Early-Transition-Metal Silyl Complexes Free of Anionic π-Ligands. A Comparison of Alkyl and Silyl Ligands

Abstract

A new family of silyl complexes, (RCH₂)₃MSiR'₃ (M = Ti, Zr), (RCH₂)₂Ta(=CHR)SiR'₃ and (RCH₂)₂W(=CR)SiR'₃, which are free of cyclopentadienyl (Cp) or other supporting anionic Ω-ligands, have been prepared and characterized. The silyl ligands are found to be more reactive than the alkyl ligands in the silyl alkyl and silyl alkylidene complexes. Silane (HSiR'₃) elimination to form metal—carbon multiple bonds is preferred over alkane (RCH₃) elimination. In the formation of (Me₃SiCH₂)₂Ta(=CHSiMe₃)Si(SiMe₃)₃ from the reaction of (Me₃SiCH₂)₃TaCl₂ with 2 equiv of Li(THF)₃Si(SiMe₃)₃, an intermediate (Me₃SiCH₂)₃Ta(Cl)Si(SiMe₃)₃ was identified. The first step in the conversion of this intermediate to (Me₃SiCH₂)₂Ta(=CHSiMe₃)Si(SiMe₃)₃ is a silane [HSi(SiMe₃)₃] elimination reaction to form “(Me₃SiCH₂)₂Ta(=CHSiMe₃)Cl”. The dimer of “(Me₃SiCH₂)₂Ta(=CHSiMe₃)Cl”, (Me₃SiCH₂)₄(Cl)₂Ta₂(=CHSiMe₃)₂, was observed in the formation of (Me₃SiCH₂)₂Ta(=CHSiMe₃)Si(SiMe₃)₃ and in the decomposition of (Me₃SiCH₂)₃Ta(Cl)Si(SiMe₃)₃. Subsequent Cl⁻ substitution by Si(SiMe₃)₃ ⁻ leads to the formation of the silyl alkylidene complex. In comparison, the first step in the reaction of (RCH₂)₄TaCl (R = CMe₃, SiMe₃) with LiCH₂R to form alkyl alkylidene complexes (RCH₂)₃Ta=CHR is Cl⁻ substitution by CH₂R⁻ to form Ta(CH₂R)₅. Subsequent alkane elimination gives (RCH₂)₃ Ta=CHR The kinetic and mechanistic studies of these processes will be discussed.