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.