Protonation equilibria of DL-3-amino-2-hydroxypropanoic acid and its complexes with cobalt(II), nickel(II), copper(II), and zinc(II)
The protonation constants of DL-3-amino-2-hydroxypropanoic acid [H3L+=+H3N·CH2·CH(OH)·CO2H], as determined potentiometrically at 25 °C and I= 0.1 mol dm–3 KCl, are log K1 13.2 log K2 9.14, and log K3 2.66 for the hydroxyl, amine, and carboxyl groups respectively. With cobalt(II), copper(II), and zinc(II) the main species are [M2L2] and [ML2]2–, the outstanding one being the dimer, whereas nickel(II) forms only the mononuclear complexes [NiL] and [NiL2]2–. The structure of the dimers probably consists of two ML units held together by bridging oxygen atoms. The ligand is bound to the metal through chelation of the aminoethanolatomoiety, while the carboxylate anion is bound via a water molecule of the co-ordination sphere. The complexing capacity of the present ligand is enhanced with respect to DL-4-amino-3-hydroxybutanoic acid.