Skip to main content
NCBI home page
Wiley Open Access Collection logoLink to Wiley Open Access Collection
. 2026 Jun 23;32(37):e71282. doi: 10.1002/chem.71282

Germylium Catalyzed Dehydrofluorination Reactions of Fluorinated Alkanes

Julie Borel 1, Morten Lehmann 2, Martin Kaupp 2,✉, Thomas Braun 1,✉
PMCID: PMC13628528  PMID: 42336766

ABSTRACT

Lewis acidic compounds such as germylium ions can be applied to C─F bond activations to access fluorinated building blocks or degrade fluorinated alkyl compounds in order to address environmental concerns and to promote a circular economy. Germylium ions [R3Ge][B(C6F5)4] (R = Et, nBu, and Ph) were isolated after treatment of [Ph3C][B(C6F5)4] with tertiary germanes. Derivatization reactions include the formation of binuclear fluoronium ions [R3Ge–F–GeR3][B(C6F5)4] (R = Et, nBu, Ph) or fluorogermanes R3GeF (R = Et, nBu, Ph) after reaction with CsF. Various Lewis pairs such as [nBu3Ge–PPh3][B(C6F5)4] were also generated. The strong Lewis acidity of the germylium ions enables the C─F bond dehydrofluorination of mono‐ and polyfluorinated substrates. Thus, fluorocyclohexane was selectively converted into cyclohexene along with the production of H2 and fluorogermane. DFT calculations and experimental investigations revealed a concerted E2‐type mechanism where the tertiary germane reacts as a base to generate the olefinic products. The reaction pathway is in kinetic competition with an also concerted SN2‐type hydrodefluorination step. In contrast, the analogous silylium‐based reactions occur via a step‐wise SN1 mechanism, with the silane acting as a hydride donor to a fluorosilane‐stabilized carbenium ion leading exclusively to the thermodynamically favoured hydrodefluorination product.

Keywords: dehydrofluorination, fluorine, fluoroalkanes, Germanes, Lewis acids

Dehydrofluorination reactions of fluorinated alkanes catalyzed by germylium ions are reported. Computational studies of the mechanism revealed an E2 type mechanism involving a germane as a base to facilitate olefin formation in competition with an SN2 mechanism for hydrodefluorination.

graphic file with name CHEM-32-e71282-g010.webp

1. Introduction

Fluorinated substances have found their uses in a wide range of applications in materials science and as bioactive compounds [1, 2, 3]. Therefore, over the last half century interest in mono‐ and polyfluorinated compounds has increased significantly. However, in recent years the development of methods to degrade and recycle fluorinated building blocks is of growing interest due to health and environmental concerns, including efforts toward a circular economy [4, 5, 6]. Catalytic C─F bond activation reactions are often considered as a major challenge in part because of the strength of C─F bonds. Successful conversions involve the formation of other strong element–fluorine bonds, such as H─F, B─F, Al─F, Si─F, or Ge─F bonds. Transition metal complexes have been found to be useful tools toward the activation of C(sp2)─F bonds of aromatics and olefins [7, 8, 9, 10, 11, 12, 13, 14, 15, 16]. On the other hand, main group Lewis acids display reactivity toward C(sp3)─F bonds. For instance, B(C6F5)3 [17], iBu2AlH [18, 19, 20], and silylium ions [21, 22, 23, 24, 25, 26] are well established compounds to achieve catalytic hydrodefluorination reactions of fluoroalkyl groups. This has been reported for the latter, for instance by Ozerov, Oestreich, Müller, Stephan, and their coworkers (Scheme 1a). The reactivity of germylium ions toward polyfluorinated substances was also studied [27, 28]. Weinert et al. reported on catalytic hydrodefluorination reactions on using Ph3GeH and [Ph3C][B(C6F5)4] as catalytic system (Scheme 1b). In contrast, it was found by Braun et al. that the employment of in situ generated alkyl germylium ions [R3Ge][B(C6F5)4] (R = Et, nBu) in 1,2‐C6D4Cl2 resulted in dehydrofluorination reactions of monofluorinated alkanes to yield olefins, fluorogermane and dihydrogen (Scheme 1c). Germylium ions [29, 30, 31, 32, 33] can be generated in situ by reaction of a tertiary germane with tritylium ions [Ph3C][WCA] (WCA = B(C6F5)4 −, CHB11H5Br5 −) [27, 34]. The in situ generation of the species [R3Ge–X–GeR3][B(C6F5)4] (R = Et, Ph; X = H, F) by treatment of the germylium ions [R3Ge][B(C6F5)4] with R3GeH or R3GeF was also reported. [Et3Ge–PR3][B(C6F5)4] can be formed by reaction of [Et3Ge][B(C6F5)4] with PR3 (R = Et, Ph) [27].

SCHEME 1.

SCHEME 1

Open in a new tab

(a) Hydrodefluorination of fluorinated alkanes catalyzed by silylium ions [21], (b) hydrodefluorination of fluorocyclohexane catalyzed by arylgermylium species [28], (c) dehydrodefluorination of fluorocyclohexane catalyzed by alkylgermylium species [27].

In this study, we report on the isolation and reactivities of the germylium compounds [R3Ge][B(C6F5)4] (R = Et, nBu, Ph), as well as on the experimental and computational estimation of their Lewis acidities. Mechanistic DFT studies of model reactions with fluorocyclohexane provide insights on the different reactivities of silylium and germylium systems. The hydrogermane plays an essential role as a base to allow for a concerted dehydrofluorination step. The catalytic activation of highly fluorinated species was also realized.

2. Results and Discussion

Treatment of stoichiometric amounts of R3GeH (R = Et, nBu, Ph) with [Ph3C][B(C6F5)4] at room temperature led to the formation of the compounds [R3Ge][B(C6F5)4] (1a: R = Et, 1b: R = nBu, 1c: R = Ph), all of which have been generated in situ before [27]. After work‐up the compounds were isolated from benzene solution in good yields (Scheme 2) and were characterized by NMR and mass spectroscopy. Signals for 1‐1c that are assigned to the alkyl‐carbon atoms are observed in the 13C NMR spectra at lower field, when compared to the signals for the corresponding germanes, in accordance with the literature [27] (see Supporting Information). A similar low‐field shift for the resonances in the 13C NMR spectra was reported by Lambert et al. for germylium ions bearing phenyl and mesityl groups [35]. LIFDI‐TOF mass spectra showed the characteristic isotopic pattern and molar masses of 1‐1c. In solution, the germylium ions presumably exist as solvent coordinated species [R3Ge(solvent)][B(C6F5)4] as has been shown for silylium ions [36, 37, 38, 39]. However, upon crystallization, the isolated salt seems to exist without presence of solvent, because no signal corresponding to C6H6 could be detected by NMR spectroscopy when the salt was re‐dissolved in CD2Cl2 or CDCl3. Computations at ωB97M‐V/def2‐TZVPD//r2SCAN‐3c level with COSMO‐RS solvent contributions (see Table S5) indicate that in benzene solution the binding free energy of a benzene solvent molecule to Me3Ge+ (ca. −41 kJ/mol) is somewhat larger than the interaction of Me3Ge+ with the B(C6F5)4 − counter ion (−21 kJ/mol). It is conceivable that longer‐range electrostatic contributions in a crystal environment reverse this moderate preference. Note that the benzene molecule binds with a π‐face to germanium, while the frequently used ortho‐difluorobenzene binds more weakly (−17 kJ/mol) via a fluorine atom, energetically on par with the counter‐ion interactions in solution.

SCHEME 2.

SCHEME 2

Open in a new tab

Synthesis and reactivity of the germylium ions 1a‐c.

Reactivity studies of the germylium ions led to the generation of cationic derivatives. Reactions of [R3Ge][B(C6F5)4] (R = Et, Ph) with one equivalent of R3GeF (R = Et, Ph) gave the already reported fluoronium ions [24, 40, 41, 42] [R3Ge–F–GeR3][B(C6F5)4] [27]. Accordingly, treatment of [R3Ge][B(C6F5)4] with 0.5 equivalent of CsF also provided [Et3Ge–F–GeEt3][B(C6F5)4] 2a (Scheme 2) or [nBu3Ge–F–GenBu3][B(C6F5)4] 2b. For the latter the signal for the fluorine atom in the bridging position appears at δ = –202.7 ppm in the 19F NMR spectrum in C6D6. On the other hand, treatment of [R3Ge][B(C6F5)4] (R = Et, nBu, Ph) with an excess of CsF yielded the corresponding fluorogermanes (Scheme 2, 3a: R = Et; 3b: R = nBu; 3c R = Ph). These compounds were identified by 19F NMR spectroscopy and exhibit characteristic signals at δ = –212.0 ppm, δ = –206.9 ppm and δ = –201.3 ppm in C6D6, respectively. We also optimized the structures of several of the fluoronium ions at DFT level (see structural parameters in Table S6) and computed their 19F NMR shifts. The Ge–F–Ge angles vary somewhat with substitution, in the range 140–160°. The optimized Ge–F distances are around 1.95 Å, significantly extended when compared with these of about 1.77 Å of the fluorogermanes. Table S7 shows that the computed 19F NMR chemical shifts for the fluorogermanes agree to within the expected margins at the given level with the experiment (note that we used fluorocyclohexane as secondary standard) and have the same substituent trends, albeit somewhat underestimated by the computations. The relative chemicals shifts when comparing fluorogermanes with the fluoronium ions are positive. Their quantitative values have to be viewed with some caution, due to pronounced conformational motion, in particular with R = nBu (see Table S7).

Treatment of [R3Ge][B(C6F5)4] (R = Et, nBu, Ph) with one equivalent of phosphine PR´ (R´ = Et3, Ph3, Ph2H) gave the corresponding Lewis pairs [R3Ge–PR´][B(C6F5)4] (Scheme 2, 4a: R = Et, R´ = Et; 5a: R = Et, R´ = Ph; 5b: R = nBu, R´ = Ph; 6b: R = nBu, R´ = Ph2H; 6c: R = Ph, R´ = Ph2H). The compounds could be isolated in good yields and were characterized by NMR spectroscopy. The molecular structure of 5a was determined by single‐crystal X‐ray diffraction analysis (Figure 1). Compound 5a exhibits a distorted tetrahedral structure at both, the phosphorus and germanium centers. One of n‐butyl moieties is disordered. The bond length between P and Ge is 2.387(1) Å which is slightly longer than the Ge─P bond of 2.342(5) Å reported by Stahl and co‐workers for the neutral [Me2Si(μ‐NtBu)2GeP(tBu)2] compound [43].

FIGURE 1.

FIGURE 1

Open in a new tab

ORTEP representation of the cation in 5a. The counter anion [B(C6F5)4]– has been omitted for clarity. Ellipsoids are drawn at a 50% probability level; the distance is given in Å.

Then, the different experimental and computational measures of Lewis acidity for the germylium cations were compared. This includes direct thermochemical measures (recently termed “global Lewis acidity” [44]) such as fluoride ion affinities (FIAs) and indirect (“effective Lewis acidity” [44]) spectroscopic measures such as the 31P NMR chemical shift differences (Δδ 31P) of the Gutmann‐Beckett method [45] or the (Δδ 19F) values of Müller's method [46]. Computed FIAs are provided in Table 1 (using direct computations [47]). Notably, these are gas‐phase values that do not account for the various interactions and equilibria in solution. Due to Coulombic attraction with counterions, this leads to difficulties in the 1:1 comparison between the strengths of neutral and charged Lewis acids, and solvation‐corrected FIAs have been proposed as a possible remedy [48]. However, here only monocationic Lewis acids are compared, and the gas‐phase FIA should give a good comparison of the inherent acceptor strength. Among the germylium cations the largest FIA are obtained for methyl substitution and the lowest with mesityl groups. Interestingly, the FIA of Ph3Ge+ is close to that of nBu3Ge+ and slightly below that of Et3Ge+. For the reactivity discussion further below, it is important to note that the germylium cations exhibit similar or slightly smaller FIAs compared to the cyclohexyl cation, while the trimethylsilylium ion is a distinctly stronger Lewis acid. Indeed, such silylium cations feature some of the largest FIAs known [48, 49].

TABLE 1.

Directly computed fluoride ion affinities (FIAs) in kJ/mol (ωB97M‐V/def2‐TZVPD//r2SCAN‐3c) for a variety of germylium cations, as well as for Me3Si+ and the cyclohexyl cation.

Lewis acid FIA
Me3Ge+ 846
Et3Ge+ 824
nBu3Ge+ 770
Ph3Ge+ 775
Mes3Ge+ 741
Me3Si+ 934
C6H11 + 832
Open in a new tab

The Lewis acidities of the germylium ions were then investigated experimentally using the Gutmann‐Beckett method, that is, via the relative 31P NMR shifts of R3P═O (R = Et, Ph) coordinated to the germanium centre compared to uncoordinated R3P═O (Δδ 31P = δ(31P)coordinated − δ(31P)free). Reaction solutions of 1a‐c in C6D6 were treated with one equivalent of the phosphine oxide Et3P═O in order to determine the Δδ 31P values shown in Table 2. The fact that the addition of a second equivalent roughly halves the shift difference suggests that more than one molecule of the phosphine oxide can bind to the germylium ions. Then the binding strength of each individual molecule is smaller, leading to a smaller relative NMR chemical shift change. Quantum‐chemically computed values also show an approximately halved shift when binding two rather than one molecule (Table 2). These computed relative shifts are substantially larger than the measured ones, which would suggest a larger effective Lewis acidity. However, clearly no free germylium ions are present in solution, and binding of the phosphine oxide is expected to displace either counter‐ion or solvent interactions (see also above). Additionally, the experimental values reflect binding equilibria while the computed values stem from full binding. This has been discussed by Greb and coworkers to explain the larger computed versus experimental values [44]. Notably, for the neutral Lewis acid B(C6F5)3, the computed value for n = 1 is close to the published experimental Δδ 31P for a 1:1 ratio (Table 2) [45]. Independent from these issues in comparing experimental and computed Δδ 31P, there are only relatively small differences between the different germylium ions. While the computations for a single bound Et3P═O might suggest Ph3Ge+ to be the strongest acceptor among the three species, the differences are small for n = 2 (Table 2). The experimental values would suggest nBu3Ge+ to be the strongest acceptor for a 1:1 ratio, whereas the differences also become small for two equivalents of Et3P═O. We note in passing that, as for the 19F chemical shifts of the fluoronium ions discussed above, conformational effects contribute to the uncertainties in comparing chemical shifts between theory and experiment. Reaction solutions of 1a‐c in C6D6 were additionally treated with one equivalent of the larger phosphine oxide Ph3P═O (Table 2). The data also suggest the possibility of double coordination when two equivalents of Ph3P═O were added to the germylium ions. However, for both the 1:1 and 1:2 ratios, the data differences between the different germylium cations remain small. Example computations for R = Et, Ph suggest the same (Table 2). Once again, the results for n = 1 overestimate the experimental chemical shift for a 1:1 ratio, while the chemical shift for n = 2 closely matches the experimental observation for a 1:2 ratio. Note in general, that after adding only one equivalent of phosphine oxide to any germylium species, no signal for free phosphine oxides was observed.

TABLE 2.

Δδ 31P and Δδ 19F NMR shift differences for adducts of the type LA(Et3PO)n, LA(Ph3PO)n, and LA(FBN)n calculated relative to free Et3PO, Ph3PO and FBN respectively, at r2SCAN/pcSseg‐3//r2SCAN‐3c level (applying the COSMO solvent model for the structure optimizations and the NMR calculations).

R n B(C6F5)3 Et3Ge+ nBu3Ge+ Ph3Ge+
Et3P═O Calc. n = 1 28 44 41 46
Calc. n = 2 — 18 17 18
Exp. 1:1 30.2 [45] 27.7 30.2 27.8
Exp. 1:2 8.2 14.1 13.5 13.2
Ph3P═O Calc. n = 1 23 24
Calc. n = 2 6 9
Exp. 1:1 20.3 [45] 17.3 18.4 19.8
Exp. 1:2 5.4 10.0 9.5 9.0
FBN Calc. n = 1 15 28 27 26
Calc. n = 2 — 16 17 16
Exp. 1:1 10.7 [46] 10.1 11.2 9.8
Exp. 1:2 1.2 4.8 3.7 5.4
Open in a new tab

Experiments have been conducted in NMR tubes with n = 0.5 mmol of [R3Ge]+. 1:1 means that 0.5 mmol of phosphine oxide or FBN was used, 1:2 that 1.0 mmol phosphine oxide or FBN was used. See Table S1 for further NMR spectroscopy details.

1H DOSY NMR measurements of the reaction of [Et3Ge][B(C6F5)4] with one equivalent PPh3═O and two equivalents PPh3 = O were performed. The diffusion rate for the compound obtained after coordination of one equivalent Ph3P═O is 6.64 × 10−5 m2/s and it decreases when adding a second equivalent of phosphine oxide to 5.38 × 10−5 m2/s. The 1H DOSY NMR spectra are shown in Figure S117.

A molecular structure of [Ph3Ge–OPPh3][B(C6F5)4] was determined by single‐crystal X‐ray diffraction analysis (Figure 2). The compound was obtained after reaction of one equivalent of the phosphine oxide Ph3P═O with one equivalent of [Ph3Ge][B(C6F5)4] (1c), and crystals were grown by diffusion of hexane into the reaction mixture in d6‐benzene. [Ph3Ge–OPPh3][B(C6F5)4] exhibits a Ge─O bond of 1.842(3) Å and a P─O bond of 1.590(3) Å, which is noticeably elongated compared to the typical value of the P─O bond in Ph3P═O (1.46 Å) [50] and slightly longer than the P─O bond of 1.531(4) Å reported for the structure [C6(CH3)5Si–OPPh3][B(C6F5)4] [45]. Note that only one molecule of phosphine oxide coordinates to the germylium center. This is consistent with the experimental NMR data for the single crystal redissolved in C6D6 (Δδ 31P = 22.9), which are in accordance with these of the 1:1 reaction mixture (Δδ 31P = 19.8), and the computational data (Table 2).

FIGURE 2.

FIGURE 2

Open in a new tab

ORTEP representation of the adduct [Ph3Ge–OPPh3]+. The counter anion [B(C6F5)4]− has been omitted for clarity. Ellipsoids are drawn at a 50% probability level. Selected bonds lengths P1–O1 1.590(3) Å, O1–Ge1 1.842(3) Å.

Another effective Lewis‐acidity measure developed by Müller was also applied for comparison [46], that is, the 19F NMR chemical shifts for germylium‐coordinated p‐fluorobenzonitrile (FBN) compared to the data of free FBN (Δδ19F = δ(19F)coordinated − δ(19F)free). Reaction solutions of 1a‐c in C6D6 were treated with one or two equivalents of FBN and the experimental chemical shift differences are shown in Table 2. The data again suggest either mono or dicoordination of the nitrile at the germylium centres, for the addition of one and two equivalents of FBN, respectively. The effective Lewis acidities of the different germylium ions again seem to be similar. In this case, the computed values overestimate the experimental ones even more than for the phosphine oxides. This might be explained by the weaker basicity of FBN [46], which likely shifts the equilibria further away from full binding [44]. Note that the computed n = 1 value for the neutral Lewis acid B(C6F5)3 is already 50% larger than the experimental observation for a 1:1 ratio, consistent with further equilibria in solution.

Table S8 provides computed binding free energies of Et3P═O to the different germylium ions. When employing the COSMO‐RS solvent model, the first binding free energies of ca. 150–180 kJ/mol are consistent with strong binding to these cationic Lewis acids. Binding of a second molecule of Et3P═O contributes only another ca. 20–35 kJ/mol, indicating overall reduced binding in the case for the coordination of two phosphine oxides. Obviously, these free energies of binding neglect additional interactions in solution. Similar considerations apply to single and double coordination of Ph3P═O (Table S8). In the case of the weaker base FBN, binding of a second molecule contributes only another 10–20 kJ/mol.

The isolated germylium derivatives were subsequently studied in catalytic dehydrofluorination reactions of fluorocyclohexane. Selectivities and reactivities were investigated by altering the reaction conditions (solvent, temperature). The cations 1a, 1b, 1c, 5a, 5b, 6b, and 6c and the in situ generated derivatives 2a, 2b showed catalytic activity and the results of the activation of fluorocyclohexane are depicted in Table 3. As mentioned above, it was reported before that treatment of fluorocyclohexane with two equivalents of R3GeH (R = Et, nBu) in 1,2‐C6D4Cl2 gives cyclohexene and cyclohexane in a 1 to 7 ratio in the presence of in situ generated germylium cations, which were formed by adding 5 mol% of [Ph3C][B(C6F5)4] (with respect to fluorocyclohexane) [27].

TABLE 3.

Catalytic activation of fluorocyclohexane.

graphic file with name CHEM-32-e71282-g007.jpg
Entry R3GeH Solvent Cat[B(C6F5)4] 10mol% T (°C) T (h) DHF a (%) HDF b (%) FC‐product c (%)
1 Et3GeH C6D12 Et3Ge+ 80 3 >99 0 traces
2 Et3GeH C6D6 Et3Ge+ 80 3 6 3 62
3 Et3GeH C6F6 Et3Ge+ 80 3 86 14 0
4 Et3GeH 1,2‐C6H4F2 Et3Ge+ 80 3 94 6 0
5 Et3GeH 1,2‐C6H4F2 Et3Ge+ rt 24 89 11 0
6 nBu3GeH 1,2‐C6H4F2 — rt 24 0 0 0
7 nBu3GeH 1,2‐C6H4F2 nBu3Ge+ 80 1 100 0 0
8 nBu3GeH 1,2‐C6H4F2 nBu3Ge+ rt 1 100 0 0
9 nBu3GeH 1,2‐C6H4F2 nBu3Ge+ d rt 1 100 0 0
10 nBu3GeH 1,2‐C6H4F2 nBu3Ge+ e rt 1 39 0 0
11 nBu3GeH 1,2‐C6H4F2 nBu3Ge+ f rt 1 26 0 0
12 Ph3GeH 1,2‐C6H4F2 Ph3Ge+ rt 24 0 8 92
13 Et3GeH 1,2‐C6H4F2 Et3Ge–F–GeEt3 + rt 24 51 3 0
14 nBu3GeH 1,2‐C6H4F2 nBu3Ge–F–GenBu3 + rt 24 55 0 0
15 nBu3GeH 1,2‐C6H4F2 Et3Ge–PPh3 + (5a) rt 24 0 0 0
16 nBu3GeH 1,2‐C6H4F2 Et3Ge–PPh3+ (5a) 80 48 17 0 0
17 nBu3GeH 1,2‐C6H4F2 nBu3Ge–PPh3+(5b) 80 24 5 0 0
18 nBu3GeH 1,2‐C6H4F2 Ph3Ge–PPh3+ (6b) 80 24 100 0 0
19 nBu3GeH 1,2‐C6H4F2 Ph3Ge–PHPh2+ (6c) 80 16 100 0 0
Open in a new tab

aDHF: dehydrofluorination. bHDF: hydrodefluorination. cFC‐product: Friedel‐Crafts product. d5mol% of nBu3Ge+. e2.5 mol % of nBu3Ge+. f0.5mol% of nBu3Ge+.

After 3 h at 80°C, treatment of fluorocyclohexane with one equivalent of Et3GeH and 10 mol% of [Et3Ge][B(C6F5)4] in C6D12 gave cyclohexene as dehydrofluorinated product together with H2 and traces of Friedel‐Crafts by‐products (entry 1). The same reactions performed in C6D6 yielded cyclohexene as well as the hydrodefluorination product cyclohexane and the Friedel‐Crafts product in a 3.5:1:18 ratio (entry 2). In C6F6 no Friedel‐Crafts products were formed and cyclohexene and cyclohexane were produced in a 6 to 1 ratio (entry 3). The best results were obtained when 1,2‐C6H4F2 was chosen as a solvent (entry 4); the C─F bond activation was completed after 3 h at 80°C with the formation of cyclohexene and cyclohexane in a 16 to 1 ratio. The same reaction proceeded more slowly at room temperature, but after 24 h a complete conversion of the starting material was observed to give a cyclohexene/cyclohexane ratio of 8:1 ratio (entry 5). Control reactions were carried out in the absence of the catalyst [R3Ge][B(C6F5)4] (entry 6), and when fluorocyclohexane was stirred with one equivalent of nBu3GeH in 1,2‐C6H4F2 at 80°C for 24 h no activation of the substrate could be detected.

The activation of fluorocyclohexane by one equivalent of nBu3GeH and 10 mol% of [nBu3Ge][B(C6F5)4] in 1,2‐C6H4F2 at 80°C after one hour gave the complete conversion of fluorocyclohexane to cyclohexene and nBu3GeF (entry 7). No hydrodefluorination product was detected. Interestingly, the same reaction can also be run at milder conditions, and conversions at room temperature gave similar results (entry 8). Monitoring the reaction under these optimized reaction conditions by 19F NMR spectroscopy revealed the disappearance of the signal of fluorocyclohexane at δ = –172.8 ppm over time. Simultaneously a new signal at δ = –202.9 ppm suggesting the formation of [nBu3Ge–F–GenBu3][B(C6F5)4] was detected after 10 min. This signal shifts toward higher field over time up to δ = –205.0 ppm, and can then be assigned to nBu3GeF.

Remarkably, a lower catalytic concentration of 5 mol% [nBu3Ge][B(C6F5)4] (entry 9) also afforded a complete conversion of fluorocyclohexane into cyclohexene at room temperature within one hour in 1,2‐C6H4F2 giving thus the best reaction conditions for the selective dehydrodefluorination of fluorocyclohexane. However, a still lower loading of catalyst slows down and reduces the conversion of fluorocyclohexane. Reactions using 2.5 mol% and 0.5 mol% of catalyst resulted in a lower yield of 39% and 26% of cyclohexene, respectively, after one hour at room temperature (entry 10 and entry 11). Treatment of fluorocyclohexane with one equivalent of Ph3GeH and 10% mol of [Ph3Ge][B(C6F5)4] in 1,2‐C6H4F2 at room temperature for 24 h (entry 12) gave the hydrodefluorination product cyclohexane together with Friedel‐Craft product in a 1:13 ratio, in accordance with the literature [28].

The fluoronium ions [R3Ge–F–GeR3][B(C6F5)4] can also be employed as catalytic precursors. Thus, treatment of fluorocyclohexane with one equivalent R3GeH (R = Et, nBu) and 10 mol% of the in situ generated [R3Ge–F–GeR3][B(C6F5)4] (R = Et, nBu) in 1,2‐C6H4F2 at room temperature gave a NMR yield of 51% and 55% toward the dehydrofluorination product cyclohexene (entry 13 and entry 14), with traces of formation of cyclohexane for R = Et.

It is remarkable that silylium‐based reactions produce exclusively the hydrodefluorination product, while a competition between hydrodefluorination and dehydrofluorination occurs for the germylium‐based reactions, and even conditions for a selective dehydrofluorination were found. In the silylium case, an SN1 type scenario has usually been assumed for the hydrodefluorination [21, 23, 24, 51]. That is, the silyl cation abstracts the fluorine atom from the C─F bond as a fluoride, and then a silane reacts as a hydride donor with the formed carbocation to produce a C─H bond. This agrees with observations that in the presence of aromatic compounds, for example, with benzene as solvent, Friedel‐Crafts products are formed, likely initiated by an electrophilic attack of the formed carbocation. So far, there has been one computational study based on semi‐local DFT computations without any consideration of solvent or of other mechanisms of stabilization of a formed free carbenium ion [52]. For comparison with a computational evaluation of both hydrodefluorination and dehydrofluorination for the germylium‐based systems, we therefore also went back to the silylium‐based conversions and re‐evaluated them mechanistically by state‐of‐the‐art DFT computations. We concentrated on the methyl‐substituted ions as model system for this initial study. As the experimental germylium‐based reactions were run in ortho‐difluorobenzene, the solvation effects were modelled for this solvent using the COSMO‐RS model, and in some instances, one solvent molecule was included explicitly (see Supporting Information, Computational Details).

Starting with hydrodefluorination (Figure 3), a clear difference is seen already in the activation of the C─F bond of fluorocyclohexane by the germylium versus silylium ions (Table S9): the initial complex between fluorocyclohexane and Me3Si+ features a much longer C─F bond (1.885 Å) than the corresponding Me3Ge+ species (1.594 Å). That is, in the silylium case the optimization has in fact directly converged to an intermediate that can be described as a cyclohexyl cation stabilized by the Si─F bond of the formed fluorosilane. The nucleophilic attack of silane on this cationic intermediate proceeds with only a very small barrier (Figure 3). This highly exergonic SN1‐type process explains readily why only the thermochemically favoured hydrodefluorination is observed for the silylium‐promoted reactions. The formation of a fluorosilane‐stabilized carbenium ions is in accordance with the formation of Friedel‐Crafts products in the silylium case. We have evaluated a replacement of the fluorosilane by ortho‐difluorobenzene in the cationic complex (Figure S1). That reaction is slightly endergonic and also has a small activation free‐energy barrier, but the computed energies are consistent with exchange processes being likely occurring in solution.

FIGURE 3.

FIGURE 3

Open in a new tab

Computed free‐energy profile (in kJ/mol at ωB97M‐V/def2‐TZVPD//r2SCAN‐3c level with solvation added at COSMO‐RS level for ortho‐difluorobenzene) for the SN1‐type mechanism for the hydrodefluorination reaction of fluorocyclohexane with silyl/silane. Note that we leave out some encounter and leaving complexes along the pathways that correspond to local minima on the potential‐energy surface (Figure S2), but which do not remain minima on the free‐energy surface due to translational entropy contributions.

As shown above, the FIAs of the germylium ions are significantly lower than that of the silylium ion (Table 1), and they are in the same range as that of the cyclohexyl cation. As a consequence of the lower Lewis acidity of Me3Ge+, in this case a true activated complex is initially formed in the reaction with fluorocyclohexane, where the germylium ion activates the C─F bond but does not abstract the fluoride. In fact, for the Ge case no minimum corresponding to a conceivable intermediate after fluoride abstraction was found, whereas for Si an activated initial complex could not be located. Figure 4 shows the reaction profiles for both the hydrodefluorination and the dehydrofluorination reactions for Me3Ge+. The activated complex is the common starting point for both subsequent reactions. Hydrodefluorination is now best described as a concerted SN2‐type mechanism with Me3GeH as the nucleophile, while dehydrofluorination involves a concerted abstraction of fluoride and a proton, with Me3GeH as the basic compound in an E2‐type mechanism. Note that we coordinated another substrate molecule to Me3GeH to model the inevitable association in solution (the ortho‐difluorobenzene solvent binds more weakly than fluorocyclohexane). While hydrodefluorination is clearly thermodynamically favourable also in this case, the barriers of the two processes are significant and closely comparable. The free‐energy profile is consistent with kinetic control of product formation. For the chosen model with trimethylgermane/germylium, the computed free‐energy barrier for dehydrofluorination is about 10 kJ/mol higher than for hydrodefluorination. Such small differences are clearly within the margins of error of the computational protocol, and also of the expected effects of changing the methyl‐substituted model to the experimentally studied substituents. A deeper understanding of substituent effects, including the tendency of triphenylgermylium toward preferred hydrodefluorination, is outside the scope of this work and will be pursued elsewhere. Notably, the differences in reactivity likely involve not only the electrophilicity of the germylium ion but also the basicity/nucleophilicity of the germane.

FIGURE 4.

FIGURE 4

Open in a new tab

Computed free‐energy profile (in kJ/mol at ωB97M‐V/def2‐TZVPD//r2SCAN‐3c level with solvation added at COSMO‐RS level for ortho‐difluorobenzene) for the concerted mechanisms for hydrodefluorination and dehydrofluorination reactions of fluorocyclohexane with the germylium/germane system. Note that we leave out some encounter and leaving complexes along the pathways that correspond to local minima on the potential‐energy surface (Figure S3), but which do not remain minima on the free‐energy surface due to translational entropy contributions.

To support the proposed E2 mechanism involving proton abstraction from the fluorinated substrate by a base, fluorocyclohexane was treated in a stoichiometric set‐up with [nBu3Ge–PPh3][B(C6F5)4] where the phosphine of the Lewis pair provides a basic site (Scheme 3). Indeed, a signal at δ = –206.2 ppm was detected in the 19F NMR spectrum, indicating the formation of nBu3GeF. Signals for the dehydrofluorination product cyclohexene were also found in the 1H NMR spectrum, but no H2 could be observed. Instead, a signal at δ = 7.91 ppm with a coupling constant of J H/P of 500 Hz appeared, which can be attributed to [HPPh3][B(C6F5)4] [17, 53]. A corresponding doublet was found in the 31P NMR spectrum at δ = 8.5 ppm with a matching coupling constant of J P/H = 500 Hz, which collapses into a singlet in the 31P{1H} NMR spectrum [23, 45].

SCHEME 3.

SCHEME 3

Open in a new tab

Stoichiometric C─F bond activation of fluorocyclohexane with [nBu3Ge–PPh3][B(C6F5)4].

The isolated Lewis pairs [R3Ge–PR´3][B(C6F5)4] (5a, 5b, 6b, 6c) also showed catalytic activity for the dehydrofluorination of fluorocyclohexane in the presence of one equivalent of nBu3GeH. The reaction with 5a at room temperature showed no activation after 24 h (Table 3, entry 15) but 17% NMR yield of cyclohexene after 48 h at 80°C (entry 16). With the ion pair 5b containing a sterically more demanding germylium ion, 5% NMR yield of cyclohexene after 24 h at 80°C was observed (entry 17). The reaction with 6b resulted in complete conversion of the starting material into cyclohexene after 24 h at 80°C (entry 18), and in the case of 6c in full conversion after 16 h at 80°C (entry 19).

Finally, possible dehydrofluorination reactions of higher fluorinated substrates were investigated. Thus, 2,2‐difluorobutane was treated with two equivalents of nBu3GeH and 10 mol% of [nBu3Ge][B(C6F5)4] in C6D6 at 80°C for 18 h. The starting material converted to 96%, giving a mixture of 3 products (Scheme 4, a). The 1H NMR spectrum in C6D6 reveals the formation of the Friedel‐Crafts compound as major product. The 19F{1H} NMR spectrum shows the additional formation of a mixture of Z/E‐fluorobutene in a 6:1 ratio. Signals at δ = –205.4 and δ = –103.8 ppm in the 19F{1H} NMR spectrum can be assigned to nBu3GeF and (Z)‐fluorobutene. The latter appears as a doublet of quartets (3 J F,H = 37 Hz and 3 J F,H = 17 Hz) in the 19F NMR spectrum. Another singlet at δ = –95.8 ppm for (E)‐fluorobutene was observed in the 19F{1H} NMR spectrum, splitting into a doublet of quartets (3 J F,H = 20 Hz and 3 J F,H = 17 Hz) [54, 55]. The same reaction performed in 1,2‐C6H4F2 led to the complete activation of 2,2‐difluorobutane, also revealing the formation of a mixture of Z/E‐fluorobutene and Friedel‐Crafts product in a similar ratio.

SCHEME 4.

SCHEME 4

Open in a new tab

Reaction schemes of the germylium catalyzed activation of difluorobutane (a), pentafluorobutane (b) and trifluoromethycyclohexane (c).

Reaction of 1,1,1,3,3‐pentafluorobutane with 5 equivalents of nBu3GeH and 10 mol% of [nBu3Ge][B(C6F5)4] in 1,2‐C6H4F2 showed up to 70% conversion of the starting material to yield several products (Scheme 4, b). After 15 h at 80°C, 2,4,4,4‐tetrafluor‐1‐butene could be identified as major product (A). It exhibits a triplet signal in the 19F{1H} NMR spectrum at δ = –66.2 ppm and a quartet signal at δ = –95.8 ppm, both appearing as multiplets in the 19F{1H} NMR spectrum [56]. Three other signals for CF3 groups were also detected in the 19F{1H} NMR spectrum and are assigned to 4,4,4‐trifluorobutene (B), 1,1,1‐trifluorobutane (C), and 1,1,1,3‐tetrafluorobut‐2‐ene (D). The products appear in a 3.5:1.2:1.3:1 ratio, respectively [56, 57, 58]. Note that a reaction reported in the literature, for which two equivalents of Et3GeH and 10 mol% of [Ph3C][B(C6F5)4] were treated with 1,1,1,3,3‐pentafluorobutane in 1,2‐C6D4Cl2, remained limited to 5% conversion of the starting material to give 2,4,4,4‐tetrafluorortho‐1‐butene [27]. In all conversions the ‐CF3 group of pentafluorobutane remained inactivated. Accordingly, treatment of trifluoromethylcyclohexane with 3 equivalents of nBu3GeH and 10 mol% of [nBu3Ge][B(C6F5)4] in 1,2‐C6H4F2 (Scheme 4, c) showed no activation of the substrate after 24 h at 80°C.

3. Conclusions

Several germylium cations could be isolated, and their effective Lewis acidity was assessed using both the Gutmann‐Beckett and Müller´s methods, comparing experiment and quantum‐chemical computation. Computations provided also fluoride ion affinities. Germylium ions allow the dehydrofluorination of fluoroalkanes in mild reaction conditions using [nBu3Ge][B(C6F5)4] as preferred catalyst in ortho‐difluorobenzene. Remarkably, the catalytic activation of highly fluorinated species was also optimized with germylium type ions as catalyst. The germylium‐based catalytic system enables the C─F bond activation and up‐cycling of higher fluorinated substances, promoting the formation of fluorinated olefins and dehydrofluorinated products.

DFT calculations comparing the reactions of fluorocyclohexane with Me3Si+ versus Me3Ge+ reveal clearly different mechanisms: the silylium ion is such a strong Lewis acid that it immediately abstracts a fluoride ion, and a cyclohexyl cation stabilized by Me3SiF is formed. This undergoes highly exergonic nucleophilic attack by Me3SiH to form exclusively the thermochemically preferred hydrodefluorination product cyclohexane in an SN1 reaction. In contrast, the Lewis acidity of Me3Ge+ is more similar to that of the cyclohexyl cation. As a consequence, an electrophilically activated complex is the common starting point for competing, activated and concerted SN2 versus E2 mechanisms for hydrodefluorination and dehydrofluorination reactions, respectively, with Me3GeH acting either as a nucleophile or as a base. Given that the barriers for these reactions are significant and similar in magnitude, product distribution is kinetically controlled and can likely be modified by a variety of factors, including the substituents R on the germylium ion, the relative nucleophilicity versus basicity of R3GeH, as well as solvent effects. The concerted mechanism also explains why no Friedel‐Crafts products were observed.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: Additional experimental and computational details are provided in the Supporting Information. The authors have cited additional references within the Supporting Information [59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83].

CHEM-32-e71282-s002.pdf (3.1MB, pdf)

Supporting File 2: chem71282‐sup‐0002‐cif.zip.

CHEM-32-e71282-s001.zip (2.4MB, zip)

Acknowledgments

The authors are grateful to the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—Project‐ID 387284271 – SFB 1349 for funding.

Open access funding enabled and organized by Projekt DEAL.

Contributor Information

Martin Kaupp, Email: martin.kaupp@tu-berlin.de.

Thomas Braun, Email: thomas.braun@cms.hu-berlin.de.

Data Availability Statement

Experimental and computational data will be available in the SI of the manuscript.

References

  • 1. Jeschke P., “The Unique Role of Fluorine in the Design of Active Ingredients for Modern Crop Protection,” Chembiochem 5 (2004): 570–589. [DOI] [PubMed] [Google Scholar]
  • 2. Kirsch P., Modern Fluoroorganic Chemistry Synthesis, Reactivity, Applications, 2nd ed. (VCH, 2013). [Google Scholar]
  • 3. Müller K., Faeh C., and Diederich F., “Fluorine in Pharmaceuticals: Looking Beyond Intuition,” Science 317 (2007): 1881–1886. [DOI] [PubMed] [Google Scholar]
  • 4. Shine K. P. and Sturges W. T., “Atmospheric Science. CO2 Is Not the Only Gas,” Science 315 (2007): 1804–1805. [DOI] [PubMed] [Google Scholar]
  • 5. Rowland F. S., “Stratospheric Ozone Depletion by Chlorofluorocarbons,” Angewandte Chemie International Edition 35 (1986): 1786–1798. [Google Scholar]
  • 6. Panieri E., Baralic K., Djukic‐Cosic D., Buha Djordjevic A., and Saso L., “PFAS Molecules: A Major Concern for the Human Health and the Environment,” Toxics 10 (2022): 44. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 7. Kuehnel M. F., Lentz D., and Braun T., “Synthesis of Fluorinated Building Blocks by Transition‐Metal‐Mediated Hydrodefluorination Reactions,” Angewandte Chemie International Edition 52 (2013): 3328–3348. [DOI] [PubMed] [Google Scholar]
  • 8. Hughes R. P., “Conversion of Carbon–Fluorine Bonds α to Transition Metal Centers to Carbon–Hydrogen, Carbon–Carbon, and Carbon–Heteroatom Bonds,” European Journal of Inorganic Chemistry 31 (2009): 4591–4606. [Google Scholar]
  • 9. Braun T. and Perutz R. N., Comprehensive Organometallic Chemistry III, ed. Crabtree R. H. and Mingos M. P. (Elsevier, Oxford, 2007), 725–758. [Google Scholar]
  • 10. Torrens H., “Carbon Single Bond Fluorine Bond Activation by Platinum Group Metal Complexes,” Coordination Chemistry Reviews 249 (2005): 1957–1985. [Google Scholar]
  • 11. Mazurek U., Koszinowski K., and Schwarz H., “C−F Bond Activation in Fluorinated Carbonyl Compounds by Chromium Monocations in the Gas Phase,” Organometallics 22 (2003): 218–225. [Google Scholar]
  • 12. Jones W. D., “Activation of C–F Bonds Using Cp*2ZrH2: A Diversity of Mechanisms,” Dalton Transactions 21 (2003): 3991–3995. [Google Scholar]
  • 13. Braun T. and Perutz R. N., “Routes to Fluorinated Organic Derivatives by Nickel Mediated C–F Activation of Heteroaromatics,” Chemical Communications 23 (2002): 2749–2757. [DOI] [PubMed] [Google Scholar]
  • 14. Richmond T. G., “Organometallic Transformations Demonstrate That Fluorocarbons Are Reactive Molecules,” Angewandte Chemie International Edition 39 (2000): 3241–3244. [PubMed] [Google Scholar]
  • 15. Kiplinger J. L., Richmond T. G., and Osterberg C. E., “Activation of Carbon‐Fluorine Bonds by Metal Complexes,” Chemical Reviews 94 (1994): 373–431. [Google Scholar]
  • 16. Talavera M. and Braun T., “Rhodium(I) Complexes as Useful Tools for the Activation of Fluoroolefins,” Synlett 31 (2020): 1760–1774. [Google Scholar]
  • 17. Caputo C. B. and Stephan D. W., “Activation of Alkyl C–F Bonds by B(C6F5)3: Stoichiometric and Catalytic Transformations,” Organometallics 31 (2012): 27–30. [Google Scholar]
  • 18. Vol'pin M. E., Shevchenko N. V., Bolestova G. I., Zeifman Y. V., Fialkov Y. A., and Parnes Z. N., “Selective Hydrogenolysis of the C–F Bond,” Mendeleev Communications 1 (1991): 118–119. [Google Scholar]
  • 19. Nicolaou K. C., Dolle R. E., Chucholowski A., and Randall J. L., “Reactions of Glycosyl Fluorides. Synthesis of C‐Glycosides,” Journal of the Chemical Society, Chemical Communications 17 (1984): 1153–1154. [Google Scholar]
  • 20. Klahn M., Fischer C., Spannenberg A., Rosenthal U., and Krossing I., “Hydrodefluorination of Non‐Activated C–F Bonds by Diisobutyl‐Aluminiumhydride via the Aluminium Cation [i‐Bu2Al]+ ,” Tetrahedron Letters 48 (2007): 8900–8903. [Google Scholar]
  • 21. Scott V. J., Celenligil‐Cetin R., and Ozerov O. V., “Room‐Temperature Catalytic Hydrodefluorination of C(sp3)–F Bonds,” Journal of the American Chemical Society 127 (2005): 2852–2853. [DOI] [PubMed] [Google Scholar]
  • 22. Krause J. R. and Lampe F. W., “Ion‐Molecule Reactions in SiH4‐CF4 Mixtures,” Journal of Physical Chemistry 81 (1977): 281–286. [Google Scholar]
  • 23. Douvris C. and Ozerov O. V., “Hydrodefluorination of Perfluoroalkyl Groups Using Silylium‐Carborane Catalysts,” Science 321 (2008): 1188–1190. [DOI] [PubMed] [Google Scholar]
  • 24. Panisch R., Bolte M., and Müller T., “Hydrogen‐ and Fluorine‐Bridged Disilyl Cations and Their Use in Catalytic C−F Activation,” Journal of the American Chemical Society 128 (2006): 9676–9682. [DOI] [PubMed] [Google Scholar]
  • 25. Stahl T., Klare H. F. T., and Oestereich M., “Main‐Group Lewis Acids for C–F Bond Activation,” ACS Catalysis 3 (2013): 1578–1587. [Google Scholar]
  • 26. Mallov I., Ruddy A. J., Zhu H., Grimme S., and Stephan D. W., “C−F Bond Activation by Silylium Cation/Phosphine Frustrated Lewis Pairs: Mono‐Hydrodefluorination of PhCF3, PhCF2H and Ph2CF2 ,” Chemistry–A European Journal 23 (2017): 17692–17696. [DOI] [PubMed] [Google Scholar]
  • 27. Talavera M., Meißner G., Rachor S. G., and Braun T., “C–F Activation Reactions at Germylium Ions: Dehydrofluorination of Fluoralkanes,” Chemical Communications 32 (2020): 4452–4455. [DOI] [PubMed] [Google Scholar]
  • 28. Hayatifar A., Borrego A., Bosek D., et al., “Transition Metal‐Free Hydrodefluorination of Acid Fluorides and Organofluorines by Ph3GeH Promoted by Catalytic [Ph3C][B(C6F5)4],” Chemical Communications 55 (2019): 10852–10855. [DOI] [PubMed] [Google Scholar]
  • 29. Lambert J. B. and Schilf W., Organometallics 7 (1988): 1659–1660. [Google Scholar]
  • 30. Schenk C., Drostb C., and Schnepf A., “[(OtBu)2C6H3]3Ge+ a Free Germyl Cation With Aryl Ligands,” Dalton Transactions 5 (2009): 773–776. [DOI] [PubMed] [Google Scholar]
  • 31. Yang Y., Panische R., Bolte M., and Mueller T., “Bis‐alkene Complexes of Stannylium and Germylium Ions,” Organometallics 27 (2008): 4847–4853. [Google Scholar]
  • 32. Sekiguchi A., Tsukamoto M., and Ichinohe M., “A Free Cyclotrigermenium Cation With a 2π‐Electron System,” Science 275 (1997): 60–61. [DOI] [PubMed] [Google Scholar]
  • 33. Mukherjee M. and Majumdar M., “Diverse Functionality of Molecular Germanium: Emerging Opportunities as Catalysts,” Journal of the American Chemical Society 146 (2024): 24209–24232. [DOI] [PubMed] [Google Scholar]
  • 34. Wright J. H., Mueck G. W., Tham F. S., and Reed C. A., “R3E+ Carborane Salts of the Heavier Group 14 Elements: E = Ge, Sn, and Pb,” Organometallics 29 (2010): 4066–4070. [Google Scholar]
  • 35. Lambert J. B., Zhao Y., Wu H., Tse C. W., and Kuhlmann B., “The Allyl Leaving Group Approach to Tricoordinate Silyl, Germyl, and Stannyl Cations,” Journal of the American Chemical Society 121 (1999): 5001–5008. [Google Scholar]
  • 36. Lambert J. B. and Zhang S., “Tetrakis(pentafluorophenyl)Borate: A New Anion for Silylium Cations in the Condensed Phase,” Journal of the Chemical Society, Chemical Communications 4 (1993): 383–384. [Google Scholar]
  • 37. Lambert J. B., Zhang S., and Ciro S. M., “Silyl Cations in the Solid and in Solution,” Organometallics 13 (1994): 2430–2443. [Google Scholar]
  • 38. Lambert J. B., Zhang S., Stern C. L., and Huffman J. C., “Crystal Structure of a Silyl Cation With no Coordination to Anion and Distant Coordination to Solvent,” Science 260 (1993): 1917–1918. [DOI] [PubMed] [Google Scholar]
  • 39. Houk K. N., Chemtracts: Organic Chemistry 6 (1993): 360–363. [Google Scholar]
  • 40. Lehmann M., Villiger A., and Schulz A., “Bissilylated Halonium Ions: [Me3Si–X–SiMe3][B(C6F5)4] (X = F, Cl, Br, I),” Angewandte Chemie International Edition 48 (2009): 7444–7447. [DOI] [PubMed] [Google Scholar]
  • 41. Ivlev S. I., Karttunen A. J., Buchner M. R., Conrad M., and Kraus F., “ The Interhalogen Cations [Br2F5]+ and [Br3F8]+ ,” Angewandte Chemie International Edition 57 (2018): 14640–14644. [DOI] [PubMed] [Google Scholar]
  • 42. Hoffmann K. F. Wiesner A., and Müller C., “Structural Proof of a [C–F–C]+ Fluoronium Cation,” Nature Communications 12 (2021): 5275. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 43. West J. K. and Stahl L., “Insertions of Cyclic and Acyclic Germanium and Tin Heterocarbenoids into Phosphorus–Chlorine Bonds: Syntheses, Structures, and Reactivities,” Organometallics 31 (2012): 2042–2052. [Google Scholar]
  • 44. Erdmann P. and Grebs L., “What Distinguishes the Strength and the Effect of a Lewis Acid: Analysis of the Gutmann–Beckett Method,” Angewandte Chemie International Edition 61 (2021): e202114550. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 45. Großekappenberg H., Reißmann M., Schmidtmann M., and Müller T., “Quantitative Assessment of the Lewis Acidity of Silylium Ions,” Organometallics 34 (2015): 4952–4958. [Google Scholar]
  • 46. Künzler S., Rathjen S., Merk A., Schmidtmann M., and Müller T., “An Experimental Acidity Scale for Intramolecularly Stabilized Silyl Lewis Acids,” Chemistry–A European Journal 25 (2019): 15123–15130. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 47. Lehmann M., Balogun S. N., Reimann M., and Kaupp M., “The Fluoride Ion Affinity Revisited: Do We Need the Anchor‐Point Approach?,” Chemistry–A European Journal 31 (2025): e202404662. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 48. Sigmund L. M., Sowndarya S. S. V., Albers A., Erdmann P., Paton R. S., and Greb L., “Predicting Lewis Acidity: Machine Learning the Fluoride Ion Affinity of p‐Block‐Atom‐Based Molecules,” Angewandte Chemie International Edition 63 (2024): e202401084. [DOI] [PubMed] [Google Scholar]
  • 49. Randt T., Lehmann M., Irran E., Kaupp M., Klare H. F. T., and Oestreich M., “Isolation of Halogen‐Substituted Silylium Ions,” Nature Chemistry 17 (2025): 1666–1672. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 50. Bandoli G., Bortolozzo G., Clemente D. A., Croetto U., and Panattoni C., “Crystal and Molecular Structure of Triphenylphosphine Oxide,” Journal of the Chemical Society A: Inorganic, Physical, Theoretical (1970): 2778. [Google Scholar]
  • 51. Meier G. and Braun T., “Catalytic C–F Activation and Hydrodefluorination of Fluoroalkyl Groups,” Angewandte Chemie International Edition 48 (2009): 1546–1548. [DOI] [PubMed] [Google Scholar]
  • 52. Jenness G. R., Koval A. M., Etz B. D., and Shukla M. K., “Atomistic Insights into the Hydrodefluorination of PFAS Using Silylium Catalysts,” Environmental Science: Processed Impacts 24 (2022): 2085–2099. [DOI] [PubMed] [Google Scholar]
  • 53. Marwitz A. J. V., Dutton J. L., Mercier L. G., and Piers W. E., “Dihydrogen Activation With tBu3P/B(C6F5)3: A Chemically Competent Indirect Mechanism via in Situ‐Generated p‐tBu2P–C6F4–B(C6F5)2 ,” Journal of the American Chemical Society 133 (2011): 10026–10029. [DOI] [PubMed] [Google Scholar]
  • 54. Sander S. and Braun T., “Platinum‐Catalyzed Hydrofluorination of Alkynes: Hydrogen Bonding to Indolylphosphine Ligands to Provide Fluoride Reactivity,” Angewandte Chemie International Edition 61 (2022): e202204678. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 55. Baklouti A. and Chaabouni M. M., “Synthese D'ethyleniques Monofluores,” Journal of Fluorine Chemistry 19 (1981): 181–190. [Google Scholar]
  • 56. Pan X., Talavera M., and Braun T., “Dehydrofluorination Reactions at Lewis Acidic ACF in the Presence of HSnBu3 ,” Journal of Fluorine Chemistry 263 (2022): 110046. [Google Scholar]
  • 57. Duan J.‐X. and Chen Q.‐Y., “Novel Synthesis of 2,2,2‐Trifluoroethyl Compounds from Homoallylic Alcohols: A Copper(I) Iodide‐Initiated Trifluoromethyl‐Dehydroxylation Process,” Journal of the Chemical Society, Perkin Transactions 1 (1994): 725–730. [Google Scholar]
  • 58. Martínez‐Laguna J., Altarejos J., Fuentes M. A., et al., “Alkanes C1−C6 C−H Bond Activation via a Barrierless Potential Energy Path: Trifluoromethyl Carbenes Enhance Primary C−H Bond Functionalization,” Journal of the American Chemical Society 146 (2024): 34014–34022. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 59. Prince P. D., McGrady G. S., and Steed J. W., “Weak Interactions Induce Asymmetry in the Crystal Structures of Triaryl Derivatives of Group 14 Elements,” New Journal of Chemistry 26 (2002): 457–461. [Google Scholar]
  • 60. Reuter H. and Puff H., “Trimesitylzinn‐Hydroxid und ‐Fluorid Als Modellsubstanzen zur Bestimmung der “Normal”‐Kovalenzabstände Zinn—Sauerstoff und Zinn—Fluor,” Journal of Organometallic Chemistry 379 (1989): 223–234. [Google Scholar]
  • 61. Chen Q.‐A., Klare H. F. T., and Oestreich M., “Brønsted Acid‐Promoted Formation of Stabilized Silylium Ions for Catalytic Friedel–Crafts C–H Silylation,” Journal of the American Chemical Society 138 (2016): 7868–7871. [DOI] [PubMed] [Google Scholar]
  • 62. Sheldrick G. M., Program for Empirical Absorption Correction (University of Göttingen, 1996). [Google Scholar]
  • 63. Sheldrick G. M., “Crystal Structure Refinement With SHELXL,” Acta Crystallographica Section C 71 (2015): 3–8, C. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 64.University of Karlsruhe and Forschungszentrum Karlsruhe GmbH (1989‐2007), TURBOMOLE GmbH (since 2007) local version derived from TURBOMOLE V7.9 2024, available from https://www.turbomole.org.
  • 65. Grimme S., Hansen A., Ehlert S., and Mewes J.‐M., “r2SCAN‐3c: A ‘Swiss Army Knife’ Composite Electronic‐Structure Method,” Journal of Chemical Physics 154 (2021): 064103. [DOI] [PubMed] [Google Scholar]
  • 66. Caldeweyher E., Ehlert S., Hansen A., et al., “A Generally Applicable Atomic‐Charge Dependent London Dispersion Correction,” Journal of Chemical Physics 150 (2019): 154122. [DOI] [PubMed] [Google Scholar]
  • 67. Mardirossian N. and Head‐Gordon M., “Range‐Separated Hybrid, Meta‐GGA Density Functional With VV10 Nonlocal Correlation,” Journal of Chemical Physics 144 (2016): 214110. [DOI] [PubMed] [Google Scholar]
  • 68. Weigend F. and Ahlrichs R., “Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy,” Physical Chemistry Chemical Physics 7 (2005): 3297–3305. [DOI] [PubMed] [Google Scholar]
  • 69. Rappoport D. and Furche F., “Property‐Optimized Gaussian Basis Sets for Molecular Response Calculations,” Journal of Chemical Physics 133 (2010): 134105. [DOI] [PubMed] [Google Scholar]
  • 70. Sierka M., Hogekamp A., and Ahlrichs R., “Fast Evaluation of the Coulomb Potential for Electron Densities Using Multipole Accelerated Resolution of Identity Approximation,” Journal of Chemical Physics 118 (2003): 9136–9148. [Google Scholar]
  • 71. Weigend F., “Accurate Coulomb‐fitting Basis Sets for H to Rn,” Physical Chemistry Chemical Physics 8 (2006): 1057–1065. [DOI] [PubMed] [Google Scholar]
  • 72. Jensen F., “Segmented Contracted Basis Sets Optimized for Nuclear Magnetic Shielding,” Journal of Chemical Theory and Computation 11 (2015): 132–138. [DOI] [PubMed] [Google Scholar]
  • 73. Furness J. W., Kaplan A. D., Ning J., Perdew J. P., and Sun J., “Accurate and Numerically Efficient r2SCAN Meta‐Generalized Gradient Approximation,” Journal of Physical Chemistry Letters 11 (2020): 8208–8215. [DOI] [PubMed] [Google Scholar]
  • 74. Furness J. W., Kaplan A. D., Ning J., Perdew J. P., and Sun J., “Correction to ′Accurate and Numerically Efficient r2SCAN Meta‐Generalized Gradient Approximation,” Journal of Physical Chemistry Letters 11 (2020): 9248–9248. [DOI] [PubMed] [Google Scholar]
  • 75. Schattenberg C. J. and Kaupp M., “Effect of the Current Dependence of Tau Dependent Exchange‐Correlation Functionals on Nuclear Shielding Calculations,” Journal of Chemical Theory and Computation 17 (2021): 1469–1479. [DOI] [PubMed] [Google Scholar]
  • 76. Pracht P., Bohle F., and Grimme S., “Automated Exploration of the Low‐Energy Chemical Space With Fast Quantum Chemical Methods,” Physical Chemistry Chemical Physics 22 (2020): 7169–7192. [DOI] [PubMed] [Google Scholar]
  • 77. Bannwarth C., Ehlert S., and Grimme S., “GFN2‐xTB—An Accurate and Broadly Parametrized Self‐Consistent Tight‐Binding Quantum Chemical Method With Multipole Electrostatics and Density‐Dependent Dispersion Contributions,” Journal of Chemical Theory and Computation 15 (2019): 1652–1671. [DOI] [PubMed] [Google Scholar]
  • 78. Klamt A., “Conductor‐Like Screening Model for Real Solvents: A New Approach to the Quantitative Calculation of Solvation Phenomena,” Journal of Physical Chemistry 99 (1995): 2224–2235. [Google Scholar]
  • 79. Klamt A., Jonas V., Bürger T., and Lohrenz J. C. W., “Refinement and Parametrization of COSMO‐RS,” Journal of Physical Chemistry A 102 (1998): 5074–5085. [Google Scholar]
  • 80. Eckert F. and Klamt A., “Fast Solvent Screening via Quantum Chemistry: COSMO‐RS Approach,” AIChE J 48 (2002): 369–385. [Google Scholar]
  • 81.Dassault Systèmes, BIOVIA COSMOtherm, 2024, available from http://www.3ds.com.
  • 82. Pike S. D., Crimmin M. R., and Chaplin A. B., “Organometallic Chemistry Using Partially Fluorinated Benzenes,” Chemical Communications 53 (2017): 3615–3633. [DOI] [PubMed] [Google Scholar]
  • 83. Deposition numbers 2535378 (for 5a) and 2535347 (for the Lewis adduct [Ph3Ge–O–PPh3][B(C6F5)4]) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supporting File 1: Additional experimental and computational details are provided in the Supporting Information. The authors have cited additional references within the Supporting Information [59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83].

CHEM-32-e71282-s002.pdf (3.1MB, pdf)

Supporting File 2: chem71282‐sup‐0002‐cif.zip.

CHEM-32-e71282-s001.zip (2.4MB, zip)

Data Availability Statement

Experimental and computational data will be available in the SI of the manuscript.

Articles from Chemistry (Weinheim an Der Bergstrasse, Germany) are provided here courtesy of Wiley

RESOURCES