The close interaction of a CF bond with a carbonyl π–system: Attractive, repulsive, or both?

• We study a molecule with an unusual interaction between a CF bond and a ketone.
• CO stretching frequency is blue-shifted compared to control molecules.
• Calculations show that the blue-shift is maximized near the equilibrium distance.
• Electron deformation density maps show a σ-hole-like region on the fluorine.

We have synthesized a molecule containing a close interaction between a CF bond and the π–orbitals of a ketone carbonyl group. Our studies have revealed that there is a combination of attractive and repulsive forces at play: the ketone’s IR stretching mode is blue-shifted, the carbonyl is bent away from the fluorine atom, and electron deformation density maps show some significant distortion of the fluorine atom’s electron density distribution. Finally, binding of the ketone to an aluminum-based Lewis acid deshields the fluorine nucleus. IR and NMR spectroscopy, single crystal X-ray crystallography, and quantum mechanical calculations were used to investigate this unusual interaction.

In this report, we investigate a molecule synthesized by our lab that contains an unusual interaction between a CF bond and the π–system of a ketone. It appears that the fluorine atom is both acting as a donor to the electropositive carbon atom and experiencing electron repulsion and steric compression from the oxygen atom in the ketone. Key observations that led to this conclusion include a distortion in the electron density about the fluorine atom and a significant blue-shift in the CO stretch. Calculations indicate that, perhaps not coincidentally, the blue shift is maximized very near the equilibrium CF⋯CO distance.Figure optionsDownload as PowerPoint slide