SPECIATION STUDY OF A BIS-(3-HYDROXY-4-PYRIDINONE) TOWARDS M2+

This contribution is the result of a speciation study on the complexing ability of a bis-(3-hydroxy-4-pyridinone) ligand (NTAPr(3,4-HP)2), derivative of nitrilotriacetic acid (NTA), towards divalent metal cations of biological interest, namely Ca2+, Cu2+ and Zn2+. The 3-hydroxy-4-pyridinones are a class of bidentate compounds derivatives of deferiprone and represent good chelating agents for the detoxification of the human body from hard metal cations [1]. The synthesis and the acid-base properties of NTAPr(3,4-HP)2 were already presented [2] and the protonation data confirmed by 1H NMR spectroscopic measurements at I = 0.15 mol L-1 in NaCl(aq) and T = 298.15 K. On the sequence of the study of this ligand as a strong sequestering agent for hard metal cations (Al3+, Fe3+) [2], it appeared also relevant to assess its affinity towards Ca2+, Cu2+ and Zn2+. Therefore, the complexing ability of the ligand towards this set of metal cations was investigated by means of UV-Vis spectrophotometric titrations at the same experimental conditions of the protonation studies. The speciation models obtained consisted of protonated and simple metal-ligand species; the stability constants refined for the ML complex follow the trend: Cu2+ > Zn2+ > Ca2+. Concerning the Zn2+/NTAPr(3,4-HP)2 complexation, some 1H NMR spectroscopic measurements at I = 0.15 mol L-1 in NaCl(aq) and T = 298.15 K and computational studies were performed to gain information on the metal-ligand coordination. Furthermore, the sequestering ability of the ligand towards the metal cations of interest was investigated by the determination of the pL0.5 [3] and pM [4] parameters calculated at different pHs and pH = 7.4, respectively.