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The Key Role of the Latent N-H Group in Milstein's Catalyst for Ester Hydrogenation

submitted on 20.01.2021, 21:21 and posted on 22.01.2021, 05:24 by John Pham, Cole Jarczyk, Eamon Reynolds, Sophie Kelly, Thao Kim, Tianyi He, Jason Keith, Anthony Chianese

We previously demonstrated that Milstein’s seminal diethylamino-substituted PNN-pincer-ruthenium catalyst for ester hydrogenation is activated by dehydroalkylation of the pincer ligand, releasing ethane and eventually forming an NHEt-substituted derivative that we proposed is the active catalyst. In this paper, we present a computational and experimental mechanistic study supporting this hypothesis. Our DFT analysis shows that the minimum-energy pathways for hydrogen activation, ester hydrogenolysis, and aldehyde hydrogenation rely on the key involvement of the nascent N-H group. We have isolated and crystallographically characterized two catalytic intermediates, a ruthenium dihydride and a ruthenium hydridoalkoxide, the latter of which is the catalyst resting state. A detailed kinetic study shows that catalytic ester hydrogenation is first-order in ruthenium and hydrogen, shows saturation behavior in ester, and is inhibited by the product alcohol. A global fit of the kinetic data to a simplified model incorporating the hydridoalkoxide and dihydride intermediates and three kinetically relevant transition states showed excellent agreement with the results from DFT.


NSF CHE-1665144

NSF CHE-1954924

NSF CHE-1726308


Email Address of Submitting Author


Colgate University


United States

ORCID For Submitting Author


Declaration of Conflict of Interest

The authors declare no conflict of interest.