A theoretical study of the mechanisms of oxidation of ethylene by manganese oxo complexes

Abstract

The mechanisms of oxidation of ethylene by manganese–oxo complexes of the type MnO₃L (L = O⁻, Cl, CH₃, OCH₃, Cp, NPH₃) have been explored on the singlet, doublet, triplet and quartet potential energy surfaces at the B3LYP/LACVP* level of theory and the results discussed and compared with those of the technetium and rhenium oxo complexes we reported earlier, thereby drawing group trends in the reactions of this important class of oxidation catalysts. In the reactions of MnO₃L (L = O⁻, Cl⁻, CH₃, OCH₃, Cp, NPH₃) with ethylene, it was found that the formation of the dioxylate intermediate along the concerted [3 + 2] addition pathway on the singlet potential energy is favored kinetically and thermodynamically over its formation by a two-step process via the metallaoxetane by [2 + 2] addition. The activation barriers for the formation of the dioxylate and the product stabilities on the singlet PES for the ligands studied are found to follow the order: NPH₃ < Cl⁻ < CH₃O⁻ < Cp < O⁻ < CH₃. On the doublet PES, the activation barriers for the formation of the dioxylate intermediate for the ligands are found to follow the order: CH₃O⁻ < Cl⁻ < Cp < CH₃ while the order of product stabilities is: Cl⁻ < CH₃O⁻ < Cp < CH₃. The order of dioxylate product stabilities on the triplet surface for the ligands studied is: Cl⁻ < CH₃O⁻ < Cp < CH₃ < NPH₃ < O⁻ and the order on the quartet surface is O⁻ < Cp < CH₃ < NPH₃ < Cl⁻ < CH₃O⁻. The re-arrangement of the metallaoxetane intermediate to the dioxylate is not a feasible reaction for all the ligands studied. Of the group VII B metal oxo complexes studied, MnO₄⁻ and MnO₃(OCH₃) appear to be the best catalysts for the exclusive formation of the dioxylate intermediate, MnO₃(OCH₃) being better so on both kinetic and thermodynamic grounds. The best epoxidation catalyst for the Mn complexes is MnO₃Cl; the formation of the epoxide precursor will not result from the reaction of LMnO₃ (L = O⁻, Cp) with ethylene on any of the surfaces studied. The trends observed for the oxidation reactions of the Mn complexes with ethylene compare closely with those reported by us for the ReO₃L and TcO₃L (L = O⁻, Cl, CH₃, OCH₃, Cp, NPH₃) complexes, but there is far greater similarity between the Re and Tc complexes than between Mn and either of the other two. There does not appear to be any singlet–triplet or doublet–quartet spin-crossover in any of the pathways studied.