Spectroscopic Characterization of a [C−F−C]⁺ Fluoronium Ion in Solution

Abstract

We report the first spectroscopic evidence for a [C-F-C] fluoronium ion in solution. Extensive NMR studies ( F, H, C) characterize a symmetric cage-like species in which fluorine exhibits substantial covalent bonding to each of the two carbon atoms involved in the three-center interaction. Experimental NMR data comport well with calculated values to lend credence to the structural assignment. As the culminating experiment, a Saunders isotopic perturbation test confirmed the symmetric structure. Congruent with the trend in other types of onium ions, the calculated charge at fluorine moves in a more positive (less negative) direction from the neutral. It is this important trend that explains in part the extraordinary historical difficulty in making theoretical predictions of fluoronium ions come true in solution, and why it takes fluorine captured in a cage to produce, finally, a stable ion and complete the historical arc of the organic halonium ion story.

Quest for a Symmetric [C–F–C]+ Fluoronium Ion in Solution: A Winding Path to Ultimate Success

“…The 19 F NMR signal appears as a triplet of pentets, consistent with strong geminal coupling and remote "W-coupling." The 1 H NMR spectrum shows a highly diagnostic downfield-shifted doublet (2H, 8.11 ppm) representing the expected reciprocal geminal coupling to 19 F. 27 On the other hand, we were alarmed upon seeing the 13 C NMR spectrum. Instead of the expected C 2v symmetry of the desired ion, the observed species revealed extra signals consistent with overall C s symmetry.…”

“…−30 °C. The 19 F NMR signal appears as a triplet of pentets, consistent with strong geminal coupling and remote “W-coupling.” The 1 H NMR spectrum shows a highly diagnostic downfield-shifted doublet (2H, 8.11 ppm) representing the expected reciprocal geminal coupling to 19 F …”

Catalyzed and Promoted Aliphatic Fluorination

“…For example, we were interested in developing a selective fluorination in order to obtain a suitable precursor for the generation of a symmetrical fluoronium ion in solution (Scheme ). Although this particular transformation never practically came to fruition, it helped send us on our way.…”

Noncovalent Interactions of Fluorine with Amide and CH2 Groups in N-Phenyl γ-Lactams: Covalently Identical Fluorine Atoms in Nonequivalent Chemical Environments

“…Such a signal shifting was well-documented for F nuclei that participated in close contacts with C–H groups, regardless of the interactions tracking weak hydrogen bonding , or steric deshielding mechanisms. , 19 F NMR signals reflect the behavior of its p-electrons, and the deshielding effects imply that the p-electron cloud in F is either shared or restricted . Stronger interactions between F and CH 2 group led to larger 19 F downfield shift; however, such contacts may not be entirely attractive interactions. , …”