Fe–Pt Twisted Heterometallic Bicyclic Supramolecules via Multicomponent Self-Assembly

Assembly of Six Types of Heteroleptic Pd2L4 Cages under Kinetic Control

Heteroleptic assemblies composed of several kinds of building blocks have been seen in nature. It is still unclear how natural systems design and create such complicated assemblies selectively. Past efforts on multicomponent self-assembly of artificial metal-organic cages have mainly focused on finding a suitable combination of building blocks to lead to a single multicomponent self-assembly as the thermodynamically most stable product. Here, we present another approach to selectively produce multicomponent Pd(II)-based self-assemblies under kinetic control based on the selective ligand exchanges of weak Pd-L coordination bonds retaining the original orientation of the metal centers in a kinetically stabilized cyclic structure and on local reversibility given in certain areas of the energy landscape in the presence of the assist molecule that facilitates error correction of coordination bonds. The kinetic approach enabled us to build all six types of Pd2L4 cages and heteroleptic tetranuclear cages composed of three kinds of ditopic ligands. Although the cage complexes thus obtained are metastable, they are stable for 1 month or more at room temperature.

Phenylthiol-BODIPY-based supramolecular metallacycles for synergistic tumor chemo-photodynamic therapy

The development of more effective tumor therapy remains challenging and has received widespread attention. In the past decade, there has been growing interest in synergistic tumor therapy based on supramolecular coordination complexes. Herein, we describe two triangular metallacycles (1 and 2) constructed by the formation of pyridyl boron dipyrromethene (BODIPY)-platinum coordination. Metallacycle 2 had considerable tumor penetration, as evidenced by the phenylthiol-BODIPY ligand imparting red fluorescent emission at ∼660 nm, enabling bioimaging, and transport visualization within the tumor. Based on the therapeutic efficacy of the platinum(II) acceptor and high singlet oxygen (¹O₂) generation ability of BODIPY, 2 was successfully incorporated into nanoparticles and applied in chemo-photodynamic tumor therapy against malignant human glioma U87 cells, showing excellent synergistic therapeutic efficacy. A half-maximal inhibitory concentration of 0.35 μM was measured for 2 against U87 cancer cells in vitro. In vivo experiments indicated that 2 displayed precise tumor targeting ability and good biocompatibility, along with strong antitumor effects. This work provides a promising approach for treating solid tumors by synergistic chemo-photodynamic therapy of supramolecular coordination complexes.

Tetratopic Terpyridine Building Unit as a Precursor to Wheel-Like Metallo-Supramolecules

In an effort to construct molecules with distinct shapes and functions, the design and synthesis of multitopic ligands are often able to play an important role. Here, we report the synthesis of a novel tetratopic organic ligand LA, which can be viewed as a bis-tenon with successive angular orientations in space. The particular ligand has been treated with different tailored metal-organic ligands to afford new members of the molecular wheel family (multi-rhomboidal-shaped wheel and bis-trapezium-shaped wheel) that show enhanced stability. Two-dimensional (2D) diffusion nuclear magnetic resonance (NMR) spectroscopy (DOSY), electrospray ionization (ESI) mass spectrometry, traveling wave ion mobility (TWIM), and gradient tandem mass spectrometry (gMS₂) experiments, as well as molecular modeling, have been employed to provide structural information and differentiate the isomeric separation process. In addition, considering that LA has rotational properties, it is expected to open the door to functional supramolecules and stimuli-responsive materials.

A host-guest semibiological photosynthesis system coupling artificial and natural enzymes for solar alcohol splitting

Development of a versatile, sustainable and efficient photosynthesis system that integrates intricate catalytic networks and energy modules at the same location is of considerable future value to energy transformation. In the present study, we develop a coenzyme-mediated supramolecular host-guest semibiological system that combines artificial and enzymatic catalysis for photocatalytic hydrogen evolution from alcohol dehydrogenation. This approach involves modification of the microenvironment of a dithiolene-embedded metal-organic cage to trap an organic dye and NADH molecule simultaneously, serving as a hydrogenase analogue to induce effective proton reduction inside the artificial host. This abiotic photocatalytic system is further embedded into the pocket of the alcohol dehydrogenase to couple enzymatic alcohol dehydrogenation. This host-guest approach allows in situ regeneration of NAD⁺/NADH couple to transfer protons and electrons between the two catalytic cycles, thereby paving a unique avenue for a synergic combination of abiotic and biotic synthetic sequences for photocatalytic fuel and chemical transformation.

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

Better Together: Functional Heterobimetallic Macrocyclic and Cage-like Assemblies

Metallosupramolecular chemistry has attracted the interest of generations of researches due to the versatile properties and functionalities of oligonuclear coordination complexes. Quite a number of different discrete cages were investigated, mostly consisting of only one type of ligand and one type of metal cation. Looking for ever more complex structures, heterobimetallic complexes became more and more attractive, as they give access to new structural motifs and functions. In the last years substantial success has been made in the design and synthesis of cages consisting of more than one type of metal cations, and a rapidly growing number of functional materials has appeared in the literature. This Minireview describes recent developments in the field of discrete heterometallic macrocycles and cages focusing on functional materials that have been used as host‐systems or as magnetic, photo‐active, redox‐active, and even catalytically active materials.

Triple Self-Sorting in Constitutional Dynamic Networks: Parallel Generation of Imine-Based CuI , FeII , and ZnII Complexes

Three imine-based metal complexes, having no overlap in terms of their compositions, have been simultaneously generated from the self-sorting of a constitutional dynamic library (CDL) containing three amines, three aldehydes, and three metal salts. The hierarchical ordering of the stability of the three metal complexes assembled and the leveraging of the antagonistic and agonistic relationships existing between the constituents within the constitutional dynamic network corresponding to the CDL were pivotal in achieving the sorting. Examination of the process by NMR spectroscopy showed that the self-sorting of the FeII and ZnII complexes depended on an interplay between the thermodynamic driving forces and a kinetic trap involved in their assembly. These results also exemplify the concept of "simplexity"-the fact that the output of a self-assembling system may be simplified by increasing its initial compositional complexity-as the two complexes could self-sort only in the presence of the third pair of organic components, those of the CuI complex.

From Dimeric to Octameric Metallo-Supramolecular Macrocycles Based on Sterically Congested Ligand-assisted Self-Assembly with Zn(II), Cd(II), and Fe(II)

Two sterically congested 2,2':6',2″-terpyridine-based ligands LA and LB, composed of asymmetrically contiguous terpyridine units, are designed and synthesized for metallo-supramolecular architectures. The significant advantage of this design is that the terpyridines in these ligands have different chemical environments and show a selective coordination ability with each other. For ditopic ligand LA, the self-assembly with Zn(II), Cd(II), and Fe(II) gave the rhombic dimers, which have the same sets of terpyridine signals as ligand LA. The self-assembly behavior of tritopic ligand LB with Cd(II) and Zn(II) are observed for discrete tetramers under thermodynamic control, whereas ligand LB and Fe(II) are assembled to generate a mixture of tetramer, hexamer, and octamer, which are successfully isolated using regular chromatographic separation. Moreover, the sterically congested ligands and metal ions formed stable intermediates to drive the formation of discrete structures, which is also proved by mixing LB and Cd(II) with a precise stoichiometric ratio of 1:1. These supramolecular complexes are thoroughly characterized by NMR spectroscopy, electrospray ionization-mass spectrometry, and 2D traveling-wave ion mobility-mass spectrometry. This conceptually new design of sterically congested ligands provides a novel strategy for precisely controlled supramolecular complexes with diverse architectures.

Construction of Supramolecular Liquid-Crystalline Metallacycles for Holographic Storage of Colored Images

Design and construction of new functionalized supramolecular coordination complexes (SCCs) via coordination-driven self-assembly strategy is highly important in supramolecular chemistry and materials science. Herein, we present a family of well-defined metallacycles decorated with mesogenic forklike dendrons through the strategy of coordination-driven self-assembly. Due to the existence of mesogenic forklike dendrons, the obtained metallacycles displayed the smectic A liquid crystal phase at room temperature while their precursors exhibited the rectangular columnar liquid crystal phase. Interestingly, by taking advantage of the electrostatic interactions between the positively charged metallacycle and the negatively charged heparin, the doping of heparin induced a significant change of the liquid-crystalline behaviors of metallacycles. More importantly, the prepared liquid-crystalline metallacycles could be further applied for holographic storage of colored images. Notably, the rhomboidal metallacycle and hexagonal metallacycle gave rise to different holographic performances although they featured a similar liquid crystal phase behavior. Therefore, this research not only provides the first successful example of supramolecular liquid-crystalline metallacycles for holographic storage of colored images but also opens a new door for supramolecular liquid-crystalline metallacycles toward advanced optical applications.

Coordination-driven self-assembly of discrete supramolecular double-metallacycles

This review comprehensively summarizes the recent advances in the coordination-driven self-assembly of discrete supramolecular double-metallacycles.

Self-sorting of two imine-based metal complexes: balancing kinetics and thermodynamics in constitutional dynamic networks

A major hurdle in the development of complex constitutional dynamic networks (CDNs) is the lack of strategies to simultaneously control the output of two (or more) interconnected dynamic processes over several species, namely reversible covalent imine bond formation and dynamic metal–ligand coordination. We have studied in detail the self-sorting process of 11 constitutional dynamic libraries containing two different amines, aldehydes and metal salts into two imine-based metal complexes, having no overlap in terms of their compositions. This study allowed us to determine the factors influencing the fidelity of this process (concentration, electronic and steric parameters of the organic components, and nature of the metal cations). In all 11 systems, the outcome of the process was primarily determined by the ability of the octahedral metal ion to select its pair of components from the initial pool of components, with the composition of the weaker tetrahedral complex being imposed by the components rejected by the octahedral metal ions. Different octahedral metal ions required different levels of precision in the “assembling instructions” provided by the organic components of the CDN to guide it towards a sorted output. The concentration of the reaction mixture, and the electronic and steric properties of the initial components of the library were all found to influence the lifetime of unwanted metastable intermediates formed during the assembling of the two complexes.

Constitutional dynamic networks have been developed to simultaneously control the output of two interconnected dynamic processes over several species of the same reaction mixture.

Self-Healing Heterometallic Supramolecular Polymers Constructed by Hierarchical Assembly of Triply Orthogonal Interactions with Tunable Photophysical Properties

Here, we present a method for the building of new bicyclic heterometallic cross-linked supramolecular polymers by hierarchical unification of three types of orthogonal noncovalent interactions, including platinum(II)-pyridine coordination-driven self-assembly, zinc-terpyridine complex, and host-guest interactions. The platinum-pyridine coordination provides the primary driving force to form discrete rhomboidal metallacycles. The assembly does not interfere with the zinc-terpyridine complexes, which link the discrete metallacycles into linear supramolecular polymers, and the conjugation length is extended upon the formation of the zinc-terpyridine complexes, which red-shifts the absorption and emission spectra. Finally, host-guest interactions via bis-ammonium salt binding to the benzo-21-crown-7 (B21C7) groups on the platinum acceptors afford the cross-linked supramolecular polymers. By continuous increase of the concentration of the supramolecular polymer to a relatively high level, supramolecular polymer gel is obtained, which exhibits self-healing properties and reversible gel-sol transitions stimulated by various external stimuli, including temperature, K⁺, and cyclen. Moreover, the photophysical properties of the supramolecular polymers could be effectively tuned by varying the substituents of the precursor ligands.

[2.2]Paracyclophane bis(pyridine)-based metallosupramolecular rhombs in the gas phase: Competitive cleavage of non-covalent and weak covalent bonds

The gas-phase fragmentation behavior of self-assembled metallo-supramolecular rhombs based on an unusual chiral [2.2]paracyclophane bis(pyridine) ligand is studied by collision-induced dissociation mass spectrometry. The fragmentation patterns strongly depend on the charge state of the respective mass-selected aggregate. For the doubly charged ions, simple symmetric fragmentation is observed in full accordance with previous results reported for related metallo-supramolecular species. The triply charged species cleaves unsymmetrically which can be rationalized by a preferred formation of ions with low charge density. CID of the quadruply charged rhomb reveals a complex fragmentation. Besides ligand oxidation to the radical cation, facile cleavage of the central covalently bound part of the [2.2]paracyclophane ligand takes place which is even preferred over rupture of the weak dative pyridine-Pd bond.

Multisite Binding of Drugs and Natural Products in an Entropically Favorable, Heteroleptic Receptor

The cavities of artificial receptors are defined by how their components fit together. The encapsulation of specific molecules can thus be engineered by considering geometric principles; however, intermolecular interactions and steric fit scale with receptor size, such that the ability to bind multiple guests from a specific class of compounds remains a current challenge. By employing metal-organic self-assembly, we have prepared a triangular prism from two different ligands that is capable of binding more than 20 different natural products, drugs, and steroid derivatives within its prolate cavity. Encapsulation inflates the host, enhancing its ability to bind other guests in peripheral pockets and thus enabling our system to bind combinations of different drug and natural product cargoes in different locations simultaneously. This new mode of entropically favorable self-assembly thus enables central encapsulation to amplify guest-binding events around the periphery of an artificial receptor.

Self-Organization into Preferred Sites by MgII, MnII, and MnIII in Brucite-Structured M19 Cluster

The search for functional materials, for example those aiming at microelectronics, magnetic recording, and catalysis, often ventures into mixed metal systems to achieve optimization of the properties. Thus, understanding site preference and self-organization is crucial but hard to implement. Herein, we present a system whereby Mg^II, Mn^II, and Mn^III ions selectively locate exact positions within the Brucite-structured cluster, Mn₁₃Mg₆, [Mn^III⊂Mg^II₆⊂Mn^II₉Mn^III₃( L)₁₈(OH)₁₂(N₃)₆](ClO₄)₆·12CH₃CN, H L = 1-(hydroxymethyl)-3,5-dimethylpyrazolate). The Mn^III being small (78 pm) takes up the core position; while 6 Mg^II (86 pm) are located in the inner ring, and the 9 large Mn^II (97 pm) and 3 Mn^III occupy the outer ring. The factors (a) ionic radii, (b) regularity in coordination geometry, oxophilicity, and softness of Mg^II compared to Mn^II, and (c) Jahn-Teller distortion of Mn^III may all be implicated synergistically. Electrospray ionization mass spectrometry reveals the M₁₉ disc remains an integral unit when crystals are dissolved, and exchange between Mg and Mn occurs within the disc during its formation. Diamagnetic Mg^II doping insulates the magnetic exchange between the central Mn^III and those in the outer ring, thus giving an overall antiferromagnetic exchange interaction between nearest-neighbors of the outer ring. The work reveals the underlying rule for site-preference of main group metal versus transition metal in disc-like Brucite-structured cluster and provides an elegant new avenue to assemble heterometallic clusters in a stepwise fashion.

Facile synthesis of multicomponent heterobimetallic metallomacrocycles through selective metal–ligand coordination

Through selective coordination, three heterobimetallic Zn^II–Pd^II/Pt^II metallomacrocycles have been constructed by either a stepwise or a one-pot self-assembly protocol.

Coordination-driven self-assembly of discrete Ru6-Pt6 prismatic cages

The coordination-driven self-assembly of two new Ru6-Pt6 hexanuclear trigonal prismatic cages comprising arene-ruthenium(II) clips (1a(NO3)2 and 1b(NO3)2 ) and a tritopic platinum(II) metalloligand 2 has been performed in methanol at room temperature. The [3 + 2] hexanuclear cages 3a and 3b were isolated in good yields and characterized by well-known spectroscopic techniques including multinuclear NMR, mass spectrometry, UV-vis and infrared studies. Geometry optimization revealed the shapes and sizes of these hexanuclear prismatic cages. The combination of ruthenium and platinum metal center in a one-pot self-assembly reaction showcases the construction of aesthetically elegant heterometallic structures in supramolecular chemistry leading to the formation of a single major product.

How electrostatic networks modulate specificity and stability of collagen

One-quarter of the 28 types of natural collagen exist as heterotrimers. The oligomerization state of collagen affects the structure and mechanics of the extracellular matrix, providing essential cues to modulate biological and pathological processes. A lack of high-resolution structural information limits our mechanistic understanding of collagen heterospecific self-assembly. Here, the 1.77-Å resolution structure of a synthetic heterotrimer demonstrates the balance of intermolecular electrostatics and hydrogen bonding that affects collagen stability and heterospecificity of assembly. Atomistic simulations and mutagenesis based on the solved structure are used to explore the contributions of specific interactions to energetics. A predictive model of collagen stability and specificity is developed for engineering novel collagen structures.

Multiple Pathways in the Self-Assembly Process of a Pd4L8 Coordination Tetrahedron

The self-assembly of a Pd₄1₈ coordination tetrahedron (Tet) from a ditopic ligand, 1, and palladium(II) ions, [PdPy*₄]²⁺ (Py* = 3-chloropyridine), was investigated by a ¹H NMR-based quantitative approach (quantitative analysis of self-assembly process, QASAP), which allows one to monitor the average composition of the intermediates not observed by NMR spectroscopy. The self-assembly of Tet takes place mainly through three pathways and about half of the Tet structures were produced through the reaction of a kinetically produced Pd₃L₆ double-walled triangle (DWT) and 200-nm-sized large intermediates (Int^L). In two of the three pathways, the leaving ligand (Py*), which is not a component of Tet, catalytically assisted the self-assembly. Such a multiplicity of the self-assembly process of Tet suggests that molecular self-assembly takes place on an energy landscape like a protein-folding funnel.

Quantitative Analysis of the Self-Assembly Process of Hexagonal PtII Macrocyclic Complexes: Effect of the Solvent and the Components

The self-assembly process of three Pt^II -linked hexagonal macrocycles consisting of dinuclear Pt^II complexes and organic ditopic ligands was investigated in polar and less polar solvents by a recently developed approach: quantitative analysis of the self-assembly process (QASAP). In polar CD₃ NO₂ , for all the three macrocycles, an ML₂ complex was the dominant intermediate during self-assembly, as a result of high positive allosteric cooperativity for the ligand exchange on the Pt^II centers of the dinuclear Pt^II complexes. On the other hand, in less polar CD₂ Cl₂ , the self-assembly process was affected by the components. For two of the three macrocycles, the chainlike oligomers that contain fewer metals and ligands than the corresponding macrocycles grew with time and the type of the chainlike intermediates formed correlated with the allostericity of the two binding sites in the organic ditopic ligands. In every case, no long oligomers containing more components than the macrocycles themselves were produced during the self-assembly even though free rotation around single bonds in the chainlike oligomers allows them to adopt various conformations that do not facilitate the cyclization. This result suggests that electrostatic and/or steric factors besides rigidity of the components make the cyclization advantageous not only thermodynamically but also kinetically.

A H-shaped heterometallic Sn4Au4 system with guest-tuneable multicolour and selective luminescence sensing properties

A H-shaped heterometallic Sn₄Au₄ system displays guest-dependent optical properties and its stimuli-responsive methylred-rich crystals are sensitive towards pH variations.

Terpyridine-based metallo-organic cages and supramolecular gelation by coordination-driven self-assembly and host–guest interaction

Dimetallo-cages connected with dialkylammonium salts via host–guest interactions resulted in thermodynamic polymer metallo-gels.

Terpyridine-based metallosupramolecular constructs: tailored monomers to precise 2D-motifs and 3D-metallocages

Comprehensive summary of the recent developments in the growing field of terpyridine-based, discrete metallosupramolecular architectures.

A metalloligand appended with benzimidazole rings: tetranuclear [CoZn3] and [CoCd3] complexes and their catalytic applications

A novel Co³⁺-based metalloligand, offering appended benzimidazole rings, has been utilized for the synthesis of catalytically active tetranuclear [CoZn₃] and [CoCd₃] heterometallic coordination complexes.

The Effect of Solvent and Coordination Environment of Metal Source on the Self-Assembly Pathway of a Pd(II)-Mediated Coordination Capsule

The effect of reaction environment on the self-assembly process of an octahedron-shaped Pd₆L₈ capsule was investigated. Quantitative analysis of self-assembly process with ¹H NMR spectroscopy revealed that the self-assembly pathway of the capsule was altered by solvent and a leaving ligand coordinating to the metal source, which are not the components of the final self-assembly. Solvents definitively determine the pathway of the self-assembly at a very early stage of the self-assembly. Contrary to the expectation that the weaker the coordination ability of the leaving ligand is, the faster the formation of the final assembly becomes, a leaving ligand with weak coordination ability tends to generate a kinetically trapped species to prevent the capsule formation under mild conditions.

Syntheses, Structures, and Solution Studies of Multicomponent Macrocycles and Cages Based on Versatile Ligands

Different types of multinuclear half-sandwich rhodium macrocycles and cages were designed and synthesized by using two similar multifunctional hydroxamate ligands (pyrazine-2-hydroxamic acid (NaHL¹ ) and 4,4'-bipyridine-2-hydroxamic acid (KHL² )) featuring one monodentate site and two pairs of chelating sites. The Rh^III -Pd^II heterometallic macrocycles were constructed by using the semi-open palladium(II) source [Pd(en)Cl₂ ] with two free acceptor sites. However, only one kind of macrocycle was found when the shorter ligand L¹ was used, while in for the larger ligand, various spectroscopic techniques demonstrated the coexistence of hexanuclear and octanuclear macrocycles in solution and the proportions of both components depended on concentration and temperature. The palladium salt Pd(NO₃ )₂ , as a source of "naked" Pd²⁺ , was introduced to assemble the cuboid-shaped cage composed of two types of metal ions and three types of organic ligands. In addition, two silver(I)-containing mixed-metal complexes bridged by pyrazine were obtained, in which two forms of decanuclear complex with C_2v and C_2h point symmetry cocrystallized-one is a polymeric structure and the other is a discrete cage. However, the third form, with D₂ point symmetry, was found in the larger cage.

A Truncated Molecular Star

A pentanuclear coordination complex assembled from any palladium(II) component and non-chelating ligands is hitherto unreported. The pentanuclear complex [Pd₅ (L1)₅ (L2)₅ ](BF₄ )₁₀ , 1 reported here was prepared by the spontaneous complexation of [Pd(DMSO)₄ ](BF₄ )₂ with the non-chelating bidentate ligands 1,4-phenylenebis(methylene) diisonicotinate, L1 and 4,4'-bipyridine, L2 in a one-pot method at room temperature. The planar polycyclic complex 1 with outer diameters of ≈3 nm is termed as a "molecular star" owing to its resemblance with a pentagram shape. Interim paths leading to the star were also probed to decipher related dynamics of the system.

Supersnowflakes: Stepwise Self-Assembly and Dynamic Exchange of Rhombus Star-Shaped Supramolecules

With the goal of increasing the complexity of metallo-supramolecules, two rhombus star-shaped supramolecular architectures, namely, supersnowflakes, were designed and assembled using multiple 2,2':6',2″-terpyridine (tpy) ligands in a stepwise manner. In the design of multicomponent self-assembly, ditopic and tritopic ligands were bridged through Ru(II) with strong coordination to form metal-organic ligands for the subsequent self-assembly with a hexatopic ligand and Zn(II). The combination of Ru(II)-organic ligands with high stability and Zn(II) ions with weak coordination played a key role in the self-assembly of giant heteroleptic supersnowflakes, which encompassed three types of tpy-based organic ligands and two metal ions. With such a stepwise strategy, the self-sorting of individual building blocks was prevented from forming the undesired assemblies, e.g., small macrocycles and coordination polymers. Furthermore, the intra- and intermolecular dynamic exchange study of two supersnowflakes by NMR and mass spectrometry revealed the remarkable stability of these giant supramolecular complexes.

Synthesis and characterization of a dipyriamethyrin–uranyl complex

A bench stable uranyl complex of a hexaazadipyriamethryin macrocyclic ligand has been prepared and characterized.