KINETICS AND MECHANISM OF COMPLEX FORMATION BETWEEN [PtCl(DIEN)]+ AND THIOLS AND THIOETHERS

Abstract

Reactions of the monofunctional platinum(II) complex, [PtCl(dien)]+, with different thiols and thioethers, including biologically important molecules, have been studied as a function of temperature (288.2–308.2K) using conventional electronic spectrophotometry in 0.10 M aqueous hydrochloric acid and by 1H NMR spectroscopy. The second-order rate constants, k2, are similar, varying between 1.43 × 10−3 and 46.1 × 10−3 M−1s−1 at 25°C. The reactivity follows the sequences: D-penicillamine ≤ L-cysteine ≤ glutathione ≤ thiodiglycolic acid ≤ thioglycolic acid ≤ L-methionine ≤ S-methylthioglycolic acid ≤ glycyl-D,L-methionine. However, variation in size, bulkiness and solvation of the entering ligands reflect in their properties as nucleophiles. Large negative values of the entropy of activation (ΔS≠), between −140 and −190 J K−1 mol−1, indicate that all thiols and thioethers react via the same associative mechanism. Results have been analyzed in relation to the antitumor activity and toxicity of platinum(II) complexes.