Multiple one-electron oxidation and reduction of trinuclear bis(2,4-pentanedionato)ruthenium complexes with substituted diquinoxalino[2,3-a:2′,3′-c]phenazine ligands |
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| Authors: | Sandeep Ghumaan Biprajit Sarkar Mahendra P Patil Jan Fiedler Raghavan B Sunoj Wolfgang Kaim Goutam Kumar Lahiri |
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| Institution: | 1. Department of Chemistry, Indian Institute of Technology – Bombay, Powai, Mumbai 400076, India;2. Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D – 70550 Stuttgart, Germany;3. J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejškova 3, CZ – 18223 Prague, Czech Republic |
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| Abstract: | The complexes (μ3-L1/L2)Ru(acac)2]3, acac− = 2,4-pentanedionato, L1 = 2,3,8,9,14,15-hexachlorodiquinoxalino2,3-a:2′,3′-c]phenazine and L2 = 2,3,8,9,14,15- hexamethyldiquinoxalino2,3-a:2′,3′-c]phenazine, undergo stepwise one-electron oxidation involving a total of three electrons and stepwise one-electron reduction with three (L2) or four electrons (L1). All reversibly accessible states were characterized by UV–Vis–NIR spectroelectrochemistry. Oxidation leads to mixed-valent intermediates {(μ3-L)Ru(acac)2]3}+ and {(μ3-L)Ru(acac)2]3}2+ of which the RuIIIRuIIRuII combinations exhibit higher comproportionation constants Kc than the RuIIIRuIIIRuII states – in contrast to a previous report for the unsubstituted parent systems {(μ3-L3)Ru(acac)2]3}+/2+, L3 = diquinoxalino2,3-a:2′,3′-c]phenazine. No conspicuous inter-valence charge transfer absorptions were observed for the mixed-valent intermediates in the visible to near-infrared regions. The monocations and monoanions were characterized by EPR spectroscopy, revealing rhombic ruthenium(III) type signals for the former. Electron addition produces ruthenium(II) complexes of the reduced forms of the ligands L, a high resolution EPR spectrum with 14N and 35,37Cl hyperfine coupling and negligible g anisotropy was found for {(μ3-L1)Ru(acac)2]3}−. DFT calculations of (μ3-L1)Ru(acac)2]3 confirm several ligand-centered low-lying unoccupied MOs for reduction and several metal-based high-lying occupied MOs for electron withdrawal, resulting in low-energy metal-to-ligand charge transfer (MLCT) transitions. |
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| Keywords: | DFT calculations EPR Radical complexes Ruthenium Electrochemistry Spectroelectrochemistry |
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