Physical Chemistry

A. E. Borisov, N. V. Novikova, and N. A. Chumaevskii

Infrared Absorption Spectra of Organometallic Compounds of the Ethylene Series

On the cis- and trans-configurations of propenylantimony compounds

($\mathrm{Sb}^{\mathrm{III}}$ and $\mathrm{Sb}^{\mathrm{V}}$)

(Presented by Academician I. V. Obreimov, July 18, 1960)

The present communication is devoted to an investigation of the infrared absorption spectra of the cis- and trans-isomers of propenyl compounds of trivalent and pentavalent antimony. The synthesis of these substances has been described$^{(1)}$. As far as we know from the literature, there are no special studies of cis- and trans-isomerism in organometallic compounds by means of infrared absorption spectra.

In the present work we discuss the results of studying the i.r. absorption spectra of the above-mentioned compounds. A number of communications$^{(2,3)}$ indicate the possibility of identifying cis- and trans-isomers in organic compounds by i.r. absorption bands at 690 cm$^{-1}$ (for cis-) and 960–970 cm$^{-1}$ for trans-configurations; these data apply mainly to disubstituted hydrocarbons of the ethylene series, of the type $\mathrm{R}_1-\mathrm{CH}=\mathrm{CH}-\mathrm{R}_2$ (cis- and trans-).

The distinction between configurations according to these works is based in most cases on the presence of a 970 cm$^{-1}$ band in the trans- and its absence in the cis-configurations. We note at once that the calculations of work$^{(4)}$ show a small difference in the frequencies of the absorption bands of out-of-plane vibrations of CH groups for

Fig. 1

double bond (the interval 965–980 cm\(^{-1}\) is given for both cis- and trans-configurations, the type of vibrations

\[ \begin{array}{c} \mathrm{H}^{+}\quad +\mathrm{H}\\[-2pt] \quad \backslash \ \ /\\[-2pt] \mathrm{C}=\mathrm{C}\\[-2pt] \quad / \ \ \backslash\\[-2pt] \mathrm{R}\quad \mathrm{R} \end{array} ,\qquad \begin{array}{c} \mathrm{R}\quad +\mathrm{H}\\[-2pt] \quad \backslash \ \ /\\[-2pt] \mathrm{C}=\mathrm{C}\\[-2pt] \quad / \ \ \backslash\\[-2pt] \mathrm{H}^{+}\quad \mathrm{R} \end{array} ), \]

the authors of that work used experimental data \((^{5})\), from which it is also evident that, for cis- and trans-configurations, the frequencies of the absorption bands associated with out-of-plane vibrations of CH bonds in disubstituted butenes differ very little in practice.

It follows from what has been said that in a number of cases, relying only on the above-mentioned criteria, it is impossible to give a completely unambiguous answer concerning the type of geometrical configuration; and the interpretation of the results obtained with respect to cis-, trans-configurations must be treated with extreme caution.

The infrared absorption spectra were recorded by us on a two-beam IR spectrometer based on the VIKS M-3 spectrometer with a NaCl prism (700–1800 cm\(^{-1}\)) and on a single-beam IKS-12 spectrometer with a KBr prism (400–700 cm\(^{-1}\)). Liquid substances were recorded in cells of constant thickness (0.016 mm with KBr windows and 0.050 mm with NaCl windows). Solid compounds were pressed with KBr.

Fig. 2

The assignment of the configurations of the stereoisomers under discussion was made by comparing the infrared absorption spectra \((^{6})\) and on the basis of the regularity, previously established by us together with A. N. Nesmeyanov on extensive material, concerning the non-inversion of the configuration of cis–trans ethylene isomers in reactions of electrophilic and homolytic exchange \((^{1,7,8})\). In all the propenyl antimony compounds investigated by us (\(\mathrm{Sb}^{\mathrm{III}}\) and \(\mathrm{Sb}^{\mathrm{V}}\)), a fairly appreciable difference is seen between the cis- and trans-configurations in their infrared absorption spectra (Figs. 1 and 2, Table 1).

For all compounds with trans-configurations (trans arrangement of hydrogens with respect to the double bond), the presence of intense absorption bands in the region 945–970 cm\(^{-1}\) is characteristic. If for tri- and pentapropenylantimony the frequency of the out-of-plane vibrations of the CH groups lies at 971 cm\(^{-1}\), then for

halogen derivatives, namely: for dichloro-, dibromo-, and diiodotripropenylantimony the frequency decreases regularly to 945 cm\(^{-1}\); for bromotetrapropenylstibonium the frequency lies in the range 967 cm\(^{-1}\) (see Table 1).

Table 1

Frequencies of propenyl compounds of Sb\(^{III}\) and Sb\(^{V}\)

Another essential indication of the trans configuration of the compounds we investigated is the presence in their infrared spectra of absorption bands at 718–726 cm\(^{-1}\) (Table 1, Figs. 1 and 2), whereas for the cis configurations these bands are absent.

Fig. 3

For the cis configurations of the compounds investigated, the infrared spectra contain absorption bands at about 920–940 cm\(^{-1}\), of significantly lower intensity than the absorption bands for the trans configurations at 945–970 cm\(^{-1}\). Only cis-tripropenylantimony and cis-pentapropenylantimony have out-of-plane CH-vibration frequencies close to the value for the trans form (970 cm\(^{-1}\)), but the intensity of this band in the cis form (estimated from band areas) is approximately three times lower than in the trans configuration. The same picture is observed for propenyl bromide (cis and trans) (see Table 2, Fig. 3): here, for the cis configuration, the intensity of the band at 930 cm\(^{-1}\) is also lower than in the trans form, by approximately a factor of three.

For both cis and trans compounds, the infrared spectra contain absorption bands at 655–660 cm\(^{-1}\); for the cis form the intensity of these bands is 2—

2.5 times greater than for the trans- (estimated from the example of tripropenylstibine and the bromide of cis- and trans-propenyl). Among the low-frequency bands one should note the rather intense bands at 452 cm\(^{-1}\), which appear in the IR spectra of the cis compounds of dichloro-, dibromo-, diiodotripropenyl- and bromotetrapropenylstibonium; for the analogous trans compounds there are no absorption bands at 452 cm\(^{-1}\). These bands are also absent both in cis- and in trans-tri- and pentapropenylstibine.

Table 2

Frequencies of propenyl bromide

As for the absorption bands caused by out-of-plane* vibrations of the CH groups at the double bond (1200–1300 cm\(^{-1}\)), they are practically indistinguishable in frequency, but between the pairs of bands 1) 1200 cm\(^{-1}\) and 2) 1300 cm\(^{-1}\) in the cis and trans compounds a different ratio of intensities is observed (intensity at the band maximum).

If for the trans isomers the intensities of the absorption bands at 1300 cm\(^{-1}\) are less than at 1200 cm\(^{-1}\), then for the cis compounds, conversely, the absorption bands at 1200 cm\(^{-1}\) are less intense than those at 1300 cm\(^{-1}\) in the cis. In the region of deformation vibrations of the carbon—hydrogen bonds (1380–1450 cm\(^{-1}\)) no substantial differences are observed. The frequency of the double bonds is practically identical for the cis and trans isomers; the absorption bands have approximately the same intensity.

The authors express their gratitude to Academician I. V. Obreimov for his interest in the work, and also to R. A. Isaeva and E. D. Vlasov for assistance in recording the IR absorption spectra.

Institute of Organoelement Compounds
Academy of Sciences of the USSR

Received
16 VI 1960

REFERENCES CITED

1. A. N. Nesmeyanov, A. E. Borisov, N. V. Novikova, Izv. AN SSSR, OKhN, 1959, 1216.
2. L. Bellamy, Infrared Spectra of Molecules, IL, 1957, p. 53.
3. N. Sheppard, J. Inst. Petrol., 37, No. 327, 99 (1951).
4. N. Sheppard, A. B. B. M. Sutherland, Proc. Roy. Soc., A196, 195 (1949).
5. H. Gerschinowitz, E. B. Wilson, J. Chem. Phys., 6, 247 (1938).
6. A. N. Nesmeyanov, A. E. Borisov, N. V. Novikova, Izv. AN SSSR, OKhN, 1960, No. 1, 147; Tetrahedron Letters, No. 8, 23 (1960).
7. A. N. Nesmeyanov, A. E. Borisov, DAN, 60, 67 (1948).
8. A. N. Nesmeyanov, A. E. Borisov, N. V. Novikova, DAN, 119, 504 (1958).

* Vibrations without displacement out of the plane of the C=C bond.