Self-Assembly of Chiral Metal–Organic Tetartoid

Fine-Tuning of the Sequential Self-Assembly of Entangled Polyhedra by Exploiting the Side-Chain Effect

The control of the sequential self-assembly processes of highly entangled (Ag3L2)n (n=2,4,6,8) and Ag21L12 coordination polyhedra using side-chain effects was studied via the introduction of linear or branched side chains into the tripodal ligands. In addition to changes in the intermediate polyhedral species affording the multi- pathway process, disruption of the kinetic control of the sequential self-assembly was observed, thus demonstrating the utility of steric control for the construction of 3D-entangled molecular materials on the 5 nm scale with high molecular complexity.

Interplay of Stereochemistry and Charge Governs Guest Binding in Flexible ZnII4L4 Cages

Here, we report the synthesis of a family of chiral ZnII4L4 tetrahedral cages by subcomponent self-assembly. These cages contain a flexible trialdehyde subcomponent that allows them to adopt stereochemically distinct configurations. The incorporation of enantiopure 1-phenylethylamine produced Δ4 and Λ4 enantiopure cages, in contrast to the racemates that resulted from the incorporation of achiral 4-methoxyaniline. The stereochemistry of these ZnII4L4 tetrahedra was characterized by X-ray crystallography and chiroptical spectroscopy. Upon binding the enantiopure natural product podocarpic acid, the ZnII stereocenters of the enantiopure Δ4-ZnII4L4 cage retained their Δ handedness. In contrast, the metal stereocenters of the enantiomeric Λ4-ZnII4L4 cage underwent inversion to a Δ configuration upon encapsulation of the same guest. Insights gained about the stereochemical communication between host and guest enabled the design of a process for acid/base-responsive guest uptake and release, which could be followed by chiroptical spectroscopy.

High-Nuclearity Ln210Al140 Clusters: Neonates of Open Hollow Dodecahedral Cage Families

Open hollow dodecahedral cage clusters have long been a coveted target in synthetic chemistry, yet their creation poses immense challenges. Here we report two open hollow dodecahedral lanthanide-aluminum (Ln-Al) heterometallic cage clusters, namely, [Ln210Al140(μ2-OH)210(μ3-OH)540(OAc)180(H2O)156](ClO4)120·(MeCN)x·(H2O)y, (Ln = Dy and x = 27, y = 300 for 1; Ln = Y and x = 28, y = 420 for 2). Remarkably, the 350 metal atoms in 1 and 2 display a Keplerate-type four-shell structure of truncated icosidodecahedron@dodecahedron@dodecahedron@icosidodecahedron. The diameter of the cationic cluster in 1 is approximately 5.0 nm, with an inner cavity diameter of about 2.8 nm and a window diameter of roughly 0.66 nm. The cluster in 1 boasts an accessible inner void volume of up to 15,000 Å3. Notably, these cage clusters maintain stability in water, and the truncated icosidodecahedrons in 1 and 2 are the first of their kind synthesized to date. Given that the open hollow dodecahedral Ln-Al cage cluster has never been reported before, this work represents a member in the family of hollow open dodecahedral cages.

Chiral Ligand-Concentration Mediating Asymmetric Transformations of Silver Nanoclusters: NIR-II Circularly Polarized Phosphorescence Lighting

Near infrared (NIR) emitter with circularly polarized phosphorescence (CPP), known as NIR CPP, has emerged as a key part in the research of cutting-edge luminescent materials. However, it remains a challenge to obtain nanoclusters with NIR CPP activity. Here, we propose an asymmetric transformation approach to efficiently synthesize two pairs of chiral silver nanoclusters (R/S-Ag29 and R/S-Ag16) using an achiral Ag10 nanocluster as starting material in the presence of different concentration chiral inducer (R/S)-1,1'-binaphthyl-2,2'-diyl hydrogenphosphate (R/S-BNP). R/S-Ag29, formed in the low-concentration R/S-BNP, exhibits a unique kernel-shell structure consisting of a distorted Ag13 icosahedron and an integrated cage-like organometallic shell with a C3 symmetry, and possesses a superatomic 6-electron configuration (1S2|1P4). By contrast, R/S-Ag16, formed in the high-concentration R/S-BNP, features a sandwich-like pentagram with AgI-pure kernel. Profiting from the hierarchically chiral structures and superatomic kernel-dominated phosphorescence, R/S-Ag29 exhibits infrequent CPP activity in the second near-infrared (975 nm) region, being the first instance of NIR-II CPP observed among CPL-active metal nanoclusters. This study presents a new approach to reduce the difficulty of de novo synthesis for chiral silver nanomaterials, and facilitates the design of CPP-active superatomic nanoclusters in NIR region.

Recent progress for chiral stationary phases based on chiral porous materials in high-performance liquid chromatography and gas chromatography separation

Chirality is a fundamental property of nature. Separation and analysis of racemates are of great importance in the fields of medicine and the production of chiral biopharmaceutical intermediates. Chiral chromatography has the characteristics of a wide separation range, fast separation speed, and high efficiency. The development and preparation of novel chiral stationary phases with good chiral recognition and separation capacity is the core and key of chiral chromatographic separation and analysis. In this work, the representative research progress of novel chiral porous crystal materials including chiral covalent organic frameworks, chiral porous organic cages, chiral metal-organic frameworks, and chiral metal-organic cages used as chiral stationary phases of capillary gas chromatography and high-performance liquid chromatography over the last 4 years is reviewed in detail. The chiral recognition and separation properties of the representative studies in this review are also introduced and discussed.

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.

Metal-Organic Layers with Photosensitizer and Pyridine Pairs Activate Alkyl Halides for Photocatalytic Heck-Type Coupling with Olefins

Photochemical generation of alkyl radicals from haloalkanes often requires strong energy input from ultraviolet light or a strong photoreductant. Haloalkanes can alternatively be activated with nitrogen-based nucleophiles through a sequential SN2 reaction and single-electron reduction to access alkyl radicals, but these two reaction steps have opposite steric requirements on the nucleophiles. Herein, we report the design of Hf12 metal-organic layers (MOLs) with iridium-based photosensitizer bridging ligands and secondary-building-unit-supported pyridines for photocatalytic alkyl radical generation from haloalkanes. By bringing the photosensitizer and pyridine pairs in proximity, the MOL catalysts allowed facile access to the pyridinium salts from SN2 reactions between haloalkanes and pyridines and at the same time enhanced electron transfer from excited photosensitizers to pyridinium salts to facilitate alkyl radical generation. Consequentially, the MOLs efficiently catalyzed Heck-type cross-coupling reactions between haloalkanes and olefinic substrates to generate functionalized alkenes. The MOLs showed 4.6 times higher catalytic efficiency than the homogeneous counterparts and were recycled and reused without a loss of catalytic activity.

Steric and Geometrical Frustration Generate Two Higher-Order CuI12L8 Assemblies from a Triaminotriptycene Subcomponent

The use of copper(I) in metal-organic assemblies leads readily to the formation of simple grids and helicates, whereas higher-order structures require complex ligand designs. Here, we report the clean and selective syntheses of two complex and structurally distinct CuI12L8 frameworks, 1 and 2, which assemble from the same simple triaminotriptycene subcomponent and a formylpyridine around the CuI templates. Both represent new structure types. In T-symmetric 1, the copper(I) centers describe a pair of octahedra with a common center but whose vertices are offset from each other, whereas in D3-symmetric 2, the metal ions form a distorted hexagonal prism. The syntheses of these architectures illustrate how more intricate CuI-based complexes can be prepared via subcomponent self-assembly than has been possible to date through consideration of the interplay between the subcomponent geometry and solvent and electronic effects.

Stereocontrolled Self-Assembly of a Helicate-Bridged CuI12L4 Cage That Emits Circularly Polarized Light

Control over the stereochemistry of metal-organic cages can give rise to useful functions that are entwined with chirality, such as stereoselective guest binding and chiroptical applications. Here, we report a chiral CuI12L4 pseudo-octahedral cage that self-assembled from condensation of triaminotriptycene, aminoquinaldine, and diformylpyridine subcomponents around CuI templates. The corners of this cage consist of six head-to-tail dicopper(I) helicates whose helical chirality can be controlled by the addition of enantiopure 1,1'-bi-2-naphthol (BINOL) during the assembly process. Chiroptical and nuclear magnetic resonance (NMR) studies elucidated the process and mechanism of stereochemical information transfer from BINOL to the cage during the assembly process. Initially formed CuI(BINOL)2 thus underwent stereoselective ligand exchange during the formation of the chiral helicate corners of the cage, which determined the overall cage stereochemistry. The resulting dicopper(I) helicate corners of the cage were also shown to generate circularly polarized luminescence.

Dynamic Stereochemistry of M8 Pd6 Supramolecular Cages Based on Metal-Center Lability for Differential Chiral Induction, Resolution, and Recognition

A series of isostructural supramolecular cages with a rhombic dodecahedron shape have been assembled with distinct metal-coordination lability (M8 Pd6 -MOC-16, M=Ru2+ , Fe2+ , Ni2+ , Zn2+ ). The chirality transfer between metal centers generally imposes homochirality on individual cages to enable solvent-dependent spontaneous resolution of Δ8 /Λ8 -M8 Pd6 enantiomers; however, their distinguishable stereochemical dynamics manifests differential chiral phenomena governed by the cage stability following the order Ru8 Pd6 >Ni8 Pd6 >Fe8 Pd6 >Zn8 Pd6 . The highly labile Zn centers endow the Zn8 Pd6 cage with conformational flexibility and deformation, enabling intrigue chiral-Δ8 /Λ8 -Zn8 Pd6 to meso-Δ4 Λ4 -Zn8 Pd6 transition induced by anions. The cage stabilization effect differs from inert Ru2+ , metastable Fe2+ /Ni2+ , and labile Zn2+ , resulting in different chiral-guest induction. Strikingly, solvent-mediated host-guest interactions have been revealed for Δ8 /Λ8 -(Ru/Ni/Fe)8 Pd6 cages to discriminate the chiral recognition of the guests with opposite chirality. These results demonstrate a versatile procedure to control the stereochemistry of metal-organic cages based on the dynamic metal centers, thus providing guidance to maneuver cage chirality at a supramolecular level by virtue of the solvent, anion, and guest to benefit practical applications.

Spontaneous Resolution of Chiral Janus-Type Double-Layered Metallocyclic Strips Incorporating Möbius Ring and Circular Helicate

Homochiral metal-organic macrocyclic complexes are of great significance owing to their chirality and well-defined internal cavities that potentially have the ability to mimic complicated biological processes. Here we report a novel metal/anion-coordination co-driven strategy for the formation of nanoscale supramolecular metallocycles with unique topology, large size, and desired chirality. The enantiomeric Janus-type metallocyclic strips are assembled based on the synergistic coordination of sulfate anions and CoII ions to a bifunctional achiral ligand combining the o-phenylene-(bis)urea anion-chelating and 8-hydroxyquinoline metal-coordinating sites. The inherent chirality arises from two types of helical chiralities (triply twisted Möbius ring and circular helicate), which is observed for the first time for metal-organic complex systems. Notably, spontaneous chiral resolution by conglomerate crystallization into a pair of enantiomers (P- or M-Co9) is realized, which is attributed to the multiple weak intermolecular interactions facilitating the hierarchically helical superstructure.

Allosterically Regulated Guest Binding Determines Framework Symmetry for an FeII 4 L4 Cage

Self-assembly of a flexible tritopic aniline and 3-substituted 2-formylpyridine subcomponents around iron(II) templates gave rise to a low-spin FeII 4 L4 capsule, whereas a high-spin FeII 3 L2 sandwich species formed when a sterically hindered 6-methyl-2-formylpyridine was used. The FeII 4 L4 cage adopted a new structure type with S4 symmetry, having two mer-Δ and two mer-Ʌ metal vertices, as confirmed by NMR and X-ray crystallographic analysis. The flexibility of the face-capping ligand endows the resulting FeII 4 L4 framework with conformational plasticity, enabling it to adapt structurally from S4 to T or C3 symmetry upon guest binding. The cage also displayed negative allosteric cooperativity in simultaneously binding different guests within its cavity and at the apertures between its faces.

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 Pd_n L_2n 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.

Subtle Stereochemical Effects Influence Binding and Purification Abilities of an FeII4L4 Cage

A tetrahedral Fe^II₄L₄ cage assembled from the coordination of triangular chiral, face-capping ligands to iron(II). This cage exists as two diastereomers in solution, which differ in the stereochemistry of their metal vertices, but share the same point chirality of the ligand. The equilibrium between these cage diastereomers was subtly perturbed by guest binding. This perturbation from equilibrium correlated with the size and shape fit of the guest within the host; insight as to the interplay between stereochemistry and fit was provided by atomistic well-tempered metadynamics simulations. The understanding thus gained as to the stereochemical impact on guest binding enabled the design of a straightforward process for the resolution of the enantiomers of a racemic guest.

Metallo-Supramolecular Hexagonal Wreath with Four Switchable States Based on a pH-Responsive Tridentate Ligand

In biological systems, many biomacromolecules (e.g., heme proteins) are capable of switching their states reversibly in response to external stimuli, endowing these natural architectures with a high level of diversity and functionality. Although tremendous efforts have been made to advance the complexity of artificial supramolecules, it remains a challenge to construct metallo-supramolecular systems that can carry out reversible interconversion among multiple states. Here, a pH-responsive tridentate ligand, 2,6-di(1H-imidazole-2-yl)pyridine (H₂DAP), is incorporated into the multitopic building block for precise construction of giant metallo-supramolecular hexagonal wreaths with three metal ions, i.e., Fe(II), Co(II), and Ni(II), through coordination-driven self-assembly. In particular, a Co-linked wreath enables in situ reversible interconversion among four states in response to pH and oxidant/reductant with highly efficient conversion without losing structural integrity. During the state interconversion cycles, the physical properties of the assembled constructs are finely tuned, including the charge states of the backbone, valency of metal ions, and paramagnetic/diamagnetic features of complexes. Such discrete wreath structures with a charge-switchable backbone further facilitate layer-by-layer assembly of metallo-supramolecules on the substrate.

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.

Enantiomorphic Single Crystals of Linear Lead(II) Bromide Perovskitoids with White Circularly Polarized Emission

Chiroptical hybrid organic-inorganic perovskites are emerging as a new class of promising materials with mirror optical signal responses for optoelectronic applications. However, chiroptical white-emission materials have been scarcely unearthed. Herein, four pairs of hybrid lead(II) bromide perovskitoids were obtained, namely, (R)- and (S)-(H₂ MPz)PbBr₄ (R/S-MPz=(R)-(-)/(S)-(+)-2-methylpiperazine) (1 and 2), (R)- and (S)-(H₂ MPz)₃ Pb₂ Br₁₀ ⋅2 DMAc (3 and 4), (R)- and (S)-(H₂ MPz)PbBr₄ ⋅0.5 MeCN (5 and 6) and (R)- and (S)-(H₂ MPz)₂ Pb₂ Br₈ ⋅DCM (7 and 8). Notably, they all exhibit ultrabroadband emission and chiroptical signals. Perovskitoids 3-6 even achieve white circularly polarized emission with a high dissymmetric factor (g_lum ) (±3×10^-3 for 3 and 4; ±8×10^-3 for 5 and 6). This new type of hybrid perovskitoids will attract attention and find applications in chiroptical fields because of the extensively and easily tunable photophysical properties.

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.

Metal-organic cages against toxic chemicals and pollutants

The continuous release of toxic chemicals and pollutants into the atmosphere and natural waters threatens, directly and indirectly, human health, the sustainability of the planet, and the future of society. Materials capable of capturing or chemically inactivating hazardous substances, which are harmful to humans and the environment, are critical in the modern age. Metal-organic cages (MOCs) show great promise as materials against harmful agents both in solution and in solid state. This Highlight features examples of MOCs that selectively encapsulate, adsorb, or remove from a medium noxious gases, toxic organophosphorus compounds, water pollutant oxoanions, and some emerging organic contaminants. Remarkably, the toxicity of interacting contaminants may be lowered by MOCs as well. Specific cases pertaining to the use of these cages for the chemical degradation of some harmful substances are presented. This Highlight thus aims to provide an overview of the possibilities of MOCs in this area and new methodological insights into their operation for enhancing their activity and the engineering of further remediation applications.

Self-Assembly of Chiral Ferrocene-Functionalized Polyoxotitanium Clusters for Photocatalytic Selective Sulfide Oxidation

Here, we systematically studied the self-assembly behavior of chiral polyoxytitanium clusters for the first time. Through the cooperative assembly of ferrocenecarboxylic acid and ketoxime ligands, we successfully incorporated the planar chirality of ferrocene (Fc) into the layered {Ti₅} building blocks. The resulting {Ti₅Fc} clusters can be used as structural units to assemble into large ordered structures in various ways; either a pair of {Ti₅Fc} enantiomers are bridged by organic adhesive to form sandwich structures or two homochiral {Ti₅Fc} units participate in the assembly to form the large clusters. Depending on the assembly modes, the chirality of {Ti₅Fc} can be transferred to large nanoclusters or disappear to form mesostructures. The difference of the assembly modes between the {Ti₅Fc} units can also tune the photoelectric activity of the resulting clusters, which has been verified by using {Ti₁₀Fc-6/7} as catalysts for photocatalytic selective sulfide oxidation. This work not only is an important breakthrough in the study of the self-assembly of chiral nanoclusters but also provides an important reference for understanding of chiral transfer on the nanoscale.

Adaptive coordination assemblies based on a flexible tetraazacyclododecane ligand for promoting carbon dioxide fixation

Coordination hosts based on flexible ligands have received increasing attention due to their inherent adaptive cavities that often show induced-fit guest binding and catalysis like enzymes. Herein, we report the controlled self-assembly of a series of homo/heterometallic coordination hosts (Me₄enPd)_2n(ML)_n [n = 2/3; M = Zn(ii)/Co(ii)/Ni(ii)/Cu(ii)/Pd(ii)/Ag(i); Me₄en: N,N,N′,N′-tetramethylethylenediamine] with different shapes (tube/cage) from a flexible tetraazacyclododecane-based pyridinyl ligand (L) and cis-blocking Me₄enPd(ii) units. While the Ag(i)-metalated ligand (AgL) gave rise to the formation of a (Me₄enPd)₄(ML)₂-type cage, all other M(ii) ions led to isostructural (Me₄enPd)₆(ML)₃-type tubular complexes. Structural transformations between cages and tubes could be realized through transmetalation of the ligand. The buffering effect on the ML panels endows the coordination tubes with remarkable acid–base resistance, which makes the (Me₄enPd)₆(ZnL)₃ host an effective catalyst for the CO₂ to CO₃²⁻ conversion. Control experiments suggested that the integration of multiple active Zn(ii) sites on the tubular host and the perfect geometry match between CO₃²⁻ and the cavity synergistically promoted such a conversion. Our results provide an important strategy for the design of adaptive coordination hosts to achieve efficient carbon fixation.

A series of coordination hosts were prepared and their applications in CO₂ fixation were studied.

Encapsulation of Metal Clusters within Porous Organic Materials: From Synthesis to Catalysis Applications

Metal clusters (MCs) with dimensions between a single metal atom and nanoparticles of >2 nm usually possess distinct geometric and electronic structures, their outstanding performance in catalysis applications have underpinned a broad research interest. However, smaller-sized MCs are easily deactivated by migration coalescence during the catalysis process because of their high surface energy. Therefore, the search of an appropriate stabilizer for MCs is urgently demanded. In recent years, porous organic polymers (POPs) and organic molecular cages (OMCs), as emerging functional materials, have attracted significant attention. Benefiting from the spatial confinement, encapsulating MCs into these porous organic materials is a promising approach to guarantee the uniform size distribution and stability. In this review, we aim to provide a comprehensive summary of the recent progress in the synthetic strategies and catalysis applications of the encapsulated MCs, and seek to uncover promising ideas that can stimulate future developments at both the fundamental and applied levels.

Morphology-Dependent Peroxidase Mimicking Enzyme Activity of Copper Metal-Organic Polyhedra Assemblies

The morphology of nanomaterials (geometric shape and dimension) play a significant role in its various physical and chemical properties. Thus, it is essential to link morphology with performance in specific applications. For this purpose, the morphology of copper metal-organic polyhedra (Cu-MOP) can be modulated through distinct assembly process, which facilitates the exploration of the relationship between morphology and catalytic performance. In this work, the assemblies of Cu-MOP with three different morphologies (nanorods, nanofibers and nanosheets) were facilely prepared by the variation of solvent mixture of N, N-dimethylformamide (DMF) and methanol, revealed the important role of the interaction between the surface group and the solvent on the morphology of these assemblies. Cu-MOP nanofibers exhibited the highest mimetic peroxidase enzyme activity over the Cu-MOP nanosheets and nanorods, which have been utilized in the detection of glucose. Cu-MOPs assemblies with tunable morphology accompanied with adjustable mimic peroxidase activity, had great potential applications in the field of bioanalytical chemistry and biomedicals.

Artificial Metal-Peptide Assemblies: Bioinspired Assembly of Peptides and Metals through Space and across Length Scales

The exploration of chiral crystalline porous materials, such as metal-organic complexes (MOCs) or metal-organic frameworks (MOFs), has been one of the most exciting recent developments in materials science owing to their widespread applications in enantiospecific processes. However, achieving specific tight-affinity binding and remarkable enantioselectivity toward important biomolecules is still challenging. Perhaps most critically, the lack of adaptability, compatibility, and processability in these materials severely impedes practical applications in chemical engineering and biological technology. In this Perspective, artificial metal-peptide assemblies (MPAs), which are achieved by the assembly of peptides and metals with nanometer-sized cavities or pores, is a new development that could address the current bottlenecks of chiral porous materials. Bioinspired assembly of pore-forming MPAs is not foreign to biological systems and has granted scientists an unprecedented level of control over the chiral recognition sites, conformational flexibility, cavity sizes, and hydrophilic segments through ultrafine-tuning of peptide-derived linkers. We will specifically discuss exemplary MPAs including structurally well-defined metal-peptide complexes and highly crystalline metal-peptide frameworks. With insights from these structures, the peptide assembly and folding by the closer cooperation of metal coordination and noncovalent interactions can create adaptable protein-like nanocavities undergoing a myriad of conformational variations that is reminiscent of enzymatic pockets. We also consider challenges to advancing the field, where the deployment of side-chain groups and manipulation of amino acid sequences are more likely to access the programmable, genetically encodable peptide-mediated porous materials, thus contributing to the enhanced enantioselective recognition as well as enabling key biochemical processes in next-generation versatile biomimetic materials.

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.

Symmetry Breaking of Atomically Precise Fullerene-like Metal Nanoclusters

Here we report a neutral fullerene-like core-shell homosilver Ag₁₃@Ag₂₀ nanocluster that is fully protected by an achiral bidentate thiolate ligand (9,12-dimercapto-1,2-closo-carborane, C₂B₁₀H₁₀S₂H₂), which crystallizes in centrosymmetric space group R3̅. Continuous Cu doping in the dodecahedral shell first induced symmetry breaking to generate chiral Ag₁₃@Ag_20-nCu_n (6 ≥ n ≥ 2) containing two acetonitrile ligands in space group P2₁2₁2₁, and then produced symmetric all-thiolated Ag₁₃@Ag_20-nCu_n (20 ≥ n ≥ 13) in the higher space group Im3̅. The selectively copper-doped Ag₁₃@Ag_20-nCu_n (6 ≥ n ≥ 2) cluster has its structure reorganized to a lower symmetry that shows chiroptical activity. Moreover, structural distortion of Ag₁₃@Ag_20-nCu_n (6 ≥ n ≥ 2) further expanded in chiral R-/S-propylene oxide, which induced a more prominent core-based CD response. This work revealed a novel mechanism of chirality generation at the atomic level through asymmetric shell-doping of metal nanoclusters, which provides new insight into the origin of chirality in inorganic nanostructures.

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.

Sterics and Hydrogen Bonding Control Stereochemistry and Self-Sorting in BINOL-Based Assemblies

Here we demonstrate how the hydrogen-bonding ability of a BINOL-based dialdehyde subcomponent dictated the stereochemical outcome of its subsequent self-assembly into one diastereomeric helicate form when bearing free hydroxy groups, and another in the case of its methylated congener. The presence of methyl groups also altered the self-sorting behavior when mixed with another, short linear dialdehyde subcomponent, switching the outcome of the system from narcissistic to integrative self-sorting. In all cases, the axial chirality of the BINOL building block also dictated helicate metal center handedness during stereospecific self-assembly. A new family of stereochemically pure heteroleptic helicates were thus prepared using the new knowledge gained. We also found that switching from Fe^II to Zn^II, or the incorporation of a longer linear ligand, favored heteroleptic structure formation.

Precise Implantation of an Archimedean Ag@Cu12 Cuboctahedron into a Platonic Cu4Bis(diphenylphosphino)hexane6 Tetrahedron

Precision loading of nanoclusters in confined spaces, which has been enthusiastically pursued in the scientific realm, is still associated with some mysteries of "how", "when", and "why". Here, we isolated two similar heterometallic cluster-in-cage compounds, [Ag@Cu₁₂S₈@Cu₄(dpph)₆]X (X = OH, SD/AgCu16a and X = PF₆, SD/AgCu16b; SD = SunDi), by use of an antigalvanic reaction between organometallic [PhC≡CCu]_n and Ph₃CSH with elemental silver. Both compounds are formed by fitting an Archimedean Ag@Cu₁₂ cuboctahedral cluster into a Platonic Cu₄(dpph)₆ tetrahedral cage [dpph = bis(diphenylphosphino)hexane]. The Ag@Cu₁₂ cluster is a hollow cuboctahedral Cu₁₂ cage filled with a central Ag^I atom, and all eight triangular faces of the Ag@Cu₁₂ cuboctahedron are triply capped by eight S^2- ions, four of which in a tetrahedral array further internally pillar four Cu vertices of the outer Cu₄(dpph)₆ tetrahedron, fixing the cluster in the cage. Both compounds can be deemed as molecular fragments excised from porous nanomaterials filled with discrete nanoclusters, thus providing more details for understanding the confined growth of atomically precise nanoclusters. Electrospray ionization mass spectrometry (ESI-MS) reveals that the AgCu₁₆ cluster is quite stable in CH₂Cl₂ and can stepwise lose dpph ligand in the gas phase under increased collision energy. This work not only presents a precise aggregation of metal atoms in a confined cavity to form a cluster-in-cage compound but also provides deep insights into the binding and geometry matching between clusters and cages in one entity.

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.

Role of the Auxiliary Ligand in the Spontaneous Resolution of Enantiomers in Three-Dimensional Coordination Polymers

Four pairs of chiral 3D coordination polymers (CPs), [Zn₂(BDC)(lac)(DMF)]·guest (2) (H₂BDC = benzene dicarboxylic acid; H₂lac = lactic acid; guest = 1.5DMF + i-PrOH), [Co₂(BDC)(lac)(DMF)]·guest (3) (guest = DMF + 2H₂O), [Fe₄(BDC)₃(lac)₂(DMF)₂](CO₃)·guest (4) (guest = DMF + 2H₂O), and {Zn₁₁(BPDC)₆(lac)₆[NH₂(CH₃)₂]₂}·guest (H₂BPDC = 3,3'-biphenyldicarboxylic acid; guest = 6DMF + 18H₂O) (5), are prepared through the reactions of racemic lactic acid (rac-H₂lac) with different metal ions and auxiliary ligands. Structural analyses and DFT calculations reveal that forming more and stronger coordination bonds between the auxiliary ligand and metal ions is more conducive to the spontaneous resolution of enantiomers in 3D CPs than simply increasing the entropy of the auxiliary ligand itself.

Self-Assembly of a Ti4(embonate)6 Cage toward Silver

Herein we report a variety of supramolecular architectures that are self-assembled by the highly charged anionic Ti₄L₆ (L = embonate) cages and noble-metal Ag⁺ ions in the presence of the different ligands, including six Ti₄L₆-Ag(PPh₃) cages in whose structures the Ti₄L₆ cage catches various in situ formed [Ag(PPh₃)]⁺ moieties by a coordination bond and one cocrystal superstructure of a Ti₄L₆ cage with an in situ generated [Ag₂(Ph₂P(CH₂)₅PPh₂)₃]²⁺ cage via supramolecular interactions. In addition, the third-order nonlinear-optical properties of these compounds are investigated in detail.

Coordination-Assembled Water-Soluble Anionic Lanthanide Organic Polyhedra for Luminescent Labeling and Magnetic Resonance Imaging

Lanthanide-containing functional complexes have found a variety of applications in materials science and biomedicine because of their unique electroptical and magnetic properties. However, the poor stability and solubility in water of multicomponent lanthanide organic assemblies significantly limit their practical applications. We report here a series of water-stable anionic Ln_2nL_3n-type (n = 2, 3, 4, and 5) lanthanide organic polyhedra (LOPs) constructed by deprotonation self-assembly of three fully conjugated ligands (H₄L¹ and H₄L^2a/b) featuring a 2,6-pyridine bitetrazolate chelating moiety. The outcomes of the LOPs formation reactions were found to be very sensitive toward the reaction conditions including base, metal source, solvents, and concentrations as characterized by a combination of NMR, high-resolution ESI-MS and X-ray crystallography. Ligands H₄L^2a/b manifested an excellent sensitization toward lanthanide ions (Ln = Eu^III and Tb^III), with high luminescent quantum yields for Tb₈L^2a₁₂ (Φ = 11.2% in water) and Eu₈L^2b₁₂ (Φ = 76.8% in DMSO) measured in polar solvents. Furthermore, due to the giant molecular weight and rigidity of the polyhedral skeleton, Gd₈L^2b₁₂ showed a very high longitudinal relaxivity (r₁) of 400.53 mM^-1S^-1. The performance of Gd₈L^2b₁₂ as potential magnetic resonance imaging contrast agents (CAs) in vivo was evaluated with much longer retention time in the tumor sites compared with the commercial Gd^III-based CAs. Dual-modal imaging potential has also been demonstrated with the mixed Eu/Gd LOPs. Our results not only provide a new design route toward water-stable multinuclear lanthanide organic assemblies but also offer potential candidates of supramolecular-edifices for bioimaging and drug delivery.

Equi-size nesting of Platonic and Archimedean metal-organic polyhedra into a twin capsid

Inspired by the structures of virus capsids, chemists have long pursued the synthesis of their artificial molecular counterparts through self–assembly. Building nanoscale hierarchical structures to simulate double-shell virus capsids is believed to be a daunting challenge in supramolecular chemistry. Here, we report a double-shell cage wherein two independent metal–organic polyhedra featuring Platonic and Archimedean solids are nested together. The inner (3.2 nm) and outer (3.3 nm) shells do not follow the traditional “small vs. large” pattern, but are basically of the same size. Furthermore, the assembly of the inner and outer shells is based on supramolecular recognition, a behavior analogous to the assembly principle found in double-shell viruses. These two unique nested characteristics provide a new model for Matryoshka–type assemblies. The inner cage can be isolated individually and proves to be a potential molecular receptor to selectively trap guest molecules.

Supramolecular constructs that mimic complex biological assemblies are synthetically challenging. Here, the authors present a double-shell cage wherein two independent metal-organic polyhedra are nested together in a manner analogous to that found in double-shell virus capsids.

Coordinative-to-covalent transformation, isomerization dynamics, and logic gate application of dithienylethene based photochromic cages

Photochromic coordinative cages containing dynamic C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N imine bonds are assembled from a dithienylethene-based aldehyde and tris-amine precursors via metallo-component self-assembly. The resulting metal-templated cages are then reduced and demetalated into pure covalent-organic cages (COCs), which are otherwise difficult to prepare via de novo organic synthesis. Both the obtained coordinative and covalent cages can be readily interconverted between the ring-open (o-isomer) and one-lateral ring-closed (c-isomer) forms by UV/vis light irradiation, demonstrating distinct absorption, luminescence and photoisomerization dynamics. Specifically, the ring-closed c-COCs show a blue-shifted absorption band compared with analogous metal-templated cages, which can be applied in photoluminescence (PL) color-tuning of upconversion materials in different ways, showing potential for constructing multi-readout logic gate systems.

Metal-templated component self-assembly and then demetalation affords photochromic covalent organic cages applicable for upconversion PL-color tuning for logic gates.

An S10-Symmetric 5-Fold Interlocked [2]Catenane

The reaction of sym-pentakis(4-aminothiophenyl)corannulene with 2-formyl-6-methylpyridine and Cu^I or 2-formyl-1,10-phenanthroline and M^II (M = Co, Zn) yields an S₁₀-symmetric 5-fold interlocked [2]catenane of two interpenetrating [Cu^I₅L₂]⁵⁺ cages or D₅-symmetric [M^II₅L₂]¹⁰⁺ cages, respectively. The new structures were characterized by X-ray crystallography, NMR spectroscopy, and mass spectrometry. Density functional theory computations point to dispersive energies on par with traditional covalent bond energies. Subcomponent exchange reactions favored formation of the [Co^II₅L₂]¹⁰⁺ cage over the [Cu^I₁₀L₄]¹⁰⁺ catenane. The single cage and catenane each cocrystallized with a corannulene guest to form a bowl-in-bowl substructure.