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.