Complexation Behaviour And Stability Of Schiff Bases In Aqueous Solution. The Case Of An Acyclic Diimino(Amino) Diphenol And Of Its Reduced Triamine Derivative.

. The complexes of copper(II), nickel (II), and zinc(II) with the acyclic Schiff base H2LA, derived from the condensation 1:2 of 1,2-ethanediamine,N-(2-aminoethyl)-N-methyl with 3-ethoxy-2-hydroxybenzaldehyde, and with the related polyamine derivative H2LB, obtained by reduction of H2LA with NaBH4, have been prepared and their properties studied by ir, nmr, and SEM-EDS. Furthemore the coordination behaviour of H2LA and H2LB toward H+, Cu2+, Ni2+ and Zn2+ was investigated by potentiometric, nmr, and UV-VIS measurements. In particular, potentiometric equilibrium studies indicate that H2LA is not enough stable to have a pH range where it is the only species in aqueous solution. As a matter of fact, the Schiff base forms in aqueous solution in a relatively short p[H+] range, between 6 and 10, with a maximum formation percentage of 30% at p[H+] ƒî9. On the other hand, the involvement of the iminic nitrogens in the complexes markedly stabilizes the azomethylenic linkage so that the metal complexes of H2LA, particularly those of copper(II), are the species largely prevailing in solutions with p[H+] > 3.5. The stability constants of the complexes formed by the metal ions with H2LA and H2LB (ƒÝ = 0.1 NaClO4 and 25¢XC ) follows the order Cu(II) >> Ni(II) > Zn(II). Moreover, a comparison of their corresponding values shows that copper(II) gives complexes more stable with H2LA, whereas Ni(II) and Zn(II) prefer the reduced ligand, H2LB.