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]-.