Mechanistic Investigation Into Catalytic Hydrosilylation with a High-Valent Ruthenium(VI)–Nitrido Complex: A DFT Study

Density functional theory calculations
with the B3LYP-D function
have been performed to investigate the mechanism of carbonyl hydrosilylation
reactions catalyzed by the high-valent nitridoruthenium­(VI) complex
[RuN­(saldach)­(CH3OH)]+[ClO4]− (1; saldach is the dianion of racemic N,N′-cyclohexanediylbis­(salicylideneimine)).
Our computational results indicate a favored ionic outer-sphere mechanistic
pathway. This pathway initiates with a silane addition to the RuVI center, which proceeds through a SN2-Si transition
state corresponding to the nucleophilic attack of the carbonyl on
the silicon center. This attack then prompts the heterolytic cleavage
of Si–H bond. The rate-determining energy of the SN2-Si transition state is calculated to be 22.9 kcal/mol with benzaldehyde.
In contrast, our calculations indicate that the initial silane addition
to the nitrido ligand does not represent an intermediate of the catalytic
process leading to the silyl ether products, since it involves high-energy
transition states (29.2 and 37.8 kcal/mol) in the reduction of carbonyls.
Moreover, the computational results show that the RuIII–saldach species afforded by N–N coupling (with an
activation barrier of 24.2 kcal/mol) of the nitridoruthenium­(VI) complex
provides a competitive hydrosilylation reaction by favoring the ionic
outer-sphere mechanistic pathway, associated with a significantly
small activation barrier (3.7 kcal/mol). This study provides theoretical
insight into the novel properties of the high-valent transition-metal
RuVI–nitrido catalyst in catalytic reduction reactions.