Isomeric Diruthenium Complexes Of A Heterocyclic And Quinonoid Bridging Ligand: Valence And Spin Alternatives For The Metal/Ligand/ Metal Arrangement

5,7,12,14-Tetraazapentacene-6,13-quinone (L) reacts with 2 equiv of [Ru(acac)(2)(CH3CN)(2)] to form two linkage isomeric bis(chelate) compounds, [{Ru-II(acac)(2)}(2)( mu-L)], blue 1, with 5,6;12,13 coordination and violet 2 with 5,6;13,14 coordination. The linkage isomers could be separated, structurally characterized in crystals as rac diastereomers (Delta Delta/ Lambda Lambda), and studied by voltammetry (CV, DPV), EPR, and UV-vis-NIR spectroelectrochemistry (meso-1, rac-2). DFT and TD-DFT calculations support the structural and spectroscopic results and suggest a slight energy preference (Delta E = 263 cm(-1)) for the rac-isomer 1 as compared to 2. Starting from the Ru-II-(mu-L-0)-Ru-II configurations of 1 and 2 with low-lying metal-to-ligand charge transfer (MLCT) absorptions, the compounds undergo two reversible one-electron oxidation steps with open-shell intermediates 1(+) (K-c = 4 X 10(4)) and 2(+) (K-c = 6 X 10(5)). Both monocations display metal-centered spin according to EPR, but the DFT-calculated spin densities suggest a Rum(III)(mu-L center dot-)Ru-III three-spin situation with opposite spin density at the bridging ligand for the meso form of 1(+), estimated to lie 1887 cm(-1) lower in energy than rac-1(+), which is calculated with a Class II mixed-valent situation Ru-III-(mu-L-0)-Ru-II. A three-spin arrangement Ru-III-(mu-L center dot-)-Ru-III with negative spin density at one metal site is suggested by DFT for rac-(2+) which is more stable by Delta E = 890 cm(-1) than rac-1(+). Reduction of 1 or 2 (Kc = 10(7)-10(8)) occurs mainly at the central bridging ligand with notable contributions (30%) from the metals in 1(-) and 2(-). The mixed-valent Ru-III(mu-L)Ru-II versus radical-bridged Ru-III(mu-L center dot-)Ru-III alternative is discussed comprehensively in comparison with related valence-ambiguous cases.