On the behaviour of Ru(I) and Ru(II) carbonyl acetates in the presence of H2 and/or acetic acid and their role in the catalytic hydrogenation of acetic acid

The reactivity of phosphine substituted ruthenium carbonyl carboxylates Ru(CO)2(MeCOO)2(PBu3)2, Ru2(CO)4(μ-MeCOO)2(PBu3)2, Ru4(CO)8(μ-MeCOO)4(PBu3)2 with H2 and/or acetic acid was investigated by IR and NMR spectroscopy to clarify their role in the catalytic hydrogenation of acetic acid. Evidences were collected to suggest hydride ruthenium complexes as the catalytically active species. Equilibria among ruthenium hydrides and carboxylato complexes take place in the presence of hydrogen and acetic acid, that is in the conditions of the catalytic reaction. Nevertheless the presence of acetic acid reduces the rate of the formation of hydrides. Working at a very high temperature (180°C) polynuclear phosphido hydrides such as [Ru6(μ-H)6(CO)10(μ-PHBu)(μ-PBu2)2(PBu3)2(μ6-P)] were formed. These phosphido clusters are suggested as the resting state of the catalytic system. Furthermore the bi- or tetranuclear Ru(I) carboxylato complexes react with acetic acid giving a mononuclear ruthenium complex Ru(CO)2(MeCOO)(μ-MeCOO)(PBu3), containing a monodentate and a chelato acetato ligands. This complex was spectroscopically characterised. Its identity and structure were confirmed by its reactivity with stoichiometric amount of PPh3 to give Ru(CO)2(MeCOO)2(PBu3)(PPh3), a new mononuclear ruthenium carbonyl carboxylate containing two different phosphines, that was fully characterised.