Proton-stabilized three-dimensional anionic framework in H[Zn₆O₂(BO₃)₃]

A three-dimensional anionic framework built up from [ZnO₄] tetra­hedra and planar [BO₃] groups, stabilized by H atoms, has been found for hydrogen zinc oxide borate, H[Zn₆O₂(BO₃)₃]. Boron and one of the borate O atoms are on 18e (2) positions. Triple units of [ZnO₄] tetra­hedra sharing a common oxygen vertex on a 12c (3) site and strong asymmetrical linear hydrogen bonds with the H atom [on a 12c (3) position] disordered over a twofold axis are specific structural features of this zincoborate. There is evidence that the reported Zn₄O(BO₃)₂ [Harrison, Gier & Stuky (1993). Angew. Chem. Int. Ed. Engl. 32, 724-726] corresponds to this structure.

S1. Comment

Borates show a large structural diversity from isolated anions to three-dimensional frameworks containing [BO₃]³⁻ and/or [BO₄]⁵⁻ moieties. Many of the framework structures are built in combination with mono- or bivalent metals. In the course of investigations on borate frameworks, such as Na₃(NO₃)(B₆O₁₀) (Yakubovich et al., 2002), we became interested in mixed anionic systems containing zinc. Interestingly, among more than 200 known borate minerals no Zn compound is reported, but several synthetic Zn borates have been structurally characterized so far, namely ZnB₄O₇ (Martinez-Ripoll et al. 1971), Zn₃(BO₃)₂ (Baur & Tillmanns 1970), Zn(B₃O₃(OH)₅·H₂O (Ozols et al. 1971), Zn₂(BO₃)[(OH)_0.75F_0.25] (Corbel et al., 2001), Zn(H₂O)(B₂O₄)(H₂O)_0.12 (Choudhury et al. 2002), Zn₄O(BO₃)₂ (Harrison et al. 1993) and Zn₄O(BO₂)₆ (Bondareva et al., 1978; Smith-Verdier & Garcia-Blanco, 1980). The last one is a borate analogue of the microporous aluminosilicate sodalite Na₄Cl(AlSiO₄)₃. Zn[B₃O₄(OH)₃] is the most important commercial Zn borate today, used primarily as a polymer additive and preservative in wood composites with a worldwide annual production exceeding 10 000 metric tons (Schubert et al., 2003).

In the course of investigating the products of soft hydrothermal syntheses in the system ZnO–B₂O₃–NaBr–H₂O, we found crystals with the previously unreported composition H[Zn₆O₂(BO₃)₃].

In the structure (Fig. 1), the Zn²⁺ ions are surrounded by O atoms in tetrahedral coordination at distances from 1.938 (2) to 1.963 (1) Å. Boron forms only triangular oxo-complexes with B—O bonds between 1.372 (4) and 1.378 (2) Å. Each O atom is surrounded by three cations, viz. three Zn atoms for atom O1, and two Zn and one B atoms for atoms O2 and O3. Thus, a three-dimensional framework is formed. The main feature is triple units of [ZnO₄] tetrahedra sharing one common O1 vertex that are connected by an equivalent unit over three corners (O2) and three [BO₃] units to form a cage, in the center of which a disordered H atom is localized (Fig. 2). This H atom was found in a difference Fourier map close to the 6a position (site symmetry 32) and was refined with a free isotropic displacement parameter. It forms a strong asymmetric linear hydrogen bond between two µ³-bridging O1 atoms with an O···O distance of 2.450 (6) Å. The O—H···O axis is on the threefold axis, and the H atom is disordered by operation of the twofold axis over both alternative positions of a double-minimum potential. An electron density map calculated by a difference Fourier synthesis after removing the H atom from the model (Fig. 3) clearly shows a double maximum. The crystal chemical function of the H atoms seems to be the stabilization of the unusual cage unit shown in Fig. 2. Hydrogen bonds of similar strength [O···O = 2.4853 (5) Å] have been reported for γ-metaboric acid (Freyhardt et al. 2000) and are well known from structures of hydrogen sulfates, hydrogen phosphates and hydrogen arsenates. In a recent paper (Schwendtner & Kolitsch 2005), a similar example with split H positions and an O···O distance of 2.508 (4) Å was found for CsCr(H_1.5AsO₄)₂(H₂AsO₄).

The three-dimensional structure can be described as formed by layers of the aforementioned B₆ cages (Fig. 2) interconnected by [BO₃] units to form a porous net (Fig. 4a). Along the c axis, equivalent layers are stacked according to the space-group symmetry in six orientations (`6L structure') and interconnected by sharing common vertices of [ZnO₄] tetrahedra and via [BO₃] units (Fig. 4b).

Our compound has similar unit-cell parameters and the same space-group type as the Zn borate Zn₄O(BO₃)₂ published by Harrison et al. (1993). Its crystal structure was solved from powder data and corresponds roughly to our model, but the authors could achieve a charg- balanced formula for the compound only by assumption of vacancies in the position of one O atom, the occupancy of which was fixed to 0.833 (giving a formula comparable to our setting [Zn₆O_1.5(BO₃)₃]). Probably, these authors had the same compound H[Zn₆O₂(BO₃)₃] in hand but, of course, were unable to detect the H atom by powder methods.

S2. Experimental

Colourless crystals of H[Zn₆O₂(BO₃)₃] with rhombohedral shape up to 1 mm large were formed by hydrothermal synthesis in the system ZnO–B₂O₃–NaBr–H₂O (at a temperature of 553 K and pressure of 70 bar, for a period of 20 days, with a ZnO/B₂O₃/NaBr/H₂O ratio of 2:2:1:30) in a PTFE-lined stainless steel autoclave. The presence of Zn in the samples was confirmed by qualitative X-ray spectral analysis.

S3. Refinement

For the disordered H atom, localized on a 12c position close to 6a, the z parameter and the isotropic displacement parameter were refined without any constraints. No significant correlations were observed for these parameters and the refinement converged well.

Computing details

Data collection: Win-Xpose in X-AREA (Stoe & Cie, 2000); cell refinement: Win-Cell in X-AREA; data reduction: Win-Integrate in X-AREA; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg 2006).

Figures

	Fig. 1. : The main structural elements of the title compound, with the atomic labelling scheme. Displacement ellipsoids are drawn at the 90% probability level; the H atom is shown in one of the alternative positions only and with arbitrary radius. [Symmetry codes: (i) y, −x + y, −z; (ii) −y + 1/3, −x + 2/3, z + 1/6; (iii) x − y, x, −z; (iv) y + 1/3, x − 1/3, 1/6 − z.]
	Fig. 2. : The cage of six [ZnO4] tetrahedra, including the strong hydrogen bond. [Symmetry codes: (i) y + 1/3, x − 1/3, −z + 1/6; (ii) x + 1, y, z; (iii) x − y + 1, x, −z; (iv) −y + 4/3, −x + 2/3, z + 1/6.]
	Fig. 3. : A difference electron density map around the H-atom position. Isolines at 0.1 e Å^−3.
	Fig. 4. : (a) The layer around z = 1/6 of [BO3]-connected `Zn6-cages' in [001] projection. (b) A sequence of layers along the c axis. In the higher layers only the Zn1 and O1 atoms are drawn. Large grey spheres: B; small grey spheres: Zn; large white spheres: O; small white spheres: H; polyhedra: [ZnO4].

zinc hydrogen borate oxide

Crystal data

H[Zn6O2(BO3)3]	Dx = 3.976 Mg m^−3
Mr = 601.66	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3c	Cell parameters from 4205 reflections
a = 8.1558 (5) Å	θ = 2.3–32.0°
c = 26.171 (4) Å	µ = 14.14 mm^−1
V = 1507.6 (3) Å^3	T = 193 K
Z = 6	Rhombohedron, colourless
F(000) = 1704	0.22 × 0.15 × 0.15 mm

Data collection

Stoe IPDS-II diffractometer	541 independent reflections
Radiation source: fine-focus sealed tube	516 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.026
Detector resolution: 6.7 pixels mm^-1	θmax = 31.0°, θmin = 3.3°
ω scans	h = −11→11
Absorption correction: multi-scan (Blessing 1995)	k = −11→11
Tmin = 0.047, Tmax = 0.104	l = −37→31
3364 measured reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.019	All H-atom parameters refined
wR(F^2) = 0.044	w = 1/[σ^2(Fo^2) + (0.020P)^2 + 4.P] where P = (Fo^2 + 2Fc^2)/3
S = 1.25	(Δ/σ)max < 0.001
541 reflections	Δρmax = 0.56 e Å^−3
35 parameters	Δρmin = −0.72 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.00133 (9)

Special details

Experimental. X-ray spectral analysis: CamScan 4DV + EDA Link AN 1000

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	Occ. (<1)
Zn1	0.39230 (3)	0.22346 (3)	0.028616 (9)	0.01285 (11)
O1	0.6667	0.3333	0.03652 (11)	0.0138 (5)
O2	0.3129 (3)	0.3333	0.0833	0.0155 (4)
O3	0.3163 (2)	0.2652 (2)	−0.03807 (6)	0.0158 (3)
B1	0.1447 (4)	0.3333	0.0833	0.0141 (6)
H1	0.6667	0.3333	0.068 (6)	0.06 (5)*	0.50

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.01286 (14)	0.01287 (15)	0.01293 (14)	0.00651 (10)	−0.00054 (8)	−0.00053 (7)
O1	0.0126 (7)	0.0126 (7)	0.0163 (11)	0.0063 (3)	0.000	0.000
O2	0.0143 (7)	0.0184 (10)	0.0150 (9)	0.0092 (5)	−0.0023 (4)	−0.0045 (8)
O3	0.0211 (7)	0.0193 (7)	0.0120 (6)	0.0139 (6)	−0.0007 (5)	−0.0001 (5)
B1	0.0136 (10)	0.0126 (13)	0.0158 (13)	0.0063 (7)	−0.0003 (5)	−0.0006 (10)

Geometric parameters (Å, º)

Zn1—O3	1.9383 (15)	O1—H1	0.82 (16)
Zn1—O3^i	1.9565 (16)	O2—B1	1.372 (4)
Zn1—O1	1.9616 (4)	O2—Zn1^v	1.9630 (13)
Zn1—O2	1.9630 (13)	O3—B1^i	1.378 (2)
Zn1—Zn1^i	3.1576 (4)	O3—Zn1^ii	1.9565 (16)
Zn1—Zn1^ii	3.1576 (4)	B1—O3^vi	1.378 (2)
O1—Zn1^iii	1.9616 (4)	B1—O3^ii	1.378 (2)
O1—Zn1^iv	1.9616 (4)
O3—Zn1—O3^i	106.38 (8)	Zn1^iii—O1—Zn1^iv	118.90 (3)
O3—Zn1—O1	113.99 (9)	Zn1^iii—O1—Zn1	118.90 (3)
O3^i—Zn1—O1	109.88 (5)	Zn1^iv—O1—Zn1	118.90 (3)
O3—Zn1—O2	111.66 (7)	Zn1^iii—O1—H1	96.05 (8)
O3^i—Zn1—O2	108.86 (5)	Zn1^iv—O1—H1	96.05 (9)
O1—Zn1—O2	106.02 (9)	Zn1—O1—H1	96.05 (9)
O3—Zn1—Zn1^i	74.65 (5)	B1—O2—Zn1^v	123.93 (5)
O3^i—Zn1—Zn1^i	35.64 (4)	B1—O2—Zn1	123.93 (5)
O1—Zn1—Zn1^i	109.44 (4)	Zn1^v—O2—Zn1	112.14 (11)
O2—Zn1—Zn1^i	137.08 (3)	B1^i—O3—Zn1	125.50 (15)
O3—Zn1—Zn1^ii	36.03 (5)	B1^i—O3—Zn1^ii	125.21 (14)
O3^i—Zn1—Zn1^ii	117.91 (5)	Zn1—O3—Zn1^ii	108.33 (7)
O1—Zn1—Zn1^ii	128.76 (5)	O2—B1—O3^vi	120.23 (14)
O2—Zn1—Zn1^ii	75.69 (5)	O2—B1—O3^ii	120.23 (14)
Zn1^i—Zn1—Zn1^ii	99.351 (11)	O3^vi—B1—O3^ii	119.5 (3)

Symmetry codes: (i) y, −x+y, −z; (ii) x−y, x, −z; (iii) −y+1, x−y, z; (iv) −x+y+1, −x+1, z; (v) x−y+1/3, −y+2/3, −z+1/6; (vi) −y+1/3, −x+2/3, z+1/6.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O1^vii	0.82 (16)	1.63 (17)	2.450 (6)	180

Symmetry code: (vii) y+1/3, x−1/3, −z+1/6.