Theoretical insights on the C-C bond reductive elimination from Co(III) center

The density-functional theory (DFT) calculations using B3LYP functional were performed to inspect the mechanism of the reductive elimination of ethane from a cobalt (III) dimethyl complex, (PMe₃)₃Co(CH₃)₂I. Three different pathways, i.e., radical mechanism (path A), concerted C-C bond formation (path B) and α-hydride elimination (path C) were studied for the reductive elimination reaction. A PMe₃ ligand of the hexa-coordinated cobalt complex dissociates to form penta-coordinated complex. The penta-coordinated complex undergoes reductive elimination reaction. The calculated Gibbs free energy for the formation of methyl radical is 27.6 kcal/mol. Path B involving concerted C-C bond formation shows the activation energy barrier of 12.8 kcal/mol for the reductive elimination reaction. The calculated activation energy barrier for α-hydride elimination mechanism is 53.1 kcal/mol. Path B shows the lowest activation energy barrier of 12.8 kcal/mol, however, this value is much lower than the experimentally determined activation energy (25.0 kcal/mol). With MN12SX functional, the activation energy value improves to 20.8 kcal/mol which is close to the experimental value.