Asymmetrical Diruthenium Complex Bridged by a Redox-Active Ligand

The asymmetrical dinuclear complex [(acac)2Ru1­(μ-abpy)­Ru2­(Cym)­Cl]­PF6 ([2]­PF6), with acac– = acetylacetonato = 2,4-pentanedionato, abpy = 2,2′-azobis­(pyridine), and Cym = p-cymene = 1-isopropyl-4-methylbenzene, has been obtained from the mononuclear precursors [Ru­(acac)2(abpy)] and [Ru­(Cym)­Cl2]2. X-ray crystal structure analysis suggests the oxidation state formulation [(acac)2Ru1III(μ-abpy•–)­Ru2II(Cym)­Cl]+ for 2+, with antiferromagnetic coupling between one RuIII center and the radical-anion bridging ligand (abpy•–), based on the N–N distance of 1.352(3) Å. As appropriate references, the newly synthesized mononuclear [(abpy)­RuII(Cym)­Cl]­PF6 ([1]­PF6) with an unreduced NN double bond at d(NN) = 1.269(4) Å and the symmetrical dinuclear [(acac)2Ru2.5(μ-abpy•–)­Ru2.5(acac)2] with d(NN) = 1.372(4) Å (rac isomer) support the above assignment for 2+ as an asymmetrical mixed-valent configuration bridged by a radical ligand. Reversible one-electron oxidation leads to a dication, 22+, with largely metal-centered spin (EPR: g1 = 2.207, g2 = 2.155, and g3 = 1.929), and a weak intervalence charge-transfer absorption at 1700 nm, as observed by spectroelectrochemistry. These results support a description of 22+ as [(acac)2Ru1III(μ-abpy0)­Ru2II(Cym)­Cl]2+. Density functional theory (DFT) calculations suggest that the first reduction of [2]­PF6 also involves the bridging ligand, leading to [(acac)2Ru1III(μ-abpy2–)­Ru2II(Cym)­Cl] (2). Experimentally, the first reduction of 2+ is not fully reversible, with evidence for the loss of chloride to form [(acac)2Ru1­(μ-abpy)­Ru2­(Cym)]+ (2a+; g1 = 2.454, g2 = 2.032, and g3 = 1.947). Further reduction produces [(acac)2Ru1II(μ-abpy2–)­Ru2II(Cym)] (2a), which forms [(acac)2Ru1II(μ-abpy2–)­Ru2I(Cym)]−/[(acac)2RuII(μ-abpy•–)­Ru0(Cym)]− (2a–) in yet another one-electron step (g1 = 2.052, g2 = 2.008, and g3 = 1.936). The major electronic transitions for each redox state have been assigned by time-dependent DFT calculations.