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 (CH3 CN)2 ] to form two linkage isomeric bis(chelate) compounds, [{RuII (acac)2 }2 (μ-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 (ΔΔ/ΛΛ), 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 (ΔE = 263 cm-1 ) for the rac-isomer 1 as compared to 2. Starting from the RuII -(μ-L0 )-RuII 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+ (Kc = 4 × 104 ) and 2+ (Kc = 6 × 105 ). Both monocations display metal-centered spin according to EPR, but the DFT-calculated spin densities suggest a RuIII (μ-L•- )RuIII 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 RuIII -(μ-L0 )-RuII . A three-spin arrangement RuIII -(μ-L•- )-RuIII with negative spin density at one metal site is suggested by DFT for rac-2+ which is more stable by ΔE = 890 cm-1 than rac-1+ . Reduction of 1 or 2 (Kc = 107 -108 ) occurs mainly at the central bridging ligand with notable contributions (30%) from the metals in 1- and 2- . The mixed-valent RuIII (μ-L)RuII versus radical-bridged RuIII (μ-L•- )RuIII alternative is discussed comprehensively in comparison with related valence-ambiguous cases.