Biimidazole complexes of ML22+ [M = Ru or Os, L = 2-(phenylazo)pyridine]. Synthesis, structure and redox properties of mono- and di-nuclear complexes ‡

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Abstract
A series of mono- and di-nuclear complexes of ruthenium and osmium containing the terminal ligands 2-(phenylazo)pyridine (pap), 2,2′-bipyridine (bpy) and the bridging ligand 2,2′-biimidazole(H2biim) have been prepared. The mononuclear complex [Ru(pap)2(H2biim)][ClO4]2·3CH2Cl2 was characterised by a single-crystal structure determination. The mononuclear complexes [M(pap)2(H2biim)]2+ (M = Ru or Os) act as dibasic acids and undergo deprotonation to afford [M(pap)2(Hbiim)]+ and [M(pap)2(biim)]. The completely deprotonated complexes behave as reactive building units (‘metal complex as ligand’) which react smoothly with metal complex units to produce homo- and hetero-dinuclear complexes. The structure of the dinuclear complex [Ru2(bpy)4(biim)][ClO4]2 was determined. Cyclic voltammetric studies on the above complexes were performed. The quasi-reversible half-wave potentials of the oxidation processes of the dinuclear complexes are dependent on the metal as well as on the nature of the terminal ligands. These undergo two one-electron oxidation processes centered at the metal. Partial oxidation of the dinuclear complexes led to unsymmetrical bridge cleavage to form monomeric complexes. Large numbers of ligand reduction responses were observed. In some cases the first ligand reduction occurs close to zero potential (with respect to the SCE).