Efficient Hydrogenation of N-Heterocycles Catalyzed by NNP-Manganese(I) Pincer Complexes at Ambient Temperature

In summary, several imidazolylphosphine pincer ligands and the corresponding Mn complexes were synthesized, fully characterized, and studied by a combination of experimental and theoretical investigations. The complexes were tested in several hydrogenation reactions with good to excellent performance. Especially, the reduction of diverse N-heterocycles can be performed in excellent yields and high selectivity under very mild conditions using catalyst Mn-2 b. Spectroscopic studies support a classical metal-ligand cooperative mechanism for the activation of the catalyst, revealing amido as well as hydrido species. A third species could also be observed, tentatively assigned as the tert-butoxy species (MnOBu)-Bu-t-NH-2 b. DFT supported these findings showing the possibility of equilibrium between all three catalyst species in the reaction. It further revealed higher kinetic reactivity as well as lower formation barrier for MnH-NH-2 b over MnH-NH-1 b as likely causes for its improved reactivity.