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 BF4- 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 BF4- 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.

Graphical abstractThe 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. We investigated the nature of bonding of 1 on silica with X-ray photoelectron spectroscopy, B-11 solid state NMR, Mössbauer spectroscopy and diffuse reflectance infrared spectroscopy. The combined data suggests fluorination of the silica surface by the BF4- anion.Figure optionsDownload full-size imageDownload as PowerPoint slide