Fluorenyl cation trapped

22 February 2015 - Organic physical chemistry

Fluorene is a typical aromatic compound and so is it's anion. The fluorenyl cation on the other hand is antiaromatic and any attempt to synthesising it would seem a futile exercise. Where some of the giants in chemistry like Olah and Schleyer tried anyway and failed some 30 years ago actual synthesis now seems in the bag although with the usual caveat that is concerns low temperature matrix isolation. Costa et al. (DOI) report that it can be made by photolysis of diazofluorene in an argon matrix in presence of a substance called amorphous ice. At 3 kelvin the first reaction product is the carbene fluorenylidene and after heating it up to 50K this carbene has reacted with water to the cation. This identification is based on IR spectroscopy and supported by theoretical calculations. The reaction of the cation with water is highly exothermic but at these sort of temperatures that reaction is at a standstill. According to Costa the cation is also stabilized because the hydroxyl counteranion is allowed to diffuse away. After further heating the ultimate product is fluorenol.