Peroxo and alkylperoxidic molybdenum(VI) complexes as intermediates in the epoxidation of olefins by alkyl hydroperoxides

Novel oxoperoxomolybdenum(VI) complexes with the general formula MoO(O2)L2X2 (III, L = DMF, HMPT) and MoO(O2)Cl(ON)L(IV, ON) = pyridin-2-carboxylate (Pic), 8-hydroxyquinolinate (Quin) were prepared from the reaction of Ph3COOH or H2O2 with the corresponding cis-dioxo complexes. In the reaction with Ph3COOH both oxygen atoms of the peroxo moiety were found, by 18O labeling experiments, to come from the hydroperoxide. The X-ray crystal structure of MoO(O2)Cl(Pic)(HMPT) revealed a bipyramidal pentagonal surounding with a rather short OO distance (1.41 Å). Complexes III were found to be more reactive than MoO(O2)2,HMPT for the epoxidation of olefins (oxidative cleavage products are consecutively formed) but react by the same cyclic peroxymetalation mechanism. The absence of reaction in the case of complexes IV illustrates the necessity for the metal to possess an equatorial releasable coordination site adjacent to the peroxo group for the oxygen transfer to occur. Catalytic oxidation of olefins using Ph3COOH gave a selectivity in oxygenated products very different from that using t-BuOOH, and 18O labeling studies showed that alkyl-peroxidic rather than peroxo species are intermediates in this latter reaction. The mechanism of epoxidation of olefins by alkyl hydroperoxides catalyzed by d0 metal complexes is discussed.