Influence of H-bond competitors on the solvent-dependent structures of an octaurea-calix[4]tube.

A novel octaurea-calix[4]tube (UC4T) has been synthesized in three steps from the original Beer's p-tert-butylcalix[4]tube ionophore. In a polar solvent (DMSO-d6), UC4T rapidly interconverts between two identical conformations with C2v symmetry for the two calix[4]arene subunits. However, in a less polar solvent mixture (CDCl3/DMSO-d6, 98 : 2), UC4T adopts a highly distorted asymmetric structure, which hinders the formation of typical tetraurea calix[4]arene capsular assemblies. The complexation of potassium (or barium) cations inside the dioxyethylene ionophoric binding site of UC4T triggers a C2v to C4v symmetry rearrangement of the two calix[4]arene subunits. This rearrangement leads to the formation of a transient capsular dimeric species observed in solution upon the addition of KI or BaCl2·2H2O to a solution (CDCl3/DMSO-d6, 98 : 2) of the macrocycle. X-ray studies confirm UC4T's ability to adopt different asymmetric conformations, depending on its interactions with solvent molecules. Two distinct crystal forms (α and ß) of UC4T have been obtained, each displaying divergent calix[4]arene subunits with pinched-cone conformations. These conformations exhibit distinctive head-to-tail (α) or head-to-head/tail-to-tail (ß) orientations of the ureido groups, which are involved in hydrogen bonding with solvent molecules. Notably, the pseudo-capsular 1D supramolecular polymeric arrays observed in the ß form of UC4T assemble to create large parallel solvent channels.

Proton transfer mediated recognition of amines by ionizable macrocyclic receptors.

Ammonium ion/carboxylate ion pairing is a key interaction ubiquitous in biological systems, but amine recognition by ionizable molecular receptors, mediated by host-to-guest proton transfer, has too often been overlooked as a design element for molecular recognition. This survey will show that proton transfer mediated recognition is a powerful and versatile tool that can be made to work with different amines and diverse macrocyclic scaffolds, such as crown ethers, calixarenes or pillararenes. We will trace the history of this recognition motif since Cram's first report half a century ago down to the latest applications in supramolecular sensing, drug-delivery and materials science, highlighting along the way the impact of host-to-guest proton transfer on self-assembly and molecular recognition.

Self-Assembly of Discrete Porphyrin/Calix[4]tube Complexes Promoted by Potassium Ion Encapsulation.

The pivotal role played by potassium ions in the noncovalent synthesis of discrete porphyrin-calixarene nanostructures has been examined. The flattened-cone conformation adopted by the two cavities of octa-cationic calix[4]tube C4T was found to prevent the formation of complexes with well-defined stoichiometry between this novel water-soluble calixarene and the tetra-anionic phenylsulfonate porphyrin CuTPPS. Conversely, preorganization of C4T into a C4v-symmetrical scaffold, triggered by potassium ion encapsulation (C4T@K+), allowed us to carry out an efficient hierarchical self-assembly process leading to 2D and 3D nanostructures. The stepwise formation of discrete CuTPPS/C4T@K+ noncovalent assemblies, containing up to 33 molecular elements, was conveniently monitored by UV/vis spectroscopy by following the absorbance of the porphyrin Soret band.

Stimuli-Responsive Internally Ion-Paired Supramolecular Polymer Based on a Bis-pillar[5]arene Dicarboxylic Acid Monomer.

A novel bis-pillar[5]arene dicarboxylic acid self-assembles in the presence of 1,12-diaminododecane to yield overall neutral, internally ion-paired supramolecular polymers. Their aggregation, binding mode, and morphology can be tuned by external stimuli such as solvent polarity, concentration, and base treatment.

Long-Range Chiral Induction by a Fully Noncovalent Approach in Supramolecular Porphyrin-Calixarene Assemblies.

The hierarchical assembly, in aqueous solution, of a new multi-metalloporphyrin/calixarene aggregate has been accomplished. In this supramolecular system transfer of chirality, from the outermost components to the central porphyrin reporter, takes place as a result of favorable and fully noncovalent long-range electronic communication.

Antiadhesive and antibacterial properties of pillar[5]arene-based multilayers.

A new type of coating, based on carboxylato-pillar[5]arene/poly(allylamine hydrochloride) multilayer films, for the sustained release of antibiotics with in vitro antiadhesive and antimicrobial activity against Gram-positive and Gram-negative bacteria is described.

Recognition and optical sensing of amines by a quartz-bound 7-chloro-4-quinolylazopillar[5]arene monolayer.

Covalent bonding of 7-chloro-4-quinolylazo-octamethoxypillar[5]arene molecules to silylated quartz substrates readily produced a new chromogenic reusable pillararene-coated quartz slide, for the direct UV detection of "transparent" analytes in solution. This device provides an analyte-selective optical response towards linear (di)amines with a highly reproducible optical read-out.

Ring/Chain Morphology Control in Overall-Neutral, Internally Ion-Paired Supramolecular Polymers.

The self-assembly of internally ion-paired, neutral AA/BB-type supramolecular polymers composed of complementary di-ionizable homoditopic pairs of monomers is reported. Host-to-guest double-proton transfer mediates the recognition between bis-calix[5]arenedicarboxylic acids and α,ω-diaminoalkanes to yield cyclic, doughnut-shaped assemblies with morphologies (i.e., cyclic vs. linear) that can be controlled by means of external chemical stimuli. The behavior of these intriguing aggregates, both in solution and on surfaces, has been investigated by a combination of 1 H and DOSY NMR spectroscopy, light-scattering, and atomic force microscopy.

A water-soluble pillar[5]arene as a new carrier for an old drug.

The remarkable affinity of deca-carboxylatopillar[5]arene WP5 towards the aminoglycoside antibiotic, amikacin, in aqueous media is reported; in vitro studies on Gram-positive bacteria (Staphylococcus aureus) show that drug entrapment inside WP5 also takes place in the presence of the microrganisms, thus pointing to WP5 as an appealing carrier for amikacin targeted delivery.

Calix[5]arene Through-the-Annulus Threading of Dialkylammonium Guests Weakly Paired to the TFPB Anion.

Through-the-annulus threading of calix[5]arene penta-O-ethers by dialkylammonium cations coupled to the loosely coordinating superweak TFPB- anion has been achieved. 1H NMR titration data show that preorganization of the calix[5]arene scaffold leads to great thermodynamic stability of the pseudorotaxane complexes as well as to a favorable threading kinetic. Thus, calix[5]arene 1c, bearing tert-butyl groups at the wide rim, was threaded by all of the cations under study (with the exception of the dibenzylammonium 2b+) more tightly than the other derivatives under investigation (Ka's up to 2.02 ± 0.2 × 105 M-1) because of its preorganized cone conformation. According to DFT calculations, van der Waals interactions between the tert-butyl groups of 1c and the alkyl chain of the cationic axle are likely responsible for the remarkable stability observed. The threading of the calix[5]arene wheels with the asymmetric pentylbenzylammonium axle 2c+ led to the toposelective formation of the endo-pentylpseudorotaxane stereoisomer in agreement with the known "endo-alkyl rule". Owing to the steric hindrance of the axle phenyl group, the threading of the guest was seen to occur in a unidirectional fashion through the calixarene narrow rim.

Unique binding behaviour of water-soluble polycationic oxacalix[4]arene tweezers towards the paraquat dication.

The first water-soluble polycationic oxacalix[4]arene molecular tweezers able to recognize - under pH control - the paraquat dication as a result of a delicate balance between electrostatic repulsion, Coulombic shielding and attractive π-stacking interactions are reported.

Self-assembly of amphiphilic anionic calix[4]arenes and encapsulation of poorly soluble naproxen and flurbiprofen.

Supramolecular aggregates formed through the association of an amphiphilic tetra-O-butylsulfonate calix[4]arene 1 were investigated in aqueous solution by a combination of different techniques (NMR, DLS and AFM). The ability of the micellar aggregates of calixarene 1 to increase the solubility of poorly water-soluble drugs was studied.

Probing the inner space of salt-bridged calix[5]arene capsules.

A combined DOSY and XRD study indicates that a carboxylcalix[5]arene receptor is able to encapsulate α,ω-diamines of appropriate length by means of a proton-transfer-mediated recognition process followed by salt-bridge-assisted bis-endo-complexation.

[5,11,17,23-Tetra-tert-butyl-25,27-(3,6-dioxaoctan-1,8-di-oxy)-26,28-bis-(pyridin-2-ylmeth-oxy)calix[4]arene]sodium iodide-1,2,4,5-tetra-fluoro-3,6-diiodo-benzene-methanol (2/3/4).

The title compound, [Na(C62H76N2O6)]I·1.5C6F4I2·2CH3OH, is composed of five components: a calix[4]arene derivative (hereinafter C4), a sodium cation, an iodide anion, a 1,2,4,5-tetra-fluoro-3,6-diiodo-benzene (tFdIB) mol-ecule and a methanol mol-ecule in a 1:1:1:1.5:2 ratio. The complex shows several inter-esting features: (i) the polyoxygenated loop of C4 effectively chelates a sodium cation in the form of a distorted octahedron and separates it from the iodide counter-ion, the shortest Na(+)⋯I(-) distance being greater than 6.5 Å; (ii) the cavity of C4 is filled by a methanol mol-ecule; (iii) a second methanol mol-ecule is hydrogen-bonded to the N atom of a pyridinyl substituent pendant of C4 and halogen-bonded to the I atom of a tFdIB mol-ecule; (iv) the two I atoms of another tFdIB mol-ecule are halogen-bonded to two iodide anions, which act as monodentate halogen-bond acceptorss; (v) one of the two tFdIB molecules is located about a centre of inversion.

Selective amine recognition driven by host-guest proton transfer and salt bridge formation.

The stepwise synthesis of ionizable p-tert-butylcalix[5]arenes 1a·H and 1b·H, featuring a fixed cone cavity endowed with a carboxyl moiety at the narrow rim, is described. Single-crystal X-ray analyses have shown that in the solid state 1a·H and 1b·H adopt a cone-out conformation with the carboxylic OH group pointing in, toward the bottom of the aromatic cavity, as a result of a three- or two-center hydrogen-bonding pattern between the carboxyl group and the phenolic oxygen atom(s). The affinity of amines for calix[5]arene derivatives 1a·H and 1b·H was probed by (1)H NMR spectroscopy and single-crystal X-ray diffraction studies. These carboxylcalix[5]arenes are shown to selectively recognize linear primary amines--over branched, secondary, and tertiary amines--by a two-step process involving a proton transfer from the carboxyl to the amino group to provide the corresponding alkylammonium ion, followed by binding of the latter inside the cavity of the ionized calixarene. Proton transfer occurs only with linear primary amines, that is, when the best size and shape fit between host and substrate is achieved, while the other amines remain in their noncompeting unprotonated form. The role of the solvent in the ionization/complexation process is discussed. Structural studies on the n-BuNH(2) complexes with 1a·H and 1b·H provide evidence that binding of the in situ formed n-BuNH(3)(+) substrate to the cavity of the ionized macrocycle is ultimately secured, in the case of 1a·H, by the formation of an unprecedented salt-bridge interaction.

Induction of chirality in porphyrin-(bis)calixarene assemblies: a mixed covalent-non-covalent vs a fully non-covalent approach.

CD and UV-vis measurements were carried out to demonstrate chirality induction-promoted by chiral covalently linked groups and non-covalently bound Δ- and Λ-[Ru(phen)(3)](2+) enantiomers-in metallo-porphyrin-calixarene and metallo-porphyrin-bis-calixarene non-covalent assemblies, respectively.

Sequence, stoichiometry, and dimensionality control in porphyrin/bis-calix[4]arene self-assemblies in aqueous solution.

The use of a water-soluble octacationic bis-calix[4]arene with divergent cavities (BC(4)) as a templating agent for the assembly of a tetraanionic porphyrin (CuTPPS) has allowed the noncovalent synthesis of 2D or 3D multiporphyrin assemblies. Self-assembly of CuTPPS and BC(4) molecules proceeded under hierarchical control in a stepwise fashion to yield discrete and isolable supramolecular nanostructures containing up to 33 molecular elements (i.e., the CuTPPS/BC(4) 17:16 assembly, obtained in less than three hours). The formation of these species could be conveniently monitored by means of UV/Vis spectroscopy by following the absorbance of the Soret band at 412 nm. In particular, the attainment of the pivotal CuTPPS/BC(4) 5:4 species with a cruciform structure, as the key fork-point intermediate for the subsequent formation of the higher 2D and 3D assemblies, has been demonstrated by light-scattering studies and by an unequivocal synthesis of mixed-porphyrin/calixarene 5:4 species involving the use of two different types of metallated porphyrins, namely CuTPPS and MnTPPS. The remarkable stability of these assemblies permits a stepwise synthesis that makes it possible to choose the desired porphyrin sequence.