Nitrido complex of high-valent Ru(VI) -catalyzed reduction of imines and alkynes with hydrosilanes: A theoretical study of the reaction mechanism

Abstract

The reaction mechanism for hydrosilylation of imines and alkynes catalyzed by a nitrido complex of high-valent Ru(VI) has been studied using density functional theory. Four different reaction routes for RuVIN complex catalyzed hydrosilylation have been probed in detail, spanning those involving N, RuVI, or RuIII as active center in activating SiH bonds. The activation of SiH bond at the nitrido ligand can be discounted because they would lead to large activation barrier for reduction of imines and alkynes. Our computational results support an ionic outer-sphere mechanism involving silane initially binding with ruthenium metal. This proposed ionic outer-sphere mechanism is characterized by a rate-determining SiH bond cleavage upon imines nitrogen or alkynes carbon atoms backside attacking the silane-ruthenium complex, featuring a SN2@Si transition state. In addition, we found organic substrate is crucial in controlling the catalytic cycle. With imines, the ionic outer-sphere cycle occurs at the high-valent RuVI metal center is preferable. The highest energy barrier must overcome is 21.9 kcal/mol. With alkynes, the low-valent RuIII complex formed by NN coupling of RuVIN complex is the real catalytic species. The rate-determining energy barrier is calculated to be 12.7 kcal/mol.