Azomethine metal complexes

There is another structure which, although it does not represent a typical azomethine compound, is synthesized from nickel complexes of the anilide of di-imino butyric acid (35). 

The synthesis involves nitrosating the corresponding substituted acetoacetic anilide with sodium nitrite in acetic acid and subsequently, by adding hydroxyl-amine to the same reaction vessel, converting the compound to the oxime. Finally, complexation is achieved by means of a Ni(II) salt. 

Yet another structural principle is represented by metal complex pigments based on isoindolinones. Condensation of amino-iminoisoindolinones (imino-phthalimide) with 2-aminobenzimidazole in a high boiling solvent affords an azomethine (36). This compound reacts with salts of divalent metals, such as Co, Cu, Ni, to yield yellow azomethine metal complex pigments [10]  

Pigment Yellow 177,48120 is synthesized by using 2-cyanomethylene benzimidazole instead of 2-amino benzimidazole to react with iminophthalimide. For more information about cobalt complexes, refer to [11], 

Commercially available azo and azomethine metal complex pigments cover the spectral range from considerably greenish to reddish yellow and yellowish orange. Compared to their parent structures (the corresponding azo and azomethine compounds), azomethine metal complexes frequently exhibit a distinctly duller shade. Formation of the metal complex often shifts the color of an originally yellow material in the greenish yellow direction. 

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Azomethine metal complex pigments replace the metal with tin stabilizers, resulting in a change in shade. In the case of manganese-laked pigments trouble can also be expected in the presence of epoxy compounds. Pigment preparations based on epoxidized soya bean oil are normally used instead of diisodecyl phthalate pastes in the automotive sector e.g. for coloring PVC roofs etc. 

Table 25 Examples of azo and azomethine metal complex pigments.

It is only for reasons of simplified classification that the pigments which are described in this section, like azomethine metal complex pigments (Sec. 2.10.1.2), are listed in Chapter 2. Actually, rather than being azo pigments, these are azo methine and methine pigments, which in the classification system adopted in this book are formally located between azo pigments and polycyclic pigments. 

Sakaki S (2005) Theoretical Studies of C-H s-Bond Activation and Related by Transition-Metal Complexes. 12 31-78 Satoh T, see Miura M (2005) 14 1-20 Satoh T, see Miura M (2005) 14 55-84 Savoia D (2005) Progress in the Asymmetric Synthesis of 1,2-Diamines from Azomethine Compounds. 15 1-58 Schmalz HG, Gotov B, Bbttcher A (2004) Natural Product Synthesis. 7 157-180 Schmidt B, Hermanns J (2004) Olefin Metathesis Directed to Organic Synthesis Principles and Applications. 13 223-267... 

Metal complex formation considerably improves both lightfastness and weatherfastness in o,o -dihydroxyazo and o,o -dihydroxy azomethine pigments, usually compromised by distinctly dull color. 

Special consideration should be paid to metal complexes such as azomethine pigments (Sec. 2.10). At high temperatures, the yellow copper complex with the chemical constitution 10, incorporated in PVC, will exchange its chelated copper atoms with the metal atoms present in the application medium. Stabilizers containing barium/cadmium or lead produce yellow shades, while dibutyl tin thiogly-colate or other tin compounds produce a brilliant medium red. Color change is slow at low temperatures, but at 160°C the effect is rapid [108],... 

The commercially most interesting metal complex pigments within the azo series are those obtained from aromatic o,o -dihydroxyazo compounds, while important products within the azomethine series are nickel or copper complexes of aromatic o,o -dihydroxyazomethine compounds. 

The aromatic moieties are possibly substituted benzene or napthaline rings. In azomethine pigments, only one form of metal complex is possible. This is in contrast to azo metal complexes, which may assume either structure 31 or 32 ... 

Metal complex pigments are mainly used in paints. The products are fast enough to be applied especially in industrial finishes. Some representatives, particularly azomethine copper complex pigments, are very weatherfast, which makes them suitable candidates for automotive finishes. High transparency in combination with good weatherfastness is an asset for use in metallic finishes. It is not uncommon for metal complexes to lose much of their brilliance in white reductions. Some are also recommended for use in architectural paints, especially for emulsion paints. 

Recent developments in this class prefer azomethine complexes as chemical structures rather than azo metal complexes. The list of commercially available types includes Pigment Green 8 and 10, Pigment Yellow 117,129,150,153,177,179, and Pigment Orange 59,65, and 68, as well as P.R.257. 

Vidali, M. et al J. Inorg. Nucl. Chem., 1975, 37, 1715-1719 A series of uranyl complexes of macrocyclic azomethines were used as ligands for transition metal ions, with perchlorate anions. Raman spectra of the uranyl-metal complexes could not be recorded because the samples exploded during attempted measurements. 

Similar to those of oxygen and sulfur ylide, ammonium ylide or azomethine ylide can be generated by the interaction of metal carbene and amine or imine, respectively. As is the case of sulfur, nitrogen also has a strong coordinating ability to a metal complex. Consequently, metal complex-catalyzed diazo decomposition in the presence of an amine or imine usually requires high reaction temperatures (Figure 6). 

The crystal and molecular structures of isomeric cluster complexes, HFe3(MeC=NH)(CO)9 and HFe3(N=CHMe)(CO)9, derived from reduction of acetonitrile have been reported.149,150 In addition to these, a wide variety of complexes with ligands containing azomethine groups (>C=N—) along with other functional groups (e.g. metal complexes of salicylaldiimine) are known and these will be discussed in Chapter 20. 

Azomethines bear a formal resemblance to azo compounds and many parallels exist in the coordination chemistry of the two series of compounds. Thus the bidentate azomethines (191) behave in a strictly comparable manner to the bidentate azo compounds (20) (Section 58.2.2.1). The isomeric, bidentate azomethines (192), however, form metal complexes which undergo very facile hydrolysis as a result of polarization of the azomethine linkage. The difference between the two types of complex is dramatically illustrated by the results of a study135 of metal complex formation by the bis(azomethine) (193). This cannot function as a tetradentate ligand for steric reasons and reacts with copper, nickel and cobalt halides in cold ethanolic solution to form the five-coordinate complexes (194). Crystallization of these products from ethanol gives the five-... 

In general, the stability of metal complexes of bidentate azomethines of the type (191) is comparable to that of metal complexes of the analogous bidentate azo compounds (14) and is inadequate for practical application in the dyestuffs field. Limited numbers of such complexes have, however, been claimed to be suitable for pigment applications. 

Metal complexes of tetradentate azomethines, e.g. (199), are reported136 to have very high light-fastness but to be tinctorially weak and dull in hue. Despite this they are of technical interest as very fast yellow to brown pigments. They find no application as dyestuffs, however, because of these deficiencies for example, the chromium complex of (200) gives dull, tinctorially weak dyeings on wool possessing poor wet-fastness properties. 

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