Azo Derivatives of Aminosalicylic Acids and Their Intracomplex Salts

L. P. Kulev, L. I. Aristov

In the last decade, considerable progress has been achieved in the search for new antitubercular agents.

Along with the synthesis of new antitubercular substances, attention is also being devoted to elucidating the mechanism of their action, since the resolution of this question opens up new prospects for the deliberate synthesis of medicinal preparations of a definite structure.

However, despite the large body of experimental material on the synthesis and comprehensive study of antitubercular preparations, the question of their mechanism of action still remains little studied.

With regard to the mechanism of action of p-aminosalicylic acid, most investigators [1] proceed from the standpoint of biological antagonism, i.e., they associate its action with the displacement from enzymatic systems of p-aminobenzoic acid—one of the most important growth factors of microbes.

Although this hypothesis is supported by some experimental data, it nevertheless is not able to explain many facts.

It is unclear, for example, why 3-hydroxy-4-aminobenzoic acid has low activity in comparison with “PASK,” from which it differs by the position of the phenolic hydroxyl. The same result is obtained if, in the benzene ring of p-aminosalicylic acid, hydrocarbon radicals or an amino group are introduced in place of the hydroxyl group [2].

A serious objection to this assumption is also the circumstance that “PASK,” while exerting a destructive effect on tubercle bacteria, does not exert this influence on other microbes for which p-aminobenzoic acid is also a characteristic growth factor [3].

Other investigators (Erlenmeyer, Buu-Hoï) [4, 5] associate the mechanism of action of p-aminosalicylic acid with its chemical properties, in particular with its tendency to form intracomplex compounds with heavy metals. However, no systematic investigation in this direction was carried out by the authors, although the point of view they expressed, as it seems to us, undoubtedly deserves attention.

There are contradictory data on the role of the complex-formation reaction in the mechanism of the antibacterial action of substances. Some investigators associate the physiological activity of certain compounds (oxyquinoline and its derivatives, p-aminosalicylic acid, isonicotinic acid hydrazide, etc.) with their complex-forming ability. Other investigators do not confirm this point of view [6, 7].

The subject of the present work was the preparation of azo derivatives of aminosalicylic acids and the elucidation, in this series of compounds, of the relationship between antitubercular activity and complex-forming ability.

The compounds obtained are characterized, above all, by the presence in the benzene ring of the diazo component of hydroxyl and carboxyl groups, located—

…arranged relative to one another in the ortho position, which condition the formation of intracomplex compounds with a whole series of metals. The interaction of salicylic acid itself with salts of copper, iron, and nickel has been studied by many investigators; it has been established that the composition of intracomplex compounds is substantially affected by the pH of the medium. However, the structure of the complex salts still cannot be regarded as definitively established [8, 9].

Various interpretations of the structure of intracomplex compounds of azo derivatives of salicylic acid are also encountered when considering the question of the structure of chrome dyes. Nevertheless, when comparing the fastness of chrome azo dyes and alizarin lakes, one may come to the conclusion that the carbonyl group participates in the formation of the intracomplex compound of an azo dye with a metal.

In the compounds obtained, in addition to the indicated grouping of salicylic acid, there are also other complex-forming groups, which can likewise react with metals to form intracomplex compounds.

Thus, for example, for nickel the characteristic groupings are the azo group with hydroxyls situated in the ortho position to it.

However, the azo derivative with salicylic acid residues, upon examination of solutions of this compound by the colorimetric method, also forms with nickel salts a complex compound of composition 1:2. This is confirmed by the noticeable deepening of the color when fabric mordanted with a nickel salt is dyed with this dye.

A considerable change in the intensity of the coloration of a dye having two complex-forming groupings—oxynaphthol and salicylic acid—as compared with other azo derivatives studied, occurs upon interaction with salts of cobalt and especially copper. This indicates that, in the present case, the grouping of 8-oxynaphthol is the most specific for these metals.

Both the structure of the azo derivatives and the nature of the metal entering into the intracomplex compound determine the stability of the complexes. Here, too, a definite dependence is observed between the structure of the azo compounds and the stability of the complexes they form with copper and nickel. Complexes containing nickel possess the greatest stability when the hydroxyl of the azo component is situated next to the azo group (1, 2, 6, 12, 13, 15, 17, 18, 19 of the table). Shifting the hydroxyl to the para position relative to the azo group entails a considerable decrease in the stability of the complex, even when the azo component contains a second strong complex-forming group, for example, the nitrogen of a heterocycle (4, 5).

For the copper complexes of the same azo derivatives (4 and 5), the opposite picture is observed, i.e., an increase in stability. The introduction of fatty hydrocarbon radicals into the benzene ring of the azo component has a noticeable effect on the stability of both copper and nickel complexes; namely, in all cases an increase in stability is observed (13, 18, 19).

Colorimetric studies of the stability of intracomplex compounds of azo derivatives of aminosalicylic acids show that the various complex-forming groupings, according to the specificity of their action and decreasing activity, can be arranged in the following order:

\[ \text{for Cu:}\quad \begin{matrix} \text{chelate grouping}\\[-2pt] \left(\begin{array}{c} \mathrm{OH}\\[-2pt] \mathrm{N} \end{array}\right) \end{matrix} > \mathrm{R}\!\left(\mathrm{OH}\right)-\mathrm{N}=\mathrm{N}-\mathrm{R}\!\left(\mathrm{NO_2}\right) > \mathrm{R}\!\left(\begin{array}{c}\mathrm{OH}\\ \mathrm{COOH}\end{array}\right) > \mathrm{R}-\mathrm{N}=\mathrm{N}-\mathrm{R}\!\left(\mathrm{NO_2}\right); \]

\[ \text{for Fe:}\quad \mathrm{R}\!\left(\begin{array}{c}\mathrm{OH}\\ \mathrm{COOH}\end{array}\right) = \begin{matrix} \text{chelate grouping}\\[-2pt] \left(\begin{array}{c} \mathrm{OH}\\[-2pt] \mathrm{N} \end{array}\right) \end{matrix} > \mathrm{R}-\mathrm{N}=\mathrm{N}-\mathrm{R}\!\left(\mathrm{NO_2}\right); \]

\[ \text{for Ni:}\quad \mathrm{R}\!\left(\mathrm{OH}\right)-\mathrm{N}=\mathrm{N}-\mathrm{R}\!\left(\mathrm{NO_2}\right) > \mathrm{R}-\mathrm{N}=\mathrm{N}-\mathrm{R}\!\left(\mathrm{NO_2}\right) > \begin{matrix} \text{chelate grouping}\\[-2pt] \left(\begin{array}{c} \mathrm{OH}\\[-2pt] \mathrm{N} \end{array}\right) \end{matrix} > \mathrm{R}\!\left(\begin{array}{c}\mathrm{OH}\\ \mathrm{COOH}\end{array}\right). \]

Some properties and analytical data for the azo derivatives obtained are given in Table 1.

Experimental Part

The coupling reaction was carried out in a weakly alkaline medium with all azo components, except α-naphthylamine, for which it was carried out in an acidic medium. To accelerate the azo-coupling reaction, in individual cases pyridine or glycerin was added. The coupling products were carefully freed from the starting substances and recrystallized from alcohol to a constant melting point. In the case of the formation of isomers, their separation was carried out by chromatography. The melting point was determined on a block. The azo derivatives are sparingly soluble in water, slightly soluble in hot alcohol, and better soluble in hot pyridine.

The colorimetric determination of the composition of intracomplex compounds of azo dyes with heavy metals was carried out by Job’s method.

Preparation of intracomplex salts. A weighed portion of 0.5–1 g of the substance was dissolved in 300–350 ml of a 15% pyridine solution; the solution was filtered. To the hot filtrate, 20 ml of a 0.1 M alcoholic solution of cupric chloride was gradually added. The reaction mixture was cooled; the precipitate that separated was filtered off and washed with hot water. Copper intracomplex salts are poorly soluble in organic solvents and have a decomposition temperature lower than that of the pure dyes. The percentage copper content was determined by ignition, and the nitrogen content by the modified Kjeldahl method.

The comparative stability of the intracomplex compounds was determined as follows. To 20 ml of a 0.001 M solution of the dye under investigation, placed in a separatory funnel, 1 ml of a 0.01 M solution of a salt of copper, or nickel, cobalt, or iron was added. The contents were shaken, and a reagent characteristic for the metal was added to the reaction mixture. A 1% solution of sodium diethyldithiocarbamate was chosen as such a reagent; with the given metals it also forms a molecular compound. The contents of the separatory funnel were shaken, and the compound formed by the metal with sodium diethyldithiocarbamate was extracted with chloroform. The colored chloroform solution was transferred to a 10 ml cuvette and, using a red light filter and a standard solution (pure chloroform), was colorimetered. Calibration curves had first been constructed for the salts of copper, nickel, and cobalt with sodium diethyldithiocarbamate. From these, the percentage of metal bound by the reagent was determined, and by difference the metal bound by the azo dye under investigation was determined. The results of the determinations are presented in a summary table.

Table 1.

Properties of the compounds obtained

Conclusions

1. Nineteen azo derivatives of isomeric aminosalicylic acids were obtained, and their chemical analysis was carried out.

2. The composition of the metal-containing complexes of the azo derivatives was determined by colorimetric and chemical methods.

It was established that, for most intracomplex salts, a 1:2 composition is characteristic.

1. The stability of the intracomplex compounds was determined by the colorimetric method. It was shown that the stability of the complexes formed by azo dyes with heavy metals depends on the nature of the metal and on the position of the complex-forming groups in the azo-dye molecule.

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