Mesoporous silica grafted with multiply bonded dimolybdenum cations: XAFS analysis and catalytic activity in cyclopentadiene polymerisationElectronic Supplementary Information available. See http://www.rsc.org/suppdata/cp/b1/b108320a/

Molybdenum K-edge X-ray absorption fine structure (XAFS) spectroscopy has been used to characterise the local structure around the metal centres in mesoporous silica MCM-41 derivatised with the complex salts [Mo2(µ-O2CCH3)2(CH3CN)6](BF4)2, [Mo2(µ-CH3CONH)2(CH3CN)4](BF4)2, [Mo2(CH3CN)10](BF4)4 and [Mo2(µ-O2CCH3)2(dppa)2(CH3CN)2](BF4)2. Analysis of the EXAFS reveals that the molybdenum–molybdenum quadruple bond is retained in the grafted materials and that the equatorially coordinated ligands are not displaced. The results support the assumption that the complexes are immobilised on the silica surface by an interaction with silanol groups in axial positions. Consistent with these findings, catalysis tests show that the activities of the supported dimolybdenum complexes as initiators for the polymerisation of cyclopentadiene are comparable with those observed for the free complexes in the homogeneous phase. The XAFS results also reveal that air-oxidation of the supported catalysts leads to the rapid formation of surface-fixed dioxo-bridged binuclear species, characterised by a molybdenum–molybdenum interatomic distance of 2.56 ± 0.02 Å.