Investigation of Molybdenum Tetracarbonyl Complexes As Precursors to Mo^VI Catalysts for the Epoxidation of Olefins

The one-dimensional organic−inorganic hybrid material [MoO3(bipy)] (3) (bipy = 2,2′-bipyridine) is obtained rapidly and in quantitative yield by the reaction of the complex cis-[Mo(CO)4(bipy)] (1) with excess tert-butylhydroperoxide (TBHP) in n-decane/dichloromethane at room temperature. A similar oxidative decarbonylation of the complex cis-[Mo(CO)4(di-t-Bu-bipy)] (2) (di-t-Bu-bipy = 4,4′-di-tert-butyl-2,2′-bipyridine) leads to the isolation of the polynuclear complex [Mo8O24(di-t-Bu-bipy)4] (4). The structure of 4, as the CH2Cl2 solvate, was determined by X-ray crystallography. The unit cell contains two crystallographically independent octameric windmill-type complexes, formulated as [Mo8O24(di-t-Bu-bipy)4], both of which contain a central cubane-type Mo4(μ3-O)4 core. Four peripheral [MoO2(di-t-Bu-bipy)]2+ units cap the long edges of the Mo4 tetrahedron of the central cubane. The close packing of these complexes via weak offset π−π contacts involving the organic ligands leads to a structure having large channels (occupied by solvent molecules) running in various directions of the unit cell. Compounds 3 and 4 can be used as the basis for active catalytic systems for the liquid-phase epoxidation of cis-cyclooctene with TBHP as the oxidant, giving the corresponding epoxide as the only product. Notably higher activities, with no change in selectivity, are possible by using microwave-assisted heating instead of conventional oil bath heating and/or by increasing the reaction temperature from 55 °C to 75 °C. The excellent stability of these MoVI catalytic systems was confirmed by carrying out six consecutive reaction runs at 75 °C under microwave-assisted heating. The stable parent carbonyls (1 and 2) can be used as catalyst precursors since they are transformed into 3 and 4 under the operating catalytic conditions.