DINUCLEAR AND DENDRITIC POLYNUCLEAR RUTHENIUM(II) AND OSMIUM(II) POLYPYRIDINE COMPLEXES - ELECTROCHEMISTRY AT VERY POSITIVE POTENTIALS IN LIQUID SO2

The electrochemistry in liquid SO2 of Ru-II and Os-II dendritic polynu clear complexes (two hexanuclear species, having as the core a bis-che lating ligand, and one decanuclear species, built around a metal core) has been performed together with that of several of their lower-nucle arity analogues (five dinuclear and one trinuclear species), which can be considered as components of the larger dendritic species. All of t he compounds contain 2,3- or 2,5-bis(2-pyridyl)pyrazine (2,3- or 2,5-d pp) as bridging ligands and 2,2'-bipyridine (bpy) as terminal ligands. The identification of the odor sites for the compounds with high nucl earity 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 similar to 4.3 V vs SCE under our experimental conditions) in conjunc tion with tretrabutylammonium hexafluoroarsenate as supporting electro lyte, several metal- and ligand-centered oxidations not previously obs erved for these compounds are reported. In particular, we observed (i) the second oxidation of Os ions (Os-III/Os-IV), (ii) the inner Ru ion oxidations (Ru-II/Ru-III) for complexes with higher-than-2 nuclearity (reported for the first time in dendrimers), and (iii) bpy and dpp ox idation. Metal-metal interaction in the Os dinuclear compound, inferre d from the electrochemical data, depends on the oxidation state of the metals. For the two investigated hexanuclear compounds, ([(bpy)(2)Ru( mu-2,3-dpp)](2)Ru(mu-2,3 -dpp)Ru [mu-2,3-dpp)Ru(bpy)(2)](2))(12+) and )Ru(mu-2,5-dpp)Ru[mu-2,3-dpp)Ru(bpy)(2)](2)}(12+), two sets of electro chemically equivalent Ru ions were found: the four external Ru ions gi ve rise to a four-electron-transfer peak, and the two internal Ru ions correspond to a bielectronic peak. The decanuclear compound (mu-2,3-d pp)Ru[mu-2,3-dpp)Ru(bpy)(2)](2)}(3)](20+) presents three sets of Ru-ba sed 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 brid ge or a terminal bpy identifies the easier-to-oxidize ligand and gives information about the electronic distribution in the complex.