Structural studies of mono- and dimetallic MoVI 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 kappa(2)-N,O (V) and neutral kappa(2)-N,N (VI), have been prepared and fully characterised by NMR spectroscopy and single-crystal X-ray diffraction studies, Dimetallic dioxo(mu-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). ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).