Sensitivity of the valence structure in diruthenium complexes as a function of terminal and bridging ligands

Abstract

The compounds [(acac)2RuIII(μ-H2L 2-)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 (H 2L0 → → → →H2L 4-) 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•-)Ru III(acac)2]+ (1+) → [(acac)2RuIII(μ-H2L2-)Ru III(acac)2] (1) → [(acac)2Ru III(μ-H2L•3-)RuIII(acac) 2]-/[(acac)2RuIII(μ-H 2L2-)RuII(acac)2]- (1-) → [(acac)2RuIII(μ-H 2L4-)RuIII(acac)2] 2-/[(acac)2RuII(μ-H2L 2-)RuII(acac)2]2- (12-) and [(bpy)2RuIII(μ-H2L•-) RuII(bpy)2]4+ (24+) → [(bpy)2RuII(μ-H2L•-)Ru II(bpy)2]3+/[(bpy)2Ru II(μ-H2L2-)RuIII(bpy) 2]3+ (23+) → [(bpy)2Ru II(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 H2L n as compared to O causes a cathodic shift of redox potentials with corresponding consequences for the valence structure. © 2014 American Chemical Society.