A computational study on ligand assisted vs. ligand participation mechanisms for CO2 hydrogenation: importance of bifunctional ligand based catalysts

Abstract

CO₂ hydrogenation products are not only useful chemical sources but also promising hydrogen storage materials. A DFT study has been carried out on the CO₂ hydrogenation reaction catalyzed by a series of bifunctional aminomethyl based Mn(I) complexes. We find that the N–H functionality in the aminomethyl ligand shows a metal–ligand cooperation (MLC) mechanism for the CO₂ hydrogenation reaction. Here, the N–H functionality assists the MLC mechanism by stabilizing the formate anion via N–H⋯O hydrogen bonding interactions. This is opposite to the MLC mechanism proposed by Noyori for ketone hydrogenation, where the N–H functionality actively participates in the reaction mechanism via cleavage/formation of N–H/M–H bonds. Furthermore, the stabilized formate anion initiates heterolytic H₂ cleavage, which requires a very low barrier compared to external base/ligand participation heterolytic H₂ cleavage. Therefore, the bifunctional aminomethyl based Mn(I) complexes are promising for the CO₂ hydrogenation reaction and our study may be very helpful for experimentalists for the development of efficient bifunctional ligand-based catalysts for the CO₂ hydrogenation reaction.