Organometallic Compounds of the Alkali Metals with Phenylsilyl Substituents at the Carbanionic Center. Crystal Structures of LiC(SiMe3)2(SiMe2Ph)·Et2O, NaC(SiMe3)2(SiMe2Ph)·TMEDA, NaC(SiMe3)(SiMe2Ph)2·TMEDA, {LiCH(SiMe2Ph)2}2, RbC(SiMe2Ph)3, and CsC(SiMe2Ph)3 (TMEDA = Tetramethylethylenediamine)

Abstract

The syntheses of a number of alkali metal compounds MC(SiMe3)3-n(SiMe2Ph)n, are described (n = 1, M = Li(TMEDA) (3a), Li(Et2O) (3b), or Na(TMEDA) (3c); n = 2, M = Li(THF)2 (6a), Li(TMEDA) (6b), Na(TMEDA) (6c), or K (6d); n = 3, M = Rb (7d) or Cs (7e); THF = tetrahydrofuran, TMEDA= tetramethylethylenediamine). The compounds 3b, 3c, and 6c adopt molecular structures with intramolecular metal−phenyl interactions. The lithium compound LiCH(SiMe2Ph)2, 5, is dimeric with electron-deficient μ-Li bridges and intramolecular metal−phenyl interactions. The compounds MC(SiMe2Ph)3, M = Na (7b), K (7c), Rb (7d), or Cs (7e), form polymeric chains in the solid state with both intra- and intermolecular metal−phenyl interactions. For the lighter alkali metals the interaction is unsymmetrical and mainly with the ipso and ortho carbon atoms, but for the heavier metals there are almost equal metal−carbon distances to all six atoms of the phenyl ring. The long M−C and short Si−C bond lengths and the wide Si−C−Si angles indicate that the structures are highly ionic. The presence of ionic species in solution is revealed by multinuclear NMR data, in particular by the low frequency shifts associated with the central carbon atoms, by the high carbon−silicon coupling constants, and by the low barriers to inversion compared with those in the trisilylmethanes from which the carbanions are derived.