Mononuclear and Binuclear Cyclopentadienyl Oxo Molybdenum and Tungsten Complexes: Syntheses and Applications in Olefin Epoxidation Catalysis

New half-sandwich molybdenum complexes [Mo(CpBz)Cl 2 (O)] ( 1 ), [{Mo(CpBz)(O) 2 } 2 (μ-Ο)] ( 5 ), and [Mo(CpBz)Cl 2 ] 2 ( 6 ) (CpBz = C 5 (CH 2 Ph) 5 ), the tungsten derivative [W(CpBz)Cl(O) 2 ] ( 3 ), and a high yield synthesis of [Mo(CpBz)Cl(O) 2 ] ( 2 ) are described. The molecular structures and cyclic voltammetry measurements of 1 and 2 and a comparative study of various molybdenum dioxo complexes as olefin epoxidation catalyst precursors are also presented. The performance of [Mo(CpPr i 4 )Cl(O) 2 ] ( 9 ) as cyclooctene epoxidation catalyst using TBHP as oxidant was determined and compared with other [MoCp‘Cl(O) 2 ] complexes (Cp‘ = C 5 H 5 ( 7 ), C 5 Me 5 ( 8 ), CpBz ( 2 )) to assess the activity dependence on the ring substituents. Under the same experimental conditions, bimetallic compounds [{MoCp‘(O) 2 } 2 (μ-Ο)] (Cp‘ = CpBz ( 5 ), Cp* ( 10 ), CpBu t 3 ( 11 ), CpPr i 4 ( 12 )) have been tested and [CpMoO 2 ] 2 O was checked in aqueous solution, with H 2 O 2 and TBHP as oxidant. Finally, [Mo(CpBz)(CO) 3 CH 3 ] ( 4 ) was used as a catalyst precursor for cyclooctene epoxidation in the presence of TBHP. The analogy between the behaviors displayed by this complex and complex 2 is discussed.