PNN Acetate Ruthenium(II) Complexes as Active Catalysts for Fatty Ester Hydrogenation

The pincer acetate complexes trans ‐[Ru(η 1 ‐OCOCH 3 ) 2 (PNN)(PPh 3 )] (PNN = N‐(2‐(diphenylphosphaneyl)benzyl)‐1‐(pyridin‐2‐yl)methanamine (PNN’) 1, 2‐(diphenylphosphaneyl)‐N‐(pyridin‐2‐ylmethyl)ethan‐1‐amine (PNN G ) 2 ) are synthesized from [Ru(η 2 ‐OCOCH 3 ) 2 (PPh 3 ) 2 ] and the corresponding PNN ligands in acetone. Reaction of [RuH(η 2 ‐OCOCH 3 )(PPh 3 ) 3 ] with the PNN ligands in n ‐heptane gives trans ‐[RuH(η 1 ‐OCOCH 3 )(PNN)(PPh 3 )] (PNN = PNN’ 3 , PNN G 4 ). Substitution of PPh 3 in 1 with PPh 2 Me gives trans ‐[Ru(η 1 ‐OAc) 2 (PNN’)(PPh 2 Me)] ( 5 ) in toluene, whereas starting from 2 results in trans ‐[Ru(η 1 ‐OAc) 2 (PNN G )(PPh 2 Me)] ( 6 ) and cis ‐[Ru(η 2 ‐OAc)(PNN G )(PPh 2 Me)]OAc ( 7 ) (in 1:1 ratio). The monocarbonyl [Ru(η 2 ‐OAc)(CO)(PNN’)]OAc ( 8 ) and cis ‐[Ru(η 1 ‐OAc) 2 (CO)(PNN’)] ( 9 ) (in 3:1 ratio) are formed from [Ru(η 1 ‐OAc)(η 2 ‐OAc)(CO)(PPh 3 ) 2 ] with PNN’, whereas reaction of the latter precursor with 1‐(2‐(diphenylphosphaneyl)phenyl)‐N‐(2‐(pyridin‐2‐yl)ethyl)methanimine (PNN(C 2 ) imine ) affords cis ‐[Ru(η 1 ‐OAc) 2 (CO)(PNN(C 2 ) imine )] ( 10 ). Acetate complexes 1 ‐ 9 catalyze the homogeneous hydrogenation of methyl decanoate to 1‐decanol with NaOMe under mild reaction conditions (T = 40 °C, H 2 pressure 27.5 bar) and low catalyst loading (S/C 1000–100 000) in the absence of solvents. Complex 2 displays extremely high catalytic activity for the reduction of fatty acid methyl and ethyl esters at S/C of 50 000–100 000, with high chemoselectivity and without CC bond reduction. In addition, 1 ‐ 4 and 8/9 are found to catalyze the base‐free hydrogenation of aldehydes. NMR studies show that 1 reacts with H 2 (5 bar), affording the monohydride 3 , which, with KO t Bu and hydrogen, reduces methyl benzoate to benzyl alcohol.