Encapsulated Dye in Coordination-Assembled Octahedron for Visible-Light-Driven Proton Reduction and Nitroaromatic Hydrogenation

To mimic the finely tuned natural photosynthetic systems, a large metal-organic octahedron was synthesized by one-pot self-assembly with modified triphenylamine ligands and redox-active cobalt ions. By encapsulating an organic dye, fluorescein (Fl), within the inner cavity of the octahedron, the host-guest supramolecular system was provided for light-driven hydrogen production. The intimate distance between the redox site and the photosensitizer in the supramolecular metal-organic cage allowed the photoinduced electrons to transfer from the excited state Fl* to the redox cobalt center in a pseudo-intramolecular pathway. The supramolecular system showed good performance in light-driven hydrogen production and the reduction of nitroaromatic compounds. Control experiments based on a mononuclear compound resembling a cobalt corner of the octahedron and inhibitor competition provided evidence of enzyme-like catalytic behavior. The supramolecular reaction pathways within confined spaces contribute to the superior activity of the host-guest system.

Supramolecular Coordination Cages for Artificial Photosynthesis and Synthetic Photocatalysis

Because sunlight is the most abundant energy source on earth, it has huge potential for practical applications ranging from sustainable energy supply to light driven chemistry. From a chemical perspective, excited states generated by light make thermodynamically uphill reactions possible, which forms the basis for energy storage into fuels. In addition, with light, open-shell species can be generated which open up new reaction pathways in organic synthesis. Crucial are photosensitizers, which absorb light and transfer energy to substrates by various mechanisms, processes that highly depend on the distance between the molecules involved. Supramolecular coordination cages are well studied and synthetically accessible reaction vessels with single cavities for guest binding, ensuring close proximity of different components. Due to high modularity of their size, shape, and the nature of metal centers and ligands, cages are ideal platforms to exploit preorganization in photocatalysis. Herein we focus on the application of supramolecular cages for photocatalysis in artificial photosynthesis and in organic photo(redox) catalysis. Finally, a brief overview of immobilization strategies for supramolecular cages provides tools for implementing cages into devices. This review provides inspiration for future design of photocatalytic supramolecular host-guest systems and their application in producing solar fuels and complex organic molecules.

Binding of Dual-Function Hybridized MetalOrganic Capsules to Enzymes for Cascade Catalysis

The combination of chemo- and biocatalysis for multistep syntheses provides attractive advantages in terms of evolvability, promiscuity, and sustainability striving for desirable catalytic performance. Through the encapsulation of flavin analogues by both NADH and heme mimics codecorated heteroleptic metal-organic capsules, herein, we report a progressive host-guest strategy to imitate cytochrome P450s catalysis for cascade oxidative coupling catalysis. Besides the construction of stable dual-function metal-organic capsules and the modification of cofactor-decorated capsules at the domain of enzymes, this supramolecular strategy involves multistage directional electron flow, affording reactive ferric peroxide species for inducing oxygenation. Under light irradiation, the metal-organic capsule selectively converts stilbene to oxidative coupling products (including 2-oxo-1,2-diphenylethyl formate, 2-alkoxy-1,2-diphenylethanone) in tandem with enzymatic reactions respectively, at the domain of natural enzymes. The ingenious combination of capsules and enzymes with the in situ-regenerated capsule-loaded NADH cofactor promises non-native coupling reactions by forming regional cooperation and division. This abiotic-biotic conjugated host-guest strategy is conducive to the de novo creation of multifunctional components approaching active enzymatic sites for reinforced matter and energy transporting, demonstrating a key role of multicomponent supramolecular catalysts for one-pot integrated catalytic conversions.

Beyond Platonic: How to Build Metal-Organic Polyhedra Capable of Binding Low-Symmetry, Information-Rich Molecular Cargoes

The field of metallosupramolecular chemistry has advanced rapidly in recent years. Much work in this area has focused on the formation of hollow self-assembled metal-organic architectures and exploration of the applications of their confined nanospaces. These discrete, soluble structures incorporate metal ions as ‘glue’ to link organic ligands together into polyhedra.Most of the architectures employed thus far have been highly symmetrical, as these have been the easiest to prepare. Such high-symmetry structures contain pseudospherical cavities, and so typically bind roughly spherical guests. Biomolecules and high-value synthetic compounds are rarely isotropic, highly-symmetrical species. To bind, sense, separate, and transform such substrates, new, lower-symmetry, metal-organic cages are needed. Herein we summarize recent approaches, which taken together form the first draft of a handbook for the design of higher-complexity, lower-symmetry, self-assembled metal-organic architectures.

Modifying electron injection kinetics for selective photoreduction of nitroarenes into cyclic and asymmetric azo compounds

Modifying the reactivity of substrates by encapsulation is essential for microenvironment catalysts. Herein, we report an alternative strategy that modifies the entry behaviour of reactants into the microenvironment and substrate inclusion thermodynamics related to the capsule to control the electron injection kinetics and the selectivity of products from the nitroarenes photoreduction. The strategy includes the orchestration of capsule openings to control the electron injection kinetics of electron donors, and the capsule’s pocket to encapsulate more than one nitroarene molecules, facilitating a condensation reaction between the in situ formed azanol and nitroso species to produce azo product. The conceptual microenvironment catalyst endows selective conversion of asymmetric azo products from different nitroarenes, wherein, the estimated diameter and inclusion Gibbs free energy of substrates are used to control and predict the selectivity of products. Inhibition experiments confirm a typical enzymatic conversion, paving a new avenue for rational design of photocatalysts toward green chemistry.

Coordination-Directed Self-Assembly of Functional Polynuclear Lanthanide Supramolecular Architectures

Lanthanide supramolecular chemistry is a fast growing and intriguing research field due to the unique photophysical, magnetic, and coordination properties of lanthanide ions (Ln^III). Compared with the intensively investigated mononuclear Ln-complexes, polymetallic lanthanide supramolecular assemblies offer more structural superiority and functional advantages. In recent decades, significant progress has been made in polynuclear lanthanide supramolecules, varying from structural evolution to luminescent and magnetic functional materials. This review summarizes the design principles in ligand-induced coordination-driven self-assembly of polynuclear Ln-structures and intends to offer guidance for the construction of more elegant Ln-based architectures and optimization of their functional performances. Design principles concerning the water solubility and chirality of the lanthanide-organic assemblies that are vital in extending their applications are emphasized. The strategies for improving the luminescent properties and the applications in up-conversion, host-guest chemistry, luminescent sensing, and catalysis have been summarized. Magnetic materials based on supramolecular assembled lanthanide architectures are given in an individual section and are classified based on their structural features. Challenges remaining and perspective directions in this field are also briefly discussed.

Recent advances and perspectives on supramolecular radical cages

Supramolecular radical chemistry has been emerging as a cutting-edge interdisciplinary field of traditional supramolecular chemistry and radical chemistry in recent years. The purpose of such a fundamental research field is to combine traditional supramolecular chemistry and radical chemistry together, and take the benefit of both to eventually create new molecules and materials. Recently, supramolecular radical cages have been becoming one of the most frontier and challenging research focuses in the field of supramolecular chemistry. In this Perspective, we give a brief introduction to organic radical chemistry, supramolecular chemistry, and the emerging supramolecular radical chemistry along with their history and application. Subsequently, we turn to the main part of this topic: supramolecular radical cages. The design and synthesis of supramolecular cages consisting of redox-active building blocks and radical centres are summarized. The host-guest interactions between supramolecular (radical) cages and organic radicals are also surveyed. Some interesting properties and applications of supramolecular radical cages such as their unique spin-spin interactions and intriguing confinement effects in radical-mediated/catalyzed reactions are comprehensively discussed and highlighted in the main text. The purpose of this Perspective is to help students and researchers understand the development of supramolecular radical cages, and potentially to stimulate innovation and creativity and infuse new energy into the fields of traditional supramolecular chemistry and radical chemistry as well as supramolecular radical chemistry.

A host-guest 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.

Abiotic–biotic hybrid systems are promising to trap light for fuel and chemical transformation with high efficacy and selectivity. This study reports a coenzyme-mediated supramolecular host-guest semibiological system combining supramolecular catalyst and enzymes for solar alcohol splitting.

Radii-dependent self-assembly of chiral lanthanide complexes: synthesis, chirality, and single-molecule magnet behavior

A pair of 3-methoxysalicylhydrazone-based homochiral ligands constructed chiral trinuclear and pentanuclear complexes with La^III and Dy^III ions, respectively, which indicates that the radii controlled the self-assembled structures. Chiral transfer during the self-assembly processes was confirmed by crystal structure analysis and CD spectroscopy. Then, magnetic investigations demonstrated that the chiral Dy₅ complexes exhibited typical single-molecule magnet behavior.

A host-guest approach to combining enzymatic and artificial catalysis for catalyzing biomimetic monooxygenation

Direct transfer of protons and electrons between two tandem reactions is still a great challenge, because overall reaction kinetics is seriously affected by diffusion rate of the proton and electron carriers. We herein report a host–guest supramolecular strategy based on the incorporation of NADH mimics onto the surface of a metal-organic capsule to encapsulate flavin analogues for catalytic biomimetic monooxygenations in conjunction with enzymes. Coupling an artificial catalysis and a natural enzymatic catalysis in the pocket of an enzyme, this host–guest catalyst–enzyme system allows direct proton and electron transport between two catalytic processes via NADH mimics for the monooxygenation of both cyclobutanones and thioethers. This host–guest approach, which involves the direct coupling of abiotic and biotic catalysts via a NADH-containing host, is quite promising compared to normal catalyst–enzyme systems, as it offers the key advantages of supramolecular catalysis in integrated chemical and biological synthetic sequences.

Combining artificial and natural enzymes is a strategy to mimic biocatalytic processes with high efficiency and selectivity. This study reports a dual catalytic system composed of flavin adenine dinucleotide model and NADH mimics to catalyze the monooxygenation of cyclobutanones and thioethers.

Supramolecular Photocatalysis by Utilizing the Host-Guest Charge-Transfer Interaction: Visible-Light-Induced Generation of Triplet Anthracenes for [4+2] Cycloaddition Reactions

Supramolecular photocatalysis via charge-transfer excitation of a host-guest complex was developed by use of the macrocyclic boronic ester [2+2]_BTH-F containing highly electron-deficient difluorobenzothiadiazole moieties. In the presence of a catalytic amount of [2+2]_BTH-F , the triplet excited state of anthracene was generated from the charge-transfer excited state of anthracene@[2+2]_BTH-F by visible-light irradiation, and cycloaddition of the excited anthracene with several dienes and alkenes proceeded in a [4+2] manner in high yields.

Electron transfer in the confined environments of metal–organic coordination supramolecular systems

In this review, we overview regulatory factors and diverse applications of electron transfer in confined environments of supramolecular host–guest systems.

Synthesis, crystal structures, and luminescent properties of Zn(ii), Cd(ii), Eu(iii) complexes and detection of Fe(iii) ions based on a diacylhydrazone Schiff base

Acylhydrazone Schiff bases are rich in N and O atoms to coordinate with metal ions to form multidentate complexes.

Photochemical Properties of Host-Guest Supramolecular Systems with Structurally Confined Metal-Organic Capsules

Inspired by natural photosynthesis, researchers have designed symmetric metal-organic hosts with large inner pockets that are spontaneously generated through preorganized ligands and functionalized metallocorners to construct dye-containing host-guest systems. The abundant noncovalent interaction sites in the pockets of the hosts facilitated substrate-catalyst interactions for possible enrichment, fixation, and activation of substrates/reagents, providing special electron transfer pathways for regio- or stereoselectively photocatalytic chemical transformations. In this Account, we focus our attention on metal-organic hosts that contain photoactive or redox-active units to evaluate electron transfer and charge separation between host and guest units in these supramolecular systems and elucidate the related photoinduced chemical reactions controlled by these electron transfer processes within the structurally confined pockets of these interesting metal-organic hosts. We have been engaged in developing methods to isolate a series of chromophores for charge separation in supramolecular systems, incorporating organic dyes as photosensitizers in metal-organic hosts with electron acceptor/donor guests is a promising way to enable typical enzyme-like photocatalytic transformations within a confined microenvironment. Related to these inter- and intramolecular photoinduced electron transfer (PET) processes, the formation of host-guest supramolecular systems to fix and isolate the donor-acceptor pair with a short through-space distance provided a new PET pathway to stabilize the charge-separated ion pair. Highly efficient photosynthetic systems can be obtained when charge transfer to electron donors/acceptors occurs faster than the charge recombination. This Account starts with a brief summary of the potential approaches for constructing photoactive metal-organic hosts through the incorporation of dye molecules within ligand backbones or as a part of the metal nodes of the architecture. Following the methodological summary is a discussion on the mechanisms governing the photoinduced charge separation and electron transfer pathways within the dye-incorporated metal-organic hosts. We also searched for strategies for constructing photoactive supramolecular systems through encapsulating dye molecules within the inner space of redox-active hosts. The photochemistry of these systems demonstrated the following advantages due to the structural confinement: avoiding excited state quenching caused by other chemical species, including aggregated dyes, stabilizing the radical intermediate and tuning the absorption or emission of the guest through electron/energy transfer pathways. The photoinduced dye to redox-active host electron transfer is a new and efficient pathway that is meaningful for chemists to realize and understand many important enzymatic processes and to reveal the secrets of a substance and energy metabolism in biological systems. The confined interactions between the host and the guest have shown fascinating effects of promoting and controlling light-induced chemical transformations.

Catalytic reactions within the cavity of coordination cages

Natural enzymes catalyze reactions in their substrate-binding cavities, exhibiting high specificity and efficiency. In an effort to mimic the structure and functionality of enzymes, discrete coordination cages were designed and synthesized. These self-assembled systems have a variety of confined cavities, which have been applied to accelerate conventional reactions, perform substrate-specific reactions, and manipulate regio- and enantio-selectivity. Many coordination cages or cage-catalyst composites have achieved unprecedented results, outperforming their counterparts in different catalytic reactions. This tutorial review summarizes recent developments of coordination cages across three key approaches to coordination cage catalysis: (1) cavity promoted reactions, (2) embedding of active sites in the structure of the cage, and (3) encapsulation of catalysts within the cage. Special emphasis of the review involves (1) introduction of the structure and property of the coordination cage, (2) discussion of the catalytic pathway mediated by the cage, (3) elucidation of the structure-property relationship between the cage and the designated reaction. This work will summarize the recent progress in supramolecular catalysis and attract more researchers to pursue cavity-promoted reactions using discrete coordination cages.

Negatively charged metal–organic hosts with cobalt dithiolene species: improving PET processes for light-driven proton reduction through host–guest electrostatic interactions

Negatively charged metal–organic hosts combined with cationic Ru-based photosensitizers by electrostatic interactions to improve PET processes for efficient photocatalytic proton reduction.

Photocatalytic H2 production using a hybrid assembly of an [FeFe]-hydrogenase model and CdSe quantum dot linked through a thiolato-functionalized cyclodextrin

It is a great challenge to develop iron-based highly-efficient and durable catalytic systems for the hydrogen evolution reaction (HER) by understanding and learning from [FeFe]-hydrogenases. Here we report photocatalytic H₂ production by a hybrid assembly of a sulfonate-functionalized [FeFe]-hydrogenase mimic (1) and CdSe quantum dot (QD), which is denoted as 1/β-CD-6-S-CdSe (β-CD-6-SH = 6-mercapto-β-cyclodextrin). In this assembly, thiolato-functionalized β-CD acts not only as a stabilizing reagent of CdSe QDs but also as a host compound for the diiron catalyst, so as to confine CdSe QDs to the space near the site of diiron catalyst. In addition, another two reference systems comprising MAA-CdSe QDs (HMAA = mercaptoacetic acid) and 1 in the presence and absence of β-CD, denoted as 1/β-CD/MAA-CdSe and 1/MAA-CdSe, were studied for photocatalytic H₂ evolution. The influences of β-CD and the stabilizing reagent β-CD-6-S^- on the stability of diiron catalyst, the fluorescence lifetime of CdSe QDs, the apparent electron transfer rate, and the photocatalytic H₂-evolving efficiency were explored by comparative studies of the three hybrid systems. The 1/β-CD-6-S-CdSe system displayed a faster apparent rate for electron transfer from CdSe QDs to the diiron catalyst compared to that observed for MAA-CdSe-based systems. The total TON for visible-light driven H₂ evolution by the 1/β-CD-6-S-CdSe QDs in water at pH 4.5 is about 2370, corresponding to a TOF of 150 h^-1 in the initial 10 h of illumination, which is 2.7- and 6.6-fold more than the amount of H₂ produced from the reference systems 1/β-CD/MAA-CdSe and 1/MAA-CdSe. Additionally, 1/β-CD-6-S-CdSe gave 2.4-5.1 fold enhancement in the apparent quantum yield and significantly improved the stability of the system for photocatalytic H₂ evolution.

Redox-active copper triangles as an enzymatic molecular flask for light-driven hydrogen production

A redox-active Cu-based triangle was developed to encapsulate fluorescein for photocatalytic hydrogen production. Control experiments and inactive ATP as an inhibitor were performed to confirm this enzymatic photocatalytic behaviour.

Metal–organic redox vehicles to encapsulate organic dyes for photocatalytic protons and carbon dioxide reduction

By encapsulation of an organic dye, a supramolecular nickel–organic macrocycle for the photocatalytic reduction of protons and CO₂ has been reported.

An allosteric photoredox catalyst inspired by photosynthetic machinery

Biological photosynthetic machinery allosterically regulate light harvesting via conformational and electronic changes at the antenna protein complexes as a response to specific chemical inputs. Fundamental limitations in current approaches to regulating inorganic light-harvesting mimics prevent their use in catalysis. Here we show that a light-harvesting antenna/reaction centre mimic can be regulated by utilizing a coordination framework incorporating antenna hemilabile ligands and assembled via a high-yielding, modular approach. As in nature, allosteric regulation is afforded by coupling the conformational changes to the disruptions in the electrochemical landscape of the framework upon recognition of specific coordinating analytes. The hemilabile ligands enable switching using remarkably mild and redox-inactive inputs, allowing one to regulate the photoredox catalytic activity of the photosynthetic mimic reversibly and in situ. Thus, we demonstrate that bioinspired regulatory mechanisms can be applied to inorganic light-harvesting arrays displaying switchable catalytic properties and with potential uses in solar energy conversion and photonic devices.

Photosynthetic systems regulate light harvesting via structural and electronic control of antenna proteins. Here, the authors report a light-harvesting antenna/reaction centre mimic that can be allosterically regulated using mild and redox-inactive inputs, via a coordination framework with hemilabile ligands.

A metal-organic tetrahedron as a redox vehicle to encapsulate organic dyes for photocatalytic proton reduction

The design of artificial systems that mimic highly evolved and finely tuned natural photosynthetic systems is a subject of intensive research. We report herein a new approach to constructing supramolecular systems for the photocatalytic generation of hydrogen from water by encapsulating an organic dye molecule into the pocket of a redox-active metal-organic polyhedron. The assembled neutral Co4L4 tetrahedron consists of four ligands and four cobalt ions that connect together in alternating fashion. The cobalt ions are coordinated by three thiosemicarbazone NS chelators and exhibit a redox potential suitable for electrochemical proton reduction. The close proximity between the redox site and the photosensitizer encapsulated in the pocket enables photoinduced electron transfer from the excited state of the photosensitizer to the cobalt-based catalytic sites via a powerful pseudo-intramolecular pathway. The modified supramolecular system exhibits TON values comparable to the highest values reported for related cobalt/fluorescein systems. Control experiments based on a smaller tetrahedral analogue of the vehicle with a filled pocket and a mononuclear compound resembling the cobalt corner of the tetrahedron suggest an enzymatic dynamics behavior. The new, well-elucidated reaction pathways and the increased molarity of the reaction within the confined space render these supramolecular systems superior to other relevant systems.

Dynamic imine chemistry in metal–organic polyhedra

This review highlights the intercession of Schiff base ligands in the preparation of self-assembled architectures mainly metal–organic polyhedra and describes their unprecedented role in various key applications.

Fluorophore incorporation allows nanomolar guest sensing and white-light emission in M4L6 cage complexes

M₄L₆ cages built from BODIPY- and pyrene-containing subcomponents perform multiple functions: sensing anions and amino acids, and forming a white-light emitting complex with perylene.

Giant electroactive M4L6 tetrahedral host self-assembled with Fe(II) vertices and perylene bisimide dye edges

Self-assembly of octahedral Fe(II) ions and linear perylene bisimide (PBI) dyes with 2,2'-bipyridine groups covalently attached at the imide positions quantitatively yields an Fe4(PBI)6 tetrahedron by the directional bonding approach. With an edge length of 3.9 nm and estimated internal volume >950 Å(3), tetrahedron T is one of the largest M4L6 tetrahedra ever reported. Importantly, many of the desirable photo- and electroactive properties of the PBI ligands are transferred to the nanoscale metallosupramolecule. Tetrahedron T absorbs strongly across the visible spectrum out to 650 nm and exhibits a total of 7 highly reversible electrochemical oxidation and reduction waves spanning a 3.0 V range. This facile cycling of 34 electrons between +18 and -16 charged species is likely enabled due to the porous nature of the tetrahedron that allows the necessary counterions to freely flow in and out of the host. Host-guest encapsulation of C60 by T in acetonitrile was studied by (13)C NMR spectroscopy, UV-vis spectroscopy, and ESI-MS, confirming that the tetrahedron is a suitable host for large, functional guest molecules.