Variable Kinetic Isotope Effect Reveals a Multistep Pathway for Protonolysis of a Pt-Me Bond

The reaction of (cod)PtMe2 (cod = 1,5-cyclo-octadiene) with trifluoroacetic acid to release methane is an important system because it represents the microscopic reverse of desirable methane activation and because it has an unusually large kinetic isotope effect (KIE) that has been tentatively attributed to proton tunneling. A detailed kinetic and mechanistic investigation of this system was conducted using stopped-flow and traditional time-dependent UV-vis spectroscopy, supported by NMR and density functional theory studies. Consistently large KIE values (-14) in line with previous reports were obtained over a large range of reactant concentrations (0.1-1.6 mM (cod)PtMe2 and 3.2 mM to 6.0 M acid). At lower concentrations of acid, the KIE decreased significantly (KIE = -6 for 0.1 mM (cod)PtMe2 and 0.2 mM acid). This concentration-dependent KIE suggests a multistep reaction mechanism, eliminating the need to invoke proton tunneling. The reaction exhibits first-order dependence on (cod)PtMe2 and approximately second-order dependence on acid, with at least 2 equiv of acid required for complete conversion. Overall, the kinetic data indicate a multimolecular, multistep reaction mechanism for the protonolysis of (cod)PtMe2, thus ruling out the previously accepted bimolecular single-step mechanism. A mechanistic alternative consistent with the kinetic data is proposed, in which sequential oxidative addition and reductive elimination occur, and the second equivalent of acid serves to stabilize the trifluoroacetate anion in solution.