Tautomeric Switching and Metal‐Cation Sensing of Ligand‐Equipped 4‐Hydroxy‐/4‐oxo‐1,4‐dihydroquinolines

Novel 4-hydroxyquinoline (4HQ) based tautomeric switches are reported. 4HQs equipped with coordinative side arms (8-arylimino and 3-piperidin-1-ylmethyl groups) were synthesized to access O- or N-selective chelation of Zn2+ and Cd2+ ions by 4HQ. In the case of the monodentate arylimino group, O chelation of metal ions induces concomitant switching of phenol tautomer to the keto form in nonpolar or aprotic media. This change is accompanied by selective and highly sensitive fluorometric sensing of Zn2+ ions. In the case of the bidentate 8-(quinolin-8-ylimino)methyl side arm, NMR studies in CD3OD indicated that both Cd2+ and Zn2+ ions afford N chelation for 4HQ, coexisting with tautomeric switching from quinolin-4(1H)-one to quinolin-4-olate. In corroboration, UV/Vis-monitored metal-ion titrations in toluene and methanol implied similar structural changes. Additionally, fluorescence measurements indicated that the metal-triggered tautomeric switching is associated with compound signaling properties. The results are supported by DFT calculations at the B3LYP 6-31G* level. Several X-ray structures of metal-free and metal-chelating 4HQ are presented to support the solution studies.