Complexes of Some Polyheterocyclic Amines with Phenols

B. V. Tronov, O. A. Terekhova

For the most readily available simple heterocyclic amines—pyridine and piperidine, and also for quinoline, in the molecule of which we have a combination of a pyridine ring with a benzene ring—the reactions of complex formation with various substances, both inorganic and organic, have already been studied quite well. A large number of complex compounds have been found, especially for pyridine, and certain regularities have been established in the dependence of the ability to form complexes on the composition and structure of the amines and of the other components of the systems under investigation [1].

It was of interest to study in this respect also more complex amines, namely those containing two pyridine rings.

In the present work we used 4,4′-dipyridyl, 2,3′-diquinolyl, and 6,6′-diquinolyl. Phenols, which have long been known as good complex-forming agents, were chosen as the second components of the complex-forming systems. For comparison (with nitrophenols), experiments were also carried out with m-dinitrobenzene and with symmetrical trinitrotoluene. The investigation was conducted by the preparative method and by the method of thermal analysis.

Experimental Part

Isolation and analysis of complex compounds.

The synthesis of the complexes was carried out by mixing saturated solutions of the starting substances in alcohol or benzene. In the present work alcohol proved to be the better solvent.

In most cases, after the mixed solutions had been cooled, a precipitate separated. Sometimes precipitation began after the solution had stood for some time. The precipitate was separated from the solution, washed, and dried at room temperature. If the melting point was broad, the complex was recrystallized from benzene or alcohol. The homogeneity of the crystals was checked under a microscope.

The composition of the complexes was established from the nitrogen content and also from the halogen content, if a halogen-containing phenol had been taken for the reaction. In complexes of nitrophenols it was sometimes difficult, from the nitrogen content, to draw a conclusion about the composition of the complex. In such cases the amine was additionally determined by the gravimetric method in the form of a salt with chromic acid. The method was first developed on specially prepared mixtures. It was found that the error in determining amines in a mixture with phenols and mononitrophenols did not exceed 1%, and with dinitrophenols reached up to 3%. However, the above-mentioned errors did not prevent a confident judgment regarding the composition of the complexes. For example, in the system 6,6′-diquinolyl–p-nitrophenol, for a molar composition of 1:1 the percentage of amine is 64.3, and for a composition of 1:2 it is 47.9. Meanwhile, the difference in nitrogen content (10.65% and 10.48%) lies within the possible error of ordinary analytical determinations.

Halogen was determined by the Robertson method [6].

The following compounds were isolated (Table 1).

Table 1

Results of analysis

All the compounds obtained are stable in air. They are poorly soluble in organic solvents and are decomposed by acids into their constituent parts. The melting point in most cases lies above the melting points of both starting substances.

Complexes with phenols that do not contain a nitro group are colorless; with mononitrophenols and 2,4-dinitrophenol they are yellow; with 2,6-dinitrophenol and 2,6-dibromo-4-nitrophenol they are bright orange. With phenol, as well as with meta-dinitrobenzene, it was not possible to isolate complexes. Only 6,6′-diquinolyl reacted with nitrotoluene.

Thermal Analysis

The study was carried out by the method proposed by Klochko-Zhovnir Yu. F. [5], in which an apparatus is used for determining the melting temperature in a capillary. A flask with sulfuric acid served as the bath.

The components were taken in ratios of 10, 20, 30, 40, etc., mole percent of one of them to 90, 80, 70, 60, etc., mole percent of the other. Near the eutectic point, in order to refine its composition and melting temperature, mixtures of intermediate compositions were additionally examined.

The melting (crystallization) temperature of the eutectic was taken to be the temperature at which a transparent liquid first appeared on the surface of the solid phase. The temperature at which the last crystal of the solid phase disappeared and the contents of the capillary became completely transparent was taken as the temperature at which crystallization of the mixture began.

This method makes it possible to work with a small quantity of substance. Table 2 gives a summary of the data on the composition of the complexes found.

Table 2

Discussion of the Results

The results of our study show, first of all, that with lengthening of the molecules of heterocyclic amines their ability to form complexes with phenols becomes weaker. Thus, pyridine with phenol in benzene solution gives complexes of composition 1:1 and 2:1 (6, 7), which were detected by an electrochemical method. Experiments with 4,4′-dipyridyl under these conditions did not give a positive result. For quinoline, complexes with resorcinol and hydroquinone were isolated (6, 7). 2,3′-Diquinolyl under analogous conditions does not form complexes, while 6,6′-diquinolyl gave a complex only with hydroquinone. In general, 2,3′-diquinolyl, in which the bond between the two quinoline groupings is through the pyridine rings, proved to be less reactive than 6,6′-diquinolyl. This is also confirmed by the fact that compounds with mononitrophenols were isolated only for 6,6′-diquinolyl.

A comparison of different phenols with respect to their capacity for complex formation shows that accumulation of hydroxyls and nitro groups for the most part increases the reactivity of the phenol.

It may be considered that the reactivity of phenols in the reaction studied by us is only partly connected with the constant of electrical dissociation. The same was observed in other works for amines [8].

Besides simple electrostatic attraction, mutual polarization of the components of the complex-forming [[unclear: continuation cut off]]

of the forming system. The course of the reaction is also influenced by the polarizing action of the solvent. Polar alcohol creates more favorable conditions for complex formation than nonpolar benzene.

As for the structure of the complexes that arise, in most of the systems we studied complex formation must proceed through the formation of a hydrogen bond. Such should be the structure of the colorless compounds with phenol, naphthols, dioxybenzenes, trioxybenzenes, and of the pale-yellow complexes with mononitrophenols and 2,4-dinitrophenol. The complexes of 2,6-dinitrophenol and 2,6-dibromo-4-nitrophenol have a bright orange color. The bond between the molecules of nitrophenol and amine in them is probably effected through the amino nitrogen (electron donor) and the positively charged nitrogen of the nitro group (electron acceptor).

Conclusions

1. Sixteen complex compounds of 4,4′-dipyridyl, 2,3′-diquinolyl, and 6,6′-diquinolyl with organic substances, chiefly with phenols, have been obtained.

2. By the method of thermal analysis, 11 systems of the above-mentioned amines with phenols and nitrophenols have been studied. In 9 of them the formation of fairly stable compounds was found; in two systems complexes with an incongruent melting point were detected.

3. It has been shown that doubling of pyridine and quinoline molecules weakens the ability of the nitrogen of the pyridine ring to form complexes with phenols, while the mutual influence of the pyridine rings in 2,3′-diquinolyl is stronger than in 6,6′-diquinolyl.

4. Considerations have been expressed concerning the structure of the complex compounds found.

LITERATURE

1. Dionis’ev D. E., Kirillova A. D. ZhOKh, 22, 2090, 1952.
2. Thorpe J., Whiteley M. Practical Guide to Organic Analysis. ONTI, p. 63, 1937.
3. Klochko-Zhovnir Yu. F. ZhPХ, 21, issue 3, 1948.
4. Tronov B. V., Kulev L. P. Izv. Tomsk Polytechnic Institute, 64, 3, 1948.
5. Tronov B. V. and Bortovoi I. M. ZhOKh, 24, 1750, 1954.
6. Nosc K. Ber. 16, 886, 1883.
7. Baeyer A., Villiger V. Ber. 35, 1208; 1902.
8. Chugaev L. A. ZhRKhO, 41, 285; 1909.