Ti3+-, V2+/3+-, Cr2+/3+-, Mn2+-, and Fe2+-Substituted MOF-5 and Redox Reactivity in Cr- and Fe-MOF-5

Redox-promoted associative assembly of metal-organic materials

We develop an associative synthesis of metal-organic materials that combines solid-state metal oxidation and coordination-driven self-assembly into a one-step, waste-free transformation. The methodology hinges on the unique reactivity of ortho-quinones, which we introduce as versatile oxidants for mechanochemical synthesis. Our strategy opens a previously unexplored route to paramagnetic metal-organic materials from elementary metals.

Solvothermal Metal Metathesis on a Metal-Organic Framework with Constricted Pores and the Study of Gas Separation

Metal-organic frameworks (MOFs) with constricted pores can increase the adsorbate density of gas and facilitate effective CO2 separation from flue gas or natural gas due to their enhanced overlapping of potential fields of the pores. Herein, an MOF with constricted pores, which was formed by narrow channels and blocks of functional groups, was fabricated from the assembly of a methyl-functionalized ligand and Zn(II) centers (termed NPC-7-Zn). Structural analysis of the as-synthesized NPC-7-Zn reveals a series of zigzag pores with pore diameters of ∼0.7 nm, which could be favorable for CO2 traps. For reinforcing the framework stability, a solvothermal metal metathesis on the pristine MOF NPC-7-Zn was performed, and a new Cu(II) MOF (termed NPC-7-Cu) with an identical framework was produced. The influence of the reaction temperatures on the metal metathesis process was investigated. The results show that the constricted pores in NPC-7-Zn can induce kinetic issues that largely slow the metal metathesis process at room temperature. However, this kinetic issue can be solved by applying higher reaction temperatures. The modified MOF NPC-7-Cu exhibits significant improvements in framework stability and thus leads to a permanent porosity for this framework. The constricted pore structure enables enhanced potential fields for these pores, rendering this MOF with high adsorbate densities for CO2 and high adsorption selectivity for a CO2/N2 gas mixture. The adsorption kinetic studies reveal that CH4 has a faster diffusion rate constant than CO2, showing a surface diffusion controlled mechanism for CO2 and CH4 adsorption.

A versatile synthetic route for the preparation of titanium metal-organic frameworks

Through a versatile synthetic strategy, High Valence Metathesis and Oxidation (HVMO), a series of Ti-MOFs with predesigned topologies and structures were synthesized.

Exploitation of new titanium metal–organic frameworks (Ti-MOFs) with high crystallinity has been attracting great attention due to their vast application potential in photocatalysis. Herein a versatile synthetic strategy, namely, High Valence Metathesis and Oxidation (HVMO), is developed to synthesize a series of Ti-MOFs with predesigned topologies and structures. The crystallinity of these Ti-MOFs was well maintained throughout, as confirmed by powder X-ray diffraction and gas adsorption measurements. Significantly, there were only a few examples of Ti-MOFs, not to mention a general synthetic strategy for various kinds of Ti-MOFs in the literature. This contribution also illustrates the intriguing potential of Ti-MOF platforms in photocatalysis.

A Flexible Photoactive Titanium Metal-Organic Framework Based on a [Ti(IV)3(μ3-O)(O)2(COO)6] Cluster

The synthesis of titanium-carboxylate metal-organic frameworks (MOFs) is hampered by the high reactivity of the commonly employed alkoxide precursors. Herein, we present an innovative approach to titanium-based MOFs by the use of titanocene dichloride to synthesize COK-69, the first breathing Ti MOF, which is built up from trans-1,4-cyclohexanedicarboxylate linkers and an unprecedented [Ti(IV)3(μ3-O)(O)2(COO)6] cluster. The photoactive properties of COK-69 were investigated in depth by proton-coupled electron-transfer experiments, which revealed that up to one Ti(IV) center per cluster can be photoreduced to Ti(III) while preserving the structural integrity of the framework. The electronic structure of COK-69 was determined by molecular modeling, and a band gap of 3.77 eV was found.

Dynamic DMF Binding in MOF-5 Enables the Formation of Metastable Cobalt-Substituted MOF-5 Analogues

Multinuclear solid-state nuclear magnetic resonance, mass spectrometry, first-principles molecular dynamics simulations, and other complementary evidence reveal that the coordination environment around the Zn(2+) ions in MOF-5, one of the most iconic materials among metal-organic frameworks (MOFs), is not rigid. The Zn(2+) ions bind solvent molecules, thereby increasing their coordination number, and dynamically dissociate from the framework itself. On average, one ion in each cluster has at least one coordinated N,N-dimethylformamide (DMF) molecule, such that the formula of as-synthesized MOF-5 is defined as Zn4O(BDC)3(DMF) x (x = 1-2). Understanding the dynamic behavior of MOF-5 leads to a rational low-temperature cation exchange approach for the synthesis of metastable Zn4-x Co x O(terephthalate)3 (x > 1) materials, which have not been accessible through typical high-temperature solvothermal routes thus far.

Mixed-Metal Strategy on Metal-Organic Frameworks (MOFs) for Functionalities Expansion: Co Substitution Induces Aerobic Oxidation of Cyclohexene over Inactive Ni-MOF-74

Different amounts of Co-substituted Ni-MOF-74 have been prepared via a post-synthetic metal exchange. Inductively coupled plasma mass spectrometry, powder X-ray diffraction (XRD), N2 adsorption/desorption, and extended X-ray absorption fine structure (EXAFS) analyses indicated the successful metathesis between Co and Ni in Ni-MOF-74 to form the solid-solution-like mixed-metal Co/Ni-MOF-74. It was found that introduction of active Co into the Ni-MOF-74 framework enabled the inert Ni-MOF-74 to show activity for cyclohexene oxidation. Since Co was favorably substituted at positions more accessible to the substrate, the mixed-metal Co/Ni-MOF-74 showed superior catalytic performance, compared with pure Co-MOF-74 containing a similar amount of Co. This study provides a facile method to develop solid-solution-like MOFs for heterogeneous catalysis and highlights the great potential of this mixed-metal strategy in the development of MOFs with specific endowed functionalities.

Synthesis of 3D structured graphene as a high performance catalyst support for methanol electro-oxidation

A simple process for preparing 3D structured graphene (3D-G) by a solution combustion method is reported. The product was deposited with platinum and used for methanol electro-oxidation. The catalyst shows a considerable enhancement in both the activity and stability towards methanol electro-oxidation reaction. Characterization reveals that the Pt/3D-G catalyst has a more negative onset potential as well as a higher electrochemically active specific surface area than a commercial Pt/C catalyst. Moreover, the catalyst exhibits higher tolerance to corrosion than carbon black. This work provides an efficient way for preparing 3D-G as a promising support for the oxidation of small organic molecules in fuel cells.

Defect-Engineered Metal-Organic Frameworks

Defect engineering in metal-organic frameworks (MOFs) is an exciting concept for tailoring material properties, which opens up novel opportunities not only in sorption and catalysis, but also in controlling more challenging physical characteristics such as band gap as well as magnetic and electrical/conductive properties. It is challenging to structurally characterize the inherent or intentionally created defects of various types, and there have so far been few efforts to comprehensively discuss these issues. Based on selected reports spanning the last decades, this Review closes that gap by providing both a concise overview of defects in MOFs, or more broadly coordination network compounds (CNCs), including their classification and characterization, together with the (potential) applications of defective CNCs/MOFs. Moreover, we will highlight important aspects of "defect-engineering" concepts applied for CNCs, also in comparison with relevant solid materials such as zeolites or COFs. Finally, we discuss the future potential of defect-engineered CNCs.

Metal-Organic Frameworks via Emissive Metal-Carboxylate Zwitterion Intermediates

Pyridinemethanol-carboxylate esters form octahedral complexes with Zn(NO₃ )₂ in aqueous DMF that subsequently undergo hydrolysis at elevated temperatures to form metal-carboxylate zwitterions. In situ deprotonation of the hydroxy group leads to thermally robust, neutral MOFs. This stepwise synthesis can be controlled by temperature and is made possible by the subtle difference in reactivity of the functional groups.

Hierarchical heteroaggregation of binary metal-organic gels with tunable porosity and mixed valence metal sites for removal of dyes in water

Hierarchical heteronuclear metal-organic gels (MOGs) based on iron (Fe) and aluminium (Al) metal-organic framework (MOF) backbones bridged by tri-carboxylate ligands have firstly been synthesized by simple solvothermal method. Monometallic MOGs based on Fe or Al give homogenous monoliths, which have been tuned by introduction of heterogeneity in the system (mismatched growth). The developed gels demonstrate that surface areas, pore volumes and pore sizes can be readily tuned by optimizing heterogeneity. The work also elaborates effect of heterogeneity on size of MOG particles which increase substantially with increasing heterogeneity as well as obtaining mixed valence sites in the gels. High surface areas (1861 m²/g) and pore volumes (9.737 cc/g) were obtained for heterogeneous gels (0.5Fe-0.5Al). The large uptakes of dye molecules (290 mg/g rhodamine B and 265 mg/g methyl orange) with fast sorption kinetics in both neutral and acidic mediums show good stability and accessibility of MOG channels (micro and meso-/macropores), further demonstrating their potential applications in catalysis and sorption of large molecules.

Heterometallic cluster-based indium-organic frameworks

With the help of the ligand-oriented method, we have successfully embedded independent copper-based units into the indium-organic framework system for the first time, in which the Cu4I4 clusters and In3O(CO2)6 clusters coexist. This heterometallic cluster-based framework has a large porosity with extra-open channels along the c-axis, and its sorption capacity has also been investigated.

Cation exchange at the secondary building units of metal-organic frameworks

Cation exchange is an emerging synthetic route for modifying the secondary building units (SBUs) of metal-organic frameworks (MOFs). This technique has been used extensively to enhance the properties of nanocrystals and molecules, but the extent of its applications for MOFs is still expanding. To harness cation exchange as a rational tool, we need to elucidate its governing factors. Not nearly enough experimental observations exist for drawing these conclusions, so we provide a conceptual framework for approaching this task. We address which SBUs undergo exchange, why certain ions replace others, how the framework influences the process, the role of the solvent, and current applications. Using these guidelines, certain trends emerge from the available data and missing experiments become obvious. If future studies follow this framework, then a more comprehensive body of observations will furnish a deeper understanding of cation exchange and inspire future applications.

Introduction of functionality, selection of topology, and enhancement of gas adsorption in multivariate metal-organic framework-177

Metal-organic framework-177 (MOF-177) is one of the most porous materials whose structure is composed of octahedral Zn4O(-COO)6 and triangular 1,3,5-benzenetribenzoate (BTB) units to make a three-dimensional extended network based on the qom topology. This topology violates a long-standing thesis where highly symmetric building units are expected to yield highly symmetric networks. In the case of octahedron and triangle combinations, MOFs based on pyrite (pyr) and rutile (rtl) nets were expected instead of qom. In this study, we have made 24 MOF-177 structures with different functional groups on the triangular BTB linker, having one or more functionalities. We find that the position of the functional groups on the BTB unit allows the selection for a specific net (qom, pyr, and rtl), and that mixing of functionalities (-H, -NH2, and -C4H4) is an important strategy for the incorporation of a specific functionality (-NO2) into MOF-177 where otherwise incorporation of such functionality would be difficult. Such mixing of functionalities to make multivariate MOF-177 structures leads to enhancement of hydrogen uptake by 25%.

Porous coordination polymers with ubiquitous and biocompatible metals and a neutral bridging ligand

The design of inexpensive and less toxic porous coordination polymers (PCPs) that show selective adsorption or high adsorption capacity is a critical issue in research on applicable porous materials. Although use of Group II magnesium(II) and calcium(II) ions as building blocks could provide cheaper materials and lead to enhanced biocompatibility, examples of magnesium(II) and calcium(II) PCPs are extremely limited compared with commonly used transition metal ones, because neutral bridging ligands have not been available for magnesium(II) and calcium(II) ions. Here we report a rationally designed neutral and charge-polarized bridging ligand as a new partner for magnesium(II) and calcium(II) ions. The three-dimensional magnesium(II) and calcium(II) PCPs synthesized using such a neutral ligand are stable and show selective adsorption and separation of carbon dioxide over methane at ambient temperature. This synthetic approach allows the structural diversification of Group II magnesium(II) and calcium(II) PCPs.

Inexpensive porous materials synthesized from Group II metals may be useful for industrial applications. Here, the authors demonstrate that neutral bridging ligands can be used for the synthesis of magnesium(II) and calcium(II) porous coordination polymers.

"Heterogeneity within order" in metal-organic frameworks

Metal-organic frameworks (MOFs) are constructed by linking inorganic units with organic linkers to make extended networks. Though more than 20 000 MOF structures have been reported most of these are ordered and largely composed of a limited number of different kinds building units, and very few have multiple different building units (heterogeneous). Although heterogeneity and multiplicity is a fundamental characteristic of biological systems, very few synthetic materials incorporate heterogeneity without losing crystalline order. Thus, the question arises: how do we introduce heterogeneity into MOFs without losing their ordered structure? This Review outlines strategies for varying the building units within both the backbone of the MOF and its pores to produce the heterogeneity that is sought after. The impact this heterogeneity imparts on the properties of a MOF is highlighted. We also provide an update on the MOF industry as part of this themed issue for the 150th anniversary of BASF.

De facto methodologies toward the synthesis and scale-up production of UiO-66-type metal–organic frameworks and membrane materials

The recent development in the synthetic methods and scale-up production of UiO-66-type MOFs and their related composites is presented.

Porous ZnCo2O4 nanoparticles derived from a new mixed-metal organic framework for supercapacitors

ZnCo₂O₄ nanoparticles were synthesized by thermal annealing of a new mixed-metal MOF and show high capacitance for electrochemical capacitors.

Crystal engineering, structure–function relationships, and the future of metal–organic frameworks

After twenty years of vigorous R&D, where are MOFs headed?

Single crystalline hollow metal–organic frameworks: a metal–organic polyhedron single crystal as a sacrificial template

Single crystalline hollow MOFs with cavity dimensions on the order of several micrometers and hundreds of micrometers were prepared using a metal–organic polyhedron single crystal as a sacrificial hard template.

A DFT/TDDFT mission to probe push–pull vinyl coupled thiophene oligomers for optoelectronic applications

VCTO encompassing pull–pull and push–pull categories has been appraised for fine tuning of band gaps and optoelectronic application through DFT/TDDFT calculations.

Introduction of a mediator for enhancing photocatalytic performance via post-synthetic metal exchange in metal–organic frameworks (MOFs)

Introduction of Ti as a mediator in Ti substituted NH₂-Uio-66(Zr) is an effective way to improve its photocatalytic performance.

Arsenic removal from aqueous solutions by adsorption using novel MIL-53(Fe) as a highly efficient adsorbent

A MIL-53(Fe) analogue was successfully synthesized by a HF free-solvothermal method.

Thermodynamic parameters of cation exchange in MOF-5 and MFU-4l

We present a method for approximating thermodynamic parameters , ΔH, and ΔS for the cation exchange process in metal–organic frameworks, as exemplified by Ni²⁺ exchange into Zn₄O(1,4-benzenedicarboxylate)₃ (MOF-5) and Co²⁺ exchange into MOF-5 and Zn₅Cl₄(bis(1H-1,2,3-triazolo-[4,5-b],[4′,5′-i])dibenzo-[1,4]-dioxin)₃ (MFU-4l).

Peculiar phenomena of structural transformations triggered from a nickel coordination polymer

Four multi-dimensional nickel-based coordination polymers have been prepared, and their solvent-mediated and solid-state structural transformations have been observed and studied systematically.

Reversible conversion of valence-tautomeric copper metal–organic frameworks dependent single-crystal-to-single-crystal oxidation/reduction: a redox-switchable catalyst for C–H bonds activation reaction

Reversible structural transformations proceeded between a Cu^I framework and a Cu^ICu^II framework, which endowed them as a redox-convertible catalyst.

Electronic structure modulation of metal-organic frameworks for hybrid devices

The study of metal-organic frameworks has largely been motivated by their structural and chemical diversity; however, these materials also possess rich physics, including optical, electronic, and magnetic activity. If these materials are to be employed in devices, it is necessary to develop an understanding of their solid-state behavior. We report an approach to calculate the effect of strain on the band structure of porous frameworks. The origin of the bidirectional absolute deformation potentials can be described from perturbations of the organic and inorganic building blocks. The unified approach allows us to propose several uses for hybrid materials, beyond their traditionally posited applications, including gas sensing, photoelectrochemistry, and as hybrid transistors.

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 Mn(2+) and three to four orders of magnitude greater sensitivity for metal ions than previously reported luminescent MOFs.