An Efficient Method for the Preparation of Oxo Molybdenum Salalen Complexes and Their Unusual Use as Hydrosilylation Catalysts

A number of molybdenum(VI) dioxo salalen complexes were prepared from the reaction of Mo(CO)6 and salen ligands containing bulky substituents, providing a novel and facile entry to Mo−salalen compounds. Two of the complexes were characterized by single-crystal X-ray diffraction. Reduction with organic phosphines or silanes afforded the monooxo molybdenum(IV) complexes, along with dinuclear molybdenum(V) species featuring a bridged oxo ligand (μ-O). One of the dinuclear complexes as well as a molybdenum(VI) dioxo salan complex was characterized structurally. All of the molybdenum compounds except the monooxo molybdenum(IV) were fully characterized by NMR, mass spectrometry, and elemental analyses. Investigations of acetophenone and 4-Ph-2-butanone reduction with PhSiH3 showed that all of these molybdenum oxo complexes could serve as catalysts at reasonably low loading (1 mol % Mo) and ∼110 °C. The time profiles and efficacy of catalysis varied depending on the precursor form of the catalyst, MoVI(O)2 vs (O)MoV−O−MoV(O) vs MoIV(O). Solvent effects, radical scavenger probes, and other mechanistic considerations reveal that the monooxo molybdenum(IV) is the most likely active form of the catalyst.