Three distinct redox states of an oxo-bridged dinuclear ruthenium complex

A series of [{(terpy)(bpy)Ru}(μ-O){Ru(bpy)(terpy)}](n+) ([RuORu](n+), terpy=2,2';6',2''-terpyridine, bpy=2,2'-bipyridine) was systematically synthesized and characterized in three distinct redox states (n=3, 4, and 5 for Ru(II,III)2, Ru(III,III)2, and Ru(III,IV)2, respectively). The crystal structures of [RuORu](n+) (n=3, 4, 5) in all three redox states were successfully determined. X-ray crystallography showed that the Ru-O distances and the Ru-O-Ru angles are mainly regulated by the oxidation states of the ruthenium centers. X-ray crystallography and ESR spectra clearly revealed the detailed electronic structures of two mixed-valence complexes, [Ru(III)ORu(IV)](5+) and [Ru(II)ORu(III)](3+), in which each unpaired electron is completely delocalized across the oxo-bridged dinuclear core. These findings allow us to understand the systematic changes in structure and electronic state that accompany the changes in the redox state.