Potentiometric Study of the Dissociation Constants of Some Schiff Bases from Ortho-Hydroxynaphthaldehyde and 2,4-Substituted Aniline and Stability Constants of Their Cu(II) Complexes in Mixed Solvents

Abstract

The dissociation constants of Schiff bases, (E)-1-[(2,4-dichlorophenylimino)methyl]naphthalen-2-ol, (E)-1-[(2-chloro-4-nitrophenylimino)methyl]naphthalen-2-ol, (E)-1-[(2,4-dimethoxyphenylimino)methyl]naphthalen-2-ol, and their stability constants with the copper(II) metal ion were determined potentiometrically. The experimental conditions were arranged to achieve coordination in aqueous-ethanol solutions [ethyl alcohol (30%) and water (70%) at 25.0 +/- 0.1(degrees)C and a constant ionic strength of 0.1 mol/dm(-3) NaCl]. It was detected that each ligand has two dissociation constants, which are associated to the protonated imine nitrogen and the hydroxyl group. The dissociation constants of these ligands and their stability constants of Cu(II) complexes were computed and discussed in relation to their structural configuration, substituents and basicity. The difference in basicity or acidity of these Schiff base ligands were explained in terms of the inductive effect, resonance and steric effect. Keto-enol tautomerism was discussed in the context of hydrogen bonding, basicity of the imine group and hydrogen acidity of the hydroxyl group; a zwitterionic tautomer might develop as well. The chemical species existing in the solution at experimental conditions used were presented with speciation diagrams. The SUPERQUAD program was used to compute dissociation and stability constants as well as the speciation diagrams. The aims of this study was to determine potentiometrically the dissociation constants of the Schiff base ligands used and the stability constants of their complexes with Cu(II).