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

The diruthenium(0) complex [Ru-2(mu -CO)(CO)(4)(mu -dppm)(2)] (1) (dppm = Ph2PCH2PPh2), is a catalyst for the transfer hydrogenation, using formic acid as hydrogen donor, of the alkynes Ph=CPh, PhC=CMe, EtC=CEt, and PrC=CPr but not of the terminal alkynes HC=CH, PhC=CH, BuC=CH, or the alkynes containing one or two electron-withdrawing substituents PhC=CCO2Me and MeO2CC=CCO2Me. 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 PhC=CH, it is shown that the alkyne reacts with protonated 1 to give first [Ru-2(mu -CPh=CH2)(CO)(4)(mu -dppm)(2)][HCO2], which then isomerizes to give the catalytically inactive, stable complex [Ru-2(mu -CH=CHPh)(CO)(4)(mu -dppm)(2)][HCO2]. This complex has been structurally characterized and both of the mu -styrenyl complexes are shown to be fluxional in solution.