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

摘要

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 LReO_3 (L = O~-, Cl, CH_3, OCH_3, Cp, NPH_3) 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 LReO _3 (L = O~-, Cl, CH_3, OCH_3, Cp, NPH_3) 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~- > CH_3 > NPH_3 > CH_3O~- > Cl~- > Cp and the reaction energies follow the order CH_3 > O~- > NPH_3 > CH_3O~- > 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 = CH_3, CH_3O~-, Cl~-. The activation barriers for the formation of the metallaoxetane intermediate are found to increase for the ligands in the order CH_3 < Cl - < CH_3O~- while the reaction energies follow the order Cl~- < CH_3O~- < CH_3. The subsequent re-arrangement of the metallaoxetane intermediate to the dioxylate is only feasible in the case of ReO_3(OCH_3). Of all the complexes studied, the best dioxylating catalyst is ReO_3Cp (singlet surface); the best epoxidation catalyst is ReO_3Cl (singlet surface); and the best metallaoxetane formation catalyst is ReO _3(NPH_3) (triplet surface). This journal is