Stabilization of Carboxylate Anion with a NH···O Hydrogen Bond: Facilitation of the Deprotonation of Carboxylic Acid by the Neighboring Amide NH Groups

The formation of the NH···O hydrogen bonds of carboxylic acids, 2,6-(t-BuCONH)2C6H3COOH (1) and 2-t-BuCONH-6-MeC6H3COOH (2), carboxylate, [NEt4][2,6-(t-BuCONH)2C6H3COO] (3), and a mixed complex, [N(n-Pr)4][H{2,6-(t-BuCONH)2C6H3(COO)}2] (4), were determined by X-ray structure analysis, 1H NMR, and IR spectroscopy, both in the solid state and in solution. The amide NH group forms weak intramolecular hydrogen bond between the NH and O=C group, and no NH···OH hydrogen bond is formed in the carboxylic acid state. Carboxylate anion 3 forms a strong, intramolecular, partially-covalent hydrogen bond between NH···O− (anion). The strength of the NH···O hydrogen bonds is also maintained in a solution having a low dielectric constant. The pKa values for 1 and 2, measured in a micellar solution, indicated that one of the important triggers for deprotonation is a direct interaction toward the oxygen atom of the OH group with the amide NHs.