A Star of David catenane

The Topological Transformation of 41 Knot to 412 Link through Supramolecular Fusion

Realizing topological transformation through supramolecular fusion is particularly challenging, as the self-assembly of disparate components often results in the orthogonal assembly of building blocks into distinct structures rather than the formation of a heteroleptic architecture. This study introduces a topological transformation, transitioning from a figure-eight knot (41 knot) to a Solomon link (412 link) through a supramolecular fusion process. By employing two structurally similar amino acid ligands (L1 and L3) of varying lengths as bridge ligands, we obtained figure-eight knot 1 and a molecular tweezer-like compound 3 when individually complexed with binuclear Cp*Rh acceptor B1. Our results revealed that subtle modifications to bridge ligands can lead to dramatic changes in their structures and recognition properties. Moreover, we successfully achieved the targeted formation of a heteroleptic Solomon link 4 by blending figure-eight knot 1 and compound 3 in a 1:1 ratio without the need for templates. This procedure effortlessly converted the 41 knot into a 412 link, thus marking a significant advancement in the topological transformation. This work not only marks the construction of the first heteroleptic Solomon link comprising two distinct metallamacrocycles but also demonstrates a process of supramolecular fusion-based topological transformation involving three distinct topological structures.

Folding a Molecular Strand into a Trefoil Knot of Single Handedness with Co(II)/Co(III) Chaperones

We report the synthesis of a right-handed (Δ-stereochemistry of strand crossings) trefoil knot from a single molecular strand containing three pyrazine-2,5-dicarboxamide units adjacent to point-chiral centers and six pyridine moieties. The oligomeric ligand strand folds into an overhand (open-trefoil) knot through the assistance of coordinatively dynamic Co(II) "chaperones" that drive the formation of a three-metal-ion circular helicate. The entangled structure is kinetically locked by oxidation to Co(III) and covalently captured by ring-closing olefin metathesis to generate a trefoil knot of single topological handedness. The stereochemistry of the strand crossings in the metal-coordinated overhand knot is governed by the stereochemistry of the point-chiral carbon centers in the ligand strand. The overhand and trefoil knots were characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography. Removal of the metal ions from the knot, followed by hydrogenation of the alkene, yielded the wholly organic trefoil knot. The metal-free knot and parent ligand were investigated by circular dichroism (CD) spectroscopy. The CD spectra indicate that the topological stereochemistry of the knot has a greater effect on the asymmetry of the chromophore environment than do the point-chiral centers of the strand.

Epitaxially Grown Mechanically Robust 2D Thin Film of Secondary Interactions Led Molecularly Woven Material

Molecularly woven materials with striking mechanical resilience, and 2D controlled topologies like textiles, fishing nets, and baskets are highly anticipated. Molecular weaving exclusively apprehended by the secondary interactions expanding to laterally grown 2D self-assemblies with retained crystalline arrangement is stimulating. The interlacing entails planar molecules screwed together to form 2D woven thin films. Here, secondary interactions led 2D interlaced molecularly woven material (2°MW) built by 1D helical threads of organic chromophores twisted together via end-to-end CH···O connections, held strongly at inter-crossing by multiple OH···N interactions to prevent slippage is presented. Whereas, 1D helical threads with face-to-face O-H···O connections sans interlacing led the non-woven material (2°NW). The polarity-driven directionality in 2°MW led the water-actuated epitaxial growth of 2D-sheets to lateral thin films restricted to nano-scale thickness. The molecularly woven thin film is self-healing, flexible, and mechanically resilient in nature, while maintaining the crystalline regularity is attributed to the supple secondary interactions (2°).

The Topological Transformation of Trefoil Knots to Solomon Links via Diels-Alder Click Reaction

The quest for more efficient, user-friendly, and less wasteful topological transformations remains a significant challenge in the realm of postassembly modifications. In this article, high yields of two molecular trefoil knots (Rh-1, Ir-1) were obtained using ligand 3,6-bis(3-(pyridin-4-yl)phenyl)-1,2,4,5-tetrazine (L1) with reactive tetrazine units and binuclear half-sandwich organometallic units [Cp*2M2(μ-TPPHZ)(OTf)2](OTf)2 (Rh-B, M = RhIII; Ir-B, M = IrIII). 2,5-Norbornadiene was used as an inducer of the Diels-Alder click reaction to modulate rapidly and efficiently the transformation of Trefoil knots to Solomon links. However, the key to achieving this topological structural change is the subtle increase in site steric of the pyridazine fragments (L2), which allows the molecular structures to spread and bend in three-dimensional space, as confirmed by single-crystal X-ray diffraction, ESI-TOF/MS, elementary analysis and detailed solution-state NMR techniques.

Giant Expanded Porous Metallo-Hexagons

Molecular self-assembly is a widely recognized approach for fabricating biomimetic functional nanostructures. Here, we report the synthesis of two giant hollow coronoid-like supramolecular hexagons, H1 and H2. These hexagons feature large cavities, showcasing unique inner and outer hexagons fixed by specific connectivities for enhanced stability and high metal center density. H1 exhibits properties that can be transformed through the thermodynamic conversion of the metallopolymer formed by L1 and L2. With an edge length of 6.8 nm, H2 is one of the largest hexagons reported to date. 1D and 2D NMR, TEM, ESI-MS, and TWIM-MS experiments provided conclusive evidence for the composition and structure of the assembled hexagons. This work demonstrates the feasibility of constructing giant supramolecular architectures with precise control over their size and shape, opening up new possibilities for the design and synthesis of sophisticated supramolecules and nonbiological materials.

Advancements and strategic approaches in catenane synthesis

Catenanes, a distinctive category of mechanically interlocked molecules composed of intertwined macrocycles, have undergone significant advancements since their initial stages characterized by inefficient statistical synthesis methods. Through the aid of molecular recognition processes and principles of self-assembly, a diverse array of catenanes with intricate structures can now be readily accessed utilizing template-directed synthetic protocols. The rapid evolution and emergence of this field have catalyzed the design and construction of artificial molecular switches and machines, leading to the development of increasingly integrated functional systems and materials. This review endeavors to explore the pivotal advancements in catenane synthesis from its inception, offering a comprehensive discussion of the synthetic methodologies employed in recent years. By elucidating the progress made in synthetic approaches to catenanes, our aim is to provide a clearer understanding of the future challenges in further advancing catenane chemistry from a synthetic perspective.

A CuI6L4 Cage Dynamically Reconfigures to Form Suit[4]anes and Selectively Bind Fluorinated Steroids

Simple organic ligands can self-assemble with metal ions to generate metal-organic cages, whose cavities bind guests selectively. This binding may enable new methods of chemical separation or sensing, among other useful functions. Here we report the preparation of a CuI6L4 pseudo-octahedral metal-organic cage, the ligands of which self-assemble from simple organic building blocks. Temperature, solvent, and the presence of different guests governed which structure predominated from a dynamic mixture of cage diastereomers with different arrangements of right- or left-handed metal vertices. Dissolution in dimethyl sulfoxide or the binding of tetrahedral guests led to a chiral tetrahedral T-symmetric framework, whereas low temperatures favored the achiral S4-symmetric diastereomer. Tetrahedral guests with long arms were encapsulated to form mechanically bonded suit[4]anes, with guest arms protruding out through host windows. The cage was also observed to bind fluorinated steroids, an important class of drug molecules, but not non-fluorinated steroids, providing the basis for new separation processes.

Unraveling the Influence of Topology and Spatial Confinement on Equilibrium and Relaxation Properties of Interlocked Ring Polymers

We use Langevin dynamics simulations to study linked ring polymers in channel confinement. We address the in- and out-of-equilibrium behavior of the systems for varying degrees of confinement and increasing topological and geometrical complexity of the interlocking. The main findings are three. First, metric observables of different link topologies collapse onto the same master curve when plotted against the crossing number, revealing a universal response to confinement. Second, the relaxation process from initially stretched states is faster for more complex links. We ascribe these properties to the interplay of several effects, including the dependence of topological friction on the link complexity. Finally, we show that transient forms of geometrical entanglement purposely added to the initial stressed state can leave distinctive signatures in force-spectroscopy curves. The insight provided by the findings could be leveraged in single-molecule nanochannel experiments to identify geometric entanglement within topologically linked rings.

Unravelling the Roles of Solvophobic Effects and π⋅⋅⋅π Stacking Interactions in the Formation of [2]Catenanes Featuring Di-(N-Heterocyclic Carbene) Building Blocks

A series of [2]catenanes has been prepared from di-NHC building blocks by utilizing solvophobic effects and/or π⋅⋅⋅π stacking interactions. The dinickel naphthobiscarbene complex syn-[1] and the kinked biphenyl-bridged bipyridyl ligand L2 yield the [2]catenane [2-IL](OTf)4 by self-assembly. Solvophobic effects are pivotal for the formation of the interlocked species. Substitution of the biphenyl-linker in L2 for a pyromellitic diimide group gave ligand L3 , which yielded in combination with syn-[1] the [2]catenane [3-IL](OTf)4 . This assembly exhibits enhanced stability in diluted solution, aided by additional π⋅⋅⋅π stacking interactions. The π⋅⋅⋅π stacking was augmented by the introduction of a pyrene bridge between two NHC donors in ligand L4 . Di-NHC precursor H2 -L4 (PF6 )2 reacts with Ag2 O to give the [Ag2 L4 2 ]2 [2]catenane [4-IL](PF6 )4 , which shows strong π⋅⋅⋅π stacking interactions between the pyrene groups. This assembly was readily converted into the [Au2 L4 2 ]2 gold species [5-IL](PF6 )4 , which exhibits exceptional stability based on the strong π⋅⋅⋅π stacking interactions and the enhanced stability of the Au-CNHC bonds.

Challenges and Opportunities of Functionalized Cucurbiturils for Biomedical Applications

Cucurbit[n]uril (CB[n]) macrocycles (especially CB[5] to CB[8]) have shown exceptional attributes since their discovery in 2000. Their stability, water solubility, responsiveness to several stimuli, and remarkable binding properties have enabled a growing number of biological applications. Yet, soon after their discovery, the challenge of their functionalization was set. Nevertheless, after more than two decades, a myriad of CB[n] derivatives has been described, many of them used in cells or in vivo for advanced applications. This perspective summarizes key advances of this burgeoning field and points to the next opportunities and remaining challenges to fully express the potential of these fascinating macrocycles in biology and biomedical sciences.

Cross-coupling by a noncanonical mechanism involving the addition of aryl halide to Cu(II)

Copper complexes are widely used in the synthesis of fine chemicals and materials to catalyze couplings of heteroatom nucleophiles with aryl halides. We show that cross-couplings catalyzed by some of the most active catalysts occur by a mechanism not previously considered. Copper(II) [Cu(II)] complexes of oxalamide ligands catalyze Ullmann coupling to form the C-O bond in aryl ethers by concerted oxidative addition of an aryl halide to Cu(II) to form a high-valent species that is stabilized by radical character on the oxalamide ligand. This mechanism diverges from those involving Cu(I) and Cu(III) intermediates that have been posited for other Ullmann-type couplings. The stability of the Cu(II) state leads to high turnover numbers, >1000 for the coupling of phenoxide with aryl chloride electrophiles, as well as an ability to run the reactions in air.

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.

Helical fluxionality: numerical frustration drives concerted low-barrier screw motions of a tricopper cluster

Uneven allocation of resources creates frustration, tension, and conflicts. Confronted with an apparent mismatch between the number of donor atoms and the number of metal atoms to be supported, helically twisted ligands cleverly come up with a sustainable symbiotic solution. As an example, we present a tricopper metallohelicate exhibiting screw motions for intramolecular site exchange. A combination of X-ray crystallographic and solution NMR spectroscopic studies revealed thermo-neutral site exchange of three metal centres hopping back and forth inside the helical cavity lined by a spiral staircase-like arrangement of ligand donor atoms. This hitherto unknown helical fluxionality is a superimposition of translational and rotational movements of molecular actuation, taking the shortest path with an extraordinarily low energy barrier without compromising the overall structural integrity of the metal-ligand assembly.

Orthogonal, modular anion-cation and cation-anion self-assembly using pre-programmed anion binding sites

Subcomponent self-assembly relies on cation coordination whereas the roles of anions often only emerge during the assembly process. When sites for anions are instead pre-programmed, they have the potential to be used as orthogonal elements to build up structure in a predictable and modular way. We explore this idea by combining cation (M⁺) and anion (X^-) binding sites together and show the orthogonal and modular build up of structure in a multi-ion assembly. Cation binding is based on a ligand (L) made by subcomponent metal-imine chemistry (M⁺ = Cu⁺, Au⁺) while the site for anion binding (X^- = BF₄ ^-, ClO₄ ^-) derives from the inner cavity of cyanostar (CS) macrocycles. The two sites are connected by imine condensation between a pyridyl-aldehyde and an aniline-modified cyanostar. The target assembly [LM-CS-X-CS-ML],⁺ generates two terminal metal complexation sites (LM and ML) with one central anion-bridging site (X) defined by cyanostar dimerization. We showcase modular assembly by isolating intermediates when the primary structure-directing ions are paired with weakly coordinating counter ions. Cation-directed (Cu⁺) or anion-bridged (BF₄ ^-) intermediates can be isolated along either cation-anion or anion-cation pathways. Different products can also be prepared in a modular way using Au⁺ and ClO₄ ^-. This is also the first use of gold(i) in subcomponent self-assembly. Pre-programmed cation and anion binding sites combine with judicious selection of spectator ions to provide modular noncovalent syntheses of multi-component architectures.

Organometallic Borromean Rings and [2]Catenanes Featuring Di-NHC Ligands

We report herein a series of organometallic Borromean rings (BRs) and [2]catenanes prepared from benzobiscarbene ligands. The reaction of dinickel complexes of the benzobiscarbenes 1 a-1 c with a thiazolothiazole bridged bipyridyl ligand L² led by self-assembly to a series of organometallic BRs. Solvophobic effects played a crucial role in the formation and stability of the interlocked species. The stability of BRs is related to the N-alkyl substituents at the precursors 1 a-1 c, where longer alkyl substitutes improve stability and inter-ring interactions. Solvophobic effects are also important for the stability of [2]catenanes prepared from 1 a-1 c and a flexible bipyridyl ligand L³ . In solution, an equilibrium between the [2]catenanes and their macrocyclic building blocks was observed. High proportions of [2]catenanes were obtained in concentrated solutions or polar solvents. The proportion of [2]catenanes in solution could be further enhanced by lengthening of the N-alkyl substitutes.

Speed Tuning of the Formation/Dissociation of a Metallorotaxane

Switching between the formation/dissociation of rotaxanes is important to control the function of various types of rotaxane-based materials. We have developed a convenient and simple strategy, the so-called "accelerator addition", to make a static rotaxane dynamic without apparently affecting the chemical structure. As an interlocked molecule that enables tuning of the formation/dissociation speed, a metallorotaxane was quantitatively generated by the complexation of a triptycene-based dumbbell-shaped mononuclear complex, [PdL₂ ]²⁺ (L=2,3-diaminotriptycene), with 27C9. As a result of the inertness of the Pd²⁺ -based coordination structure, the metallorotaxane was slowly formed (the static state). This rotaxane formation was accelerated 27 times simply by adding Br^- as an accelerator (the dynamic state). A similar drastic acceleration was also demonstrated during the dissociation process when Cs⁺ was added to the metallorotaxane to form the free axle [PdL₂ ]²⁺ and the 27C9-Cs⁺ complex.

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.

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.

The Story of the Little Blue Box: A Tribute to Siegfried Hünig

The tetracationic cyclophane, cyclobis(paraquat-p-phenylene), also known as the little blue box, constitutes a modular receptor that has facilitated the discovery of many host-guest complexes and mechanically interlocked molecules during the past 35 years. Its versatility in binding small π-donors in its tetracationic state, as well as forming trisradical tricationic complexes with viologen radical cations in its doubly reduced bisradical dicationic state, renders it valuable for the construction of various stimuli-responsive materials. Since the first reports in 1988, the little blue box has been featured in over 500 publications in the literature. All this research activity would not have been possible without the seminal contributions carried out by Siegfried Hünig, who not only pioneered the syntheses of viologen-containing cyclophanes, but also revealed their rich redox chemistry in addition to their ability to undergo intramolecular π-dimerization. This Review describes how his pioneering research led to the design and synthesis of the little blue box, and how this redox-active host evolved into the key component of molecular shuttles, switches, and machines.

Shape-Dependent Complementary Ditopic Terpyridine Pair with Two Levels of Self-Recognition for Coordination-Driven Self-Assembly

Molecular recognition in biological systems plays a vital role in the precise construction of biomacromolecules and the corresponding biological activities. Such recognition mainly relies on the highly specific binding of complementary molecular pairs with complementary sizes, shapes, and intermolecular forces. It still remains challenging to develop artificial complementary motif pairs for coordination-driven self-assembly. Herein, a series of shape-dependent complementary motif pairs, based on ditopic 2,2':6',2″-terpyridine (TPY) backbone, are designed and synthesized. The fidelity degrees of self-assemblies from these motifs are carefully evaluated by multi-dimensional mass spectrometry, nuclear magnetic resonance spectroscopy, and molecular modeling. In addition, two levels of self-recognition in both homoleptic and heteroleptic assembly are discovered in the assembled system. Through finely tuning the shape and size of the ligands, a complementary pair is developed with error-free narcissistically self-sorting at two levels of self-recognition, and the intrinsic principle is carefully investigated.

Topologically Controlled Syntheses of Unimolecular Oligo[n]catenanes

Catenanes are a well-known class of mechanically interlocked molecules that possess chain-like architectures and have been investigated for decades as molecular machines and switches. However, the synthesis of higher-order catenanes with multiple, linearly interlocked molecular rings has been greatly impeded by the generation of unwanted oligomeric byproducts and figure-of-eight topologies that compete with productive ring closings. Here, we report two general strategies for the synthesis of oligo[n]catenanes that rely on a molecular "zip-tie" strategy, where the "zip-tie" is a central core macrocycle precursor bearing two phenanthroline (phen) ligands to make odd-numbered oligo[n]catenanes, or a preformed asymmetric iron(II) complex consisting of two macrocycle precursors bearing phen and terpyridine ligands to make even-numbered oligo[n]catenanes. In either case, preformed macrocycles or [2]catenanes are threaded onto the central "zip-tie" core using metal templation prior to ring-closing metathesis (RCM) reactions that generate several mechanical bonds in one pot. Using these synthetic strategies, a family of well-defined linear oligo[n]catenanes were synthesized, where n = 2, 3, 4, 5, or 6 interlocked molecular rings, and n = 6 represents the highest number of linearly interlocked rings reported to date for any isolated unimolecular oligo[n]catenane.

Systematic Synthesis of Macrocycles Bearing up to Six 2,2'-Bipyridine Moieties through Self-Assembled Double Helix Structure

A new synthetic strategy for macrocycles bearing multiple coordination moieties was developed. A self-assembled double helix structure, composed of two linear strands bearing 2,2'-bipyridine units and Cu(I) ions, provided access to macrocycles bearing a defined number of 2,2'-bipyridine moieties and a defined ring size, via an olefin-metathesis reaction between two linear strands in the helix. The double helix structure improved the selectivity of the macrocycle synthesis by bringing the reaction points in close proximity even in the case of large macrocycles.

Knotting matters: orderly molecular entanglements

Entangling strands in a well-ordered manner can produce useful effects, from shoelaces and fishing nets to brown paper packages tied up with strings. At the nanoscale, non-crystalline polymer chains of sufficient length and flexibility randomly form tangled mixtures containing open knots of different sizes, shapes and complexity. However, discrete molecular knots of precise topology can also be obtained by controlling the number, sequence and stereochemistry of strand crossings: orderly molecular entanglements. During the last decade, substantial progress in the nascent field of molecular nanotopology has been made, with general synthetic strategies and new knotting motifs introduced, along with insights into the properties and functions of ordered tangle sequences. Conformational restrictions imparted by knotting can induce allostery, strong and selective anion binding, catalytic activity, lead to effective chiral expression across length scales, binding modes in conformations efficacious for drug delivery, and facilitate mechanical function at the molecular level. As complex molecular topologies become increasingly synthetically accessible they have the potential to play a significant role in molecular and materials design strategies. We highlight particular examples of molecular knots to illustrate why these are a few of our favourite things.

A trefoil knot self-templated through imination in water

The preparation of topologically nontrivial molecules is often assisted by covalent, supramolecular or coordinative templates that provide spatial pre-organization for all components. Herein, we report a trefoil knot that can be self-assembled efficiently in water without involving additional templates. The direct condensation of three equivalents of a tetraformyl precursor and six equivalents of a chiral diamine produces successfully a [3 + 6] trefoil knot whose intrinsic handedness is dictated by the stereochemical configuration of the diamine linkers. Contrary to the conventional wisdom that imine condensation is not amenable to use in water, the multivalent cooperativity between all the imine bonds within the framework makes this trefoil knot robust in the aqueous environment. Furthermore, the presence of water is proven to be essential for the trefoil knot formation. A topologically trivial macrocycle composed of two tetraformyl and four diamino building blocks is obtained when a similar reaction is performed in organic media, indicating that hydrophobic effect is a major driving force behind the scene.

Altering the binding affinities of tetraruthenocycles for polycyclic aromatic hydrocarbons by post-assembly modification

We developed a strategy for manipulating the binding strength of polycyclic aromatic hydrocarbons (PAHs) via covalent post-assembly modification (PAM) of tetranuclear ruthenium macrocycles containing s-tetrazine ligands. The macrocycles act as efficient receptors for various PAHs. Inverse electron demand Diels-Alder (IEDDA) reaction of the macrocycles was applied to reduce the binding ability significantly.

Distinctive features and challenges in catenane chemistry

From being an aesthetic molecular object to a building block for the construction of molecular machines, catenanes and related mechanically interlocked molecules (MIMs) continue to attract immense interest in many research areas. Catenane chemistry is closely tied to that of rotaxanes and knots, and involves concepts like mechanical bonds, chemical topology and co-conformation that are unique to these molecules. Yet, because of their different topological structures and mechanical bond properties, there are some fundamental differences between the chemistry of catenanes and that of rotaxanes and knots although the boundary is sometimes blurred. Clearly distinguishing these differences, in aspects of bonding, structure, synthesis and properties, between catenanes and other MIMs is therefore of fundamental importance to understand their chemistry and explore the new opportunities from mechanical bonds.

Molecular weaving

Historically, the interlacing of strands at the molecular level has mainly been limited to coordination polymers and DNA. Despite being proposed on a number of occasions, the direct, bottom-up assembly of molecular building blocks into woven organic polymers remained an aspirational, but elusive, target for several decades. However, recent successes in two-dimensional and three-dimensional molecular-level weaving now offer new opportunities and research directions at the interface of polymer science and molecular nanotopology. This Perspective provides an overview of the features and potential of the periodic nanoscale weaving of polymer chains, distinguishing it from randomly entangled polymer networks and rigid crystalline frameworks. We review the background and experimental progress so far, and conclude by considering the potential of molecular weaving and outline some of the current and future challenges in this emerging field.

Interfacial Charge Transfer Complexes in TiO2-Enediol Hybrids Synthesized by Sol-Gel

Metal oxide-organic hybrid semiconductors exhibit specific properties depending not only on their composition but also on the synthesis procedure, and particularly on the functionalization method, determining the interaction between the two components. Surface adsorption is the most common way to prepare organic-modified metal oxides. Here a simple sol-gel route is described as an alternative, finely controlled strategy to synthesize titanium oxide-based materials containing organic molecules coordinated to the metal. The effect of the molecular structure of the ligands on the surface properties of the hybrids is studied using three enediols able to form charge transfer complexes: catechol, dopamine, and ascorbic acid. For each system, the process conditions driving the transition from the sol to chemical, physical, or particulate gels are explored. The structural, optical, and photoelectrochemical characterization of the amorphous hybrid materials shows analogies and differences related to the organic component. In particular, electron paramagnetic resonance (EPR) spectroscopy at room temperature reveals the presence of organic radical species with different evolution and stability, and photocurrent measurements prove the effective photosensitization of TiO₂ in the visible range induced by interfacial ligand-to-metal charge transfer.

Topological Friction and Relaxation Dynamics of Spatially Confined Catenated Polymers

We study catenated ring polymers confined inside channels and slits with Langevin dynamics simulations and address how the contour position and size of the interlocked or physically linked region evolve with time. We show that the catenation constraints generate a drag, or topological friction, that couples the contour motion of the interlocked regions. Notably, the coupling strength decreases as the interlocking is made tighter, but also shorter, by confinement. Though the coupling strength differs for channel and slit confinement, the data outline a single universal curve when plotted against the size of the linked region. Finally, we study how the relaxation kinetics changes after one of the rings is cut open and conclude that considering interlocked circular polymers is key for isolating the manifestations of topological friction. The results ought to be relevant for linked biomolecules in experimental or biological confining conditions.

Boroxine template for macrocyclization and postfunctionalization

Boroxine-templated macrocyclization: Olefin metathesis of boronic acid substrates in the presence of MS4A followed by workup with pinacol yields the desired macrocyclic compounds with modifiable three boron units.

Macrocyclic vs. [2]catenane btp structures: influence of (aryl) substitution on the self templation of btp ligands in macrocyclic synthesis

The synthesis of four 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) olefin based ligands 3, 4, 11 and 12 is described and their attempted use to form mechanically interlocked molecules using ring closing metatheses (RCM) reactions. The btp ligands were modified in two ways, in 3 and 4 the aryl substitution pattern was changed from 4^th position to 3^rd position and in the case of 11 and 12, the arms were replaced with aliphatic chains. Our study demonstrates that for all four ligands, the RCM reactions only result in the formation of macrocyclic structures, which in three of the cases, were structurally characterised in both solution (using NMR and HRMS) and in the solid-state using X-ray crystallography. NMR studies were also carried out to investigate if these ligands could preorganise in solution via hydrogen bonding interactions. This study provides a handle of how such precursor substitution can be used to direct the formation of macrocycles or mechanically interlocked structures.

Topological and physical links in soft matter systems

Linking, or multicomponent topological entanglement, is ubiquitous in soft matter systems, from mixtures of polymers and DNA filaments packedin vivoto interlocked line defects in liquid crystals and intertwined synthetic molecules. Yet, it is only relatively recently that theoretical and experimental advancements have made it possible to probe such entanglements and elucidate their impact on the physical properties of the systems. Here, we review the state-of-the-art of this rapidly expanding subject and organize it as follows. First, we present the main concepts and notions, from topological linking to physical linking and then consider the salient manifestations of molecular linking, from synthetic to biological ones. We next cover the main physical models addressing mutual entanglements in mixtures of polymers, both linear and circular. Finally, we consider liquid crystals, fluids and other non-filamentous systems where topological or physical entanglements are observed in defect or flux lines. We conclude with a perspective on open challenges.

An "All-in-One" Synthetic Strategy for Linear Metalla[4]Catenanes

One fascinating and challenging synthetic target in the field of mechanically interlocked molecules is the family of linear [4]catenanes, which are topologically identical to the logo of automobile maker Audi. Herein, we report an "all-in-one" synthetic strategy for the synthesis of linear metalla[n]catenanes (n = 2-4) by the coordination-driven self-assembly of Cp*Rh-based (Cp* = η⁵-pentamethylcyclopentadienyl) organometallic rectangle π-donors and tetracationic organic cyclophane π-acceptors. We selected the pyrenyl group as the π-donor unit, leading to homogeneous metalla[2]catenanes and cyclic metalla[3]catenanes via π-stacking interactions. By taking advantage of the strong electrostatic interactions between π-donor units and π-acceptor units, a heterogeneous metalla[2]catenanes and linear metalla[3]catenanes, respectively, could be obtained by the simple stirring of homogeneous metalla[2]catenanes with a suitable tetracationic cyclophane. On this basis, this "all-in-one" synthetic strategy was further used to realize a quantitative one-step synthesis of a linear metalla[4]catenanes via the self-assembly of cyclic metalla[3]catenanes and tetracationic cyclophanes. All heterogeneous metalla[n]catenanes (n = 2-4) were fully characterized by single-crystal X-ray analysis, NMR spectroscopy and electrospray ionization mass spectrometry.

Self-Assembly of a Purely Covalent Cage with Homochirality by Imine Formation in Water

Self-assembly of host molecules in aqueous media via metal-ligand coordination is well developed. However, the preparation of purely covalent counterparts in water has remained a formidable task. An anionic tetrahedron cage was successfully self-assembled in a [4+4] manner by condensing a trisamine and a trisformyl in water. Even although each individual imine bond is rather labile and apt to hydrolyze in water, the tetrahedron is remarkably stable or inert due to multivalence. The tetrahedral cages, as well as its neutral counterparts dissolved in organic solvent, have homochirality, namely that their four propeller-shaped trisformyl residues adopt the same rotational conformation. The cage is able to take advantage of hydrophobic effect to accommodate a variety of guest molecules in water. When a chiral guest was recognized, the formation of one enantiomer of the cage became more favored relative to the other. As a consequence, the cage could be produced in an enantioselective manner. The tetrahedron is able to maintain its chirality after removal of the chiral guest-probably on account of the cooperative occurrence of intramolecular forces that restrict the intramolecular flipping of phenyl units in the cage framework.

A molecular endless (74) knot

Current strategies for the synthesis of molecular knots focus on twisting, folding and/or threading molecular building blocks. Here we report that Zn(II) or Fe(II) ions can be used to weave ligand strands to form a woven 3 × 3 molecular grid. We found that the process requires tetrafluoroborate anions to template the assembly of the interwoven grid by binding within the square cavities formed between the metal-coordinated criss-crossed ligands. The strand ends of the grid can subsequently be joined through within-grid alkene metathesis reactions to form a topologically trivial macrocycle (unknot), a doubly interlocked [2]catenane (Solomon link) and a knot with seven crossings in a 258-atom-long closed loop. This 7₄ knot topology corresponds to that of an endless knot, which is a basic motif of Celtic interlace, the smallest Chinese knot and one of the eight auspicious symbols of Buddhism and Hinduism. The weaving of molecular strands within a discrete layer by anion-template metal-ion coordination opens the way for the synthesis of other molecular knot topologies and to woven polymer materials.

A Giant [8+12] Boronic Ester Cage with 48 Terminal Alkene Units in the Periphery for Postsynthetic Alkene Metathesis

Dynamic covalent chemistry (DCC) is a powerful synthetic tool to construct large defined molecules in one step from rather simple precursors. The advantage of the intrinsic dynamics of the applied reversible reaction steps is a self‐correction under the chosen conditions, to achieve high yields of the target compound. To date, only a few examples are known, in which DCC was used to build up a molecular defined but larger product that was chemically transferred to a more stable congener in a second (irreversible) step. Here, we present a nanometer‐sized [8+12] boronic ester cage containing 48 peripheral terminal alkene units which allows to put a hydrocarbon exoskeleton around the cage via alkene metathesis.

Suit[3]ane

Suitanes are a class of mechanically interlocked molecules (MIMs) that consist of two components: a body with limbs protruding outward and a suit that fits appropriately around it, so that there is no easy way for the suit to be removed from the body. Herein, we report the synthesis and characterization of a suit[3]ane, which contains a benzotrithiophene derivative (THBTT) with three protruding hexyl chains as the body and a 3-fold symmetric, extended pyridinium-based cage, namely, HexaCage⁶⁺, as the suit. Central to its realization is effective templation, provided by THBTT during cage formation, an observation that has been supported by the strong binding constant between benzotrithiophene (BTT) and the empty cage. The solid-state structure of the suit[3]ane reveals that the body is confined within the suit's cavity with its alkyl chains protruding outward through the orifices in the cage. Notably, such a seemingly unstable molecule, having three flexible alkyl chains as its only protruding limbs, does not dissociate after prolonged heating in CD₃CN at 100 °C under pressure for 7 days. No evidence for guest exchange with the host was observed at this temperature in a 2:1 mixture of THBTT and HexaCage⁶⁺ in CD₃CN. The results indicate that flexible protruding limbs are sufficient for a suit[3]ane to remain mechanically stable even at high temperatures in solution.

Construction of Macromolecular Pinwheels Using Predesigned Metalloligands

Developing a methodology to build target structures is one of the major themes of synthetic chemistry. However, it has proven to be immensely challenging to achieve multilevel elaborate molecular architectures in a predictable way. Herein, we describe the self-assembly of a series of pinwheel-shaped starlike supramolecules through three rationally preorganized metalloligands L1-L3. The key octa-uncomplexed terpyridine (tpy) metalloligand L3, synthesized with an 8-fold Suzuki coupling reaction to metal-containing complexes, has four different types of terpyridines connected with three ⟨tpy-Ru²⁺-tpy⟩ units, making this the most subunits known so far for a preorganized module. Based on the principle of geometric complementation and the high "density of coordination sites", these metalloligands were assembled with Zn²⁺ ions to form a pinwheel-shaped star trigon P1, pentagram P2, and hexagram P3 with precisely controlled shapes in nearly quantitative yields. With molecular weights ranging from 16756 to 56053 Da and diameters of 6.7-13.6 nm, the structural composition, shape, and rigidity of these pinwheel-shaped architectures have been fully characterized by 1D and 2D (NMR), electrospray ionization mass spectrometry, traveling-wave ion mobility mass spectrometry, and transmission electron microscopy.

Transmembrane Ion Channels Formed by a Star of David [2]Catenane and a Molecular Pentafoil Knot

A (FeII)6-coordinated triply interlocked ("Star of David") [2]catenane (612 link) and a (FeII)5-coordinated pentafoil (51) knot are found to selectively transport anions across phospholipid bilayers. Allostery, topology, and building block stoichiometry all play important roles in the efficacy of the ionophoric activity. Multiple FeII cation coordination by the interlocked molecules is crucial: the demetalated catenane exhibits no anion binding in solution nor any transmembrane ion transport properties. However, the topologically trivial, Lehn-type cyclic hexameric FeII helicates-which have similar anion binding affinities to the metalated Star of David catenane in solution-also display no ion transport properties. The unanticipated difference in behavior between the open- and closed-loop structures may arise from conformational restrictions in the linking groups that likely enhances the rigidity of the channel-forming topologically complex molecules. The (FeII)6-coordinated Star of David catenane, derived from a hexameric cyclic helicate, is 2 orders of magnitude more potent in terms of ion transport than the (FeII)5-coordinated pentafoil knot, derived from a cyclic pentamer of the same building block. The reduced efficacy is reminiscent of multisubunit protein ion channels assembled with incorrect monomer stoichiometries.

Covalent Post-Assembly Modification: A Synthetic Multipurpose Tool in Supramolecular Chemistry

The use of covalent post-assembly modification (PAM) in supramolecular chemistry has grown significantly in recent years, to the point where PAM is now a versatile synthesis tool for tuning, modulating, and expanding the functionality of self-assembled complexes and materials. PAM underpins supramolecular template-synthesis strategies, enables modular derivatization of supramolecular assemblies, permits the covalent 'locking' of unstable structures, and can trigger controlled structural transformations between different assembled morphologies. This Review discusses key examples of PAM spanning a range of material classes, including discrete supramolecular complexes, self-assembled soft nanostructures and hierarchically ordered polymeric and framework materials. As such, we hope to highlight how PAM has continued to evolve as a creative and functional addition to the synthetic chemist's toolbox for constructing bespoke self-assembled complexes and materials.