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)[sub 2](bpbimH[sub 2])M(L)[sub 2]][sup 4+] (M = Ru, Os; L = bpy, phen; bpbimH[sub 2] = 2,2[prime]-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[sub a] values reflect the metal oxidation states, M(II) and M(III). The pK[sub a] values of the mixed-valence dinuclear complexes are quite close to those of each M(II) and M(III) component, which suggests that mixed-valence complexes bridged by the protonated ligand bpbimH[sub 2] exhibit the intervalence (IT) band at 7300 cm[sup [minus]1] for M = Ru and at 9100 cm[sup [minus]1] for M = Os, respectively. When the bridging ligand is deprotonated, this IT band is shifted to lower energy at 5880 cm[sup [minus]1] for M = Ru and 7700 cm[sup [minus]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 themore » bpbimH[sub 2] bridging dinuclear complexes can be utilized to serve as a trigger signal for switching the metal-metal interaction.« less