X-ray structure and AM1 studies of the proton-transfer adduct between 2,5-dihydroxy-p-quinone and 4-(N,N-dimethylamino)pyridine

The structure of the crystalline 1:1 adduct obtained by reacting 2,5-dihydroxy-1-quinone (DHpQ) and 4-(N,N-dimethylamino)pyridine (DAPY) has been determined by X-ray single-crystal diffraction. The compound consists of ribbons of the two components linked by proton-transfer bonds. Each bond is due to a proton of the 4-(N,N'-dimethylamino)pyridinium cation (r(N-H) = 1.13 Angstrom), bifurcated between two oxygen atoms of the monoprotonated anion. Antiparallel stacks of DHpQ-DAPY adducts are present in the solid. According to AM1 and molecular mechanics calculations, it has been shown that the interaction between stacked adducts is the prerequisite for the establishing of proton-transfer bonds (O-...HN+) between DHpQ and DAPY. It is suggested that electrostatic interactions such as the dipole-dipole coupling play a major role in establishing this type of hydrogen bond within each stack.