Molybdenum η3-Allyl Dicarbonyl Complexes as a New Class of Precursors for Highly Reactive Epoxidation Catalysts with tert-Butyl Hydroperoxide

New Mo(II) diimine derivatives of [Mo(η3-allyl)X(CO)2(CH3CN)2] (allyl = C3H5 and C5H5O; X = Cl, Br) were prepared, and [Mo(η3-C3H5)Cl(CO)2(BIAN)] (BIAN = 1,4-(4-chloro)phenyl-2,3-naphthalenediazabutadiene) (7) was structurally characterized by single-crystal X-ray diffraction. This complex adopted an equatorial−axial arrangement of the bidentate ligand (axial isomer), in contrast with the precursors, found as the equatorial isomer in the solid and fluxional in solution. The new complexes of the type [Mo(η3-allyl)X(CO)2(N−N)] (N−N is a bidentate chelating dinitrogen ligand) were tested for the catalytic epoxidation of cyclooctene using tert-butyl hydroperoxide as oxidant. All catalytic systems were 100% selective toward epoxide formation. While their turnover frequencies paralleled those of related Mo(II) carbonyl compounds or Mo(VI) compounds bearing similar N-donor ligands, they exhibited similar olefin conversions in consecutive catalytic runs. The acetonitrile precursors were generally more active than the diimine complexes, and the chloro derivatives more active than the bromo ones. Combined vibrational and NMR spectroscopy and computational studies (DFT) were used to investigate the nature of the molybdenum species formed in the catalytic system with [Mo(η3-C3H5)Cl(CO)2{1,4-(2,6-dimethyl)phenyl-2,3-dimethyldiazabutadiene}] (4) and to propose that the resulting species may be dimeric bearing oxide bridges.