Anion Complexes with Calixarenes

For every metal cation there is a counter anion, the latter considered to be merely a passive passenger in most of the complexation phenomena discussed above. 

Anion complexing agents need not be restricted to those containing positively 

have been prepared. The first set of compounds binds anions in the order Cl Br CN, has only a small affinity for 1 and SCN, and has no affinity for F and H2P04. . The second set of compounds, possessing C3 symmetry and containing three urea or thiourea groups, shows selectivity for Br over Cl and a high affinity for tricarboxylate anions such as benzene-1,3,5-tricarboxylate — 87,000 when X = O 190,000 M when X = S), 

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Complexes of calixarenes with bipyridyl chromophores can be stabilized by the addition of anionic side arms, such as iminodiacetate units (85). Whilst the lanthanide complexes of ligands [L51]4- and [L52]4- are not soluble in water, their photophysical properties in... 

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... 

Contrary to derivative 3, calixarene 5 was found to create a 1 2 complex with chloride anion in the solid state. Both amidic functions introduced into the upper rim of calixarene operated independently of each other and held one chloride anion with a Cl... HN distance of approx. 2.37 A and two CH... Cl con-... 

The positive charge on the cobaltocenium ion makes it an excellent guest for water-soluble, anionic cahx[6]arene hosts. Specifically, the hexasulfonatocalix[6]arene (1 ), with its structure given in Scheme 3, was shown to bind two Cob" ions on the equivalent binding sites indicated by the arrows. Therefore, one can think of an aqueous solution containing cobaltocenium, and the two hosts, P-CD and calixarene as a redox-switchable system in which, initially, two Cob" ions form a 2 1 complex with... 

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