A new bipodal carboxy-bis(hydroxypyridinonate) ligand

A new tetradentate ligand having two 3-hydroxy-4-pyridinonate moieties and one carboxylate group appended to a cyclohexane backbone is reported, together with the corresponding bidentate hydroxypyridinonate pendant arm. The synthesis and characterization of these ligands, as well as their complexation properties towards the first transition series of metal ions M(II) (M=Cu, Ni, Zn) in aqueous solution, are described and discussed herein. The tetradentate ligand forms quite stable complexes with this series of metal ions and in solution, it is proved to be a much more effective chelator than the corresponding bidentate derivative. The stability constants of the complexes formed with this set of bivalent ions follow the Irving–Williams order Ni Zn. The carboxylic group has no interaction with the metal centre, thus remaining free for potential interaction with biological ligands (e.g. proteins).