Substitution reactions in dinuclear platinum(II) complexes: an evaluation of the influence of the diazine-bridging ligand on reactivity

Abstract

A kinetic study of aqua substitution in dinuclear Pt(II) complexes, [{cis-Pt(OH2)(NH3)2}2-μ-pmn](ClO4)2 (pmn), [{cis-Pt(OH2)(NH3)2}2-μ-pdn](ClO4)2 (pdn), [{cis-Pt(OH2)(NH3)2}2-μ-qzn](ClO4)2 (qzn), [{cis-Pt(OH2)(NH3)2}2-μ-pht](ClO4)2 (pht) and [{cis-Pt(OH2)(NH3)2}2-μ-pzn](ClO4)2 (pzn) (pmn = pyrimidine, pdn = pyridazine, qzn = quinazoline, pht = phthalazine, pzn = pyrazine) by different sulphur-donor nucleophiles, thiourea (TU), N,N–dimethylthiourea (DMTU) and N,N,N,N–tetramethylthiourea (TMTU) was carried out. The reactions were followed under pseudo-first-order conditions as a function of nucleophile concentration and temperature using stopped-flow and UV–Vis spectrophotometric methods. The reactivity of the nucleophiles follows the order TU > DMTU > TMTU. The general order of reactivity for the aqua complexes follows pzn > qzn > pmn > pdn > pht which is confirmed by the obtained pK a values and the quantum chemical calculated NBO charges at the metal centre. The negative values reported for the activation entropy confirm the associative nature of the substitution process. The results demonstrate the strong connection between structural and electronic characteristics of the diazine-bridging ligand and reactivity of the dinuclear Pt(II) complexes.