Ternary complex formation in the system Ni(II) with picolinic acid and selected amino acids: Solution studies, isolation and computational calculations

The formation of mixed ligand complexes of Ni(II) with picolinic acid (Hpic) in presence of selected amino acids (HL) (serine (Hser), threonine (Hthr), methionine (Hmet) and phenylalanine (Hphe)) has been studied by pH-metric titrations. The pH-titrations of the reaction mixtures are shown to yield the complexes Ni(pic)L, [Ni(pic)(L)(OH)] and [Ni(pic)(L)(2)] in the systems studied with the amino acids ser, met and phe, while in the Ni(II)-Hpic-Hthr system only the complexes Ni(pic)L and [Ni(pic)(L)(2)] were formed. The equilibrium and formation constants of the resulting ternary complexes have been calculated at I = 1.0 mol.dm(-3) of NaCl. The stability of the ternary complexes was quantitatively compared with their corresponding binary complexes in terms of the parameters Delta log(10) K ''. The concentration distributions of various species formed in solution were also evaluated as a function of pH. The synthesis of the ternary species Ni(II)-Hpic-HL, in which HL corresponds to the amino acids ser and phe, was performed, and the solids were characterized using a combination of FT-IR, UV-Vis, elemental analysis, TGA, powder DRX and H-1 NMR. The crystallization of the isolated ternary complexes was attempted, yielding only the corresponding binary species Ni(pic)(2). Calculations on the stability of the ternary complexes were performed, to account for the impossibility to crystallize them. (C) 2017 Elsevier B.V. All rights reserved.