Quantum-chemical calculations of the tautomeric forms of azo derivatives of acetylacetone and determination of the stability constants of their complexes with rare-earth metals

Abstract

The effective atomic charges in the tautomeric forms (enol-azo, keto-azo, and hydrazo) of 3-(2-hydroxy-5-nitro-3-sulfophenylazo)pentane-2,4-dione (L1), 3-(2-hydroxy-3,5-disulfophenylazo)pentane-2,4-dione (L2), 3-(5-chloro-2-hydroxy-3-sulfophenylazo)pentane-2,4-dione (L3), 3-(2-hydroxy-4-nitrophenylazo)pentane-2,4-dione (L4), and 3-(2-hydroxyphenylazo)pentane-2,4-dione (L5) were calculated by the Huckel method (MO LCAO). It was found that the hydrazo form is most reactive for meta- and meta’-substituted derivatives (L1–3) and the keto-azo form is most reactive for para-substituted (L4) and unsubstituted ones (L5). The stability constants of complexes of rare-earth metals (La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) with L1–5 determined by potentiometric titration decrease in the order: Lu > Yb > Tm > Er > Ho > Dy > Tb > Gd > Eu > Sm > Nd > Ce > La. Functionalization of the aromatic part of ligands L affected neither the rare-earth metal: L ratio (1: 2) nor the above order of the stability constants.