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

Mechanical interlocking of metal organic cages

[2]-Catenanes formed from three-dimensional (3D) metal organic cages (MOCs) self-assembled from transition metals and organic ligands, hereafter called interlocked MOCs, constitute a relatively new class of mechanically interlocked materials (MIMs) that is receiving considerable research attention. This interest is mainly due to their esthetic synthetic aspects and mechanical properties, but also due to their potential applications in nanotechnological areas, including magnetic materials, guest selectivity, allosteric binding, or thermoresponsive behavior in elastomeric materials. In this article, a perspective on the research on interlocked MOCs (i.e., [2]-catenanes), covering from the first examples of interlocked MOCs to the latest research in this area is presented. The emphasis is in the synthetic methods used for their preparation and the structure‒function correlation aspects. The combination of experimental techniques in the solution state (i.e., NMR, ESI-MAS, etc.) and solid-state X-ray structural data, in combination with theoretical calculations have been very important to getting a rationalization of the experimental results. The described [2]-catenanes are formed from mechanically bonded hollow cages, therefore they can be exploited in host-guest chemistry applications typical from MOCs, with enhanced physical properties ranging from the mechanical bonds like enhanced cage's strength and dynamic behavior, which are needed for developing new smart functional materials.

Pincer-Ligand-Stabilized Group 10 Metal Nanoclusters: Chirality and Boat-Chair Structural Transformation

Atomically precise metal nanoclusters bridge the gap between metal atoms and metal nanoparticles, providing an ideal platform for correlating the macroscopic properties of metal nanoparticles to their microstructures. In comparison to the rapid development of coin metal nanoclusters, the knowledge on group 10 metal nanoclusters is relatively limited because of their active nature, resulting in the synthetic challenge. In this work, we successfully synthesize three group 10 metal nanoclusters, that is, Ni3(SNS)3, Ni3(SNOS)3, and Pd3(SNS)3, by using the SNS pincer ligands, which well-stabilize the nickel and palladium metal kernels. Intrinsic chirality of the nanoclusters that originated from the asymmetric arrangement of the surface SNS pincer ligands is discovered. Oxidation-induced irreversible transformation from boat-like Ni3(SNS)3 to chair-like Ni3(SNOS)3 is observed, and the boat/chair structure-dependent reactivity is revealed. The structural features and intriguing properties of Ni3 and Pd3 nanoclusters stabilized by the pincer ligand will inspire further research on group 10 metal nanoclusters.

Cooperative Multiscale-Assembly for Directional and Hierarchical Growth of Highly Oriented Porous Organic Cage Single-Crystal Microtubes and Arrays

The directional assembly of porous organic molecules into long-range ordered architectures, featuring controlled hierarchical porosity and oriented pore channels with defined spatial arrangements, is a fundamental challenge in chemistry and materials science. Herein, using porous organic cages as starting units, we present a cooperative multiscale-assembly strategy enabling the simultaneous alignment of pore channels and directional hierarchical growth in a single step. At the microscopic level, we employed double solvents to manipulate the intermolecular packing of microporous tetrahedral [4+6] imine cages (CC1 and CC3), resulting in pore channel orientation. Concurrently, at the mesoscopic level, convective flow in the double-solvent system directed the spatial distribution of nuclei species, followed by diffusion limited growth, leading to the directional formation of single-crystal microtubes. By precisely controlling the direction of convective flow, the nanocages were successfully organized into 2D and 3D single-crystal microtube arrays while maintaining oriented micropores. This hierarchical porous architecture enhanced mass transfer, as confirmed by adsorption measurements. Interestingly, such 3D hierarchical microtube arrays can be utilized to immobilize Pd clusters and enzymes (lipase or Glucose oxidase) within the micro- and macropores, respectively, showing a 3.8- to 4-fold enhancement in one-pot tandem reaction activity compared to physical mixtures of individual analogues.

Dispersion Forces-Driven Hierarchical Assembly of Protein-Like Lanthanide Octamers and Emergent CPL

Hierarchical self-assembly driven by non-covalent interactions is a prevalent strategy employed by nature to construct sophisticated biomacromolecules, such as proteins. However, the construction of protein-like superstructures that rely on weaker dispersion forces-driven hierarchical assembly remains largely unexplored. Here, we report the first example of dispersion forces driving the high-order assembly of the lanthanide trinuclear circular helicate [HNEt₃]₃[Eu₃(LL)₆] (ΔΔΔ-1) into a protein-like lanthanide octamer ((ΔΔΔ-1)₈-2). Within the octamer, the forty-eight (48) menthol groups on the ligands and eighty-four (84) 1,4-dioxane solvent molecules contribute to enhanced dispersion forces through conformational adaptation and size-matching effects. These enhanced dispersion forces not only drive the formation of the hierarchical superstructure but also result in a four-level chirality transfer from the menthol to the octamer. Benefiting from the homochirality of Eu3+, the octamer is endowed the strong circularly polarized emission (|glum|=0.34, Φoverall=41 %). This understanding of how dispersion forces drive hierarchical self-assembly provides a foundation for the directed fabrication of more fascinating superstructures.

A pseudo-cubic metal-organic cage with conformationally switchable faces for dynamically adaptive guest encapsulation

The creation of hosts capable of accommodating different guest molecules may enable these hosts to play useful roles in chemical purifications, among other applications. Metal-organic cages are excellent hosts for various guests, but they generally incorporate rigid structural units that hinder dynamic adaptation to specific guests. Here we report a conformationally adaptable pseudo-cubic cage that can dynamically increase its cavity volume to fit guests with differing sizes. This pseudo-cube incorporates a tetramine subcomponent with 2,6-naphthalene arms that cooperatively adopt a non-planar conformation, enabling the cage faces to switch between endo and exo states. A wide range of guest molecules were observed to bind within the cavity of this cage, spanning a range of sizes from 46% to 154% of the cavity volume of the empty cage. Experimental and computational evidence characterizes the flipping of cage faces from endo to exo, expanding the cavity upon binding of larger guests.

Higher Assemblies of Coordination Cage-Catenanes Linked by Copper(II) Chloride Clusters: Networks and Transformations

Cage-catenanes are chemical constructs where two or more cage-like molecules or assemblies are mechanically interlocked together. We report a new class of cage-catenanes where dimeric metal-organic cage-catenanes are linked into larger assemblies through additional bridging metal chloride links. These crystalline materials are obtained from the reaction of tris(nicotinoyl)cyclotriguaiacylene (L1) with Cu(II) salts, and all feature a tetramer of cages where two {Cu3(L1)2(X)6} cages (X=anion) are mechanically interlocked, and link to each other and to another {Cu3(L1)2(X)6}2 cage-catenane through a planar, linear tetranuclear {Cu4(μ-Cl)6Cl2} cluster. The complex of discrete tetrameric {Cu3(L1)2(X)6}4 assemblies (dimers of cage-catenanes) transforms through solvent-exchange processes to 1D coordination chain structures through additional {Cu2(μ-Cl)2} bridges between the tetrameric {Cu3(L1)2(X)6}4 assemblies. Complex [Cu6(L1)4Cl12(H2O)3] ⋅ (H2O) ⋅ 15(DMF) C2 features a 2D coordination network of 63 topology linked through three different Cu(II) clusters, namely {Cu4(μ-Cl)6}, {Cu2(μ-Cl)2} and a rare linear {Cu2(μ-Cl)} linkage. Break-down of C2 in water likely proceeds through hydrolysis of this unusual linear Cu-Cl-Cl bond.

The Designs and Applications of Tetraphenylethylene Macrocycles and Cages

Macrocycles and cages are very attractive for the development of functional materials due to their unique inner cavities. Building blocks with interesting functions and synthetic conveniences are especially attractive. Tetraphenylethylene (TPE) is such an entity with C2 symmetry and tetrakis-functional groups easily modifiable. As a typical aggregation-induced emission (AIE) active compound, TPE perfectly unites the functions of fluorescence and structural building blocks together. The unique marriage of the two roles into one component makes TPE an ideal platform for the development of functional molecular systems including macrocycles and cages. The TPE macrocycles and cages are not merely a simple combination of those two but also generate added values unseen in either component alone. The fluorescence properties of TPE in macrocycles/cages are greatly improved or modulated, which makes them more suitable for various applications compared to their linear counterparts. In this review, the chemistry and design principles of TPE macrocycles/cages are surveyed first. The unique properties of those compounds are also discussed to provide general guidance for their functionalization. A brief discussion of their applications focusing on the utilization of their unique fluorescence is also presented. In the last, outlooks and future perspectives of TPE macrocycles/cages are provided for further developments.

Chiral Self-Assembly of Twisted Prisms, Cuboids, and Polyhedral Capped Cages with Tartrate Ligands

Homochiral triangular prisms, cuboid cages, and capped polyhedral cages are successfully synthesized via coordination-driven self-assembly. Typical tartrate ligands demonstrated notable torsional flexibility and variable coordination numbers, allowing for diverse coordination patterns, including saturated chelation and terminal mono-coordination with half-sandwich rhodium and iridium fragments. The ligand lengths, molar ratios, and metal vertices are meticulously designed and fine-tuned to yield chiral cages with entirely distinct architectures. Tartrate ligand exhibits abundant hydrogen bonding interactions and chiral induction capabilities, these supramolecular assemblies are characterized by single-crystal X-ray diffraction, nuclear magnetic resonance, and circular dichroism spectroscopy. An efficient method is developed for constructing chiral structurally versatile cage-like entities, facilitating self-assembly in complicated multi-component systems.

Solvent- and Concentration-Induced Topological Transformation of a Ruthenium(II)-Based Trigonal Prism to a Triply Interlocked [2] Catenane

Synthesis of interlocked supramolecular cages has been a growing field of interest due to their structural diversity. Herein, we report the template-free synthesis of a Ru(II) triply interlocked [2] catenane using coordination-driven self-assembly. The self-assembly of a triazine-based tripyridyl donor L (2,4,6-tris(5-(pyridin-4-yl)thiophen-3-yl)-1,3,5-triazine) with a dinuclear Ru(II) acceptor M (Ru2(dhnq)(η6-p-cymene)2)(CF3SO3)2) yielded two distinct structures depending on the solvent and concentration. In methanol, a triply interlocked metalla [2] catenane (MC2) was formed, whereas in nitromethane, a non-interlocked cage (MC1) was obtained. The non-interlocked cage MC1 was gradually converted to MC2 in nitromethane by the increase in the concentration of cage MC1 from 0.5 to 9 mM. The interlocked cage (MC2) was stable after formation and was unaffected by the change in concentration. Notably, the free cage (MC1) exhibited host-guest interactions with polycyclic aromatic aldehydes, stabilizing the non-interlocked structure even at higher concentrations. In contrast, the triply interlocked [2] catenane (MC2) remains stable due to self-penetration and does not encapsulate guest molecules. This work showcases the stimuli-induced irreversible structural transformation of a triangular prismatic cage to its triply interlocked [2] catenane by employing metal-ligand coordination chemistry.

Supramolecular and molecular capsules, cages and containers

Stemming from early seminal notions of molecular recognition and encapsulation, three-dimensional, cavity-containing capsular compounds and assemblies have attracted intense interest due to the ability to modulate chemical and physical properties of species encapsulated within these confined spaces compared to bulk environments. With such a diverse range of covalent motifs and non-covalent (supramolecular) interactions available to assemble building blocks, an incredibly wide-range of capsular-type architectures have been developed. Furthermore, synthetic tunability of the internal environments gives chemists the opportunity to engineer systems for uses in sensing, sequestration, catalysis and transport of molecules, just to name a few. In this tutorial review, an overview is provided into the design principles, synthesis, characterisation, structural facets and properties of coordination cages, porous organic cages, supramolecular capsules, foldamers and mechanically interlocked molecules. Using seminal and recent examples, the advantages and limitations of each system are explored, highlighting their application in various tasks and functions.

Paracetamol Inclusion in Mechanically Interlocked Nanocages

The solid-state synthesis and fast crystallization under kinetic control of poly-[n]-catenanes self-assembled of mechanically interlocked metal organic cages (MOCs) is virtually unexplored. This is in part, due to the lack of suitable crystals for single crystal X-ray diffraction (SC-XRD) analysis which limits their progress as advanced functional materials. Here we report the unprecedented inclusion of paracetamol in the cavities of amorphous materials constituted of M12L8, interlocked MOCs synthesized by mechanochemistry under kinetic control. Full structure determination of a low-crystallinity and low-resolution powders of the M12L8 poly-[n]-catenane including paracetamol has been carried out combining XRD data and Density Functional Theory (DFT) calculations using a multi-step approach. Each M12L8 cage contains six paracetamol guests which is confirmed by thermal analysis and NMR spectroscopy. The paracetamol loading has been also carried out by the instant synthesis method using a saturated paracetamol solution in which TPB and ZnI2 self-assemble immediately (i. e., 1-5 seconds) encapsulating ~7 paracetamol molecules in the M12L8 nanocages under kinetic control also giving a good selectivity. Benzaldehyde has been included in the M12L8 cages using amorphous M12L8 polycatenanes showing that the icosahedral cages can serve as potential nanoreactors for instance to study Henry reactions in the solid-state.

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.

Triply Interlocked [2]catenanes: Rational Synthesis, Reversible Conversion Studies and Unprecedented Application in Photothermal Responsive Elastomer

Triply interlocked [2]catenane complexes featuring two identical, mechanically interlocked units are extraordinarily rare chemical compounds, whose properties and applications remain open to detailed studies. Herein, we introduce the rational design of a new ligand precursor, L1, suitable for the synthesis of six triply interlocked [2]catenanes by coordination-driven self-assembly. The interlocked compounds can be reversibly converted into the corresponding simple triangular prism metallacage by addition of H2O or DMF solvents to their CH3OH solutions, thereby demonstrating the importance of π⋅⋅⋅π stacking and hydrogen bonding interactions in the formation of triply interlocked [2]catenanes. Moreover, extensive studies have been conducted to assess the remarkable photothermal conversion performance. Complex 6 a, exhibiting outstanding photothermal conversion performance (conversion efficiency in solution : 31.82 %), is used to prepare novel photoresponsive elastomer in combination with thermally activated liquid crystal elastomer. The resultant material displays robust response to near-infrared (NIR) laser and the capability of completely reforming the shape and reversible actuation, paving the way for the application of half-sandwich organometallic units in photo-responsive smart materials.

Spontaneous Multi-scale Supramolecular Assembly Driven by Noncovalent Interactions Coupled with the Continuous Marangoni Effect

Reported herein is the multi-scale supramolecular assembly (MSSA) process along with redox reactions driven by supramolecular interactions coupled with the spontaneous Marangoni effect in ionic liquid (IL)-based extraction systems. The black powder, the single sphere with a black exterior, and the single colorless sphere were formed step by step at the interface when an aqueous solution of KMnO4 was mixed with the IL phase 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (C2OHmimNTf2) bearing octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO). The mechanism of the whole process was studied systematically. The phenomena were related closely to the change in the valence state of Mn. The MnO4- ion could be reduced quickly to δ-MnO2 and further to Mn2+ slowly by the hydroxyl-functionalized IL C2OHmimNTf2. Based on Mn2+, Mn(CMPO)32+, elementary building blocks (EBBs), and [EBB]n clusters were generated step by step. The [EBB]n clusters with the large enough size that were transferred to the interface, together with the remaining δ-MnO2, assembled into the single sphere with a black exterior, driven by supramolecular interactions coupled with the spontaneous Marangoni effect. When the remaining δ-MnO2 was used up, the mixed single sphere turned completely colorless. It was found that the reaction site of C2OHmim+ with Mn(VII) and Mn(IV) was distributed mainly at the side chain with a hydroxyl group. The MSSA process presents unique spontaneous phase changes. This work paves the way for the practical application of the MSSA-based separation method developed recently. The process also provides a convenient way to observe in situ and characterize directly the continuous Marangoni effect.

All-catecholate-stabilized black titanium-oxo clusters for efficient photothermal conversion

The controlled synthesis of titanium-oxo clusters (TOCs) completely stabilized by organic dye ligands with high stability and superior light absorption remains a significant challenge. In this study, we report the syntheses of three atomically precise catechol (Cat)-functionalized TOCs, [Ti2(Cat)2(OEgO)2(OEgOH)2] (Ti2), [Ti8O5(Cat)9(iPrO)4(iPrOH)2] (Ti8), and [Ti16O8(OH)8(Cat)20]·H2O·PhMe (Ti16), using a solvent-induced strategy (HOEgOH = ethylene glycol; iPrOH = isopropanol; PhMe = toluene). Interestingly, the TiO core of Ti16 is almost entirely enveloped by catechol ligands, making it the first all-catechol-protected high-nuclearity TOC. In contrast, Ti2 and Ti8 have four weakly coordinated ethylene glycol ligands and six weakly coordinated iPrOH ligands, respectively, in addition to the catechol ligands. Ti16 is visually evident in its distinctively black appearance, which belongs to black TOCs (B-TOCs) and exhibits an ultralow optical band gap. Furthermore, Ti16 displays exceptional stability in various media/environments, including exposure to air, solvents, and both acidic and alkaline aqueous solutions due to its comprehensive protection by catechol ligands and rich intra-cluster supramolecular interactions. Ti16 has superior photoelectric response qualities and photothermal conversion capabilities compared to Ti2 and Ti8 due to its ultralow optical band gap and remarkable stability. This discovery not only represents a huge step forward in the creation of all-catecholate-protected B-TOCs with ultralow optical band gaps and outstanding stability, but it also gives key valuable mechanistic insights into their photothermal/electric applications.