Reactions of Bromine Fluoride Dioxide, BrO₂F, for the Generation of the Mixed‐Valent Bromine Oxygen Cations Br₃O₄ ⁺ and Br₃O₆ ⁺

Abstract

A reliable synthesis of unstable and highly reactive BrO₂F is reported. This compound can be converted into BrO₂ ⁺SbF₆ ⁻, BrO₂ ⁺AsF₆ ⁻ _, and BrO₂ ⁺AsF₆ ⁻⋅2 BrO₂F. The latter decomposes into mixed‐valent Br₃O₄⋅Br₂ ⁺AsF₆ ⁻ with five‐, three‐, one‐, and zero‐valent bromine. BrO₂ ⁺ H(SO₃CF₃)₂ ⁻ is formed with HSO₃CF₃. Excess BrO₂F yields mixed‐valent Br₃O₆ ⁺OSO₃CF₃ ⁻ with five‐ and three‐valent bromine. Reactions of BrO₂F and MoF₅ in SO₂ClF or CH₂ClF result in Cl₂BrO₆ ⁺Mo₃O₃F₁₃ ⁻. The reaction of BrO₂F with (CF₃CO)₂O and NO₂ produces O₂Br‐O‐CO‐CF₃ and the known NO₂ ⁺Br(ONO₂)₂ ⁻. All of these compounds are thermodynamically unstable.

Thermodynamically unstable: A reliable synthesis of unstable and highly reactive BrO₂F is reported. This compound can be converted into BrO₂ ⁺AsF₆ ⁻⋅2 BrO₂F, which decomposes into Br₃O₄⋅Br₂ ⁺AsF₆ ⁻ with five‐, three‐, one‐, and zero‐valent bromine. Br₃O₆ ⁺SO₃CF₃ ⁻ contains three‐ and five‐valent bromine.

Bromine fluoride dioxide (bromyl fluoride) has long been known,1 and its pyramidal structure has been established by spectroscopic methods.2 It is a very reactive and unstable species that decomposes above 10 °C, often with explosion. Herein, we present a reliable and safe procedure for its high‐yielding preparation in a PFA tube system between −78° and −10 °C in amounts of 100–200 mg [Eq. (1)].

$${\displaystyle 2\mathrm{NaBrO}{}_3+\mathrm{BrF}{}_5+2\mathrm{HF}\to 3\mathrm{BrO}{}_2\mathrm{F}+2\mathrm{NaHF}{}_2}$$ (1)

A previous single‐crystal determination had suffered from O/F disorder.3 However, recrystallization from acetone at low temperatures produced several adducts. In the adduct 3 BrO₂F⋅4 acetone, the bond lengths are undisturbed by disorder: r _BrO=1.587–1.620(2) and r _BrF=1.781–1.822(2) Å. Solutions in SO₂ClF or CH₂ClF are stable at low temperature if all reductive reagents (H₂O!) are excluded. Even in anhydrous HF slow decomposition occurs (Scheme 1).

Scheme 1.

Reactions of BrO₂F. [a] See Ref. 4. [b] See Ref. 5.

SbF₅ and BrO₂F form BrO₂ ⁺SbF₆ ⁻. This product is identical to the one that has been obtained recently in the reaction of BrO₃F with SbF₅ under loss of oxygen.4 AsF₅ works in the same way as SbF₅, giving BrO₂ ⁺AsF₆ ⁻. This compound can be sublimed with some decomposition in vacuum at 10 °C. This indicates that the fluoride ion affinity of AsF₅ is just large enough for the formation of this ionic species. AsF₅ as a gas can easily be applied in various amounts relative to BrO₂F: In a reaction with excess BrO₂F, crystals of BrO₂ ⁺AsF₆ ⁻⋅2 BrO₂F are formed. These turned into dark‐red Br₃O₄⋅Br₂ ⁺AsF₆ ⁻ under loss of oxygen after standing for days at −30 °C.

The cation Br₃O₄ ⁺⋅Br₂ of this salt is shown in Figure 1. The Br₂ part of the cation can be described as a Br₂ molecule attached to the Br‐O part of the cation: The Br−Br bond length of 2.280(1) Å), the Br−Br⋅⋅⋅Br bond angle of 104.8(1)°, and the corresponding Raman line of 297.5 cm⁻¹ are typical for molecular bromine bonded through halogen bonding. The Br₃O₄ ⁺ cation can be viewed as a combination of BrO₂ ⁺ and neutral O=Br‐O‐Br or as O₂Br‐O‐Br⁺‐O‐Br. In each description, it contains one‐, three‐, and five‐valent bromine (in addition to the zero‐valent Br₂).

Figure 1.

Cation 1 in Br₃O₄ ⁺⋅Br₂AsF₆ ⁻. Cation 2 (almost identical) and anions are omitted. Displacement parameters (also in all figures below) set at 50 %. Distances given in Å. Angles: O1‐Br1‐O2 110.6°, O3‐Br2‐O4 103.5°, O4‐Br3⋅⋅⋅Br4 177.0°.

HSO₃CF₃ dissolves BrO₂F under formation of BrO₂ ⁺ H(SO₃CF₃)₂ ⁻. The anion H(SO₃CF₃)₂ ⁻ has only occasionally been observed;6 the non‐symmetric O−H⋅⋅⋅O bridge here is 2.515 Å long, as compared to 2.410 Å in Ref. 6.

When an excess of BrO₂F relative to HSO₃CF₃ was applied, brown crystals of Br₃O₆ ⁺SO₃CF₃ ⁻ were obtained. The cation of Br₃O₆ ⁺SO₃CF₃ ⁻ can be described as a combination of two BrO₂ ⁺ units and one BrO₂ ⁻ that weekly interact. The geometries of the two BrO₂ ⁺ units are very similar to those observed in the neat BrO₂ ⁺ compounds. Little is known about bromite, BrO₂ ⁻: The preparation of NaBrO₂ is quite tedious.7 A crystal structure determination on NaBrO₂⋅3 H₂O reveals r _Br‐O=1.701(2), 1.731(2) Å, and δ _O‐Br‐O=105.3(1)°.8 For our BrO₂ ⁻ unit, these data are r _Br‐O=1.733(1), 1.739(1) Å, and δ _O‐Br‐O=102.7(1)°. The Br₃O₆ ⁺ cation is overall close to C ₂ symmetry. Aside from the description as BrO₂ ⁺⋅BrO₂ ⁻⋅BrO₂ ⁺, this cation could also be described as a Br^III–dibromate(V) cation, albeit with two extreme long central bromine–oxygen bonds (Figure 2).

Figure 2.

The cation Br₃O₆ ⁺ in Br₃O₆ ⁺ OSO₂CF₃ ⁻; distances in Å. Angles: O1‐Br1‐O2 110.3°, O3‐Br2‐O4 102.8°, O5‐Br3‐O6 108.9°.

BrO₂F and (CH₃)₃Si‐OSO₂CF₃ in SO₂ClF also react to Br₃O₆ ⁺SO₃CF₃ ⁻, now in the form of a yellow fine powder, as confirmed by its identical Raman spectrum (see the Supporting Information).

In speculations about the formation of these mixed‐valent cations, the intermediacy of the free radical ^.BrO₂ could be considered. In contrast to long‐known ^.ClO₂, it has never been isolated. It has been detected in matrices,9 by microwave,10 and UV/Vis spectroscopy,11 and it has been postulated as a central intermediate in the Belousov–Zhabotinsky oscillating reaction.12 We often observed violet solutions in our reactions, although always for only a short period of time. This species seems to dimerize at low temperature, similar to ^.ClO₂.13 A dimer Br₂O₄ might dissociate into BrO₂ ⁺BrO₂ ⁻, which in turn could react with BrO₂ ⁺ to Br₃O₆ ⁺. Obviously not many cases of such a radical dimer dissociation into an ion pair are known; the dissociation of N₂O₄ into solid NO⁺NO₃ ⁻ in the presence of IF₅ is one example.14

The reaction of BrO₂F with MoF₅ in SO₂ClF or CH₂ClF offers another surprise: Aside from an ochre‐colored powder and colorless crystals, a red‐brown crop of crystals was always obtained, with the composition Cl₂BrO₆ ⁺Mo₃O₃F₁₃ ⁻. The cation can be formulated as ClO₂ ⁺⋅BrO₂ ⁻⋅ClO₂ ⁺, similar to BrO₂ ⁺⋅BrO₂ ⁻⋅BrO₂ ⁺. Because of the extreme oxidation power of BrO₂F, a lot of atom scrambling has obviously occurred with the solvents (Figure 3).

Figure 3.

The cation BrCl₂O₆ ⁺ in BrCl₂O₆ ⁺OSO₂CF₃ ⁻; distances in Å. Angles: O1‐Cl1‐O2 116.0°, O3‐Br1‐O4 105.1°, O5‐Cl2‐O6 115.7°.

The reaction of BrO₂F with neat (CF₃‐CO)₂O affords O₂Br‐O‐CO‐CF₃ as a pale‐yellow solid that melts at −12 °C, and inevitably explodes upon further warming (Figure 4).

Figure 4.

Molecule 1 in the crystal structure of O₂Br‐O‐CO‐CF₃; distances in Å. Angles: O1‐Br1‐O2 110.3°, O1‐Br1‐O3 98.5°, O2‐Br1‐O3 97.3°. The three independent molecules in the unit cell differ mainly only in the torsion of the CF₃ group.

The reaction of BrO₂F with NO₂ gives the known compound NO₂ ⁺Br(ONO₂)₂ ⁻ in quantitative yield as a colorless crystalline solid, formerly made from N₂O₅ and Br‐ONO₃.5 The central Br^I is linearly bonded to two oxygen atoms, as expected, and the overall structure is centrosymmetric (Figure 5).

Figure 5.

The anion Br(NO₃)₂ ⁻ in NO₂ ⁺Br(NO₃)₂ ⁻; distances in Å. Angles: Br1‐O1‐N1 116.2°; sum of angles at N1: 360.0°.

The structures of the cations Br₃O₄ ⁺, Br₃O₆ ⁺, BrCl₂O₆ ⁻, of the compound O₂Br‐OCO‐CF₃, and of the anion Br(NO₃)₂ ⁻ have been calculated by the methods B3LYP, MP2, and B97D. Whereas the direct bonds and angles were satisfactorily reproduced, the contact lengths between the units in Br₃O₄ ⁺, Br₃O₆ ⁺, and BrCl₂O₆ ⁺ were too long. The B3LYP method gives the best results among the three methods. However, the long‐distance interactions are still so far off from the experimental values that the calculations of the vibrational spectra are unreliable (see the Supporting Information).

The generation of a thus far non‐reproducible by‐product Cl₂BrO₆ ⁺ ClO₄ ⁻ in a reaction of BrO₂F/HSO₃CF₃ ⁻/SO₂ClF is reported in the Supporting Information, only to show that more of these compounds can exist. Long ago, a compound described as BrO₂ ⁺ClO₄ ⁻ was made by ozonization of BrOClO₃ in CFCl_3, but solely characterized by Cl/Br analysis.15

Experimental Section

The generation of BrO₂F from NaBrO₃, BrF₅, and HF is most easily performed on a metal vacuum line in a PFA tube (poly(perfluoroethene perfluorovinyl ether) co‐polymer) at −78 °C, and subsequent sublimation at −10 °C into a second PFA trap cooled to −78 °C. The product obtained is completely colorless. The same reaction without a metal vacuum line is described in detail in the Supporting Information, as are the reactions of BrO₂F with SbF₅, AsF₅, HSO₃CF₃, (CH₃)₃Si‐OSO₂CF₃, MoF₅, (CF₃‐CO)₂O, and NO₂.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

K. Seppelt, Angew. Chem. Int. Ed. 2019, 58, 18928.