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

Abstract

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 ironalkoxide 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.