Abstraction of Hydride from Alkanes and Dihydrogen by the Perfluorotrityl Cation

Abstract

Lewis acids play a central role in a large variety of chemical transformations. The reactivity of the strongest Lewis acids is typically studied in the context of affinity towards hard bases, such as fluoride or oxygenous species. Carbocations can be viewed as soft Lewis acids, possessing significant affinity for softer bases, such as hydride. This work presents the ambient‐temperature isolation of salts of the perfluorotrityl cation ((C₆F₅)₃C⁺ or F₁₅Tr⁺) in combination with halogenated carborane anions. The F₁₅Tr⁺ cation exhibits remarkable hydride affinity, illustrated by the observation of hydride abstraction from dihydrogen, and of the rapid abstraction of hydride from −CH₂−groups in alkanes. Theoretical studies support the favorability of hydride abstraction from dihydrogen, and indicate that the hydride abstraction from alkanes proceeds via a concerted hydride transfer process that is sensitive to steric effects.

This work presents the isolation of a perfluorotrityl cation (F₁₅Tr⁺) as a salt with halogenated carborane anions. These salts are stable at ambient temperature in the solid state. As predicted by theoretical calculations, F₁₅Tr⁺ displays remarkable propensity for abstraction of a hydride. It extracts a hydride rapidly from alkanes and slowly even from dihydrogen.

Introduction

The triphenylmethyl or trityl cation (Ph₃C⁺ or Tr⁺ ) is widely used as a relatively stable carbocation that can be isolated and used as a strong Lewis acid with propensity to abstract a hydride or an alkide (alkyl anion) from other molecules. ^[1] Alkide abstraction by Tr⁺ has been frequently studied as a way to generate cationic transition metal catalysts for olefin polymerization. ^[2] Hydride abstraction by Tr⁺ has been used to prepare a variety of reactive main‐group cations,[ ³ , ⁴ ] and especially silylium cations.[ ⁵ , ⁶ ] Studies of hydride transfer with Tr⁺ have provided valuable thermodynamic and kinetic information on the reactivity of metal hydride catalysts in the various hydrogenation/dehydrogenation transformations.[ ⁷ , ⁸ , ⁹ ] Tr ⁺ is also an interesting one‐electron oxidant, ^[10] and has been recently implicated in the formation of frustrated radical pairs. ^[11] Trityl cation derivatives substituted with electron‐donating groups are relatively easy to isolate and have found use as dyes. ^[12] Triarylmethyl cations that are more electron‐poor than the parent Tr⁺ have proven more difficult to prepare in a pure form. The fully fluorinated trityl cation (C₆F₅)₃C⁺ (F₁₅Tr⁺ , Figure 1) has attracted attention as early as in 1960s,[ ¹³ , ¹⁴ , ¹⁵ ] and was studied more recently by Mayr et al. ^[16] and Dutton et al. ^[17] While these studies demonstrated that generation of F₁₅Tr⁺ and various partially fluorinated trityls is possible in oleum, SbF₅, triflic acid, or other superacidic media, they were only studied in solution and these conditions were conducive to cation degradation.[ ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ ] Until recently, the only isolation of a polyfluorinated trityl was the X‐ray structure of *F₆Tr⁺ (isolated in an unknown yield) in the report by Reed in 2007. ^[18] In 2022, our group reported the isolation and full characterization of F₆Tr⁺ as a salt with the carborane [HCB₁₁Cl₁₁]⁻ anion. ^[19] At about the same time, F₁₅Tr⁺ was isolated in the solid state by Riedel et al., ^[20] partnered with the weakly coordinating ^[21] [Al(OTeF₅)₄]⁻. Although Riedel's work permitted X‐ray crystallographic characterization and solution observations at −40 °C, the F₁₅Tr[Al(OTeF₅)₄] salt did not appear to be stable to isolation at ambient temperature. The perchlorotrityl cation has also been isolated. ^[22]

Figure 1.

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

Wilson and Dutton studied the hydride affinity (HA) of a series of fluorinated trityl cations using density functional theory. ^[23] Their calculations showed that while F₆Tr⁺ possessed 11 % higher gas‐phase HA (212 kcal/mol) than Tr⁺ (191 kcal/mol), the value for F₁₅Tr⁺ (227 kcal/mol) was 19 % higher. The disparity was even greater for the computed values in the CH₂Cl₂ solvent continuum, where F₁₅Tr⁺ possessed a remarkable 33 % higher HA than Tr⁺ ! It should also be noted that HA of F₁₅Tr⁺ is more than double that of its widely used[ ²⁴ , ²⁵ ] isoelectronic relative (C₆F₅)₃B (108 kcal/mol in the gas phase). In 2023, an antiaromatic cyclopentadienyl cation [cyclo‐C₅(C₆F₅)₅]⁺ was calculated to possess an even higher (by ca. 5 %) hydride affinity than F₁₅Tr⁺ . ^[26] However, the reported reactivity of its salt [cyclo‐C₅(C₆F₅)₅][Sb₃F₁₆] reflected its preference to act as a strong single‐electron oxidant rather than a hydride abstractor. ^[26] Our group has been attracted to the highly reactive carbo‐ and main‐group cations in the context of our work on silylium and alumenium cation‐catalyzed activation of aliphatic C−F bonds,[ ²⁷ , ²⁸ , ²⁹ , ³⁰ ] which permitted exhaustive defluorination of perfluoroalkyl groups under mild conditions. We saw F₁₅Tr⁺ not only as a worthy target in its own right, but were especially interested in examining the hydride abstracting power of F₁₅Tr⁺ . We reasoned that a halogenated carborane anion such as [HCB₁₁Cl₁₁]⁻ (or its close relatives) ^[31] may be a better partner because of its well‐established compatibility with a variety of highly reactive carbo‐ and silylium cations, ^[6] as well as Brønsted superacids ^[32] in solvents of modest polarity. In the present work, we report successful isolation of bulk quantities of analytically pure F₁₅Tr⁺ salts at ambient temperature and the study of their extraordinary hydride abstracting power, including abstraction of hydride from dihydrogen and rapid abstraction of hydride from equimolar alkanes.

Results and Discussion

Synthesis and Characterization of F₁₅Tr⁺ Salts

The Riedel group accessed F₁₅Tr⁺ via abstraction of chloride from F₁₅TrCl using either the [Al(OTeF₅)₃]₂ Lewis acid or the proton of the protonated difluorobenzene in [H−C₆F₂H₄][Al(OTeF₅)₄]. ^[20] Because Riedel et al. reported difficulties converting F₁₅TrOH to F₁₅TrCl, we sought a more easily accessed pseudohalide derivative and were able to prepare F₁₅Tr−O₂CCF₃ in good yield. For the abstraction of trifluoroacetate from it, we utilized [(Me₃Si)₂OTf][HCB₁₁Cl₁₁] (OTf=O₃SCF₃), a reactive but conveniently isolable “Me₃Si⁺” synthon, which we previously used for successful chloride and fluoride abstractions.[ ¹⁹ , ³³ ] Given the expected high reactivity of F₁₅Tr⁺ towards C−H bonds, the choice of solvent for the reaction was a challenge. In the end, we selected silicon tetrachloride as an inert solvent; it allowed the reactants to mix and react homogeneously in solution, but the [F₁₅Tr][HCB₁₁Cl₁₁] product proved insoluble and precipitated. It was easily collected by filtration and washing with excess SiCl₄.

The surprisingly limited solubility of [F₁₅Tr][HCB₁₁Cl₁₁] made it somewhat difficult to explore its reactions and we decided to target a more solubilizing anion. The [HCB₁₁Cl₁₁]⁻ anion can be easily derivatized at the carbon via deprotonation/alkylation to give [RCB₁₁Cl₁₁]⁻ anions (R=a primary alkyl group). ^[34] However, in light of the studied reactivity (see below), alkyl groups were not a viable option. After some consideration, we prepared derivatives of the new [C₆F₅SCB₁₁Cl₁₁]⁻ anion in the anticipation (proven correct) that the −SC₆F₅ substituent will be inert and meaningfully increase the solubility. [(Me₃Si)₂OTf][C₆F₅SCB₁₁Cl₁₁] was prepared via analogous routes and used to abstract trifluoroacetate from F₁₅Tr−O₂CCF₃ in a similar fashion in SiCl₄ solvent, providing a high yield of [F₁₅Tr][C₆F₅SCB₁₁Cl₁₁].

[F₁₅Tr][HCB₁₁Cl₁₁] and [F₁₅Tr][C₆F₅SCB₁₁Cl₁₁] were isolated as pure solids at ambient temperature and showed no signs of decomposition for at least a month when stored in the solid state in an argon‐filled glovebox free of volatile alkyl‐containing compounds. We examined a series of solvents for their suitability to dissolve the F₁₅Tr⁺ salts without reacting with them. Unlike [F₆Tr][HCB₁₁Cl₁₁], the F₁₅Tr⁺ salts proved incompatible with arenes containing C−H bonds at ambient temperature, including 1,2‐difluorobenzene reported by Riedel at −40 °C for [F₁₅Tr][Al(OTeF₅)₄] and even 1,2,3,4‐tetrafluorobenzene used by Krossing for various reactive cations. ^[35] CH₂Cl₂ and CHCl₃ underwent rapid chloride abstraction upon dissolution, while plausibly unreactive candidates CFCl₃, CFBr₃, Cl₃CCOCl, C₆F₅Cl, o‐C₆F₄Br₂, and CHBr₃ did not dissolve either of the F₁₅Tr⁺ salts. Eventually, it was determined that SO₂Cl₂, C₆F₅Br, and surprisingly, CH₂Br₂ provide some solubility while remaining inert to F₁₅Tr⁺ (no change after 1–5 days at ambient temperature). Of these three, SO₂Cl₂ was the most solubilizing and C₆F₅Br was the least solubilizing. The cation in [F₁₅Tr][C₆F₅SCB₁₁Cl₁₁] (but not [F₁₅Tr][HCB₁₁Cl₁₁]) suffered some degradation in SO₂Cl₂ after 24 h at ambient temperature (¹⁹F NMR evidence). Recrystallization from CH₂Br₂ afforded single crystals of [F₁₅Tr][C₆F₅SCB₁₁Cl₁₁] that were subjected to an X‐ray diffractometry study (Figure 2). The metrics within the F₁₅Tr⁺ cation are similar to those reported by Riedel with the [Al(OTeF₅)₄] anion, except the closest approach to the central carbon is by the bromines of the two cocrystallized dibromomethane molecules. The C−Br distances (ca. 3.9 and 4.4 Å), however, are far outside the bonding range and exceed the sum of van der Waals radii. The sum of angles about the central carbon in the cation is ca. 360°, consistent with the expected sp²‐hybridization.

Figure 2.

Synthesis of F₁₅Tr⁺ salts and POV‐Ray rendition of the ORTEP (50 % probability ellipsoids) of F₁₅Tr[Cl₁₁]. Selected bond lengths (Å) and angles (°): C1−C2: 1.445(4); C1−C8: 1.445(4); C1−C14: 1.447(4); C2−C1−C8: 119.7(2); C8−C1−C14: 120.0(2); C14−C1−C2: 120.3(2).

Abstraction of Hydride from Alkanes

We previously reported that [F₆Tr][HCB₁₁Cl₁₁] abstracted a hydride from methylcyclohexane (>98 % F₆Tr−H formation after 96 h), an alkane containing a tertiary C−H bond. ^[19] Riedel et al. also noted the abstraction of hydride by [F₁₅Tr][(Al(OTeF₅)₃Cl] from a tertiary C−H in isobutane, with some anion redistribution observed. ^[20] Here, we investigated the reaction of [F₁₅Tr][HCB₁₁Cl₁₁] with cyclohexane, which contains only secondary C−H bonds (Figure 3). Remarkably, the hydride abstraction was 92 % complete in a 1 : 1 reaction at 0.017 M concentration after 5 min. For proper comparison, we studied the analogous reaction of [F₆Tr][HCB₁₁Cl₁₁] with cyclohexane, as well (see Supporting Information). Although it also proceeded (to give F₆Tr−H), it was orders of magnitude slower (apparent half‐life of a few days vs a few minutes for F₁₅Tr⁺). In addition, the reaction of [F₆Tr][HCB₁₁Cl₁₁] eventually generated a greater variety of products than just F₆Tr−H, ostensibly because the latter, unlike F₁₅Tr−H, is subject to the Friedel–Crafts substitution by the carbocations resulting from the abstraction of hydride from cyclohexane.

The abstraction of hydride from pentane with either [F₁₅Tr][HCB₁₁Cl₁₁] in SO₂Cl₂ or of [F₁₅Tr][C₆F₅SCB₁₁Cl₁₁] in C₆F₅Br was slower than with cyclohexane, and was complicated by the apparent side reactions following hydride abstraction, but still proceeded readily at ambient temperature (24 % F₁₅Tr−H yield in 1 h at 0.013 M). On the other hand, treatment of [F₁₅Tr][C₆F₅SCB₁₁Cl₁₁] in C₆F₅Br or in CH₂Br₂ with 2,2,4,4‐tetramethylpentane, an alkane containing a more sterically encumbered −CH₂− group, did not proceed at all. Similar reactions with 2,2‐dimethylbutane also did not result in a detectable amount of F₁₅Tr−H after 120 h, although 2,2‐dimethylbutane itself was eventually consumed, possibly in an isomerization or other reaction mediated by the cations resulting from hydride abstraction below NMR‐detectable concentration. Examination of the reactions of [F₁₅Tr][C₆F₅SCB₁₁Cl₁₁] with methane and ethane in C₆F₅Br showed no evidence for hydride abstraction after 5–10 days. These results suggest that F₁₅Tr⁺ readily abstracts a hydride from unencumbered secondary C−H bonds, but not from primary or those in methane. The lack of reaction with −CH₂‐containing, but sterically encumbered substrates further suggests that the reaction requires a close approach of F₁₅Tr⁺ to the C−H bond (and is not, for example, initiated by electron transfer; see below). Abstraction of a hydride from a primary −CH₃ position did take place in a reaction of [F₁₅Tr][C₆F₅SCB₁₁Cl₁₁] with Et₄Si in C₆F₅Br (21 % conversion in 30 min at 0.13 M, >97 % after 24 h). That is also much faster than the previously reported reaction of [F₆Tr][HCB₁₁Cl₁₁] with Et₄Si. The β‐silyl effect ^[36] presumably makes the hydride abstraction from R₃SiCH₂‐CH₃ more favorable than from R₃CCH₂‐CH₃.

Abstraction of Hydride from Dihydrogen

Reactions of [F₁₅Tr][C₆F₅SCB₁₁Cl₁₁] with H₂ or D₂ were examined in C₆F₅Br solvent (Figure 3). They were notably slower than the reactions with cyclohexane and required heating at 70 °C to proceed at a practical rate. Quantitative monitoring of the reaction by ¹⁹F NMR spectroscopy in C₆F₅Br is not viable, but the resonance of the F₁₅Tr−H or F₁₅Tr−D product could be unambiguously quantified using ¹H or ²H NMR analysis. In five separate experiments, we observed 70–90 % yield of F₁₅Tr−H or F₁₅Tr−D after 4–5 days.

Figure 3.

Hydride abstraction reactions F₁₅Tr⁺ alkanes, tetraethylsilane, and dihydrogen.

Theoretical Analysis

To get further insight into the reactivity profile we utilized dispersion‐corrected density functional theory to study the abstraction of hydride of cyclohexane, as a representative example, by the F₁₅Tr⁺ cation (see Supporting Information for more details) without explicit inclusion of the counterion. As shown in Figure 4, we examined two approaches to cyclohexane by F₁₅Tr⁺ – either from the axial (green pathway, left), or the equatorial C−H positions (maroon pathway, right). The initial complexation between F₁₅Tr⁺ and cyclohexane is enthalpically favorable, but disfavored entropically in both cases by ~5 kcal/mol. Notably, both C−H abstractions were calculated to be only slightly endoergic (0.2 and 0.7 kcal/mol uphill) with respect to the separated cyclohexane and “naked” F₁₅Tr⁺ cation, but downhill in energy with respect to the complexes. The final complexed products INT2‐A and INT2‐E have slightly different energies because they are non‐covalent adducts of F₁₅Tr−H with different conformers of the cyclohexyl cation; they can be assumed to interconvert with negligible barrier. Notably, we found that the abstraction of the less sterically hindered equatorial hydride proceeded via a much lower barrier (19.2 kcal/mol) than the corresponding axial hydride abstraction (23.3 kcal/mol). This is consistent with the observations of ostensibly sterically inhibited abstraction from the CH₂ groups of 2,2‐dimethylbutane and 2,2,4,4‐tetramethylpentane. The calculated barrier for hydride abstraction from cyclohexane is also remarkably consistent with the experimental observations – the apparent half‐life of 1–2 min at RT corresponds to ca. 20 kcal/mol in the free energy barrier. Moreover, the calculated KIE value of 4.64 (see Supporting Information for additional details) for the equatorial abstraction agreed well with the experimental determination of 3.9(4) (from reactions with mixtures of c‐C₆H₁₂/c‐C₆D₁₂), which supports the notion of C−H bond breaking being the rate‐limiting step of the reaction. Finally, we note that the favorable and fast isomerization of the cyclohexyl cation into the tertiary methylcyclopentyl cation is well established,[ ³⁷ , ³⁸ ] and both cations can undergo further potentially favorable transformations via loss of proton, and isomerization or oligomerization of the ensuing olefins. We have not attempted to establish the full experimental fate of the cyclohexyl cation, although it is clear from the ¹H NMR spectra that multiple species were formed.

Figure 4.

Equatorial vs axial hydride abstraction from cyclohexane by F₁₅Tr⁺ calculated at the UB3LYP‐D3/6‐311+g(d,p)‐CPCM(Chlorobenzene) level of theory. Gibbs energies and enthalpies are in kcal/mol, and the C−H distances (in red) given for the TS geometries are in Å.

For the reaction with dihydrogen, we performed calculations (Figure 5) on the overall thermodynamics of the process. F₁₅Tr[C₆F₅SCB₁₁Cl₁₁] and H₂ were calculated to form a slightly unfavorable encounter complex INT3. For the product INT4, we chose to place the proton on the sulfur atom in the anion. The most obvious contact between the protonated, sulfonium zwitterion [(C₆F₅)SH−CB₁₁Cl₁₁] and F₁₅Tr−H in INT4 is an H…F hydrogen bond (d_HF=2.21 Å). The conversion to INT4 was calculated to be exoergic by −10.4 kcal/mol. The calculation in Figure 5 is consistent with the experimental observation in showing thermodynamic favourability, however exhaustive attempts at finding the transition state to calculate the barrier for this reaction were unsuccessful. There are of course multiple other choices for treating the fate of the proton in this system. For example, in the superacid H[HCB₁₁Cl₁₁], it resides on the chlorine atoms of the carborane. ^[39] The C₆F₅Br solvent or the F₁₅Tr−H product may also in principle serve as a Brønsted base.

Figure 5.

Top: Gibbs energies and enthalpies (in parentheses), in kcal/mol, for the abstraction of hydride from H₂ by F₁₅Tr[C₆F₅S‐CB₁₁Cl₁₁] calculated at the UB3LYP‐D3/6‐311+g(d,p)‐CPCM(Chlorobenzene) level of theory. Bottom: Typical heterolytic splitting of H₂ by a borane/phosphine frustrated Lewis pair.

Overall, the reaction of F₁₅Tr[C₆F₅SCB₁₁Cl₁₁] with H₂ can be viewed as an extreme example of heterolytic dihydrogen splitting by a frustrated Lewis pair (FLP). ^[40] The thermodynamic favorability of H₂ splitting has been analyzed as a function of the HA of the Lewis acid and the basicity of the base.[ ⁴¹ , ⁴² ] (C₆F₅)₃B is a prototypical Lewis acid in conventional FLP reactions (Figure 5, bottom), and in order for the H₂ splitting to be favorable, it needs to be complemented by a base at least as strong as a triorganophosphine.[ ⁴⁰ , ⁴¹ , ⁴² ] Since HA of F₁₅Tr⁺ is massively greater than that of (C₆F₅)₃B (by ca. 120 kcal/mol in the gas phase), ^[23] it can drive favorable H₂ splitting with a base as weak as a halogenated carborane anion.

Conclusions

In summary, we have demonstrated the synthesis of the perfluorotrityl cation (F₁₅Tr⁺ ) in combination with halogenated carborane anions. These salts are stable under inert atmosphere at ambient temperature. In solution, they display remarkable propensity for hydride abstraction from secondary positions in alkanes and also from H₂. Theoretical analysis corroborates the experimental findings and indicates that the hydride abstraction proceeds via a concerted transfer of a hydride to F₁₅Tr⁺ . The high potency of F₁₅Tr⁺ as a hydride abstractor should characterize it as a soft Lewis superacid.

Note Added in Proof

Since the publication of this article online as an Accepted Article, a closely relevant article by Riedel et al. has been published. ^[43] It details the synthesis of F₁₅Tr⁺ and the related perfluoro/halo‐substituted trityl cations with the Ga₂Cl₇ ^‐ counterion, and includes their reactivity in hydride abstraction from alkanes.

Conflict of Interests

The authors declare no conflict of interest.

Supporting information

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Supporting Information

Leong D. W., Gogoi A. R., Maity T., Lee C.-I, Bhuvanesh N., Gutierrez O., Ozerov O. V., Angew. Chem. Int. Ed. 2025, 64, e202422190. 10.1002/anie.202422190

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article. Deposition Number (https://www.ccdc.cam.ac.uk/services/structures?id=doi:10.1002/anie.2024XXXXX) CCDC 2374369 contains the supplementary crystallographic data for this paper. These data are provided free of chargeby the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe (http://www.ccdc.cam.ac.uk/structures).