A sterically demanding molybdenum(VI) dioxo complex was found to catalytically activate molecular oxygen and to transfer its oxygen atoms to phosphines. Intermediate peroxo as well as reduced mono-oxo complexes were isolated and fully characterized. Monomeric Mo(IV) monooxo species proved to be of an unusual nature with the coordinated phosphine trans to the oxo group. The reduced molybdenum centers can activate O₂ to form a stable Mo(VI) oxo–peroxo complex unambiguously characterized by single crystal X-ray diffraction analysis. NMR experiments demonstrate that both oxygen atoms of the peroxo unit are transferred to an accepting substrate, generating the Mo(IV) intermediate and restarting the catalytic cycle.

Molybdenum(IV) monooxo compound that contains bis(β-ketiminato) ligands activates molecular oxygen forming a molybdenum(VI) monooxo peroxo compound, representing a new entry into molybdenum peroxo derivatives.

The preparation of the tridentate phenol based amine ligands HL1–HL4 is achieved via a convenient one-pot synthesis by reductive amination in quantitative yield in an autoclave under 7 bar H₂ gas. Reaction of [MoO₂(acac)₂] and the corresponding ligand HLX (X = 1, 2 and 4) in methanol–H₂O results in the formation of orange to red dimeric μ-oxo bridged [{MoO₂(LX)}₂(μ-O)] (X = 1, 2 and 4) complexes 1–3 in high yield and high purity. Complexes 1–3 are stable towards air and water. Both ligands coordinate via the phenolic O atom, the amine N atom and the third donor atom in the side chain (OMe for 1 and NMe₂ for 2 and 3) in a fac mode to the metal center. The molybdenum atoms are linked by a bridging μ-oxo moiety to each other as confirmed by X-ray diffraction analyses of complexes 2 and 3. All complexes have been tested in the epoxidation of several internal and terminal alkenes using TBHP as an oxidant. Depending on the nature of the substrate, the epoxides are obtained in moderate to good yields and high selectivities. In the epoxidation of cyclooctene a TOF = 467 h⁻¹ with complex 1 has been observed, significantly higher compared to other dimeric complexes reported in the literature. In the more challenging epoxidation of styrene, complexes 1 and 2 have proven to be highly selective as only the formation of styrene oxide is observed. The OMe based complex 1 has also proven to be more active than the NMe₂ based counterparts 2 and 3. The basic conditions induced by the NMe₂ groups in complexes 2 and 3 lower their catalytic activity.