Photolysis

Hierarchically porous metal–organic frameworks (HP-MOFs) facilitate mass transfer due to mesoporosity while preserving the advantage of microporosity. This unique feature endows HP-MOFs with remarkable application potential in multiple fields. Recently, new methods such as linker labilization for the construction of HP-MOFs have emerged. To further enrich the synthetic toolkit of MOFs, we report a controlled photolytic removal of linkers to create mesopores within microporous MOFs at tens of milliseconds. Ultraviolet (UV) laser has been applied to eliminate “photolabile” linkers without affecting the overall crystallinity and integrity of the original framework. Presumably, the creation of mesopores can be attributed to the missing-cluster defects, which can be tuned through varying the time of laser exposure and ratio of photolabile/robust linkers. Upon laser exposure, MOF crystals shrank while metal oxide nanoparticles formed giving rise to the HP-MOFs. In addition, photolysis can also be utilized for the fabrication of complicated patterns with high precision, paving the way towards MOF lithography, which has enormous potential in sensing and catalysis.     

Stepwise synthesis of metal-organic frameworks: replacement of structural organic linkers

We demonstrate how a single-crystal to single-crystal transformation resulting from bridging-linker replacement is possible in extended 2D and 3D metal-organic frameworks (MOFs) by introducing pillared paddlewheel MOF structures into a solution containing dipyridyl linkers. No lateral movement of the layers was observed during this transformation, creating a templating effect from the "parent" structure to the "daughter" structure. A previously unattainable structure was obtained by a two-step synthetic method utilizing the bridging-linker replacement transformation method. Additionally, a bridging-linker insertion was observed when excess linker was used with the 2D MOF structure, inducing an overall 2D to 3D transformation.     

Stereoselective Halogenation of Integral Unsaturated C‐C Bonds in Chemically and Mechanically Robust Zr and Hf MOFs

Metal–organic frameworks (MOFs) containing Zr^IV‐based secondary building units (SBUs), as in the UiO‐66 series, are receiving widespread research interest due to their enhanced chemical and mechanical stabilities. We report the synthesis and extensive characterisation, as both bulk microcrystalline and single crystal forms, of extended UiO‐66 (Zr and Hf) series MOFs containing integral unsaturated alkene, alkyne and butadiyne units, which serve as reactive sites for postsynthetic modification (PSM) by halogenation. The water stability of a Zr–stilbene MOF allows the dual insertion of both ?OH and ?Br groups in a single, aqueous bromohydrination step. Quantitative bromination of alkyne‐ and butadiyne‐containing MOFs is demonstrated to be stereoselective, as a consequence of the linker geometry when bound in the MOFs, while the inherent change in hybridisation and geometry of integral linker atoms is facilitated by the high mechanical stabilities of the MOFs, allowing bromination to be characterised in a single‐crystal to single‐crystal (SCSC) manner. The facile addition of bromine across the unsaturated C?C bonds in the MOFs in solution is extended to irreversible iodine sequestration in the vapour phase. A large‐pore interpenetrated Zr MOF demonstrates an I₂ storage capacity of 279 % w/w, through a combination of chemisorption and physisorption, which is comparable to the highest reported capacities of benchmark iodine storage materials for radioactive I₂ sequestration. We expect this facile PSM process to not only allow trapping of toxic vapours, but also modulate the mechanical properties of the MOFs.     

Cooperative effect of temperature and linker functionality on CO2 capture from industrial gas mixtures in metal-organic frameworks: a combined experimental and molecular simulation study

In this work, the cooperative effect of temperature and linker functionality on CO(2) capture in metal-organic frameworks (MOFs) was investigated using experimental measurements in combination with molecular simulations. To do this, four MOFs with identical topology but different functional groups on the linkers and three important CO(2)-containing industrial gas mixtures were adopted. The interplay between linker functionality and temperature was analyzed in terms of CO(2) storage capacity, adsorption selectivity, working capacity of CO(2) in temperature swing adsorption (TSA) processes, as well as sorbent selection parameter (S(ssp)). The results show that the effect of linker functionality on CO(2) capture performance in the MOFs is strongly interconnected with temperature: up to moderate pressures, the lower the temperature, the larger the effect of the functional groups. Furthermore, the modification of a MOF by introducing more complex functional groups can not only improve the affinity of framework for CO(2), but also reduce the free volume, and thus may contribute negatively to CO(2) capture capability when the packing effect is obvious. Therefore, when we design a new MOF for a certain CO(2) capture process operated at a certain temperature, the MOF should be designed to have maximized affinity for CO(2) but with a negligible or small effect caused by the reduction of free volume at that temperature and the corresponding operating pressure.     

Functionality proportion and corresponding stability study of multivariate metal-organic frameworks

Zn-based multivariate metal-organic frameworks (MTV-MOFs) with different functionality proportions and with different thermal and chemical stabilities can be obtained by employing the appropriate synthesis method.     

Molecular Vise Approach to Create Metal‐Binding Sites in MOFs and Detection of Biomarkers

We report a new approach to create metal‐binding site in a series of metal–organic frameworks (MOFs), where tetratopic carboxylate linker, 4′,4′′,4′′′,4′′′′‐methanetetrayltetrabiphenyl‐4‐carboxylic acid, is partially replaced by a tritopic carboxylate linker, tris(4‐carboxybiphenyl)amine, in combination with monotopic linkers, formic acid, trifluoroacetic acid, benzoic acid, isonicotinic acid, 4‐chlorobenzoic acid, and 4‐nitrobenzoic acid, respectively. The distance between these paired‐up linkers can be precisely controlled, ranging from 5.4 to 10.8 Å, where a variety of metals, Mg²⁺, Al³⁺, Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺, Cd²⁺ and Pb²⁺, can be placed in. The distribution of these metal‐binding sites across a single crystal is visualized by 3D tomography of laser scanning confocal microscopy with a resolution of 10 nm. The binding affinity between the metal and its binding‐site in MOF can be varied in a large range (observed binding constants, K_obs from 1.56×10² to 1.70×10⁴ L mol^?1), in aqueous solution. The fluorescence of these crystals can be used to detect biomarkers, such as cysteine, homocysteine and glutathione, with ultrahigh sensitivity and without the interference of urine, through the dissociation of metal ions from their binding sites.     

In Situ Generation of an N‐Heterocyclic Carbene Functionalized Metal–Organic Framework by Postsynthetic Ligand Exchange: Efficient and Selective Hydrosilylation of CO2

The reported metal–organic framework (MOF) catalyst realizes CO₂ to methanol transformation under ambient conditions. The MOF is one rare example containing metal‐free N‐heterocyclic carbene (NHC) moieties, which are installed using an in situ generation strategy involving the incorporation of an imidazolium bromide based linker into the MOF by postsynthetic ligand exchange. Importantly, the resultant NHC‐functionalized MOF is the first catalyst capable of performing quantitative hydrogen transfer from silanes to CO₂, thus achieving quantitative (>99 %) methanol yield. Density‐functional theory calculations indicate the high catalytic activity of the NHC sites in MOFs are attributed to the decreased reaction barrier of a reaction route involving the formation of an NHC‐silane adduct. In addition, the MOF‐immobilized NHC catalyst shows enhanced stability for up to eight cycles without base activation, as well as high selectivity towards the desired silyl methoxide product.     

Competitive Excimer Formation and Energy Transfer in Zr‐Based Heterolinker Metal–Organic Frameworks

The spectroscopy and dynamics of a series of Zr‐based MOFs in dichloromethane suspension are reported. These Zr‐NADC MOFs were constructed by using different mixtures of 2,6‐naphthalenedicarboxylate (NDC) and 4‐amino‐2,6‐naphthalenedicarboxylate (NADC) as organic linkers. The fraction of NADC relative to NDC in these heterolinker MOFs ranges from 2 to 35 %. The results indicate two competitive photoprocesses: NDC excimer formation and an energy transfer (ET) from excited NDC linkers to NADC linkers. Increasing the fraction of NADC linkers in the Zr‐NADC nanostructure decreases the mean time constant of NDC excimer formation, while the NADC emission intensity experiences a drop at the highest fraction of this linker in the MOF. The first observation is explained by an increase in the energy‐transfer probability between the two linkers, and the second by emission quenching in the NADC linkers due to ultrafast charge transfer assisted by the amino group. Femtosecond time‐resolved emission studies showed that the ET process (recorded as decaying and rising components) from excited NDC to NADC takes place in 1.2 ps. Direct excitation of the NADC linkers (at 410 nm) shows a decaying, but not rising, component of 250–480 fs, which could reflect the formation of a nonemissive charge‐separation state. The results show that by using MOFs having heterolinkers it is possible to trigger and tune excimer formation and ET processes.     

Self‐Generation of Surface Roughness by Low‐Surface‐Energy Alkyl Chains for Highly Stable Superhydrophobic/Superoleophilic MOFs with Multiple Functionalities

We transformed the hydrophilic metal–organic framework (MOF) UiO‐67 into hydrophobic UiO‐67‐R s (R=alkyl) by introducing alkyl chains into organic linkers, which not only protected hydrophilic Zr₆O₈ clusters to make the MOF interspace superoleophilic, but also led to a rough crystal surface beneficial for superhydrophobicity. The UiO‐67‐R s displayed high acid, base, and water stability, and long alkyl chains offered better hydrophobicity. Good hydrophobicity/oleophilicity were also possible with mixed‐ligand MOFs containing metal‐binding ligands. Thus, a (super)hydrophobic MOF catalyst loaded with Pd centers efficiently catalyzed Sonogashira reactions in water at ambient temperature. Studies of the hydrophobic effects of the coordination interspace and the outer surface suggest a simple de novo strategy for the synthesis of superhydrophobic MOFs that combine surface roughness and low surface energy. Such MOFs have potential for environmentally friendly catalysis and water purification.     

Expanding Linker Dimensionality in Metal-organic Frameworks for sub-Ångstrom Pore Control for Separation Applications

Metal-organic frameworks (MOFs) are a class of porous materials with high surface areas, which are acquiring rapid attention on an exponential basis. A significant characteristic of MOFs is their ability to act as adsorbents to selectively separate component mixtures of similar size, thereby addressing the technological need for an alternative approach to conventional distillation methods. Recently, MOFs comprising a 3-Dimensional (3D) linker have shown outstanding capabilities for difficult separations compared to the parent 2-Dimensional (2D) analogue. 3D-linkers with a polycyclic core are underrepresented in the MOF database due to the widespread preferred use of 2D-linkers and the misconceived high-cost of 3D linkers. We summarize the recent research of 3D-linker MOFs and highlight their beneficial employment for selective gas and hydrocarbon adsorption and separation. Furthermore, we outline forecasts in this area to create a platform for widespread adoption of 3D-linkers in MOF synthesis.     

A Metal–Organic Framework Containing Unusual Eight‐Connected Zr–Oxo Secondary Building Units and Orthogonal Carboxylic Acids for Ultra‐sensitive Metal Detection

Two metal–organic frameworks (MOFs) with Zr–oxo secondary building units (SBUs) were prepared by using p,p′‐terphenyldicarboxylate (TPDC) bridging ligands pre‐functionalized with orthogonal succinic acid (MOF‐ 1 ) and maleic acid groups (MOF‐ 2 ). Single‐crystal X‐ray structure analysis of MOF‐ 1 provides the first direct evidence for eight‐connected SBUs in UiO‐type MOFs. In contrast, MOF‐ 2 contains twelve‐connected SBUs as seen in the traditional UiO MOF topology. These structural assignments were confirmed by extended X‐ray absorption fine structure (EXAFS) analysis. The highly porous MOF‐ 1 is an excellent fluorescence sensor for metal ions with the detection limit of <0.5 ppb for Mn²⁺and three to four orders of magnitude greater sensitivity for metal ions than previously reported luminescent MOFs.     

Multicomponent Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are constructed from metal ions or clusters and organic linkers. Typical MOFs are rather simple, comprising just one type of joint and linker. An additional degree of structural complexity can be introduced by using multiple different components that are assembled into the same framework In the early days of MOF chemistry, conventional wisdom held that attempting to prepare frameworks starting from such a broad set of components would lead to multiple different phases. However, this review highlights how this view was mistaken and frameworks comprising multiple different components can be deliberately designed and synthesized. When coupled to structural order and periodicity, the presence of multiple components leads to exceptional functional properties that can be understood at the atomic level.     

Microporous Metal–Organic Frameworks for Gas Separation

Microporous metal–organic frameworks (MOFs) are comparatively new porous materials. Because the pores within such MOFs can be readily tuned through the interplay of both metal‐containing clusters and organic linkers to induce their size‐selective sieving effects, while the pore surfaces can be straightforwardly functionalized to enforce their different interactions with gas molecules, MOF materials are very promising for gas separation. Furthermore, the high porosities of such materials can enable microporous MOFs with optimized gas separation selectivity and capacity to be targeted. This Focus Review highlights recent significant advances in microporous MOFs for gas separation.     

Metal-organic frameworks: structural, energetic, electronic, and mechanical properties

The structural, energetic, electronic, and mechanical properties of a series of metal-organic framework (MOF) materials have been systematically studied with the density functional based tight-binding method. The cubic array of Zn(4)O(CO2)6 units (connectors) connected by different types of organic secondary building blocks (linkers) was considered. The results show that these materials are stable with bulk moduli ranging from 0.5 to 24 GPa with decreasing size of the linker. All MOFs are semiconductors or insulators with band gaps between 1.0 and 5.5 eV, mainly determined by highest occupied molecular orbital-lowest unoccupied molecular orbital gaps of the linker molecules. The atomic charges are nearly the same for free building blocks and the solid MOFs.