Manganese Pincer Complexes for the Base-Free, Acceptorless Dehydrogenative Coupling of Alcohols to Esters: Development, Scope, and Understanding

Aliphatic
PNP pincer-supported earth-abundant manganese­(I) dicarbonyl
complexes behave as effective catalysts for the acceptorless dehydrogenative
coupling of a wide range of alcohols to esters under base-free conditions.
The reaction proceeds under neat conditions, with modest catalyst
loading and releasing only H2 as byproduct. Mechanistic
aspects were addressed by synthesizing key species related to the
catalytic cycle (characterized by X-ray structure determination, multinuclear
(1H, 13C, 31P, 15N, 55Mn) NMR, infrared spectroscopy, inter alia), by studying
elementary steps connected to the postulated mechanism, and by resorting
to DFT calculations. As in the case of related ruthenium and iron
PNP catalysts, the dehydrogenation results from cycling between the
amido and amino-hydride forms of the PNP-Mn­(CO)2 scaffold.
For the dehydrogenation of alcohols into aldehydes, our results suggest
that the highest energy barrier corresponds to the hydrogen release
from the amino-hydride form, although its value is close to that of
the outer-sphere dehydrogenation of the alcohol into aldehyde. This
contrasts with the ruthenium and iron catalytic systems, where dehydrogenation
of the substrate into aldehyde is less energy-demanding compared to
hydrogen release from the cooperative metal–ligand framework.