Novel tetradentate chelators derived from 3-hydroxy-4-pyridinone units: synthesis, characterization and aqueous solution properties

The synthesis and characterization of three novel tetradentate ligands (T1, T2 and T3) based on 3-hydroxy-4-pyridinone chelating units are described. The three ligands exhibit different flexibility due to the use of two different anchor molecules, piperazine and 1,2-diaminobenzene, and to the diverse length of the 3-hydroxy-4-pyridinone arms. All reactions were performed using both conventional heating and microwave irradiation in order to evaluate the possibility of using greener methods in these synthetic procedures. The results showed that, in all cases, microwave irradiation provides the desired ligands reducing reaction time in ca. 97%. The structure of ligand T3 was resolved by X-ray crystallography, showing significant hydrogen bonding and interesting pep interaction between the benzene and pyridinone rings. The use of potentiometric and spectroscopic methods allowed determination of acidity constants and unequivocal assignment of proton loss for each pKa value. Interaction of the ligands with divalent metal ions was assessed by spectroscopic methods.