Cyclosilanes anion radicals

The cyclic peralkylsilane oligomers, (R2Si) with n = 4-6, manifested especially strong electron delocalization.5 These rings are structurally analogous to those of the cycloalkanes, since the silicon atoms form four sigma bonds. However, the electronic properties of the cyclosilanes more nearly resemble those of aromatic hydrocarbons such as benzene. One example of such behavior is their reduction to anion radicals. Aromatic hydrocarbons such as naphthalene can be reduced, electrolytically or with alkali metals, to deeply colored anion radicals in which an unpaired electron occupies the lowest unoccupied molecular orbital (LUMO) of the hydrocarbon (equation (2)). 

The product consists of various amounts of high polymer (.r is very large) and discrete cyclosilanes with n = 5-35. This is the largest homologous series of cyclic compounds now known except for the cycloalkanes. Although these compounds are formally saturated, they behave in some ways as aromatic hydrocarbons. They can be reduced to anion radicals, and EPR spectra indicate that the unpaired electron is delocalized over the entire ring. O ... 

Electron delocalization in cyclic polysilanes has been studied rather extensively. In these compounds the -electrons play the role of the delocalized rc-electrons in cyclic polyenes9. Cyclosilanes therefore show some properties resembling those of aromatic hydrocarbons. In particular, they undergo one-electron oxidation or reduction to form cation radicals or anion radicals, in which (from the ESR spectra) the unpaired electrons are fully delocalized. 

Numerous substituted cyclosilanes have also been reduced to anion radicals and studied by ESR62-64. A few examples are compounds 9-12. In 9, the ESR spectrum is... 

For the last few years we have tried to synthesize larger two-dimensional silicon frameworks. We wanted to find out how large a molecule must be in order to show delocalization effects of electrons as in elemental silicon. It is well known that radical anions are possible in simple cyclosilanes in which the additional electron is completely delocalized in the cycle [11]. 

Besides some new cyclosilanes with side chains and cage-like compounds, we were able to isolate the decalin analogue cyclosilane Si Me in a yield of about 20%. We characterized the compound by the usual spectroscopic methods, investigated the structure, and found that this compound is able to form a radical anion [12]. In former investigations we were not able to form such radical anions with linearly connected polycyclic systems [13]. 

The most intriguing difference between the chemical properties of cyclopolysilanes and those of cycloalkanes is the ability of the former to form either anion or cation radicals upon one-electron reduction or oxidation, respectively. For example, the cyclic pentamer (Mc2Si)5 is reduced to the corresponding radical anion by sodium-potassium alloy in diethyl ether [see eqn (4.1) in Section 4.1.3], whereas the hexamer (Me2Si)6 is oxidised by aluminium trichloride in dichlor-omethane to the corresponding cation radical. In both cases the EPR spectra of the radical ions can be interpreted in terms of a-electron delocalisation over the entire polysilane ring (see Section 10.1.4.1). In this respect, the cyclosilanes resemble aromatic hydrocarbons rather than their aliphatic analogues. 

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