Ligand substitution in chromium(III)-aqua complexes by l -histidine: kinetic resolution of two long-lived intermediates

The kinetics of the reaction of substitution of aqua ligands in Cr(III) complexes by the amino acid l -histidine, in aqueous media and under acidic conditions (pH 3.60–5.79), has been studied with the aid of a spectrophotometric technique (at 530 nm). The rate-time profiles showed an initial acceleration period followed by a deceleration one. A model of three consecutive reactions has been applied, involving two long-lived intermediates (not reactive enough to be in steady state) and three rate constants: k 1 , k 2 and k 3 , corresponding to the successive decays of the reactant, the first long-lived intermediate and the second, respectively. The three rate constants increased when the medium ionic strength was raised using KNO 3 as background electrolyte, whereas the effect of KCl was of minor intensity, and showed base catalysis. The activation parameters for the reactions of Cr(III) with l -histidine, l -histidine methyl ester, l -arginine, l -lysine and 2-picolinic acid were also determined. The UV–Vis spectrum of the first long-lived intermediate was rather close to that of the inorganic reactant, whereas the spectrum of the second long-lived intermediate was somehow in between those corresponding to the inorganic reactant and the reaction product. The spectra of the final reactant mixtures revealed the co-existence of at least two complexes differing in the number of organic ligands, along with the corresponding protonated forms. The proposed mechanism involves the activation of the Cr(III) starting complex by deprotonation, as well as three rate-determining (slow) steps in which the breakage of a Cr(III)-aqua chemical bond takes place, thus leaving a vacant site to which the organic ligand can coordinate.