DFT study on mechanism of carbonyl hydrosilylation catalyzed by high-valent molybdenum (IV) hydrides
2018
Tan, Hong Chye | Wang, Wenmin | Wei, Haiyan
Density functional theory (DFT) calculations have been employed to investigate hydrosilylation of carbonyl compounds catalyzed by three high-valent molybdenum (VI) hydrides: Mo(NAr)H(Cp)(PMe₃) (1A), Mo(NAr)H(PMe₃)₃ (1B), and Mo(NAr)H (Tp)(PMe₃) (Tp = tris(pyrazolyl) borate) (1C). Three independent mechanisms have been explored. The first mechanism is “carbonyl insertion pathway”, in which the carbonyls insert into Mo−H bond to give a metal alkoxide complex. The second mechanism is the “ionic hydrosilylation pathway”, in which the carbonyls nucleophilically attacks η¹-silane molybdenum adduct. The third mechanism is [2 + 2] addition mechanism which was proposed to be favorable for the high-valent di-oxo molybdenum complex MoO₂Cl₂ catalyzing the hydrosilylation. Our studies have identified the “carbonyl insertion pathway” to be the preferable pathway for three molybdenum hydrides catalyzing hydrosilylation of carbonyls. For Mo(NAr)H (Tp)(PMe₃) (Tp = tris(pyrazolyl) borate), the proposed nonhydride mechanism experimentally is calculated to be more than 32.6 kcal/mol higher than the “carbonyl insertion pathway”. Our calculation results have derived meaningful mechanistic insights for the high-valent transition metal complexes catalyzing the reduction reaction.
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