New insight into the catalytic cycle about epoxidation of alkenes by N2O over a Mn–substituted Keggin-type polyoxometalate

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Although epoxidation of alkenes by N2O 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 [PW11O39MnIIIH2O]4−. In the presence of N2O pressure, the formation of the active catalytic species [PW11O39MnVO]4− involves a ligand–substituted reaction about replacement of the aqua ligand with N2O to generation of POM/N2O adduct [PW11O39MnIIION2]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 [PW11O39MnIIION2]4− involves crossing of the quintet state with a low-lying triplet state. Due to the high reactivity, the high–valent MnV–oxo species, [PW11O39MnVO]4−, may react with the excess N2O and alkenes. Thus, two alternative reaction pathways corresponding to activation of N2O 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/N2O adduct according to our DFT–M06L calculations.