Catalytic Hydrogenation of CO2 by Fe Complexes Containing Pendant Amines: Role of Water and Base

The role of the outer sphere ligand is reported to be very crucial for the hydrogenation of CO2. We have investigated a series of Fe complexes with flexible pendant amines and predicted their potentials for CO2 hydrogenation to formic acid using density functional theory calculations. Among the modeled Fe complexes, the Fe complex [(CpFe)-Fe-c6F5-(p(2)(tsu)N(2)(tBu))](+) containing the electron-withdrawing Cp-C6F5 ligand is the most active catalyst with a total free energy barrier of only 14.9 kcal/mol. Besides, we find that the rate determining steps (heterolytic cleavage and hydride transfer) of CO2 hydrogenation are improved significantly when catalyzed by the pendant amine based [(CpFe)-Fe-C6F5(p(2)(tBu)N(2)(tBu))](+) complex. The roles of water and base have been found to be very crucial for the whole catalytic cycle. The ligand-assisted pathway is very favorable toward one of the rate-determining steps, and therefore such flexible outer sphere ligands are crucial for the design of catalyst for CO2 hydrogenation.