Kinetics and mechanism of the reaction between chromium(III) and 3,4-dihydroxy-phenyl-propenoic acid (caffeic acid) in weak acidic aqueous solutions

Our study of the complexation of 3,4-dihydroxy-phenyl-propenoic acid by chromium(III) could give information on the way that this metal ion is available to plants. The reaction between chromium(III) and 3,4-dihydroxy-phenyl-propenoic acid in weak acidic aqueous solutions has been shown to take place by at least three stages. The first stage corresponds to substitution (Id mechanism) of water molecule from the Cr(H2O)(5)OH2+ coordination sphere by a ligand molecule. A very rapid protonation equilibrium, which follows, favors the aqua species. The second and the third stages are chromium(III) and ligand concentration independent and are attributed to isomerisation and chelation processes. The corresponding activation parameters are Delta H-2(obs)(not equal) = 28.6 +/- 2.9 kJ mol(-1), Delta S-2(obs)(not equal) = -220 +/- 10 J K-1 mol(-1), Delta H-3(obs)(not equal) = 62.9 +/- 6.7 kJ mol(-1) and Delta S-3(obs)(not equal) = -121 +/- 22 J K-1 mol(-1). The kinetic results suggest associative mechanisms for the two steps. The associatively activated substitution processes are accompanied by proton release causing pH decrease. Copyright (C) 2008 Vladimiros Thoma et al.