CNN Pincer Ruthenium Catalysts for Hydrogenation and Transfer Hydrogenation of Ketones: Experimental and Computational Studies

Reaction of [RuCl(CNN)(dppb)] (1-Cl) (HCNN=2-aminomethyl-6-(4-methylphenyl)pyridine; dppb=Ph2P(CH2)(4)PPh2) with NaOCH2CF3 leads to the amine-alkoxide [Ru(CNN)(OCH2CF3)(dppb)] (1-OCH2CF3), whose neutron diffraction study reveals a short RuOHN bond length. Treatment of 1-Cl with NaOEt and EtOH affords the alkoxide [Ru(CNN)(OEt)(dppb)](EtOH)(n) (1-OEtnEtOH), which equilibrates with the hydride [RuH(CNN)(dppb)] (1-H) and acetaldehyde. Compound 1-OEtnEtOH reacts reversibly with H-2 leading to 1-H and EtOH through dihydrogen splitting. NMR spectroscopic studies on 1-OEtnEtOH and 1-H reveal hydrogen bond interactions and exchange processes. The chloride 1-Cl catalyzes the hydrogenation (5atm of H-2) of ketones to alcohols (turnover frequency (TOF) up to 6.5x10(4)h(-1), 40 degrees C). DFT calculations were performed on the reaction of [RuH(CNN)(dmpb)] (2-H) (HCNN=2-aminomethyl-6-(phenyl)pyridine; dmpb=Me2P(CH2)(4)PMe2) with acetone and with one molecule of 2-propanol, in alcohol, with the alkoxide complex being the most stable species. In the first step, the Ru-hydride transfers one hydrogen atom to the carbon of the ketone, whereas the second hydrogen transfer from NH2 is mediated by the alcohol and leads to the key amide intermediate. Regeneration of the hydride complex may occur by reaction with 2-propanol or with H-2; both pathways have low barriers and are alcohol assisted.