An NMR study on the 6,6′-(2-(diethylamino)ethylazanediyl)bis(methylene)bis(5-hydroxy-2-hydroxymethyl-4H-pyran-4-one) interaction with AlIII and ZnII ions

• NMR characterization of the tetradentate ligand L9
• NMR study of complex formation equilibria of L9 with AlIII and ZnII
• Rigid and symmetric structure of L9–AlIII complexes up to pH 7.4 revealed by NMR data
• Several ZnII species in solution of ML stoichiometry pointed out by NMR experiments

Here we report about the complex formation among an amine-bearing bis-kojic acid, 6,6′-(2-(diethylamino)ethylazanediyl)bis(methylene)bis(5-hydroxy-2-hydroxymethyl-4H-pyran-4-one) and two metal ions, the trivalent hard and not essential metal ion AlIII and the borderline and essential divalent metal ion ZnII. We carried out a thorough NMR study in order to reach the indispensable structural information on the behavior of these complexes in solution. A combination of 1D, 2D total correlation spectroscopy, heteronuclear single quantum coherence spectroscopy, nuclear Overhauser enhancement spectroscopy and rotating-frame Overhauser effect spectroscopy experiments was used to assign the signals of both free and metal-bound ligand at different pH values. Our results highlighted the different coordination behaviors of the ligand towards the different metal ions, depending on their hard or borderline character. The trivalent metal ion, AlIII, mainly forms dinuclear helicate complexes of M2L3 stoichiometry, and the coordination only involves both hydroxypyrone (O,O)-donor atoms. NMR data are in agreement with the presence of a rigid and symmetric structure of L9–AlIII complexes up to physiological pH. On the contrary, with the divalent metal ion, NMR data showed the coexistence of several species in solution though ZnII forms complexes of ML stoichiometry at physiological pH, where the metal coordination involves the nitrogen atoms of both the linker and the side-chain amine groups together with the oxygen atoms of phenolate groups. The in solution study will be of interest for providing an insight on the ligand bioavailability and on its behavior in the chelation treatments.

A NMR study of complex formation among an amine-bearing bis-kojic acid and the trivalent AlIII and the divalent ZnII. Different experiments were used to assign the signals of free and metal-bound ligand, highlighting the different coordinations with the two metal ions, depending on their hard or borderline character.Figure optionsDownload as PowerPoint slide