Synthesis and Characterization of (Me3ECH2)2Ta(CHEMe3)Si(SiMe3)3 (E = C, Si). Kinetic and Mechanistic Studies of the Formation of a Silyl Alkylidene Complex through Preferential Silane Elimination
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Abstract
Silyl alkylidene complexes (Me3ECH2)2Ta(CHEMe3)Si(SiMe3)3 (E = C, 1; E = Si, 2), which are free of anionic π-ligands such as cyclopentadienyl (Cp), were prepared through the reactions of (Me3ECH2)3TaCl2 with 2 equiv of LiSi(SiMe3)3(THF)3. An unprecedented preferential elimination of silane HSi(SiMe3)3 leads to the formation of the alkylidene bonds in 1 and 2. An intermediate (Me3SiCH2)3Ta(Cl)Si(SiMe3)3 (9) was observed in the formation of 2. 9 was found to react with LiSi(SiMe3)3(THF)3 and LiCH2SiMe3 to form 2 and (Me3SiCH2)3TaCHSiMe3 (6), respectively. The reaction of 9 with LiSi(SiMe3)3(THF)3 to form the silyl alkylidene complex 2 follows a pathway different from the reaction of (Me3ECH2)4TaCl (E = C, Si) with LiCH2EMe3 to form alkyl alkylidene complexes (Me3ECH2)2TaCHEMe3. The decomposition of 9 was found to follow first-order kinetics, with ΔH⧧ = 17.2(1.0) kcal/mol and ΔS⧧ = −4(4) eu, and give an unstable dimeric alkylidene complex (Me3SiCH2)4(Cl)2Ta(CHSiMe3)2 (12). The reaction between 9 and LiSi(SiMe3)3(THF)3 to form 2 and HSi(SiMe3)3 was observed to follow first-order kinetics, and the reaction rates were independent of the concentration of LiSi(SiMe3)3(THF)3. In addition, the rates of this reaction (k2) are almost equal to the rates of the decomposition of 9 (k1). These results are consistent with the presence of “(Me3SiCH2)2Ta(CHSiMe3)Cl” as an intermediate in the conversion of 9 to 2. Kinetic and mechanistic studies of the formation of 2 will be discussed. A thermodynamic analysis of the preferential silane elimination shows that this preference may not be thermodynamic in origin, and could be attributed to a kinetic effect.
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