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

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 ((HLO)-O-3), with [Ru(PPh3)(2)Cl-2] in a 2:1 molar ratio in the presence of a base gives complexes of type [Ru(PPh3)(2)(L)(2)] (L = L-1, L-2 and L-3). The 2,4,6-tribromophenolate ligand (L-1) binds to ruthenium as a bidentate O,Br-donor, while the 2-nitrophenolate ligand (L-2) acts as a bidentate O,O-donor. 2,4,6-Trinitrophenol ((HLO)-O-3) undergoes oxygen loss from one nitro group at the ortho position and coordinates to ruthenium in the 2-nitroso-4,6-dinitrophenolate (L-3) form through the nitroso nitrogen and phenolate oxygen. The structures of the [Ru(PPh3)(2)(L-1)(2)] and [Ru(PPh3)(2)(L-3)(2)] complexes have been solved by X-ray crystallography. In [Ru(PPh3)(2)(L-1)(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)(L-3)(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 d(6), 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. (C) 2000 Elsevier Science Ltd. All rights reserved.