Thermodynamic anion templating

Such complexes form a precursor to a full discussion of the vast and highly topical field of self-assembly (Chapter 10). We consider them here since they resemble structurally the types of compounds discussed in Section 4.7, but unlike metal-based anion receptors the simple thermodynamic equilibrium between host, anion and complex is not the only process occurring in solution. In fact multiple equilibria are occurring covering all possible combinations of interaction between anions, cations and ligands. These systems have the appeal that the formation of particular metal coordination complexes are thus subject to thermodynamic anion templating (cf. the thermodynamic template effect in macrocycle synthesis, Section 3.9.1) and vice versa. 

The linear helicate forms first as a kinetic product, upon heating for a few minutes at 170°C in ethylene glycol. Longer reaction times resulted in disassembly of the initial structure and the formation of the thermodynamic product consisting of a circular pentanuclear helicate. This structure is apparently templated by the chloride anion, which is bound strongly inside the cavity of the helical macrocycle. 

A rare example of electropolymerisation employed for the formation of an anion selective crosslinked polymer matrix was introduced by Kamata et al. [153,154]. Reductive coupling between trifunctional p-cyanopyridinium crosslinkers (Fig. 15) in water containing the required counteranion led to stable polymer networks which showed thermodynamic and kinetic anion selectivity. A size exclusion effect was observed whereby only the imprint halogen (and any smaller halogen anion) was electrochemically recognised. Diffusion of the counterion was reduced in the case of the same template anion MIP, but increased with the size of the templating anion. Experiments clearly demonstrating anion selectivity have yet to be carried out. 

Fluorescence Sensing of Anions, p. 566 Guanidium-Based Anion Receptors, p. (575 Halogen Bonding, p. 628 Macrocyclic Synthesis, p. 830 Molecular Squares, Boxes, and Cubes, p. 909 Naked Anion Effect, p. 939 Organometallic Anion Receptors, p. 1006 Rotaxanes and Pseudorotaxanes, p. 1194 Self-Assembly Definition and Kinetic and Thermodynamic Considerations, p. 1248 The Template Effect, p. 1493... 

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