Lithiation of Diphenyl(triorganosilyl)methanes

The lithiation of several triorganosilyl-substituted diphenylmethanes, (R₃Si)Ph₂CH, in THF has been studied. The structures of two of these compounds have been determined by X-ray diffraction studies and shown to consist of solvent-separated ion pairs [Li(THF)₄][Li{C(SiR₃)Ph₂}₂] (4, R₃ = Me₃; 5, R₃ = Me₂Ph), in which the anions are lithates, each containing lithium bound to two η²-coordinated, silylated diphenylmethyl subunits. Despite contact with lithium the organic residues are almost planar (sum of angles around C(1) for both structures, 356.7°), indicating a high degree of anionic charge delocalization. This is further reflected by the collated multinuclear NMR data. Reaction of [Li(THF)₄][Li{C(SiMe₃)Ph₂}₂] with TMEDA in hexane was shown by an X-ray diffraction study to have resulted in complete removal of lithium from the anion coordination sphere to give [Li(TMEDA)₂][(Me₃Si)Ph₂C]. This consists of discrete planar [(Me₃Si)Ph₂C]^- carbanions and [Li(TMEDA)₂]⁺ countercations. The behavior of the lithiated diphenylmethanes was rationalized to be a result of both electronic (by extensive charge delocalization) and kinetic (by the bulky R₃Si substituent) stabilization.