[Xe(OTeF₅)(py^F)]⁺: a strong oxidizing xenonium(II) teflate cation with N-donor bases

Abstract

Herein we report on the formation of the adduct salts [Xe(OTeF₅)(py^F)][Al(OTeF₅)₄] (py^F = C₅F₅N, C₅H₃F₂N) by abstraction of an –OTeF₅ group from Xe(OTeF₅)₂ with the Lewis superacid Al(OTeF₅)₃ and subsequent adduct formation of the generated [XeOTeF₅]⁺ cation with fluorinated pyridines. These salts represent the first xenonium cations with the weakly coordinating [Al(OTeF₅)₄]⁻ anion. The strong oxidizing property of these compounds is further assessed.

---

The strong oxidizing [XeF]⁺ cation exhibits a high Lewis acidity and therefore readily forms adducts with e.g. nitrogen bases that are resistant to oxidation.^1,2 In fact, the chemistry of Xe–N species has mostly been realized by coordination of nitrogen-containing bases to stabilize the [XeF]⁺ cation, although they can be produced by the reaction of XeF₂ with a suitable protic acid and the release of HF.³

The pentafluoroorthotellurate (teflate, –OTeF₅) group resembles the fluoride ligand in its high electron withdrawing properties, but with a higher steric demand.^4,5 Therefore, a variety of reactive species, including noble-gas compounds,⁶e.g. Xe(OTeF₅)₂ and Kr(OTeF₅)₂, have been synthesized.^7,8 The teflate analogue of the [XeF]⁺ cation, namely [XeOTeF₅]⁺, was first reported by Sladky in the reaction of FXeOTeF₅ in combination with the Lewis acid AsF₅, which forms the [XeOTeF₅][AsF₆] salt.⁹ Later it was shown, that this salt has a significant cation–anion interaction through a fluoride bridge.¹⁰ This interaction between the xenon(II) centre and the [AsF₆]⁻ anion is indicative of the high Lewis acidity of the [XeOTeF₅]⁺ cation, similar to its well-known fluoride analogue [XeF]⁺. Efforts to isolate the free [XeOTeF₅]⁺ cation in the solid state by utilizing weakly coordinating anions (WCAs) such as [Sb(OTeF₅)₆]⁻ were so far unsuccessful. Instead, the solvent adduct [Xe(OTeF₅)(SO₂ClF)][Sb(OTeF₅)₆] was always obtained, indicating the near-linear alignment at the xenon centre.^11,12 Furthermore, the [XeOTeF₅]⁺ cation has been proved to be a strong two-electron oxidizer, which is e.g. a useful synthon for the preparation of trihalomethyl carbocations.¹³

Additionally, due to its chemical robustness, the teflate group has been utilized to prepare well-performing WCAs.¹⁴ Our group reported on the [Al(OTeF₅)₄]⁻ anion in 2017,¹⁵ which has subsequently allowed the stabilization of highly reactive cations, such as [P₄H]⁺, [(CH₃)₂Cl]⁺, [(C₅H₅P)CH₃]⁺ and [(C(C₆F₅)₃)]⁺.^16–19 Based on these achievements the teflate-based aluminate is a promising candidate for resisting the strong oxidizing property of a xenonium(II) cation. The lack of N-donor adducts of the [XeOTeF₅]⁺ cation, contrary to the well investigated ones for [XeF]⁺, prompted us to extend the chemistry of the teflate derivative, while pushing the limits of the [Al(OTeF₅)₄]⁻ anion to withstand strong oxidizing cations.

Herein we report on the formation and characterization of the cationic pentafluoroorthotellurato xenonium(II) adducts with fluorinated pyridines, namely [Xe(OTeF₅)(py^F)]⁺ (py^F = C₅F₅N, C₅H₃F₂N), as their salts of the weakly coordinating [Al(OTeF₅)₄]⁻ anion. Furthermore, the oxidizing properties of the cation [Xe(OTeF₅)(C₅F₅N)]⁺ have been investigated. Moreover, the molecular structure of [Xe(OTeF₅)(NC₅F₅)][Sb(OTeF₅)₆] was determined via single-crystal X-ray diffraction.

Based on the literature-known synthesis of the [FXe(NC₅F₅)]⁺ cation, starting from [HNC₅F₅][AsF₆] and XeF₂,²⁰ Xe(OTeF₅)₂ was reacted with our recently prepared Brønsted acid [HNC₅F₅][Al(OTeF₅)₄].²¹ However, various attempts to obtain [Xe(OTeF₅)(NC₅F₅)][Al(OTeF₅)₄] (1) with this method were unsuccessful, resulting in multiple teflate- and pentafluoropyridine-containing species. Unfortunately, no xenon-compound was observed by ¹²⁹Xe NMR spectroscopy.

Alternatively, we performed the abstraction of a teflate group from Xe(OTeF₅)₂ with the Lewis superacid Al(OTeF₅)₃,²² whereby both the [Xe(OTeF₅)]⁺ cation and the corresponding WCA, [Al(OTeF₅)₄]⁻, should be formed. The equimolar reaction of Xe(OTeF₅)₂ with Al(OTeF₅)₃ in SO₂ClF at −50 °C successfully results in a non-isolable intermediate with a distinct yellow colour, presumably [Xe(OTeF₅)][Al(OTeF₅)₄]. Characterization of this intermediate was not possible with the low-temperature spectroscopic measurements. By subsequent addition of one equivalent of a fluorinated pyridine (py^F = C₅F₅N, C₅H₃F₂N), the cationic adducts [Xe(OTeF₅)(NC₅F₅)]⁺ (1) and [Xe(OTeF₅)(NC₅H₃F₂)]⁺ (2) as salts of the [Al(OTeF₅)₄]⁻ anion are formed (Scheme 1).

Scheme 1 Synthesis of [Xe(OTeF₅)(py^F)][Al(OTeF₅)₄] (py^F = C₅F₅N, C₅H₃F₂N).

The temperature-sensitive compounds 1 and 2 were characterized by NMR spectroscopy at −50 °C in SO₂ClF, confirming the successful formation of the desired species (Table 1). Variable temperature NMR studies showed that compounds 1 and 2 decompose at temperatures higher than −10 °C. In the ¹⁹F NMR spectrum of 1, two magnetically inequivalent –OTeF₅ groups with overlapping signals are observed. Simulation of the spectrum confirmed the expected two AB₄ spin systems assigned to one teflate group in the cation and the four teflate groups located at the aluminium centre forming the WCA (Fig. 1a). The spectroscopic parameters of the latter are in agreement with those previously reported for the [Al(OTeF₅)₄]⁻ anion.¹⁵ The formation of this WCA to stabilize the xenonium(II) cation is additionally proved by ²⁷Al NMR spectra, as both adduct salts 1 and 2 show the characteristic resonance at δ(²⁷Al) ≈ 46 ppm for the [Al(OTeF₅)₄]⁻ anion.

Table 1 ¹⁹F, ²⁷Al and ¹²⁹Xe NMR parameters for [Xe(OTeF₅)(py^F)][Al(OTeF₅)₄] (py^F = C₅F₅N, C₅H₃F₂N)^a

Species	Chemical shift (δ)^b^c [ppm]							Coupling constant^b [Hz]
	^19FA	^19FB	^129Xe	^19Fo	^19Fm	^19Fp	^27Al	^1 J(^19FA,^125Te)	^2 J(^19FA,^19FB)	^3 J(^19Fo,^129Xe)
a All NMR spectra were recorded in SO2ClF at −50 °C with [D6]acetone external locking. b The ^19F NMR data are reported according to the simulated spectra obtained with the gNMR software (see ESI). c The symbols FA and FB denote equatorial and axial fluorine atoms, respectively, within the AB4 spin system of the –OTeF5 group. The symbols Fo, Fm, Fp represent ortho-, meta- and para-fluorine atoms of the fluorinated pyridine moieties.
[Xe(OTeF5)(NC5F5)]^+	−45.4	−41.6	−2241	−86.7	−152.0	−108.3	—	3598	177	69
[Al(OTeF5)4]^−	−38.4	−45.7	—	—	—	—	45.9	3353	188	—
[Xe(OTeF5)(NC5H3F2)]^+	−43.1	−42.5	−2433	−67.6	—	—	—	3488	180	57
[Al(OTeF5)4]^−	−38.1	−45.5	—	—	—	—	46.2	3338	186	—

---

Fig. 1 NMR spectra of [Xe(OTeF₅)(NC₅F₅)][Al(OTeF₅)₄] (1) in SO₂ClF ([D₆]acetone (external lock), −50 °C). (a) ¹⁹F NMR (282 MHz) spectra showing the teflate region of (top to bottom) experimental 1, simulated 1, simulated [Al(OTeF₅)₄]⁻ anion and simulated [Xe(OTeF₅)(NC₅F₅)]⁺ cation. (b) ¹⁹F NMR (282 MHz) spectrum of 1 depicting the ortho fluorine atoms of the pentafluoropyridine moiety, with the corresponding ¹²⁹Xe satellites. (c) ¹²⁹Xe NMR (83 MHz) spectrum of 1 depicting a broad singlet.

The AB₄ pattern of the cations is inverted compared to the one of the [Al(OTeF₅)₄]⁻ anion, but in agreement with the observed ¹⁹F NMR spectrum of the previously reported [Xe(OTeF₅)(SO₂ClF)]⁺ cation.¹² The chemical shifts of the [Xe(OTeF₅)(NC₅F₅)]⁺ cation in 1 are found to be δ(¹⁹F_A) = −45.4 ppm and δ(¹⁹F_B) = −41.6 ppm, with the corresponding ²J(¹⁹F_A,¹⁹F_B) coupling constant of 177 Hz (Table 1). The resonances of the teflate group in the [Xe(OTeF₅)(NC₅H₃F₂)]⁺ cation in 2 are less separated, appearing at δ(¹⁹F_A) = −43.1 ppm and δ(¹⁹F_B) = −42.5 ppm, with the ²J(¹⁹F_A,¹⁹F_B) coupling constant being 180 Hz (Table 1).

Furthermore, the ¹⁹F NMR chemical shifts arising from the C₅F₅N moiety in the [Xe(OTeF₅)(NC₅F₅)]⁺ cation in 1 (Table 1) are found to be downfield shifted with respect to those of neat C₅F₅N (Δδ(¹⁹F_ortho) = 3.0 ppm, Δδ(¹⁹F_meta) = 10.8 ppm, Δδ(¹⁹F_para) = 26.6 ppm). The ortho-fluorine resonance is accompanied by ¹²⁹Xe (I = ½, 26.4%) satellites, arising from the ³J(¹⁹F,¹²⁹Xe) spin–spin coupling (Fig. 1b). The presence of xenon satellites unambiguously demonstrates the coordination of C₅F₅N to the xenon(II) centre. In the ¹⁹F NMR spectrum of 2, two different C₅H₃F₂N species are observed. The signal corresponding to the cation of product 2 is easily identified, as it exhibits the characteristic ¹²⁹Xe satellites with a ³J(¹⁹F,¹²⁹Xe) coupling constant of 57 Hz. The secondary signal, resonating at a higher field, δ(¹⁹F_ortho) = −77.8 ppm, than neat C₅H₃F₂N is found to be [HNC₅H₃F₂]⁺ cation by a control experiment.

The ¹²⁹Xe NMR spectra of 1 and 2 consist of one xenon resonance appearing as a broad singlet at δ(¹²⁹Xe) = −2241 ppm (Fig. 1c) and δ(¹²⁹Xe) = −2433 ppm, respectively, indicating the presence of a single xenon species in each case. When compared to the previously reported [XeOTeF₅]⁺ cation in SO₂ClF at −50 °C, which shows a chemical shift of δ(¹²⁹Xe) = −1459.5 ppm,¹² the ¹²⁹Xe chemical shifts of 1 and 2 are found to be shifted to lower frequencies upon adduct formation with nitrogen bases.

To further characterize our new xenonium cations, concentrated reaction mixtures of 1 and 2 were analysed by low-temperature Raman spectroscopy. The presence of strong fluorescence interference in the spectrum of 1 rendered its interpretation difficult. In the case of 2 (see Fig. S3.1 in ESI†), characteristic bands can be observed, which are in good agreement with the computed wavenumbers for the [Xe(OTeF₅)(NC₅H₃F₂)]⁺ cation at the B3LYP/def2-TZVPP level of theory. In particular, the prominent bands observed at [small nu, Greek, tilde] = 140, 241 and 581 cm⁻¹ are assigned to the stretching modes of O–Xe–N, Te–O–Xe and Xe–N, respectively. According to our experimental measurements, bands at [small nu, Greek, tilde] = 303, 432 and 1221 cm⁻¹ belong to remaining free SO₂ClF in the sample. On the other hand, the Te–O and Te–F bands are overlapped with those from the [Al(OTeF₅)₄]⁻ anion, therefore hampering the assignment of the bands in the teflate region, 600–780 cm⁻¹.

All our crystallization attempts for 1 and 2 to investigate the molecular structure of the new xenonium(II) cationic adducts in the solid state were so far unsuccessful. Consequently, we changed the anion from [Al(OTeF₅)₄]⁻ to [Sb(OTeF₅)₆]⁻, which resulted useful to crystallize the [Xe(OTeF₅)(SO₂ClF)]⁺ cation.^11,12 The salt [Xe(OTeF₅)(NC₅F₅)][Sb(OTeF₅)₆] (3) was obtained by a similar procedure as 1 and 2. Namely, the salt [Xe(OTeF₅)(SO₂ClF)][Sb(OTeF₅)₆] was treated with one equivalent of C₅F₅N, forming a yellow solution. Characterization by multinuclear NMR spectroscopy showed that the ¹⁹F and ¹²⁹Xe NMR data of 3 are in good agreement with those of the cation in the salt 1 (see ESI† for the complete data set). Unfortunately, the Raman spectrum of 3 shows strong fluorescence, which did not allow a detailed analysis, likewise in the case of 1.

Upon slowly cooling down the reaction mixture from −50 °C to −80 °C, single crystals of 3 suitable for X-ray diffraction could be obtained. Compound 3 crystallizes in the triclinic space group P1̄. The O1, Xe1 and N1 atoms of the cation are linearly aligned with an angle of 179.2(2)° (Fig. 2), which is consistent with the AX₂E₃ VSEPR molecular structure for a xenon(II) species and closer to 180° than the previously reported O–Xe–O angle in the molecular structure of [Xe(OTeF₅)(SO₂ClF)][Sb(OTeF₅)₆] (174.2(2)°).¹² The Te1–O1–Xe1 angle in 3 is 121.4(2)°, which is similar to the related SO₂ClF adduct salt (120.8(2)°).¹² The distance between the xenon and the oxygen atom of the teflate group in 3 is slightly elongated (Xe1–O1 = 202.8(4) pm), and the distance from the xenon to the nitrogen atom is shortened (Xe1–N1 = 233.4(5) pm), when compared with [Xe(OTeF₅)(SO₂ClF)]⁺ (Xe–O_teflate = 196.9(4) pm, Xe–O_solvent = 247.9(4) pm), and is in agreement with the higher basicity of C₅F₅N than of SO₂ClF. The measured O1–Te1 distance of 188.3(5) pm is found to be in between the values reported for Xe(OTeF₅)₂ (184.3(11) pm)¹⁰ and the [Xe(OTeF₅)(SO₂ClF)]⁺ cation (193.8(5) pm).¹²

Fig. 2 Molecular structure of [Xe(OTeF₅)(NC₅F₅)][Sb(OTeF₅)₆] (3) in the solid state. Displacement ellipsoids set at 50% probability. Selected bond lengths [pm] and angles [°] are depicted. For crystallographic details see ESI.†

Finally, to demonstrate the oxidation potential of [Xe(OTeF₅)(NC₅F₅)][Al(OTeF₅)₄] (1), the compound was reacted with an excess of tris(2,4,6-tribromophenyl)amine in order to oxidize it to the corresponding ammoniumyl radical cation, which is a variation of tris(4-tribromophenyl)amine (“magic blue”) with an even higher standard oxidation potential.²³ An immediate colour change from brown to deep-purple with gas evolution was observed upon addition of the amine, indicating the formation of the tris(2,4,6-tribromophenyl)ammoniumyl radical cation. This was proved by EPR spectroscopy showing one broad signal with a g value of 2.009, assigned to the radical cation (see Fig. S5, ESI†). Furthermore, the oxidation potential of xenonium cations has been evaluated by adiabatic ionization energy calculations at the B3LYP/def2-TZVPP level of theory and resulted to be XeF/XeF⁺ 10.7, Xe(OTeF₅)/[Xe(OTeF₅)]⁺ 10.6, and Xe(OTeF₅)(NC₅F₅)/[Xe(OTeF₅)(NC₅F₅)]⁺ 9.2 eV (see also ESI,† Table S4).

In conclusion, we have demonstrated that the Lewis superacid Al(OTeF₅)₃ is able to abstract a teflate group from Xe(OTeF₅)₂ to presumably form the [Xe(OTeF₅)]⁺ cation and the weakly coordinating [Al(OTeF₅)₄]⁻ anion. This intermediate could be subsequently stabilized upon coordination of the oxidation-resistant nitrogen bases C₅F₅N and C₅H₃F₂N. This way, the [Xe(OTeF₅)(py^F)][Al(OTeF₅)₄] (py^F = C₅F₅N, C₅H₃F₂N) salts were prepared and characterized by low-temperature NMR and Raman spectroscopy, entailing the first xenonium(II) cations stabilized by the [Al(OTeF₅)₄]⁻ WCA. Also, the synthesis of the related [Sb(OTeF₅)₆]⁻ salt of the [Xe(OTeF₅)(NC₅F₅)]⁺ cation enabled us to structurally characterize this unprecedented cation for the first time. Moreover, we have experimentally shown the high oxidation potential of the adduct salt [Xe(OTeF₅)(NC₅F₅)][Al(OTeF₅)₄] (1), which oxidizes tris(2,4,6-tribromophenyl)amine to form the corresponding ammoniumyl radical cation.

This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project ID: 387284271 (SFB 1349: Fluorine-Specific Interactions) and the European Research Council (ERC) – Project HighPotOx. A. P.-B. thanks the Fonds der Chemischen Industrie for a Liebig Fellowship. Computing resource was provided by the Zentrum für Elektronische Datenbearbeitung (ZEDAT) at Freie Universität Berlin. The authors thank the core facility Biosupramol for analytical measurements and acknowledge MSc Liza Richter (Humboldt-Universität zu Berlin), MSc Deniz Meyer (Freie Universität Berlin) and MSc Johanna Schlögl (Freie Universität Berlin) for performing spectroscopic measurements.

Conflicts of interest

There are no conflicts to declare.

Notes and references

1. A. A. A. Emara and G. J. Schrobilgen, J. Chem. Soc., Chem. Commun., 1987, 21, 1644.
2. G. L. Smith, H. P. A. Mercier and G. J. Schrobilgen, Inorg. Chem., 2007, 46, 1369.
3. J. F. Sawyer, G. J. Schrobilgen and S. J. Sutherland, J. Chem. Soc., Chem. Commun., 1982, 4, 210.
4. K. Seppelt, Angew. Chem., Int. Ed. Engl., 1982, 21, 877.
5. M. Gerken, H. P. A. Mercier and G. J. Schrobilgen, in Advanced Inorganic Fluorides, ed. T. Nakajima, B. Žemva and A. Tressaud, Elsevier, Lausanne, 2000, pp. 117–174.
6. D. S. Brock, G. J. Schrobilgen and B. Žemva, in Comprehensive Inorganic Chemistry II, ed J. Reedijk and K. Poeppelmeier, Elsevier, Oxford, 2nd edn, 2013, ch. 1.25, vol. 1, pp. 755–822.
7. F. Sladky, Angew. Chem., Int. Ed. Engl., 1969, 8, 523.
8. J. C. P. Sanders and G. J. Schrobilgen, J. Chem. Soc., Chem. Commun., 1989, 20, 1576.
9. F. Sladky, Angew. Chem., Int. Ed. Engl., 1970, 9, 375.
10. B. A. Fir, H. P. A. Mercier, J. C. P. Sanders, D. A. Dixon and G. J. Schrobilgen, J. Fluorine Chem., 2001, 110, 89.
11. P. Ulferts and K. Seppelt, Z. Anorg. Allg. Chem., 2004, 630, 1589.
12. H. P. A. Mercier, M. D. Moran, J. C. P. Sanders, G. J. Schrobilgen and R. J. Suontamo, Inorg. Chem., 2005, 44, 49.
13. H. P. A. Mercier, M. D. Moran, G. J. Schrobilgen, C. Steinberg and R. J. Suontamo, J. Am. Chem. Soc., 2004, 126, 5533.
14. I. M. Riddlestone, A. Kraft, J. Schaefer and I. Krossing, Angew. Chem., Int. Ed., 2018, 57, 13982.
15. A. Wiesner, T. W. Gries, S. Steinhauer, H. Beckers and S. Riedel, Angew. Chem., Int. Ed., 2017, 56, 8263.
16. A. Wiesner, S. Steinhauer, H. Beckers, C. Müller and S. Riedel, Chem. Sci., 2018, 9, 7169.
17. S. Hämmerling, G. Thiele, S. Steinhauer, H. Beckers, C. Müller and S. Riedel, Angew. Chem., Int. Ed., 2019, 58, 9807.
18. L. Fischer, F. Wossidlo, D. Frost, N. T. Coles, S. Steinhauer and S. Riedel, Chem. Commun., 2021, 57, 9522.
19. K. F. Hoffmann, D. Battke, P. Golz, S. M. Rupf, M. Malischewski and S. Riedel, Angew. Chem., Int. Ed., 2022, 61, e202203777.
20. A. A. A. Emara and G. J. Schrobilgen, J. Chem. Soc., Chem. Commun., 1988, 4, 257.
21. S. Kotsyuda, A. N. Toraman, P. Voßnacker, M. A. Ellwanger, S. Steinhauer, C. Müller and S. Riedel, Chem. – Eur. J., 2023, 29, e202202749.
22. K. F. Hoffmann, A. Wiesner, S. Steinhauer and S. Riedel, Chem. – Eur. J., 2022, 28, e202201958.
23. W. Schmidt and E. Steckhan, Chem. Ber., 1980, 113, 577.

---

Footnote

† Electronic supplementary information (ESI) available. CCDC 2302485. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3cc05560d

---