Hydrogenation of ketones with β-diketimine manganese(I) catalysts at room temperature

• Effective hydrogenation of ketones at room temperature. • First catalytic application of beta-diketiminat manganese(I) complexes. • Experiments indicate that an anionic manganese complex is the catalytically active species, with the potassium counter ion playing an important role in the catalytic cycle. • Addition of alcohol results in increased reaction rate. The hydrogenation of organic substrates typically requires elevated temperatures, high H 2 pressures, and extended reaction times. To enable more favorable reaction conditions, a deeper understanding of the underlying mechanism is essential. The in literature proposed pathways often involve either a metal–ligand cooperative mechanism or a mechanism proceeding via an anionic metal center, in which the counterion plays a significant role. Herein, we present β-diketimine manganese(I) catalysts of the type [MnBr(CO) 3 (HL)] (HL = H 2 C[C(Me)NAr] 2 or HC(Me)[C(Me)NAr] 2 ) capable of efficiently reducing ketones at room temperature. One equivalent of base results in the deprotonation of the β-diketimine ligand and abstraction of the bromine, resulting in neutral five coordinate manganese species with an anionic ligand [Mn(Lx)(CO) 3 ]. Additional base is required for the hydrogenation to proceed, suggesting that the active catalyst operates as an anionic species. Mechanistic studies revealed a strong dependence on the nature of the counterion introduced by the added base. Moreover, the addition of MeOH to the catalytic reaction led to a significant increase in conversion for all substrates. This effect is attributed to solvation of the counterion by MeOH which modulates the formation of the catalytically active ion pair and thereby accelerating the catalytic process.