Migratory CO insertion and aldehyde formation in carbonylation of methane by the Rh(PH3)(2)Cl catalyst. A dynamical density functional study

Sakakura and Tanaka have recently developed an alkane carbonylation process with the Vaska type complex Rh(PR(3))(2)Cl(CO) (1) as the (pre)catalyst. We have previously studied the initial steps of the alkane carbonylation process involving C-H activation, Here, we present density functional calculations on the remaining part of the catalytic cycle, This part involves migratory insertion of CO into the Rh-CH3, bond of Rh(PH3)(2)Cl(H)(CH3) (CO) (6), generating the rhodium acyl Rh(PH3)(2)Cl(H)(CH3CO) (11), and further the addition of another CO molecule to 11, yielding Rh(PH3)(2)Cl(CO)(H)(CH3CO) (12). Finally, acetaldehyde is eliminated front 12 to regenerate 1. The present investigation combines ''static'' calculations of the stationary points on the potential surface with first principles molecular dynamics calculations based on the Car-Parrinello-Projector-Augmented-Wave method. We find that the rate limiting step in the carbonylation of methane is the migratory insertion of coordinated CO into the Rh-CH3 bond, 6 --> 11, with a barrier of 129 (trans) and 114 kJ/mol (cis), respectively, whereas the reductive elimination of methane from 6 has a lower calculated barrier of 72 (trans) and 57 kJ/mol (cis) according to our previous work. Therefore, the carbonylation will be seriously retarded by the concurrent reductive alkane elimination. We find that the overall productivity of the methane carbonylation process is determined by the migratory insertion step.