Asymmetrical diruthenium complex bridged by a redox-active ligand.

The asymmetrical dinuclear complex [(acac)(2)Ru1(μ-abpy)Ru2(Cym)Cl]PF(6) ([2]PF(6)), 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)Cl(2). X-ray crystal structure analysis suggests the oxidation state formulation (acac)(2)Ru1(III)(μ-abpy(•-))Ru2(II)(Cym)Cl for 2(+), with antiferromagnetic coupling between one Ru(III) 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)Ru(II)(Cym)Cl]PF(6) ([1]PF(6)) with an unreduced N═N double bond at d(NN) = 1.269(4) Å and the symmetrical dinuclear [(acac)(2)Ru(2.5)(μ-abpy(•-))Ru(2.5)(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, 2(2+), with largely metal-centered spin (EPR: g(1) = 2.207, g(2) = 2.155, and g(3) = 1.929), and a weak intervalence charge-transfer absorption at 1700 nm, as observed by spectroelectrochemistry. These results support a description of 2(2+) as (acac)(2)Ru1(III)(μ-abpy(0))Ru2(II)(Cym)Cl. Density functional theory (DFT) calculations suggest that the first reduction of [2]PF(6) also involves the bridging ligand, leading to [(acac)(2)Ru1(III)(μ-abpy(2-))Ru2(II)(Cym)Cl] (2). Experimentally, the first reduction of 2(+) is not fully reversible, with evidence for the loss of chloride to form (acac)(2)Ru1(μ-abpy)Ru2(Cym) (2a(+); g(1) = 2.454, g(2) = 2.032, and g(3) = 1.947). Further reduction produces [(acac)(2)Ru1(II)(μ-abpy(2-))Ru2(II)(Cym)] (2a), which forms (acac)(2)Ru1(II)(μ-abpy(2-))Ru2(I)(Cym)/(acac)(2)Ru(II)(μ-abpy(•-))Ru(0)(Cym) (2a(-)) in yet another one-electron step (g(1) = 2.052, g(2) = 2.008, and g(3) = 1.936). The major electronic transitions for each redox state have been assigned by time-dependent DFT calculations.