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(BiimH₂)](PF₆)₂ (1, bpy is bipyridine, BiimH₂ is 2,2′-biimidazole), [Ru(bpy)₂-(BbimH₂)](PF₆)₂ (2, BbimH₂ is 2,2′-bibenzimidazole), and [Ru(bpy)₂(DMBbimH₂)](PF₆)₂ (3, DMBbimH₂ is 7,7′-dimethyl-2,2′-bibenzimidazole) and [Ru(bpy)₂(TMBbimH₂)]²⁺ (4, TMBbimH₂ 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.