Osmium(II) bipyridine (bpy) complexes containing O,O-donor ligands and X-ray crystal structure of the acetylacetonato(acac) complex [Os(bpy)2(acac)](PF6)

The new complexes [Os11(bpy)2L](PF6) [bpy = 2,2′-bipyridine; HL = acetylacetone (Hacac), trifluoroacetylacetone (Htfacac), hexafluoroacetylacetone, dibenzoylmethane or tropolone] have been prepared and the chemical oxidation of the [OsII(bpy)2L]+ species (L = acac or tfacac) by aqueous S2O82- solution in the presence of (NH4)PF6 gave the reddish brown complexes [OsIII(bpy)2L](PF6)2. These complexes were characterized by spectroscopic measurements and also investigated by cyclic voltammetry. The X-ray crystal structure of [Os(bpy)2(acac)](PF6) (1) has been determined and showed that the complex has a distorted octahedral geometry in an environment of N4O2 donors. The reactivity toward oxidation of benzyl alcohol by complex (1) using H2O2 or other co-oxidants has been studied and compared with that of the related ruthenium(II) complexes.

► Careful synthesis of the complexes [OsII(bpy)2L](PF6) was performed under N2 and use of the heterogeneous base catalyst (CaCO3).
► The structural data were obtained.
► The electrochemical data showed that the O,O-donor ligand (L) stabilizes the high oxidation state of osmium(III) and confirms that OsII is easier to oxidize than RuII and the general trend is consistent with the inherent stability of the third-row vs second-row transition metals in higher oxidation states.
► Osmium complexes are weaker oxidanmolstrts than those of ruthenium.