Quantum chemical studies of the addition of transition metal oxides of the type LMO3(L = O- , Cl-, NPH3, CH3, OCH3, Cp) to ethylene
The oxo complexes of group VII (Mn, Tc, and Re) are of great interest for their potential toward epoxidation and dihydroxylation. In this work, the mechanisms of oxidation of ethylene by the group VII transition metal-oxo complexes of the type LMO3 (M= Mn, Tc, Re and L= O-, Cl, CH3, OCH3, Cp) are explored at the B3LYP/LACVP* level of theory. The activation and reaction energies for the stepwise and concerted addition pathways along spin states other than the singlet which could ultimately lead to exploration of the extent of the two state reactivity was also investigated. In the reaction of LMnO3 (L= Cp, CH3), LTcO3 (L= O- , Cl, Cp, OCH3) and LReO3 (L= Cp, O-) with ethylene, it was found that the direct [3+2] addition pathway on the doublet potential energy surfaces leading to the formation of the dioxylate intermediate is favored kinetically and thermodynamically over the two-step process via the metallaoxetane intermediate to form the dioxylate. However, in the CH3TcO3 system, the formation of the doublet epoxide precursor is kinetically the most favorable pathway. The [2+2] addition pathway leading to the formation of the four membered metallacycle was found to be the kinetically and thermodynamically most viable reaction path for the NPH3TcO3. The formation of the four membered metallacycle intermediate is kinetically and thermodynamically more favorable than the direct [3+2] addition pathway for LReO3 (L= CH3, OCH3, Cl, NPH3) on the doublet surface. However, in the ReO3(OCH3) system, the reaction occurs on the doublet potential energy surface. The activation energy for the formation of the dioxylate via the metallaoxetane intermediate and the direct [3+2] addition of the ethylene to the oxo-complex were found to be comparable along the doublet potential energy surface. The formation of the epoxide precursor will not result from the reaction of LMO3 (M= Mn, Tc, Re, L= O- , Cp) with ethylene on all the surfaces studied.
A Thesis submitted to the School of Graduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirements for the Degree of Master of Science in Physical Chemistry, June, 2012