Ruthenium(II) 2,2′-Bibenzimidazole Complex as a Second-Sphere Receptor for Anions Interaction and Colorimeter

A ruthenium(II) complex [Ru(bpy)₂(H₂bbim)](PF₆)₂ (1) as anions receptor has been exploited, where Ru(II)-bpy moiety acts as a chromophore and the H₂bbim ligand as an anion binding site. A systematic study suggests that 1 interacts with the Cl⁻, Br⁻, I⁻, NO₃⁻, HSO₄⁻, and H₂PO₄⁻ anions via the formation of hydrogen bonds. Whereas 1 undergoes a stepwise process with the addition of F⁻ and OAc⁻ anions: formation of the monodeprotonated complex [Ru(bpy)₂(Hbbim)] with a low anion concentration, followed by the double-deprotonated complex [Ru(bpy)₂(bbim)], in the presence of a high anion concentration. These stepwise processes concomitant with the changes of vivid colors from yellow to orange brown and then to violet can be used for probing the F⁻ and OAc⁻ anions by naked eye. The deprotonation processes are not only determined by the basicity of the anion but also related to the strength of hydrogen bonding, as well as the stability of the formed compounds. Moreover, a double-deprotonated complex [Ru(bpy)₂(bbim)]·CH₃OH·H₂O (3) has been synthesized, and the structural changes induced by the deprotonation has also been investigated. In addition, complexes [Ru(bpy)₂(Hbbim)]₂(HOAc)₃Cl₂·12H₂O (2), [Ru(bpy)₂(Hbbim)](HCCl₃CO₂)(CCl₃CO₂)·2H₂O (4), and [Ru(bpy)₂(H₂bbim)](CF₃CO₂)₂·4H₂O (5) have been synthesized to observe the second sphere coordination between the Ru(II)-H₂bbim moiety and carboxylate groups via hydrogen bonds in the solid state.