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.