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

1,3-bis(4-nitrophenyl)urea (<b>1</b>) 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 (CH<sub>3</sub>COO<sup>-</sup> > C<sub>6</sub>H<sub>5</sub>COO<sup>-</sup> > H<sub>2</sub>PO<sub>4</sub><sup>-</sup> > NO<sub>2</sub><sup>-</sup> > HSO<sub>4</sub><sup>-</sup> > NO<sub>3</sub><sup>-</sup>). The [Bu<sub>4</sub>N][<b>1</b>·CH<sub>3</sub>COO] 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<sup>-</sup> ion first establishes a hydrogen-bonding interaction with <b>1</b> to give the most stable 1:1 complex, and then on addition of a second equivalent, induces urea deprotonation, due to the formation of HF<sub>2</sub><sup>-</sup>. The orange-red deprotonated urea solution uptakes carbon dioxide from air to give the tetrabutylammonium salt of the hydrogencarbonate H-bond complex, [Bu<sub>4</sub>N][<b>1</b>·HCO<sub>3</sub>], whose crystal and molecular structures have been determined.