Aliphatic Mn–PNP complexes for the CO2 hydrogenation reaction: a base free mechanism

Abstract

Using density functional theory (DFT) calculations, a series of aliphatic amido Mn–PNP-based complexes was studied for base free CO₂ hydrogenation. The amido group present in the aliphatic PNP ligand favours a heterolytic H₂ cleavage and proton transfer mechanism contrary to any active catalyst for CO₂ hydrogenation, where base facilitates this process. Further, a set of σ-donor (PH₃ and PMe₃) and π-acceptor (CO and PF₃) ligand-based Mn–PNP complexes was studied to understand the electronic and steric effects on the rate determining steps. Our reaction free-energy profile suggests that Mn1 is a promising base-free catalyst for CO₂ hydrogenation with an overall free energy barrier of 23.3 kcal mol⁻¹. We found that the proper combination of σ-donor and π-acceptor ligands may not be a necessary criteria to improve the overall activity of the catalyst although π-acceptor ligands favour heterolytic H₂-cleavege and σ-donor ligands favour the hydride transfer mechanism.