Thermodynamic and kinetic behaviour of [Pt(2-methylthiomethylpyridine)(OH2)2]2+

Abstract

The diaqua complex [Pt(2-methylthiomethylpyridine)(OH₂)₂]²⁺, Pt(mtp), was synthesized and investigated thermodynamically as well as kinetically. Spectrophotometric acid–base titrations were performed to determine the pK_a values of the two coordinated water ligands. A low pK_a1 value of 3.15 was observed for the water molecule trans to the pyridine donor, whereas a pK_a2 value of 6.84 was found for the water molecule trans to the labilising sulphur donor. The substitution of coordinated water by a series of sterically hindered S-containing nucleophiles, viz.thiourea (tu), N,N′-dimethylthiourea (dmtu) and N,N,N′,N′-tetramethylthiourea (tmtu), was studied under pseudo first-order conditions as a function of nucleophile concentration, pH (2, 4.75, 7.4), temperature and pressure, using stopped-flow techniques and UV-vis spectroscopy. In general the first substitution reaction takes place trans to the sulphur donor. At pH 2 the nucleophiles react in the order tu (634 ± 10) > dmtu (507 ± 5) ≫ tmtu (165 ± 3 M⁻¹ s⁻¹ at 25 °C), which is caused by steric hindrance. The second observed reaction involves two steps, viz. the displacement of the second water ligand and dechelation of the pyridine ring with the third-order rate constants 73.3 ± 0.8 (tu), 22.1 ± 0.1 (dmtu) and 6.8 ± 0.2 M⁻² s⁻¹ (tmtu) at 25 °C. At pH 4.75 the reactions are in general slower due to the presence of aqua-hydroxo species. The same order in reactivity was found, viz. tu (106 ± 1) > dmtu (72 ± 1) ≫ tmtu (14.1 ± 0.5 M⁻¹ s⁻¹ at 25 °C). No evidence for ring-dechelation could be observed under these conditions. At pH 7.4 the inert dihydroxo species is predominantly present in solution and consequently no substitution reaction was observed. Quantum chemical calculations were performed to support the interpretation and discussion of the experimental results.