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

The title complexes were obtained as MIIM′II species [(bpy)2M(μ-abpy)M′(bpy)2](PF6)4, M,M′ = Ru or Os, using the new mononuclear precursor [(bpy)2Os(abpy)](PF6)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 g1 − g3 = 0.25 in frozen solution. Further reduction was monitored by UV/Vis spectroelectrochemistry. Oxidation produced OsIII EPR signals for [(bpy)2Os(abpy)]3+ and [(bpy)2Os(μ-abpy)Ru(bpy)2]5+, indicating a RuIIOsIII 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 Kc for the mixed valent diosmium(III,II) state (Kc > 1015) in comparison to the diruthenium(III,II) analogue with Kc = 1010 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.

Graphical abstractRadical 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 Kc > 1015 for the diosmium(III,II) species confirms the electron transfer alternative for the valence exchange mechanism.Figure optionsDownload full-size imageDownload as PowerPoint slide