Synthesis, structure and tautomerism of two benzothiazolyl azo derivatives of 2-naphthol: A crystallographic, NMR and computational study

The efficient synthesis, 1H and 13C NMR characterization, crystal and molecular structure, DFT calculations of 6-[(2-hydroxy-1-naphthyl)diazenyl]-2-methylbenzothiazole, as well as DFT calculations of 6-[(2-hydroxy-1-naphthyl)diazenyl]benzothiazole, are reported. Single-crystal X-ray diffraction data show that crystals of the azo dye derived from 2-methylbenzothiazole is principally composed of the hydrazone form. 2D NMR techniques (COSY, HMQC and HMBC) were used to assign 1H and 13C chemical shifts for each azo dye in DMSO and CHCl3 solution. Azo–hydrazone tautomerism of the dyes was investigated jointly by 13C NMR and DFT calculations. All three approaches show that in solution the equilibrium is only slightly shifted towards the hydrazone form. The very good agreement between the three differently obtained equilibrium constants suggest M06-2X/6-311+G(d,p)-SMD as the level of theory of choice for quantum-mechanical calculations of these and similar compounds in solution.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights▶ Two benzothiazolyl azo derivatives of 2-naphthol were prepared and fully characterized. ▶ In the crystalline phase azo dyes are dominantly present in the hydrazone form. ▶ Azo–hydrazone tautomerism was studied in solution. ▶ Equilibrium constants were estimated by NMR, DFT calculations and their combination. ▶ In the solution, the tautomeric equilibrium is slightly shifted towards the hydrazone form.