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

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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.