Energy-level tailoring in a series of redox-rich quinonoid-bridged diruthenium complexes containing tris2-pyridylmethyl)amine as a co-ligand.

Reactions of [{Ru(tmpa)}2(μ-Cl)2][ClO4]2, (2[ClO4]2, tmpa=tris(2-pyridylmethyl)amine) with 2,5-dihydroxy-1,4-benzoquinone (L1), 2,5-di-[2,6-(dimethyl)-anilino]-1,4-benzoquinone (L2), or 2,5-di-[2,4,6-(trimethyl)-anilino)]-1,4-benzoquinone (L3) in the presence of a base led to the formation of the dinuclear complexes [{Ru(tmpa)}2(μ-L1-2H)][ClO4]2 (3[ClO4]2), [{Ru(tmpa)}2(μ-L2-2H)][ClO4]2 (4[ClO4]2), and [{Ru(tmpa)}2(μ-L3-2H)][ClO4]2 (5[ClO4]2). Structural characterization of 5[ClO4]2 showed the localization of the double bonds within the quinonoid ring and a twisting of the mesityl substituents with respect to the quinonoid plane. Cyclic voltammetry of the complexes show two reversible oxidation and quinonoid-based reduction processes. Results obtained from UV/Vis/NIR and EPR spectroelectrochemistry are invoked to discuss ruthenium- versus quinonoid-ligand-centered redox activity. The complex 3[ClO4]2 is compared to the reported complex [{Ru(bpy)}2(μ-L1-2 H)]2+ (12+, bpy=2,2′-bipyridine). The effects of substituting the bidentate and better π-accepting bpy co-ligands with tetradentate tmpa ligands [pure σ-donating (amine) as well as σ-donating and π-accepting (pyridines)] on the redox and electronic properties of the complexes are discussed. Comparisons are also made between complexes containing the dianionic forms of the all-oxygen-donating L1 ligand with the L2 and L3 ligands containing an [O,N,O,N] donor set. The one-electron oxidized forms of the complexes show absorption in the NIR region. The position as well as the intensity of this band can be tuned by the substituents on the quinonoid bridge. In addition, this band can be switched on and off by using tunable redox potentials, making such systems attractive candidates for NIR electrochromism.