# Assessing Fluorosulfonyl Pentafluorooxosulfate (FSO2-OSF5) Reservoir Capacity: Selective SOF4, SO2F2, and [OSF5]- Anion Release

Alexandre Millanvois, Carlota Bahri, Thomas Drews, Simon Steinhauer, Sebastian Riedel

► To cite this version:

Alexandre Millanvois, Carlota Bahri, Thomas Drews, Simon Steinhauer, Sebastian Riedel. Assessing Fluorosulfonyl Pentafluorooxosulfate (FSO2-OSF5) Reservoir Capacity: Selective SOF4, SO2F2, and [OSF5]- Anion Release. Angewandte Chemie, 2025, 137 (36), 10.1002/ange.202510796. hal-05264598

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# Assessing Fluorosulfonyl Pentafluorooxosulfate (FSO₂–OSF₅) Reservoir Capacity: Selective SOF₄, SO₂F₂, and [OSF₅]⁻ Anion Release

Alexandre Millanvois,* Carlota Bahri, Thomas Drews, Simon Steinhauer, and Sebastian Riedel*

**Abstract:** Herein, the investigation of inorganic reagents containing the pentafluorooxosulfate (OSF₅) moiety is presented. First, the gram-scale preparation of the reactive yet stable pentafluorosulfur hypofluorite (FOSF₅) as well as its spectroscopic data are reported. Then, the preparation and full characterization of the corresponding fluorosulfonyl pentafluorooxosulfate (FSO₃SF₅, FSPO) by the fluoro-pentafluorooxosulfanylation of SO₂ are highlighted. This liquid reagent (b.p. 39.6 °C) was later successfully used for modulable release of [cat][OSF₅] ion-pair. The selective, in-solution release of SOF₄ and SOF₄/SO₂F₂ species was also demonstrated. Finally, the SuFEx reactivity was investigated and showed interesting reservoir ability. Indeed, we report the one-pot synthesis of sulfonyl fluoride (-SO₂F) and sulfurimidoyl fluoride (-N=S(O)F₂) species.

## Introduction

Extending the scope of biologically active fluorinated moieties has been a long indirect collaboration between inorganic and organic chemistry. One recent example is the investigation of the pentafluorosulfanyl (–SF₅) group.[1,2] Indeed, from the first synthesis using elemental fluorine (F₂), numerous synthetic improvements were reported up until recently, with the efficient TCICA/KF combination approach.[3] This new accessibility has allowed pentafluorosulfanylation reactions and related (bio)physical applications to be extensively studied.[4–16] For polyfluorinated R–S<sup>V1</sup>F₅

systems, the currently investigated gap concerns group 16 linkers insertion (R–Y–S<sup>V1</sup>F₅) similarly to the development of trifluoromethyl (–CF₃) chemistry.[17–19] Indeed, addition of oxygen (–OCF₃),[20,21] sulfur (–SCF₃),[22–27] or selenium (–SeCF₃)[28–30] bridges have allowed beneficial (bio)physical modulation of the latter moiety.

The strong electron withdrawing effect (–I) of the pentafluorosulfanyl moiety can be a drawback for biology-oriented applications.[31,32] Therefore, it is expected that the use of pentafluorooxosulfate (–OSF₅) can positively modify those properties by reducing electronic effects while inducing flexibility and oxygen interactions. The modern pentafluorooxosulfanylation chemistry[33,34] relies on the generation of silver(I) pentafluorooxosulfate (Ag[OSF₅])[35,36] and the cation exchange strategy,[37] as utilized by Pitts[38] and Qing[39] with a nucleophilic approach. While this method greatly restricts experimental optimization (solvent and concentration) for reaction design, it also permits to avoid handling of the scarcely available thionyl tetrafluoride (SOF₄).

The latter species was introduced by Moissan in 1901 and strongly studied by inorganic chemists for a century. More recently, Sharpless[40–48] demonstrated its use as a versatile precursor for multidimensional SuFEx click chemistry.[49–53] We newly re-investigated this reagent[54] and its synthesis allowing the handling of thionyl tetrafluoride in 100 mmol scale.[37] We then studied OSF₅ ion-pairs and the influence of the cations in organic solvents. This methodology is complementary to the generation previously mentioned. Additionally, we questioned the design of a non-gaseous reagent that would bring modularity to the pentafluorooxosulfate chemistry. We aimed to develop a reservoir able to release [OSF₅]⁻ ions in a controlled manner. To meet these specifications, we considered the chemistry of trifluoromethoxy anionic precursors (Figure 1). Indeed, numerous well-established reagents allow trifluoromethoxy

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Nucleophilic trifluoromethoxy anion precursors.</figcaption></figure>

<footnote>[*] Dr. A. Millanvois, C. Bahri, T. Drews, Dr. S. Steinhauer, Prof. Dr. S. Riedel
Fachbereich Biologie, Chemie, Pharmazie Institut für Chemie und Biochemie – Anorganische Chemie Freie Universität Berlin, Fabeckstr. 34/36, 14195 Berlin, Germany
E-mail: alexml@zedat.fu-berlin.de
s.riedel@fu-berlin.de</footnote>

<footnote>Additional supporting information can be found online in the Supporting Information section</footnote>

<footnote>© 2025 The Author(s). Angewandte Chemie published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
<figure><img src="image_4.png" /><figcaption>**Scheme 1.** Reported synthesis of fluorosulfonyl pentafluorooxosulfate 3.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Scheme 2.** Synthesis of pentafluorosulfur hypofluorite 5.</figcaption></figure>

anion release by reaction with a fluoride anion ($X^{+}F^{-}$) or base.[55-60]

We found in fluorosulfonyl pentafluorooxosulfate (3, $FSO_{3}SF_{5}$) a potential candidate for this chemistry. This “high boiling” (39.6 °C) and water stable[61] liquid has been synthetically studied since 1963 but only partially characterised (IR, $^{19}$F NMR,[62-64] and GED[65]). Interestingly, no applications were reported and its potential for modern (bio)organic chemistry was to be shown. Regarding its synthesis, two main pathways have been described (Scheme 1).

First, the reaction of fluorosulfate hypobromide (1, $FSO_{3}Br$) with sulfur bromide pentafluoride (2, $SF_{5}Br$) which allows access to the desired analogue 3 in high yield (Scheme 1).[66] Unfortunately, this pathways requires the use of the poorly accessible $S_{2}O_{6}F_{2}$ notably known for its potential hazardous synthesis [67] as well as the use of the $BrSF_{5}$ 2 which is not commercially available.

The preparation of $FSO_{3}SF_{5}$ 3 was also reported from ($OSF_{5}$)$_2$ peroxide,[68] fluorine-fluorosulfate $FOSO_{2}F$[69] and corona discharges of wet $SF_{6}$.[70] Additionally, the one-pot oxidative pentafluorooxosulfanylation and fluorination of sulfur dioxide (4, $SO_{2}$, 2 equiv., Scheme 1) was reported by Pass and Roberts in 1963 by thermal activation at 50 °C.[61] In their report, the reaction was performed at medium temperature but high pressure (148 bars). In 1964, Ruff reported a similar reaction using light irradiation affording 3 in 30% yield.[71] Since this route would allow the perfect atom economy reaction and $SOF_{4}$ synthesis is now available in large scale in our laboratory, we decided to re-investigate it for the preparation of 3.

## Results and Discussion

The synthesis of the required hypofluorite 5 was achieved by using a slight excess of *fluorine* (1.1 equiv.) and thionyl tetrafluoride 6 (Scheme 2).[71] The preparation of 5 can also be achieved directly from $SOF_{2}$.[72] This catalysed reaction (using CsF allows fluorination of $CsOSF_{5}$), led to the quantitative gram-scale formation of pentafluorosulfur hypofluorite (5, 21.5 mmol, 3.5 g per batch) after fluorine

<figure><img src="image_6.png" /><figcaption>**Figure 2.** IR spectra at 5 K in neon matrix; Asterisk (*) indicate peaks originating from decomposition of 5 to $SOF_{4}$ in the stainless-steel line connected to the matrix chamber.</figcaption></figure>

removal. ! $F_{2}$ and $FOSF_{5}$ are strong oxidizers and react violently with organic material (see Supporting Information).

With this reagent in hand, we decided to update its structural and theoretical depiction. We first performed gas-phase IR and the spectrum agreed with the literature data (Figure S10). To add further clarity in the IR characterization, we also recorded the IR spectrum by matrix-isolation technique in a neon matrix at 5 K to reduce linewidth and overlaps (Figure 2).

The experimental bands were then assigned, supported by density functional theory (DFT) calculation, to $v_{as}$(OF) 961 cm$^{-1}$, $v_{as}$(SF$_{eq}$) 957 cm$^{-1}$ and 934 cm$^{-1}$, $v_{as}$(SF$_{ax}$) 931 cm$^{-1}$ and $v_{s}$(SF$_{eq}$) 887 cm$^{-1}$. At lower frequencies, we observed rocking ($\rho$) and scissoring ($\delta$): $\rho$(SF$_{eq}$) 612 cm$^{-1}$, $\delta$(SF$_{ax}$) 590 cm$^{-1}$ and $\delta$ (SF$_{eq}$) 578 cm$^{-1}$.

We then recorded (neat, -80 °C, uncorrected, see Supporting Information) the $^{19}$F, $^{17}$O, and $^{33}$S NMR of 5. In the refined[64,73] $^{19}$F NMR spectrum (Figure 3), the -OSF$_{5}$ group appears as an AB$_{4}$ pattern, with a high order quintet ($S-F_{ax}$) $\delta$ = 52.2 ppm and a high order doublet of doublet ($S-F_{eq}$) at $\delta$ = 50.3 ppm with a measured $^{2}$J(F, F) of 153.5 Hz. At 183.9 ppm, we observe a first order quintet for the FOSF$_{5}$ with a $^{3}$J(F, F$_{eq}$) coupling of 17.5 Hz. A $^{1}$ΔF($^{32}$S-$^{33}$S) isotope shift of -0.028 and -0.029 ppm was respectively observed for the equatorial and axial fluorines of -OSF$_{5}$. Regarding the hypohalite (F-O), the isotope shift is -0.014 ppm. In the $^{17}$O NMR spectrum, we observed a broad doublet at $\delta$ = 485.4 ppm with measured couplings $^{1}$J(O,F) of 454.2 Hz (Figure 3). The $^{33}$S NMR showed a broad sextet at -166.8 ppm with a $^{1}$J(S, F) of 256.6 Hz (Figure 3).

We then performed DFT calculations on the hypofluorite 5 and its higher group 16 analogue (FOTeF$_{5}$) at B3LYP/def2-TZVP level using the ORCA 6 program package (Figure 4).

With the minimum structure of both species, we used the natural bond orbital (NBO 7.0) program to compare them. In both cases ($FOYF_{5}$; Y = S, Te), the natural population analysis (NPA) charge showed a small negative one for both reactive fluorine atoms (F-OYF$_{5}$) and that both chalcogen centers display large positive charges. Additional similarities can be observed in the comparable hypofluorite bond distances. The angles' variations could be attributed to the steric hinderance induced by the center's radius. Overall, these results agree with the previous theoretical study of the electron-withdrawing effect for OYF$_{5}$ derivatives.[74]
<figure><img src="image_7.png" /><figcaption>**Figure 3.** Multinuclear NMR Spectroscopic data of 5: ¹⁹F (unscaled); ¹⁷O; ³³S; Experimental spectra (top), simulation (bottom) performed with gNMR 5.0.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 4.** DFT calculations at the B3LYP/def2-TZVP level, minimum structures of FOSF₅ and FOTeF₅, with italic values corresponding to the npa charges. Electrostatic potential mapped onto the electron density isosurface (0.025 a.u.).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Scheme 3.** Preparation of fluorosulfonyl pentafluorooxosulfate 3.</figcaption></figure>

We then performed the reaction of 5 with sulfur dioxide 4 at different pressures. Firstly, the reaction was conducted at 5 bara with liquified SO₂ at 50 °C to mimic the work of Pass (Scheme 3).

After 14 h, trap-to-trap distillation and further purification allowed the isolation of the desired FSO₃SF₅ in 26% yield as a clear liquid and was confirmed by IR spectroscopy (Figure 5).

The main subproducts observed are thionyl tetrafluoride 6 and sulfuryl fluoride (7, SO₂F₂). Compared to the initial study, no large amount of sulfur hexafluoride SF₆ was formed and the yield was slightly higher than the reported 19%. Therefore, we envisaged that further reducing the pressure could allow a more selective synthesis. At 2 bara with 1.4 equiv. of SO₂, we recovered 3 in 15% yield with fluorosulfonic acid (FSO₃H) contamination. Further attempts have not

permitted to increase the yield so far. With the FSPO in hand, we completed its missing characterization (neat ¹⁹F, ¹⁷O and ³³S NMR). We first attempted to elucidate the solid-state structure of FSO₃SF₅ by in-situ crystallization on the diffractometer in a capillary tube at 166 K. Unfortunately, we observed glass transition that deeply complexified the preparation of a single crystal. Therefore, the elucidation is still ongoing in the laboratory.

In the refined ¹⁹F NMR spectrum (Figure 6), the fluorine atoms of the -OSF₅ group appear as a high order doublet of doublet (S-F<sub>eq</sub>) δ = 70.9 ppm and a high order quintet (S-F<sub>ax</sub>) at δ = 54 ppm with the measured ²J(F,F) of 153.7 Hz. In the case of a strong electron withdrawing substituent (e.g. -CF₃[75]) on OYF₅ species (Y = S, Se, Te), an inversion from AB₄ to A₄B pattern is noted for [OSF₅]. We also observed for the axial position a ¹ΔF(³²S-³³S) isotope shift of -0.027 ppm, while the equatorial position is shifted by -0.028 ppm. For the pentafluorooxosulfate group of 3, the average ¹ΔF(³²S-³³S) is -0.0275 ppm. To our delight we also observed the rare quadrupole effect as small satellites lines of ¹⁹F-³³S (0.7% abundance). For the fluorosulfonyl (-SO₂F) moiety, we observed a first-order quintet at δ = 43.8 ppm and a spatial ⁴J(F, F) of 7.5 Hz with the -SF₇eq. A ¹ΔF(³²S-³³S) isotope shift of -0.025 ppm is observed.

Then, we managed to identify two distinct S-F coupling in ³³S NMR at 30.7 MHz for the -OSF₅ moiety. The -OSF₅ is seen at δ = -180.7 ppm as a doublet of quintet. The ¹J(S, F₇eq) is 258.4 Hz and ¹J(S, F₆ₐₓ) 254.1 Hz. Concerning -SO₂F,
<figure><img src="image_10.png" /><figcaption>**Figure 6.** Multinuclear NMR Spectroscopic data of 3: ¹⁹F (unscaled); ¹⁷O; ³³S; Experimental spectra (top), simulation (bottom) performed with gNMR 5.0.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 7.** DFT calculation at the B3LYP/def2-TZVP level, minimum structures of FSO₃SF₅, with italic values corresponding to the npa charges. Electrostatic potential mapped onto the electron density isosurface (0.025 a.u.).</figcaption></figure>

it displays a broad singlet at δ = −46.9 ppm (not depicted in Figure 6). The ¹⁷O NMR spectrum at 54.3 MHz showed a broad singlet for −OSF₅ and a doublet for −SO₂F respectively at δ = 259.7 and δ = 164.7 ppm with measured couplings ²J(O, F) of 32.2 Hz (Figure 6).

To further study the FSO₃SF₅ species and the influence of the fluorosulfonyl group, we performed DFT calculation allowing the depiction of the ESP map and NBO analysis provided the NPA charges (Figure 7).

When compared to 5 (Figure 4, top), we observe a stronger electronic influence of the fluorosulfonyl on the −OSF₅. Interestingly, 3 exhibits charge uniformity for each atom type. Both sulfurs display large positive charges and each oxygen bears most negative ones. Fluorine atoms charges show slight variations across the −SF₅ attributed to spatial/steric interactions.

Finally, inspired by the OCF₃ chemistry and the initial work of Demaerel, De Borggraeve and Ismalaj, we hypothesized that 3 could act as a non-gaseous, OSF₅, SO₂F₂, and SOF₄ reservoir (Scheme 4). First, the generation of Cs[OSF₅] and SO₂F₂ was achieved in quantitative and rapid manner using an excess of CsF in deuterated acetone (Scheme 4.1). This reactivity was previously observed using CsF at 100 °C by Ruff in a solid-gas interface reaction.[71] Then, we decided to compare the formation of NEt₃Me[OSF₅] from FSO₃SF₅ and NEt₃Me[Cl] versus Ag[OSF₅] first published by our group[37] and then by Qing[39] who demonstrated its long term solid-state stability at −30 °C. To our delight, we observed the

formation of the desired ion-pair 10 in 90% yield and sulfuryl chloride fluoride 9, highlighting the versatility of 3 for the controlled formation of OSF₅ anions. A small part (10%) was also converted to SO₂F₂/SOF₄ (Scheme 4.2).

We then envisaged that using sodium fluoride (NaF) could trigger the decomposition of the supposedly instable NaOSF₅ ion-pair (Equation 1).

$$Na[OSF_5] \rightarrow NaF + SOF_4 \quad (1)$$

As the lattice energy is high and the solubility of NaF is low, the reaction would proceed at the solid-liquid interface. After shaking the reaction medium, we first observed traces of SOF₄. We then attempted the addition of dimethylformamide (DMF) to increase the solubility of the fluoride salt (Scheme 4.3). Almost immediately the decomposition of 3 and the release of SOF₄ 6 in 88% yield was observed. We also formed 12% of an unknown OSF₅ ion-pair. Sulfuryl fluoride (33.2 ppm) was formed as traces with a major compound observed at 37.2 ppm. This is close to organic compounds bearing a fluorosulfate moiety or the [FSO₃]⁻ anion. Therefore, we first envisaged the nucleophilic attack on the −SO₂F by the DMF affording a non-stabilized OSF₅ and a cationic species. The [OSF₅]⁻ could then decompose in the desired SOF₄ and the onium in a more stable [FSO₃]⁻ anion. Therefore, a selective release of 6 was obtained and one could directly use the generated thionyl tetrafluoride in a one-pot synthesis. We also observed that Cs₂CO₃ containing low amount of CsOH could generate the sulfuryl fluoride free formation of CsOSF₅ with possibly FSO₃Cs and CO₂.

Using an excess of NaF (17 equiv. shaken in the NMR tube, Scheme 4.4) we observed the slow SOF₄ and SO₂F₂ generation. This reaction was performed in a Young NMR tube; faster reaction rate should be obtained with proper stirring. We also managed to spot the previously unknown Na[OSF₅] ion-pair (up to 11% during the release, ¹⁹F NMR) probably stabilized by MeCNₓ•Na adducts as previously reported for silver salts.[37]

We also examined our reagent for the preparation of new complexes (Equation 2) avoiding the potential use of AgOSF₅ (Equation 3).[31,32]
<figure><img src="image_12.png" /><figcaption>**Scheme 4.** Reservoir ability of FSO₃SF₅: 1) Release of sulfuryl fluoride and caesium pentafluorooxosulfate anion; 2) Release of Sulfuryl chloride fluoride and ammonium pentafluorooxosulfate anion; 3) Selective release thionyl tetrafluoride; 4) Direct release of thionyl tetrafluoride and sulfuryl fluoride; 5) Unsuccessful preparation of novel transition metal bearing the pentafluorooxosulfate ligand.</figcaption></figure>

$$[M-F] + SOF_4 \rightarrow [M - OSF_5] \quad (2)$$

$$[M - Cl] + AgOSF_5 \rightarrow [M - OSF_5] + AgCl \quad (3)$$

Indeed, the direct insertion of thionyl tetrafluoride in metal-fluoride bond would allow the formation of transition-metal bearing the rare pentafluorooxosulfate ligand. We then attempted to extend the scope of known transition metal starting from titanocene difluoride **11**. According to the literature and the related bond dissociation energies (BDE: Ti—F 569 versus Ti—O 662 kJ mol⁻¹), the extrusion of SO₂F₂ and insertion of OSF₅ could proceed. Surprisingly, the desired reactivity was not observed even at high temperature (75 °C) but **3** showed high stability (Scheme 4.5). Further discussion on the thermal stability of **3** can be found in the Supporting Information (DSC analysis, Figures S3–S6). We then envisaged that the S—O bond cleavage of **3** must disfavor the formation of **12** and therefore performed a control experiment using an excess of SOF₄. Unfortunately, we also did not observe the insertion of SOF₄ in the Ti—F bond in contrast to what was reported for Cu or Ni.

Finally, we believed that FSO₃SF₅ must have similar selectivity toward 3-aminophenol **13** than SO₂F₂/SOF₄ previously described by Sharpless (Scheme 4, top left). Therefore, we started investigating the dual-SuFEx reactivity of **3** (Scheme 5). First, we attempted a one-pot experiment that led to the formation of **15** in 25% but several subproducts (including —NH—SO₂F and —N(—SO₂F)₂) and continuous scrambling were observed due to the phenol moiety.

<figure><img src="image_13.png" /><figcaption>**Scheme 5.** One-pot gases release and successive formation of 3-sulfurimidoylphenol **14** and 3-sulfurimidoyl sulfonyl fluoride **15**, NMR yield. Y = S(O)F₂.</figcaption></figure>

To increase the selectivity, we slowly generated SOF₄/SO₂F₂ from **3** using NaF. The addition of 3-aminophenols **13** led to the formation of the sulfurimidoyl (—N=S(O)F₂) **14** almost immediately with the formation of sodium bifluoride. We observed the described selectivity of aniline for SOF₄ and no competition of the phenol. Then, N,N-Diisopropylethylamine (DIPEA, 2 equiv.) was used for the formation of the sulfonyl fluoride moiety (—SO₂F) affording the desired product **15** in 91% yield without the need of large excesses of **7** and **6**.

Using benzylamine **16**, we confirmed the selectivity of primary amine with thionyl tetrafluoride **6** in the generated 1:1 gases mixture (Scheme 6, top). We observed the formation of the sulfurimidoyl **17** in 76% yield. Over-reaction with SO₂F₂ **7** forming Bn-NH—SO₂F was observed in 3% yield.

Finally, phenol **18** was added to the mixture of gases. To our delight, we observed the formation of phenoxythionyl trifluoride **19** in 75% yield without adding complementary base (i.e., DIPEA). In the refined ¹⁹F spectra,[76] we observed a doublet of doublet for the equatorial fluorine with the measured ²J(F,F) of 200.9 and 196.5 Hz and two overlapped doublets for axial fluorines. We also observed
<figure><img src="image_14.png" /><figcaption>**Scheme 6.** Selectivity of primary amine (top) and phenol (bottom) with $SOF_4/SO_2F_2$ gases mixture.</figcaption></figure>

for the equatorial position a ¹ΔF(³²S⁻³³S) isotope shift of -0.030 ppm, while the axial positions are shifted by -0.021 and -0.020 ppm. These results also confirmed the higher electrophilic behavior and reactivity of the valuable thionyl tetrafluoride (Schemes 5 and 6). We also confirmed the higher reactivity of aniline/amine with 6.

## Conclusion

To summarize, we have re-investigated the fluoro-sulfonyl pentafluorooxosulfate ($FSO_3SF_5$) and the pentafluorosulfur hypofluorite ($FOSF_5$) species. For both compounds, we provide the missing NMR characterization (¹⁷O and ³³S) and thoroughly refined the known ¹⁹F NMR data.

With this work, we propose the first alternative to the reported $OSF_5$ chemistry concerning the generation of key anions and gases by establishing $FSO_3SF_5$ ($FSPO$) as a convenient liquid reagent. Indeed, its use as reservoir for Sulfuryl Fluoride/Thionyl Tetrafluoride ($SO_2F_2/SOF_4$) and pentafluorooxosulfate (⁰OSF₅) anion was shown. Not only does this provide modularity for the emerging $OSF_5$ chemistry, it allows the introduction of a new generation of reagents for multidimensional SuFEx click chemistry. We hope that with such reagent in hand, further improvements of the existent literature will be possible. So far, we found that the use of inexpensive NaF as hydrogen fluoride scavenger permits the base free (and glass etching free) formation of sulfurimidoyl fluoride. Further optimization and scale up are ongoing to access better yields. We believe that this class of practical reagents are necessary for the (bio)organic chemistry and for PFAS replacement research. We also actively investigate its use for the development of new distinct pentafluorooxo-sulfanylation methodologies.

Finally, both the uses of $FOSF_5$ as a direct transfer reagent and the synthesis and application of the corresponding ($OSF_5$)₂ peroxide are being developed by our group.

## Acknowledgements

The authors would like to thank the HPC Service of FUB-IT and the Core Facility BioSupraMol supported by the DFG. This work was supported by Freie Universität Berlin, A.M. thanks the University for a postdoctoral Rising Star fellowship. C.B. thanks Dahlem Research School for a PhD scholarship. The authors thank Dr. Paul Golz for useful

discussions and acknowledge Holger Pernice for his help and useful discussions. The authors thank Xiya Xia for recording the matrix IR spectra. Further support is acknowledged by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 387284271 – SFB 1349. The authors thank Solvay Fluor for providing elemental fluorine.

Open access funding enabled and organized by Projekt DEAL.

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article.

**Keywords:** Click chemistry • Pentafluorooxosulfate salts ($OSF_5$) • Storage • SuFEx • Thionyl tetrafluoride

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Manuscript received: May 16, 2025

Revised manuscript received: June 26, 2025

Accepted manuscript online: June 30, 2025

Version of record online: July 30, 2025