Hydrogen transfer from formic acid to alkynes catalyzed by a diruthenium complex

The diruthenium(0) complex [Ru 2 (µ-CO)(CO) 4 (µ-dppm) 2 ] (1) (dppm = Ph 2 PCH 2 PPh 2 ), is a catalyst for the transfer hydrogenation, using formic acid as hydrogen donor, of the alkynes PhCºCPh, PhCºCMe, EtCºCEt, and PrCºCPr but not of the terminal alkynes HCºCH, PhCºCH, BuCºCH, or the alkynes containing one or two electron-withdrawing substituents PhCºCCO 2 Me and MeO 2 CCtriple bondCCO 2 Me. In the successful reactions, the formic acid is first decomposed to carbon dioxide and hydrogen, which then hydrogenates the alkynes in a slower reaction. In the unsuccessful reactions, the decomposition of formic acid is strongly retarded by the alkyne. In the case with the alkyne PhCºCH, it is shown that the alkyne reacts with protonated 1 to give first [Ru 2 (µ-CPh=CH 2 )(CO) 4 (µ-dppm) 2 ][HCO 2 ], which then isomerizes to give the catalytically inactive, stable complex [Ru 2 (µ-CH=CHPh)(CO) 4 (µ-dppm) 2 ][HCO 2 ]. This complex has been structurally characterized and both of the µ-styrenyl complexes are shown to be fluxional in solution.Key words: ruthenium, hydrogenation, catalysis, binuclear..