Chemoselectivity in σ bond activation by lanthanocene complexes from a DFT perspective:: reactions of Cp2LnR (R = CH3, H, SiH3) with SiH4 and CH3-SiH3

The pathways for the sigma bond metathesis reactions between Cp(2)LnCH(3) and SiH4 to give either Cp(2)LnSiH(3) and CH4 (CH3/SiH3 exchange) or Cp(2)LnH and H3C - SiH3 (Si - C coupling) have been studied using DFT(B3PW91) calculations. It is shown that the nature of the lanthanide atom has essentially no influence on the free enthalpy profile. All reactions that could occur between H3C - SiH3, formed from the reaction between the initial reagents (Cp(2)LnCH(3) and SiH4), and the lanthanocene complexes (Cp(2)LnH or Cp(2)LnSiH(3)), have been then studied for La only. The activation of the Si - H bond is preferred over that of Si - C or C - H bonds. In addition, in the reaction of Cp2LaH with SiH3CH3, the silyl group favours the formation of the C-bonded alkylsilyl complex. The activation of the Si - H bond is not selective, i.e. Si can be at either the alpha or beta sites, with respect to the metal center, in the 4-center transition state. The reaction between Cp2LaSiH3 and SiH4 is preferentially the exchange of SiH3 groups over the Si - Si coupling, mostly for thermodynamic reasons. The Si - Si coupling is however not strongly disfavoured by thermodynamics and has an accessible activation energy.