Nature of Urea−Fluoride Interaction:  Incipient and Definitive Proton Transfer

1,3-bis(4-nitrophenyl)urea (1) interacts through hydrogen bonding with a variety of oxoanions in an MeCN solution to give bright yellow 1:1 complexes, whose stability decreases with the decreasing basicity of the anion (CH₃COO^- > C₆H₅COO^- > H₂PO₄^- > NO₂^- > HSO₄^- > NO₃^-). The [Bu₄N][1·CH₃COO] complex salt has been isolated as a crystalline solid and its molecular structure determined, showing the formation of a discrete adduct held together by two N−H···O hydrogen bonds of moderate strength. On the other hand, the F^- ion first establishes a hydrogen-bonding interaction with 1 to give the most stable 1:1 complex, and then on addition of a second equivalent, induces urea deprotonation, due to the formation of HF₂^-. The orange-red deprotonated urea solution uptakes carbon dioxide from air to give the tetrabutylammonium salt of the hydrogencarbonate H-bond complex, [Bu₄N][1·HCO₃], whose crystal and molecular structures have been determined.