Journal
ACS CATALYSIS
Volume 7, Issue 9, Pages 5847-5855Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acscatal.7b00764
Keywords
iron-pincer; hydrogenation; chemoselectivity; mechanism; DFT
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Funding
- Research Council of Norway through a FRINATEK [231706]
- Centre of Excellence Grant [179568]
- Tromso research foundation [TFS2016KHH]
- Notur - The Norwegian Metacenter for Computational Science [nn9330k, nn4654k]
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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.
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