Directional Shuttling of a Stimuli‐Responsive Cone‐Like Macrocycle on a Single‐State Symmetric Dumbbell Axle

Rotaxane‐based molecular shuttles are often operated using low‐symmetry axles and changing the states of the binding stations. A molecular shuttle capable of directional shuttling of an acid‐responsive cone‐like macrocycle on a single‐state symmetric dumbbell axle is now presented. The axle contains three binding stations: one symmetric di(quaternary ammonium) station and two nonsymmetric phenyl triazole stations arranged in opposite orientations. Upon addition of an acid, the protonated macrocycle shuttles from the di(quaternary ammonium) station to the phenyl triazole binding station closer to its butyl groups. This directional shuttling presumably originates from charge repulsion and an orientational binding preference between the cone‐like cavity and the nonsymmetric phenyl triazole station. This mechanism for achieving directional shuttling by manipulating only the wheels instead of the tracks is new for artificial molecular machines.     

Desymmetrization of an Octahedral Coordination Complex Inside a Self‐Assembled Exoskeleton

The synthesis of a centrally functionalized, ribbon‐shaped [6]polynorbornane ligand L that self‐assembles with Pd^II cations into a {Pd₂ L ₄} coordination cage is reported. The shape‐persistent {Pd₂ L ₄} cage contains two axial cationic centers and an array of four equatorial H‐bond donors pointing directly towards the center of the cavity. This precisely defined supramolecular environment is complementary to the geometry of classic octahedral complexes [M(XY)₆] with six diatomic ligands. Very strong binding of [Pt(CN)₆]^2? to the cage was observed, with the structure of the host–guest complex {[Pt(CN)₆]@Pd₂L₄} supported by NMR spectroscopy, MS, and X‐ray data. The self‐assembled shell imprints its geometry on the encapsulated guest, and desymmetrization of the octahedral platinum species by the influence of the D_4h‐symmetric second coordination sphere was evidenced by IR spectroscopy. [Fe(CN)₆]^3? and square‐planar [Pt(CN)₄]^2? were strongly bound. Smaller octahedral anions such as [SiF₆]^2?, neutral carbonyl complexes ([M(CO)₆]; M=Cr, Mo, W) and the linear [Ag(CN)₂]^? anion were only weakly bound, showing that both size and charge match are key factors for high‐affinity binding.     

Halogen Bonding in Solution: Anion Recognition,Templated Self‐Assembly,and Organocatalysis

The halogen bond is a supramolecular interaction between a Lewis‐acidic region of a covalently bound halogen and a Lewis base. It has been studied widely in silico and experimentally in the solid state; however, solution‐phase applications have attracted enormous interest in the last few years. This Minireview highlights selected recent developments in halogen bond interactions in solution, with a focus on the use of receptors based on halogen bonds in anion recognition and sensing, anion‐templated self‐assembly, as well as in organocatalysis.     

Multicomponent Self‐Assembly with a Shape‐Persistent N‐Heterotriangulene Macrocycle on Au(111)

Multicomponent network formation by using a shape‐persistent macrocycle ( MC6 ) at the interface between an organic liquid and Au(111) surface is demonstrated. MC6 serves as a versatile building block that can be coadsorbed with a variety of organic molecules based on different types of noncovalent interactions at the liquid–solid interface. Scanning tunneling microscopy (STM) reveals the formation of crystalline bicomponent networks upon codeposition of MC6 with aromatic molecules, such as fullerene (C₆₀) and coronene. Tetracyanoquinodimethane, on the other hand, was found to induce disorder into the MC6 networks by adsorbing on the rim of the macrocycle. Immobilization of MC6 itself was studied in two different noncovalently assembled host networks. MC6 assumed a rather passive role as a guest and simply occupied the host cavities in one network, whereas it induced a structural transition in the other. Finally, the central cavity of MC6 was used to capture C₆₀ in a complex three‐component system. Precise immobilization of organic molecules at discrete locations within multicomponent networks, as demonstrated here, constitutes an important step towards bottom‐up fabrication of functional surface‐based nanostructures.     

Time‐Resolved Assembly of Cluster‐in‐Cluster {Ag12}‐in‐{W76} Polyoxometalates under Supramolecular Control

We report the time‐resolved supramolecular assembly of a series of nanoscale polyoxometalate clusters (from the same one‐pot reaction) of the form: [H_(10+m)Ag₁₈Cl(Te₃W₃₈O₁₃₄)₂]_n, where n=1 and m=0 for compound 1 (after 4 days), n=2 and m=3 for compound 2 (after 10 days), and n=∞ and m=5 for compound 3 (after 14 days). The reaction is based upon the self‐organization of two {Te₃W₃₈} units around a single chloride template and the formation of a {Ag₁₂} cluster, giving a {Ag₁₂}‐in‐{W₇₆} cluster‐in‐cluster in compound 1 , which further aggregates to cluster compounds 2 and 3 by supramolecular Ag‐POM interactions. The proposed mechanism for the formation of the clusters has been studied by ESI‐MS. Further, control experiments demonstrate the crucial role that TeO₃^2?, Cl^?, and Ag⁺ play in the self‐assembly of compounds 1 – 3 .     

Prismarene: An Emerging Naphthol‐Based Macrocyclic Arene

Phenol‐based macrocyclic arenes have been widely used in supramolecular chemistry, significantly enriching the toolbox of the field. In contrast, naphthol‐based macrocyclic arenes are rather underdeveloped. Very recently, Gaeta and co‐workers successfully synthesized such macrocycles (referred to as prism[n]arenes) with good guest‐binding ability by reacting 2,6‐dimethoxynaphthalene with paraformaldehyde under optimized conditions. In view of the simple synthesis and good host–guest chemistry, we anticipate that this macrocycle will find similar success and wide applications as the phenol‐based macrocyclic arenes.     

A Macrocycle Based on a Heptagon‐Containing Hexa‐peri‐hexabenzocoronene

A cyclophane is reported incorporating two units of a heptagon‐containing extended polycyclic aromatic hydrocarbon (PAH) analogue of the hexa‐peri‐hexabenzocoronene (HBC) moiety (hept‐HBC). This cyclophane represents a new class of macrocyclic structures that incorporate for the first time seven‐membered rings within extended PAH frameworks. The saddle curvature of the hept‐HBC macrocycle units induced by the presence of the nonhexagonal ring along with the flexible alkyl linkers generate a cavity with shape complementarity and appropriate size to enable π interactions with fullerenes. Therefore, the cyclophane forms host–guest complexes with C₆₀ and C₇₀ with estimated binding constants of K_a=420±2 m ^?1 and K_a=(6.49±0.23)×10³ m ^?1, respectively. As a result, the macrocycle can selectively bind C₇₀ in the presence of an excess of a mixture of C₆₀ and C₇₀.     

Designed Enclosure Enables Guest Binding Within the 4200 Å3 Cavity of a Self‐Assembled Cube

Metal–organic self‐assembly has proven to be of great use in constructing structures of increasing size and intricacy, but the largest assemblies lack the functions associated with the ability to bind guests. Here we demonstrate the self‐assembly of two simple organic molecules with Cd^II and Pt^II into a giant heterometallic supramolecular cube which is capable of binding a variety of mono‐ and dianionic guests within an enclosed cavity greater than 4200 ų. Its structure was established by X‐ray crystallography and cryogenic transmission electron microscopy. This cube is the largest discrete abiological assembly that has been observed to bind guests in solution; cavity enclosure and coulombic effects appear to be crucial drivers of host–guest chemistry at this scale. The degree of cavity occupancy, however, appears less important: the largest guest studied, bound the most weakly, occupying only 11 % of the host cavity.     

A Bambusuril Macrocycle that Binds Anions in Water with High Affinity and Selectivity

Synthetic receptors that function in water are important for the qualitative and quantitative detection of anions, which may act as pollutants in the environment or play important roles in biological processes. Neutral receptors are particularly appealing because they are often more selective than positively charged receptors; however, their affinity towards anions in pure water is only in range of 1–10³ L mol^?1. The anion‐templated synthesis of a water‐soluble bambusuril derivative is shown to be an outstanding receptor for various inorganic anions in pure water, with association constants of up to 10⁷ L mol^?1. Furthermore, the macrocycle discriminates between anions with unprecedented selectivity (up to 500 000‐fold). We anticipate that the combination of remarkable affinity and selectivity of this macrocycle will enable the efficient detection and isolation of diverse anions in aqueous solutions, which is not possible with current supramolecular systems.     

Host–Guest Binding‐Site‐Tunable Self‐Assembly of Stimuli‐Responsive Supramolecular Polymers

Despite the remarkable progress made in controllable self‐assembly of stimuli‐responsive supramolecular polymers (SSPs), a basic issue that has not been consideration to date is the essential binding site. The noncovalent binding sites, which connect the building blocks and endow supramolecular polymers with their ability to respond to stimuli, are expected to strongly affect the self‐assembly of SSPs. Herein, the design and synthesis of a dual‐stimuli thermo‐ and photoresponsive Y‐shaped supramolecular polymer (SSP2) with two adjacent β‐cyclodextrin/azobenzene (β‐CD/Azo) binding sites, and another SSP (SSP1) with similar building blocks, but only one β‐CD/Azo binding site as a control, are described. Upon gradually increasing the polymer solution temperature or irradiating with UV light, SSP2 self‐assemblies with a higher binding‐site distribution density; exhibits a flower‐like morphology, smaller size, and more stable dynamic aggregation process; and greater controllability for drug‐release behavior than those observed with SSP1 self‐assemblies. The host–guest binding‐site‐tunable self‐assembly was attributed to the positive cooperativity generated among adjacent binding sites on the surfaces of SSP2 self‐assemblies. This work is beneficial for precisely controlling the structural parameters and controlled release function of SSP self‐assemblies.     

Subcomponent Self‐Assembly of a 4 nm M4L6 Tetrahedron with ZnII Vertices and Perylene Bisimide Dye Edges

Formation of a tetrahedron with >4 nm perylene bisimide (PBI) dye edges and Zn^II vertices in a one‐pot 22 component self‐assembly reaction is reported. The luminescent polyhedron equilibrates to a Zn₂L₃ helicate and disassembles upon dilution. Insights into the subcomponent self‐assembly of extended PBI ligands help to refine design rules for constructing large photofunctional metallosupramolecular hosts.     

Host–Guest Recognition and Fluorescence of a Tetraphenylethene‐Based Octacationic Cage

We report the synthesis and characterization of a three‐dimensional tetraphenylethene‐based octacationic cage that shows host–guest recognition of polycyclic aromatic hydrocarbons (e.g. coronene) in organic media and water‐soluble dyes (e.g. sulforhodamine 101) in aqueous media through CH???π, π–π, and/or electrostatic interactions. The cage?coronene exhibits a cuboid internal cavity with a size of approximately 17.2×11.0×6.96 ų and a “hamburger”‐type host–guest complex, which is hierarchically stacked into 1D nanotubes and a 3D supramolecular framework. The free cage possesses a similar cavity in the crystalline state. Furthermore, a host–guest complex formed between the octacationic cage and sulforhodamine 101 had a higher absolute quantum yield (Φ_F=28.5 %), larger excitation–emission gap (Δλ_ex‐em=211 nm), and longer emission lifetime (τ=7.0 ns) as compared to the guest (Φ_F=10.5 %; Δλ_ex‐em=11 nm; τ=4.9 ns), and purer emission (Δλ_FWHM=38 nm) as compared to the host (Δλ_FWHM=111 nm).     

Encapsulation of Metalloporphyrins in a Self‐Assembled Cubic M8L6 Cage: A New Molecular Flask for Cobalt–Porphyrin‐Catalysed Radical‐Type Reactions

The synthesis of a new, cubic M₈L₆ cage is described. This new assembly was characterised by using NMR spectroscopy, DOSY, TGA, MS, and molecular modelling techniques. Interestingly, the enlarged cavity size of this new supramolecular assembly allows the selective encapsulation of tetra(4‐pyridyl)metalloporphyrins (M^II(TPyP), M=Zn, Co). The obtained encapsulated cobalt–porphyrin embedded in the cubic zinc–porphyrin assembly is the first example of a catalytically active encapsulated transition‐metal complex in a cubic M₈L₆ cage. The substrate accessibility of this system was demonstrated through radical‐trapping experiments, and its catalytic activity was demonstrated in two different radical‐type transformations. The reactivity of the encapsulated Co^II(TPyP) complex is significantly increased compared to free Co^II(TPyP) and other cobalt–porphyrin complexes. The reactions catalysed by this system are the first examples of cobalt–porphyrin‐catalysed radical‐type transformations involving diazo compounds which occur inside a supramolecular cage.