Tetrazoles are potent anion recognition elements that emulate the disfavored anti conformations of carboxylic acids

We report here the first study of the protonated, neutral form of tetrazoles as anion binding functional groups. Our studies reveal them to be capable of binding anions with extremely high potency in polar solutions. In studying carboxylic acid-containing congeners, we find a remarkable discrepancy: a strictly analogous acid-containing host binds anions > or = 50,000-fold more weakly than the tetrazole under study. We can explain this functional difference by considering tetrazole tautomerization equilibria and carboxylic acid conformational preferences.