Complexation behaviour and stability of Schiff bases in aqueous solution. The case of an acyclic diimino(amino) diphenol and its reduced triamine derivative

Abstract

The copper(II), nickel(II), and zinc(II) complexes of the acyclic Schiff base H₂L_A, obtained by [1 + 2] condensation of 1,2-ethanediamine,N-(2-aminoethyl)-N-methyl with 3-ethoxy-2-hydroxybenzaldehyde, and of H₂L_B, the reduced derivative of H₂L_A, 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 H₂L_A and H₂L_B 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 H₂L_A 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 H₂L_A, 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 H₂L_A and H₂L_B follow the order Cu²⁺ ≫ Ni²⁺ > Zn²⁺; distribution plots show that copper(II) gives complexes more stable with H₂L_A, whereas Ni²⁺ and Zn²⁺ prefer the reduced ligand, H₂L_B.