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

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 (H 3 L), the experimental data were consistent with the formation of six mononuclear Al−ligand species, each involving catecholate coordination: AlHL +, AlL, Al(OH)L -, AlL 2 3-, Al(OH)L 2 4-, and AlL 3 6- . For DHB (H 2 L), the data were consistent with the formation of six monomeric and one dimeric Al species: AlHL 2+, Al(HL) 2 +, Al(HL) 3 0, Al(HL) 2 L -, Al(HL)L 2 2-, AlL 3 3-, and (OH) 2 Al 2 (HL) 2 2+, in which HL has a protonated amino group. On the basis of the DASA (H 2 L - ) titration data, three monomeric and one dimeric Al−ligand species were proposed: AlL 0, AlL 2 3-, Al(OH)L 2 4-, and Al 2 (OH) 2 L 4 8- . 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 −SO 3 - < −CH CH·CO·O·(C 6 H 10 O 3 )·COOH < −CH CH·COOH < −NH 3 + .