Metal–Covalent Organic Frameworks (MCOFs): A Bridge Between Metal–Organic Frameworks and Covalent Organic Frameworks

Many sophisticated chemical and physical properties of porous materials strongly rely on the presence of the metal ions within the structures. Whereas homogeneous distribution of metals is conveniently realized in metal–organic frameworks (MOFs), the limited stability potentially restricts their practical implementation. From that perspective, the development of metal–covalent organic frameworks (MCOFs) may address these shortcomings by incorporating active metal species atop highly stable COF backbones. This Minireview highlights examples of MCOFs that tackle important issues from their design, synthesis, characterization to cutting-edge applications.     

Cooperative Carbon Dioxide Adsorption in Alcoholamine- and Alkoxyalkylamine-Functionalized Metal–Organic Frameworks

A series of structurally diverse alcoholamine- and alkoxyalkylamine-functionalized variants of the metal–organic framework Mg₂(dobpdc) are shown to adsorb CO₂ selectively via cooperative chain-forming mechanisms. Solid-state NMR spectra and optimized structures obtained from van der Waals-corrected density functional theory calculations indicate that the adsorption profiles can be attributed to the formation of carbamic acid or ammonium carbamate chains that are stabilized by hydrogen bonding interactions within the framework pores. These findings significantly expand the scope of chemical functionalities that can be utilized to design cooperative CO₂ adsorbents, providing further means of optimizing these powerful materials for energy-efficient CO₂ separations.     

Catalytic Performance of Zr-Based Metal–Organic Frameworks Zr-abtc and MIP-200 in Selective Oxidations with H2O2

The catalytic performance of Zr-abtc and MIP-200 metal–organic frameworks consisting of 8-connected Zr₆ clusters and tetratopic linkers was investigated in H₂O₂-based selective oxidations and compared with that of 12-coordinated UiO-66 and UiO-67. Zr-abtc demonstrated advantages in both substrate conversion and product selectivity for epoxidation of electron-deficient C=C bonds in α,β-unsaturated ketones. The significant predominance of 1,2-epoxide in carvone epoxidation, coupled with high sulfone selectivity in thioether oxidation, points to a nucleophilic oxidation mechanism over Zr-abtc. The superior catalytic performance in the epoxidation of unsaturated ketones correlates with a larger amount of weak basic sites in Zr-abtc. Electrophilic activation of H₂O₂ can also be realized, as evidenced by the high activity of Zr-abtc in epoxidation of the electron-rich C=C bond in caryophyllene. XRD and FTIR studies confirmed the retention of the Zr-abtc structure after the catalysis. The low activity of MIP-200 in H₂O₂-based oxidations is most likely related to its specific hydrophilicity, which disfavors adsorption of organic substrates and H₂O₂.     

Molecular Encapsulation of Trimeric Chromium Carboxylate Clusters in Metal–Organic Frameworks and Propylene Sorption

Oxo-bridged trimeric chromium acetate clusters [Cr₃O(OOCCH₃)₆(H₂O)₃]NO₃ have been encapsulated for the first time in the mesoporous cages of the chromium terephthalate MIL-101(Cr). The isolated clusters in MIL-101(Cr) have increased affinity towards propylene compared to propane, due to generation of a new kind of pocket-based propylene-binding site, as supported by DFT calculations.     

Fast and Selective Semihydrogenation of Alkynes by Palladium Nanoparticles Sandwiched in Metal–Organic Frameworks

The semihydrogenation of alkynes into alkenes rather than alkanes is of great importance in the chemical industry. Unfortunately, state-of-the-art heterogeneous catalysts hardly achieve high turnover frequencies (TOFs) simultaneously with almost full conversion, excellent selectivity, and good stability. Here, we used metal–organic frameworks (MOFs) containing Zr metal nodes (“UiO”) with tunable wettability and electron-withdrawing ability as activity accelerators for the semihydrogenation of alkynes catalyzed by sandwiched palladium nanoparticles (Pd NPs). Impressively, the porous hydrophobic UiO support not only leads to an enrichment of phenylacetylene around the Pd NPs but also renders the Pd surfaces more electron-deficient, which leads to a remarkable catalysis performance, including an exceptionally high TOF of 13835 h⁻¹, 100 % phenylacetylene conversion 93.1 % selectivity towards styrene, and no activity decay after successive catalytic cycles. The strategy of using molecularly tailored supports is universal for boosting the selective semihydrogenation of various terminal and internal alkynes.     

Decorating Channel Walls in Metal–Organic Frameworks with Crown Ethers for Efficient and Selective Separation of Radioactive Strontium(II)

Nuclear accidents and the improper disposal of nuclear wastes have led to serious environmental radioactive pollutions. The rational design of adsorbents for the highly efficient separation of strontium(II) is essential in treating nuclear waste and recovering radioactive strontium resources. Metal–organic frameworks (MOFs) are potential materials for the separation of aqueous metal ions owing to their designable structure and tunable functionality. Herein, a novel 3D MOF material MOF-18Cr6, in which 1D channels are formed using 18-crown-6-ether-containing ligands as channel walls, is fabricated for strontium(II) separation. In contrast to traditional MOFs designed by grafting functional groups in the framework pores, MOF-18Cr6 possesses regular 18-crown-6-ether cavities on the channel walls, which not only can transport and intake strontium(II) via the channels, but also prevent blockage of the channels after the binding of strontium(II). Consequently, the functional sites are fully utilized to achieve a high strontium(II) removal rate of 99.73 % in simulated nuclear wastewater. This study fabricates a highly promising adsorbent for the separation of aqueous radioactive strontium(II), and more importantly, can provide a new strategy for the rational design of high-performance MOF adsorbents for separating target substances from complex aqueous environments.     

Highly Selective and Stable Zn (II)-Based Metal–Organic Frameworks for the Detections of Tetracycline Antibiotic and Acetone in Aqueous System

Tetracycline (TC) is one of the most widely used antibiotics in aquaculture, and its good water solubility makes it a major contaminant in seawater. Therefore, it is very necessary and challenging to develop an efficient detection method. In this work, two novel metal–organic frameworks [Zn (bpydb)(bimmb)_0.5]_n ( 1 ), {[Zn₂(bpydb)₂(bimb)]·[Zn (bpydb)(bimb)]·H₂O}_n ( 2 ), (bimb = 1,4-bis (lmidazol) butane, H₂bypdb = 4,4′-(4,4’-Bipyridine-2,6-diyl)dibenzoic acid, bimmb = 1,4-bis (imidazol-1-ylmethyl)benzene) were successfully synthesized under solvothermal conditions. Zn-MOF 1 – 2 were characterized by X-ray powder diffraction (PXRD), Fourier transform infrared (FT-IR) spectroscopy and thermogravimetric analysis (TGA). As expected, 1 – 2 have excellent fluorescence properties, thermal stability and good structural stability in water. TC in water can be detected by fluorescence quenching with high selectivity. At the same time, the fluorescence quenching efficiency remains unchanged in the presence of other interfering antibiotics and in aqueous solutions of different pH values (pH = 3–10). The detection ability of 1 in real seawater has not changed substantially, showing considerable practical application prospects. Interestingly, 1 – 2 also efficiently detected traces of acetone in solution with detection limits of 0.07 μM (4.38 ppb) and 0.18 μM (10.85 ppb), respectively. In addition, the mechanism of fluorescence quenching is further discussed.     

Tin(II)-Based Metal–Organic Frameworks Enabling Efficient,Selective Reduction of CO2 to Formate under Visible Light

Certain metal complexes are known as high-performance CO₂ reduction photocatalysts driven by visible light. However, most of them rely on rare, precious metals as principal components, and integrating the functions of light absorption and catalysis into a single molecular unit based on abundant metals remains a challenge. Metal-organic frameworks (MOFs), which can be regarded as intermediate compounds between molecules and inorganic solids, are potential platforms for the construction of a simple photocatalytic system composed only of Earth-abundant nontoxic elements. In this work, we report that a tin-based MOF enables the conversion of CO₂ into formic acid with a record high apparent quantum yield (9.8 % at 400 nm) and >99 % selectivity without the need for any additional photosensitizer or catalyst. This work highlights a new MOF with strong potential for photocatalytic CO₂ reduction driven by solar energy.     

The Significance of Metal Coordination in Imidazole-Functionalized Metal–Organic Frameworks for Carbon Dioxide Utilization

The grafting of imidazole species onto coordinatively unsaturated sites within metal–organic framework MIL-101(Cr) enables enhanced CO₂ capture in close proximity to catalytic sites. The subsequent combination of CO₂ and epoxide binding sites, as shown through theoretical findings, significantly improves the rate of cyclic carbonate formation, producing a highly active CO₂ utilization catalyst. An array of spectroscopic investigations, in combination with theoretical calculations reveal the nature of the active sites and associated catalytic mechanism which validates the careful design of the hybrid MIL-101(Cr).     

Single-Crystal Transformation Engineering the Spin Change of Metal–Organic Frameworks via Cluster Deconstruction

Spin crossover (SCO) materials with new architectures will expand and enrich the research in the SCO field. Here, we report two metal–organic frameworks (MOFs) containing tetradentate organic ligands and hexatopic linkers [Ag₈X₈(CN)₆]⁶⁻ (X=Br and I), which represents the first SCO MOF with clusters as building blocks. The silver halide cluster can be further removed after reacting with lithium tetracyanoquinodimethan (LiTCNQ). Such post-synthetic modification (PSM) is realized via single-crystal to single-crystal (SCSC) transformation from urk to nbo topology. Accordingly, the spin state and fluorescence properties are greatly modified by cluster deconstruction. Therefore, these achievements will provide new ideas for the design of new SCO systems and the development of PSM methods.     

Thermally Activated Adsorption in Metal–Organic Frameworks with a Temperature-Tunable Diffusion Barrier Layer

Modification of the external surfaces of metal–organic frameworks offers a new level of control over their adsorption behavior. It was previously shown that capping of MOFs with ethylenediamine (EDA) can effectively retain small gaseous molecules at room temperature. Reported here is a temperature-induced variation in the capping-layer gate-opening mechanism through a combination of in situ infared experiments and ab initio simulations of the capping layer. An atypical acceleration and increase in the loading of weakly adsorbed molecules upon raising the temperature above room temperature is observed. These findings show the discovery of novel temperature-dependent kinetics that goes beyond standard kinetics and suggest a new avenue for tailoring selective adsorption by thermally tuning the surface barrier.     

Microscopic and Mesoscopic Dual Postsynthetic Modifications of Metal–Organic Frameworks

We report the dual postsynthetic modification (PSM) of a metal–organic framework (MOF) involving the microscopic conversion of C−H bonds into C−C bonds and the mesoscopic introduction of hierarchical porosity. MOF crystals underwent single-crystal-to-single-crystal transformations during the electrophilic aromatic substitution of Co₂(m-DOBDC) (m-DOBDC⁴⁻=4,6-dioxo-1,3-benzenedicarboxylate) with alkyl halides and formaldehyde. The steric hindrance caused by the proximity of the introduced functional groups to the coordination bonds reduced bond stability and facilitated the transformation into hierarchically porous mesostructures by etching with in situ generated protons (hydroniums) and halides. The numerous defect sites in the mesostructural MOFs are potential water-sorption sites. However, since the introduced functional groups are close to the main adsorption sites, even methyl groups are able to considerably decrease water adsorption, whereas hydroxy groups increase adsorption at low vapor pressures.     

Accelerated Dynamic Reconstruction in Metal–Organic Frameworks with Ligand Defects for Selective Electrooxidation of Amines to Azos Coupling with Hydrogen Production

Electrosynthesis coupled hydrogen production (ESHP) mostly involves catalyst reconstruction in aqueous phase, but accurately identifying and controlling the process is still a challenge. Herein, we modulated the electronic structure and exposed unsaturated sites of metal–organic frameworks (MOFs) via ligand defect to promote the reconstruction of catalyst for azo electrosynthesis (ESA) coupled with hydrogen production overall reaction. The monolayer Ni-MOFs achieved 89.8 % Faraday efficiency and 90.8 % selectivity for the electrooxidation of 1-methyl-1H-pyrazol-3-amine (Pyr−NH₂) to azo, and an 18.5-fold increase in H₂ production compared to overall water splitting. Operando X-ray absorption fine spectroscopy (XAFS) and various in situ spectroscopy confirm that the ligand defect promotes the potential dependent dynamic reconstruction of Ni(OH)₂ and NiOOH, and the reabsorption of ligand significantly lowers the energy barrier of rate-determining step (*Pyr−NH to *Pyr−N). This work provides theoretical guidance for modulation of electrocatalyst reconstruction to achieve highly selective ESHP.     

Dominant Role of Hole Transport Pathway in Achieving Record High Photoconductivity in Two-Dimensional Metal–Organic Frameworks

Metal–organic frameworks (MOFs) with mobile charges have attracted significant attention due to their potential applications in photoelectric devices, chemical resistance sensors, and catalysis. However, fundamental understanding of the charge transport pathway within the framework and the key properties that determine the performance of conductive MOFs in photoelectric devices remain underexplored. Herein, we report the mechanisms of photoinduced charge transport and electron dynamics in the conductive 2D M−HHTP (M=Cu, Zn or Cu/Zn mixed; HHTP=2,3,6,7,10,11-hexahydroxytriphenylene) MOFs and their correlation with photoconductivity using the combination of time-resolved terahertz spectroscopy, optical transient absorption spectroscopy, X-ray transient absorption spectroscopy, and density functional theory (DFT) calculations. We identify the through-space hole transport mechanism through the interlayer sheet π–π interaction, where photoinduced hole state resides in HHTP ligand and electronic state is localized at the metal center. Moreover, the photoconductivity of the Cu−HHTP MOF is found to be 65.5 S m⁻¹, which represents the record high photoconductivity for porous MOF materials based on catecholate ligands.     

Structures and Structural Evolution of Sublayer Surfaces of Metal–Organic Frameworks

The structural characterization of sublayer surfaces of MIL-101 is reported by low-dose spherical aberration-corrected high-resolution transmission electron microscopy (HRTEM). The state-of-the-art microscopy directly images atomic/molecular configurations in thin crystals from charge density projections, and uncovers the structures of sublayer surfaces and their evolution to stable surfaces regulated by inorganic Cr₃(μ₃-O) trimers. This study provides compelling evidence of metal–organic frameworks (MOFs) crystal growth via the assembly of sublayer surfaces and has important implications in understanding the crystal growth and surface-related properties of MOFs.     

Isoreticular Expansion of Metal–Organic Frameworks via Pillaring of Metal Templated Tunable Building Layers: Hydrogen Storage and Selective CO2 Capture

The deliberate construction of isoreticular eea-metal–organic frameworks (MOFs) ( Cu-eea-1 , Cu-eea-2 and Cu-eea-3 ) and rtl-MOFs ( Co-rtl-1 and Co-rtl-2 ) has been accomplished based on the ligand-to-axial pillaring of supermolecular building layers. The use of different metal ions resulted in two types of supermolecular building layers (SBLs): Kagome (kgm) and square lattices (sql) which further interconnect to form anticipated 3D-MOFs. The isoreticular expansion of (3,6)-connected Cu-MOFs has been achieved with desired eea-topology based on kgm building layers. In addition, two (3,6)-connected Co-rtl-MOFs were also successfully constructed based on sql building layers. The Cu-eea-MOFs were shown to act as hydrogen storage materials with appreciable amount of hydrogen uptake abilities. Moreover Cu-eea-MOFs have also exhibited remarkable CO₂ capture ability at ambient condition compared to nitrogen and methane, due to the presence of amide functionalities.     

Solvent-Assisted Metal Metathesis: A Highly Efficient and Versatile Route towards Synthetically Demanding Chromium Metal–Organic Frameworks

Chromium(III)-based metal–organic frameworks (Cr-MOFs) are very attractive in a wide range of investigations because of their robustness and high porosity. However, reports on Cr-MOFs are scarce owing to the difficulties in their direct synthesis. Recently developed postsynthetic routes to obtain Cr-MOFs suffered from complicated procedures and a lack of general applicability. Herein, we report a highly efficient and versatile strategy, namely solvent-assisted metal metathesis, to obtain Cr-MOFs from a variety of Fe^III-MOFs, including several well-known MOFs and a newly synthesized one, through judicious selection of a coordinating solvent. The versatility of this strategy was demonstrated by producing Cr-MIL-100, Cr-MIL-142A/C, Cr-PCN-333, and Cr-PCN-600 from their Fe^III analogues and Cr-SXU-1 from a newly synthesized MOF precursor, Fe-SXU-1, in acetone as the solvent under very mild conditions. We have thus developed a general approach for the preparation of robust Cr-MOFs, which are difficult to synthesize by direct methods.     

Chiral Motifs in Highly Interpenetrated Metal–Organic Frameworks Formed from Achiral Tetrahedral Ligands**

Formation of highly interpenetrated frameworks is demonstrated. An interesting observation is the presence of very large adamantane-shaped cages in a single network, making these crystals new entries in the collection of diamondoid-type metal–organic frameworks (MOFs). The frameworks were constructed by assembling tetrahedral pyridine ligands and copper dichloride. Currently, the networks’ degree of interpenetration is among the highest reported and increases when the size of the ligand is increased. Highly interpenetrated frameworks typically have low surface contact areas. In contrast, in our systems, the voids take up to 63 % of the unit cell volume. The MOFs have chiral features but are formed from achiral components. The chirality is manifested by the coordination chemistry around the metal center, the structure of the helicoidal channels, and the motifs of the individual networks. Channels of both handednesses are present within the unit cells. This phenomenon shapes the walls of the channels, which are composed of 10, 16, or 32 chains correlated with the degree of interpenetration 10-, 16-, and 32-fold, respectively. By changing the distance between the center of the ligand and the coordination moieties, we succeeded in tuning the diameter of the channels. Relatively large channels were formed, having diameters up to 31.0 Å×14.8 Å.     

Spin Manipulation in a Metal–Organic Layer through Mechanical Exfoliation for Highly Selective CO2 Photoreduction

Spin manipulation of transition-metal catalysts has great potential in mimicking enzyme electronic structures to improve activity and/or selectivity. However, it remains a great challenge to manipulate room-temperature spin state of catalytic centers. Herein, we report a mechanical exfoliation strategy to in situ induce partial spin crossover from high-spin (s=5/2) to low-spin (s=1/2) of the ferric center. Due to spin transition of catalytic center, mixed-spin catalyst exhibits a high CO yield of 19.7 mmol g⁻¹ with selectivity of 91.6 %, much superior to that of high-spin bulk counterpart (50 % selectivity). Density functional theory calculations reveal that low-spin 3d-orbital electronic configuration performs a key function in promoting CO₂ adsorption and reducing activation barrier. Hence, the spin manipulation highlights a new insight into designing highly efficient biomimetic catalysts via optimizing spin state.