Characterization with 95Mo, 17O, 1H NMR and EPR of alkylperoxo, alkoxo, peroxo and diolo molybdenum (VI) complexes formed in the course of catalytic epoxidation of cyclohexene with organic hydroperoxides

Abstract

Using 95Mo, 1H, 17O NMR and EPR, we have characterized molybdenum (VI) complexes formed in the course of catalytic epoxidation of cyclohexene with tert-butyl and cumene hydroperoxides. Dioxobis (acetylacetonato) molybdenum (VI) and hexacarbonyl molybdenum (0) were used as starting materials for the catalysts. Six new molybdenum (VI) complexes ( I–VI) were detected. Complexes I, II and III are formed when ROOH is added to solutions of MoO 2(acac) 2 in pure benzene. In benzene:cyclohexene:hydroperoxide mixtures complexes I–III are observed only at the initial stage of the epoxidation process. Then they are gradually replaced by complexes IV–VI that drive the catalytic cycle of epoxidation. NMR spectra of complexes IV–VI are the same, irrespective of whether MoO 2(acac) 2 or Mo(CO) 6 have been used as the starting material for the catalysts. On the basis of the 95Mo, 1H, 17O NMR, EPR spectra and reactivity studies, the following compositions for I–VI are suggested: I is the alkylperoxo complex MoO 2(OOR) 2, II the alkoxo complex MoO 2(OR) 2, III the monoperoxo complex MoO(O 2) (acac) 2, IV and V are two types of MoO 2(1,2-diolo) 2 complexes (where 1,2-diolo is trans-cyclohexane-1,2-diol), and VI the alkylperoxo complex MoO 2(1,2-diolo) (OOR). The latter complex serves as the active particle of epoxidation.