Polysilane, oxidation

Strong oxidizing agents such as m-chloroperbenzoic (MCPBA) acid also react with polysilanes, to insert oxygen atoms between the silicons (17)... 

As these experiments indicate, polysilanes can in some cases be converted to silicon carbide directly, without the necessity for formation of polycarbosilane, fractionation, or oxidation. For example, polysilastyrene copolymers can be formed into films or fibers and then crosslinked by irradiation with UV light. The crosslinked polysilane forms silicon carbide when heated to 1100°C in vacuum. (1U This method can be used in a "printing" mode, if a film of polysilane is cast onto a ceramic or metal substrate, then... 

The spun polysilane fibers are cured by oxidation in air at a temperature of 160 - 200°C. The curing process is necessary to permit the conversion of the green fiber to the ceramic fiber without softening during pyrolysis. It is presumed that the oxidation mechanism results in the formation of Si-O-C and Si-O-Si bonds by the reaction of Si-CH3 and Si-Si in the polysilane. 

The ladder polysilanes are highly a-conjugated systems, and they are easily oxidized and reduced to give unique oxidation products and persistent radical anions. 

Oxidation of Si-Si bonds with peracids is one of the fundamental reactions of polysilanes.47 The ladder polysilanes have several unequivalent Si-Si bonds, and it seems interesting to study the selectivity of their oxidation positions.48,49... 

When anti-2 was oxidized with 3 equivalents of MCPBA, the trioxidation product 11 was obtained in 81% yield (Scheme 7). Similarly, the oxidation of anti,anti-3 and anti,anti,anti-4 with 4 and 5 equivalents of MCPBA gave the tetraoxidation product 12 and the pentaoxidation product 13, respectively, in moderate yields. Therefore, these ladder polysilanes were found to be oxidized in a unique manner one of two polysilane main chains was oxidized selectively, and novel ladder compounds consisting of polysiloxane and polysilane chains were formed. 

In Fig. 14, the UV spectra of the domino oxidation products are shown. As the number of oxatetrasilacyclopentane rings increases, the lowest energy absorption maximum shifts bathochromically, and the molecular extinction coefficient becomes far larger (10 270 nm (s 3200), 11 273 nm (s 7600), 12 292 nm (s 25000), 13 297 nm (s 51900). The intense absorption of the order 104 is remarkable because these molecules contain no obvious chromo-phores which should give such intense absorption. Since the intense absorption is not observed in the ladder polysilanes, it is apparently due to the electronic effect of the oxygen atoms on the Si-Si a conjugation systems. 

The structures of the radical anions were confirmed by the following experiment (Scheme 9). The reduction of the ladder polysilanes was monitored by UY-visible-NIR spectroscopy. When the absorption of the ladder polysilanes was completely replaced by the absorption of the radical anions, the sealed tube was opened. The radical anions were immediately oxidized, and the starting ladder polysilanes were recovered in high isolated yields. It is reasonable to conclude that the radical anions of the ladder polysilanes retain the ladder structure, and the Si-Si bond cleavage or skeletal rearrangement does not occur. 

Fundamental knowledge on the structures and properties of the ladder polysilanes has accumulated in our research for the past 15 years. Some results were unpredictable, including the silicon double helix structure, the domino oxidation, the formation of persistent radical anions, the Diels-Alder reactions at the 1,4-positions of anthracene, etc. These results let us recognize that the construction of novel structures will open the new chemistry. 

Oxidation state, nickel, 170, 198, 202 Oxidative addition, 11, 20, 21, 293 Oxidative cleavage, polysilane, 153-154 Oxidative coupling, 171-173, 179-182 in C12-oligomers, 173, 188, 207, 209-210 electronic and steric factors, 198, 200 modelling, 176... 

The redox properties of cyclic polysilanes are interesting because they resemble those of aromatic hydrocarbons. For instance, cyclic polysilanes can be reduced to anion radicals or oxidized to cation radicals. ESR spectra for both the cation and anion radicals indicate that the unpaired electron is fully delocalized over the ring [17,19,20]. The aromatic properties of the cyclic polysilanes are ascribed to a high energy delocalized HOMO and a relatively low energy LUMO. Because the HOMO and LUMO levels lie at similar level to those of benzene, cyclic polysilanes can serve either as electron donors or electron acceptors. 

The electrochemical oxidation of cyclic polysilanes has also been investigated [64], Cyclic polysilanes display at least two anodic waves separated by 0.2-0.4 V. The first oxidation potential is 1.1 to 1.4 V (vs SCE) which depends... 

The cyclic voltammetry of polysilanes adsorbed on the electrode surface has also been investigated [65]. The oxidation potentials depend upon the nature of the organic groups on silicon. The electrochemical oxidation is irreversible to give soluble products which are liberated from the surface of the anode. 

Polysilane co-polymers with pendant siloxane groups were also synthesized, aiming at air oxidation-resistant polysilanes, utilizing the partial tendency for phase separation of polar side chain and apolar main chain, resulting in surface accumulation of siloxane groups,154 155 as shown in Scheme 20. 

Other than this system, metallated polysilanes contain the metal in low-valent oxidation states. Such systems have been reported by two groups. In 1995, an alternative functionalization route starting from poly[methyl(H)silylene] or poly[methyl(H)silylene-fo-methylphenylsilylene], 37, was reported, in which the polysilane Si-H moiety was hydro-silated using 1,3,5-hexatriene, affording the diene-modified polymer 67, which was metal functionalized using triiron dodecacarbonyl to give the iron tricarbonyl-polysilane coordination complex, 68.177... 

Scheme 37 m-CPBA oxidation of ladder polysilanes to hybrid silane/siloxane polymers (substituents /-Pr). 

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