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]−.