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Mechanistic Insights into the Catalytic Carbonyl Hydrosilylation by Cationic [CpM(CO)2(IMes)]+ (M = Mo, W) Complexes: The Intermediacy of η1-H(Si) Metal Complexes

Abstract

The mechanism of carbonyls hydrosilylation by cationic cyclopentadienyl molybdenum/tungsten complexes, [CpM(CO)2(IMes)]+ (M = Mo, W) have 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 the Bullock’s proposed ionic catalytic cycles are calculated to be moderate of 24.2 (Mo) and 20.6 (W) kcal/mol, respectively. However, our calculations support an alternative ionic mechanism which features an SN2@Si transition state be the preferred reaction pathway instead of the ionic mechanism proposed by Bullock. The ionic SN2 type mechanistic pathway is initiated by silane end-on coordination on the metal centers, forming η1-H(Si) Mo/W complexes. Then carbonyls oxygen backside attacks the η1-silane metal adducts to prompt the cleavage of SiH bond via SN2@Si transition states, giving silyl carbenium ion and metal hydrides. The rate determining steps along the ionic SN2 type mechanistic pathways are corresponding to heterolytic cleavage of the η1 coordinated SiH bond, which are calculated to be significantly low, 8.7 (Mo) and 7.4 (W) kcal/mol, respectively. In this regard, our calculations reveal that silane end-on coordination on the metal center leads to stable η1-H(Si) Mo/W adducts, which are more stable ( by 10.7 and 5.1 kcal/mol) than the Mo(IV)/W(IV) silyl hydrides. Furthermore, the η1-H(Si) Mo/W adducts represent the intermediates of the catalytic hydrosilylation reactions by two cationic molybdenum/tungsten complexes.

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Publication details

The article was received on 09 Oct 2017, accepted on 28 Nov 2017 and first published on 28 Nov 2017


Article type: Paper
DOI: 10.1039/C7NJ03856A
Citation: New J. Chem., 2017, Accepted Manuscript
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    Mechanistic Insights into the Catalytic Carbonyl Hydrosilylation by Cationic [CpM(CO)2(IMes)]+ (M = Mo, W) Complexes: The Intermediacy of η1-H(Si) Metal Complexes

    S. Fang , H. Chen, W. Wang and H. Wei, New J. Chem., 2017, Accepted Manuscript , DOI: 10.1039/C7NJ03856A

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