Elucidating the role of amine donors in manganese catalyzed transfer hydrogenation

Catalysts for transfer hydrogenation (TH) reactions based on the earth abundant transition metal manganese are an emerging field of research. A common moiety found in most reported phosphine-free catalysts is a primary or secondary amine donor, which is generally considered to be crucial within an outer-sphere mechanism. Here, we investigated the role of the NH-motif in the mechanism in catalytic transfer hydrogenation of ketones. Therefore, we synthesized Mn(I) complexes with a pyridyl-pyrazole or a pyridyl-imidazole ligand, which carry an NH-motif at different positions. Subsequent investigation of their catalytic activity points towards an inner-spere mechanism in which the NH-motif of the amino ligand is deprotonated throughout the catalytic cycle. The resulting amido ligand increases the electron density at the metal which facilitates the hydride transfer to the substrate. The presented alternative inner-sphere mechanism may aid in the future development of manganese-based catalysts for hydrogen transfer reactions.