PHYSICAL CHEMISTRY

A. N. RODIONOV, T. V. TALALAEVA, D. N. SHIGORIN, G. N. TYUMOFEYUK,
Corresponding Member of the Academy of Sciences of the USSR K. A. KOCHESHKOV

STUDY OF THE STRUCTURE OF COMPLEXES OF ALIPHATIC ORGANOLITHIUM COMPOUNDS

It is known that organolithium compounds are associated; the nature of these associates has not yet been definitively clarified, but it is evidently connected with the interaction of the groups CH$_2$—Li, leading to the formation of multicenter electron orbitals. The capacity of organolithium compounds for association is apparently decisive in the behavior of this class of compounds in numerous reactions.

With the aim of further investigating the structure of this class of compounds, we studied the IR spectra of aliphatic molecules R—Li. Their molecular weights were determined by the ebullioscopic method. In order to be able to assign the bands in the IR spectra accurately to definite vibrations and to clarify the participation of individual groups and atoms in the association, the IR spectra of isotopically substituted ethyllithium molecules were measured

\[ (\mathrm{C}_2\mathrm{H}_5\mathrm{Li}^7,\ \mathrm{C}_2\mathrm{H}_5\mathrm{Li}^6,\ \mathrm{CH}_3\mathrm{CD}_2\mathrm{Li}^7,\ \mathrm{CD}_3\mathrm{CH}_2\mathrm{Li}^7,\ \mathrm{CD}_3\mathrm{CD}_2\mathrm{Li}^7) \]

and of n-amyllithium (n-C$_5$H$_{11}$Li$^7$, n-C$_5$H$_{11}$Li$^6$) in the region from 4000 to 400 cm$^{-1}$. Ethyllithium derivatives were investigated in the vapor phase (beginning of distillation at $p \approx 8 \cdot 10^{-3}$ mm Hg and $t = 90$–98°), in solutions (benzene, hexane, diethyl and dibutyl ethers), and in crystals (in vaseline oil and in benzene solution at −70°).

n-Butyllithium, n-amyllithium, and n-dodecyllithium were studied only in solutions (benzene, hexane, cyclohexane, diethyl ether).

The IR spectra were measured on a Hilger H-800 double-beam spectrometer with LiF, NaCl, and KBr prisms. Ebullioscopic measurements were carried out on the apparatus described by A. I. Shatenshtein et al. ($^1$).

Because of the great sensitivity of organolithium compounds to oxygen and moisture, all operations for preparing the substances and assembling the cuvettes were carried out in an atmosphere of dry argon.

Table 1 gives the principal bands observed in the IR spectra of isotopically substituted ethyllithium molecules in the vapor phase and in benzene solutions. From these data it is evident that the spectra for one type of molecule in the vapor phase and in benzene solution differ little from one another, whereas isotopic substitutions sharply change the spectrum.

Thus, the intense band near 500–550 cm$^{-1}$ is noticeably shifted, upon replacement of Li$^7$ by Li$^6$, toward shorter wavelengths, and in the opposite direction upon replacement of H by D in the $\alpha$-CH$_2$ group. Approximate calculations for the valence vibration of the CH$_2$—Li group under the above-mentioned substitutions give an analogous change in the frequency of this vibration. This confirms the correctness of assigning the band near 500–550 cm$^{-1}$ to the vibration of the CH$_2$—Li group in the complex ($^{2,3}$). The frequency of this vibration, moreover, is somewhat increased on going from ethyllithium to longer-chain organolithium compounds and decreases when neutral solvents are replaced by donor ones (Table 2). This is apparently connected mainly with the different strength, geometry, and type of complexes, which are affected by the polarity of the bond, steric hindrance of the radicals, the donor properties of the solvents, etc. The degree of association in this case noticeably

Table 1

IR spectra of isotopically substituted ethyllithium molecules

Table 2

Vibrational frequencies of CH₂—Li groups in complexes

has no effect on the position of the band of the CH₂—Li group. Thus, although the degree of association decreases as the radical becomes more complex (ethyllithium and n-dodecyllithium in benzene, Table 3), the position of the absorption band of the CH₂—Li group differs by 30 cm⁻¹ in the hexameric complexes of ethyllithium and n-butyllithium in benzene and nearly coincides for hexameric n-butyllithium and the tetrameric complex of n-dodecyllithium in benzene. The lowering of the CH₂—Li vibrational frequency in ether solutions is apparently connected with the formation of complex acceptor–donor complexes between the initial complexes of the organolithium compound and ether molecules. At the same time, preservation of the hexameric complex in ether makes n-butyllithium more stable in this solvent than ethyllithium, which lowers the degree of association in ether at low concentrations to 4.5–5.

Table 3

Association factors of organolithium compounds

It was noted earlier that in the spectra of ethyllithium solutions there is an intense band near 920 cm\(^{-1}\), the origin of which was unknown (Table 1). Investigation of the infrared spectra of isotopically substituted molecules showed that this band is shifted upon various substitutions of H by D, but is not changed when Li\(^7\) is replaced by Li\(^6\). An approximate calculation for the C—C stretching vibration in five isotopically substituted ethyllithium molecules gave values close to the experimental ones. This made it possible to assign the bands in the region 925—850 cm\(^{-1}\), observed in the spectra of ethyllithium vapors and solutions, predominantly to the C—C stretching vibration.

In the region of the C—H and C—D stretching vibrations, along with absorption lying within the limits usual for aliphatic radicals, intense bands are observed that are shifted by 50—100 cm\(^{-1}\) toward longer wavelengths (Table 1). The use of isotopic substitution in individual groups of the ethyl radical made it possible to establish that the frequencies of the absorption bands of the CH\(_3\) groups do not differ noticeably from their usual values. The shifted bands, however, belong to vibrations of the CH\(_2\) groups located in the α-position to the lithium atom. Moreover, these lowered bands are also observed in the spectra of n-butyllithium, n-dodecyllithium, etc.

An approximate calculation shows that the distortion of the H—C—Li angles possible upon formation of complexes cannot explain such a strong lowering of the frequency of the C—H stretching vibration in the α-CH\(_2\) group. Apparently, this is connected with a change in the valence state of the α-carbon atom as a result of an increase in the negative charge on it due to the inductive effect of the neighboring electropositive lithium atom. According to Bent’s data (\(^4\)), an increase in the negative charge on the α-C atom should lead to some lengthening and weakening of the C—H bond located in the α-position relative to Li, and consequently to a lowering of the frequency of the stretching vibration of these bonds.

The calculation shows that, in order to lower the frequency of the C—H stretching vibration by 50—100 cm\(^{-1}\), the force constant of this bond must be decreased from \(8 \cdot 10^6\) to \(7 \div 7.5 \cdot 10^6\) cm\(^{-2}\), which, in comparison with Linnett’s data (\(^5\)), is entirely possible for such an electropositive element as lithium. On going from solution to the crystalline state (in the case of ethyllithium), there occurs a considerable lowering of the vibration frequency of the CH\(_2\)—Li groups (Table 2) and a decrease in the difference in the positions of the C—C vibration band upon various isotopic substitutions. Substances that do not pass under the experimental conditions (benzene solution at −70°) into the crystalline state change their spectrum only slightly (n-butyllithium, n-dodecyllithium).

It is characteristic that in the spectra of ethyllithium derivatives, on going from solution to the crystalline state, only the isotopic shift upon replacement of Li\(^7\) by Li\(^6\) remains almost constant \(\left(\frac{\nu_{\mathrm{CH}_2-\mathrm{Li}^6}}{\nu_{\mathrm{CH}_2-\mathrm{Li}^7}}\right.\) in solutions 1.037; in crystals 1.040), whereas replacement of H by D leads to a larger change in it \(\left(\frac{\nu_{\mathrm{CH}_2\mathrm{Li}^7}}{\nu_{\mathrm{CD}_2-\mathrm{Li}^7}}\right.\) in solutions 1.029, in crystals 1.016).

The strong shift of the vibrational bands of the \(\mathrm{CH_2—Li}\) groups, observed on passing from one aggregate state to another, and also the noticeable decrease in the isotopic shift upon replacement of H by D, are apparently connected with a change in the structure of the complexes. The valence vibrations \(\mathrm{C—H}\) (\(\mathrm{C—D}\)) change only slightly in these transitions.

Physicochemical Institute
named after L. Ya. Karpov

Received
6 V 1963

CITED LITERATURE

1. A. I. Shatenshtein, Yu. P. Vyrskii et al., Temporary instruction on methods for determining molecular weight, Physicochemical Institute named after Karpov, Moscow, 1961.
2. R. West, W. Glaze, J. Am. Chem. Soc., 83, 3580 (1961).
3. A. N. Rodionov, D. N. Shigorin et al., DAN, 143, No. 1, 137 (1962).
4. H. Bent, J. Chem. Phys., 33, 1258 (1960).
5. I. Linnett, Trans. Farad. Soc., 41, 223 (1945).