Molecular structure -: Activity relationships for the oxidation of organic compounds using mesoporous silica catalysts derivatised with bis(halogeno)dioxomolybdenum(VI) complexes

Mo K-edge XAFS spectra have been measured for ordered mesoporous silica MCM-41 grafted with the complexes [MoO2X2(thf)(2)] (X = Cl, Br). For grafting reactions in the absence of triethylamine, materials with 1 wt. % Mo are obtained; the Mo K-edge EXAFS results indicate the co-existence of isolated surface-fixed monomeric species {MoO2[(-O)(3)SiO](2)(thf)(n)} and {MoO2[(-O)(3)SiO]X(thf)(n)}. When Et3N is used in the grafting reactions, materials with 4 wt.% Mo are obtained. The EXAFS data for the material prepared using [MoO2Cl2(thf)(2)] and Et3N indicate the presence of dinuclear species with two Mo-VI centres, each with two Mo=O groups and each linked by one or two oxo bridges (Mo...Mo 3.27 Angstrom). The molybdenum centres in the material prepared using the dibromo complex comprise mainly isolated four-coordinate dioxomolybdenum(vi) and trioxomolybdenum(vi) monomeric species, with a small contribution from dimeric species. All materials were further characterised in the solid state by powder X-ray diffraction, N-2 adsorption analysis, MAS NMR (C-13, Si-29) and FTIR spectroscopy. The derivatised MCMs perform differently as catalysts in the liquid-phase oxidation of various olefins and alcohols with tert-butyl hydroperoxide. The highest alkene epoxidation activity was recorded for the catalysts with low metal loading, whereas the material containing oxo-bridged dimers, had the highest activity for oxidation of alcohols. The recyclability of all the catalysts was tested: the catalytic activity of the derivatised materials tended to stabilize with ageing.