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Tautomeric Switching and Metal‐Cation Sensing of Ligand‐Equipped 4‐Hydroxy‐/4‐oxo‐1,4‐dihydroquinolines
Author(s) -
Todorov Aleksandar R.,
Nieger Martin,
Helaja Juho
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201103868
Subject(s) - tautomer , chelation , chemistry , denticity , metal ions in aqueous solution , metal , titration , ligand (biochemistry) , fluorescence , ion , toluene , inorganic chemistry , medicinal chemistry , organic chemistry , receptor , biochemistry , physics , quantum mechanics
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(1 H )‐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.