Faculté des sciences et de médecine

Tautomerism in azo dyes: Border cases of azo and hydrazo tautomers as possible NMR reference compounds

Deneva, Vera ; Lyčka, Antonin ; Hristova, S. ; Crochet, Aurélien ; Fromm, Katharina M. ; Antonov, Liudmil M.

In: Dyes and Pigments, 2019, vol. 165, p. 157–163

In order to meet the need for NMR reference compounds in the study of tautomeric azo dyes, two series of azo dyes, derived from 3-methyl-1-phenyl-4- (phenyldiazenyl)-1H-pyrazol-5-amine and 5-methyl-2-phenyl-4-(2- phenylhydrazono)-2,4-dihydro-3H-pyrazol-3-one, have been studied by using molecular spectroscopy (UV–Vis and NMR) and quantum-chemical calculations (M06- 2X/TZVP) in solution. The... More

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    Summary
    In order to meet the need for NMR reference compounds in the study of tautomeric azo dyes, two series of azo dyes, derived from 3-methyl-1-phenyl-4- (phenyldiazenyl)-1H-pyrazol-5-amine and 5-methyl-2-phenyl-4-(2- phenylhydrazono)-2,4-dihydro-3H-pyrazol-3-one, have been studied by using molecular spectroscopy (UV–Vis and NMR) and quantum-chemical calculations (M06- 2X/TZVP) in solution. The results clearly indicate that 3-methyl-1-phenyl-4- (phenyldiazenyl)-1H-pyrazol-5-amines are present in pure azo tautomeric form, while 5-methyl-2-phenyl-4-(2-phenylhydrazono)-2,4-dihydro-3H-pyrazol-3-ones exist as hydrazone tautomers. The tautomeric state is not substantially influenced by nitro group substitution in the phenyl ring. Consequently, the studied compounds can be used as model tautomers in the NMR evaluation of the tautomeric state in various azo dyes in solution. According to the crystallographic data (obtained by us or available in the literature) the conclusions about the tautomerism of the studied compounds in solution are valid in solid state as well.