Computational Mechanistic Study of the Hydrogenation and Dehydrogenation Reactions Catalyzed by Cobalt Pincer Complexes

The mechanisms of hydrogenation and dehydrogenation reactions catalyzed by a series of aliphatic PNP cobalt pincer complexes, [(PNRP^iPr)­CoH]⁺ (R = H and CH₂X; X = H, Me, NH₂, OMe, OH, F, and Cl) and [(PNP^iPr)­CoH], are studied by density functional theory calculations. In the hydrogenation of propylene catalyzed by [(PNRP^iPr)­CoH]⁺, a propylene molecule first inserts into the Co–H bond to form a Co–C bond. Then a H₂ molecule is inserted into the Co–C bond for the formation and release of propane. The influence of different substituents on the N atom of the pincer ligand for the hydrogenation process is investigated. The relations between the field/inductive effect (σ_F) and total free energy barriers and the properties of the lowest energy intermediates, including the Co–N bond lengths, the lowest unoccupied molecular orbital energies, and the Wiberg bond indices of Co–N bonds, are analyzed. The results show that σ_F plays a crucial role in the substituent effect. The mechanism of acceptorless dehydrogenation of alcohols is also elucidated with a detailed free energy profile for the whole catalytic cycle. We found that the very low catalytic activity of [(PNP^iPr)­CoH] is caused by the easily transfer of H from cobalt to nitrogen to form stable intermediates