Complex formation between transition metal ions and salicylglycine, a metabolite of aspirin

Ionization constant and complex formation constants of salicylglycine (1) in aqueous solution at 25°C and ionic strength 0.2 mol dm KCl have been determined. The pK values for the ligand are 3.44(1) and 8.24(4) and H NMR studies show that there is no further ionization at higher pH as was previously suggested. In the case of copper(II) there is no evidence for complex formation below pH 4 and the main species in solution at pH5 according to pH-metric and UV-vis spectrophotometric evidence in MLH in which the ligand is bonded to the metal through the phenolate and carboxylate oxygen atoms and the deprotonated peptide nitrogen. The species ML also exists albeit in low concentrations. At pH10 the formation of MLH resulting from ionization of an equatorial aquo ligand is observed. In the case of nickel(II) and zinc(II) no complex formation occurs below pH 6 but above this pH the species ML and MLH, the latter involving deprotonation of an aquo ligand, are observed although at pHca 8 in the case of nickel(II) and pH ~ 7.5 in the case of zinc(II) precipitation occurs.