Metal complexes of phenols are important in nature and useful in the laboratory. The metals involved usually include iron, aluminum and magnesium. In nature the flavonoids account for most red, blue, and violet -and to some extent yellow - colors. The majority of yellow colors are the result of the presence of carotenoids and aurones.
The precise color of anthocyanins depends on the substitution pattern, the pH (red in acid, blue in base), but also on the formation of complexes formed with iron, aluminum and magnesium ions. Such complexes can be rather large, and chemically diverse. An example is the blue pigment protocyanin (2.15) from cornflower. This is a cyanidin 3,5-diglucoside complex of two molecules of cyanidin linked through their o-diphenol groups with one Al3+ or Fe3+ ion. Cyanocentaurin is even more complex: four molecules of cyanidin 3,5-diglucoside, iron, and three molecules of biflavone glycoside.
Several structures are capable of forming metal complexes: o-dihydroxy phenols (2.16), 3-hydroxy chromones (2.17), 5-hydroxychromones (2.18), and o-hydroxycarbonyls (2.19).
The overall structure of the molecule determines the reactivity of the molecule with the metal, and the presence of the metal ion will impact the chemical properties of the complex. The degree to which the chemical
properties are altered as a result of complex formation depends on the structure of the phenolic compound. For example, aluminum chloride has less effect on the absorption spectrum of catechol (2.20) than on that of 3,4-dihydroxychalcone (2.21).
Metal complexes are used for compound identification. They can shift or change absorption spectra, change the Rf of compounds in thin layer chromatography, and change visual colors used in chromatography.
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