Synthesis and characterization of methyltrioxorhenium(VII) immobilized in bipyridyl-functionalized mesoporous silica

Methyltrioxorhenium(VII) (MTO) has been immobilized in the mesoporous silica MCM-41 functionalized with pendant bipyridyl (bpy) groups of the type 4-[(-O)₃Si(CH₂)₄]- 4′-CH₃-2,2′-bipyridine. Powder X-ray diffraction and N₂ adsorption-desorption studies confirm that the regular hexagonal symmetry of the host is retained during the grafting reaction and that the channels remain accessible. Solid-state magic angle spinning NMR spectroscopy (²⁹Si, ¹³C) was also used to monitor the coupling reactions. The formation of a tethered Lewis base adduct of the type CH₃ReO₃·bpy CH3Re03-bpy is confirmed by IR, Raman, and X-ray absorption fine structure (XAFS) spectroscopy. The XAFS results do, however, indicate that not all rhenium ions are present in this form, and this agrees with elemental analysis which gives an Re/N ratio of 1:1.1 (0.55 mmol Re g^-1). It is likely that the excess rhenium atoms are present as uncoordinated MTO molecules assembled in the MCM channels. The model complexes (4-alkyl-4′-methyl-2,2′-bipyridine)(methyl)trioxorhenium (alkyl = methyl, n-butyl) were also prepared and characterized.