Covalent Post-Synthetic Modification of Metal-Organic Cages: Concepts and Recent Progress.

Metal-organic cages (MOCs) are supramolecular coordination complexes that have internal cavities for hosting guest molecules and exhibiting various properties. However, the functions of MOCs are limited by the choice of the building blocks. Post-synthetic modification (PSM) is a technique that can introduce new functional groups and replace existing ones on the MOCs without changing their geometry. Among many PSM methods, covalent PSM is a promising approach to modify MOCs with tailored structures and functions. Covalent PSM can be applied to either the internal cavity or the external surface of the MOCs, depending on the functionality expected to be customized. However, there are still some challenges and limitations in the field of covalent PSM of MOCs, such as the balance between the stability of MOCs and the harshness of organic reactions involved in covalent PSMs. This concept article introduces the organic reaction types involved in covalent PSM of MOCs, their new applications after modification, and summarizes and provides an outlook of this research field.

Precise Post-Synthetic Modification of Heterometal-Organic Capsules for Selectively Encapsulating Tetrahedral Anions.

Post-synthetic modification plays a crucial role in precisely adjusting the structure and functions of advanced materials. Herein, we report the self-assembly of a tubular heterometallic Pd3Cu6L16 capsule that incorporates Pd(II) and CuL1 metalloligands. This capsule undergoes further modification with two tridentate anionic ligands (L2) to afford a bicapped Pd3Cu6L16L22 capsule with an Edshammer polyhedral structure. By employing transition metal ions, acid, and oxidation agents, the bicapped capsule can be converted into an uncapped one. This uncapped form can then revert back to the bicapped structure on the addition of Br- ions and a base. Interestingly, introducing Ag+ ions leads to the removal of one L2 ligand from the bicapped capsule, yielding a mono-capped Pd3Cu6L16L2 structure. Furthermore, the size of the anions critically influences the precise control over the post-synthetic modifications of the capsules. It was demonstrated that these capsules selectively encapsulate tetrahedral anions, offering a novel approach for the design of intelligent molecular delivery systems.

Facile Fabrication of Hollow Nanoporous Carbon Architectures by Controlling MOF Crystalline Inhomogeneity for Ultra-Stable Na-Ion Storage.

Hollow nanoporous carbon architectures (HNCs) present significant utilitarian value for a wide variety of applications. Facile and efficient preparation of HNCs has long been pursued but still remains challenging. Herein, we for the first time demonstrate that single-component metal-organic frameworks (MOFs) crystals, rather than the widely reported hybrid ones which necessitate tedious operations for preparation, could enable the facile and versatile syntheses of functional HNCs. By controlling the growth kinetics, the MOFs crystals (STU-1) are readily engineered into different shapes with designated styles of crystalline inhomogeneity. A subsequent one-step pyrolysis of these MOFs with intraparticle difference can induce a simultaneous self-hollowing and carbonization process, thereby producing various functional HNCs including yolk-shell polyhedrons, hollow microspheres, mesoporous architectures, and superstructures. Superior to the existing methods, this synthetic strategy relies only on the complex nature of single-component MOFs crystals without involving tedious operations like coating, etching, or ligand exchange, making it convenient, efficient, and easy to scale up. An ultra-stable Na-ion battery anode is demonstrated by the HNCs with extraordinary cyclability (93 % capacity retention over 8000 cycles), highlighting a high level of functionality of the HNCs.

Ultrafast Photocatalytic Detoxification of Mustard Gas Simulants by a Mesoporous Metal-Organic Framework with Dangling Porphyrin Molecules.

Developing effective catalysts to degrade chemical warfare agents is of great significance. Herein, a mesoporous MIL-101(Cr) composite material dangled with porphyrin molecules (denote as TCPP@MIL-101(Cr), TCPP = tetra(4-carboxyphenyl)porphyrin) is reported, which can be used as a heterogeneous photocatalyst for detoxification of mustard gas simulants 2-chloroethyl ethyl sulfide (CEES) to 2-chloroethyl ethyl sulfoxide (CEESO) with a half-life of 1 min. The catalytic performance of TCPP@MIL-101(Cr) is comparable to that of homogeneous molecular porphyrin. Mechanistic studies reveal that both 1 O2 and O2 •- are efficiently generated and play vital roles in the oxidation reaction. Gold nanoparticles (AuNPs) are attached to the TCPP@MIL-101(Cr) to further enhance the catalytic activity with a benchmark half-life of 45 s, which is the fastest record so far. A medical mask loaded TCPP@MIL-101(Cr) is fabricated for practical applications, which can selectively photoxidize CEES to CEESO under sunlight and air atmosphere, exhibiting the best degradation performance among the reported fabric-like composite materials.

Self-Assembly of Two Tubular Metalloligand-Based Palladium-Organic Cages as Hosts for Polycyclic Aromatic Hydrocarbons.

Herein we report two tubular metal-organic cages (MOCs), synthesized by the self-assembly of bidentate metalloligands with different lengths and PdII. These two MOCs feature Pd4L8-type square tubular and Pd3L6-type triangular cage structures, respectively. Both MOCs have been fully characterized by NMR spectroscopy, mass spectrometry, and theoretical calculation. Both cages can be employed for encapsulating polycyclic aromatic hydrocarbons and show high binding affinity toward coronene.

Tailor-Made Pdn L2n Metal-Organic Cages through Covalent Post-Synthetic Modification.

Post-synthetic modification (PSM) is an effective approach for the tailored functionalization of metal-organic architectures, but its generalizability remains challenging. Herein we report a general covalent PSM strategy to functionalize Pdn L2n metal-organic cages (MOCs, n=2, 12) through an efficient Diels-Alder cycloaddition between peripheral anthracene substituents and various functional motifs bearing a maleimide group. As expected, the solubility of functionalized Pd12 L24 in common solvents can be greatly improved. Interestingly, concentration-dependent circular dichroism and aggregation-induced emission are achieved with chiral binaphthol (BINOL)- and tetraphenylethylene-modified Pd12 L24 , respectively. Furthermore, Pd12 L24 can be introduced with two different functional groups (e.g., chiral BINOL and achiral pyrene) through a step-by-step PSM route to obtain chirality-induced circularly polarized luminescence. Moreover, similar results are readily observed with a smaller Pd2 L4 system.

Achiral Au(I) Cyclic Trinuclear Complexes with High-Efficiency Circularly Polarized Near-Infrared TADF.

Realizing high photoluminescence quantum yield (PLQY) in the near-infrared (NIR) region is challenging and valuable for luminescent material, especially for thermally activated delay fluorescence (TADF) material. In this work, we report two achiral cyclic trinuclear Au(I) complexes, Au3 (4-Clpyrazolate)3 and Au3 (4-Brpyrazolate)3 (denoted as Cl-Au and Br-Au), obtained through the reaction of 4-chloro-1H-pyrazole and 4-bromo-1H-pyrazole with Au(I) salts, respectively. Both Cl-Au and Br-Au exhibit TADF with high PLQY (>70 %) in the NIR I (700-900 nm) (λmax = 720 nm) region, exceeding other NIR-TADF emitters in the solid state. Photophysical experiments and theoretical calculations confirmed the efficient NIR-TADF properties of Cl-Au and Br-Au were attributed to the small energy gap ΔE(S1-T2) (S = singlet, T = triplet) and the large spin-orbital coupling induced by ligand-to-metal-metal charge transfer of molecular aggregations. In addition, both complexes crystallize in the achiral Pna21 space group (mm2 point group) and are circularly polarized light (CPL) active with maxima luminescent dissymmetry factor |glum | of 3.4 × 10-3 (Cl-Au) and 2.7 × 10-3 (Br-Au) for their crystalline powder samples, respectively. By using Cl-Au as the emitting ink, 3D-printed luminescent logos are fabricated, which own anti-counterfeiting functions due to its CPL behavior dependent on the crystallinity.

A Self-Assembled Capsule for Propylene/Propane Separation.

The development of energy-saving technology for the efficient separation of olefin and paraffin is highly important for the chemical industry. Herein, we report a self-assembled Fe4 L6 capsule containing a hydrophobic cavity, which can be used to encapsulate and separate propylene/propane. The successful encapsulation of propylene and propane by the Fe4 L6 cage in a water solution was documented by NMR spectroscopy. The binding constants K for the Fe4 L6 cage toward propylene and propane were determined to be (5.0±0.1)×103  M-1 and (2.1±0.7)×104  M-1 in D2 O at 25 °C, respectively. Experiments and theoretical studies revealed that the cage exhibited multiple weak interactions with propylene and propane. The polymer-grade propylene (>99.5 %) can be obtained from a mixture of propylene and propane by using the Fe4 L6 cage as a separation material in a U-shaped glass tube. This work provides a new strategy for the separation of olefin/paraffin.

Increasing the Stability of Metal-Organic Frameworks by Coating with Poly(tetrafluoroethylene).

When compared to industrially stable zeolites, the instability of metal-organic frameworks (MOFs) has been denounced by researchers. Boosting the stability of existing MOFs is highly important for practical applications. In this report, we develop a new strategy to prepare MOFs/poly(tetrafluoroethylene) (PTFE) composites, which can highly improve the chemical, pressure, and photostabilities of zeolitic imidazolate framework (ZIF)-8. Composite materials were prepared by a physical blending of ZIF-8 and PTFE emulsion with different ratios and annealing at 370 °C. Transmission electron microscopy (TEM) studies reveal that the nanoparticles of ZIF-8 are coated by PTFE to form the composite materials. Upon mixing with 20 or 50 wt % PTFE, the ZIF-8/PTFE materials show a superhydrophobic property with water contact angles of around 156°. Pristine ZIF-8 is not stable in water with stirring under acidic, basic, and irradiation conditions, while the ZIF-8/PTFE materials are stable under the same conditions. The ZIF-8/PTFE materials can also maintain their crystalline structure after being compressed with a 10 MPa pressure, while pristine ZIF-8 changes to an amorphous solid after the same pressure treatment. Using water as a solvent, ZIF-8/PTFE can be used as a highly efficient and recyclable catalyst for Knoevenagel reaction at room temperature. The successful preparation of stable ZIF-8/PTFE composite materials provides a useful method to enhance the chemical, pressure, and photostabilities of MOFs.

Self-Assembly of a Pd4Cu8L8 Cage for Epoxidation of Styrene and Its Derivatives.

Herein we report a discrete heterometallic Pd4Cu8L8 cage with a tubular structure, which was synthesized by the assembly of copper metalloligands and PdII ions in a stepwise manner. The Pd4Cu8L8 cage has been unequivocally characterized by single-crystal X-ray diffraction, electrospray ionization-mass spectroscopy, and energy dispersive spectroscopy. The cage showed excellent catalytic activity in the epoxidation of styrene and its derivatives under conditions without using additional solvent, providing potential material for catalyzing the oxidation reactions.

Building a Pyrazole-Benzothiadiazole-Pyrazole Photosensitizer into Metal-Organic Frameworks for Photocatalytic Aerobic Oxidation.

Charge separation plays a crucial role in regulating photochemical properties and therefore warrants consideration in designing photocatalysts. Metal-organic frameworks (MOFs) are emerging as promising candidates for heterogeneous photocatalysis due to their structural designability and tunability of photon absorption. Herein, we report the design of a pyrazole-benzothiadiazole-pyrazole organic molecule bearing a donor-acceptor-donor conjugated π-system for fast charge separation. Further attempts to integrate such a photosensitizer into MOFs afford a more effective heterogeneous photocatalyst (JNU-204). Under visible-light irradiation, three aerobic oxidation reactions involving different oxygenation pathways were achieved on JNU-204. Recycling experiments were conducted to demonstrate the stability and reusability of JNU-204 as a robust heterogeneous photocatalyst. Furthermore, we illustrate its applications in the facile synthesis of pyrrolo[2,1-a]isoquinoline-containing heterocycles, core skeletons of a family of marine natural products. JNU-204 is an exemplary MOF platform with good photon absorption, suitable band gap, fast charge separation, and extraordinary chemical stability for proceeding with aerobic oxidation reactions under visible-light irradiation.

Iron(II) Metal-Organic Framework with unh Topology and Tetrazole-Padded Helical Channels.

A unique metal-organic framework (MOF) with tetrazole-padded helical channels has been successfully synthesized in one pot from iron(II) trifluoromethanesulfonate, 4-formylimidazole, hydrazine, and sodium azide under solvothermal conditions and features a rare unh topology and porous structure for gas adsorption. Transformations of condensation, cycloaddition, and coordination occurred during the synthetic process, in which a 1,5-disubstituted tetrazole ligand was formed in situ.

Heterometal-Organic Cages as Photo-Fenton-like Catalysts.

Metal-organic cages, a class of supramolecular containers constructed by the self-assembly of metal ions and organic ligands, show great promise as catalytic agents. In this work, we designed and synthesized a series of rhombic dodecahedral Ni-Cu heterometal imidazolate cages (Ni8Cu6L24) that can act as highly active photo-Fenton-like catalysts. These cages possess a high ability to generate hydroxyl radicals (•OH) under visible light in the presence of H2O2, which can rapidly degrade organic pollutants (e.g., rhodamine B, methylene blue, and methyl orange) into CO2 and H2O. Besides, they are robust catalysts, with high catalytic activity and reusability under conditions in high H2O2 concentration, providing potentially advanced materials for degrading persistent organic pollutants.

Cobalt-Based Metal-Organic Cages for Visible-Light-Driven Water Oxidation.

Water oxidation to molecular oxygen is indispensable but a challenge for splitting H2O. In this work, a series of Co-based metal-organic cages (MOCs) for photoinduced water oxidation were prepared. MOC-1 with both bis(µ-oxo) bridged dicobalt and Co-O (O from H2O) displays catalytic activity with an initial oxygen evolution rate of 80.4 mmol/g/h and a TOF of 7.49 × 10-3 s-1 in 10 min. In contrast, MOC-2 containing only Co-O (O from H2O) in the structure results in a lower oxygen evolution rate (40.8 mmol/g/h, 4.78 × 10-3 s-1), while the amount of oxygen evolved from the solution of MOC-4 without both active sites is undetectable. Isotope experiments with or without H218O as the reactant successfully demonstrate that the molecular oxygen was produced from water oxidation. Photophysical and electrochemical studies reveal that photoinduced water oxidation initializes via electron transfer from the excited [Ru(bpy)3]2+* to Na2S2O8, and then, the cobalt active sites further donate electrons to the oxidized [Ru(bpy)3]3+ to drive water oxidation. This proof-of-concept study indicates that MOCs can work as potential efficient catalysts for photoinduced water oxidation.

Chiral 3D Coordination Polymers Consisting of Achiral Terpyridyl Precursors: from Spontaneous Resolution to Enantioenriched Induction.

A pair of enantiomers of three-dimensional (3D) chiral coordination polymers (CCPs) were successfully constructed by using achiral components 4,2':6',4''-terpyridyl precursors and Cu2+ through spontaneous resolution (1 a). By utilising feeding controlled chiral-templated induction and chiral auxiliary behaviour of optically pure camphor sulfonate (CSA), the enantioenriched (1 b-P and 1 b-M) and CSA captured (1 c-P and 1 c-M) CCPs were successfully synthesised, respectively. The chiral information of the corresponding products was confirmed by X-ray single crystal diffraction and solid-state CD spectra. Meanwhile, the formation processes of 1 b-P and 1 b-M were monitored through solution CD spectra, UV/Vis spectra and ESI-TOF MS. Based on these results, a reasonable chiral-templated induction mechanism of forming 1 b-P and 1 b-M was proposed.

Self-assembly of a Mixed Valence Copper Triangular Prism and Transformation to Cage Triggered by an External Stimulus.

A triangular prismatic metal-organic cage based on mixed valence copper ions has been designed and synthesized by using metallocycle panels and pillar ligands. The triangular prism will be quickly transformed to a 10-nuclear cage upon an external chemical stimulus, which features a bicapped square antiprism structure. This prismatic cage can act as a catalyst for oxidation of aromatic alcohols to their corresponding aromatic aldehydes with high yields at room temperature under O2 atmosphere.

Fine-Tuning Apertures of Metal-Organic Cages: Encapsulation of Carbon Dioxide in Solution and Solid State.

Metal-organic cages are potential artificial models for mimicking biological functions due to their capability of selective encapsulation for certain guest molecules. In this work, we designed and synthesized a series of rhombic dodecahedral Ni-imidazolate cages (Ni14L24) with precisely controlled aperture for CO2 encapsulation. The aperture of the cages can be tuned by the strategies of ligand decoration and metal-ion hybridization. Similar to the breathing function of alveoli, CO2 passes through the dynamic aperture into the cages under a pressure of 2.0-3.0 bar in methanol solution, and slowly move out of the cages when the pressure goes down. In the solid state, CO2 is encapsulated and prisoned in the cages under a high pressure of 15.0-30.0 bar or supercritical conditions. By replacing the square-coordinated Ni2+ with Cu2+, the resulting Ni-Cu heteronuclear cage lost the capability of physically encapsulating CO2 even though the aperture's size is only slightly changed.

Assembly of Metal-Organic Frameworks of SiF62- in Situ Formed from Borosilicate Glass.

The SiF62- anions are in situ formed in the reactions of MF2 (M = Cu2+, Zn2+, Ni2+, and Co2+) salts and nitrogen-containing ligands in borosilicate glass tubes under solvothermal conditions and then used to further construct a family of metal-organic frameworks (MOFs). This in situ reaction demonstrates a new and facile strategy for the fabrication of MOFs based on SiF62-.

Self-Assembly of Chiral Metal-Organic Tetartoid.

Chiral coordination cages feature both chirality and defined inner space, providing advanced molecular materials. A series of chiral 20-nucleus cobalt-imidazolate cages were synthesized by self-assembly of 72 subcomponents, featuring a novel tetartoid (tetragonal pentagonal dodecahedron) structure. Spontaneous resolution of racemic tetartoidal cages (Δ and Λ) into a conglomerate of homochiral crystals are observed, while both homochiral Δ and Λ tetartoidal cages can be obtained through chiral induction of (d)- and (l)-enantiomers of menthol, respectively. The 2-methyl substituent on imidazolyl is critical to the formation of a tetartoidal cage, and the absence of such steric effect will switch the final structure to a cubic cage.

Boosting Luminescence of Planar-Fluorophore-Tagged Metal-Organic Cages Via Weak Supramolecular Interactions.

A variety of planar organic fluorophores (R=phenyl, naphthalenyl, pyrenyl) and luminescence promotors (X=Cl, Br, I) were equipped, respectively, on the alternatively arranged vertices of cubic ZnII -imidazolate cages through orthogonal subcomponent self-assembly. It was found that supramolecular interactions, especially weak C-H⋅⋅⋅X interactions, play a key role in activating an efficient radiative pathway, coupled with reduced nonradiative decay rate through metal coordination, therefore significantly boosting the emission quantum yields of the system. This finding provides a strategy that utilizes molecular geometry and supramolecular interactions to modulate the emission efficiency of luminescent cage-based materials.