Asymmetrical Diruthenium Complex Bridged by a Redox-Active Ligand

The asymmetrical dinuclear complex [(acac)(2)Ru1(mu-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)Cl-2](2). X-ray crystal structure analysis suggests the oxidation state formulation [(acac)(2)Ru1(III)(mu-abpy(center dot-))Ru2(II)(Cym)Cl](+) for 2(+), with antiferromagnetic coupling between one Ru-III center and the radical-anion bridging ligand (abpy(center dot-)), based on the N-N distance of 1.352(3) angstrom. As appropriate references, the newly synthesized mononuclear [(abpy)Ru-II(Cym)Cl]PF6 ([1]PF6) with an unreduced N=N double bond at d(NN) = 1.269(4) angstrom and the symmetrical dinuclear [(acac)(2)Ru-2.5(mu-abpy(center dot-))Ru-2.5(acac)(2)] with d(NN) = 1.372(4) angstrom (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)(mu-abpy(0))Ru2(II)(Cym)Cl](2+). Density functional theory (DFT) calculations suggest that the first reduction of [2]PF6 also involves the bridging ligand, leading to [(acac)(2)Ru1(III)(mu-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(mu-abpy)Ru2(Cym)](+) (2a(+); g(1) = 2.454, g(2) = 2.032, and g(3) = 1.947). Further reduction produces [(acac)(2)Ru1(II)(mu-abpy(2-))Ru2(II)(Cym)] (2a), which forms [(acac)(2)Ru1(II)(mu-abpy(2-))Ru2(I)(Cym)](-)/[(acac)(2)Ru-II(mu-abpy(center dot-)) 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.