Syntheses, structural, electrochemical and optical studies of heterobinuclear ruthenium–osmium alkynyl complexes

The complexes trans-[Os(Ctriple bond; length of mdashCC6H4-4-Ctriple bond; length of mdashCR)Cl(dppe)2] (R = SiPri31, H 2), trans,trans-[(dppe)2ClOs(Ctriple bond; length of mdashCC6H4-4-Ctriple bond; length of mdashC)RuX(dppe)2] (X = Cl 3, Ctriple bond; length of mdashCC6H4-4-Ctriple bond; length of mdashCSiPri34), trans-[Os(Ctriple bond; length of mdashCC6H4-4-Ctriple bond; length of mdashCC6H4-4-Ctriple bond; length of mdashCR)Cl(dppe)2] (R = SiPri35, H 6), and trans,trans-[(dppe)2ClOs(Ctriple bond; length of mdashCC6H4-4-Ctriple bond; length of mdashCC6H4-4-Ctriple bond; length of mdashC)RuCl(dppe)2] (7) have been synthesized, and the identities of 1, 2, and 6 confirmed by single-crystal X-ray diffraction studies. Cyclic voltammetry shows that the mononuclear complexes 1, 2, 5, and 6 are oxidized at potentials within a narrow range (0.45–0.49 V), in processes centered on the osmium ethynyl neighbourhood and for simplicity assigned as OsII/III, while the heterobinuclear complexes 3, 4, and 7 exhibit lower oxidation potentials for OsII/III and a second oxidation process assigned in a similar fashion to RuII/III; the difference in potential between the Os- and Ru-localized processes decreases as the π-bridge is lengthened. UV–vis–NIR spectroelectrochemical studies on 1 and 5 reveal the appearance on oxidation of a low-energy band ascribed to chloro to metal-ethynyl charge transfer. Osmium-centered oxidation at the heterobinuclear complexes 4 and 7 results in appearance of a low-energy band, which blue-shifts and increases in intensity on further oxidation to 42+ and 72+.