Securing a Supramolecular Architecture by Tying a Stopper Knot

We report on a rotaxane‐like architecture secured by the in situ tying of an overhand knot in the tris(2,6‐pyridyldicarboxamide) region of the axle through complexation with a lanthanide ion (Lu³⁺). The increase in steric bulk caused by the knotting locks a crown ether onto the thread. Removal of the lutetium ion unties the knot, and when the axle binding site for the ring is deactivated, the macrocycle spontaneously dethreads. When the binding interaction is switched on again, the crown ether rethreads over the 10 nm length of the untangled strand. The overhand knot can be retied, relocking the threaded structure, by once again adding lutetium ions.     

Coordination‐Driven Self‐Assembly of a Molecular Knot Comprising Sixteen Crossings

Molecular knots have become highly attractive to chemists because of their prospective properties in mimicking biomolecules and machines. Only a few examples of molecular knots from the billions tabulated by mathematicians have been realized and molecular knots with more than eight crossings have not been reported to date. We report here the coordination‐driven [8+8] self‐assembly of a higher‐generation molecular knot comprising as many as sixteen crossings. Its solid‐state X‐ray crystal structure and multinuclear 2D NMR findings confirmed its architecture and topology. The formation of this molecular knot appears to depend on the functionalities and geometries of donor and acceptor in terms of generating appropriate angles and strong π‐π interactions supported by hydrophobic effects. This study shows coordination‐driven self‐assembly offers a powerful potential means of synthesizing more and more complicated molecular knots and of understanding differences between the properties of knotted and unknotted structures.     

The Self‐Sorting Behavior of Circular Helicates and Molecular Knots and Links

We report on multicomponent self‐sorting to form open circular helicates of different sizes from a primary monoamine, Fe^II ions, and dialdehyde ligand strands that differ in length and structure by only two oxygen atoms. The corresponding closed circular helicates that are formed from a diamine—a molecular Solomon link and a pentafoil knot—also self‐sort, but up to two of the Solomon‐link‐forming ligand strands can be accommodated within the pentafoil knot structure and are either incorporated or omitted depending on the stage that the components are mixed.     

A Novel Supramolecular System: Combination of Two Switchable Processes in a [2]Rotaxane

A novel supramolecular system, which is made up of a dibenzo[24]crown‐8 (DB24C8) ring component linked with a calix[4]arene derivative, a dumbbell component, containing a secondary ammonium center (‐NR₂H₂⁺‐) and a 4,4′‐bipyridinium (BIPY²⁺) unit, and stoppered with two 3,5‐di‐tert‐butylphenyl groups on the two termini of the dumbbell component, has been synthesized. The system displays a combination of two processes: the pH‐induced shuttling of a DB24C8 ring and the complexation/decomplexation of K⁺ ions. The switching process of this supramolecular system was investigated in detail by ¹H NMR spectroscopy. The results showed that the supramolecular system can only switch smoothly in CD₃CN. The two separated switchable processes can run together smoothly in this supramolecular system.     

Strategies and tactics for the metal-directed synthesis of rotaxanes, knots, catenanes, and higher order links

More than a quarter of a century after the first metal template synthesis of a [2]catenane in Strasbourg, there now exists a plethora of strategies available for the construction of mechanically bonded and entwined molecular level structures. Catenanes, rotaxanes, knots and Borromean rings have all been successfully accessed by methods in which metal ions play a pivotal role. Originally metal ions were used solely for their coordination chemistry; acting either to gather and position the building blocks such that subsequent reactions generated the interlocked products or by being an integral part of the rings or "stoppers" of the interlocked assembly. Recently the role of the metal has evolved to encompass catalysis: the metal ions not only organize the building blocks in an entwined or threaded arrangement but also actively promote the reaction that covalently captures the interlocked structure. This Review outlines the diverse strategies that currently exist for forming mechanically bonded molecular structures with metal ions and details the tactics that the chemist can utilize for creating cross-over points, maximizing the yield of interlocked over non-interlocked products, and the reactions-of-choice for the covalent capture of threaded and entwined intermediates.     

Diversity Oriented Preparation of Pillar[5]arene-Containing [2]Rotaxanes by a Stopper Exchange Strategy

Rotaxane building blocks bearing 3,5-bis(trifluoromethyl) benzenesulfonate (BTBS) stoppers have been efficiently prepared from a pillar[5]arene derivative, 3,5-bis(trifluoromethyl) benzenesulfonyl chloride (BTBSCl) and different diols, namely 1,10-decanediol and 1,12-dodecanediol. The BTBS moieties of these compounds are good leaving groups and stopper exchange reactions could be achieved by treatment with different nucleophiles thus affording rotaxanes with ester, thioether or ether stoppers.     

Bistable or Oscillating State Depending on Station and Temperature in Three‐Station Glycorotaxane Molecular Machines

High‐yield, straightforward synthesis of two‐ and three‐station [2]rotaxane molecular machines based on an anilinium, a triazolium, and a mono‐ or disubstituted pyridinium amide station is reported. In the case of the pH‐sensitive two‐station molecular machines, large‐amplitude movement of the macrocycle occurred. However, the presence of an intermediate third station led, after deprotonation of the anilinium station, and depending on the substitution of the pyridinium amide, either to exclusive localization of the macrocycle around the triazolium station or to oscillatory shuttling of the macrocycle between the triazolium and monosubstituted pyridinium amide station. Variable‐temperature ¹H NMR investigation of the oscillating system was performed in CD₂Cl₂. The exchange between the two stations proved to be fast on the NMR timescale for all considered temperatures (298–193 K). Interestingly, decreasing the temperature displaced the equilibrium between the two translational isomers until a unique location of the macrocycle around the monosubstituted pyridinium amide station was reached. Thermodynamic constants K were evaluated at each temperature: the thermodynamic parameters ΔH and ΔS were extracted from a Van′t Hoff plot, and provided the Gibbs energy ΔG. Arrhenius and Eyring plots afforded kinetic parameters, namely, energies of activation E_a, enthalpies of activation ΔH^≠, and entropies of activation ΔS^≠. The ΔG values deduced from kinetic parameters match very well with the ΔG values determined from thermodynamic parameters. In addition, whereas signal coalescence of pyridinium hydrogen atoms located next to the amide bond was observed at 205 K in the oscillating rotaxane and at 203 K in the two‐station rotaxane with a unique location of the macrocycle around the pyridinium amide, no separation of ¹H NMR signals of the considered hydrogen atoms was seen in the corresponding nonencapsulated thread. It is suggested that the macrocycle acts as a molecular brake for the rotation of the pyridinium–amide bond when it interacts by hydrogen bonding with both the amide NH and the pyridinium hydrogen atoms at the same time.     

pH‐Controllable Supramolecular Systems

Proton, all that matters! This Focus Review surveys representative examples of pH‐controllable supramolecular systems with interesting features and state‐of‐the‐art applications, which can lead to the construction of meaningful molecular machines for electronic and biological applications that can be controlled by simple perturbation with acid and base.

     

Modular Columnar Supramolecular Polymers as Scaffolds for Biomedical Applications

Self‐assembly of discotic molecules into supramolecular polymers offers a flexible approach for the generation of multicomponent one‐dimensional columnar architectures with tuneable biomedical properties. Decoration with ligands induces specific binding of the self‐assembled scaffold to biological targets. The modular design allows the easy co‐assembly of different discotics for the generation of probes for targeted imaging and cellular targeting with adjustable ligand density and composition.