Anion‐Induced Urea Deprotonation

Abstract

The urea-based receptor 1 (1-(7-nitrobenzo[1,2,5]oxadiazol-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 (Bu₄N[L]) was isolated and its crystal and molecular structure determined.