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 (mu-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 similar to 110 degrees C. The time profiles and efficacy of catalysis varied depending on the precursor form of the catalyst, Mo-VI(O)(2) vs (O)Mo-V-O-Mo-V(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.