MCM-41 functionalized with bipyridyl groups and its use as a support for oxomolybdenum(vi) catalysts

Abstract

The ordered mesoporous silica MCM-41 was covalently grafted with (3-chloropropyl)trimethoxysilane. Chloro substitution by the anion [4-CH2-4′-Me-2,2′-bipyridine]− gave a ligand-silica containing ca. 0.3 mmol bipyridyl groups per gram. Powder X-ray diffraction and nitrogen adsorption–desorption analysis demonstrated that the textural characteristics of the support were preserved during the grafting experiments and that the channels remained accessible, despite sequential reductions in surface area, pore volume and pore size. The coupling reactions were monitored by 29Si MAS NMR and 13C CP MAS NMR spectroscopy. Bipyridyl-functionalized MCM-41 exhibits a high encapsulating ability, as evidenced by its interaction with a dichloromethane solution of MoO2Cl2(THF)2. A material with a metal loading of 8.3 mass% was obtained. Molybdenum K-edge EXAFS analysis could not substantiate the formation of a tethered complex of the type MoO2Cl2(N–N), but instead indicated the formation of unidentate-bridged entities of the type [O2Mo–X–MoO2] with a metal–metal separation of 3.28 Å. The molybdenum-containing MCM was active as a catalyst for the epoxidation of cyclooctene by tert-butyl hydroperoxide. However, this activity is due, at least in part, to leached molybdenum species in solution.