Metal complex pigments

Only a few azo metal complexes are available as pigments. Most of these are very lightfast and weatherfast. The chelating metal is usually nickel, and less commonly, cobalt or iron(II). 

The azo group (—N=N—) may be replaced by the analogous (—CH=N—) moiety to form an azomethine complex pigment, usually with copper as a chelating metal. The number of commercially available products in this group is also restricted. They typically afford yellow shades. Those species that provide the required lightfastness and weather resistance are used in automotive finishes and other industrial coatings. 

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However, there are metal complex pigments. Without doubt the most important metal complex pigment is copper phthalocyanine (4). The phthalocyanines were discovered by accident in 19286 and now represent the second most important class of colorants after the azo colorants. Copper phthalocyanine itself exists in several polymorphic forms and gives beautiful blue and cyan colors with outstanding fastness properties.5-7 Halogenated copper phthalocyanines provide green pigments (see Section 9.12.4.3). 

Metal complex dyes and pigments perform key functions in both the image generation step and the toner development step. It is the metal complex pigment that produces the positive hole in the organic photoconductor. 

Organic semiconductors are becoming increasingly important in the fabrication of electronic devices. For electron transport, metal complex pigments, such as hexa-deca-fluoro copper phthalocyanine (76), are showing potential.79... 

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. 

Apart from alizarin lake , which is now being formulated as an aluminum/cal-cium complex [3] (Sec. 3.6.2), the oldest known metal complex pigment is an iron complex. In 1885,0. Hoffmann reported on the iron complex of l-nitroso-2-naph-thol, which under the name of Pigmentgriin B (Pigment Green 8,10006) was first industrially exploited in 1921 by BASF. 

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 commercially interesting metal complex pigments usually contain the co-ordinative tetravalent Cu+ + or Ni+ + ions, less commonly Co++ ions. The fourth coordination site is typically occupied by a solvent molecule with a free electron pair. It may also be engaged by the second nitrogen atom of a different pigment molecule, a phenomenon which is observed in azo complexes and similar materials. In the latter case, sandwich structures are obtained [5]. The copper and nickel complexes are mostly planar molecules. 

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] ... 

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. 

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. 

Besides, metal complex pigments are also used in printing inks as well as in other areas of application. 

Table 25 shows examples of the above type of metal complex pigments. 

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. 

Kondo, T., Oyama, K., and Yoshida, K., Chiral molecular recognition on formation of a metalloanthocyanins a supramolecular metal complex pigment from blue flowers of Salvia patens, Angew. Chem. Int. Ed. Engl, 40, 894, 2001. 

Redox Catalysts. Metallized redox catalysts are closely related to organic metal-complex pigments and distinguished, in general, by a polycyclic structure. The... 

The most important class of organic pigments in terms of production are the azo pigments. Other important pigment classes include metal-complex pigments (e.g., copper phthalocyanine) and higher polycyclic compounds (e.g., anthraquinone, quinacridone, isoindolinone, and perylene). 

Further, stable 2 1 transition metal complex pigments 27 (C.I. Pigment Yellow 177) and 28 (C.I. Pigment Yellow 179), featuring six-ring chelates, have been prepared from methine and azomethine type isoindolinones l... 

Analogs of metal complex pigments 27 and 28 featuring other heterocyclic amines and methine compounds have also been reported in the hterature. ... 

Methine isoindolinone metal complex pigments, in general, have so far met with very limited commercial success. This is primarily due to their dull and weak shades, as well as low fastness properties. 

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