(Benzylideneamino)thioureas – Chromogenic Interactions with Anions andN–H Deprotonation

A family of neutral N-(R1-substituted-benzylideneamino)-N- (R2-substituted-phenyl)thioureas (LH) were designed as anion receptors, and their interactions with anions in MeCN solution were investigated through spectrophotometric and 1H NMR titration experiments. While oxo anions (e.g., CH3COO–, H2PO4 –) form genuine H-bond complexes based on complementary N–H···O interactions with LH receptors, the fluoride ion undergoes a two-step interaction, involving (i) formation of the [LH···F]– complex, and (ii) release of an HF molecule to give [HF2]– and the deprotonated form of the receptor (L–). Deprotonation takes place at the N–H fragment closer to the R2-substituted phenyl ring, as indicated by 1H NMR spectroscopy. The logK values for the formation of the [LH···CH3COO]– H-bond complexes vary over the 3.1–3.8 range and are scarcely affected by the natures of the R1 and R2 substituents. The investigated systems may be of interest in the design of molecular devices in which the optical properties of different and distant substituents are modulated through the interaction of a chosen anion at the thiourea site.