Basic Promotors Impact Thermodynamics and Catalyst Speciation in Homogeneous Carbonyl Hydrogenation

Homogeneously catalyzed
reactions often make use of additives and
promotors that affect reactivity patterns and improve catalytic performance.
While the role of reaction promotors is often discussed in view of
their chemical reactivity, we demonstrate that they can be involved
in catalysis indirectly. In particular, we demonstrate that promotors
can adjust the thermodynamics of key transformations in homogeneous
hydrogenation catalysis and enable reactions that would be unfavorable
otherwise. We identified this phenomenon in a set of well-established
and new Mn pincer catalysts that suffer from persistent product inhibition
in ester hydrogenation. Although alkoxide base additives do not directly
participate in inhibitory transformations, they can affect the equilibrium
constants of these processes. Experimentally, we confirm that by varying
the base promotor concentration one can control catalyst speciation
and inflict substantial changes to the standard free energies of the
key steps in the catalytic cycle. Despite the fact that the latter
are universally assumed to be constant, we demonstrate that reaction
thermodynamics and catalyst state are subject to external control.
These results suggest that reaction promotors can be viewed as an
integral component of the reaction medium, on its own capable of improving
the catalytic performance and reshaping the seemingly rigid thermodynamic
landscape of the catalytic transformation.