Hierarchical Assembly of an Interlocked M8 L16 Container

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.

Polycatenanes Formed of Self-Assembled Metal-Organic Cages

Poly-[n]-catenanes (PCs) self-assembled of three-dimensional (3D) metal organic cages (MOCs) (hereafter referred to as PCs-MOCs) are a relatively new class of mechanically interlocked molecules (MIMs) that combine the properties of MOCs and polymers. The synthesis of PCs-MOCs is challenging because of the difficulties associated with interlocking MOCs, the occurrence of multiple weak supramolecular electrostatic interactions between cages, and the importance of solvent templating effects. The high density of mechanical bonds interlocking the MOCs endows the MOCs with mechanical and physical properties such as enhanced stability, responsive dynamic behavior and low solubility, which can unlock new functional properties. In this Minireview, we highlight the benefit of interlocking MOCs in the formation of PCs-MOCs structures as well as the synthetic approaches exploited in their preparation, from thermodynamic to kinetic methods, both in the solution and solid-states. Examples of PCs-MOCs self-assembled from various types of nanosized cages (i.e., tetrahedral, trigonal prismatic, octahedral and icosahedral) are described in this article, providing an overview of the research carried out in this area. The focus is on the structure-property relationship with examples of functional applications such as electron conductivity, X-ray attenuation, gas adsorption and molecular sensing. We believe that the structural and functional aspects of the reviewed PCs-MOCs will attract chemists in this research field with great potential as new functional materials in nanotechnological disciplines such as gas adsorption, sensing and photophysical properties such as X-ray attenuation or electron conductivity.

High-order layered self-assembled multicavity metal--organic capsules and anti-cooperative host-multi-guest chemistry

The construction and application of metal-organic cages with accessible internal cavities have witnessed rapid development, however, the precise synthesis of complex metal-organic capsules with multiple cavities and achievement of multi-guest encapsulation, and further in-depth comprehension of host-multi-guest recognition remain a great challenge. Just like building LEGO blocks, herein, we have constructed a series of high-order layered metal-organic architectures of generation n (n = 1/2/3/4 is also the number of cavities) by multi-component coordination-driven self-assembly using porphyrin-containing tetrapodal ligands (like plates), multiple parallel-podal ligands (like clamps) and metal ions (like nodes). Importantly, these high-order assembled structures possessed different numbers of rigid and separate cavities formed by overlapped porphyrin planes with specific gaps. The host-guest experiments and convincing characterization proved that these capsules G2-G4 could serve as host structures to achieve multi-guest recognition and unprecedentedly encapsulate up to four C60 molecules. More interestingly, these capsules revealed negative cooperation behavior in the process of multi-guest recognition, which provides a new platform to further study complicated host-multi-guest interaction in the field of supramolecular chemistry.

Chiral Truxene-Based Self-Assembled Cages: Triple Interlocking and Supramolecular Chirogenesis

Incorporating chiral elements in host-guest systems currently attracts much attention because of the major impact such structures may have in a wide range of applications, from pharmaceuticals to materials science and beyond. Moreover, the development of multi-responsive and -functional systems is highly desirable since they offer numerous benefits. In this context, we describe herein the construction of a metal-driven self-assembled cage that associates a chiral truxene-based ligand and a bis-ruthenium complex. The maximum separation between both facing chiral units in the assembly is fixed by the intermetallic distance within the lateral bis-ruthenium complex (8.4 Å). The resulting chiral cavity was shown to encapsulate polyaromatic guest molecules, but also to afford a chiral triply interlocked [2]catenane structure. The formation of the latter occurs at high concentration, while its disassembly could be achieved by the addition of a planar achiral molecule. Interestingly the planar achiral molecule exhibits induced circular dichroism signature when trapped within the chiral cavity, thus demonstrating the ability of the cage to induce supramolecular chirogenesis.

Hetero- and Homointerlocked Metal-Organic Cages

Interlocked molecular assemblies constitute a captivating ensemble of chemical topologies, comprising two or more separate components that exhibit remarkably intricate structures. The interlocked molecular assemblies are typically identical, and heterointerlocked systems that comprise structurally distinct assemblies remain unexplored. Here, we demonstrate that metal-templated synthesis can be exploited to afford not only a homointerlocked cage but also a heterointerlocked cage. Treatment of a carboxylated 2,9-dimethyl-1,10-phenanthroline (dmp) or Cu(I) bis-dmp linker with a Ni4-p-tert-butylsulfonylcalix[4]arene cluster affords noninterlocked octahedron and quadruply interlocked double cages consisting of two identical tetragonal pyramids, respectively. In contrast, when a mixture of dmp and Cu(I) bis-dmp linkers is used, a quadruply heterointerlocked cage is produced, consisting of a tetragonal pyramid and an octahedron. With photoredox-active [Cu(dmp)2]+ in the structures, both interlocked cages exhibit remarkable performance as photocatalysts for atom transfer radical addition (ATRA) reactions of trifluoromethanesulfonyl chloride with alkenes or oxo-azidations of vinyl arenes. These interlocked structures serve the dual purpose of stabilizing photocatalytically active components against deactivation and encapsulating substrates within the cavity, resulting in yields comparable to or even surpassing those of their molecular counterparts. This work thus provides a new strategy that combines metal templating and nontemplating approaches to design new types of interlocked assemblies with intriguing architectures and properties.

Self-Assembly of Cluster-Mediated 3D Catenanes with Size-Specific Recognition Behavior

Although interlocked three-dimensional molecules display unique properties associated with their spatial structures, their synthesis and study of their host-guest properties remain challenging. We report the formation of a novel [2]catenane, [Et4N]@[(Tp*WS3Cu3Cl)2(cis-bpype)3]2(OTf)5 ([Et4N][1](OTf)5), by self-assembly of the cluster node [Tp*WS3Cu3Cl]+ and the organic linker (Z)-1,2-diphenyl-1,2-bis(4-(pyridin-4-yl)phenyl)ethene (cis-bpype). Single-crystal X-ray and NMR analyses established that [1]4+ is formed by the interpenetration of two cluster-organic cages. Unique cation-in-cation host-guest complexes were observed with this catenane. The crystalline, empty catenane was formed by taking advantage of the electrostatic repulsion-induced weak binding of the host. Encapsulation experiments also reveal that the empty catenane can adaptively encapsulate cations such as [Et4N]+ and [Pr4N]+ in the cross cavity but is unable to encapsulate [Bu4N]+ and [Me4N]+, although the size of the latter is compatible with that of the cavity. Theoretical calculations and volume analysis allow to unravel the ingenious role of catenane structures and the interplay between electrostatic repulsion and attractive noncovalent interactions for size-specific recognition behavior in host-guest systems involving species with similar electric charges.

Guest-Induced Reversible Transformation between an Azulene-Based Pd2L4 Lantern-Shaped Cage and a Pd4L8 Tetrahedron

Azulene, a blue structural isomer of naphthalene, is introduced as the backbone for a new family of Pd(II)-based self-assemblies. Three organic ligands, equipped with varying donor groups, produce three [Pd2L4] cages of different cavity dimensions. Unexpectedly, the addition of organic disulfonate guests to the smallest lantern-shaped cage (featuring pyridine donors) led to a rapid and quantitative transformation to a distorted-tetrahedral [Pd4L8] species. On the contrary, [Pd2L4] cages formed from ligands with isoquinoline donors either just encapsulated the guests or showed no interaction. The tetrahedral species could be fully reverted back to its original [Pd2L4] topology by capturing the guest by another, stronger binding [Pd2L'4] coordination cage, narcissistically self-sorting from the first cage. The azulenes, serving as colored hydrocarbon backbones of minimal atom count, allow one to follow cage assembly and guest-induced transformation by the naked eye. Furthermore, we propose that their peculiar electronic structure influences the system's assembly behavior.

Synthesis of an Adaptable Molecular Barrel and Guest Mediated Stabilization of Its Metastable Higher Homologue

Structural and functional modulation of three-dimensional artificial macromolecular systems is of immense importance. Designing supramolecular cages that can show stimuli mediated reversible switching between higher-order structures is quite challenging. We report here construction of a Pd₆ trifacial barrel (1) by coordination self-assembly. Surprisingly, barrel 1 was found to exhibit guest-responsive behavior. In presence of fullerenes C₆₀ and C₇₀, 1 unprecedentedly transformed to its metastable higher homologue Pd₈ tetrafacial barrel (2), forming stable host-guest complexes (C₆₀)₃⊂2 and (C₇₀)₂⊂2, respectively. Again, encapsulated fullerenes could be extracted from the cavity of 2 using 1,2-dichlorobenzene, leading to its facile conversion to the parent trifacial barrel 1. Such reversible structural interconversion between an adaptable molecular barrel and its guest stabilized higher homologue is an uncommon observation.

Solvent-Controlled Quadruple Catenation of Giant Chiral [8+12] Salicylimine Cubes Driven by Weak Hydrogen Bonding

Mechanically interlocked structures are fascinating synthetic targets and the topological complexity achieved through catenation offers numerous possibilities for the construction of new molecules with exciting properties. In the structural space of catenated organic cage molecules, only few examples have been realized so far, and control over the catenation process in solution is still barely achieved. Herein, we describe the formation of a quadruply interlocked catenane of giant chiral [8+12] salicylimine cubes. The formation could be controlled by the choice of solvent used in the reaction. The interlocked structure was unambiguously characterized by single crystal X-ray diffraction and weak hydrogen bonding was identified as a central driving force for the catenation. Furthermore, scrambling experiments using partially deuterated cages were performed, revealing that the catenane formation occurs through mechanical interlocking of preformed single cages.

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

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

Dimeric and trimeric catenation of giant chiral [8 + 12] imine cubes driven by weak supramolecular interactions

Mechanically interlocked structures, such as catenanes and rotaxanes, are fascinating synthetic targets and some are used for molecular switches and machines. Today, the vast majority of catenated structures are built upon macrocycles and only a very few examples of three-dimensional shape-persistent organic cages forming such structures have been reported. However, the catenation in all these cases was based on a thermodynamically favoured π-π-stacking under certain reaction conditions. Here, we show that catenane formation can be induced by adding methoxy or thiomethyl groups to one of the precursors during the synthesis of chiral [8 + 12] imine cubes, giving dimeric and trimeric catenated organic cages. To elucidate the underlying driving forces, we reacted 11 differently 1,4-disubstituted terephthaldehydes with a chiral triamino tribenzotriquinacene under various conditions to study whether monomeric cages or catenated cage dimers are the preferred products. We find that catenation is mainly directed by weak interactions derived from the substituents rather than by π-stacking.

Transformation networks of metal-organic cages controlled by chemical stimuli

The flexibility of biomolecules enables them to adapt and transform as a result of signals received from the external environment, expressing different functions in different contexts. In similar fashion, coordination cages can undergo stimuli-triggered transformations owing to the dynamic nature of the metal-ligand bonds that hold them together. Different types of stimuli can trigger dynamic reconfiguration of these metal-organic assemblies, to switch on or off desired functionalities. Such adaptable systems are of interest for applications in switchable catalysis, selective molecular recognition or as transformable materials. This review highlights recent advances in the transformation of cages using chemical stimuli, providing a catalogue of reported strategies to transform cages and thus allow the creation of new architectures. Firstly we focus on strategies for transformation through the introduction of new cage components, which trigger reconstitution of the initial set of components. Secondly we summarize conversions triggered by external stimuli such as guests, concentration, solvent or pH, highlighting the adaptation processes that coordination cages can undergo. Finally, systems capable of responding to multiple stimuli are described. Such systems constitute composite chemical networks with the potential for more complex behaviour. We aim to offer new perspectives on how to design transformation networks, in order to shed light on signal-driven transformation processes that lead to the preparation of new functional metal-organic architectures.

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

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

Nonaqueous Emulsion Polycondensation Enabled by a Self‐Assembled Cage‐like Surfactant

Nonaqueous emulsions are crucial for a range of applications based on water‐sensitive systems such as controlled polymerizations requiring anhydrous reaction conditions and the stabilization of readily hydrolyzable reagents or pharmacologically active components. However, defined molecular surfactants to stabilize such nonaqueous emulsions are scarce. We introduce a self‐assembled coordination cage, decorated with cholesterol functionalities, to serve as a molecular surfactant for various oil‐in‐oil emulsions of immiscible organic solvents. While the positively charged cage forms the amphiphile's polar moiety, the non‐polar cholesterol appendices can bend in a common direction to stabilize the emulsion. Templated by the droplets, polycondensation reactions were carried out to produce microstructured polyurethane and polyurea materials of different particle sizes and morphologies. Further, the amphiphilic cage can encapsulate a guest molecule and the resulting host‐guest assembly was also examined as a surfactant. In addition, the aggregation behavior of the amphiphilic cage in an aqueous medium was examined.

A class of organic cages featuring twin cavities

A variety of organic cages with different geometries have been developed during the last decade, most of them exhibiting a single cavity. In contrast, the number of organic cages featuring a pair of cavities remains scarce. These structures may pave the way towards novel porous materials with emergent properties and functions.We herein report on rational design of a three-dimensional hexaformyl precursor 1, which exhibits two types of conformers, i.e. Conformer-1 and -2, with different cleft positions and sizes. Aided by molecular dynamics simulations, we select two triamino conformation capturers (denoted CC). Small-sized CC-1 selectively capture Conformer-1 by matching its cleft size, while the large-sized CC-2 is able to match and capture both conformers. This strategy allows the formation of three compounds with twin cavities, which we coin diphane. The self-assembly of diphane units results in superstructures with tunable proton conductivity, which reaches up to 1.37×10^-5 S cm^-1.

The preparation of nanocages with unprecedented architectures may lead to new functions. Here the authors report the self-assembly of organic cages featuring twin cavities; the geometry and pocket size determine the molecular packing and the proton conductivity performance.

Anion binding properties of a hollow PdL-cage

The hollow [PdL][BArF]2 complex 1 of a tetra-pyridyl (py) ligand (L) has a [Pd(py)4]2+ coordination environment. Addition of coordinating anions resulted in the formation of a neutral species with Pd(py)2(anion)2 coordination environment (12A). These species bind further to the coordinating anions in the order Cl- > N3- > Br- > I- > AcO- with Ka1 : 1 ≤ 414 M-1. With relatively non-coordinating anions 1 remains intact and displays 1 : 2 binding behaviour dominated by the 1 : 1 stoichiometry in the order NO3- (∼105 M-1) » ClO4- and BF4- (∼103 M-1). As evidenced by crystal structure data, DFT calculations and {1H-19F}-HOESY NMR with BF4-, the anions are bound by charge assisted [C-H]+···anion interactions.

Supramolecular Coordination Cages Based on N-Heterocyclic Carbene-Gold(I) Ligands and Their Precursors: Self-Assembly, Structural Transformation and Guest-Binding Properties

The incorporation of functional groups into the cavity of discrete supramolecular coordination cages (SCCs) will bring unique functions and applications. Here, three dicarboxylate ligands (H₂ L1Cl, H₂ L2Cl and H₂ L3Cl) containing N-heterocyclic carbene (NHC) precursors as linkers were introduced to construct SCCs by combining with two C₃ -symmertic (CpZr)₃ (μ₃ -O)(μ₂ -OH)₃ clusters as three-connect vertices, resulted in a series of rugby-like V₂ E₃ (V=vertex, E=edge) type homoleptic cages (SCC-1, SCC-2 and SCC-3). However, V₄ E₆ -type tetrahedral cages (SCC-4 and SCC-5), incorporating six Au-NHC moieties, were obtained when the corresponding NHC-gold(I) functionalized ligands (H₂ L1^Au , H₂ L2^Au ) were applied. For the first time, we present a trackable CpZr-involved cage to cage conversion to generate a heteroleptic V₂ E₃ cage (SCC-6) from two homoleptic cages (SCC-2 and SCC-5) with different geometries of V₂ E₃ and V₄ E₆ . The heteroleptic assembly SCC-6 can also be formed upon a subcomponent displacement strategy. The structural transformation and reassembly processes were detected and monitored by ¹ H NMR spectroscopy and electrospray-ionization mass spectrometry. The formation of heteroleptic assembly was further supported by single crystal X-ray diffraction analysis. Moreover, homoleptic cage SCC-2 possesses a trigonal bipyramidal cationic cavity allowing the encapsulation of a series of sulfonate anionic guests.

Increasing structural and functional complexity in self-assembled coordination cages

Progress in metallo-supramolecular chemistry creates potential to synthesize functional nano systems and intelligent materials of increasing complexity. In the past four decades, metal-mediated self-assembly has produced a wide range of structural motifs such as helicates, grids, links, knots, spheres and cages, with particularly the latter ones catching growing attention, owing to their nano-scale cavities. Assemblies serving as hosts allow application as selective receptors, confined reaction environments and more. Recently, the field has made big steps forward by implementing dedicated functionality, e.g. catalytic centres or photoswitches to allow stimuli control. Besides incorporation in homoleptic systems, composed of one type of ligand, desire arose to include more than one function within the same assembly. Inspiration comes from natural enzymes that congregate, for example, a substrate recognition site, an allosteric regulator element and a reaction centre. Combining several functionalities without creating statistical mixtures, however, requires a toolbox of sophisticated assembly strategies. This review showcases the implementation of function into self-assembled cages and devises strategies to selectively form heteroleptic structures. We discuss first examples resulting from a combination of both principles, namely multicomponent multifunctional host-guest complexes, and their potential in application in areas such as sensing, catalysis, and photo-redox systems.

Russian-Doll-Like Molecular Cubes

Nanosized cage-within-cage compounds represent a synergistic molecular self-assembling form of three-dimensional architecture that has received particular research focus. Building multilayered ultralarge cages to simulate complicated virus capsids is believed to be a tough synthetic challenge. Here, we synthesize two large double-shell supramolecular cages by facile self-assembly of presynthesized metal-organic hexatopic terpyridine ligands with metal ions. Differing from the mixture of prisms formed from the inner tritopic ligand, the redesigned metal-organic hexatopic ligands bearing high geometric constraints that led to the exclusive formation of discrete double-shell structures. These two unique nested cages are composed of inner cubes (5.1 nm) and outer huge truncated cubes (12.0 and 13.2 nm) with six large bowl-shape subcages distributed on six faces. The results with molecular weights of 75 232 and 77 667 Da were among the largest synthetic cage-in-cage supramolecules reported to date. The composition, size and shape were unambiguously characterized by a combination of ¹H NMR, DOSY, ESI-MS, TWIM-MS, TEM, AFM, and SAXS. This work provides an interesting model for functional recognition, delivery, and detection of various guest molecules in the field of supramolecular materials.

Dynamic Stereoselection of Peptide Helicates and Their Selective Labeling of DNA Replication Foci in Cells*

Although largely overlooked in peptide engineering, coordination chemistry offers a new set of interactions that opens unexplored design opportunities for developing complex molecular structures. In this context, we report new artificial peptide ligands that fold into chiral helicates in the presence of labile metal ions such as Fe^II and Co^II . Heterochiral β-turn-promoting sequences encode the stereoselective folding of the peptide ligands and define the physicochemical properties of their corresponding metal complexes. Circular dichroism and NMR spectroscopy in combination with computational methods allowed us to identify and determine the structure of two isochiral ΛΛ-helicates, folded as topological isomers. Finally, in addition to the in-vitro characterization of their selective binding to DNA three-way junctions, cell-microscopy experiments demonstrated that a rhodamine-labeled Fe^II helicate was internalized and selectively stains DNA replication factories in functional cells.

Can we predict materials that can be synthesised?

The discovery of materials is an important element in the development of new technologies and abilities that can help humanity tackle many challenges. Materials discovery is frustratingly slow, with the large time and resource cost often providing only small gains in property performance. Furthermore, researchers are unwilling to take large risks that they will only know the outcome of months or years later. Computation is playing an increasing role in allowing rapid screening of large numbers of materials from vast search space to identify promising candidates for laboratory synthesis and testing. However, there is a problem, in that many materials computationally predicted to have encouraging properties cannot be readily realised in the lab. This minireview looks at how we can tackle the problem of confirming that hypothetical materials are synthetically realisable, through consideration of all the stages of the materials discovery process, from obtaining the components, reacting them to a material in the correct structure, through to processing into a desired form. In an ideal world, a material prediction would come with an associated 'recipe' for the successful laboratory preparation of the material. We discuss the opportunity to thus prevent wasted effort in experimental discovery programmes, including those using automation, to accelerate the discovery of novel materials.

A New Mechanically-Interlocked [Pd2 L4 ] Cage Motif by Dimerization of two Peptide-based Lemniscates

Most metallo-supramolecular assemblies of low nuclearity adopt simple topologies, with bridging ligands spanning neighboring metal centers in a direct fashion. Here we contribute a new structural motif to the family of host compounds with low metal count (two) that consists of a pair of doubly-interlocked, Figure-eight-shaped subunits, also termed "lemniscates". Each metal is chelated by two chiral bidentate ligands, composed of a peptidic macrocycle that resembles a natural product with two pyridyl-terminated arms. DFT calculation results suggest that dimerization of the mononuclear halves is driven by a combination of 1) Coulomb interaction with a central anion, 2) π-stacking between intertwined ligand arms and 3) dispersive interactions between the structure's compact inner core bedded into an outer shell composed of the cavitand-type macrocycles. The resulting cage-like architecture was characterized by NMR, MS and X-ray structure analyses. This new mechanically bonded system highlights the scope of structural variety accessible in metal-mediated self-assemblies composed of only a few constituents.

A covalent deprotection strategy for assembling supramolecular coordination polymers from metal-organic cages

A Cu₄L₄ metal-organic cage (MOC) composed of amine-protected ligands forms supramolecular coordination polymers (SCPs) upon covalent post-assembly deprotection. The amorphous SCPs form by virtue of aniline-copper coordination and possess a tunable porosity based on the rate of deprotection.

A hierarchical assembly strategy for near-infrared photothermal conversion: unconventional heterogeneous metalla[2]catenanes

Herein, we report a hierarchical assembly strategy for constructing heterogeneous half-sandwich organometallic D-A (D = π-donor, A = π-acceptor) interlocked structures, and their application in near-infrared (NIR) photothermal conversion. Thienothiophene and diketopyrrolopyrrole groups were selected as the D and A units, leading to two homogeneous metalla[2]catenanes with D-D-D-D and A-A-A-A stacks, respectively. By the ordered secondary assembly of homogeneous metalla[2]catenanes, two unprecedented heterogeneous D-A metalla[2]catenanes comprising an unusual mixed D-A-D-D and unconventional D-A-A-A stacks were realized by the combination of multiple noncovalent interactions, as all demonstrated by a detailed X-ray crystallographic study. Benefiting from the mixed D-A stacking modes, NIR absorption of heterogeneous D-A metalla[2]catenanes is significantly enhanced in contrast to homogeneous metalla[2]catenanes. Thanks to the enhanced NIR absorption and the fluorescence quenching effect from half-sandwich organometallic fragments, heterogeneous D-A metalla[2]catenanes displayed high-performance NIR photothermal conversion properties (η = 27.3%).

Balancing Ligand Flexibility versus Rigidity for the Stepwise Self-Assembly of M12 L24 via M6 L12 Metal-Organic Cages

Non-covalent interactions are important for directing protein folding across multiple intermediates and can even provide access to multiple stable structures with different properties and functions. Herein, we describe an approach for mimicking this behavior in the self-assembly of metal-organic cages. Two ligands, the bend angles of which are controlled by non-covalent interactions and one ligand lacking the above-mentioned interactions, were synthesized and used for self-assembly with Pd²⁺ . As these weak interactions are easily broken, the bend angles have a controlled flexibility giving access to M₂ (L1)₄ , M₆ (L2)₁₂ , and M₁₂ (L2)₂₄ cages. By controlling the self-assembly conditions this process can be directed in a stepwise fashion. Additionally, the multiple endohedral hydrogen-bonding sites on the ligand were found to play a role in the binding and discrimination of neutral guests.

Coordination-Directed Construction of Molecular Links

Since the emergence of the concept of chemical topology, interlocked molecular assemblies have graduated from academic curiosities and poorly defined species to become synthetic realities. Coordination-directed synthesis provides powerful, diverse, and increasingly sophisticated protocols for accessing interlocked molecules. Originally, metal ions were employed solely as templates to gather and position building blocks in entwined or threaded arrangements. Recently, metal centers have increasingly featured within the backbones of the integral structural elements, which in turn use noncovalent interactions to self-assemble into intricate topologies. By outlining ingenious recent examples as well as seminal classic cases, this Review focuses on the role of metal-ligand paradigms in assembling molecular links. In addition, the ever-evolving approaches to efficient assembly, the structural features of the resulting architectures, and their prospects for the future are also presented.

A cyclic bis[2]catenane metallacage

Catenated cages represent chemistry’s challenging synthetic targets because a three-dimensional assembly is necessary for their formation. Herein, a cyclic bis[2]catenane is constructed through the coordination-driven self-assembly of the interlocked bis-metallacage, by the 90° Pt(II) heteroligation of the endo-functionalized double-bridged tweezer bearing pyridyl moieties and the tetra-carboxylated linker. NMR spectrometry, X-ray crystallography and mass spectrometry confirm the formation of a cyclic bis[2]catenane with “∞”-shaped topology via a 14-component self-assembly. Particularly, reversibly responsive transformation between the bis[2]catenane and the bis-metallacage can be realized by guest exchange, concentration effect and solvent effect. This work represents a novel example of a cyclic cage-based [2]catenane oligomer.

Catenated cages are challenging synthetic targets in chemistry. Here, the authors employ a multi-component coordination strategy using a Pt(II) heteroligation to construct a cyclic bis[2]catenane metallacage, which could be reversibly transformed between the catenated structure and the bis-metallacage.

Kinetically Controlled Fast Crystallization of M12L8 Poly-[n]-catenanes Using the 2,4,6-Tris(4-pyridyl)benzene Ligand and ZnCl2 in an Aromatic Environment

Kinetic control in the presence of six aromatic solvents has been successfully applied in the synthesis of a poly-[n]-catenane composed of interlocked M₁₂L₈ icosahedral nanometric cages (i.e., internal voids of 2500 ų). When the exotridentate tris-pyridyl benzene ligand and ZnCl₂ with appropriate templating molecules because of good ligand aromatic interactions are used, the metal-organic cages can be synthesized very fast, homogeneously, and in large quantities as microcrystalline materials. Synchrotron single-crystal X-ray data (100 K) allowed the resolution of nitrobenzene guest molecules at the internal walls of the M₁₂L₈ nanocages, whereas in the central part of the cages the solvent is highly disordered. The guest release occurs in two steps with the disordered nitrobenzene guests released in the first step (lower temperatures) because of the absence of strong cage-guest interactions. Density functional theory calculations provided a rationalization of these outcomes and, in particular, solid-state approaches, showed theoretical evidence of the kinetic nature in the formation of the poly-[n]-catenane by the analysis of the packing energy in terms of monomeric and dimeric cages.

Embedding and Positioning of Two FeII4 L4 Cages in Supramolecular Tripeptide Gels for Selective Chemical Segregation

An unreported d,l-tripeptide self-assembled into gels that embedded FeII4 L4 metal-organic cages to form materials that were characterized by TEM, EDX, Raman spectroscopy, rheometry, UV/Vis and NMR spectroscopy, and circular dichroism. The cage type and concentration modulated gel viscoelasticity, and thus the diffusion rate of molecular guests through the nanostructured matrix, as gauged by 19 F and 1 H NMR spectroscopy. When two different cages were added to spatially separated gel layers, the gel-cage composite material enabled the spatial segregation of a mixture of guests that diffused into the gel. Each cage selectively encapsulated its preferred guest during diffusion. We thus present a new strategy for using nested supramolecular interactions to enable the separation of small molecules.

Probing the Dynamics of the Imine-Based Pentafoil Knot and Pentameric Circular Helicate Assembly

We investigate the self-assembly dynamics of an imine-based pentafoil knot and related pentameric circular helicates, each derived from a common bis(formylpyridine)bipyridyl building block, iron(II) chloride, and either monoamines or a diamine. The mixing of circular helicates derived from different amines led to the complete exchange of the N-alkyl residues on the periphery of the metallo-supramolecular scaffolds over 4 days in DMSO at 60 °C. Under similar conditions, deuterium-labeled and nonlabeled building blocks showed full dialdehyde building block exchange over 13 days for open circular helicates but was much slower for the analogous closed-loop pentafoil knot (>60 days). Although both knots and open circular helicates self-assemble under thermodynamic control given sufficiently long reaction times, this is significantly longer than the time taken to afford the maximum product yield (2 days). Highly effective error correction occurs during the synthesis of imine-based pentafoil molecular knots and pentameric circular helicates despite, in practice, the systems not operating under full thermodynamic control.

The chemistry of multi-component and hierarchical framework compounds

This review is expected to provide a library of multi-component hierarchically porous compounds, which shall guide the state-of-the-art design of future porous materials with unprecedented tunability, synergism and precision.

A luminescent edge-interlocked prismatic heteroleptic metallocage assembled through a ligand replacement reaction

A luminescent edge-interlocked heteroleptic metallocages based on Cu₃(pyrazolate)₃ was prepared through a ligand replacement reaction from a homoleptic metallocage and a new ligand.

Waterproof architectures through subcomponent self-assembly

Construction of metal–organic containers that are soluble and stable in water can be challenging – we present diverse strategies that allow the synthesis of kinetically robust water-soluble architectures via subcomponent self-assembly.

Metal–organic containers are readily prepared through self-assembly, but achieving solubility and stability in water remains challenging due to ligand insolubility and the reversible nature of the self-assembly process. Here we have developed conditions for preparing a broad range of architectures that are both soluble and kinetically stable in water through metal(ii)-templated (M^II = Co^II, Ni^II, Zn^II, Cd^II) subcomponent self-assembly. Although these structures are composed of hydrophobic and poorly-soluble subcomponents, sulfate counterions render them water-soluble, and they remain intact indefinitely in aqueous solution. Two strategies are presented. Firstly, stability increased with metal–ligand bond strength, maximising when Ni^II was used as a template. Architectures that disassembled when Co^II, Zn^II and Cd^II templates were employed could be directly prepared from NiSO₄ in water. Secondly, a higher density of connections between metals and ligands within a structure, considering both ligand topicity and degree of metal chelation, led to increased stability. When tritopic amines were used to build highly chelating ligands around Zn^II and Cd^II templates, cryptate-like water-soluble structures were formed using these labile ions. Our synthetic platform provides a unified understanding of the elements of aqueous stability, allowing predictions of the stability of metal–organic cages that have not yet been prepared.

Hierarchical Assemblies of Supramolecular Coordination Complexes

Hierarchical self-assembly (HAS) is a multilevel organization process that first assembles elementary molecular units into ordered secondary structures via noncovalent interactions, which further act as the building blocks to form more complex multifunctional superstructures at the next level(s). The HAS strategy has been used as a versatile method for the preparation of soft-matter nanoarchitectures of defined size and morphologies, tunable luminescence, and biological importance. However, such preparation can be greatly simplified if well-defined dynamic structures are employed as the cores that upon linking form the desired nanoarchitectures. Discrete supramolecular coordination complexes (SCCs) with well-defined shapes, sizes, and internal cavities have been widely employed to construct hierarchical systems with functional diversity. This Account summarizes the prevailing strategies used in recent years in the preparation of SCC-based HASs and illustrates how the combination of dynamic metal-ligand coordination with other interactions was used to obtain hierarchical systems with interesting properties. HASs with dual orthogonal interactions involving coordination-driven self-assembly and hydrogen bonding/host-guest interaction generally result in robust and flexible supramolecular gels. Likewise, hybridization of SCCs with a suitable dynamic covalent network via a hierarchical strategy is useful to prepare materials with self-healing properties. The intrinsic positive charges of the SCCs also make them suitable precursors for the construction of HASs via electrostatic interactions with negatively charged biological/abiological molecules. Furthermore, the interplay between the hydrophilic and lipophilic characters of HASs by varying the number and spacial orientation of alkyl/oxyethylene chains of the SCC is a simple yet controllable approach to prepare ordered and tunable nanostructures. Certain SCC-cored hierarchical systems exhibit reversible polymorphism, typically between micellar, nanofiber, and vesicular phases, in response to various external perturbations: heat, photoirradiation, pH-variance, redox-active agents, etc. At the same time, multiple noncovalent interaction mediated HASs are growing in numbers and are promising candidates for obtaining functionally diverse materials. The photophysical properties of SCC-based HASs have been used in many analytical applications. For example, embedding tetraphenylethene (TPE)-based pyridyl ligands within metallo-supramolecular structures partially restricts the molecular rotations of its phenyl rings, endowing the resultant SCCs with weak emissions. Further aggregation of such HASs in suitable solvents results in a marked enhancement in emission intensity along with quantum yields. They act as sensitive sensors for different analytes, including pathogens, drugs, etc. HASs are also useful to develop multidrug systems with cooperative chemotherapeutic effects. Hence, the use of HASs with theranostic SCCs combining cell-imaging agents and chemotherapeutic scaffolds is a promising drug delivery strategy for cancer theranostics. At the same time, their responsiveness to stimuli, oftentimes due to the dynamic nature of the metal-ligand interactions, play an important role in drug release via a disassembly mechanism.