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

The reaction of 3,4-dihydroxyphenylpropionic acid (dihydrocaffeic acid, hydcafH(3)) 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 k(l(obs)) = k(1)K(0)([H+])(')([hydcaffH3]) where K-0(') corresponds to the Cr(H2O)(6)(3+) 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 DeltaH(2)(not equal) = 78 +/- 3 kJ mol(-1), DeltaS(2)(not equal) = -49 +/- 9 J K-1 mol(-1), DeltaH(3)(not equal) = 60 +/- 9 kJ mol(-1) and DeltaS(3)(not equal) = - 112 +/- 39 J K-1 mol(-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. (C) 2003 Elsevier Inc. All rights reserved.