Intrinsic Dynamics and Condition Dependence of Homogeneous Ester Hydrogenation Revealed by Mn-CNC Pincers

Homogeneous catalytic hydrogenations often operate in dynamic conditions. For example, in hydrogenation of esters reaction medium changes its polarity and becomes protic as reaction proceeds. As a result, the nature of the catalytic and reactive species in catalysis can change throughout the reaction, making it impossible to draw a static picture describing the catalyst performance. Herein we report on the molecular origins of such a complexity in the catalytic hydrogenation of esters. Using a new bis-N-heterocyclic carbene manganese (I) pincer catalyst we perform operando FTIR spectroscopy and kinetic studies to reveal the highly dynamic nature of the Mn intermediates formed in the catalytic mixture. Furthermore, we identify persistent inhibition phenomena caused by the reversible interaction of catalytically competent species with alcohol products. Pronounced strongly in Mn-promoted hydrogenation, this inhibitory pathway can principally affect any transition metal catalyst in ester hydrogenation. Finally, we show that the catalyst inhibition can be suppressed by using basic alkoxide promotors common for the majority of polar hydrogenations. We provide the first experimental and computational evidence that alkoxide promotion can alter the shape of the free energy surface and render catalyst inhibition unfavorable. At large, it implies a marked change to the values of the standard free energies of the key steps in the catalytic cycle that are universally assumed to be constant. While allowing to promote transformations that would otherwise be thermodynamically prohibited, these phenomena make a case for reconsideration of a traditional static viewpoint on homogeneous hydrogenation reactions.