Issue 23, 2012

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 KB 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 KA are 2.74 for 1, 5.19 for 2, 4.54 for 3, and 3.78 for 4. The pKa1 values of the complexes in acetonitrile solution are calculated by subtracting log KA from pKa (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 (DPT) 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 KB (7.26) and DPT value (0.57) both reach their maxima at a critical point, where pKa1 for the complex is 17.8 and ΔpKa for the adduct is 4.5 (ΔpKa = pKa(HOAc) − pKa1, in acetonitrile solution). Moreover, the binding constant log KB shows linear correlation with the degree of proton transfer DPT.

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

Supplementary files

Article information

Article type
Paper
Submitted
31 Jan 2012
Accepted
04 Apr 2012
First published
04 Apr 2012

Dalton Trans., 2012,41, 7026-7036

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

H. Mo, J. Wu, Z. Qiao and B. Ye, Dalton Trans., 2012, 41, 7026 DOI: 10.1039/C2DT30225J

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