Hetero‐Epitaxial Approach by Using Labile Coordination Sites to Prepare Catenated Metal–Organic Frameworks with High Surface Areas

A solid‐state approach that takes advantage of the ordered 3D arrangement of active secondary building units allows the preparation of new interlocked MOFs that grow hetero‐epitaxially on the crystal faces of a precursor phase that acts as a “topological blueprint”. The synthetic strategy is exemplified by using rigid acetylene‐based ligands to produce highly augmented Cu^II acetate‐based MOFs.     

Instant Visual Detection of Picogram Levels of Trinitrotoluene by Using Luminescent Metal–Organic Framework Gel‐Coated Filter Paper

There is an ongoing need for explosive detection strategies to uncover threats to human security including illegal transport and terrorist activities. The widespread military use of the explosive trinitrotoluene (TNT) for landmines poses another particular threat to human health in the form of contamination of the surrounding environment and groundwater. The detection of explosives, particularly at low picogram levels, by using a molecular sensor is seen as an important challenge. Herein, we report on the use of a fluorescent metal–organic framework hydrogel that exhibits a higher detection capability for TNT in the gel state compared with that in the solution state. A portable sensor prepared from filter paper coated by the hydrogel was able to detect TNT at the picogram level with a detection limit of 1.82 ppt (parts per trillon). Our results present a simple and new means to provide selective detection of TNT on a surface or in aqueous solution, as afforded by the unique molecular packing through the metal–organic framework structure in the gel formation and the associated photophysical properties. Furthermore, the rheological properties of the MOF‐based gel were similar to those of a typical hydrogel.     

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.     

Stabilizing and Organizing Bi3Cu4 and Bi7Cu12 Nanoclusters in Two‐Dimensional Metal–Organic Networks

Multinuclear heterometallic nanoclusters with controllable stoichiometry and structure are anticipated to possess promising catalytic, magnetic, and optical properties. Heterometallic nanoclusters with precise stoichiometry of Bi₃Cu₄ and Bi₇Cu₁₂ can be stabilized in the scaffold of two‐dimensional metal–organic networks on a Cu(111) surface through on‐surface metallosupramolecular self‐assembly processes. The atomic structures of the nanoclusters were resolved using scanning tunneling microscopy and density functional theory calculations. The nanoclusters feature highly symmetric planar hexagonal shapes and core–shell charge modulation. The clusters are arranged as triangular lattices with a periodicity that can be tuned by choosing molecules of different size. This work shows that on‐surface metallosupramolecular self‐assembly creates unique possibilities for the design and synthesis of multinuclear heterometallic nanoclusters.     

Selective and Sensitive Aqueous‐Phase Detection of 2,4,6‐Trinitrophenol (TNP) by an Amine‐Functionalized Metal–Organic Framework

Highly selective and sensitive aqueous‐phase detection of nitro explosive 2,4,6‐trinitrophenol (TNP) by a hydrolytically stable 3D luminescent metal–organic framework is reported. The compound senses TNP exclusively even in the presence of other nitro‐compounds, with an unprecedented sensitivity in the MOF regime by means of strategic deployment of its free amine groups. Such an accurate sensing of TNP, widely recognized as a harmful environmental contaminant in water media, establishes this new strategic approach as one of the frontiers to tackle present‐day security and health concerns in a real‐time scenario.     

Versatile Tailoring of Paddle‐Wheel ZnII Metal–Organic Frameworks through Single‐Crystal‐to‐Single‐Crystal Transformations

A new tetracarboxylate ligand having short and long arms formed 2D layer Zn^II coordination polymer 1 with paddle‐wheel secondary building units under solvothermal conditions. The framework undergoes solvent‐specific single crystal‐to‐single crystal (SC‐SC) transmetalation to produce 1_Cu . With a sterically encumbered dipyridyl linker, the same ligand forms non‐interpenetrated, 3D, pillared‐layer Zn^II metal–organic framework (MOF) 2 , which takes part in SC‐SC linker‐exchange reactions to produce three daughter frameworks. The parent MOF 2 shows preferential incorporation of the longest linker in competitive linker‐exchange experiments. All the 3D MOFs undergo complete SC‐SC transmetalation with Cu^II, whereby metal exchange in different solvents and monitoring of X‐ray structures revealed that bulky solvated metal ions lead to ordering of the shortest linker in the framework, which confirms that the solvated metal ions enter through the pores along the linker axis.     

A Calixarene‐Based Metal–Organic Framework for Highly Selective NO2 Detection

A calixarene‐based metal–organic framework (Zr‐cal, [Zr₆O₄(OH)₄(FA)₆]₂(cal)₃], FA=formate, cal=1,3‐alt‐25,26,27,28‐tetrakis[(carboxy)methoxy]calixarene) was synthesized and characterized by single‐crystal X‐ray diffraction. The three‐dimensional framework is a 4,6‐connected network of gar topology and exhibits two equal but nonintersecting three‐dimensional pore systems. It has a specific BET surface area of 670 m² g^?1, and the calixarene cavities are accessible through the pore systems. The exposed calixarenes can be used for the visual detection and encapsulation of NO₂ through the formation of deeply colored charge–transfer complexes inside the MOF. The highly selective complexation was analyzed by UV/Vis and IR spectroscopy, and the stability of the material was confirmed by powder X‐ray diffraction and ¹H NMR spectroscopy. Finally, the MOF was used as a sensor material in a home‐made sensor cell and showed high sensitivity for NO₂.     

Theoretical Insights into the Tuning of Metal Binding Sites of Paddlewheels in rht‐Metal–Organic Frameworks

Theoretical investigations of CO₂ sorption are performed in four members of the highly tunable rht‐metal–organic framework (MOF) platform. rht‐MOFs contain two Cu²⁺ ions that comprise the metal paddlewheels and both are in chemically distinct environments. Indeed, one type of Cu²⁺ ion faces toward the center of the linker whereas the other type faces away from the center of the linker. Electronic structure calculations on the series of rht‐MOFs demonstrate that one of the Cu²⁺ ions has a consistently higher charge magnitude relative to the other. As a consequence, the Cu²⁺ ion with the higher partial positive charge acts as the favored sorbate binding site at initial loading as revealed by grand canonical Monte Carlo (GCMC) simulations that include many‐body polarization. It was found that the charge distribution about the copper paddlewheels is dependent on the type of functional groups present on the linker. This study demonstrates how the binding site about the metal paddlewheels in the rht‐MOF platform can be controlled by changing the functionality on the organic ligand.     

Scorpionate‐Type Coordination in MFU‐4l Metal–Organic Frameworks: Small‐Molecule Binding and Activation upon the Thermally Activated Formation of Open Metal Sites

Postsynthetic metal and ligand exchange is a versatile approach towards functionalized MFU‐4l frameworks. Upon thermal treatment of MFU‐4l formates, coordinatively strongly unsaturated metal centers, such as zinc(II) hydride or copper(I) species, are generated selectively. Cu^I‐MFU‐4l prepared in this way was stable under ambient conditions and showed fully reversible chemisorption of small molecules, such as O₂, N₂, and H₂, with corresponding isosteric heats of adsorption of 53, 42, and 32 kJ mol^?1, respectively, as determined by gas‐sorption measurements and confirmed by DFT calculations. Moreover, Cu^I‐MFU‐4l formed stable complexes with C₂H₄ and CO. These complexes were characterized by FTIR spectroscopy. The demonstrated hydride transfer to electrophiles and strong binding of small gas molecules suggests these novel, yet robust, metal–organic frameworks with open metal sites as promising catalytic materials comprising earth‐abundant metal elements.     

Supramolecular Assembly of Interfacial Nanoporous Networks with Simultaneous Expression of Metal–Organic and Organic‐Bonding Motifs

The formation of 2D surface‐confined supramolecular porous networks is scientifically and technologically appealing, notably for hosting guest species and confinement phenomena. In this study, we report a scanning tunneling microscopy (STM) study of the self‐assembly of a tripod molecule specifically equipped with pyridyl functional groups to steer a simultaneous expression of lateral pyridyl–pyridyl interactions and Cu–pyridyl coordination bonds. The assembly protocols yield a new class of porous open assemblies, the formation of which is driven by multiple interactions. The tripod forms a purely porous organic network on Ag(111), phase α, in which the presence of the pyridyl groups is crucial for porosity, as confirmed by molecular dynamics and Monte Carlo simulations. Additional deposition of Cu dramatically alters this scenario. For submonolayer coverage, three different porous phases coexist (i.e., β, γ, and δ). Phases β and γ are chiral and exhibit a simultaneous expression of lateral pyridyl–pyridyl interactions and twofold Cu–pyridyl linkages, whereas phase δ is just stabilized by twofold Cu–pyridyl bonds. An increase in the lateral molecular coverage results in a rise in molecular pressure, which leads to the formation of a new porous phase (ε), only coexisting with phase α and stabilized by a simultaneous expression of lateral pyridyl–pyridyl interactions and threefold Cu–pyridyl bonds. Our results will open new avenues to create complex porous networks on surfaces by exploiting components specifically designed for molecular recognition through multiple interactions.     

Turn‐On Luminescence Sensing and Real‐Time Detection of Traces of Water in Organic Solvents by a Flexible Metal–Organic Framework

The development of efficient sensors for the determination of the water content in organic solvents is highly desirable for a number of chemical industries. Presented herein is a Mg²⁺ metal–organic framework (MOF), which exhibits the remarkable capability to rapidly detect traces of water (0.05–5 % v/v) in various organic solvents through an unusual turn‐on luminescence sensing mechanism. The extraordinary sensitivity and fast response of this MOF for water, and its reusability make it one of the most powerful water sensors known.     

Hybridized Polyoxometalate‐Based Metal–Organic Framework with Ketjenblack for the Nonenzymatic Detection of H2O2

The rational design and development of efficient and affordable enzyme‐free electrocatalysts for electrochemical detection are of great significance for the large‐scale applications of sensor materials, and have aroused increasing research interest. Herein, we report that a typical polyoxometalate (POM)‐based metal–organic framework (NENU5) that was hybridized with ketjenblack (KB) was a highly efficient electrochemical catalyst that could be used for the highly sensitive nonenzymatic detection of H₂O₂. The composite catalyst exhibited superb electrochemical detection performance towards H₂O₂, including a broad linear range from 10–50 mm , a low detection limit of 1.03 μm , and a high sensitivity of 33.77 μA mm ⁻¹, as well as excellent selectivity and stability. These excellent electrocatalytic properties should be attributed to the unique redox activity of the POM, the high specific surface area of the metal–organic framework (MOF), the strong conductivity of KB, and the synergistic effects of the multiple components in the composites during the electrolysis of H₂O₂. This work provides a new pathway for the exploration of nonenzymatic electrochemical sensors.     

Luminescent Metal–Organic Frameworks (MOFs) as a Chemopalette: Tuning the Thermochromic Behavior of Dual‐Emissive Phosphorescence by Adjusting the Supramolecular Microenvironments

Two classical copper(I)‐cluster‐based luminophores, namely, Cu₄I₄ and [Cu₃Pz₃]₂ (Pz=pyrazolate), are immobilized in a supramolecular system through the formation of metal–organic framework (MOF) materials. This series of luminescent MOF materials, namely, [Cu₄I₄(NH₃)Cu₃( L1 )₃]_n, [Cu₄I₄(NH₂CH₃)Cu₃( L1 )₃]_n, and [Cu₄I₄Cu₃( L2 )₃]_n ( L1 =3‐(4‐pyridyl)‐5‐(p‐tolyl)pyrazolate; L2 =3‐(4‐pyridyl)‐5‐(2,4‐dimethylphenyl)pyrazolate), exhibit diverse thermochromism attributed to the relative functioning efficacy of the two coordination luminophores. Such an intriguing chemopalette effect is regulated by the different supramolecular microenvironments between the two‐dimensional layers of these MOFs, and in particular, by the fine‐tuned Cu–Cu distances in the excimeric [Cu₃Pz₃]₂ luminophore. The structure–property elucidation of the thermochromic behavior allows one to understand these optical materials with unusual dual‐emissive properties.     

The Uncommon Channel‐Based Ln‐MOFs for Highly Selective Fe3+ Detection and Superior Rhodamine B Adsorption

Two new isostructural 3D lanthanide–organic frameworks [H₂N(Me)₂] [Ln₃(OH)(bpt)₃(H₂O)₃] (DMF)₂?(H₂O)₄ ( 1‐Ln ; Ln=Sm and Eu) with a 1D channel (25 Å) have been successfully assembled from the rare trinuclear [Ln₃(OH)(COO)₉] clusters and biphenyl‐3,4′,5‐tricarboxylic acid (H₃bpt) and exhibit high stability towards water in the pH range 3–10. MOF 1‐Eu is a promising luminescent probe for sensing Fe³⁺ in aqueous solution and is also selective towards rhodamine B (RhB) with a superior adsorption capacity of 735 mg g^?1, which is the highest among the reported Ln‐MOFs for RhB removal so far. Periodic DFT calculations further confirmed the selective adsorption of rhodamine B over other dyes.     

Simultaneous Presence of Both Open Metal Sites and Free Functional Organic Sites in a Noncentrosymmetric Dynamic Metal–Organic Framework with Bimodal Catalytic and Sensing Activities

Assimilation of open metal sites (OMSs) and free functional organic sites (FOSs) with a framework strut has opened up a new route for the fabrication of novel metal–organic materials, thereby providing a unique opportunity to explore their multiple functionalities. A new metal–organic framework (MOF), {[Cu(ina)₂(H₂O)][Cu(ina)₂(bipy)]?2 H₂O}_n ( 1 ) (ina=isonicotinate, bipy=4,4′‐bipyridine), has been synthesized and characterized. Complex 1 is crystallized in the orthorhombic noncentrosymmetric space group Aba2 and consists of two different 2D coordination polymers, [Cu(ina)₂(H₂O)]_n and [Cu(ina)₂(bipy)]_n, with entrapped solvent water molecules. Hydrogen‐bonding interactions assemble these two different 2D coordination layers in a single‐crystal structure with interdigitation of pendant 4,4′‐bipy from one layer into the groove of another. Upon removal of guest molecules, 1 undergoes a structural transformation in single‐crystal‐to‐single‐crystal fashion with expansion of the effective void space. Each metal center is five‐coordinated and thus can potentially behave as an OMS, and the free pyridyl groups of pendant 4,4′‐bipy moieties and free ? C?O groups can act as free FOSs. Thus, owing to presence of both OMSs and free FOSs, the framework exhibits multifunctional properties. Owing to the presence of OMSs, the framework can act as a Lewis acid catalyst as well as a small‐molecule sensor material, and in a similar way, owing to the presence of free FOSs, it performs as a Lewis base catalyst and a cation sensor material. Furthermore, owing to noncentrosymmetry with large polarity along a particular direction, it shows strong second‐harmonic generation/nonlinear optical (SHG‐NLO) activity.     

Hydrogen Storage in a Potassium‐Ion‐Bound Metal–Organic Framework Incorporating Crown Ether Struts as Specific Cation Binding Sites

To develop a metal–organic framework (MOF) for hydrogen storage, SNU‐200 incorporating a 18‐crown‐6 ether moiety as a specific binding site for selected cations has been synthesized. SNU‐200 binds K⁺, NH₄⁺, and methyl viologen(MV²⁺) through single‐crystal to single‐crystal transformations. It exhibits characteristic gas‐sorption properties depending on the bound cation. SNU‐200 activated with supercritical CO₂ shows a higher isosteric heat (Q_st) of H₂ adsorption (7.70 kJ mol^?1) than other zinc‐based MOFs. Among the cation inclusions, K⁺ is the best for enhancing the isosteric heat of the H₂ adsorption (9.92 kJ mol^?1) as a result of the accessible open metal sites on the K⁺ ion.     

Metal–Organic Frameworks (MOFs) as Safer,Structurally Reinforced Energetics

Second‐generation cobalt and zinc coordination architectures were obtained through efforts to stabilize extremely sensitive and energetic transition‐metal hydrazine perchlorate ionic polymers. Partial ligand substitution by the tridentate hydrazinecarboxylate anion afforded polymeric 2D‐sheet structures never before observed for energetic materials. Carefully balanced reaction conditions allowed the retention of the noncoordinating perchlorate anion in the presence of a strongly chelating hydrazinecarboxylate ligand. High‐quality X‐ray single‐crystal structure determination revealed that the metal coordination preferences lead to different structural motifs and energetic properties, despite the nearly isoformulaic nature of the two compounds. Energetic tests indicate highly decreased sensitivity and DFT calculations suggest a high explosive performance for these remarkable structures.