Formazan, structure

Both routes were found to give the same product their melting points were identical and mixed melting-point determinations gave no depression of the melting point. From this, it follows that, by the modern conception of formazan structure, the structure can be expressed by the following chelate. 

Accordingly, the identity of the two products affords decisive evidence for the formazan structure of the new sugar derivatives. "... 

Another class of metal complex dyes is derived from the formazan structure. These dyes are applied to wool and nylon from a neutral or weakly acidic dyebath analogous to the 2 1 premetallized 0,0 -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. 

Although the formation of tetrazolium salts of structure 92 by the oxidation of formazans under acid conditions has been known for many years, it is only recently that the quatemization of 1,5-disubstituted... 

Solutions of tetrazolium salts, e.g., 53, have been reported to both become colored and bleached under the influence of both UV and visible light. Several workers have attributed this phenomenon to photoreduction to the corresponding formazan (51) and the formation of a fluorescent colorless compound (152) through photooxidation.240- 243 The reduction of 152 under UV or blue light to the intense green radical structure (153) has also been reported (Scheme 21).244 A one-electron reduction product (154) is proposed as a short-lived intermediate in the photoreduction.245... 

The chemistry of tetrazolium thiolates and other mesoionic tetra-zoliums has been extensively reviewed.297,298 Tho ugh there are no reports of the chemical reduction of thiolates to formazan-like structures, the polarographic reduction of the complex betaines (146) to formazans has been reported.655... 

Formazans can be characterized as tautomeric structures 178 and 179. Schiele proposed that, in some cases at least, formazans are better represented bv the delocalized structure 180 implying coplanarity among substituents. 302... 

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... 

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... 

Copper and nickel complexes of the tridentate l-(2-carboxyphenyl)-3,5-diphenyl- (169 X = C02 R = R = Ph) and 1-(2-hydroxyphenyl)-3,5-diphenyl-(169 X = 0 R = R = Ph) formazans were prepared118 by the interaction of the formazan and the appropriate metal acetate in alcohol and were assigned the three-coordinate structures (170 X = O, C02 R = R = Ph M = Ni, Cu) since the diamagnetic nickel complexes were found to be unimolecular in benzene solution. Treatment of the nickel complex (170 X = O, R = R = Ph M = Ni) with pyridine gave a violet crystalline adduct which was assigned the four-coordinate structure (171 X = O R = R = Ph M = Ni). A product similar to the latter could not be obtained from the nickel complex of l-(2-carboxyphenyl)-3,5-diphenylformazan but nickel complexes of this type were obtained from both l-(2-hydroxyphenyl)- (169 X = O, R = CN R = Ph) and l-(2-carboxyphenyl)- (169 X = C02 R = CN R = Ph) 3-cyano-5-phenylformazans. In all three cases a considerable shade change occurred on going from the three-coordinate complex to the pyridine adduct. 

More recent work117 has brought to light other, surprising, differences between l-(2-carboxy-phenyl- and l-(2-hydroxyphenyl)-formazans and cast some light on the structures of their copper complexes. Like 1,3,5-triphenylformazan, both l-(2-carboxyphenyl)- and l-(2-hydroxyphenyl)-... 

The preparation of chromium(III) and cobalt(III) complexes of tetradentate formazans in which the coordination sphere of the metal is completed by a variety of neutral ligands has been reported116 and these (185) were claimed to be very stable towards acids. By analogy with copper complexes, however, some doubt must exist regarding the structures assigned to those complexes derived from l,5-bis(2-carboxyphenyl)formazans. Various dyestuff and pigment applications have... 

Reactive Dyes. These dyes form a covalent bond with the fiber, usually cotton, although they are used to a small extent on wool and nylon. This class of dyes, first introduced commercially in 1956 by ICI, made it possible to achieve extremely high washfastness properties by relatively simple dyeing methods. A marked advantage of reactive dyes over direct dyes is that their chemical structures are much simpler, their absorption spectra show narrower absorption bands, and the dyeings are brighter. The principal chemical classes of reactive dyes are azo (including metallized azo), triphendioxazine, phthalocyanine, formazan, and anthraquinone (see Section 3.1). 

Formazan dyes are closely related to azo dyes and are derived from the following basic structure ... 

Complexes of formazans having one or two carboxylic groups in the 2- or 2,2-positions of the aryl substituents, however, appear to be bicyclic. Thus, a hydrate was obtained on reaction of l,5-bis(2-carboxyphenyl)-3-cyanoformazan with copper acetate [76], The coordinated water could not be removed by drying and produced a broad band in the IR spectrum at 3450 cm-1. The copper complexes of formazans containing carboxyl groups were then assigned the structure 42. 

Formazan-like structures with a 6/5/6-ring system are exemplified by 45 [79] and 46 [80] ... 

Formazan dyes bear a formal resemblance to azo dyes, since they contain an azo group, but have sufficient structural dissimilarities to be considered as a separate class. The most important formazan dyes are the metal complexes, particularly copper complexes, of tetradentate formazans (see Section 2.10.3). They are used as reactive dyes for cotton (12) is a representative example. 

Formazanes, whose general structure is given by 227, form chelates with metal-cycles of different sizes depending on the nature of substituents R1, R3, and R5... 

In general, six-member metal-cycles are characteristic of the complexes of aromatic formazanes 228 [417-427]. However, a five-member structure 229 with the coordination unit MN4 is formed if R3=NOz [428], The same structure is observed in the hetarylformazanes 230 [340,342,418,420,429-435] and 231 [341] ... 

The nature of the donor X atom and metal (917 923) are factors regulating the linkage of metal-cycles in the ICC of the examined azo compounds. A comparison of structures 228-231 shows that the linkage of formazane chelates is determined by the character of substituents at C and N atoms 227. Six-member metal-cycles are formed in structures of aromatic formazanes 228 (except 229), while the ICC containing live-member metal-cycles are typical for hetarylformazanes 230 and 231 [11,274,275]. 

Superoxide radicals are another factor in oxidative damage. They can be determined with nitrobluetetrazolium (NBT), which then forms the colourless formazan. When melanoidins scavenge the superoxide radicals, the colour of the NBT persists.490,491 The activity of a glucose-glycine melanoidin on superoxide radicals is equivalent to the effect of 16 units of superoxide dismutase. The effect of the HMM and LMM fractions of this melanoidin is almost the same. The reaction rate constant of the melanoidin was markedly higher than that of ascorbic acid. If this were due to the reductone structures embedded in the melanodin, it is difficult to explain why the reducing power of the melanoidins is only 0.7 that of ascorbic acid.490... 

Chromogens of different classes, viz., arylazo derivatives of resorcinol, naphthols, naphthylamines, chromotropic acid and its cyclic modification, and also some formazans, were investigated by Fedorov et al.95 using 13C CP/MAS NMR. Usually only negligible changes of 6(13C) were found on comparing solution and solid-state spectra. Table 14 shows solution and the solid-state data for azo resorcinols in alkaline media existing in paro-quinoidic structure 33, not in the ortho one typical of some other compounds. 

Burns, G. R., Cunningham, C. W. and McKee, V. (1988). Photochromic formazans Raman specta, X-ray crystal structure and C magnetic resonance spectra of the orange and red isomers of 3-ethyl-l,5-diphenylformazan J. Chem. Soc., Perkin Trans. II, 1275-80. [216]... 

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