Interactions of N-heteroalkylaminomethylenebisphosphonic acids with Cd(II) ions: Electrochemical and spectroscopic investigations

Abstract The aminomethylenebisphosphonates are known to be excellent chelators for many metal ions. In this work, the coordination properties of three different aminomethylenebisphosphonates with N -substituted heteroalkyl moieties ( L1 , L2 and L3 ) and their N -pyridyl derivative ( L4 ) toward cadmium(II) ions are described. Due to their coordination abilities over a broad range of pH, the compounds of this group are good candidates for heavy metal detoxification. To determine the stability constants and the coordination mode of formed aminomethylenebisphosphonate-cadmium(II) complexes, four analytical methods were employed: potentiometry, pulse polarography (DPP), nuclear magnetic resonance spectroscopy (NMR) and electrospray ionization mass spectrometry (ESI-MS). The studies performed revealed that ligands L1 , L2 and L3 possessed similar coordination properties, with Cd(II) ions binding below pH 2 to form equimolar complexes and above pH 6 to form bis-complexes. The ligand L4 behaved in a different way, forming six complexes of varied stoichiometry: CdH 4 L 3 , CdH 3 L 3 , CdHL 2 , CdL 2 , CdH 2 L and Cd 3 H 4 L 3 . The hypothetical competition plot between common complexone d -penicillamine and the studied ligands showed better complexation properties for the aminomethylenebisphosphonates in the pH range 2–6 than for neutral pH, where above neutral pH d -penicillamine bound Cd(II) ions significantly better.