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 Ka = 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”.