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Dynamics of mechanically bonded macrocycles in radial hetero[4]catenane isomers
Abstract
A pair of radial [4]catenane isomers interlocked with two CB[6]s and one β-CD is reported. Due to the different positions of the tightly bound CB[6]s, shuttling dynamics of the β-CD between the two biphenyl stations are different in the isomers.
... Thereafter, some CD-based catenanes were fabricated. consisting of two cucurbit [6]urils and one β-CD (Fig. 4f) [34]. This catenane is expected to be an advanced material in molecular machines. ...
... Wu et al. prepared a β-CD-based [2]catenane with a macrocyclic axile compound consisting of 4,4'-bipyridine and 1,6-dibromohexane ( Fig. 5b) [36]. Interestingly, this catenane worked as a chiral sensor by treatment with Cu(II) ions because the Fig. 4 Examples of CD-based catenanes [26,[29][30][31][32][33][34] Content courtesy of Springer Nature, terms of use apply. Rights reserved. ...
In recent years, a number of interlocked molecules such as rotaxanes, catenanes, polyrotaxanes, and polycatenanes have been actively synthesized. Cyclodextrins (CDs) are representative building blocks of these interlocked molecules; however, very few reports are available on CD-based catenanes and polycatenanes. In this review article, we provide an overview of CD-based interlocked molecules and introduce examples of CD-based catenanes and polycatenanes. Finally, the perspectives of research on CD-based catenanes and polycatenanes are discussed.
... More recently, similar [4]catenane isomers with one interlocked β-CD and central macrocycles with or without a CB[6] "speed bump" between two degenerated biphenyl stations have also been prepared and their dynamics were studied by detail NMR analysis. [32] Apart from the radial [n]catenanes described above, CBAAC has also been used in the synthesis of branched [n]catenanes, in which a branched arrangement of the interlocked macrocycles is much less common than other radial or linear [n]catenanes. In 2016, our group has reported the one-pot synthesis of the branched [6]catenane Cat-4 from building blocks 25 f and 26 a in the presence of a Cu + template in aqueous DMF. ...
The chemistry of mechanically interlocked molecules (MIMs) such as catenane and rotaxane is full of new opportunities for the presence of a mechanical bond, and the efficient synthesis of these molecules is therefore of fundamental importance in realizing their unique properties and functions. While many different types of preorganizing interactions and covalent bond formation strategies have been exploited in MIMs synthesis, the use of cucurbit[6]uril (CB[6]) in simultaneously templating macrocycle interlocking and catalyzing the covalent formation of the interlocked components is particularly advantageous in accessing high‐order catenanes and rotaxanes. In this review, catenane and rotaxane obtained from CB[6]‐catalyzed azide‐alkyne cycloaddition will be discussed, with special emphasis on the synthetic strategies employed for obtaining complex [n]rotaxanes and [n]catenanes, as well as their properties and functions.
Design and synthesis of higher order catenane are unexpectedly complex and involve precise cooperation among the precursors overcoming competing and opposing interactions. We achieved synthesis of [2], linear [3], radial [4] in a one‐pot reaction by consecutive ring closing through click reactions. This synthesis gave three isolable products due to two, four, and six‐click reactions between suitable coupling partners. Yields of the isolate templated‐catenane decrease from lower to higher‐ordered catenane (40 %, 12 %, and 4 %), probably due to the bite angle as well as the flexibility of the reacting partners. Removal of templating cobalt(III) ion leads to the formation of fully organic [2], linear [3], and radial [4]catenane. These synthesized catenanes were purified by column chromatography and characterized by ¹H‐NMR, ¹³C‐NMR, and ESI‐MS spectroscopy. The synthesized catenanes have free binding sites suitable for post‐functionalization and may be used for the synthesis of higher‐ordered catenane.
Dynamics and stimuli-responsiveness of two hetero[4]rotaxanes containing interlocked cucurbit[6]uril and cyclodextrin (β-CD or γ-CD) were studied and reported. Due to the different relative strength of the various inter-component interactions involving the interlocked β-CD and γ-CD, different co-conformations, dynamic features and response to temperature and solvent polarity change, and the presence of base and external guest were revealed by NMR for the two homologous [4]rotaxanes.
From being an aesthetic molecular object to a building block for the construction of molecular machines, catenanes and related mechanically interlocked molecules (MIMs) continue to attract immense interest in many research areas. Catenane chemistry is closely tied to that of rotaxanes and knots, and involves concepts like mechanical bonds, chemical topology and co-conformation that are unique to these molecules. Yet, because of their different topological structures and mechanical bond properties, there are some fundamental differences between the chemistry of catenanes and that of rotaxanes and knots although the boundary is sometimes blurred. Clearly distinguishing these differences, in aspects of bonding, structure, synthesis and properties, between catenanes and other MIMs is therefore of fundamental importance to understand their chemistry and explore the new opportunities from mechanical bonds.
A branched [8]catenane from an efficient one‐pot synthesis (72% HPLC yield, 59% isolated yield) featuring the simultaneous use of three kinds of templates and cucurbit[6]uril‐mediated azide‐alkyne cycloaddition (CBAAC) for ring‐closing is reported. Design and assembly of the [8]catenane precursors are unexpectedly complex that can involve cooperating, competing and non‐influencing interactions. Due to the branched structure, dynamics of the [8]catenane can be modulated in different extent by rigidifying/loosening the mechanical bonds at different regions by using solvent polarity, acid‐base and metal ions as the stimuli. This work not only highlights the importance of understanding the delicate interplay of the weak and non‐obvious supramolecular interactions in the synthesis of high‐order [n]catenane, but also demonstrates a complex control of dynamics and flexibility for exploiting [n]catenanes applications.
A branched [8]catenane from an efficient one‐pot synthesis (72 % HPLC yield, 59 % isolated yield) featuring the simultaneous use of three kinds of templates and cucurbit[6]uril‐mediated azide–alkyne cycloaddition (CBAAC) for ring‐closing is reported. Design and assembly of the [8]catenane precursors are unexpectedly complex that can involve cooperating, competing and non‐influencing interactions. Due to the branched structure, dynamics of the [8]catenane can be modulated in different extent by rigidifying/loosening the mechanical bonds at different regions by using solvent polarity, acid‐base and metal ions as the stimuli. This work not only highlights the importance of understanding the delicate interplay of the weak and non‐obvious supramolecular interactions in the synthesis of high‐order [n]catenane, but also demonstrates a complex control of dynamics and flexibility for exploiting [n]catenanes applications.
Novel sulfonamide [2]catenanes were successfully prepared through a self-templation approach. The catenated skeletons were confirmed by traveling-wave ion-mobility spectrometry (TWIMS) combined with gradient tandem mass spectroscopy (gMS2). Moreover, two pyrene units were further introduced into the [2]catenane through the alkylation reaction of the sulfonamide moieties, resulting in the successful synthesis of a novel pyrene-functionalized [2]catenane which displayed a switchable optical output controlled by cation-induced conformation transformation.
Mechanically interlocked molecules have fascinated chemists for decades. Initially a tantalising synthetic challenge, interlocked molecules have continued to capture the imagination for their aesthetics and, increasingly, for their potential as molecular machines and use in materials applications. Whilst preliminary statistical attempts to prepare these molecules were exceedingly inefficient, a raft of template-directed strategies have now been realised, providing a vast toolbox from which chemists can access interlocked structures in excellent yields. For many envisaged applications it is desirable to move away from small, discrete interlocked molecules and turn to oligomers and polymers instead, either due to the need for multiple mechanical bonds within the desired material, or to exploit an extended scaffold for the organisation and arrangement of individual mechanically interlocked units. In this tutorial-style review we outline the synthetic strategies that have been employed for the synthesis of mechanically interlocked oligomers and polymers, including oligo-/polymerisation of (pseudo)interlocked precursors, metal–organic self-assembly, the use of orthogonal template motifs, iterative approaches and grafting onto polymer backbones.
A pair of radial [5]catenanes, with either an isomeric cyclic ‐AABB‐ or ‐ABAB‐ type sequence of the interlocked β‐cyclodextrin (β‐CD) and cucurbit[6]uril (CB[6]) units, has been efficiently synthesized. Because of a marked difference in the binding strength and interlocking sequence of the peripheral macrocycles, interesting sequence‐dependent properties, characteristic of mechanically bonded macrocycles, were realized. Variable‐temperature ¹H NMR studies showed that the ‐ABAB‐ isomer has a more independent β‐CD dynamic, whereas the β‐CD motions in the ‐AABB‐ isomer are coupled. Dynamics of the pH‐insensitive β‐CD can also be further modulated upon base‐triggered mobilization of the CB[6]. These unique properties of the mechanical bond expressed in a sequence‐specific fashion and the transmission of the control on the macrocycle dynamics from one interlocked component to another, highlight the potential of similar complex hetero[n]catenanes in the design of advanced, multicomponent molecular machines.
A series of hetero [4]-, [5]- and [6]rotaxanes containing both cucurbit[6]uril (CB[6]) and γ-cyclodextrin (γ-CD) as the macrocyclic components have been synthesized via a threading-followed-by-stoppering approach. Due to the orthogonal binding of CB[6] to ammonium and γ-CD to biphenylene/tetra(ethylene glycol), the [n]rotaxanes display a specific sequence of the interlocked macrocycles. In addition, despite of the asymmetry of γ-CD with respect to the orthogonal plane of the axle, only one stereoisomer of the [6]rotaxane was obtained.
We report here the efficient synthesis of a series of [3]catenanes featuring the use of cucurbit[6]uril to simultaneously mediate the mechanical and covalent bond formations. By coupling the mechanical interlocking with covalent macrocyclization, formation of topological isomers is eliminated and the [3]catenanes are formed exclusively in good yields. The efficient access to these [3]catenanes and the presence of other recognition units render them promising building blocks for the construction of other high-order interlocked structures.
As a major class of mechanically interlocked molecules, not only are catenanes topologically intriguing targets that challenge the chemical synthesis to the efficient formation of mechanical bonds, but also the mechanical properties arising from the topology offer unique and attractive features for the development of novel functional molecular materials. Despite advancements in templated methods for different types of interlocked architectures, [n]catenane possessing multiple numbers of interlocked macrocycles still remains a difficult synthetic target with very few reported examples. If the unique mechanical properties of catenanes are to be fully exploited, reliable, controllable, and efficient strategies for accessing [n]catenanes will be necessary. In this Viewpoint, challenges, considerations, and strategies to [n]catenanes are discussed.
In this article, a six-component self-sorting process that involves three types of crown ether macrocycles and three sorts of cation guest molecules, was carefully and thoroughly investigated. The six components include three kinds of crown ethers, namely bis(p-phenylene-34-crown-10) (BPP34C10), dibenzo-24-crown-8 (DB24C8), benzo-21-crown-7 (B21C7), and their corresponding cation guest molecules, namely, 4,4’-bipyridine dications (BPY2+), dibenzylammonium (DBA) and benzylalkylammonium (BAA) ions, respectively. Based on this well-established highly selective six-component self-sorting process, a hetero[6]rotaxane bearing three different kinds of crown ether macrocycles was designed and successfully synthesized through a facile and efficient one-pot “click” stoppering strategy. Such a work is supposed to be a significant advancement in the construction of mechanically interlocked molecules with high structural complexity as well as a good supplement in the areas of multi-component self-sorting and noncovalent self-assembly.
A water soluble [6]catenane consists of two interlocking [3]catenane was synthesised in 91% yield using readily accessible precursors. The new strategy features the simultaneous use of orthogonal Cu+-phenanhroline and CB[6]-ammonium interactions for preorganising the precursors and the efficient CB[6]-catalysed azide-alkyne cycloaddition as the bond forming reactions for ring closing, such that high structural complexity and fidelity of the products is resulted without compromising interlocking efficiency. A related [4]catenane with three different types of macrocycles was also obtained in good yield.
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.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
The supramolecular chemistry of cucurbituril, a synthetic receptor, is fascinating because of the remarkable guest binding behavior of the host. Although cucurbituril is potentially as useful as crown ethers, cyclodextrins, and calixarenes in many applications, its chemistry has not been developed much until recently because of several shortcomings. Recently we synthesized cucurbituril homologues and derivatives. These new members of the cucurbituril family have expanded the scope further, and interest in them has grown enormously. The diversity in guest binding behavior has led to many interesting studies such as redox control of guest binding, stabilization of charge-transfer complexes inside the host cavity, encapsulation of drug molecules, formation of redox-controllable vesicles, and so on. The cucurbituril homologues and derivatives thus provide new opportunities in many areas of supramolecular chemistry including recognition, catalysis, separation, transport, and many others.
The efficient synthesis of well-defined, linear oligocatenanes possessing multiple mechanical bonds remains a formidable challenge in the field of mechanically interlocked molecules. Here, a one-pot synthetic strategy is described to prepare a linear [4]catenate using orthogonal metal templation between a macrocycle precursor, composed of terpyridine and phenanthroline ligands spaced by flexible glycol linkers, and a closed phenanthroline-based molecular ring. Implementation of two simultaneous ring-closing metathesis reactions after metal complexation resulted in the formation of three mechanical bonds. The linear [4]catenate product was isolated in 55% yield as a mixture of topological diastereomers. The intermediate metal complexes and corresponding interlocked products (with and without metals) were characterized by nuclear magnetic resonance, mass spectrometry, gel permeation chromatography, and UV-vis absorption spectroscopy. We envision that this general synthetic strategy may pave the way for the synthesis of higher order linear oligocatenates/catenanes with precise molecular weights and four or more interlocking molecular rings.
Sequence isomerism is fundamental to the storage, transfer and expression of information in (bio)macromolecules and artificial polymers. Realization of sequence-specific properties in mechanically bonded molecules is extremely challenging due to the huge synthetic difficulty in the precise and controllable non-covalent interlocking of constitutionally different macrocycles in specific orders within a rotaxane or catenane. This Synpacts article highlights how sequence isomers of multicomponent rotaxanes and catenanes can be obtained by novel synthetic strategies and careful building block design.
We report here that the product topology in a copper-templated catenane synthesis can be controlled by favouring a particular macrocyclisation pathway, offering an additional strategy for improving the efficiency of...
Room temperature phosphorescent (RTP) γ-CD-CB[6]-cowheeled [4]rotaxanes were synthesized by implanting a naphthalene axle into the cavity of iodine-substituted γ-CDs. The strong green RTP was quenched exclusively by Trp while no...
Heterorotaxanes, in which at least two types of macrocycles were introduced as the wheel components in rotaxanes, have attracted more and more attentions during the past decades owing to their unique structural features and intriguing properties. The coexistence of varied macrocycles endows the resultant heterorotaxanes not only versatile shuttling and switching behaviors but also great potential for the construction of functional rotaxane systems for applications. In this feature article, classified by the various strategies towards the synthesis of heterorotaxanes, i.e. orthogonal binding approach, self-sorting approach, cooperative capture approach, and active metal template approach etc, a survey of the successful synthesis of heterorotaxanes will be provided.
A little zinc makes the rings all link
Although polymer strands are often informally called chains, molecular topologies that actually resemble extended macroscopic chain links have proven surprisingly challenging to make. Most approaches have settled for tethering pairs of interlocked rings amid spacer segments. Wu et al. now report successful synthesis of polycatenanes in which tens of rings are consecutively interlinked. The key was using zinc ions to template the threading of one macrocycle precursor through flanking preformed macrocycles, after which metathesis catalysis closed up the first ring, and the metal could be flushed out.
Science , this issue p. 1434
Motor proteins are nature's solution for directing movement at the molecular level. The field of artificial molecular motors takes inspiration from these tiny but powerful machines. Although directional motion on the nanoscale performed by synthetic molecular machines is a relatively new development, significant advances have been made. In this review an overview is given of the principal designs of artificial molecular motors and their modes of operation. Although synthetic molecular motors have also found widespread application as (multistate) switches, we focus on the control of directional movement, both at the molecular scale and at larger magnitudes. We identify some key challenges remaining in the field.
Mechanically interlocked molecules (MIMs) which contain two or more different recognition sites in the shape of noncovalent bonds between their component parts provide the perfect prototypes for the design and synthesis of molecular switches and machines. In example, namely bistable catenanes and rotaxanes, it is convenient to identify a ground state co-conformation (GSCC) and a higher energy metastable state co-conformation (MSCC) where the GSCC:MSCC ratio at room temperature in solution is ideally for most purposes in the range of at least 10:1. The chapter discusses the most of the parameters that influence ring translation in n-donor/n-acceptor mechanomolecules and discusses competitive binding interactions that lead to mechanical switching motions as a result of molecular recognition. It also describes the single-molecule experiments and many of the molecular machines that involve molecular motion in condensed phases and at interfaces, motivating the pursuit of a more fundamental understanding of how these phases influence the thermodynamics and kinetics.
The conceptual basis for mechanostereochemistry, both static and dynamic, can be appreciated most readily by drawing analogies with the stereochemistry of compounds whose molecules possess only covalent bonds, for example, cyclohexane and its derivatives. Since compounds such as catenanes and rotaxanes contain mechanically interlocked entities, it is convenient to call these entities component parts. This chapter reviews many dynamic mechanostereoisomers that interconvert between different co-conformations by intramolecular translation. The static mechanostereoisomers arising from differences in orientation, ring sequence, mechanical chirality or topology can sometimes be interconverted by dynamic processes, e.g., the inversion of mechanical helicity by dynamic rocking processes, the inversion of mechanically planar chirality by bowl-to-bowl inversion of cyanostar macrocycles and calixarenes, and the inversion of conditional topological chirality as a result of the dynamic rearrangement of coordinative bonds in Zn2C5524+. The promise of robust dynamics in solid-state materials and at interfaces may support the development of novel functions for these mechanically bonded systems.
Click chemistry describes a family of modular, efficient, versatile and reliable reactions which have acquired a pivotal role as one of the most useful synthetic tools with a potentially broad range of applications. While copper(i)-catalysed alkyne-azide cycloaddition is the most widely adopted click reaction in the family, the fact that it is cytotoxic restricts its practice in certain situations, e.g., bioconjugation. Consequently, researchers have been exploring the development of copper-free click reactions, the most popular example so far being strain-promoted alkyne-azide cycloadditions. An early example of copper-free click reactions that is rarely mentioned in the literature is the cucurbit[6]uril (CB6) catalysed alkyne-azide cycloaddition (CB-AAC). Despite the unique ability of CB-AAC to generate mechanically interlocked molecules (MIMs) - in particular, rotaxanes - its slow reaction rate and narrow substrate acceptance limit its scope. In this Tutorial Review, we describe our efforts of late in developing the fundamental principles and practical applications of a new copper-free click reaction - namely, cooperative capture synthesis, whereby introducing a cyclodextrin (CD) as an accelerator in CB-AAC, hydrogen bonding networks are formed between the rims of CD and CB6 in a manner that is positively cooperative, giving rise to a high level of pre-organisation during efficient and quick rotaxane formation. For example, [4]rotaxanes can be prepared nearly quantitatively within a minute in water. Furthermore, we have demonstrated that CB-AAC can accommodate a wider substrate tolerance by introducing pillararenes as promoters. To date, we have put cooperative capture synthesis into practice by (i) preparing polyrotaxanes containing up to 200 rings in nearly quantitative yields, (ii) trapping conformational isomers of polymacrocycles as rings in rotaxanes, (iii) demonstrating solid-state fluorescence and Förster resonance energy transfer (FRET) processes by fixing the fluorophores in a rotaxane and (iv) establishing the principle of supramolecular encryption in the preparation of dynamically and reversibly tunable fluorescent security inks.
A topologically complex peptide [4]catenane with the crossing number of 12 was synthesized by a folding and assembly strategy wherein the folding and metal-directed self-assembly of a short peptide fragment occur simultaneously. The latent Ω-looped conformation of the Pro-Gly-Pro sequence was found only when pyridines at the C- and N-termini coordinatively bind metal ions (Ag(I) or Au(I) ). Crystallographic studies revealed that the Ω-looped motifs formed four M3 L3 macrocycles that were intermolecularly entwined to generate an unprecedented peptide [4]catenane topology.
[5]Catenanes were synthesized by olefin metathesis dimerization. The reaction of pseudorotaxanes, which were derived from a [2]catenane and one equivalent of an ammonium salt bearing two terminal olefins in dichloromethane, with a catalytic amount of Grubbs catalyst afforded linear [5]catenanes in 12% yield. Intermolecular and intramolecular olefin metathesis reactions were controlled by the length of the alkyl chain of the ammonium salts.
The first example of utilizing halogen-bonding anion recognition to facilitate molecular motion in an interlocked structure is described. A halogen-bonding and hydrogen-bonding bistable rotaxane is prepared and demonstrated to undergo shuttling of the macrocycle component from the hydrogen-bonding station to the halogen-bonding station upon iodide recognition. In contrast, chloride-anion binding reinforces the macrocycle to reside at the hydrogen-bonding station.
A new azobenzene-based dithiol building block was developed which, upon oxidation, forms predominantly a [2]catenane consisting of two interlocked trimers. In the presence of cyclodextrin templates a series of [2] and [3]catenanes was formed instead. We developed a method that enabled estimating the equilibrium constants for catenation in all of these systems. The formation of the [3]catenanes is either cooperative or anticooperative, depending on the nature of the cyclodextrin. Using molecular dynamics simulations, we linked positive and negative cooperativity to, respectively, burying and exposure of hydrophobic surfaces upon catenation. Our results underline the importance of directly quantifying noncovalent interactions within catenanes, as the corresponding pseudo-rotaxane model systems were found to be poor predictors of binding interactions in the actual catenane.
Mechanistic understanding of the translational movements in molecular switches is essential for designing machine-like prototypes capable of following set pathways of motion. To this end, we demonstrated that increasing the station-to-station distance will speed up the linear movements forward and slow down the movements backward in a homologous series of bistable rotaxanes. Four redox-active rotaxanes, which drove a cyclobis(paraquat-p-phenylene) (CBPQT(4+)) mobile ring between a tetrathiafulvalene (TTF) station and an oxyphenylene station, were synthesized with only variations to the lengths of the glycol linker connecting the two stations (n = 5, 8, 11, and 23 atoms). We undertook the first mechanistic study of the full cycle of motion in this class of molecular switch using cyclic voltammetry. The kinetics parameters (k, ΔG(⧧)) of switching were determined at different temperatures to provide activation enthalpies (ΔH(⧧)) and entropies (ΔS(⧧)). Longer glycol linkers led to modest increases in the forward escape (t1/2 = 60 to <7 ms). The rate-limiting step involves movement of the tetracationic CBPQT(4+) ring away from the singly oxidized TTF(+) unit by overcoming one of the thiomethyl (SMe) speed bumps before proceeding on to the secondary oxyphenylene station. Upon reduction, however, the return translational movement of the CBPQT(4+) ring from the oxyphenylene station back to the neutral TTF station was slowed considerably by the longer linkers (t1/2 = 1.4 to >69 s); though not because of a diffusive walk. The reduced rate of motion backward depended on folded structures that were only present with longer linkers.
The formation of catenated systems can be simplified greatly if one or more rings are generated via self-assembly. Herein we exploit the orthogonality of coordination-driven self-assembly and crown-ether host/guest complexation to obtain a [4]molecular necklace and a [7]molecular necklace based on a well-developed recognition motif of 1,2-bis(pyridinium)ethane/dibenzo[24]crown-8. By adapting the bis(pyridinium) motif into the backbone of a donor building block, the resulting semi-rigid dipyridyl species can serve both as a structural element in the formation of metallacycles and as a site for subsequent host/guest chemistry. The pseudo-linear nature of the donor precursor lends itself to the formation of triangular and hexagonal central metallacycles based on the complementary acceptor unit used. This exemplary system organizes up to eighteen molecules from three unique species in solution to afford a single supramolecular ensemble.
[2]Rotaxanes consisting of butadiyne-linked porphyrin dimers threaded through a phenanthroline-containing macrocycle, can be synthesised by an active-metal template directed copper-mediated Glaser coupling, in yields of up to 61%, without requiring a large excess of the macrocycle. The crystal structure of one of these rotaxanes confirms that the macrocycle is clasped around the centre of the porphyrin dimer. A radial hexa-pyridyl template was used to convert the alkyne-terminated [2]rotaxane into a [4]catenane cyclic porphyrin hexamer in 62% yield, via palladium-catalysed Glaser coupling. The related [7]catenane porphyrin dodecamer complex was also isolated as a by-product of this cyclisation, in 6% yield. These results illustrate the scope of template-directed synthesis for creating complex interlocked structures directly from simple starting materials.
A pH-responsive polypseudorotaxane has been synthesized via cucurbit[6]uril (CB6)-catalyzed 1,3-dipolar cycloaddition using diazide and dialkyne monomers, which contain a long aliphatic-spacer. The polypseudorotaxane was characterized by spectroscopic techniques (1H, 13C NMR and FT-IR) and matrix assisted laser desorption time-of-flight mass spectrometry (MALDI-TOF MS). The experimental results reveal that this polypseudorotaxane behaves as a pH-driven polymeric switch. Thus, when amine groups are protonated at an appropriate pH, CB6s are located on the triazole rings due to ion–dipole interaction, whereas at high pH they move onto the hydrophobic aliphatic spacer rather than slipping off the polypseudorotaxane.
The construction of catenanes, comprised of between two and seven interlocked rings, has been achieved. Two tris-1,5-naphtho-57-crown-15 macrocycles template the formation of cyclobis(paraquat-4,4‘-biphenylene) to give a [3]catenane, which acts as a template for the construction of one and then another cyclobis(paraquat-p-phenylene) to give a [4]- and [5]catenane (Olympiadane). When high pressure was used in these templated syntheses, a [6]- and [7]catenane, as well as a [5]catenane that is topologically isomeric with Olympiadane, were also obtained. X-ray analyses of the [3]-, [5]-, and [7]catenanes reveal an optimum use of electrostatic, π−π stacking, [C−H···π] interactions, and [C−H···O] hydrogen bonds in the organization of the component rings within the molecules. Noteworthy in the [7]catenane structure is the location of four of the PF6- anions within voids present in the 20+ ion. Temperature-dependent 1H NMR spectroscopic studies and electrochemical investigations have revealed the dynamic and redox behavior of these catenanes in solution.
Two large and five small rings interlock to give the [7]catenane depicted on the right. Every potential π-donor and π-acceptor recognition site within this molecule is utilized. Only the application of high pressure (12 kbar) made possible the efficient assembly of this exotic molecular compound, for which a solid-state structure was obtained by X-ray crystallography.
A novel synthetic approach is described for the construction of catenanes in aqueous solution from a partially methylated cyclodextrin (CD)-namely, heptakis(2,6-di-O-methyl-β-cyclodextrin) (DM-β-CD)-and a range of substrate molecules that contain a hydrophobic central core in the form of a 4,4′-disubstituted biphenyl unit (usually bitolyl) carrying two hydrophilic polyether side chains terminated by primary amine functions. In water, the amphiphilic catenane precursors form 1:1 complexes with β-CD and DM-β-CD and 2:1 (guest: host) complexes with the larger γ-CD. Macrocyclizations of the biphenyl-containing substrates with aromatic diacid chlorides in aqueous solution and in the presence of DM-β-CD under Schotten-Baumann conditions afforded-in low yields-a range of [2]- and [3]catenanes. When a consitutionally asymmetrical diamine was employed as the substrate, orientational isomers of a [2]catenane were obtained. A [3]catenene incorporating a macrocyclic tetralactam was found to exist as a mixture of head-to-head and head-to-tail isomers, which could be separated by high pressure liquid chromatography and identified unambiguously by nuclear magnetic resonance spectroscopy. One of the [2]catenanes afforded good single crystals from which the solid state structure was determined by X-ray crystallography. Other techniques which aided the characterization of these novel compounds included ultraviolet/visible and luminescence spectroscopy, dynamic nuclear magnetic resonance spectroscopy and fast atom bombardment mass spectrometry. Generally speaking, the catenated cyclodextrins are soluble in halogenated and aromatic hydrocarbons as well as in hydroxylic solvents. The existence of these new compounds gives us a unique insight into the nature of the noncovalent bonding interactions that cyclodextrins employ in binding substrate molecules.
Upon encapsulating the ligand PhNH(CH2)6NH(CH2)4NH2, the nonadecacyclic molecular receptor cucurbituril (C36H36N24O12) exhibits a bimodal binding pattern contingent upon pH.
The design and synthesis of a molecular shuttle, in which the two components are a cyclobis(paraquat-p-phenylene) macro-cycle and a linear polyether chain intercepted by two symmetrically-located hydroquinol units around a central bis(2-oxypropylenedithio)tetrathiafulvalene residue, and terminated by two 4-tritylphenyl ether functions, are reported.
Five interlocked rings in a linear arrangement form the familiar symbol of the Olympic Games and also describe olympiadane, a [5]catenane which was assembled from eight components in two steps at ambient temperature and pressure. This self-assembly process is possible as sufficient molecular recognition was programmed into the components. The fragmentation patterns in the mass spectrum and the ¹H NMR spectrum support the structure shown schematically on the right.
Seven of the best: A dynamic combinatorial library of polycatenated tetrahedra was prepared by complexation between a dynamic Fe4 L6 tetrahedral cage, constructed from ligands containing an electron-deficient naphthalenediimide core, and an electron-rich aromatic crown ether, 1,5-dinaphtho[38]crown-10. The highest order species in the library is the tetrahedral [7]catenane.
THE developing field of nanotechnology has generated wide interest across a broad range of scientific disciplines1. In particular, the realization of nanoscale switching devices might have far-reaching implications for computing and biomimetic engineering2–4. But miniaturization of existing semiconductor technology may not be the best approach to the fabrication of structures whose dimensions are smaller than the wavelength of the radiation used in optical lithography and etching techniques5. The approach observed in the natural world, whereby nanostructures are built up through the self-assembly6–9 of smaller molecular entities, holds substantial promise. Nature abounds with molecular switching devices which perform a variety of functions, such as the transport of metabolites across cell membranes or the signalling of nerve impulses. These processes are commonly controlled by stimuli such as changes in ion concentrations and electrical potentials. Here we report the synthesis of a supramolecular structure (compound 1-[PF6]4, Fig. 1A) that can be reversibly switched between two states by proton
concentration changes or by electrochemical means. The super-molecule is a rotaxane comprising a molecular ring threaded on an axle containing two ‘docking points’. We can effect controlled switching of the ring from one of these positions to the other. We use 1H NMR and ultra violet/visible spectroscopy to characterize the dynamics of the bead's movement along the thread before and after switching.
Evidence for inclusion complexation in solution between cucurbituril (C36H36N24O12) and various alkyl- and aryl-substituted ammonium ions is elaborated. Induced NMR chemical shifts and UV spectral perturbations of guests complexed within cucurbituril are noted and may be used to quantitate binding. Dissociation constants (kd) for over 50 guests are recorded, and the kinetics of complexation have also been investigated. On the basis of selectivity among bound species, a detailed model for the structure of the host-guest complexes is deduced. The cavity within cucurbituril has dimensions equivalent to the size of a para-disubstituted benzene ring. Successful inclusion is attributable to hydrophobic interactions (freeing of solvent molecules upon complexation) and to a charge-dipole attraction between ammonium cations and the electronegative oxygens of the urea moieties in cucurbituril.
The novel cage substance cucurbituril encapsulates and tightly binds substituted ammonium ions having dimensions smaller than a para-disubstituted benzene ring.
Three different multicomponent molecular systems have been synthesized by means of the three-dimensional template effect of copper(I). These systems incorporate both a coordinating ring (2,9-diphenyl-1,10-phenanthroline-containing 30-membered ring) and a molecular string which consists of two different coordination sites (2,9-disubstituted-1,10-phenanthroline and 5,5‘ ‘-disubstituted-2,2‘:6‘,2‘ ‘-terpyridine unit). Each end of the string could be functionnalyzed by a small group or by a bulky stopper (tris(p-tert-butylphenyl)(4-hydroxyphenyl)methane), leading to an unstoppered compound, to a semi-rotaxane, or to a real rotaxane. As in the case of a disymmetrical copper [2]-catenane, large reversible molecular motions have been induced both electrochemically and photochemically. The driving force of the rearrangement processes is the high stability of two markedly different coordination environments for the copper(I) and copper(II) ions. In the copper(I) state, two phenanthroline units (one of the ring, one of the string) interact with the metal ion in a tetrahedral geometry (CuI(4)), whereas in the copper(II) state, one phenanthroline belonging to the ring and the terpyridine of the string afford a five-coordinate geometry (CuII(5)). The rates of the molecular motion processes (from CuII(4) to CuII(5) and from CuI(5) to CuI(4)) are respectively faster and slower (minutes time scale) as compared to those for the catenane species. This result could be interpreted on the basis of structural differences between the rotaxane and catenane systems.
By applying the three-dimensional template effect of copper(I), previously used for making various interlocking ring systems, a new disymmetrical [2]-catenate has been made which consists of two different interlocking rings. One ring contains a 2,9-diphenyl-1,10-phenanthroline (dpp) unit whereas the other cycle incorporates both a dpp motif and a 2,2‘,6‘,2‘‘-terpyridine (terpy) fragment, the coordination site of these two chelates pointing toward the inside of the ring. Depending on the oxidation state of the central metal (Cu(I) or Cu(II)), and thus on its preferred coordination number, two distinct situations have been observed. With monovalent copper, the two dpp units interact with the metal and the terpy fragment remains free, at the outside of the molecule. By contrast, when the catenate is complexed to divalent copper, the terpy motif is bonded to the metal and it is now a dpp ligand which lies at the periphery of the complex. This dual coordination mode leads to dramatically different molecular shapes and properties for both forms. The molecular motion which interconverts the four- and the five-coordinate complexes can be triggered chemically, electrochemically, or photochemically by changing the oxidation state of the copper center (II/I). The process has been studied by electrochemistry and by UV-vis spectroscopy. Interestingly, once the stable 4-coordinate copper(I) complex has been oxidized to a thermodynamically unstable pseudo-tetrahedral copper(II) species, the rate of the gliding motion of the rings which will afford the stable 5-coordinate species (copper(II) coordinated to dpp and terpy) can be controlled at will. Under certain experimental conditions, the changeover process is extremely slow (weeks), and the 4-coordinate complex is more or less frozen. By contrast, addition of a coordinating counterion to the medium (Cl-) enormously speeds up the rearrangement and leads to the thermodynamically stable 5-coordinate complex within minutes.
For the synthetic receptor cucurbituril, the rate of inclusion complex formation correlates with the molecular diameter of alkylammonium ion ligands but not with the thermodynamic stability of the complexes formed. Measurements of 13C NMR spin-lattice relaxation allow comparison of molecular tumbling motions of the receptor with those of bound ligands, by determination at their respective correlation times. Guest ions appear to rotate relatively freely within cucurbituril, irrespective of the stability of the complexes. Results are interpreted in terms of shape complementarity between receptor and ligand.
Transition-metal-containing rotaxanes can behave as linear motors at the molecular level. The molecules are set into motion either by an electrochemical reaction or using a chemical signal. In a first example, a simple rotaxane is described that consists of a ring threaded by a two-coordination-site axle. The ring contains a bidentate ligand, coordinated to a copper center. The axle incorporates both a bidentate and a terdentate ligand. By oxidizing or reducing the copper center to Cu(II) or Cu(I) respectively, the ring glides from a given position on the axle to another position and vice versa. By generalizing the concept to a rotaxane dimer, whose synthesis involves a quantitative double-threading reaction triggered by copper(I) complexation, a molecular assembly reminiscent of a muscle is constructed. By exchanging the two metal centers of the complex (copper(I)/zinc(II)), a large-amplitude movement is generated, which corresponds to a contraction/stretching process. The copper(I)-containing rotaxane dimer is in a stretched situation (overall length 8 nm), whereas the zinc(II) complexed compound is contracted (length 6.5 nm). The stretching/contraction process is reversible and it is hoped that, in the future, other types of signals can be used (electrochemical or light pulse) to trigger the motion.
In one fell swoop, polyrotaxanes comprising up to 64 rings can be synthesized as a result of cucurbit[6]uril-templated 1,3-dipolar azide-alkyne cycloadditions accelerated in the presence of cyclodextrins as a consequence of self-sorting and positive cooperativity, brought about by hydrogen bonding. Mixing six components in one pot affords a hetero[4]rotaxane in one minute in quantitative yield.
Like with a string of pearls, four molecular “beads” are threaded on a molecular rectangle to form a molecular necklace. This rectangular species is synthesized from two L-shaped, preorganized pseudorotaxanes with two molecular beads each (cucurbituril, schematically symbolized by the barrels), held together by Cu2+ ions [Eq. (1)].
Water soluble [5]rotaxane and [5]pseudorotaxane based on cucurbit[6]uril and anchored to a meso-tetraphenyl porphyrin have
been synthesized and characterized by spectroscopic methods (1H-NMR, 13C-NMR and UV), and by elemental analysis, and mass spectrometry. The preliminary results of the pH-driven switching properties
of [5]rotaxane investigated through 1H-NMR spectroscopy are reported. These results were compared with those obtained from a model porphyrin, which was prepared
by the de-threading cucurbit[6]uril from [5]pseudorotaxane under basic conditions.
We report the template-directed synthesis of a well-defined, kinetically stable [5]molecular necklace with dialkylammonium ion (R(2)NH(2)(+)) as recognition site and DB24C8 as macrocycle. A thread containing four dialkylammonium ions with olefin at both ends was first synthesized and then subjected to threading with an excess amount of DB24C8 to form pseudo[5]rotaxane, which in situ undergoes ring closing metathesis at the termini with second generation Grubbs catalyst to yield the desired [5]molecular necklace. The successful synthesis of [5]molecular necklace is mainly attributed to the self-assembly and dynamic covalent chemistry which allows the formation of thermodynamically most stable product. The self-assembly of the DB24C8 ring onto the recognition site known as templating effect was driven by noncovalent stabilizing interactions like [N(+)-H···O], [C-H···O] hydrogen bonds as well as [π···π] interactions which is facilitated in non-polar solvents. The reversible nature of olefin metathesis reaction makes it suitable for dynamic covalent chemistry since proof-reading and error-checking operates until it generates thermodynamically the most stable interlocked molecule. Riding on the success of [5]molecular necklace, we went a step further and attempted to synthesize [7]molecular necklace using the same protocol. This led to the synthesis of another thread with olefin at both ends but having six dibenzylammonium ions along the thread. However, the extremely poor solubility of this thread containing six secondary ammonium ions limits the self-assembly process even after we replaced the typical PF(6)(-) counter anion with a more lipophilic BPh(4)(-) anion. Although the poor solubility of the thread remains the bottleneck for making higher order molecular necklaces yet this approach of "threading-followed-by-ring-closing-metathesis" for the first time produces kinetically and thermodynamically stable, well-defined, homogeneous molecular necklace which was well characterized by one-dimensional, two-dimensional, variable temperature proton NMR spectroscopy and ESI mass spectroscopy.