Ruthenium(II) diphosphine/diamine/diimine complexes and catalyzed hydrogen-transfer to ketones

An in situ product, presumed to be RuCl2(DPPF)(PPh3), formed in CH2Cl2 from a 1:1 mixture of 1,1’-bis(diphenylphosphino)ferrocene (DPPF) and RuCl2(PPh3)(3), reacts with 1 equiv of a diamine or a diimine (N-N donors) dissolved in MeOH to generate RuCl2(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 1S,2S-diaminocyclohexane (1S,2S-dach). Diethylenetriamine (dien), a tridentate N-donor, generates a monochloro cationic complex. The isolated complexes are trans-RuCl2(DPPF)(en) (1), trans-RuCl2(DPPF)(dimen) (2), [RuCl(DPPF)(dien)]Cl (3), trans-RuCl2(DPPF)(diap) (4), cis-RuCl2(DPPF)(bipy) (5), cis-RuCl2(DPPF)(phen) (6), and trans-RuCl2(DPPF)(1S,2S-dach) (7). The known complex trans-RuCl2(DPPB)(en) (8) was similarly made using RuCl(DPPB)(2)(mu-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 similar to 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 degrees 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.