Thermal and spectral properties of alkali metal complexes of 2-hydroxy-1,4-naphthoquinone

Thermal and spectral studies of reactions between 2-hydroxy-1,4-naphthoquinone (lawsone) and sodium metal (Lw-1, Lw-1A, Lw-1B), CH₃COONa (Lw-2), NaOH (Lw-3), KOH (Lw-4), K₂CO₃ (Lw-5), Tris buffer (Lw-6), ammonia (Lw-7) are studied. Red solids of Lw-1 to Lw-7, Lw-1A, and Lw-1B were isolated and are characterized by elemental analysis, FT-IR, ¹H NMR and UV–Visible spectroscopy. FT-IR spectra of Lw-1A and Lw-1B show, ν _OH of adsorbed as well as coordinated water molecules between 3,600–3,100 cm^?1 and decrease in ν _C=O frequency of lawsone ligand. The benzenoid ring protons C(5)H, C(8)H, C(6)H and C(7)H in Lw-1A and Lw-1B show upfield shift in ¹H NMR spectra. Hypsochromic shift and bathochromic shift is observed to π–π* transition band (~329 nm) and charge transfer band (~455 nm), respectively in UV–Visible spectra of all compounds. Pyrolytic decomposition of all compounds is studied by nonisothermal TG studies in air. Step I in all compounds leads to loss of adsorbed water molecules. Decomposition of lawsone anion in all compounds occurs in two or more steps. Thermodynamically Lw-1 to Lw-7, Lw-1A and Lw-1B are different compounds and their decomposition mechanisms are varied. The respective metal oxide residue viz. (Na₂O or K₂O) obtained after complete decomposition of Lw-1 to Lw-5, Lw-1A, and Lw-1B, is analyzed by powder X-ray diffraction. The adsorbed as well as coordinated water molecules are revealed by DTA and DSC studies as endothermic peak at ~100 °C. Decomposition mechanisms for lawsone anion are proposed based on LC–MS, GC–MS, and TG studies. Thermal and spectral studies reveal the coordination of lawsone ligand in its naphthosemiquinone form with alkali metal ions.