Carboxylato-triphenylphosphine complexes of ruthenium, cationic triphenylphosphine complexes derived from them, and their behaviour as homogeneous hydrogenation catalysts for alkenes

The two-electron reduction of the oxo-centred triangular acetato-complex, Ru3O(CO2Me)6(PPh3)3, in which the metal has mean oxidation state 2⅔, gives a yellow ruthenium(II) species, probably Ru(CO2Me)2PPh3. In methanol containing a non-complexing strong acid and additional triphenylphosphine to give a minimum PPh3 : Ru ratio of 2 : 1, the complex acts as a catalyst for the hydrogenation of alkenes. Similar catalyst solutions in methanol are obtained by the action of acids on the hydrido-acetate, RuH(CO2Me)(PPh3)3, or the dihydride, Ru(H)2(PPh3)4; aqueous fluoroboric acid, p-toluenesulphonic acid hydrate or trifluoromethylsulphonic acid have been used. The initial product is evidently the (solvated) cation [Ru(PPh3)3]2+, but in presence of alkene a bis species, such as [Ru(PPh3)2(alkene)]2+ is formed. On standing, or on treatment with H2 or CO in absence of alkene, the orange-red tris species are converted to catalytically inactive yellow species, this reaction being faster in presence of water. From aged solutions the salt [(Ph3P)2Ru(OH)2Ru(PPh3)2](BF4)2 has been isolated. The yellow solutions of Ru(CO2Me)2PPh3 are oxidised by air to a new type of oxo-bridged ruthenium(III) complex, Ru2O(CO2Me)4(PPh3)2, which is shown to be an intermediate in the preparation of RuH(CO2Me)(PPh3)3 from commercial hydrated ruthenium chloride. The diacetate, Ru(CO2Me)2(PPh3)2 has also been prepared and the reduction of the tetra-µ-acetato-complex [Ru2(CO2Me)4]Cl investigated. Analogues of some of the compounds with other carboxylate groups are described.