Single crystal X-ray structure of Lawsone anion: Evidence for coordination of alkali metal ions and formation of naphthosemiquinone radical in basic media

Abstract: 2-hydroxy-1,4-naphthoquinone; Lawsone (Lw) is a natural compound found in henna leaves. The reaction of lawsone with ‘Na’ metal (Lw-1), CH3COONa (Lw-2), NaOH (Lw-3), KOH (Lw-4), K2CO3 (Lw-5) and Tris(hydroxymethyl)aminomethane (Lw-6) were studied. Red orange solids obtained for Lw-1 to Lw-6 are characterized by Elemental Analysis, FTIR, 1HNMR and EPR studies. The results reveal the coordination of alkali metals ‘Na’ and ‘K’ to lawsone anion. The single crystal X-ray structure of Lw-6 was solved and it crystallizes in triclinic space group P-1 with extensive hydrogen bonding network of CO, NO and OO between cations and anions. Polycrystalline powder X-band EPR spectra of Lw-1 to Lw-5 shows signals ∼2.004 at 133K, while Lw-6 is EPR silent. The naphthosemiquinone (NSQ − ) radical formed in Lw-2 to Lw-5, is due to disproportion reaction of catechol and naphthoquinone. [Copyright &y& Elsevier]