Mixed-valent and radical states of complexes [(bpy) 2M(μ-abpy)M′(bpy) 2] n+ , M,M′ = Ru or Os, abpy = 2,2′-azobispyridine: Electron transfer vs. hole transfer mechanism in azo ligand-bridged complexes

• Published in: Inorganica Chimica Acta 2006-02, Vol.359 (3), p.821-829
• Main Authors: Heilmann, Martina, Frantz, Stéphanie, Kaim, Wolfgang, Fiedler, Jan, Duboc, Carole
• Format: Article
• Language: English
• Subjects: Mixed valency; Osmium compounds; Radical ligands; Ruthenium compounds; Spectroelectrochemistry
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/j.ica.2005.04.044

Radical complexes [(bpy) 2M(abpy)] + and [(bpy) 2M(μ-abpy)M′(bpy) 2] 3+ as well as metal(III) species [(bpy) 2M(abpy)] 3+ and mixed-valent ions [(bpy) 2M(μ-abpy)M′(bpy) 2] 5+ were studied by EPR and UV/Vis/NIR spectroelectrochemistry. Differences in symmetry and in spin–orbit coupling contributions determine the spectroscopic response. The higher stability constant K c > 10 15 for the diosmium(III,II) species confirms the electron transfer alternative for the valence exchange mechanism. The title complexes were obtained as M IIM′ II species [(bpy) 2M(μ-abpy)M′(bpy) 2](PF 6) 4, M,M′ = Ru or Os, using the new mononuclear precursor [(bpy) 2Os(abpy)](PF 6) 2 for the osmium-containing dinuclear complexes. One-electron reduction produces radical complexes [(bpy) 2M(μ-abpy)M′(bpy) 2] 3+ and [(bpy) 2M(abpy)] + with significant contributions from the metals, as evident from the EPR effects on successive replacement of ruthenium by osmium with its much higher spin–orbit coupling constant. The diruthenium and diosmium radical complexes were also studied by EPR at high-frequency (285 GHz), the latter shows an unusually large g anisotropy g 1 − g 3 = 0.25 in frozen solution. Further reduction was monitored by UV/Vis spectroelectrochemistry. Oxidation produced Os III EPR signals for [(bpy) 2Os(abpy)] 3+ and [(bpy) 2Os(μ-abpy)Ru(bpy) 2] 5+, indicating a Ru IIOs III species for the latter. The diosmium(III,II) and diruthenium(III,II) mixed-valent species remained EPR silent at 4 K, however, they exhibit weak inter-valence charge transfer (IVCT) bands at about 1460 nm. Whereas the cyclic voltammetric response towards reduction is only marginally different for the three dinuclear complexes, successive replacement of ruthenium by osmium causes the first oxidation potential to decrease. The much higher comproportionation constant K c for the mixed valent diosmium(III,II) state ( K c > 10 15) in comparison to the diruthenium(III,II) analogue with K c = 10 10 confirms the electron transfer alternative for the valence exchange mechanism, in contrast to the hole transfer established for analogous dinuclear complexes with the formally related diacylhydrazido(2−) bridging ligands.