Electrochemical and spectroscopic investigations of selected N-heteroalkylaminomethylenebisphosphonic acids with Pb(II) ions

Abstract The coordination properties of aminomethylenebisphosphonic acids towards various metal ions have been exploited during the last decade. These compounds are good chelators for biologically important metal ions, such as Cu(II), Fe(II), Zn(II), Ca(II), and Mg(II), as well as a toxic ion (i.e., Cd(II)). In this study, the interactions of five aminomethylenebisphosphonates with N-substituted heteroalkyl moieties ( L1, L2 and L3 ) and their N -pyridyl ( L4 ) and N -benzylpiperazine ( L5 ) derivatives with Pb(II) ions were investigated. A wide range of analytical methods were employed, including potentiometry, pulse polarography (DPP), nuclear magnetic resonance (NMR) spectroscopy and electrospray ionization mass spectrometry (ESI-MS), and the obtained data were correlated. Ligands L1–L3 exhibited similar coordination properties with the formation of dinuclear species (i.e., Pb 2 H 4 L 2 and Pb 2 H 3 L 2 ) at a low pH and equimolar complexes above a pH of ca. 8. Despite their structural differences, L4 and L5 also form dinuclear complexes, and an increase in the pH leads to the formation of bis-complexes (i.e., PbH 2 L 2 (only L4 ), PbHL 2 and PbL 2 ). The hypothetical competition plot between the studied ligands and two metal ions (i.e., Pb(II) and Ca(II)) revealed that Pb(II) ions are more strongly bound than Ca(II) ions. Additionally, the L4 ligand, which has the strongest coordination properties among the studied ligands, exhibited better binding to Pb(II) ions than a common complexone (i.e., D-penicillamine) in a wide pH range. The modes of formation of complexes between Pb(II) and bisphosphonic acids are also discussed compared to a wide spectrum of other metals belonging to different groups in the periodic table.