The fluorenyl cation is a very unstable 4π-antiaromatic compound that has a lifespan of only a few picoseconds in protic solvents. Previous attempts at synthesizing it have been largely unsuccessful.

Wolfram Sander, Ruhr-Universität Bochum, Germany, Elsa Sanchez-Garcia, Max Planck Institute for Coal Research, Mülheim an der Ruhr, Germany, and co-workers have isolated the cation in a matrix of low-density amorphous (LDA) water ice at temperatures below 30 K. Using photolysis of diazofluorene, they generated the carbene fluorenylidene, which is protonated to give the fluorenyl cation.

The researchers characterized the product by IR and visible-light spectroscopy. The experimental spectra were in almost perfect agreement with QM/MM simulations of the fluorenyl cation in an environment of water molecules. Using deuterated water (D₂O) and observing isotopic shifts in the IR spectra, the team was able to confirm that protons were incorporated into the product, thereby further supporting the assignment.

• The Fluorenyl Cation,
  Paolo Costa, Iris Trosien, Miguel Fernandez-Oliva, Elsa Sanchez-Garcia, Wolfram Sander,
  Angew. Chem. Int. Ed. 2015.
  DOI: 10.1002/anie.201411234