Hydrogenation of CO2 to methanol catalyzed by a manganese pincer complex: insights into the mechanism and solvent effect

Abstract

Herein, density functional theory (DFT) calculations were employed to explore the reaction mechanism of three cascade cycles for the hydrogenation of carbon dioxide to methanol (CO₂ + 3H₂ → CH₃OH + H₂O) catalyzed by a manganese pincer complex [Mn(Ph₂PCH₂SiMe₂)₂N(CO)₂]. The three cascade cycles involve: the hydrogenation of CO₂ to formic acid, the hydrogenation of formic acid to methanediol and the decomposition of methanediol to formaldehyde and water, and the hydrogenation of formaldehyde to methanol. The calculated results demonstrate that hydrogen activation is the rate-determining step of each catalytic cycle under solvent-free conditions, and the energy span of the whole reaction is 27.1 kcal mol⁻¹. Furthermore, the solvent was found to be of importance in this reaction. In three different solvents, the rate-determining steps of this reaction are all the hydrogen transfer step of the formic acid hydrogenation stage, and the corresponding energy spans in water, toluene and THF solvents are 21.3, 20.8 and 20.4 kcal mol⁻¹, respectively. Such a low energy span implies that this manganese complex could be a promising catalyst for the efficient conversion of CO₂ and H₂ to methanol at temperatures below 100–150 °C.