Ruthenium Pincer Nitrosyl Complex Salts in Catalytic Transfer Hydrogenations Under Mild Conditions

Abstract The nitrosyl ligand exhibits interesting features for catalysis, such as strong π‐acidity and potential noninnocent behavior. Pincer complexes are the extraordinarily stable yet tunable and often catalytically highly active. However, even though pincer nitrosyl complexes could open new possibilities within, they have been scarcely explored within catalysis. In this study, we have described the synthesis of a novel family of ruthenium PNP nitrosyl complex salts ([RuPNP(Cl) 2 NO]X) and their use for catalytic transfer hydrogenation. For the transfer hydrogenation of ketones, all catalysts (Ru‐1 to Ru‐4) are highly active at 90 °C, reaching up to full conversion already within 5 min. At 35 °C, Ru‐1 (X = Cl) and Ru‐2 (X = BF 4 ) were more efficient than Ru‐3 (X = BPh 4 ) and Ru‐4 (X = PF 6 ). Interestingly, for the transfer hydrogenation of aldehydes, higher temperatures and longer reaction times were required. Thus, Ru‐1 and Ru‐3 performed better with the aldehydes, leading to full conversion after 7 h at 70 °C. Overall, a large substrate scope is presented for the first time with a pincer nitrosyl catalyst. Lastly, competition studies revealed that the hydrogenation of a ketone is suppressed by the presence of an aldehyde. The data indicates two different mechanisms for aldehyde and ketone transfer hydrogenation.