A theoretical study of the mechanisms of oxidation ofethylene by manganese oxo complexes†

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Royal Society of Chemistry
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 potentialenergy surfaces at the B3LYP/LACVP* level of theory and the results discussed and compared with thoseof the technetium and rhenium oxo complexes we reported earlier, thereby drawing group trends in thereactions 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 con-certed [3 + 2] addition pathway on the singlet potential energy is favored kinetically and thermodynami-cally over its formation by a two-step processviathe metallaoxetane by [2 + 2] addition. The activationbarriers for the formation of the dioxylate and the product stabilities on the singlet PES for the ligandsstudied are found to follow the order: NPH3<Cl−<CH3O−<Cp<O−<CH3. On the doublet PES, the acti-vation barriers for the formation of the dioxylate intermediate for the ligands are found to follow theorder: CH3O−<Cl−<Cp<CH3while the order of product stabilities is: Cl−<CH3O−<Cp<CH3. The orderof 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-arrange-ment of the metallaoxetane intermediate to the dioxylate is not a feasible reaction for all the ligandsstudied. Of the group VII B metal oxo complexes studied, MnO4−and MnO3(OCH3) appear to be the bestcatalysts for the exclusive formation of the dioxylate intermediate, MnO3(OCH3) being better so on bothkinetic and thermodynamic grounds. The best epoxidation catalyst for the Mn complexes is MnO3Cl; theformation of the epoxide precursor will not result from the reaction of LMnO3(L = O−, Cp) with ethyleneon any of the surfaces studied. The trends observed for the oxidation reactions of the Mn complexeswith 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 betweenMn and either of the other two. There does not appear to be any singlet–triplet or doublet–quartetspin-crossover in any of the pathways studied.
An article published by Royal Society of Chemistry and also available at DOI: 10.1039/c3dt51700d
DaltonTrans., 2013,42, 14411