New insight into the catalytic cycle about epoxidation of alkenes by N 2 O over a Mn-substituted Keggin-type polyoxometalate.

Although epoxidation of alkenes by N2 O catalyzed by Mn-substituted polyoxometalates (POMs) has been studied both experimental and theoretical methods, a complete catalytic cycle has not been established currently. In the present paper, density functional theory (DFT) calculations were employed to explore possible reaction mechanism about this catalytic cycle. Our DFT studies reveal that the reaction pathway starts from a low-valent Keggin-type POM aquametal derivative [PW11 O39 MnIII H2 O]4- . In the presence of N2 O pressure, the formation of the active catalytic species [PW11 O39 MnV O]4- involves a ligand-substituted reaction about replacement of the aqua ligand with N2 O to generation of POM/N2 O adduct [PW11 O39 MnIII ON2 ]4- and dissociation of N2 from this adduct. The calculated free energy indicates that the ligand-substituted reaction is endergonic both in gas phase or various solvents. The partial optimization method reveals that the dissociation of N2 from [PW11 O39 MnIII ON2 ]4- involves crossing of the quintet state with a low-lying triplet state. Due to the high reactivity, the high-valent MnV -oxo species, [PW11 O39 MnV O]4- , may react with the excess N2 O and alkenes. Thus, two alternative reaction pathways corresponding to activation of N2 O and epoxidation of alkenes have been considered in this work. The calculated free energy profile indicates that epoxidation of alkenes pathway is the favorable routes. Finally, a complete catalytic cycle for this reaction has been proposed. The rate-determining step in this catalytic cycle is the dissociation of N2 from the low-valent POM/N2 O adduct according to our DFT-M06L calculations.