A Dihydride Mechanism Can Explain the Intriguing Substrate Selectivity of Iron-PNP-Mediated Hydrogenation

Iron-PNP pincer complexes are efficient catalysts for the hydrogenation of aldehydes and ketones. A variety of hydrogenation mechanisms have been proposed for these systems, but there appears to be no clear consensus on a preferred pathway. We have employed high-level quantum chemical calculations to evaluate various mechanistic possibilities for iron-PNP catalysts containing either CH2, NCH3, or NH in the PNP linker. For all three catalyst types, we propose that the active species is a trans-dihydride complex. For CH2- and NH-containing complexes, we predict a dihydride mechanism involving a dearomatization of the backbone. The proposed mechanism proceeds through a metal-bound alkoxide intermediate, in excellent agreement with experimental observations. Interestingly, the relative stability of the iron-alkoxide can explain why complexes with NCH3 in the PNP linker are chemoselective for aldehydes, whereas those with CH2 or NH in the linker do not show a clear substrate preference. As a general concept in computational catalysis, we recommend to employ known substrate selectivities as a diagnostic factor to evaluate the probability of proposed mechanisms.