Verknoten und Durchfädeln von Molekülen: Chemie und Chiralität molekularer Knoten und ihrer Ensembles

Self-Assembly of Molecular Trefoil Knots Featuring Pentadecanuclear Homoleptic AuI -, AuI /AgI -, or AuI /CuI -Alkynyl Coordination

Metal-free and metal-containing molecular trefoil knots are fascinating ensembles that are usually covalently assembled, the latter requiring the rational design of di- or multidentate/multipodal ligands as connectors. In this work, we describe the self-assembly of pentadecanuclear Au^I trefoil knots [Au₁₅ (C≡CR)₁₅ ] from monoalkynes HC≡CR (R=9,9-X₂ -fluorenyl with X=nBu, n-hexyl) and [Au^I (THT)Cl]. Hetero-bimetallic counterparts [Au₉ M₆ (C≡CR)₁₅ ] (M=Cu/Ag) were self-assembled by reactions of [Au₁₅ (C≡CR)₁₅ ] with [Cu(MeCN)₄ ]⁺ /AgNO₃ and HC≡CR. The type of pentadecanuclear trefoil knots described herein is characterized by X-ray crystallography, 2D NMR and HR-ESI-MS. [Au₉ Cu₆ (C≡CR)₁₅ ] is relatively stable in hexane; its excited state properties were investigated. DFT calculations revealed that non-covalent metal-metal and metal-ligand interactions, together with longer alkyl chain-strengthened inter-ligand dispersion interactions, govern the stability of the trefoil knot structures.

A Hydrogen-Bonded Ravel Assembled by Anion Coordination

Anion-coordination-driven assembly (ACDA) is showing increasing power in the construction of anionic supramolecular architectures. Herein, by expanding the anion centers from oxoanion (phosphate or sulfate) to organic tris-carboxylates, an Archimedean solid (truncated tetrahedron) and a highly entangled, double-walled tetrahedron featuring a ravel topology have been assembled with tris-bis(urea) ligands. The results demonstrate the promising ability of tris-carboxylates as new anion coordination centers in constructing novel topologies with increasing complexity and diversity compared to phosphate or sulfate ions on account of the modifiable size and easy functionalization character of these organic anions.

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.

Induction of Single-Handed Helicity of Polyacetylenes Using Mechanically Chiral Rotaxanes as Chiral Sources

Effective induction of preferred-handed helicity of polyacetylenes by pendant mechanically chiral rotaxanes is discussed. Polyacetylenes possessing optically active mechanically chiral rotaxanes in the side chains were synthesized by the polymerization of the corresponding enantiopure [2]rotaxane-type ethynyl monomers prepared by the chiral-phase HPLC separations. The CD Cotton effects revealed that the polyacetylenes took preferred-handed helical conformations depending on the rotaxane chirality. The preferred-handed helix was not disturbed by an additional chiral substituent on the rotaxane side chain. These results demonstrate the significance and utility of mechanically chiral rotaxanes for the effective construction of asymmetric fields.

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.

Daisy Chain Rotaxanes Made from Interlocked DNA Nanostructures

We report the stepwise assembly of supramolecular daisy chain rotaxanes (DCR) made of double-stranded DNA: Small dsDNA macrocycles bearing an axle assemble into a pseudo-DCR precursor that was connected to rigid DNA stoppers to form DCR with the macrocycles hybridized to the axles. In presence of release oligodeoxynucleotides (rODNs), the macrocycles are released from their respective hybridization sites on the axles, leading to stable mechanically interlocked DCRs. Besides the expected threaded DCRs, certain amounts of externally hybridized structures were observed, which dissociate into dumbbell structures in presence of rODNs. We show that the genuine DCRs have significantly higher degrees of freedom in their movement along the thread axle than the hybridized DCR precursors. Interlocking of DNA in DCRs might serve as a versatile principle for constructing functional DNA nanostructures where the movement of the subunits is restricted within precisely confined tolerance ranges.

A trefoil knotted polymer produced through ring expansion

A synthetic strategy is reported for the production of a trefoil knotted polymer from a copper(I)-templated helical knot precursor through ring expansion. The expected changes in the properties of the knotted polymer compared to a linear analogue, for example, reduced hydrodynamic radius and lower intrinsic viscosity, together with an atomic force microscopy (AFM) image of individual molecular knots, confirmed the formation of the resulting trefoil knotted polymer. The strategies employed here could be utilized to enrich the variety of available polymers with new architectures.

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.

Phenyl-(2-pyridyl)-(3-pyridyl)-(4-pyridyl)methane: synthesis, chiroptical properties, and theoretical calculation of its absolute configuration

The title compound, a prototypical chiral molecule based on a tetraarylmethane framework, has been synthesized in five steps from (2-pyridyl)-(3-pyridyl)ketone. X-ray crystallographic analysis revealed the tetraarylmethane framework of the molecule but did not determine the positions of the nitrogen atoms because the crystal is a racemic compound and the aryl groups are disordered in the crystal. The optical resolution of the title compound was achieved by chiral HPLC with a Chiralcel OD column. The CD spectra of the two fractions in acetonitrile exhibited opposite signs as expected for a pair of enantiomers. Their CD spectra are changed in 2 M HCl due to protonation. The calculated CD curve for the target molecule based on time-dependent density functional theory (TDDFT) reproduces the experimental result very well, thus suggesting that the first eluted fraction is the R isomer in terms of absolute configuration.

Topological study of pseudo-cubic hydrogen-bond networks in a binary system composed of primary ammonium carboxylates: an analogue of an ice cube

Hierarchical classification and single-crystal X-ray analysis of unique pseudo-cubic hydrogen-bond networks composed of primary ammonium carboxylates were carried out. The networks consist of four carboxylate anions and four primary ammonium cations at the corners of the cube, and twelve charge-assisted N--H...O hydrogen bonds on the sides of the cube. The configuration of the carboxylate anions generates topological diversity in the network. The results of this hierarchical classification showed that pseudo-cubic hydrogen-bond networks composed of primary ammonium carboxylates can form nine topologically different networks. These pseudo-cubic networks are a subset of the networks formed by octameric water in the form of an "ice cube". The classification system can be applied to other pseudo-cubic networks in a similar way. A survey of crystal structures based on combinations of triphenylacetic acid with various alkylamines (carbon numbers up to eight) and examination of the CSD (Cambridge Structural Database) showed eight salts that form such networks in their crystal structures. These structures are classified into six topologically different networks. Similar networks composed of other salts are also discussed from a topological point of view.

Nondegenerate π-Donor/π-Acceptor [2]Catenanes Containing Proton-Ionizable 1H-1,2,4-Triazole Subunits: Synthesis and Spontaneous Resolution

Chirality can hold the key to inducing directionality of motion in components of molecular devices. With this idea in mind, we describe here 1) the template-directed synthesis of two [2]catenanes wherein cyclobis(paraquat-p-phenylene) is interlocked with polyether macrocycles containing, in addition to one 3,5-bis(oxymethylene)-1H-1,2,4-triazole unit, either one 1,4-dioxybenzene or one 1,5-dioxynaphthalene ring system. We also report 2) the full characterization of both [2]catenanes by fast atom bombardment mass spectrometry (FABMS), X-ray crystallography, and dynamic (1)H NMR spectroscopy. We reveal 3) the fact that the [2]catenanes not only exist, both in the solution-state and in the solid-state, as strictly one of the two possible translational isomers, but that they also exhibit spontaneous resolution on crystallization leading to formation of homochiral crystals, as indicated by X-ray crystallography and circular dichroism (CD) experiments. Finally, we comment 4) on the chances of switching these catenanes chemically.

Anion-Templated Calix[4]arene-Based Pseudorotaxanes and Catenanes

We present the rational design and anion-binding properties of the first anion-templated pseudorotaxanes and catenanes in which the "wheel" component is provided by a calix[4]arene macrobicyclic unit. The designs and syntheses of two new calix[4]arene macrobicycles, 2 and 3, are presented, and the abilities of these new species both to bind anions and to undergo anion-dependent pseudorotaxane formation are demonstrated. Furthermore, it is shown that performing ring-closing metathesis reactions on some of these pseudorotaxane assemblies gives novel catenane species 14 and 15, in which the yield of interlocked molecule obtained is critically dependent on the presence of a suitable anion template, namely, chloride. Exchange of the chloride anion in catenane 14 a for hexafluorophosphate gives catenane 14 d, which contains a unique anion-binding domain defined by the permanently interlocked hydrogen-bond-donating calix[4]arene macrobicycle and pyridinium macrocycle fragments. The anion-binding properties of this domain are presented, and shown to differ from non-interlocked components.

Modular Synthesis and Dynamics of a Variety of Donor–Acceptor Interlocked Compounds Prepared by Click Chemistry

A series of donor-acceptor [2]-, [3]-, and [4]rotaxanes and self-complexes ([1]rotaxanes) have been synthesized by a threading-followed-by-stoppering approach, in which the precursor pseudorotaxanes are fixed by using Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition to attach the required stoppers. This alternative approach to forming rotaxanes of the donor-acceptor type, in which the donor is a 1,5-dioxynaphthalene unit and the acceptor is the tetracationic cyclophane cyclobis(paraquat-p-phenylene), proceeds with enhanced yields relative to the tried and tested synthetic strategies, which involve the clipping of the cyclophane around a preformed dumbbell containing pi-electron-donating recognition sites. The new synthetic approach is amenable to application to highly convergent sequences. To extend the scope of this reaction, we constructed [2]rotaxanes in which one of the phenylene rings of the tetracationic cyclophane is perfluorinated, a feature which significantly weakens its association with pi-electron-rich guests. The activation barrier for the shuttling of the cyclophane over a spacer containing two triazole rings was determined to be (15.5+/-0.1) kcal mol(-1) for a degenerate two-station [2]rotaxane, a value similar to that previously measured for analogous degenerate compounds containing aromatic or ethylene glycol spacers. The triazole rings do not seem to perturb the shuttling process significantly; this property bodes well for their future incorporation into bistable molecular switches.

Probing Structural Effects on Replication Efficiency through Comparative Analyses of Families of Potential Self-Replicators

A formidable synthetic apparatus for the creation of nanoscale molecular structures and supramolecular assemblies through molecular structures can potentially be created from systems that are capable of parallel automultiplication (self-replication). In order to achieve this goal, a detailed understanding of the relationship between molecular structure and replication efficiency is necessary. Diastereoisomeric templates that are capable of specific and simultaneous autocatalysis have been synthesised. A systematic experimental and theoretical evaluation of their behaviour and that of structurally-related systems reveals the key determinants that dictate the emergence of self-replicative function and defines the structural space within which this behaviour is observed.

Amide-Based Molecular Knots as Platforms for Fluorescent Switches

A series of amide-based molecular knots equipped selectively with fluorescent dansyl and/or pyrenesulfonyl moieties were synthesized from the readily available tris(allyloxy)knotane. UV/Vis absorption spectra, emission spectra, and the emission lifetimes of the fluorescent knotanes were investigated in chloroform at 298 K. The absorption spectra of the knotanes correspond to those of mixtures of their UV-active constituents. The fluorescence quantum yields and lifetimes of the dansyl and pyrenesulfonyl moieties are partly quenched by the knotane platform. In the KN(Da)(2)(Py) species, the fluorescent excited state of the dansyl units (lambda(max)=510 nm) lies at lower energy than the fluorescent excited state of the pyrenesulfonyl unit (lambda(max)=385 nm), the emission of which is accordingly quenched with sensitization of the dansyl fluorescence. In the KN(Ao)(2)(Da), KN(Ao)(Da)(2), and KN(Da)(3) species, the addition of acids causes the protonation of their dansyl units with a consequent decrease in the intensity of the dansyl band at 510 nm and appearance of the emission band of the protonated dansyl unit (lambda(max)=340 nm). Each dansyl unit of KN(Ao)(Da)(2) and KN(Da)(3) undergoes the independent protonation. In these incompletely protonated knotanes the fluorescence of the protonated dansyl units is partly quenched by nonprotonated ones. These processes can be quantitatively reversed upon addition of a base. In KN(Da)(2)(Py), an increase of the fluorescence of its pyrenesulfonyl group is observed when the dansyl groups are protonated. The results obtained show that the readily available and easily functionalizable amide-knotanes can be used as an interesting scaffold to obtain fluorescent switches.