Dioxomolybdenum( VI )‐Modified Mesoporous MCM‐41 and MCM‐48 Materials for the Catalytic Epoxidation of Olefins

The dioxomolybdenum(VI) complex [MoO2Cl2(L‐L)], containing the bidentate 1,4‐diazabutadiene ligand RN=C(Ph)−C(Ph)=NR [R = (CH2)3Si(OEt)3], was immobilised in the ordered mesoporous silica MCM‐41 by carrying out a grafting reaction in dichloromethane. The grafted material (3.3 wt.‐% Mo) was tested as a catalyst for the epoxidation of cyclooctene with tert‐butyl hydroperoxide (TBHP) at 55 °C. Selectivity to the epoxide was very high and the observed kinetic profile was similar to that of the complex [MoO2Cl2(L‐L)] in the homogeneous phase. On recycling several times, some activity was lost from the first to second runs, but thereafter stabilised. Tethered complexes of this type were also prepared by a stepwise approach. The mesoporous silicas MCM‐41 and MCM‐48 were first treated with a toluene solution of the ligand L‐L under reflux. Before isolating the grafted materials, the silylating agent Me3SiCl was added to remove the residual Si−OH groups on the surface of the mesoporous materials. Oxomolybdenum species were subsequently introduced into the ligand‐silicas by pore volume impregnation of a solution of the complex [MoO2Cl2(THF)2] in CH2Cl2 at room temperature. The modified materials have been characterised by powder X‐ray diffraction, N2 adsorption, and solid‐state MAS NMR (13C, 29Si). Despite having lower metal loadings (1 wt.‐% Mo), the materials exhibited similar catalytic activities for the epoxidation of cyclooctene to that of the modified MCM‐41 with 3.3 wt.‐% of Mo. However, selectivities to the epoxide were lower. The MCM supported catalysts have also been tested for the epoxidation of cyclododecene and α‐pinene with TBHP. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)