Immobilisation of methyltrioxorhenium on functionalised MCM-41

Methyltrioxorhenium (MTO) was immobilised in the mesoporous silica MCM-41 functionalised with a pyrazolylpyridine ligand. Elemental analysis indicated that the rhenium loading in the final material (3.6 wt.%, 0.19 mmol g ^-1) was about 35% of the ligand content. Powder X-ray diffraction confirmed that the regular hexagonal symmetry of the host was preserved during the grafting experiments. The functionalised material was also characterised by ¹³C and ²⁹Si magic-angle spinning NMR, FTIR and Raman spectroscopy. Analysis of the vibrational spectra was supported by preparing a model complex containing the ligand ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate and by carrying out ab initio calculations. The results indicate that bidentate and monodentate coordination of the N-base ligand to the rhenium centre coexist for the model complex in the solid state. In solution, ¹⁷O-NMR spectroscopy for the model complex indicates a dynamic behaviour without a solely bidentate coordination even at low temperature. The coordination seems to be relatively weak, even for a monodentate adduct. For MTO immobilised in MCM-41 functionalised with the pyrazolylpyridine ligand, monodentate coordination of the pyridyl nitrogen atom to the metal centre was also found to be the major form present.