Polysilanes ladder

Polycyclopolysilanes, 133-135 Polysilanes, ladder, 133-166 Polysiloxane, 145-148 Potential energy surfaces (PES), 8, 70-78, 85-88... 

Silicon-based polymers form a dimensional hierarchy from disilanes, to crystal silicon, and through polysilanes, ladder polymers, siloxenes, polysilane alloys, clusters, and amorphous silicons and include unsaturated systems, such as polysilenes, hexasilabenzenes, and so on. Their properties depend basically on the network dimensions and can vary from conducting (metallic) and semiconducting to insulating. 

In this review, we explain the synthesis, structures, and properties of ladder polysilanes.18 The ladder polysilanes have the unique structure consisting of catenated and fused cyclotetrasilane rings. This system has remarkable features ... 

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

Fig. 5. HPLC analysis of ladder polysilanes (ODS, CH3OH-THF (7 3), UV detection at...

Selective synthetic routes to a specific ladder polysilane have also been developed. Anti-1,2,5,6-tetra-/er -butyl-3,3,4,4,7,7,8,8-octaisopropyltricy-... 

The structures of ladder polysilanes were determined by X-ray crystallography.41,42 In Fig. 6, molecular structures of the ladder polysilanes with the anti structure are shown. A remarkable feature of the anti ladder polysilanes is a helical structure. These molecules can be regarded as silicon double helices, in which two polysilane main chains twist in one way and are bridged by Si-Si bonds. The twist angles between the terminal rungs are... 

On the other hand, the ladder polysilanes containing a syn moiety have more strained structures (Fig. 8). The Si-Si bond lengths of syn-2... 

Fig. 9. UV-visible spectra of ladder polysilanes in hexane at room temperature.

Fig. 10. Lowest transition energies for ladder and linear polysilanes. Values for linear poly silanes are cited from reference 44.

From these results, the electronic properties of the ladder polysilanes are found to be highly affected by their stereochemistry. 

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 radical anions of polysilanes have been studied as unique a-conjugated radical ion species.2,4,56,57 Although many radical anions of polysilanes have been reported, most of them are unstable species that can only be observed at low temperatures. Quite recently, we found that the radical anions of longer ladder polysilanes are persistent at room temperature.58... 

The radical anions 1, anti-2, anti,anti-3, and anti,anti,anti-4 were generated by the reduction of the corresponding ladder polysilanes with potassium (Scheme 8). When the ladder poly silanes were treated with potassium in tetrahydrofuran (THF) at ca. — 70 °C, the solutions were immediately colored 1, purple anti-2, brown anti,anti-3, blue and anti,anti,anti-4, black. In the UV-visible-NIR spectra, several absorption bands appeared (Fig. 15). The intense absorption of anti,anti-3 and anti,anti,anti-4 in the near-infrared region is noted because it has been... 

The ESR spectra of the radical anions of the ladder polysilanes show a relatively broad signal with satellites (Fig. 16). From the intensity, the satellites are attributed to the 13C nuclei at the -positions of the isopropyl groups rather than the 29Si nuclei. The number of spin couplings is equal to the number of equivalent carbon atoms at the a-positions, indicating that the spin is highly delocalized. These results resemble those of the radical anions of other cyclopolysilanes observed by ESR57 and can be explained by... 

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. 

The Si-Si bonds of the ladder polysilanes are cleaved by electrophiles.39,60,61 When 1 was allowed to react with an excess amount of PdCl2(PhCN)2, trans-1 and cis-1 were obtained in 36 and 15% yields, respectively (Scheme 10).39 Other chlorinated products were not formed, indicating the bridgehead Si-Si bond was selectively cleaved. The selectivity is rationalized by the coordination of the bridgehead Si-Si bond, in which... 

The ladder polysilanes can be used as useful probes for obtaining information on the stereochemistry of Si-Si bond cleavage reactions.60 Watanabe and co-workers64 and West and co-workers65 reported that the strained Si-Si bonds of cyclotetrasilanes such as [(z-Pr)2Si]4 and (Et2Si)4 are cleaved by hydrogen chloride, hydrochloric acid, and hydrobromic acid. However, to the best of our knowledge, no report has dealt with the... 

The reaction of 1 with hydrobromic acid gave quantitatively the cis adduct 14 (Scheme 11). The reaction of 1 with hydrochloric acid gave the cis adduct 15 in 79% yield and the trans adduct 16 in 18% yield. In these reactions, no other Si-Si bond cleavage products were obtained. The ladder polysilane 1 did not react with hydrofluoric acid. The reactions of 1,4-di-ter -butyl-2,2,3,3,5,5,6,6-octaisopropylbicyclo[2.2.0]hexasilane with hydrobromic acid and hydrochloric acid were attempted, but no reactions took place. This result is ascribed to steric hindrance by the tert-butyl groups on the bridgehead silicon atoms. 

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