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

Reaction of [MoO2(η2-tBu2pz)2] with Schiff base ligands
HLX (X = 1–5) gave molybdenum­(VI) dioxo complexes
of the type cis-[MoO2(LX)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 (R2 = 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.