Metal-organic Cages Research Articles

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.

Modeling Kinetics and Thermodynamics of Guest Encapsulation into the [M4L6]12- Supramolecular Organometallic Cage.

The encapsulation of molecular guests into supramolecular hosts is a complex molecular recognition process in which the guest displaces the solvent from the host cavity, while the host deforms to let the guest in. An atomistic description of the association would provide valuable insights on the physicochemical properties that guide it. This understanding may be used to design novel host assemblies with improved properties (i.e., affinities) toward a given class of guests. Molecular simulations may be conveniently used to model the association processes. It is thus of interest to establish efficient protocols to trace the encapsulation process and to predict the associated magnitudes ΔGbind and ΔGbind⧧. Here, we report the calculation of the Gibbs energy barrier and Gibbs binding energy by means of explicit solvent molecular simulations for the [Ga4L6]12– metallocage encapsulating a series of cationic molecules. The ΔGbind⧧ for encapsulation was estimated by means of umbrella sampling simulations. The steps involved were identified, including ion-pair formation and naphthalene rotation (from L ligands of the metallocage) during the guest’s entrance. The ΔGbind values were computed using the attach–pull–release method. The results reveal the sensitivity of the estimates on the force field parameters, in particular on atomic charges, showing that higher accuracy is obtained when charges are derived from implicit solvent quantum chemical calculations. Correlation analysis identified some indicators for the binding affinity trends. All computed magnitudes are in very good agreement with experimental observations. This work provides, on one side, a benchmarked way to computationally model a highly charged metallocage encapsulation process. This includes a nonstandard parameterization and charge derivation procedure. On the other hand, it gives specific mechanistic information on the binding processes of [Ga4L6]12– at the molecular level where key motions are depicted. Taken together, the study provides an interesting option for the future design of metal–organic cages.

Guest‐Induced Transformations in Metal‐Organic Cages

AbstractStimuli‐responsive structural transformations are common in artificial coordination assemblies and can help expand the engineering of programmable functional materials. In particular, chemical guests have proven to act as stimuli and signals inducing structural interconversions in Metal‐Organic Cages (MOCs). In addition to cages, other related classes of coordination assemblies can express structural reorganization upon chemical binding to produce new MOC constructs, many of which do not form otherwise, and exhibit properties and functions not available before the transformation. This review highlights recent examples of such a class of guest‐induced transformations. The connection between changes and host–guest interactions is examined for both neutral and anionic guests, pointing out that in‐situ formed molecular cargoes may also exert conversions. Cases of mixtures of MOCs that redistribute upon encapsulation of complementary species are also featured. This review thus aims to provide an update and new methodological insights into guest‐responsive MOCs for enhancing their design and applications.

A host\u2013guest semibiological photosynthesis system coupling artificial and natural enzymes for solar alcohol splitting

Development of a versatile, sustainable and efficient photosynthesis system that integrates intricate catalytic networks and energy modules at the same location is of considerable future value to energy transformation. In the present study, we develop a coenzyme-mediated supramolecular host-guest semibiological system that combines artificial and enzymatic catalysis for photocatalytic hydrogen evolution from alcohol dehydrogenation. This approach involves modification of the microenvironment of a dithiolene-embedded metal-organic cage to trap an organic dye and NADH molecule simultaneously, serving as a hydrogenase analogue to induce effective proton reduction inside the artificial host. This abiotic photocatalytic system is further embedded into the pocket of the alcohol dehydrogenase to couple enzymatic alcohol dehydrogenation. This host-guest approach allows in situ regeneration of NAD+/NADH couple to transfer protons and electrons between the two catalytic cycles, thereby paving a unique avenue for a synergic combination of abiotic and biotic synthetic sequences for photocatalytic fuel and chemical transformation.

Endohedral functionalization of chiral metal-organic cages for encapsulating achiral dyes to induce circularly polarized luminescence

<h2>Summary</h2> Circularly polarized luminescence (CPL) is useful for 3D optical imaging and optical storage, but it remains a big challenge to prepare chiral compounds with both strong emission and high-dissymmetry factor (<i>g</i>_lum). Here, we demonstrate that encapsulation of achiral dyes into hollow cages can inhibit the aggregation-caused quenching (ACQ) behavior and promote CPL induction and amplification. This leads to the synthesis of two chiral hexahedral Pd₆<b>L</b>₁₂ cages with crown ether or methoxymethyl moieties. Complexation of K⁺ with crown ethers allows inclusion of anionic achiral boron-dipyrromethene (BODIPY) dyes into the cage cavities to form stable host-guest complexes by cation-anion and host-guest interactions. Although both the cage and dyes exhibit no CPL, their host-guest complexes display strong CPL emissions with the <i>g</i>_lum factors in the order of 10⁻³ in solution. Upon crystallization, one order of magnitude amplification of the glum values in the cage-dye adducts (up to 2.4 × 10⁻²) is obtained.

Phototriggered Self-Adaptive Functionalized MOC-Based Drug Delivery Platform Promises High Antitumor Efficacy.

Due to their great stability and special cavities, metal-organic cages (MOCs) are increasingly considered as promising nanocarriers for drug delivery. However, the size and surface dilemmas restrict their further biomedical applications. The ultrasmall size of MOCs facilitates tumor penetration but suffers from quick clearance and poor accumulation at the tumor site. Hydrophobicity of MOC surfaces improves internalization into tumor cells while causing low blood circulation time as well as poor biocompatibility. Therefore, it remains challenging for the MOC-based drug delivery nanoplatform to realize high therapeutic efficacy because it requires different or even opposite dimensions and surface characteristics in different steps of circulation, penetration, accumulation, and internalization processes. In this study, an unprecedented phototriggered self-adaptive platform (ZnPc@polySCage) is developed by integrating functionalized MOCs and a photodynamic therapy based reactive oxygen species responsive strategy to realize high-efficiency tumor-specific therapy. ZnPc@polySCage remains hydrophilic and stealthy during circulation, and retains its small original size for tumor penetration, while transforming to a larger size for effective accumulation and hydrophobic for enhanced internalization under laser irradiation in tumor tissue. With these essential transitions, ZnPc@polySCage demonstrates prominent antitumor effects. Overall, the work provides an advantageous strategy for functional MOC-based platforms and biomedical applications.

Homoporous hybrid membranes containing metal-organic cages for gas separation

Hybridization of porous fillers and polymers is an effective strategy for fabricating composite membranes, affording synergic properties contributed by every single component. However, it has high requirements on the dispersivity of porous fillers and compatibility between porous fillers and the polymeric matrix. Herein, we report the fabrication of homoporous hybrid membranes (HHMs) by chemically cross-linking polymerizable metal-organic cages (MOCs) with polymeric precursors via UV-induced polymerization. The polymerizable MOC molecules, synthesized by heterogeneous postassembly modification, are soluble in polymeric precursors and can be cross-linked with in-situ formed polymeric matrix at a molecular-level dispersivity. The cross-linking between MOCs and polymeric matrix can improve the interfacial compatibility and enhance the thermal stability of the membranes. Compared with the pristine polymeric membrane, the HHM with 1 wt% of MOC loading shows an enhanced CO2 permeability (338 Barrer) and a comparable CO2/CH4 selectivity (16.7) due to the increased CO2 solubility and diffusivity with the addition of MOCs. This strategy inspires the design of hybrid membranes with uniform yet tunable pore sizes for efficient molecular separations.

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.

Boosting Enantioselectivity of Chiral Molecular Catalysts with Supramolecular Metal–Organic Cages

The search for new methodologies to tune and control the enantioselectivities of molecular catalysts is of great importance in the field of asymmetric catalysis. Here, we have illustrated that chir...

Structural Flexibility in Metal-Organic Cages.

Metal-organic cages (MOCs) have emerged as a diverse class of molecular hosts with potential utility across a vast spectrum of applications. With advances in single-crystal X-ray diffraction and economic methods of computational structure optimisation, cavity sizes can be readily determined. In combination with a chemist’s intuition, educated guesses about the likelihood of particular guests being bound within these porous structures can be made. Whilst practically very useful, simple rules-of-thumb, such as Rebek’s 55% rule, fail to take into account structural flexibility inherent to MOCs that can allow hosts to significantly adapt their internal cavity. An often unappreciated facet of MOC structures is that, even though relatively rigid building blocks may be employed, conformational freedom can enable large structural changes. If it could be exploited, this flexibility might lead to behavior analogous to the induced-fit of substrates within the active sites of enzymes. To this end, in-roads have already been made to prepare MOCs incorporating ligands with large degrees of conformational freedom. Whilst this may make the constitution of MOCs harder to predict, it has the potential to lead to highly sophisticated and functional synthetic hosts.

Tetrazole-Functionalized Zirconium Metal-Organic Cages for Efficient C2 H2 /C2 H4 and C2 H2 /CO2 Separations.

Isoreticular functionalization is a well-elucidated strategy for pore environment tuning and the basis of gas separation performance in extended frameworks. The extension of this approach to discrete porous molecules such as metal-organic cages (MOCs) is conceptually straightforward but hindered by synthetic complications, especially stability concerns. We report the successful isoreticular functionalization of a zirconium MOC with tetrazole moiety by bottom-up synthesis. The title compound (ZrT-1-tetrazol) shows promising C2 H2 /CO2 and C2 H2 /C2 H4 separation performance, as demonstrated by adsorption isotherms, breakthrough experiments, and density functional theory calculations. The design analogy between MOFs and highly stable MOCs may guide the synthesis of novel porous materials for challenging separation applications.

Tetrazole‐Functionalized Zirconium Metal‐Organic Cages for Efficient C2H2/C2H4 and C2H2/CO2 Separations

AbstractIsoreticular functionalization is a well‐elucidated strategy for pore environment tuning and the basis of gas separation performance in extended frameworks. The extension of this approach to discrete porous molecules such as metal‐organic cages (MOCs) is conceptually straightforward but hindered by synthetic complications, especially stability concerns. We report the successful isoreticular functionalization of a zirconium MOC with tetrazole moiety by bottom‐up synthesis. The title compound (ZrT‐1‐tetrazol) shows promising C2H2/CO2 and C2H2/C2H4 separation performance, as demonstrated by adsorption isotherms, breakthrough experiments, and density functional theory calculations. The design analogy between MOFs and highly stable MOCs may guide the synthesis of novel porous materials for challenging separation applications.

A Robust Photocatalytic Hybrid Material Composed of Metal-Organic Cages and TiO2 for Efficient Visible-Light-Driven Hydrogen Evolution.

The design of photochemical molecular devices (PMDs) for photocatalytic H2 production from water is a meaningful but challenging subject currently. Herein, a Pd2 L4 type metal-organic cage (denoted as MOC-Q2) is designed as a PMD, which consists of two catalytic centers (Pd2+ ) and four photosensitive ligands (L-2) with four pyridine anchoring groups. Subsequently, the MOC-Q2 is combined with TiO2 to form TiO2 -MOC-Q2 hybrid materials with different MOC-Q2 contents by a facile sol-gel method, which have micro/mesoporous structures and large surface areas. The optimized TiO2 -MOC-Q2 (6.5 wt%) exhibits high H2 production activity (7.9 mmol g-1 h-1 within 5 h) and excellent durability, giving a TON value of 23477 or 11739 (based on MOC-Q2 or Pd moles) after recycling for 7 rounds. By contrast, the pure MOC-Q2 only shows an ordinary photocatalytic H2 production rate (0.84 mmol g-1 h-1 within 5 h) in the homogeneous system. It can be deduced that TiO2 drives the photocatalysis and simultaneously acts as the structure promoter. This study presents a meaningful and distinctive attempt of a new approach for the design and development of MOC-based heterogeneous photocatalysts.

Single Metal–Organic Cage Decorated with an Ir(III) Complex for CO2 Photoreduction

Single Metal–Organic Cage Decorated with an Ir(III) Complex for CO₂ Photoreduction

Supramolecular Catalysis of Acyl Transfer within Zinc Porphyrin-Based Metal-Organic Cages.

To illustrate the supramolecular catalysis process in molecular containers, two porphyrinatozinc(II)-faced cubic cages with different sizes were synthesized and used to catalyze acyl-transfer reactions between N-acetylimidazole (NAI) and various pyridylcarbinol (PC) regioisomers (2-PC, 3-PC, and 4-PC). A systemic investigation of the supramolecular catalysis occurring within these two hosts was performed, in combination with a host-guest binding study and density functional theory calculations. Compared to the reaction in a bulk solvent, the results that the reaction of 2-PC was found to be highly efficient with high rate enhancements (kcat/kuncat = 283 for Zn-1 and 442 for Zn-2), as well as the different efficiencies of the reactions with various ortho-substituted 2-PC substrates and NAI derivates should be attributed to the cages having preconcentrated and preoriented substrates. The same cage displayed different catalytic activities toward different PC regioisomers, which should be mainly attributed to different binding affinities between the respective reactant and product with the cages. Furthermore, control experiments were carried out to learn the effect of varying reactant concentrations and product inhibition. The results all suggested that, besides the confinement effect caused by the inner microenvironment, substrate transfer, including the encapsulation of the reactant and the release of products, should be considered to be a quite important factor in supramolecular catalysis within a molecular container.

Direct identification of HMX via guest-induced fluorescence turn-on of molecular cage

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is one of the most widely used powerful explosives. The direct and selective detection of HMX, without the requirement of specialized equipment, remains a great challenge due to its extremely low volatility, unfavorable reduction potential and lack of aromatic rings. Here, we report the first chemical probe of direct identification of HMX at ppb sensitivity based on a designed metal-organic cage (MOC). The cage features two unsaturated dicopper units and four electron donating amino groups inside the cavity, providing multiple binding sites to selectively enhance host-guest events. It was found that compared to other explosive molecules the capture of HMX inside the cavity would strongly modulate the emissive behavior of the host cage, resulting in highly induced fluorescence “turn-on” (160 folds). Based on the density functional theory (DFT) simulation, the mutual fit of both size and binding sites between host and guest leads to the synergistic effects that perturb the ligand-to-metal charge-transfer (LMCT) process, which is probably the origin of such selective HMX-induced turn-on behavior.

Constructing Heterogeneous Direct Z-Scheme Photocatalysts Based on Metal-Organic Cages and Graphitic-C3N4 for High-Efficiency Photocatalytic Water Splitting.

The development of artificial devices that mimic the highly efficient and ingenious photosystems in nature is worthy of in-depth study. A metal-organic cage (MOC) Pd2(M-4)4(BF4)4, denoted as MOC-Q1, integrating four organic photosensitized ligands M-4 and two Pd2+ catalytic centers is designed for a photochemical molecular device (PMD). MOC-Q1 is successfully immobilized on graphitic carbon nitride (g-C3N4) by hydrogen bonds to obtain a robust heterogeneous direct Z-scheme g-C3N4/MOC-Q1 photocatalyst for H2 generation under visible light. The optimized g-C3N4/MOC-Q1 (2 wt %) system shows high hydrogen evolution activity (4495 μmol g-1 h-1 based on the catalyst mass) and exhibits stable performances for 25 h (a turnover number of 19,268 based on MOC-Q1), significantly outperforming pure MOC-Q1, g-C3N4, and comparsion materials Pd/g-C3N4/M-4, which is the highest one of all reported heterogeneous MOC-based photocatalysts under visible irradiation. This enhancement can be ascribed to the synergistic effects of high-efficient electron transfer, extended visible-light response region, and good protective environment for MOC-Q1 arising from an efficient direct Z-scheme heterostructure of g-C3N4/MOC-Q1. This rationally designed and synthesized MOC/g-C3N4-based heterogeneous PMD is expected to have great potential in photocatalytic water splitting.

Assembly and Covalent Cross-Linking of an Amine-Functionalised Metal-Organic Cage.

The incorporation of reactive functional groups onto the exterior of metal-organic cages (MOCs) opens up new opportunities to link their well-defined scaffolds into functional porous solids. Amine moieties offer access to a rich catalogue of covalent chemistry; however, they also tend to coordinate undesirably and interfere with MOC formation, particular in the case of Cu2 paddlewheel-based MOCs. We demonstrate that tuning the basicity of an aniline-functionalized ligand enables the self-assembly of a soluble, amine-functionalized Cu4L4 lantern cage (1). Importantly, we show control over the coordinative propensity of the exterior amine of the ligand, which enables us to isolate a crystalline, two-dimensional metal-organic framework composed entirely of MOC units (2). Furthermore, we show that the nucleophilicity of the exterior amine of 1 can be accessed in solution to generate a cross-linked cage polymer (3) via imine condensation.

Robust Synthetic Route toward Anisotropic Metal-Organic Cages with Tunable Surface Chemistry.

Metal-organic cages with well-defined interior cavities and tunable surface chemistry serve as attractive building blocks for new types of soft nanoporous materials. While a compositionally diverse repertoire of metal-organic cages exists, the vast majority feature highly symmetric cores. Here, we report a robust, generalizable synthetic route toward anisotropic copper paddlewheel-based cages with tunable pendant amide groups. An isostructural family with increasingly hydrophobic surface properties has been synthesized and characterized by single-crystal X-ray diffraction, gas sorption analysis, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and 1H NMR digestion experiments. The metal-organic cages reported here may enable a deeper study of how anisotropy influences the long-range structure and emergent function of soft nanoporous materials.

Stabilizing Porosity in Organic Cages through Coordination Chemistry.

The number of studies concerning the permanent porosity of molecular materials, especially porous organic cages (POCs) and porous coordination cages (PCCs), have increased substantially over the past decade. The work presented here outlines novel approaches to the preparation of porous molecular structures upon metalation of nonporous, amine-based organic cages. Reduction of the well-known CC3 and CC1 imine-based POCs affords nonporous, highly flexible amine cages. These materials can be endowed with significant levels of structural rigidity via post-synthetic metalation of their ethylenediamine-type binding pockets. The hybrid metal-organic cages accessed through this approach combine aspects of POC and PCC chemistry, with structures of this type providing a potentially promising new direction for the design and development of porous molecular materials with tunability in overall charge, metal cation, porosity, and solubility.