Stability Constants for Aluminum(III) Complexes with the 1,2-Dihydroxyaryl Ligands Caffeic Acid, Chlorogenic Acid, DHB, and DASA in Aqueous Solution

Abstract

Equilibrium constants are reported for the aluminum(III) complexes with four ligands that contain the 1,2-dihydroxyaryl group: trans-3-(3‘,4‘-dihydroxyphenyl)propenoic acid (caffeic acid), 3-(3‘,4‘-dihydroxycinnamoyl)-1,3,4,5-tetrahydroxycyclohexanecarboxylic acid (chlorogenic acid), 3,4-dihydroxybenzylamine (DHB), and 1,2-dihydroxyanthraquinone-3-sulfonic acid (DASA). The protonation and complexation reactions were studied by potentiometric and spectrophotometric titrations in aqueous solution at 25 °C (I = 0.10 M KCl). The complexation reactions were studied for ligand/metal ratios in the range from 1.3 to 5.9. For caffeic and chlorogenic acids (H3L), the experimental data were consistent with the formation of six mononuclear Al−ligand species, each involving catecholate coordination: AlHL+, AlL, Al(OH)L-, AlL23-, Al(OH)L24-, and AlL36-. For DHB (H2L), the data were consistent with the formation of six monomeric and one dimeric Al species: AlHL2+, Al(HL)2+, Al(HL)30, Al(HL)2L-, Al(HL)L22-, AlL33-, and (OH)2Al2(HL)22+, in which HL has a protonated amino group. On the basis of the DASA (H2L-) titration data, three monomeric and one dimeric Al−ligand species were proposed: AlL0, AlL23-, Al(OH)L24-, and Al2(OH)2L48-. The relative stability of the 1:1 complexes, AlL, was DASA > chlorogenic acid > caffeic acid > DHB. This indicates the progressive electron-withdrawing effects of the ring substituents −SO3- < −CHCH·CO·O·(C6H10O3)·COOH < −CHCH·COOH < −NH3+.