Multiple Transformations among Anion-based A2nL3n Assemblies: Bicapped Trigonal Antiprism A8L12, Tetrahedron A4L6, and Triple Helicate A2L3 (A = Anion)

Peripheral Control of the Assembly and Chirality of Anion-Based Octanuclear Cubes by Cation-π Networks

Self-assembly of sophisticated polyhedral cages has drawn much attention because of their elaborate structures and potential applications. Herein, we report the anion-coordination-driven assembly of the first A8L12 (A = anion, L = ligand) octanuclear cubic structures from phosphate anion and p-xylylene-spaced bis-bis(urea) ligands via peripheral templating of countercations (TEA+ or TPA+). By attaching terminal aryl rings (phenyl or naphthyl) to the ligand through a flexible (methylene) linker, these aryls actively participate in the formation of plenty of "aromatic pockets" for guest cation binding. As a result, multiple peripheral guests (up to 22) of suitable size are bound on the faces and vertices of the cube, forming a network of cation-π interactions to stabilize the cube structure. More interestingly, when chiral ligands were used, either diastereomers of mixed Λ- and Δ-configurations (with TEA+ countercation) for the phosphate coordination centers or enantiopure cubes (with TPA+) were formed. Thus, the assembly and chirality of the cube can be modulated by remote terminal groups and peripheral templating tetraalkylammonium cations.

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

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

Incorporation of an Anion-Coordinated Triple Helicate into a Thin Film for Choline Recognition in an Aqueous System

Functional thin films, being fabricated by incorporating discrete supramolecular architectures, have potential applications in research areas such as sensing, energy storage, catalysis, and optoelectronics. Here, we have determined that an anion-coordinated triple helicate can be solution-processed into a functional thin film by incorporation into a polymethyl methacrylate (PMMA) matrix. The thin films fabricated by the incorporation of the anion-coordinated triple helicate show multiple optical properties, such as fluorescence, CD, and CPL. In addition, the film has the ability to recognize choline and choline derivatives in a water system. The successful recognition of Ch+ by the film represents the first example of utilizing 'aniono'-supramolecular architectures for biomolecule detection in aqueous solution and opens up a new route for designing biocompatible functional materials.

Anion-encapsulating, discrete prism and extended frusta, from trimetallated triangular macrocycles and linkers

Reaction of a trinuclear triangular macrocyclic complex Pb3L(CF3SO3)6 with bidentate linkers in a ratio of 3 equiv. of linker per 2 equiv. of complex, produces a prismatic structure with 4,4'-dipyridyl, and two unprecedented, extended 3D frustum-like structures with 1,2-di(4-pyridyl)ethylene and 1,4-di(4-pyridyl)benzene. The cavities of these structures encapsulate triflate anions.

Excited-Multimer Mediated Supramolecular Upconversion on Multicomponent Lanthanide-Organic Assemblies

Upconversion (UC) is a fascinating anti-Stokes-like optical process with promising applications in diverse fields. However, known UC mechanisms are mainly based on direct energy transfer between metal ions, which constrains the designability and tunability of the structures and properties. Here, we synthesize two types of Ln8L12-type (Ln for lanthanide ion; L for organic ligand L1 or L2R/S) lanthanide-organic complexes with assembly induced excited-multimer states. The Yb8(L2R/S)12 assembly exhibits upconverted multimer green fluorescence under 980 nm excitation through a cooperative sensitization process. Furthermore, upconverted red emission from Eu3+ on the heterometallic (Yb/Eu)8L12 assemblies is also realized via excited-multimer mediated energy relay. Our findings demonstrate a new strategy for designing UC materials, which is crucial for exploiting photofunctions of multicomponent lanthanide-organic complexes.

Formation of Metallosupramolecular Helicates and Mesocates from Poly-N-Heterocyclic Carbene Ligands

In this work, a series of poly-NHC-based tetranuclear silver helicates and mesocates were synthesized from the silver-mediated self-assembly of the ligands involving multiple tridentate CNC-type pincer units and NHC coordination sites. The silver helicate was found to be transferred to a gold mesocate upon metal exchange reaction. The metallosupramolecular helicates and mesocates have been fully characterized by single-crystal X-ray crystallography, mass spectrometry, and multinuclear nuclear magnetic resonance spectroscopies. This study provides an example of the selective preparation of poly-NHC-based helicates or mesocates depending on the size of metal ions and the steric effect of ligands.

Solvato-Controlled Assembly and Structural Transformation of Emissive Poly-NHC-Based Organometallic Cages and Their Applications in Amino Acid Sensing and Fluorescence Imaging

Stimuli-induced structural transformation of supramolecular cages has drawn increasing attention because of their sensitive feature to external variations as model systems to simulate biological processes. However, combining structural transformation and useful functions has remained a difficult task. This study reports the solvato-controlled self-assembly of two unique topologies with different emission characteristics, a water-soluble Ag₈ L₄ cage (A) and an Ag₄ L₂ cage (B), produced from the same sulfonate-pendant tetraphenylethene (TPE) bridged tetrakis-(1,2,4-triazolium) ligand. Both cages show interesting solvent-responsive reversible structural transformation, and the change of fluorescence signals can efficiently track the process. Additionally, water-soluble cage A exhibits unique properties in thermochromism, thiol amino acid sensing, and subcellular imaging in aqueous media.

Anion-Coordination-Driven Assembly

The assembly of discrete architectures has been an important subject in supramolecular chemistry because of their elegant structures and fascinating properties. During the last several decades, supramolecular chemists have developed manifold strategies for hierarchical assembly, which are normally classified by two main types of driving force: covalent and noncovalent interactions. Typical noncovalent interactions include metal coordination, hydrogen bonding, and other weak forces. These approaches have achieved great progress in the construction of various supramolecular structures, such as macrocycles, cages, polyhedra, and interlocked systems. Among these methods, metal-coordination-driven assembly is attractive due to the well-defined coordination properties of metal ions. Indeed, in terms of supramolecular chemistry, the concept of "coordination" has been expanded beyond transition metals. In particular, anion coordination chemistry, which was first proposed by Lehn in 1978 [ Acc. Chem. Res. 1978, 11, 49] and then elucidated in detail by Bowman-James two decades later [ Acc. Chem. Res. 2005, 38, 671], has grown up to a subfield of supramolecular chemistry. It is noticeable that anions also show "dual valencies" like transition metals, wherein the "primary valence" is the charge balance for anions by countercations while the "secondary valence", i.e., the coordination, refers to hydrogen bonding interactions where the electron flow is from the electron-rich anion (the coordination center) to hydrogen bonding donors (the ligands). Thus, anions also display certain coordination numbers and specific coordination geometries. Although such features are far less regular than those of transition metals, they are sufficient to allow anion coordination to serve as the driving force for assembling discrete supramolecular architectures. In this Account, the anion-coordination-driven assembly (ACDA), a new assembling strategy established by us during the past decade, will be presented. We summarize our work in the construction of a series of "aniono" supramolecular structures, especially triple helicates and tetrahedral cages, based on the coordination between oligourea ligands and anions (mostly phosphate). In particular, we will detail the considerations in the design of ligands, the assembling process including structural transformation, and functionalization of the systems toward guest inclusion, supramolecular catalysis, photoswitches, and molecular devices. These results demonstrate the great potential of ACDA in fabricating novel anion-based systems. Although the design concept was originally loaned from traditional coordination chemistry of transition metals, and structures of anion complexes bear some resemblance to metal complexes, there are significant differences of the aniono supramolecular assemblies from the metallo analogues. For example, these metal-free systems are held together by multiple hydrogen bonds (dozens to nearly 100), thus facilitating assembly/disassembly under mild conditions and relatively flexible structures for adaptive guest inclusion. To this end, intriguing applications (supramolecular chirality, catalysis, energy storage, etc.) may be expected for aniono systems. We hope the current Account will attract more attention from researchers in supramolecular assembly and inspire more efforts in this fascinating area.

Anion-Coordination-Driven Assembly: From Discrete Supramolecular Self-Assemblies to Functional Soft Materials

Anion templated assembly of supramolecular systems has been extensively explored in previous reports, whereas anions serve only as an auxiliary and spectator role. With the development of anion coordination chemistry in recent years, anion coordination-driven assembly (ACDA) has emerged as a new strategy for the construction of supramolecular self-assemblies. Anions are proved to exist as the main actors in the construction of supramolecular architectures, i. e., serve as the coordination center. This Review will focus on the recent progress in anion-coordination-driven assembly of discrete supramolecular architectures, such as helicates, polyhedrons and polygons, and the various applications of 'aniono'-systems. At the end of this Review, we highlight current challenges and opportunities for future research of anion-coordination-driven self-assembly.

Templation and Concentration Drive Conversion Between a FeII12L12 Pseudoicosahedron, a FeII4L4 Tetrahedron, and a FeII2L3 Helicate

We report the construction of three structurally distinct self-assembled architectures: Fe^II₁₂L₁₂ pseudoicosahedron 1, Fe^II₂L₃ helicate 2, and Fe^II₄L₄ tetrahedron 3, formed from a single triazatriangulenium subcomponent A under different reaction conditions. Pseudoicosahedral capsule 1 is the largest formed through subcomponent self-assembly to date, with an outer-sphere diameter of 5.4 nm and a cavity volume of 15 nm³. The outcome of self-assembly depended upon concentration, where the formation of pseudoicosahedron 1 was favored at higher concentrations, while helicate 2 exclusively formed at lower concentrations. The conversion of pseudoicosahedron 1 or helicate 2 into tetrahedron 3 occurred following the addition of a CB₁₁H₁₂^- or B₁₂F₁₂^2- template.

Metal Ions Determined Self-Assembly Using Terpyridine Building Blocks

Due to the dynamic reversibility of coordination-driven force, the structures of metallo-cages are sensitive to many stimulus, including ligand geometry, temperature, concentration, anions, pH, light, and so on. Among them,...

A Hydrogen-Bonded Ravel Assembled by Anion Coordination

Anion-coordination-driven assembly (ACDA) is showing increasing power in the construction of anionic supramolecular architectures. Herein, by expanding the anion centers from oxoanion (phosphate or sulfate) to organic tris-carboxylates, an Archimedean solid (truncated tetrahedron) and a highly entangled, double-walled tetrahedron featuring a ravel topology have been assembled with tris-bis(urea) ligands. The results demonstrate the promising ability of tris-carboxylates as new anion coordination centers in constructing novel topologies with increasing complexity and diversity compared to phosphate or sulfate ions on account of the modifiable size and easy functionalization character of these organic anions.

Acid-Tolerant Sulfate Tetrahedral Cages from Anion-Coordination-Driven Assembly

Supramolecular cages have been constructed by anion-coordination-driven assembly (ACDA) in recent years and have shown unique host-guest interactions. However, most of the reported cages are made of the phosphate ion (PO₄ ^3- ); other anions have rarely been explored. Here we show for the first time that the sulfate ion (SO₄ ^2- ) is also able to form the A₄ L₄ tetrahedral motif with tris-bis(urea) ligands, but this is dependent on the stoichiometry of the sulfate ion (in solution). Notably, the sulfate cages display enhanced resistance for both Brønsted (pH as low as 4.3 in acetone containing 15 % water) and Lewis acids (metal complexes) compared to the corresponding phosphate cages, and thus could find applications where an acidic (proton or metals) environment is required.

Narcissistic self-sorting in anion-coordination-driven assemblies

Three tris-bis(urea) ligands with triphenylamine-based C₃-symmetric spacers were synthesized, which assembled with sulfate or phosphate to form anionic A₃L₂ pinwheel helices (A = anion and L = ligand) and A₄L₄ tetrahedra, respectively. Interestingly, narcissistic self-sorting was observed in both structures from the mixture of the ligands, wherein each assembly contains only one type of ligand with no detectable mixed-ligand product as confirmed by the NMR and MS studies.

Hexameric Lanthanide-Organic Capsules with Tertiary Structure and Emergent Functions

Biological macromolecules always function through a collective behavior of the aggregated constituents, which usually are self-assembled together via noncovalent interactions. Likewise, artificial supramolecular assemblies, whose properties and functions are mainly derived from their primary and secondary structures, may also aggregate into high-order architectures with emergent functions not available on the individual components. Here we report the first example of an insulin-like hexamerization of lanthanide triple helicates toward a 4 nm diameter hexameric capsule via consecutive metal-directed and anion-directed assembly processes. Hierarchical chiral-sorting self-assembly endows hexamers with aggregation-induced stability and emission enhancement. Furthermore, emergent guest-encapsulation function and enantioselectivity toward terpene drugs have been realized in the late-formed central cavity of the hexamers. This study not only provides a feasible strategy for constructing sophisticated and multifunctional lanthanide-organic materials but also sheds some light on the self-assembly processes in nature.

Self-assembled luminescent Cu(i) tetranuclear metallacycles based on 3,3′-bipyridine ligands

Three luminescent tetranuclear macrocycles are obtained selectively, applying coordination-driven supramolecular processes to the reaction of 3,3′-bipyridine ligand with in situ formed Cu(i) bimetallic units bearing a coordination angle of ca. 120°.

Dinuclear helicate or mononuclear pincer lanthanide complexes from one ligand: stereo-controlled assembly and catalysis

By varying the metal/ligand ratio, either dinuclear triple helicate or mononuclear pincer can be stereo-selectively obtained from the same chiral ligand. Their catalytic properties have been demonstrated with the asymmetric FC alkylation reactions.