Kinetics and mechanism of the reaction between chromium(III) and 3,4-dihydroxyphenylpropionic (dihydrocaffeic) acid in weak acidic aqueous solutions

Abstract

The reaction of 3,4-dihydroxyphenylpropionic acid (dihydrocaffeic acid, hydcafH3) with chromium(III) in weak acidic aqueous solutions has been shown to take place through various oxygen-bonded intermediates. The formation of the oxygen-bonded complexes upon substitution of water molecules of the chromium(III) coordination sphere takes place in at least three stages, the first of which has an observed rate constant k1(obs)=k1K0′[hydcafH3][H+] where K0′ corresponds to the Cr(H2O)63+ complex dissociation equilibrium. The second and third stages are ligand concentration independent and are thus attributed to isomerisation and chelation processes. The corresponding activation parameters are ΔH2≠=78±3 kJmol−1, ΔS2≠=−49±9 JK−1mol−1, ΔH3≠=60±9 kJmol−1 and ΔS3≠=−112±39 JK−1mol−1. The kinetic results support associative mechanisms and the nature of the electronic spectra a catecholic-type of coordination at the pH and concentration range studied and reported in this paper. The associatively activated substitution processes are accompanied by proton release causing a pH decrease. At lower acid concentration oxidation of the ligand takes place with concomitant high increase in the UV and VIS absorbance.