A density functional theory study of the mechanisms of oxidation of ethylene by rhenium oxide complexes

Abstract

The oxo complexes of group VII B are of great interest for their potential toward epoxidation and dihydroxylation. In this work, the mechanisms of oxidation of ethylene by rhenium-oxo complexes of the type LReO₃ (L = O⁻, Cl, CH₃, OCH₃, Cp, NPH₃) have been explored at the B3LYP/LACVP* level of theory. The activation barriers and reaction energies for the stepwise and concerted addition pathways involving multiple spin states have been computed. In the reaction of LReO₃ (L = O⁻, Cl, CH₃, OCH₃, Cp, NPH₃) with ethylene, it was found that the concerted [3 + 2] addition pathway on the singlet potential energy surfaces leading to the formation of a dioxylate intermediate is favored over the [2 + 2] addition pathway leading to the formation of a metallaoxetane intermediate and its re-arrangement to form the dioxylate. 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 and the reaction energies follow the order CH₃ > O⁻ > NPH₃ > CH₃O⁻ > Cl⁻ > Cp. On the doublet PES, the [2 + 2] addition leading to the formation the metallaoxetane intermediate is favored over dioxylate formation for the ligands L = CH₃, CH₃O⁻, Cl⁻. The activation barriers for the formation of the metallaoxetane intermediate are found to increase for the ligands in the order CH₃ < Cl⁻ < CH₃O⁻ while the reaction energies follow the order Cl⁻ < CH₃O⁻ < CH₃. The subsequent re-arrangement of the metallaoxetane intermediate to the dioxylate is only feasible in the case of ReO₃(OCH₃). Of all the complexes studied, the best dioxylating catalyst is ReO₃Cp (singlet surface); the best epoxidation catalyst is ReO₃Cl (singlet surface); and the best metallaoxetane formation catalyst is ReO₃(NPH₃) (triplet surface).