Tautomeric azo/hydrazone

In theory, azo dyes can undergo tautomerism azo/hydrazone for hydroxyazo dyes azo/imino for aminoazo dyes, and azonium/ammonium for protonated azo dyes. A more detailed account of azo dye tautomerism can he found elsewhere. 

Azo-hydrazone tautomerism Azo-hydrazone tautomerism (83) (Section 58.2.3.2) is a well-established phenomenon in o,o -dihydroxydiarylazo compounds, the various species existing in solution as an equilibrium mixture the composition of which is determined by a number of factors including substituents in the azo compound, the nature of the solvent, etc. It has been invoked to account for the formation of isomeric 2 1 cobalt(III) complexes by symmetrical o,o -dihydroxydi-... 

The structure of sugar phenylosazones, derivatives first prepared by Fischer in 1884, has remained undefined. Besides Fischer s open-chain structure, a tautomeric azo-hydrazone structure and ring structures have been proposed. Although work in the last two decades involving ultraviolet spectra studies - and examination of the acetylated - and... 

Taft substituent constants 219 Tautomerism, azo-hydrazone 366 azo-hydrazone—ene-hydrazine 360 in Schiff base, base-catalyzed 360 of pyrroles 108... 

Azocrown ethers pyrazoles, 5, 228 Azo dyes, 1, 328-331 colour and constitution, 1, 342 heterocyclic, 1, 325-326 Azo-hydrazone tautomerism, 1, 331, 334 Azoles acetic acids decarboxylation, 5, 92 acetoxymercurio reactions, 5, 107 acetyl... 

There are two main factors to consider in order to understand the bonding and structures of metal complex azo colorants, namely (i) azo/hydrazone tautomerism and (ii) the nature of the azo-to-metal bonding. 

Dyes based on 4-phenylazo-l-naphthol (6) have been used extensively to study azo/hydrazone tautomerism since they exist as an equilibrium mixture of both the azo and hydrazone tautomers.8 However, they are of little use commercially and of no use whatsoever for metal complex azo dyes since the hydroxy group is not ortho to the azo group so these cannot act as chelating ligands. 

The solvent effect on the azo-hydrazone equilibrium of 4-phenylazo-l-naphthol has been modelled using ab initio quantum-chemical calculations. The hydrazone form is more stable in water and in methylene chloride, whereas methanol and iso-octane stabilise the azo form, The calculated results were in good agreement with the experimental data in these solvents. Similar studies of l-phenylazo-2-naphthol and 2-phenylazo-l-naphthol provided confirmation. Substituent effects in the phenyl ring were rationalised in terms of the HOMO-LUMO orbital diagrams of both tautomeric forms [53]. 

The problem of the tautomerism between hydrazones and azo compounds has been the subject of considerable controversy. The details of the problem have been reviewed recently [104]. Among the causes of the dispute are the fact that phenylhydrazones are readily oxidized in air to give phenylazohydro-peroxides which resemble azo compounds in their spectra (both visible and ultraviolet) and the fact that the hydroperoxides may decompose to azo compounds [105]. 

The phenomenon of azo—hydrazone tautomerism is firmly established75 in the benzeneazo-phenol, -naphthol, and -anthranol series, the azo form predominating in the first of these (65), the hydrazone form in the second (66), whilst the third exists exclusively in the hydrazone form (67). The situation is less clear in the analogous o-aminodiarylazo series. Thus, whilst it is known that o-aminoazobenzene (68) exists exclusively in the azo form, it has not been established whether... 

Copper and nickel Chromium and cobalt N -Np Na-Np Meridional/facial coordination Azo/hydrazone tautomerism Non-planarity due to azo/hydrazone tautomerism Formation of two copper and two nickel complexes by unsymmetrical o.o -dihydroxyazobenzenes78 Detection of isomeric 2 1 chromium complexes 79 proton magnetic resonance80 Isolation of isomeric 2 1 chromium and cobalt complexes 86 87 limited X-ray crystallographic89 90 Proton magnetic resonance80 Detection of five isomeric 2 1 chromium complexes of symmetrical o.o -dihydroxydiarylazo compounds... 

Azo/hydrazone tautomerism was discovered in 1884 [3], The same orange dye was obtained by coupling benzenediazonium chloride with 1-naphthol and by condensing phenylhydrazine with 1,4-naphthoquinone. The expected products were the azo dye (62) (R=H) and the hydrazone (63) (R=H). It was correctly assumed that there was an equilibrium between the two forms, i.e., tautomerism. 

The discovery prompted extensive research into azo/hydrazone tautomerism, a phenomenon which is not only interesting but also extremely important as far as commercial azo dyes are concerned because the tautomers have different colors, different properties (e.g., lightfastness), different toxicological profiles, and, most importantly, different tinctorial strengths. Since the tinctorial strength of a dye primarily determines its cost-effectiveness, it is desirable that commercial azo dyes should exist in the strongest tautomeric form. This is the hydrazone form. 

From the viewpoint of stereochemistry the most interesting metal complexes are the octahedrally coordinated 1 2 chromium and cobalt complex dyes, which are medially metallized azo and azomethine compounds with functional groups in the o- and o -positions. Three types of isomerism can be discriminated geometrical, N-a, 3, and that arising from azo-hydrazone tautomerism. 

Azo-hydrazone tautomerism is a property that is indivisibly linked to azo dyes. NMR techniques used for its characterization were described in the previous review.1 Since its publication, data on several model compounds have been measured. 

Mazzola et a/.114 studied azo-hydrazone tautomerism and acid-base equilibria of FD C Yellow 6 (52). The compound exists as a hydrazone below pH 9 and as an azo anion species at pH 14. A dynamic NMR effect was observed at pH 12 (corresponding to pKa according to potentiometric titration), where the midpoint of the azo-hydrazone equilibrium occurs. The observed NMR line broadening is due to slow proton transfer between... 

C chemical shifts of C — OH/C=0 groups were used with the aim of determining azo-hydrazone tautomeric equilibrium in derivatives of squaric acid and in hydrazones of ortho-hydroxy aldehydes.116... 

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