Urea vs. thiourea in anion recognition

Neutral anion receptors (LH) form stable 1 : 1 H-bond LH...X]- complexes with carboxylates, halides and phosphate (X-). Some of the LH...X]- complexes, in presence of an excess of X-, release an HX fragment, with formation of HX2]- and the deprotonated receptor L-. The tendency towards deprotonation increases with the acidity of the receptor and with the stability of the HX2]- self-complex. Thus, the more acidic thiourea containing receptor deprotonates in the presence all the investigated anions except chloride, whereas the less acidic urea containing receptor undergoes deprotonation only in the presence of fluoride, due to the high stability of HF2]-.