Photophysical, electrochemical and anion sensing properties of Ru(ii) bipyridine complexes with 2,2′-biimidazole-like ligand

Abstract

A new anion sensor [Ru(bpy)₂(DMBbimH₂)](PF₆)₂ (3) (bpy is 2, 2′-bipyridine and DMBbimH₂ is 7,7′-dimethyl-2,2′-bibenzimidazole) has been developed. Its photophysical, electrochemical and anion sensing properties are compared with two previously investigated systems, [Ru(bpy)₂(BiimH₂)](PF₆)₂ (1) and [Ru(bpy)₂(BbimH₂)](PF₆)₂ (2) (BiimH₂ is 2,2′-biimidazole and BbimH₂ is 2,2′-bibenzimidazole). The high acidity of the N–H fragments in these complexes make them easy to be deprotonated by strong basic anions such as F⁻ and OAc⁻, and they form N–H⋯X hydrogen bonds with weak basic anions like Cl⁻, Br⁻, I⁻, NO₃⁻, and HSO₄⁻. Complex 3 displays strong hydrogen bonding with these 5 weak basic anions, with binding constants between 17 000 and 21 000, which are larger than those observed in complex 1, with binding constants between 3300 and 5700, and in complex 2, which shows no hydrogen bonding toward Cl⁻, Br⁻, I⁻, and NO₃⁻, and forms considerable hydrogen bonds with HSO₄⁻ with a binding constant of 11 209. These hydrogen bonding behaviours give different NMR, emission and electrochemical responses. The different anion binding affinity of these complexes may be mainly attributed to their different pK_a1 values, 7.2 for 1, 5.7 for 2, and 6.2 for 3. The additional methyl groups at the 7 and 7′ positions of complex 3 may also play an important role in the enhancement of anion binding strength.