Anion-Selective Interaction and Colorimeter by an Optical Metalloreceptor Based on Ruthenium(II) 2,2‘-Biimidazole: Hydrogen Bonding and Proton Transfer

A new anion sensor [Ru(bpy)2(H2biim)](PF6)2 (1) (bpy = 2,2‘-bipyridine and H2biim = 2,2‘-biimidazole) has been
developed, in which the Ru(II)-bpy moiety acts as a chromophore and the H2biim ligand as an anion receptor via
hydrogen bonding. A systematic investigation shows that 1 is an eligible sensor for various anions. It donates
protons for hydrogen bonding to Cl-, Br-, I-, NO3-, HSO4-, H2PO4-, and OAc- anions and further actualizes
monoproton transfer to the OAc- anion, changing color from yellow to orange brown. The fluoride ion has a high
affinity toward the N−H group of the H2biim ligand for proton transfer, rather than hydrogen bonding, because of
the formation of the highly stable HF2- anion, resulting in stepwise deprotonation of the two N−H fragments.
These processes are signaled by vivid color changes from yellow to orange brown and then to violet because of
second-sphere donor−acceptor interactions between Ru(II)-H2biim and the anions. The significant color changes
can be distinguished visually. The processes are not only determined by the basicity of anion but also by the
strength of hydrogen bonding and the stability of the anion−receptor complexes. The design strategy and remarkable
photophysical properties of sensor 1 help to extend the development of anion sensors.