Structural Studies of Mono‐ and Dimetallic Mo VI Complexes − A New Mechanistic Contribution in Catalytic Olefin Epoxidation Provided by Oxazoline Ligands

New seven‐coordinate oxo(peroxo)molybdenum( VI ) complexes with bidentate chiral oxazolines, anionic κ 2 ‐ N , O ( V ) and neutral κ 2 ‐ N , N ( VI ), have been prepared and fully characterised by NMR spectroscopy and single‐crystal X‐ray diffraction studies. Dimetallic dioxo(μ‐oxo)molybdenum( VI ) compounds containing oxazolinylpyridine ligands II − IV have also been synthesised. NMR studies showed the presence of several isomers (up to six species) due to the Mo−O−Mo bridge angle and the unsymmetrical nature of the N,N ligand. The roles of mono‐ ( I , V and VI ) and dimetallic ( II ) oxomolybdenum( VI ) complexes in the catalytic epoxidation of cyclooctene, norbornene and ( R )‐limonene have been studied. The high activity of the oxomono(peroxo) precursor V containing the hemilabile oxazolinylphenolate, in contrast to the unexpected low activity for the oxobis(peroxo) species VI , could be justified by the inertness observed for the oxazolinylpyridine ligand. In addition, the dimetallic system II afforded high selectivity towards limonene epoxide formation ( trans / cis ‐ 10 ratio up to 9:1). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)