Calixarenes 1.3- alternate conformation

When these crowned, ionizable calixarenes contain no t-Bu groups in the 4-position (MCI1, MCI2, MCI3, MCI4, and MCI5), the increased molecular flexibility causes the extraction to shift to higher pH (cone conformer), the selectivity for Ba2+ to disappear (partial cone conformer) or to be less pronounced (1,3-alternate conformer).141... 

Calixarenes are potential platforms on which specific binding arms can be grafted. The extractive properties of these molecules for metallic ions depend on the cavity size, the conformation, and the nature of the ligating groups. Different calix[4] arene-crown-6 derivatives in the 1,3-alternate conformation have been studied for Cs recovery from both basic and acidic solutions (257-262). Calixarene-based picol-inamide ligands have been proposed as candidates for separating actinides from lanthanides (263, 264). 

The related monodentate ligands 5.33 and 5.34 form MT4 type complexes with the relatively inert platinum(II). The complex [Pt(5.33)4]2+ adopts a 1,2-alternate conformation (by analogy with the calixarenes, Section 3.14) giving two anion-binding pockets that can bind planar anions such as nitrate and acetate in a 1 2 ratio. Acetate displays a positive allosteric effect with Kn being more than twice Ku. Tetrahedral anions are bound in a 1 1 ratio, however, and it is likely the complex can adopt a variety of conformations in solution. Related to [Pt(5.33)4]2+ is [Pt(5.34)4]2+. Like the complex with 5.33, X-ray diffraction results show that the chloride complex adopts a 1,2-alternate conformation with a... 

The structures and abbreviations used for designating the calix[4]arenes are shown in (5). These calix[4]arenes, have both a wide (upper) and a narrow (lower) rim that can be chemically modified to produce complexants that are selective for particular metal ions. In the simple calixarene framework the wide rim has hydrocarbon functionalities, and the narrow rim phenolic groups. Calixarenes are conformationally mobile, and the extreme structures for the calix[4]arenes have been termed the cone, partial cone, 1,3-alternate, and 1,2-alternate conformations (6). Because of the conical geometry of the calix[4]arene structure, the cavity size of the wide rim is larger than that of the narrow rim. 

Reaction of the tetra-/-butyl calixarene 103 with PCI5 gave rise to an unusual calixarene 104 containing 4-, 5- and 6-coordinate phosphorus. The isolated molecule, which was characterised by P nmr and X-ray crystallography, adopted a non-standard geometry between partial cone and 1,2-alternate conformations. ... 

Numerous examples of 1,3-alternate conformers of calix[4]arenes are known, but relatively few have been established by X-ray crystallography. In addition to earlier examples, more recent ones include the tetra-O-substituted 92a and 92b as a K complex) (see Figure 4.9), the doubly-bridged 92c,the bis-calixarene 93, and the calix[4]arene 40 containing four extraannular OH groups. 

The conformation of the calixarene derivative also plays a role, and thus, the 1,3-alternate conformation of calix[4]arene can interact with other metals such as silver. 

Another family of calixarene-based ditopic receptors are the l,3-calix[4]arene-Z w(crown ethers), where the calixarene is locked into the 1,3-alternate conformation, and substrate binding normally occurs at the crown ether loops. These may form mononuclear or dinuclear complexes with alkali inetal cations, or may bind neutral guest molecules. The unsymmetrical 1.3-alternate calix[4]ar-ene-l,3-crown-6 2,4-(l,2-phenylene)-crown-6 forms a 1 1 complex with Cs. The crystal structure shows that the cation is bound only within the adapted phenylene-crown-6 loop, and a molecule of acetonitrile is bound within the unmodified crown-ether loop. ... 

The crystal structure of the calix[4Jindole 26a was elucidated and it was found that the molecule adopts a 1,3-alternate conformation in the solid state forming a "square" channel (Figure 29) that is analogous to what is seen in the 1,3-altemate conformation of calixarenes. ... 

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