The crucial role of ligand deprotonation in manganese catalyzed hydrogenation reactions

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Hydrogenation catalysts based on the abundant early transition metal manganese recently received growing attention. A common motif found in most of the reported systems is a deprotonatable hydrogen atom in the ligand, usually considered necessary within an outer-sphere mechanism. Here we investigated the mechanism of the catalytic hydrogenation of ketones, focusing on the crucial function of the deprotonatable H atom in the ligand. Therefore, we synthesized phosphine-free Mn(I) complexes of the type [(L)MnBr(CO)3)] utilizing pyridyl-imidazole ligands, having a deprotonatable H atom within the imidazole ring. They were found to be active catalysts for the hydrogenation of ketones with molecular hydrogen. Kinetic and spectroscopic studies together with DFT calculations point towards an inner-sphere mechanism with the hydride transfer to the substrate being the rate determining step. Most importantly, we demonstrate that the NH-motif remains deprotonated throughout the catalytic cycle maintaining an anionic manganese species. Thereby, the anionic ligand is a more potent electron donor which enhances the rate determining hydride transfer.