Br₂F₇⁻ and Br₃F₁₀⁻: peculiar anions showing μ₂- and μ₃-bridging F-atoms

Abstract

RbCl and CsCl react with BrF₃ yielding the corresponding decafluoridotribromates(III), MBr₃F₁₀ (M = Rb, Cs), which were structurally characterized for the first time. The Br₃F₁₀⁻ anion is surprisingly not linear but contains a μ₃-bridging fluorine atom and seems to be the first example of μ³-F bridging of Br atoms. The compounds are highly reactive and cannot be handled in glassware. As for the tetrafluoridobromates themselves, they are powerful oxidizers and thus suitable for the dry-chemical recycling of precious metals and additionally feature a significantly higher BrF₃ content.

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Under appropriate conditions fluorine is able to react with almost any element and a rich diversity of compounds result. In fluorine-containing molecules and molecular ions, the fluorine atom clearly prefers terminal positions, although many coordination compounds incorporate fluorine atoms as bridging ligands.¹ The most common type of F-bridging is the μ₂-connection (either straight mono(μ-F), or bent di(μ-F)).^2–13 The highly strained tri(μ-F)-bridges between various elements¹ are scarce with only some examples in the literature.^2–13 Bridges among three atoms (μ₃-F) are even more rare;^14–21 however, μ₄-F^22,23 and even cage-like μ₆-F^22,24,25 coordination types are known. In the vast majority of these compounds the fluorine atom bridges either metal atoms (thus, forming homo- or hetero (oligo-)nuclear complexes), or, more rarely, metal and nonmetal atoms. In comparison, compounds featuring μ₂-F bridges between two nonmetal atoms are even more uncommon; examples are known for H,²⁶ Kr,^27,28 Xe,²⁹ Br,^4,30 and I,³¹ as well as for several metalloid atoms: B,³² As,³³ Sb,³⁴ and Si.³⁵ To the best of our knowledge, the μ₃-F type among nonmetals was fully established only for Xe³⁶ and I.^20,21

Here, we report the results of our investigations on the non-common fluorine bridging in rubidium and cesium decafluoridotribromates(III): RbBr₃F₁₀ (compound 1) and CsBr₃F₁₀ (compound 2), respectively, as well as in rubidium heptafluoridodibromate(III), RbBr₂F₇ (compound 3). The compounds were synthesized during our research on the metal tetrafluoridobromate(III) series – powerful oxidizers which are promising for the dry-chemical recycling of noble metals.^4,37,38 MBr₃F₁₀ and MBr₂F₇ (M = Rb, Cs) were first reported by Stein;³⁰ however, their structures could only be deduced by Raman spectroscopy. Also, military personnel tried the synthesis of Br₃F₁₀⁻ but did not succeed.⁴¹ The Br₃F₁₀⁻ (and Br₂F₇⁻) anion was reported to be chainlike with Br–μ-F–Br connections. We continued our previous works on BrF₄⁻ compounds⁴ and started to investigate compounds of higher BrF₃ content to reveal how the chain elongation influences the structure of the anion and its reactivity. However, single crystal X-ray diffraction analyses, vibrational spectroscopy, as well as quantum chemical calculations showed that the Br₃F₁₀⁻ anions are not chain-like but contain μ₃-bridging F atoms.

Compounds 1, 2 and 3 were synthesized using stoichiometric amounts of MCl (M = Rb, Cs) and BrF₃, according to eqn (1), which was previously applied in the synthesis of RbBrF₄:³⁸

6MCl + (6n + 2)BrF₃ → 6MBr_nF_(3n+1) + Br₂ + 3Cl₂ (1)

RbBr₃F₁₀ and CsBr₃F₁₀ were obtained as yellowish-colored crystalline solids. Since both compounds are structurally isotypic, we present a detailed structural description only for compound 1 (Rb). Further details of the compounds presented here, such as powder X-ray patterns, Rietveld refinements, thermal analyses, experimentally observed as well as calculated Raman and IR spectra and band assignments, are available from the ESI.†

Compound 1 crystallizes in the monoclinic space group type P2₁ (no. 4) with a = 7.6219(3) Å, b = 8.2593(4) Å, c = 8.4645(4) Å, β = 114.414(1)°, V = 485.21(4) ų, and Z = 4 at 110 K. Further crystallographic details are given in Table 2. The asymmetric unit and the crystal packing of compound 1 are shown in Fig. 1 and Fig. S10 (ESI†), respectively.

Fig. 1 The asymmetric unit of RbBr₃F₁₀. Displacement ellipsoids are shown at 70% probability level at 110 K.

An unexpected and unusual feature of compounds 1 and 2 is the shape of the Br₃F₁₀⁻ anion. It contains a μ₃-bridging fluorine atom and not two μ₂-bridging fluorine atoms as was previously deduced from Raman spectroscopic investigations on the compound.³⁰ To the best of our knowledge, RbBr₃F₁₀ and CsBr₃F₁₀ are the first documented compounds which incorporate fluorine as a μ₃-bridging ligand between Br atoms.

The bromine atoms in RbBr₃F₁₀ are coordinated by four fluorine atoms, each in an almost square-planar shape. The F–Br–F angles are observed in the range of 86.8(1) to 95.8(1)°, which is typical for known tetrafluoridobromates(III): KBrF₄,^12,39 RbBrF₄,³⁸ CsBrF₄,⁴ CsBr₂F₇,⁴ and Ba(BrF₄)₂.³⁷ The Br atoms are located almost exactly in the virtual planes formed by the corresponding fluorine atoms (the distances from the least-squares planes are only 0.0026(4), 0.0033(5), and 0.0095(4) Å). The μ₃-F atom is slightly (0.458(2) Å for compound 1, 0.370(8) Å for compound 2) above the virtual plane built by the three Br atoms. The Br–μ₃-F–Br angles are observed in the interval from 113.2(1) to 120.1(1)°. All three planar BrF₃-units are tilted towards each other, and the angles between the virtual planes are equal to 65.46(6), 66.22(7), and 69.10(7)°. The point group of the Br₃F₁₀⁻ anion is C₁; however, its symmetry is very close to D₃. In DFT calculations on the isolated anion point group D₃ is obtained. We also carried out two gas-phase DFT calculations on the Br₃F₁₀⁻ anion to compare the relative energetics of the μ₃-bridging, experimentally observed structure and the previously postulated chain-like structure with Br–μ-F–Br connections. At the DFT-PBE0/def2-TZVP level of theory, the D₃-symmetric, μ₃-bridging structure is energetically 15 kJ mol⁻¹ more favorable than the C_2h-symmetric, chainlike structure (structural data are available in the ESI†). As expected, the Br-F bonds in the trans-position with respect to the μ₃-F atom are shortened by circa 0.1 Å (for both compounds) in comparison to the other terminal F atoms. The coordination sphere of the Br₃F₁₀⁻ anion is also interesting and is discussed in the ESI† for reasons of brevity. Selected atomic distances and angles of the Br₃F₁₀⁻ anions, experimentally observed as well as theoretically predicted, are given in Table 1.

Table 1 Selected bond lengths and angles for the Br₃F₁₀⁻ anion. The atom labels correspond to those in Fig. 1. The DFT-PBE0 data for the ideal D₃-symmetric Br₃F₁₀⁻ anion have been calculated for the gas-phase (see the ESI for computational details)

Parameter	Value (Å/°)
	RbBr3F10	CsBr3F10	Br3F10^− (DFT-PBE0)
Br(1)–μ3-F	2.243(3)	2.238(10)	2.30
Br(2)–μ3-F	2.248(2)	2.246(9)	2.30
Br(3)–μ3-F	2.320(3)	2.329(10)	2.30
Br–trans-F (F7, F8, F10)	1.745(2)⋯1.752(2)	1.746(6)⋯1.767(7)	1.76
Br–F (other)	1.837(2)⋯1.874(2)	1.824(10)⋯1.878(7)	1.84
Br(1)–μ3-F–Br(2)	120.1(1)	122.0(3)	120
Br(2)–μ3-F–Br(3)	114.7(1)	115.0(4)	120
Br(3)–μ3-F–Br(1)	113.2(1)	115.1(4)	120
Distance from μ3-F to Br(1)–Br(2)–Br(3) plane	0.458(2)	0.370(8)	0
Tilting angles between F3M–μ-F–MF3 least-squares planes	65.46(6) 66.22(7) 69.10(7)	62.1(3) 66.2(2) 72.0(2)	81.8

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It is interesting to note that the cation type seems to have a rather low impact on the molecular structure of the Br₃F₁₀⁻ anion. All corresponding Br–F bond lengths (including those to μ₃-F as well as to trans-F atoms) do not change significantly (3σ) if the Rb atoms are exchanged by Cs atoms. However, the bond angles seem to be more susceptible to the cation size and undergo a noticeable change resulting also in the change in the μ₃-F distance from the virtual Br(1)–Br(2)–Br(3) plane.

Compound 3, rubidium heptafluoridodibromate(III), appears as a yellowish crystalline solid. However, the product of reaction (1) contains a mixture of RbBrF₄, RbBr₂F₇, and BrF₃ rather than pure compound 3 (see the ESI†), which confirms the results obtained by Stein.³⁰ RbBr₂F₇ crystallizes in the monoclinic space group type P2₁/c (no. 14) with a = 7.5109(3) Å, b = 7.8759(3) Å, c = 13.6898(5) Å, β = 123.118(2)°, V = 678.26(5) ų, and Z = 4 at 110 K. It is structurally isotypic to CsBr₂F₇.⁴ The crystallographic details of RbBr₂F₇ are given in Table 2. The asymmetric unit and the crystal packing of compound 3 are shown in Fig. 2 and Fig. S11 (ESI†) respectively.

Table 2 Crystallographic details of the title compounds

	Compound 1	Compound 2	Compound 3
Empirical formula	Br3F10Rb	Br3F10Cs	Br2F7Rb
Color and appearance	Colorless plates, yellowish powders
M/g mol^−1	515.20	562.60	378.29
Crystal system	Monoclinic
Space group	P21 (no. 4)		P21/c (no. 14)
a/Å	7.6219(3)	7.4399(16)	7.5109(3)
b/Å	8.2593(4)	8.4037(17)	7.8759(3)
c/Å	8.4645(4)	8.8365(18)	13.6898(5)
β/°	114.414(1)	115.31(3)	123.118(2)
V/Å^3	485.21(4)	519.4(2)	678.26(5)
Z	2		4
ρ calc/g cm^−3	3.526	3.597	3.705
ρ exp/g cm^−3	3.687 ± 0.006	3.460 ± 0.026	3.786 ± 0.007
λ/Å	0.71073 (Mo-Kα)
T/K	110	100	110
R int, Rσ	0.0444, 0.0465	0.0472, 0.1449	0.0533, 0.0309
R(F^2) (all data), wR(F^2) (all data)	0.0294, 0.0509	0.0730, 0.0732	0.0336, 0.0507
S (all data)	1.024	0.790	1.055
Flack parameter x	0.050(6)	0.00(2)	—
No. of reflections, parameters, constraints, restraints	4132, 128, 0, 1	3195, 128, 0, 1	2035, 92, 0, 0
2θ range refined (min, max)	2.643, 35.843	2.912, 31.900	3.138, 30.362
Δρmax, Δρmin/e Å^−3	1.22, −1.45	1.43, −1.21	0.94, −0.94
(Δσ)max	0.001	0.000	0.001
ICSD number	431741	431740	431739

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Fig. 2 The asymmetric unit of RbBr₂F₇. Displacement ellipsoids are shown at 70% probability level at 110 K.

RbBr₂F₇ contains a Br₂F₇⁻ anion with the fluorine atom F(1) acting as a μ₂-bridging ligand between the two bromine atoms. The Br–μ-F distances are 2.115(2) and 2.145(2) Å, and are essentially equal to 2.113(1) and 2.143(1) Å as observed in CsBr₂F₇;⁴ however, the Br–μ-F–Br angle is 134.75(9)° being circa 6° smaller than the analogous angle of 140.27(6)° reported for the Cs compound. The trans-F–Br distances are equal to 1.767(2) and 1.780(2) Å, while the other terminal Br–F bond lengths lie in the range of 1.845(2) to 1.890(2) Å, therefore, being equal to the corresponding bonds in CsBr₂F₇ within the 3σ criterion. The F–Br–F angles are observed within the interval from 87.46(7) to 95.65(7)° (87.74(6) to 95.12(5)° in CsBr₂F₇). These facts imply again that these counter-ions do not significantly influence the molecular structure of the fluoridobromate anions. The cell parameters together with the selected bond lengths and angles for RbBr₂F₇ and CsBr₂F₇ as well as for CsAu₂F₇ (which is similar in terms of the anion geometry but is not structurally isotypic)⁴⁰ are shown in Table 3.

Table 3 Selected bond lengths and angles in RbBr₂F₇, CsBr₂F₇, and CsAu₂F₇. The atom labels correspond to those in Fig. 2, and were changed in the cases of CsBr₂F₇ and CsAu₂F₇ to make the comparison possible

Parameter	Value (Å/°)
	RbBr2F7 (P21/c)	CsBr2F7 (P21/c)^4	CsAu2F7 (C2/c)^40
M = Br (RbBr2F7, CsBr2F7); Au (CsAu2F7).
a/Å	7.5109(3)	7.7078(1)	11.365(6)
b/Å	7.8759(3)	8.0218(2)	10.820(15)
c/Å	13.6898(5)	14.1584(3)	7.374(3)
β/°	123.118(2)	122.742(2)	123.40(3)
M(1)–F(1)	2.145(2)	2.143(1)	1.988(8)
M(1)–F(3)	1.871(2)	1.868(2)	1.89(2)
M(1)–F(4)	1.855(2)	1.858(2)	1.92(2)
M(1)–F(5)	1.767(2)	1.769(1)	1.86(2)
M(2)–F(1)	2.115(2)	2.112(2)	1.988(8)
M(2)–F(2)	1.890(2)	1.884(2)	1.89(2)
M(2)–F(6)	1.845(2)	1.849(1)	1.92(2)
M(2)–F(7)	1.780(2)	1.779(2)	1.86(2)
M(1)–F(1)–M(2)	134.75(9)	140.27(6)	130.08(4)
F(5)–M(1)–F(1)	175.17(8)	176.18(6)	176.0(6)
F(7)–M(2)–F(1)	176.04(8)	176.44(7)	176.0(6)
Tilting angle between F3M–μ-F–MF3 planes	65.31(6)	61.90(4)	48.1(3)

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In comparison to the Br₃F₁₀⁻ anions, a shortening of the Br–μ-F–Br bridges of circa 0.1 to 0.2 Å is observed which may be due to smaller Coulomb repulsion between the two Br atoms. The Br–trans-F distances in the Br₂F₇⁻ anion are on average 0.025 Å longer than those in Br₃F₁₀⁻. The other terminal fluorine atoms are not so susceptible to the change of the inner environment and the corresponding Br–F bonds show only a slight elongation of approximately 0.01 Å.

All compounds reported in this work may be convenient carriers for BrF₃ due to its high mass content (72.4, 79.7, 73.0% by mass in RbBr₂F₇, RbBr₃F₁₀ and CsBr₃F₁₀ respectively). So, the “problematic” BrF₃ can be replaced by these comparatively easy to handle solid compounds. To date there has been no indication for the existence of such interhalide anions in compounds with cations other than Rb or Cs, which is possibly due to the cation size. Also, it is unknown if suitable reaction conditions can be found that would lead to the formation of the highly symmetrical Br₄F₁₃⁻ anion, with a μ₄-bridging F atom. Investigations in those directions are ongoing.

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Footnote

† Electronic supplementary information (ESI) available: Experimental and computational details and results from quantum chemical calculations. CCDC 1499499–1499501. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6cc06761a

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