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

Abstract

A B S T R A C TIn 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-(2phenylhydrazono)-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 3methyl-1-phenyl-4-(phenyldiazenyl)-1H-pyrazol-5-amines are present in pure azo tautomeric form, while 5methyl-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.

Experimental and Computational Study of Novel Pyrazole Azo Dyes as Colored Materials for Light Color Paints

“…Crystallographic X-ray data of some pyrazole, triazole, and tetrazole azo dyes were reported in azo-hydrazo tautomerism studies [ 51 , 52 ]. Intramolecular hydrogen bonds are present in most of the dyes included in these studies.…”

“…Intramolecular hydrogen bonds are present in most of the dyes included in these studies. H-bond parameters of the new azo dyes were compared to those of the dyes reported in the above mentioned articles: the N–H ····· A, (A=O or N atoms) hydrogen bond distance values were between 2.62 Å and 2.73 Å in the case of the azo tautomers (for A=N) (and between 2.27 Å and 2.34 Å for the azo and 2.62 Å for hydrazo tautomers [ 51 , 52 ]), between 1.86 Å and 2.2 Å (for A=O) (and between 1.83 Å and 2.06 Å [ 51 , 52 ]) for the hydrazo tautomers. The donor-acceptor H-bond lengths (d N(H)…O for hydrazo and d (H)N…N for azo tautomers) had values between 2.35 Å and 2.43 Å (and between 2.37 Å and 2.87 Å [ 51 , 52 ]) for the azo, and between 2.58 Å and 2.87 Å (and between 2.55 Å and 2.77 Å [ 51 , 52 ]) for the hydrazo tautomers, respectively.…”

Tautomerism and Self-Association in the Solution of New Pinene-Bipyridine and Pinene-Phenanthroline Derivatives

“…The NMR chemical shieldings were calculated using the GIAO approximation [44] at the B3LYP/6-311+G(2d,p) level of theory. This level of theory was recommended in the pioneering work by Cheeseman et al [45] focused on the comparison of different models for calculating nuclear magnetic resonance shielding tensors and showed very good results in predicting the NMR spectra of azo-hydrazone tautomerism [46]. The calculated absolute shieldings were transformed to chemical shifts using the reference compound tetramethylsilane, Si(CH 3 ) 4 , for hydrogen: δ = δ calc (ref) − δ calc .…”

A detailed UV–Vis spectral investigation of six azo dyes derived from benzoic- and cinnamic acids: experimental and theoretical insight

“…Azo-hydrazone tautomerism, a phenomenon intrinsic to molecules containing the protic functional group conjugated with the azo linkage, has been studied intensely for over a century. Ever since the coexistence of two isomeric species of 1-(phenyldiazenyl)naphthalene-2-ol in solution has been discovered, a myriad of papers has been reported debating this subject for numerous compounds, by means of both various chemometric and spectroscopic tools [1][2][3]. It is a feature of great importance, since the different tautomers of a particular compound bear different characteristics, regarding dyeing abilities and physicochemical properties.…”

“…The strength of these specific and nonspecific interactions is a limiting factor that influences further solvent-tautomeric proton interactions, and there-fore, the prototropy. On the other hand, there is a drawback in this field of research regarding the inability of spectroscopic characterization techniques, such as UV-Vis, FT-IR and NMR spectroscopy, to identify individual tautomeric species of certain molecules in solution [1,5]. In order to overcome the lack of information given by spectroscopic methods for a pure, single tautomer, various mathematical approximations provided by quantum-chemical models are often employed in the research [3].…”