Chemistry

Academician A. N. NESMEYANOV, E. I. FEDIN, P. V. PETROVSKII,
V. A. DUBOVITSKII, O. V. NOGINA, N. A. LAZAREVA

APPLICATION OF THE N.M.R. METHOD TO THE STUDY OF THE NATURE OF THE TITANIUM–CYCLOPENTADIENYL BOND IN MONO- AND DICYCLOPENTADIENYL DERIVATIVES OF TITANIUM

It was reported earlier \((^1)\) that some, but not all, of the cyclopentadienyl (CPD) derivatives of titanium that we studied form ferrocene in reaction with \(\mathrm{Fe}^{2+}\). These same compounds are readily hydrolyzed and alcoholyzed at the Ti—CPD bond. This gave us grounds to conclude that the Ti—CPD bond has different degrees of ionic character in different CPD derivatives of titanium. Other compounds studied by us do not possess these properties \((^1)\).

Table 1

It turned out, moreover, that the i.r. spectra of compounds that transfer the CPD ring to \(\mathrm{Fe}^{2+}\) differ from the spectra of substances that do not possess this property \((^2)\). (In the former case a shift is observed of the frequencies of the out-of-plane deformation vibrations of the C—H bonds of the CPD ring and of the antisymmetric vibrations of the Ti—CPD bond toward lower frequencies.) For \(\mathrm{C_5H_5Ti(OC_2H_5)_3}\) and \(\mathrm{Ti(OC_2H_5)_4}\), the electrical conductivity* was measured using the apparatus described in \((^3)\) at a frequency of 1000 Hz. Cells of the Jones and Bollinger type were used, with constants 0.0212 and 0.0431 cm\(^{-1}\). It was found that for \(\mathrm{C_5H_5Ti(OC_2H_5)_3}\) the specific electrical conductivity \(\chi_{25^\circ} = 7.5 \cdot 10^{-10}\ \Omega^{-1}\cdot\mathrm{cm}^{-1}\), while for \(\mathrm{Ti(OC_2H_5)_4}\) \(\chi_{25^\circ}\) is practically equal to zero.

The data obtained do not contradict our assumption that in \(\mathrm{C_5H_5Ti(OC_2H_5)_3}\) there is an ionic bond, or a bond close to it, between titanium and the CPD ring.

In the present work we report that CPD derivatives of titanium, both those that form and those that do not form ferrocene in reaction with \(\mathrm{Fe}^{2+}\), differ in the value of the chemical shift of the protons of the CPD rings in high-resolution n.m.r. spectra.

We proceeded from the assumption that changes in chemical shifts in compounds of one metal (in the present case titanium), differing in the nature of the atoms or groups attached to the metal, can basically be explained by changes in the electron density at the C—H bonds, and not by the magnetic anisotropy of the metal atom.

* Measurements of electrical conductivity were carried out at the Institute of Electrochemistry by Yu. M. Kessler and N. M. Allatova, to whom the authors express their deep gratitude.

Table 2

As is evident from the results presented in Tables 1 and 2, a decrease in the number of electron-acceptor chlorine atoms attached to titanium leads to an increase in the electron density on the CPD ring. (The signal of the protons of the CPD ring is shifted substantially into the region of a stronger field.)

Fig. 1. Change in the chemical shift on the δ scale of the protons of the ring (solid line, left scale) and of the methyl protons in the ethoxy group (dashed line, right scale) in the series CH₃C₅H₄Ti(OC₂H₅)₃₋ₙClₙ (A), and in the series C₅H₅Ti(OC₂H₅)₃₋ₙ·Clₙ (B).

Naturally, in the substances listed in Table 2, an additional increase in the electron density on the CPD ring is observed under the influence of the methyl group bonded to it.

The striking linearity of the changes in chemical shifts shown in Fig. 1A is noteworthy. However, in the series C₅H₅Ti·(OC₂H₅)₃₋ₙClₙ \((n = \text{from } 0 \text{ to } 3)\), deviations from additivity were observed (Fig. 1B). The nonobligatory nature of the additivity of chemical shifts in the compounds studied is also evident from the coincidence of the signal positions of the ring protons in methyl- and ethylcyclopentadienyltriethoxytitaniums.

From consideration of the data in Tables 3 and 4 it follows that substances in which, judging by the value of the chemical shift, the electron density at the C—H bonds of the CPD rings is increased form ferrocene in reaction with Fe²⁺. In them, hydrolysis and alcoholysis of the Ti—CPD-ring bond proceed readily (only compounds with unsubstituted rings were studied).

Thus, the data obtained by the high-resolution NMR method are in agreement with the chemical behavior and with the conclusions drawn in the investigation of the IR spectra of the substances studied by us. Apparently, it may be considered established that, as the electron-acceptor properties of the groups attached to the titanium atom decrease, the Ti—CPD-ring bond becomes more ionic, and the covalent character of the bond correspondingly decreases.

The chemical shifts in high-resolution NMR spectra for the protons of the CPD ring in compounds of the type C₅H₅Fe(CO)₂R were investigated by King (4). It was shown that, with weakening of the electron-acceptor properties of R, the signal of the protons of the CPD ring shifts into the region of a stronger field, which is in good agreement with the data of the present work.

It should be noted that, with an increase in the number of chlorine atoms in \( \mathrm{C_5H_5Ti(OC_2H_5)_{3-n}Cl_n} \) (\(n = 0, 1, 2, 3\)), the electron density decreases not only on the protons of the ring, but also on the methylene and methyl protons in the ethoxy group. In order to make sure that there is no substantial contribution to this effect from the magnetic anisotropy of the electron shell of titanium and the electrons of the cyclopentadienyl ring, we measured the chemical shifts of the protons of the \(\mathrm{CH_2}\)- and \(\mathrm{CH_3}\)-groups in the series \((\mathrm{C_2H_5O})_{4-n}\mathrm{TiCl}_n\), where \(n = 0, 1, 2, 3\). The values \(\delta(\mathrm{CH_3})\) and \(\delta(\mathrm{CH_2})\) are 1.16 and 4.34 for \(n=0\); 1.39 and 4.64

Table 3

Table 4

* Substances marked with an asterisk were obtained by us for the first time. A description of their synthesis and properties will be published in the near future.

for \(n=1\); 1.55 and 4.81 for \(n=2\); 1.68 and 4.93 for \(n=3\). As can be seen, in this series an increase in the number of chlorine atoms leads to approximately the same decrease in electron density on the \(\mathrm{CH_2}\)- and \(\mathrm{CH_3}\)-protons of the ethoxy group as in the analogous cyclopentadienyl derivatives of titanium (see Tables 1 and 2).

We consider extremely interesting the fact that the electron-acceptor properties of the chlorine atom are transmitted in the presence of a long chain of single bonds that includes a titanium atom, an oxygen atom, and one or two carbon atoms.

The proton-resonance spectra were recorded on a TsLA-5535 spectrometer with an operating frequency of 40 MHz and proton stabilization of the resonance conditions. The accuracy of measurement of the chemical shifts was $\pm 1 \cdot 10^{-8}$. Sealed cylindrical ampoules 4.7 mm in diameter were used; tetramethylsilane was used as the internal standard. Two of the substances we studied had previously been investigated by the NMR method ($^5$). For $(\mathrm{C}_5\mathrm{H}_5)_2\mathrm{TiCl}_2$ an absolute coincidence of the chemical shift was established; the data for $\mathrm{C}_5\mathrm{H}_5\mathrm{TiCl}_3$ differ by $0.07 \cdot 10^{-6}$, which is apparently connected with insufficient purity of the sample studied in ($^5$) (as the authors of that work later reported ($^6$)), and also with insufficient stability of the position of the tetrahydrofuran peak taken as the standard. The spin–spin coupling constant in the ethyl- and ethoxy-substituted compounds proved to be close to 7 Hz and did not reveal any specific dependence on the structure of the molecule.

Splitting of the signals of the CPD protons was observed only in titanium CPD derivatives substituted in the ring. The magnitude of this splitting is characteristic of substituted aromatic systems. In all other cases the CPD protons gave a narrow singlet, unambiguously indicating their equivalence.

Institute of Organoelement Compounds
Academy of Sciences of the USSR

Received
2 III 1963

CITED LITERATURE

$^1$ A. N. Nesmeyanov, O. V. Nogina, V. A. Dubovitskii, Izv. AN SSSR, OKhN, 1962, 1481.
$^2$ G. G. Dvoryantseva, Yu. N. Sheinker et al., DAN, 161, No. 3 (1965).
$^3$ E. Sevastyanov, A. I. Gorbanev, Yu. M. Kessler, ZhFKh, 36, 644 (1962).
$^4$ R. B. King, J. Am. Chem. Soc., 85, 1918 (1963).
$^5$ C. L. Sloan, W. A. Barber, J. Am. Chem. Soc., 81, 1364 (1959).
$^6$ C. L. Sloan, W. A. Barber, Chimica e industria, 42, 877 (1960).