Molecular mechanical and quantum chemical study on the species involved in the hydrolysis of cis-diamminedichloroplatinum(II) and substituted bis(ethylenediamine)dichloroplatinum(II) complexes. Part I. Reactants and products

Abstract

Cisplatin and its substituted ethylenediamine derivs., cis-PtCl2(R2en) (en = ethylenediamine, R = H, Ph, 2-, 3- and 4-PhOH) have been studied with respect to the first step of their hydrolysis reaction. The geometry of the reactants and products was detd. by mol. mechanics (MM). The MM optimized structures were used to calc. by the extended Hueckel method the charge distribution and relative electronic energies. The MM and EH calcns. were carried out with different ligand conformations. Due to increased non-bonded repulsion, with increasing ligand bulkiness, the square planar arrangement is the preferred geometry also by the MM results. This addnl. (to the electronic) stabilization of the square planar arrangement around Pt(II) is unfavorable for the aquation process. The thermodn. stabilities correlate with the rate of hydrolysis of meso-, (+)- and (-)-[1,2-bis(2-hydroxyphenyl)ethylenediamine]dichloroplatinum(II) (3-PtCl2). The slower rate of hydrolysis of the meso diastereoisomer as compared with that of the D, L species of 3-PtCl2 is explained by the presence of a 5th Pt-O contact in the meso diastereoisomer which hinders the entrance of the water mol. and makes the hydrolysis slower.