Synthesis, characterization and molecular structures of homologated analogs of 2-bromo-3-(n-alkylamino)-1,4-napthoquinone

Four analogues of 2-bromo-3-(n-alkylamino)-1,4-napthoquinone (where n-alkyl is methyl in L-1Br, ethyl in L-2Br, propyl in L-3Br and butyl in L-4Br) are synthesized and characterized. A reaction mechanism is proposed for the formation of L-1Br to L-4Br from the starting material 2,3-dibromo-1,4-naphthoquinone. The νNH frequency in the FT-IR spectra is affected by the intramolecular hydrogen bonding in L-1Br to L-4Br and is observed ∼3267 cm−1 in L-2Br. A shift of ∼25 cm−1 is observed in the νCBr frequency in all the compounds as compared to 2,3-dibromo-1,4-naphthoquinone (627 cm−1). A broad charge transfer band is observed between 400 and 600 nm in the UV–Vis spectra, which imparts red colour to all the compounds. Molecular structures of L-2Br and L-3Br were studied by single crystal X-ray diffraction studies. Molecules of L-2Br crystallize in Pca21, whereas the molecule L-3Br crystallizes in the P-1 space group. Molecules of L-2Br forms a polymeric chain through NH⋯O interaction and forms beautiful butterfly like arrangement of molecules when viewed down the ‘a’ axis. Ladder like polymeric chain of molecules is observed in L-3Br via CH⋯O and NH⋯O interactions. Every alternating neighbouring chains of L-3Br, show π–π stacking interactions between the quinonoid rings of the molecules, however this interaction is not observed in L-2Br.