Heavy Alkali Metal Tris(trimethylsilyl)silanides:  A Synthetic and Structural Study

Abstract

Treatment of lithium tris(trimethylsilyl)silanide, Li(THF) 3 Si(SiMe 3 ) 3, with heavier alkali metal tert -butoxides yielded the alkali metal silanides MSi(SiMe 3 ) 3 (M = K, Rb, or Cs) in a simple, high-yielding, one-step procedure. Separation of the two solid reaction products was achieved by addition of crown ether, which also determines the formation of contact or separated ion pairs in the solid state. Here we report the synthesis and structural characterization of the contact ion K(18-crown-6)Si(SiMe 3 ) 3, 2, in addition to the separated [K(12-crown-4) 2 ][Si(SiMe 3 ) 3 ], 1, [Rb(15-crown-5) 2 ][Si(SiMe 3 ) 3 ], 3, and [Cs(18-crown-6) 2 ][Si(SiMe 3 ) 3 ], 5 . [Rb(18-crown-6) 2 ][Si(SiMe 3 ) 3 ][Rb(18-crown-6)Si(SiMe 3 ) 3 ] 2, 4, is a rare example where both contact and separated ions are observed in the solid state. The investigation of synthetic routes toward the target compounds also examined the previously published metalation of Si(SiMe 3 ) 4 with potassium tert -butoxide. This route proved to be temperamental: depending on reaction conditions and solvent systems, either adducts between product and unreacted starting material, namely, [{K(THF)Si(SiMe 3 ) 3 }{KO t Bu} 3 ], 6, or the target compound KSi(SiMe 3 ) 3 was isolated. All compounds were characterized by X-ray crystallography and NMR spectroscopy.