Mechanistic insights into the catalytic carbonyl hydrosilylation by cationic [CpM(CO)(2)(IMes)](+) (M = Mo, W) complexes: the intermediacy of eta(1)-H(Si) metal complexes

摘要

The mechanism of carbonyl hydrosilylation by cationic cyclopentadienyl molybdenum/tungsten complexes, [CpM(CO)(2)(IMes)](+) (M = Mo, W), has been investigated using density functional calculations. Earlier studies by Bullock and co-workers proposed that the ionic mechanism with the intermediacy of oxidative addition complexes of Mo(IV)/W(IV) silyl hydrides accounts for the catalytic reactions. The activation energies of the turnover-limiting steps along Bullock's proposed ionic catalytic cycles are calculated to be moderate, at 24.2 (Mo) and 20.6 (W) kcal mol(-1), respectively. However, our calculations support an alternative ionic mechanism which features a S(N)2@Si transition state as the preferred reaction pathway instead of the ionic mechanism proposed by Bullock. The ionic S(N)2-type mechanistic pathway is initiated by the silane end-on coordination on metal centers, forming eta(1)-H(Si) Mo/W complexes. Then, the carbonyl oxygen backside attacks the eta(1)-silane metal adducts to prompt the cleavage of Si-H bond via S(N)2@Si transition states, giving silyl carbenium ion and metal hydrides. The rate-determining steps along the ionic S(N)2-type pathways correspond to heterolytic cleavage of the eta(1) coordinated Si-H bond and are calculated to be quite low, at 8.7 (Mo) and 7.4 (W) kcal mol(-1), respectively. In this regard, our calculations reveal that silane end-on coordination on the metal center leads to stable eta(1)-H(Si) Mo/W adducts, which are more stable, by 10.7 and 5.1 kcal mol(-1), than the Mo(IV)/W(IV) silyl hydrides. Furthermore, the eta(1)-H(Si) Mo/W adducts represent the intermediates of catalytic hydrosilylation reactions by two cationic molybdenum/tungsten complexes.