Rubidium metaborate, Rb₃B₃O₆

Rubidium metaborate, Rb₃B₃O₆, was obtained by the reaction of Rb₂CO₃ and BN using a radiofrequency furnace at a maximum reaction temperature of 1173 K. The crystal structure has been determined by single-crystal X-ray diffraction. The space group is , with all atoms positioned on a twofold axis (Wyckoff site 18e). The ionic compound is isotypic with Na₃B₃O₆, K₃B₃O₆ and Cs₃B₃O₆.

S1. Comment

A wide variety of alkali borates have been reported. Rb₃B₃O₆ belongs to the series of alkali metaborates M₃B₃O₆ (M = Li, Na, K, Rb and Cs). Surprisingly, a single-crystal structure determination of Rb₃B₃O₆ has not been published as yet. For many years, Schneider (1970) and Hoppe (1994) assumed Rb₃B₃O₆ to be isotypic with the other alkali metaborates, but the structure has never been examined by single-crystal X-ray diffraction. We present here the crystal structure of Rb₃B₃O₆, solved and refined from X-ray diffraction data.

We confirm that Rb₃B₃O₆ is isotypic with Na₃B₃O₆ (Marezio et al., 1963), K₃B₃O₆ (Schneider, 1970) and Cs₃B₃O₆ (Hoppe, 1994).

The lattice parameters increase from Na to Cs, along with the size of the cations. The already published lattice parameters of Rb₃B₃O₆ (Schneider, 1970) are nearly equivalent to those found in this investigation [a = 13.157 (2) Å and b = 7.744 (1) Å]. The characteristic building units are cyclic planar B₃O₆³⁻ anions, which can be described as three corner-sharing BO₃³⁻ groups. The B—O bond length for the terminal atom O1 is 1.315 (6) Å, while the B—O bond to bridging atom O2 [1.407 (3) Å] is significantly longer. The B—O distances of the M₃B₃O₆ (M = Na, K, Rb and Cs) series are compared in Table 1. The B₃O₆³⁻ rings are stacked in a staggered manner along [001], rotated against each other by 60° (Fig. 1 and 2). Each Rb atom is surrounded by seven O atoms, with distances of about 300 pm.

S2. Experimental

Rb₃B₃O₆ was obtained by the high-temperature reaction of Rb₂CO₃ (2.0 mmol) and BN (2.0 mmol) using a radiofrequency (rf) furnace. Details of the experimental setup are given by Schnick et al. (1999). Under an atmosphere of pure argon, the starting compounds were placed in a tungsten crucible, which was positioned at the center of the induction coil of an rf furnace. The reaction was performed under an atmosphere of pure nitrogen (purified by silica gel, potassium hydroxide, molecular sieve, P₄O₁₀ and a BTS catalyst). The reaction batch was heated to 1173 K at a rate of 7.3 K min⁻¹. The temperature was maintained for 2 h, and then the product was cooled at 0.2 K min⁻¹ to 473 K. Subsequently, the mixture was quenched to room temperature. Rb₃B₃O₆ was obtained as a coarse crystalline white solid mixed with RbCN.

Computing details

Data collection: Collect (Nonius, 1997–2000); cell refinement: HKL SCALEPACK (Otwinowski & Minor, 1997); data reduction: HKL DENZO (Otwinowski & Minor, 1997) and SCALEPACK; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: PLATON (Spek, 2003) and WinGX (Farrugia, 2003).

Figures

	Fig. 1. The crystal structure of Rb3B3O6, viewed along the crystallographic c axis, with 99% probability displacement ellipsoids.
	Fig. 2. The Rb atom surrounded by B3O6^3− rings, with 99% probability displacement ellipsoids.

(I)

Crystal data

Rb3B3O6	Dx = 3.303 Mg m^−3
Mr = 384.85	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3c	Cell parameters from 8606 reflections
Hall symbol: -R32"c	θ = 3.1–40.3°
a = 13.1572 (19) Å	µ = 18.87 mm^−1
c = 7.7434 (15) Å	T = 293 K
V = 1160.9 (3) Å^3	Block, colourless
Z = 6	0.38 × 0.19 × 0.17 mm
F(000) = 1044

Data collection

Nonius KappaCCD diffractometer	231 independent reflections
Vertically mounted graphite crystal monochromator	226 reflections with I > 2σ(I)
Detector resolution: 9 pixels mm^-1	Rint = 0.082
ϕ and ω scans	θmax = 25°, θmin = 5.4°
Absorption correction: numerical X-SHAPE (Stoe & Cie, 1999),	h = −15→15
Tmin = 0.056, Tmax = 0.154	k = −15→15
5001 measured reflections	l = −9→9

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0139P)^2 + 8.1199P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.019	(Δ/σ)max < 0.001
wR(F^2) = 0.046	Δρmax = 0.43 e Å^−3
S = 1.22	Δρmin = −0.40 e Å^−3
231 reflections	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
22 parameters	Extinction coefficient: 0.00105 (19)

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.

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

	x	y	z	Uiso*/Ueq
Rb1	0.43824 (3)	0.0000	0.2500	0.0190 (3)
O1	0.2091 (2)	0.0000	0.2500	0.0185 (8)
O2	0.1046 (2)	0.1046 (2)	0.2500	0.0198 (8)
B1	0.1092 (4)	0.0000	0.2500	0.0171 (12)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Rb1	0.0178 (3)	0.0169 (3)	0.0221 (4)	0.00846 (16)	0.00203 (9)	0.00407 (17)
O1	0.0132 (12)	0.0190 (18)	0.0252 (19)	0.0095 (9)	−0.0003 (8)	−0.0005 (15)
O2	0.0104 (12)	0.0104 (12)	0.036 (2)	0.0036 (13)	−0.0013 (7)	0.0013 (7)
B1	0.017 (2)	0.019 (3)	0.015 (3)	0.0095 (14)	0.0004 (10)	0.001 (2)

Geometric parameters (Å, º)

Rb1—O1^i	2.9082 (16)	O1—Rb1^vi	2.9081 (5)
Rb1—O1^ii	2.908 (3)	O1—Rb1^vii	2.9081 (5)
Rb1—O2^iii	2.911 (3)	O1—Rb1^v	2.9970 (11)
Rb1—O2^ii	2.911 (2)	O1—Rb1^iv	2.9971 (11)
Rb1—O1^iv	2.9971 (11)	O2—B1	1.407 (3)
Rb1—O1^v	2.9971 (11)	O2—B1^viii	1.407 (3)
Rb1—O1	3.014 (3)	O2—Rb1^vii	2.911 (2)
Rb1—B1^i	3.245 (2)	O2—Rb1^iii	2.911 (2)
Rb1—B1^ii	3.245 (5)	B1—O2^ix	1.407 (3)
Rb1—Rb1^iv	3.5182 (7)	B1—Rb1^vi	3.245 (2)
Rb1—Rb1^v	3.5183 (7)	B1—Rb1^vii	3.245 (2)
Rb1—B1^v	3.632 (3)	B1—Rb1^v	3.632 (3)
O1—B1	1.315 (6)	B1—Rb1^iv	3.632 (3)
O1^i—Rb1—O1^ii	82.39 (10)	Rb1^iv—Rb1—Rb1^v	107.899 (18)
O1^i—Rb1—O2^iii	48.35 (6)	O1^i—Rb1—B1^v	87.34 (8)
O1^ii—Rb1—O2^iii	126.89 (7)	O1^ii—Rb1—B1^v	90.42 (4)
O1^i—Rb1—O2^ii	126.89 (7)	O2^iii—Rb1—B1^v	71.60 (6)
O1^ii—Rb1—O2^ii	48.35 (6)	O2^ii—Rb1—B1^v	108.26 (6)
O2^iii—Rb1—O2^ii	175.10 (6)	O1^iv—Rb1—B1^v	161.60 (6)
O1^i—Rb1—O1^iv	89.37 (2)	O1^v—Rb1—B1^v	20.10 (9)
O1^ii—Rb1—O1^iv	107.09 (9)	O1—Rb1—B1^v	91.49 (4)
O2^iii—Rb1—O1^iv	92.81 (5)	B1^i—Rb1—B1^v	73.56 (12)
O2^ii—Rb1—O1^iv	88.11 (5)	B1^ii—Rb1—B1^v	104.98 (8)
O1^i—Rb1—O1^v	107.09 (9)	Rb1^iv—Rb1—B1^v	107.32 (2)
O1^ii—Rb1—O1^v	89.37 (2)	Rb1^v—Rb1—B1^v	74.50 (7)
O2^iii—Rb1—O1^v	88.11 (5)	B1—O1—Rb1^vi	92.59 (6)
O2^ii—Rb1—O1^v	92.81 (5)	B1—O1—Rb1^vii	92.59 (6)
O1^iv—Rb1—O1^v	158.34 (7)	Rb1^vi—O1—Rb1^vii	174.82 (12)
O1^i—Rb1—O1	138.81 (5)	B1—O1—Rb1^v	108.36 (6)
O1^ii—Rb1—O1	138.81 (5)	Rb1^vi—O1—Rb1^v	90.63 (2)
O2^iii—Rb1—O1	92.45 (3)	Rb1^vii—O1—Rb1^v	87.74 (2)
O2^ii—Rb1—O1	92.45 (3)	B1—O1—Rb1^iv	108.36 (6)
O1^iv—Rb1—O1	79.17 (3)	Rb1^vi—O1—Rb1^iv	87.74 (2)
O1^v—Rb1—O1	79.17 (3)	Rb1^vii—O1—Rb1^iv	90.63 (2)
O1^i—Rb1—B1^i	23.88 (11)	Rb1^v—O1—Rb1^iv	143.28 (11)
O1^ii—Rb1—B1^i	101.80 (9)	B1—O1—Rb1	180.0
O2^iii—Rb1—B1^i	25.70 (8)	Rb1^vi—O1—Rb1	87.41 (6)
O2^ii—Rb1—B1^i	149.40 (9)	Rb1^vii—O1—Rb1	87.41 (6)
O1^iv—Rb1—B1^i	96.76 (4)	Rb1^v—O1—Rb1	71.64 (6)
O1^v—Rb1—B1^i	93.41 (8)	Rb1^iv—O1—Rb1	71.64 (6)
O1—Rb1—B1^i	118.14 (8)	B1—O2—B1^viii	124.3 (5)
O1^i—Rb1—B1^ii	101.80 (9)	B1—O2—Rb1^vii	90.54 (19)
O1^ii—Rb1—B1^ii	23.88 (11)	B1^viii—O2—Rb1^vii	137.35 (18)
O2^iii—Rb1—B1^ii	149.40 (9)	B1—O2—Rb1^iii	137.35 (18)
O2^ii—Rb1—B1^ii	25.70 (8)	B1^viii—O2—Rb1^iii	90.54 (19)
O1^iv—Rb1—B1^ii	93.41 (8)	Rb1^vii—O2—Rb1^iii	74.36 (8)
O1^v—Rb1—B1^ii	96.77 (4)	O1—B1—O2^ix	122.1 (2)
O1—Rb1—B1^ii	118.15 (8)	O1—B1—O2	122.1 (2)
B1^i—Rb1—B1^ii	123.71 (16)	O2^ix—B1—O2	115.7 (5)
O1^i—Rb1—Rb1^iv	87.24 (4)	O1—B1—Rb1^vi	63.53 (9)
O1^ii—Rb1—Rb1^iv	159.027 (14)	O2^ix—B1—Rb1^vi	63.76 (14)
O2^iii—Rb1—Rb1^iv	52.82 (4)	O2—B1—Rb1^vi	156.38 (6)
O2^ii—Rb1—Rb1^iv	130.890 (9)	O1—B1—Rb1^vii	63.53 (9)
O1^iv—Rb1—Rb1^iv	54.41 (6)	O2^ix—B1—Rb1^vii	156.38 (6)
O1^v—Rb1—Rb1^iv	111.15 (2)	O2—B1—Rb1^vii	63.76 (14)
O1—Rb1—Rb1^iv	53.950 (9)	Rb1^vi—B1—Rb1^vii	127.06 (18)
B1^i—Rb1—Rb1^iv	73.65 (5)	O1—B1—Rb1^v	51.55 (7)
B1^ii—Rb1—Rb1^iv	146.80 (7)	O2^ix—B1—Rb1^v	92.98 (12)
O1^i—Rb1—Rb1^v	159.028 (14)	O2—B1—Rb1^v	127.55 (9)
O1^ii—Rb1—Rb1^v	87.25 (4)	Rb1^vi—B1—Rb1^v	75.02 (8)
O2^iii—Rb1—Rb1^v	130.888 (8)	Rb1^vii—B1—Rb1^v	72.79 (7)
O2^ii—Rb1—Rb1^v	52.82 (4)	O1—B1—Rb1^iv	51.54 (7)
O1^iv—Rb1—Rb1^v	111.15 (2)	O2^ix—B1—Rb1^iv	127.55 (9)
O1^v—Rb1—Rb1^v	54.41 (6)	O2—B1—Rb1^iv	92.98 (12)
O1—Rb1—Rb1^v	53.949 (9)	Rb1^vi—B1—Rb1^iv	72.79 (7)
B1^i—Rb1—Rb1^v	146.80 (7)	Rb1^vii—B1—Rb1^iv	75.02 (8)
B1^ii—Rb1—Rb1^v	73.65 (5)	Rb1^v—B1—Rb1^iv	103.09 (14)

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