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

Abstract

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.

Functional Dimming of Pincer‐Shaped Bibenzimidazole‐Ruthenium(II) Complexes with Improved Anion‐Sensitive Luminescence

“…[5,6] A potent approach to detect anions through hydrogen bonds is to connect one or more acidic H-donor functions with a luminophore in a way that the absorption and emission properties of the latter are visibly influenced by the appearance and strength of the Hbond interaction. [7,8] On the one hand, polypyridyl-ruthenium(II) complexes are widely applicable molecular tools for the introduction of light in technological and biological applications [9][10][11][12] because of their well-known photophysical properties and generally long-lived excited states, in which light energy is absorbed and provided for chemical conversions. [13,14] On the other hand, the vast synthetic variability of the general tris(diimine) framework allows the adjustment of a chromophore for a distinct application.…”

Interaction of an Iridium(III)–Bibenzimidazole Complex with Anions – Implications for Luminescent Sensing

“…[1,3,[6][7][8][9][10][11][12][13][14][15][16][17][18] Regarding these observations, many workgroups have used Ru IIpolypyridyl complexes as chromophores, in which functional groups such as amide, pyrrole, pyrimidine, thiourea, urea, and imidazole ligands, [2,[19][20][21] but also other H-bond-donor functionalities, are appended and form hydrogen bonds with anions. [1,3,[6][7][8][9][10][11][12][13][14][15][16][17][18] Regarding these observations, many workgroups have used Ru IIpolypyridyl complexes as chromophores, in which functional groups such as amide, pyrrole, pyrimidine, thiourea, urea, and imidazole ligands, [2,[19][20][21] but also other H-bond-donor functionalities, are appended and form hydrogen bonds with anions.…”

[Ru(tpy)(H4bbdip)]2+ as an anion-selective colorimetric sensor

“…The presence of multiple isosbestic points in the UV-vis adsorption titrations implied that a couple of species coexisted at the equilibrium. Based on the usual behavior of F − and OAc − [8][9][10], we deduced that they might interact with complex 1 via two simultaneous processes: hydrogen bonding and proton transfer on the N\H group of ligand H 4 bbdip to F − and OAc − .…”