Interaction between biimidazole complexes of ruthenium and acetate: hydrogen bonding and proton transfer

Abstract

The hydrogen bonding and deprotonation processes between four ruthenium biimidazole complexes, namely [Ru(bpy)(2)(BiimH(2))](PF(6))(2) (1, bpy is bipyridine, BiimH(2) is 2,2'-biimidazole), [Ru(bpy)(2)-(BbimH(2))](PF(6))(2) (2, BbimH(2) is 2,2'-bibenzimidazole), and [Ru(bpy)(2)(DMBbimH(2))](PF(6))(2) (3, DMBbimH(2) is 7,7'-dimethyl-2,2'-bibenzimidazole) and [Ru(bpy)(2)(TMBbimH(2))](2+) (4, TMBbimH(2) is 5,6,5',6'-tetramethyl-2,2'-bibenzimidazole), and acetate are investigated. Their hydrogen bonded adducts are indeed trapped and observed by absorption spectra and electrochemical experiments in acetonitrile solution in the presence of an excess of acetic acid for the first time. The binding constants log K(B) for these adducts are 6.74 for 1·OAc, 7.11 for 2·OAc, 7.26 for 3·OAc, and 6.99 for 4·OAc. A new approach to calculate the deprotonation constant is also developed by establishing a set of circular equilibria. The equilibrium constants for the first deprotonation step of the complexes log K(A) are 2.74 for 1, 5.19 for 2, 4.54 for 3, and 3.78 for 4. The pK(a1) values of the complexes in acetonitrile solution are calculated by subtracting log K(A) from pK(a) (HOAc in acetonitrile), giving 19.6 for 1, 17.1 for 2, 17.8 for 3, and 18.5 for 4. The degree of proton transfer (D(PT)) can be quantified by the calculation of absorption spectral and redox data, which is 0.41 for 1·OAc, 0.53 for 2·OAc, 0.57 for 3·OAc, and 0.47 for 4·OAc. Interestingly, the binding constant log K(B) (7.26) and D(PT) value (0.57) both reach their maxima at a critical point, where pK(a1) for the complex is 17.8 and ΔpK(a) for the adduct is 4.5 (ΔpK(a) = pK(a)(HOAc) - pK(a1), in acetonitrile solution). Moreover, the binding constant log K(B) shows linear correlation with the degree of proton transfer D(PT).

Protonation‐Dependent Luminescence of an Iridium(III) Bibenzimidazole Chromophore

“…This luminescent character renders this complex a convenient candidate for applications in photocatalysis, organic light-emitting diode (OLED) technology (e.g., as a chromophore), and even in anion sensing. [5][6][7][14][15][16] Preliminary time-resolved emission investigations showed the expected complex behavior for the Ir(tmBBI)-H 2 chromophore. Here, a short-lived high-energy emitting state progresses to a long-lived lower-energy excited state.…”

“…Ye and coworkers have confirmed the capability of biimidazole complexes to act as hydrogen-bond donors through NMR and UV/Vis spectroscopy. [5][6][7] The absorption features in the visible region change significantly upon deprotonation or hydrogen bonding to fluoride or acetate ions. In their fully deprotonated state, these complexes act as metalloligands, that is, the deprotonated biimidazole sphere can bind a second metal center such as zinc(II), nickel(II), or copper(I).…”

Ruthenium Imidazophenanthrolinium Complexes with Prolonged Excited‐State Lifetimes

“…[11][12][13][14] Owing to their remarkably stable photoredox chemistry, many ruthenium polypyridyl complexes are potent chromophores for lightdriven catalysis, [15,16] dye-sensitized solar cells, [17][18][19] and photocatalytic water splitting. [20,21] Crucial factors for their application herein are (1) high chemical stability, (2) intense absorption of visible light, (3) efficient population of a reactive charge-transfer (CT) excited state, and (4) long excitedstate lifetimes (up to the μs range).…”

A Macrocyclic 2,2′‐Bibenzimidazole Ruthenium(II) Chromophore as a Versatile Building Block for Supramolecular Devices

“…[28] These effects influence the photophysical properties of the ruthenium chromophore, which can lead to cation-driven molecular light switches [29] and luminescent cation and anion sensors. [30][31][32] To introduce the versatile supramolecular properties of bibenzimidazoles (BBIs) into a functional molecular architectures, synthetic access to 4,4Ј-disubstituted 2,2Ј-bibenzimidazoles is necessary (Scheme 1). The introduction of variable substituents is possible.…”