DO NONCLASSICAL SILABENZENE ANIONS EXIST - SYNTHESIS AND X-RAY CRYSTAL-STRUCTURE OF CROWN ETHER STABILIZED LITHIUM SILACYCLOHEXADIENIDES, LI(12-CROWN-4)2ME2SIC5H5 AND LI(12-CROWN-4)2-TERT-BU(H)SIC5H5

Metalation of 1,1-dimethyl-1-silacyclohexa-2,4-diene (10) and 1-tert-butyl-1-hydro-1-silacyclohexa-2,4-diene (11) with n-butyllithium leads to the corresponding silacyclohexadienyl lithium compounds, which on further treatment with the crown ether 12-crown-4 afford the deep red to violet crystalline title compounds [Li(12-crown-4)2] [Me2SiC5H5] (12) and [Li(12-crown-4)2] [t-Bu(H)SiC5H5] (13) as solvent-separated ion pairs with free silacyclohexadienide anions. The question whether the anions in 12 and 13 may be regarded as silicon-bridged pentadienide species or as nonclassical silabenzenes is answered on the basis of 1H, 13C, 29Si NMR, and IR data of 10-13 and on the basis of an X-ray crystal structure determination of 12. The π-bonding in these R2SiC5H5- anions is found as quasiaromatic. In this context, the most intriguing structural and spectroscopic features are as follows: (I) planarity of the SiC5 framework, short endocyclic silicon-carbon bonds, and rather long exocyclic Si-C bonds in 12, (II) NMR signals for the ring-carbon hydrogens in 12 and 13 at comparatively low field, (III) dramatic low field shift in the 1H NMR spectrum for the Si-H proton on going from 11 to 13, and (IV) shift to lower wave numbers for the Si-H stretching mode (IR) and smaller Si-H coupling constant (29Si NMR) on going from 11 to 13. The π-bonding in R2SiC5H5- anions is qualitatively rationalized using the MO model (interaction of π-pentadienide with σ⋆SiR2 orbitals, negative hyperconjugation) and the valence bond approach (participation of the resonance structures 8c, 8d). © 1990, American Chemical Society. All rights reserved.