Transition metal complexes of aminophosphonic acid analogues of methionine and alanine in aqueous solution

The stoichiometries and stability constants of the nickel(II), copper(II) and zinc(II) complexes of 1-amino-3-methylthiopropanephosphonic acid (MetP) and 1-amino-ethanephosphonic acid (alpha-AlaP) have been determined pH-metrically at 25 degrees C at an ionic strength of 0.2 mol dm(-3) (KCl). From the stability data and the absorption spectra of the complexes it has been established that simple aminophosphonic acids coordinate to the nickel(II) and copper(II) ions forming chelate complexes in which the metal binding mode is bidentate with the [NH2, PO32-} donor set. P-31 and H-1 NMR measurements showed that MetP and alpha-AlaP exhibit similar properties in the presence of zinc(II)) ions, but the ligand reacts to form a cyclic phosphonoamidate in neutral and slightly alkaline solution in the Zn(II)-alpha-AlaP system and at slightly acidic conditions in the Zn(II)-MetP system. This difference reveals that the latter ligand at pH > 7 prefers Zn(II) coordination involving all possible (amino, phosphonate and thioether sulfur) donor groups.