Correlated translational motions in pseudo-rotaxane complexes controlled by a single chemical stimulus.

Coordinated motions are essential in the operation of molecular machines. This feature can be achieved by landscaping the energy surface along the movement coordinates. Herein, we present an approach of using a single stimulus to modify the free energy curve describing the threading and shuttling of a ring along a linear molecule. This approach has been realized by locating two identical ring-binding sites near the axle termini.

An Operative Electrostatic Slipping Mechanism along Macrocycle Flexibility Accelerates Guest Sliding during pseudo-Rotaxane Formation.

A pseudo-rotaxane is a host-guest complex composed of a linear molecule encircled by a macrocyclic ring. These complexes can be assembled by sliding the host over the guest terminal groups. If there is a close match between the molecular volume of the flanking groups on the guest and the cavity size of the macrocycle, the slipping might occur slowly or even become completely hindered. We have previously shown that it is possible to overcome the restraints imposed by steric effects on the sliding process by integrating electrostatic attractive interactions during the slipping step. In this work, we extend our electrostatically assisted slipping approach (EASA) to a new host-guest system featuring a flexible macrocyclic ring and a series of asymmetric guests containing a cyclic tertiary ammonium group. Compelling evidence for pseudo-rotaxane formation is presented, along with thermodynamic and kinetic data. Experimental results suggests that the higher conformational flexibility of 24-crown-8 significantly increases the sliding rate, compared with the more rigid dibenzo-24-crown-8, without affecting complex stability. Furthermore, by combining the EASA and macrocycle flexibility, we were capable to slip a large eight-membered cyclic group across the 24-crown-8 annulus, setting a new limit on the ring molecular size that can pass through a 24-membered crown ether.

An Anionic Ring Locked into an Anionic Axle: A Metastable Rotaxane with Chemically Activated Electrostatic Stoppers.

The use of the electrostatic stoppers concept in the field of mechanically interlocked molecules is reported; these stoppers are chemically sensitive end groups on a linear guest molecule that allows for the conversion of a pseudo-rotaxane species into a rotaxane complex by a change in the medium acidity. The chemical stimulus causes the appearance of negative charges on both ends of the linear component, passing from cationic to anionic, and causing a significant ring-to-axle electrostatic repulsion. This phenomenon has two different and simultaneous effects: 1) destabilizes the complex as a consequence of confining an anionic ring into an anionic axle, and 2) increases the dissociation energy barrier, thus impeding ring extrusion. This newly formed metastable rotaxane species is resistant to solvent and temperature effects and performs as a two-state degenerated molecular shuttle in solution.

Energetics and the molecular structure of an ion-paired supramolecular system in water.

The forces that bind the components of a host-guest complex to generate a stable supramolecular system are noncovalent interactions. The enthalpy of this association, ΔasH°, usually measured using calorimetry, quantifies the magnitude of such interactions and is directly related to the stability of the supramolecular complex formed. Using Calvet calorimetry to determine the enthalpies of solution and reaction in water, the enthalpy of association was derived for a supramolecular system formed by the anionic macrocycle anti-disulfodibenzo[24]crown-8 ([DSDB24C8]2-) and the dicationic guest paraquat [PQT]2+. The calorimetric results show an exothermic association process, which indicates the generation of strong interactions between the components of the ion pair. This is consistent with the formation of a stable supramolecular complex [PQT][DSDB24C8], whose spatial arrangement in aqueous solution is proposed based on spectroscopic analysis.

Shuttling Motion in a Host-Guest Complex Triggered by Spiropyran to Merocyanine Reversible Chemical Transformation.

A stimulus-responsive guest-containing spiropyran and viologen unit assembles with a 24-membered crown ether into a stable host-guest complex displaying a partially threaded geometry. Acid addition induces guest transformation to a merocyanine species activating a second recognition site, suitable for the formation of a pseudorotaxane. The simultaneous presence of two recognition sites produces a small-amplitude macrocycle shuttling motion, from the viologen to the merocyanine moiety. Base addition returns the guest to its spiropyran form, and concurrently the translation motion stops.

Self-assembly of a supramolecular network with pseudo-rotaxane cross-linking nodes and its transformation into a mechanically locked structure by rotaxane formation.

A main chain poly(crown ether) macromolecule has been cross-linked with a two binding site guest to afford a stimuli-responsive supramolecular polymeric network with pseudo-rotaxane nodes. This network forms gels and films that are responsive to temperature and solvent polarity; however, the materials can be shielded against both effects by transforming the pseudo-rotaxane junctions into rotaxane links, creating a mechanically locked structure.

Self-assembly of pseudo-rotaxane and rotaxane complexes using an electrostatic slippage approach.

The protonation of a cyclic tertiary amine, integrated into the structure of a dumbbell-shaped guest molecule, accelerates the sliding of the guest through the cavity of a crown ether macrocycle to yield a stable pseudo-rotaxane complex. The use of an amine with the appropriate ring size followed by a proton transfer reaction leads to the formation of an interlocked rotaxane species.

Coupled molecular motions driven by light or chemical inputs: spiropyran to merocyanine isomerisation followed by pseudorotaxane formation.

We have designed and prepared a new dual stimuli-responsive guest molecule containing a spiropyran fragment and a pyridinium moiety. Acid addition or UV-light irradiation induces guest transformation to a merocyanine isomer, promoting the threading motion through a 24-crown-8 macrocycle and the formation of a [2]pseudorotaxane complex.

Electrostatic kinetic barriers in the threading/dethreading motion of a rotaxane-like complex.

A new rigid cationic thread, based on the 1,2-bis(bipyridinium)ethane motif, and a 24-crown-8 anionic macrocycle self-assemble into a pseudo-rotaxane complex in aqueous solution. The presence of pH-responsive end groups on the thread, remote from the recognition motif, allows controlling the threading/dethreading rate without perturbing the complex structure or stability. The difference in the rate is attributed to the activation or deactivation of electrostatic barriers on the thread during the sliding process at different pH values.

Complexation of imidazopyridine-based cations with a 24-crown-8 ether host: [2]pseudorotaxane and partially threaded structures.

A new series of linear molecules derived from 1,2-bis(imidazopyridin-2-yl)ethane can fully or partially penetrate the cavity of the dibenzo-24-crown-8 macrocycle to produce a new family of host-guest complexes. Protonation or alkylation of the nitrogen atoms on the pyridine rings led to an increase in the guest total positive charge up to 4+ and simultaneously generated two new recognition sites (pyridinium motifs) that are in competition with the 1,2-bis(benzimidazole)ethane motif for the crown ether. The relative position of the pyridine ring and the chemical nature of the N-substituent determined the preferred motif and the host-guest complex geometry: (i) for linear guests with relatively bulky groups (i.e., a benzyl substituent), the 1,2-bis(benzimidazole)ethane motif is favored, leading to a fully threaded complex with a [2]pseudorotaxane geometry; (ii) for small substituents, such as -H and -CH3 groups, regardless of the guest shape, the pyridinium motifs are preferred, leading to external partially threaded complexes in a 2:1 host to guest stoichiometry.

Synthesis and structural characterization of fluorinated thiosemicarbazones.

Six new fluorinated thiosemicarbazones R­C(R')=N-NH-C(S)NH(2) (R = 2,4-C(6)H(3)F(2), R' = H (1); R = 2,5-C(6)H(3)F(2), R' = H (2); R = 2,6-C(6)H(3)F(2), R' = H (3); R = 3,4-C(6)H(3)F(2), R' = H (4); R = 3,5-C(6)H(3)F(2), R' = H (5) and R = 4-C(6)H(4)F, R' = C(6)H(5), (6)) have been prepared. The molecular structures of compounds 1 to 6 have been determined.

Co-conformational isomerism in a neutral ion-paired supramolecular system.

Two different counter-ion-free host-guest complexes have been prepared and isolated. These compounds were formed from two equally and opposite doubly-charged species, the viologen guests 1 a(2+) and 1 b(2+) and the anti-disulfodibenzo[24]crown-8 [DSDB24C8](2-) host, which gave rise to the 1:1 neutral complexes [1 a⋅DSDB24C8] and [1 b⋅DSDB24C8]. These species are held together by hydrogen bonding and π stacking, as well as strong electrostatic interactions. The investigation of these neutral ion-paired supramolecular systems in solution and in the solid state allowed us to establish their co-conformational preferences. Compound [1 a⋅DSDB24C8], with small methyl groups as substituents on the viologen unit, may adopt three different geometries, 1) an exo nonthreaded, 2) a partially threaded, and 3) a threaded arrangement, depending on the relative spatial orientation between the host and guest: The partially-threaded structure is preferred in solution and in the solid state. The presence of bulky tert-butylbenzyl groups in the viologen moiety in compound [1 b⋅DSDB24C8] restricts the possible geometrical arrangements to one: The exo nonthreaded arrangement. This structure was confirmed in the solid state by X-ray crystallography. The stability of the neutral complexes in solution was determined by UV/Vis spectrophotometry. The stoichiometry of the complexes was established by continuous variation experiments, and overall equilibrium constants and ΔG° values were determined on the basis of dilution experiments. The results observed are a consequence of only the intrinsic stability of the complexes as there are no additional contributions from counter ions.

Direct, facile synthesis of N-acyl-alpha-amino amides from alpha-keto esters and ammonia.

N-acyl-alpha-amino amides were prepared, without the necessity of chromatographic purification, in a single step by heating the corresponding alpha-keto ester in methanolic ammonia.

Optically sensed, molecular shuttles driven by acid-base chemistry.

A pair of bistable [2]rotaxane, molecular shuttles were prepared that combine 1,2-bis(pyridinium)ethane and benzylanilinium recognition sites; acid-base controlled shuttling of DB24C8 was accompanied by a change in colour and/or fluorescence intensity.

Chemically controlled self-assembly of [2]pseudorotaxanes based on 1,2-bis(benzimidazolium)ethane cations and 24-crown-8 macrocycles.

Cations derived from 1,2-bis(benzimidazolium)ethane can penetrate the cavity of dibenzo-24-crown-8 macrocycles to produce a new family of [2]pseudorotaxanes. These supramolecular structures are held together by a series of charge-assisted hydrogen bonds (+)N-H[dot dot dot]O, ion-dipole and pi-stacking interactions. These new adducts were fully characterised by NMR spectroscopy, ESI mass spectrometry and single-crystal X-ray diffraction. The effect of electron-donating and electron-withdrawing groups on the association constants was also analyzed. Chemical control of the threading/unthreading process was acheived by the alternate addition of acid and base.

Push-pull [2]pseudorotaxanes. Electronic control of threading by switching ON/OFF an intramolecular charge transfer.

[reaction: see text] Addition of acid and base allows switching between two different structures in a push-pull bis(pyridinium)ethane axle, controlling its ability to form [2]pseudorotaxanes with a dibenzo-24-crown-8 ether wheel.

A mechanical "flip-switch". Interconversion between co-conformations of a [2]rotaxane with a single recognition site.

[2]Rotaxanes utilising the 1,2-bis(pyridinium)ethane, 24-crown-8 motif can exist in two distinct co-conformations whose relative abundances are solvent dependent.

A versatile template for the formation of [2]pseudorotaxanes. 1,2-Bis(pyridinium)ethane axles and 24-crown-8 ether wheels.

Linear 1,2-bis(pyridinium)ethane 'axles' and macrocyclic 24-membered crown ether 'wheels' (, and ) combine to form [2]pseudorotaxanes. These interpenetrated adducts are held together by N+...O ion-dipole interactions, a series of C-H...O hydrogen bonds and pi-stacking between electron-poor pyridinium rings of the axle and electron-rich catechol rings of the wheel. 1H NMR spectroscopy was used to identify the structural details of the interaction and to determine the thermodynamics of the binding process in solution. Analysis of nine of these adducts by single crystal X-ray crystallography allowed a detailed study of the non-covalent interactions in the solid state. A wide variety of structural changes could be made to the system. The versatility and potential of the template for the construction of permanently interlocked structures such as rotaxanes and catenanes is discussed.

Diastereoselective allylation and crotylation of N-unsubstituted imines derived from ketones.

A wide variety of tertiary carbinamines are synthesized in high yields via diastereoselective allylation and crotylation of in situ generated N-unsubstituted ketimines.