Isolable Silylene Anion Radical: Structural Characteristics in the Solid State and in Solution

Tetrakis(di-tert-butylmethylsilyl)disilene (1) was treated with 2.2 equiv of metal naphthalenide (M = Li, Na) in THF. When a crown ether was added to the reaction mixture, the silylene anion radical ((tBuMe2Si)2Si:•- M(crown ether)2+) (2a:  M = Li, 2b:  M = Na) was formed. They were isolated in the form of the lithium or sodium salts as extremely air- and moisture-sensitive red crystals. The structures of 2 were unambiguously characterized by ESR spectroscopic data as well as by X-ray crystallographic analysis. The X-ray crystal structure of 2a reveals free silylene anion radical, showing no interaction with counterion (solvent-separated ion pair). The structures of 2 in solution were also deduced by ESR spectroscopy. The ESR spectra of 2a and 2b in a polar solvent (THF or DME) show a solvent-separated ion pair, whereas the ESR spectrum of 2b in toluene shows contact ion pair owing to the interaction of the counterion with the anionic silicon atom.