Heavy alkali metal tris(trimethylsilyl)silanides: A synthetic and structural study

Treatment of lithium tris(trimethylsilyl)silanide, Li(THF)(3)Si(SiMe3)(3), with heavier alkali metal tert-butoxides yielded the alkali metal silanides MSi(SiMe3)(3) (M = K, Rb, or Cs) in a simple, high-yielding, one-step proc edure. Separation of the two solid reaction products was achieved by additi on of crown ether, which also determines the formation of contact or separa ted ion pairs in the solid state. Here we report the synthesis and structur al characterization of the contact ion K(18-crown-6)Si(SiMe3)(3), 2, in add ition to the separated [K(12-crown-4)(2)][Si(SiMe3)(3)], 1, [Rb(15-crown-5) (2)][Si(SiMe3)(3)], 3, and [Cs(18-crown-6)(2)][Si(SiMe3)(3)], 5. [Rb(18-cro wn-6)(2)][Si(SiMe3)(3)][Rb(18-crown-6)Si(SiMe3)(3)](2), 4, is a rare exampl e where both contact and separated ions are observed in the solid state. Th e investigation of synthetic routes toward the target compounds also examin ed the previously published metalation of Si(SiMe3)4 with potassium tert-bu toxide. This route proved to be temperamental: depending on reaction condit ions and solvent systems, either adducts between product and unreacted star ting material, namely, [(K(THF)Si(SiMe3)(3))((KOBu)-Bu-t)(3)], 6, or the ta rget compound KSi(SiMe3)(3) was isolated. All compounds were characterized by X-ray crystallography and NMR spectroscopy.