Strontium tetra­fluoridoborate and barium tetra­fluoridoborate

In Sr(BF₄)₂, which is isomorphous with the previously published Ca(BF₄)₂, the metal atom possesses a coordination number of 8 with a square-anti­prismatic environment. Each tetra­fluorido­borate anion is bonded to four metal centers. In the barium derivative, the metal center, with symmetry 2/m, is surrounded by 14 F atoms. The B atom and two of the three independent F atoms occupy special positions with symmetry m. Each anion is connected to five Ba atoms. This structure differs significantly from an earlier published structure of Ba(BF₄)₂ [published as Ba₂(BF₄)₄; Lin, Cheng, Chen & Huang (1998). Jiegon Huaxue, 17, 245]. The radial distribution functions for the present Ba(BF₄)₂ and earlier Ba₂(BF₄)₄ structures differ significantly.

S1. Comment

Sr(BF₄)₂ appears to be isomorphous with the previously reported compound Ca(BF₄)₂ (Jordan et al., 1975). In this structure, the metal atom possesses coordination number 8 with a square-antiprismatic coordination polyhedron. The Sr—F distances lie in the narrow range of 2.294 (5)–2.359 (4) Å compared with 2.33–2.401 Å in Ca(BF₄)₂. The metal center is bonded to eight BF₄^- units. In turn, each anion is connected to four Sr atoms. All four F atoms in each anion act as µ₂ bridges between B and Sr atoms, resulting in very similar B—F bond lengths of 1.389 (9)–1.399 (9) Å [please check range of values].

Because of the much greater coordination ability of barium compared with strontium, the structure of Ba(BF₄)₂ appears to be more complicated. The Ba atom, located on special position 2a (2/m), possesses coordination number 14 (or 10 + 4). Ten Ba—F bond lengths lie in the range 2.690 (5)–2.886 (2) Å, and there are four longer Ba—F distances of 3.361 (5) Å. Each Ba atom is surrounded by ten BF₄^- anions. The coordination polyhedron of barium shares two edges and two rectangular planes with four other metal centers. The shortest Ba···Ba distances are 4.763 (2) Å for two Ba atoms with a shared plane and 5.014 (1) Å for a pair of metal atoms connected by a shared edge (Fig. 1).

The B atom and two of the three crystallographically independent F atoms of each BF₄^- unit lie on mirror planes (4i Wyckoff positions). Each tetrafluoridoborate anion is bonded to five Ba atoms. The B—F distances clearly correlate with the bridging function of each F atom. The shortest B—F distance [B1—F2 = 1.369 (9) Å] is associated with the µ₂ bridging function of atom F2. Two B1—F3 bond lengths of 1.393 (5) Å correspond to the µ₃ bridging function of atom F3, which forms a shared corner between two Ba atoms. The longest B—F distance [B1—F1 = 1.420 (9) Å] corresponds to the location of atom F1 on an edge shared by two barium polyhedra. The bridging functions of both cations and anions result in the formation of a three-dimensional network (Fig. 2).

The results obtained for Ba(BF₄)₂ differ strongly from those from a previously published structure of Ba(BF₄)₂ [published as Ba₂(BF₄)₄ (Lin et al., 1998); space group P2₁/n, a = 8.339 (3) Å, b = 16.530 (7) Å, c = 10.212 (4) Å, β = 106.64 (3)°, V = 1349 (2) ų], in which the Ba atom possesses the unusually low coordination number 9 and the Ba—F distances are in the range 2.69 (2)–2.91 (3) Å. The radial distribution functions for the Ba(BF₄)₂ and Ba₂(BF₄)₄ structures are noticeably different.

Related literature

For related literature, see: Jordan et al. (1975); Lin et al. (1998).

S2. Experimental

Sr(BF₄)₂ was prepared by the reaction between SrF₂ (Alfa Aesar, 99.99%) and excess BF₃ (Union Carbide, 99.5%) in anhydrous HF (Fluka, purum) as solvent. When the reaction was complete, the solvent and excess BF₃ were removed on a vacuum line. Ba(BF₄)₂ was prepared in a similar way starting from BaF₂ (Alfa Aesar, 99.99%). The bulk samples were characterized by Raman spectroscopy [793, 533, 353 cm^-1 for Sr(BF₄)₂; 783, 536, 355 cm^-1 for Ba(BF₄)₂] and X-ray powder diffraction. Samples for diffraction analysis were grown by crystallization of an M(BF₄)₂ (M = Sr and Ba) solution in anhydrous HF with a temperature gradient of 10 ° in an FEP crystallization vessel. Crystals were immersed in perfluoronated oil (ABCR, FO5960) in a dry box, selected under the microscope and transferred into the cold nitrogen stream of the diffractometer.

S3. Refinement

The highest peaks of residual density are 0.02 Å from atom Sr1 and 0.89 Å from atom Ba1.

Computing details

For both compounds, data collection: CrystalClear (Rigaku Corporation, 1999); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SIR92 (Altomare et al., 1993) and TEXSAN (Molecular Structure Corporation, 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: Diamond (Pennington, 1999); software used to prepare material for publication: WinGX (Version 1.70; Farrugia, 1999) and enCIFer (Version 1.2; Allen et al., 2004).

Figures

	Fig. 1. The coordination surrounding the Ba atom (left) and the BF4^- anion (right) in the structure of Ba(BF4)2. Large grey spheres represent Ba atoms, the tetrahedra are BF4^- units, and B atoms are shown as small dark spheres.
	Fig. 2. The packing of the structure of Ba(BF4)2.

(I) Barium tetrafluoridoborate

Crystal data

Ba(BF4)2	F(000) = 276
Mr = 310.96	Dx = 3.724 Mg m^−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -C 2y	Cell parameters from 51 reflections
a = 12.513 (9) Å	θ = 3.5–28.7°
b = 5.0135 (13) Å	µ = 7.27 mm^−1
c = 4.7628 (15) Å	T = 200 K
β = 111.854 (4)°	Chunk, colourless
V = 277.3 (2) Å^3	0.1 × 0.08 × 0.06 mm
Z = 2

Data collection

Rigaku Mercury CCD diffractometer	342 reflections with I > 2σ(I)
ω scans	Rint = 0.049
Absorption correction: multi-scan (Blessing, 1995)	θmax = 28.6°, θmin = 3.5°
Tmin = 0.471, Tmax = 0.650	h = −15→6
596 measured reflections	k = −6→6
342 independent reflections	l = −6→6

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.029	Secondary atom site location: difference Fourier map
wR(F^2) = 0.070	w = 1/[σ^2(Fo^2) + (0.0297P)^2 + 0.5815P] where P = (Fo^2 + 2Fc^2)/3
S = 1.19	(Δ/σ)max < 0.001
342 reflections	Δρmax = 1.14 e Å^−3
32 parameters	Δρmin = −1.11 e Å^−3

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

	x	y	z	Uiso*/Ueq
Ba1	0.0000	0.0000	1.0000	0.0151 (2)
F1	0.1010 (4)	0.5000	0.9273 (9)	0.0152 (9)
F2	0.2704 (3)	0.5000	0.8507 (10)	0.0156 (9)
F3	0.1154 (3)	0.2718 (7)	0.5323 (7)	0.0206 (7)
B1	0.1531 (7)	0.5000	0.7084 (17)	0.0118 (15)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ba1	0.0149 (3)	0.0145 (4)	0.0135 (3)	0.000	0.0026 (2)	0.000
F1	0.0160 (19)	0.017 (2)	0.014 (2)	0.000	0.0076 (16)	0.000
F2	0.0106 (19)	0.017 (2)	0.018 (2)	0.000	0.0038 (16)	0.000
F3	0.0223 (15)	0.0168 (16)	0.0189 (15)	0.0011 (13)	0.0034 (12)	−0.0093 (13)
B1	0.016 (4)	0.009 (4)	0.008 (3)	0.000	0.003 (3)	0.000

Geometric parameters (Å, º)

Ba1—F2^i	2.690 (5)	F1—B1	1.420 (9)
Ba1—F3^ii	2.765 (3)	F2—B1	1.369 (9)
Ba1—F1	2.886 (2)	F3—B1	1.393 (5)
Ba1—Ba1^iii	4.7628 (15)
F2^i—Ba1—F3^ii	112.60 (10)	F3^vii—Ba1—F1	115.82 (11)
F2^iv—Ba1—F3^ii	67.40 (10)	F1—Ba1—F1^viii	120.60 (15)
F3^ii—Ba1—F3^v	59.07 (15)	F1^ix—Ba1—F1^viii	59.40 (15)
F3^vi—Ba1—F3^v	120.93 (15)	Ba1—F1—Ba1^x	120.60 (15)
F2^i—Ba1—F1	63.85 (8)	F2—B1—F3	111.0 (4)
F2^iv—Ba1—F1	116.15 (8)	F3—B1—F3^xi	110.4 (6)
F3^ii—Ba1—F1	114.89 (11)	F2—B1—F1	109.7 (6)
F3^vi—Ba1—F1	65.11 (11)	F3—B1—F1	107.3 (4)
F3^v—Ba1—F1	64.18 (11)

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

(II) Strontium tetrafluoridoborate

Crystal data

Sr(BF4)2	F(000) = 960
Mr = 261.24	Dx = 3.313 Mg m^−3
Orthorhombic, Pbca	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -P 2ac 2ab	Cell parameters from 77 reflections
a = 9.121 (7) Å	θ = 1.6–28.7°
b = 8.714 (6) Å	µ = 10.41 mm^−1
c = 13.180 (9) Å	T = 200 K
V = 1047.6 (13) Å^3	Chunk, colourless
Z = 8	0.07 × 0.07 × 0.05 mm

Data collection

Rigaku Mercury CCD (2 × 2 bin mode) diffractometer	1253 independent reflections
Radiation source: fine-focus sealed tube	1132 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.016
Detector resolution: 14.7059 pixels mm^-1	θmax = 28.7°, θmin = 3.1°
dtprofit.ref scans	h = −10→12
Absorption correction: multi-scan (Blessing; 1995)	k = −11→10
Tmin = 0.486, Tmax = 0.595	l = −17→14
4207 measured reflections

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.046	Secondary atom site location: difference Fourier map
wR(F^2) = 0.153	w = 1/[σ^2(Fo^2) + (0.0951P)^2 + 10.4343P] where P = (Fo^2 + 2Fc^2)/3
S = 1.09	(Δ/σ)max = 0.001
1253 reflections	Δρmax = 2.11 e Å^−3
100 parameters	Δρmin = −1.48 e Å^−3

Special details

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

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq
Sr1	1.03288 (5)	0.78148 (6)	0.60416 (3)	0.0087 (2)
F11	0.8753 (5)	0.5772 (5)	0.6205 (3)	0.0288 (9)
F12	0.6983 (5)	0.3937 (5)	0.6186 (3)	0.0319 (10)
F13	0.6772 (5)	0.5992 (5)	0.5156 (3)	0.0345 (10)
F14	0.8550 (5)	0.4215 (5)	0.4832 (3)	0.0322 (10)
F21	0.9248 (6)	0.7683 (5)	0.4418 (3)	0.0303 (9)
F22	0.7504 (5)	0.7765 (5)	0.3172 (3)	0.0324 (10)
F23	0.9134 (6)	0.9745 (5)	0.3345 (3)	0.0364 (10)
F24	0.9899 (5)	0.7414 (6)	0.2754 (4)	0.0303 (10)
B1	0.7770 (8)	0.4970 (9)	0.5596 (6)	0.0241 (14)
B2	0.8943 (9)	0.8161 (10)	0.3424 (6)	0.0241 (14)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sr1	0.0082 (3)	0.0086 (4)	0.0092 (3)	−0.00038 (15)	0.00023 (14)	0.00025 (15)
F11	0.029 (2)	0.029 (2)	0.0282 (19)	−0.0052 (18)	−0.0043 (17)	0.0008 (16)
F12	0.029 (2)	0.034 (2)	0.033 (2)	−0.0095 (19)	−0.0014 (17)	0.0058 (18)
F13	0.035 (2)	0.035 (2)	0.034 (2)	0.0085 (19)	−0.0038 (18)	0.0020 (18)
F14	0.029 (2)	0.031 (2)	0.037 (2)	0.0013 (18)	0.0027 (18)	−0.0075 (18)
F21	0.032 (2)	0.039 (3)	0.021 (2)	−0.0011 (19)	0.0001 (19)	0.0017 (17)
F22	0.0226 (19)	0.047 (3)	0.027 (2)	−0.0050 (18)	0.0010 (18)	−0.0036 (18)
F23	0.047 (3)	0.029 (2)	0.033 (2)	−0.004 (2)	−0.001 (2)	−0.0035 (18)
F24	0.029 (2)	0.037 (2)	0.025 (2)	0.004 (2)	0.0037 (19)	−0.0014 (19)
B1	0.021 (3)	0.026 (3)	0.025 (4)	−0.001 (3)	−0.004 (3)	0.002 (3)
B2	0.024 (3)	0.028 (4)	0.020 (3)	0.001 (3)	−0.003 (3)	−0.006 (3)

Geometric parameters (Å, º)

Sr1—F22^i	2.294 (5)	F11—B1	1.392 (9)
Sr1—F11	2.298 (5)	F12—B1	1.389 (9)
Sr1—F24^ii	2.299 (5)	F13—B1	1.399 (8)
Sr1—F13^i	2.303 (4)	F14—B1	1.397 (9)
Sr1—F23^iii	2.327 (5)	F21—B2	1.402 (9)
Sr1—F12^iv	2.333 (5)	F22—B2	1.398 (9)
Sr1—F14^v	2.345 (4)	F23—B2	1.395 (10)
Sr1—F21	2.359 (5)	F24—B2	1.401 (9)
Sr1—Sr1^iii	4.733 (2)
F22^i—Sr1—F11	109.12 (17)	F13^i—Sr1—F21	68.86 (17)
F22^i—Sr1—F24^ii	71.64 (17)	F23^iii—Sr1—F21	116.60 (16)
F11—Sr1—F24^ii	74.58 (16)	F12^iv—Sr1—F21	73.53 (16)
F22^i—Sr1—F13^i	85.11 (17)	F14^v—Sr1—F21	72.55 (16)
F11—Sr1—F13^i	140.48 (16)	B1—F11—Sr1	137.5 (4)
F24^ii—Sr1—F13^i	144.01 (17)	B1—F12—Sr1^vi	133.8 (4)
F22^i—Sr1—F23^iii	82.09 (17)	B1—F13—Sr1^vii	160.6 (4)
F11—Sr1—F23^iii	143.80 (17)	B1—F14—Sr1^v	158.6 (4)
F24^ii—Sr1—F23^iii	76.93 (17)	B2—F21—Sr1	156.3 (5)
F13^i—Sr1—F23^iii	72.86 (17)	B2—F22—Sr1^vii	139.4 (4)
F22^i—Sr1—F12^iv	146.84 (15)	B2—F23—Sr1^iii	154.7 (4)
F11—Sr1—F12^iv	75.63 (18)	B2—F24—Sr1^viii	139.9 (4)
F24^ii—Sr1—F12^iv	78.57 (16)	F12—B1—F11	109.5 (6)
F13^i—Sr1—F12^iv	112.55 (17)	F12—B1—F14	111.2 (6)
F23^iii—Sr1—F12^iv	77.22 (18)	F11—B1—F14	108.9 (6)
F22^i—Sr1—F14^v	71.26 (16)	F12—B1—F13	108.0 (6)
F11—Sr1—F14^v	74.61 (16)	F11—B1—F13	109.8 (6)
F24^ii—Sr1—F14^v	119.42 (18)	F14—B1—F13	109.4 (6)
F13^i—Sr1—F14^v	75.79 (16)	F23—B2—F22	110.1 (6)
F23^iii—Sr1—F14^v	140.16 (17)	F23—B2—F24	109.6 (7)
F12^iv—Sr1—F14^v	138.64 (16)	F22—B2—F24	108.7 (6)
F22^i—Sr1—F21	139.50 (17)	F23—B2—F21	109.8 (6)
F11—Sr1—F21	77.64 (16)	F22—B2—F21	109.6 (6)
F24^ii—Sr1—F21	144.57 (17)	F24—B2—F21	109.1 (6)

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