Kinetics and Mechanism of the Interaction of Di-μ-hydroxobis (1,10-phenanthroline)dipalladium(II) Perchlorate with Thioglycolic Acid and Glutathione in Aqueous Solution

Abstract

The kinetics of interaction between di-l-hydroxobis(1,10-phenanthroline) dipalladium(II) perchlorate and thioglycolic acid and with glutathione has been studied spectrophotometrically in aqueous medium as a function of the complex concentration as well as the ligand concentrations, pH, and temperature at constant ionic strength. The observed pseudo-first-order rate constants kobs (s-1) obeyed the equation kobs = k1[Nu] (Nu = nucleophile). At pH = 6.5, the interaction with thioglycolic acid shows two distinct consecutive steps and both steps are dependent on the concentration of thioglycolic acid. The rate constants for the process are: k1 & 10-5 s-1 and k2 & 10-3 dm3 _ mol-1 _ s-1. The association equilibrium constant (KE) for the outer sphere complex formation has been evaluated together with the rate constants for the two subsequent steps. The other bio-active ligand, glutathione, showed a single step reaction depending on [ligand] with a second-order anation rate constant: the 102 (k2) values are (61.72, 79.20, 109.24 and 154.33) dm3 . mol-1 . s-1 at 20, 25, 30 and 35 _C, respectively. On the basis of the kinetic observations and evaluated activation parameters, plausible associative mechanisms are proposed for both interaction processes.