Reactions of Bromine Fluoride Dioxide, BrO2 F, for the Generation of the Mixed-Valent Bromine Oxygen Cations Br3 O4 + and Br3 O6.

A reliable synthesis of unstable and highly reactive BrO2 F is reported. This compound can be converted into BrO2 + SbF6 - , BrO2 + AsF6 - , and BrO2 + AsF6 - ⋅2 BrO2 F. The latter decomposes into mixed-valent Br3 O4 ⋅Br2 + AsF6 - with five-, three-, one-, and zero-valent bromine. BrO2 + H(SO3 CF3 )2 - is formed with HSO3 CF3 . Excess BrO2 F yields mixed-valent Br3 O6 + OSO3 CF3 - with five- and three-valent bromine. Reactions of BrO2 F and MoF5 in SO2 ClF or CH2 ClF result in Cl2 BrO6 + Mo3 O3 F13 - . The reaction of BrO2 F with (CF3 CO)2 O and NO2 produces O2 Br-O-CO-CF3 and the known NO2 + Br(ONO2 )2 - . All of these compounds are thermodynamically unstable.

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)].

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 BrIII–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 BrI 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 CFCl3, 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.

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Supplementary

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