Rubidium tetra­fluorido­bromate(III): redetermination of the crystal structure from single-crystal X-ray diffraction data

Single crystals of rubidium tetra­fluorido­bromate(III), RbBrF₄, were grown by melting and recrystallizing RbBrF₄ from its melt. This is the first determination of the crystal structure of RbBrF₄ using single-crystal X-ray diffraction data. We confirmed that the structure contains square-planar [BrF₄]⁻ anions and rubidium cations that are coordinated by F atoms in a square-anti­prismatic manner. The compound crystallizes in the KBrF₄ structure type. Atomic coordinates and bond lengths and angles were determined with higher precision than in a previous report based on powder X-ray diffraction data [Ivlev et al. (2015). Z. Anorg. Allg. Chem. 641, 2593–2598].

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Structure description

The first attempt to determine the lattice parameters of rubidium tetra­fluorido­bromate(III) from powder X-ray diffraction data was undertaken by Popov et al. (1987). They reported the following tetra­gonal I-centered unit cell: a = 6.401 (3), c = 11.1538 (7) Å, V = 472.7 (6) ų at room temperature. The authors stated that the structure of RbBrF₄ is isotypic to that of KBrF₄ but did not provide further crystallographic details. The next report on the crystal structure of RbBrF₄ was published by Seppelt and coworkers, stating that the crystal structure is not isotypic to KBrF₄ (Mahjoub et al., 1989). In a later study we showed that the structure model by Mahjoub et al. (1989) was not correct. Indeed, the crystal-structure model obtained from powder X-ray diffraction data [I4/mcm, a = 6.37181 (15), c = 11.4934 (3) Å, V = 466.63 (2) ų at 293 K; Ivlev et al., 2015] revealed isotypism to KBrF₄. On basis of the obtained powder X-ray diffraction data, only the Rb and Br atoms could be refined with anisotropic displacement parameters. We were now able to grow single crystals of RbBrF₄ and present our results on the basis of single-crystal X-ray diffraction data at 100 K, which allowed for anisotropic refinement of all atoms and confirmed our previous model with higher precision.

The lattice parameters of RbBrF₄ obtained from the current single-crystal X-ray diffraction data (Table 1) are, as expected, slightly smaller than those of the room temperature powder X-ray data given above. In the structure, all atoms are located on special positions: Rb1 occupies Wyckoff position 4a (site symmetry 422), Br1 4d (m.mm), and F1 16l (..m). The rubidium cation is coordinated in a square-anti­prismatic manner by fluorine atoms (Fig. 1), whereas the bromine(III) atom shows a square-planar coordination by fluorine atoms.

Table 1
Experimental details

Crystal data
Chemical formula	RbBrF4
Mr	241.38
Crystal system, space group	Tetragonal, I4/mcm
Temperature (K)	100
a, c (Å)	6.2991 (5), 11.4659 (10)
V (Å^3)	454.95 (8)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	19.61
Crystal size (mm)	0.19 × 0.15 × 0.03
Data collection
Diffractometer	Bruker D8 QUEST area detector
Absorption correction	Multi-scan (SADABS; Krause et al., 2015)
Tmin, Tmax	0.151, 0.480
No. of measured, independent and observed [I > 2σ(I)] reflections	5872, 322, 189
Rint	0.070
(sin θ/λ)max (Å^−1)	0.833
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.020, 0.038, 1.06
No. of reflections	322
No. of parameters	13
Δρmax, Δρmin (e Å^−3)	0.66, −0.73
Computer programs: APEX3 (Bruker, 2018), SHELXT (Sheldrick, 2015a), SHELXL (Sheldrick, 2015b), DIAMOND (Brandenburg, 2018) and publCIF (Westrip, 2010).

Figure 1 The square-anti­prismatic coordination sphere of the Rb^+ cation by F^− anions. Atoms are shown with arbitrary radii.

The resulting Br—F bond length of 1.8905 (16) Å is comparable with the value of 1.932 (8) Å obtained from powder X-ray diffraction data at 293 K, as well as with the Br—F bond lengths reported for other tetra­fluorido­bromates(III) (Ivlev et al., 2015). The Rb—F distance amounts to 2.8447 (10) Å, likewise in good agreement with powder X-ray data (2.851 (7) Å).

A section of the crystal structure of RbBrF₄ is shown in Fig. 2.

Figure 2 The crystal structure of the title compound in a projection along the a axis. Displacement ellipsoids are shown at the 70% probability level.

Synthesis and crystallization

Rubidium tetra­fluorido­bromate(III) was synthesized by direct reaction of bromine trifluoride with rubidium chloride. The reaction was carried out under Freon-113, which acted as a protective layer against hydrolysis and as a heat absorber. The mixture of RbCl and BrF₃ was kept in a closed Teflon vessel. After three days, the Freon was removed by vacuum distillation and RbBrF₄ was obtained as a solid white residue. The powder was melted at 523 K and subsequently cooled down to room temperature. Single crystals of RbBrF₄ were obtained as small plates after crushing the solid lumps.

Refinement

Crystal data, data collection and structure refinement details are given in Table 1.

Structural data

Computing details

Data collection: APEX3 (Bruker, 2018); cell refinement: APEX3 (Bruker, 2018); data reduction: APEX3 (Bruker, 2018); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2018); software used to prepare material for publication: publCIF (Westrip, 2010).

Rubidium tetrafluoridobromate(III)

Crystal data

RbBrF4	Dx = 3.524 Mg m^−3 Dm = 3.33 Mg m^−3 Dm measured by helium pycnometry
Mr = 241.38	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I4/mcm	Cell parameters from 285 reflections
a = 6.2991 (5) Å	θ = 4.5–31.3°
c = 11.4659 (10) Å	µ = 19.61 mm^−1
V = 454.95 (8) Å^3	T = 100 K
Z = 4	Plate, colorless
F(000) = 432	0.19 × 0.14 × 0.03 mm

Data collection

Bruker D8 QUEST area detector diffractometer	322 independent reflections
Radiation source: microfocus sealed X-ray tube, Incoatec Iµs	189 reflections with I > 2σ(I)
Detector resolution: 7.9 pixels mm^-1	Rint = 0.070
ω and φ scans	θmax = 36.3°, θmin = 3.6°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −9→10
Tmin = 0.151, Tmax = 0.480	k = −9→10
5872 measured reflections	l = −19→19

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.004P)^2 + 1.2951P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.020	(Δ/σ)max < 0.001
wR(F^2) = 0.038	Δρmax = 0.66 e Å^−3
S = 1.06	Δρmin = −0.73 e Å^−3
322 reflections	Extinction correction: SHELXL (Sheldrick, 2015b), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
13 parameters	Extinction coefficient: 0.0016 (3)
0 restraints

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Br1	0.500000	0.000000	0.500000	0.00663 (14)
Rb1	0.500000	0.500000	0.750000	0.00956 (15)
F1	0.6501 (2)	0.1501 (2)	0.61660 (13)	0.0138 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Br1	0.00693 (19)	0.00693 (19)	0.0060 (2)	0.0008 (2)	0.000	0.000
Rb1	0.0099 (2)	0.0099 (2)	0.0088 (2)	0.000	0.000	0.000
F1	0.0147 (5)	0.0147 (5)	0.0120 (7)	−0.0003 (8)	−0.0047 (5)	−0.0047 (5)

Geometric parameters (Å, º)

Br1—F1	1.8905 (16)	Rb1—F1^viii	2.8447 (10)
Br1—F1^i	1.8906 (16)	Rb1—F1^ix	2.8447 (9)
Br1—F1^ii	1.8906 (16)	Rb1—F1^x	2.8447 (9)
Br1—F1^iii	1.8906 (16)	Rb1—F1	2.8447 (9)
Rb1—F1^iv	2.8447 (10)	Rb1—Rb1^xi	4.4541 (4)
Rb1—F1^v	2.8447 (10)	Rb1—Rb1^xii	4.4541 (4)
Rb1—F1^vi	2.8447 (10)	Rb1—Rb1^xiii	4.4541 (4)
Rb1—F1^vii	2.8447 (10)	Rb1—Rb1^vi	4.4541 (4)
F1—Br1—F1^i	180.0	F1^viii—Rb1—Rb1^xi	141.53 (2)
F1—Br1—F1^ii	90.01 (10)	F1^ix—Rb1—Rb1^xi	71.76 (3)
F1^i—Br1—F1^ii	89.99 (10)	F1^x—Rb1—Rb1^xi	108.24 (3)
F1—Br1—F1^iii	89.99 (10)	F1—Rb1—Rb1^xi	71.76 (3)
F1^i—Br1—F1^iii	90.01 (10)	F1^iv—Rb1—Rb1^xii	38.47 (2)
F1^ii—Br1—F1^iii	180.0	F1^v—Rb1—Rb1^xii	141.53 (2)
F1^iv—Rb1—F1^v	143.52 (7)	F1^vi—Rb1—Rb1^xii	71.76 (3)
F1^iv—Rb1—F1^vi	73.20 (3)	F1^vii—Rb1—Rb1^xii	141.53 (2)
F1^v—Rb1—F1^vi	141.19 (6)	F1^viii—Rb1—Rb1^xii	38.47 (2)
F1^iv—Rb1—F1^vii	114.95 (5)	F1^ix—Rb1—Rb1^xii	108.24 (3)
F1^v—Rb1—F1^vii	76.95 (5)	F1^x—Rb1—Rb1^xii	71.76 (3)
F1^vi—Rb1—F1^vii	73.20 (3)	F1—Rb1—Rb1^xii	108.24 (3)
F1^iv—Rb1—F1^viii	76.95 (5)	Rb1^xi—Rb1—Rb1^xii	180.0
F1^v—Rb1—F1^viii	114.95 (5)	F1^iv—Rb1—Rb1^xiii	71.76 (3)
F1^vi—Rb1—F1^viii	78.41 (7)	F1^v—Rb1—Rb1^xiii	71.76 (3)
F1^vii—Rb1—F1^viii	143.52 (7)	F1^vi—Rb1—Rb1^xiii	141.53 (2)
F1^iv—Rb1—F1^ix	73.20 (3)	F1^vii—Rb1—Rb1^xiii	108.24 (3)
F1^v—Rb1—F1^ix	78.41 (7)	F1^viii—Rb1—Rb1^xiii	108.24 (3)
F1^vi—Rb1—F1^ix	114.95 (5)	F1^ix—Rb1—Rb1^xiii	38.47 (2)
F1^vii—Rb1—F1^ix	73.20 (3)	F1^x—Rb1—Rb1^xiii	38.47 (2)
F1^viii—Rb1—F1^ix	141.19 (6)	F1—Rb1—Rb1^xiii	141.53 (2)
F1^iv—Rb1—F1^x	78.41 (7)	Rb1^xi—Rb1—Rb1^xiii	90.0
F1^v—Rb1—F1^x	73.20 (3)	Rb1^xii—Rb1—Rb1^xiii	90.0
F1^vi—Rb1—F1^x	143.52 (7)	F1^iv—Rb1—Rb1^vi	108.24 (3)
F1^vii—Rb1—F1^x	141.19 (6)	F1^v—Rb1—Rb1^vi	108.24 (3)
F1^viii—Rb1—F1^x	73.20 (3)	F1^vi—Rb1—Rb1^vi	38.47 (2)
F1^ix—Rb1—F1^x	76.95 (5)	F1^vii—Rb1—Rb1^vi	71.76 (3)
F1^iv—Rb1—F1	141.19 (6)	F1^viii—Rb1—Rb1^vi	71.76 (3)
F1^v—Rb1—F1	73.19 (3)	F1^ix—Rb1—Rb1^vi	141.53 (2)
F1^vi—Rb1—F1	76.95 (5)	F1^x—Rb1—Rb1^vi	141.53 (2)
F1^vii—Rb1—F1	78.41 (7)	F1—Rb1—Rb1^vi	38.47 (2)
F1^viii—Rb1—F1	73.20 (3)	Rb1^xi—Rb1—Rb1^vi	90.0
F1^ix—Rb1—F1	143.52 (7)	Rb1^xii—Rb1—Rb1^vi	90.0
F1^x—Rb1—F1	114.95 (5)	Rb1^xiii—Rb1—Rb1^vi	180.0
F1^iv—Rb1—Rb1^xi	141.53 (2)	Br1—F1—Rb1^vi	126.98 (3)
F1^v—Rb1—Rb1^xi	38.47 (2)	Br1—F1—Rb1	126.98 (3)
F1^vi—Rb1—Rb1^xi	108.24 (3)	Rb1^vi—F1—Rb1	103.05 (5)
F1^vii—Rb1—Rb1^xi	38.47 (2)

Symmetry codes: (i) −x+1, −y, −z+1; (ii) x, y, −z+1; (iii) −x+1, −y, z; (iv) y+1/2, −x+3/2, −z+3/2; (v) y, −x+1, z; (vi) −x+3/2, −y+1/2, −z+3/2; (vii) −y+1/2, x−1/2, −z+3/2; (viii) −y+1, x, z; (ix) x−1/2, y+1/2, −z+3/2; (x) −x+1, −y+1, z; (xi) −x+1/2, −y+1/2, −z+3/2; (xii) −x+3/2, −y+3/2, −z+3/2; (xiii) −x+1/2, −y+3/2, −z+3/2.