Redetermination of the crystal structure of caesium tetra­fluorido­bromate(III) from single-crystal X-ray diffraction data

The crystal structure of CsBrF₄ was determined from single-crystal X-ray diffraction data collected at 100 K and is compared with previous models based on powder X-ray diffraction data.

Abstract

Caesium tetra­fluorido­bromate(III), CsBrF₄, was crystallized in form of small blocks by melting and recrystallization. The crystal structure of CsBrF₄ was redetermined from single-crystal X-ray diffraction data. In comparison with a previous study based on powder X-ray diffraction data [Ivlev et al. (2013 ▸). Z. Anorg. Allg. Chem. 639, 2846–2850], bond lengths and angles were determined with higher precision, and all atoms were refined with anisotropic displacement parameters. It was confirmed that the structure of CsBrF₄ contains two square-planar [BrF₄]⁻ anions each with point group symmetry mmm, and a caesium cation (site symmetry mm2) that is coordinated by twelve fluorine atoms, forming an anti­cubocta­hedron. CsBrF₄ is isotypic with CsAuF₄.

Structure description

The first report of unit-cell parameters of CsBrF₄ from powder X-ray diffraction data was given by Popov et al. (1987 ▸). They indexed the powder pattern using a primitive tetra­gonal unit cell with lattice parameters of a = 9.828 (3), c = 7.166 (5) Å, V = 692.2 (3) ų (temperature not given). These lattice parameters are quite different compared to those of other known alkali metal tetra­fluorido­bromates(III) that crystallize in the KBrF₄ structure type [KBrF₄, I4/mcm (No. 140), a = 6.174 (2), c = 11.103 (2) Å, V = 423 ų; Siegel, 1956 ▸], and consequently CsBrF₄ is not isotypic with KBrF₄ on basis of the data provided by Popov et al. (1987 ▸). However, neither the crystal structure nor other crystallographic details of CsBrF₄ were given at that time.

Recently, we have determined the crystal structure of CsBrF₄ from powder X-ray diffraction (PXRD) data where we could only refine the F atoms isotropically (Ivlev et al., 2013 ▸). We have shown that CsBrF₄ is isotypic with CsAuF₄ (Schmidt & Müller, 2004 ▸) and crystallizes in the space group Immm (No. 71) with lattice parameters a = 5.6413 (8), b = 6.8312 (9), c = 12.2687 (17) Å, V = 472.79 (11) ų, Z = 4 at 293 K. These lattice parameters are not related to the unit cell reported by Popov et al. (1987 ▸). We assume that their powder pattern probably contained impurity lines, e.g. from possible hydrolysis products, which led to erroneous indexing. Here we present the results of a redetermination of the crystal structure of CsBrF₄ from single-crystal X-ray diffraction data at 100 K, leading to bond lengths and angles with higher precision, and with all atoms refined with anisotropic displacement parameters.

The unit-cell parameters of CsBrF₄ obtained from single-crystal X-ray diffraction data (Table 1 ▸) are expectedly smaller than those from the PXRD data at 293 K. The crystal structure contains two different square-planar [BrF₄]⁻ anions, the planes of which are parallel and rotated by about 45° with respect to each other. The first anion consists of one bromine(III) atom (Br1) on the special 2d (mmm) Wyckoff position and two fluorine atoms F1 and F3 on the special 4j (mm2) and 4g (m2m) Wyckoff positions, respectively. As a result of symmetry restrictions, the F—Br—F angle is exactly 90°. The Br1—F bond lengths are 1.8852 (13) and 1.9020 (15) Å [cf. 1.94 (4) and 1.97 (4) Å from PXRD data]. The second [BrF₄]⁻ anion contains one bromine(III) atom (Br2) on the special 2b (mmm) Wyckoff position and one fluorine atom (F2) on the special 8l (m..) Wyckoff position. The anion is slightly distorted in-plane, resulting in an almost rectangular structure with F2—Br2—F2 angles of 87.96 (7) and 92.04 (7)° and a Br2—F2 bond length of 1.8907 (10) Å [cf. 87.6 (13) and 92.4 (13)°, 1.96 (3) Å from PXRD data]. In general, the bond lengths and angles of the [BrF₄]⁻ anions in CsBrF₄ are in good correspondence with other known tetra­fluorido­bromates(III) [see Table 2 in Ivlev & Kraus (2018 ▸), and references therein]. The caesium cation occupies the special 4i (mm2) Wyckoff position and is coordinated by twelve fluorine atoms. The resulting coordination polyhedron is an anti­cubocta­hedron (Fig. 1 ▸). The Cs⋯F distances are in the range 2.9615 (11) to 3.4784 (4) Å [cf. 3.011 (1) to 3.605 (1) from PXRD data].

Table 1. Experimental details.

Crystal data
Chemical formula	CsBrF4
M r	288.82
Crystal system, space group	Orthorhombic, I m m m
Temperature (K)	100
a, b, c (Å)	5.5075 (3), 6.7890 (3), 12.2572 (6)
V (Å3)	458.30 (4)
Z	4
Radiation type	Mo Kα
μ (mm−1)	16.75
Crystal size (mm)	0.11 × 0.09 × 0.06
Data collection
Diffractometer	Bruker D8 QUEST
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 ▸)
T min, T max	0.330, 0.558
No. of measured, independent and observed [I > 2σ(I)] reflections	8570, 669, 622
R int	0.029
(sin θ/λ)max (Å−1)	0.835
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.012, 0.022, 1.12
No. of reflections	669
No. of parameters	26
Δρmax, Δρmin (e Å−3)	1.20, −0.88

Computer programs: APEX3 and SAINT (Bruker, 2018 ▸), SHELXL (Sheldrick, 2015 ▸), DIAMOND (Brandenburg, 2019 ▸) and publCIF (Westrip, 2010 ▸); coordinates taken from previous refinement.

Figure 1.

The anti­cubocta­hedron around the caesium cation. Displacement ellipsoids are shown at the 70% probability level. [Symmetry codes: (i) −x + 1, −y + 1, −z + 1; (ii) x, y, −z + 1; (iii) x +  , y +  , z +  ; (iv) x  −  , y +  , z +  ; (v) −x +  , −y +  , z +  ; (vi) −x +  , −y +  , z +  ; (vii) −x + 1, −y, −z + 1.]

The crystal structure of CsBrF₄ is shown in Fig. 2 ▸.

Figure 2.

The crystal structure of CsBrF₄ in a projection along the a axis. Displacement ellipsoids are shown at the 70% probability level.

Synthesis and crystallization

Caesium tetra­fluorido­bromate(III) was synthesized by direct reaction of bromine trifluoride with caesium 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 CsCl and BrF₃ was kept in a closed Teflon vessel. After three days the Freon was removed by vacuum distillation and CsBrF₄ was obtained as a solid white residue. The powder was melted at 483 K and cooled down to room temperature. Single crystals of CsBrF₄ were obtained as small blocks after crushing the solid lumps.

Refinement

Details of data collection and structure refinement are given in Table 1 ▸. Coordinates and atom labelling were taken from the previous refinement from PXRD data (Ivlev et al., 2013 ▸).

Supplementary Material

CCDC reference: 1980292

Additional supporting information: crystallographic information; 3D view; checkCIF report

full crystallographic data

Crystal data

CsBrF4	Dx = 4.186 Mg m−3
Mr = 288.82	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Immm	Cell parameters from 2913 reflections
a = 5.5075 (3) Å	θ = 3.3–36.8°
b = 6.7890 (3) Å	µ = 16.75 mm−1
c = 12.2572 (6) Å	T = 100 K
V = 458.30 (4) Å3	Block, colorless
Z = 4	0.11 × 0.09 × 0.06 mm
F(000) = 504

Data collection

Bruker D8 QUEST diffractometer	622 reflections with I > 2σ(I)
Radiation source: microfocus X-ray tube	Rint = 0.029
ω and φ scans	θmax = 36.4°, θmin = 3.3°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −9→9
Tmin = 0.330, Tmax = 0.558	k = −11→11
8570 measured reflections	l = −20→20
669 independent reflections

Refinement

Refinement on F2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	w = 1/[σ2(Fo2) + (0.0065P)2 + 0.5686P] where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.012	(Δ/σ)max = 0.001
wR(F2) = 0.022	Δρmax = 1.20 e Å−3
S = 1.12	Δρmin = −0.88 e Å−3
669 reflections	Extinction correction: SHELXL-2018/3 (Sheldrick 2015), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
26 parameters	Extinction coefficient: 0.00232 (16)
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
Cs1	0.500000	0.500000	0.71714 (2)	0.01008 (4)
Br1	0.500000	0.000000	0.500000	0.00720 (6)
Br2	0.500000	0.000000	0.000000	0.00760 (6)
F1	0.500000	0.000000	0.34482 (12)	0.0155 (3)
F2	0.500000	0.19339 (16)	0.11100 (9)	0.0169 (2)
F3	0.500000	0.2777 (2)	0.500000	0.0141 (3)

Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
Cs1	0.01130 (6)	0.01075 (6)	0.00821 (7)	0.000	0.000	0.000
Br1	0.00756 (12)	0.00827 (11)	0.00578 (13)	0.000	0.000	0.000
Br2	0.00816 (12)	0.00841 (11)	0.00624 (13)	0.000	0.000	0.000
F1	0.0179 (7)	0.0216 (7)	0.0069 (6)	0.000	0.000	0.000
F2	0.0201 (5)	0.0160 (4)	0.0146 (5)	0.000	0.000	−0.0073 (4)
F3	0.0164 (6)	0.0087 (6)	0.0172 (7)	0.000	0.000	0.000

Geometric parameters (Å, º)

Cs1—F2i	2.9615 (11)	Cs1—F1vii	3.4784 (4)
Cs1—F2ii	2.9615 (11)	Cs1—F1i	3.4784 (4)
Cs1—F3i	3.0597 (7)	Br1—F3	1.8852 (13)
Cs1—F3	3.0597 (7)	Br1—F3vii	1.8853 (13)
Cs1—F1iii	3.1674 (8)	Br1—F1vii	1.9020 (15)
Cs1—F1iv	3.1674 (8)	Br1—F1	1.9020 (15)
Cs1—F2v	3.3166 (7)	Br2—F2viii	1.8907 (10)
Cs1—F2iii	3.3166 (7)	Br2—F2ix	1.8907 (10)
Cs1—F2iv	3.3166 (7)	Br2—F2x	1.8907 (10)
Cs1—F2vi	3.3166 (7)	Br2—F2	1.8907 (10)
F2i—Cs1—F2ii	89.32 (4)	F1iv—Cs1—F1i	96.194 (16)
F2i—Cs1—F3i	105.79 (3)	F2v—Cs1—F1i	107.50 (2)
F2ii—Cs1—F3i	164.90 (3)	F2iii—Cs1—F1i	61.838 (19)
F2i—Cs1—F3	164.90 (3)	F2iv—Cs1—F1i	61.838 (19)
F2ii—Cs1—F3	105.79 (3)	F2vi—Cs1—F1i	107.50 (2)
F3i—Cs1—F3	59.11 (4)	F1vii—Cs1—F1i	154.77 (5)
F2i—Cs1—F1iii	69.423 (18)	F3—Br1—F3vii	180.0
F2ii—Cs1—F1iii	69.423 (18)	F3—Br1—F1vii	90.0
F3i—Cs1—F1iii	115.46 (2)	F3vii—Br1—F1vii	90.0
F3—Cs1—F1iii	115.46 (2)	F3—Br1—F1	90.0
F2i—Cs1—F1iv	69.423 (18)	F3vii—Br1—F1	90.0
F2ii—Cs1—F1iv	69.423 (18)	F1vii—Br1—F1	180.0
F3i—Cs1—F1iv	115.46 (2)	F2viii—Br2—F2ix	87.96 (7)
F3—Cs1—F1iv	115.46 (2)	F2viii—Br2—F2x	92.04 (7)
F1iii—Cs1—F1iv	120.78 (5)	F2ix—Br2—F2x	180.00 (6)
F2i—Cs1—F2v	123.869 (16)	F2viii—Br2—F2	180.0
F2ii—Cs1—F2v	90.05 (2)	F2ix—Br2—F2	92.04 (7)
F3i—Cs1—F2v	81.61 (2)	F2x—Br2—F2	87.96 (7)
F3—Cs1—F2v	57.553 (17)	F2viii—Br2—Cs1xi	120.01 (2)
F1iii—Cs1—F2v	156.305 (19)	F2ix—Br2—Cs1xi	120.005 (19)
F1iv—Cs1—F2v	58.13 (3)	F2x—Br2—Cs1xi	59.995 (19)
F2i—Cs1—F2iii	90.05 (2)	F2—Br2—Cs1xi	59.99 (2)
F2ii—Cs1—F2iii	123.868 (16)	F2viii—Br2—Cs1xii	59.99 (2)
F3i—Cs1—F2iii	57.553 (17)	F2ix—Br2—Cs1xii	59.995 (19)
F3—Cs1—F2iii	81.61 (2)	F2x—Br2—Cs1xii	120.005 (19)
F1iii—Cs1—F2iii	58.13 (3)	F2—Br2—Cs1xii	120.01 (2)
F1iv—Cs1—F2iii	156.305 (19)	Cs1xi—Br2—Cs1xii	180.0
F2v—Cs1—F2iii	133.81 (3)	F2viii—Br2—Cs1xiii	120.01 (2)
F2i—Cs1—F2iv	90.05 (2)	F2ix—Br2—Cs1xiii	120.005 (19)
F2ii—Cs1—F2iv	123.868 (17)	F2x—Br2—Cs1xiii	59.995 (19)
F3i—Cs1—F2iv	57.553 (17)	F2—Br2—Cs1xiii	59.99 (2)
F3—Cs1—F2iv	81.61 (2)	Cs1xi—Br2—Cs1xiii	91.951 (5)
F1iii—Cs1—F2iv	156.305 (19)	Cs1xii—Br2—Cs1xiii	88.049 (5)
F1iv—Cs1—F2iv	58.13 (3)	F2viii—Br2—Cs1xiv	59.99 (2)
F2v—Cs1—F2iv	46.64 (4)	F2ix—Br2—Cs1xiv	59.995 (19)
F2iii—Cs1—F2iv	112.26 (3)	F2x—Br2—Cs1xiv	120.005 (19)
F2i—Cs1—F2vi	123.868 (17)	F2—Br2—Cs1xiv	120.01 (2)
F2ii—Cs1—F2vi	90.05 (2)	Cs1xi—Br2—Cs1xiv	88.049 (5)
F3i—Cs1—F2vi	81.61 (2)	Cs1xii—Br2—Cs1xiv	91.951 (5)
F3—Cs1—F2vi	57.553 (17)	Cs1xiii—Br2—Cs1xiv	180.0
F1iii—Cs1—F2vi	58.13 (3)	Br1—F1—Cs1xiii	119.61 (2)
F1iv—Cs1—F2vi	156.305 (19)	Br1—F1—Cs1xi	119.61 (2)
F2v—Cs1—F2vi	112.26 (3)	Cs1xiii—F1—Cs1xi	120.78 (5)
F2iii—Cs1—F2vi	46.64 (4)	Br1—F1—Cs1vii	102.61 (2)
F2iv—Cs1—F2vi	133.81 (3)	Cs1xiii—F1—Cs1vii	83.807 (16)
F2i—Cs1—F1vii	147.27 (3)	Cs1xi—F1—Cs1vii	83.807 (16)
F2ii—Cs1—F1vii	57.95 (3)	Br1—F1—Cs1i	102.61 (2)
F3i—Cs1—F1vii	106.94 (3)	Cs1xiii—F1—Cs1i	83.807 (16)
F3—Cs1—F1vii	47.83 (3)	Cs1xi—F1—Cs1i	83.807 (16)
F1iii—Cs1—F1vii	96.194 (16)	Cs1vii—F1—Cs1i	154.78 (5)
F1iv—Cs1—F1vii	96.194 (16)	Br2—F2—Cs1i	179.32 (6)
F2v—Cs1—F1vii	61.838 (19)	Br2—F2—Cs1xiii	90.42 (3)
F2iii—Cs1—F1vii	107.50 (2)	Cs1i—F2—Cs1xiii	89.95 (2)
F2iv—Cs1—F1vii	107.50 (2)	Br2—F2—Cs1xi	90.42 (3)
F2vi—Cs1—F1vii	61.838 (19)	Cs1i—F2—Cs1xi	89.95 (2)
F2i—Cs1—F1i	57.95 (3)	Cs1xiii—F2—Cs1xi	112.26 (3)
F2ii—Cs1—F1i	147.27 (3)	Br1—F3—Cs1i	119.56 (2)
F3i—Cs1—F1i	47.83 (3)	Br1—F3—Cs1	119.56 (2)
F3—Cs1—F1i	106.94 (3)	Cs1i—F3—Cs1	120.89 (4)
F1iii—Cs1—F1i	96.194 (16)

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