pH-Triggered Assembly of Organometallic Receptors for Lithium Ions

The reaction of half-sandwich complexes of ruthenium, rhodium, and iridium with amino-substituted 3-hydroxy-2-pyridone ligands in aqueous solution gives monomeric O,O‘-chelate complexes. Upon addition of base, the complexes assemble to form trimeric metallamacrocycles, as evidenced by NMR spectroscopy and single-crystal X-ray analyses. The macrocycles are able to act as highly selective receptors for lithium ions. The binding constants depend on the nature of the half-sandwich complex, the ligand, and the pH. With a commercially available (cymene)Ru complex, a receptor with a Li⁺ binding constant of K_a = 5.8 (±1.0) × 10⁴ M^-1 and a Li⁺−Na⁺ selectivity of 10 000:1 can be obtained. The fact that the assembly process of the receptor is pH-dependent can be used to detect the presence of lithium ions by a pH measurement. Furthermore, it is possible to transduce the binding of Li⁺ into a change of color by means of a chemical reaction with FeCl₃. This allows the detection of Li⁺ in the pharmacologically relevant concentration range of 0.5−1.5 mM by the “naked eye”.