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

Abstract

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.