Two azo pigments based on β-naphthol

Abstract

There has been much discussion in the literature of the azo-hydrazone tautomerism of pigments. All commercial azo pigments with beta-naphthol as the coupling compound adopt the hydrazone tautomeric form (Ph-NH-N=C) in the solid state. In contrast, the red pigments 1-[4-(dimethylamino)phenyldiazenyl]-2-naphthol, C(18)H(17)N(3)O, (1a), and 1-[4-(diethylamino)phenyldiazenyl]-2-naphthol, C(20)H(21)N(3)O, (1b), have been reported to be azo tautomers or a mixture of azo and hydrazone tautomers in the solid state. To prove these observations, both compounds were synthesized, recrystallized and their crystal structures redetermined by single-crystal structure analysis. Difference electron-density maps show that the H atoms of the hydroxyl groups are indeed bonded to the O atoms. Nevertheless, a small amount of the hydrazone form seems to be present. Hence, the compounds are close to being ;real' azo compounds. Compound (1a) crystallizes with a herring-bone structure and compound (1b) forms a rare double herring-bone structure.

First crystal structure of a Pigment Red 52 compound: DMSO solvate hydrate of the monosodium salt

“…The pigment exhibits the hydrazone tautomeric form, like all industrial hydrazone pigments (formerly known as 'azo pigments') (Gilli et al, 2005;Schmidt et al, 2008;Hunger & Schmidt, 2018). The N-H group forms two intramolecular [S 1 1 (6)] N-HÁ Á ÁO hydrogen bonds (Table 1).…”

Conformational analysis, tautomerization, IR, Raman, and NMR studies of 3-phenylazo-2,4-pentanedione

“…This tautomer, AE4, is the next most stable tautomer; therefore, it is concluded that existing of any other tautomer or conformer beside HA1 in a significant amount in the sample is unlikely. The preference for the hydrazone tautomeric forms has been explained by the fact that generally a C@N double bond is energetically more stable than a N@N double bond [19]. However, theoretical calculations [4] suggest that azo-enol tautomers contribute in formation of metal complexes.…”

Substituent and Solvent Polarity on the Spectroscopic Properties in Azo Derivatives of 2-Hydroxynaphthalene and Their Difluoroboranes Complexes

“…This tautomerization is quite interesting from a theoretical standpoint because the two tautomers have different technical properties. In azonaphthols, the equilibrium depends on temperature, the solvent, and the substitution pattern [ 18 , 19 , 20 , 21 , 22 ].…”