Dinuclear and Dendritic Polynuclear Ruthenium(II) and Osmium(II) Polypyridine Complexes: Electrochemistry at Very Positive Potentials in Liquid SO

The electrochemistry in liquid SO2 of RuII and OsII dendritic polynuclear complexes (two hexanuclear species, having as the core a bis-chelating ligand, and one decanuclear species, built around a metal core) has been performed together with that of several of their lower-nuclearity analogues (five dinuclear and one trinuclear species), which can be considered as components of the larger dendritic species. All of the compounds contain 2,3or 2,5-bis(2-pyridyl)pyrazine (2,3or 2,5-dpp) as bridging ligands and 2,2′-bipyridine (bpy) as terminal ligands. The identification of the redox sites for the compounds with high nuclearity was made possible by a bottom-up approach based on an extensive comparison of their electrochemical behavior with that of the simpler species. Owing to the large anodic potential window of liquid SO2 (to ∼4.3 V vs SCE under our experimental conditions) in conjunction with tetrabutylammonium hexafluoroarsenate as supporting electrolyte, several metaland ligand-centered oxidations not previously observed for these compounds are reported. In particular, we observed (i) the second oxidation of Os ions (OsIII/OsIV), (ii) the inner Ru ion oxidations (RuII/ RuIII) for complexes with higher-than-2 nuclearity (reported for the first time in dendrimers), and (iii) bpy and dpp oxidation. Metal-metal interaction in the Os dinuclear compound, inferred from the electrochemical data, depends on the oxidation state of the metals. For the two investigated hexanuclear compounds, {[(bpy)2Ru(μ-2,3-dpp)]2Ru(μ-2,3-dpp)Ru[(μ-2,3-dpp)Ru(bpy)2]2} and {[(bpy)2Ru(μ-2,3-dpp)]2Ru(μ-2,5-dpp)Ru[(μ-2,3-dpp)Ru(bpy)2]2}, two sets of electrochemically equivalent Ru ions were found: the four external Ru ions give rise to a four-electron-transfer peak, and the two internal Ru ions correspond to a bielectronic peak. The decanuclear compound [Ru{(μ-2,3-dpp)Ru[(μ-2,3-dpp)Ru(bpy)2]2}3] presents three sets of Rubased oxidations: a six-electron process due to the oxidation of the six peripheral Ru centers, a one-electron process assigned to the central metal, and a three-electron process involving oxidation of the three intermediate metal centers. The location of ligand oxidation on a bridge or a terminal bpy identifies the easier-to-oxidize ligand and gives information about the electronic distribution in the complex.