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Isomeric Diruthenium Complexes of a Heterocyclic and Quinonoid Bridging Ligand: Valence and Spin Alternatives for the Metal/Ligand/Metal Arrangement

Isomeric Diruthenium Complexes of a Heterocyclic and Quinonoid Bridging Ligand: Valence and Spin Alternatives for the Metal/Ligand/Metal Arrangement

Inorganic Chemistry 55(23): 12357-12365

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, [{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.

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Accession: 058176184

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PMID: 27934410

DOI: 10.1021/acs.inorgchem.6b02245

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