(Benzylideneamino)thioureas - Chromogenic interactions with anions and N-H deprotonation

A family of neutral N-(R-1-substituted-benzylideneamino)-N'(R-2-substituted-phenyl)thioureas (LH) were designed as anion receptors, and their interactions with anions in MeCN solution were investigated through spectrophotometric and H-1 NMR titration experiments. While oxo anions (e.g., CH3COO-, H2PO4-) form genuine H-bond complexes based on complementary N-H center dot center dot center dot O interactions with LH receptors, the fluoride ion undergoes a two-step interaction, involving (i) formation of the [LH center dot center dot center dot 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 R-2 -substituted phenyl ring, as indicated by 1H NMR spectroscopy. The log K values for the formation of the ILH center dot center dot center dot CH3COO](-) H-bond complexes vary over the 3.1-3.8 range and are scarcely affected by the natures of the R1 and R-2 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. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006.