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

The pincer acetate complexes trans-[Ru(η1-OCOCH3)2(PNN)(PPh3)] (PNN = N-(2-(diphenylphosphaneyl)benzyl)-1-(pyridin-2-yl)methanamine (PNN’) 1, 2-(diphenylphosphaneyl)-N-(pyridin-2-ylmethyl)ethan-1-amine (PNNG) 2) are synthesized from [Ru(η2-OCOCH3)2(PPh3)2] and the corresponding PNN ligands in acetone. Reaction of [RuH(η2-OCOCH3)(PPh3)3] with the PNN ligands in n-heptane gives trans-[RuH(η1-OCOCH3)(PNN)(PPh3)] (PNN = PNN’ 3, PNNG 4). Substitution of PPh3 in 1 with PPh2Me gives trans-[Ru(η1-OAc)2(PNN’)(PPh2Me)] (5) in toluene, whereas starting from 2 results in trans-[Ru(η1-OAc)2(PNNG)(PPh2Me)] (6) and cis-[Ru(η2-OAc)(PNNG)(PPh2Me)]OAc (7) (in 1:1 ratio). The monocarbonyl [Ru(η2-OAc)(CO)(PNN’)]OAc (8) and cis-[Ru(η1-OAc)2(CO)(PNN’)] (9) (in 3:1 ratio) are formed from [Ru(η1-OAc)(η2-OAc)(CO)(PPh3)2] with PNN’, whereas reaction of the latter precursor with 1-(2-(diphenylphosphaneyl)phenyl)-N-(2-(pyridin-2-yl)ethyl)methanimine (PNN(C2)imine) affords cis-[Ru(η1-OAc)2(CO)(PNN(C2)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, H2 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 H2 (5 bar), affording the monohydride 3, which, with KOtBu and hydrogen, reduces methyl benzoate to benzyl alcohol.