Ruthenium(II) Diphosphine/Diamine/Diimine Complexes and Catalyzed Hydrogen-Transfer to Ketones

Abstract

An in situ product, presumed to be RuCl 2 (DPPF)(PPh 3 ), formed in CH 2 Cl 2 from a 1:1 mixture of 1,1‘-bis(diphenylphosphino)ferrocene (DPPF) and RuCl 2 (PPh 3 ) 3, reacts with 1 equiv of a diamine or a diimine (N−N donors) dissolved in MeOH to generate RuCl 2 (DPPF)(N−N) complexes: N−N is ethylenediamine (en), N, N ‘-dimethyl(ethylenediamine) (dimen), 1,3-diaminopropane (diap), 2,2‘-bipyridine (bipy), 1,10-phenanthroline (phen), and 1 S, 2 S -diaminocyclohexane (1 S,2 S -dach). Diethylenetriamine (dien), a tridentate N-donor, generates a monochloro cationic complex. The isolated complexes are trans -RuCl 2 (DPPF)(en) ( 1 ), trans -RuCl 2 (DPPF)(dimen) ( 2 ), [RuCl(DPPF)(dien)]Cl ( 3 ), trans -RuCl 2 (DPPF)(diap) ( 4 ), cis -RuCl 2 (DPPF)(bipy) ( 5 ), cis -RuCl 2 (DPPF)(phen) ( 6 ), and trans -RuCl 2 (DPPF)(1 S,2 S -dach) ( 7 ). The known complex trans -RuCl 2 (DPPB)(en) ( 8 ) was similarly made using RuCl(DPPB) 2 (μ-Cl) 3 as precursor, where DPPB is 1,4-bis(diphenylphosphino)butane. Complexes 1, 2, 5, and 8 were characterized crystallographically. Complexes 1 − 8 are effective precursor catalysts in basic 2-propanol solutions for the hydrogen-transfer hydrogenation of acetophenone; the chiral phosphine system ( 7 ) gives only ∼12% ee at high conversions to 1-phenylethanol, while at 25% conversion the ee reaches 36%. Greater activity for precursor catalyst 1 versus that of 2 qualitatively supports the “metal−ligand bifunctional” mechanism for such diphosphine/diamine systems; however, the “NH-free” diimine bipy and phen systems are as active at 80 °C as the diamine systems and must operate by a different mechanism. Complex 8 is also an effective precursor hydrogen-transfer catalyst for other alkyl−aryl and dialkyl ketones, which were used as model substrates for components of lignin; a substituted styrene was not hydrogenated.