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. (2nd March 2018)

• Record Type: Journal Article
• Authors: Fang, Shaoqin; Chen, Hongcai; Wang, Wenmin; Wei, Haiyan
• Is Part Of: New journal of chemistry. Volume 42:Number 7(2018)
• Page Start: 4923
• Page End: 4932
• Publication Date: 2018-03-02
• Subjects: Chemistry -- Periodicals; Chimie -- Périodiques; 540
• DOI: 10.1039/c7nj03856a
• Languages: English
• ISSNs: 1144-0546

Abstract

Abstract : The ionic SN 2-type mechanistic pathway initiated by silane end-on coordination on the metal centers, forming η 1 -H(Si) Mo/W complexes, is the preferred reaction pathway for the two cationic cyclopentadienyl molybdenum/tungsten complexes, [CpM(CO)2 (IMes)] + (M = Mo, W) in catalyzing carbonyl hydrosilylation. Abstract : 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 SN 2@Si transition state as the preferred reaction pathway instead of the ionic mechanism proposed by Bullock. The ionic SN 2-type mechanistic pathway is initiated by the silane end-on coordination on metal centers, forming η 1 -H(Si) Mo/W complexes. Then, the carbonyl oxygen backside attacks the η 1 -silane metal adducts to prompt the cleavage of Si–H bond via SN 2@Si transition states, giving silyl carbenium ion and metal hydrides. The rate-determining steps along the ionic SN 2-type pathways correspond to heterolytic cleavage of the η 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 η 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 η 1 -H(Si) Mo/W adducts represent the intermediates of catalytic hydrosilylation reactions by two cationic molybdenum/tungsten complexes.