Direct dynamics analysis of the cationic Cp*(PMe₃)Ir(CH₃) methane C-H activation mechanism

Area of Science:

• Organometallic Chemistry
• Computational Chemistry
• Reaction Dynamics

Background:

• The σ-bond metathesis reaction involving methane and cationic Cp*(PMe3)IrIII(CH3) was previously studied using static Density Functional Theory (DFT) calculations.
• DFT calculations indicated a two-step mechanism involving oxidative addition and reductive elimination, with an iridium(V)-hydride (IrV-H) intermediate.
• Prior quasiclassical direct molecular dynamics simulations suggested a minor but significant dynamical pathway bypassing the IrV-H intermediate.

Purpose of the Study:

• To investigate the reaction dynamics of the σ-bond metathesis between methane and cationic Cp*(PMe3)IrIII(CH3) using direct dynamics simulations.
• To assess the influence of simulation time step size on the observed reaction mechanism.
• To evaluate the effect of a continuum solvent model on the reaction dynamics.

Main Methods:

• Direct dynamics simulations employing a microcanonical temperature sampling method.
• Trajectory initialization and propagation using the Gaussian program.
• Examination of trajectories with varying average time steps (1.5 fs, 0.75 fs, 0.25 fs) and reverse trajectories.
• Simulation of 30 trajectories using a SMD continuum dichloromethane solvent model.

Main Results:

• A time step of approximately 1.5 fs was found to be too large, potentially overestimating the number of trajectories bypassing the IrV-H intermediate.
• Trajectories simulated with 0.75 fs and 0.25 fs time steps yielded results qualitatively similar to previous simulations using a 1 fs time step.
• Reverse trajectories propagated with a 0.25 fs time step demonstrated complete skipping of the methane σ-complex.
• Simulations incorporating a SMD continuum dichloromethane solvent model showed behavior comparable to gas-phase trajectories.

Conclusions:

• The σ-bond metathesis reaction can proceed via a dynamical one-step mechanism, bypassing the IrV-H intermediate, particularly at shorter time steps.
• Simulation time step size is a critical parameter influencing the accuracy of dynamical simulations for this reaction.
• The presence of dichloromethane solvent does not significantly alter the fundamental reaction dynamics observed in the gas phase.