Theoretical Study of the Reversible Hydrogenation of Carbon Dioxide to Formate on Mn-Pincer Complexes

Recently, Beller and coworkers reported a study on the reversible hydrogenation of CO₂ to formic acid using a Mn(I)-PN⁵P complex. In this paper, we performed DFT calculations to understand the mechanism for this reversible reaction occurring on the Mn-PN⁵P, Mn-PN³P, and Mn-PNP catalysts. Through investigating in detail two possible routes for CO₂ hydrogenation to formic acid, we noticed that the production of formic acid is not thermodynamically favorable. The CO₂ reversible hydrogenation actually takes place between CO₂/H₂ and formate rather than formic acid, although in the beginning of the process formic acid is used. By comparing three pincer ligands (PN⁵P, PN³P, and PNP), it can be found that the Mn-PN³P and Mn-PNP complexes are favorable for the dehydrogenation of formic acid, but relatively unfavorable for the hydrogenation of CO₂. Only the PN⁵P ligand can balance the forward and reverse reactions, showing the best catalytic activity for CO₂ reversible hydrogenation to formic acid. Our computational results support the experimental work of Beller and coworkers and can explain the advantage of the PN⁵P ligand. This paper could be of benefit for promoting the application of the PN⁵P ligand in realizing the reversible H₂ storage-release cycle.