Monometallic and Bimetallic Ruthenium(II) Complexes Derived from 4,5-Bis(benzimidazol-2-yl)imidazole (H3Imbzim) and 2,2′-Bipyridine as Colorimetric Sensors for Anions: Synthesis, Characterization, and Binding Studies

Mixed-ligand monometallic and bimetallic ruthenium(II) complexes of compositions [(bpy)(2)Ru(H(3)Imbzim)](ClO(4))(2) x 2 H(2)O (1) and [(bpy)(2)Ru(H(2)Imbzim)Ru(bpy)(2)](ClO(4))(3) x CH(2)Cl(2) (2), where H(3)Imbzim = 4,5-bis(benzimidazol-2-yl)imidazole and bpy = 2,2'-bipyridine, have been synthesized and characterized using standard analytical and spectroscopic techniques. The X-ray crystal structures of both compounds have been determined and showed that 1 crystallized in the triclinic form with space group P1 and 2 is in the monoclinic form with space group P2(1)/m. The anion binding properties of complexes 1 and 2, as well as those of the parent H(3)Imbzim, were thoroughly investigated in an acetonitrile solution using absorption, emission, and (1)H NMR spectral studies, which revealed that both of the metalloreceptors act as sensors for F(-), for AcO(-), and, to some extent, for H(2)PO(4)(-). At a relatively lower concentration of anions, a 1:1 hydrogen-bonded adduct was formed; however, in the presence of an excess of anions, stepwise deprotonation of the two benzimidazole NH fragments occurred, an event that was signaled by the development of vivid colors visible with the naked eye. Double deprotonation was also observed in the presence of hydroxide. Less basic anions (AcO(-) and H(2)PO(4)(-)) induce deprotonation of only one NH. The effect of solvents on the absorption and emission spectral behavior has also been studied in detail. The binding affinities of different anions toward the receptors were evaluated and showed that the binding constants of 1 and 2 are substantially enhanced relative to free H(3)Imbzim because upon coordination to the Ru(II) center(s), H(3)Imbzim/H(2)Imbzim(-) becomes electron-deficient, thereby rendering the imidazole NH protons more available for hydrogen bonding to the anions. Cyclic voltammetry studies carried out in acetonitrile provided evidence of an anion-dependent electrochemical response with F(-) and AcO(-). Anion-induced lifetime shortening makes complex 2 a suitable lifetime-based sensor for anions.