title: Isomeric Diruthenium Complexes of a Heterocyclic and Quinonoid Bridging Ligand: Valence and Spin Alternatives for the Metal/Ligand/Metal Arrangement.
abstract: 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)-RuIIconfigurations 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•-)RuIIIthree-spin situation with opposite spin density at the bridging ligand for the meso form of 1+, estimated to lie 1887 cm-1lower 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•-)-RuIIIwith negative spin density at one metal site is suggested by DFT for rac-2+which is more stable by ΔE = 890 cm-1than 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)RuIIversus radical-bridged RuIII(μ-L•-)RuIIIalternative is discussed comprehensively in comparison with related valence-ambiguous cases.
authors.affiliation: Department of Chemistry, Indian Institute of Technology Bombay , Powai, Mumbai 400076, India.
authors.forename: Mohd Asif
authors.initials: MA
authors.lastname: Ansari
authors.affiliation: Department of Chemistry, Indian Institute of Technology Bombay , Powai, Mumbai 400076, India.
authors.forename: Abhishek
authors.initials: A
authors.lastname: Mandal
authors.affiliation: Institut für Anorganische Chemie, Universität Stuttgart , Pfaffenwaldring 55, Stuttgart D-70550, Germany.
authors.forename: Alexa
authors.initials: A
authors.lastname: Paretzki
authors.affiliation: Institut für Anorganische Chemie, Universität Stuttgart , Pfaffenwaldring 55, Stuttgart D-70550, Germany.
authors.forename: Katharina
authors.initials: K
authors.lastname: Beyer
authors.affiliation: Institut für Anorganische Chemie, Universität Stuttgart , Pfaffenwaldring 55, Stuttgart D-70550, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Kaim
authors.affiliation: Department of Chemistry, Indian Institute of Technology Bombay , Powai, Mumbai 400076, India.
authors.forename: Goutam Kumar
authors.initials: GK
authors.lastname: Lahiri