Synthesis and proton-coupled redox properties of mononuclear or asymmetric dinuclear complexes of ruthenium, rhodium and/or osmium containing 2,2′-bis(2-pyridyl)-6,6′-bibenzimidazole

New mononuclear and heterodinuclear complexes, [ML2(H2L1)]2+ and [L′2M(H2L1)M′L″2]4+[M, M′= Ru, Rh and/or Os; L′ or L″= 2,2′-bipyridine (bipy), 1,10-phenanthroline or 4,4′-dimethylbipyridine], containing the dinucleating ligand 2,2′-bis(2-pyridyl)-6,6′-bibenzimidazole (H2L1) have been prepared. The metal-to-ligand charge-transfer bands are almost unaltered when changing from the mono- to di-nuclear complexes, indicating that the bridging H2L1 ligand has slightly lower π* orbital energy than that of bipy. The bridging H2L1 ligand acts as a σ/π-donor ligand. Both the absorption spectra and the oxidation potentials of the complexes are strongly dependent on the solution pH, which determines the NH deprotonation of the co-ordinated ligand H2L1. The mononuclear complexes of Ru and Os act not only as basic acids but also as diacidic bases, while the heterodinuclear complexes essentially act as dibasic acids. The proton-coupled redox reaction was demonstrated by plots of E½vs. pH (Pourbaix diagrams). The pka values of the complexes reflect on both the type of metals and their oxidation states, MII and MIII. The introduction of asymmetry in the dinuclear complexes containing H2L1 can provide not only a potential difference between the two metal sites but also a preferential protonation (or deprotonation) site.