Molybdenum(VI) Dioxo Complexes Employing Schiff Base Ligands with an Intramolecular Donor for Highly Selective Olefin Epoxidation

Reaction of [MoO2(eta(2)-tBu(2)pz)(2)] with Schiff base ligands HLX (X = 1-5) gave molybdenum(VI) dioxo complexes of the type cis-[MoO2(L-X)(2)] as yellow to light brown solids in moderate to good yields. All ligands coordinate via its phenolic O atom and the imine N atom in a bidentate manner to the metal center. The third donor atom (R-2 = OMe or NMe2) in the side chain in complexes 1-4 is not involved in coordination and remains pendant. This was confirmed by X-ray diffraction analyses of complexes 1 and 3. Complexes 1, 3, and 5 exist as a mixture of two isomers in solution, whereas complexes 2 and 4 with sterically less demanding substituents on the aromatics only show one isomer in solution. All complexes are active catalysts in the epoxidation of various internal and terminal alkenes, and epoxides in moderate to good yields with high selectivities are obtained. In the challenging epoxidation of styrene, complexes 1 and 2 prove to be very active and selective. The selectivity seems to be influenced by the pendant donor arm, as complex 5 without additional donor in the side chain is less selective. Experiments prove that the addition of n-butyl methyl ether as intermolecular donor per se has no influence on the selectivity. The basic conditions induced by the NMe2 groups in complexes 3 and 4 lead to lower activity.