Placido Neri

Pseudorotaxanes with self-sorted sequence and stereochemical orientation.

Partner preferences in pseudorotaxane formation were exploited to establish an integrative self-sorting system able to discriminate simultaneously at the sequence and stereochemical level (see picture). It was found that calix[6]arenes were threaded selectively with a preferred orientation onto bisammonium axles, even when the structural differences between the possible building blocks were small and located remote from the binding sites.

Fixed or Invertible Calixarene-Based Directional Shuttles

The first examples of rotaxanes based on calixarenes threaded by dialkylammonium ions, which also represent the first examples of calixarene-based molecular shuttles, are reported. The base/acid treatment demonstrated that these systems act as molecular shuttles, which move between three sites on the axle. When small OMe groups are appended at the calix[6]arene lower rim an unprecedented inversion of its shuttling direction is observed, which occurs through a cone-to-cone inversion of the macrocycle.

5,5′‐Bicalix[4]arene: The Bridgeless Prototype of Double Calix[4]arenes of the “Head‐to‐Head” Type

Two molecular bowls can be directly linked to give an open-and-shut capsule simply by heating p-H-calix[4]arene in CH3CN at reflux in the presence of FeCl3⋅6 H2O. Double calixarenes of this type could in principle be present as syn or anti conformers (shown schematically on the right). Whereas the anti form was observed in the solid state, there is presumably an equilibrium of the two forms in solution. The globular architecture of the syn conformation suggests the potential for cooperation between two cone binding sites in the endo-calix complexation of suitable guests.

Synthesis of glycidyl calixarenes, versatile substrates for the preparation of chiral calixarene-based ligands

Abstract The first calixarenes bearing chiral glycidyl groups at the lower rim have been obtained by treatment of p-tert -butylcalix[4]arene ( 1a ) and p-tert -butylcalix[6]arene ( 1b ) with glycidyl tosylate or 3-nitrobenzenesulfonate in the presence of a base. Their structure was established as syn -1,3-diglycidyl calix[4]arene 2 , tetraglycidyl calix[4]arene 3 (partial cone, 1,2-alternate and 1,3-alternate conformers), and 1,2,4,5-tetraglycidyl calix[6]arene 4 , respectively, by 1 H and 13 C NMR spectroscopy. Regio- and stereoselective epoxide ring-opening of 2 with amines led to chiral β-aminoalcohols ( 5 and 6 ).

Stereoprogrammed direct synthesis of calixarene-based [3]rotaxanes.

Directional calixarene wheels were threaded along a bis(benzylalkylammonium) axle in a stereoprogrammed way to obtain the first examples of calixarene-based [3]rotaxanes. The base/acid treatment demonstrated that these systems act as nanosized molecular shuttles. An unprecedented switching between the tail-to-tail and head-to-head relative orientation of the calix-wheels was observed.

Absolute configuration assignment of inherently chiral calix[4]arenes using DFT calculations of chiroptical properties.

The racemate of an inherently chiral meta-substituted calix[4]arene derivative 3 has been resolved via enantioselective HPLC. Measured optical rotation dispersion and electronic circular dichroism have been compared with DFT theoretical predictions. The comparison indicates that the absolute configuration of the dextrorotatory enantiomer (+)-3 is cS. The procedure has been successfully tested against a literature precedent confirming the validity of the method.

Methane Adsorption in a Supramolecular Organic Zeolite

Gas storage in solids is an increasingly important research area with applications in the field of energy, environment, biology and medicine. Because it concerns the adsorption of methane, the main component of natural gas fuels, the storage target for any solid to be effective in energy applications and compete with the compressed form is approximately 35 wt % or 180 v/v. Several different types of porous materials such as carbon materials, zeolites, silica gels, MOFs and polymers have been recently examined in addition to organic crystalline microporous solids and nonporous ones, but no ideal candidates have emerged so far. In this paper we wish to report on the methane adsorption properties of a new microporous organic zeolite based on 1,2-dimethoxy-p-tert-butylcalix[4]dihydroquinone (Scheme 1), which has been recently characterised by our research group. The crystalline solid has a cubic structure (a= 36.412(4) ) with networked channels and hydrophobic cages. The channels have minimum and maximum diameter of 3.9 and 8.5 , respectively, excluding van der Waals radii, and they are filled with easily removable water molecules. The hydrophobic cages show a gate diameter of 2.2 and can reach a maximum diameter of 11.2 (excluding van der Waals radii), and the estimated cage volume is about 1000 . Interestingly, the supramolecular framework is also preserved after the removal of channel water molecules. Previous studies showed that the BET surface area corresponds to 230 m g 1 (N2 at 77 K) and that carbon dioxide can be absorbed at room temperature inside its nanochannels with high selectivity with respect to H2 gas, [9] encouraging further studies towards methane adsorption. Methane adsorption measurements were performed by using a home-made volumetric system. Sorption isotherms have been carried out at 298, 278 and 203 K. Data have been collected at initial pressures between 0.5 and 3 atm. The adsorption isotherm at 203 K is reported in Figure 1, (see the Supporting Information for adsorption isotherms at 298 and 278 K). At 1 atm and 298 K our compound adsorbs 0.11 methane molecules for each calixarene molecule. For comparison, p-tert-butylcalix[4]arene adsorbs 0.14 methane molecules for each calixarene molecule under the same conditions. Under the mild conditions of ambient pressure and temperature our sample shows an efficiency for methane storage of 0.29 wt %, whereas in the case of CO2 molecules the value was 0.5 wt %. The adsorption isotherm at 203 K is not of type I. The adsorption step at 2 atm could be interpreted by considering that at first the methane molecules enter the channels, interacting with the most favourite sites, and then upon increasing the pressure the channels are progressively filled. Note that there are 90 methane molecules in the unit cell at 3 atm pressure, which approximately corresponds to a 1:2 ratio between calixarene molecules and methane molecules. [a] Dr. C. Tedesco, Dr. L. Erra, Dr. V. Cipolletti, Dr. C. Gaeta, Prof. P. Neri Department of Chemistry and Nano_MATES Research Center University of Salerno, 84084 Fisciano (Italy) Fax: (+39) 089-969603 E-mail : ctedesco@unisa.it neri@unisa.it [b] Dr. M. Brunelli, Prof. A. N. Fitch European Synchrotron Radiation Facility 38043 Grenoble (France) [c] Prof. J. L. Atwood Department of Chemistry University of Missouri-Columbia Columbia, Missouri 65211 (USA) [d] Dr. M. Brunelli Current address: ILL Institut Laue-Langevin BP 156, 38042 Grenoble Cedex 9 (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200903218. Scheme 1. 1,2-Dimethoxy-ptert-butylcalix[4]dihydroquinone.

Interconnected water channels and isolated hydrophobic cavities in a calixarene-based, nanoporous supramolecular architecture

An interesting interplay of H-bond and van der Waals-like interactions in a proximal calix[4]arene dihydroquinone derivative gives rise to a nanoporous supramolecular architecture with the simultaneous existence of water channels and very large hydrophobic cavities.

Conformational features and recognition properties of a conformationally blocked calix[7]arene derivative.

The shaping of a calix[7]arene macrocycle into cone-like structure 3, through exhaustive alkylation of doubly bridged calix[7]arene derivative 2 with bulky groups, has been investigated. Conformational details about the structure adopted by calix[7]arene derivative 3 in solution have been obtained by using chemical shift surface maps, as previously reported by our group. Thus, chemical shift contour plots indicated that 3 adopted a cone-shaped structure in solution analogous to that adopted by the known p-tert-butylcalix[7]arene heptacarboxylic acid derivative 4. Interestingly, the X-ray structure of derivative 3 showed a high degree of similarity to the theoretical structure, which confirmed the validity of the contour plots method. The preorganized calix[7]arene host 3 showed interesting recognition abilities toward both organic and alkali cations. In fact, an unprecedented endo-cavity complexation of linear and branched alkyl ammonium cations with a larger calix[7]arene host was evidenced. A comparable affinity for branched tBuNH(3)(+) and linear nBuNH(3)(+) guests was observed.

Resolution of inherently chiral calix[4]arenes with AABB and CDCD substitution patterns on the upper and lower rims, respectively

Abstract Proximal di- tert -butylcalix[4]arene (5,11-di- tert -butylcalix[4]arene-25,26,27,28-tetrol) 1b , obtained by direct partial removal of tert- butyl groups from p-tert- butylcalix[4]arene, gave high yields of inherently chiral derivatives upon ‘symmetry breaking’ by syn -distal di- O -alkylation or di- O -acylation in the presence of K 2 CO 3 . The chirality of these compounds was proven by the splitting of 1 H NMR signals in the presence of Pirkles reagent and in some cases by HPLC enantiomeric resolution using chiral stationary phases and corroborated by mirror-image CD spectra.