Zouhair Asfari

Binding of acetylcholine and other quaternary ammonium cations by sulfonated calixarenes. Crystal structure of a [choline-tetrasulfonated calix[4]arene] complex

Abstract The water-soluble tetra- and hexasulfonated calix[4] and calix[6]arenes bind very strongly the neurotransmitter acetylcholine and other quaternary ammonium cations with association constants Ks up to 4×105 M−1 for NEt4 +. The high affinities for choline and acetylcholine (Ks=5×104 to 8×104 M−1) are comparable to those of the biological recognition sites. The crystal structure of the [choline-tetrasulfonated calix[4]arene] complex was determined. Two complexes A and B were observed in the asymmetric unit. In each one, the choline inserts its N-terminal inside the cavity of the receptor. The calix[4]arene adopts a single cone conformation but the choline substrate is in an extended conformation in complex A and adopts two other folded conformations in complex B (75 % –25 %). The very high association constants and the crystal structure provide important information about the nature of the binding in this biologically most relevant complex.

An anthracene-based fluorescent sensor for transition metal ions derived from calix[4]arene

The preparation of anthracene-based fluorescent calix[4]arene 1 combining calix[4]arene element and 9-anthryl unit is reported. The presence of a dioxotetraaza unit allows the chelation of transition metal cations such as zinc and nickel.

Bis(crown ether) and Azobenzocrown Derivatives of Calix[4]arene. A Review of Structural Information from Crystallographic and Modelling Studies

The association within one molecule ofcalix[4]arene and crown ether moieties leads toligands with new complexing properties. In particular,calix[4]arene bis(crown-6) and some of itsderivatives have been shown to be highly selectiveextractants for caesium ions. This review presents thebackground of the study and the results of crystalstructure determinations and molecular modellingcalculations performed during the investigation of twomolecular families, the bis(crown ether) and theazobenzocrown derivatives of calix[4]arene.

Synthesis of tripodal aza crown ether calix[4]arenes and their supramolecular chemistry with transition-, alkali metal ions and anions

Abstract Tripodal aza crown ether calix[4]arenes, 5a , 5b , 6a and 6b , have been synthesized. The structure of protonated 5a was elucidated by X-ray crystallography to be a self-threaded rotaxane. Complexation studies of 5a and 5b towards anions using Na+ as countercation were carried out by 1H NMR titration in dimethylsulfoxide-d6 and the mixture of chloroform-d and methanol-d4, respectively. Ligands 5a and 5b were able to form 1:1 complexes with Br−, I− and NO3− and the complexation stability varied as follows: NO3−>I−>Br−. The effect of countercation on anion complexation was also investigated. The results showed that the association constants of 5a towards Br− in the presence of various cations varied as K+>Bu4N+>Na+. The enhancement in anion complexation ability of 5a may result from the rearrangement of the tripodal ammonium unit in the presence of K+. The neutral forms, 6a and 6b , were able to form complexes with transition metal ions such as Co2+, Ni2+, Cu2+ and Zn2+. The stability of the complexes followed the sequence: Ni2+ 6a and 6b may, therefore, potentially be used as either transition metal ion or anion receptors that can be controlled by pH of the solution.

Calixcrowns and related molecules

The synthesis of 1,2- and 1,3-calix(4)-bzs-crowns, double calix(4)arenes and double-calix-crowns have been shown to depend on the reaction conditions (nature of the base, structure of the ditosylates, and the stoichiometry of the reactants). The 1,3-alternate conformation of the 1,3-calix(4)-bis-crowns was shown to be favourable to the selective complexation of cesium cation. The observed Na+/Cs+ selectivity was exploited in separation processes using them as carriers in transport throught supported liquid membranes (SLMs). The best Na+/Cs+ selectivity (1/49000) was observed for the naphtyl derivative 2. Calix(aza)crowns and 1,3-~alix(4)-bZs-(aza)-crowns were also produced throught the preliminary formation of the Schiff-base-calixarenes which were hrther hydrogenated. The syntheses consisted or via the 1,3-selective alkylation of calixarenes followed by cyclization into a 1,3-bridge either the direct 1,3-capping of the calixarene with appropriate ditosylates. Soft metal complexation by these ligands is also presented.

Cation binding properties of a novel 1,3-alternate calix[4]biscrown. Formation of 1:1 and 1:2 complexes and unique cation tunneling across a calix[4]arene cavity

Abstract A novel 1,3-alternate calix[4]biscrown was synthesized by the reaction of calix[4]arene and tetraethyleneglycol ditosylate in the presence of K 2 CO 3 . It formed a 1:2 complex as well as a 1:1 complex with alkali metal and ammonium cations and the cation tunneling phenomenon across the calix[4]arene cavity was detectable with 1 H NMR spectroscopy even at room temperature.

Simultaneous self-assembly of a [2]catenane, a trefoil knot, and a Solomon link from a simple pair of ligands.

A topological triptych: Three molecular links, a [2]catenane, a trefoil knot, and a Solomon link, were obtained in one pot through the self-assembly of two simple ligands in the presence of Zn(II). The approach relied on dynamic covalent chemistry and metal templation.

Synthesis and complexing properties of a double-calix[4]arene crown ether

Treatment ofp-tert-butylcalix[4]arene with tetraethyleneglycol ditosylate in the presence of potassium carbonate in acetonitrile afforded the first example of a double-calixcrown (1) in 52% yield. The double-calixcrown (1) was shown to form 1: 1 complexes with potassium and rubidium picrates in the solid state and in solution in chloroform. The stability constants of the alkali complexes have been determined in acetonitrile, as well as the phase transfer parameters from water to dichloromethane.

Calixarenes as Polyhapto-aromatic Ligands: Alkali Metal Ions and Sulfonated Calixarenes

Crystal structure determinations for the hexarubidium and octacesium derivatives of calix[6]arene hexasulfonic acid, as well as of a new solvate of the pentasodium derivative of calix[4]arene tetrasulfonic acid, and comparison with structural data from the literature for alkali metal complexes of sulfonated calixarenes, have provided further evidence for the importance of interactions with aromatic ~ -electrons for the heavier alkali metal cations. There is evidence as well that this interaction can be enhanced by extended ionization of the sulfonated calixarene.

The synthesis of double-calixarenes

Abstract Macrocyclic molecules consisting of two calix[4]arene moieties linked via their p-positions by one 1, two 2 or four 3 aliphatic chains were synthetized for the first time.