Calixarenes neutral molecules complexation

Complex Formation Involving Calixarenes 7.1 Solid State Complexes with Neutral Molecules... 

The search for artificial host mimics possessing specific weak interaction and complexation properties to ions and neutral molecules has inspired scientists to work with great effort during the past several years and much success can be reported. Different subareas of research related to this matter have developed (crown chemistry, cyclodextrins, calixarenes etc.). They are partly a subject of former volumes of this series (cf. Top. Curr. Chem. 98, 101,121,123,125,128,132, and 136). 

The earliest reported examples of a system binding cations and neutral molecules simultaneously employed traditional inacrocyclic host molecules, such as calixarenes 1, which are described more fully below, and cyclic polyethers. Incorporating pyridyl or Schiff base groups into crown ethers leads to macrocycles with two possible binding modes. "" in the complex 2,6-pyrido[27]crown-9.LiC104.urea, both the Li" cation and urea are encapsulated within the crown ether cavity with interactions between them. In contrast, the complex of a Schiff-base-derived crown ether 2 has the Ni immobilized at the Schiff-base binding sites in a square-planar geometry. 

The simultaneous binding of cations and neutral molecules can be achieved by different types of molecular hosts or assemblies. Traditional host molecules such as crown ethers and calixarenes can complex both a cation and neutral molecule within different receptor sites. Metallohosts, where a metal cation is an intrinsic part of the host assembly, can complex neutral molecules within... 

The chemical behavior of clays is also mimicked by the title complexes in two respects the presence of cation-exhange capabilities and the reduced water content in the ammonium salt [20]. Currently, other similarities between clays and the water-soluble calixarenes are being probed. In particular, the size- and polarity-selective cavity of the calix[4]arene sulfonates is finding utility in the separation of organic substances, cations, anions, and neutral molecules, from aqueous feed streams. 

Continuing our studies in the functionalization of calixarene macrocycles to build up new specific ligands for ions and neutral molecules we have synthesized three new carboxylic acid derivatives (3-5) of p.t-butyIcalix[4]arene (1), determined their structures and checked their complexing ability. 

Solid-state inclusion complexes of parent calixarenes with neutral molecules... 

Calixarenes, resorcinarenes, and their simple ether derivatives have an electron-rich, 7r-basic cavity, which is able to host cations and neutral molecules but not anions, which, therefore, do not form en Jo-cavity inclusion complexes with these receptors. However, direct TT-metallation of the exterior of the macrocycles provides a vacant cavity with altered electrostatic properties. Indeed, the ability of certain transition metal ions... 

It is hard to find a topic in supramolecular chemistry that has not been touched, even briefly, by calixarenes or related macrocycles. This justifies the fact that we were compelled to give only a short summary of the properties of classical calixarenes as molecular receptors for ions and neutral molecules or as scaffolds for the construction of multivalent ligands able to interact with biomacromolecules. We believe, indeed, that these two aspects are the most peculiar and remarkable features of calixarenes, on which a variety of other complex supramolecular functions can be built. The future will certainly be bright for this class of synthetic macrocycles as their molecular recognition and scaffolding properties are likely to be exploited in the classical ways and also in novel forms within the framework of the third developing phase of supramolecular chemistry, namely, constitutional dynamic chemistry and adaptive chemistry. ... 

R. Ungaro, A. Arduini, A. Casnati, et al., Complexation of ions and neutral molecules by functionalized calixarenes, in Computational Approaches in Supramolecular Chemistry, ed. G. Wipff, Kluwer Academic Publishers, Dordrecht, 1994, pp. 277-300. 

Ungaro, R. Arduini, A. Casnati, A. Ori, 0. Pochini, A. Ugozzoli, F. (1994) Complexation of Ions and Neutral Molecules by Functionalized Calixarenes in Computational Approaches in... 

With the aim of obtaining evidences for the inclusion of neutral molecules inside the apolar cavity of calixarenes we previously synthesized a tetracarboxylic acid derivative 23 of p-rm-butylcalix[4]arene in the fixed cone conformation [13a]. The alkali metal salts of this acid were water soluble up to 5 x 10 M, but they were unable to complex neutral molecules such as benzene, toluene, xylenes and methanol which were easily included in the apolar cavity of this class of macrocycles in the solid state showing a well defined stoichiometry [6]. 

Later Shinkai and coll, have reported several upper rim sulfonated calixarenes (e.g. 24), which have a much higher water solubility than 23 and are able to complex organic cations and neutral molecules in water solution [28]. Therefore, we synthesized a tetracarboxylic acid tetrasulfonate 25 in the fixed cone structure in order to compare its binding properties with those of the flexible tetrasulfonate 24, also studied by Atwood and coll. [33]. 

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