On the structures of free-base lepidine and some mineral acid salts

Abstract

The crystal and molecular structures of free-base lepidine (4-methylquinoline, 1) and the mineral acid salts lepidinium chloride (2), nitrate (3) and dihydrogen phosphate (4) are reported. Compound 1 was studied by differential scanning calorimetry and in situ capillary cryocrystallography. In the crystal structures, the molecules of free-base lepidine in 1 and the lepidinium cations in 2–4 are face-to-face π⋅⋅⋅π-stacked. The quinoline nitrogen atom is not involved in directional intermolecular interactions such as weak hydrogen bonds in 1. In the salts 2–4, proton transfer to the quinoline nitrogen atom and N−H⋅⋅⋅Cl (in 2) and N−H⋅⋅⋅O (in 3 and 4) hydrogen bonding is observed. Hirshfeld surface analysis reveals that lepidinium salts 2 and 3 form hydrogen-bonded dimers in the crystal and confirms that the packing of 4 is dominated by hydrogen-bonded dihydrogen phosphate anion chains.