Catalytic hydrogenation of CO₂ by manganese complexes: Role of π-acceptor ligands

We have carried out computational studies on the CO₂ hydrogenation reaction catalyzed by three different Mn-based complexes (Mn1, Mn2, and Mn3) to understand the role of σ-donor (PMe₃) and π-acceptor (CO) character of the ligands. Further, the role of a different set of σ-donor and π-acceptor ligands is studied as the studied CO (π-acceptor and PMe₃ (σ-donor) ligands have the differences not only in electronic properties but also in steric effects. Here, we find that the σ-donor ligands (PMe₃/PH₃) favor the hydride transfer, whereas the π-acceptor ligands (CO/PF₃) favor the heterolytic H₂-cleavege. The energy profile diagram shows that the hydride transfer is the rate-determining step when the CO₂ hydrogenation reaction is catalyzed by a Mn-complex containing σ-donor (PMe₃/PH₃) and π-acceptor (CO/PF₃) ligands.