A comparative DFT study on the mechanism of olefin epoxidation catalyzed by substituted binuclear peroxotungstates ([SeO₄WO(O₂)₂MO(O₂)₂]ⁿ⁻ (M = Ti^IV, V^V, Ta^V, Mo^VI, W^VI, Tc^VII, and Re^VII))

Abstract

The selective epoxidation of olefins catalyzed by substituted binuclear peroxotungstates ([SeO 4 WO(O 2 ) 2 MO(O 2 ) 2 ] n2 (M 5 Ti IV , V V , Ta V , Mo VI , W VI , Tc VII , and Re VII )) are investigated at the density functional theory level. The computational results reveal that the activation barrier corresponding to the oxygen transfer to the ethylene step decreases with M 5 V > Ti > Ta > Mo > W > Tc > Re. The Re and Tc substituted species can effectively improve the catalytic activity with lower Gibbs free energy barriers of 22.53 and 25.82 kcal/mol relative to the others under normal conditions. This suggests that Re and Tc center peroxo complexes would improve the catalytic performance. The higher activity of the substituted species is directly attributed to the lower energy of the r*(OAO) orbital. The reaction barriers in epoxidation process are rationalized by analyzing the atomic charge, the OAO bond length, and the interaction between the substituted metal and the peroxo group of the precursor complexes.

Alkene Epoxidation Catalyzed by Ti-Containing Polyoxometalates: Unprecedented β-Oxygen Transfer Mechanism

“…Then, the more reactive hydroperoxo species transfers the electrophilic α-oxygen to the alkene, yielding the epoxide. Theoretical studies have also been reported for H 2 O 2 -based epoxidations by other type of POMs: lacunary polyoxotungstates, Se-containing dinuclear peroxotungtates, and divanadium-substituted polyoxotungstates . For the latter two systems, the peroxo group is identified as the active species, , whereas for lacunary polyoxotungstates, both hydroperoxo- and peroxo-type mechanisms have been proposed.…”

“…Theoretical studies have also been reported for H 2 O 2 -based epoxidations by other type of POMs: lacunary polyoxotungstates, Se-containing dinuclear peroxotungtates, and divanadium-substituted polyoxotungstates . For the latter two systems, the peroxo group is identified as the active species, , whereas for lacunary polyoxotungstates, both hydroperoxo- and peroxo-type mechanisms have been proposed. We showed that the nature of the substituted metal has a significant effect on the mechanism …”