Structural proof of a [C-F-C]+ fluoronium cation

Abstract

Organic fluoronium ions can be described as positively charged molecules in which the most electronegative and least polarizable element fluorine engages in two partially covalent bonding interactions to two carbon centers. While recent solvolysis experiments and NMR spectroscopic studies on a metastable [C-F-C]⁺ fluoronium ion strongly support the divalent fluoronium structure over the alternative rapidly equilibrating classical carbocation, the model system has, to date, eluded crystallographic analysis to confirm this phenomenon in the solid state. Herein, we report the single crystal structure of a symmetrical [C-F-C]⁺ fluoronium cation. Besides its synthesis and crystallographic characterization as the [Sb₂F₁₁]^- salt, vibrational spectra are discussed and a detailed analysis concerning the nature of the bonding situation in this fluoronium ion and its heavier halonium homologues is performed, which provides detailed insights on this molecular structure.

Fig. 1. Overview of fluoronium ions in the condensed phase.

a Crystallographically characterized fluoronium ions^,,–. Note that the formal charges shown inside a circle do not represent the actual charge of the corresponding atoms. b Synthesis of the fluoronium salt [1][Sb₂F₁₁].

Fig. 2. Molecular structure of the fluoronium ion 1 as its [Sb₂F₁₁]⋅(SO₂ClF)₃ salt in the solid state.

Anion and solvent molecules are not depicted. Thermal ellipsoids set to 50% probability. Selected bond lengths [pm] and angles [°]: F1–C1 156.6(3), F1–C2 158.7(3), C1–F1–C2 115.64(17), O2–C14–O1–Sb1 19.0(4).

Fig. 3. Vibrational analysis of fluoronium 1.

Left: Experimental infrared spectra of [1][Sb₂F₁₁] (black) and [Cs][Sb₂F₁₁] (blue) at −40 °C, as well as calculated spectra of cation 1 (red) and anion [Sb₂F₁₁]⁻ (green) at B3LYP/def2-TZVPP level of theory. Bands of the anion are denoted with an asterisk in the experimental spectrum and bands associated with the carbonyl group are denoted with a dagger. Right: Approximate representation and assignment of selected C–F–C normal modes of 1 (see text, only displacements involving the C–F–C unit are shown).

Fig. 4. Electron localization in Fluoro- and Chloronium.

Electron localization function in the C–X–C plane (X = F, Cl) of 1 (left) and its chloronium analog (right) and in their C–O–C planes containing the halogen’s lone pairs, perpendicular to the former one. Both planes are merged at the molecule’s O–X axis (dashed red line). ELF is defined from 0.0 (white) to 1.0 (red); contours are drawn in intervals of 0.1.