Kinetics and mechanism of interaction of some bioactive ligands with cis-diaqua(cis-1,2-diaminocyclohexane)platinum(II) in aqueous medium

The substitution reaction of cis-[Pt(cis-dach)(H2O)(2)](2 +) (where 'dach' is cis-1,2-diaminocyclohexane) with 2-thiouracil (S, N), 1,2-cyclohexanedionedioxime (N, N) and acetylacetone (O, O) were studied in aqueous solution in 0.10 M NaClO4 under pseudo-first order conditions as a functions of concentration, pH and temperature using UV-Vis spectrophotometry. The substitution reaction proceeds via rapid outer sphere association complex formation, followed by two slow consecutive steps. The first of these involves ligand-assisted deaquation, while second involves chelation as the second aqua ligand is displaced. The association equilibrium constant (K-E) for the outer sphere complex formation has been evaluated together with rate constants for the two subsequent steps. The rate constants increase with increasing ligand concentration and the evaluated activation parameters for all reactions suggest an associative substitution mechanism for both the aqua ligand substitution processes. The product of the reaction has been characterized by IR, NMR and ESI-MS spectral analysis; which throws more light on the mechanistic behaviour of platinum(II) antitumour complexes.