A kinetic and mechanistic study of analogous bifunctional dialkylamine platinum(ii) complexes

This study was aimed at investigating the comparative substitution behaviour of analogous cis/trans-Pt(ii) complexes with inert dialkylamine ligands. The rate of substitution of the aqua ligands by three nucleophiles, viz. thiourea (TU), 1,3-dimethylthiourea (DMTU) and 1,1,3,3-tetramethylthiourea (TMTU), for the complexes: [cis-Pt(OH2)(2)(NH3)(2)](ClO4)(2) (cPt), [cis-Pt(OH2)(2)(NH2CH3)(2)](ClO4)(2) (cPtM), [cis-Pt(OH2)(2){NH2CH(CH3)(2)}(2)](ClO4)(2) (cPtR), [trans-Pt(OH2)(2)(NH3)(2)](ClO4)(2) (tPt), [trans-Pt(OH2)(2)(NH2CH3)(2)](ClO4)(2) (tPtM) and [trans-Pt(OH2)(2){NH2CH(CH3)(2)}(2)](ClO4)(2) (tPtR) was investigated under pseudo first-order conditions as a function of concentration and temperature by stopped-flow and UV/visible spectrophotometry. The reactions of the cis-complexes proceed in two concerted steps whereas those of the trans-complexes followed a stepwise mechanism involving rate determining substitution of the first chloride followed by a fast second substitution step, with no intermediates being detected. The pseudo first-order rate constants, k(obs) obeyed the rate law: k(obs) = k(2)[nucleophile]. The trans-complexes were observed to be approximately 10(3) times more reactive than their corresponding cis-analogues. The electronic and the steric hindrance due to the geometries of the complexes controlled the overall reaction pattern. The order of reactivity of the complexes is tPt > tPtM > tPtR > cPt > cPtM > cPtR. The reactivity of the nucleophiles with the complexes decreases with an increase in steric demand in the order: TU > DMTU > TMTU. The trans-dialkylamine complexes formed distinctly different kinetic products relative to their cis-analogues. Pt-195 NMR spectroscopic results confirmed the final kinetic products of each of the cis- and trans-complexes. The negative entropy of activation (Delta S-not equal) values in all the complexes investigated assert an associative substitution mechanism, with the mechanism being more pronounced in the cis-complexes than in their trans-analogues.