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

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 CH⋯O, NH⋯O and OH⋯O between cations and anions. Polycrystalline powder X-band EPR spectra of Lw-1 to Lw-5 shows signals ∼2.004 at 133 K, 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.

► Naphthosemiquinone radicals of 2-hydroxy-1,4-naphthoquinone is isolated in solid state and characterized.
► Reduction takes place in presence of sodium metal and in basic solution of alkali metal salts.
► Lawsone anion obtained in tris buffer shows extensive hydrogen bonding network between cations, cation–anion and anions.
► Coordination of lawsone anion to sodium and potassium is evidenced.
► The radical signatures were confirmed by EPR spectroscopy.