Caged oxoanions.
The association between azacryptand hosts and oxoanion guests is reviewed. Positively charged hosts are the most effective; we focus on protonated azacryptands. Assessment of quantitative data suggests an anion cryptate effect and provides clear evidence for charge-based selectivity. Crystal structures show both cavity and cleft binding sites for anions within the series of cryptands studied. These two binding modes exhibit different pH dependence offering the possibility for design of monitoring/clean-up strategies based on a variation of appropriate host(s) and pH conditions.