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The urea-based receptor 1 (1-(7-nitrobenzo1,2,5oxadiazol-4-yl)-3-(4-nitrophenyl)urea, L--H), interacts with X- ions in MeCN, according to two consecutive steps: 1) formation of a hydrogen-bond complex L--H...X-; 2) deprotonation of L--H to give L- and HX2-, as shown by spectrophotometric and 1H NMR titration experiments. Step 2) takes place with more basic anions (fluoride, carboxylates, dihydrogenphosphate), while less basic anions (Cl-, NO2-, NO3-) do not induce proton transfer. On crystallisation from a solution containing L--H and excess Bu4NF, the tetrabutylammonium salt of the deprotonated urea derivative (Bu4NL) was isolated and its crystal and molecular structure determined.
Boiocchi et al. (Sat,) studied this question.