Cation-binding hosts calixarenes

As with cation binding hosts, it is also possible to use calixarene frameworks (Section 3.14) to organise anion binding moieties. Addition of a cobaltocinium bis(amide) moiety to the upper rim of a calix[4]arene with p-toluenesulfonate functionalities to enhance solubility results in a rigid host 4.64... 

Long Reviews (a) Pochini, A. Ungaro, R. Calixarenes and Related Hosts, in Comprehensive Supramolecular Chemistry Vogtle, F., Ed. Pergamon Press Oxford 1996, Vol. 2, pp. 103-142 (b) McKervey, M. A. Schwing-Weill, M.-J. Arnaud-Neu, F. Cation Binding by Calixarenes , in... 

The hemispherands, spherands, calixarenes, and related derivatives. A number of hosts for which the pre-organization criterion is half met (the hemispherands) (Cram et al., 1982) or fully met (the spherands) (Cram, Kaneda, Helgeson Lein, 1979) have been synthesized. An example of each of these is given by (251) and (252), respectively. In (251), the three methoxyl groups are conformationally constrained whereas the remaining ether donors are not fixed but can either point in or out of the ring. This system binds well to alkali metal ions such as sodium and potassium as well as to alkylammonium ions. The crystal structure of the 1 1 adduct with the f-butyl ammonium cation indicates that two linear +N-H - 0... 

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 synthesis and nomenclature of calixarenes such as 3.118 has already been in introduced (Section 3.14 - For an introduction to calixarenes and the history of their development see Box 3.4) in the context of their widespread applications as hosts for cations, generally binding to metals through... 

For binding to occur, a host must possess suitable binding sites with favourable electronic properties such as polarity and hydrogen bond donor/acceptor abilities that complement those of the guest. Cyclophanes such as I, calixarenes II, homocalixarenes IH, and resor-cinarenes FV fulfil these requirements (Scheme 12.1). Cucurbiturils V are water soluble container molecules without aromatic subunits. Therefore, both electrostatic interaction other than cation- r interaction and hydrophobic interaction play a dominant role. 

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