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

Iron-PNP pincer complexes are efficient catalystsfor the hydrogenation of aldehydes and ketones. A varietyof hydrogenation mechanisms have been proposed for thesesystems, but there appears to be no clear consensus on apreferred pathway. We have employed high-level quantumchemical calculations to evaluate various mechanistic possibilitiesfor iron-PNP catalysts containing either CH2, NCH3, orNH in the PNP linker. For all three catalyst types, we proposethat the active species is a trans-dihydride complex. For CH2-and NH-containing complexes, we predict a dihydridemechanism involving a dearomatization of the backbone. Theproposed mechanism proceeds through a metal-boundalkoxide intermediate, in excellent agreement with experimental observations. Interestingly, the relative stability of the ironalkoxidecan explain why complexes with NCH3 in the PNP linker are chemoselective for aldehydes, whereas those with CH2 orNH in the linker do not show a clear substrate preference. As a general concept in computational catalysis, we recommend toemploy known substrate selectivities as a diagnostic factor to evaluate the probability of proposed mechanisms