Comparison of hydration reactions for "piano-stool" RAPTA-B and [Ru(η6-arene)(en)Cl]+ complexes: density functional theory computational study.

The hydration process for two Ru(II) representative half-sandwich complexes: Ru(arene)(pta)Cl(2) (from the RAPTA family) and Ru(arene)(en)Cl (further labeled as Ru_en) were compared with analogous reaction of cisplatin. In the study, quantum chemical methods were employed. All the complexes were optimized at the B3LYP/6-31G(d) level using Conductor Polarizable Continuum Model (CPCM) solvent continuum model and single-point (SP) energy calculations and determination of electronic properties were performed at the B3LYP∕6-311++G(2df,2pd)/CPCM level. It was found that the hydration model works fairly well for the replacement of the first chloride by water where an acceptable agreement for both Gibbs free energies and rate constants was obtained. However, in the second hydration step worse agreement of the experimental and calculated values was achieved. In agreement with experimental values, the rate constants for the first step can be ordered as RAPTA-B > Ru_en > cisplatin. The rate constants correlate well with binding energies (BEs) of the Pt∕Ru-Cl bond in the reactant complexes. Substitution reactions on Ru_en and cisplatin complexes proceed only via pseudoassociative (associative interchange) mechanism. On the other hand in the case of RAPTA there is also possible a competitive dissociation mechanism with metastable pentacoordinated intermediate. The first hydration step is slightly endothermic for all three complexes by 3-5 kcal∕mol. Estimated BEs confirm that the benzene ligand is relatively weakly bonded assuming the fact that it occupies three coordination positions of the Ru(II) cation.