Host-guest complexes calixarenes. Charged

Owing to the unique host-guest properties of calixarene macrocycles, calixarene-functionalized silica particles were used as cation-selective receptor colloids [81], and separator of lanthanides and actinides [82]. Calixarenes were also used for the syntheses of Ru, Pt and Pd NPs [83-85]. Furthermore, photoluminescence and charge-transfer complexes of calixarenes and calix[4]arene-based glycoclusters were grafted onto the surface of Ti02 NPs [64, 86]. 

The relatively low thermodynamic stability of complexes of hemicarcerands or other container-type hosts is a direct consequence of structural aspects of the walls that make up the inner surface of such compounds. These walls are lined by aromatic subunits while free electron pairs of heteroatoms such as those of the ether oxygen atoms are preferentially oriented to the outside. Complexes are therefore enthalpically stabilized only by weak dispersive interactions. In the case of positively charged guests cation-re interactions can contribute to binding enthalpy as in a self-assembled calixarene-derived capsule [9], but directed interactions such as hydrogen-bonding interactions are usually absent. 

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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