Complexation behaviour and stability of Schiff bases in aqueous solution. The case of an acyclic diimino(amino) diphenol and its reduced triamine derivative†
The copper(II), nickel(II), and zinc(II) complexes of the acyclic Schiff base H2LA, obtained by [1 + 2] condensation of 1,2-ethanediamine,N-(2-aminoethyl)-N-methyl with 3-ethoxy-2-hydroxybenzaldehyde, and of H2LB, the reduced derivative of H2LA, were prepared and their properties studied by IR, NMR and SEM-EDS. In these complexes, the metal ion is always located in the coordination chamber of the ligand delimited by two phenol oxygens and nitrogen atoms (either aminic or iminic). The coordination behaviour of H2LA and H2LB towards H+, Cu2+, Ni2+ and Zn2+ in aqueous solution at 298 K and µ = 0.1 mol dm−3 (Na)ClO4 was also studied by potentiometric, NMR and UV-VIS measurements. In particular, potentiometric equilibrium studies indicate that H2LA is not stable enough to have a pH range in which it is the sole species in aqueous solution. In such a solution, the Schiff base forms over a limited pH range, between 6 and 10, with a maximum formation percentage at pH ∼9. In addition, the involvement of imine nitrogens in the complexes markedly stabilises the azomethylene linkage, so that the metal complexes of H2LA, particularly those of copper(II), are the species largely prevailing in solutions with pH >3.5. The stability constants of the complexes formed by metal ions with H2LA and H2LB follow the order Cu2+ ≫ Ni2+ > Zn2+; distribution plots show that copper(II) gives complexes more stable with H2LA, whereas Ni2+ and Zn2+ prefer the reduced ligand, H2LB.