A density functional theory study of the mechanisms of oxidation of ethylene by technetium oxo complexes

The mechanisms of oxidation of ethylene by transition metal-oxo complexes of the type LTcO3 (L = O−, Cl, CH3, OCH3, Cp, NPH3) have been explored by computing the activation barriers and reaction energies for the concerted and stepwise addition pathways at the density functional theory B3LYP/LACVP* level of theory. The results indicate that in the reaction of LTcO3 (L = O−, Cl, CH3, OCH3, Cp, NPH3) with ethylene, the formation of the dioxylate intermediate through the concerted [3 + 2] addition pathway on the singlet potential energy surface is favored kinetically and thermodynamically over its formation through the two-step process via the metallaoxetane intermediate. The activation barrier for the formation of the dioxylate on the singlet PES for the ligands studied is found to follow the order: O− > CH3 > NPH3 > CH3O− > Cl− > Cp while the reaction energies follow the order: Cl− > O− > CH3 > NPH3 > CH3O− > Cp. On the doublet PES, the [2 + 2] addition leading to the formation of the four-membered metallacycle intermediate is favored kinetically and thermodynamically for the ligands when L = NPH3. The direct [2 + 1] addition of ethylene across the oxo- ligand of doublet TcO3(CH3) to form the epoxide precursor is favored when L = CH3. The activation barriers for the formation of the dioxylate intermediate are found to follow the order: Cl− < CH3O− < CH3 whiles the reaction energies follow the order Cl− < CH3O− < CH3. The re-arrangement of the metallaoxetane intermediate to the dioxylate is not a feasible pathway for the formation of the dioxylate. The formation of the epoxide precursor will not result from the reaction of LTcO3 (L = O−, Cp) with ethylene on all the surfaces explored. There does not appear to be a spin-crossover in any of the pathways studied.