Kinetic Isotope Effects as a Probe for the Protonolysis Mechanism of Alkylmetal Complexes: VTST/MT Calculations Based on DFT Potential Energy Surfaces

Abstract

Protonolysis by platinum or palladium complexes has been extensively studied because it is the microscopic reverse of the C-H bond activation reaction. The protonolysis of (COD)PtIIMe2, which exhibits abnormally large kinetic isotope effects (KIEs), is proposed to occur via a concerted pathway (SE2 mechanism) with large tunneling. However, further investigation of KIEs for the protonolysis of ZnMe2 and others led to a conclusion that there is no noticeable correlation between the mechanism and magnitude of KIE. In this study, we demonstrated that variational transition state theory including multidimensional tunneling (VTST/MT) could accurately predict KIEs and Arrhenius parameters of the protonolysis of alkylmetal complexes based on the potential energy surfaces generated by density functional theory. The predicted KIEs, EaD - EaH values, and AH/AD ratios for the protonolysis of (COD)PtIIMe2 and ZnIIMe2 by TFA agreed very well with experimental values. The protonolysis of ZnMe2 with the concerted pathway has a very flat potential energy surface, which produces a very small tunneling effect and therefore a small KIE. The predicted KIE for the stepwise protonolysis (SE(ox) mechanism) of (COD)PtIIMe2 was much smaller than that of the concerted pathway, but greater than the KIE of the concerted protonolysis of ZnMe2. A large KIE, which entails a significant tunneling effect, could be used as an experimental probe of the concerted pathway. However, a normal or small KIE should not be used as an indicator of the stepwise mechanism, and the interplay between experiments and reliable theory including tunneling would be essential to uncover the mechanism correctly.