Influence of solvent on ligand-substitution reactions of PtII complexes as function of the π-acceptor properties of the spectator chelate

A detailed kinetic study of the substitution reactions of [Pt-II(L-3)Cl](+) complexes, where L-3 represents a tridentate N-donor chelate which contains different numbers of pi-acceptor pyridine units, with thiourea as a function of nucleophile concentration, temperature and pressure was undertaken in a series of solvents of different polarity, viz. 1-pentanol, 1-propanol, ethanol, methanol and water. The substitution behaviour of [Pt(diethylenetriamine)Cl](+) (aaa), [Pt{2,6bis(aminomethyl)pyridine}Cl](+) (apa), [Pt{bis(2-pyridylmethyl)amine}Cl](+) (pap) and [Pt(terpy)Cl](+) (ppp) was studied in the selected solvents, In the case of alcoholic solutions, an increase in the substitution rate with increasing length of the carbon chain was observed, which is ascribed to the possible role of van der Waals forces in stabilizing the transition state. In the case of methanol and water as solvent, the secondorder rate constants correlate with the polarity of the solvent. The activation parameters for the substitution of the aaa, apa, pap, and ppp complexes in methanol and water were determined, from which evidence for an associative mechanism was obtained, The introduction of pi-acceptor ligands leads to a higher electrophilicity of the platinum(II) centre and consequently a higher lability of the complex. The reactivity of the complexes is controlled by a combination of solvent and pi-acceptor effects. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).