Complete Transmetalation in a Metal–Organic Framework by Metal Ion Metathesis in a Single Crystal for Selective Sensing of Phosphate Ions in Aqueous Media

A complete transmetalation has been achieved on a barium metal–organic framework (MOF), leading to the isolation of a new Tb‐MOF in a single‐crystal (SC) to single‐crystal (SC) fashion. It leads to the transformation of an anionic framework with cations in the pore to one that is neutral. The mechanistic studies proposed a core–shell metal exchange through dissociation of metal–ligand bonds. This Tb‐MOF exhibits enhanced photoluminescence and acts as a selective sensor for phosphate anion in aqueous medium. Thus, this work not only provides a method to functionalize a MOF that can have potential application in sensing but also elucidates the formation mechanism of the resulting MOF.     

Sequential Transformation of Zirconium(IV)‐MOFs into Heterobimetallic MOFs Bearing Magnetic Anisotropic Cobalt(II) Centers

Heterometallic metal–organic frameworks (MOFs) allow the precise placement of various metals at atomic precision within a porous framework. This new level of control by MOFs promises fascinating advances in basic science and application. However, the rational design and synthesis of heterometallic MOFs remains a challenge due to the complexity of the heterometallic systems. Herein, we show that bimetallic MOFs with MX₂(INA)₄ moieties (INA=isonicotinate; M=Co²⁺ or Fe²⁺; X=OH^?, Cl^?, Br^?, I^?, NCS^?, or NCSe^?) can be generated by the sequential modification of a Zr‐based MOF. This multi‐step modification not only replaced the linear organic linker with a square planar MX₂(INA)₄ unit, but also altered the symmetry, unit cell, and topology of the parent structure. Single‐crystal to single‐crystal transformation is realized so that snapshots for transition process were captured by successive single‐crystal X‐ray diffraction. Furthermore, the installation of Co(NCS)₂(INA)₄ endows field‐induced slow magnetic relaxation property to the diamagnetic Zr‐MOF.     

Giant Enhancement of Second Harmonic Generation Accompanied by the Structural Transformation of 7‐Fold to 8‐Fold Interpenetrated Metal–Organic Frameworks (MOFs)

Interpenetration in metal–organic frameworks (MOFs) is an intriguing phenomenon with significant impacts on their properties, and functional applications. Herein, we show that a 7‐fold interpenetrated MOF ( 1 ) is transformed into an 8‐fold interpenetrated MOF by the loss of DMF in a single‐crystal‐to‐single‐crystal manner. This is accompanied by a giant enhancement of the second harmonic generation (SHG ca. 125 times) and two‐photon photoluminescence (ca. 14 times). The strengthened π–π interaction between the individual diamondoid networks and intensified oscillator strength of the molecules aid the augment of dipole moments and boost the nonlinear optical conversion efficiency. Large positive and negative thermal expansions of 1 occur at 30–150 °C before the loss of DMF. These results offer an avenue to manipulate the NLO properties of MOFs using interpenetration and provide access to tunable single‐crystal NLO devices.     

Formation of a Syndiotactic Organic Polymer Inside a MOF by a [2+2] Photo‐Polymerization Reaction

Getting suitable crystals for single‐crystal X‐ray crystallographic analysis still remains an art. Obtaining single crystals of metal–organic frameworks (MOFs) containing organic polymers poses even greater challenges. Here we demonstrate the formation of a syndiotactic organic polymer ligand inside a MOF by quantitative [2+2] photopolymerization reaction in a single‐crystal‐to‐single‐crystal manner. The spacer ligands with trans,trans,trans‐conformation in the pillared‐layer MOF with guest water molecules in the channels, undergo pedal motion to trans,cis,trans‐conformation prior to [2+2] photo‐cycloaddition reaction and yield single crystals of MOF containing two‐dimensional coordination polymers fused with the organic polymer ligands. We also show that the organic polymer in the single crystals can be depolymerized reversibly by cleaving the cyclobutane rings upon heating. These MOFs also show interesting photoluminescent properties and sensing of small organic molecules.     

A Tale of Copper Coordination Frameworks: Controlled Single‐Crystal‐to‐Single‐Crystal Transformations and Their Catalytic CH Bond Activation Properties

Metal–organic frameworks (MOFs), as a class of microporous materials with well‐defined channels and rich functionalities, hold great promise for various applications. Yet the formation and crystallization processes of various MOFs with distinct topology, connectivity, and properties remain largely unclear, and the control of such processes is rather challenging. Starting from a 0D Cu coordination polyhedron, MOP‐1, we successfully unfolded it to give a new 1D‐MOF by a single‐crystal‐to‐single‐crystal (SCSC) transformation process at room temperature as confirmed by SXRD. We also monitored the continuous transformation states by FTIR and PXRD. Cu MOFs with 2D and 3D networks were also obtained from this 1D‐MOF by SCSC transformations. Furthermore, Cu MOFs with 0D, 1D, and 3D networks, MOP‐1, 1D‐MOF, and HKUST‐1, show unique performances in the kinetics of the C?H bond catalytic oxidation reaction.     

Magnetic porous carbon derived from Co‐doped metal–organic frameworks for the magnetic solid‐phase extraction of endocrine disrupting chemicals

Metal–organic frameworks‐5 (MOF‐5) was explored as a template to prepare porous carbon due to its high surface area, large pore volume, and permanent nanoscale porosity. Magnetic porous carbon, Co@MOF‐5‐C, was fabricated by the one‐step direct carbonization of Co‐doped MOF‐5. After carbonization, the magnetic cobalt nanoparticles are well dispersed in the porous carbon matrix, and Co@MOF‐5‐C displays strong magnetism (with the saturation magnetization intensity of 70.17emu/g), high‐specific surface area, and large pore volume. To evaluate its extraction performance, the Co@MOF‐5‐C was applied as an adsorbent for the magnetic solid‐phase extraction of endocrine disrupting chemicals, followed by their analysis with high‐performance liquid chromatography. The developed method exhibits a good linear response in the range of 0.5–100 ng/mL for pond water and 1.0–100 ng/mL for juice samples. The limits of detection (S/N  = 3) for the analytes were in the range of 0.1–0.2 ng/mL.     

Chemical Patterning in Single Crystals of Metal–Organic Frameworks by [2+2] Cycloaddition Reaction

Conventional chemical patterning involves films of polymeric materials. Herein, we demonstrate that the presence or absence of guest solvents in the crystal voids modulates the patterning of the cyclobutane rings in highly strained metal–organic frameworks (MOFs) under UV light. The olefin pairs of the spacer ligands, which resemble a ladder‐like structure, in the MOF, undergo a [2+2] cycloaddition reaction in a single‐crystal‐to‐single‐crystal manner. For instance, a partial photoreaction in the MOFs in the absence of a guest solvent as well as with dimethylacetamide in the voids generated two different patterns of the cyclobutane rings. Surprisingly, the MOF with the lattice dimethylformamide undergoes 100 % photoreaction, but the photoproduct contains broken chains. Such chemical patterning at the molecular level represents a next step in crystal engineering.     

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‐DOBDC^4?=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.     

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.     

Single Crystal‐to‐Single Crystal Site‐Selective Postsynthetic Metal Exchange in a Zn–MOF Based on Semi‐Rigid Tricarboxylic Acid and Access to Bimetallic MOFs

The metal ions in a neutral Zn–MOF constructed from tritopic triacid H₃L with inherent concave features, rigid core, and peripheral flexibility are found to exist in two distinct SBUs, that is, 0D and 1D. This has allowed site‐selective postsynthetic metal exchange (PSME) to be investigated and reactivities of the metal ions in two different environments in coordination polymers to be contrasted for the first time. Site‐selective transmetalation of Zn ions in the discrete environment is shown to occur in a single crystal‐to‐single crystal (SCSC) fashion, with metal ions such as Fe³⁺, Ru³⁺, Cu²⁺, Co²⁺, etc., whereas those that are part of 1D SBU sustain structural integrity, leading to novel bimetallic MOFs, which are inaccessible by conventional approaches. To the best of our knowledge, site‐selective postsynthetic exchange of an intraframework metal ion in a MOF that contains metal ions in discrete as well as polymeric SBUs is heretofore unprecedented.     

Host–Guest [2+2] Cycloaddition Reaction: Postsynthetic Modulation of CO2 Selectivity and Magnetic Properties in a Bimodal Metal–Organic Framework

Simultaneous tuning of permanent porosity and modulation of magnetic properties by postsynthetic modification (PSM) with light in a metal–organic framework is unprecedented. With the aim of achieving such a photoresponsive porous magnetic material, a 3D photoresponsive biporous framework, MOF1, which has 2D channels occupied by the guest 1,2‐bis(4‐pyridyl)ethylene (bpee), H₂O, and EtOH molecules, has been synthesized. The guest bpee in 1 is aligned parallel to pillared bpee with a distance of 3.9 Å between the ethylenic groups; this allows photoinduced PSM of the pore surface through a [2+2] cycloaddition reaction to yield MOF2. Such photoinduced PSM of the framework structure introduces enhanced CO₂ selectivity over that of N₂. The higher selectivity in MOF2 than that of MOF1 is studied through theoretical calculations. Moreover, MOF2 unveils reversible changes in T_c with response to dehydration–rehydration. This result demonstrates that photoinduced PSM is a powerful tool for fabricating novel functional materials.     

Crystal Dynamics in Multi‐stimuli‐Responsive Entangled Metal–Organic Frameworks

An understanding of solid‐state crystal dynamics or flexibility in metal–organic frameworks (MOFs) showing multiple structural changes is highly demanding for the design of materials with potential applications in sensing and recognition. However, entangled MOFs showing such flexible behavior pose a great challenge in terms of extracting information on their dynamics because of their poor single‐crystallinity. In this article, detailed experimental studies on a twofold entangled MOF ( f‐MOF‐1) are reported, which unveil its structural response toward external stimuli such as temperature, pressure, and guest molecules. The crystallographic study shows multiple structural changes in f‐MOF‐1 , by which the 3 D net deforms and slides upon guest removal. Two distinct desolvated phases, that is, f‐MOF‐1 a and f‐MOF‐1 b , could be isolated; the former is a metastable one and transformable to the latter phase upon heating. The two phases show different gated CO₂ adsorption profiles. DFT‐based calculations provide an insight into the selective and gated adsorption behavior with CO₂ of f‐MOF‐1 b . The gate‐opening threshold pressure of CO₂ adsorption can be tuned strategically by changing the chemical functionality of the linker from ethanylene (?CH₂?CH₂?) in f‐MOF‐1 to an azo (?N=N?) functionality in an analogous MOF, f‐MOF‐2 . The modulation of functionality has an indirect influence on the gate‐opening pressure owing to the difference in inter‐net interaction. The framework of f‐MOF‐1 is highly responsive toward CO₂ gas molecules, and these results are supported by DFT calculations.     

Highly Selective Water Adsorption in a Lanthanum Metal–Organic Framework

We present a new metal–organic framework (MOF) built from lanthanum and pyrazine‐2,5‐dicarboxylate (pyzdc) ions. This MOF, [La(pyzdc)_1.5(H₂O)₂] ? 2 H₂O, is microporous, with 1D channels that easily accommodate water molecules. Its framework is highly robust to dehydration/hydration cycles. Unusually for a MOF, it also features a high hydrothermal stability. This makes it an ideal candidate for air drying as well as for separating water/alcohol mixtures. The ability of the activated MOF to adsorb water selectively was evaluated by means of thermogravimetric analysis, powder and single‐crystal X‐ray diffraction and adsorption studies, indicating a maximum uptake of 1.2 mmol g^?1 MOF. These results are in agreement with the microporous structure, which permits only water molecules to enter the channels (alcohols, including methanol, are simply too large). Transient breakthrough simulations using water/methanol mixtures confirm that such mixtures can be separated cleanly using this new MOF.     

A novel biosensor for early diagnosis of liver cancer cases using smart nano‐magnetic metal–organic framework

For the first time, alpha‐fetoprotein (AFP) quantification as an early test for liver cancer diagnosis competing with other interfering analytes and factors based on a novel nano‐magnetic metal–organic framework (MOF) was investigated. The novel nano‐magnetic MOF was synthesized via a simple preparation method, and characterized using various techniques. The X‐ray photoelectron spectrum of the nano‐magnetic MOF shows Cu 2p, O 1s, N 1s and C 1s peaks that evidence the presence of these elements in the sample without any impurities. Scanning and transmission electron microscopy images show the nano‐magnetic MOF with average size between 22 and 43 nm. The results of a photoluminescence study show that the nano‐magnetic MOF exhibits strong emission at 418 nm after excitation at 354 nm. Moreover, a Stern–Völmer graph shows a linear calibration curve over an AFP concentration range 1.0–520 ng ml^?1 with a correlation coefficient of 0.997, detection limit of 1.18 ng ml^?1 and quantitation limit of 3.58 ng ml^?1. According to the marked quenching of the photoluminescence intensity of the nano‐magnetic MOF using various concentrations of AFP, it was successfully used as a biosensor for AFP in serum samples collected from hepatitis patients in addition to healthy males and females. The quenching mechanism was well studied, and found to be a dynamic type. The present work offers a simple, low‐cost, room‐temperature and rapid technique, being non‐time‐consuming, highly accurate, selective and highly sensitive compared with most published methods.     

Multi‐shelled Hollow Metal–Organic Frameworks

Hollow metal–organic frameworks (MOFs) are promising materials with sophisticated structures, such as multiple shells, that cannot only enhance the properties of MOFs but also endow them with new functions. Herein, we show a rational strategy to fabricate multi‐shelled hollow chromium (III) terephthalate MOFs (MIL‐101) with single‐crystalline shells through step‐by‐step crystal growth and subsequent etching processes. This strategy relies on the creation of inhomogeneous MOF crystals in which the outer layer is chemically more robust than the inner layer and can be selectively etched by acetic acid. The regulation of MOF nucleation and crystallization allows the tailoring of the cavity size and shell thickness of each layer. The resultant multi‐shelled hollow MIL‐101 crystals show significantly enhanced catalytic activity during styrene oxidation. The insight gained from this systematic study will aid in the rational design and synthesis of other multi‐shelled hollow structures and the further expansion of their applications.     

Adsorption behaviour of a CdII–triazole MOF for butan‐2‐one in a single‐crystal‐to‐single‐crystal (SCSC) fashion: the role of hydrogen bonding and C—H…π interactions

The adsorption behaviour of the Cd^II–MOF {[Cd(L)₂(ClO₄)₂]·H₂O ( 1 ), where L is 4‐amino‐3,5‐bis[3‐(pyridin‐4‐yl)phenyl]‐1,2,4‐triazole, for butan‐2‐one was investigated in a single‐crystal‐to‐single‐crystal (SCSC) fashion. A new host–guest system that encapsulated butan‐2‐one molecules, namely poly[[bis{μ₃‐4‐amino‐3,5‐bis[3‐(pyridin‐4‐yl)phenyl]‐1,2,4‐triazole}cadmium(II)] bis(perchlorate) butanone sesquisolvate], {[Cd(C₂₄H₁₈N₆)₂](ClO₄)₂·1.5C₄H₈O}_n, denoted C₄H₈O@Cd‐MOF ( 2 ), was obtained via an SCSC transformation. MOF 2 crystallizes in the tetragonal space group P4₃2₁2. The specific binding sites for butan‐2‐one in the host were determined by single‐crystal X‐ray diffraction studies. N—H…O and C—H…O hydrogen‐bonding interactions and C—H…π interactions between the framework, ClO₄^? anions and guest molecules co‐operatively bind 1.5 butan‐2‐one molecules within the channels. The adsorption behaviour was further evidenced by ¹H NMR, IR, TGA and powder X‐ray diffraction experiments, which are consistent with the single‐crystal X‐ray analysis. A ¹H NMR experiment demonstrates that the supramolecular interactions between the framework, ClO₄^? anions and guest molecules in MOF 2 lead to a high butan‐2‐one uptake in the channel.     

In situ Synchrotron IR Microspectroscopy of CO2 Adsorption on Single Crystals of the Functionalized MOF Sc2(BDC‐NH2)3

Synchrotron radiation (SR) IR microspectroscopy has enabled determination of the thermodynamics, kinetics, and molecular orientation of CO₂ adsorbed in single microcrystals of a functionalized metal–organic framework (MOF) under conditions relevant to carbon capture from flue gases. Single crystals of the small‐pore MOF, Sc₂(BDC‐NH₂)₃, (BDC‐NH₂=2‐amino‐1,4‐benzenedicarboxylate), with well‐defined crystal form have been investigated during CO₂ uptake at partial pressures of 0.025‐0.2 bar at 298–373 K. The enthalpy and diffusivity of adsorption determined from individual single crystals are consistent with values obtained from measurements on bulk samples. The brilliant SR IR source permits rapid collection of polarized spectra. Strong variations in absorbance of the symmetric stretch of the NH₂ groups of the MOF and the asymmetric stretch of the adsorbed CO₂ at different orientations of the crystals relative to the polarized IR light show that CO₂ molecules align along channels in the MOF.     

Switchable Surface Hydrophobicity–Hydrophilicity of a Metal–Organic Framework

Materials with surfaces that can be switched from high/superhydrophobicity to superhydrophilicity are useful for myriad applications. Herein, we report a metal–organic framework (MOF) assembled from Zn^II ions, 1,4‐benzenedicarboxylate, and a hydrophobic carborane‐based linker. The MOF crystal‐surface can be switched between hydrophobic and superhydrophilic through a chemical treatment to remove some of the building blocks.     

A Metal–Organic Framework Based Methane Nano‐trap for the Capture of Coal‐Mine Methane

As a major greenhouse gas, methane, which is directly vented from the coal‐mine to the atmosphere, has not yet drawn sufficient attention. To address this problem, we report a methane nano‐trap that features oppositely adjacent open metal sites and dense alkyl groups in a metal–organic framework (MOF). The alkyl MOF‐based methane nano‐trap exhibits a record‐high methane uptake and CH₄/N₂ selectivity at 298 K and 1 bar. The methane molecules trapped within the alkyl MOF were crystalographically identified by single‐crystal X‐ray diffraction experiments, which in combination with molecular simulation studies unveiled the methane adsorption mechanism within the MOF‐based nano‐trap. The IAST calculations and the breakthrough experiments revealed that the alkyl MOF‐based methane nano‐trap is a new benchmark for CH₄/N₂ separation, thereby providing a new perspective for capturing methane from coal‐mine methane to recover fuel and reduce greenhouse gas emissions.