Diastereoselective Amplification of a Mechanically Chiral [2]Catenane.

Achiral [2]catenanes composed of rings with inequivalent sides may adopt chiral co-conformations. Their stereochemistry depends on the relative orientation of the interlocked rings and can be controlled by sterics or an external stimulus (e.g., a chemical stimulus). Herein, we have exploited this stereodynamic property to amplify a mechanically chiral (P)-catenane upon binding to (R)-1,1'-binaphthyl 2,2'-disulfonate, with a diastereomeric excess of 85%. The chirality of the [2]catenane was ascertained in the solid state by single crystal X-ray diffraction and in solution by NMR and CD spectroscopies. This study establishes a robust basis for the development of a new synthetic approach to access enantioenriched mechanically chiral [2]catenanes.

Untying the Photophysics of Quinolinium-Based Molecular Knots and Links.

Complex molecular knots and links are still difficult to synthesize and the properties arising from their topology are mostly unknown. Here, we report on a comparative photophysical study carried out on a family of closely related quinolinium-based knots and links to determine the impact exerted by topology on the molecular backbone. Our results indicate that topology has a negligible influence on the behavior of loosely braided molecules, which mostly behave like their unbraided equivalents. On the other hand, tightly braided molecules display distinct features. Their higher packing density results in a pronounced ability to resist deformation, a significant reduction in the solvent-accessible surface area and favors close-range π-π interactions between the quinolinium units and neighboring aromatics. Finally, the sharp alteration in behavior between loosely and tightly braided molecules sheds light on the factors contributing to braiding tightness.

Synchronized On/Off Switching of Four Binding Sites for Water in a Molecular Solomon Link.

A molecular Solomon link adopts different conformations in acetonitrile (1) and in water (2). Contrary to expectations, the main driving force of the transformation is not the change in medium polarity, but the cooperative binding of about four molecules of water, forming a tiny droplet in the central cavity of 2. Mechanistic studies reveal that the four binding sites can simultaneously switch between an inactive state (unable to bind water) and an active state (able to bind water) during the transformation. Spatial and temporal coordination of switching events is commonly observed in biological systems but has been rarely achieved in artificial systems. Here, the concerted activation of the four switchable sites is controlled by the topology of the whole molecule.

Ion-Mobility Mass Spectrometry for the Rapid Determination of the Topology of Interlocked and Knotted Molecules.

A rapid screening method based on traveling-wave ion-mobility spectrometry (TWIMS) combined with tandem mass spectrometry provides insight into the topology of interlocked and knotted molecules, even when they exist in complex mixtures, such as interconverting dynamic combinatorial libraries. A TWIMS characterization of structure-indicative fragments generated by collision-induced dissociation (CID) together with a floppiness parameter defined based on parent- and fragment-ion arrival times provide a straightforward topology identification. To demonstrate its broad applicability, this approach is applied here to six Hopf and two Solomon links, a trefoil knot, and a [3]catenate.

A Strategy to Synthesize Molecular Knots and Links Using the Hydrophobic Effect.

Conventional approaches to the synthesis of molecular knots and links mostly rely on metal templation. We present here an alternative strategy that uses the hydrophobic effect to drive the formation of complex interlocked structures in water. We designed an aqueous dynamic combinatorial system that can generate knots and links. In this system, the self-assembly of a topologically complex macrocycle is thermodynamically favored only if an optimum packing of all its components minimizes the hydrophobic surface area in contact with water. Therefore, the size, geometry, and rigidity of the initial building blocks can be exploited to control the formation of a specific topology. We illustrate the validity of this concept with the syntheses of a Hopf link, a Solomon link, and a trefoil knot. This latter molecule, whose self-assembly is templated by halides, binds iodide with high affinity in water. Overall, this work brings a fresh perspective on the synthesis of topologically complex molecules: Solvophobic effects can be intentionally harnessed to direct the efficient and selective self-assembly of knots and links.

Subtle Ligand Modification Inverts Guest Binding Hierarchy in M(II)8L6 Supramolecular Cubes.

Zinc(II), a dimolybdenum(II) paddlewheel tetramine A, and 2-formylpyridine self-assembled to generate a cubic Zn(II)8(L(A))6 assembly. The paddlewheel faces of this assembly exhibited two distinct conformations, whereas the analogous Fe(II)8(L(A))6 framework displayed no such perturbation to its structure. This variation in behavior is attributed to the subtle difference in ligand rotational freedom between the Zn(II)- and Fe(II)-cornered cubes. The incorporation of a fluorinated Mo(II)2 paddlewheel, B, into analogous Zn(II)8(L(B))6 and Fe(II)8(L(B))6 structures resulted in changes to the rotational dynamics of the ligands. These differing dynamics perturbed the energies of the frontier orbitals of these structures, as determined through spectroscopic and electrochemical methods. The result of these perturbations was an inversion of the halide binding preference of the Zn(II)8(L(B))6 host as compared to its Zn(II)8(L(A))6 congener, whereas the Fe(II)8(L(B))6 host maintained a similar binding hierarchy to Fe(II)8(L(A))6.

Imine-based [2]catenanes in water.

We report the efficient condensation of imine-based macrocycles from dialdehyde A and aliphatic diamines B n in pure water. Within the libraries, we identified a family of homologous amphiphilic [2]catenanes, whose self-assembly is primarily driven by the hydrophobic effect. The length and odd-even character of the diamine alkyl linker dictate both the yield and the conformation of the [2]catenanes, whose particular thermodynamic stability further shifts the overall equilibrium in favour of imine condensation. These findings highlight the role played by solvophobic effects in the self-assembly of complex architectures.