Synthesis and characterization of 2-(n-alkylamino)-1,4-napthoquinone: Molecular structures of ethyl and hexyl derivatives

• π–π Crystal structures of ethyl and hexyl derivatives of 2-(n-alkylamino)-1,4-aminonaphthoquinone are discussed.
• Molecules shows NH⋯O and CH⋯O hydrogen bonding interactions.
• Hexyl derivative shows π–π stacking interactions.
• 13C chemical shifts are assigned to all seven derivatives.

We would like to introduce seven analogues of 2-(n-alkylamino)-1,4-napthoquinone (where n-alkyl is methyl in LH-1, ethyl in LH-2, propyl in LH-3, butyl in LH-4, pentyl in LH-5, hexyl in LH-6 and heptyl in LH-7). All the said analogues have been successfully synthesized from 1,4-naphthoquinone and well characterised using different spectroscopic techniques. Furthermore, in order to understand the mechanistic aspects of formation of LH-1–LH-7 compounds we could propose the mechanism. The FT-IR spectroscopic analysis of LH-1–LH-7 compounds indicating that the presence of characteristic band of NH group at ∼3340 cm−1. This band could assigned to existence of intramolecular hydrogen bonding interactions. The 13C-NMR spectroscopic technique has been used to study structural feature of LH-1–LH-7 compounds via HSQC, COSY and DEPT experiments. Finally, the structural aspects of the LH-2–LH-6 compounds have been confirmed by single crystal X-ray diffraction studies. We could observed that LH-2 compound crystallises in monoclinic space group P21/c, whereas LH-6 crystallises in triclinic P-1 space group. The molecule of LH-2 and LH-6 compounds forms dimers via NH⋯O hydrogen bonding interaction while polymeric chains of dimers have been seen via CH⋯O hydrogen bonding. It is very interesting to note that the molecules of LH-6 possessing a π–π stacking interaction between C⋯N of the neighbouring chains.