Polysilanyl anions

These species may fall into two classes isolable or spectroscopically detectable polysilanyl anions, and polysilanyl anions postulated as reaction intermediates. 

Polysilanyl anions, H3SiH2SiK, (HaSOaHSiK, and (HaSOjSiK, are prepared, respectively, (a) from H,SiK with SiH4, (b) from HaSiK and SiaHg, and (c) from SiaH or SiaHg and K in HMPA, and identified by H NMR and IR spectroscopy 46) (see also Section V). 

The Si-Si bond cleavage of trisilanes by MeLi in THF is applicable to the preparation of disilanyllithiums, in which a phenyl or mesityl group is attached to a-silicon [Eq. (30)] (69). A similar cleavage reaction occurs with a Na/K alloy to afford disilanylpotassium. These species couple with AdCOCl (Ad = 1-adamanthyl) to give acyldisilane in modest yields. 

The Si NMR signals of MeaSiMe2SiLi are observed at -74.9 ppm for a-silicon (bonded to Li) and at -12.4 ppm for /3-silicon (in trimethysilyl) in THF-dg solution. These data indicate that the negative charge is highly localized on the a-silicon atom. At 180 K, the a-silicon is observed as a 

The resulting dianion decomposes at room temperature with a half-life of 13.5 hours at 30°C. The Si resonance is observed at -1.24 ppm, shifted upheld relative to that of the starting dichlorodisilane (8.99 ppm). The C NMR spectrum shows that the ipso carbon on the naphthalene ring (172.94 ppm) shifts remarkably downfield relative to the starting material (136.23 ppm). This may be caused by the 7r-polarization effect of the naphthyl ring as observed in the case of (phenylsilyl)lithiums (see section III-B). 

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Polysilanyl anions are reactive intermediates in various synthetic routes to polysilanes. 

In the Wurtz coupling reaction of dichlorosilanes with sodium to poly silanes [Eq. (42)], polysilanyl anions are considered to be crucial reaction intermediates in the propagation step and in the cyclization via back-biting (Scheme 23) 88). 

Anionic polymerization of masked disilenes has opened up a novel route to polysilanes (95). I-Phenyl-7,8-disilabicyclo[2.2.2]octa-2,5-dienes can be used as masked disilenes. n-BuLi works as an initiator. The polymerization may involve the attack of the polysilanyl anions on a silicon atom of the monomer, resulting in the formation of the new propagating polymer anion and biphenyl. This method is applicable to aminopolysilane synthesis (Scheme 28). 

Figure 5. Degradation of Living Polysilanyl Anion (GPC Profiles).

In another elegant application of the masked disilene method, the anionic polysilanyl chain ends were chemically bound to substrate surfaces to form end-graft polysilanes. These systems are discussed in Section 3.11.4.2.3. 

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