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)₄PPh₂) with NaOCH₂CF₃ leads to the amine-alkoxide [Ru(CNN)(OCH₂CF₃)(dppb)] (1-OCH₂CF₃), whose neutron diffraction study reveals a short RuO⋯HN bond length. Treatment of 1-Cl with NaOEt and EtOH affords the alkoxide [Ru(CNN)(OEt)(dppb)]·(EtOH)_n (1-OEt·n EtOH), which equilibrates with the hydride [RuH(CNN)(dppb)] (1-H) and acetaldehyde. Compound 1-OEt·n EtOH reacts reversibly with H₂ leading to 1-H and EtOH through dihydrogen splitting. NMR spectroscopic studies on 1-OEt·n EtOH and 1-H reveal hydrogen bond interactions and exchange processes. The chloride 1-Cl catalyzes the hydrogenation (5 atm of H₂) of ketones to alcohols (turnover frequency (TOF) up to 6.5×10⁴ h^-1, 40 C). DFT calculations were performed on the reaction of [RuH(CNN′)(dmpb)] (2-H) (HCNN′=2-aminomethyl-6-(phenyl)pyridine; dmpb=Me₂P(CH₂)₄PMe₂) 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 H₂; both pathways have low barriers and are alcohol assisted.