Polyammonium-based anion receptors

The work of Park and Simmons [92] has inspired many efforts into the research of ammonium- and polyammonium-based anion receptors and are the subject of numerous reviews [93,94] Here we will only look at receptors containing quaternary ammonium centers. These groups bind anions via electrostatic interactions only. 

The pH dependence of many anions may cause problems in designing an appropriate host. For example, a receptor based upon polyammonium moieties may not be sufficiently protonated in the pH region where the anion is present in the desired form (vide infra). 

Many anions exist only in a relatively narrow pH window, which can cause problems especially in the case of receptors based upon polyammonium salts where the host may not be fully protonated in the pH region in which the anion is present in the desired form. 

Unfortunately, polyammonium hosts are limited by the pH range over which they are protonated. This pH range is the same as that at which anions (such as phosphate and carboxylate) also begin to protonate. Consequently, the utility of this class of receptor was limited, thus causing the development of guanidin-ium based hosts. 

---