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

Abstract

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 = k₂[nucleophile]. The trans-complexes were observed to be approximately 10³ 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. ¹⁹⁵Pt NMR spectroscopic results confirmed the final kinetic products of each of the cis- and trans-complexes. The negative entropy of activation (ΔS^≠) 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.