Aluminium(III) and Iron(III) 1,2-Diphenolato Complexes: a Potentiometric Study

Abstract

The equilibrium reactions between aluminium(III) and the phenols catechol (LH₂), protocatechuic acid (LH₃) and catechin (LH₄), and between iron(III) and protocatechuic acid have been studied by potentiometric titration in aqueous solution, I 0.10M ( KCl ), 25°C. Stability constants are reported for the mononuclear diphenolato complexes AlLH_n-2, Al(LH_n-2)₂and Al(LH_n-2)₃, and the hydroxo complexes Al(OH)LH_n-2 and Al(OH)(LH_n-2)₂, n = 2 ( catechol ) or 4 ( catechin ). For protocatechuic acid, the carboxylate -coordinated species lLH₂²⁺ and the species AlLH ⁺ (carboxyl- protonated ) are also postulated. Analogous species were characterized for the iron(III)- protocatechuic acid system. Stoichiometric end-points were obtained only if an excess of ligand was used (L/M > 4). Monomeric and polymeric aluminium- hydroxo species and ternary metal- ligand - hydroxo species were found to be unimportant under the experimental conditions employed.