Host design anion hosts

Recently considerable attention has been directed at anion binding ligands. Macrobicyclic 27 29) and macrotricyclic amines 30,31) were topologically designed to host anions such as spherical Cl-, linear Nf 32). These anion substrates are incorporated into macrocyclic cavities lined with appropriate anion-binding sites capable of forming hydrogen bonds like those of protonated amines (see /, below). 

List similarities and differences in the design of selective supramolecular hosts for anions and cations. What are the key considerations in each case Build on these ideas by suggesting reasons why the supramolecular chemistry of anions has developed more slowly than that of cations. 

One important reason for the mediocre performance of many artificial hosts may originate from the ambiguity of the binding modes. If we take host-guest binding of anions in water as an example, the blueprint of host design almost always relies on the creation of polar interactions which should attract the guest... 

The issues relevant in the construction of anion hosts cannot be grouped in a hierarchical order as there is an intimate interplay between all of them. Successful design mandates a balanced compromise in weighting the individual influences. Advice can and should be sought from pertinent calculations (molecular modelling/dynamics), from the successes and the very few examples of failure accumulated in the literature and collected in recent reviews [21-24], from inspiration provided by abundant biological examples and crystal structures, and not least from personal experience in the handling... 

Hay BP, Firman TK, Moyer BA (2005) Structural design criteria for anion hosts strategies for achieving anion shape recognition through the complementary placement of urea donor groups. J Am Chem Soc 127 1810-1819... 

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