Tautomeric switching and metal-cation sensing of ligand-equipped 4-hydroxy-/4-oxo-1,4-dihydroquinolines

Aleksandar R Todorov; Martin Nieger; Juho Helaja

doi:10.1002/chem.201103868

Tautomeric switching and metal-cation sensing of ligand-equipped 4-hydroxy-/4-oxo-1,4-dihydroquinolines

Chemistry. 2012 Jun 4;18(23):7269-77. doi: 10.1002/chem.201103868. Epub 2012 Apr 19.

Authors

Aleksandar R Todorov¹, Martin Nieger, Juho Helaja

Affiliation

¹ Laboratory of Organic Chemistry, Department of Chemistry, University of Helsinki, A.I.Virtasen aukio 1, P.O. Box. 55, University of Helsinki, 00014 Helsinki, Finland.

PMID: 22517539
DOI: 10.1002/chem.201103868

Abstract

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 Zn(2+) and Cd(2+) 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 Zn(2+) ions. In the case of the bidentate 8-(quinolin-8-ylimino)methyl side arm, NMR studies in CD(3) OD indicated that both Cd(2+) and Zn(2+) 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.