Coordination Chemistry of Pigments and Dyes of Historical Interest

Chapter 13

Coordination Chemistry of Pigments and Dyes of Historical Interest 1

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Mary Virginia Orna , Adrienne W. Kozlowski , Andrea Baskinger , and Tara Adams 1

Downloaded by UNIV ILLINOIS URBANA on May 12, 2013 | http://pubs.acs.org Publication Date: November 4, 1994 | doi: 10.1021/bk-1994-0565.ch013

1

Department of Chemistry, College of New Rochelle, New Rochelle, NY 10805 Department of Chemistry, Central Connecticut State University, New Britain, CT 06050

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Many pigments and dyes of historical interest exhibit the capability of forming Werner-type coordination complexes. The relationships between the structures and color characteristics of these colorants are reviewed in this paper, with particular attention to the work of Paul Pfeiffer, a student and long-time assistant of Alfred Werner's who did extensive work in the characterization and application of Werner's coordination theory to alizarin-related dyes. The production and use of pigments and dyes in ancient, medieval, and modern times until the middle of the nineteenth century gradually evolved from the craftsman's art into a science. In the dyeing of textiles it was recognized even in prehistoric times that plant ashes and other materials such as lime and clay were important auxiliaries that conferred fastness and often color variation. Gradually dyers came to realize that these natural materials contained metals such as aluminum, tin, and iron and that it was the metals' presence that determined the desirable properties of the dyebath. Pigments, on the other hand, were essentially insoluble colorants applied to a substrate with the use of a medium such as water, egg yolk, or oil. Many colorants are used as either a dye or a pigment, the simplified generalized rule of thumb being that a pigment is water-insoluble and applied via a medium whereas a dye is water-soluble and penetrates or adheres directly to its substrate when applied from an aqueous medium (7). Natural Colorants Before Perkin Yellow Colorants. The most important yellow dye in ancient and medieval times was weld, a flavone (Figure la) derivative extracted from the seeds, stems, and leaves of Reseda luteola L., commonly known as dyer's rocket. This colorant is resistant to atmospheric oxidation, rendering it quite lightfast and hence extremely popular and useful. In combination with the blue dye woad it was used to produce the Lincoln green made famous by Robin Hood and his merry men. Unlike weld, quercitron, a flavonol derivative (Figure lb), is much more susceptible to degradation by light and was not as important (2). Safflower yellow, derived from carthamin, was often used as a surrogate for Spanish saffron, a polyene extracted from the stigmas of Crocus sativus (3). 0097-6156/94/0565-0165$08.00/0 © 1994 American Chemical Society In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


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COORDINATION CHEMISTRY

Figure l a . Flavone (2-Phenyl-4H-lbenzopyran-4-one)

Figure l b . Flavonol (3-Hydroxyflavone)

Table I lists some important examples of each type of yellow dye discussed above. The structures of both luteolin and quercetin, the principal coloring matters of weld and quercitron, respectively, suggest that 4,5-type chelates could form with metal ions impregnated in the fibers to be dyed. Undoubtedly, the formation of these chelates lent stability to the colors of the dyed fibers, but their importance as chelates seems to be as limited at the importance of their respective dyes. Blue and Purple Colorants. The only natural blue dyes in antiquity were indigo and woad, both containing the identical principal coloring matter, indigo, or indigotin (Figure 2), probably the oldest coloring matter known. The chemical identity of this colorant was, of course, unknown until the advent of modern chemistry, and both materials were thought to be distinctly different from one another. Although woad has always been associated with its vegetable origin, it was once thought, at least in England, that indigo was of mineral origin. The indigo-bearing plant, Indigofera tinctoria, was formerly grown all over the world, but the synthetic product has replaced the vegetable product since 1900. Woad is obtained from Isatis tinctoria, a herbaceous biennial indigenous to southern Europe (4), and contains as little as 1/30 the amount of indigo in its coloring matter. Although indigo does not form coordination compounds, it is mentioned here because of its historical importance. It is to this day one of the few naturally occurring dyes in wide use. 0 - - H

Figure 2. Indigo [2-(l,3-Dihydro-3-oxo-2H-indol-2-ylidene)l,2-dihydro-3H-indol-3-one], blue colorant used as a vat dye

Prussian blue is a notable coordination compound in that it is the first modern pigment to have a known history and established date of preparation (4). The most

In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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Pigments and Dyes of Historical Interest

Table I. Some Important Yellow Dyes Dye

Chemical Compound


Weld

Luteolin

Quercitron

Quercetin

Source Flavone dye extracted from the seeds, stems, and leaves of Reseda luteola L. (Dyer's Rocket) Flavonol dye from the bark of the North American oak, Quercus tinctoria nigra

Structure

OH

HO.

HO.

OH Safflower

Carthamin

Chalcone dye from dried petals of Carthamus tinctorius (Dyer's Thistle)

Ο

CH^OH

OH

OH

OH


168


commonly used formula is Fe4[Fe(CN)6]3. The intense blue color arises from an intervalence charge transfer band involving F e + F e -» F e + F e . 2 +

3 +

3 +

2+


The purple of antiquity was obtained from various genera of Mediterranean shellfish. The most important of these were Phyllonotus, Thais, Dicathais, and Bolinus (Linnaean terminology), but known commonly as Murex and Purpura. The one dye that is common to all of these muricids is 6,6'-dibromoindigo, a deep purple indigotin derivative which differs from the parent compound by the presence of two bromine atoms. The dye is extracted as a yellowish secretion from the hypobranchial gland of the shellfish (12,000 molluscs yield approximately 1.5 g of the dye), which contains the dye precursor, indoxyl sulfate, usually as the potassium salt (Figure 3) with a variety of substituents at the 2 and 6 positions, depending upon the species of mollusc (5). Under the influence of light and air, the precursor changes from yellow-

H Figure 3. Potassium indoxyl sulfate, dye precursor of Tyrian Purple (X = H , Br; Y = H, SCH , S 0 C H ) 3

2

3

ish, to green, to blue, and finally to purple within a few minutes. The colored matter is fairly insoluble in water and must be reduced to the water-soluble leuco form in the presence of fibers and then reoxidized to form the colored dye on the fiber. The whole process is known as vat dyeing (6). Denomination of the purple dye yields the parent compound, indigotin. Varying proportions of the dibromoindigo, monobromoindigo, and nonbrominated indigo yield varying shades of purple to blue. The historic interest in this dye is evidenced by the fact that Pliny the Elder (A.D. 23-79) devotes six chapters of his celebrated Natural History to its production, including treatises on the nature and kinds of shellfish, the history of its appearance in Roman outerwear, the high cost of the dye, and the methods used to obtain and process it (7). Furthermore, volumes, and indeed, doctoral dissertations have been written on its role in the production of the Biblical blue, or tekhelet, but the nature of this mysterious blue remains doubtful to this day (8). Black Colorants. Satisfactory synthetic black dyes are scarce even to this day. Most modern black shades are obtained by mixing two or more dyes together. Thus logwood, the only important black dye of the pre-Perkin era, still enjoys usage in the dyeing of silks and leather. Logwood, derived from the heartwood of the Central American tree, Hematoxylon campechiancum L., was imported to Europe in the early sixteenth century. It was a relatively unimportant red dye until the French chemist Michel Eugène Chevreul (1786-1889) discovered that it combined with metallic salts to give colored lakes. The principal coloring constituent is hematein (Figure 4), which, when combined with criromium, yields the black shades for which logwood is renowned. The structure of the hematein-chromium chelate complex is yet to be elucidated, but it is thought that it has a macromolecular structure in which the cmOmium ions link hematein molecules together by chelation (Figure 5).


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OH


Figure 4. Hematein [6a,7-Dihydro-3,4,6a,10-tetrahydroxybenz [b]indeno[ 1,2-d]pyran-9(6H)-one]

Figure 5. Logwood, a black chroman-type dye (L = Ligand)

Red Colorants. The most important class of pre-Perkin colorants, from the coordination chemistry point of view, is the red colorants. A l l of the major red colorants, whether of animal or vegetable origin, are derivatives of anthraquinone (See Table II). The principal red coloring matter of madder is alizarin, or 1,2dihydroxyanthraquinone. Natural madder contains a considerable amount of another colorant, purpurin, or 1,2,4-trihydroxyanthraquinone, which accounts for the various shades of red to purplish-red that can be obtained when different sources of madder are used. In almost every source that describes dyeing with anthraquinone derivatives, the use of a metallic salt is mentioned. The principal metals used were aluminum, iron, tin, and in later years, chromium. The resulting color on the fiber depended in large part upon the metal used since the dyes chelated with the metal ions. The colors of different chelate compounds with alizarin and cochineal are shown in Table III.


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Table II. Anthraquinone-Based Red Colorants

Madder or Alizarin. Roots of the Rubia tinctorum plant. Roots were known as "alizari," hence alizarin

Cochineal (Carminic Acid). Female insect, Coccus cacti, which lives on Prickly Pear cactus, found in Mexico. 200,000 insects yield 1 kg. of dye

OH H

Ο

n

CH2OH

OH Kermès (Kermesic Acid). Female scale insects, Coccus ilicis, which infect the Kermes oak

OH

Ο

OH

Ο

Ο CH

3

H.C

COOH


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Cochineal is a red colorant derived from female scale-insects; its chief coloring matter is carminic acid, the structure of which is shown in Table Π.


Table III. Colors of Chelate Compounds of Alizarin and Cochineal Mordant

Alizarin

Cochineal

None Aluminum Tin Chromium Iron Copper SOURCE: Adapted from ref.

Brownish-Yellow Red Pink Puce Brown Brown Yellow Brown 9.

Scarlet Red (aq. soin.) Crimson Scarlet Purple Purple

Figure 6 illustrates the formation of 2:1 neutral complexes of divalent metal ions and 1-hydroxyanthraquinones. Figure 7 illustrates the possibility of the formation of polymeric complexes with 1,4- and 1,5-dihydroxyanthraquinones with divalent metal ions. A n interesting variation on the chelate formation of alizarin derivatives is the production of Turkey red, a brilliant red dye known from ancient times. Recipes for its production insist not only on aluminum, but on lime, in order to achieve the redpurplish color and the high fastness properties for which it is famed. Its probable structure is shown in Figure 8 (10). Some modern mordant dyestuffs derived from alizarin are shown in Table IV.

Figure 6. 1-Hydroxyanthraquinones form neutral 2:1 complexes with divalent metals such as Cu & Zn




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Figure 8. Probable Structure of Turkey Red 1:2 Complex of Aluminum with Alizarin


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Table IV. Mordant Dyestuffs Derived from Alizarin Dye

Structure


Alizarin Orange A

Brilliant Alizarin Bordeaux R

OH

OH

Ο

Alizarin Red PS

S0 Na 3

Borate ester of 1,2-dihydroxyanthraquinone. Treatment with acetic anhydride yields l-hydroxy-2acetoxyanthraquinone, the 1-hydroxy group being protected against electrophilic attack because of coordination to boron


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Application of Werner's Coordination Theory to Alizarin-Related Dyes


Paul Pfeiffer (1875-1951), Alfred Werner's student and long-time assistant, published in Liebig's Annalen in 1913 (11) an extensive paper describing the application of Werner's coordination theory to alizarin-related dyes. Through a detailed series of experiments involving various substituted quinones, he showed that alizarin coordinates to tin through both the carbonyl oxygen and the adjacent 1hydroxyl group, as illustrated in Figure 9. He particularly pointed out how this reaction is an example of Heinrich Ley's inner complexes.

Ο Figure 9. Metal complex formation with hydroxyanthraquinones

Ο Figure 10. Compound used by Pfeiffer in dyeing experiments

Pfeiffer's Experimental Observations. Pfeiffer knew that SnCU reacts smoothly on heating with a variety of 1-hydroxyanthraquinones. For example, alizarin (Table Π) reacted to give a greenish-brown solution from which a violet-black powder crystallized. His analysis showed that only three chlorides remained per tin atom, as represented by his structure shown in Figure 9. The dark powder dissolved in aqueous ammonia (NH3) to give a dark violet solution; in alcohol it gave an orange solution which yielded orange needles upon addition of water. The chlorides in the original complex (Figure 9) could be replaced by oxide and hydroxide by dissolving the powder in pyridine and adding a small amount of water. Figure 10 illustrates the structure that Pfeiffer assigned to the orange-red needles obtained from this reaction. This product dissolved in ammonia to give a deep orange-red solution which was stable on heating. Pfeiffer determined that silk and wool were dyed red by this solution, but that cotton was not. Pfeiffer's observation that alizarin dye lakes (soluble dyes precipitated on insoluble substrates) contained alkaline earth metals (notably calcium from lime) as well as coordinated tin or other mordant metals such as chromium, aluminum, or iron led to his investigations of the role of the 2-hydroxyl groups in anthraquinone dyes. He collected a large body of data showing that strong bases neutralize 2-hydroxyl groups in preference to 1-hydroxyl groups. Furthermore, he noted that the color of the dye is deepened when a 2-hydroxyl group is present in its ionized form. When the 2hydroxyl group is absent, the tin chloride derivatives of 1-hydroxyanthraquinone are red, while alizarin derivatives which possess an ionized 2-hydroxyl group are violetblack. This study of Pfeiffer's neatly fit mordant dyes into Werner's coordination theory by differentiating between inner complex formation with tin and ionic salt formation with strong bases. Pfeiffer did not pursue any further research on dyes.


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Metal Ions as Mordants


The effect of metal ions as mordants for anthraquinone dyes has been noted for many years. Alizarin was usually dyed on wool mordanted with aluminum to give red shades, or mordanted with chromium which produced browns and violet-browns. A typical dyeing recipe involved the slow heating of wool, alum and potassium acid tartrate, followed by addition of alizarin and calcium acetate. Chrome Mordants. Chrome mordants have been used extensively, giving a brighter color than other transition metals. The usual recipe involved heating the wool with potassium dichromate (K^C^Ov) in a dilute sulfuric acid solution before adding the dye. In the dyeing process amino acid residues are coordinated to Cr(III) in the mordanted wool, but the Cr(III) can be removed with oxalate. Hartley (12) has proposed that the mechanism for this chrome process involves a series of two-electron reductions, from Cr(VI) to Cr(IV) and from Cr(IV) to Cr(II). Cr(II) is labile and coordinates to the wool. It is subsequently oxidized to Cr(III) in air. It has long been known by dyers that the chromium uptake by wool is much faster from Cr(VI) solutions that from Cr(III) solutions. The reduction of Cr(VI) is believed to occur by reaction with disulfide linkages in the wool, although sometimes reducing agents such as formic or lactic acids or potassium hydrogen tartrate can also play a role. Spectroscopic Evidence. In the case of M = Cr(III) infrared spectra suggest that the Cr(III) is coordinated to the protein carboxyl groups (13); in addition, Hartley's data show that sulfur is not coordinated. The esterification of the carboxylic acid group reduces chromium uptake, but blocking amines by dinitrophenylation does not inhibit this process (14). While the effects of mordanting on the color properties of alizarin and cochineal dyes have been widely noted, spectra of the mordanted dyes and the shifts induced by various metals do not appear to have received much attention. Uncoordinated alizarin shows a broad, intense ultraviolet envelope with a low energy absorption at 430 nm. This has been assigned to an η -> π* transition. Labhart (15) studied the effects of various substituents on the spectrum of anthraquinone and noted that groups in the 1-position, which are capable of hydrogen bonding to the carbonyl group, cause the absorption to move to longer wavelengths - what dyers call the bathochromic shift. This shift is the same effect observed with metallized derivatives of alizarin. In either case electron derealization is enhanced, and the absorption wavelength maximum moves farther into the visible region. Ionizing a second hydroxyl group in the adjacent 2-position further enhances electron donation into the ring system and increases the intensity of the absorption as well as its bathochromic shift. These observations help to explain the empirically discovered benefits of using alkaline earth salts of the mordanted dye. Use of alizarin dyes continues today with substituted molecules such as those given in Table IV. Conclusion Although the chemistry of ancient dyes and mordanting processes was developed empirically, today we understand much of it in terms of modern coordination theory. However, many aspects of color shifts still await investigation.


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Literature Cited 1. Price, R. In Comprehensive Coordination Chemistry: The Synthesis, Reactions, Properties & Applications of Coordination Compounds; Wilkinson, G., Ed.; Pergamon Press: New York, NY, 1987; Vol. 6; pp 35-94. 2. Gregory, P. F.; Gordon, P. Organic Chemistry in Colour; Springer-Verlag: New York, NY, 1983. 3. The Merck Index, 11th Ed.; Budavari, S., Ed.; Merck & Co., Inc.: Rahway, NJ, 1989. 4. Gettens, R. J.; Stout, G. L. Painting Materials: A Short Encyclopaedia; Dover Publications: New York, NY, 1966. 5. Elsner, O. In Dyes in History and Archaeology, No. 10; Rogers, P. W., Ed.; Textile Research Associates: York, UK, 1992; pp 11-16. 6. Koren (Kornblum), Z. C. In Colors from Nature: Natural Colors in Ancient Times; Sorek, C.; Ayalon, E., Eds.; Eretz Israel Museum: Tel Aviv, Israel, 1993; pp 15*-31.* 7. Meldola, R. J. Soc. Dyers Colour. 1910, 26, pp. 103-111. 8. The Royal Purple and the Biblical Blue; Spanier, E., Ed.; Keter Publishing House: Jerusalem, Israel, 1987. 9. Peters, R. H. Textile Chemistry, Vol. III: The Physical Chemistry of Dyeing; Elsevier Scientific Publishing Co.: New York, NY, 1975;p649. 10. Kiel, E. G.; Heertjes, P. M. Rec. Trav. Chim. 1965, 84, 89 ff. 11. Pfeiffer, P. Justus Liebig's Annalen der Chemie 1913, 398, 137 ff. 12. Hartley, F. J. Soc. Dyers Colour. 1969, 85, 66 ff; 1970, 86, 209 ff; Aust. J. Chem. 1969, 22, 229 ff. 13. Hartley, F. Aust. J. Chem. 1968, 21,2723 ff. 14. Meekel, L. Textilveredlung 1967, 2, 715 ff. 15. Labhart, H. Helv. Chim. Acta 1957, 40, 1410 ff. RECEIVED October 18, 1993


Coordination Chemistry of Pigments and Dyes of Historical Interest

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