Preparation and catalytic properties of a new dioxomolybdenum(VI) complex covalently anchored to mesoporous MCM-48

Reaction of the solvent adduct MoO₂Cl₂(THF)₂ with the 1,4-diazabutadiene ligands RNC(Ph)C(Ph)NR R = (CH₂)₂CH₃, (CH₂)₃Si(OMe)₃] leads to complexes of the type MoO₂Cl₂(LL) in good yields at room temperature within a few minutes. The complex bearing the trimethoxysilyl groups was immobilized in the ordered mesoporous silica MCM-48 by covalent bonding. Solid state MAS NMR spectroscopy (¹³C, ²⁹Si) confirms that the structural integrity of the complex was retained during immobilization, except for loss of methoxide groups due to the reaction with surface silanols. Powder X-ray diffraction (XRD) and N₂ adsorption studies of the derivatized material indicate that the textural properties of the support were preserved during the grafting experiment and that the channels remained accessible. The modified material is active and selective in the epoxidation of cyclooctene at 55 °C using tert-butyl hydroperoxide as the oxidant. Stability was checked by recycling the solid catalyst several times. Some activity is lost from the first to second runs, but thereafter stabilizes. The catalytic behavior of the heterogenized catalyst was also compared with that observed in homogeneous phase for the complex with R=(CH₂)₂CH₃.