Simultaneous Self-Assembly of a [2]Catenane, a Trefoil Knot, and a Solomon Link from a Simple Pair of Ligands

Molecular Knots

The first synthetic molecular trefoil knot was prepared in the late 1980s. However, it is only in the last few years that more complex small-molecule knot topologies have been realized through chemical synthesis. The steric restrictions imposed on molecular strands by knotting can impart significant physical and chemical properties, including chirality, strong and selective ion binding, and catalytic activity. As the number and complexity of accessible molecular knot topologies increases, it will become increasingly useful for chemists to adopt the knot terminology employed by other disciplines. Here we give an overview of synthetic strategies towards molecular knots and outline the principles of knot, braid, and tangle theory appropriate to chemistry and molecular structure.

Pseudopeptidic compounds for the generation of dynamic combinatorial libraries of chemically diverse macrocycles in aqueous media

The combination of pseudopeptidic dithiol building blocks leads to the generation of highly diverse dynamic libraries of macrocycles in aqueous media.

Topological transformation of a trefoil knot into a [2]catenane

Kinetic and thermodynamic investigation of topological transformation of a trefoil knot into a [2]catenane in water.

A folding-directed catalytic microenvironment in helical dynamic covalent polymers formed by spontaneous configuration control

Helical dynamic covalent polymers with a folding-directed hollow tubular structure were investigated for catalysis.

Tying a Molecular Overhand Knot of Single Handedness and Asymmetric Catalysis with the Corresponding Pseudo-D3-Symmetric Trefoil Knot

We report the stereoselective synthesis of a left-handed trefoil knot from a tris(2,6-pyridinedicarboxamide) oligomer with six chiral centers using a lanthanide(III) ion template. The oligomer folds around the lanthanide ion to form an overhand knot complex of single handedness. Subsequent joining of the overhand knot end groups by ring-closing olefin metathesis affords a single enantiomer of the trefoil knot in 90% yield. The knot topology and handedness were confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. The pseudo-D3-symmetric knot was employed as an asymmetric catalyst in Mukaiyama aldol reactions, generating enantioselectivities of up to 83:17 er, which are significantly higher than those obtained with a comparable unknotted ligand complex.

Optimal Self-Assembly of Linked Constructs and Catenanes via Spatial Confinement

How to direct the self-assembly of simple templates toward constructs with complex shape and topology is still an open problem. Recent advancements have made it possible to self-assemble various types of knotted constructs, but targeting general multicomponent topologies, that is, links and catenanes, has proved much harder. Here, we study how the yield and complexity of self-assembled links depends on both intrinsic and extrinsic system properties and, particularly, template shape and spatial confinement. We show that slit confinement does not necessarily suppress linking but can rather enhance it significantly thanks to entropic effects. We also found that only a limited set of binary links are recurrent for different template shapes. These privileged topologies include all those experimentally realized so far plus a few additional ones, such as the 7₇² and 7₈² links that, hence, ought to be ideal candidates for broadening the current class of constructs with addressable topology.

[C-H···anion] interactions mediate the templation and anion binding properties of topologically non-trivial metal-organic structures in aqueous solutions

Two synthetic approaches-temperature variation and anion templation-allowed for the selective formation of a [2]catenane ([2]C4+ ) or a trefoil knot (TK6+ ), or for the enhanced formation of a Solomon link (SL8+ ), all from a simple set of starting materials (Zn(ii) acetate, diformylpyridine (DFP) and a diamino-2,2'-bipyridine (DAB)) in mixed aqueous solutions. The catenane formed exclusively at 90 °C in a 1 : 1 mixed solvent of D2O and MeOD. In the presence of bromide ion as template, TK6+ formed exclusively at 50 °C in the same solvent. In the solid state, TK6+ hosts two bromide ions in its central cavity by forming six Csp2 -H hydrogen bonds. In D2O, TK6+ , which was originally prepared as a trifluoroacetate (TFA) salt, was found to exchange two TFA counterions for two monovalent anions of different sizes and shapes, which lodged within the knot's central cavity. In contrast to bromide, the larger triflate anion (CF3SO3-) promoted the formation of SL8+ , which was characterized by 1H NMR spectroscopy and mass spectrometry. Two dimensional heteronuclear 19F-1H-HOSEY NMR experiments detected CH···F interactions inside the cavity of SL8+ . Thus, the product distribution of this dynamic link forming system is sensitive to temperature and the size and shape of the anion template, and one of the products, TK6+ , is capable of binding a variety of monovalent anions in D2O with high affinity (with log β2 values of 4 to 6 being typical).

Chloride-Anion-Templated Synthesis of a Strapped-Porphyrin-Containing Catenane Host System

The synthesis, structure and anion-recognition properties of a new strapped-porphyrin-containing [2]catenane anion host system are described. The assembly of the catenane is directed by discrete chloride anion templation acting in synergy with secondary aromatic donor-acceptor and coordinative pyridine-zinc interactions. The [2]catenane incorporates a three-dimensional, hydrogen-bond-donating anion-binding pocket; solid-state structural analysis of the catenane⋅chloride complex reveals that the chloride anion is encapsulated within the catenane's interlocked binding cavity through six convergent CH⋅⋅⋅⋅Cl and NH⋅⋅⋅Cl hydrogen-bonding interactions and solution-phase (1) H NMR titration experiments demonstrate that this complementary hydrogen-bonding arrangement facilitates the selective recognition of chloride over larger halide anions in DMSO solution.

Lanthanide Template Synthesis of Trefoil Knots of Single Handedness

We report on the assembly of 2,6-pyridinedicarboxamide ligands (1) with point chirality about lanthanide metal ion (Ln(3+)) templates, in which the helical chirality of the resulting entwined 3:1 ligand:metal complexes is covalently captured by ring-closing olefin metathesis to form topologically chiral molecular trefoil knots of single handedness. The ligands do not self-sort (racemic ligands form a near-statistical mixture of homoleptic and heteroleptic lanthanide complexes), but the use of only (R,R)-1 leads solely to a trefoil knot of Λ-handedness, whereas (S,S)-1 forms the Δ-trefoil knot with complete stereoselectivity. The knots and their isomeric unknot macrocycles were characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography and the expression of the chirality that results from the topology of the knots studied by circular dichroism.

A Solomon link through an interwoven molecular grid

A molecular Solomon link was synthesized through the assembly of an interwoven molecular grid consisting of four bis(benzimidazolepyridyl)benzthiazolo[5,4-d]thiazole ligands and four zinc(II), iron(II), or cobalt(II) cations, followed by ring-closing olefin metathesis. NMR spectroscopy, mass spectrometry, and X-ray crystallography confirmed the doubly interlocked topology, and subsequent demetalation afforded the wholly organic Solomon link. The synthesis, in which each metal ion defines the crossing point of two ligand strands, suggests that interwoven molecular grids should be useful scaffolds for the rational construction of other topologically complex structures.

Catenanes: fifty years of molecular links

Half a century after Schill and Lüttringhaus carried out the first directed synthesis of a [2]catenane, a plethora of strategies now exist for the construction of molecular Hopf links (singly interlocked rings), the simplest type of catenane. The precision and effectiveness with which suitable templates and/or noncovalent interactions can arrange building blocks has also enabled the synthesis of intricate and often beautiful higher order interlocked systems, including Solomon links, Borromean rings, and a Star of David catenane. This Review outlines the diverse strategies that exist for synthesizing catenanes in the 21st century and examines their emerging applications and the challenges that still exist for the synthesis of more complex topologies.

Dynamic stereoisomerization in inherently chiral bimetallic [2]catenanes

Stereoisomerization and the unprecedented phenomenon of metal translocation in the absence of redox processes were probed in two inherently chiral bimetallic [2]catenanes by using a combination of variable-temperature (1)H NMR and CD spectroscopies, X-ray crystallography, and DFT calculations.

A simple and highly effective ligand system for the copper(I)-mediated assembly of rotaxanes

A [2]rotaxane was produced through the assembly of a picolinaldehyde, an amine, and a bipyridine macrocycle around a Cu(I) template by imine bond formation in close-to-quantitative yield. An analogous [3]rotaxane is obtained in excellent yield by replacing the amine with a diamine, thus showing the suitability of the system for the construction of higher order interlocked structures. The rotaxanes are formed within a few minutes simply through mixing the components in solution at room temperature and they can be isolated through removal of the solvent or precipitation.

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.