Sensitivity of the Valence Structure in Diruthenium Complexes As a Function of Terminal and Bridging Ligands

The compounds [(acac)(2)Ru-III(mu-H2L2-)Ru-III(acac)(2)] (rac, 1, and meso, 1') and [(bpy)(2)Ru-III(mu-H2L center dot-)Ru-III(bpy)(2)](C1O(4))(3) (meso, [2](ClO4)(3)) have been structurally, magnetically, spectroelectrochemically, and computationally characterized (acac(-) = acetylacetonate, bpy = 2,2'-bipyridine, and H4L = 1,4-diamino-9,10-anthraquinone). The N,O;N',O'-coordinated mu-H2Ln- forms two beta-ketiminato-type chelate rings, and 1 or 1' are connected via NH center dot center dot center dot O hydrogen bridges'in the crystals. 1 exhibits a complex magnetic behavior, while [2](ClO4)(3) is a radical species with mixed ligand/metal-based spin. The combination of redox noninnocent bridge (H2L0 -> -> -> -> H2L4-) and {(acac)(2)Ru-II} -> ->{(acac)(2)Ru-IV} or {(bpy)(2)Ru-II} -> {(bpy)(2)Ru-III} in 1/1' or 2 generates alternatives regarding the oxidation state formulations for the accessible redox states (1(n) and 2(n)), which have been assessed by UV-vis-NIR, EPR, and DFT/TD-DFT calculations. The experimental and theoretical studies suggest variable mixing of the frontier orbitals of the metals and the bridge, leading to the following most appropriate oxidation state combinations: [(acac)(2)Ru-III(mu-H2L center dot-)Ru-III(acac)(2)](+) (1(+)) -> [(acac)(2)Ru-III(mu-H2L2-)Ru-III(acac)(2)] (1) -> [(acac)(2)Ru-III mu-H2L center dot 3-)Ru-III(acac)(2)](-)/[(acac)(2)](-) Ru-III(mu-H2L2-)Ru-III(acac)(2)](-) (1(-)) -> [(acac)(2)Ru-III(mu-H2L4-)Ru-III(acac)(2)](2-)/[(acac)(2)Ru-III(mu-H2L2-)Ru-III(acac)(2)](2-) (1(2-)) and {(bpy)(2)Ru-III(mu-H2L2-)Ru-III(bpy)(2)](4+) (2(4+)) -> [(bpy)(2)Ru-III(mu-H2L center dot-)Ru-II(bpy)(2)](3+)/[(bpy)(2)Ru-III(mu-H2L2-)Ru-III(bpy)(2))(3+) (2(3+)) -> [(bpy)(2)Ru-II(mu-H2L2-)Ru-II(bpy)(2)](2+) (2(2+)). The favoring of Ru-III by sigma-donating acac(-) and of Ru-II by the pi-accepting bpy coligands shifts the conceivable valence alternatives accordingly. Similarly, the introduction of the NH donor function in H2Ln as compared to 0 causes a cathodic shift of redox potentials with corresponding consequences for the valence structure.