Bonding of the iron Lewis acid-THF adduct CpFe(CO)2 · THF to silica gel and its implication for cyclopropanation reactions

The adsorption of the iron Lewis acid-THF adduct CpFe(CO)2 · THF (1) onto the silica gel has been observed to dramatically alter the cis :trans ratio for cyclopropanation reactions versus the homogeneous catalyzed reactions. To better understand this dramatic change in selectivity, we investigated the nature of bonding of 1 on silica with a number of analytical techniques. X-ray photoelectron spectroscopy showed the presence of a new peak at 687.7 eV for the silica-supported catalyst, which indicated possible fluorination by the View the MathML sourceBF4- anion. Further experiments using solid state NMR showed that a new boron species was also generated by the adsorption onto the silica gel. Mössbauer spectroscopy showed that adsorption of the iron Lewis acid-THF adduct onto the silica gel did not change the oxidation state to the iron; however, diffuse reflectance infrared spectroscopy showed the loss of surface hydroxyl groups and a shift in one of the C–O absorptions to higher wave numbers. The combined data suggest fluorination of the silica surface by the View the MathML sourceBF4- anion. This theory was tested by adsorption of the iron Lewis acid-THF adduct onto a polytrimethyl hydrosilylsilicate resin and sodium perchlorate treated silica. Analysis showed that both fluorination and physical adsorption of the catalysts occur, although fluorination was found to predominate for binding.