Synthesis, Structural Characterization, and Photophysical, Electrochemical, Intercomponent Energy-Transfer, and Anion-Sensing Studies of Imidazole 4,5-Bis(benzimidazole)-Bridged OsIIOsII and RuIIOsII Bipyridine Complexes

Homo- and heterobimetallic complexes of composition [(bpy)2MII(H2Imbzim)M′II(bpy)2](ClO4)3·nH2O, where MII = M′II = Os (1), MII = Ru and M′II = Os (2), H3Imbzim = 4,5-bis(benzimidazole-2-yl)imidazole, and bpy = 2,2′-bipyridine, have been synthesized and characterized using standard analytical and spectroscopic techniques. Both of the complexes crystallized in monoclinic form with the space group P21/m for 1 and P21/n for 2. The absorption spectra, redox behavior, and luminescence properties of the complexes have been thoroughly investigated. The complexes display very intense, ligand-centered absorption bands in the UV region and moderately intense metal-to-ligand charge-transfer (MLCT) bands in the visible region. The bimetallic complexes show two successive one-electron reversible metal-centered oxidations. The strong fluorescence of free H3Imbzim is completely quenched in the metal complexes by energy transfer to the metal-based units, which exhibit their characteristic MLCT phosphorescence. The luminescence data of the heterometallic complex 2 show that electronic energy transfer takes place from the ruthenium center to the osmium-based component. The anion binding properties of the complexes have been studied in solutions using absorption, emission, and 1H NMR spectral measurements. The metalloreceptors act as sensors for F− and AcO− ions. Sensing studies indicate the presence of two successive anion-induced deprotonation steps, leading to the formation of [(bpy)2M(HImbzim)M′(bpy)2]2+ and [(bpy)2M(Imbzim)M′(bpy)2]+ species. Double deprotonation is also observed in the presence of hydroxide. The binding affinities of different anions toward the receptors have been evaluated. Cyclic voltammetry measurements carried out in acetonitrile have provided evidence in favor of anion-dependent electrochemical responses of the bimetallic metalloreceptors with F− and AcO− ions.