Dimethylplatinum(II) complexes: Computational insights into Pt-C bond protonolysis

摘要

A detailed density functional theory (DFT) study of the protonation and subsequent methane elimination reactions of dimethylplatinum(II) complexes in presence of triflic acid in various solvents has been undertaken to contribute to the debate concerning the mechanism of the electrophilic cleavage of the Pt-C bond in Pt(II) complexes. Both mechanisms of direct one-step proton attack at the Pt-C bond (S_E2) and stepwise oxidative-addition on the central metal followed by reductive elimination (S_E(ox)) have been explored for a series of dimethylplatinum(II) complexes changing the nature of the ancillary ligands and the solvent. Theoretical calculations show that the most likely mechanism cannot be predicted on the basis of spectator ligands donating properties only. A one-step protonolysis pathway is characteristic for complexes containing P based ligands, whereas for complexes containing N based and, in general, hard poor-donor ligands a common behavior cannot be indicated. Solvent nucleophilicity can influence the rate of the S_E(ox) rate mechanism, whereas its steric hindrance can induce a change of the preferred mechanism. The hypothesis that five-coordinate methyl hydrido platinum(IV) intermediates might be formed along the S_E(ox) pathway is not supported. Only six-coordinate Pt(IV) hydride complexes are calculated to be stable intermediates generated by direct protonation at the platinum center. Formation and experimental detection of six-coordinate Pt(IV) hydrides, nevertheless, cannot be considered a definite evidence that a S_E(ox) mechanism is operative because such intermediates can be also generated by a hydrogen migration to Pt from the carbon atom of the σ-complex methane molecule formed by a S_E2 attack. For all the examined complexes methane loss occurs by an associative mechanism. Both solvent and anion of the acid can assist methane displacement. Calculations have been also carried out to probe whether the preference for a concerted or a stepwise mechanism should be predicted on the basis of two proposed criteria: metal-complex charge distribution as a consequence of the Pt-C bond polarization and the nature of the highest occupied molecular orbital (HOMO).