Molecular mechanical and quantum chemical study of the species involved in the hydrolysis of cis-diamminedichloroplatinum(II) and substituted bis(ethylenediamine)dichloroplatinum(II) complexes .2. Simulated transition

摘要

cis-Diamminedichloroplatinum(II) (cisplatin) and its substituted ethylenediamine derivatives cis-PtCl2(R(2)en) (en = ethylenediamine, R = H, Ph, 2-, 3-, and 4-PhOH) have been investigated with respect to the possible structures of the hypothetical Transition State Complexes (TSC) of the hydrolytic S(N)2 reaction in which one Cl is replaced by H2O. TSCs with trigonal bipyramid (TBP) and square pyramid (SP) geometry (coordination number 5), have been studied by Molecular Mechanics (MM) and Extended Huckel (EH) methods. The EH and MM energies as well as the number of occurrence (entropy factor) for the cisplatinum compound point to a preferred TBP TSC geometry with NH3 and Cl in axial positions. However, for en and substituted en compounds, TSCs with SP geometries (Cl in apical position:) are preferred. The calculated EH and MM energies of the TBP and SP structures do not differ significantly and TBP <-> SP interconversions may play an essential role in TSC formation. To improve the discrimination, the MM-optimized geometries were treated in terms of displacement coordinates for D-3h (TBP) and C-4v (SP) by calculating the total distortion vectors (DV). DV identified once again the TBP with NH3 and Cl in axial position as the least-distorted conformer, but it also revealed the combinations of displacement coordinates which shape the TSC geometry.