Molybdenum(vi) catalysts obtained from η3-allyl dicarbonyl precursors: Synthesis, characterization and catalytic performance in cyclooctene epoxidation

Abstract

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 tert-butylhydroperoxide (TBHP) or H₂O₂ gave the following compounds in good to excellent yields: the oxo-bridged 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).