Dewar silabenzene

UV irradiation (A>320 nm) of matrix-isolated silabenzene [137a] led to the disappearance of its absorptions and the appearance of bands of other unstable molecules, which were assigned to Dewar silabenzene [138]. This assumption is reasonable because sp hybridization of the silicon atom is preferable to sp hybridization and favourable to the stability of [138]. Besides, according to calculations the energy differences between mono-cyclic [137a] and bicyclic [138] structures decrease from CeHe (314 kJ moP ) to silabenzene CsSiHe (125 kJ moP ). 

The silaaromatics 36 and 38 reveal in the observed UV spectra an additional bathochromic shift compared to silabenzene 24. Both, 36 and 38, disappear in an irreversible process upon irradiation with X = 290-420 nm, probably due to the formation of the corresponding Dewar silabenzenes. The kinetically stabilized silaaromatics 36 and 38 turned out to be stable up to 90 K even without an argon cage, but decomposed unspecifically at higher temperatures. 

Silabenzene results from the pyrolysis of silacyclohexadlenes and is best considered as a symmetry distorted 67c-electron ring. Photolysis Induces interconversion of silabenzene with Dewar-silabenzene,... 

Theoretical studies have shown that the resonance energy of silabenzene should be 2/3 that of benzene . Even though other isomers of CsHeSi such as Dewar silabenzene may be competitive in stability, silabenzene appears to have all the attributes expected for an analogue of benzene. Nevertheless, silaaromatics were for a long time thought to be elusive molecules despite the relative stability of similar heteroatomic derivatives of benzene such as phosphabenzene, arsabenzene and stibabenzene . 

Comparison of the energy of para-disilabcnzcne (11) with that of the isomeric Dewar disilabenzene (23) and the silylenes 24 and 25 follows the trends discussed above for the isomers of silabenzene. At IIF/3-21 G( )//I H7STO-3G the most stable isomer is the... 

Silabenzene 24 reveals a characteristic Si—H stretching vibration at 2217 cm-1, as expected for hydrogen attached to a sp2-hybridized silicon atom. Compound 24 shows a typical benzene-type UV spectrum with absorptions at X = 217, 272 and 320 nm, which fit into the series of the already known donor-substituted heterobenzenes30. An additional structural proof was the partially reversible photochemical conversion of 24 into Dewar... 

The dehydrogenation of 30 turned out to be more difficult than the analogous conversion of 27 to 24. The IR spectrum of the matrix-isolated products showed only one absorption at 1273 cm-1, which disappeared upon irradiation with X = 405 nm probably yielding Dewar disilabenzene 32. The UV spectrum proved to be of higher diagnostic value. 1,4-Disilabenzene 31 shows a typical heterobenzene electronic spectrum with absorptions at X = 408, 340 and 275 nm displaying another bathochromic shift compared to silabenzene 24. 

Irradiation of matrix-isolated silabenzene at 320 nm causes a transformation to the Dewar isomer318,324. This conversion is readily followed by the shift of the Si-H stretching frequency from that characteristic of an sp2 to that characteristic of an sp3 hybridized silicon. Irradiation of the Dewar isomer with 240 nm light318 causes partial reversion to silabenzene. A similar photostationary state can be reached in the case of 1-phenyl-1-silabenzene315. Since there are no wavelengths at which the Dewar isomer absorbs while the silaaromatic itself does not, it is not possible to achieve complete conversion of the Dewar form into the silaaromatic form. 

A computational investigation has rationalized the different course of the photoreaction of silabenzene with respect to the carbocyclic analogue. The reaction leads to the silabenzvalene, rather than to Dewar... 

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