Chemistry of some ruthenium phenolates: synthesis, structure and redox properties

Abstract

Reaction of three phenolate ligands, viz. 2,4,6-tribromophenol (HL¹, where H stands for the phenolic proton), 2-nitrophenol (HL²) and 2,4,6-trinitrophenol (HL³O), with [Ru(PPh₃)₃Cl₂] in a 2¦1 molar ratio in the presence of a base gives complexes of type [Ru(PPh₃)₂(L)₂] (L=L¹, L² and L³). The 2,4,6-tribromophenolate ligand (L¹) binds to ruthenium as a bidentate O,Br-donor, while the 2-nitrophenolate ligand (L²) acts as a bidentate O,O-donor. 2,4,6-Trinitrophenol (HL³O) undergoes oxygen loss from one nitro group at the ortho position and coordinates to ruthenium in the 2-nitroso-4,6-dinitrophenolate (L³) form through the nitroso nitrogen and phenolate oxygen. The structures of the [Ru(PPh₃)₂(L¹)₂] and [Ru(PPh₃)₂(L³)₂] complexes have been solved by X-ray crystallography. In [Ru(PPh₃)₂(L¹)₂] the coordination sphere around ruthenium is O₂P₂Br₂ with a trans-cis-cis disposition of the three sets of donor atoms, respectively. In [Ru(PPh₃)₂(L³)₂] ruthenium has a N₂O₂P₂ coordination sphere with a cis-cis-trans arrangement of the three sets of donor atoms, respectively. The [Ru(PPh₃)₂(L)₂] complexes are diamagnetic (low-spin d⁶, S=0) and in acetonitrile solution show intense MLCT transitions in the visible region. Cyclic voltammetry on the [Ru(PPh₃)₂(L)₂] complexes shows a reversible ruthenium(II)-ruthenium(III) oxidation within 0.63-0.71 V versus SCE followed by an irreversible ruthenium(III)-ruthenium(IV) oxidation near 1.5 V versus SCE.