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

The nitrosyl ligand exhibits interesting features for catalysis, such as strong π acidity, excellent stability, and potential non innocent behavior. Pincer complexes are known to be extraordinarily stable yet tunable and catalytically highly active. However, even though pincer nitrosyl complexes could potentially open new possibilities within catalysis, they have been scarcely explored and witnessed little development within this area. Herein, we describe the synthesis of a novel family of ruthenium PNP nitrosyl complex salts (PNP = (iPr2PCH2CH2)2NH and X = Cl, BF4, BPh4 and PF6) and their use for catalytic transfer hydrogenation of a range of carbonyl compounds. For the transfer hydrogenation of ketones, an optimization study showed that all catalysts (Ru-1 to Ru-4) are highly active at 90 °C, reaching high to full conversion already within 5 minutes, a behavior that was comparable to the well-known commercially available Ru-MACHO derivative Ru-5. At 35 °C, Ru 1 (X = Cl) and Ru 2 (X = BF4) were more efficient than Ru 3 (X = BPh4) and Ru-4 (X = PF6), likely due to their higher solubility in isopropanol. 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 hours at 70 °C. Overall, a large scope of ketones and aldehydes are 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.