Platinum(II)-Mediated C–H Bond Activation of Benzene Revisited: Mechanistic Insights by Microkinetic Modeling

Abstract

The ion/molecule reaction of the coordinatively unsaturated, cationic Pt(II) complex [(bipy)Pt(CH3)]+ (bipy = 2,2′-bipyridine) with benzene─originally investigated in mass-spectrometric gas-phase studies─has been successfully translated into solution. [(bipy)Pt(CH3)(solv)]+ (solv = H2O and 2,2,2-trifluoroethanol (TFE)) is generated in situ by protonation of [(bipy)Pt(CH3)2] in TFE in the presence of benzene. The resulting C–H bond activation reaction proceeds by an order of magnitude faster than for a closely related Pt(II)-diimine complex. In reactions with C6D6, kinetic isotope effects are not observed, and the data identify substrate coordination as the rate-determining step. The isotopologue distributions of the reaction products are analyzed by a microkinetic model, enabling quantification of the relative energetics of methane loss and reversible C–H bond activation. Moreover, subtle insights such as the number of back-and-forth steps prior to the finalizing methane loss are elucidated. Counterions and the water concentration do not have an influence on the observed isotopic distributions, thus indicating that the isotopologue-determining steps are strictly intramolecular. Extensive H/D scrambling provides clear evidence for the pronounced stability of the involved methane complexes [(L)Pt(C6H5)(CH4)]+. Moreover, degenerate benzene exchange in the initially formed benzene complexes [(L)Pt(CH3)(C6H6)]+ significantly influences the produced isotopologue distributions.