We describe here the first Mn(I) complexes with P-stereogenic PN(H)P pincer ligands (1) as catalysts for the asymmetric hydrogenation of ketones. Complex 1b (1 mol%, 50 bar H₂, toluene, 80 °C) hydrogenates acetophenone to 1-phenylethanol with 99% yield and 55% ee after 16 h reaction time.

Furthermore, we studied 1a for the sake of comparison with the iron(II) analogue 2a, which has been previously prepared in our group.¹ The comparison between the isoelectronic manganese(I) and iron(II) hydrogenation catalysts has rapidly grown into a hot topic,² but no quantitative analysis has been reported to date. DFT calculations, NMR spectroscopic and kinetic studies indicate that the lower catalytic activity of Mn(I) is due to the large energetic span between the transition state of dihydrogen activation (2) and the alkoxide complex that is formed as resting species (4).³

[1] R. Huber, A. Passera, E. Gubler, A. Mezzetti, Adv. Synth. Catal., 2018, 360, 2900.
[2] a) G. A. Filonenko, R. van Putten, E. J. M. Hensen, E. A. Pidko, Chem. Soc. Rev., 2018, 47, 1459; b) F. Kallmeier, R. Kempe, Angew. Chem. Int. Ed., 2018, 57, 46; c) N. Gorgas, K. Kirchner, Acc. Chem. Res., 2018, 51, 1558; d) L. Alig, M. Fritz, S. Schneider, Chem. Rev., 2019, 119, 2681.
[3] L. de Luca, A. Passera, A. Mezzetti, J. Am. Chem. Soc., 2019, 141, 2545.