Structural Changes of Di(2‐pyridyl) Ketone on Single and Twofold Protonation

Di(2-pyridyl) ketone, intricately twisted due to its two nNσ and one nOσ lone pairs, exhibits tremendous structural differences on single and double protonation to salts containing either the anion [B(C6H5)4]−, which cannot be protonated, or Cl−, which accepts two hydrogen bonds. In the monocation, generated in acetic acid with added sodium tetraphenylborate, the pyridine rings of di(2-pyridyl) ketone are planarized due to the formation of an intermolecular hydrogen bond. Aqueous hydrogen chloride produces a dication with both nitrogen centers protonated and the resulting di(2-pyridyl) ketone hydrate embedded in a complex hydrogen-bond network. Extensive quantum-chemical calculations based on the experimental structural data allow to rationalize the unexpected results.