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

Abstract

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 E_½vs. 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 H₂L¹ can provide not only a potential difference between the two metal sites but also a preferential protonation (or deprotonation) site.