Abstract
The osmium compound trans,cis‐[OsCl~2~(PPh~3~)~2~(Pyme)] (1) (Pyme=1‐(pyridin‐2‐yl)methanamine), obtained from [OsCl~2~(PPh~3~)~3~] and Pyme, thermally isomerizes to cis,cis‐[OsCl~2~(PPh~3~)~2~(Pyme)] (2) in mesitylene at 150 °C. Reaction of [OsCl~2~(PPh~3~)~3~] with Ph~2~P(CH~2~)~4~PPh~2~ (dppb) and Pyme in mesitylene (150 °C, 4 h) leads to a mixture of trans‐[OsCl~2~(dppb)(Pyme)] (3) and cis‐[OsCl~2~(dppb)(Pyme)] (4) in about an 1:3 molar ratio. The complex trans‐[OsCl~2~(dppb)(Pyet)] (5) (Pyet=2‐(pyridin‐2‐yl)ethanamine) is formed by reaction of [OsCl~2~(PPh~3~)~3~] with dppb and Pyet in toluene at reflux. Compounds 1, 2, 5 and the mixture of isomers 3/4 efficiently catalyze the transfer hydrogenation (TH) of different ketones in refluxing 2‐propanol and in the presence of NaO__i__Pr (2.0 mol %). Interestingly, 3/4 has been proven to reduce different ketones (even bulky) by means of TH with a remarkably high turnover frequency (TOF up to 5.7×10^5^ h^−1^) and at very low loading (0.05–0.001 mol %). The system 3/4 also efficiently catalyzes the hydrogenation of many ketones (H~2~, 5.0 atm) in ethanol with KO__t__Bu (2.0 mol %) at 70 °C (TOF up to 1.5×10^4^ h^−1^). The in‐situ‐generated catalysts prepared by the reaction of [OsCl~2~(PPh~3~)~3~] with Josiphos diphosphanes and (±)‐1‐alkyl‐substituted Pyme ligands, promote the enantioselective TH of different ketones with 91–96 % ee (ee=enantiomeric excess) and with a TOF of up to 1.9×10^4^ h^−1^ at 60 °C.