Cavity-Directed Chromism of Phthalein Dyes

Subcomponent self-assembly construction of tetrahedral cage FeII4L4 for high-resolution gas chromatographic separation

Metal organic cages (MOCs), as an emerging discrete supramolecular compounds, have received widespread attention in separation, biomedicine, gas capture, catalysis, and molecular recognition due to their porosity, adjustability and stability. Herein, we present a new chiral MOC FeII4L4 coated capillary column prepared for gas chromatographic (GC) separation of different types of organic compounds, including n-alkanes, n-alcohols, alkylbenzenes, isomers, especially for racemic compounds. There are 20 different kinds of racemates (e.g., alcohols, ethers, epoxides, esters, alkenes, and aldehydes) were well resolved on the FeII4L4 chiral column and a maximum resolution value for 1-phenyl-1-propanol reaches 6.17. The FeII4L4 coated column exhibited high column efficiency (3100 plates m-1 for n-dodecane) and good enantiomeric resolution complementary to that of a commercial β-DEX 120 column and the previously reported chiral MOC [Fe4L6] (ClO4)8 coated column. The relative standard deviation (RSDs) of the peak area and retention time of glycidol and nitrotoluene were below 1.2 %. This study reveals that chiral MOCs have good application prospects in chromatographic separation.

Guest Encapsulation Alters the Thermodynamic Landscape of a Coordination Host

The architecture of self-assembled host molecules can profoundly affect the properties of the encapsulated guests. For example, a rigid cage with small windows can efficiently protect its contents from the environment; in contrast, tube-shaped, flexible hosts with large openings and an easily accessible cavity are ideally suited for catalysis. Here, we report a "Janus" nature of a Pd6L4 coordination host previously reported to exist exclusively as a tube isomer (T). We show that upon encapsulating various tetrahedrally shaped guests, T can reconfigure into a cage-shaped host (C) in quantitative yield. Extracting the guest affords empty C, which is metastable and spontaneously relaxes to T, and the T⇄C interconversion can be repeated for multiple cycles. Reversible toggling between two vastly different isomers paves the way toward controlling functional properties of coordination hosts "on demand".

Photocatalysis in Water-Soluble Supramolecular Metal Organic Complex

As an emerging subset of organic complexes, metal complexes have garnered considerable attention owing to their outstanding structures, properties, and applications. In this content, metal-organic cages (MOCs) with defined shapes and sizes provide internal spaces to isolate water for guest molecules, which can be selectively captured, isolated, and released to achieve control over chemical reactions. Complex supramolecules are constructed by simulating the self-assembly behavior of the molecules or structures in nature. For this purpose, massive amounts of cavity-containing supramolecules, such as metal-organic cages (MOCs), have been extensively explored for a large variety of reactions with a high degree of reactivity and selectivity. Because sunlight and water are necessary for the process of photosynthesis, water-soluble metal-organic cages (WSMOCs) are ideal platforms for photo-responsive stimulation and photo-mediated transformation by simulating photosynthesis due to their defined sizes, shapes, and high modularization of metal centers and ligands. Therefore, the design and synthesis of WSMOCs with uncommon geometries embedded with functional building units is of immense importance for artificial photo-responsive stimulation and photo-mediated transformation. In this review, we introduce the general synthetic strategies of WSMOCs and their applications in this sparking field.

Embedding Lanthanide Organic Polyhedra into Mesoporous Silica Nanoparticles for the Photocatalytic Degradation of Organic Dyes

Organic pollutants cause severe environmental problems because of their damage to human health and ecological systems. Photocatalytic degradation of persistent organic pollutants is of great importance to address these hazards. Herein, we report a lanthanide organic polyhedra-based hybrid material Gd₈ L₁₂ ⊂MSN with the capability of photocatalytic dye degradation. Gd₈ L₁₂ ⊂MSN was prepared by embedding the Gd₈ L₁₂ complex into mesoporous silica nanoparticles (MSNs) using a "ship-in-a-bottle" strategy. Photocurrent response tests revealed that this hybrid material is a potential semiconductor and could generate a rapid and steady photocurrent upon irradiation. Further dye degradation experiments indicated that it could photocatalyze the degradation of familiar organic dyes. Thereinto, compared with the critical Gd₈ L₁₂ complex, the hybrid material exhibited an acceleration of 2.4 times and realized reusability. This not only offers a potential advanced photocatalyst for degrading persistent organic pollutants, but also provides a strategy for the application of supramolecular materials in environmental science.

Reactivity and Selectivity of the Diels-Alder Reaction of Anthracene in [Pd6L4]12+ Supramolecular Cages: A Computational Study

The field of supramolecular metal-organic cage catalysis has grown rapidly in recent years. However, theoretical studies regarding the reaction mechanism and reactivity and selectivity controlling factors for supramolecular catalysis are still underdeveloped. Herein, we demonstrate a detailed density functional theory study on the mechanism, catalytic efficiency, and regioselectivity of the Diels-Alder reaction in bulk solution and within two [Pd₆L₄]¹²⁺ supramolecular cages. Our calculations are consistent with experiments. The origins of the catalytic efficiency of the bowl-shaped cage 1 have been elucidated to be the host-guest stabilization of the transition states and the favorable entropy effect. The reasons for the switch of the regioselectivity from 9,10-addition to 1,4-addition within the octahedral cage 2 were attributed to the confinement effect and the noncovalent interactions. This work would shed light on the understanding of [Pd₆L₄]¹²⁺ metallocage-catalyzed reactions and provide a detailed mechanistic profile otherwise difficult to obtain from experiments. The findings of this study could also aid to the improvement and development of more efficient and selective supramolecular catalysis.

Ternary host-guest complexes with rapid exchange kinetics and photoswitchable fluorescence

Confinement within molecular cages can dramatically modify the physicochemical properties of the encapsulated guest molecules, but such host-guest complexes have mainly been studied in a static context. Combining confinement effects with fast guest exchange kinetics could pave the way toward stimuli-responsive supramolecular systems-and ultimately materials-whose desired properties could be tailored "on demand" rapidly and reversibly. Here, we demonstrate rapid guest exchange between inclusion complexes of an open-window coordination cage that can simultaneously accommodate two guest molecules. Working with two types of guests, anthracene derivatives and BODIPY dyes, we show that the former can substantially modify the optical properties of the latter upon noncovalent heterodimer formation. We also studied the light-induced covalent dimerization of encapsulated anthracenes and found large effects of confinement on reaction rates. By coupling the photodimerization with the rapid guest exchange, we developed a new way to modulate fluorescence using external irradiation.

Morpholine-Functionalized Multicomponent Metallacage as a Vector for Lysosome-Targeted Cell Imaging

Metallacages with suitable cavities and specific functions are promising delivery vectors in biological systems. Herein, we report a morpholine-functionalized metallacage for lysosome-targeted cell imaging. The efficient host-guest interactions between the metallacage and dyes prevent them from aggregation, so their emission in aqueous solutions is well maintained. The fluorescence quantum yield of these host-guest complexes reaches 74.40%. Therefore, the metallacage is further employed as a vector to deliver dyes with different emission colors (blue, green, and red) into lysosomes for targeted imaging. This research affords a type of vector for the delivery of various cargos toward biological applications, which will enrich the usage of metallacages in biomedical engineering.

A self-assembled coordination cage enhances the reactivity of confined amides via mechanical bond-twisting

Self-assembled coordination cages composed of metal cations and ligands can enhance the hydrolysis of non-covalently trapped amides in mild conditions as demonstrated in recent experiments. Here, we reveal the mechanism that accelerates base-catalyzed amide hydrolysis inside the octahedral coordination cage, by means of a quantum mechanics/molecular mechanics/polarizable continuum model. The calculated activation barrier of the nucleophilic OH^- addition to a planar diaryl amide drastically decreases in the cage because of mechanical bond-twisting due to host-guest π-stacking. By contrast, the OH^- addition to an N-acylindole, which possesses a twisted amide bond in bulk water, is not enhanced in the cage. Even though the cage hinders OH^- collisions with the confined amide, the cage can twist the dihedral angle of the planar amide so as to mimic the transition state of OH^- addition.

Computational Modeling of Supramolecular Metallo-organic Cages-Challenges and Opportunities

Self-assembled metallo-organic cages have emerged as promising biomimetic platforms that can encapsulate whole substrates akin to an enzyme active site. Extensive experimental work has enabled access to a variety of structures, with a few notable examples showing catalytic behavior. However, computational investigations of metallo-organic cages are scarce, not least due to the challenges associated with their modeling and the lack of accurate and efficient protocols to evaluate these systems. In this review, we discuss key molecular principles governing the design of functional metallo-organic cages, from the assembly of building blocks through binding and catalysis. For each of these processes, computational protocols will be reviewed, considering their inherent strengths and weaknesses. We will demonstrate that while each approach may have its own specific pitfalls, they can be a powerful tool for rationalizing experimental observables and to guide synthetic efforts. To illustrate this point, we present several examples where modeling has helped to elucidate fundamental principles behind molecular recognition and reactivity. We highlight the importance of combining computational and experimental efforts to speed up supramolecular catalyst design while reducing time and resources.

Metal–Organic Cages: Applications in Organic Reactions

Supramolecular metal–organic cages, a class of molecular containers formed via coordination-driven self-assembly, have attracted sustained attention for their applications in catalysis, due to their structural aesthetics and unique properties. Their inherent confined cavity is considered to be analogous to the binding pocket of enzymes, and the facile tunability of building blocks offers a diverse platform for enzyme mimics to promote organic reactions. This minireview covers the recent progress of supramolecular metal–organic coordination cages for boosting organic reactions as reaction vessels or catalysts. The developments in the utilizations of the metal–organic cages for accelerating the organic reactions, improving the selectivity of the reactions are summarized. In addition, recent developments and successes in tandem or cascade reactions promoted by supramolecular metal–organic cages are discussed.

Conformation-Selective Self-Assembly of Pd6 Trifacial Molecular Barrels Using a Tetrapyridyl Ligand

A conformationally flexible tetrapyridyl ligand L was assembled separately with three cis-blocked 90° Pd^II acceptors (M1, M2, and M3) containing different blocking diamines. Surprisingly, different conformations of the donor L were arrested by the acceptors depending on the nature of the blocking amine, leading to the formation of isomeric Pd₆ barrels (B1, B2, and B3). B2 and B3 with larger windows have been used to encapsulate polyaromatic hydrocarbons.

Encapsulation within a coordination cage modulates the reactivity of redox-active dyes

Confining molecules within well-defined nanosized spaces can profoundly alter their physicochemical characteristics. For example, the controlled aggregation of chromophores into discrete oligomers has been shown to tune their optical properties whereas encapsulation of reactive species within molecular hosts can increase their stability. The resazurin/resorufin pair has been widely used for detecting redox processes in biological settings; yet, how tight confinement affects the properties of these two dyes remains to be explored. Here, we show that a flexible Pd^II₆L₄ coordination cage can efficiently encapsulate both resorufin and resazurin in the form of dimers, dramatically modulating their optical properties. Furthermore, binding within the cage significantly decreases the reduction rate of resazurin to resorufin, and the rate of the subsequent reduction of resorufin to dihydroresorufin. During our studies, we also found that upon dilution, the Pd^II₆L₄ cage disassembles to afford Pd^II₂L₂ species, which lacks the ability to form inclusion complexes - a process that can be reversed upon the addition of the strongly binding resorufin/resazurin guests. We expect that the herein disclosed ability of a water-soluble cage to reversibly modulate the optical and chemical properties of a molecular redox probe will expand the versatility of synthetic fluorescent probes in biologically relevant environments.

Coexistence of 1 : 1 and 2 : 1 inclusion complexes of indigo carmine

We show that the optical properties of indigo carmine can be modulated by encapsulation within a coordination cage. Depending on the host/guest molar ratio, the cage can predominantly encapsulate either one or two dye molecules. The 1 : 1 complex is fluorescent, unique for an indigo dye in an aqueous solution. We have also found that binding two dye molecules stabilizes a previously unknown conformation of the cage.

We show that the optical properties of indigo carmine can be modulated by encapsulation within a coordination cage.

Self-Assembly of Octanuclear PtII/PdII Coordination Barrels and Uncommon Structural Isomerization of a Photochromic Guest in Molecular Space

Two tetragonal molecular barrels TB1 and TB2 were successfully synthesized by coordination-driven self-assembly of a tetrapyridyl donor (L) of the thiazolo[5,4-d]thiazole backbone with cis-blocked 90° Pd(II) and Pt(II) acceptors, respectively. The single-crystal structure analysis of TB1 revealed the formation of a two-face opened tetragonal Pd8 molecular barrel architecture. In contrast, the isostructural Pt(II) barrel (TB2) is water-soluble. The large confined hydrophobic molecular cavity including wide open windows and good water solubility of the barrel TB2 made it a potential molecular container for the encapsulation of guests with different sizes and properties. This has been exploited to encapsulate and stabilize the open form of a photochromic molecule (G2) in water, while the same photochromic molecule exists exclusively in a cyclic zwitterionic form in aqueous medium in the absence of the barrel TB2. This cyclic form is very stable in water and does not go back to its parent open form under common external stimuli. Surprisingly, reverse switching of the cyclic form to a colored hydrophobic open form was also possible instantly in water upon addition of the solid barrel TB2 into an aqueous solution of G2. Such a fast reverse isomerization of an irreversible process in aqueous medium by utilizing host-guest interaction of the barrel TB2 and the guest G2 is interesting. The barrel TB2 was also capable of encapsulating the water-insoluble radical initiator G1 in aqueous medium.

Design and Applications of Water-Soluble Coordination Cages

Compartmentalization of the aqueous space within a cell is necessary for life. In similar fashion to the nanometer-scale compartments in living systems, synthetic water-soluble coordination cages (WSCCs) can isolate guest molecules and host chemical transformations. Such cages thus show promise in biological, medical, environmental, and industrial domains. This review highlights examples of three-dimensional synthetic WSCCs, offering perspectives so as to enhance their design and applications. Strategies are presented that address key challenges for the preparation of coordination cages that are soluble and stable in water. The peculiarities of guest binding in aqueous media are examined, highlighting amplified binding in water, changing guest properties, and the recognition of specific molecular targets. The properties of WSCC hosts associated with biomedical applications, and their use as vessels to carry out chemical reactions in water, are also presented. These examples sketch a blueprint for the preparation of new metal-organic containers for use in aqueous solution, as well as guidelines for the engineering of new applications in water.

Fluorescent Metallacycle-Cored Amphiphilic Nanoparticles Formed by β-Cyclodextrin-Based Host-Guest Interactions towards Cancer Theranostics

Theranostic agents, taking the advantages of both imaging and therapeutic functions, are anticipated to be key components in the development of personalized medicine in which the therapeutic response can be real-time monitored. Herein, three metallacycles with pendent adamantane groups are prepared by coordination-driven self-assembly of Pt^II ligands with anticancer activities and tetraphenylethylene derivatives with emission. β-Cyclodextrin, which shows good host-guest interactions with adamantane moieties, was added to form amphiphilic supramolecular nanoparticles with the aim to enhance the aqueous solubilities and bioactivities of these metallacycles. Moreover, when rhodamine-modified β-cyclodextrin was used as the carrier, the release of the metallacycles from the nanoparticles could be monitored in situ through the fluorescence changes owing to the efficient fluorescence resonance energy transfer from the metallacycles to rhodamine-modified β-cyclodextrin. In vitro and in vivo studies showed that these nanoparticles not only served as cell imaging contrast agents but also displayed improved anticancer activities, allowing them to serve as potential candidates for cancer theranostics. This study provides a simple and efficient method to prepare theranostic agents by hierarchical supramolecular self-assembly, which will pave the way for image-guided cancer therapy, targeted cancer therapy, and related biomedical fields.

Improving Fatigue Resistance of Dihydropyrene by Encapsulation within a Coordination Cage

Photochromic molecules undergo reversible isomerization upon irradiation with light at different wavelengths, a process that can alter their physical and chemical properties. For instance, dihydropyrene (DHP) is a deep-colored compound that isomerizes to light-brown cyclophanediene (CPD) upon irradiation with visible light. CPD can then isomerize back to DHP upon irradiation with UV light or thermally in the dark. Conversion between DHP and CPD is thought to proceed via a biradical intermediate; bimolecular events involving this unstable intermediate thus result in rapid decomposition and poor cycling performance. Here, we show that the reversible isomerization of DHP can be stabilized upon confinement within a Pd^II₆L₄ coordination cage. By protecting this reactive intermediate using the cage, each isomerization reaction proceeds to higher yield, which significantly decreases the fatigue experienced by the system upon repeated photocycling. Although molecular confinement is known to help stabilize reactive species, this effect is not typically employed to protect reactive intermediates and thus improve reaction yields. We envisage that performing reactions under confinement will not only improve the cyclic performance of photochromic molecules, but may also increase the amount of product obtainable from traditionally low-yielding organic reactions.

Enhanced reactivity of twisted amides inside a molecular cage

When an amide group is distorted from its planar conformation, the conjugation between the nitrogen lone pair and the π* orbital of the carbonyl is disrupted and the reactivity towards nucleophiles is enhanced. Although there are several reports on the synthesis of activated twisted amides, amide activation through mechanical twisting is much less common. Here, we report twisted amides that are stabilized through their inclusion in a self-assembled coordination cage. When secondary aromatic amides are included in a T_d-symmetric cage, the cis-twisted conformation is favoured over the trans-planar one-as evidenced by single-crystal X-ray diffraction analysis-revealing that the amide can twist by up to 34°. As a consequence of this distortion, the hydrolysis of amides is significantly accelerated upon inclusion.

Chemical reactivity under nanoconfinement

Confining molecules can fundamentally change their chemical and physical properties. Confinement effects are considered instrumental at various stages of the origins of life, and life continues to rely on layers of compartmentalization to maintain an out-of-equilibrium state and efficiently synthesize complex biomolecules under mild conditions. As interest in synthetic confined systems grows, we are realizing that the principles governing reactivity under confinement are the same in abiological systems as they are in nature. In this Review, we categorize the ways in which nanoconfinement effects impact chemical reactivity in synthetic systems. Under nanoconfinement, chemical properties can be modulated to increase reaction rates, enhance selectivity and stabilize reactive species. Confinement effects also lead to changes in physical properties. The fluorescence of light emitters, the colours of dyes and electronic communication between electroactive species can all be tuned under confinement. Within each of these categories, we elucidate design principles and strategies that are widely applicable across a range of confined systems, specifically highlighting examples of different nanocompartments that influence reactivity in similar ways.

Chemistry of photoswitching molecules in the confined nanospace of aqueous molecular vessels

This Frontier article highlights the photoswitching behaviour of azobenzenes, spiropyrans and DASA molecules inside the nano cavity of self-assembled coordination molecular vessels in aqueous medium and in the solid state.

Cavity-based applications of metallo-supramolecular coordination cages (MSCCs)

This review describes cavity-based applications of cage-like SCCs such as molecular recognition and separation, stabilization of reactive species by encapsulation, as drug delivery systems and as molecular flasks.

Otherwise Unstable Structures Self-Assemble in the Cavities of Cuboctahedral Coordination Cages

We present a method for the directed self-assembly of interlocked structures and coordination complexes in a set of metal-organic hosts. New homo- and heteroleptic metal complexes-species that cannot be prepared outside-form within the cavities of cuboctahedral coordination cages. When linear bidentate guests and macrocycles are sequentially introduced to the host, a rotaxane is threaded internally; the resulting ternary host-guest complex is a new kind of molecular gyroscope. Tetradentate guests segregate the cavities of these cages into distinct spaces, promoting new stoichiometries and modes of ligand binding to metal ions. The behaviors of bound complexes were observed to alter markedly as a result of confinement: In situ oxidations and spin transitions, neither of which occur ex situ, were both observed to proceed. By providing a tailored space for new modes of coordination-driven self-assembly, the inner phases of cuboctahedral coordination cages provide a new medium for synthetic coordination chemistry.

Nanoparticles of metal-organic cages designed to encapsulate platinum-based anticancer agents

We present a novel design to use metal-organic cages (MOCs) to encapsulate Pt-based anticancer agents for delivery. A fluorescein-conjugated Pt(iv) prodrug of cisplatin is developed for its encapsulation in a cationic MOC via host-guest interactions, which then forms drug-loaded nanoparticles with an anionic polymer.

Synthesis and Characterization of Ferrocene Based Hemicages

We present a series of tripodal ligands L1-3, which fold into hemicages C1-3 by using coordination-driven dynamic combinational chemistry. The identities of these hemicages were characterized using ¹H NMR, ¹H-¹H COSY, DOSY, and ESI-TWIM-MS. Free rotation of the ferrocene structural units in the ligands affords an adaptable directionality, which is essential for the construction of these hemicages. Encapsulation of adamantane by C2 indicates the presence of a well-defined inner cavity as the binding pocket.

Reversible chromism of spiropyran in the cavity of a flexible coordination cage

Confining molecules to volumes only slightly larger than the molecules themselves can profoundly alter their properties. Molecular switches-entities that can be toggled between two or more forms upon exposure to an external stimulus-often require conformational freedom to isomerize. Therefore, placing these switches in confined spaces can render them non-operational. To preserve the switchability of these species under confinement, we work with a water-soluble coordination cage that is flexible enough to adapt its shape to the conformation of the encapsulated guest. We show that owing to its flexibility, the cage is not only capable of accommodating-and solubilizing in water-several light-responsive spiropyran-based molecular switches, but, more importantly, it also provides an environment suitable for the efficient, reversible photoisomerization of the bound guests. Our findings pave the way towards studying various molecular switching processes in confined environments.

Construction of half-sandwich multinuclear complexes including tunnel architectures via C–H-activation-directed assembly

Half-sandwich bi-, tetra-, hexa- and octanuclear complexes were prepared via C–H-activation-directed assembly based on three aromatic ligands. A series of tunnel architectures were observed in the complexes, with guest molecules in certain parts.

Anion Encapsulation Drives the Formation of Dimeric GdIII[15-metallacrown-5]3+ Complexes in Aqueous Solution

Metallacrown complexes capable of sequestering dianions, as shown in the solid state, also exist in aqueous solution at neutral pH, as demonstrated by calorimetric and mass spectrometric data. The driving forces for the formation of these dimeric complexes in solution strongly depend on the chain length of the guest rather than its degree of unsaturation.

Controllable construction of half-sandwich octanuclear complexes based on pyridyl-substituted ligands with conjugated centers

Based on three tetradentate ligands with conjugated centers, seven half-sandwich octanuclear complexes were selectively obtained. Several subsequent structural conversions were also successfully conducted.

Switching of the π-electronic conjugations in the reduction of a dithienylethene-fused p-benzoquinone

Visible light unlocks the π-electronic conjugation of a dithienylethene-fused p-benzoquinone derivative to cause a light-driven oxidation reaction.