Bifunctional aliphatic PNP pincer catalysts for hydrogenation: Mechanisms and scope

Hydrogenation is one of the important transformation methodologies in academia and applied research; and homogeneous hydrogenation by using defined transition metal complexes provides the opportunity to fine-tuning the catalytic activity via either metal substitution or ligand modification. Recent studies in the synthesis, characterization and evaluation of transition metal based aliphatic HN(CH2CH2R2) chelating ligand complexes show excellent catalytic performances in the reactions of hydrogenation, transfer hydrogenation and isomerization of carbonyl compounds, as well as aqueous methanol dehydrogenation which includes formic acid dehydrogenation in the last step. It is found that B3PW91 computed gas phase kinetic and thermodynamic data have the closest and best agreement with the experiments, while those including solvation effects and/or dispersion corrections differ strongly from the experiments. Current solvation models and dispersion corrections applied in these hydrogenation reactions are not sufficient enough for quantitative comparison. In this review, the results from earth-abundant (Fe, Mn) to precious (Ru, Os, Ir) metals based PNP complexes used in the hydrogenation of esters and aldehydes are summarized. It shows clearly that Fe- and Mn-based PNP catalysts can be as effective as Ru-, Os- and Ir-based PNP catalysts. The B3PW91 results perfectly explained the experimentally observed selectivity of transfer hydrogenation of alpha,beta-unsaturated aldehydes and ketones, as well as the isomerization of allyl alcohol on the basis of self-transfer hydrogenation. Such interplay between computation and experiment not only streamlines the experimentally observed results, but also offers the understanding of the reaction mechanisms and reaction conditions.