Molybdenum(VI) Catalysts obtained from eta3-Allyl Dicarbonyl Precursors: Synthesis, Characterization and Catalytic Performance in Cyclooctene Epoxidation

The oxidative decarbonylation of the η3-allyl dicarbonyl complexes [Mo(η3-C3H5)Cl(CO)2(L)] (L = 2,2′- bipyridine (bipy) (1), 4,4′-di-tert-butyl-2,2′-bipyridine (di-tBu-bipy) (2)) by reaction with aqueous tertbutylhydroperoxide (TBHP) or H2O2 gave the following compounds in good to excellent yields: the oxobridged dimers [MoO2Cl(L)]2O (L = bipy (3), di-tBu-bipy (6)) using TBHP(10 equiv.)/CH3CN/r.t.; the molybdenum oxide/bipyridine hybrid material {[MoO3(bipy)][MoO3(H2O)]}n (4) and the octanuclear complex [Mo8O24(di-tBu-bipy)4] (7) using TBHP(50 equiv.)/H2O/70 °C; the oxodiperoxo complexes MoO(O2)2(L) (L = bipy (5), di-tBu-bipy (8)) using H2O2(10 equiv.)/CH3CN/r.t. The structure of 7·x (solvent) (where solvent = CH2Cl2 and/or diethyl ether) was determined by single crystal X-ray diffraction. Despite possessing the same windmill-type complex as that described previously for 7·10CH2Cl2, the crystal structure of 7·x(solvent) is unique due to differences in the crystal packing. Compounds 1–8 were examined as catalysts or catalyst precursors for the epoxidation of cyclooctene using aqueous TBHP or H2O2 as oxidant at 55 or 70 °C. Reactions were performed without co-solvent or with the addition of water, ethanol or acetonitrile. Cyclooctene oxide was always the only reaction product. Solids recovered after 24 h reaction at 70 °C were identified by FT-IR spectroscopy as the hybrid 4 from (1,3–5)/TBHP, complex 5 from (1,3–5)/H2O2, and complex 8 from (2,6–8)/H2O2. With TBHP as oxidant, the highest epoxide yields (for 24 h reaction at 70 °C) were obtained using excess H2O as solvent (28–38% for 1,3–5; 87–98% for 2,6–8), while with H2O2 as oxidant, the highest epoxide yields were obtained using CH3CN as solvent (54–81% for 3–8).