Tuning the Electronic Properties in Ruthenium-Quinone Complexes through Metal Coordination and Substitution at the Bridge

A rare example of a mononuclear complex [(bpy)(2)Ru(L--H(1))](ClO4), 1(ClO4) and dinuclear complexes [(bpy)(2)Ru(-L--2H(1))Ru(bpy)(2)](ClO4)(2), 2(ClO4)(2), [(bpy)(2)Ru(-L--2H(2))Ru(bpy)(2)](ClO4)(2), 3(ClO4)(2), and [(bpy)(2)Ru(-L--2H(3))Ru(bpy)(2)](ClO4)(2), 4(ClO4)(2) (bpy=2,2-bipyridine, L-1=2,5-di-(isopropyl-amino)-1,4-benzoquinone, L-2=2,5-di-(benzyl-amino)-1,4-benzoquinone, and L-3=2,5-di-[2,4,6-(trimethyl)-anilino]-1,4-benzoquinone) with the symmetrically substituted p-quinone ligands, L, are reported. Bond-length analysis within the potentially bridging ligands in both the mono- and dinuclear complexes shows a localization of bonds, and binding to the metal centers through a phenolate-type O- and an immine/imminium-type neutral N donor. For the mononuclear complex 1(ClO4), this facilitates strong intermolecular hydrogen bonding and leads to the imminium-type character of the noncoordinated nitrogen atom. The dinuclear complexes display two oxidation and several reduction steps in acetonitrile solutions. In contrast, the mononuclear complex 1(+) exhibits just one oxidation and several reduction steps. The redox processes of 1(1+) are strongly dependent on the solvent. The one-electron oxidized forms 2(3+), 3(3+), and 4(3+) of the dinuclear complexes exhibit strong absorptions in the NIR region. Weak NIR absorption bands are observed for the one-electron reduced forms of all complexes. A combination of structural data, electrochemistry, UV/Vis/NIR/EPR spectroelectrochemistry, and DFT calculations is used to elucidate the electronic structures of the complexes. Our DFT results indicate that the electronic natures of the various redox states of the complexes in vacuum differ greatly from those in a solvent continuum. We show here the tuning possibilities that arise upon substituting [O] for the isoelectronic [NR] groups in such quinone ligands.