| Abstract
| - The well-characterized complexes cis- and trans-RuCl2(P−P)(N−N), where P−P is usually a chelating ferrocenyl diphosphine and N−N is a chelating diamine or diimine, act as catalyst precursors in basic 2-propanol for the transfer hydrogenation of acetophenone and other ketones; the “NH-free” imine systems show activity comparable to that of the amine systems, implying the presence of a non-bifunctional mechanism. The precursor [RuCl(P−P)(N−N−N)]Cl containing a triamine is slightly less active.
- An in situ product, presumed to be RuCl2(DPPF)(PPh3), formed in CH2Cl2 from a 1:1 mixture of1,1‘-bis(diphenylphosphino)ferrocene (DPPF) and RuCl2(PPh3)3, reacts with 1 equiv of a diamine or adiimine (N−N donors) dissolved in MeOH to generate RuCl2(DPPF)(N−N) complexes: N−N isethylenediamine (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). Theknown complex trans-RuCl2(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 characterizedcrystallographically. Complexes 1−8 are effective precursor catalysts in basic 2-propanol solutions forthe 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 activityfor precursor catalyst 1 versus that of 2 qualitatively supports the “metal−ligand bifunctional” mechanismfor such diphosphine/diamine systems; however, the “NH-free” diimine bipy and phen systems are asactive at 80 °C as the diamine systems and must operate by a different mechanism. Complex 8 is alsoan effective precursor hydrogen-transfer catalyst for other alkyl−aryl and dialkyl ketones, which wereused as model substrates for components of lignin; a substituted styrene was not hydrogenated.
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