Isolation and Characterization, Including by X-ray Crystallography, of Contact and Solvent-Separated Ion Pairs of Silenyl Lithium Species

Reaction of bromoacylsilane 1 (pink solution) with tBu(2)MeSiLi (3.5 equiv) in a 4:1 hexane: THF solvent mixture at -78 degrees C to room temperature yields the solvent separated ion pair (SSIP) of silenyl lithium E-[(tBuMe(2)Si)(tBu(2)MeSi) C=Si(SiMetBu(2))](-) [Li center dot 4THF](+) 2a (green-blue solution). Removal of the solvent and addition of benzene converts 2a into the corresponding contact ion pair (CIP) 2b (violet-red solution) with two THF molecules bonded to the lithium atom. The 2a(sic)2b interconversion is reversible upon THF(sic)benzene solvent change. Both 2a and 2b were characterized by X-ray crystallography, NMR and UV/Vis spectroscopy, and theoretical calculations. The degree of dissociation of the Si-Li bond has a large effect on the visible spectrum (and thus color) and on the silenylic Si-29 NMR chemical shift, but a small effect on the molecular structure. This is the first report of the X-ray molecular structure of both the SSIP and the CIP of any R2E=E'RM species (E=C, Si; E'=C, Si; M=metal).