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, [(PNRPiPr)­CoH]+ (R = H and CH2X; X = H, Me, NH2, OMe, OH, F, and Cl) and [(PNPiPr)­CoH], are studied by
density functional theory calculations. In the hydrogenation of propylene
catalyzed by [(PNRPiPr)­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 (σF) 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 σF 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
[(PNPiPr)­CoH] is caused by the easily transfer of H from
cobalt to nitrogen to form stable intermediates.