Chemistry

I. P. Gol’dshtein, Z. F. Il’icheva, N. A. Slovokhotova,
E. N. Gur’yanova, and Corresponding Member of the USSR Academy of Sciences K. A. Kocheshkov

Spectroscopic Study of the Complexes of Thiophane and Thiophene with Tin Tetrachloride

Using the method of dielectrometric titration it was shown (¹) that the interaction of certain heterocyclic compounds with metal halides leads either to the formation of donor–acceptor complexes (thiophane), or to the formation of π-complexes (furan, thiophene). In the present work this question was investigated spectroscopically. The infrared spectra of the systems thiophane + SnCl₄ and thiophene + SnCl₄ were measured on an H-800 double-beam spectrophotometer in sealed NaCl cells in the region 1800–650 cm⁻¹. The electronic spectra of these systems were recorded in benzene solution on an SF-4 spectrophotometer. Preparation of the mixtures and filling of the cells were carried out in a sealed chamber under an argon atmosphere. The complex of thiophane with SnCl₄ is a white crystalline substance of composition SnCl₄·2C₄H₈S with a melting point of 154–155°C. For measurement of the infrared spectra, the samples were prepared as a paste in Vaseline oil.

The thiophene + SnCl₄ system was studied in solution in excess SnCl₄. Several days after preparation, the pale-yellow solution turned bright orange, and then a red precipitate of composition SnCl₄ : C₄H₄S = 1 : 4 separated from the solution. In order to remove SnCl₄, the precipitate was dissolved in dioxane (with which SnCl₄ gives stable complexes), and then a yellowish precipitate was again deposited from the dioxane solution by hexane. The infrared spectra of these precipitates were measured in Vaseline oil.

In the spectrum of the complex of thiophane with SnCl₄, in comparison with the spectrum of pure

Table 1

Infrared spectra of thiophane and of a solution of thiophane in SnCl₄

* Estimate of band intensity: v.s. — very strong, strong — strong, med. — medium, weak — weak.

thiophane (Table 1), a shift of the band at 685 cm⁻¹ into the region of 660 cm⁻¹ is observed. The band at 685 cm⁻¹ belongs to vibrations of the C—S bond in the thiophane ring (²). The lowering of the vibration frequency of this bond by 25 cm⁻¹ in the complex indicates that the complex is formed through donor–acceptor interaction of SnCl₄ with the unshared electron pair of the sulfur atom. This conclusion agrees with the conclusion drawn from measurements of dipole moments

Table 2

Infrared spectra of thiophene, an aged* solution of thiophene in SnCl₄, and the complex of thiophene oligomers with SnCl₄

* Measured two days after preparation.

(¹). In the infrared spectrum of a freshly prepared solution of thiophene in SnCl₄ (Fig. 1a, Table 2), the position of the 715 cm⁻¹ band, assigned to vibrations of the C—S bond in the ring (³), does not change in comparison with the position of this band in the spectrum of pure thiophene. This fact gives grounds for asserting that the complex of thiophene with SnCl₄ is not formed through donor–acceptor interaction of SnCl₄ with the sulfur atom in the ring. All the other absorption bands in the infrared spectrum of this solution likewise do not change, except for the 1595 cm⁻¹ band, whose intensity increases and whose maximum is shifted by approximately 10 cm⁻¹ toward the low-frequency region. The 1595 cm⁻¹ band is assigned to vibration of the thiophene ring (³).

A similar change in the intensity of the absorption band of benzene-ring vibrations in the region of 1596 cm⁻¹ was observed in the spectrum of the complex of antimony trichloride with benzene (⁴), and it was shown that the SbCl₃ complex with benzene belongs to π-complexes. Consequently, the complex formed in a freshly prepared solution of thiophene with SnCl₄ may also be regarded as a π-complex. Such an interpretation agrees with the results of measuring the dipole moment of this complex (¹). Moreover, this is confirmed by the appearance of an absorption band in the region of 510 mμ in the electronic spectrum of a solution of thiophene and SnCl₄ in benzene (Fig. 2), since we previously showed (⁵) that, upon formation of the π-complex of SnCl₄ with diphenylethylene, an absorption band appears in its electronic spectrum in the visible region. However, in the infrared spectrum of a solution of thiophene in SnCl₄ measured several days after its preparation, obser—

Fig. 1. IR spectra: a — solid line — thiophene (d = 40 mμ), dotted line — fresh solution of thiophene in SnCl₄ (d = 60 mμ); b — solution of thiophene in SnCl₄ two days after preparation.

very substantial changes are observed in comparison with the spectrum of pure thiophene, as well as with the spectrum of a freshly prepared solution (Fig. 1b, Table 2): some absorption bands shift and a number of new bands appear at 1500, 1440, 1175, and 980 cm(^{-1}), while the intensity of the newly appearing bands at 1500 and 1440 cm(^{-1}) increases noticeably with time. In the region 1400–1450 cm(^{-1}) lie the absorption bands of deformation vibrations of the CH bond; in the spectrum of thiophene the corresponding band is in the region of 1415 cm(^{-1}), and in the spectrum of thiophane it is in the region of 1445 cm(^{-1}). Consequently, the appearance of a band in the region of 1440 cm(^{-1}) in the spectrum of a solution of thiophene in SnCl(_4), measured two days after preparation, indicates a strong change in the thiophene molecules during aging of the solution.

Fig. 2. Electronic spectrum of a solution of thiophene and SnCl(4) in benzene.
(C = 0.431) mol/l. Ratio}
(C_{\mathrm{C_4H_4S}} : C_{\mathrm{SnCl_4}} = 2)

The spectrum of the solid red precipitate that separated from a solution of thiophene in SnCl(_4) after several days is very similar to the spectrum of the aged solution, but in addition to the already indicated new bands, a band appears in the spectrum of the precipitate in the region of 687 cm(^{-1}) (Fig. 3), i.e., in the same region where the band assigned to the C—S bond lies in the spectrum of thiophane. This is also an indication that, in the system under consideration, a process occurs that changes the thiophene ring. Such a process is polymerization, which takes place in thiophene in the presence of SnCl(_4) ((^{6})).

As Korsiak et al. ((^{7})) and also Topchiev et al. ((^{8})) have shown, during the polymerization of furan and silvan under the influence of SnCl(_4), the conjugated system of double bonds in the furan ring disappears and a polymer with one isolated double bond in each ring constituting the chain is formed. There is every reason to believe that the polymer formed during the polymerization of thiophene in the presence of the same catalyst must have a similar structure. The intense coloration of the precipitate separating from a solution of thiophene in SnCl(_4), and its composition, indicate that this precipitate is a complex of thiophene oligomers with SnCl(_4). Only in this way can one explain the absence, in the infrared spectrum of the precipitate, of the band corresponding to an isolated double bond in the region 1650–1670 cm(^{-1}), and the appearance of a new band in the region of 1500 cm(^{-1}). The latter band should be assigned to the vibration frequency of the double bond in (\pi)-complexes of thiophene oligomers with SnCl(_4), lowered by 150–170 cm(^{-1}), similar to what was observed in the formation of a (\pi)-complex of diphenylethylene and its dimer with SnCl(_4) ((^{5})). A band in the region of 1650 cm(^{-1}) appeared in the spectrum of the precipitate after it had been washed with dioxane to remove SnCl(_4), while the band in the region of 1500 cm(^{-1}) disappeared. This serves as proof that the precipitate separating from a solution of thiophene in SnCl(_4) is a complex of thiophene oligomers with SnCl(_4).

Fig. 3. IR spectra in the region 800–650 cm(^{-1}) of thiophene (1) and of the precipitate separated from a solution of thiophene in SnCl(_4) (2)

The small second maximum in the region of 560 mµ in the electronic spectrum of a solution of thiophene with SnCl(_4) in benzene (Fig. 2) can also be explained by the formation of complexes of thiophene dimers or trimers with SnCl(_4).

Consequently, in solutions of thiophene in SnCl(_4), (\pi)-complexes are first formed through interaction of SnCl(_4) with the (\pi)-electrons of the thiophene ring,

which leads to the gradual polymerization of thiophene, and the thiophene oligomers formed also give $\pi$-complexes with $\mathrm{SnCl}_4$.

Physicochemical Institute
named after L. Ya. Karpov

Received
3 III 1962

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