PROTON-INDUCED SWITCHING OF METAL METAL INTERACTIONS IN DINUCLEAR RUTHENIUM AND OSMIUM COMPLEXES BRIDGED BY 2,2'-BIS(2-PYRIDYL)BIBENZIMIDAZOLE

New dinuclear complexes [M(L)2(bpbimH2)M(L)2]4+ (M = Ru, Os; L = bpy, phen; bpbimH2 = 2,2'-bis(2-pyridyl)bibenzimidazole) act as dibasic acids. Both the absorption spectra and oxidation potentials are strongly dependent on the solution pH, which is responsible for the deprotonation of the N-H group on the coordinated bridging ligand. The pK(a) values reflect the metal oxidation states, M(II) and M(III). The pK(a) values of the mixed-valence dinuclear complexes are quite close to those of each M(II) and M(III) component, which suggests that the mixed-valence complexes have a localized valence structure. The mixed-valence complexes bridged by the protonated ligand bpbimH2 exhibit the intervalence (IT) band at 7300 cm-1 for M = Ru and at 9100 cm-1 for M = Os, respectively. When the bridging ligand is deprotonated, this IT band is shifted to lower energy at 5880 cm-1 for M = Ru and 7700 cm-1 for M = Os and intensified. The degree of metal-metal interaction of the deprotonated dinuclear complexes becomes 4-6 times larger than that of the protonated complexes. This proton-induced change of metal-metal interaction can be rationalized by change of HOMO energy levels on deprotonation or protonation in the bridging ligands. Thus, proton transfer in the bpbimH2 bridging dinuclear complexes can be utilized to serve as a trigger signal for switching the metal-metal interaction.