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

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