Tetranuclear polypyridyl complexes of RuII and FeII:: Synthesis, electrochemical, photophysical and photochemical behaviour

Three heterotetranuclear complexes [{Ru-II(bpy)(2)(LLn)}(3)Fe-II](8+) [bpy = 2,2'-bipyridine, n = 2, 4, 6; denoted [{Ru(LLn)}(3)Fe](8+)), in which one iron centre is complexed by three Ru-II-tris-bipyridine-like moieties containing covalently bridging bis-bipyridine LLn ligands, have been synthesised and characterised. The stability and the electrochemical, photophysical and photochemical properties of these complexes have been investigated in CH3CN. The cyclic voltammograms of all complexes exhibit two successive reversible oxidation processes in the positive region, corresponding to the Fe-II/Fe-III and Ru-II/Ru-III redox couples. These systems are clearly separated (Delta E-1/2 about 300 mV), which indicates the absence of an electronic connection between the two subunits. In the negative region, three successive reversible four-electron systems are observed, corresponding to the ligand-based reduction processes. The two oxidized forms of the complexes [{Ru-II(LLn)}(3)Fe-III](9+) and [(Ru-III(LLn))(3)Fe-III](12+), which are obtained by two successive exhaustive electrolyses, are very stable. The [Ru(LLn))(3)Fe](8+) complexes are luminescent, which shows that the covalent linkage between the Ru-II-trisbipyridine and Fe-II-tris-bipyridine subunits leads to an only partial quenching of the Ru-II* excited states by energy transfer to the Fe-II centre. The luminescence lifetime and quantum yield are found to be independent of the complexes' concentration, thus indicating that the energy-transfer process is only due to an intramolecular electron-exchange mechanism. Quantitative photoinduced oxidation of the tetranuclear complexes has been performed by continuous photolysis experiments in the presence of a large excess of a diazonium salt, which plays the role of a sacrificial oxidant. Two successive oxidation processes (Fell -> Fe-III and Ru-II -> Ru-III) are observed.((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).