First-Principles Insights into the Mechanism of CO 2 Hydrogenation Reactions by Fe-PNP Pincer Complex.

Using the state of the art theoretical methods, we have provided a comprehensive mechanistic understanding of the CO 2 hydrogenation into HCOOH, H 2 CO, and CH 3 OH by 2,6-bis(diisopropylphosphinomethyl)pyridine (PNP)-ligated Fe pincer complex, featuring one CO and two H as co-ligands. For the computational investigation, a verified structural model containing methyl groups in place of the experimental isopropyl groups was used. Three catalytic conversions involving hydrogenation of CO 2 into formic acid (HCOOH), HCOOH into formaldehyde and methanol were studied in different solvent medium. Our modelled complex appears to be a viable base-free catalyst for the conversion of CO 2 into HCOOH and HCOOH into H 2 CO, based on the free energy profiles, which show apparent activation energy barriers of 16.28 kcal/mol and 23.63 kcal/mol for the CO 2 to HCOOH and HCOOH to H 2 CO conversion, respectively. However, the computed results show that, due to the huge energy span of H 2 CO to CH 3 OH conversion, complete hydrogenation of CO 2 into methanol could not occur under moderate conditions. Morpholine co-catalyst, which can lower the hydrogenation barrier by taking part in a simultaneous H-atom donation-acceptance process, could have assisted in completing this step.