Dimeric capsules by the self-assembly of triureidocalix

Supramolecular Handshakes: Characterization of Urea-Carboxylate Interactions Within Calixarene Frameworks

The research of molecular capsules offers high application potential and numerous benefits in various fields. With the aim of forming supramolecular capsules which can be reversibly assembled and dissociated by simple external stimuli, we studied interactions between calixarenes containing urea and carboxylate moieties. To this end two ureido-derivatives of p-tert-butylcalix[4]arene comprising phenylureido-moieties and diacetate-calix[4]arenes were prepared. The binding of acetate by ureido-derivatives of calixarene in acetonitrile was characterized, revealing high affinity of ureido-calixarenes for carboxylates. This suggested high potential for uniting the complementary calix[4]arenes via H-bonds between carboxylic groups and urea moieties. The assembly of calixarenes was examined in detail by means of UV, 1H NMR, ITC, DOSY, MS, and conductometry providing insight in the structure-stability relationship. The tetraureido-calixarene derivative formed the most stable heterodimers with diacetate-calix[4]arenes featuring practically quantitative association upon mixing the two calixarene counterparts. The possibility of controlling the formation of the heterodimer by protonating the carboxylates, thereby hindering the interactions critical for capsule assembly, was investigated. Indeed, the reversibility of breaking and re-forming the heterodimer by addition of an acid and base to the solution containing urea- and carboxylate-derivative calix[4]arene was demonstrated using NMR spectroscopy.

Calix[6]arene-Based [3]Rotaxanes as Prototypes for the Template Synthesis of Molecular Capsules

In this work, the ability of several bis-viologen axles to thread a series of heteroditopic tris(N-phenylureido)calix[6]arene wheels to give interwoven supramolecular complexes to the [3]pseudorotaxane type was studied. The unidirectionality of the threading process inside these nonsymmetric wheels allows the formation of highly preorganised [3]pseudorotaxane and [3]rotaxane species in which the macrocycles phenylureido moieties, functionalised with either ester, carboxylic, or hydroxymethyl groups, are facing each other. As verified by NMR and semiempirical computational studies, these latter compounds possess the correct spatial arrangement of their subcomponents, which could lead, in principle, upon proper bridging reaction, to the realisation of upper-to-upper molecular capsules that are based on calix[6]arene derivatives.

Calixarene-based artificial ionophores for chloride transport across natural liposomal bilayer: Synthesis, structure-function relationships, and computational study

An amphiphilic calix[6]arene, alone or complexed with an axle to form a pseudo-rotaxane, has been embedded into liposomes prepared from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and the permeability of the membrane-doped liposomes towards Cl^- ions has been evaluated by using lucigenin as the fluorescent probe. The pseudo-rotaxane promotes transmembrane transport of Cl^- ions more than calix[6]arene does. Surprisingly, the quenching of lucigenin was very fast for liposomes doped with the positively charged axle alone. Molecular dynamics (MD) simulations and quantum-chemical calculations were also carried out for providing a semi-quantitative support to the experimental results.

Heteroditopic Calix[6]arene Based Intervowen and Interlocked Molecular Devices

Since the dawn of supramolecular chemistry, calixarenes have been employed as platforms onto which functional groups and binding sites can be loaded in a regio- and stereocontrolled manner for the recognition of charged and neutral species. Despite their wider annulus, potentially suitable to bind larger guests, the larger members of the calixarene series have been relatively less employed, mainly because of the synthetic difficulties to control their conformational flexibility and their regioselective functionalization. In this account, we will present the achievements gained during the last two decades on the use of the calix[6]arene as a platform to build-up structures in which the macrocycle acts as a wheel for the synthesis of oriented (pseudo)rotaxanes. We also account on how these calix[6]arene hosts affect the reactivity or spectroscopic properties of their bound guests.

Synthesis and properties of a redox-switchable calix[6]arene-based molecular lasso

A self-complexing lasso-like molecule has been synthesised as the first example of a calix[6]arene-based [1]pseudorotaxane. This artificial molecular lasso can be switched between self-threaded and dethreaded structures by redox stimulation.

Plugging a Bipyridinium Axle into Multichromophoric Calix[6]arene Wheels Bearing Naphthyl Units at Different Rims

Tris-(N-phenylureido)-calix[6]arene derivatives are heteroditopic non-symmetric molecular hosts that can form pseudorotaxane complexes with 4,4'-bipyridinium-type guests. Owing to the unique structural features and recognition properties of the calix[6]arene wheel, these systems are of interest for the design and synthesis of novel molecular devices and machines. We envisaged that the incorporation of photoactive units in the calixarene skeleton could lead to the development of systems the working modes of which can be governed and monitored by means of light-activated processes. Here, we report on the synthesis, structural characterization, and spectroscopic, photophysical, and electrochemical investigation of two calix[6]arene wheels decorated with three naphthyl groups anchored to either the upper or lower rim of the phenylureido calixarene platform. We found that the naphthyl units interact mutually and with the calixarene skeleton in a different fashion in the two compounds, which thus exhibit a markedly distinct photophysical behavior. For both hosts, the inclusion of a 4,4'-bipyridinium guest activates energy- and/or electron-transfer processes that lead to non-trivial luminescence changes.

Citric acid encapsulation by a double helical foldamer in competitive solvents

A new double helical aromatic oligoamide capsule able to bind to citric acid in polar and protic solvents was prepared. Aromatic amino acids in the sequence encode both structural (strand curvature and double helix formation) and functional features (recognition pattern) of the assembled capsule.

Anion-tuned supramolecular gels: a natural evolution from urea supramolecular chemistry

This tutorial review looks at the formation of low molecular weight gels from molecular principles using the well-explored supramolecular chemistry of ureas as an example. Synthesising lessons learned from classical urea inclusion chemistry, ureas in crystal engineering, ureas in self-assembly, urea functional groups in anion binding and sensing, and ureas as organocatalysts lead to the development and understanding of a new class of anion-tunable, urea-based soft materials. This review concludes with a look at emerging application areas for tunable gel-phase materials as controlled crystal growth media, both in templating metallic nanoparticles and in the growth and isolation of high quality crystals of molecular organic compounds, including polymorphic pharmaceuticals.

Molecular behavior in small spaces

The study of physical organic chemistry in solution is a mature science, over a century old, but over the last 10 years or so, reversible encapsulation has changed the way researchers view molecular interactions. It is now clear that the behavior of molecules in dilute solution is really quite different from their behavior in capsules. Molecules isolated from bulk media in spaces barely large enough to accommodate them and a few neighbors show new phenomena: their activities resemble those of molecules inside biochemical structures--pockets of enzymes, interiors of chaperones, or the inner space of the ribosome--rather than conventional behavior in solution. In this Account, we recount the behavior of molecules in these small spaces with emphasis on structures and reactivities that have not been, and perhaps cannot be, seen in conventional solution chemistry. The capsules self-assemble through a variety of forces, including hydrogen bonds, metal-ligand interactions, and hydrophobic effects. Their lifetimes range from milliseconds to hours, long enough for NMR spectroscopy to reveal what is going on inside. We describe one particular capsule, the elongated shape of which gives rise to many of the effects and unique phenomena. Molecular guests that are congruent to the space of the host can be tightly packed inside and show reduced mobilities such as rotation and translation within the capsule. These mobilities depend strongly on what else is encapsulated with them. We also relate how asymmetric spaces can be created inside the capsule by using a chiral guest. In contrast to the situation in dilute solution, where rapid exchange of solute partners and free molecular motion average out the steric and magnetic effects of chirality, the long lifetimes of the encounters in the capsules magnify the effects of an asymmetric environment. The capsule remains achiral, but the remaining space is chiral, and coencapsulated molecules respond in an amplified way. We probe the various regions of the capsule with guests of different shape. Primary acetylenes, the narrowest of functional groups, can access the tapered ends of the capsule that exclude functions as small as methyl groups. The shape of the capsule also has consequences for aromatic guests, gently bending some and straightening out others. Flexible structures such as normal alkanes can be compressed to fit within the capsule and conform to its shape. We obtain a measure of the internal pressure caused by the compressed guests by determining its effect on the motion of the capsule's components. These forces can also drive a spring-loaded device under the control of external acids and bases. We show that spacer elements can be added to give self-assembled capsules of increased complexity, with 15 or more molecules spontaneously coming together in the assembly. In addition, we analyze the behavior of gases, including the breakdown of ideal gas behavior, inside these capsules. The versatility of these capsule structures points to possible applications as nanoscale reaction chambers. The exploration of these confined spaces and of the molecules within them continues to open new frontiers.

Second generation of calix[6]aza-Cryptands: synthesis of heteroditopic receptors for organic ion pairs

The efficient syntheses of calix[6]azacryptands decorated with anion-binding groups on the narrow rim have been achieved from an 1,3,5-tris-protected calix[6]hexa-amine. These heteroditopic receptors can bind ammonium ions or organic ion pair salts with a positive cooperativity. In regard to their functionalization at the 1,3,5-phenolic positions, these compounds constitute the first examples of a second generation of C3 v symmetrical calix[6]azacryptands.

Synthesis and study of calix[6]cryptamides: A new class of heteroditopic receptors that display versatile host-guest properties toward neutral species and organic associated ion-pair salts

The synthesis of a new family of molecular receptors, namely the calix[6]cryptamides, was achieved through an original [1+1] macrocyclization step that consists of a peptide-coupling reaction between tripodal triscarboxylic acids and a calix[6]trisamine subunit. Several C3- or C3v-symmetrical calix[6]arene-based compounds capped by a trisamido cryptand unit on the narrow rim have been obtained, with the more flexible partners leading to the best yields. These calix[6]cryptamides exhibit two favorably positioned binding sites for the complexation of organic-associated ion pairs in close proximity: a well-defined calix[6]arene cavity suitable for the inclusion of ammonium ions and a cryptamide unit for the coordination of anions. We demonstrate one example, chiral calix[6]cryptamide 12, that constitutes a heteroditopic receptor capable of cooperatively binding both a primary ammonium ion and its chloride counterion, thanks to a combination of polarization and induced-fit effects. In addition, the hydrophobic calixarene cavity of 12 can strongly bind neutral guests through hydrogen bonding and is capable of discriminating between different enantiomers. All these versatile host-guest properties differ greatly from those observed in the parent calix[6]azacryptands.

Encapsulation of small polar guests in molecular apple peels

Three aromatic oligoamides have been prepared that have alternating 1,6-diaminopyridine and 1,6-pyridinedicarboxylic acid units at the center of the sequence and two 8-amino-2-quinolinecarboxylic acid units at each extremity. The three oligomers differ in the number--3, 5, or 7-of pyridine units in the sequence. They were designed to adopt helically folded conformations in solution and in the solid state. The sequence of monomers was chosen so that the diameter of the helix is larger in the center than at each extremity, and hence they resemble helically wrapped apple peels. According to modeling studies, the pyridine units were expected to define a polar hollow within the helix that is large enough to accommodate small polar guests, whereas the quinoline units at each end of the oligomeric sequences were expected to completely cap the hollow and transform the helix cavities into a closed shell that may act as a capsule. Crystallographic studies demonstrate that the oligomers do fold into helices that define a cavity isolated from the surrounding medium in the solid state. Depending on the number of pyridine rings, one or two water molecules are bound within the capsules. The crystal structure of a capsule fragment shows that MeOH can also be hosted by the largest oligomer. Solution NMR studies confirm that binding of water also occurs in solution with the same stoichiometry as observed in the solid state. The capsules have distinct signals depending on whether they are empty, half-full, or full, and these species are in slow exchange on the NMR timescale at low temperature. Indeed, the binding and release of water molecules requires a significant conformational distortion of the helix that slows down these processes. The addition of small polar molecules such as methanol, hydrazine, hydrogen peroxide, or formic acid to the largest capsule leads to the observation of new sets of NMR signals of the capsules that were assigned to complexes with these guests. However, water appears to be the preferred guest.