UDC 542.957

R. G. KOSTYANOVSKII, A. K. PROKOF’EV

THREE-MEMBERED RINGS WITH A COORDINATION BOND

(Presented by Academician N. N. Semenov, 1 VII 1965)

In previous communications we discussed the “lateral” electronic interaction in the system

\[ >\mathrm{N}-\mathrm{C}-\mathrm{X}, \]

where X is an atom with an unshared electron pair \((^{1,2})\). Most naturally, the N—X interaction in this system may be represented when atom X has vacant orbitals. In this case a three-membered ring with a coordination bond is possible in principle:

\[ \begin{array}{c} >\mathrm{N}\\[-2mm] \downarrow\\[-1mm] \mathrm{X} \end{array} \]

An unambiguous proof of such an effect is of interest not only in itself. It would have far-reaching consequences for considering numerous transformations of geminal systems in a unified way.

By the method described earlier \((^{3})\), we obtained and studied a series of geminal derivatives with a tin atom (Table 1). The structure of the products was confirmed by NMR data (Table 2), IR and mass spectra (Fig. 1)*.

Table 1

Compounds: I. \((\mathrm{CH_3})_3\mathrm{SnCH_2N}\)(morpholine), II. \((\mathrm{CH_3})_3\mathrm{SnCH_2N}^{+}(\mathrm{CH_3})\)(morpholine) \(\mathrm{I}^{-}\), III. \((\mathrm{CH_3})_3\mathrm{SnCH_2}\)—\( \mathrm{N}^{+}\mathrm{H}\)(morpholine) \(\mathrm{Cl}^{-}\), IV. \((\mathrm{CH_3})_3\mathrm{SnCH_2N}\)(aziridine), V. \((\mathrm{CH_3})_3\mathrm{SnCH_2N}\)(pyrrole), VI. \((\mathrm{CH_3})_3\mathrm{SnCH_2OCH_3}\), VII. \((\mathrm{C_2H_5})_3\mathrm{SnCH_2OCH_3}\), VIII. \((\mathrm{C_2H_5})_3\mathrm{SnCH_2CN}\), IX. \((\mathrm{CH_3})_3\mathrm{SnCH_2OCOCH_3}\), X. \((\mathrm{C_2H_5})_3\mathrm{SnCH_2OCOCH_3}\), XI. \((\mathrm{CH_3})_3\mathrm{SnCH_2Cl}\), XII. \((\mathrm{CH_3})_3\mathrm{SnCH_2F}\).

* Literature data: b.p. 44–48/15; \(n_D^{20}\) 1.4860; \(d_4^{25}\) 1.5560 \((^{4})\).

Determination of the basicity of aminomethylstannanes (Table 3) shows that the basicity constant \((K_b)\) is 4 orders of magnitude lower than in the corresponding carbon analogs. A less pronounced decrease in \(K_b\) is observed for aminomethylgermanes \((^{5})\) and contradictory data for silanes \((^{5,6})\). Lower—

* In the first spectrum of Fig. 1 the maximum peak is reduced 10-fold.

Table 2

Chemical shifts in NMR spectra relative to the signal of hexamethyldisiloxane
(measured on a JNM-C-60 instrument) and in nuclear gamma-resonance (NGR) spectra
relative to SnO\(_2\) \(^{(10)}\)

Table 3

Basicity of aminomethylstannanes, -germanes, and -silanes
(determined by potentiometric titration with respect to pH during
half-neutralization in aqueous dioxane, \(C \sim 10^{-2}\) mole/l at 20°
on an LPU-01 instrument)

…tion of the basic properties is manifested in the iodomethylation reaction of I, which proceeds with an almost quantitative yield only after 27 days (28 hr, 37.6%; 94 hr, 67.6%; 195 hr, 80%; 648 hr, 94.4%). Aminomethylgermanes and silanes are iodomethylated completely in 8–10 hr \(^{(5)}\).

A considerable decrease in the basic properties of nitrogen can be explained by a change in the coordination number of tin and by the formation of an intramolecular coordination bond \(\mathrm{N \to Sn}\), with some transfer of the unshared electron pair of nitrogen to the vacant orbital of tin with partial \(sp^3d\) character.

From the decrease in basicity one can estimate the upper limit of the strength of the \(\mathrm{N \to Sn}\) bond. From

\[ \frac{k_1}{k_2}=\exp\left(-\frac{\Delta\Delta E}{RT}\right); \qquad \Delta\Delta F=\Delta\Delta H-T\Delta\Delta S^\circ, \]

neglecting the change in the entropy term, we obtain \(\Delta\Delta H \sim 5.5\) kcal/mole. A close value (4.5 kcal/mole) was found for the energy of the \(\mathrm{O \to Sn}\) interaction in a distannoxane of the type

\[ \begin{array}{c} \mathrm{Sn}\\[-2mm] /\!\!\!\backslash\\[-1mm] \mathrm{O}\quad \mathrm{O}\\[-1mm] \backslash\!\!\!/\\[-2mm] \mathrm{Sn} \end{array} \]

\(^{(7)}\) and \(\mathrm{N \to B}\) in dimers of aminomethyldimethylborane (4.49 kcal/mole) \(^{(8)}\):

\[ \begin{array}{c} >\mathrm{N}\backslash \mathrm{B}<\\ \downarrow \quad \uparrow\\ >\mathrm{B}\backslash \mathrm{N}< \end{array} \]

The \(\mathrm{N \to Sn}\) interaction is confirmed by the slowing of nitrogen inversion in ethyleneiminomethylstannanes. For a change in configuration in this case, in addition to overcoming the inversion barrier, rupture of the coordination bond is necessary.

Fig. 1

Coalescence of the doublet signal of the ethyleneimine protons in the spectra of IV is observed at \(t_c > 140^\circ\), whereas in N-alkylethyleneimines the signals coalesce into a narrow singlet at \(t_c = 60—80^\circ\) \((^2)\). The inversion barrier in N-alkylethyleneimines is 10 kcal, which is close to the barrier of hindered rotation in dimethylformamide (7 kcal) and dimethylacetamide (12 kcal; \(t_c = 63^\circ\)), while in nitrosodimethylamine \(t_c = 193^\circ\) and the rotational barrier is 23 kcal. Estimation of the strength of the \(N \to Sn\) bond from these data leads to a value of the same order as from the decrease in basicity.

In the UV spectra of aminomethylstannanes, in contrast to tetraalkylstannanes, a small bathochromic shift and a significant increase in the extinction coefficient are observed (Table 4).

The possibility of intermolecular interaction with the formation, for example, of a dimer

\[ \begin{array}{ccc} >N & \diagup & Sn< \\ \downarrow & & \uparrow \\ >Sn & \diagdown & N< \end{array} \]

is excluded by the constancy of the molecular weight when it is determined for compound I by the cryoscopic method (in benzene at concentrations of \(0.2 \div 2.8\%\)) and by the electrothermal method \((^9)\) (in cyclohexane and methyl ethyl ketone in the range \(1.2 \div 11\%\)).

The absence of quadrupole splitting and the small changes in chemical shifts in the Mössbauer spectra (Table 2) \((^{10})\) can be explained by insufficient sensitivity of the method to weak interactions. Thus, for the limiting cases, when the coordination number of tin is 4 in the case of \((CH_3)_3SnCl\), \(\delta = 1.40\), \(\Delta = 3.09\), and for the corresponding complex with pyridine, when the coordination number is 5, \(\delta = 1.42\); \(\Delta = 3.35\) \((^{11})\). In addition, dialkyldi-

carboxylates of tin in the \(\delta\) and \(\Delta\) values differ hardly at all from the analogs of distannoxanes \(^{(12)}\), for which the presence of an \(\mathrm{O}\to\mathrm{Sn}\) interaction has been clearly proved \(^{(7)}\); in the NMR spectra the effect appears only when the number of distannoxane units in the molecule is increased \(^{(10)}\).

Thus, despite the absence of convincing data on the coordinating ability of tetraalkylstannanes, it must be admitted that in aminomethylstannanes there is an intramolecular coordination interaction, energetically close to a hydrogen bond. The phenomenon is evidently due to the fixed arrangement of the tin and nitrogen atoms (position effect), which favors interaction with a partial change in hybridization and configuration and with the approach of \(\mathrm{N}\to\mathrm{Sn}\) from the initial state:

\[ \begin{array}{c} \mathrm{Sn}\; \cdots\cdots\cdots\cdots\; \mathrm{N}\\[-2mm] \quad \diagup\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\diagdown\\[-1mm] 2.18\ \text{\AA}\quad 109^\circ 28'\quad 1.47\ \text{\AA}\\[-1mm] \mathrm{Sn}\cdots\cdots\cdots\cdots\cdots\mathrm{N}\\ 3.00\ \text{\AA} \end{array} \]

An analogous interaction can explain the increase in the mobility of halide in trialkylchloromethylstannanes \(^{(3,5)}\). The effect increases in the presence of electronegative substituents at tin; thus, in trichloromethylchlorostannane \((\nu_{\mathrm{CH}} 3015\ \mathrm{cm}^{-1})\) and in halomethyltrihalogermanes \((\nu_{\mathrm{CH}}>3000\ \mathrm{cm}^{-1})\) \(^{(5)}\), the valence vibrations of the \(\mathrm{C-H}\) bonds lie in the region characteristic of a three-membered ring. It is interesting that the chemical shift of the protons of the \(\mathrm{CH_2}\) group of aminomethyldimethylborane lies in the region characteristic of three-membered rings \(^{(7)}\). The ejection of difluorocarbene from \((\mathrm{CH_3})_3\mathrm{SnCF_3}\) can likewise be explained by an \(\mathrm{F}\to\mathrm{Sn}\) interaction \(^{(13)}\).

Table 4

UV spectra in hexane

The reason for the enhancement of the effects in the series

\[ \mathrm{Sn}>\mathrm{Ge}>\mathrm{Si} \]

should evidently be sought not in the difference in the electronegativities of the elements (which is too small!), but in an increase in coordinating ability \(^{(14)}\).

The formation of an intramolecular coordination bond is evidently a general property of geminal systems with atoms \((\mathrm{E})\) having vacant \(p\)- or \(d\)-orbitals and unshared electron pairs \((\mathrm{X})\): for \(\mathrm{BCH_2N}<\) there is evidence in favor of an \(\mathrm{N}\to\mathrm{B}\) interaction \(^{(8)}\), while in \(-\mathrm{ZnCH_2X}\) \(^{(15)}\), \(-\mathrm{HgCH_2X}\) \(^{(16)}\), \(>\mathrm{SiCH_2X}\) \(^{(17)}\), \(>\mathrm{PCH_2X}\) \(^{(18)}\) one may propose a unified mechanism of carbene ejection:

\[ >\!\begin{matrix} \mathrm{E}\\[-1mm] \uparrow\\[-1mm] \mathrm{X} \end{matrix} \ \to\ \mathrm{E}-\mathrm{X}+>\mathrm{C:} \]

Institute of Chemical Physics
Academy of Sciences of the USSR

Received
30 VI 1965

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