Co‐Conformational Distribution of Nanosized [2]Catenanes Determined by Pulse EPR Measurements

The co‐conformational ensembles of three differently sized [2]catenanes were studied by measuring pair correlation functions corresponding to the separation of nitroxide spin labels—one attached to each of the two macrocycles—with the double electron–electron resonance (DEER) experiment. A geometric model for the [2]catenanes was derived that approximates the macrocycles by circles and takes into account the topological constraint. Comparison of the experimental to the theoretically predicted pair correlation functions gives insight into the co‐conformational distribution and the size of the macrocycles. It was found that the macrocycles of the medium‐ and large‐sized catenanes in chloroform are close to fully expanded, while they are partially collapsed in glassy o‐terphenyl. For the small‐sized catenane, moderate interaction between the unsaturated sections of the macrocycles in chloroform is indicated by a slight overrepresentation of short label‐to‐label separations in the pair correlation function.     

When Self‐Assembly Fails: Stepwise Metal‐Directed Synthesis of [2]Catenanes

On the attempted synthesis of a series of homo‐ and heterotrimetallic [2]catenanes by the self‐assembly of a 2‐(pyridin‐4‐ylmethyl)‐2,7‐diazapyrenium ligand, (ethylenediamine)palladium(II) or platinum(II) nitrate, and a dioxoaryl bis(N‐monoalkyl‐4,4′‐bipyridinium) salt as building blocks, both the one‐pot direct self‐assembly of the components and the so called “magic ring” approach fail to produce the expected trinuclear [2]catenanes under thermodynamically driven conditions. However, one of the target supramolecules is obtained by following a stepwise protocol, consisting of the threading of a dinuclear Pt^II metallacycle and the dioxoaryl bis(N‐monoalkyl‐4,4′‐bipyridinium) axle, followed by kinetically controlled Pt^II‐directed cyclization of the corresponding pseudorotaxane.     

Surface‐Bound Cucurbit[8]uril Catenanes on Magnetic Nanoparticles Exhibiting Molecular Recognition

We demonstrate the preparation of surface‐bound cucurbit[8]uril (CB[8]) catenanes on silica nanoparticles (NPs), where CB[8] was employed as a tethered supramolecular “handcuff” to selectively capture target guest molecules. In this catenane, CB[8] was threaded onto a methyl viologen (MV²⁺) axle and immobilized onto silica NPs. The formation of CB[8] catenanes on NPs were confirmed by UV/Vis titration experiments and lithographic characterization, demonstrating a high density of CB[8] on the silica NPs surface, 0.56 nm^?2. This CB[8] catenane system exhibits specific molecular recognition towards certain aromatic molecules such as perylene bis(diimide), naphthol and aromatic amino acids, and thus it can act as a nanoscale molecular receptor for target guests. Furthermore, we also demonstrate its use as an efficient and recyclable nano‐platform for peptide separation. By embedding magnetic NPs inside silica NPs, separation could be achieved by simply applying an external magnetic field. Moreover, the peptides captured by the catenanes could be released by reversible single‐electron reduction of MV²⁺. The entire process demonstrated high recoverability.     

Coupling of the Decarboxylation of 2‐Cyano‐2‐phenylpropanoic Acid to Large‐Amplitude Motions: A Convenient Fuel for an Acid–Base‐Operated Molecular Switch

The decarboxylation of 2‐cyano‐2‐phenylpropanoic acid is fast and quantitative when carried out in the presence of 1 molar equivalent of a [2]catenane composed of two identical macrocycles incorporating a 1,10‐phenanthroline unit in their backbone. When decarboxylation is over, all of the catenane molecules have experienced large‐amplitude motions from neutral to protonated catenane, and back again to the neutral form, so that they are ready to perform another cycle. This study provides the first example of the cyclic operation of a molecular switch at the sole expenses of the energy supplied by the substrate undergoing chemical transformation, without recourse to additional stimuli.     

Halogen‐ and Hydrogen‐Bonding Catenanes for Halide‐Anion Recognition

Halogen‐bonding (XB) interactions were exploited in the solution‐phase assembly of anion‐templated pseudorotaxanes between an isophthalamide‐containing macrocycle and bromo‐ or iodo‐functionalised pyridinium threading components. ¹H NMR spectroscopic titration investigations demonstrated that such XB interpenetrated assemblies are more stable than analogous hydrogen bonding (HB) pseudorotaxanes. The stability of the anion‐templated halogen‐bonded pseudorotaxane architectures was exploited in the preparation of new halogen‐bonding interlocked catenane species through a Grubbs’ ring‐closing metathesis (RCM) clipping methodology. The catenanes’ anion recognition properties in the competitive CDCl₃/CD₃OD 1:1 solvent mixture revealed selectivity for the heavier halides iodide and bromide over chloride and acetate.     

Controllable donor-acceptor neutral [2]rotaxanes

In pursuit of a neutral bistable [2]rotaxane made up of two tetraarylmethane stoppers--both carrying one isopropyl and two tert-butyl groups located at the para positions on each of three of the four aryl rings--known to permit the slippage of the pi-electron-donating 1,5-dinaphtho[38]crown-10 (1/5DNP38C10) at the thermodynamic instigation of pi-electron-accepting recognition sites, in this case, pyromellitic diimide (PmI) and 1,4,5,8-naphthalenetetracarboxylate diimide (NpI) units separated from each other along the rod section of the rotaxane's dumbbell component, and from the para positions of the fourth aryl group of the two stoppers by pentamethylene chains, a modular approach was employed in the synthesis of the dumbbell-shaped compound NpPmD, as well as of its two degenerate counterparts, one (PmPmD) which contains two PmI units and the other (NpNpD) which contains two NpI units. The bistable [2]rotaxane NpPmR, as well as its two degenerate analogues PmPmR and NpNpR, were obtained from the corresponding dumbbell-shaped compounds NpPmD, PmPmD, and NpNpD and 1/5DNP38C10 by slippage. Dynamic 1H NMR spectroscopy in CD2Cl2 revealed that shuttling of the 1/5DNP38C10 ring occurs in NpNpR and PmPmR, with activation barriers of 277 K of 14.0 and 10.9 kcal mol(-1), respectively, reflecting a much more pronounced donor-acceptor stabilizing interaction involving the NpI units over the PmI ones. The photophysical and electrochemical properties of the three neutral [2]rotaxanes and their dumbbell-shaped precursors have also been investigated in CH2Cl2. Interactions between 1/5DNP38C10 and PmI and NpI units located within the rod section of the dumbbell components of the [2]rotaxane give rise to the appearance of charge-transfer bands, the energies of which correlate with the electron-accepting properties of the two diimide moieties. Comparison between the positions of the visible absorption bands in the three [2]rotaxanes shows that, in NpPmR, the major translational isomer is the one in which 1/5DNP38C10 encircles the NpI unit. Correlations of the reduction potentials for all the compounds studied confirm that, in this non-degenerate [2]rotaxane, one of the translational isomers predominates. Furthermore, after deactivation of the NpI unit by one-electron reduction, the 1/5DNP38C10 macrocycle moves to the PmI unit. Li+ ions have been found to strengthen the interaction between the electron-donating crown ether and the electron-accepting diimide units, particularly the PmI one. Titration experiments show that two Li+ ions are involved in the strengthening of the donor-acceptor interaction. Addition of Li+ ions to NpPmR induces the 1/5DNP38C10 macrocycle to move from the NpI to the PmI unit. The Li+-ion-promoted switching of NpPmR in a 4:1 mixture of CD2Cl2 and CD3COCD3 has also been shown by 1H NMR spectroscopy to involve the mechanical movement of the 1/5DNP38C10 macrocycle from the NpI to the PmI unit, a process that can be reversed by adding an excess of [12]crown-4 to sequester the Li+ ions.     

Backbone‐Directed Self‐Assembly of Interlocked Molecular Cyclic Metalla[3]Catenanes

The efficient backbone‐directed self‐assembly of cyclic metalla[3]catenanes by the combination of tetrachloroperylenediimide (TCPDI)‐based dinuclear rhodium(III) clips and 4,4′‐diazopyridine or 4,4′‐dipyridylethylene ligands is realized in a single‐step strategy. The topology and coordination geometry of the cyclic metalla[3]catenanes are characterized by NMR spectroscopy, ESI‐TOF‐MS spectrometry, UV/Vis‐NIR spectroscopy, and X‐ray diffraction studies. The most remarkable feature of the formed cyclic metalla[3]catenane is that it contains π‐aggregates (ca. 2.6 nm) incorporating six TCPDIs. Further studies revealed that cyclic metalla[3]catenanes can be converted reversibly to their corresponding sodium adducts and precursor building blocks, respectively. This strategy opens the possibility of generating unique supramolecular structures from discrete functional π‐aggregates with precise arrangements.     

[2]Catenanes Built Around Octahedral Transition‐Metal Complexes that Contain Two Intertwined Endocyclic but Non‐sterically Hindering Tridentate Ligands

Sterically hindering bidentate chelates, such as 2,9‐diphenyl‐1,10‐phenanthroline, form entwined complexes with copper(I) and other tetrahedrally coordinated transition‐metal centres. To prepare octahedral complexes containing two entwined tridentate ligands and thus apply a strategy similar to that used for making catenanes with tetrahedral metal centres, the use of the classical terpy ligand (terpy=2,2′:6′,2′′‐terpyridine) appears to be attractive. In fact, 6,6′′‐diphenyl‐2,2′:6′,2′′‐terpyridine (dp‐terpy) is not appropriate due to strong “pinching” of the organic backbone by coordination to the metal and thus stable entwined complexes with this ligand cannot be obtained. Herein, we report the synthesis and coordination properties of a new family of tridentate ligands, the main features of which are their endocyclic nature and non‐sterically hindering character. The coordinating fragment consists of two 8′‐phenylisoquinolin‐3′‐yl groups attached at the 2 and 6 positions of a pyridine nucleus. Octahedral complexes containing two such entangled ligands around an octahedral metal centre, such as Fe^II, Ru^II or Co^III, are highly stable, with no steric congestion around the metal. By using functionalised ligands bearing terminal olefins, double ring‐closing metathesis leads to [2]catenanes in good yield with Fe^II or Co^III as the templating metal centre. The X‐ray crystallography structures of the Fe^II precursor and the Fe^II catenane are also reported. These show that although significant pinching of the ligand is observed in both Fe^II complexes, the system is very open and no steric constraints can be detected.     

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.