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. [ML(2)(H(2)L(1))](2+) and [L(2)'M(H(2)L(1))M'L(2)''](4+) [M, M' = Ru, Rh and/or Os; L' or L'' = 2.2'-bipyridine (bipy), 1,10-phenanthroline or 4,4'-dimethytbipyridine], containing the dinucleating ligand 2,2'-bis(2-pyridyl)-6,6'-bibenzimidazole (H(2)L(1)) have been prepared. The metal-to-ligand charge-transfer bands are almost unaltered when changing from the mono- to dinuclear complexes, indicating that the bridging H(2)L(1) ligand has slightly lower pi* orbital energy than that of bipy. The bridging H(2)L(1) ligand acts as a sigma/pi-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 H(2)L(1). 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 E, vs. pH (Pourbaix diagrams). The pK(a) values of the complexes reflect on both the type of metals and their oxidation states, M(II) and M(III). The introduction of asymmetry in the dinuclear complexes containing H(2)L(1) can provide not only a potential difference between the two metal sites but also a preferential protonation (or deprotonation) site.