Heteroditopic Calix Research Articles

Selenoureido Calix[6]arenes: A Novel Platform for Pseudorotaxane Synthesis

AbstractHeteroditopic calix[6]arenes are widely employed macrocycles for the synthesis of pseudo‐oriented interlocked systems and stimuli‐induced molecular machines, among others. Herein, we introduce a new calix[6]arene receptor functionalised with three phenylselenoureido groups. These hydrogen‐bonding donor moieties are able to promote the threading of viologen‐based organic axles inside the macrocyclic cavity to form a stable pseudorotaxane species. Preliminary investigations using 1H‐NMR measurements and semi‐empirical and DFT studies suggested its further use as a platform for the synthesis of rotaxanes.

Supramolecular hydrogelation via host-guest anion recognition: Lamellar hydrogel materials for the release of cationic cargo

Supramolecular hydrogelation via host-guest anion recognition: Lamellar hydrogel materials for the release of cationic cargo

A thermoresponsive supramolecular polymer gel from a heteroditopic calix[4]pyrrole.

Supramolecular polymerization of calix[4]pyrroles bearing tethered carboxylate functional groups was reported in the form of their tetrabutylammonium salts. At high concentrations one of the calix[4]pyrroles with a shorter linker between the calixpyrrole core and the carboxylate unit was found to give a thermoresponsive supramolecular polymer gel.

Solvent‐ and Light‐Controlled Unidirectional Transit of a Nonsymmetric Molecular Axle Through a Nonsymmetric Molecular Wheel

The development of a pseudorotaxane motif capable of performing unidirectional threading and dethreading processes under control of external stimuli is particularly important for the construction of processive linear motors based on rotaxanes and, at least in principle, it discloses the possibility to access to rotary motors based on catenanes. Here, we report a strategy to obtain the solvent-controlled unidirectional transit of a molecular axle through a molecular wheel. It is based on the use of appropriately designed molecular components, the essential feature of which is their non-symmetric structure. Specifically they are an axle containing a central electron-acceptor 4,4'-bipyridinium core functionalized with a hexanol chain at one side, and a stilbene unit connected through a C6 chain at the other side, and a heteroditopic tris(phenylureido)-calix[6]arene wheel. In apolar solvents the axle threads into the wheel from its upper rim and with the end carrying the OH group, giving an oriented pseudorotaxane structure. After a stoppering reaction, which replaces the small hydroxy group with a bulky diphenylacetyl moiety, and replacement of the apolar solvent with a polar one, dethreading occurs through the slippage of the stilbene unit from the lower rim of the wheel, that is, in the same direction of the threading process. The essential role played by the stilbene unit to achieve the unidirectional transit of the axle through the wheel, and to tune the dethreading rate by light is also demonstrated.

Ion‐Pair Recognition by a Heteroditopic Triazole‐Containing Receptor

A new heteroditopic calix[4]diquinone triazole containing receptor capable of recognising both cations and anions through Lewis base and C-H hydrogen-bonding modes, respectively, of the triazole motif has been prepared. This ion-pair receptor cooperatively binds halide/monovalent-cation combinations in an aqueous mixture, with selectivity trends being established by (1)H NMR and UV/Vis spectroscopy. Cation binding by the calix[4]diquinone oxygen and triazole nitrogen donors enhances the strength of the halide complexation at the isophthalamide recognition site of the receptor. Conversely, anions bound in the receptor's isophthalamide cavity enhance cation recognition. (1)H NMR investigations in solution suggest that the receptor's triazole motifs are capable of coordinating simultaneously to both cation and anion guest species. Solid-state X-ray crystallographic structural analysis of a variety of receptor ion-pair adducts further demonstrates the dual cation-anion binding role of the triazole group.

Extending the family of heteroditopic calix[4]diquinone receptors for cooperative AND ion-pair recognition

New heteroditopic calix[4]diquinone receptors with different anion binding units and calix[4]diquinone scaffolds have been synthesised. Ion-pair binding studies using UV/visible and 1H NMR spectroscopies reveal that receptors 1, 2 and 4 are cooperative AND ion-pair receptors, which display little affinity for ‘free’ ions but enhanced binding of the ion-pairs NaCl, NH4Cl and KCl. The more preorganised receptor 2 binds the ion-pairs more weakly than receptor 1. Varying the nature of the calix[4]diquinone scaffold appears to have little effect on ion-pair binding, although the calix[4]diquinone framework is more conformationally flexible than the tert-butylcalix[4]diquinone one and can be synthesised using the milder oxidant chlorine dioxide.

Acid–base modulation of a versatile heteroditopic calix[6]arene based receptor

A new calix[6]crypturea (3) has been efficiently synthesized through a domino Staudinger/aza-Wittig reaction followed by a [1 + 1] macrocyclization step. In comparison to the previously reported tren-based calix[6]crypturea, this heteroditopic receptor 3 displays a more flexible and larger tris-ureido cap. Due to this structural alteration, 3 exhibits unique host-guest properties: (i) the protonation of its basic cap leads to a rigidification of the whole structure and, thus, allosteric control of the binding properties and selective guest switching processes are possible, (ii) its versatility is unprecedented in the literature since it can bind either neutral molecules, anions, primary/secondary ammonium ions, quaternary ammonium ions or contact ion pairs according to different modes of recognition and with a remarkable selectivity within each family of guest, (iii) cascade complexes even stable in a protic environment can be obtained. These remarkable features are nicely illustrated by the fact that, according to the nature of its counterion, an ammonium ion R(1)R(2)NH(2)(+) can be accommodated into the cavity either as an independent guest, as a contact ion-pair or as a cascade complex. All these results are reminiscent of biological receptors and validate the strategy that consists of designing receptors presenting a high flexibility that can be controlled by an external stimulus.

Molecular Dynamics Study of a Heteroditopic-Calix[4]diquinone-Assisted Transfer of KCl and Dopamine Through a Water−Chloroform Liquid−Liquid Interface

The ability of two heteroditopic calix[4]diquinone receptors to transport a KCl ion-pair and a dopamine zwitterion through a water-chloroform interface was investigated via molecular dynamics (MD) simulations. Gas-phase conformational analysis has been carried on KCl and dopamine receptor binding associations and the lowest energy structures found in both cases show that the recognition of KCl and dopamine zwitterion occurs through multiple and cooperative N-H...anion and O...cation bonding interactions, with the receptor adopting equivalent folded conformations stabilized by pi-stacking interactions. The unconstrained MD simulations performed on KCl and dopamine complexes inserted in either the chloroform or water phase revealed that receptors are preferentially located at the interface with the hydrophobic tert-butyl groups of the calix[4]diquinone moiety immersed in the chloroform bulk while the polar anion binding cavity is directed toward the water phase. When the KCl complex is placed in chloroform, the release of the ion-pair occurs only after the first contact with the water interface, being a nonsimultaneous event, with the chloride anion leaving the receptor before the potassium cation. The dopamine, via the -NH(3)(+) binding entity, remains bound to the receptor during the entire time of the MD simulation (10 ns). In contrast, when both complexes were inserted in the water bulk, the full release of KCl and dopamine are fast events. The potentials of mean force (PMFs), associated with the migration of the complexes from chloroform to water through the interface, were calculated from steered molecular dynamics (SMD) simulations. The PMFs for the free KCl and zwitterionic dopamine migrations were also obtained for comparison purposes. The transport of KCl from water to chloroform (the reverse path) mediated by the receptor has a free energy barrier estimated in 6.50 kcal mol(-1), which is 3.0 kcal mol(-1) smaller than that found for the free KCl. The transport of dopamine complex along the reverse path is characterized by downhill energy profile, with a small free energy barrier of 6.56 kcal mol(-1).

Monotopic and heteroditopic calix[4]arene receptors as hosts for pyridinium and viologen ion pairs: a solution and solid-state study

The binding efficiency of a series of monotopic () and heteroditopic () calix[4]arene-based receptors has been evaluated in chloroform solution toward N-methylpyridinium ion pairs using NMR and UV/vis spectroscopic techniques. These experiments provided evidence that, due to a positive cooperative effect, the presence of a phenylurea H-bond donor group on the upper rim of the calix[4]arene macrocycle increases the recognition abilities of the heteroditopic receptors by up to two orders of magnitude with respect to the monotopic receptors. The heteroditopic receptors are also able to form 2:1 host-guest inclusion complexes with dimethylviologen salts both in low polarity solvents and in the solid state. These complexes are stabilized through the cooperation of weak (CH-pi and cation-pi) and strong (hydrogen bonding) intramolecular interactions between the binding domains of the calix[4]arene host and the two ions of the guest ion pair.

Self-assembly of heteroditopic calix[4]arene capsules through ion-pair recognition

The chemical information stored in a rigid heteroditopic calix[4]arene receptor allows the formation in the solid state of an extended 1D superstructure of self-assembled capsules through ion-pair recognition processes of N-methylpyridinium chloride.

Cooperative AND Ion‐Pair Recognition by Heteroditopic Calix[4]diquinone Receptors

A new family of heteroditopic calix[4]diquinone receptors capable of the cooperative recognition of ion-pair species through a contact binding mechanism has been developed. The receptors bind contact ion pairs cooperatively, with an unprecedented AND recognition phenomenon being observed to operate in certain cases, in which receptors display no affinity for either of the individual "free" cation or anion, but bind the cation and anion ion-pair strongly. X-ray crystallographic, solution-state, and computational methods rationalize the observed recognition behavior of the receptors. It is shown that the contact ion-pair interaction occurs through a pi-stacking-mediated folding of the receptors such that the anion and cation binding sites are arranged in close proximity, while in the solid state an unusual ion-mediated receptor dimerization is observed. Molecular dynamics simulations are further used to explain the observed trends in the association constants of different ion-pair species and the mechanism of interaction.

Tuning the strength and selectivity of ion-pair recognition using heteroditopic calix[4]arene-based receptors

The syntheses, characterisation and ion binding properties of two novel heteroditopic calix[4]arene receptors are reported. These systems were found to demonstrate cooperative binding of certain ion-pairs, with the selectivity depending critically on the structure of the receptors. Furthermore, a macrocyclic effect for ion-pair recognition was observed to operate.

Cooperative AND Receptor for Ion‐Pairs.

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Cooperative AND receptor for ion-pairs

A novel heteroditopic calix[4]diquinone receptor capable of binding an anion and cation simultaneously in a cooperative fashion is shown only to recognise halide anions in the presence of a suitable cobound cationic guest species, and displays affinity for certain ion-pairs where no affinity for either of the free ions is observed.

Recognition of guests bearing donor and acceptor hydrogen bonding groups by heteroditopic calix[4]arene receptors

The synthesis of new hosts specifically designed for the recognition of neutral guests bearing donor–acceptor hydrogen bonding groups is described. These hosts are characterized by the presence of two distinct binding region in close proximity: the rigid π-donor cavity and the H-bond donor N-methylene-N′-phenylureido group inserted onto the upper rim of the calix[4]arene skeleton. The binding abilities of these receptors were investigated toward a series of neutral ditopic organic molecules in CDCl3 solution by 1H NMR spectroscopy. The results obtained show that rigidity of the calix[4]arene apolar cavity is the control element in determining efficiency. In fact, compared with the more rigid 2, host 10, where the rigidity of the cone structure is maintained by hydrogen bonding of the OH of the lower rim, a decrease of efficiency of almost one order of magnitude was observed. The cooperative effect of the two binding region of host 2 was verified with different classes of ditopic guests. Good efficiency in the recognition of urea derivatives and dimethylsulfoxide was achieved.

Potassium cation cooperative anion recognition by heteroditopic calix[4]arene bis(benzo-15-crown-5) receptor molecules

New upper-rim functionalised heteroditopic calix[4]arene receptors containing amide linked bis(benzo-15-crown-5) ether, bis(benzo-15-crown-5)-bis(ferrocene) and tetrakis(benzo-15-crown-5) groups have been prepared. 1H NMR titration studies reveal the bis(benzo-15-crown-5) potassium cation complexed calix[4]arene receptors enhance the strength of chloride, benzoate and dihydrogen phosphate binding in polar organic solvents via favourable preorganisation and electrostatic effects. Cyclic and square wave voltammetric investigations demonstrate that the bis(ferrocene) containing calix[4]arene receptor is capable of electrochemical anion recognition. In the presence of potassium cations the electrochemical response to benzoate anions was amplified significantly.

A novel self-assembled supramolecular architecture involving cation, anion and a calix[4]arene heteroditopic receptor

In the presence of potassium acetate the heteroditopic calix[4]arene receptor 1 , in the 1,3- alternate structure and bearing two pentafluorophenyl amide groups as anion binders and a crown-5 polyether for cation complexation, self-assembles in an unexpected 2:2:2 (calixarene:cation:anion) supramolecular structure, as shown by X-ray crystal structure.

Anion effects on the recognition of ion pairs by calix[4]arene-based heteroditopic receptors.

A novel heteroditopic receptor (5) based on a rigid calix[4]arene cavity bearing at the upper rim four arylsulfonamido binding sites has been synthesized. The binding abilities of this new host have been investigated in apolar solvents toward a series of tetramethylammonium salts (tosylate, chloride, acetate, trifluoroacetate, and picrate) and compared with those of monotopic and heteroditopic calix[4]arene-bis(crown-3)-based receptors 1 and 2 in order to evaluate the role of the anion on ion-pair recognition. While monotopic host 1 shows an efficiency order toward the different salts that increases when the anion is less interactive (Hofmeister trend), an opposite role of the counteranion on the recognition process was observed with host 5 (anti-Hofmeister trend). A more complex behavior is experienced by host 2, which shows a high and leveled efficiency for all the anions tested. The results obtained were explained on the basis of the different types of ion pairs present in the recognition process. Further information on the role of the anion were obtained by the "dual host" strategy utilizing the tri-n-butylthioureido derivative of tren 7, which forms a stable complex with chloride anion. The very high efficiency shown by these heteroditopic hosts opens new routes in supramolecular projects and is a very interesting tool in the molecular recognition of ion-pairs and its applications.