Calixarenes simultaneous binding

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... 

Appropriately strapped calixarenes have also been shown to complex anions and cations simultaneously. Such a system is the ditopic, calixarene-based receptor 11 (Scheme 9.4) reported by Tumcharern and coworkers [51[. Receptor 11, in which the strap incorporates two different binding motifs (urea and amide), showed selectivity for the tetrahedral phenylphosphinate anion (PPhH02) over simple Y-shaped anions, such as acetate. After initial screening studies, quantitative analyses were carried out using NMR spectroscopic methods. Specifically, titrations were carried out in CD3CN using the TBA salts of three promising anions, namely acetate, phenylphosphinate, and diphenylphosphate. Standard analyses of the... 

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. 

Simultaneous receptors exist that can bind neutral and ionic guests. These types of receptors are large macrocycles, such as calixarenes, in which the ion is bound to the functional groups at the lower rim of the calixarene and the neutral (often solvent) molecule is encapsulated within the cavity of fhe bowl, e.g. tetramethoxy-p-f-butylcalix[4]arene (2.116). In the solid state, an Na ion is coordinated to the oxygen atoms of the lower rim of fhe calix[4]arene and the complex contains one molecule of toluene within the hydrophobic cavity (Figure 2.27). 

Calixarenes can play a relevant role in molecular recognition not only as receptors but also as ligands. In this latter case, they mainly work as scaffolds, which suitably organize several identical substituents in space linked to the lower or the upper rim and simultaneously orient these moieties toward the surface areas of large macromolecules or toward the recognition points of multiple binding site receptors. In other words, they act as core for the construction of multivalent ligands (see Multivalency, Concepts). 

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