Anion-Induced Urea Deprotonation

The urea-based receptor 1 (1-(7-nitrobenzo[1,2,5]oxadiazol-4-yl)- 3-(4-nitrophenyl)urea, LH), interacts with X- ions in MeCN, according to two consecutive steps: 1) formation of a hydrogen-bond complex [L-H···X]-; 2) deprotonation of LH 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 LH and excess Bu4NF, the tetrabutylammonium salt of the deprotonated urea derivative (Bu4N[L]) was isolated and its crystal and molecular structure determined.