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5,7,12,14-Tetraazapentacene-6,13-quinone (L) reacts with 2 equiv of [Ru(acac)(CHCN)] to form two linkage isomeric bis(chelate) compounds, [{Ru(acac)}(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) for the rac-isomer 1 as compared to 2. Starting from the Ru-(μ-L)-Ru 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 = 4 × 10) and 2 (K = 6 × 10). Both monocations display metal-centered spin according to EPR, but the DFT-calculated spin densities suggest a Ru(μ-L)Ru three-spin situation with opposite spin density at the bridging ligand for the meso form of 1, estimated to lie 1887 cm lower in energy than rac-1, which is calculated with a Class II mixed-valent situation Ru-(μ-L)-Ru. A three-spin arrangement Ru-(μ-L)-Ru with negative spin density at one metal site is suggested by DFT for rac-2 which is more stable by ΔE = 890 cm than rac-1. Reduction of 1 or 2 (K = 10-10) occurs mainly at the central bridging ligand with notable contributions (30%) from the metals in 1 and 2. The mixed-valent Ru(μ-L)Ru versus radical-bridged Ru(μ-L)Ru alternative is discussed comprehensively in comparison with related valence-ambiguous cases.

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The doubly deprotonated bridging ligand L12- derived from 2,6-bis(2-pyridyl)-1,5-dihydro-1’,4’-benzoquinono[2’,3’-d:5’,6’-d’]diimidazole H2L1 forms coordination compounds with two bis(2,2’-bipyridine)osmium(II) complex fragments in anti ([1](ClO4)2) and syn configuration ([2](ClO4)2) of {(μ-L1)[Os(bpy)2]2}(ClO4)2, as evident from...

The dinuclear complexes {(μ-H2L)[Ru(bpy)2]2}(ClO4)2 ([3](ClO4)2), {(μ-H2L)[Ru(pap)2]2}(ClO4)2 ([4](ClO4)2), and the asymmetric [(bpy)2Ru(μ-H2L)Ru(pap)2](ClO4)2 ([5](ClO4)2) were synthesized via the mononuclear species [Ru(H3L)(bpy)2]ClO4 ([1]ClO4) and [Ru(H3L)(pap)2]ClO4 ([2]ClO4), where H4L is the centrosymmetric 1,5-diamino-9,10-anthraquinone, bpy is 2,2'-bipyridine, and pap is 2-phenylazopyridine. Electrochemistry of the structurally characterized [1]ClO4, [2]ClO4, [3](ClO4)2, [4](ClO4)2, and [5](ClO4)2 reveals multistep oxidation and reduction processes, which were analyzed by electron paramagnetic resonance (EPR) of paramagnetic intermediates and by UV-vis-NIR spectro-electrochemistry. With support by time-dependent density functional theory (DFT) calculations the redox processes could be assigned. Significant results include the dimetal/bridging ligand mixed spin distribution in 3(3+) versus largely bridge-centered spin in 4(3+)-a result of the presence of Ru(II)-stabilizig pap coligands. In addition to the metal/ligand alternative for electron transfer and spin location, the dinuclear systems allow for the observation of ligand/ligand and metal/metal site differentiation within the multistep redox series. DFT-supported EPR and NIR absorption spectroscopy of the latter case revealed class II mixed-valence behavior of the oxidized asymmetric system 5(3+) with about equal contributions from a radical bridge formulation. In comparison to the analogues with the deprotonated 1,4-diaminoanthraquinone isomer the centrosymmetric H2L(2-) bridge shows anodically shifted redox potentials and weaker electronic coupling between the chelate sites.

The redox-active ligand 5,7,12,14-tetraazapentacene-6,13-quinone = L forms structurally characterised compounds with three (1) or four (2) [Ru(acac)] complex fragments in which each of the metals is N,O-chelated. The new tris- and tetrakis-bidentate chelate compounds exhibit ruthenium centres bridged at about 4 Å by quinone O atoms which are then situated across the pentacene π system at about 6-8 Å distance. Several electron transfer processes were observed by voltammetry (CV, DPV) and the intermediates identified by EPR and UV-Vis-NIR spectroelectrochemistry. TD-DFT calculations were applied to assign the proper oxidation states within the multistep redox systems {(μ-L)[Ru(acac)]}, n = 3 or 4, revealing both metal and ligand based electron transfer.