Metal Formazan Dyes

Another class of metal complex dyes is derived from the formazan stmcture. These dyes are appHed to wool and nylon from a neutral or weakly acidic dyebath analogous to the 2 1 premetallized OjO -dihydroxyazo complexes. The bluish-gray dye Cl Acid Black 180 [11103-91-6] (61) (Cl 13710) is a 2 1 cobalt complex of the formazan type. 

Theoretically, the dye or chromogen can be any colored species. Of course, requirements for fastness, solubiUty, tinctorial value, ecology, and economy must be met. Most commonly used chromophores parallel those of other dye classes. Azo dyes (qv) represent the largest number with anthraquiaone and phthalocyanine making up most of the difference. Metallized azo and formazan dyes are important and have gained ia importance as a chromophore for blue dyes duriag receat years (6) (see Dyes and dye intermediates). 

There are bi-, tri-, and tetradentate formazan dyes7 but only the tetradentate copper complex formazan dyes have found use as commercial products. The dye Cl Reactive Blue 160, of generic structure (41), is a representative example. Because of the intensity of their colors, the metal complex formazan dyes have also found application in analysis. Thus, the dye (42) detects zinc at a concentration of 1 part in 50 million.31... 

Formazan dyes bear a formal resemblance to azo dyes, since they contain an azo group. The most important formazan dyes are the metal complexes, particularly copper complexes, of letradentate formazans. They are used as reactive dyes far cotton. 

Blue dyes are derived from anthraquinone, phthalocyanine, or metallized formazan. See also Anthraquinone (Figs. I and 2). There are also oxazine and thiazine dyes reported (Fig. 2). 

Fig. 2. Phlhalocyanine (S) and metallized formazan (6,7) and azine (8) blue reactive dyes. Reactive Blue 15 (Cl 74459) (5) Reactive Blues (6) and (7) Reactive Blue 204 (8). Other blue oxazine dyes ate other fluorotriazines and a dichlorotriazine.

As far as dyes are concerned, chelates of type Cb are termed 1 2 or symmetrical 1 2 metal complexes if the two tridentate ligands are equal, and mixed or unsymmetrical 1 2 metal complexes in the other case. Chelates of type Bb represent 1 1 metal complexes. The types Cb and Bb include, in general, azo and azo-methine metal complex dyes, whereas chelates of the quadridentate types Ac and Be are derived predominantly from formazan and phthalocyanine chromophores. 

Formazan dyes have become important reactive dyes for cotton. Formazan dyes are also used in analytical chemistry because of the high color intensity of many of their metal complexes. The chemistry of formazans was first exploited in 1962 by MacDonald to produce color photographs [81],... 

Virtually every conceivable chromophore has been used in the synthesis of reactive dyes, including monoazo and disazo species, metal complexes of azo dyes, formazan dyes, anthraquinones, triphenodioxazines, and phthalocyanines. The product lines offered by the major dye producers in most cases feature comparable chromophores, differing primarily in the nature of the reactive systems and the particular substitution patterns adopted. 

Metal-Complex (Formazan) Dyes. The hydrazone from 2-carboxyphenylhydra-zine-4-sulfonic acid and benzaldehyde is suspended in water and then dissolved by adding aqueous sodium hydroxide to obtain pH 6.5 -7.0. This solution is added to the aqueous diazonium salt solution obtained from a typical aqueous diazotiza-tion of 4-(2-sulfooxyethylsulfonyl)-2-aminophenyl-6-sulfonic acid. The mixture is then dripped into an aqueous solution of copper sulfate, while the pH is maintained with soda at 5.5 - 6.5. After complete coupling the pH is adjusted to 1 with concentrated hydrochloric acid. The strongly acidic solution is then neutralized with alkali to pH 5.5. The copper - formazan complex is salted out along with sodium chloride, filtered, washed with dilute aqueous sodium chloride solution, and dried. A dark powder results which gives a dark blue solution in water. It consists of an electrolyte-containing powdered sodium salt of the acid 25 ... 

THF and 2 g 20% Me2CO soln. of polysulfonamide binder and 7 mg formazan dye were added. The soln. was coated 0.15-mm-thick onto poly(ethylene terephthalate) film and dried to give a coating which was completely bleached when contacted a few s. with a metal block at 150°. 

The preparation of optical recording elements having recording layers containing mixtures of metallized formazan, (I), and cyanine dyes, (II), is provided (3). 

T. Poiger, S.D. Richardson and G.L. Baughman, Analysis of anionic metallized azo and formazan dyes by capiUary electrophoresis-mass spectrometry, J. Chromatogr. A, 886, 259-270, 2000. 

Dyes 8-12 belong to the family of azo conqiounds, while 13 is a formazan dye. Formazan dyes are more commonly used as dyes for cellulose-based fibers, in which case they would contain a reactive moiety, and are 1 1 Cu-complexed dyes (1 metal per dye molecule) (6). Metal complexes of the 1 2 type (1 metal per 2 dye molecules) are used for polyamide and protein fibers. In the latter case, Cr and Co complexes... 

Formazan, l-(2-hydroxy-5-suIfophenyl)-3-phenyl-5-(2-carboxyphenyl)-zinc detection, 6, 83 Formazans cobalt complexes dyes, 6, 81 metal complexes bidentate, 6, 78 color photography, 6,111 dyes, 6, 77 tetradentate, 6, 81 tridentate, 6, 79... 

The physical and chemical properties of formazans have been reviewed in detail.1,2 In this discussion only those properties that directly or indirectly affect the choice of a tetrazolium salt as a leuco dye will be discussed. Spectral, acid—baseyedox, and metal complexing properties will be stressed. 

Triphenylformazan behaves as a bidentate ligand forming 2 1 complexes (217) with divalent copper, nickel, and cobalt.377 Formazan metal complexes can be compared to complexes of azo dyes or beta diketones due to structural similarity.301,302 In general, formazan metal complexes have low stability toward acids. However, when electron-donating substituents are added to the aromatic ring, a considerable enhancement in stability is observed. Cationic complexes of type 218 are also known. The complexation of formazan with metal cation can be accompanied by oxidation to the tetrazolium salt and the formation of a complex... 

Formazans and their metal complexes are used as textile dyes by direct application. The in situ reduction of tetrazolium salts has not been used to introduce the dyes to their substrates. Treatment with triaryl mono and bis tetrazolium salts followed by a reducing agent such as ascorbic acid or thioglycolic acid has been claimed as a method of introducing formazans as permanent hair dyes.641 There are some references to their use as therapeutic agents.642 544... 

Formazans or their precursors, tetrazolium salts, have been used in dye-bleach systems. Certain formazans102 such as (37) can complex with metal ions to give dye images with improved light stability. 

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