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

The
compounds [(acac)2RuIII(μ-H2L2–)­RuIII(acac)2]
(rac, 1, and meso, 1′) and [(bpy)2RuII(μ-H2L•–)­RuII(bpy)2]­(ClO4)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
μ-H2Ln– forms
two β-ketiminato-type chelate rings, and 1 or 1′ are connected via NH···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)2RuII} → →{(acac)2RuIV} or {(bpy)2RuII} → {(bpy)2RuIII} in 1/1′
or 2 generates alternatives regarding the oxidation state
formulations for the accessible redox states (1n and 2n),
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)2RuIII(μ-H2L•–)­RuIII(acac)2]+ (1+) → [(acac)2RuIII(μ-H2L2–)­RuIII(acac)2]
(1) → [(acac)2RuIII(μ-H2L•3–)­RuIII(acac)2]−/[(acac)2RuIII(μ-H2L2–)­RuII(acac)2]− (1–) → [(acac)2RuIII(μ-H2L4–)­RuIII(acac)2]2–/[(acac)2RuII(μ-H2L2–)­RuII(acac)2]2– (12–) and [(bpy)2RuIII(μ-H2L•–)­RuII(bpy)2]4+ (24+) → [(bpy)2RuII(μ-H2L•–)­RuII(bpy)2]3+/[(bpy)2RuII(μ-H2L2–)­RuIII(bpy)2]3+ (23+) → [(bpy)2RuII(μ-H2L2–)­RuII(bpy)2]2+ (22+). The favoring of RuIII by
σ-donating acac– and of RuII by
the π-accepting bpy coligands shifts the conceivable valence
alternatives accordingly. Similarly, the introduction of the NH donor
function in H2Ln as compared
to O causes a cathodic shift of redox potentials with corresponding
consequences for the valence structure.