Formazan unsymmetrical

Unsymmetrically substituted formazans form a complex mixture of valence tautomers and geometric isomers with different conformations. The tautomerism is further complicated when substituents on the aryl ring are... 

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. 

Ester and cyano groups generally resist substitution by aryl azo groups and must first be hydrolyzed to produce carboxyl and carbonamide groups respectively, which can be more easily eliminated [62], Unsymmetrical formazans can be produced according to Scheme 2.15. 

In addition to synthesizing symmetrical 1 2 Fe complexes of dyes 21, we prepared unsymmetrical dyes such as 30>32. In these examples, 23 and 25 were combined with 24 (cf. 30-31) and dye 22 was combined with 25 (cf. 32). Dyes 30 (violet) and 31 (bluish-violet) possessed low lightfastness, suggesting that formazan 24 should not be used alone or in combination with another formazan ligand when lightfastness in the Fe-complex is important. On the other hand, unsymmetrical dye 32 is a further example of a very lightfast black formazan for wool and nylon that arises from the use of dye ligand 25. 

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