Isolable fluorinated triphenylmethyl cation salts of [HCB11Cl11]-: demonstration of remarkable hydride affinity

Abstract

Significantly fluorinated triarylmethyl cations have long attracted attention as potentially accessible highly reactive carbocations, but their isolation in a convenient form has proved elusive. We show that abstraction of chloride with a cationic silylium reagent leads to the facile formation of di-, tetra-, and hexafluorinated trityl cations, which could be isolated as analytically pure salts with the [HCB₁₁Cl₁₁]^- counterion and are compatible with (halo)arene solvents. The F₆Tr⁺ cation carrying six meta-F substituents was computationally predicted to possess up to 20% higher hydride affinity than the parent triphenylmethyl cation Tr⁺. We report that indeed F₆Tr⁺ displays reactivity unmatched by Tr⁺. F₆Tr⁺ at ambient temperature abstracts hydrides from the C-H bonds in tetraethylsilane, mesitylene, methylcyclohexane, and catalyzes Friedel-Crafts alkylation of arenes with ethylene, while Tr⁺ does none of these.

Fig. 1. The parent trityl cation Tr⁺, selected literature examples of fluorinated trityls, and the fluorinated trityl salts prepared and studied in this work.

Fig. 2. Synthesis of fluorinated trityl cation salts and their appearance.

Fig. 3. POV-Ray rendition of the ORTEP (50% probability ellipsoids) drawing of F₂Tr[Cl11] (top) and F₆Tr[Cl11] (bottom). Only one cation and one anion from each asymmetric unit is shown. Solvent and disorder are omitted for clarity.

Fig. 4. Reactions of Tr⁺ and F₆Tr⁺ with a substoichiometric amount of HSiEt₃.

Fig. 5. Reactions of F₆Tr[Cl11] resulting in the abstraction of a hydride from C(sp³)–H bonds.