Active‐Metal Template Synthesis of a Halogen‐Bonding Rotaxane for Anion Recognition

The synthesis of an all‐halogen‐bonding rotaxane for anion recognition is achieved by using active‐metal templation. A flexible bis‐iodotriazole‐containing macrocycle is exploited for the metal‐directed rotaxane synthesis. Endotopic binding of a Cu^I template facilitates an active‐metal CuAAC iodotriazole axle formation reaction that captures the interlocked rotaxane product. Following copper‐template removal, exotopic coordination of a more sterically demanding rhenium(I) complex induces an inversion in the conformation of the macrocycle component, directing the iodotriazole halogen‐bond donors into the rotaxane’s interlocked binding cavity to facilitate anion recognition.     

Stepwise Halide‐Triggered Double and Triple Catenation of Self‐Assembled Coordination Cages

A simple self‐assembled [Pd₂ L ₄] coordination cage consisting of four carbazole‐based ligands was found to dimerize into the interpenetrated double cage [3 X@Pd₄ L ₈] upon the addition of 1.5 equivalents of halide anions (X=Cl^?, Br^?). The halide anions serve as templates, as they are sandwiched by four Pd^II cations and occupy the three pockets of the entangled cage structure. The subsequent addition of larger amounts of the same halide triggers another structural conversion, now yielding a triply catenated link structure in which each Pd^II node is trans‐coordinated by two pyridine donors and two halide ligands. This simple system demonstrates how molecular complexity can increase upon a gradual change of the relative concentrations of reaction partners that are able to serve different structural roles.     

A Dynamic and Responsive Host in Action: Light‐Controlled Molecular Encapsulation

The rational design of a flexible molecular box, oAzoBox ⁴⁺, incoporating both photochromic and supramolecular recognition motifs is described. We exploit the E?Z photoisomerization properties of azobenzenes to alter the shape of the cavity of the macrocycle upon absorption of light. Imidazolium motifs are used as hydrogen‐bonding donor components, allowing for sequestration of small molecule guests in acetonitrile. Upon E→Z photoisomerization of oAzoBox ⁴⁺ the guest is expelled from the macrocyclic cavity.     

A Triphasic Sorting System: Coordination Cages in Ionic Liquids

Host–guest chemistry is usually carried out in either water or organic solvents. To investigate the utility of alternative solvents, three different coordination cages were dissolved in neat ionic liquids. By using ¹⁹F NMR spectroscopy to monitor the presence of free and bound guest molecules, all three cages were demonstrated to be stable and capable of encapsulating guests in ionic solution. Different cages were found to preferentially dissolve in different phases, allowing for the design of a triphasic sorting system. Within this system, three coordination cages, namely Fe₄L₆ 2 , Fe₈L₁₂ 3 , and Fe₄L₄ 4 , each segregated into a distinct layer. Upon the addition of a mixture of three different guests, each cage (in each separate layer) selectively bound its preferred guest.     

Guest‐Triggered Supramolecular Isomerism in a Pillared‐Layer Structure with Unusual Isomers of Paddle‐Wheel Secondary Building Units by Reversible Single‐Crystal‐to‐Single‐Crystal Transformation

Solvothermal reaction of Zn(NO₃)₂ ? 4 H₂O, 1,4‐bis[2‐(4‐pyridyl)ethenyl]benzene (bpeb) and 4,4′‐oxybisbenzoic acid (H₂obc) in the presence of dimethylacetamide (DMA) as one of the solvents yielded a threefold interpenetrated pillared‐layer porous coordination polymer with pcu topology, [Zn₂(bpeb)(obc)₂] ? 5 H₂O ( 1 ), which comprised an unusual isomer of the well‐known paddle‐wheel building block and the trans–trans–trans isomer of the bpeb pillar ligand. When dimethylformamide (DMF) was used instead of DMA, a supramolecular isomer [Zn₂(bpeb)(obc)₂] ? 2 DMF ? H₂O ( 2 ), with the trans–cis–trans isomer of the bpeb ligand with a slightly different variation of the paddle‐wheel repeating unit, was isolated. In MeOH, single crystals of 2 were transformed by solvent exchange in a single‐crystal‐to‐single‐crystal (SCSC) manner to yield [Zn₂(bpeb)(obc)₂] ? 2 H₂O ( 3 ), which is a polymorph of 1 . SCSC conversion of 3 to 2 was achieved by soaking 3 in DMF. Compounds 1 and 2 as well as 2 and 3 are supramolecular isomers.     

Size‐Selective Encapsulation of Hydrophobic Guests by Self‐Assembled M4L6 Cobalt and Nickel Cages

Subtle differences in metal–ligand bond lengths between a series of [M₄L₆]^4? tetrahedral cages, where M=Fe^II, Co^II, or Ni^II, were observed to result in substantial differences in affinity for hydrophobic guests in water. Changing the metal ion from iron(II) to cobalt(II) or nickel(II) increases the size of the interior cavity of the cage and allows encapsulation of larger guest molecules. NMR spectroscopy was used to study the recognition properties of the iron(II) and cobalt(II) cages towards small hydrophobic guests in water, and single‐crystal X‐ray diffraction was used to study the solid‐state complexes of the iron(II) and nickel(II) cages.     

Cucurbit[7]uril: Surfactant Host–Guest Complexes in Equilibrium with Micellar Aggregates

In order to compare the formation of host–guest complexes between β‐cyclodextrin (β‐CD) or cucurbit[7]uril (CB7) and cationic surfactants we studied the hydrolysis of 4‐methoxybenzenesulfonyl chloride (MBSC). The selected surfactants allowed the length of the hydrocarbon chain to be varied between 6 and 18 carbon atoms. Contrary to the expected behaviour, the values of the binding constants between CB7 and surfactants are independent of the alkyl chain length of the surfactant. In the case of β‐CD, however, a clear dependence of the binding constant on the hydrophobic character of the surfactant was observed. The values obtained with CB7 are significantly higher than those obtained with β‐CD and these differences are explained to be a consequence of electrostatic interactions of the surfactants with the portals of CB7. It was found that a small percentage of uncomplexed CB7 was in equilibrium with the cationic micelles and this percentage increased on increasing the hydrophobic character of the surfactant.     

A Cobalt Metallacrown Anion Host with Guest‐Dependent Redox Activity

Two bowl‐shaped cavities , each having three OH^? hydrogen‐bond donors at its base, are present in double‐cone‐shaped metallacrown anion host [Co₆(μ‐OH)₆(μ‐L)₆]^m+ ( 1 ^{m +} ; HL=3{5}‐(pyrid‐2‐yl)‐5{3}‐(tert‐butyl)pyrazole). Depending on its affinity for the anions present, it can be isolated in its Co^III₃Co^II₃ (m=3; e.g., 1 (ClO₄)₃) and Co^III₂Co^II₄ (m=2; e.g., 1 (BF₄)₂ ? n H₂O) oxidation states. See picture for photographs of isolated salts.

     

Imine Macrocycle with a Deep Cavity: Guest‐Selected Formation of syn/anti Configuration and Guest‐Controlled Reconfiguration

A dynamic covalent bond is one of the ideal linkages for the construction of large and robust organic architectures. In the present article, we show how organic templates can efficiently transform a complex dynamic imine library into a dynamic imine macrocycle. Not only is the constitution well controlled, but also the syn/anti host configuration is efficiently selected and even the orientation of the guest in the asymmetric cavity of the host can be well aligned. This is attributed to the delicate balance and the cooperation of multiple noncovalent interactions between the hosts and the guests. Through sequential additions of three guests in appropriate amounts, controlled structural reconfiguration of dynamic covalent architectures has been achieved for the first time.     

A Light‐Controlled Release System Based on Molecular Recognition of Cyclodextrins

This Communication describes a new light‐controlled release system based on molecular recognition of cyclodextrins. Azobenzene (Azo) residue is employed as a photoresponsive guest residue because it can switch the partner from α‐cyclodextrin (αCD) to β‐cyclodextrin (βCD) by irradiation with UV light. Poly(sodium acrylate)s possessing αCD, βCD, and Azo residues (pAαCD, pAβCD, and pAAzo, respectively) are mixed in aqueous solutions to form aggregates through the formation of inclusion complexes of Azo with αCD and/or βCD. A chemical cargo, 1‐pyrenemethylammonium chloride (PyMA), is contained in the aggregates, and its release behavior is investigated by dialysis experiments under UV irradiation. These data indicate that the amount of PyMA released for the pAαCD/pAβCD/pAAzo ternary mixture is approximately three times as high as those for the pAαCD/pAAzo and pAβCD/pAAzo binary mixtures because of the light‐controlled rearrangement of inclusion complexes.

     

Stimuli‐Responsive Cucurbit[7]uril‐Mediated BSA Nanoassembly for Uptake and Release of Doxorubicin

We report the construction of a non‐toxic nanoassembly of bovine serum albumin (BSA) protein and the cucurbit[7]uril macrocycle as well as its stimuli‐responsive breakage with adamantylamine or pH, which restores the protein structure and recognition properties. The assembly showed efficient loading and controlled release of a standard drug, doxorubicin (DOX), and the same was validated in live cells. The cell viability studies documented that the DOX‐loaded assembly mask the cytotoxicity of DOX and the toxicity can be revived at the target on demand, triggering its therapeutic activation. This is found to be more effective in the cancer cells. In addition, such host‐assisted protein assemblies are also highly promising for stabilizing/protecting the native protein structure, a viable approach to prevent/inhibit protein misfolding and aggregation.     

Counterion‐Controlled Self‐Sorting in an Amphiphilic Calixarene Micellar System

Molecular recognition of small molecules and ions by artificial receptors in microheterogeneous media such as micelles and vesicles can, in principle, provide better models of biological systems in comparison with bulk solutions. In this work we have investigated the complexation of an organic fluorescent probe with amphiphilic calixarene receptor below and above the critical micelle concentration (CMC). For concentrations below the CMC, the probe forms a host–guest complex with the calixarene behaving like a traditional host–guest system operating in bulk solution. Above the CMC, multiple equilibrium processes are established and the probe can exchange between the recognition site of the calixarene in the monomeric state, micellized state and/or the micellar hydrophobic core. Careful analysis of the results obtained from NMR spectroscopy and fluorescence experiments allowed us to propose a quantitative model to describe the system. The increment of the local concentration of Na⁺ counterions at the Stern layer displace the dye to the micelle core through competitive binding of Na⁺ in the cavity of the receptor and is decisive for the observed self‐sorting behavior.